12.3.1 Color-center model

The color-center model was firstly proposed by Hand and Russell (1990), and supported by several others reporting similar results for the case of exposed germanosilicate (Atkins et al., 1993; Dong et al., 1995; Williams et al., 1992) and other types of glasses (Roman and Winick, 1993). In short, Hand, Russell and co-workers proposed that optical absorption changes related to the Ge oxygen deficiency center defects, peaking at the 240 nm spectral band (Yuen, 1982), and translated to GeE’ centers by the laser irradiation (Nishii et al., 1995), are translated into refractive index changes by employing the Kramers–Kronig parity transformation (see Eq. [12.1]).

[12.1]

In this hypothesis ion displacements were disregarded, letting only electronic rearrangements be accounted (Othonos and Kalli, 1999). Results related to the color-center model had been also reported for fused silica exposed to 193 nm excimer laser radiation (Rothschild et al., 1989), where a part of the refractive index changes was attributed to SiE’ center generation peaking at 215 nm (Hosono et al., 1996).

Other most recent examples related to the photosensitivity of silver doped and undoped phosphate glasses (Pissadakis and Michelakaki, 2008; Pissadakis et al., 2004) showed that the color-center model can be used for obtaining a reliable estimation of the minimum refractive index changes inscribed inside an optical matrix using laser radiation of the same wavelength but of different pulse duration. In short, the color-center model succeeded in describing partially the photosensitivity of Ge-doped silicate glasses (including hydrogenated species) where strong absorption precursors prompt efficient color-center transformations, predicting index changes of few parts of 10⁻⁴ (Dong et al., 1995; Tsai et al., 1997). However, the photosensitivity of low-defect concentration optical matrices such as fused silica (Albert et al., 1999), or refractive index changes induced into glasses for prolonged exposures after the color-center annihilation or transformation has been saturated, cannot be described accurately by this model.

12.3.2 The volume modification model

The volume modification model is one of the most fundamental hypotheses in glass photosensitivity, formed since the early years of investigations (Primak, 1972; Primak and Kampwirth, 1968). Correlated refractive index (n) and volume (V) changes are well described by the Lorentz–Lorentz equation (see Eq. [12.2]), accounting the molar refractivity R = 4/3πNα (N number of molecules, α polarizability) of the exposed glass matrix (Born and Wolf, 1999).

[12.2]

Both for the cases of exposed fused silica (Borrelli et al., 1997; Fiori and Devine, 1986; Rothschild et al., 1989) and germanosilicate glasses (Borrelli et al., 1999; Cordier, 1997) volume modification effects have been observed mostly in the form of compaction/densification (Douay et al., 1997). In that model, the silicon or germanium-oxygen deficiency bonds forming higher order defect rings and voids are dissociated by the laser radiation to lower order structures, but also to modifications of these bonds’ intermediate oxygen angle, leading to microscopic void annihilation and macroscopic volume compaction (Piao et al., 2000). Refractive index changes Δn induced in silicate glasses are well described by a universal power law rule of the form:

[12.3]

where F is the energy density, N the number of pulses, τ the pulse duration and b the power index laying between 0.5 and 0.7 (Borrelli et al., 1999).

Such densification effects in silica glass have also been investigated using 157 nm excimer (Smith and Borrelli, 2006), 213 nm, quintupled Nd:YAG (Schenker, 1994) laser and 800 nm femtosecond (Bellouard et al., 2006) laser radiation. Opposite sign, thus volume dilation effects have been measured in the case of hydrogenated silica glass (Smith et al., 2001), as well as for the case of phosphate (Michelakaki and Pissadakis, 2009; Yliniemi et al., 2006b) and fluoride (Sramek et al., 2000) glasses under 193 nm irradiation. Especially, for the case of phosphate glass the role of the P–O bond and other ion modifiers existing in the glass dominate the progression of negative index changes, in direct dependence on the accumulated energy density doses delivered into the glass. The volume modification model has been successfully used for describing refractive index changes induced in silicate glasses under irradiation using deep ultraviolet lasers (Albert et al., 1999; Borrelli et al., 1999; Pissadakis and Konstantaki, 2005b); and partially for the refractive index changes induced by infrared femtosecond irradiations (Streltsov and Borrelli, 2002).

12.3.3 The stress-relief model

The third model, that of stress-relief, was proposed by Limberger and Fonjallaz (Fonjallaz et al., 1995) for describing the refractive index changes induced in high Ge-doped silicate glass fibers exposed using pulsed 240 nm laser radiation. In the stress-relief model, the compaction induced by the irradiation in Type I gratings generates axial and radial stresses in the bright fringes of interference; which contribute negative refractive index changes in the overall photosensitivity through the photoelastic effect (Frocht). Stress birefringence, or photoelasticity, refractive index changes Δn in isotropic materials are described by the equation:

[12.4]

where n_x is the refractive index of the unstressed material, q₁₁ and q₁₂ are stress optical coefficients and P_xx, P_yy arestress components (Born and Wolf, 1999). Under this scheme the refractive index changes associated with compaction are considered as inelastic, while those associated with secondary stress-relief are considered elastic.

Moreover, for B/Ge-codoped fibers where compressive or tensile, radial and axial stresses are formed due to the large difference between the core–cladding compositions and thus thermal expansion coefficients (Kim et al., 2000; Raine et al., 1999), exposure to ultraviolet radiation can relieve these stresses, contributing with according sign to the refractive index changes. Such hypothesis as the above, employing stress relaxation and high compaction, has already been used to better understand the Type IIA (see Section 12.4.3) Bragg grating photosensitivity, in B/Ge-codoped fibers (Ky et al., 2003); but also stress reduction in inscriptions in hydrogenated fibers (Ky et al., 1999; Limberger et al., 2007). The stress-relief model is volume modification driven and can be considered as a secondary mechanism following extensive exposed glass volume changes of the exposed glass.

12.3.4 The phase-changes model

Exposures performed under extreme intensities and energy densities can induce phase changes inside the exposed glass, and the refractive index changes formed cannot be described accurately by any of the models described above or their combination. Such phase or damage changes may be of the form of filamentation (Watanabe et al., 2003), void formation (Kazansky, 2007; Taylor et al., 2008), extreme compaction (Mihailov et al., 2003), crystallization/amorphization (Fisette, 2006) and ion-migration (Pissadakis et al., 2004), generated by the combination of rapid transformation effects such as shock waves, melting and resolidification and crack-propagation. Photoluminescence (Dianov et al., 1996; Nishikawa et al., 1992) and micro-Raman spectroscopy (Chan et al., 2001; Dianov et al., 1997b; Fletcher et al., 2009) are the most favorable tools for investigating such abrupt material changes, together with topological investigation techniques such as scanning electron microscopy, atomic force microscopy, or micro-/nano-indentation (Aashia et al., 2009; Bellouard et al., 2006).

12.4.1 Type I photosensitivity

Type I photosensitive gratings are the most common category, being fabricated in low and medium concentration Ge concentration fibers under modest exposure intensities, using either CW or pulsed (Albert et al., 1995) laser sources. Their refractive index changes evolution during recording and follows a monotonic power law trend, while usually reaching saturation after the defect states have been exhausted for the single-photon absorption process; multiphoton absorption inscriptions in Ge-doped fiber can also be of Type I under controlled intensities for avoiding volume damage effects. Hydrogenated germanosilicate (Lemaire et al., 1993) fibers under low accumulated energy dose exposures also follow a Type I photosensitivity evolution. Pristine and hydrogen loaded, all-silica fibers, under 193 nm excimer laser exposure or ultraviolet and infrared femtosecond laser irradiation can also exhibit Type I photosensitivity changes (Albert et al., 2002; Smelser et al., 2005; Zagorulko et al., 2004). Type I refractive index changes are usually associated with a single underlying photosensitivity mechanism (Othonos and Kalli, 1999), or with co-occurring photosensitivity processes that contribute to the overall refractive index changes under the same sign.

12.4.2 Type II photosensitivity

Volume damage gratings induced under extreme exposure conditions by means of high energy densities (i.e. ~ 1 J/cm² at 248 nm) (Archambault et al., 1993) or intensities (several TW/cm² at 800 nm) (Smelser et al., 2005) are classified as Type II. In such Type II photosensitivity gratings refractive index engineering is associated with phase changes induced (Archambault, 1994) as a result of the rapid heating and resolidificaton of the exposed material, which in turn induce thermal fictive effects in the exposed glass. Type II gratings were initially demonstrated as ‘single-pulse’ gratings (Archambault et al., 1993); however, similar damage modifications can be obtained under multi pulse femtosecond laser exposures (Mihailov et al., 2003). Since such phase changes are formed above the T_g or even the melting point of the exposed glass, their thermal stability is massively increased compared to the standard Type I photosensitive gratings. Type II photosensitivity has also been observed in exposures of silica blanks (Zhang et al., 2006) using sub-MHz repetition laser sources, due to heat accumulation effects.

12.4.3 Type IIA photosensitivity

The irradiation through a phase mask of non-hydrogenated, high Ge-concentration and B/Ge-co-doped optical fibers at energy densities below the phase changes threshold revealed the complex nature Type IIA photosensitivity. First indications of the behavior of such photosensitivity were presented by Xie et al. (1993); however, a clearer demonstration of Type IIA behavior was reported by Dong et al. (1996) when a B/Ge co-doped fiber was exposed to 193 nm excimer laser radiation. In Type IIA photosensitivity both the average and modulated refractive index changes probed by the Bragg reflector inscribed follow a non-monotonic trend. Initially, the modulated refractive index changes increase similar to Type I photosensitivity, while after reaching a short plateau point, follow a declining trend leading to a turning point when the Bragg grating strength minimizes or even vanishes. Upon energy dose provided to the system the grating strength grows again, reaching a final plateau of saturation. The average refractive index changes induced into the fiber red-shift in wavelength up to the turning point of the Bragg grating strength, and then become either stable or negative/blue-shifted (Dong et al., 1996; Riant and Haller, 1997). In the broader family of Type IIA photosensitivity are included most of the complex refractive index changes induced in fiber and planar waveguide samples, where underlying photosensitivity processes are manifold and negative component refractive index changes emerge (Canning et al., 1998; Wiesmann et al., 1999). Type IIA photosensitivity inscriptions in Ge-doped fibers withstand much higher temperatures than standard Type I counterparts, exhibiting minor decay up to 800°C in specific cases (Groothoff and Canning, 2004).

12.4.4 Type IA photosensitivity

The most recent kind of distinct photosensitivity behavior is that of Type IA, observed in hydrogenated germanosilicate fibers under prolonged exposures using CW or pulsed laser sources, however, applying intensities which can form Type IIA gratings in the same pristine fibers (Liu et al., 2002; Simpson et al., 2004). These Type IA gratings undergo significant red-shifts of the Bragg wavelength, mostly greater than 10 nm for the whole duration of the exposure. Such massive wavelength shifts correspond to average refractive index changes of the order of few parts of ×10⁻², substantially modifying the guiding properties of the exposed fiber, by means of higher mode cut-off and fundamental mode confinement. The modulated refractive index behavior of Type IA gratings resembles that of Type IIA photosensitivity, where after a rapid increase the grating strength decreases to a minimum turning point, followed by a slower increase toward stabilization (Kalli et al., 2006).

12.5.1 Ultraviolet nanosecond and picosecond laser inscriptions

A significant demonstration of the straightforward inscription of Bragg reflectors into low-photosensitivity fibers was presented by Albert et al. (1995), after exploiting two-photon absorption effects and laser radiation of high photon energy (Fig. 12.1a). In that work, 193 nm excimer laser radiation (6.42 eV photon) was used to inscribe refractive index changes of ~ 10⁻³ in a hydrogen unsensitized SMF-28 standard telecom fiber, while the same irradiation produced less than half the index changes in a high Ge-doped fiber. Such refractive index changes were enough to produce Type I short and strong Bragg reflectors, in standard telecom fibers, while simultaneously avoiding the obstacles of hydrogenation. In a similar manner, Herman et al. exploited the much shorter photon wavelength of 157 nm (7.9 eV) for inscribing long-period gratings in pristine SMF-28 fibers (Chen et al., 2001), providing energy to the system above the germanosilicate core glass bandgap, while allowing single-photon ionization processing (Dyer et al., 2001) and refractive index changes of the order of ~ 4 × 10⁻⁴. Compaction was the primary underlying physical mechanism in the 157 nm exposures, while cladding absorption defined a rather narrow envelope for optimum inscription conditions. The above two deep ultraviolet wavelengths were also used for locking photosensitivity in hydrogen loaded fibers (Chen et al., 2002b), as well as for Bragg grating amplification (Dyer et al., 1994). Later, Dyer et al. (2008) used 157 nm nanosecond laser radiation and a custom-made CaF₂ phase mask for recording Bragg reflectors in low-defect SMF-28 and Hi-980 Corning fibers (Fig. 12.1b), leading to refractive index changes of ~ 2.8 × 10⁻⁴, extending the work of Herman and Chen (Chen et al., 2001).

In 2005 there was presented the first inscription of Bragg reflectors in a germanosilicate fiber using 213 nm, 150 ps frequency quintupled Nd:YAG laser radiation, targeting the low absorption spectral valley formed between the Ge oxygen deficiency centers peaking at 5 eV, and the GeE’ centers absorption band tail initiated at this wavelength regime (Pissadakis and Konstantaki, 2005b). Spectro-photometric measurements of 9% mole Ge-doped fiber performs presented by Archambault (1994) reveal that the absorption at the valley of the 213 nm wavelength is more than 10× lower that that measured at the 242 nm germanium-oxygen deficiency band, heavily exploited in grating recording using 248 nm excimer lasers.

Average index changes Δn ≈ 8.5 × 10⁻⁴ were obtained after 2 hours’ exposure of the Nufern GF1B fiber with a cumulative energy density of 2.9 kJ/cm² (Fig. 12.2a and 12.2b), while employing an elliptical Talbot interferometer (Pissadakis and Reekie, 2005). The Talbot interferometer configuration imposed adequate separation between the same mask and the fiber, allowing high energy density delivery in the fiber complex, without significant absorption in the phase mask due to the SiE’ centers formation located at the 5.8 eV band (Skuja, 1998). The refractive index growth data followed the universal power law trend applied in the case of compaction in silicate glasses, exhibiting a b factor of ~ 0.66 (Borrelli et al., 1999; Schenker, 1994), described by Eq. [12.3]. The GW/cm² magnitude intensities used in the inscription (Pissadakis and Konstantaki, 2005b) in conjunction with the high two-photon absorption coefficient of the germanosilicate core led to a nonlinear absorption coefficient similar or even higher than the linear absorption at this wavelength (Kalachev et al., 2005), promoting two-photon effects. Exposures performed using different energy densities in the Nufern GF1B fiber support further the above assertion, by leading to refractive index changes such that their ratio is proportional to the second power of the corresponding ratio of the energy densities per pulse (see Fig. 12.3).

Using 213 nm wavelength and 150 ps pulse duration the recording of Type IIA Bragg reflectors in the commercial PS1250/1500 Fibercore B/Ge co-doped fiber was also presented, leading to refractive index changes of 9.0 × 10⁻⁴ (Pissadakis and Konstantaki, 2005a). Again, the refractive index changes data referring to the Type IIA evolution regime were well fitted to a power law growth (see Eq. [12.3]), with b factor 0.65 (Fig. 12.4).

Both the results obtained for the Nufern GF1B and the PS1250/1500 Fibercore optical fibers during Bragg grating inscriptions using 213 nm, 150 ps Nd:YAG radiation, provide evidence that the primary underlying physical mechanism was that of volume compaction. The first surprising finding that supports the compaction model emerges from the power law fitting of the refractive index data for both fibers used and exposed using the 213 nm, 150 ps. Both fibers exhibit almost identical b-factor value after fitting their photosensitivity data using Eq. [12.3]; similar b-values have been also reported by Borrelli (Borrelli et al., 1999) and Schenker (1994). Another experimental finding in support of the compaction model for the 213 nm, 150 ps irradiation is obtained by producing Bragg reflectors using 248 nm, nanosecond excimer laser radiation, utilizing extreme energy densities (~ 1 J/cm² per pulse). By comparing the data for the 213 and 248 nm exposures under the dose conditions of Fig. 12.5, one can see that initially the 248 nm exposure exhausts 242 nm peaking germanium-oxygen deficiency centers and their corresponding refractive index contribution; then follows an identical growth trend with the 213 nm exposure. Since the 213 nm, 150 ps radiation does not primarily rely on the exhausting of color centers for inducing refractive index changes, both laser wavelengths produce the same type of changes in the glass (Pissadakis and Konstantaki, 2005b), after specific dose threshold has been surpassed.

Shortly after the above reports on 213 nm photosensitivity, 211 nm 250 fs radiation was used for long-period grating recording in hydrogenated SMF-28 and B/Ge co-doped fibers, following a clear Type I photosensitivity behavior for both fibers exposed (Kalachev et al., 2005). Recently 30 KHz, 213 nm Nd:VO₄, 7 ns laser radiation was used for recording Bragg ratings in hydrogen-free, SMF-28 fiber, achieving refractive index changes of the order of 10⁻³ (Gagné and Kashyap, 2010). Gagné and Kashyap supported that the primary physical mechanism dominating the photosensitivity of SMF-28 fiber using 213 nm nanosecond radiation was of a single-photon absorption nature, prompting color-center generation. The single-photon color-center generation was associated with the longer pulse duration employed.

12.5.2 Infrared femtosecond laser inscriptions

There have been early investigations on the use of femtosecond laser radiation in the refractive index engineering of optical fibers, starting from the work of Saifi et al. (1989), who measured permanent refractive index changes of few parts of ×10⁻⁵ induced in a twin core, germanosilicate optical fiber, directional coupler by exposure to 620 nm, 100 fs laser radiation. Saifi et al. quantified these changes by measuring changes in the beat-length of the directional coupler, by power coupling between the two cores, while attributing for the first time the photosensitivity obtained in multiphoton generated structural changes (Griscom, 2011). A decade later, Cho et al. (1999) observed increase of the SiE’ centers after the formation of plasma channelling in a multimode silica fiber using 790 nm, 110 fs Ti:Sapphire laser radiation. The formation of plasma channelling induced a permanent double cladding structure into the multimode silica fiber of 2 × 10⁻² refractive index contrast. Then, Fertein et al. (2001) measured 6 × 10⁻³ refractive index changes using in the cavity length a Bragg gratings Fabry-Perot in Corning fiber SMF-28, exposed using 800 nm femtosecond laser, at a focal spot of 0.4 mm.

The landscape of fiber photosensitivity and grating recording was redrawn after the first Bragg reflectors inscribed in SMF-28 and all-silica depressed cladding optical fibers by Mihailov et al. (2003, 2004). In this work, both SMF-28 and all-silica fibers were exposed using a modified design phase mask for forming short length and highly scattering Bragg reflectors (Fig. 12.6), which maintained their reflectivities in temperatures greater than 1000°C.

The refractive index changes were a product of a four/five-photons (depending upon the material bandgap) nonlinear absorption at intensities of 1.2 × 10¹³ W/cm². That group exposed hydrogenated SMF-28 fibers using the same setup and conditions revealing reduction of compaction effects in the silica glass cladding (Limberger et al., 2007), while the gratings exhibited annealing behavior similar to those inscribed using ultraviolet radiation (Smelser et al., 2004). By varying exposure energy densities pure Type I-IR or Type II-IR gratings could be formed in SMF-28 fibers, where Type I-IR are possibly associated with color-center accumulation, and Type II-IR are products of extreme ionization and plasma formation processes. Type I, ultrabroad bandwidth, chirped Bragg reflectors were fabricated using Ti:Sapphire 800 nm, 1 KHz femtosecond laser radiation, in hydrogenated and pristine standard telecom fibers, using highly chirped phase masks, achieving FWHM bandwidths greater than 200 nm (Bernier et al., 2009).

Alternatively to the use of custom design phase masks for controlling spatial dispersion and beam de-condensation effects that can detrimentally affect the high intensities required during femtosecond laser refractive index engineering (Mihailov et al., 2004), point-by-point Bragg grating recordings were performed in standard telecom fibers (Martinez et al., 2004). The nonlinear refractive index engineering at sub-wavelength volumes using 800 nm femtosecond lasers (Glezer et al., 1996) offers significant advantages in the point-by-point Bragg grating processing, allowing ease in tailoring the topology of the periodic structure as well as its polarization characteristics. Martinez et al. inscribed such reflectors operating in the first diffraction and also in higher orders, using high magnification objectives and an 800 nm, 1 kHz, 150 fs Ti:Sapphire laser (Fig. 12.7). The Bragg reflectors inscribed were almost 25 dB in strength, exhibiting typical birefringence of ~ 3 × 10⁻⁵ due to the highly asymmetrical nature of the inscription process. Using the same method Martinez et al. (2006) also presented Bragg grating inscription in a jacket unstripped standard telecom fiber, succeeding in grating strengths greater than 25 dB. The above was a significant improvement compared to the previous art, where special coatings were used for inscribing Bragg reflectors through the fiber jackets using 244 nm laser radiation (Chao et al., 1999). Infrared femtosecond laser point-by-point Bragg grating inscription technique was also used for recording apodized geometry structures in SMF-28e fibers by adopting a slanted scanning technique with respect to the fiber core (Williams et al., 2011b) or complex sampled and phase-shifted Bragg reflectors (Marshall et al., 2010).

Different kinds of Bragg reflectors were inscribed in standard telecom fibers exploiting filamentation effects, induced under Ti:Sapphire 800 nm femtosecond laser radiation (Bernier et al., 2011). High energy density inscriptions can lead to filamentation generation and propagation inside the fiber complex, inducing refractive index changes as high as ×10⁻² (Sudrie et al., 2002; Yamada et al., 2001). Due to the nature of filament generation and propagation, the modifications induced in the hosting glass matrix are spatially localized in typical dimensions below a few microns, allowing high refractive index contrasts, thus increased diffraction efficiencies. Bernier et al. (2011) reported grating inscription by cross-sectioning the fiber core with the periodic filament generated by a phase mask, inducing refractive index changes of ~ 2.5 × 10⁻³.

12.5.3 Ultraviolet femtosecond laser inscriptions

While in IR femtosecond exposures several photons were required for covering the large bandgap of germanosilicate glasses and fused silica, at the expense of tight focusing and narrow envelope inscription conditions, the use of ultraviolet picoseconds and femtosecond sources could alleviate these tight inscription conditions exploiting lower-order, nonlinear effects. The scaling down rule of the damage fluence threshold of an optical material versus pulse duration (Boyd, 2003; Stuart et al., 1995) applies also in photosensitivity, questioning the efficiency of traditional laser wavelengths such as 248 and 266 nm into the inscription of higher index changes at lower energy doses. The first inscriptions of Bragg gratings in hydrogenated SMF-28 fibers were presented using a low repetition rate, frequency quadrupled 264 nm Nd:glass laser (Dragomir et al., 2003), at maximum intensities of 77 GW/cm².

Comparative results on the Bragg grating inscription process in SMF-28, phosphorus doped and all-silica fibers using 1 KHz repetition rate, tripled 267 nm, Ti:Sapphire femtosecond laser with results obtained using 157 and 193 nm ultraviolet excimer lasers (Zagorulko et al., 2004). Zagorulko et al.’s study revealed that the photosensitivity of hydrogenated, low-Ge content silicate glass fibers using femtosecond, ultraviolet laser resembles that obtained using 157 nm excimer laser, leading to similar refractive index changes (few parts of 10⁻³) and growth rates, while suffering less from saturation effects (Fig. 12.8).

The fabrication of long-period gratings in hydrogenated SMF-28 fiber variants using 352 nm, emitted from a frequency tripled Nd:glass laser and point-by-point exposure, was reported by Dubov et al. (2005). In this report a three-photon absorption process was triggered at intensities of the order up to 2000 GW/cm², while cladding damage was visible denoting asymmetrical absorption across the fiber cross-section.

Strong Bragg reflectors were also fabricated in SMF-28 fibers for modest accumulated energy densities (Livitziis and Pissadakis, 2008), using a double phase mask interferometer and 248 nm, 500 fs laser radiation (see Fig. 12.9a). These Type I gratings saturated at accumulated energy densities as low as 3.5 kJ/cm², reaching refractive index changes up to ~ 7 × 10⁻⁴, while exhibiting thermal durability up to 900°C (Fig. 12.9b). The enhanced thermal stability of these SMF-28 fiber gratings, compared to those recorded using 193 nm excimer laser radiation (Albert et al., 1995), was associated with the higher two-photon absorption coefficient of the Ge-doped core (Dragomir, 2002), which in turn may lead to a substantial increase of the local temperature levels in the bright fringes.

Bragg and long-period grating inscriptions using femtosecond lasers produced mostly Type I and Type II photosensitive refractive index changes, while no Type IIA had been produced either by 800 nm Ti:Sapphire or quadrupled Nd:glass lasers, for prolonged irradiations and energy density doses into germanosilicate glasses fibers irrespective of Ge concentration and codopands. The first Type IIA Bragg grating recording utilizing femtosecond laser radiation was presented by Violakis et al. (2006) using 248 nm, 500 fs hybrid excimer/dye laser and phase mask configuration in contact mode. The fibers exposed were the B/Ge co-doped, PS1250/1500 from Fibrecore and a Hi-Ge from FiberLogix (Fig. 12.10a,b).

Violakis et al. presented comparative data with similar inscriptions using 248 nm, 34 ns excimer laser radiation, where the femtosecond grating recording was saturated for doses 10× times smaller than those required in the nanosecond recording; while both Type I and Type IIA refractive index changes slopes were accelerated for the femtosecond exposure case (Fig. 12.11). In addition, annealing studies revealed that the Type IIA gratings recorded using 248 nm, 500 fs radiation demarcated at 700°C, 100°C higher than the nanosecond counterparts, due to structural changes formed by the irradiation (Violakis et al., 2006).

In later studies performed by the same group (Pissadakis et al., 2006), comparative Type IIA Bragg grating recordings were demonstrated using 5 ps, 500 and 120 fs, 248 nm radiation together with micro-Raman studies, investigating the effect of exposure conditions in the photosensitivity growth characteristics, by performing exposures for fixed intensity and energy density and different pulse durations (Fig. 12.12a). Exposures performed under fixed intensities for different pulse durations illustrated that the effect of the energy density plays a predominant role in the triggering of the Type IIA photosensitivity mechanism, while intensity rather affects its progression speed (Fig. 12.12b).

Additionally, the 248 nm femtosecond laser irradiation modifies the characteristics of the Boson peak (Hehlen et al., 2002) in Raman spectra obtained (Dianov et al., 1997b) from the boron-germanosilicate glass core, shifting this to longer wave numbers (Fig. 12.13), while exhibiting similarities to the response of silicate glasses subjected to hydrostatic pressure above the plasticity level (Inamura et al., 1997).

The modification of the Boson peak constitutes a strong indication that the primary underlying mechanism behind Type IIA photosensitivity is that of compaction in the bright fringes of interference, and secondary that of stress relief; that was also described by Ky et al. (2003b).

12.5.4 Regenerated gratings

Bragg grating regeneration and post-exposure amplification were first observed in Type IIA gratings fabricated using 193 nm, excimer laser radiation in a B/Ge co-doped optical fiber (Dong and Liu, 1997). In such grating type, the exposure was ceased before reaching Type IIA saturation, and then the fiber reflector was subjected to annealing processes, where amplification strength was observed. Similar abnormal thermal annealing behavior had also been observed in Bragg gratings fabricated in OH-flooded, F-depressed cladding fiber at temperatures within the range of 1000°C (Fokine, 2002a), attributed to molecular water formation and diffusion into the silica glass matrix, and subsequent oxygen reduction. Then, thermal regeneration was also observed in a hydrogenated, F, P and B/Ge co-doped fiber with high concentration of both B and Ge ions that had been exposed to 193 nm, for forming a Type I Bragg reflector (Bandyopadhyay et al., 2008).

Canning et al. (Bandyopadhyay et al., 2008) speculate that the grating writing process leaves a structural signature in the exposed glass matrix, which is not demarked by the thermal treatment, even though the refractive index changes may erase. Instead such structural signature imprinted in the glass may constitute a catalytic base for assisting chemical reactions with hydrogen for forming possible species of hydrite or hydroxyl groups that can induce local stresses in the exposed fringe level (Canning et al., 2008b). Further, the above regenerated gratings exhibited extreme durability to thermal treatment, withstanding temperatures up to 1000°C (Fig. 12.14). Similar regeneration results were obtained for gratings fabricated in high Ge content fibers under strained annealing (Lindner et al., 2011), while surviving under similar temperature conditions as those presented in (Bandyopadhyay et al., 2008).

Thermal regeneration behavior has been observed for Type IIA Bragg gratings fabricated using femtosecond and picosecond 248 nm laser radiation (Pissadakis et al., 2006), for exposures ceased before reaching the Type IIA saturation level (Fig. 12.15).

Figure 12.15a shows index modulation Δn_mod changes versus accumulated energy density for Bragg grating exposure in B-Ge optical fiber using 248 nm, 120 fs, 500 fs and 5 ps laser radiation, of similar fluence. Figure 12.15b shows normalized grating strength, during regeneration annealing at isothermal temperature steps of 100°C, of the previous Bragg gratings until erasure point is reached.

Three different Bragg gratings exposures performed in the PS1250/1500 Fibercore B/Ge codoped fiber using 5 ps, 500 fs and 120 fs while keeping similar energy densities, ceased when reaching the same refractive index change level at the Type IIA photosensitivity regime. Accordingly all gratings were annealed from room temperature up to 600°C, reaching full demarcation. These three gratings exhibited almost identical regeneration characteristics, amplifying in strength with increasing temperature, reaching the maximum strength point at 550°C approximately; then erasing until the temperature of 600°C. Such behavior reveals that the glass composition and thermal history dominates the specific regeneration process, while the exposure conditions play rather a secondary role. Therefore, the regeneration characteristics may be tuned by changing the pre-exposure thermal history of the glass (i.e. rapid heating and cooling process), for modifying the population and type of fictive defects in the matrix, that can be later seeded by the irradiation process (Bandyopadhyay et al., 2008).

12.7.1 Phosphate glass fiber gratings

The photosensitivity of phosphorus codoped silicate glass fibers has been investigated since the 1990s, when 193 and 240 nm pulsed laser radiation was used for inducing transient and permanent refractive index in those fibers (Canning et al., 1995). The permanent residue of refractive index changes obtained for hydrogen loaded, phosphorus doped silica glasses was of the order of 10⁻³, under 193 nm irradiation; unloaded fibers were quite less photosensitive. It was also known that phosphorus dopand suppresses the 242 nm germanium-oxygen deficiencies band when inserted in germanosilicate glasses, acting as a passivator for such types of wrong bonds, thus reducing photosensitivity (Dong et al., 1994).

The valence state of phosphorus allowing the formation of single and double bonds with oxygen, defines the micro-coordination of phosphate glasses while leading to a linear-like polymerization chain built in combination with oxygen and other ion modifiers; a micro-coordination structure which is substantially different from that of silicate glasses. Due to this versatile phosphorus–oxygen bond state, phosphate glasses exhibit interesting optical, chemical and mechanical properties, dependent upon the ratio between phosphorus and oxygen ion concentrations, and the incorporation of other matrix modifiers. Phosphate glasses are highly transparent and durable to ultraviolet radiation, have a high solubility of rare-earth ions and exhibit low T_g points (Ehrt et al., 1994). Rare-earth doped phosphate glasses have been drawn into standard and MOFs and planar waveguides for realizing high gain amplifiers (Hwang et al., 1999) and lasers (Li et al., 2005) of short physical lengths. Therefore, the straightforward inscription of Bragg reflectors in those guiding structures is required for realizing lasing devices and sensors (Strasser, 1996). The first studies of the photosensitivity of pure phosphate glasses were presented in the mid-1990s, mostly focused on fluorophosphates slab samples including Ce⁺ ion modifiers while being irradiated using 248 and 193 nm excimer laser radiation (Ebendorff-Heidepriem and Ehrt, 1996; Ebendorff-Heidepriem et al., 2002). In these studies spectroscopic modifications were solely examined, rather than refractive index engineering.

Generally, the exposure of phosphate glasses to ultraviolet laser and X-ray radiation results in color-center generation extended from the visible to the ultraviolet and bandgap spectral regime, associated with the transformation of the phosphorus–oxygen bond and the related defects, with most prominent the PO hole center in the visible band and the PO₄ electron center peaking at the 242 nm wavelength (Ebeling et al., 2002; Ehrt et al., 2000). Further to these first photosensitivity studies carried out using short wavelength sources and single-photon absorption excitation, there were presented refractive index engineering demonstrations using infrared (Chan et al., 2003) and near ultraviolet femtosecond lasers in pristine and silver doped phosphate glass substrates (Watanabe, 2001), respectively.

A study focusing on the refractive index engineering of the commercially available rare-earth doped IOG-1 Schott glass including the irradiation of pristine and silver ion-exchanged samples using 248 nm nanosecond excimer laser was presented in 2004 (Pissadakis et al., 2004). This study revealed that the refractive index changes obtained in a pristine glass slab for accumulated energy density doses of 12 kJ/cm² was of the order of a few parts of ×10⁻⁵, while the addition of silver boosted the photosensitivity of the glass by almost three orders of magnitude. The same glass exposed using 213 nm, 150 ps Nd:YAG laser and an elliptical Talbot interferometer exhibited similar refractive index changes (few parts ×10⁻⁵); however, non-monotonic growth effects were monitored during grating inscription (Pappas and Pissadakis, 2006).

Other investigations in the same IOG-1 glass using 193 nm laser radiation led to greater photosensitivity effects that were directly exploited for waveguide laser development (Yliniemi et al., 2006a, 2006b). Most importantly the work of Yliniemi et al. revealed that phosphate glass undergoes structural changes under 193 nm laser irradiation including volume modification, confirmed by utilizing micro-Raman spectroscopy and atomic force microscopy. Similar findings related to volume modifications induced by 193 nm (Michelakaki and Pissadakis, 2009) and 248 nm, 500 fs (Pissadakis and Michelakaki, 2008) laser radiation were presented by Michelekaki et al. where Knoop micro-hardness measurements were used for monitoring non-monotonic volume dilation effects versus energy density dose, under a single-photon absorption mechanism (Fig. 12.20a).

From the variation of the Knoop hardness data obtained for 193 nm excimer laser exposures in bulk phosphate glass samples, the corresponding changes in the elastic modulus were evaluated, confirming the volume dilation induced by the irradiation process (Fig. 12.20b). Such atypical radiation-induced volume modification effects were attributed to PO bond transformation from single to double and subsequent cleaving upon the conditions of the irradiation process.

The aforementioned investigations on the photosensitivity of bulk phosphate glass samples constituted a solid background for attempting Bragg grating inscription in all-phosphate glass fibers. The first Bragg gratings in a rare-earth doped all-phosphate glass fiber were demonstrated by Albert et al. (2006) using 193 nm excimer laser and standard phase mask inscription technique. Albert et al. inscribed strong, Type I Bragg reflectors in a phosphate glass fiber, obtaining average refractive index changes of 5 × 10⁻⁴. These gratings under low temperature annealing, amplified more than 65% in strength; this behavior was attributed to slow stress relaxation effects accelerated by the thermal treatment (Albert et al., 2006; Rodica Matei et al., 2007). Grobnic et al. (2007) followed with positive refractive index, Type I Bragg grating inscriptions in a similar phosphate glass fiber using 800 nm, femtosecond laser radiation, reaching refractive index changes greater than 1.5 × 10⁻³. These reflectors exhibited rather standard thermal stability, without post-exposure amplification effects, while maintaining their strengths up to temperatures of 400°C. Notably, the results of Grobnic et al. (2007) oppose earlier observations of Chan et al. (2003) where negative refractive index changes formed waveguides in IOG-1 glass under 800 nm, femtosecond irradiation; these negative refractive index changes demarcated at substantially lower temperatures.

Recently, Sozzi et al. exposed the same fiber as Grobnic et al. (2007) using 248 nm, 500 fs laser radiation and a double phase mask interferometer (Sozzi et al., 2011). Sozzi et al. found that at high energy densities for such pulse duration, a photosensitivity mechanism similar to Type IIA was activated in the phosphate glass fiber, following non-monotonic growth of both the average and modulated refractive index changes (Fig. 12.21a). The last was the first demonstration of Type IIA-like photosensitivity growth in a non-silicate, soft glass matrix optical fiber. The average refractive index changes obtained were greater than ×10⁻³, for accumulated energy doses of 6.5 kJ/cm². These anomalous growth Bragg gratings, maintained the greater part of their strength up to 377°C (Fig. 12.21b). Inscriptions performed at lower energy densities followed a Type I photosensitivity trend, indicating that the specific non-monotonic photosensitivity mechanism is possibly not of linear-depen-dence. Sozzi et al. further investigated the origin of the specific photosensitivity mechanism using micro-indentation Knoop hardness measurements, performed in exposed and side polished phosphate glass fibers.

These micro-indentation measurements provided further insight into the volume modifications that the glass undergoes under 248 nm, 500 fs irradiation, showing that for prolonged exposures volume dilation takes place, driving negative refractive index changes. These results are in agreement with those presented by Michelakaki et al. where the Knoop hardness of another phosphate glass initially hardens and then dilates under 193 nm, 10 ns and 248 nm, 120 fs laser exposures (Michelakaki and Pissadakis, 2009). The results of Knoop hardness variation, exhibit a universal character (Fig. 12.22), irrespective of glass composition and irradiation conditions, similarly to the compaction model studied extensively for silica glass (Borrelli et al., 1997; Primak, 1972). The non-monotonic photosensitivity behavior observed for this phosphate fiber for Bragg grating inscription using 248 nm, 500 fs radiation can be described as a mirror counterpart of the Type IIA mechanism of heavily doped germanosilicate fibers (Ky et al., 2003). The fundamental difference between the mechanisms occurring in the phosphate and the germanosilicate glass fibers is the sign of refractive index changes induced in the bright fringe of interference: in the case of the phosphate glass these changes are negative due to volume dilation, generating compressive stresses.

12.7.2 Fluoride glass fibers

While fluoride glasses were first realized in 1975 (Poulain et al., 1975), and shortly after their spectroscopic and active ion-host characteristics established them as promising materials for lasing devices (Zhu and Peyghambarian, 2010), their laser photosensitivity has been investigated in detail for almost two decades after the initial invention. The structure of fluoride glasses where the oxygen ion has been replaced by that of fluorine allows low phonon energies and thus great transparencies both at the Urbach tail band and the far-infrared regime, sketching an efficient bandwidth from 200 nm to 8 μm approximately. Poignant et al. recorded Bragg gratings in a ZBLAN fiber doped with Ce⁺³ ions as photosensitivity enhancer, reaching refractive index changes of few parts of ×10⁻⁴ after exposure to 246 nm pulsed radiation (Poignant et al., 1994; Taunay et al., 1994).

Since the photosensitivity observed was underlined by a specific sensitizer, potentially the mechanism of refractive index changes could be that of color-center formation; however, that was not concluded from the experiment (Taunay et al., 1994). Sramek et al. (2000) exposed fluorozirconate glass slabs of different compositions to 193 nm excimer laser radiation, and found that surface expansion occurs at heights of tens of nanometers for accumulated energy doses lower than 1 kJ/cm² (Fig. 12.23). The last authors justified this laser-induced expansion after considering similar crystal structures such as b-BaZr₂F₁₀ and BaZrF₆, which lead to density ratios close to unity after comparison with the fluorozirconate glasses exposed; thus exhibiting low structural defect population. The underlying photosensitivity mechanism speculated for 193 nm excitation, refers to Coulombian bond breaking between ion modifiers such as Ba with unpaired fluorine, changing the glass micro-coordination with respect to the Zr₂F₁₃ unit blocks.

Moreover, Zeller et al. (2005) exposed undoped fluoride glasses to 193 nm excimer laser radiation and investigated the radiation-induced glass disorder by examining changes in the Urbach tail of the irradiated specimens (John et al., 1986; Urbach, 1953). Zeller and co-authors further quantified the depth of radiation damage into the exposed glasses by gradually polishing them and re-evaluating refractive index changes by means of Kramers–Kronig transformation of the color centers formed. Cerium doped ZBLAN fluoride bulks were also exposed by Williams et al. (1997) using 248 nm, excimer lasers, revealing that the Ce³⁺ excitation was of a two-step absorption nature, exploiting the 4f¹–5d¹ transition leading to quadratic dependence upon intensity. In the same study, it was also found that the Cerium ion color centers created were simultaneously bleached by the 248 nm excitation.

Similar to other Bragg grating inscription in low defect glass optical fibers Grobnic et al. (2006) used 800 nm femtosecond laser for recording ~ 2.0 × 10⁻⁴ refractive index changes in an undoped ZBLAN fiber; nonetheless, without referring to the sign of the refractive index changes. These gratings resisted to temperatures up to 240°C, quite close to the T_g of the glass at 265°C. An illustrative study on the photosensitivity of ZBLAN fibers was presented by Bernier et al. (2007), where negative refractive index changes were identified locally in the fiber core and cladding areas, using fiber profilometry (Fig. 12.24). Also, positive refractive index changes were traced at the core–cladding interface, and at other areas of the cladding, providing an initial evidence that the sign of the refractive index changes is directly dependent upon the dissipated intensity at microscopic level. Moreover, these refractive index engineering results appear to be in agreement with the volume dilation measurements in bulk samples presented by Sramek et al. (2000) and the Urbach tail investigations of Zeller et al. (2005) where a radiation-induced disordering of the glass is described.

12.8.1 Bragg and long-period grating inscriptions

Photonic crystal fibers (PCFs) fuse unique light guiding properties together with useful microfluidic structures, offering great flexibility in developing photonic devices implemented with novel and high performance functionalities. Even though there was a standardized photosensitivity and refractive index engineering background related to the inscription of Bragg and long-period gratings in standard optical fibers and most of the glass materials used for drawing them, the emergence of PCFs and MOFs imposed new inscription issues as well as challenges (Knight et al., 1996; Russell, 2006). The capillary structure surrounding the guiding area affects the light penetration for side-illumination as this is principally required for periodic structure inscription using laser radiation. Specifically, the geometrically complex core and cladding shape of PCFs and MOFs induces significant scattering and refraction effects that in general reduce the average laser intensities delivered into the fiber core. Such reduced power dissipation into the fiber core during the side-illuminated grating inscription process deteriorates the photosensitivity yield by means of the magnitude of the refractive index changes formed into the guiding area and the accumulated energy densities needed for reaching saturation of the inscription. These scattering and refraction effects can dominate over the type of the photosensitivity process occurring, as well as the order of the underlying photon absorption. Many PCF and MOF designs are drawn from a single type of optical glass, often of low defect concentration such as fused silica, exhibiting low photosensitivity using conventional exposure methods. Therefore, additional issues arise during the grating recording in those high quality glass MOFs, rendering the inscription process a difficult and multiparameter materials science and light propagation problem.

Most of the methods used for inscribing gratings in standard optical fibers have been used for similar inscriptions in MOFs counterparts, revealing interesting grating recording behaviors and spectral responses. Initially, the safe path of Ge-based photosensitivity and hydrogenation was followed for forming Bragg and long-period gratings inside MOFs, by doping a small socket of the guiding area with Ge-ions.

Three years after the demonstration of PCF, Eggleton et al. (1999) presented the first Bragg and long-period grating inscription in such a solid core fiber (Fig. 12.25), which contained a Ge-doped photosensitive socket, using a 242 nm frequency-doubled excimer-pumped dye laser. For alleviating side-illumination scattering issues, Eggleton et al. hydrogen loaded the Ge-doped solid core PCF, reaching grating strengths greater than 50%. The same group presented a more detailed analysis of Bragg grating inscription and spectral resonance effects in several MOFs of different geometry and dopands, including excitation of cladding modes (Eggleton et al., 2000; Kerbage et al., 2000).

Kakarantzas et al. (2002) presented the first long-period grating recording in a solid core all-silica PCF by using 10.6 μm CO₂ laser radiation, and a scanning mirror beam focusing setup, following the technique inaugurated by Davis et al. (1998) in standard optical fibers. In this demonstration, the glass PCF was heated by the CO₂ laser above the fictive temperature of silica, resulting in the formation of both geometrical and micro-coordination changes in the rapidly cooling glass and succeeding in inscribing short length, high strength LPGs and rocking filters (Kakarantzas et al., 2003).

Accordingly, the first Bragg grating inscription in an all-silica, solid core PCF was reported by Groothoff et al. (2003) using a 193 nm, excimer laser of standard spatial coherence and a phase mask in contact mode (Fig. 12.26). The Type I refractive index changes reported in that demonstration were of the order of 2.4 × 10⁻⁴, after an exposure of cumulative fluence of 200 kJ/cm², leading to significant grating strengths. The refractive index modification mechanism was assumed to be that of two-photon resulting primarily in the dissociation of the Si-O bond, inducing matrix compaction. Such experimental observation is similar to the results presented by Rothschild (Rothschild et al., 1989), Devine (Fiori and Devine, 1986) and Borrelli (Borrelli et al., 1997). Also, these authors found that there were non-thermal, post-exposure matrix relaxation effects, resulting in spectral modifications of the inscribed reflectors. Canning et al. further exploited the above writing approach for developing DFB laser in an Er-doped silicate glass PCF, reaching slope efficiencies of 12.5% (Canning et al., 2003; Groothoff et al., 2005); but also in the recording of Bragg reflectors in a Fresnel type MOF (Martelli et al., 2005).

Other groups used 193 nm excimer laser radiation for recording Bragg gratings in hydrogenated phosphosilicate and germanosilicate PCFs, reaching refractive index changes greater than 3 × 10⁻³ for the phosphosilicate matrix, and one order of magnitude higher changes for the germanosilicate fibers (Beugin et al., 2006). In the work of Beugin et al. (2006) the PCF section was spliced both sides to a standard optical fiber prior to hydrogenation, for encapsulating hydrogen under the loading pressure into the capillaries (Sørensen et al., 2005), maintaining high concentrations of hydrogen species into the PCF core during the grating recording process, while reducing the out-diffusion rate (Liou et al., 1997).

Violakis et al. (Violakis and Pissadakis, 2007b) used a high spatial coherence 193 nm excimer laser for recording strong Bragg reflectors in a commercially available (Fig. 12.27a), solid core PCF, reaching high refractive index changes (Δn = 3 × 10⁻⁴). The use of an increased spatial coherence laser allowed the formation of high contrast interference fringes along the whole cross-section of the exposed fiber (Othonos and Lee, 1995), a condition that is significant for maintaining high intensities necessary for nonlinear absorption photosensitivity processes. In the demonstration of Violakis et al. (Violakis and Pissadakis, 2007b) the Bragg reflectors recorded using an energy density of 230 mJ/cm² exhibited a broad wide spectral valley observed at the left side of the fundamental grating notch, related with broadband coupling to radiation modes. This kind of radiation mode coupling was also confirmed when visible laser was injected into the PCF fiber and out-coupled by the inscribed Bragg grating. Such a spectral feature that is routinely observed in the case of surface relief gratings imprinted on planar waveguides (see Fig. 12.27b) may be related to a shallow, compaction relief periodic feature induced in the microstructured fiber capillaries (Schenker, 1994).

Fu et al. (2005) exposed a silica glass solid core PCF using a frequency tripled Ti:Sapphire 267 nm femtosecond laser for recording Bragg reflectors; however, the fiber was hydrogenated for increasing photosensitivity, similar to the example of Zagorulko et al. (2004). Using this approach the refractive index changes inscribed in the PCF exposed were ~ 3 × 10⁻⁴ under accumulated energy density reaching 100 kJ/cm², while in a standard allsilica fiber three times greater index changes were recorded (1.1 × 10⁻³) using 54.5 kJ/cm² (Zagorulko et al., 2004).

Similarly, Brambilla et al. (2006) used 264 nm, 220 fs laser radiation for inscribing 1 cm long and 20 dB strong LPGs into an all-silica solid core PCF. Different types of hydrogenated PCF fibers were also exposed to femtosecond and picosecond, 248 nm laser radiation using standard phase mask and double phase mask interferometer (Fig. 12.28), leading to refractive index changes of 1.2 × 10⁻⁴, for energy doses of ~ 18 kJ/cm² (Pissadakis et al., 2008).

Other demonstrations included the use of Ti:Sapphire 800 nm, femtosecond laser radiation for Bragg grating inscription in pristine and tapered PCFs (Mihailov et al., 2006); and frequency tripped Ti:Sapphire laser at 262 nm for comparative recordings in unloaded and hydrogen-loaded PCFs (Wang et al., 2009b). All of the above demonstrations led to the formation of Type I photosensitivity refractive index changes and inscription behavior; however, there were examples where Type IIA Bragg reflectors were inscribed in MOF and PCF using 193 nm excimer lasers. Cook et al. (2008) exposed a 6.7%wt Ge-doped core, 12-ring PCF for completing Type IIA saturation after typical accumulated doses of 20 kJ/cm². Others (Pissadakis et al., 2009b) exposed a 36%wt Ge-doped, small-core, highly nonlinear PCF, for observing complex Type IIA photosensitivity effects for the two guiding modes supported by the fiber (Fig. 12.29a). Both scattering modes exhibited post-fabrication, amplification effects after storing at room temperature due to stress relaxation of the small dimension 1.38 μm, highly doped core; while surviving up to 800°C following a mixed Type I and IIA decay behavior. (see Fig. 12.29b)

In parallel, there is interest in the inscription of Bragg reflectors in small and collapsed core microstructured optical fibers. Due to the specific core geometry and dimensions (< 5 μm), collapsed core fibers exhibit a guiding mode profile largely extending in the inter-capillary areas, allowing high overlap with the refractive index medium included in those. The last renders them ideal platforms for the development of high sensitivity in-fiber refractometers and biological sensors, prompting the inscription of Bragg reflectors in their minimal size cores.

Initially, there were reports on the Bragg grating inscription in hydrogenated, Ge-socket collapsed core microstructured optical fibers using CW, 244 nm, frequency doubled Ar⁺ ion laser radiation (Huy et al., 2006; Phan Huy et al., 2007) (see Fig. 12.31b, c). Becker et al. (2009) used 267 nm, femtosecond laser radiation for recording arrays of Bragg reflectors in all-silica collapsed core fibers (Fig. 12.30b,c), reaching refractive index changes of 6 × 10⁻⁴.

Recently, Ge-doped socket, collapsed core fibers were exposed using high coherence, 193 nm, excimer laser radiation (Fig. 12.31), leading to visible compaction effects in the exposed core, resulting in significant light out-scattering for shorter wavelengths (Konstantaki et al., 2011). The structural changes induced in the collapsed core and surrounding cladding are under further investigation using micro-Raman analysis.

Novel Bragg grating inscription approaches, while exploiting the absorption or transparency of the infiltrated/encapsulated material into the PCF capillaries, have been presented for all-solid PCFs. Jin et al. (2007) attempted Bragg grating inscriptions using 248 nm excimer laser, in a hydrogenated, all-solid silica PCF containing into the core surrounding capillaries 1%wt Ge-doped silica glass, reaching ~ 1 dB reflectivities, and cladding mode inversion at the red-side of the spectrum. In another fashion, Bigot et al. (2009) designed an all-solid PCF, where the capillaries contained a low-photosensitivity, phosphosilicate glass, while the grating host material was a F/Ge co-doped core. In such an arrangement, the low photosensitivity glass infused into the capillaries did not impose absorption issues for side irradiation at the recording wavelength, allowing significant intensities reaching the fiber core and thus inducing refractive index modulation.

12.8.2 Studies on the issue of the side-illumination scattering during grating inscription

In all of the above Bragg and long-period grating demonstrations utilizing ultraviolet laser sources, there was not any special preparation related to the relative rotational position of the fibers with respect to the side-illuminating laser beam, therefore, the photosensitivity and refractive index engineering results presented there were not absolutely accurate or repeatable. The first study on the impact of the capillary structure and its rotational or transversal position along the fiber axis with respect to the laser beam on the Bragg grating recording processing was presented by Marshall et al. (2007).

Experimentally, Marshall et al. utilized 267 nm, femtosecond radiation coupled under different conditions into the PCF under exposure for different relative orientations and positions, and accordingly the 1.9 eV photoluminescence (Stathis and Kastner, 1984) induced was measured through the PCF fiber core. The same group used a finite element method for simulating the propagation of a small size beam for the aforementioned side-illuminating conditions evaluating an average field intensity (Fig. 12.32), and correlating such measurements with those obtained from photoluminescence experiments for different angles of rotation (Marshall et al., 2007). Similar investigations were presented by Canning et al. (2008a) using finite difference time domain (FDTD) simulations for estimating the intensities reaching the PCF core for a variety of rotational angles during Bragg grating side-illumination, for 193 nm excimer laser radiation. Geernaert et al. (2008) inscribed Bragg gratings in a highly asymmetric germanium-doped, birefringent microstructured optical fiber using a 248 nm excimer laser that had been subjected to hydrogenation; thus, strong single-photon absorption effects dominated its photosensitivity. Geernaert et al. found that the rotational state of the fiber around its axis during this single-photon absorption inscription did not significantly affect the reflection strength of the Bragg grating, while having a greater impact in its wavelength.

In another approach the FDTD method was used again for illustrating in greater detail the average, maximum and minimum intensities reaching the core of all-silica standard and microstructured optical fibers (Fig. 12.33), leading to better insights with respect to the impact to the material photosensitivity (Pissadakis et al., 2009a). In that study, the wavelength investigated was that of 248 nm, for 5 ps pulse duration and for propagation into hydrogenated silica glass fibers. The simulation results obtained depicted that while the mean intensities occurring in a side-illuminated PCF were 30%–40% lower than those found in a standard all-silica fiber, the maximum intensities that can be found in the core of a side-illuminated PCF can even be 200% greater (Fig. 12.34).

Scattering can greatly affect the triggering of specific photosensitivity processes, especially those of two-photon absorption which are quadratically dependent upon the local intensities, and dominate the refractive index engineering. In this study, the standard and PCF all-silica fibers exposed reconfirmed that the Bragg gratings recorded in the PCF were of lower strength than those formed in the standard all-silica fiber (Fig. 12.35).

The issue of laser beam scattering for side-illumination during Bragg grating recording has been also addressed by infiltrating inside the fiber capillaries liquids of high transparency at the irradiation wavelength and of refractive index value close to that of silica; or by suitably designing the fiber capillaries’ geometry for scattering less. This approach can lead to improved recording results, in the case where the fiber core is highly photosensitive, with linear absorption coefficient substantially higher than that of the infiltrated liquid. Sørensen et al. (2007) infiltrated organic solvents into a hydrogenated, Ge-doped core PCF for reducing the deteriorating effect of extensive side-illuminating scattering during Bragg grating recording using 248 nm pulsed laser radiation. Methanol and n-heptane reduced scattering effects which allowed significant amount of photons to reach the Ge-doped PCF core and induce significant refractive index changes, improving the grating strengths obtained by several decibels.

Following a different route, Baghdasaryan et al. (2011) used the FDTD method for simulating different candidate geometries with respect to the d/λ factor and hole diameter for reducing side-illumination scattering for 800 nm and a specific pulse duration. The results obtained in Baghdasaryan et al. (2011) provide a first indication that such optimization is possible without sacrificing the guiding properties of the custom-made PCF. Petrovic, Allsop, et al. (Allsop et al., 2011; Petrovic and Allsop, 2010) examined the effect of the numerical aperture of the focusing optics used during grating inscription in PCF using femtosecond lasers, further studying the effect of the specific recording conditions in the annealing and spectral characteristics of the inscribed gratings, thus providing a correlation with the photosensitivity mechanisms triggered.

12.9.1 End-face machining

The most frequent structures formed onto the cleaved end-face of standard optical fibers were these of diffraction gratings, micro lenses, interferometers and Fabry-Perot cavities. These optical structures combine relative straightforward etching processing protocol, while providing significant photonic operations such as tailored guiding mode out-/in-coupling, spectral analysis and optical phase reading. The simplest micromachining approach followed by several groups was that of local heating of cleaved fiber ends using 10.6 μm CO₂ laser beams, for forming end-firing, convex micro lenses (Abdelrafik et al., 2001; Presby et al., 1990); however, this achieved rather low control over the shape of the lenses formed (Fig. 12.36a). Li et al. (2007) used a 157 nm excimer laser for etching diffraction gratings of 2 μm period, micro lenses and Mach–Zehnder step-shifted interferometer onto the endface of multimode and standard telecom fibers. In this work energy densities greater than 4 J/cm² were employed for etching smooth pits and features onto the cleaved silicate glass fiber facet, reaching 5 μm flat depth pits, after single-photon absorption interaction.

Parabolic end-face lenses were formed using lathing machining schemes (Fig. 12.36b) while utilizing Schwarzschild reflecting objective and 157 nm radiation (Dou et al., 2008). Excimer laser 157 nm radiation was also employed by Machavaram et al. (Machavaram, 2006) for end-face pit and sharp tip machining in several fiber types (including sapphire fibers), in combination with selective chemical etching post-processing for improving surface smoothness and achieve specific shaping. The same wavelength was used by Ran et al. (2009) for initially forming flat-top, high smoothness, ~ 30 μm deep pits on SMF-28 optical fiber, and then enclosing these etched cavities into the fiber line by fusion splicing, forming high finesse Fabry-Perot interferometers. This in-fiber Fabry-Perot etalon exhibited spectral fringe contrast of 30 dB, allowing strain measurements at high temperatures and modulation rates. Using a more complex splicing, cleaving and re-ablating process an end-face refractometer was presented by the above group providing resolution of 1130.887 nm/RI units (Zengling et al., 2009).

Using different laser wavelength (785 nm) and pulse duration (125 fs) than nanosecond 157 nm excimer laser radiation, 33° angle (see Fig. 12.37), conical shape pits were formed into optical fibers for side-firing out-coupling (Sohn et al., 2010). The relatively rough ablated surface was polished to better quality using post-exposure arc discharging. Alternatively, by adapting a suitable length splicing buffer fiber to a PCF, and subsequent machining holes through the end-face of this buffer layer, Wang et al. (2010) presented a selective infiltration technique of the surrounding micro-capillary structure.

12.9.2 Structuring through the cladding

The structuring of the fiber end-face using laser beams led to the development of optical fiber structures exhibiting purely optical functionalities. The structuring through the cladding area of optical fibers using laser processes allowed the implementation of microfluidic functionalities in addition to the photonic structures developed until then. For a long period, the related state-of-the-art was rather primitive where simple etched holes (Gower, 2000) and jacket removal (Barnier et al., 2000) only had been presented. Then the use of 157 nm and 800 nm-femtosecond laser micromachining systems prompted the versatile structuring of small size photonic devices such as optical fibers.

Deeper through-out trenches, of several tens of micrometers’ length, were ablated in SMF-28 optical fibers using 157 nm excimer laser radiation by several authors for forming rectangular shape air cavities (Fig. 12.38a) and observing Fabry-Perot interferometric spectral modulations (Fig. 12.38b) in transmission and reflection mode (Machavaram et al., 2007; Ran et al., 2007, 2008); others used 800 nm, 1 KHz femtosecond radiation for ablating similar long length cavities (Li et al., 2008). Such demonstrations were also reported for in-fiber etalon etching in endlessly single mode PCFs allowing fringe probing for wider wavelength bandwidths (Ran et al., 2007; Rao et al., 2007). In most of these Fabry-Perot in-fiber interferometers a primary concern was related to the flatness and parallelism of the opposite walls constituting the oscillating cavities along the longitudinal axis of the fiber.

Zhou et al. (2007) used 800 nm femtosecond laser volume damage micro-machnining, in combination with HF acid selective chemical etching for forming narrow slot openings and boreholes in the cladding of optical fibers, suitably positioned within the length of pre-inscribed photosensitive Bragg gratings. These inscribed slots allowed the infiltration of these microfluidic fiber Bragg gratings using fluids with different refractive indexes, developing refractometers of 945 nm/RIU sensitivity. A similar micromachined hole design in a standard optical fiber was used for refractometric measurements where the light was refracted and out-coupled from the cylindrical shape ring upon its refractive index contrast with the fiber core (Wang et al., 2009a). Selective chemical etching of femtosecond laser-exposed Bragg gratings using a phase mask was used for forming deep periodic structures into a standard optical fiber (Fig. 12.39) and realizing infiltrated refractometer by monitoring the shift and power change of the Bragg peak scattered (Yang et al., 2011).

The laser etching of the cladding of microstructured and photonic crystal fibers provided a novel approach in the optimum exploitation of the micro-/optofluidic properties of these fibers. A first example was presented by van Braken et al. (2007), using an 800 nm femtosecond laser for drilling microchannels reaching the photonic bandgap periodic structure of a hollow core PCF; also in the same work micro-channels were drilled in collapsed core fiber claddings. Such micro-channels were exploited for side pressurized C₂H₂ gas infiltration and optical absorption measurements at the 1.5 μm band. Trench etching of the fiber cladding using femtosecond infrared laser radiation was used as an alternative method for selectively filling of the capillaries of a solid core PCF, by removing an arc slice of the out-cladding and reaching capillary channels lying underneath, exposing them for liquid immersion (Yiping et al., 2010). Controlled hole drilling with respect to the depth toward the fiber core was achieved using 800 nm femtosecond radiation, for developing long-period rejection filters in solid core all-silica PCF (Liu et al., 2010).

CO₂ lasers emitting at 10.6 μm do not provide structuring of high spatial resolution and most applications were presented on thermal lensing formation onto fiber end-faces. Nonetheless, after the demonstrations of Davis et al. (1998) and Kakarantzas et al. (2002), there several other examples were reported that laser radiation could be used for fine cladding structuring of standard and microstructured fibers. Wang et al. (2006) used CO₂ laser radiation for etching grooves onto standard optical fiber claddings for forming strongly scattering LPGs, while later presenting the thermal cladding deformation in hollow core PCFs demonstrate the first LPG in such a fiber design (Wang et al., 2008). Another group used a CO₂ laser beam in a lathing setup for inducing periodic, helical deformations outside of a standard optical fiber for realizing torque sensitive band rejection filters (Oh et al., 2004).

12.9.3 ‘In-fiber’ structuring using lasers

The use of laser radiation for modifying/etching the inner walls/capillaries of PCFs and MOFs has not been broadly investigated yet. In such an approach the laser beam passes through the cladding area and interacts selectively with the channels enclosed; the fiber core can be also modified. The main problems related to the in-fiber etching approach are those of extensive damage of the structure due to the explosive nature of ablation, deteriorating transmission signal and increasing losses; as well as the difficulty in controlling the exposure conditions and gas infiltration pressure for maintaining etching near the fiber core. Shujing et al. (2010) exploit the debris products of in-fiber ablation generated by 800 nm, femtosecond laser beam for selectively filling the capillaries of a solid core PCF and form a kind of damaged long-period gratings in those fibers; while achieving diffraction efficiencies of more than 20 dB over few (< 15) grating periods.

Violakis et al. (Violakis and Pissadakis, 2007a) attempted for the first time the etching of Bragg gratings into the capillaries of solid core PCFs infiltrated with fluorinated gas, using 193 nm excimer laser radiation. For simple gas infiltration inside the fiber capillaries under atmospheric pressure, some exposure results indicated that the existence of SF₆ inside the fiber capillaries can promote relief structure generation. While SF₆ exhibits high selectivity over silica glass etching, attacking preferentially silicon (Chuang, 1981), its photo-dissociation inside the PCF channels could offer controlled etching of the capillary walls (Batool et al., 2004). However, the method did not lead to easily reproducible results, due to extensive redeposition of ablated glass and reaction products inside the capillaries (Batool et al., 2004). Also, the uncontrolled SF₆ gas pressure conditions, where no buffer gas was used, while maintaining high pressures, increased recombination rates of excited species of fluorine.

Gratings fabricated in SF₆ infiltrated PCFs, exhibited increased strengths and faster saturation under specific exposure conditions by means of energy density and number of pulses (Fig. 12.40). Nonetheless, SEM pictures of cleaved fibers at the grating exposed section, revealed extensive debris redeposition and hole blocking. The last can be responsible for the saturation of the Bragg grating etching process earlier than inscriptions performed in empty PCF using identical conditions.

Recently, Sozzi et al. (forthcoming) have employed fluorinated organic liquids infiltrated inside MOFs and PCFs for etching Bragg and long-period gratings into their capillary structure using 248 nm, 5 ps laser radiation. By suitably tailoring the absorption coefficient of the infiltrated medium with respect to the capillary structure and geometry, poor etching and non-repeatability issues reported by Violakis et al. can be minimized, allowing efficient in-fiber periodic structuring (Fig. 12.41).

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