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Since its demonstration in 2003, Bragg grating inscription with high-power femtosecond pulse duration infrared sources and phase masks has proven to be an effective and far more versatile approach to grating fabrication than the conventional ultraviolet laser technique. The ultrafast IR laser-based process allows for the creation of grating structures in glassy and crystalline material waveguides that are not typically UV-photosensitive, thereby creating new applications for Bragg gratings. In this paper we will review studies that have been performed on the development and applications of the ultrafast laser technique to fabricate gratings in various optical fibers and waveguides using phase masks.
©2011 Optical Society of America
1. Introduction
Regeneratively amplified femtosecond pulse duration infrared (fs-IR) laser systems are ideal tools for laser-material processing and have been used to fabricate photonic devices in glasses such as imbedded waveguides [1]. The ultrahigh peak power radiation generated by these systems can induce large index changes in glasses, through multiphoton absorption and ionization processes that result in material compaction and/or defect formation depending on the intensity of the exposure. Above a material-dependent ionization threshold intensity I th, multiphoton ionization (MPI) causes dielectric breakdown that likely results in localized melting, material compaction or void formation. The resultant index change is permanent up to the glass transition temperature tg, of the material. Below I th, another regime of induced index change is observed that can be erased by annealing with sub-tg temperatures [2]. In this regime, multiphoton absorption likely results in defect formation and material compaction similar to the Type I UV-induced index change seen in Ge-doped silica. Index changes occurring with intensities greater than I th, are referred to here as Type II index changes.
Laser-induced index change is the defining attribute that makes fabrication of fiber Bragg gratings (FBGs) possible [3]. When considering fs-IR laser induced index change, two techniques are widely used to produce an index modulation (Δn) in waveguides and fibers: point-by-point exposure [4] and multi-beam interference using a phase mask [5]. Point-by-point FBG inscription with fs-IR lasers uses tight beam focusing and sophisticated precision translation systems. When well controlled, it has demonstrated itself to be a versatile approach to FBG inscription allowing for complex grating designs such as chirped structures and phase shifts [6]. By localization of index change within specific areas of the core region, birefringence can be increased and specific cladding modes resonances can be enhanced [7]. Cladding modes resulting from tilted gratings have been used for multiparameter sensing from a single grating structure [8].
The phase mask is a self-aligning interferometer that reliably produces a highly repeatable spatially modulated interference field, which can be imprinted waveguides [5]. Optical path lengths of the generated beams are automatically matched for an incoming beam that is at normal incidence to the mask. Combining the fs-IR laser and the phase mask approach, efficient FBG writing in standard telecom and pure silica core single mode fibers was demonstrated [5,9]. A material dependent intensity threshold for grating formation was observed (Type I) where the Δn had similar thermal stability when compared to Type I UV-induced gratings. At higher power levels that are consistent with white light generation in the fibers, thermally ultra-stable defects/damage sites may be produced (Type II) [10]. Because the fs-IR processes for induced index change are nonlinear, the FBG Δn that results from the sinusoidal interference field is non-sinusoidal. Unlike unsaturated UV-induced FBGs, fs-IR laser induced FBGs not only have the fundamental Bragg resonance λB but also higher order resonances at different wavelengths such that,
where m is the order number neff is the effective index of the fiber and ΛG is the pitch of the grating. Using higher order resonance gratings is advantageous from a characterization perspective as gratings can be directly viewed under an optical microscope.High Δn FBG inscription with a fs-IR laser and a phase mask is as simple as the standard UV-laser writing technique without being limited to materials that are UV-photosensitive. Using this technique, FBG inscription has been demonstrated in many different kinds of fibers and waveguides: pure silica core fibers [9], borosilicate glass fibers [11], fluoride (ZBLAN) glass fibers [12,13], radiation hardened fluorine-doped silica fibers [14], actively doped silica and phosphate glass fibers [15–18], pure silica photonic crystal fibers and tapers [19], highly non-linear bismuth oxide and chalcogenide optical fibers [20,21], and finally in crystalline waveguide materials such as sapphire [22,23] and YAG optical fiber [24].
In this paper, we present results and applications of FBGs induced with fs-IR lasers and phase masks in various optical materials and waveguide geometries. Applications include cladding mode suppression, direct FBG fabrication through protective fiber coatings, FBG fabrication in exotic fibers and materials for laser cavities and high temperature stable sensors.
2. FBG Fabrication and Characterization
Various optical fibers and waveguides were illuminated using Fourier transform limited 800 nm 125 fs duration pulses from a regeneratively amplified Ti:sapphire laser. The 6.4 mm diameter Gaussian beam was focused through various cylindrical lenses (f = 12 to 160 mm) and phase masks as shown in Fig. 1(a) . Pulse energies and repetition rates were varied between 0.2 to 2 mJ/pulse and 1 to 1000 Hz respectively depending on the study. Using tight focusing geometries, vertical positioning/scanning of the irradiating beam was performed within the waveguide normal to its axis with the incident beam centered onto the fiber and then scanned vertically over the fiber cross-section using a piezo-actuated translation stage with ± 10 μm travel and a 20 second period.
Fig. 1 (a) Schematic representation of the lens-fiber-phase mask geometry for FBG inscription, (b) schematic of diffractive order walk-off for a phase mask with corrugation period Λ..
The fiber-phase mask distance d is a very important parameter as it allows for the writing of pure two-beam interference gratings with a femtosecond pulse and a phase mask, even one possessing poor zero order suppression [25,26]. The pulses diffracted by the phase mask have different arrival times at the distance d normal to the phase mask. For large d, the diffracted order pairs (0, ± 1, ± 2 etc.) no longer overlap spatially resulting in a diffracted order walk-off effect (See Fig. 1(b)). The resulting gratings have periods that are half the pitch of the phase mask. Grating spectra were typically measured using an Er+ white light source and an optical spectrum analyzer or a swept tunable C-L band laser. Annealing experiments of FBGs were typically performed in air with a tube furnace or a ceramic microheater.
3. Applications of fs-IR Induced FBGs in Silica-Based Fibers and Claddings
Telecommunication applications of UV-laser induced FBGs, such as pump laser stabilization, dispersion compensation, narrow band filtering etc., can all be achieved using the fs-IR approach but without the necessity for fiber processing like photosensitization by hydrogen loading or polymer jacket removal. The ability to easily photoinduce structures in silica claddings or various silica waveguide geometries has resulted in a number of interesting applications of the fs-IR process that previously were not easily achievable.
3.1 Cladding Mode Suppression
UV-induced high Δn FBGs suffer from out-of-band losses on the short wavelength side of λB due to strong coupling of the forward propagating core mode (LP01) into discrete backward-propagating cladding modes [27]. When localized to the fiber core alone, an FBG doesn't fully overlap the LP01 mode that extends into the cladding region. This mismatch results in cladding mode coupling. As fs-IR FBGs are easily induced in both standard and pure silica core fibers [9], high quality grating structures were produced simultaneously in both the core and cladding of SMF-28 fiber. The propagating LP01 mode was completely subtended by a grating structure resulting in almost complete suppression of cladding mode coupling [28].
3.2 Cladding Structures: Induction of Modal Birefringence, Bend Sensing
For several applications such as single polarization fiber lasers or multi-parameter FBG sensing, a given amount of birefringence is desirable [29,30]. Direct exposure of silica with fs-IR laser intensities that are in the Type II regime but below those required for void formation produce birefringent nanoporous structures that are dependent on the polarization of the writing beam [31,32]. Such birefringent structures when induced in waveguides, are often associated with undesirable scattering loss.
Type II FBGs made with the fs-IR laser/phase mask in SMF-28 fiber have birefringence values of ~10−4 [33]. When Type II grating structures are made in the cladding near the core, large amounts of birefringence are locally induced in the core region without generating excessive loss in the core [34]. Birefringence as high as ~8 × 10−4 was obtained in the core of SMF-28 fiber when Type II structures were written in the fiber cladding on each side of the core near the core/cladding interface. The reflection spectrum splitting of a weak probe grating written in the core, which does not resonantly interact with the cladding structure, is shown in Fig. 2 along with a photomicrograph of the structure induced in the cladding.
Fig. 2 Probe FBG spectra before (red) and after (blue) cladding exposure, inset is the photomicrograph of a Type II exposure in fiber cladding near the core. Image taken along the exposure beam axis.
3.3 FBGs in Silica Tapers and Solid Core PCF, Microfluidic Sensing
In certain waveguide structures, such as adiabatically tapered fibers or solid core photonic crystal fibers (PCF), the guided mode couples evanescently into the surrounding environment. The λB of a FBG present in the waveguide is dependent upon the neff seen by the guided mode. If the surrounding environment changes, for example by immersing the device in a fluid, a shift in λB, ΔλB is observed. In this way, refractometers based on FBGs can be easily realized. In Fig. 3 , FBGs are fabricated in 30 and 50 μm diameter tapers made from low cutoff wavelength Ge-doped silica fiber (Corning HI780) and are used as refractometer-fluid level sensors [35]. Refractive index can be measured both by the shift in λB as well as by the amount of signal loss as light is coupled out of the fiber.
Fig. 3 Variation in reflected signal power with the ambient refractive index na for tapers with 30 μm (◯) and 50 μm diameters (). ΔλBragg with na denoted by triangles (△). Inset: photomicrograph of FBG in a) the 50 μm and b) the 30 μm diameter taper.
In Fig. 4 , an FBG is fabricated in continuously single mode silica core PCF (ESM-12-01 from Crystal Fibre/NKT Photonics). The cladding air holes tend to scatter the incoming IR radiation during inscription. By rotating the fiber axis in order to optimize IR beam transmission through the fiber, a third order grating (ΛG = 1.6μm) is written possessing an estimated index modulation Δn of ~4 × 10−4 [19]. This device could be used as a refractometer by wicking fluid into the fiber air holes surrounding the grating and monitoring the ΔλB. If the fiber is tapered with a hydrogen flame such that there is asymmetric partial closure of the air holes about the core, the induced FBG has extremely high birefringence above 10−3 [36].
Fig. 4 Transmission (red), reflection (blue) and modeled (dotted) spectra of a grating written in ESM-12-01 fiber with 1200 μJ pulses and 200 Hz. Exposure time: 5 minutes. Inset picture is a microscope image of the ESM-12-01 fiber cross-section.
4. Gratings in Highly Nonlinear Fibers
4.1 Bismuth Oxide Optical Fibers
Bismuth oxide fibers have thermal expansion and nonlinear Kerr coefficients almost 3 and 500 times larger respectively than standard telecom fiber [37,38]. FBGs were made in bismuth oxide fiber from Asahi Glass Co [20], which had a step index structure with two claddings and a core, the refractive indices being 2.21 for the core and 2.11 and 2.12 for the first and the second claddings respectively. It was exposed using a 12 mm focal length cylindrical lens through a 2.14 μm period phase mask with 200 μJ/pulse and a repetition rate of 20 Hz. The FBG spectrum after a 3-minute exposure is shown in Fig. 5 . The 1.07 μm grating period corresponds to a third order Bragg resonance at 1539.5 nm.
Fig. 5 Transmission (red) and reflection (blue) spectra of a bismuth oxide FBG. Inset photomicrograph is of the FBG made with the 4.28 μm pitched mask.
The high fiber nonlinearity decreases the optical power required for FBG-assisted all optical switching to the level of watts making it attractive for real applications. Exploiting the strong χ(3) -nonlinearity of this fiber in a cross-phase modulation scheme, transmission of a probe near the grating stop band was changed from 90% to 20%, producing a 6.5 dB extinction ratio at optical powers of only 55 W, resulting in an 18 × improvement in the switching power when compared to silica [39].
4.2 Chalcogenide fibers
Multimode chalcogenide fibers with core/cladding diameters of 60/165 μm were exposed to fs irradiation through the 4.28 μm period phase mask [21]. Compared to all other waveguides tested with fs-IR radiation, the chalcogenide fiber had very low surface ablation threshold intensities. FBGs made with 100 μJ/pulse at 10 Hz repetition rate and a 30 mm focal length lens are shown in Fig. 6 . Observable refractive index fringes are indicative of very high Δn which are likely a consequence of 1) the low optical bandgap Eg at 2.35 eV (compared to that of Ge-doped fused silica of ~Eg = 7.5 eV), and 2) the large non-linearity of the refractive index perhaps resulting in strong self-focusing effects.
Fig. 6 Photomicrograph of an FBG made in chalcogenide fiber. Image taken perpendicular to the direction of the beam.
5. Grating Inscription through the Polymer Jacket
UV-laser FBG inscription requires that the UV absorbing protective polymer jacket be removed prior to and replaced after the FBG fabrication. These processes are time-consuming and threaten the reliability of the fiber making them undesirable from a manufacturing viewpoint. For some high-powered fiber laser applications, the polymer jacket is used as a second cladding in order to guide pump light within the fiber. Its removal is also undesirable.
FBGs were written through the acrylate coating of SMF-28 fiber using fs-IR radiation and the point-by-point method [40]. A tight focusing geometry produced a large intensity differential between the fiber coating and core resulting in high Type-II Δn values in the core without coating damage (Δn = 3 × 10−4).
Using the phase mask technique, FBGs were also inscribed through the acrylate coating of SMF-28 fiber [41] and high numerical aperture (NA, high Ge-doped) fiber [42] in the Type I regime, when the fiber had been loaded with hydrogen. H2-loading of Ge-doped silica fibers increases the fibers’ photosensitivity to fs-IR light in a manner similar to the UV photosensitization process [43]. Index modulations of up to 1.4 × 10−3 were induced in the high NA fibers with mechanical strengths remaining at 75% to 85% of the pristine fiber value. By using a novel two-lens exposure geometry, high differentials in intensity between the fiber coating and core were obtained. Direct FBG inscription through the acrylate coating of SMF-28 fiber with a phase mask but without the need for H2 loading was then realized [44].
For high strain and high temperature sensing applications, it is desirable to have the FBG in polyimide-coated bend insensitive fibers, as mechanical information is better transferred through the thin polyimide coating than the thicker and softer acrylate coatings. Polyimide is also more thermally stable with operating temperatures up to 400 °C. With a 1.071 μm pitched phase mask and a single lens, FBGs were written through the polyimide coating of H2-loaded high NA fibers from FiberLogix; Δn of 1 × 10−4 were generated with mechanical strengths remaining at ~50% of the pristine fiber value [45].
6. Gratings in Active Fibers for Fiber Lasers
Fiber lasers have many useful applications because of their high efficiency, gain and good beam quality. Writing FBGs directly into the core of active single mode optical fibers is a convenient way to create compact, fiber compatible, low loss laser resonator cavities however traditional UV laser FBGs are not appropriate for short cavity fiber lasers where high levels of Erbium–Ytterbium doping (Er3+ and Yb3+) are required. Ge co-doping of active fibers, which is needed to enhance UV photosensitivity, causes ion clustering of Er3+ and Yb3+ dopants resulting in low laser efficiency [46].
Using fs-IR lasers, grating reflectors were inscribed directly into conventional Ge-free Er:Yb-codoped silica active fibers using both the point-by-point and the phase mask techniques [14,47]. High Δn (2 to 3 × 10−3) can be achieved in Er3+ and Yb3+ doped silica fibers however the maximum Δn that one can generate in the Type I regime is dependent on doping level and is less than the maximum achievable Δn in pure silica core fiber [16].
Other glasses, such as fluoride or phosphate, are more desirable substrates as higher concentrations of Er3+ and Yb3+ can be achieved. Dissimilarity with silica however makes coupling of these fibers to external resonator mirrors difficult. Direct inscription of FBG resonators in these fibers is preferable. Short FBGs were induced in heavily doped Er-Yb phosphate glass fiber producing cavity mirror reflectivities > 99.99% for a 6-mm-long grating corresponding to Δn > 1.5 × 10−3 [18]. Femtosecond IR laser induced gratings have also been fabricated in thullium-doped fluoride fibers [13] producing a fiber laser [48]. Because FBGs are easily written in multimode or large mode area (LMA) fibers [49], this technique is ideally suited for fabrication of resonator mirrors within actively doped LMA fibers for high average power (kW) laser cavities.
7. Applications of fs-IR FBGs for Harsh Environmental Sensing
7.1 Thermally Stable Type II FBGs in SiO2 Based Fibers
FBGs are often used as sensing elements. Strain and temperature imposed on the grating structure can be directly correlated to variations in the FBG’s spectral response [50]. UV-laser induced FBGs are restricted to lower temperatures (< 600 °C) because of the instability of the Δn at higher temperatures. FBGs written with fs-IR radiation exhibit high thermal stability when written in the Type II regime [10] making them suitable for sensing applications such as monitoring furnaces, combustion situations, etc. A long-term study of Type II fs-IR FBG stability was performed by writing large Δn Type II gratings in SMF-28 and pure silica core fibers that were then heated to 1000 °C [51]. FBG temperature was maintained for 150 hours while monitoring the FBG's reflectivity and resonant wavelength (Fig. 7 ). There was no degradation of the FBG strength (Δn = 1.7 × 10−3) for the duration of the test. An increase in Δn was observed and is likely a result of both Type I and Type II index change being written simultaneously. The peaks of the complex interference pattern are sufficiently intense to ionize the glass producing an index change that is durable with temperature. In the valleys of the interference pattern, the intensity is below the Type II threshold, however some Type I index change is generated. As the device is annealed, the permanent Type II index change remains while the annealable Type I index change is erased resulting in a higher Δn contrast. The temperature of the fs grating was subsequently increased and kept at 1050 °C for 100 hours during which the Δn decreased slightly from 1.7 × 10−3 to 1.6 × 10−3. Spectra taken initially at room temperature and after a further 100 hours at 1050 °C are shown in inset of Fig. 7. A drift of the Bragg resonance to longer wavelengths of 0.2 nm was detected at the end of experiment. When the fiber is pre-annealed at high temperatures in order to relax residual stresses, resulting Type II fs-IR FBGs are then operable up to 1200 °C [52].
Fig. 7 Grating reflectivity translated into index modulation (Δn) for a thermally stable grating. Grating strength remains constant after 250 hrs at temperatures ≥ 1000 °C. Inset picture is the grating spectrum as a function of temperature.
FBGs written in the Type II regime sometimes suffer from broadband scattering losses which can limit the number of gratings that can be concatenated into a fiber sensor array. It was recently demonstrated that writing strongly overcoupled gratings in H2-loaded Ge-doped SMF-28 in the Type I regime resulted in FBGs with Type II thermal stabilities but without the associated scattering loss [53].
7.2 Sensor Applications of fs-IR Laser Induced FBGs in Sapphire
For temperatures > 1200 °C, silica based fibers are inadequate. The most successful optical fiber used for high temperature sensing is the single crystal sapphire fiber that has a tg of ~2030 °C. Unlike conventional single mode fibers, sapphire fibers are made in the form of rods absent a cladding layer making them sensitive to bending losses. With fiber diameters commercially available (see Fig. 8(a) ), guided light is highly multimode at 1550 nm. Present sapphire fiber sensors are mostly based on Fabry-Perot structures within the fiber producing a broadband interferometric signal that varies with temperature [54]. Such devices are used effectively as point sensors.
Fig. 8 Cross sections of commercially available (a) 125 μm and (b) 60 μm diameter sapphire fiber, (c) The grating structure inscribed in 150 μm diameter fiber with (d) multimode (red) and single mode (green) reflection responses. Single mode field expander shown in (e) that resulted in single mode reflection spectrum shown in green in (d).
Laser inscribed sapphire FBGs (SFBGs) are naturally multimode devices producing broad bandwidth reflections that are not as sensitive to temperature and strain as narrowband single mode FBG sensors [22]. The red trace in Fig. 8(d)) presents a typical multimode reflection response from a SFBG when interrogated with a multimode coupler and white light source. The SFBG multimode reflection spectrum is characterized by a large bandwidth having a complicated structure that is a superposition of many modes reflected by the grating. Although the SFBG sensor can be used in the multimode regime, propagation of a single mode is preferable due to the detection advantages offered by a narrowband single mode reflection response. To produce a single mode response, the SFBG was probed using an adiabatic fiber taper to expand the ~10 μm diameter single mode into a fundamental mode approaching the diameter of the sapphire fiber (as shown in Fig. 8(e)) [23]. The fundamental guided mode of the sapphire waveguide is excited producing a single mode reflection response (green trace Fig. 8(d)). In this fashion, single mode reflection responses consistent with existing FBG sensor array interrogators can be generated. SFBGs show no degradation of the grating strength at high temperatures up to 1745 °C [55]. The SFBG has definite advantages over other sapphire fiber sensors that rely on Fabry-Perot etalons at the fiber tip. Unlike Fabry-Perot sapphire sensors, SFBG sensors with different wavelengths could be multiplexed to make distributed optical sensor arrays operable up to 2000 °C.
Cross sensitivity of measurands is a potential source of error in SFBGs as with its silica based FBG counterparts. At very high temperatures, thermal blackbody radiation produces a strong background spectrum that reduces the signal-to-noise ratio (SNR) of the reflected signal [55]. By exciting the fundamental/low order modes of a SFBG, higher SNR values can be obtained. In this fashion, a dual stress/temperature sensor was developed [56]. The SFBG was used to measure strain at elevated temperatures while the blackbody radiation signal was used as a temperature reference. The portion of the wavelength shift dependent on temperature could then be decoupled from the strain.
8. Conclusions
Femtosecond infrared laser inscription of Bragg gratings in various waveguides is presented. FBG inscription with fs-IR radiation and a phase mask is as simple as with the UV laser approach but can be applied to any optical waveguide that is transparent to near IR radiation. Using our technique, FBGs have been inscribed in crystalline sapphire optical fiber, lithium niobate waveguides, glasses such as pure silica, borosilicate, actively doped silica, fluoride, phosphate and photonic crystal fibers. Depending on the intensity used, high quality gratings can be easily manufactured for telecommunication type applications such as narrowband filtering or fiber laser cavity mirrors. High reliability gratings can be directly written through standard protective coatings of fibers, removing extra processing steps in grating manufacturing. With greater intensities, high reflectivity gratings can be written that are stable up to the glass transition temperature of the material into which they are inscribed. Such gratings show potential for high temperature sensing applications. Gratings in sapphire fiber could conceivably be used in high temperature applications up to 2000 °C.
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