1. Introduction

Fiber Bragg gratings (FBGs) are usually produced by fiber exposure to UV radiation with sufficient absorption of the fiber core glass. As a rule, an increase of the UV-radiation photon energy results in higher efficiency of the grating formation. High photosensitivity in the case of 157-nm excimer F₂-laser exposure (7.9-eV photon energy) has been reported [1]. However, the use of a F₂ laser for FBGs fabrication has serious disadvantages, such as problems of deep-UV radiation delivery through the air, the fiber cladding and the phase mask.

Another method of refractive index modification is based on multiphoton absorption of high-intensity IR femtosecond pulses. This approach was used for the fabrication of long-period fiber gratings (LPFGs) by 800-nm femtosecond radiation of intensity ~10¹⁴ W/cm² [2]. Recently, FBGs were written with the help of specially made phase masks using high-intensity 800-nm femtosecond pulses [3, 4].

By using harmonics of IR radiation from femtosecond lasers, we can reduce the number of photons required for the multiphoton absorption, thereby decreasing the radiation intensity. The generation of harmonics occurs efficiently in femtosecond lasers. LPFG fabrication by 400-nm femtosecond pulses of intensity ~10¹⁰ W/cm² was demonstrated in Ref. 5. Also, efficient LPFG inscription was performed using the 264-nm fourth-harmonic of a femtosecond Nd:glass laser [6]. Recently, with the help of the 264-nm femtosecond radiation, we obtained strong FBGs in low- and high-Ge-doped fibers [7]. In this paper, we report the fabrication of FBGs in a Corning SMF-28 fiber, a P₂O₅-doped-core fiber and a pure-silica-core fiber by third-harmonic radiation of a femtosecond Ti:sapphire laser.

2. Experimental

800-nm, 45-fs pulses from a Spitfire Ti:sapphire amplifier (Spectra-Physics) with a pulse energy of 1 mJ and a pulse repetition rate of 1 kHz were converted to the third harmonic with the help of a TP-1A Tripler (Uniwave Technology). The 267-nm radiation with a pulse duration of ~150 fs, a pulse energy of 130µJ, and a beam diameter of 2.5 mm was directed on a fiber by a 38-mm fused-silica cylindrical lens through a phase mask. Polarization of the laser beam was perpendicular to the fiber axis. The fiber without polymer coating was fixed behind the phase mask at a distance of about 100 µm. The incident radiation intensity (1-2×10¹¹ W/cm²) on the fiber was varied by changing of the distance between the lens and the fiber. The intensity on the fiber was limited by a two-photon absorption in the phase mask substrate (2-mm thickness). The standard phase mask optimized for 248-nm radiation (Bragg Photonics) had a period of 1.07 µm. The transmission spectra of the written FBGs were measured during inscription with a superluminescent diode and an Ando AQ6317B optical spectrum analyzer with a resolution of 0.05 nm.

We compared the results of the 267-nm irradiation with the refractive index change induced by 193-nm and 157-nm excimer lasers. In the case of the excimer lasers, we used the method of Δn_ind measuring based on an intrafiber Mach-Zehnder interferometer consisting of two LPFGs [8]. The 193-nm ArF laser (CL-5000, Physics Instrumentation Center of the A.M. Prokhorov General Physics Institute, Russian Academy of Sciences) generated 8-ns, 30-mJ pulses at a repetition rate of 10 Hz, and the 157-nm F₂ laser (CL-7000, Physics Instrumentation Center), 18-ns, 20-mJ pulses at a repetition rate of 10 Hz.

The following fibers were used:

- H₂-loaded and pristine GeO₂-doped telecom Corning SMF-28 fiber (3-mol% GeO₂, Δn_core-clad=0.005, λ_c=1.25 µm);

H₂-loaded P₂O₅-doped “P903” fiber (12- mol% P₂O₅, Δn_core-clad=0.01, λ_c=1.07 µm);

H₂-loaded and pristine pure-silica-core fluorine-depressed cladding fiber (Δn_core-clad=0.007, λ_c=0.99 µm).

The fibers were loaded with hydrogen at a pressure of 120 atm, at 80°C during 2 days.

3. Results and discussion

In H₂-loaded SMF-28 fiber, we inscribed a 33.2-dB FBG (Fig. 1) with an intensity of 1.2×10¹¹ W/cm² and an exposure dose of 0.37 kJ/cm² (20-s irradiation time, 20000 pulses). The out-of-band insertion loss of the grating was <0.2 dB. The 33.2-dB FBG peak corresponds to the induced refractive index modulation depth Δn_ind=2.4×10^-3, assuming uniform 2.5-mm FBG.

 

Fig. 1. Transmission spectrum of FBG recorded in an H₂-loaded SMF-28 fiber

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Fig. 2. Dependence of refractive-index change on exposure dose of an H₂-loaded SMF-28 fiber.

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We did not observe any saturation of the FBG peak growth. For the FBG in the H₂-loaded SMF-28 fiber, the dependence of the Δn_ind on the exposure dose is shown in Fig. 2. The results of irradiation of H₂-loaded SMF-28 fibers by 193-nm and 157-nm excimer lasers are also depicted for comparison. It should be noted that the refractive index changes were induced by uniform UV beams in the case of 193-nm and 157-nm radiation. However, in our experiments with 193-nm ArF laser, Δn_ind calculated from FBG peaks was always less (at the same dose) than that measured with the help of a Mach-Zehnder interferometer irradiated by uniform UV beam. It should be noted that strong asymmetry of the induced refractive index in the fiber cross-section was revealed earlier in an H₂-loaded SMF-28 fiber under 157-nm irradiation [1]. As seen from Fig. 2, photosensitivities of the H₂-loaded SMF-28 fiber under irradiation at 267 and 157 nm are comparable. It is also possible to compare the efficiency of FBG inscription in this fiber at 267 and 248 nm (KrF excimer laser) [9]. To obtain Δn_ind=8×10^-4 with 248-nm nanosecond pulses (pulse fluence of 0.35 J/cm², pulse intensity of ≈2×107 W/cm²), the exposure dose of ≈⁷ kJ/cm² was used, which is 88 times larger than in the case of 267-nm femtosecond irradiation (80 J/cm²).

A 14.3-dB FBG (Δn_ind=1.3×10^-3) was written in an H₂-free SMF-28 fiber with an intensity of 1.6×10¹¹ W/cm² and an exposure dose of 58 kJ/cm² (40-min irradiation duration, 2.4×10⁶ pulses). To obtain the same Δn_ind in an H₂-loaded SMF-28 fiber, a dose of 0.14 kJ/cm² was required, which was ≈400 times lower than in the case of an H₂-free SMF-28 fiber. Thus, H₂-loading results in dramatic enhancement of the 267-nm photosensitivity of low-Ge-doped SMF-28 fibers.

 

Fig. 3. Dependence of refractive-index change on exposure dose of H₂-loaded P903 fiber.

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Strong FBGs were also fabricated in the H₂-loaded P₂O₅-doped-core P903 fiber. A 41.7-dB FBG (Δn_ind=3.5×10^-3) was written with an intensity of 1.6×10¹¹ W/cm² and an exposure dose of 0.62 kJ/cm² (26-s irradiation time, 26000 pulses). In spite of the high Δn_ind, there was no saturation of the FBG peak growth. The dependence of the Δn_ind on the exposure dose is compared in Fig. 3 with the irradiation results of this fiber by 193-nm and 157-nm lasers. The 267-nm photosensitivity of the fiber is about two times higher than in the case of 157-nm radiation and about ten times higher than in the case of 193-nm radiation. We would like to note that the P₂O₅-doped fiber core has no absorption band at 242 nm (in contrast to Ge-doped fibers), and relatively low Δn_ind=3×10^-4 was induced in other H₂-loaded P₂O₅-doped-core fibers by 248-nm nanosecond radiation with a very high dose of 160 kJ/cm² [10]. The high efficiency of FBGs inscription by 267-nm femtosecond pulses is probably explained by the high excitation energy (9.3 eV) which results from two-photon absorption. Another advantage of the femtosecond laser compared to excimer lasers is the excellent spatial coherence of a nearly TEM₀₀ laser beam.

We also inscribed strong FBG in the H₂-loaded pure-SiO₂-core fluorine-depressed cladding fiber. A 11.5-dB FBG (Δn_ind=1.1×10^-3) was written with an intensity of 1.6×10¹¹ W/cm² and an exposure dose of 54.5 kJ/cm² (38-min irradiation duration, 2.27×10⁶ pulses). In the H₂-loaded SMF-28 fiber (3 mol% GeO₂ core) the same Δn_ind=1.1×10^-3 was achieved with a dose of only ≈0.1 kJ/cm². Without H₂-loading, a very weak FBG (≈0.1 dB) was obtained in the fiber with a dose of >50 kJ/cm². To the best of our knowledge, in SiO₂-core fibers the highest Δn_ind obtained by UV irradiation without damage was reported to equal 5×10^-4 [11].

The results of FBG inscription with 267 nm femtosecond radiation are shown in Table 1. The strong dependence of the 267-nm photosensitivity on H₂-loading and core dopants points to photochemical nature of the induced refractive index, in contrast to “damage FBGs”

Table 1. Results of irradiation with 267-nm femtosecond pulses

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fabricated by 800-nm femtosecond pulses with an intensity of 2.9×10¹² W/cm² [4]. As mentioned above, the incident radiation intensity on the fibers was limited in our experiments by nonlinear absorption in the phase mask substrate. On the other hand, the laser beam width on the fibers was 100–200 µm and high accuracy of the alignment of the incident beam on the fiber core was not required. Also, we can expect the absence of damage of irradiated fibers owing to a low pulse fluence (<30 mJ/cm²) used in our experiments.

 

Fig. 4. Temperature dependence of refractive-index modulation of FBGs written in the H₂-loaded P903 fiber during isochronal annealing.

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Fig. 5. Temperature dependence of refractive-index modulation of FBGs written in an SMF-28 fiber during isochronal annealing.

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Study of thermostability of the fabricated FBGs gives some information on the type of FBGs. Isochronal annealing at each temperature was performed for 20 min, after which the temperature was raised by 100°C. The results of isochronal annealing of the FBG recorded in the H₂-loaded P903 fiber are presented in Fig. 4. The annealing data of FBG fabricated with 193-nm nanosecond pulses are shown for comparison. Thermostability of these FBGs is similar. Annealing characteristics of FBGs written in the H₂-loaded and H₂-free SMF-28 fiber with 267-nm femtosecond pulses (Fig. 5) are also typical for UV-induced Type-I FBGs (significant erasure of the index modulation at T≤500°C). In contrast, thermostability of “damage” FBGs fabricated by 800-nm femtosecond pulses is similar to that of Type II UV-induced damage gratings [4].

4. Conclusion

The fabrication of FBGs by the 267-nm third-harmonic radiation from a femtosecond Ti:sapphire laser is reported to our knowledge for the first time. Strong high-quality Bragg gratings with photoinduced refractive-index modulation of more than 10^-3 were written in an H₂-loaded and pristine telecom Corning SMF-28 fiber, an H₂-loaded phosphosilicate fiber, and an H2-loaded pure-silica-core fluorine-doped cladding fiber using a standard phase mask made for 248-nm radiation. The out-of-band insertion losses of the fabricated FBGs are <0.5 dB.

The refractive index change induced by the 267-nm femtosecond pulses was compared with the results of exposure by 193-nm and 157-nm excimer lasers. The 267-nm photosensitivity of some fibers is higher than in the case of the deep-UV 157-nm irradiation. The high photosensitivity in the case of 267-nm femtosecond pulses is probably due to the high excitation energy (9.3 eV) which results from two-photon absorption [7].

The dependence of the refractive-index change on the exposure dose, the annealing characteristics of the fabricated gratings as well as strong dependence of the 267-nm photosensitivity on H₂-loading and the core dopants are typical for Type-I UV-written FBGs.