Fiber Bragg grating inscription by high-intensity femtosecond UV laser

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J. Opt. Soc. Am. B / Vol. 22, No. 2 / February 2005

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Fiber Bragg grating inscription by high-intensity femtosecond UV laser light: comparison with other existing methods of fabrication Stephen A. Slattery and David N. Nikogosyan Department of Physics, National University of Ireland, University College Cork, Cork, Ireland

Gilberto Brambilla Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK Received June 16, 2004; revised manuscript received August 31, 2004; accepted September 8, 2004 By use of high-intensity (⬇200 GW/cm2) femtosecond 264-nm laser light and a phase mask technique, Bragg grating inscription in a range of different photosensitive and standard telecom fibers (both H2 -free and H2 -loaded) was studied. The dependences of the induced refractive index modulation versus the incident fluence as well as the thermal decay curves were compared with similar dependences for gratings fabricated by other existing methods. It was shown that with high-intensity UV laser irradiation, two-quantum photoreactions occur in the irradiated fiber core, that result in a significant photosensitivity enhancement of the investigated fibers in comparison with conventional low-intensity 248-nm exposure (by 6–128 times, depending on fiber type and irradiation intensity). © 2005 Optical Society of America OCIS codes: 140.3610, 190.4180, 220.4610, 230.1480, 060.2290.

1. INTRODUCTION For the inscription of fiber Bragg gratings (FBGs) and long-period fiber gratings (LPFGs) standard KrF* excimer lasers at 248 nm, as well as continuous UV sources at ⬇240 nm, are generally used.1–3 The refractive index change generated in fibers at these wavelengths is connected with the absorption band of defects in germanosilicate glass and is mainly of a single-photon nature. Recently it was shown that LPFG recording with deep-UV (157 nm), F2 excimer laser radiation,4,5 as a result of the high energy of the light quanta, considerably reduces (by more than two orders of magnitude) the necessary light fluence. However, it is practically impossible to use this technique for Bragg grating fabrication because of the problem of transporting deep-UV radiation through the air, phase mask and fiber cladding. Another way to deliver a high excitation energy into the fiber core is to use multiple-photon absorption of highintensity, Ti:sapphire laser radiation at 800 nm. Such an approach was used for LPFG fabrication five years ago.6 Very recently, by means of tight focusing through a phase mask, Bragg gratings were written in standard SMF-28 telecom fiber and in silica-core fiber.7,8 It was shown that mutiple-photon IR inscription could be done without fiber hydrogenation, and the recorded FBGs possessed good temperature stability. However, according to recent publications9,10 the implementation of this approach requires careful positioning of the phase mask with respect to the irradiated fiber; failure to meet this requirement immediately leads to a high value of outband (‘‘gray’’) losses near the transmission loss peak in the irradiated fiber.7,8 A third way to reach a high value of excitation energy 0740-3224/2005/020354-08$15.00

inside the germanosilicate fiber core is to use highintensity UV laser irradiation with a subsequent two-step (two-quantum absorption through an intermediate electronic state) or two-photon (two-quantum excitation through a virtual-state) excitation process (see Fig. 1). It is well known that with the rise of irradiation intensity from the megawatt to the gigawatt range, the probability of the occurrence of two-quantum photoprocesses (both two-step and two-photon) in the irradiated medium greatly increases.11 This leads to an enhancement in the photosensitivity (quantum yield of refractive index modulation) of UV irradiated fibers (indeed, their cores) because of the involvement of additional excitation routes. In the case of photosensitive fibers possessing a large linear absorption at the irradiation wavelength, these additional excitation pathways are mostly two-step photoreactions sharing their first excitation quantum with the linear one-step (single-quantum) process. In the case of standard telecommunication fibers having much lower linear absorption at UV irradiation wavelengths (e.g., ⬇2.8 cm⫺1 at 264 nm),12 the photosensitivity improvement is due to the realization of the two-photon excitation channel (see the discussion below). Although FBG recording by a phase mask technique through two-step excitation was realized a decade ago (with an excimer ArF* laser at 193 nm and a light intensity of ⬇0.1 GW/cm2),13,14 the second possible excitation channel (through two-photon absorption) was experimentally demonstrated by us recently with high-intensity (⬇100 GW/cm2) UV laser light.15 We have also investigated the photosensitivity enhancement in photosensitive and standard fibers with the change from relatively low-intensity 248-nm irradiation to high-intensity 264-nm © 2005 Optical Society of America

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Vol. 22, No. 2 / February 2005 / J. Opt. Soc. Am. B

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tronics Inc.). The longitudinal displacement of the lens with respect to the fiber allowed us to vary the incident UV irradiation intensity. In accordance with Ref. 18 the incident pulse peak intensity and total incident irradiation fluence were calculated from the following equations, respectively: I⫽

冉冊 ␲

⑀p

3/2

ln 2

E⫽ Fig. 1. Simplified scheme of photoexcitation and energy levels in germanosilicate glass.

irradiation,15,16 and have shown that in standard telecom fiber, the photosensitivity improvement is more noticeable as a result of realization of a two-photon excitation pathway. In this paper we thoroughly investigate FBG fabrication in a number of commercial, standard telecom and photosensitive fibers (both H2 -free and hydrogenated) as a result of high-intensity (100–300 GW/cm2) 264-nm femtosecond irradiation and provide comparisons with other existing methods of FBG inscription.

2. EXPERIMENTAL SETUP The scheme of the experimental setup is given in Fig. 2. For the inscription of FBGs we used the fourth-harmonic radiation of a commercial, femtosecond, Nd:glass laser (Twinkle, Light Conversion Ltd., Lithuania).17 This laser system emits femtosecond 264-nm pulses that are Gaussian in time and space and were characterized by us earlier.18 The measured pulse duration was 220 fs (FWHM), the beam diameter 0.3 cm (FWHM), the repetition rate 27 Hz, and the pulse energy up to 300 ␮J. The femtosecond UV pulses were focused by a fused silica cylindrical lens of 21.8-cm focal length through a phase mask and onto the fiber (with acrylate coating removed), which was placed behind the phase mask at a distance of ⬇100 ␮m. The phase mask was fixed together with the fiber onto a three-dimension positioner that could be aligned relative to the laser beam. In all the experiments described below we used the static phase mask technique (as opposed to the scanning phase mask technique employed in Ref. 15). A specially designed phase mask (Ibsen Photonics) with a 1.07-␮m pitch optimized for 264-nm illumination and of small thickness (1 mm) was employed in the experiments. This allowed us to reduce significantly the losses due to two-photon absorption in the substrate of the phase mask. In a separate experiment we measured the twophoton absorption coefficient of the phase mask material and found it to be equal to 2.3 ⫻ 10⫺11 W/cm2 , which is close to the best values for the Suprasil fused silica samples (1.7 ⫻ 10⫺11 cm/W) reported in Ref. 18. The incident laser pulse energy was measured with a pyroelectric detector (Ophir Optronics Inc. PE10) connected to a Laserstar energy meter system (Ophir Op-

␲w

2

2 ln 2

冉

␶w

2

4

冉

F⫺S F

⑀ pN F⫺S F

⫺

l nF

冊

⫺

,

l nF

冊

,

(1)

(2)

where ⑀ p is the pulse energy incident on the fiber; F is the focal length of the lens at 264 nm; S is the distance between the focusing lens and the fiber; l is the thickness of the phase mask; n is the refractive index of fused silica (phase mask) at 264 nm (n ⫽ 1.5007); 19 ␶ is the pulse duration at FWHM, equal to 220 ⫾ 10 fs; 18 w is the beam radius at FWHM, equal to 1.51 ⫾ 0.02 mm; 18 and N is the total number of laser pulses incident on the fiber. The relative accuracy of our fluence and intensity measurements were ⬇1% and 4%, respectively.18 The monitoring of Bragg grating transmission during inscription was done with an AQ4222 EE LED source and AQ6317C optical spectrum analyzer (both supplied by Yokogawa Europe BV). The UV exposure was computer controlled with LabVIEW software and a peripheral component interface–general-purpose interface bus card (both from National Instruments). For comparison purposes we also performed the FBG inscription with conventional 248-nm radiation from a KrF* excimer laser (GSI Lumonics). The beam size, pulse duration and repetition rate were 7 ⫻ 10 mm, 20 ns, and 20 Hz, respectively. The beam was focused by a cylindrical lens of focal length 150 mm onto a 3-mm thick phase mask optimised for 248-nm exposure with a pitch of 1.060 ␮m. The fiber was positioned within a few micrometers of the phase mask. The incident intensity was fixed at 25 MW/cm2. A small section (3 mm long) of the beam was used to write the grating. The real-time monitoring of Bragg grating transmission was done with a broadband LED source and an optical spectrum analyzer (HP 70950A). In our experiments we used three different single-mode fibers: (1) standard telecom fiber SMF-28 (Corning) with

Fig. 2. Scheme of the experimental setup. EE-LED, edgeemitting LED; OSA, optical spectrum analyzer.

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a core diameter of 8.2 ␮m and numerical aperture of 0.14; (2) photosensitive GF1 fiber from Nufern with a core diameter of 9.0 ␮m and numerical aperture of 0.13; and (3) photosensitive, boron-codoped PS1250/1500 fiber from Fibercore with a core diameter of 6.6 ␮m and numerical aperture of 0.13. Sensitization of the fibers was performed in a hydrogen atmosphere at ⬇15 MPa and 70 °C for 2 weeks. The induced refractive index modulation was calculated by ⌬n mod ⫽

⫽

␭

␲␩L

tanh⫺1 共 1 ⫺ T 兲 1/2

␭ 2␲␩L

冋

ln

1 ⫹ 共 1 ⫺ T 兲 1/2 1 ⫺ 共 1 ⫺ T 兲 1/2

册

,

(3)

where T is the transmission loss peak of a uniform grating; ␭ is the grating operation wavelength; L is the length of the grating; and ␩ is the mode overlap parameter (fraction of the fiber mode power contained in the core), which is given by

␩⫽

␲ 2d 2K 2 ␭ 2 ⫹ ␲ 2d 2K 2

,

Fig. 3. Growth of grating strength and shift of the transmission loss peak in a FBG recorded in H2 -loaded Nufern GF1 fiber with different light fluences and incident intensity of 231 GW/cm2. The linewidth values of the recorded gratings at the 3 dB level are 0.14, 0.28, and 0.59 nm at fluences 14, 38, and 120 J/cm2, respectively.

(4)

where d is the fiber core diameter and K is the numerical aperture of the fiber. The latter equation can be easily deduced from the ratio of the power of the lowest-order mode in the fiber core to that in the cladding, given in Ref. 20 as P core /P clad ⫽

␲ 2d 2K 2 ␭2

.

(5)

Temperature stability studies of recorded FBGs were performed by the isochronal annealing approach.21 The fiber was kept in a quartz cylinder inside an oven (Carbolite MTF 12/25/250). The temperature of the oven was increased from room temperature to 50 °C (or 100 °C) and kept at that temperature for 0.5 h. The changes in the grating strength and the position of the transmission loss peak were monitored by an optical spectrum analyzer (Ando AQ6317C). After that the temperature was raised in steps of 50 °C (or 100 °C) and kept at each temperature for a half hour; each time, the grating signal was recorded. The normalized refractive index modulation (proportional to the integrated coupling constant) was calculated and graphed against the temperature.

3. RESULTS AND DISCUSSION Figures 3–5 show the increase in the transmission loss peak and the simultaneous redshift of the FBG wavelength with the irradiation fluence for three different fibers under high-intensity 264-nm exposure. It is also seen from these graphs that the rise in grating strength is accompanied by an increase in the grating linewidth. Figures 6 and 7 demonstrate the experimental curves of the refractive index modulation versus the irradiation fluence in FBGs recorded in four different fiber samples with high-intensity femtosecond UV irradiation. From these

Fig. 4. Growth of grating strength and shift of the transmission loss peak in a FBG recorded in H2 -loaded Corning SMF-28 fiber with different light fluences and incident intensity of 241 GW/cm2. The linewidth values of the recorded gratings at the 3 dB level are 0.25, 0.40, and 0.62 nm at fluences 81, 115, and 228 J/cm2, respectively.

graphs it follows that the photosensitivity of the investigated fibers increases in the following order: H2 -free Nufern ⬍ H2 -free Fibercore ⬍ H2 -loaded SMF-28 ⬍ H2 -loaded Nufern. For SMF-28 the refractive index modulation is proportional to the irradiation intensity (Fig. 6); this observation confirms the two-quantum character of the photochemical reactions involved. This also agrees with our previous findings.15 Using the elementary expressions for single-quantum and two-photon absorption and the fixed-field approximation, and assuming a rectangular shape for the laser pulse, one can easily estimate the fraction of UV light absorbed in the cladding and core of SMF-28 fiber at highintensity 264-nm excitation. In our previous work12 we measured the linear and two-photon absorption coefficients for 3.5-mol.% Ge-doped fused silica preform, which corresponds in dopant concentration to the core of

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SMF-28 fiber (3 mol.% Ge). The values obtained at 264 nm were 2.8 cm⫺1 and 42 ⫻ 10⫺11 cm/W. The twophoton absorption coefficient for pure fused silica (fiber cladding) at 264 nm is 1.8 ⫻ 10⫺11 cm/W. 12 Using this data and the geometrical parameters of SMF-28 given above, we can show that with high-intensity (240 GW/cm2) 264-nm irradiation, the fiber cladding absorbs only 2.5% of the incident light energy (by the two-photon absorption mechanism), while the fiber core absorbs 8.5% of the light energy entering the core (8.3% by two-photon absorption and 0.2% by linear absorption), or 8.3% of the incident light energy. Therefore we can ignore linear absorption (and hence the two-step excitation process) in the fiber core of SMF-28 and consider the FBG inscription process in SMF-28 fiber (Fig. 6) a solely two-photon one. On the other hand, from these estimates it follows that the light energy absorbed in the fiber core exceeds by only 3.3 times the amount absorbed in the cladding. The bandgap energy value for fused silica is 9.3 eV,22 which is below the doubled quantum energy value 2h ␯

Fig. 5. Growth of grating strength and shift of the transmission loss peak in a FBG recorded in H2 -free Fibercore PS1250/1500 fiber with different light fluences and incident intensity of 205 GW/cm2. The linewidth values of the recorded gratings at the 3 dB level are 0.13, 0.28, and 0.49 nm at fluences 298, 775, and 1728 J/cm2, respectively.

Fig. 6. Refractive-index modulation versus fluence for H2 -loaded Corning SMF-28 fiber irradiated at two different incident intensities.


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Fig. 7. Comparison of refractive index modulation curves for three photosensitive fiber samples: H2 -free and H2 -loaded Nufern GF1 and H2 -free Fibercore PS1250/1500.

Fig. 8. Comparison of refractive index modulation curves for H2 -free Fibercore PS1250/1500 fiber after femtosecond (264 nm, 205 GW/cm2) and nanosecond (248 nm, 25 MW/cm2) irradiation.

⫽ 9.39 eV. Therefore, in principle there could be some refractive index modulation in fiber cladding as well; however, we never observed any significant outband (‘‘gray’’) losses near the transmission loss peak, which are characteristic of the multiple-photon IR approach.7,8 At the moment we cannot give similar estimates for photosensitive fibers irradiated with high-intensity 264-nm radiation as the corresponding two-photon absorption coefficient values for the fiber core material are unknown. However, it is clear that with an increase in dopant concentration the relative amount of light energy absorbed in the core will increase in comparison with standard telecom fiber. From Fig. 7 it is clear that hydrogenation of Nufern GF1 fiber increases its photosensitivity by ⬇40 times. Corresponding to this, we failed to record FBGs in H2 -free SMF-28 under present experimental conditions. Further increase of the irradiation intensity (above 250 GW/cm2) immediately led to a rise in two-photon absorption losses in the phase mask substrate, which limited the UV intensity incident on the fiber.

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Figures 8 and 9 present the results of a comparative study of the refractive index modulation change with light fluence for H2 -free Nufern GF1 and Fibercore PS1250/1500 fibers taken with femtosecond and nanosecond UV irradiation at wavelengths of 264 and 248 nm, respectively. From these graphs it follows that the photosensitivity of these fibers is significantly higher at the femtosecond (264 nm) irradiation than at the nanosecond (248 nm) one. Similar data were obtained for H2 -loaded Nufern GF1 and Corning SMF-28 fibers (data not shown). The observed behavior of the wavelength shift of the transmission loss peak (Figs. 10 and 11) was similar to that of the refractive index modulation curves (Figs. 6 and 7), which is not surprising because according to simple theoretical considerations for low exposure times the shift in FBG wavelength is proportional to the refractive index change.2 Our results on thermostability of FBGs recorded with high-intensity 264-nm femtosecond irradiation are presented in Figs. 12–14. From these graphs it follows that FBGs recorded in Fibercore PS1250/1500 fiber exhibit the

Fig. 9. Comparison of refractive index modulation curves for H2 -free Nufern GF1 fiber after femtosecond (264 nm, 180 GW/cm2) and nanosecond (248 nm, 25 MW/cm2) irradiation.

Fig. 10. Wavelength change of transmission loss peak versus light fluence for H2 -loaded Corning SMF-28 and Nufern GF1 fibers.

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Fig. 11. Wavelength change of transmission loss peak versus light fluence for H2 -free Fibercore PS1250/1500 and Nufern GF1 fibers.

Fig. 12. Thermal stability of FBG recorded in a H2 -free Fibercore PS1250/1500 fiber with 264-nm radiation at incident intensity of 138 GW/cm2 and incident fluence of 0.38 kJ/cm2; the initial grating strength was 12.7 dB.

lowest thermal stability. The refractive index modulation value decreases by five times at a temperature of 400 °C (Fig. 12). The refractive index modulation value measured in annealed Nufern GF1 fiber decreases by five times at a temperature of ⬇700 °C. It should be emphasized that H2 -loaded and H2 -free Nufern GF1 fibers have different decay curves. In the temperature range 100– 600 °C the H2 -free Nufern fiber is more temperature resistant than the H2 -loaded one, whereas at temperatures above 600 °C the H2 -loaded Nufern GF1 fiber is more resistant (Fig. 13). The thermal decay curves for FBGs created in hydrogenated Nufern fiber at high-intensity 264-nm and conventional 248-nm irradiations are similar (data not shown). In contrast, the thermal decay curves for FBGs recorded in H2 -loaded SMF-28 by 264-nm and 248-nm laser sources differ significantly (Fig. 14). These results show that FBGs fabricated in hydrogenated standard telecom fiber SMF-28 possess the highest thermal stability of all fibers investigated; its refractive index modulation value decreases by five times at a temperature of more than 800 °C.

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Fig. 13. Thermal stability of FBGs inscribed in H2 -free and H2 -loaded Nufern GF1 fibers with 264-nm radiation at incident intensities of 164 and 231 GW/cm2 and incident fluences of 7.18 and 0.26 kJ/cm2, respectively; the initial grating strengths were 8.4 and 31.7 dB, respectively.

Fig. 14. Thermal stability of two FBGs recorded in H2 -loaded Corning SMF-28 fiber at 248- and 264-nm irradiation. The incident intensities were 0.025 and 166 GW/cm2 and incident fluences were 16.5 and 0.24 kJ/cm2, respectively. The initial grating strengths were 10.4 and 11.6 dB, respectively.

During the annealing process the grating wavelength experiences a redshift due to thermal expansion of the fiber. The typical experimental dependence of a FBG transmission loss peak wavelength versus the temperature recorded in H2 -free Nufern GF1 fiber is given in Fig. 15. The measured thermal shift of the Bragg wavelength for all fibers used was in the range (0.9– 1.1) ⫻ 10⫺2 nm/°C, which agrees well with the estimate of (1 – 2) ⫻ 10⫺2 nm/°C given in Ref. 2. Let us compare the photosensitivity results obtained by us with a high-intensity 264-nm femtosecond source against those of a conventional 248-nm KrF* excimer laser. The results are presented in Table 1. We discuss first the non-loaded fibers (Figs. 8 and 9). To record a 9.95-dB grating in a Fibercore PS1250/1500 fiber at 248-nm irradiation, an incident laser fluence of 4.4 kJ/cm2 was needed. For the inscription of a similar grating at 264-nm irradiation with incident intensity of


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205 GW/cm2, 5.7 times less fluence was necessary. To fabricate a FBG of 6.92 dB in H2 -free, Nufern GF1 fiber with 248-nm radiation, 11.9 kJ/cm2 incident fluence was applied. This is 7.6 times more than the amount that was needed in the case of 264-nm irradiation with incident intensity of 180 GW/cm2. To create a 10.2-dB grating in photosensitive, H2 -loaded Nufern GF1 fiber, we needed to apply 18 times less fluence with 264-nm irradiation than with 248 nm. Similarly, for H2 -loaded, Corning SMF-28 fiber at 264-nm irradiation, we needed to use 85 times less fluence (if irradiating with 167 GW/cm2 intensity), or even up to 128 times less fluence value (if irradiating with 241 GW/cm2 intensity), compared with 248-nm irradiation. Further comparison could be done with literature data on FBG inscription by a low-intensity (0.6 MW/cm2) pulsed UV copper vapor laser emitting at 255 nm.23 To induce a refractive index change of 0.6 ⫻ 10⫺3 in H2 -free Fibercore PS1250/1500, it was necessary to apply a total incident fluence of 5 kJ/cm2. In our case, the same refractive index change was produced with a total incident fluence of 1.5 kJ/cm2 (Fig. 7), which is ⬇3 times smaller. When irradiating H2 -loaded SMF-28 fiber with 255-nm pulses, a refractive index modulation of 1 ⫻ 10⫺3 was produced with a total incident fluence of 12.5 kJ/cm2. In our case, irradiating at 264 nm with an intensity of 241 GW/cm2 (Fig. 6), we reached the same refractive index modulation with a total incident fluence of 0.32 kJ/cm2, which is ⬇40 times smaller. The same copper vapor laser can operate at 271 nm with a pulse intensity of 1.2 MW/cm2.23 It is interesting to compare the photosensitivity of H2 -loaded SMF-28 fiber at this wavelength with our results at 264 nm. To produce a refractive index modulation of 10⫺3 one should apply 271-nm light with a fluence of 158 kJ/cm2. For 264-nm irradiation with an intensity of 241 GW/cm2, we need a fluence value of only 0.32 kJ/cm2 (Fig. 6), which is different by 494 times. This comparison supports our suggestion made above regarding the two-photon nature

Fig. 15. The shift of the FBG wavelength versus temperature recorded in H2 -free Nufern GF1 fiber with an incident intensity of 164 GW/cm2 and an incident fluence of 7.18 kJ/cm2; the initial grating strength was 8.4 dB.

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Table 1. Comparison between High-Intensity 264-nm Approach of FBG Inscription and the Conventional One Irradiation Wavelength [nm]

Incident Intensity [GW/cm2]

Transmission Peak Loss [dB]

Incident Laser Fluence [kJ/cm2]

H2 -free Fibercore PS1250/1500

248 264

0.025 205

9.95 10.65

4.4 0.775

H2 -free Nufern GF1

248 264

0.025 180

6.92 7.02

11.9 1.573

H2 -loaded Nufern GF1

248 264

0.025 231

10.17 10.19

0.7 0.038

H2 -loaded Corning SMF-28

248 264 264

0.025 167 241

10.31 10.34 9.77

14.7 0.172 0.115

Fiber

of the photochemistry involved in the case of highintensity 264-nm irradiation of H2 -loaded SMF-28 fiber. It is also of interest to compare our results with the multiple-photon infrared approach to the fabrication of FBGs,7,8,24,25 which utilizes 120-fs, 800-nm pulses generated by a Ti:sapphire laser. The authors of Refs. 7, 8, 24, and 25 give contradictory data on UV irradiance: In Refs. 7 and 24 the incident intensity of 12,000 GW/cm2 for FBG inscription in H2 -free SMF-28 is mentioned, whereas in Refs. 8 and 25 the cited infrared peak intensity is only 2900 GW/cm2 (⬇4 times less). In Ref. 25 the authors state that for FBG inscription in H2 -loaded SMF-28 fiber they needed of an order less infrared intensity. If we combine all the data from Refs. 7, 8, 24, and 25 it follows that to induce a refractive index change of 1.85 ⫻ 10⫺3 in H2 -loaded SMF-28 by high-intensity (3000–12,000 GW/cm2) 800-nm femtosecond radiation, an incident fluence of 0.8–3.2 kJ/cm2 is needed. In our case with 264-nm irradiation at a 241-GW/cm2 incident intensity, we required a fluence value of 0.65 kJ/cm2 (see Fig. 6), which is in all cases smaller than that under the IR multiple-photon approach. We tried to fabricate Bragg gratings with the highest possible values for the transmission loss peak with highintensity 264-nm irradiation. We have shown that the values of 30 dB for photosensitive fibers and of 45 dB for hydrogenated standard telecommunication fibers are easily reachable by our technique. The experimental transmission spectra with maximal grating strength recorded in H2 -free Fibercore and H2 -loaded SMF-28 fibers are presented in Figs. 16 and 17, respectively. The further increase is limited by the sensitivity of our spectral analyser and/or by the absence of a splicer for making loss-free connections in the case of photosensitive fibers. It should be emphasized that we did not observe any saturation of the refractive index modulation, especially in the case of two-photon excitation (SMF-28 fiber). The comparison between our FBGs and those with suppressed cladding modes recently fabricated by the IR multiple-photon approach9,10 shows that in both cases the recorded gratings possess similar values of off-band losses and linewidth. On the other hand FBGs inscribed by

high-intensity 264-nm femtosecond laser pulses are thermally erased at lower temperatures (at least partly), which in the case of photosensitive fibers could be connected with the involvement of single-quantum processes.

Fig. 16. Maximal grating strength achieved in H2 -free Fibercore PS1250/1500 fiber irradiated at incident intensity of 205 GW/cm2 and incident fluence of 2.68 kJ/cm2.

Fig. 17. Maximal grating strength achieved in H2 -loaded Corning SMF-28 fiber irradiated at incident intensity of 241 GW/cm2 and incident fluence of 1.22 kJ/cm2.

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4. CONCLUSION Two-quantum high-intensity 264-nm inscription of Bragg gratings in standard telecom (Corning SMF-28) and photosensitive (Nufern GF1, Fibercore PS1250/1500) fibers was investigated. The refractive index change dependences on incident fluence at 264 nm irradiation obtained, as well as thermostability results, were compared with similar graphs obtained for low-intensity irradiation at 248 nm, 255 nm, and 271 nm, as well as with the results obtained with high-intensity, IR multiple-photon excitation at 800 nm. It was shown that for the fiber samples investigated, the efficiency of grating inscription is maximal with high-intensity femtosecond 264-nm irradiation, whereas the thermal stability of our gratings is comparable with that obtained by other techniques but less than that observed under 800-nm exposure.

ACKNOWLEDGMENTS We acknowledge the contribution of Adrian Dragomir (currently with the European Patent Office), who participated in the early stages of this study, and the fruitful cooperation with Kristian Buchwald (Ibsen Photonics). The authors are also grateful to the Science Foundation Ireland for their generous financial support (grant 0.1/ IN.1/IO58).

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Corresponding author D. N. Nikogosyan’s e-mail address is [email protected].

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