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Long-period grating inscription in hydrogen-free SMF-28 fiber by high-repetition-rate femtosecond UV pulses
Optical Fiber Technology 18 (2012) 88–92
Contents lists available at SciVerse ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Long-period grating inscription in hydrogen-free SMF-28 fiber by high-repetition-rate femtosecond UV pulses Bryan J. O’Regan a, David N. Nikogosyan a,⇑, Domas Paipulas b, Viacˇeslav Kudriašov b, Valdas Sirutkaitis b a b
Physics Department, University College Cork, Cork, Ireland Laser Research Center, Vilnius University, Vilnius, Lithuania
a r t i c l e
i n f o
Article history: Received 3 August 2011 Revised 12 December 2011 Available online 31 January 2012 Keywords: Transmission gratings Standard telecom fiber
a b s t r a c t Using femtosecond UV (258 nm) pulses, generated at high-repetition-rate (50 kHz), we managed to record high-quality long-period gratings in a number of fibers, including a standard telecommunication one, SMF-28. Along with the main grating, connected to the refractive index change in the fiber core, at the relatively high intensity of the inscribing UV radiation, I 1.5 TW/cm2, we recorded the formation of an additional long-period grating, based on the refractive index change induced in the fiber cladding. We have compared the temperature sensitivity and the thermal stability of both gratings. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Long-period fiber grating (LPFG) fabrication by femtosecond laser light is known from 1999 [1]. In this pioneering work the radiation of an IR (k = 800 nm) femtosecond laser was tightly focused onto the fiber core of a standard telecommunication fiber. Though the quality of the inscribed LPFG was not the best, annealing experiments demonstrated its increased temperature stability, above 500 °C. Later it was shown [2], that involved photo-inscription process proceeds via the five-photon absorption mechanism and therefore the tight focusing, bringing to very high irradiation intensities (10–200 TW/cm2), is essential. Recently, a number of works, utilizing LPFG fabrication by high-intensity 800 nm femtosecond pulses, were published [3–6]. LPFG inscription with UV (k = 264 nm) femtosecond pulses is known from 2002 [7]. The advantage of this approach is that photo-induction of the refractive index changes in the core of a standard telecom fiber proceeds via the two-photon mechanism [8], as a result much smaller irradiation intensities (100– 500 GW/cm2) are needed [9–11]. This eliminates the necessity of tight focusing the inscribing laser light, making the optical alignment procedure simpler, and provides the uniform illumination of fiber core. Besides, in some cases, e.g. for the photochemical LPFG fabrication in a photonic crystal fiber (PCF), the five-photon 800 nm femtosecond inscription cannot be used at all. The reason is that it is very difficult (if not impossible) to provide the tight focusing of the inscribing IR light beam onto the fiber core through numerous holey tubes inside the PCF. At the same time the twophoton 264 nm femtosecond UV irradiation does the job [12,13]. ⇑ Corresponding author. Fax: +353 21 4276949. E-mail address: [email protected] (D.N. Nikogosyan). 1068-5200/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.yofte.2012.01.004
Though the two-photon femtosecond UV inscription was successfully applied to the number of photosensitive and hydrogenated fibers, until now it was not possible to use it for LPFG creation in a standard hydrogen-free telecommunication fiber. In the current study, using the fourth harmonic radiation of the modern high-repetition-rate femtosecond Yb:KGW laser at 258 nm, we managed to record for the first time LPFGs in H2-free SMF-28 and investigate their properties. 2. Experimental set-up For LPFG fabrication we used a prototype of the commercial high-repetition-rate Yb:KGW femtosecond laser PHAROS (Light Conversion Ltd., Lithuania) [14]. The laser pulses (wavelength k = 1030 nm, average power P = 1.5 W, pulse duration sp = 340 fs at FWHM, repetition rate f = 50 kHz) were frequency-quadrupled to the fourth harmonic using HIRO frequency converter (Light Conversion Ltd., Lithuania) [14]. The LPFG inscription set-up is depicted in Fig. 1. The femtosecond UV pulses (k = 258 nm, energy per pulse ep = 1.3–2.7 lJ, sp 300 fs (FWHM), beam diameter 2w = 0.147 cm (FWHM), f = 50 kHz) were directed by a CaF2 spherical lens with focal distance 46.7 cm onto the fiber (with the polymer jacket removed). For fabrication of LPFGs with the period 475 lm and 3 cm length (63 grooves), a step-by-step inscription method was used. An adjustable slit with 237.5 lm width was placed in front of the fiber at a distance less than 300 lm. Each LPFG groove was irradiated until the necessary fluence value was reached, then the inscribing beam was closed, the fiber was moved perpendicularly to slit along its axis for a distance of 475 lm, and then the laser beam was opened and the next groove was recorded and so on, until the whole sequence of 63 grooves had been created. In each case of LPFG recording, we inscribed a series of grat-
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Fig. 1. Schematic of the experimental set-up.
ings, corresponding to different total accumulated fluence values, and after that determined the fluence value, corresponding to the maximum of the first absorption loss peak (around 22 dB). A motorized stage (Standa, Lithuania) with 1 lm translation accuracy was used to move the fiber during experiment. Fiber was translated along its axis until the maximum number of points was inscribed, after that it was returned back to the initial position. The LPFG grating inscription process and all relevant parameters like grating period, number of points and exposure time were controlled by a computer program. For the irradiation intensity and total accumulated fluence determination, the following expressions were used [15]:
I¼
ep
p
32 sp w2
ln 2
ELPFG ¼
ð1Þ
FS 2 F
2
ep Np d
ð2Þ
pw2 FS 2 ln 2
F
where ep is the pulse energy reaching the slit, Np is the total number of pulses, corresponding to LPFG inscription, d is the energy attenuation coefficient while passing through the slit, F is the focal distance of the lens, S is the distance between the principal plane of the lens and the fiber. The changes in grating transmission loss peaks and their spectral positions were monitored during the LPFG fabrication using broadband light source AQ4305 (Yokogawa Europe BV) and optical spectrum analyzer Q8384 (Advantest, Japan). For the LPFG inscription, three different non-hydrogenated fibers were used: (1) a photosensitive Ge/B co-doped Fibercore PS1250/1500 fiber (Fibercore Ltd., UK), with a Ge content of about 10 mol.%, a core diameter of 7.1 lm, and a numerical aperture of 0.13; (2) less photosensitive Flexcore fiber (Fiber Optics Research Center, Moscow, Russia) with a Ge content of about 4.5 mol.% and a core diameter of 5.4 lm; (3) non-photosensitive standard telecommunication SMF-28 fiber (Elliot Scientific, USA) with a Ge content of about 3 mol.%, a core diameter of 8.2 lm, and a numerical aperture of 0.14. All three fibers had a cladding diameter of 125 lm. As the fiber photosensitivity strongly depends on fiber
Fig. 2. Transmission loss spectra of LPFGs with 475 lm period and 3 cm length, recorded in Fibercore fiber by femtosecond UV pulses at: (a) high repetition rate and (b) low repetition rate [11]. The radiation wavelength, repetition rate, incident light intensity and total accumulated fluence for all 63 grooves are indicated. Note the drastic difference in total fluence values between both approaches.
tension [16], all the LPFG fabrication experiments were conducted at a constant tension value. To insure this, the fiber was tightened prior to the irradiation in fiber holders at the continuous load of a 214 g weight. Temperature sensitivity studies and isochronal annealing of the recorded LPFGs were made with a Carbolite oven MTF 12/25/250. The measurements of temperature sensitivity were performed by changing the temperature in the 20–220 °C range. In order to stabilize the refractive index changes inside the LPFG, the latter was preliminary annealed for 3 h at 120 °C. The temperature of the oven was then increased step by step from room temperature to 220 °C and kept at each intermediate temperature value for a half hour to get a thermal equilibrium inside the LPFG. For the isochronal annealing, the temperature of oven was varied from ambient to 800 °C.
Table 1 Irradiation parameters used at UV (258 nm) high-repetition-rate (50 kHz) high-intensity LPFG inscription in different fibers and corresponding parameters of fabricated longperiod gratings with 475 lm period and 3 cm length. Fiber no.
Fiber type
Intensity (GW/cm2)
Total accumulated fluence (MJ/cm2)
Main peak wavelength (nm)
Main peak value (dB)
1 8 3 11 10 17
Fibercore Fibercore Flexcore Flexcore SMF-28 SMF-28
4 140 280 1400 1540 1540
0.08 0.15 22 93 194 354
1593 1590 1467 1480 1614 1580
17 18 20 24 21 17
Note. The fluence values correspond to the inscription of whole LPFG.
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Fig. 5. Photograph of a section of a LPFG, recorded in SMF-28 fiber by 258 nm highrepetition-rate femtosecond pulses. The size of the bar is 100 lm. The period of areas with the changed refractive index in the fiber cladding is 475 lm.
Fig. 3. Transmission loss spectrum of LPFG with 475 lm period and 3 cm length, recorded in Flexcore fiber by 258 nm high-repetition-rate femtosecond pulses. The radiation wavelength, repetition rate, incident light intensity and total accumulated fluence for all 63 grooves are indicated. Note the additional loss peak at 1098 nm. The linewidth of this additional peak is 10.1 nm, while the linewidth of the main LPFG loss peak at 1480 nm is 30.2 nm.
For microscopy studies, we used an Olympus BX51 microscope, operating in differential interference contrast (DIC) regime. 3. Results and discussion Using 258 nm 300 fs pulses at 50 kHz repetition rate, we managed to record LPFGs with 475 lm period and 3 cm length in all
three fibers. We used for the inscription three different intensities, the lower one, 4 GW/cm2, the middle one, 140–280 GW/cm2, and the highest one, 1400–1540 GW/cm2. However, while the LPFG fabrication in Fibercore fiber proceeds well already at lowest intensity, the LPFGs in SMF-28 could be created only at the highest irradiation intensity. Table 1 summarizes the light irradiation parameters, used in our experiments. From the consideration of the results, gathered in Table 1, it follows, that the increase of irradiation intensity by two orders of magnitude does not impact on the LPFG formation in the Fibercore fiber. This is in line with a single-photon mechanism of photochemical processes, taking place at high-intensity UV excitation of this fiber [17]. Fig. 2a demonstrates the transmission loss spectrum of LPFG, recorded in Fibercore fiber by 258 nm 50 kHz femtosecond pulses.
Fig. 4. Transmission loss spectra of LPFGs with 475 lm period and 3 cm length, successively recorded in SMF-28 fiber by 258 nm high-repetition-rate femtosecond pulses (a– d). The radiation wavelength, repetition rate, incident light intensity and total accumulated fluence for all 63 grooves are indicated. Note, how with the raise of total accumulated fluence the additional loss peak at 1310 nm (a) overlaps with the main LPFG peak (b–d). The linewidth of the additional peak is 10.7 nm (b), while the linewidth of the main LPFG loss peak at 1614 nm is 18 nm (d).
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Fig. 6. Isochronal annealing curves for the two peaks at 1354 nm (additional peak) and 1614 nm (main transmission loss peak) of the LPFG, recorded in a standard telecom fiber (Fig. 4d).
Below, in Fig. 2b, a spectrum of the identical LPFG, recorded in our previous work in the same fiber with 264 nm 183 lJ 220 fs 27 Hz pulses [11], is given. From the comparison of these two spectra, their close similarity follows. The linewidth values of the peaks are also similar, i.e., 18.1 nm for 1590 nm peak and 16.2 nm for the 1589 nm peak. However, the total accumulated fluence values are drastically different, namely, the value obtained at 258 nm 50 kHz fabrication is by more than three orders higher than that obtained at 264 nm 27 Hz one. This means that the increase of repetition rate of inscribing femtosecond UV pulses brings about a much lower efficiency of the formation of refractive index changes in the core of the photosensitive fiber. Fig. 3 demonstrates the transmission loss spectrum of LPFG, recorded in Flexcore fiber by 258 nm 50 kHz femtosecond pulses. With a smaller Ge-doping value in the core of this fiber, 4.5% instead of 10% in the case of Fibercore, we observed even larger values of total irradiation fluence, 22–93 MJ/cm2, than in the case of Fibercore fiber. However, the most interesting feature is the appearance of the additional peak in the region of 1098 nm (only at highest irradiation intensity, I = 1400 Gw/cm2). The conducted studies revealed, that the temperature sensitivity of this peak, 600 pm/°C, differs significantly from the temperature sensitivity value of 80 pm/°C for the main LPFG absorption loss peak at 1480 nm. The correlation coefficient R for both values is 0.998.
Now we will discuss the spectra obtained at 258 nm 50 kHz femtosecond irradiation of a standard telecom fiber SMF-28 (Fig. 4a–d). All four subsequently taken spectra clearly indicate the appearance of the additional peak in the region of 1310– 1354 nm. It should be emphasized, that the literature spectra of LPFGs, recorded by tightly-focused high-intensity 800 nm femtosecond pulses (see, i.e., [3]), show the complete absence of such an additional peak. As during high-intensity 800 nm LPFG fabrications the inscribing radiation is commonly tightly-focused onto the fiber core, we can assign this peak to the additional long-period grating, formed in the fiber cladding. In relation to that it should be noted, that the formation of the refraction index changes, simultaneously both in the core and in the cladding was observed earlier during Bragg grating inscription experiments in a H2-free SMF-28 fiber with 264 nm 27 Hz femtosecond pulses [18]. However, to the best of our knowledge, nobody has reported the simultaneous fabrication of two LPFGs, one in the core and one in the cladding of a standard non-hydrogenated telecom fiber at high-intensity UV irradiation. In the current work, the appearance of the periodic refractive index changes in the cladding of the fiber No. 17 from Table 1 after prolonged high-intensity 258 nm 50 kHz irradiation was revealed by optical microscopy (Fig. 5). The period of the areas with the changed refractive index, deduced from the microscopic studies, agrees well with the period of main LPFG, recorded in the core. The conducted thermal studies demonstrated, that this additional loss peak and the main LPFG absorption loss peak in the region 1584–1614 nm, have different temperature sensitivity values, 140 and 66 pm/°C, respectively. But what is even more important, they have very different isochronal annealing curves. These curves are depicted in Fig. 6. It shows that the decline of main LPFG absorption peak starts at the temperature 350 °C, while the amplitude of the additional peak does not change up to 550 °C. This fact confirms the different nature of two LPFGs, recorded in the core and in the cladding of the same fiber, and agrees with our microscopic investigations. We compared our total accumulated fluence values for 258 nm 50 kHz femtosecond LPFG inscription in H2-free SMF-28 fiber, given in Table 1, with the literature data for LPFG fabrication in the same fiber by high-intensity 800 nm femtosecond pulses, compiled in Table 2. Such comparison shows, that generally the 800 nm femtosecond fabrication is energetically more efficient than the 258 nm 50 kHz one. At the same time, the literature data on 800 nm femtosecond inscriptions show a clear dependence on the laser pulse repetition rate, the higher the repetition rate, the higher the necessary total accumulated fluence value. Such observation is not accidental;
Table 2 Light radiation parameters used for 800 nm femtosecond LPFG inscription in non-hydrogenated SMF-28 fiber (literature data). Pulse energy (lJ)
Pulse duration (fs)
Repetition rate (kHz)
Intensity (TW/ cm2)
Total fluence (kJ/ cm2)
LPFG period (lm)
Number of grooves
Main peak value (dB)
Refs.
0.27 0.27 2.4 0.32
160 300 200 50
200 1 250 1
10 180 170 200
535 3.4 62 2.9
450 486 450 436
89 50 50 58
16 10 21 20
[3] [4] [5] [6]
Note: The fluence values in each case correspond to the inscription of whole LPFG.
Table 3 Temperature sensitivity values for the main LPFG absorption loss peak in H2-free SMF-28 fiber. Inscription wavelength (nm)
LPFG period (lm)
Number of grooves
Main peak amplitude (dB)
Temperature sensitivity (pm/°C)
Refs.
257.5 800 800
475 450 436
63 50 58
23 21 20
66 43 91
Our work [5] [6]
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it is in line with the above observation on the increase of the total accumulated fluence value for the LPFGs created in a photosensitive fiber with change from 264 nm 27 Hz femtosecond inscription to 258 nm 50 kHz one (cf. Fig. 2a and b). Indeed, the difference between the fibers, used in this work, lies mainly in quantity of Ge atoms incorporated in silica glass. After the UV excitation of germanium-related oxygen-deficient centers in Ge-doped fused silica, which could proceed either via the single-photon femtosecond excitation in the case of a photosensitive Fibercore fiber [17], or via the multi-photon IR (800 nm) or UV (267 nm, 258 nm) femtosecond excitations in the case of a standard telecom fiber, all the following chemical processes leading to refractive index changes in fused silica matrix should be similar. This allows us to predict the decrease of the total accumulated fluence value necessary for LPFG inscription with a rejection peak value of about 20 dB with femtosecond 258 nm pulses in hydrogen-free SMF-28 fiber with the decrease of pulse repetition rate from 50 kHz to 1 kHz or so. Finally, Table 3 presents the comparison of temperature sensitivity values for the main LPFG absorption loss peak in H2-free SMF-28 fiber obtained in our work with the literature data on 800 nm femtosecond inscription. Though the grating parameters are slightly different, the agreement between them is reasonably good. 4. Conclusions For the first time, the LPFG inscription with 50 kHz high-intensity femtosecond UV pulses in three different fibers, including a standard telecom one, was realized and the properties of the fabricated gratings were thoroughly studied. It was established that the increase in the repetition rate of the inscribing laser pulse generally leads to the increase of the total accumulated fluence. This finding is in line with the literature data on femtosecond IR (800 nm) LPFG fabrication in non-hydrogenated standard telecom fiber with different pulse repetition rates. It was also observed that, at the relatively high value of incident intensity, about 1.5 TW/cm2, the formation of an additional long-period grating, based on the refractive index changes in the fiber cladding, takes place. Acknowledgments Two of us, B.J.O’R. and D.N.N., would like to thank the Laser Lab Europe programme for the possibility to use the facilities of Laser Research Center at Vilnius University, Lithuania (Grant VULRC 001672).
References [1] Y. Kondo, K. Nouchi, T. Mitsuyu, M. Watanabe, P.G. Kazansky, K. Hirao, Fabrication of long-period fiber gratings by focused radiation of infrared femtosecond laser pulses, Opt. Lett. 24 (1999) 646–648. [2] C.W. Smelser, S.J. Mihailov, D. Grobnic, Formation of type I-IR and type II-IR gratings with an ultrafast IR laser and a phase mask, Opt. Express 14 (2005) 5377–5386. [3] F. Hindle, E. Fertein, C. Przygodzki, F. Dürr, L. Paccou, R. Bocquet, P. Niay, H.G. Limberger, M. Douay, Inscription of long-period gratings in pure silica and germano-silicate fiber cores by femtosecond laser irradiation, IEEE Photon. Technol. Lett. 16 (2004) 1861–1863. [4] N. Zhang, J.-J. Yang, M.-W. Wang, X.-N. Zhu, Fabrication of long-period fibre gratings using 800 nm femtosecond laser pulses, Chin. Phys. Lett. 23 (2006) 3281–3284. [5] Y. Yu, S. Ruan, H. Yang, C. Du, J. Zheng, Temperature sensor using a long period fiber grating fabricated by 800 nm femtosecond laser pulses, in: Proc. SPIE, vol. 7656, 2010, pp. 7650Q-1–7650Q-6. [6] B. Li, L. Jiang, S. Wang, H.-L. Tsai, H. Xiao, Femtosecond laser fabrication of long period fiber gratings and applications in refractive index sensing, Opt. Laser Technol. 43 (2011) 1420–1423. [7] A. Dragomir, D.N. Nikogosyan, A.A. Ruth, K.A. Zagorulko, P.G. Kryukov, Longperiod fibre grating formation with 264 nm femtosecond radiation, Electron. Lett. 38 (2002) 269–271. [8] A. Dragomir, D.N. Nikogosyan, K.A. Zagorulko, P.G. Kryukov, E.M. Dianov, Inscription of fiber Bragg gratings by ultraviolet femtosecond radiation, Opt. Lett. 28 (2003) 2171–2173. [9] A.I. Kalachev, V. Pureur, D.N. Nikogosyan, Investigation of long-period fiber gratings induced by high-intensity UV laser pulses, Opt. Commun. 246 (2005) 107–115; A.I. Kalachev, V. Pureur, D.N. Nikogosyan, Investigation of long-period fiber gratings induced by high-intensity UV laser pulses, Errata Opt. Commun. 251 (2005) 229. [10] D.N. Nikogosyan, Long-period gratings in a standard telecom fibre fabricated by high-intensity femtosecond UV and near-UV laser pulses, Measur. Sci. Technol. 17 (2006) 960–967. [11] B.J. O’Regan, D.N. Nikogosyan, Femtosecond UV long-period fibre grating fabrication with amplitude mask technique, Opt. Commun. 284 (2011) 5650– 5654. [12] G. Brambilla, A.A. Fotiadi, S.A. Slattery, D.N. Nikogosyan, Two-photon photochemical long-period grating fabrication in pure-fused-silica photonic crystal fiber, Opt. Lett. 31 (2006) 2675–2677. [13] A.A. Fotiadi, G. Brambilla, T. Ernst, S.A. Slattery, D.N. Nikogosyan, TPA-induced long-period gratings in a photonic crystal fiber: inscription and thermal sensing properties, J. Opt. Soc. Am. B 24 (2007) 1475–1481. [14] Light Conversion Ltd.
Contents lists available at SciVerse ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Long-period grating inscription in hydrogen-free SMF-28 fiber by high-repetition-rate femtosecond UV pulses Bryan J. O’Regan a, David N. Nikogosyan a,⇑, Domas Paipulas b, Viacˇeslav Kudriašov b, Valdas Sirutkaitis b a b
Physics Department, University College Cork, Cork, Ireland Laser Research Center, Vilnius University, Vilnius, Lithuania
a r t i c l e
i n f o
Article history: Received 3 August 2011 Revised 12 December 2011 Available online 31 January 2012 Keywords: Transmission gratings Standard telecom fiber
a b s t r a c t Using femtosecond UV (258 nm) pulses, generated at high-repetition-rate (50 kHz), we managed to record high-quality long-period gratings in a number of fibers, including a standard telecommunication one, SMF-28. Along with the main grating, connected to the refractive index change in the fiber core, at the relatively high intensity of the inscribing UV radiation, I 1.5 TW/cm2, we recorded the formation of an additional long-period grating, based on the refractive index change induced in the fiber cladding. We have compared the temperature sensitivity and the thermal stability of both gratings. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Long-period fiber grating (LPFG) fabrication by femtosecond laser light is known from 1999 [1]. In this pioneering work the radiation of an IR (k = 800 nm) femtosecond laser was tightly focused onto the fiber core of a standard telecommunication fiber. Though the quality of the inscribed LPFG was not the best, annealing experiments demonstrated its increased temperature stability, above 500 °C. Later it was shown [2], that involved photo-inscription process proceeds via the five-photon absorption mechanism and therefore the tight focusing, bringing to very high irradiation intensities (10–200 TW/cm2), is essential. Recently, a number of works, utilizing LPFG fabrication by high-intensity 800 nm femtosecond pulses, were published [3–6]. LPFG inscription with UV (k = 264 nm) femtosecond pulses is known from 2002 [7]. The advantage of this approach is that photo-induction of the refractive index changes in the core of a standard telecom fiber proceeds via the two-photon mechanism [8], as a result much smaller irradiation intensities (100– 500 GW/cm2) are needed [9–11]. This eliminates the necessity of tight focusing the inscribing laser light, making the optical alignment procedure simpler, and provides the uniform illumination of fiber core. Besides, in some cases, e.g. for the photochemical LPFG fabrication in a photonic crystal fiber (PCF), the five-photon 800 nm femtosecond inscription cannot be used at all. The reason is that it is very difficult (if not impossible) to provide the tight focusing of the inscribing IR light beam onto the fiber core through numerous holey tubes inside the PCF. At the same time the twophoton 264 nm femtosecond UV irradiation does the job [12,13]. ⇑ Corresponding author. Fax: +353 21 4276949. E-mail address: [email protected] (D.N. Nikogosyan). 1068-5200/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.yofte.2012.01.004
Though the two-photon femtosecond UV inscription was successfully applied to the number of photosensitive and hydrogenated fibers, until now it was not possible to use it for LPFG creation in a standard hydrogen-free telecommunication fiber. In the current study, using the fourth harmonic radiation of the modern high-repetition-rate femtosecond Yb:KGW laser at 258 nm, we managed to record for the first time LPFGs in H2-free SMF-28 and investigate their properties. 2. Experimental set-up For LPFG fabrication we used a prototype of the commercial high-repetition-rate Yb:KGW femtosecond laser PHAROS (Light Conversion Ltd., Lithuania) [14]. The laser pulses (wavelength k = 1030 nm, average power P = 1.5 W, pulse duration sp = 340 fs at FWHM, repetition rate f = 50 kHz) were frequency-quadrupled to the fourth harmonic using HIRO frequency converter (Light Conversion Ltd., Lithuania) [14]. The LPFG inscription set-up is depicted in Fig. 1. The femtosecond UV pulses (k = 258 nm, energy per pulse ep = 1.3–2.7 lJ, sp 300 fs (FWHM), beam diameter 2w = 0.147 cm (FWHM), f = 50 kHz) were directed by a CaF2 spherical lens with focal distance 46.7 cm onto the fiber (with the polymer jacket removed). For fabrication of LPFGs with the period 475 lm and 3 cm length (63 grooves), a step-by-step inscription method was used. An adjustable slit with 237.5 lm width was placed in front of the fiber at a distance less than 300 lm. Each LPFG groove was irradiated until the necessary fluence value was reached, then the inscribing beam was closed, the fiber was moved perpendicularly to slit along its axis for a distance of 475 lm, and then the laser beam was opened and the next groove was recorded and so on, until the whole sequence of 63 grooves had been created. In each case of LPFG recording, we inscribed a series of grat-
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Fig. 1. Schematic of the experimental set-up.
ings, corresponding to different total accumulated fluence values, and after that determined the fluence value, corresponding to the maximum of the first absorption loss peak (around 22 dB). A motorized stage (Standa, Lithuania) with 1 lm translation accuracy was used to move the fiber during experiment. Fiber was translated along its axis until the maximum number of points was inscribed, after that it was returned back to the initial position. The LPFG grating inscription process and all relevant parameters like grating period, number of points and exposure time were controlled by a computer program. For the irradiation intensity and total accumulated fluence determination, the following expressions were used [15]:
I¼
ep
p
32 sp w2
ln 2
ELPFG ¼
ð1Þ
FS 2 F
2
ep Np d
ð2Þ
pw2 FS 2 ln 2
F
where ep is the pulse energy reaching the slit, Np is the total number of pulses, corresponding to LPFG inscription, d is the energy attenuation coefficient while passing through the slit, F is the focal distance of the lens, S is the distance between the principal plane of the lens and the fiber. The changes in grating transmission loss peaks and their spectral positions were monitored during the LPFG fabrication using broadband light source AQ4305 (Yokogawa Europe BV) and optical spectrum analyzer Q8384 (Advantest, Japan). For the LPFG inscription, three different non-hydrogenated fibers were used: (1) a photosensitive Ge/B co-doped Fibercore PS1250/1500 fiber (Fibercore Ltd., UK), with a Ge content of about 10 mol.%, a core diameter of 7.1 lm, and a numerical aperture of 0.13; (2) less photosensitive Flexcore fiber (Fiber Optics Research Center, Moscow, Russia) with a Ge content of about 4.5 mol.% and a core diameter of 5.4 lm; (3) non-photosensitive standard telecommunication SMF-28 fiber (Elliot Scientific, USA) with a Ge content of about 3 mol.%, a core diameter of 8.2 lm, and a numerical aperture of 0.14. All three fibers had a cladding diameter of 125 lm. As the fiber photosensitivity strongly depends on fiber
Fig. 2. Transmission loss spectra of LPFGs with 475 lm period and 3 cm length, recorded in Fibercore fiber by femtosecond UV pulses at: (a) high repetition rate and (b) low repetition rate [11]. The radiation wavelength, repetition rate, incident light intensity and total accumulated fluence for all 63 grooves are indicated. Note the drastic difference in total fluence values between both approaches.
tension [16], all the LPFG fabrication experiments were conducted at a constant tension value. To insure this, the fiber was tightened prior to the irradiation in fiber holders at the continuous load of a 214 g weight. Temperature sensitivity studies and isochronal annealing of the recorded LPFGs were made with a Carbolite oven MTF 12/25/250. The measurements of temperature sensitivity were performed by changing the temperature in the 20–220 °C range. In order to stabilize the refractive index changes inside the LPFG, the latter was preliminary annealed for 3 h at 120 °C. The temperature of the oven was then increased step by step from room temperature to 220 °C and kept at each intermediate temperature value for a half hour to get a thermal equilibrium inside the LPFG. For the isochronal annealing, the temperature of oven was varied from ambient to 800 °C.
Table 1 Irradiation parameters used at UV (258 nm) high-repetition-rate (50 kHz) high-intensity LPFG inscription in different fibers and corresponding parameters of fabricated longperiod gratings with 475 lm period and 3 cm length. Fiber no.
Fiber type
Intensity (GW/cm2)
Total accumulated fluence (MJ/cm2)
Main peak wavelength (nm)
Main peak value (dB)
1 8 3 11 10 17
Fibercore Fibercore Flexcore Flexcore SMF-28 SMF-28
4 140 280 1400 1540 1540
0.08 0.15 22 93 194 354
1593 1590 1467 1480 1614 1580
17 18 20 24 21 17
Note. The fluence values correspond to the inscription of whole LPFG.
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Fig. 5. Photograph of a section of a LPFG, recorded in SMF-28 fiber by 258 nm highrepetition-rate femtosecond pulses. The size of the bar is 100 lm. The period of areas with the changed refractive index in the fiber cladding is 475 lm.
Fig. 3. Transmission loss spectrum of LPFG with 475 lm period and 3 cm length, recorded in Flexcore fiber by 258 nm high-repetition-rate femtosecond pulses. The radiation wavelength, repetition rate, incident light intensity and total accumulated fluence for all 63 grooves are indicated. Note the additional loss peak at 1098 nm. The linewidth of this additional peak is 10.1 nm, while the linewidth of the main LPFG loss peak at 1480 nm is 30.2 nm.
For microscopy studies, we used an Olympus BX51 microscope, operating in differential interference contrast (DIC) regime. 3. Results and discussion Using 258 nm 300 fs pulses at 50 kHz repetition rate, we managed to record LPFGs with 475 lm period and 3 cm length in all
three fibers. We used for the inscription three different intensities, the lower one, 4 GW/cm2, the middle one, 140–280 GW/cm2, and the highest one, 1400–1540 GW/cm2. However, while the LPFG fabrication in Fibercore fiber proceeds well already at lowest intensity, the LPFGs in SMF-28 could be created only at the highest irradiation intensity. Table 1 summarizes the light irradiation parameters, used in our experiments. From the consideration of the results, gathered in Table 1, it follows, that the increase of irradiation intensity by two orders of magnitude does not impact on the LPFG formation in the Fibercore fiber. This is in line with a single-photon mechanism of photochemical processes, taking place at high-intensity UV excitation of this fiber [17]. Fig. 2a demonstrates the transmission loss spectrum of LPFG, recorded in Fibercore fiber by 258 nm 50 kHz femtosecond pulses.
Fig. 4. Transmission loss spectra of LPFGs with 475 lm period and 3 cm length, successively recorded in SMF-28 fiber by 258 nm high-repetition-rate femtosecond pulses (a– d). The radiation wavelength, repetition rate, incident light intensity and total accumulated fluence for all 63 grooves are indicated. Note, how with the raise of total accumulated fluence the additional loss peak at 1310 nm (a) overlaps with the main LPFG peak (b–d). The linewidth of the additional peak is 10.7 nm (b), while the linewidth of the main LPFG loss peak at 1614 nm is 18 nm (d).
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Fig. 6. Isochronal annealing curves for the two peaks at 1354 nm (additional peak) and 1614 nm (main transmission loss peak) of the LPFG, recorded in a standard telecom fiber (Fig. 4d).
Below, in Fig. 2b, a spectrum of the identical LPFG, recorded in our previous work in the same fiber with 264 nm 183 lJ 220 fs 27 Hz pulses [11], is given. From the comparison of these two spectra, their close similarity follows. The linewidth values of the peaks are also similar, i.e., 18.1 nm for 1590 nm peak and 16.2 nm for the 1589 nm peak. However, the total accumulated fluence values are drastically different, namely, the value obtained at 258 nm 50 kHz fabrication is by more than three orders higher than that obtained at 264 nm 27 Hz one. This means that the increase of repetition rate of inscribing femtosecond UV pulses brings about a much lower efficiency of the formation of refractive index changes in the core of the photosensitive fiber. Fig. 3 demonstrates the transmission loss spectrum of LPFG, recorded in Flexcore fiber by 258 nm 50 kHz femtosecond pulses. With a smaller Ge-doping value in the core of this fiber, 4.5% instead of 10% in the case of Fibercore, we observed even larger values of total irradiation fluence, 22–93 MJ/cm2, than in the case of Fibercore fiber. However, the most interesting feature is the appearance of the additional peak in the region of 1098 nm (only at highest irradiation intensity, I = 1400 Gw/cm2). The conducted studies revealed, that the temperature sensitivity of this peak, 600 pm/°C, differs significantly from the temperature sensitivity value of 80 pm/°C for the main LPFG absorption loss peak at 1480 nm. The correlation coefficient R for both values is 0.998.
Now we will discuss the spectra obtained at 258 nm 50 kHz femtosecond irradiation of a standard telecom fiber SMF-28 (Fig. 4a–d). All four subsequently taken spectra clearly indicate the appearance of the additional peak in the region of 1310– 1354 nm. It should be emphasized, that the literature spectra of LPFGs, recorded by tightly-focused high-intensity 800 nm femtosecond pulses (see, i.e., [3]), show the complete absence of such an additional peak. As during high-intensity 800 nm LPFG fabrications the inscribing radiation is commonly tightly-focused onto the fiber core, we can assign this peak to the additional long-period grating, formed in the fiber cladding. In relation to that it should be noted, that the formation of the refraction index changes, simultaneously both in the core and in the cladding was observed earlier during Bragg grating inscription experiments in a H2-free SMF-28 fiber with 264 nm 27 Hz femtosecond pulses [18]. However, to the best of our knowledge, nobody has reported the simultaneous fabrication of two LPFGs, one in the core and one in the cladding of a standard non-hydrogenated telecom fiber at high-intensity UV irradiation. In the current work, the appearance of the periodic refractive index changes in the cladding of the fiber No. 17 from Table 1 after prolonged high-intensity 258 nm 50 kHz irradiation was revealed by optical microscopy (Fig. 5). The period of the areas with the changed refractive index, deduced from the microscopic studies, agrees well with the period of main LPFG, recorded in the core. The conducted thermal studies demonstrated, that this additional loss peak and the main LPFG absorption loss peak in the region 1584–1614 nm, have different temperature sensitivity values, 140 and 66 pm/°C, respectively. But what is even more important, they have very different isochronal annealing curves. These curves are depicted in Fig. 6. It shows that the decline of main LPFG absorption peak starts at the temperature 350 °C, while the amplitude of the additional peak does not change up to 550 °C. This fact confirms the different nature of two LPFGs, recorded in the core and in the cladding of the same fiber, and agrees with our microscopic investigations. We compared our total accumulated fluence values for 258 nm 50 kHz femtosecond LPFG inscription in H2-free SMF-28 fiber, given in Table 1, with the literature data for LPFG fabrication in the same fiber by high-intensity 800 nm femtosecond pulses, compiled in Table 2. Such comparison shows, that generally the 800 nm femtosecond fabrication is energetically more efficient than the 258 nm 50 kHz one. At the same time, the literature data on 800 nm femtosecond inscriptions show a clear dependence on the laser pulse repetition rate, the higher the repetition rate, the higher the necessary total accumulated fluence value. Such observation is not accidental;
Table 2 Light radiation parameters used for 800 nm femtosecond LPFG inscription in non-hydrogenated SMF-28 fiber (literature data). Pulse energy (lJ)
Pulse duration (fs)
Repetition rate (kHz)
Intensity (TW/ cm2)
Total fluence (kJ/ cm2)
LPFG period (lm)
Number of grooves
Main peak value (dB)
Refs.
0.27 0.27 2.4 0.32
160 300 200 50
200 1 250 1
10 180 170 200
535 3.4 62 2.9
450 486 450 436
89 50 50 58
16 10 21 20
[3] [4] [5] [6]
Note: The fluence values in each case correspond to the inscription of whole LPFG.
Table 3 Temperature sensitivity values for the main LPFG absorption loss peak in H2-free SMF-28 fiber. Inscription wavelength (nm)
LPFG period (lm)
Number of grooves
Main peak amplitude (dB)
Temperature sensitivity (pm/°C)
Refs.
257.5 800 800
475 450 436
63 50 58
23 21 20
66 43 91
Our work [5] [6]
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it is in line with the above observation on the increase of the total accumulated fluence value for the LPFGs created in a photosensitive fiber with change from 264 nm 27 Hz femtosecond inscription to 258 nm 50 kHz one (cf. Fig. 2a and b). Indeed, the difference between the fibers, used in this work, lies mainly in quantity of Ge atoms incorporated in silica glass. After the UV excitation of germanium-related oxygen-deficient centers in Ge-doped fused silica, which could proceed either via the single-photon femtosecond excitation in the case of a photosensitive Fibercore fiber [17], or via the multi-photon IR (800 nm) or UV (267 nm, 258 nm) femtosecond excitations in the case of a standard telecom fiber, all the following chemical processes leading to refractive index changes in fused silica matrix should be similar. This allows us to predict the decrease of the total accumulated fluence value necessary for LPFG inscription with a rejection peak value of about 20 dB with femtosecond 258 nm pulses in hydrogen-free SMF-28 fiber with the decrease of pulse repetition rate from 50 kHz to 1 kHz or so. Finally, Table 3 presents the comparison of temperature sensitivity values for the main LPFG absorption loss peak in H2-free SMF-28 fiber obtained in our work with the literature data on 800 nm femtosecond inscription. Though the grating parameters are slightly different, the agreement between them is reasonably good. 4. Conclusions For the first time, the LPFG inscription with 50 kHz high-intensity femtosecond UV pulses in three different fibers, including a standard telecom one, was realized and the properties of the fabricated gratings were thoroughly studied. It was established that the increase in the repetition rate of the inscribing laser pulse generally leads to the increase of the total accumulated fluence. This finding is in line with the literature data on femtosecond IR (800 nm) LPFG fabrication in non-hydrogenated standard telecom fiber with different pulse repetition rates. It was also observed that, at the relatively high value of incident intensity, about 1.5 TW/cm2, the formation of an additional long-period grating, based on the refractive index changes in the fiber cladding, takes place. Acknowledgments Two of us, B.J.O’R. and D.N.N., would like to thank the Laser Lab Europe programme for the possibility to use the facilities of Laser Research Center at Vilnius University, Lithuania (Grant VULRC 001672).
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