Hydrogenation influence on telecom fiber Bragg gratings properties

Transcription

1 Hydrogenation influence on telecom fiber Bragg gratings properties Kazimierz Jędrzejewski, Jerzy Helsztyński, Lech Lewandowski, Institute of Electronic Systems, Warsaw University of Technology, ul. Nowowiejska 15/19, Warsaw, Poland, phone: , fax: , ABSTRACT Special photosensitive high germanium boron-codoped fibers are too expensive for every-day application in the research laboratory. To lower the running costs of the fibers high-pressure hydrogenation unit was realized in form of special piping system with pressure control, easy handling and all safety requirements fulfilled. The technology and special procedures of hydrogen loading into single-mode fibers were developed and tested. Available standard single mode fiber samples used by the telecom cable companies (lengths of 2-3m) were inserted into hydrogen for different static diffusion time periods under the pressure bar in the room temperature. The optimal hydrogenation procedures for best quality gratings were found. The post-hydrogenation low-temperature storage conditions were also controlled and gratings spectral characteristics measured. No Bragg grating formation was seen in unloaded fibers under the same laser illumination. Keywords: Bragg gratings, hydrogenation, spectral properties 1. INTRODUCTION Bragg gratings inscribed directly in fiber core are at present key elements in many advancing technologies [1] including telecommunication, laser techniques and fiber sensing. Fiber Bragg grating (FBG) is a narrowband (typically nm) band-stop optical filter with low insertion losses. FBG application in a relatively simple way has solved numerous fiber transmission problems. In telecommunication DWDM systems with FBG for multi-channel transmission in III window have been designed, gain shaping and flattening characteristic in erbium-doped optical amplifiers (EDFA) has been achieved and optical pulse time compression for dispersion control was obtained. In the laser technology Bragg grating can form mirrors in fiber lasers and can stabilize pumping lasers for EDFA. Many applications have been foreseen in metrology. The geometry and transmission properties of the grating can be influenced by the external factors (e.g. temperature, pressure, strain) and used to realize the grating sensors (change of fiber length and refracting index value). The advantage is the direct wavelength change measurement without the detrimental influence of the source amplitude. With the addition of the multiplexing idea quasi-distributed sensors can be realized with the information of the measurand value and its spatial position. Photosensitivity phenomena were already observed in 1978 in special Ge-doped silica fibers under intense laser light illumination. The UV laser light absorption causes permanent local refraction index changes in germanium doped telecommunication fibers core. Several methods were developed to increase the photosensitivity, the most attractive are boron co-doping and low temperature hydrogenation processes. The idea of hydrogenation was for the first time proposed in [2]. The advantages of hydrogenation process are low price and the possibility of Bragg grating formation in many glasses also without germanium content in many cases. 2. BRAGG GRATING TECHNOLOGY In our experiments Bragg gratings were formed with the use of coherent UV illumination. Several UV light laser types have been used for stable and high local refractive index changes formation. For silica fibers two of them are most attractive: pulsed excimer lasers ArF (193 nm) and KrF (248 nm) with pulse energies extending 100 mj. The exposition time can be very short, even a single pulse shot can be sufficient to get reasonable results. The other solution is a CW double frequency argon ion laser (244 nm) with powers over 100 mw. The advantages of such laser are: very good coherence and long term stability. Its applications include small production lines but mainly they are used in research. Simple, low power Innova 90C FreD Coherent CW 100mW, 244nm UV laser was used in our experiments. The system was installed on the antivibrational high-quality optical table. Many data on Bragg grating history, theory, techniques and applications are given in monographs [3], [4]. Optical Fibers: Technology, edited by Jan Rayss, Brian Culshaw, Anna G. Mignani, Proc. of SPIE Vol. 5951, 59510I, (2005) X/05/$15 doi: / Proc. of SPIE Vol I-1

2 The phase mask method is a reasonably simple and attractive for comparison studies. Phase mask technique can be typically designated for telecommunication application. But for every wavelength a different phase mask is needed. The phase mask can be tuned to some extent (nm range) with the application, e.g. thermal expansion of the material. It works as a stamp for multiple duplication of Bragg gratings of identical properties according e.g. to the DWDM standards. The mask is a special diffraction grating made in fused silica and designed to transmit ±1 diffraction order mainly (more than 70%) and with 0-th order minimized (3-5 %) for one desired wavelength. The interference of both beams forms the intensity pattern with the period which is half of the phase mask period. The laser illumination of the phase mask and light interference on the fiber core introduce local refractive index variations via the photosensitivity process and consequently Bragg reflection in the fiber according to the formula: λ B = 2 n Λ = n δ (1) eff eff λ B is the Bragg wavelength in the fiber, Λ is the grating period and n eff - the average index value (changed under UV illumination) in the grating region and δ is the phase mask period. LASER M1 Y/Z Y/Z F PM CL M2 LTS Fig.1. Experimental set-up for Bragg grating writing with the use of the phase mask; LASER - Coherent FreD 90C, 100 mw, 244 nm argon laser, M1, M2 dielectric 244 nm mirrors, CL cylindrical lens, PM phase mask, Y/Z positioning stages, LTS linear translation (scanning) stage, F single-mode fiber The experimental system for the phase mask method is given in fig.1 [5]. Coherent dielectric 244 nm mirrors (M1, M2) redirect the laser beam. The 75 mm focal length cylindrical silica lens (CL) designed for UV applications focuses the beam on the fiber (F). The fiber was mounted in front of the phase mask (PM) in two directional linear stages (Y/Z) for precise fiber alignments. Standard phase mask of dimensions 25x3 mm was applied (O/E Land Inc.) with the period 1061 nm and 5% transmission in the zero order fringe. It was possible to write 25 mm long gratings in the system. The X movement (along the fiber) was realized by the precision linear stage (LTS) with the maximum displacement of 50 mm (Cobrabid Optica). The laser beam was scanned along the fiber by this linear stage. Stepping and DC motors were applied to operate the stage. In the interferometric method a phase mask was used instead of plate beam-splitter (Fig.2) [6]. When the UV radiation incidents perpendicularly to the mask, two main diffracted orders m = ± 1 (in properly designed mask 75 % of incident beam is transmitted in ± 1 orders) interfere. Interferometers with phase mask give full symmetry of optical path lengths in both arms. The transmitted zero order signal slightly reduces the fringe pattern contrast. It can be easily removed by proper diaphragm. The main advantage of amplitude beam splitting arrangement is the convenience of flexible period changes between fringes. Therefore grating period Λ can be varied causing the Bragg wavelength λ B changes. The incident angles ϕ on fiber can be precisely fixed by rotating and linearly translating the mirrors in both interferometer arms. It is easily expressed by eq. (2) and (3): Proc. of SPIE Vol I-2

3 and λuv Λ = (2) 2sinϕ neff λuv λb = (3) sinϕ λ uv is the incident UV laser radiation wavelength. The relative change of Bragg grating wavelength is given by: λ = ϕ ctgϕ (4) λ B It is seen from eq. (4) that the ϕ angle change of 5 0 (for λ B = 1550 nm the typical value is ϕ = 13 0 ) gives Bragg wavelength range nm. This high sensitivity of the system for these angle changes is advantageous in the sense of wavelength variation range. On the other hand, the optical system is very sensitive to any undesired environmental distortions and vibrations during the grating writing. It introduces strong precautions on equipment e.g. antivibrational table. Atmosphere cleanness and stability are also demanded during of Bragg gratings inscription. Special air purifying and conditioning system was introduced in our laboratory. UV input beam Cyllindrical lense Phase mask m=-1 m=+1 Mirror m=0 Mirror ϕ ϕ Fiber Fig.2. Configuration of the interferometer with phase mask beam splitter Fig.3. The optical set-up for the amplitude beam splitting interferometer with the phase mask; 1 - UV laser, 2 - laser output beam, 3 - colimator, 4 - colimated beam, 5 - mirrors, 6 - cylindrical lens, 7 - phase mask, 8 - interferometer mirrors, 9 two translation stages (linear and rotatable), 10 SM optical fiber Proc. of SPIE Vol I-3

4 The experimental optical arrangement for the amplitude beam splitting interferometer with the phase mask is given in fig.3. The general viev of the laser, optical table and independent optics enclosures for both methods are presented in fig.4. Three different types of SM fibers were used in experiments: matched-clad (STC, SMF-28e), Alcatel dispersion shifted (SP-DV /01F) and Corning s Leaf CPC6. The hydrogenation diffusion process was maintained for 1-14 days in bar pressure in room temperature. Fig.4. The general view of the laboratory. The Innova 90C FreD Coherent 100mW, 244nm UV laser was installed on special optical TMC LaserTop table with pneumatic isolators. The standard phase mask was used (O/E Land Inc.) with period 1061 nm, zero order signal level below 5% and predicted wavelength 1540 nm. Many different available single mode fiber samples (1 m long) were used and soaked in hydrogen for 1-14 days under the pressure bar in room temperature before the UV illumination. Bragg gratings were developed in the cores of our samples and their spectral characteristics measured. The hydrogenation system was installed in the special room equipped accordingly to the safety standards with proper ventilation system. Two separate hydrogen loading sets were installed (Linde Gases Poland). The first is intended for loading standard telecom fibers and made of four independent with internal diameter (ID) 4 mm stainless steel 600 bar pressure tested tubes of 3 m length. The second is devoted to bigger samples (planar or plate glass, fiber devices). For this purpose we have installed 3 independent stainless steel tubes 14 mm ID and 1 m length. All joints and connections were made in Swagelock standards. Additionally, the nitrogen purge installation was added to increase the system safety. The whole system was designed and tested to saturate the whole sample under the nominal hydrogen pressure of 160 bar. No hydrogen leaking was observed during the up to 3 weeks periods of fiber soaking. View of the hydrogenation system for fiber loading is given in Fig.5. The hydrogen loaded samples were kept later in freezer conditions for a long time. We have redeveloped recently the standard freezer and the storage temperature has been increased from -25 to -46 deg. C using additional Peltier cooler stage. I Fig.5. Partial view of the hydrogenation unit for telecom fiber loading Proc. of SPIE Vol I-4

5 3. MEASUREMENT SYSTEM Bragg gratings spectral characteristics in transmission mode were measured. Specially redesigned high-resolution monochromator (Carl Zeiss Jena) system (for 1550 nm window only) with homodyne detection, computer control and data acquisition was used (fig.6). Two EDD-1550 (Fermionics Lasertech Inc.) SLED single-mode fiber coupled modules emitting in nm and nm (FWHM) spectral ranges were used as the illumination source. Each module contains SLED, thermoelectric cooler (TEC) and diode temperature monitoring thermistor [7]. Output light signal was delivered via two FC/PC connectors. Each diode is operated by separate power supply block containing diode current regulator and diode current modulator for reference signal (rectangular wave, approximate frequency 1000 Hz, duty factor 50%). Each diode may be operated independently. Both diodes are powered from common supply block containing switched mode power supply (voltage pre-regulator power outlet plug type), precise compensation type voltage regulator, and reference signal modulation source. The total light power (output at connector) is approximately 50 µw (full spectrum) for each diode. SLED Monochro mator Fiber grating Fiber Fotodetector InGaAs Modulator Reference Lock-in Amplifier PC Fig.6. Spectral measurement set-up 4. RESULTS The spectral measurement results for different hydrogenation periods are given in fig Fig.7. Interferometric method, STC matched-clad fiber, 2 days in hydrogen, pressure 115 bar, Fig.8. Interferometric method, STC matched-clad fiber, 3 days in hydrogen, pressure 115 bar, Proc. of SPIE Vol I-5

6 Fig.9. Interferometric method, STC matched-clad fiber, 5 days in hydrogen, pressure 115 bar, Fig.10. Interferometric method, STC matched-clad fiber, 7 days in hydrogen, pressure 115 bar,. Fig.11. Interferometric method, STC matched-clad fiber, 16 days in hydrogen, pressure 115 bar, Fig.12. Interferometric method, STC matched-clad fiber, 21 days in hydrogen, pressure 140 bar, exposition time 200 s., 100 mw CW, 244 nm, 9 months after hydrogenation The peak and FWHM values for presented measurements are summarized in table 1. Table 1. Measurement data for 115 bar hydrogen loaded STC fiber, interferometric method, exposition time 150 s., 100 mw CW, 244 nm Hydrogen loaded [days] Bragg wavelength [nm] Depth of filter [db] FWHM [nm] Proc. of SPIE Vol I-6

7 5. CONCLUSIONS It is seen from our results that 7 days hydrogen loading under pressure of 115 bar in room temperature is sufficient to saturate the fiber core with hydrogen for good Bragg grating formation. It was observed in interferometric and as well as in the phase mask s arrangement. The Bragg grating spectral characteristic results for the phase mask method and also other types of fibers show similar behavior. In the Leaf fiber additional minimum was observed. It was probably caused by complicated refractive index profile of this fiber. We have developed procedures for cheap and ready-to-use photosensitive telecom hydrogenated fibers. No Bragg grating formation was seen in unloaded fibers under the same experimental conditions. Hydrogen loaded fibers stored for 9 months in -25 deg.c still exhibit Bragg grating formation possibility (fig.12). We wish to thank the Tele-Fonika Cable SA Myslenice for providing us samples of telecom fibers. The work was sponsored by the KBN investment grant no. 4269/IB/134/2003 and research grant no. PZB-MIN-009/T11/2003. REFERENCES [1] K. Jędrzejewski, Siatki Bragga nowy element w technice światłowodowej, Przegląd Telekomunikacyjny 2002, p (in Polish) [2] P.J. Lemaire, R.M. Atkins, V. Mizrahi, W.A. Reed, High pressure H 2 loading as a technique for achieving ultra-high UV photosensitivity and thermal sensitivity in GeO 2 doped optical fibers, Electron. Lett., vol. 29, p , 1993 [3] A. Othonos, K. Kalli, Fiber Bragg gratings, Artech House, 1999 [4] R. Kashyap, Fiber Bragg gratings, Academic Press, 1999 [5] K. Jędrzejewski, L. Lewandowski, J. Helsztyński, W. Jasiewicz, Bragg gratings in optical fibers made by the phase mask method, Proc. SPIE, vol.5576, p , 2004 [6] J. Helsztyński, L. Lewandowski W. Jasiewicz, K. Jędrzejewski, Interferometric fiber Bragg gratings, Proc. SPIE, vol. 5576, p , 2004 [7] W. Jasiewicz, Wideband light source for fiber Bragg gratings measurements, Proc. SPIE, vol.5576, p.42-44, 2004 Proc. of SPIE Vol I-7