Formation of Bragg gratings

August 1, 1989 / Vol. 14, No. 15 / OPTICS LETTERS 823 Formation of Bragg gratings in optical fibers by a transverse holographic method G. Meltz, W. W. Morey, and W. H. Glenn United Technologies Research Center, East Hartford, Connecticut 06108 Received February 6, 1989; accepted April 28, 1989 Bragg gratings have been produced in germanosilicate optical fibers by exposing the core, through the side of the cladding, to a coherent UV two-beam interference pattern with a wavelength selected to lie in the oxygen-vacancy defect band of germania, near 244 nm. Fractional index perturbations of approximately 3 X 10-5 have been written in a 4.4-mm length of the core with a 5-min exposure. The Bragg filters formed by this new technique had reflectivities of 50-55% and spectral widths, at half-maximum, of 42 GHz. In 1978, Hill et al. 1' 2 reported the formation of refrac- tive-index gratings in germanosilicate fiber by sus- tained exposure of the core to the interference pattern of oppositely propagating modes of 488- or 514.5-nm argon-ion laser radiation. Subsequent investigations by Lam and Garside3 showed that the grating strength increased as the square of the writing power, which suggested a two-photon process as the cause of the index changes. This Letter presents the first results to our knowledge that show that in-fiber Bragg grat- ings can also be formed by illuminating the core from the side of the fiber with coherent UV radiation that lies in the 244-nm germania oxygen-vacancy defect band.4-6 This intense absorption band, which is -35 nm wide, coincides with the second harmonic of both blue-green argon-ion laser lines used in previous re- search. The index modulation, which can be selected to correspond to a desired Bragg wavelength, is written within the core by exposing it to a two-beam interfer- ence pattern. The grating period is determined by the incident wavelength and the included angle between the beams. This transverse holographic method of forming gratings proves to be much more efficient and flexible than the previously reported technique. Gratings that are formed in this manner are not length limited by saturation effects3 and can be tailored to a desired transmission or reflection filter characteristic by shaping and tilting the writing pattern through control of the included angle and divergence of the beams. A grating is formed by exposing a short length of bare, photosensitive, germanosilicate fiber to a pair of overlapping coherent UV beams. The experimental arrangement is shown in Fig. 1. A tunable excimer- pumped dye laser, operated at a wavelength in the range of 486-500 nm, is used with a frequency-dou- bling crystal to provide a UV source that lies in the 244-nm band and has an adequate coherence length. The UV radiation is split into two equal-intensity beams and then recombined to produce an interfer- ence pattern within the core, normal to the fiber axis. The intensity of the pattern is increased by focusing the beams on the fiber with a pair of cylindrical lenses. The resulting focal spot is approximately rectangular, approximately 4 mm long by 125 tm wide. A filtered mercury arc source is used with a high- resolution monochromator to measure the reflection and transmission spectra of the grating. The reflect- ed signal is monitored by inserting a beam splitter at the fiber input, and the reflectivity is measured by comparing the reflected signal level to the power re- flected, at a wavelength near but out of the filter band, from a mirror placed at the output end of the fiber. The strongest gratings were written with 244-nm pulsed radiation that had an average power of 4-20 mW. Several different fibers were used, with core diameters of 2.2-2.6gum and N.A.'s of 0.17-0.24, corre- sponding to GeO2 doping of 5-12.5 mol%. Bragg grat- ings were formed with center wavelengths of 577-591 nm in (i) commercial (Spectran) 6.6-mol % germanosili- cate-core, silica-clad fiber; (ii) fiber similar to that used by Hill et al. 1' 2 ; (iii) elliptical-core, polarization- MERCURY ARC LAMP MICROSCOPE OBJ. PHOTOSENSITIVE FIBER YELLOW FILTER REFLECTOR BRAGG GRATING >o PMT ¢' I ENCLOSURE BEAM SPLITTER UV LASER BEAM-.- 1 XY RECORDER I Fig. 1. Diagram of the experimental setup. A beam split- ter (not shown) at the fiber input end is used with the monochromator to measure the reflection spectrum of the Bragg grating. PMT, photomultiplier tube. 0146-9592/89/150823-03$2.00/0 -© 1989 Optical Society of America 824 OPTICS LETTERS / Vol. 14, No. 15 / August 1, 1989 1.0 - wJ w Cc, | TRANSMISSION FWHM 42 GHz 0.5 F o _ 575 VMIRROR IN BACKGROUND 576 577 WAVELENGTH IN nm Fig. 2. Transmission and reflection spectra for a 4.4-mm- long Bragg grating filter. A 1-m narrow-band monochroma- tor with a resolution of 0.02 nm was used with a filtered arc lamp source to measure the in-fiber filter characteristics. The measured FWHM is corrected for the monochromator spectral response broadening. maintaining fiber (Andrew); and (iv) in fiber with a high germania content (N.A. = 0.24) containing a small amount (0.5-1 mol %) of phosphorus. As the periodic index modulation develops in the fiber core, a narrow notch (or peak) appears in the transmission (reflection) spectrum. The center of the peak or notch occurs at the predicted Bragg wave- length X = 2nA, where A is the grating period and n is the mode index. Figure 2 shows the reflection and complementary transmission spectra of a grating formed in a 2.6-gim-diameter core, 6.6-mol % GeO2 - doped fiber after 5-min exposure to a 244-nm interfer- ence pattern with an average power of 18.5 mW. The two spectra have similar line shapes and complemen- tary values of transmittance and reflectance. The length of the exposed region is estimated to be be- tween 4.2 and 4.6 mm, as deduced from inspection of a witness burn spot in a paper target. The FWHM of a uniformly exposed region of this length should be about 26 GHz (Refs. 3 and 7); however, the observed linewidth shown in Fig. 2 is 42 GHz, suggesting that the intensity pattern is tapered. The lack of pro- nounced sidelobes also supports this conclusion. The gratings are observed to form quickly at power levels of 10 mW and higher. For example, after 10 sec of exposure to an average pulse power of 23 mW the measured transmittance at the center of the Bragg filter decreases to 0.65, and after 30 sec of exposure it decreases to 0.55. Exposure to the UV flux in some fibers causes an immediate broadband drop in trans- mission, which then gradually recovers. In the 6.6- mol % GeO2-doped commercial fiber the transmission returned to within 6% of its previous level within 1 min. Because the spectral width of the grating filter is narrow and the index perturbation extends across the entire core cross section, it can be used to separate the fundamental HE,, mode from the higher-order modes. Figure 3 shows the measured transmission spectrum of a slightly multimode fiber. The fiber used in this experiment had a cutoff wavelength of 632 nm, corresponding to a N.A. of 0.22 (11 mol % GeO2 doping), and a core diameter of 2.2 gim. The Bragg wavelength of the fundamental occurs at 581.5 nm. At this wavelength, the value of the normalized fre- quency V is 2.62; the fiber just supports the first set of higher-order modes. Under these conditions the sep- aration of phase indices, and therefore the Bragg wavelengths, of the two modes is greatest and the individual peaks in the spectrum are easily resolvable by the in-fiber grating filter. The measured separa- tion is within 10% of the predicted value as computed from the Bragg condition XB = 2nA and the dispersion relation for step-index fibers.8 The Bragg wavelengths for the principal modes in a polarization-maintaining fiber will also be separated by the difference in their axial wave numbers, or the fiber birefringence. To show this, a weak grating was written in a commercial elliptical-core, germania- doped polarization-maintaining fiber (Andrew). The transmitted line shape, measured without a polarizing filter at the output of the fiber, consisted of the super- position of two lines. By use of a polarizer at the output, these lines could be identified with the princi- pal horizontal and vertical modes of the fibers. We can estimate the strength of the index perturba- tion (An/n) by comparing the measured peak reflectiv- ity of a grating of known length L with a prediction of the efficiency of a volume hologram within the core of the fiber. 3'7 It can be shown3 by solution of the cou- pled mode equations for the forward- and backward- traveling waves in a fiber containing a Bragg filter that the reflectivity at the Bragg wavelength is given by R = tanh2 Q, (1) where Q= irn(L/X)(An/n)rq(V). (2) The factor ti(V) a 1 - 1/V2, V > 2.4, is the fraction of the integrated fundamental mode intensity contained in the core. The measured peak reflectivities of two Bragg grat- ing filters, written in different fibers with different -J z I-c > 0 r _J 578.25 581.5 !- - 3.25 nm - 577 578 579 580 WAVELENGTH IN nm 581 582 Fig. 3. Transmission spectrum of a Bragg filter in a multi- mode fiber. The fundamental mode i8 reflected by -30% at a wavelength of 581.5 nm. The next set of higher-order modes appears at a wavelength that is 3.25 nm shorter than the notch at the fundamental. August 1, 1989 / Vol. 14, No. 15 / OPTICS LETTERS 825 6 60 - W -J 40 - LU- a: 1 ~~~~~~X 1 0 20 0 0 2 4 6 8 10 LENGTH, mm Fig. 4. Computed (solid curves) and measured reflectivity for Bragg gratings of various strengths as a function of length. Experimental points are shown for a grating written with an average power of 18.5 mW at a wavelength of 244 nm (filled square) and with an average power of 4.5 mW at a wavelength of 257.3 nm (filled circle). Two different fibers were used. power levels, are compared in Fig. 4 with theoretical predictions for gratings of various strengths as a func- tion of the length of the exposed region. The weaker 7% reflectivity filter was written with a UV laser beam at a wavelength of 257.3 nm, just on the edge of the oxygen-vacancy defect absorption band, using pulses from a mode-locked argon-ion laser and a KDP sec- ond-harmonic generator. The core was exposed for 20 min to an average power of 4.5 mW. The fiber was similar to those used by Hill et al.1,2 (core diameter 2.2 gim, N.A. = 0.22) in the first experiments on photore- fractive effects in germanosilicate fiber. Much stron- ger gratings of about the same length with reflectivi- ties of 50-55% were written in commercial germanosi- licate fiber (core diameter 2.61 gum, N.A. = 0.17) using a 5-min exposure to a pulsed crossed-beam pattern with an average power of 18.5 mW. In this case a dye laser was used with a /3-BaB204 crystal to generate second-harmonic UV radiation at 244 nm, which is close to coinciding with the center of the defect ab- sorption band. Based on the measured reflectivity, the fractional index change is estimated to be 2.8-3 X 10-5, assuming a grating of uniform strength. The peak index perturbation could be somewhat larger, however, since the linewidth measurements are wider than expected for a uniform grating. We can compare the efficiency of writing in-fiber gratings with coherent UV radiation at 244 nm to the two-photon process at 514.5 nm. To obtain an index perturbation of 3 X 10-5 using cw argon-ion radiation at 514.5 nm required a writing power of 90.7 mW with an exposure of approximately 6 min.3 This is equiva- lent to exposing the core (diameter of 2.5 gim) to an energy flux of 665 MJ/cm 2 . A grating of similar strength is obtained with an energy flux of only 1 kJ/ cm2 at a wavelength of 244 nm by directly bleaching the absorption band, an improvement of 6.7 X 105 in writing efficiency! The Bragg gratings formed by our holographic tech- nique with 257- or 244-nm radiation appear to be per- manent and stable at high temperatures. A grating in the commercial fiber was heated to 5000C and main- tained at that temperature for 18 h without a change in its reflectivity or line shape. The only variation was an expected shift in the line center due to the combi- nation of thermal expansion and a change in refractive index with temperature and stress relief. The mechanism that forms the gratings is not fully understood; all we can say is that it is related to the bleaching of the oxygen-vacancy defect band in ger- mania or germania-doped silica. Normally, germani- um is incorporated in the silicate glass in the Ge+4 oxidation state, i.e., as GeO2 ; however, Ge+2 can occur if GeO2 is dissociated into GeO and 02 in the formation of the glass, say, during the preparation of a modified chemical-vapor-deposition process preform.5'6 This process, whereby the reduced Ge+2 species is formed, is favored if the processing temperature is raised to 16500C and the molten glass is cooled quickly.9 Pre- liminary measurements of the UV absorption spectra of germanosilicate preforms suggest that the drawing process can cause the Ge+2 defect band to form. In summary, a new method for forming in-fiber Bragg gratings has been demonstrated. The grating is formed in photorefractive germanosilicate fiber by ex- posure to a coherent two-beam UV interference pat- tern. This technique provides a new means for mak- ing quasi-distributed measurements of temperature and strain by monitoring the shift in the Bragg wave- length of the sensing regions, each being individually tuned to a distinct wavelength, or by forming pairs of independent Fabry-Perot cavities. Possible applica- tions include high-efficiency distributed-feedback re- flectors, wavelength-selective couplers and taps, and dispersion-compensating filters. We acknowledge the important contributions of R. M. Elkow and J. D. Farina and the skillful technical assistance of A. L. Wilson. We also thank R. A. Weeks (Vanderbilt University), K. 0. Hill (Canadian Optical Communications Research Center), and E. Snitzer (Rutgers University) for valuable discussions of the phenomenology. This research was supported in part by the U.S. Department of the Air Force. References 1. K. 0. Hill, Y. Fujii, D. C. Johnson, and B. S. Kawasaki, Appl. Phys. Lett. 32, 647 (1978). 2. B. S. Kawasaki, K. 0. Hill, D. C. Johnson, and Y. Fujii, Opt. Lett. 3,66 (1978). 3. D. K. W. Lam and B. K. Garside, Appl. Opt. 20, 440 (1981). 4. A. J. Cohen and H. L. Smith, J. Phys. Chem. Solids 7,301 (1958). 5. P. C. Schultz, in Proceedings of the Eleventh Interna- tional Congress on Glass (North-Holland, Amsterdam, 1977), pp. 155-163. 6. M. J. Yuen, Appl. Opt. 21, 136 (1982). 7. H. Kogelnik, Bell Syst. Tech. J. 48, 2909 (1969). 8. A. W. Snyder and J. D. Love, Optical Waveguide Theory (Chapman and Hall, New York, 1983). 9. J. M. Jackson, M. E. Wells, G. Kordas, D. L. Kinser, and R. A. Wecks, J. Appl. Phys. 58, 2308 (1985).