All-fiber coherent beam combining of fiber lasers - OSA Publishing
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OPTICS LETTERS / Vol. 24, No. 24 / December 15, 1999
All-fiber coherent beam combining of fiber lasers ´ V. A. Kozlov, J. Hernandez-Cordero, and T. F. Morse Laboratory for Lightwave Technology, Division of Engineering, Brown University, Providence, Rhode Island 02912 Received July 14, 1999 One half of a 2 3 2 fused taper single-mode f iber coupler is used for the all-f iber spatial and spectral beam combining of two f iber lasers. 1999 Optical Society of America OCIS codes: 060.0060, 060.1810, 140.3510, 060.2340, 060.2330, 060.2410.
Diode-pumped single-mode fiber lasers are eff icient, compact, reliable light sources. Coherent beam combining of the outputs of several fiber lasers that preserves single-mode beam quality, spectral properties, and optical waveguide compatibility is useful in many applications. These include optical fiber communications, high-power fiber lasers, and optical fiber sensors. Multicavity, wavelength-locked operation with independent external control of each laser is of importance as a wavelength multiplexer– demultiplexer and for optical switching. Coherent beam combining of laser diodes by various techniques has been demonstrated.1 – 3 Inducing mutual coherence among the lasers and properly combining individual laser diode outputs have achieved high brightness and phase locking between the emitting elements. Spectral beam combining of fiber lasers in an external cavity was obtained only recently, at the expense of spectral brightness.4 In this Letter we report on a novel all-fiber coherent beam-combining technique for single-mode silica fiber lasers that uses a half of a 2 3 2 fused taper single-mode coupler prepared from active fibers. This approach can easily be used for coherent beam combining of a larger number of fiber lasers. Single-mode fiber fused taper couplers are low-loss components in which the fundamental modes from the input ports experience an adiabatic transformation into the fundamental modes of the coupling section. If the coupling area cross section is prepared with radial symmetry (as in the case of highly overcoupled couplers5), both fundamental modes from the coupler’s input ports will have the same spatial distribution in the center of the coupling area. If the center of the coupling area is cleaved, the coupler can serve as a common part for the cavities of several fiber lasers, and a common mirror for all fiber lasers can be used at the cleave. In our experiments, one half of a fused coupler made with active fiber was used as an output mirror –beam combiner for two fiber lasers. It was possible not only to combine the output of two fiber lasers in the same spatial mode but also to obtain injection locking between the two lasers with the same linewidth as for each independent fiber laser. N 3 N fused taper couplers can be used for coherent beam combining of N fiber lasers if the center of the coupling area is cleaved. Our experimental scheme (Fig. 1) consists of two Er-f iber lasers pumped with cw 980-nm Ti:sapphire laser radiation. The Er31 -ion concentration was ⬃300 0146-9592/99/241814-03$15.00/0
parts in 106 ; the length of each fiber cavity was approximately 8 m, which corresponded to nearly complete absorption of the pump power. The fiber was single mode at the lasing wavelength. The input mirrors for each fiber laser were Bragg grating (BG) ref lectors written in Ge-doped Accutether fiber (AT&T) with resonance wavelengths of 1536.08 nm (BG1) and 1535.80 nm (BG2) and ref lectivity of ⬃95%. These fiber gratings were then spliced to the Er-doped fibers. The resonance wavelength of grating BG2 was thermally tuned with a Peltier element, and it was possible to obtain spectral coincidence between the two Bragg gratings. The output ends of the fiber laser cavities were used to prepare a single-mode fused taper coupler that was cleaved at the midpoint, and this cleave was used as the output mirror (⬃4% ref lectivity) for the beam combiner of the two fiber lasers. The spectra of both fiber lasers were measured with an optical spectrum analyzer (Ando, Model AQ-6312B) with a spectral resolution of 0.05 nm. The fiber laser spectra at room temperature when both fibers are being pumped are shown in Fig. 2(a). The lasing wavelengths coincide with the Bragg gratings’ resonances, and their difference is equal to 0.28 nm. The peak intensities are different because of uncontrolled cavity losses, the use of a different
Fig. 1. Experimental setup. OSA, optical spectrum analyzer; WDM, wavelength-division multiplexer. 1999 Optical Society of America
December 15, 1999 / Vol. 24, No. 24 / OPTICS LETTERS
Fig. 2. Optical spectra at the all-fiber beam-combiner output. (a) Both fiber lasers are pumped for BG2 at room temperature, ( b) both fiber lasers are pumped for BG2 at a 35 ±C, (c) laser with BG2 pumped (38 ±C for BG2 temperature) and laser with BG1 pump blocked, (d) laser with BG1 pumped and laser with BG2 pump blocked.
pump level for each fiber laser, and the unknown coupling ratio for the fused fibers at these lasing wavelengths. When the temperature of Bragg grating BG2 was increased, its resonance wavelength shifted to a longer wavelength as a consequence of the change in refractive index with temperature.6 When the lasing peak of the shorter wavelength (fiber laser with BG2) approached the second lasing peak (fiber laser with BG1) we observed the process of injection locking, and only one lasing peak was obtained at the output of the beam combiner [Fig. 2(b)]. This occurred at a temperature of approximately 35 ±C (lower than that calculated from the known temperature dependence for silica fiber Bragg gratings for spectral coincidence between the two resonances). This output spectrum did not change, even when the maximum temperature was increased to 50 ±C. The injection locking began when the two laser peaks had a spectral separation of ⬃0.15 nm. This value is not too small for practical combining of fiber lasers. If one of the pump beams was blocked it was possible to measure separately the individual output spectrum of each fiber laser. Figures 2(c) and 2(d) show the spectra for BG2 at a temperature of 35 ±C: in Fig. 2(c), pumping occurs only through BG2; in Fig. 2(d), pumping occurs only through BG1. The lasing wavelengths for the two lasers are different and hence, if the pump for each fiber laser is turned on–off, a change in the output lasing wavelength of the combined lasers can be obtained, and this phenomenon can be used for optical switching or in all-fiber wavelength-division multiplexing components. The sum of the independent lasing peak intensities was approximately two times smaller than the lasing peak intensity shown in Fig. 2(b). This may be understood as a loss of approximately half of the laser power (because of the ⬃50% coupling ratio of the fused fibers at the lasing wavelengths) inside an unpumped active fiber. A coupling ratio different
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from 50% will change only the peak intensities. These additional losses are not present with the two pump beams, and the total output power is the simple sum of the individual laser outputs, as can be seen from Fig. 3(b). If the conf iguration of Fig. 1 has the Bragg gratings at different wavelengths, and if both lasers are pumped, the system behaves as if there were two independent cavities. However, if the criteria for injection locking are met, a three-mirror laser cavity is formed that contains two Bragg gratings and the cleaved 2 3 2 coupler as the output mirror. The spectrum linewidths were the same for all laser lines in Fig. 2 and were limited by the optical analyzer’s spectral resolution. The noise characteristics were similar for all these cw lasers. We investigated the spatial combination of the fiber lasers by cleaving the active fiber coupler at different points [see Fig. 3(a)] and measuring the far-f ield intensity distribution. A photodiode with a pinhole was scanned across the output beam spot when the two lasers were pumped [curves (1) in Figs. 3(b)– 3(d)]. Curves (2) and (3) in Figs. 3(b)–3(d) correspond to an output intensity distribution for the lasers with pumping only through BG1 and BG2, respectively. Figure 3(b) shows the results for the coupler cleaved at the midpoint (plane b); Figs. 3(c) and 3(d), for the consecutive cleaves of the same coupler (planes c and d) indicated in Fig. 3(a). All cleaves were prepared with a fiber hand cleaver, and the cleave quality varied. This variation may be an explanation for the asymmetry of the measured distributions and for different ref lectivities of the output mirror for the fiber lasers. It is a possible source of unequal laser output intensities as well. The midpoint cleave [Fig. 3(b)] demonstrated similar intensity distributions for all the combinations of fiber lasers.
Fig. 3. Spatial intensity distribution at the output of the cleaved end of the fused coupler – combiner. Curves (1), both fiber lasers were pumped; curves (2) and (3), the fiber lasers were pumped only through BG1 and BG2, respectively. (a) Three consecutive cleaves, b – d, of the fused coupler-combiner, ( b) output intensity distributions for cleave b, (c) output intensity distributions for cleave d, (d) output intensity distributions for cleave d.
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OPTICS LETTERS / Vol. 24, No. 24 / December 15, 1999
For these distributions no dependence on changes in temperature of BG2 was observed. Microscope analysis of the cleaved fiber coupler showed that the cross section had a circular shape. Other cleaves in planes c and d had noncircular cross sections, and the laser intensity distributions had different forms for the fiber lasers. However, for the cleave in plane c the combined laser intensity distribution was also symmetrical; i.e., the injection-locking mechanism creates a new mode structure at the output of the all-fiber beam combiner [Fig. 3(c)]. For increased asymmetry of the coupler’s cross section [Fig. 3(d)] the two fiber lasers performed independently, even for spectral coincidence of the Bragg grating resonance wavelengths. In this case the intensity distribution was the ordinary sum of the two independent light sources without coherent effects. Therefore, to obtain a coherent beam combination by use of one half of a fused coupler, the degree of fusion between the two active fibers is critical for spatial beam combining. We used two additional all-fiber optical components in our experiments to compare the performance of the combined lasers. Half of a single-mode fused taper fiber coupler was replaced by a standard Y-type singlemode coupler with a 50% coupling ratio for a broad spectral range near 1535 nm (Fig. 1, lower inset). This element is similar to the previous one, and it can be used as an all-fiber combiner for two fiber lasers. Relatively high intrinsic optical losses for this type of coupler and additional splice losses inside the fiber laser cavity make this element less interesting than a fused active fiber combiner, especially for high-power fiber laser applications or for the combination of a large number of fiber lasers. Nevertheless, the behavior of two fiber lasers spliced to the Y coupler was similar to that obtained with the fused and cleaved active fibers. A third element used in our experiments was a standard 980–1550-nm wavelength-division multiplexer that was spliced to the output fiber ends of the fiber lasers (Fig. 1, upper inset). This element cannot serve as a beam combiner for fiber lasers with similar wavelength. We used this device to demonstrate the independence of the fiber lasers used in this scheme. Because each laser has a separate output mirror (the cleaved wavelength-division multiplexer output fiber ports) the mutual inf luence of these fiber lasers on each other was relatively small. The output power was insensitive to changes in the Bragg ref lector’s resonance wavelength, but the laser power stability was
lower when grating BG2 was tuned to the resonance of grating BG1. This effect may be associated with parasitic light scattering in the wavelength-division multiplexer and the optical isolation of the output ports. In conclusion, we have demonstrated the use of half of a fused taper single-mode fiber coupler as an output mirror –beam combiner for two fiber lasers. This element (which can be generalized to an N 3 1 conf iguration) has relatively small optical losses and can be used as a means of combining highpower fiber lasers. We observed fiber laser injection locking by tuning one of the fiber laser’s Bragg grating mirrors. The spectral range for the locking process (not less than 0.15 nm) is within practical limits, and this phenomenon can be used to maintain spectral brightness of the combined fiber lasers. The spatial combination of two fiber laser beams was obtained, and we have shown that the degree of fusion of the active fibers is critical for this process. For the first time to our knowledge, both spatial and spectral combination of two fiber lasers has been demonstrated. This research was supported by the grant FQ86719900712 from the U.S. Air Force Off ice of Scientific Re´ search. J. Hernandez-Cordero acknowledges support from Direcci´on General de Asuntos del Personal Acad´emico, Universidad Nacional Aut´onoma de M´exico, Mexico. V. A. Kozlov’s e-mail address is ´ valerii [email protected]; J. Hernandez-Cordero’s e-mail address is [email protected]. References 1. N. W. Carlson, G. A. Evans, J. M. Hammer, M. Lurie, S. L. Palfrey, and A. Dholakia, Appl. Phys. Lett. 50, 1301 (1987). 2. J. R. Leger, M. L. Scott, and W. B. Veldkamp, Appl. Phys. Lett. 52, 1771 (1988). 3. C. J. Corcoran and R. H. Rediker, Appl. Phys. Lett. 59, 759 (1991). 4. C. C. Cook and T. Y. Fan, in Digest of Topical Meeting on Advanced Solid-State Lasers (Optical Society of America, Washington, D.C., 1999), postdeadline paper PD7. 5. T. F. Morse, M. Chen, L. Reinhart, and D. A. Brown, Proc. SPIE 2216, 210 (1994). 6. A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlane, K. P. Kao, C. G. Atkins, M. A. Putnam, and E. J. Friebele, J. Lightwave Technol. 15, 1442 (1997).
OPTICS LETTERS / Vol. 24, No. 24 / December 15, 1999
All-fiber coherent beam combining of fiber lasers ´ V. A. Kozlov, J. Hernandez-Cordero, and T. F. Morse Laboratory for Lightwave Technology, Division of Engineering, Brown University, Providence, Rhode Island 02912 Received July 14, 1999 One half of a 2 3 2 fused taper single-mode f iber coupler is used for the all-f iber spatial and spectral beam combining of two f iber lasers. 1999 Optical Society of America OCIS codes: 060.0060, 060.1810, 140.3510, 060.2340, 060.2330, 060.2410.
Diode-pumped single-mode fiber lasers are eff icient, compact, reliable light sources. Coherent beam combining of the outputs of several fiber lasers that preserves single-mode beam quality, spectral properties, and optical waveguide compatibility is useful in many applications. These include optical fiber communications, high-power fiber lasers, and optical fiber sensors. Multicavity, wavelength-locked operation with independent external control of each laser is of importance as a wavelength multiplexer– demultiplexer and for optical switching. Coherent beam combining of laser diodes by various techniques has been demonstrated.1 – 3 Inducing mutual coherence among the lasers and properly combining individual laser diode outputs have achieved high brightness and phase locking between the emitting elements. Spectral beam combining of fiber lasers in an external cavity was obtained only recently, at the expense of spectral brightness.4 In this Letter we report on a novel all-fiber coherent beam-combining technique for single-mode silica fiber lasers that uses a half of a 2 3 2 fused taper single-mode coupler prepared from active fibers. This approach can easily be used for coherent beam combining of a larger number of fiber lasers. Single-mode fiber fused taper couplers are low-loss components in which the fundamental modes from the input ports experience an adiabatic transformation into the fundamental modes of the coupling section. If the coupling area cross section is prepared with radial symmetry (as in the case of highly overcoupled couplers5), both fundamental modes from the coupler’s input ports will have the same spatial distribution in the center of the coupling area. If the center of the coupling area is cleaved, the coupler can serve as a common part for the cavities of several fiber lasers, and a common mirror for all fiber lasers can be used at the cleave. In our experiments, one half of a fused coupler made with active fiber was used as an output mirror –beam combiner for two fiber lasers. It was possible not only to combine the output of two fiber lasers in the same spatial mode but also to obtain injection locking between the two lasers with the same linewidth as for each independent fiber laser. N 3 N fused taper couplers can be used for coherent beam combining of N fiber lasers if the center of the coupling area is cleaved. Our experimental scheme (Fig. 1) consists of two Er-f iber lasers pumped with cw 980-nm Ti:sapphire laser radiation. The Er31 -ion concentration was ⬃300 0146-9592/99/241814-03$15.00/0
parts in 106 ; the length of each fiber cavity was approximately 8 m, which corresponded to nearly complete absorption of the pump power. The fiber was single mode at the lasing wavelength. The input mirrors for each fiber laser were Bragg grating (BG) ref lectors written in Ge-doped Accutether fiber (AT&T) with resonance wavelengths of 1536.08 nm (BG1) and 1535.80 nm (BG2) and ref lectivity of ⬃95%. These fiber gratings were then spliced to the Er-doped fibers. The resonance wavelength of grating BG2 was thermally tuned with a Peltier element, and it was possible to obtain spectral coincidence between the two Bragg gratings. The output ends of the fiber laser cavities were used to prepare a single-mode fused taper coupler that was cleaved at the midpoint, and this cleave was used as the output mirror (⬃4% ref lectivity) for the beam combiner of the two fiber lasers. The spectra of both fiber lasers were measured with an optical spectrum analyzer (Ando, Model AQ-6312B) with a spectral resolution of 0.05 nm. The fiber laser spectra at room temperature when both fibers are being pumped are shown in Fig. 2(a). The lasing wavelengths coincide with the Bragg gratings’ resonances, and their difference is equal to 0.28 nm. The peak intensities are different because of uncontrolled cavity losses, the use of a different
Fig. 1. Experimental setup. OSA, optical spectrum analyzer; WDM, wavelength-division multiplexer. 1999 Optical Society of America
December 15, 1999 / Vol. 24, No. 24 / OPTICS LETTERS
Fig. 2. Optical spectra at the all-fiber beam-combiner output. (a) Both fiber lasers are pumped for BG2 at room temperature, ( b) both fiber lasers are pumped for BG2 at a 35 ±C, (c) laser with BG2 pumped (38 ±C for BG2 temperature) and laser with BG1 pump blocked, (d) laser with BG1 pumped and laser with BG2 pump blocked.
pump level for each fiber laser, and the unknown coupling ratio for the fused fibers at these lasing wavelengths. When the temperature of Bragg grating BG2 was increased, its resonance wavelength shifted to a longer wavelength as a consequence of the change in refractive index with temperature.6 When the lasing peak of the shorter wavelength (fiber laser with BG2) approached the second lasing peak (fiber laser with BG1) we observed the process of injection locking, and only one lasing peak was obtained at the output of the beam combiner [Fig. 2(b)]. This occurred at a temperature of approximately 35 ±C (lower than that calculated from the known temperature dependence for silica fiber Bragg gratings for spectral coincidence between the two resonances). This output spectrum did not change, even when the maximum temperature was increased to 50 ±C. The injection locking began when the two laser peaks had a spectral separation of ⬃0.15 nm. This value is not too small for practical combining of fiber lasers. If one of the pump beams was blocked it was possible to measure separately the individual output spectrum of each fiber laser. Figures 2(c) and 2(d) show the spectra for BG2 at a temperature of 35 ±C: in Fig. 2(c), pumping occurs only through BG2; in Fig. 2(d), pumping occurs only through BG1. The lasing wavelengths for the two lasers are different and hence, if the pump for each fiber laser is turned on–off, a change in the output lasing wavelength of the combined lasers can be obtained, and this phenomenon can be used for optical switching or in all-fiber wavelength-division multiplexing components. The sum of the independent lasing peak intensities was approximately two times smaller than the lasing peak intensity shown in Fig. 2(b). This may be understood as a loss of approximately half of the laser power (because of the ⬃50% coupling ratio of the fused fibers at the lasing wavelengths) inside an unpumped active fiber. A coupling ratio different
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from 50% will change only the peak intensities. These additional losses are not present with the two pump beams, and the total output power is the simple sum of the individual laser outputs, as can be seen from Fig. 3(b). If the conf iguration of Fig. 1 has the Bragg gratings at different wavelengths, and if both lasers are pumped, the system behaves as if there were two independent cavities. However, if the criteria for injection locking are met, a three-mirror laser cavity is formed that contains two Bragg gratings and the cleaved 2 3 2 coupler as the output mirror. The spectrum linewidths were the same for all laser lines in Fig. 2 and were limited by the optical analyzer’s spectral resolution. The noise characteristics were similar for all these cw lasers. We investigated the spatial combination of the fiber lasers by cleaving the active fiber coupler at different points [see Fig. 3(a)] and measuring the far-f ield intensity distribution. A photodiode with a pinhole was scanned across the output beam spot when the two lasers were pumped [curves (1) in Figs. 3(b)– 3(d)]. Curves (2) and (3) in Figs. 3(b)–3(d) correspond to an output intensity distribution for the lasers with pumping only through BG1 and BG2, respectively. Figure 3(b) shows the results for the coupler cleaved at the midpoint (plane b); Figs. 3(c) and 3(d), for the consecutive cleaves of the same coupler (planes c and d) indicated in Fig. 3(a). All cleaves were prepared with a fiber hand cleaver, and the cleave quality varied. This variation may be an explanation for the asymmetry of the measured distributions and for different ref lectivities of the output mirror for the fiber lasers. It is a possible source of unequal laser output intensities as well. The midpoint cleave [Fig. 3(b)] demonstrated similar intensity distributions for all the combinations of fiber lasers.
Fig. 3. Spatial intensity distribution at the output of the cleaved end of the fused coupler – combiner. Curves (1), both fiber lasers were pumped; curves (2) and (3), the fiber lasers were pumped only through BG1 and BG2, respectively. (a) Three consecutive cleaves, b – d, of the fused coupler-combiner, ( b) output intensity distributions for cleave b, (c) output intensity distributions for cleave d, (d) output intensity distributions for cleave d.
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OPTICS LETTERS / Vol. 24, No. 24 / December 15, 1999
For these distributions no dependence on changes in temperature of BG2 was observed. Microscope analysis of the cleaved fiber coupler showed that the cross section had a circular shape. Other cleaves in planes c and d had noncircular cross sections, and the laser intensity distributions had different forms for the fiber lasers. However, for the cleave in plane c the combined laser intensity distribution was also symmetrical; i.e., the injection-locking mechanism creates a new mode structure at the output of the all-fiber beam combiner [Fig. 3(c)]. For increased asymmetry of the coupler’s cross section [Fig. 3(d)] the two fiber lasers performed independently, even for spectral coincidence of the Bragg grating resonance wavelengths. In this case the intensity distribution was the ordinary sum of the two independent light sources without coherent effects. Therefore, to obtain a coherent beam combination by use of one half of a fused coupler, the degree of fusion between the two active fibers is critical for spatial beam combining. We used two additional all-fiber optical components in our experiments to compare the performance of the combined lasers. Half of a single-mode fused taper fiber coupler was replaced by a standard Y-type singlemode coupler with a 50% coupling ratio for a broad spectral range near 1535 nm (Fig. 1, lower inset). This element is similar to the previous one, and it can be used as an all-fiber combiner for two fiber lasers. Relatively high intrinsic optical losses for this type of coupler and additional splice losses inside the fiber laser cavity make this element less interesting than a fused active fiber combiner, especially for high-power fiber laser applications or for the combination of a large number of fiber lasers. Nevertheless, the behavior of two fiber lasers spliced to the Y coupler was similar to that obtained with the fused and cleaved active fibers. A third element used in our experiments was a standard 980–1550-nm wavelength-division multiplexer that was spliced to the output fiber ends of the fiber lasers (Fig. 1, upper inset). This element cannot serve as a beam combiner for fiber lasers with similar wavelength. We used this device to demonstrate the independence of the fiber lasers used in this scheme. Because each laser has a separate output mirror (the cleaved wavelength-division multiplexer output fiber ports) the mutual inf luence of these fiber lasers on each other was relatively small. The output power was insensitive to changes in the Bragg ref lector’s resonance wavelength, but the laser power stability was
lower when grating BG2 was tuned to the resonance of grating BG1. This effect may be associated with parasitic light scattering in the wavelength-division multiplexer and the optical isolation of the output ports. In conclusion, we have demonstrated the use of half of a fused taper single-mode fiber coupler as an output mirror –beam combiner for two fiber lasers. This element (which can be generalized to an N 3 1 conf iguration) has relatively small optical losses and can be used as a means of combining highpower fiber lasers. We observed fiber laser injection locking by tuning one of the fiber laser’s Bragg grating mirrors. The spectral range for the locking process (not less than 0.15 nm) is within practical limits, and this phenomenon can be used to maintain spectral brightness of the combined fiber lasers. The spatial combination of two fiber laser beams was obtained, and we have shown that the degree of fusion of the active fibers is critical for this process. For the first time to our knowledge, both spatial and spectral combination of two fiber lasers has been demonstrated. This research was supported by the grant FQ86719900712 from the U.S. Air Force Off ice of Scientific Re´ search. J. Hernandez-Cordero acknowledges support from Direcci´on General de Asuntos del Personal Acad´emico, Universidad Nacional Aut´onoma de M´exico, Mexico. V. A. Kozlov’s e-mail address is ´ valerii [email protected]; J. Hernandez-Cordero’s e-mail address is [email protected]. References 1. N. W. Carlson, G. A. Evans, J. M. Hammer, M. Lurie, S. L. Palfrey, and A. Dholakia, Appl. Phys. Lett. 50, 1301 (1987). 2. J. R. Leger, M. L. Scott, and W. B. Veldkamp, Appl. Phys. Lett. 52, 1771 (1988). 3. C. J. Corcoran and R. H. Rediker, Appl. Phys. Lett. 59, 759 (1991). 4. C. C. Cook and T. Y. Fan, in Digest of Topical Meeting on Advanced Solid-State Lasers (Optical Society of America, Washington, D.C., 1999), postdeadline paper PD7. 5. T. F. Morse, M. Chen, L. Reinhart, and D. A. Brown, Proc. SPIE 2216, 210 (1994). 6. A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlane, K. P. Kao, C. G. Atkins, M. A. Putnam, and E. J. Friebele, J. Lightwave Technol. 15, 1442 (1997).
