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College of Optics and Photonics/CREOL, University of Central Florida, 4000 Central Florida Boulevard ... OptiGrate, 3267 Progress Drive Orlando, Florida 32826.
May 15, 2006 / Vol. 31, No. 10 / OPTICS LETTERS

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Stable coherent coupling of laser diodes by a volume Bragg grating in photothermorefractive glass George B. Venus and Armen Sevian College of Optics and Photonics/CREOL, University of Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32816-2700

Vadim I. Smirnov OptiGrate, 3267 Progress Drive Orlando, Florida 32826

Leonid B. Glebov College of Optics and Photonics/CREOL, University of Central Florida, 4000 Central Florida Boulevard, Orlando, Florida 32816-2700 Received January 3, 2006; revised February 11, 2006; accepted February 13, 2006; posted February 17, 2006 (Doc. ID 66907) Two separate laser diodes emitting near 980 nm were coupled by a thick volume Bragg grating recorded in a photothermorefractive glass. The coupled diodes emitted at the same wavelength with a spectral width narrowed to below the resolution of the spectrum analyzer 共⬍30 pm兲. Coherent emission at a pumping level greater than five times above the threshold was observed for several months with near-unity visibility of the interference pattern. © 2006 Optical Society of America OCIS codes: 140.5960, 090.7330, 160.2900.

Increasing the brightness of lasers by coherent coupling of multichannel emitters has been intensely studied for more than 25 years, and results have been published in numerous papers (see, e.g., one of the recent surveys1). There are two basic approaches for coherent coupling. The first one is to inject coherent radiation to separate lasers and force them to emit coherently.2–6 This approach allows oscillation of all components of the system in the same mode. However, dramatic changes of refractive index in semiconductors under the strong pumping current results in a significant difference of optical paths in different channels. Therefore the problem of phase measurement and control in all channels leads to high complexity and low efficiency in this approach. The second one is to design a multichannel resonator that provides coherent emission of all its components.7–16 This approach allows phase control to be obviated in the channels, but the main problem preventing stable and efficient coupling is a tendency of a multichannel system to switch between different modes of a complex resonator. A number of dispersive elements have been used to eliminate multimode oscillation, but no stable coherent coupling at high levels of pumping has been reported. This paper reports the use of thick Bragg gratings recorded in a photothermorefractive (PTR) glass17–19 for coherent coupling. These new, robust optical elements have extremely high spatial and angular dispersion, which are higher compared with those of any dispersive elements used in previous work. These diffractive elements are stable at elevated temperatures, withstand high-power laser radiation, and have low losses, thus allowing their use in laser resonators. These elements were previously used for highly efficient spectral (incoherent) combining of 0146-9592/06/101453-3/$15.00

high-power laser beams,20 dramatic narrowing of the spectral width of emission of laser diodes,21 and conversion of multimode, wide-stripe laser diodes to single-transverse-mode sources operating at high pumping levels.22 Such semiconductor lasers with external resonators, which include volume diffractive gratings, were called volume Bragg lasers. The basic idea of this work is to utilize volume Bragg gratings to create extremely dispersive external resonators for laser diodes that support only one mode and to further use the same grating for coupling two diodes. The experimental setup for coherent coupling and observation of the interference pattern between two semiconductor laser diodes is shown in Fig. 1. Two commercial single-transverse-mode 50 mW laser diodes with standard antireflection coatings 共⬃5 % 兲 emitted collimated beams near 980 nm. They were placed on separate stages mounted on the same vibration isolated optical table [Fig. 1(a)]. The optical axes of the diodes are about 10 cm above the surface of the table; the distance between the diodes is about 2 cm. Emission spectra of the diodes consist of several fluctuating lines of about 3.5 nm total spectral width [Fig. 2(a)]. The laser outputs are combined on a screen by a system of mirrors and a beam splitter [Fig. 1(a)]. Of course, these lasers are not coherent, and a combined beam exhibits a typical speckle pattern [Fig. 3(a)]. A number of PTR Bragg gratings, each having a spectral selectivity narrower than 100 pm, half-width to the first zero (HWFZ), and reflection coefficient of 98% for planar monochromatic waves were used for this experiment. First, a locking grating, working in a retroreflecting mode at 979.97 nm, was placed in the beam of laser diode LD-1 [Fig. 1(b)], causing spectral narrowing of this laser from several nanometers © 2006 Optical Society of America

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from the resonator [Fig. 1(d)], but the spectral width of the radiation of both diodes was still below 30 pm, and the interference pattern was still observed. This interference pattern was stable for a long period, which is remarkable when it is taken into account that the diodes and the coupling grating were mounted on three different stages, resulted in a resonator length of 15 cm. It is important to note that coherent coupling was observed at high levels of pumping, above five times the threshold. The total power of the coherent radiation from the coupled lasers was above 80% of the sum of the power of the independent diode lasers. The black stripe in the middle of the interference pattern in Fig. 3(b) corresponds to the angle with the

Fig. 1. Optical scheme of coherent coupling of laser diodes. (a) Combining independent diodes; (b) spectral locking LD-1; (c) phase locking two diodes; (d) after the locking grating is removed.

to less than 30 pm (Fig. 2), which is the spectral resolution of the optical spectrum analyzer used. Second, a coupling grating was placed in the beam of LD-1 at a distance of 7 cm and aligned to provide efficient diffraction of the narrowband emission at 979.97 nm from LD-1 to LD-2, thereby coupling these two lasers [Fig. 1(c)]. Both lasers emitted the narrow lines separated by less than 150 pm. A combined beam produced by the coupled narrowband lasers is still a fluctuating speckle pattern similar to that shown in Fig. 3(a) because no phase locking resulted from spectral locking of the laser diodes. After the locking grating was removed from the resonator [Fig. 1(d)], wideband emission of two laser diodes was observed, similar to that observed in the geometry of Fig. 1(a). However, it was found that in the case where the spectral width of the coupling grating (⬃40 pm HWFZ) was less than the axial mode separation of the internal resonator 共⬃70 pm兲 of the laser diodes, tuning the pumping current resulted in locking the lasers in Fig. 1(c) to the same frequency and phase. In this case the emission spectrum of both lasers is identical [Fig. 2(b)]. When these two beams were combined, the interference pattern shown in Fig. 3(b) (the dark and light lines at 45°) was produced. After this was done, the locking grating could be removed

Fig. 2. Emission spectra of laser diodes. (a) Lowresolution: light curve, original diode; dark curve, the same diode with a locking grating as an output coupler. (b) High resolution: dark curve, LD-1 locked by a locking grating; light curve, LD-2 locked by a coupling grating.

Fig. 3. Interference pattern produced by the beams of two laser diodes: (a) Isolated diodes; (b) diodes coupled by a narrowband PTR Bragg grating.

May 15, 2006 / Vol. 31, No. 10 / OPTICS LETTERS

highest diffraction efficiency for a coupling grating. Therefore radiation propagating in this direction was diffracted by the coupling grating to provide coupling between the lasers. Emission at other angles (where the interference pattern can be seen) was not diffracted by the grating, yet the interference pattern shows that the light emitted by the two diode lasers was coherent with stationary locked phases. Thus this shows that two separate lasers, coupled by a PTR Bragg grating, can behave as a single coherent source of light. This interference pattern has a visibility close to unity. It was stable for hours during continuous operation, and this process was the same for more that 11 months of repeatable experiments with the same two devices. Stable coherent emission from two separate laser diodes at high pumping levels has been demonstrated. Further development of this coherent coupling technique may lead to dramatic increases in the power of coherent emitters, and, in combination with two other PTR-glass-based technologies (spatial narrowing of wide-stripe semiconductor laser emission to a single-transverse-mode regime and spectral beam combining) may lead to high-power diode laser systems with near-diffraction-limited divergence. The work has been supported by DARPA/SHEDS contract HR-01-1041-0004. The authors express their gratitude to C. M. Stickley and E. Van Stryland for discussions, L. Glebova for the PTR glass fabrication, and I. Ciapurina for sample preparation. L. Glebov’s e-mail address is [email protected]. References 1. A. F. Glova, Quantum Electron. 33, 283 (2003). 2. J. P. Hohimer, A. Owyoung, and G. R. Hadley, Appl. Phys. Lett. 47, 1244 (1985). 3. J. Mercier and M. McCall, Opt. Commun. 138, 200 (1997). 4. A. P. Napartovich and D. V. Vysotsky, Opt. Commun. 141, 91 (1997).

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5. L. Bartelt-Berger, U. Brauch, A. Giesen, H. Huegel, and H. Opower, Appl. Opt. 38, 5752 (1999). 6. Y. Liu and Y. Braiman, IEEE J. Sel. Top. Quantum Electron. 10, 1013 (2004). 7. C. J. Chang-Hasnain, J. Berger, D. R. Scifres, W. Streifer, J. R. Whinnery, and A. Dienes, Appl. Phys. Lett. 50, 1465 (1987). 8. J. R. Leger, G. Mowry, and D. Chen, Appl. Phys. Lett. 64, 2937 (1994). 9. S. Yu. Kourtchatov, V. V. Likhanskii, A. P. Napartovich, F. T. Arecchi, and A. Lapucci, Phys. Rev. A 52, 4089 (1995). 10. J. R. Leger, G. Mowry, and L. Xu, Appl. Opt. 34, 4302 (1995). 11. G. L. Schuster and J. R. Andrews, Appl. Opt. 34, 6801 (1995). 12. V. V. Apollonov, S. I. Derzhavin, V. V. Kuzminov, D. A. Mashkovskii, A. M. Prokhorov, V. N. Timoshkin, and V. A. Filonenko, Quantum Electron. 29, 839 (1999). 13. E. K. Gorton and R. M. Jenkins, Appl. Opt. 40, 6663 (2001). 14. D. Sabourdy, V. Kermene, A. Desfarges-Berthelemot, M. Vampouille, and A. Barthelemy, Appl. Phys. B 75, 503 (2002). 15. S. Riyopoulos, Phys. Rev. A 66, 53821 (2002). 16. D. Sabourdy, V. Kermène, A. Desfarges-Berthelemot, L. Lefort, A. Barthélémy, P. Even, and D. Pureur, Opt. Express 11, 87 (2003). 17. L. B. Glebov, V. I. Smirnov, C. M. Stickley, and I. V. Ciapurin, in Proc. SPIE 4724, 101 (2002). 18. O. M. Efimov, L. B. Glebov, L. N. Glebova, and V. I. Smirnov, U.S. Patent 6,586,141 (July 1, 2003). 19. O. M. Efimov, L. B. Glebov, and V. I. Smirnov, U.S. Patent 6,673,497 (January 26, 2004). 20. I. V. Ciapurin, L. B. Glebov, L. N. Glebova, V. I. Smirnov, and E. V. Rotari, in Proc. SPIE 4974, 209 (2003). 21. G. Venus, V. Smirnov, L. Glebov, and M. Kanskar, in Proceedings of Solids State and Diode Lasers Technical Review (Directed Energy Professional, 2004), paper P-14. 22. G. B. Venus, A. Sevian, V. I. Smirnov, and L. B. Glebov, in Proc. SPIE 5711, 166 (2005).