December 15, 2001 / Vol. 26, No. 24 / OPTICS LETTERS
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Efficient frequency doubling of a pulsed laser diode by use of a periodically poled KTP waveguide crystal with Bragg gratings E. U. Rafailov, D. J. L. Birkin, and W. Sibbett School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, UK
P. Battle, T. Fry, and D. Mohatt AdvR, Inc., Suite K, 910 Technology Boulevard, Bozeman, Montana 59718 Received September 18, 2001 Blue light with an average power of as much as 7.5 mW in picosecond pulses has been generated at 486, 488, and 491 nm from a frequency-doubled, nonresonant injection seeded, gain-switched InGaAs兾GaAs diode laser by use of a periodically poled KTP waveguide crystal that incorporates a Bragg grating section. © 2001 Optical Society of America OCIS codes: 140.2020, 190.2620.
There is considerable interest in compact laser sources that operate in the blue spectral region at picosecond pulse durations1 for various applications in science and technology. Thus, in the present circumstances, the frequency doubling of near-infrared diode lasers with a waveguided nonlinear crystal represents a competitive methodology for obtaining a compact blue light source. Potassium titanyl phosphate (KTP) is especially suitable to serve as the nonlinear crystal for secondharmonic (SH) generation. This is so because of its modest cost but more importantly because it can be periodically poled to satisfy quasi-phase-matching conditions that increase the SH conversion efficiency at room temperature for a pulsed laser that exhibits single spatial and spectral mode beam characteristics. The achievement of a compact blue-light source is made practicable by the recent availability of goodbeam-quality diode lasers. Commercial diode lasers can have nearly diffraction-limited spatial outputs and narrow spectral bandwidths, for example, by use of distributed Bragg ref lector –distributed-feedback configurations.2,3 It is also possible to obtain multiwatt output powers with high beam quality by use of master oscillator – power amplif ier combinations.4 However, both of these conf igurations have drawbacks, the principal one being their high unit cost. Another approach to obtaining high power output is to use an external-cavity optical scheme,5 but to do so would require an antiref lection coating with a ref lectivity of less than 0.4% on the laser diode’s front facet. By contrast, the diode lasers used in the research described in this Letter are relatively low-cost commercially available devices (from JDS Uniphase) with cw output powers of as much as 200 mW at a wavelength of 980 nm without any extra facet coatings. A picosecond-pulse blue laser source that incorporates an InGaAs兾GaAs laser diode with a suitably designed periodically poled KTP waveguide crystal can thus be compact, stable, and self-contained and have an attractively competitive overall cost. 0146-9592/01/241961-02$15.00/0
The diode lasers that we used were 200-mW 980-nm InGaAs兾GaAs single-mode 3-mm ridge-waveguide devices. These lasers were operated either cw or gain switched, with corresponding FWHM spectral bandwidths of ,1 nm and ⬃20 nm, respectively.6 We exploited a nonresonant injection seeding effect in a gain-switched diode laser configuration7 to enhance spectral quality. A spectral linewidth reduction from 20 to 0.5 nm (Fig. 1) achieved by the nonresonant injection seeding of the gain-switched diode laser had little effect on other output laser characteristics. With this spectral narrowing, the pulse duration of the fundamental frequency mode increased from 35 to 45 ps, but the narrowing had a negligible effect on the spatial characteristics and the output power of the laser at repetition frequencies in the range 1.8– 2.7 GHz.8 The average and peak powers of the diode laser in this operation were 144 mW and ⬃1.2 W, respectively.
Fig. 1. laser.
Spectral outputs from the InGaAs兾GaAs diode
© 2001 Optical Society of America
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OPTICS LETTERS / Vol. 26, No. 24 / December 15, 2001
Fig. 2. Simplif ied optical schematic for the frequency doubling of a gain-switched diode laser.
Fig. 3. SH spectra for three Bragg grating periods.
To achieve efficient SH generation from the diode laser we designed a two-section waveguide crystal with a periodic grating structure and a Bragg grating; this grating was fabricated to facilitate nonresonant injection seeding of the diode laser. We used standard lithographic techniques to pattern a z-cut KTP wafer. The waveguides fabricated for this research had a section designed for quasi-phase-matched SH generation followed by a Bragg grating section designed to ref lect 30– 40% of the fundamental at the quasi-phase-matched wavelength. Waveguides were formed by rubidium-ion exchange when the suitably patterned KTP substrate was placed in a molten rubidium nitrate salt bath. The KTP crystal was 10 mm long and 1 mm thick and contained ⬃30 waveguides with a range of Bragg grating periods. The waveguides had dimensions of approximately 4 mm 3 4 mm and were separated by 50 mm. The periodically poled KTP crystal fabricated for this study contain three main Bragg grating periods for the fundamental wavelengths of 972, 976, and 982 nm. The experimental scheme for frequency doubling of the diode laser is shown in Fig. 2. For an average input laser power of 144 mW, the maximum SH blue light was 7.5, 5.6, and 6.8 mW for SH wavelengths centered at 486, 488, and 491 nm, respectively, as shown in Fig. 3. The temperature range was varied from 22 to 30 ±C to provide matching between the periodically poled quasi-phase-matched gratings of the KTP crystal and the Bragg gratings for the
various wavelengths. The maximum SH peak power was ⬃150 mW for the 486-nm spectral range and 25-ps pulse durations (which corresponds to ⬃12% peak power ref lected conversion eff iciency). We believe that this variation in the observed SH output powers obtained in these wavelength regions may be attributed to the optical quality of the individual waveguides. Further temporal and spectral characterizations are ongoing. Specif ic planned improvement will include (i) the use of antiref lection coatings on the periodically poled KTP waveguide crystal to reduce the facet ref lectivities from 15% to ,1%, (ii) optimization of the coupling optics to better match the focused pump beam to the confocal parameter in the crystal, and (iii) an increased frequency-doubling range of 6 –20 nm. Also, the use of gain-switched diode lasers with higher output power is under consideration. As we have demonstrated, we obtained eff icient blue light with an average power of as much as 7.5 mW and picosecond pulse durations from a frequency-doubled, nonresonant injection seeded InGaAs兾GaAs diode laser by using a periodically-poled KTP waveguide crystal with a Bragg grating section. Three different frequency-doubling ranges (486, 488, and 491 nm) for one diode laser have been generated. We thank JDS Uniphase for providing the InGaAs兾GaAs lasers used in this research. The research was supported by the UK Engineering and Physical Sciences Research Council. E. U. Rafailov’s e-mail address is
[email protected]; that of P. Battle is
[email protected]. References 1. D. J. L. Birkin, E. U. Rafailov, G. S. Sokolovskii, W. Sibbett, G. W. Ross, P. G. R. Smith, and D. C. Hanna, in Conference on Lasers and Electro-Optics (CLEO/ Europe), 2000 OSA Digest Series (Optical Society of America, Washington, D.C., 2000), paper CThB4. 2. M. Oberg, S. Nilsson, J. Wallin, D. Karlssonvarga, L. Backbom, and G. Landgren, IEEE Photon. Technol. Lett. 44, 230 (1992). 3. S. Takigawa, T. Uno, M. Kume, K. Hamada, N. Yoshikawa, H. Shimizu, and G. Kano, IEEE J. Quantum Electron. 25, 1489 (1989). 4. D. Woll, B. Beier, K. J. Boller, R. Wallenstein, M. Hagberg, and S. O’Brien, Opt. Lett. 24, 691 (1999). 5. W. J. Kozlovsky, W. P. Risk, W. Lenth, B. G. Kim, G. L. Bona, H. Jaeckel, and D. J. Webb, J. Appl. Phys. Lett. 65, 525 (1994). 6. E. U. Rafailov, D J. L. Birkin, E. A. Avrutin, W. E. Sleat, and W. Sibbett, IEE Proc. Optoelectron. 146, 51 (1999). 7. D. J. L. Birkin, E. U. Rafailov, W. Sibbett, and A. Avrutin, in Conference on Lasers and Electro-Optics (CLEO/Europe), 2000 OSA Digest Series (Optical Society of America, Washington, D.C., 2000), paper CWF76. 8. D. J. L. Birkin, E. U. Rafailov, W. Sibbett, and A. Avrutin, IEEE J. Sel. Top. Quantum Electron. 7, 287 (2001).
