/ CLEO'99 / THURSDAY MORNING
354
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&TFWHM = 3.2K L =4.2mm
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CThB3 Fig. 2. Temperature tuning curve of a
PPLT for single pass second harmonicgeneration of a cw Tisapphire laser.
22
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.loo
optical-to-optical efliciency
--.-second
harmonic output power
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cause of shorter pulses4 with higher peak power. *Fachbereich Physik, Universitiit Kaiserslautern, Kaiserslautern, Germany **Institute of Quantum Electronics, Ultrafast Laser Physics, ETH Hijnggerberg, Zurich, Switzerland 1. G.D. Miller, R.G. Batchko, W.M. Tulloch, D.R. Weise, M.M. Fejer, and R.L. Byer, Opt. Lett. 22, 1834 (1997). 2. K. Mizuuchi and K. Yamamoto, Appl. Phys. Lett. 66,2943 (1995). 3. V. Pasiskevicius, S. Wang, J.A. Tellefsen, F. Laurell, and H. Karlsson, Appl. Opt. 37, 7116 (1998). 4. T. Kellner, F. Heine, G. Huber, C. Honninger, B. Braun, F. Morier-Genoud, M. Moser, and U. Keller, J. Opt. Soc. Am. B 15, 1663 (1998). 5. C. Honninger, R. Paschotta, F. MorierGenoud, M. Moser, and U. Keller, J. Opt. Soc. Am. B 16, (1999).
= 632nm
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pump energy [pJ/pulsel
CThB4 Fig. 1. Signal pulse energy in dependence of the pump pulse energy for the A = 11.3 pm period (signal wavelength: 632 nm).
measured Theorie
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CThB3 Fig. 3. Second harmonic output power at X = 465 nm and optical-to-optical conversion efficiency vs. incident fundamental power of the mode-locked NdYAlO, laser.
16.5 W. The total fundamental power was not available for second harmonic generation due to losses at the optical diode (approximately 10% insertion losses). The investigated PPLT crystal was 5 mm in length, with an aperture of 8 mm X 0.3 mm and a grating period of A = 4.9 p m suitable for frequency doubling the NdYAlO, laser at T = 188°C. The optimum crystal length predicted by the group velocity mismatch is 10 mm for 7 ps long laser pulses. However, the crystal length was chosen to match the spectral acceptance bandwidth of the PPLT crystal (AA + L = 0.22 nm X cm) with the spectral width of the mode-locked laser pulses of AA 0.5 nm. To examine the quality of the grating we measured a temperature tuning curve with a cw Tksapphire laser. The radius of the focus of the fundamental wave inside the PPLT was wo = 30 p m which meets the plane wave approximation. From the temperature acceptance bandwidth of ATFWHM = 3.2 K we deduced an active length of the grating of 4.2 mm showing the good quality of the grating (Fig. 2). Figure 3 displays the second harmonic output power and the optical-to-optical efficiency vs. the incident fundamental power. A maximum second harmonic output power of 82 mW at an incident fundamental power of 380 mW was achieved. This corresponds to an optical-to-optical conversion efficiency of 21.5%. Future research will be focused on the improvement of the mode-locked laser system to achieve stable mode-locking without Q-switched mode-locking. This would also increase the frequency doubling efficiency be-
9:30 am ~
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Visible nanosecond PPLN optical parametric generator pumped by a passively Q-swltched single frequency Nd:YAG-laser
U. Bader, J.-P. Meyn, J. Bartschke, T. Weber, A. Borsutzky, R. Wallenstein, R.G. Batchko,' M.M. Fejer,' R.L. Byer,' Fachbereich Physik, Universitiit Kaiserslautern, Erwin-SchriidingerStraj3e 46,0-67663, Kaiserslautern, Germany; E-mail:
[email protected]
We report on an efficient visible nanosecond optical parametric generator (OPG) of periodically poled lithium niobate (PPLN). This material is well suited for realizing an efficient OPG because of the high gain which can be achieved at pump intensities well below the crystal's damage threshold. An optical parametric generator of PPLN has in fact been demonstrated in the near infrared spectral region (1.4 pm-4.3 pm) by using a 20 mm long PPLN crystal pumped with ns-pulses at 1.06 pm. This OPG provided an efficiency of 25%. The bandwidth of the IR output was several nanometers.' In our experiments we used as pump source the second harmonic 532 nm output of a diode-pumped Nd:YAG laser passively q-switched by a Cr4+:GSGGcrystal. This laser emits 2.3 ns long pulses in a single mode with an energy of 172 pJ and a repetition rate of 1.1 kHz2The second harmonic is generated in a type I1 phasematched KTP crystal. The energy of the 532 nm pulses is 90 pJ.The M2factor of the green laser beam is 1.3. The OPG consists of a PPLN crystal (dimensions 55 X 13 X 0.5 mm3) which contains 30 QPM gratings with periods in the range of 10.1 to 13 pm. The aperture of each grating is 0.5 X 0.5 mm2. The end facets of the crystal were polished with a tilt angle of 5' to prevent parametric oscillation induced by reflections. The frequency doubled radiation of the N d YAG laser was focused to a beam waist of 60 pm. The measured pump pulse energy at threshold was 13 pJ.This value is in good agreement with the pulse energy of 11 pJ cal-
0 59 1
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period A [pn]
CThB4 Fig. 2. Tuning range of the OPG for a fixed temperature of 150°C for grating periods from 10.1 pm to 11.8 pm. The solid line represents the wavelengths calculated with the Sellmeier equations given in ref. 4.
oj
T = 150'C
h, = 632nm
2000
rel. wavelength [nm]
Spectrum of the OPG signal radiation at 632 nm. The bandwidth is 0.15 nm
CThB4 Fig. 3. FWHM.
culated with an estimated de, of 12 pm/V. As seen from figure 1 the OPG provides mW average signal output power of 10.8 mW when pumped with 93 mW of 532 nm laser radiation. This output corresponds to an internal efficiency of 18%. By changing the grating period from 10.1 p m to 11.8 p m the OPG output was tunable in the range from 637 nm to 592 nm (signal wave) and 3.2 p m to 5.2 p m (idler wave). The spectrum of the signal radiation is shown in figure 3 for a wavelength of 632 nm. The measured bandwidth of 0.15 nm is in good agreement with the calculated value for single pass parametric gain. It is interesting to note that the spectrum shows no modulation or clustering effect. A reduction of the OPG bandwidth was easily achieved with the technique of injection seeding. The output of a
THURSDAY MORNING / CLEO’99 / 355 1mW HeNelaser reduced the spectral width of the 632 nm OPG output to less than 1 GHz. In present experiments the seedingis investigated in detail in respect to seed power, bandwidth and spatial beam quality. The results obtained so far clearly indicate that PPLN is- because of its high nonlinearity-is an almost ideal material for an OPG as a very simple efficient source of widely tunable radiation. *E.L. Ginzton Laboratory, Stanford University, Stanford, California 94305 USA 1. 7.1. Zayhowski, Opt. Lett. 22, 169 (1997). 2. U. Blder, J. Bartschke, I. Klimov, A. Borsutzky, R. Wallenstein, Opt. Comm. 147, 95 (1998). 3. R.L. Byer, in Quantum Electronics: A Treatise, Academic, New York (1975). 4. D.Jundt, Opt. Lett. 22, 1553 (1997)
GaAs substrate (Bottom) CThB5 Fig. 1. A cross-sectional schematic of the ZnCdSe waveguide with patterned crystal orientation.
9:45 am
CThB5
Quasi-phasematched blue-green secondharrnonlc generation in ZnCdSe waveguides with patterned crystal Orientation
S.J.B. Yoo, M.A. Koza, J.K. Furdyna,* C. Caneau,** R. Bhat,** Bellcore, 331 Newman Springs Road, Red Bank, New Jersey 07701 USA; E-mail:
[email protected] Diffraction limited coherent sources at visible blue-green wavelength impact numerous applications ranging from optical recording to flat-panel displays. Visible sources based on second-harmonic generation in perosvkites, such as LiNbO,, LiTaO,, and KTP, are promising, but they require an additional optical coupling with lasers. Semiconductor nonlinear optical waveguides are of special interest for realizing an efficient wavelength conversion device monolithically integrated with a semiconductor laser. Typical zincblende semiconductors have much larger nonlinear optical susceptibilities(e.g. = 180 pm/V1) compared to those ofLiNbO, = -68 pm/Vz). The main difficulty in realizing such a semiconductor wavelength conversion device lies in the phase-matching. Zincblende semiconductors are cubic and there is no intrinsic birefringence that can be used for phase-matching. Quasi-phase-matched3 second-harmonic generation and differencefrequency-generation in periodically domaininverted AlGaAs waveguides have been recently demon~trated.~-5 In this paper, we discuss quasi-phase-matched blue-green second-harmonic generation in a ZnCdSe waveguide grown on an AlGaAs template with patterned crystal orientation. The design and fabrication of the quasiphase-matched waveguide include those of a template with periodic domain inversion and those of a waveguide for confining both the fundamental and the second-harmonic waves. We follow the procedure similar to that discussed in Ref. 4, and apply wafer bonding6 selective etching, and epitaxial regrowth. Figure 1 shows a cross-sectional schematic of the fabricated waveguide with alternating crystal orientations. The nonlinear ZnCdSe waveguide grown by Molecular Beam Epitaxy on the patterned template
xgLr
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(b) CThB5 Fig. 2.
(a) Measured temperature tuning curve for second harmonic generation. (b) Measured second-harmonic power plotted against square of the fundamental power.
consists of undoped layers of a 1 k m thick AlAs/GaAs superlattice planarization layer, a 3 micron thick ZnSe cladding layer, a 1 micron Zn,,,,Cd,,,,Se core, and a 0.5 micron ZnSe upper cladding layer. To create rib-loaded waveguides, the wafer was masked by SiO, stripes of widths ranging from 2 microns to 20 microns and wet etched by a diffusion-limited etchant. The fabricated waveguides were cleaved into 2 mm length bars for characterization. Second harmonic generation experiments were conducted on the fabricated ZnCdSe waveguides with domain periods of 2.0 pm. Radiation from a tunable Ti:Al,O, laser was coupled into a waveguide with the stripe width of 3 micron. The radiation was linearly polarized at 45 degrees off of TE polarization for Type-I1 phase-matching. Maximum SHG occurred for fundamental wavelength of 1013.1 nm. At this optimized wavelength, blue-green emission from the waveguide was clearly visible to the naked eye at modest fun-
damental power levels of a few tens of milliwatts. Fig. 2(a) shows a wavelength tuning curve of the normalized second harmonic generation efficiency. The solid line is a curve fit to measured points shown as filled circles. Figure 2(b) shows a plot of the output secondharmonic power as a function of the square of the input fundamental power inside the waveguide. The data points line up linearly on a 2.7%/W efficiency line. In summary, we report visible blue-green second-harmonic generation in ZnCdSe waveguides with patterned crystal orientation. This work was partially supported by the National Science Foundation DMR 92-08400 during the year 1995 and 1996. *University of Notre Dame, Department of Physics, Notre Dame, Indiana 46556 USA; Email: Jacek.K.Furdyna.l k3nd.edu **Corning, Inc., 331 Newman Springs Road, Red Bank, New Jersey 07701 USA (the work completed while at Bellcore) 1. B.F. Levine and C.G. Bethea, “Nonlinear Susceptibility of Gap: Relative Measurement and Use of Measured Values to Determine a Better Absolute Value,” Appl. Phys. Lett. 20,272 (1972). 2. M.J. Weber, CRC Handbook of Laser Science and Technology, Vol. 111, CRC, Boca Raton (1986). 3. J.A.Armstrong, N.Bloembergen, J. Ducuing, and P.S. Pershan, “Interactions between Light Waves in a Nonlinear Dielectric,” Phys. Rev. 127, 1918 (1962). 4. S.J.B. Yoo,R. Bhat, C. Caneau,M.A. Koza, “Quasi-phase-matched second-harmonic generation in AlGaAs waveguides with periodic domain inversion achieved by wafer-bonding,’’ Appl. Phys. Lett. 66, 3410 (1995). 5. S.J.B.Yoo, C. Caneau, R. Bhat, M.A. Koza, A. Rajhel, N. Antoniades, “Wavelength conversion by difference-frequency generation in AlGaAs waveguides with periodic domain inversion achieved by wafer bonding,” Appl. Phys. Lett. 68, 2609 (1996). 6. Y.H. Lo, R. Bhat, D.M. Hwang, M.A. Koza, and T.P. Lee, “Bonding by atomic rearrangement of InP/InGaAs 1.5 pm wavelength lasers on GaAs substrates,” Appl. Phys. Lett. 58, 1961 (1991).
