Appl. Phys. B 87, 95–99 (2007)
Applied Physics B
DOI: 10.1007/s00340-006-2551-0
Lasers and Optics
r. hartke1,u e. heumann1 g. huber1 2 ¨ m. kuhnelt 2 ¨ u. steegmuller
Efficient green generation by intracavity frequency doubling of an optically pumped semiconductor disk laser 1 2
Institute of Laser Physics, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany OSRAM Opto Semiconductors GmbH, Wernerwerkstraße 2, 93049 Regensburg, Germany
Received: 15 November 2006 Published online: 20 January 2007 • © Springer-Verlag 2007 ABSTRACT We report the use of bismuth borate for efficient intracavity frequency doubling of an optically pumped semiconductor disk laser. An output power of 220 mW at 529 nm at room temperature has been demonstrated, corresponding to 52% conversion efficiency with respect to the fundamental power. The results are compared to frequency doubling with a potassium titanyl phosphate nonlinear crystal. PACS 42.55.Px;
1
42.70.Mp; 42.72.Bj
Introduction
Applications like laser projection displays, bioanalytical instruments, microscopy or optical storage devices require compact and efficient laser sources in the visible spectral range. Semiconductor lasers are preferred, because they can be produced at relatively low cost and small size at high production volumes. However, most of them lack either beam quality or output power. Furthermore, while laser diodes in the red and blue recently have been improved significantly, there is still no promising solution for green laser diodes. Recently, optically pumped semiconductor (OPS) disk lasers are of considerable interest as they combine the advantages of diode-pumped solid-state lasers with those of conventional edge-emitting semiconductor lasers [1, 2]. Significant continuous-wave (cw) output with brilliant beam quality has already been achieved [3]. The external cavity yields many opportunities like frequency doubling [4], single-frequency operation [5], mode locking [6] or broadband laser absorption spectroscopy [7]. The compactness and the power scalability with increasing pump spot size [8] make OPS disk lasers suitable for a variety of applications. By employing intracavity frequency doubling, a wide range from the visible to the UV spectrum can be covered. Frequency-doubled OPS disk lasers have been demonstrated for orange [9], yellow [10], green [11], cyan [4], blue [12] and UV [13] radiation. OPS disk lasers at 488 nm and 460 nm are already commercially available. Most of these lasers use the
nonlinear crystals lithium triborate (LBO) [9–12] and potassium niobate (KNbO3 ) [4, 14] for the generation of green and blue radiation. Unfortunately, LBO is not very attractive for low-power applications due to the relatively low nonlinear coefficient deff. KNbO3 has a very high deff, but the low spectral and temperature acceptance makes it impractical for use with OPS disk lasers in many applications. To achieve high efficiencies in the green at relatively low powers, optimized nonlinear conversion is required. One approach is to choose the optimum conversion material. A wellknown nonlinear crystal for second-harmonic generation (SHG) to the green is potassium titanyl phosphate (KTP). It has a rather high nonlinear optical coefficient (∼ 3.4 pm/V), a broad thermal acceptance (∼ 25 K cm) and a small walk-off angle (∼ 0.6 mrad). Furthermore, it can be grown at relatively low cost. KTP is a standard material for frequency doubling of the 1064-nm transition of Nd:YAG lasers. Monoclinic bismuth borate (BiBO) on the contrary is a relatively new crystal material. First studies of BiBO were done by Hellwig and co-workers in 1998 [15]. However, BiBO already has proven to be a good material for singlepass [16] as well as intracavity frequency doubling [17, 18]. It has a large effective nonlinear coefficient of ∼ 3.3 pm/V and conversion efficiencies of more than 63% have been demonstrated [17]. Due to its very broad spectral acceptance BiBO can be handled easily with semiconductor lasers that might shift in wavelength. Furthermore, it is non-hygroscopic and no temperature control is required. A major drawback of BiBO is its large walk-off angle of ∼ 25 mrad at 1050 nm. In this paper we report the use of type-I phase-matched BiBO for intracavity frequency doubling of an OPS disk laser with an InGaAs/GaAs gain structure. We compare the results to a setup using type-II phase-matched KTP. Characteristics of the intracavity second-harmonic generation concerning output power, beam profile and stability are presented. 2
Experimental setup
The OPS chip was grown by OSRAM Opto Semiconductors GmbH by metal organic vapor phase epitaxy on a GaAs substrate. It consisted of an AlGaAs distributed Bragg reflector centered at 1050 nm and a resonant periodic gain u Fax: +49 40 8998 5195, E-mail:
[email protected] (RPG) region. The RPG structure was composed of com-
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onator with a curved end mirror of radius R = 50 mm yielded 560 mW cw output power and a pump light conversion efficiency of 36.1%. The power characteristics for a V-type resonator can be seen in Fig. 2. The astigmatism of the folded cavity caused a smaller overlap of pump and cavity modes and therefore the infrared (IR) output was reduced when compared to a linear resonator. For 2% output coupling, 428 mW of infrared power could be achieved with the folded cavity. Additional cavity elements like a dielectric filter or a birefringent filter caused additional losses and hence reduced power was observed. The thermal roll over set in at about 1.4 W of optical pump power. In Fig. 3 the IR spectra of the OPS disk laser in the V-type setup are shown for a resonator with a birefringent filter and without any filter. Using a 2.5-mm-thick birefringent filter, the laser bandwidth could be reduced to less than 0.1 nm.
FIGURE 1 Experimental setup for intracavity second-harmonic generation. The birefringent filter can be replaced by a Brewster plate. The lengths are L 1 = 65 mm and L 2 = 107 mm. The pump beam is focused onto the chip at an angle of about 45◦
pressively strained InGaAs quantum wells at the antinodes of the standing wave pattern, AlGaAs barrier layers and straincompensating GaAsP. The gain structure was grown directly on the substrate and covered by the Bragg mirror. Afterwards the chip was mounted upside down on a heat sink. The active region was uncovered by etching off the substrate. As shown in Fig. 1, the gain structure was optically pumped at 808 nm by a single broad-area laser diode. The pump beam was collimated directly behind the diode by a fast axis collimation (FAC) lens and then focused onto the chip at an angle of about 45◦ to the surface normal of the semiconductor. The heat sink was kept at 20 ◦ C by Peltier cooling. The V-shape resonator was formed by the Bragg reflector, a curved folding mirror M1 (radius R = 75 mm) and a curved end mirror M2 ( R = 25 mm). The folding angle was about 18◦ and the mode size on the chip was 42 × 45 µm2 . The reflectivity of the mirrors M1 and M2 at 1050 nm was particularly high (> 99.95%). The second harmonic was coupled out through the folding mirror which had a high transmission (> 97%) at 525 nm. The total cavity length was 172 mm, with L 1 = 65 mm and L 2 = 107 mm. Additional optical elements, like a Brewster plate or a birefringent filter, could be inserted between the OPS chip and M1. For frequency doubling we employed the nonlinear crystal at the resonator mode waist between the folding mirror and the end mirror. Amplitude fluctuations and power stability were measured with a fast photodiode and an oscilloscope. For millisecond time scales we employed an external chopper to relate the noise to the total signal amplitude. The laser spectra were recorded with a Fourier-transform spectrometer. 3
Infrared laser characterization
The OPS disk laser offered a good efficiency. For optimized output coupling of about 2%, a linear res-
FIGURE 2 IR output characteristic in a V-resonator setup. The experimental data corresponds to IR output power for 2% output coupling for the resonator without any additional element (squares) and with a birefringent filter at Brewster’s angle (triangles), respectively
FIGURE 3 IR spectra of the OPS disk laser without nonlinear crystal in a V-type setup with a birefringent filter and without any filter. The spectrum with inserted filter was taken at random filter positions. For all other data presented in this paper the filter was adjusted for operation at 1050 nm
HARTKE et al.
Efficient green generation by intracavity frequency doubling of an optically pumped semiconductor disk laser
FIGURE 4 Spectral acceptance for SHG in a 4-mm-long BiBO crystal. The fundamental wavelength was provided by a widely tunable Yb:LSB thin-disk laser
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FIGURE 5 SHG output power versus absorbed pump power for the intracavity frequency-doubled OPS disk laser with BiBO (squares) and KTP (circles). For SHG with KTP a 2.5-mm-thick birefringent filter was added at Brewster’s angle. The inset shows the corresponding beam profiles
FIGURE 6 I. Amplitude noise and II. stability for a resonator with (a) just KTP, (b) just BiBO, (c) KTP and Brewster plate and (d) KTP and birefringent filter
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Applied Physics B – Lasers and Optics
Laser operation was observed over a tuning range of more than 10 nm. 4
Intracavity frequency doubling
The BiBO crystal was cut at (θ, ϕ) = (168◦ , 90◦ ) for type-I critical phase matching. The KTP crystal was cut at (θ, ϕ) = (90◦ , 35.1◦ ) for type-II critical phase matching. For both crystals the dimensions were 3 × 3 × 4 mm3 and the surfaces were antireflection-coated for both 1050 nm and 525 nm. A widely tunable ytterbium-doped lanthanum scandium borate (Yb:LSB) thin-disk laser was used to measure the spectral acceptance of the BiBO crystal. For external frequency doubling, the output beam was focused into the crystal by a f = 150 mm lens and the phase matching was optimized for a fundamental wavelength of 1050 nm. With a constant output power of 1.0 W, the Yb:LSB thin-disk laser was tuned from 1067.8 nm to 1033.9 nm (Fig. 4). The measured FWHM spectral acceptance was 12.5 nm, corresponding to a normalized value of 5.0 nm cm. Due to this broad bandwidth, no spectral control of the resonator was required for second-harmonic generation with BiBO. Also, no temperature control was applied. As shown in Fig. 5, 220 mW cw output power at 529 nm was achieved with the BiBO crystal. This corresponds to a conversion efficiency of 52% with respect to the maximum available infrared power. For the maximum green output power the absorbed pump power on the chip was about 1.42 W. Hence, a conversion efficiency of about 15.5% from 808 nm to 529 nm was achieved. The far field showed an interference pattern which resulted from a combined effect of the large walk-off angle and the curved end mirror (inset, Fig. 5). It disappeared in a setup with a planar end mirror. More than 80% of the energy was located in the main peak of the pattern and the beam quality factors of this peak were measured to be Mx2 < 1.45 and M 2y < 4. These results were compared to a similar setup using KTP instead of BiBO. Because of the different type of phase matching and the small spectral acceptance of KTP, a birefringent filter at Brewster’s angle had to be inserted between the OPS chip and M1 in order to fix the polarization and to limit the line width. This setup yielded a significantly lower output power of 71 mW at 525 nm, which corresponds to a conversion efficiency of 25% with respect to the maximum available infrared power. With a parameter of M 2 < 1.2, the beam quality was significantly better than for BiBO. Amplitude fluctuations (green problem) and long-term stability were measured for four different setups with a fast photodiode. Due to the very short upper-state lifetime in semiconductor materials and the reduced spatial hole burning in the RPG structure, amplitude fluctuations of frequencydoubled OPS disk lasers are reduced compared to most diodepumped solid-state lasers. On the left-hand side of Fig. 6 amplitude fluctuations on a millisecond time scale are shown. The laser was chopped externally to distinguish between noise from the photodiode and laser output noise. With simple additional intracavity elements that reduced the line width, fluctuations could be suppressed efficiently. The amplitude fluctuations for SHG with BiBO in Fig. 6b also became very small
with a birefringent filter or a Brewster plate in the resonator. Line width limiting elements furthermore show a positive influence on the long-term stability of the laser (right-hand side of Fig. 6). The laser spectra in Fig. 7 were recorded with a Fouriertransform spectrometer with a resolution of 0.5 cm−1 . In these diagrams the effect of the birefringent filter can be seen clearly. While the spectrum for the KTP setup showed a narrow line at 525 nm (Fig. 7a), the laser spectrum of the BiBO setup exhibited several lines over a range of about 0.5 nm and was shifted to a longer wavelength. 5
Conclusions
We have demonstrated 220 mW at 529 nm and a conversion efficiency of 52% with respect to the available IR power by use of a 4-mm-long BiBO crystal. This is to our knowledge the highest efficiency reported for intracavity doubled OPS disk lasers in the green. In comparison KTP showed lower power, but had advantages regarding beam quality. For both crystals amplitude fluctuations could be suppressed efficiently by inserting line width limiting elements into the cavity.
SHG spectra for frequency doubling with (a) KTP (with birefringent filter in the resonator) and (b) BiBO without additional elements in the resonator
FIGURE 7
HARTKE et al.
Efficient green generation by intracavity frequency doubling of an optically pumped semiconductor disk laser
ACKNOWLEDGEMENTS This work was supported by the Bundesministerium f¨ur Bildung und Forschung (BMBF) under FKZ 13N8592.
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