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Compact 1.3 W green laser by intracavity frequency doubling of a multi-edge-emitter laser bar using a MgO:PPLN crystal Kang Li,1,* Aiyun Yao,1 N. J. Copner,1 C. B. E. Gawith,2 Ian G. Knight,3 Hans-Ulrich Pfeiffer,4 and Bob Musk5 1
Faculty of Advanced Technology, University of Glamorgan, Pontypridd CF37 1DL, UK 2 Covesion Ltd., Romsey SO49 9AQ, UK 3 Bookham Technology plc, Caswell NN12 8EQ, UK 4 Bookham (Switzerland) AG, CH-8045 Zürich, Switzerland 5 Gooch & Housego, Torquay TQ2 7QL, UK *Corresponding author:
[email protected].
Received July 10, 2009; revised September 27, 2009; accepted October 5, 2009; posted October 15, 2009 (Doc. ID 114148); published November 4, 2009 A compact green laser of 1.3 W output at 534.7 nm is generated by intracavity frequency doubling (ICFD) of a 49 edge-emitter laser bar using a MgO-doped (PPLN) bulk crystal. The measured M2 values of green beam are 12.1 and 2.9 along the slow and fast axes, respectively. To our knowledge, this is the first demonstration of the ICFD of multi-edge-emitters laser bar. © 2009 Optical Society of America OCIS codes: 140.2020, 140.3410, 190.2620, 190.4360.
Continuous wave (CW) lasers in the green frequency range have been attracting increasing interest in various application fields such as displays, optical recording, and storage, biomedical instrumentation, etc. [1]. To date, the green laser is still not easy to be obtained by direct diode lasers operating at room temperature compared with the high-power CW output in red and blue. Several approaches have been developed for generation of green light. A diodepumped solid-state laser (DPSS) at the wavelength of 532 nm is perhaps the most widely used commercial solid-state green lasers. The optically pumped semiconductor disk laser also known as the vertical external-cavity surface-emitting laser (VECSEL) [2] is becoming increasingly attractive as it combines the benefits of semiconductor lasers with the advantages of a DPSS and external cavity flexible application. It is possible to improve the efficiency and compactness of the green laser by replacing the solid-state gain medium in DPSS system by directly using an IR diode laser and doubling it. Single-pass secondharmonic generation (SHG) of the output from a 1064 nm diode laser is the most promising solution. Green light emission with a power as high as 107 mW is generated by frequency doubling of a reliable 1060 nm DFB laser diode using a MgO:periodically pooled lithium niobate (PPLN) waveguide [3]. Recently, more than 1.5 W of green light at 531 nm has also been demonstrated by single-pass SHG in MgO:PPLN of a distributed Bragg reflector (DBR) tapered diode laser [4]. Intracavity frequency doubling (ICFD) of the electrically pumped VECSEL in quasi-phase-matched ferroelectric materials is an attractive approach for the development of power scaling, compactness, and efficiency [5]. Moreover, multiemitter arrays based on ICFD of electrically pumped VECSEL technology have been demonstrated to be suitable as a platform for projection displays [6]. Also, planar-waveguide devices combined with ICFD [7,8] dramatically improve 0146-9592/09/223472-3/$15.00
green laser efficiency, power, size, and cost. 7.6 W with a record-high electrical efficiency of 20% was demonstrated [8]. However, the challenges for laser projection still focus on the scalable greater powers and extremely low-cost requirement of the consumer electronics markets. In this Letter, we exploited ICFD of a multi-edgeemitters laser bar. A compact 1.31 W green laser is demonstrated by ICFD of a 49-emitter laser bar using a MgO: PPLN bulk crystal, which has the potential to be scalable to high production volumes and low costs with immense implication for laser-based projection displays. Figure 1 shows the schematic setup of the ICFD of 49-emitter laser bar using a MgO:PPLN bulk crystal. The multiemitter laser diode bar manufactured by the Oclaro facility in Zurich has a 3.6 mm cavity length with 49 narrow stripe emitters with 200 m pitch. The beam size from the individual single mode emitters is 0.5 m in fast axis and 4 m in slow axis with lateral far field divergence of 7 deg and vertical far field of 22 deg. Ultralow reflectivity coating on the output facet of an Oclaro 1064 nm laser diode gives less than 0.1% reflectivity in a wavelength range around 1064 nm, which eliminates the original diode laser cavity allowing the extended longer laser cavity to dominate. The laser bar is lasing even with little feedback, as the round-trip gain in the long cavity is
Fig. 1. (Color online) Schematic diagram of the ICFD of 49 edge-emitters laser bar setup. © 2009 Optical Society of America
November 15, 2009 / Vol. 34, No. 22 / OPTICS LETTERS
sufficient to overcome extremely high mirror losses. 35 W of CW power is produced at 45 A in the absence of an external cavity. A Doric cylindrical lens L1 is used to collimate the fast axis of the 49 emitters. The slow-axis microlens array L2 consists of 49 lenses in a one-dimensional array with a pitch of 200± 0.1 m. The fast and slow axes microlenses have the focal length of 121.5 m and 776 m, respectively, which are aligned with the individual emitters and resulted in 49 beams with 24 m of beam waist radius focused to the midpoint of the MgO:PPLN crystal supplied by Covesion. The bulk crystal has a length of 10 mm, a width of 13 mm, and a height of 0.5 mm with antireflection (AR) coating both centered at 1064 and 532 nm. The poling period of the crystal is 6.92 m. AR coatings of reflectivity ⬍0.5% over 20 nm centered at 1064 nm are applied to both front and back optical surfaces of L1 and L2 for all polarizations. The focus lens array L3 consists of 49 lenses in a one-dimensional array with a pitch of 200± 0.1 m and an effective focal length of 3.5± 0.18 mm, which transforms the output from each individual emitter and creates 49 parallel output beams with a symmetrical beam waist (radius) of around 43 m at the output focal plane of L3 positioned at the output coupling mirror P3. Retroreflection of the IR light is achieved at P3 coated for high reflectivity in the near-IR range and transparency for green light, providing superior stability and allowing the complete laser bar (49 separate emitters) to lase. A thin film narrow bandwidth IR filter P1 is inserted in the cavity before the MgO:PPLN to restrict the spectral laser bandwidth to ⬍0.15 nm so that optimal frequency conversion can be obtained. P1 has a reflectivity ⬎95% for unpolarized light at wavelengths of 532 nm. A half-wave plate P2 AR coated at both 1064 nm and 532 nm is inserted in the beam path for accurate control of the pump polarization. For the sake of extracting the counterpropagating SHG beam, a tilt from P1 in the cavity will provide a near collinear but translated beam output. This level of translation
Fig. 2. Output spectrum of the laser bar in the absence of an external cavity.
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would ensure no interference between the beams and allows this beam to be losslessly coupled out. The optical components alignment is of critical importance in our system and the alignment tolerances of the L1, L2, L3, and P3 are rigorous (at micrometer level). Figure 2 shows the output spectrum of the laser bar in the absence of an external cavity at an injection current of 45 A measured by an optical spectrum analyzer (Anritsu MS9710B). The broad spectrum results from the superposition of the 49 individual emitters, each of which is also not necessarily operating in single longitudinal mode. The spectrum of the green laser emission at an operating injection current of 45 A is shown in Fig. 3, which is measured using an Ocean Optics USB2000 miniature fiber optic spectrometer. A narrow peak at an emission wavelength of 534.7 nm was observed. The bandwidth of 0.75 nm is dominated by the resolution limit of the spectrometer. We have derived a model for the CW ICFD based on applicable laser rate equations [9], which describe the second-harmonic (SH) power P2 for the single emitter as P 2 = 2
−
h 4
2 2
冑
冋
共K + 兲
冑K
共K + 兲2 K
− 4共 − Ie兲
册
2
,
where h is Plank’s constant, is the fundamental frequency, Ie is pump injection current divided by electronic charge,  is the stimulated emission rate, includes losses and gain threshold, is the carrier lifetime, and K is the nonlinear coupling coefficient. Figure 4 shows that a maximum of 1.31 W green laser output is obtained at an operating injection current of 45 A. The squares are the experimental data, and the cavity loss of 70% was used to fit our calculated power (solid curve in Fig. 4) to the experimental data. Such high intracavity loss in our system is caused mainly by the coupling and reflection loss of
Fig. 3. Output spectrum of the green laser emission.
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OPTICS LETTERS / Vol. 34, No. 22 / November 15, 2009
of 1.31 W was achieved at an injection current of 45 A with an optimal phase-matching MgO:PPLN temperature of 88° C. Wavelength conversion efficiency of 3.1% and overall wall-plug efficiency of 1.5% were obtained. Further improvements are now possible through careful design of the lens used on the fast and slow axes beam waists and use of lowertemperature MgO:PPLN waveguide array. The authors wish to acknowledge Dermot Quinn and Graham Moss from Digital Projection Ltd., Rob Zaple, and Phil Henderson from Gooch & Housego Ltd. for the helpful discussion. This project was supported by the Technology Strategy Board (TSB) with the DBERR project TP/6/EPH/6/S/K2515A. References
Fig. 4. (Color online) Green light current characteristics of the ICFD of the 49-emitters laser bar.
the laser light into the laser chip, which depends mostly on the optical specification and alignment of the key microlens in the system. The output green beam was focused by a 100 mm focal length lens and the beam profiles were recorded using Thorlabs BP109 beam profiler. The M2 values of the output green beam at an injection current of 45 A were measured to be 12.1 and 2.9 along the slow and fast axes, respectively. The relatively large M2 values are caused possibly by the laser bar smile and imperfect alignment of 49 individual emitter as well. The asymmetrical green beam profiles are due to the IR beam profiles along the fast and slow axes at the beam waist. A heat management system is used for temperature control so as to achieve phase matching at the laser wavelength. The optimal phase-matching temperature of the MgO:PPLN crystal used in our work at a green output power of 1.31 W is around 88° C with an acceptance temperature bandwidth of less than 1 deg. A compact green laser of only 44.5 mm length is demonstrated by ICFD of a 49 edge-emitters laser bar using a MgO:PPLN bulk crystal. To our knowledge, this is the first time that has been demonstrated ICFD using high a brightness multi-edgeemitters laser diode bar. Green output optical power
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