A compact blue laser was generated by intracavity frequency doubling based on quasi-phase-matched second- harmonic generation (SHG) in a MgO-doped periodically poled lithium niobate bulk crystal. A 49 single-transverse- mode edge-emitters laser bar with antireflective coating was used as a pump source. An optical output power of SHG of blue lights at is generated at injection current, equivalent to an overall wall-plug efficiency of 1.33%.
© 2011 Optical Society of America
To date, several platforms have been developed for generation of compact, high-brightness, high-efficiency blue lasers. For the first technology, frequency doubling of diode-pumped solid-state (DPSS) lasers with lasing at and Nd:YAG lasing at are perhaps the most widely used commercial solid-state blue lasers [1, 2]. Mitsubishi engineers  improved this platform, using an unusual planar-waveguide configuration for replacing both the fundamental and second-harmonic bulk crystal for green-light generation.
To improve the efficiency and compactness of the blue laser, it is necessary to replace the solid-state gain medium in the DPSS system by directly using a GaAs/GaAlAs/InGaAs-based diode laser and doubling it to blue. Novalux, Inc., engineers demonstrated a  laser based on the intracavity frequency doubling (ICFD) of a diode surface-emitting laser. High-power, narrow- bandwidth, edge-emitting diode lasers directly used for second-harmonic generation (SHG) attracted many scientists’ attention recently . External-cavity diode lasers, laser diodes based on master oscillator power amplifier systems, and distributed-Bragg-reflector tapered lasers have been reported for single-pass SHG blue [6, 7, 8] and green  light.
In this Letter, we show a compact blue laser by ICFD based on a bulk-type quasi-phase-matched (QPM) SHG in a MgO-doped periodically poled lithium niobate (MgO:PPLN) crystal. A multiemitters high-performance laser bar is used as the pump laser. blue light at is generated at injection current. No diode laser facet degradation or damage is observed during a two-hour stability test of blue-laser output power.
The experiment setup is shown in Fig. 1. The laser bar on a passive Cu block, manufactured at Oclaro AG in Zurich, Switzerland, comprises 49 single-transverse-mode laser emitters laid out in a regular array with a pitch of . The cavity length is . The front facet carries a specially adapted ultralow reflectivity coating with a target reflectivity of less than 0.1% at a wavelength of to facilitate external locking of the emission wavelength as required for efficient frequency doubling. The back facet has a standard high-reflectivity coating suited to serve as an end mirror in an external-cavity arrangement. Maximum operating light output power is around with an operating current of and voltage of . The typical laser bar emission wavelength (at operating conditions) is at with lateral far field divergence (FWHM) of 4.6 deg in slow axis and 21 deg in fast axis. The threshold current is at and slope efficiency is of 0.99 ().
A cylindrical lens, (), and a microlens array, (), are used for laser bar beam shaping. consists of 49 lenses in a one-dimensional (1D) array with a pitch of . Each beam waist spot of the 49 emitters at focus is determined to have radii of and for the slow and fast axes, respectively, with a relative defocus distance of . The bulk MgO:PPLN crystal provided by Covesion, Ltd. (UK), has a length of , a width of , and a height of . It was designed to have 50 gratings with widths of and each with the grating period of . The facets of the crystal have an antireflective (AR) coating centered both at and . The MgO:PPLN crystal is temperature stabilized in order to achieve phase matching at the laser wavelength. Also, AR coatings of over centered at are applied to both the front and back optical surfaces of and for all polarizations. consists of 49 lenses in a 1D array with a pitch of and an effective focal length of , which transforms the output from each individual emitter and creates 49 parallel output beams with a symmetrical beam waist spot of around (radius) at the output focus plane of positioned at the output coupling mirror, . Retroreflection of the fundamental light is achieved at , providing excellent stability and allowing the complete laser bar to lase. is coated for high reflectivity in the near-IR range and transparency for blue light. A thin-film narrow-bandwidth IR filter, , is inserted in the cavity before the MgO:PPLN to restrict the spectral laser bandwidth so that optimal frequency conversion can be obtained. is designed as a telecom substrate with an eternity scale of . It is designed to provide minimal transmission loss () over a given wavelength range and to provide higher loss outside this wavelength range, with losses as high as at and at from the center wavelength at at all polarizations and operating environments when used with light of angle of incidence of deg. A half-wave plate, , having high transmission at and high reflection at , is inserted in the beam path. The alignment tolerances of , , , and are rigorous (at micrometer level). A blue-laser prototype was built up finally with a compact volume of . All the optics components were assembled, fixed, and mounted onto a copper heat sink.
Figure 2 shows the measured SHG power as a function of the operating injection current. A maximum of blue-laser output is obtained at a current of with the overall wall-plug efficiency of 1.33%. The filled squares in Fig. 2 are the measured values. The solid curve shows the theoretical values obtained from the equation for SHG conversion of 49 emitters for our crystal  and shows good agreement with the measured values when a cavity loss of 72% was selected. The high cavity loss is mainly caused by the coupling and reflection loss of the laser light into the MgO:PPLN chip.
The output spectrum of the laser bar shown in Fig. 3 was measured without a feedback mirror at an injection current of by an optical spectrum analyzer (Anritsu MS9710B, Anritsu Corp., Japan). The broad spectrum results from the superposition of the 49 individual emitters with AR coating, each of which is also not necessarily operating in single longitudinal mode. The spectrum of the blue-laser emission shown in Fig. 3 at an operating injection current of is measured using an Ocean Optics USB2000 (Ocean Optics, Inc., USA) miniature fiber optic spectrometer. A narrow peak at an emission wavelength of was observed. The bandwidth of less than is dominated by the resolution limit of the spectrometer.
The near-field images of 49 element arrays of fundamental and SHG light are presented in Figs. 4a, 4b, respectively. They are measured at a current of after beam shaping, which is imaged using a focal length lens. We found that each element of the array approximates a circular Gaussian beam, and they are incoherent to one another both for fundamental and SHG light. We can see in Figs. 4a, 4b that the beam size and intensity of each element of the array are not uniform, which is the result of the spherical aberration and coma of the lens we used. In Fig. 4b, the fringes positioned above and below the main 49 element arrays of blue light are the result of the scattering, reflecting, and degradation of the alignment of various optical elements in the laser cavity. Values of were observed of each element of the blue light for both axes. Figure 4c shows the beam profile of one element in the middle of the SHG light array at a current of recorded by a Thorlabs BP109 Beam Profiler (Thorlabs, Inc., USA). The values of the total output green beam at an injection current of were measured to be around 10.8 and 2.6 along the slow and fast axes, respectively. The optimal phase-matching temperature at a blue output power of is around with an acceptance temperature bandwidth FWHM of less than . Stability testing of the blue laser was carried out by monitoring the blue output power with a powermeter. At the blue output power of at a current of , the output noise was 4.06% (rms) for . The stability of blue-laser output power was obtained to be around . The center wavelength of the output spectrum of the blue- laser emission is at with a MgO:PPLN phase-matching temperature of . The blue-light beam quality and system stability were enhanced in comparison with the green laser  by the development of the alignment and assembly techniques and also the laser bar smile improvement.
A compact blue laser was demonstrated by ICFD of a 49-edge-emitters laser bar using a MgO:PPLN bulk crystal. A blue output optical power of was achieved at an injection current of with the optimum QPM temperature of , representing an overall wall-plug efficiency of 1.33%. This design for blue-laser generation can be further improved through further optimization of the design and alignment of the microlens and the use of a MgO:PPLN planar waveguide configuration.
This project was supported by the Technology Strategy Board (TSB) with DBERR Project TP/6/EPH/6/S/K2515A.
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