Correction of beam errors in high power laser diode bars and stacks J. F. Monjardin, K. M. Nowak, H. J. Baker, and D. R. Hall School of Engineering and Physical Sciences, Heriot Watt University, Edinburgh, EH14-4AS, United Kingdom
[email protected] http://www.hw.ac.uk/
Abstract: The beam errors of an 11 bar laser diode stack fitted with fastaxis collimator lenses have been corrected by a single refractive plate, produced by laser cutting and polishing. The so-called smile effect is virtually eliminated and collimator aberration greatly reduced, improving the fast-axis beam quality of each bar by a factor of up to 5. The single corrector plate for the whole stack ensures that the radiation from all the laser emitters is parallel to a common axis. Beam-pointing errors of the bars have been reduced to below 0.7 mrad. ©2006 Optical Society of America OCIS codes: (140.2010) Diode laser arrays; (140.2020) Diode lasers; (140.3300) Laser beam shaping;
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U. Brauch, P. Loosen, and H. Opower, “High-power diode lasers for direct applications,” in High Power Diode Lasers, R. Diehl, ed., (Springer-Verlag Berlin Heidelberg, 2000), pp. 303-368. I. Ghozeil, “Hartmann and other screen tests,” in Optical Shop Testing, D. Malacara, ed., (Wiley, 1992), pp. 367-396. K. M. Nowak, H. J. Baker, and D. R. Hall, “Pulsed-laser machining and polishing of silica micro-optical components using a laser and an acousto-optic modulator,” in Laser Micromachining for Optoelectronic Device Fabrication, A. Ostendorf, ed., Proc. SPIE, 4941, 107–112 (2002). G. A. J. Markillie, H. J. Baker, F. J. Villarreal, and D. R. Hall, “Effect of vaporization and melt ejection on laser machining of silica glass micro-optical components,” Appl. Opt. 41, 5660-5667 (2002). K. M. Nowak, H. J. Baker, and D. R. Hall, “Efficient laser polishing of silica micro-optic components,” Appl. Opt. 45, 162-171 (2006). T. Jitsuno, K. Tokumura, N. Nakashima, and M. Nakatsuka, “Laser ablative shaping of plastic optical components for phase control,” Appl. Opt. 38, 3338-3342 (1999). A. R. Holdsworth, H. J. Baker, “Assessment of micro-lenses for diode bar collimation,” in Laser Diode and LED Applications III, K. J. Linden, ed., Proc. SPIE 3000, 209-214 (1997). N. U. Wetter, “Three-fold effective brightness increase of laser diode bar emission by assessment and correction of diode array curvature,” Opt. Laser Technol. 33, 181-187 (2001).
The effectiveness of laser diode bar stacks in many applications, such as pumping of solidstate lasers and direct use in material processing, is limited by the poor beam quality associated with current commercial stacks. In this paper, we report the development of a technique for improving beam quality by characterising beam imperfections of a stack, and based on that information, fabricating a custom optical corrective plate that removes the effects of a series of opto-mechanical errors, which were introduced during the original fabrication of the stacks. Diode bars are typically fabricated with 25 emitters of 200 μm width along a 10 mm bar of semiconductor, which is then solder-bonded to a micro-channel watercooled heat sink. Commercially produced units, emitting 50-100W per bar, are stacked on typically 1.8 mm pitch to build up a total laser power of 500-1000 W. For many applications, an individual fast-axis collimator (FAC) lens is attached for each bar at the manufacturing stage. Currently, plano-acylindrical lenses are used to provide low aberration collimation for #73589 - $15.00 USD
(C) 2006 OSA
Received 1 August 2006; accepted 22 August 2006
4 September 2006 / Vol. 14, No. 18 / OPTICS EXPRESS 8178
the high numerical aperture fast-axis beam. Brauch et al. [1] provide a good review of the optical issues of bar stacks and their applications. The poor effective beam quality of diode bar stacks is a consequence of the practical imperfections in manufacturing and mechanical mounting for the bars and FACs at the micron level, rather than the intrinsic properties of the beams from individual lasers emitters in a bar. The cylindrical lens for each bar introduces angular aberrations that produce a local radiance loss of a factor of 2 to 3, although this may be improved with modern lens technology. The “smile” effect, whereby the semiconductor bar is bent by differential expansion during solder bonding, prevents the collimation lens being correctly positioned for all points along the bar, resulting in beams with variable pointing direction. Also, errors in attaching the FAC to the heat sink with the required positional accuracy further degrade the angular spectrum of the emitted light. There is also the problem of stacking the individual diode bars accurately enough to keep a common radiation direction for all bars. In many applications of the laser diode stacks, subsequent aperture filling, beam shaping and beam combining optics are required. The spread into a much larger volume of phase space than the theoretical minimum, mainly through errors in ray angles from the FAC, increases the design difficulty and reduces the efficiency for the subsequent beam conditioning optics, and fails to transfer the average radiance at the facet to the output application end. Once a diode stack is built, the beam errors are fixed for a given average power output and operating temperature. The fixed nature of the errors allows for correction if a suitable optical element can be made, placed close to the output plane where beam errors are predominantly angular. To demonstrate this, we report the use of a commercial diode bar stack made by OptoPower Corp., which consists of 11 bars vertically stacked with a 1.82 mm bar pitch, emitting an overall power of 440W cw. Each bar is fitted by the manufacturer with a planoacylindrical FAC which we measured to be 1.5 mm aperture and 0.9 mm effective-focallength, similar to lenses available from several manufacturers. Using the set-up in Fig. 1, we have assessed the severity of the various sources of beam error for each bar with the stack operating at full power. A slit formed by two mirrors, which are mounted on a micrometer stage (not shown), selects a single bar at a time, allowing the beam to propagate towards the screen at the left whilst deflecting the rest of the beams onto beam dumps. A cylindrical lens projects a slow-axis image of the selected bar on to a diffusing screen located 2 metres away. In the fast-axis direction, the beam is a good approximation to the far-field at this distance. A CCD camera connected to a computer is used to record the image from the screen.
lensed diode stack fast axis
Screen
beam dump bar selector mirrors slow axis imaging cylindrical lens
CCD camera to computer
beam dump
screen to stack 2m Fig. 1. Experimental setup used to measure beam properties of individual bars.
Figure 2(a) shows images for each bar in the stack for the condition as delivered by the manufacturer and Fig. 2(b) show images for the same bars after a correction has been applied, which is explained later. Each beam image has dots corresponding to the slow-axis imaging of
#73589 - $15.00 USD
(C) 2006 OSA
Received 1 August 2006; accepted 22 August 2006
4 September 2006 / Vol. 14, No. 18 / OPTICS EXPRESS 8179
Fig. 2. Slow-axis image, fast-axis far-field at 2 metres for each diode bar in the stack. (a) As delivered by manufacturer. (b) After a correction has been applied.
the 24 broad area emitters on the bar. In the fast-axis, the bar images are all significantly different due to differing combinations of opto-mechanical errors. The fast-axis M2 factor measured for the collective beam from each bar ranges from 5.5 to 15.5, with a mean value of 10.4 for the 11 bars. The varying beam shapes result from different combinations of errors for each bar. The lens attachment errors – defocus, transverse offset and twist – lead to beam pointing and beam size variations and image rotation. Imperfections in the fabrication of the cylindrical collimator lens or poor aberration correction in the lens design lead to break up into side-lobes or fringes, as clearly seen in Fig. 2(a) for bars 1, 2, 5, 9, 10 and 11. The departure of the semiconductor substrate from flatness, or microlens bending, gives rise to the bent far-field known as “smile”. As a result, the intensity distribution is modified from the expected gaussian-like profile and straight line image in the beam shapes shown in Fig. 2(a), with corresponding M2 values much larger than expected for the near diffraction-limited beams available directly at the emitting facet. 4
Slo wax 5 is (m m )
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Fig. 3. Corrective surface for bar 1. Calculated from the wavefront data.
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Transverse distance (mm) Fig. 4. Example of the fast-axis wavefront correction for one emitter in bar 1.
Since the beam errors are both pointing errors and local aberrations, we have used a wavefront sensing method based on the Hartmann technique [2] to measure the fast-axis wavefront variation at a plane close to the FAC output surface. This measurement was performed using a #73589 - $15.00 USD
(C) 2006 OSA
Received 1 August 2006; accepted 22 August 2006
4 September 2006 / Vol. 14, No. 18 / OPTICS EXPRESS 8180
sensing device built in-house, which has a spatial resolution of 40 µm, an angular sensitivity of 19 µrad and a dynamic range of ~2600. By using this method, we obtain the local wavefront tilt, which is then numerically integrated as explained in Ref. [2] to obtain the Optical Path Difference (OPD) in the fast-axis. With this information we calculate the surface depth required for a refractive correction surface fabricated in silica, which will eliminate the OPD across the fast-axis. This OPD correction is applied where significant laser intensity is detectable. The required corrective surface for bar 1 is shown in Fig. 3 as an example. This surface shape data is translated by computer into cutting instructions for a specialised, high performance CO2 laser machining/polishing workstation. As described in Ref. [3], the optical surfaces are fabricated by raster scanned, pulsed CO2 laser ablation [4] followed by laser polishing [5], using a 50μm spot diameter and 10μm raster pitch. Since publishing [4], we have significantly enhanced the machining process by developing a complex control system to keep the CO2 laser stabilised on a single line and to control the dispensed energy in each laser pulse. As a result, we are now capable of fabricating smooth surfaces of arbitrary shape up to 40 μm deep with a depth resolution of 100 nm. After laser polishing [5], the measured loss outside a scatter cone angle of 10mrad is 0.5% at 940nm. A similar optic fabrication process, but on a much smaller scale, has been reported [6] to correct a single laser emitter, using excimer laser ablation of a polymer surface on a collimator lens. The laser micro-machining produces 11 correction zones, one for each of the bars, on a single substrate. Each correction zone is a generally smoothly-varying surface over an area of 11 mm x 1.6 mm, with a surface height range of typically 10 to 15 μm. The prescription of the zones is designed to ensure all the bars have beams aligned along a common axis. The corrective plate is aligned using a micro-positioning stage at the plane of the wavefront measurement, and can be fixed satisfactorily using UV-cured glue. Relative misalignment of the stack and correction plate of ±5µm in the fast-axis, ±15µm in the slow axis and ±250 µm in the proximity to the measurement plane are observed to produce noticeable degradation in the quality of the correction. It should be noted that such tolerances are far more relaxed than for the FAC lenses. The effectiveness of the correction process for the whole stack is evident from Fig. 2(b), which shows the correction of the bar images in Fig. 2(a). The bars images are now straight, the side lobes are effectively removed and the central lobe is far brighter. Figure 4 shows an example of the uncorrected and corrected wavefront for one emitter in bar 1. The wavefront OPD range has been reduced from ~4λ to
