Continuous Tuning and Efficient Intracavity Second-Harmonic ...

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Continuous Tuning and Efficient Intracavity Second-Harmonic Generation in a Semiconductor Disk Laser With an Intracavity Diamond Heatspreader Alexander J. Maclean, Student Member, IEEE, Alan J. Kemp, Member, IEEE, Stephane Calvez, Member, IEEE, Jun-Youn Kim, Member, IEEE, Taek Kim, Martin D. Dawson, Senior Member, IEEE, and David Burns

Abstract—Using a wedged and antireflection-coated diamond heatspreader, a continuously tunable semiconductor disk laser with intracavity second-harmonic generation (SHG) is demonstrated. Output powers of 600 mW tunable over 10 nm around 530 nm are obtained. Finite-element modeling shows that the use of a diamond heatspreader for thermal management—in contrast to substrate thinning approaches—permits power scaling across the 670-2300-nm range of these lasers. Using a green laser as an exemplar, this paper details the issues involved in translating this spectral coverage to the ultraviolet and visible via SHG. Polarization and wavelength selection are discussed and the adopted approaches presented. Almost 1 W of second-harmonic light at 530 nm is demonstrated, with an efficiency of 11% with respect to the incident pump power. Index Terms—Second-harmonic generation (SHG), semiconductor disk laser, vertical external-cavity surface-emitting laser (VECSEL).

I. INTRODUCTION UNABLE semiconductor disk lasers(SDL), combining high power with good beam quality, are ideal for applications such as biomedicine [1] and chemical sensing [2] and as pump sources for other lasers [3]. This technology, otherwise known as a vertical external-cavity surface-emitting laser (VECSEL) [4], can be engineered for virtually any wavelength from 670 to 2300 nm. SDLs offer the wavelength flexibility of conventional semiconductor lasers allied to the high brightness of conventional diode-pumped solid-state lasers. This laser is based on a series of quantum wells (QWs) grown on top of a distributed Bragg reflector (DBR). The QWs are positioned at the antinodes of the standing wave field built up between the mirror and the chip surface, providing resonantly enhanced gain [5]. The wavelength of operation is determined by

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Manuscript received May 31, 2007; revised September 20, 2007. A. J. Maclean, A. J. Kemp, S. Calvez, M. D. Dawson, and D. Burns are with the Institute of Photonics, University of Strathclyde, Glasgow G4 0NW, U.K. (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). J. Y. Kim and T. Kim are with the Samsung Advanced Institute of Technology, Gyeonggi-Do, 449-712, Korea (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2007.911704

this microcavity resonance feature. The semiconductor structure is completed by adding layers to prevent oxidation and carrier recombination at the surface. The laser cavity is then formed with an external mirror(s) (see also Fig. 3); further elements can be added to enable second-harmonic generation (SHG) [6], modelocking [7] or single-frequency operation [8]. The key advantage of the SDL over conventional diode-pumped solid-state and semiconductor laser technologies is that it can be engineered for high-brightness operation from 670 nm to the midinfrared (mid-IR) 2.3 m the red wavelength [9]–[14]; by intracavity frequency doubling, this can be extended through the visible [6], [15], [16] and into the ultraviolet (UV) ( 338 nm [17]). Hence, it is advantageous if all approaches to SDL design are compatible with operation across the complete range of wavelengths and materials systems. Perhaps the most problematic area in this regard is thermal management. Two well-established approaches are to remove the substrate or to use an intracavity heatspreader window [4], [6], [11], [18]. In previous reports, green output powers of over 6 W have been reported with a thinned substrate [6], over 5 W with a heatspreader [16], and tunable over 5 nm with a thinned substrate [15]. In this study, we discuss in detail the polarization and wavelength management hurdles that must be overcome to reach these power levels and demonstrate that the heatspreader approach can be readily translated to other wavelengths [9], [17]. In Section II, the thermal limitations of SDLs are discussed and thermal management techniques are compared using finiteelement analysis. Section III describes the laser operating at its fundamental wavelength and discusses the effect of an intracavity heatspreader on tuning, polarization, and the spectral control required for SHG. Section IV discusses the choice of nonlinear crystal and presents the second-harmonic results. In all sections, techniques suitable for use at a broad range of wavelengths are emphasized. II. THERMAL MANAGEMENT A. Approaches to Thermal Management Power scaling of SDLs is limited by the temperature rise in the gain region associated with the pumping process. This shifts the resonant wavelength of the gain structure and the peak wavelength of the QW gain, but at different rates. As the pump power

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TABLE I SUMMARY OF PARAMETERS USED IN THERMAL MODEL

increases, the laser wavelength, which is tied to the micro-cavity resonance, shifts away from the peak of the QW gain, and the output becomes less efficient, eventually switching off entirely. The two main techniques used to manage the heating, and increase the output power, are thinning or completely removing the substrate and bonding a heatspreader to the surface of the chip. The first removes the thermal impedance associated with the substrate; the second bypasses this thermal impedance and that of the DBR by providing an alternative heat path. Substrate thinning is very effective in InGaAs SDLs [6] where the GaAs–AlAs DBR mirrors have low thermal impedance, but it requires considerable postprocessing of the chip, with both mechanical and chemical etching normally required to reduce the substrate thickness sufficiently. Using a high thermal conductivity transparent heatspreader removes the need for such processing. High-power results using the heatspreader approach have also been demonstrated over a much wider range of wavelengths and material systems [9], [10], [12], [14]. Finite-element analysis (FEA) has been used to model the steady-state temperature distribution in an SDL under the various approaches to thermal management [19], [20]. In this paper, FEA is used to explore the differences between substrate thinning and heatspreader-based approaches. Since heat extraction through the DBR is the main difference between the two approaches, a model was designed to determine how the different DBR mirrors required at different wavelengths affect the choice of thermal management technique. In order to keep as much comparability between wavelengths as possible, the gain regions were kept to a constant 2- m thickness with a pump power and absorption fixed to have 1 W of heat deposited in a 100- m-radius Gaussian distribution in the active region. The materials and DBR layer thicknesses chosen for each wavelength are based on real designs [8]–[10], [12], [14], [18], [21] and are summarized in Table I. To simplify the calculations, the approach outlined in [19] was used: layers of similar function are assigned an average thermal conductivity and axial symmetry is invoked. The case of substrate thinning is modeled with the substrate entirely removed; the bottom surface of the DBR and an annulus on the top of the wafer are held at a constant temperature. The case of the heatspreader uses a 250- m-thick window of single crystal natural diamond; a semiconductor substrate thickness of 300 m is assumed, and the bottom surface of the substrate and an annulus on the top of the heatspreader are held at a constant temperature. These

Fig. 1. Maximum heat rise in active region for 1 W of deposited heat comparing thermal management techniques over the wavelength range of current devices.

Fig. 2. Maximum temperature rise in the active region for heatspreaders of different thermal conductivity for 10 W of pump light absorbed in a 100-m radius.

boundary conditions ignore the effect of the way the disk is mounted. Thus, the actual temperature rise in the disk will be slightly higher than predicted, particularly in the case of substrate removal [19], [22]. The maximum temperature rise in the active region is plotted in Fig. 1. The case of the “as grown” chip, where no additional steps have been taken to reduce the heating, is included for comparison. It is clear that, around 1 m–where the materials are based on (In)GaAs–AlAs—the two thermal management techniques offer much the same performance; however, at higher and lower wavelengths, the difference becomes much greater. The temperature rise in the heatspreader case remains low, while the thinned substrate device can heat by more than half of the unmanaged value. Heatspreaders allow power-scaling techniques to be generic to the technology rather than specific to the wavelength. In a system where wavelength engineerability is a key asset, the ability to have consistent and good thermal management across the range of available wavelengths makes the heatspreader approach very attractive. This is particularly true if it can also be shown to be compatible with SHG, opening up the visible and UV spectral regions. The remainder of this paper will discuss the implication of using a heatspreader approach to thermal management in an intracavity frequency-doubled SDL and show that the issues involved can be overcome to produce a high-power high-brightness visible laser source.

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Fig. 3. Schematic of SDL cavity arrangement, with d1 = 67 mm, d2 = 350 mm, and d3 = 70 mm. Positions of the tuning element and nonlinear crystal are shown dashed. Inset: schematic of layer structure of semiconductor gain chip.

B. Choice of Heatspreader Material The choice of heatspreader material is crucial to the performance of an SDL. The original demonstration of an SDL with a heatspreader used sapphire [18]. The thermal conductivity of sapphire is similar to that of the GaAs used for the substrate. Hence, this approach only reduces the temperature rise by approximately a factor of two by providing two heat extraction routes with similar thermal impedance: through the DBR and substrate and out through the heatspreader. By increasing the thermal conductivity of the heatspreader, the heat removal path through the DBR and substrate can be bypassed, making thermal management much less dependent on the properties of the semiconductor materials system. Silicon carbide and diamond are both suitable optical materials with large thermal conductivities: diamond has the higher thermal conductivity of the two but is more expensive. A similar FEA model to that in the previous section was also used to investigate the effect of the thermal conductivity of the heatspreader material on the temperature rise in the active region of an SDL at 1060 nm (Fig. 2). While increasing the thermal conductivity beyond that of SiC (0.49 W/mm/K) has diminishing returns, the temperature rise with diamond (2 W/mm/K) is still nearly half that of SiC. Silicon carbide is birefringent, and it has been seen that diamond, although normally considered isotropic, can exhibit a degree of stress-induced birefringence as reported in [23]. This birefringence varies from sample to sample and from position to position on the sample. While in some circumstances the predictable birefringence and lower cost of SiC would be advantageous, in this study, diamond has been used because of its higher thermal conductivity; the steps taken to overcome the problems introduced by its variable birefringence effects will be reported. III. INFRARED LASER CHARACTERISTICS A. Laser Setup The gain medium was grown using low-pressure metal–organic chemical-vapor-deposition (MOCVD). The chip is made

layers of Al Ga As up of a 35-period DBR mirror with and AlAs; an active region consisting of 15 7-nm-thick compressively strained In Ga As QWs, with GaAs P strain-compensating layers, the QWs are separated by GaAs barriers to place them at the antinodes of the standing wave field; an Al Ga As window layer and a GaAs capping layer complete the structure, as described in [16], [21]. Liquid capillary bonding [24] was used to attach a 250- m -thick, 4-mm-diameter, plane-parallel piece of type IIa single crystal natural diamond to a 4 4 mm piece of the semiconductor wafer, and this composite formed one of the end mirrors of the laser resonator. The 8.8-W output power of an 808-nm fiber-coupled diode laser (100- m core diameter, 0.22 NA), was focused to a 40- m-radius spot on the semiconductor chip. As shown in Fig. 3, the laser cavity was completed by the addition of two 100 mm radius-of-curvature (ROC) folding mirrors M1 and M2 and a plane mirror M3, [all highly reflective (specified to 99.9%] at 1060 nm). The mirror separations were chosen such that the fundamental mode radius on the chip was matched to the pump spot size. The second folding mirror was also coated to have high transmission ( 97%) at 530 nm so that it could act as an output mirror for second-harmonic light. For experiments on the laser working at its fundamental wavelength, the cavity was terminated with a plane output coupler mirror M3 with either 5% or 9% transmission at 1060 nm as required. B. Etalon Effects A result of bonding a plane heatspreader to the intracavity surface of the semiconductor chip is the introduction of an etalon effect within the cavity. The effect of this etalon can clearly be seen in the free-running spectrum of the laser in Fig. 4(a). The effective gain modulation introduced into the cavity is sufficient that tuning with an intracavity birefringent filter is not continuous: the laser jumps from one etalon mode to the next. While the spectral modulation introduced by the etalon is generally undesirable, it does have the effect that another spectral filtering element in the cavity needs only to suppress the

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Fig. 4. Free-running spectrum of SDL with (a) a plane heatspreader and (b) wedged and AR-coated heatspreader, measured with a spectrometer at a resolution of 0.3 nm.

TABLE II SUMMARY OF SPECTRAL WIDTH AND POWER OF LASER OUTPUT WITH DIFFERENT BRF THICKNESSES AS MEASURED WITH A FABRY–PEROT INTERFEROMETER

Fig. 5. Power transfer characteristics of an SDL with plane and AR-coated wedged heatspreaders, with the output coupling of 9% and 5%, respectively.

etalon modes either side of the desired operating wavelength to achieve good spectral narrowing. However, since the etalon modes in this case are spaced approximately 1 nm apart, the tuning is limited to discrete wavelengths with that separation. With a wedged (2 ) and AR-coated ( 0.05 reflectivity at 1060 nm and 0 ; 4 at 808 nm and 30 ) diamond heatspreader, this problem can be eliminated [see Fig. 4(b)] and continuous tuning enabled [25]. The uncoated plane heatspreader provides an enhancement of the effective gain due to the field build-up in the subcavity formed between the DBR and the front surface of the heatspreader. When the heatspreader is wedged, this field build-up does not occur, and so the effective gain of the system is decreased. The reduced effective gain is reflected in a reduction in the output coupling giving the highest output power from 9% to 5% when a wedged heatspreader was used. Due to the limited range of mirrors available, this is not necessarily the optimal output coupling, which may partly explain the lower slope efficiency observed with the wedged output coupler. Also, the wedged heatspreader was AR-coated at the laser and pump wavelengths, decreasing the pump power reflected from the front surface (17% for the uncoated diamond). The increase in absorbed pump power will create more heat in the device which might also affect the efficiency. However, for the same incident pump power, as measured after the pump optics, a similar output was achieved in both cases, as seen in Fig. 5.

With a plane heatspreader, an output power of 3.08 W with a threshold pump power of 0.39 W and a slope efficiency of 38.3% was achieved; with the wedged heatspreader, the output was 3.0 W for 8.7-W pump power, with 35.8% slope and 0.22-W threshold. In the wedged case, the slope increases at low pump powers; however, the reason for this is not clear. The removal of the diamond etalon effect also results in a narrower unmodulated output spectrum, as seen in Fig. 4(b). Thus, by using a wedged and AR-coated heatspreader, the thermal management advantages of an intracavity diamond can be exploited while avoiding the disadvantages of a broad channeled output spectrum and discontinuous tuning. C. Spectral Control For efficient SHG, the spectrum must be constrained to be within the phase-matching bandwidth of the nonlinear crystal. To achieve the required spectral narrowing and tuning, a quartz birefringent filter (BRF) was inserted into the cavity at Brewster’s angle, shown as the dashed line in Fig. 3, and used to select the wavelength of operation; the thickness of the filter defines the oscillating bandwidth, with thicker BRFs promoting a narrower output spectrum. A plane heatspreader acts like an etalon and the depth of the loss modulation it provides is sufficient so that the birefringent filter need only suppress the neighboring modes of the heatspreader etalon in order to select a single narrow spectral peak. Different thicknesses of BRF were used, and the spectral output was measured with a scanning Fabry–Perot interferometer. Table II summarizes the spectral widths as both the full-width at half-maximum (FWHM) and the full-width at 5% of the maximum intensity (5%) for all of the filters tested (this latter figure is chosen to give an impression of the suppression of

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Fig. 6. Spectra of the SDL (a) without a BRF and (b) with a 4-mm-thick BRF taken with a scanning Fabry–Perot interferometer. The shaded area in (b) shows one repeat of the output spectrum. The diagonal line shows a voltage ramp applied to a piezo-mounted mirror.

A 4-mm-thick BRF is chosen for this work because it gives sufficient spectral narrowing for SHG and higher output power at the center wavelength.

Fig. 7. Loss from the Brewster plate, given as a percentage of the intracavity power for each Brewster surface in the cavity, as the axis of birefringence in the heatspreader is rotated with respect to the Brewster element. Modeled curve fitted to data with a phase retardation of 0.19.

modes outside the main band). Fig. 6 shows a sweep covering two or three free spectral ranges (FSRs) of the interferometer for the free-running laser and with a 4-mm BRF. Introduction of a filter reduces the output power by 12% in the case of the 2- and 4-mm-thick filters and 18.5% with the 6-mm filter. From a free-running bandwidth of 2.2 nm, comprising six modes of the heatspreader etalon, the thicker filters pick out one mode, giving a FWHM bandwidth of 0.2 nm, but do not narrow that peak. This indicates that there is little advantage in using the 6-mm filter over the 4-mm filter given the reduction in output power with the thicker filter. The loss modulation introduced by the heatspreader etalon can also be seen in the tuning of the laser with a plane heatspreader, giving rise to a quasi-discrete tuning regime where the laser jumps between modes of the heatspreader etalon as the BRF is tuned. With a wedged and AR-coated heatspreader and a 4-mmthick BRF in the cavity, continuous tunability can be achieved. The output spectrum was measured with an optical spectrum analyzer with a resolution of 0.07 nm; the filtered output is a little wider (0.51 nm) at FWHM, but without the adjacent modes of the etalon the 5% width is narrower (0.925 nm).

D. Polarization Control Using a polarizing beam-splitter cube, the polarization of the output beam was found to be linear and horizontal (with a polarization ratio 100:1) with the 4-mm birefringent filter in the cavity and a plane heatspreader. However, the power rejected by the Brewster-angled surfaces of the birefringent filter was a few percent of the intracavity circulating power. Such a poor extinction suggests that an additional birefringence is present. The most likely source for this birefringence is the diamond/semiconductor chip. Although diamond is usually considered to be isotropic, work carried out by van Loon et al. [23] showed that the diamond windows used as heatspreaders can be birefringent, with the magnitude and orientation of the birefringence varying across the sample and varying significantly between samples. When the angle of this birefringence is not aligned with the Brewster surface, it can modify the intracavity polarization, leading to losses at the Brewster surfaces. Rotation of the chip mount to align the axes of this birefringence significantly reduces this loss. Experimental data taken by rotating the mounted gain chip around the cavity axis and monitoring the percentage loss per surface from an isotropic fused silica Brewster plate are shown in Fig. 7. At the limit of the experimental setup—a rotation of 20 from the optimum—the loss is almost 2% per surface, equating to a total round-trip loss of 8% in this case, which indicates how much loss could be present for an arbitrary orientation of the gain chip if no steps were taken to ensure low-loss operation. An approach based on the Jones matrix analysis, as outlined in [26], is used to model the round-trip loss of the cavity. The Jones matrix for a round trip is the product of the matrices for the cavity elements in the order they are met. The loss is then given by ( ), where is the eigenvalue of the round-trip matrix. To model the effect of the orientation of the birefringence in the gain chip in a cavity with a nonbirefringent Brewster element, the round-trip Jones matrix is

(1)

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of 20 nm: if a wedged heatspreader is used, this tuning is continuous. The BRF also acts to constrain the laser bandwidth to about 0.5 nm, which is within the phase-matching bandwidth of typical nonlinear crystals. The birefringence associated with some diamond heatspreader samples can cause additional losses at the Brewster surfaces of the BRF; however, this can be minimized with careful rotation of the gain chip. IV. SECOND-HARMONIC GENERATION A. Crystal Choice Fig. 8. Tuning response of the SDL with an uncoated plane-parallel heatspreader, 4-mm-thick BRF, 9% output coupler (diamonds) and an AR-coated wedged heatspreader, 4-mm BRF and 5% output coupler (solid line). The diamonds represent discrete-tuning hopping between modes of the heatspreader etalon. The solid line represents continuous tuning.

where is the Jones matrix representing a rotation of the polarization axis through an angle , represents a birefringent plate with a phase retardation between the principal polarrepresents a polarizer, in this case a Brewster izations, and surface, where is the loss in the orthogonal polarization [27]. A good fit to the experimental data can be obtained for a value of the heatspreader birefringence of 0.19 of a full-wave retardation, as shown in Fig. 7. This indicates that the increased loss is due to birefringence in the heatspreader/semiconductor composite. Previous work [23] indicated that this originates in the diamond heatspreader, with negligible birefringence in the semiconductor. The two pieces of diamond used in this work were also compared: it was found that the piece cut plane-parallel had much higher and more variable birefringence than the piece cut with a wedge. It is clear that a low-birefringence specification would be advantageous in the procurement of diamond heatspreaders and that orientation of the axes of this birefringence is an important parameter in optimizing the SDL for SHG. For all other results reported in this paper, the gain chip is rotated to the minimum loss position whichever heatspreader is used. E. Tuning The tuning response of the SDL around its fundamental wavelength is shown in Fig. 8. A 4-mm-thick BRF tuning element was used, and measurements were taken for both the plane and wedged heatspreaders. With the optimal output coupler used in each case, the laser output peaks at around the free-running wavelength and gives 1 W of output power over a range of 20 nm in both cases. The points in Fig. 8 are at the peaks of the etalon modes of the heatspreader and show the range over which the laser can be discretely tuned; with the wedged heatspreader, the laser can be continuously tuned, as indicated by the solid line. This useful output gives a baseline for the potential tunability of the second harmonic laser—however, tuning at the second harmonic will be further restricted by phase-matching considerations. In summary, the introduction of a birefringent filter into the cavity allows wavelength tuning over a range in excess

The use of a birefringent filter complicates the choice of nonlinear crystal since a type II phase-matching scheme will effectively add a further frequency-selective element to the cavity: orientating the axes of the nonlinear crystal birefringence at 45 to the polarization of the intracavity light, as required by the type-II phase-matching scheme, means that, with a Brewster surface in the cavity, the nonlinear crystal acts as a second birefringent filter. Because of the length typically required of the crystal for efficient conversion to the second harmonic, this effective filter has a very narrow passband and a small FSR. This filter function—combined with the conventional birefringent filter, the etalon effects of a plane heatspreader and the effects of any birefringence in the heatspreader—can over-constrain the laser wavelength and reduce efficiency of the SHG process [28]. Again, a Jones matrix approach [26] was used to plot the round-trip loss as a function of wavelength comparing the cases of having just a BRF and having a BRF with a nonlinear crystal in one of two phase-matching regimes: type-I and type-II phase matching. (Any birefringence and the etalon effects of the heatspreader are ignored in this case.) The round-trip Jones matrices of the three cases are (2) (3) (4) , , and represent the round-trip Jones where matrices for the case of just the filter, the filter with type-I lithium triborate (LBO), and the filter with type-II potassium represents the titanyl phosphate (KTP), respectively. birefringent filter (a quartz birefringent filter with the optic axis in the plane of the plate, represented using the matrix derived is a waveplate matrix representing a 10-mm-long in [29]), LBO crystal, is a waveplate matrix representing a 10-mm-long KTP crystal. Both crystals are cut for operation at 1064 nm, and the filter is also set to this wavelength. The calculated round-trip transmission as a function of wavelength for each case is plotted in Fig. 9. It can be seen that, with just a BRF, there is a peak of transmission around 1 nm wide at 95% transmission. Adding a type-I nonlinear crystal, such as LBO, seems to broaden the filter passband somewhat and still allows for continuous tuning. However, with type-II KTP, the transmission spectrum changes to a series of sharp peaks, indicating that the laser wavelength will jump discretely between these

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Fig. 9. Calculated round-trip transmission as a function of wavelength in the case of a laser cavity containing a BRF (dotted line), a BRF, and type-I LBO (dashed line), and a BRF and type-II KTP (solid line).

TABLE III SUMMARY OF PROPERTIES OF NONLINEAR CRYSTALS FOR SHG AT 1064 nm

peaks as the birefringent filter is tuned—continuous tuning will therefore not be achieved. The properties of a range of common nonlinear crystals are given for LBO is somesummarized in Table III [30]. (The what higher than often quoted by manufacturers; however, it is in line with theoretical calculations and experimental measurements.) For tuning applications, the SHG crystal requires a wide , as well as the normal wavelength-acceptance bandwidth and requirement for high effective nonlinear coefficient low walk-off, with a preference in this case for type-I phasematching, so LBO was considered to be the best choice for this study. The lower nonlinear coefficient provided by LBO means that a longer crystal is required. For a given spot size in the crystal, the maximum useful crystal length, limited by walk-off, can be m fundamental mode radius in calculated and, for the the crystal, calculated for the cavity shown in Fig. 3, this gives mm, so a crystal much longer than this will add passive loss but not contribute to SHG. For applications where tuning in the second harmonic is required, use of a nonlinear crystal with a wide wavelength-acceptance bandwidth is key. This, combined with the low walk-off,

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Fig. 10. Tuning ranges of SHG at crystal temperatures increasing in 5 C steps from 15 C to 50 C.

allowing use of a longer crystal to compensate for the lower nonlinear coefficient, makes LBO a suitable crystal for this work. B. Experimental SHG The laser was set up as shown in Fig. 3, with a wedged and AR-coated diamond heatspreader. This heatspreader was selected to ensure continuous tuning. The optimum output coupled IR power was 2.85 W with a beam-quality parameter of in the two orthogonal planes, measured using a DataRay Inc. BeamScope™. With the 4-mm-thick BRF in the cavity, a 10-mm-long piece of LBO was inserted as close as possible to the plane mirror M3, which was coated to be HR at both wavelengths to give a single green output beam through the adjacent folding mirror M2. Closed-loop stabilization was used to keep the nonlinear crystal temperature constant at the desired value. The fundamental laser wavelength was tuned using the BRF to find the optimum phase-matching wavelength for a given crystal temperature. Output power of up to 955 mW tunnm near 532 nm was achieved with tuning limited able over by the phase-matching bandwidth of the LBO. In order to assess the potential range of wavelengths available and to find the optimum operating point, the temperature of the crystal was varied in 5 C steps, and the tuning range of the second-harmonic output was measured. Fig. 10 shows the tuning curves at each temperature. By adjusting the crystal temperature and the BRF angle, green power in excess of 600 mW could be generated at all wavelengths between 526 and 536 nm. The maximum second-harmonic output power of 955 mW was achieved at 30 C and a wavelength of 532 nm. With the crystal temperature set at 30 C, the properties of the frequency-doubled SDL were characterized. Fig. 11 shows a power transfer characteristic of the output green light, along with the fundamental IR leakage and the combined loss in both directions from the Brewster surface of the BRF. This relatively high level of loss from the Brewster surface is only present with the cavity terminated with a HR mirror. In the case of tuning , with an optimal output coupling, the round-trip loss is whereas with the cavity setup for SHG this loss increases dra. It is unclear why this was the case, but matically to clearly, having a loss of comparable magnitude to the useful output indicates a significant improvement would be possible if this issue were resolved. At higher pump powers or with a stronger nonlinearity in the cavity, where the output coupling to

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Fig. 11. Power transfer characteristic of green output with Brewster loss and IR leakage through the high-reflecting end mirror. Fig. 13. Green output from the SDL as a function of time (top line and inset), including system noise with laser input to photodiode blocked (bottom line).

Fig. 12. Spectrum of green output at peak power. Inset: false color intensity map of output beam.

the second harmonic will be stronger, the relative importance of the Brewster losses is likely to be reduced. The green output had a center wavelength of 531.9 nm, as was shown in Fig. 12, and the beam propagation factor ; the nonlinear process will be most measured to be in efficient in the fundamental mode, so the reduction in the vertical plane is expected; the increase in in the horizontal plane is due to walk-off. The output was elliptical with with the horizontal plane wider than the an eccentricity of vertical plane as seen in the inset to Fig. 12. The intracavity power is estimated to be 39 W, giving a per-pass conversion efficiency of 1.21% and a conversion from the IR laser with an optimal output coupling of 33.5%, indicating that the SHG process was relatively efficient [31]. The conversion efficiency with respect to the incident pump power was 10.8%. For pump W, the only demonstration of SHG in SDLs more powers efficient than this, that the authors are aware of, used BIBO and was not tunable [32]. The green laser output was measured over 10 s, using a photodiode with a bandwidth of 500 kHz, as shown in Fig. 13. From this measurement, the noise in the signal was calculated to have a standard deviation around the mean of 0.6%, and a calculation from a similar measurement over 100 ms gives a noise value of 0.4%. The only significant frequency present in the noise spectrum was at 100 Hz, but this was also present in a measurement with the laser input to the photodiode blocked, showing that it

originates from the detection system rather than the laser itself. It should be noted that no special efforts were made to improve the mechanical stability of the cavity nor was any electrical shielding employed, and hence improved engineering could be expected to further reduce the noise levels. The fact that the strong “green noise” [33], [34] was not present is a particular advantage of the frequency-doubled SDL. Similar results have been noted by other researchers [15], [32]. A conventional intracavity frequency-doubled solid-state laser would typically exhibit a strong amplitude modulation of the green output—and perhaps even chaotic behavior—under similar conditions. Further study is required to fully explain this insensitivity to green noise, but it presumably stems from the strong damping of relaxation oscillations that results from the very short gain lifetime in semiconductors compared with doped-dielectric materials. The relatively long cavity used in this work, and hence the large number of oscillating longitudinal modes, would also lead to averaging of any modal green noise that might be present [35]. V. CONCLUSION Intracavity SHG in an SDL with a diamond heatspreader has been shown to be a flexible and efficient means to generate visible wavelengths. In this paper, FEA was used to illustrate the broad spectral coverage that this approach enables. The means to achieve broad tunability and efficient SHG were then examined. In particular, the use of a wedged and AR-coated heatspreader to realize continuous tuning was demonstrated, and the issues associated with birefringence in the diamond heatspreader were highlighted in the context of the use of a birefringent tuning filter. In an exemplar system operating at a fundamental wavelength of 1060 nm, an output power of 3 W at an efficiency of 34.5% with respect to the incident pump power was demonstrated. This laser was continuously tunable over 20 nm with output powers in excess of 1 W over that range. The choice of nonlinear crystal was then examined with the objective of maximizing the tunability at the second harmonic, and LBO was

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chosen. An output power of almost 1 W at 530 nm was demonstrated, with a conversion efficiency of 35% with respect to the optimized fundamental laser output. The green output was tunable over 10 nm at output powers in excess of 600 mW. In addition, the second-harmonic output exhibited excellent amplitude stability, with no evidence of green noise. By using an intracavity diamond heatspreader for thermal management, SDLs have demonstrated spectral coverage from the red to the midIR. This study discusses the particular considerations involved in extending that coverage to shorter wavelengths by means of intracavity SHG: these lasers can indeed be efficient and wavelength-agile sources in the visible and UV. Such sources will simplify a range of applications, particularly in biomedicine and the pumping of high-specification scientific lasers.

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MACLEAN et al.: CONTINUOUS TUNING AND EFFICIENT INTRACAVITY SHG

[35] V. Magni, G. Cerullo, S. Desilvestri, O. Svelto, L. J. Qian, and M. Danailov, “Intracavity frequency-doubling of a CW high-power TEM Nd-Ylf laser,” Opt. Lett., vol. 18, pp. 2111–2113, 1993. Alexander J. Maclean (S’06) received the M.Eng degree from the University of Strathclyde, Glasgow, U.K., and the Engineering Diploma from the Institut National de Sciences Appliquées, Rennes, France, in 2004. He is currently working toward the Ph.D. degree at the Institute of Photonics, University of Strathclyde. His research is focused on power scaling and second harmonic generation in semiconductor disk lasers. Mr. Maclean is a member of the IET, U.K.

Alan J. Kemp (M’07) received the B.Sc. (Hons) degree from the University of Glasgow, Glasgow, U.K., in 1996, and the Ph.D. degree from the University of St. Andrews, St. Andrews, U.K., in 1999 for work on microchip lasers. From 1999 to 2002, he worked on femtosecond lasers at the University of St. Andrews. He then joined the Institute of Photonics, University of Strathclyde, Glasgow, U.K., to work on thermal effects in lasers. In 2005, he was awarded a personal research fellowship by the Royal Society of Edinburgh/Scottish Executive.

Stephane Calvez (M’05) received the Engineering Diploma from the Ecole Nationale Supérieure de Physique de Marseille, Marseille, France in 1998, and the Ph.D. degree from the Université de Franche Comte, Besançon, France, in 2002 for work on fiber lasers. Since 2002, he has been an Associate Team Leader for Semiconductor Optoelectronics with the Institute of Photonics, University of Strathclyde, Glasgow, U.K. His research focuses on novel materials, vertical-cavity devices, amplifiers, and their applications.

Jun-Youn Kim (M’03) was born in Seoul, Korea. He received the B.Sc., M.S., and Ph.D. degrees in electronics and electrical engineering from the Pohang University of Science and Technology (POSTECH), Pohang, Korea, in 1997, 1999, and 2003, respectively. His doctoral work focused on the fabrication and analysis of micro-cavity lasers, including photonic quantum-ring lasers. Since 2004, he has been with the Samsung Advanced Institute Technology (SAIT), Suwon, Korea, where he is engaged in characterization of vertical-external-cavity surface-emitting lasers and their second-harmonic generation. His general research areas are semiconductor lasers, multiple-gate CMOS for logic circuit, and reliability study of electronic devices.

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Taek Kim was born in Seoul, Korea, in 1965. He received the B.S. degree in metallurgical engineering from Sungkyunkwan University, Seoul, Korea, in 1989, and the Ph.D. degree in physics and applied physics from the University of Strathclyde, Glasgow, U.K., in 2002. Since 1989, he has been involved with III-V photonic devices including LD, LED, VCSEL, and VECSEL with the Samsung Advanced Institute of Technology, Suwon, Korea. His current research fields include III-V tandem photovoltaic devices and image sensors in addition to light-emitting devices. He is the author and coauthor of more than 60 scientific papers.

Martin D. Dawson (M’85–SM’98) received the B.Sc. (Hons) degree and the Ph.D. degree for work on semiconductor and solid-state lasers and four-wave mixing from Imperial College London, London, U.K., in 1981 and 1985, respectively. From 1985 to 1991, he was with North Texas State University and then the University of Iowa, where he was involved with ultrafast dye lasers and semiconductor spectroscopy. In 1991, he joined Sharp Laboratories of Europe Ltd., where he worked on the spectroscopy of semiconductors. He joined the Institute of Photonics, University of Strathclyde, Glasgow, U.K., in 1996 and is a Professor, Associate Director, and Team Leader for Semiconductor Optoelectronics. His research includes III-V materials studies, processing, and vertical-cavity device development. Prof. Dawson is a Fellow of the U.K. Institute of Physics and the Optical Society of America and is a member of the U.K. EPSRC Peer Review College. He has served the Scottish chapter of the IEEE Lasers and Electro-Optics Society (LEOS) since its inception and was awarded IEEE certificates for Outstanding Leadership in 2000 and 2001. He is a member of the LEOS Optical Materials and Processing Subcommittee.

David Burns received the B.Sc. (Hons) degree from the University of Glasgow, Glasgow, U.K., in 1986, and the Ph.D. degree from the University of St. Andrews, St. Andrews, U.K., in 1990 for work on semiconductor and fiber lasers. From 1990 to 1996, he was a Research Fellow with the University of St. Andrews, where he was involved with optical pump sources and ultrashort pulse generation. In 1996, he joined the Institute of Photonics, University of Strathclyde, Glasgow, U.K., where he is an Associate Director and Team Leader for Laser Development. His research interests include optically pumped semiconductor lasers, high-power SBR mode-locked solid-state lasers, stabilization techniques, and adaptive optics.