Coherent combining of high brightness tapered amplifiers for efficient non-linear conversion

We report on a coherent beam combination of three high-brightness tapered amplifiers, which are seeded by a single-frequency laser at λ = 976 nm in a simple architecture with efficiently cooled emitters. The maximal combined power of 12.9 W is achieved at a combining efficiency of > 65%, which is limited by the amplifiers’ intrinsic beam quality. The coherent combination cleans up the spatial profile, as the central lobe’s power content increases by up to 86%. This high-brightness infrared beam is converted into the visible by second harmonic generation. This results in a high non-linear conversion efficiency of 4.5%/W and a maximum power over 2 W at 488 nm, which is limited by thermal effects in the periodically poled lithium niobate (PPLN). © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
High-power high-brightness diode lasers are in strong demand for many applications and benefit from their efficiency, reliability and compactness [1]. However, the power limit of diode lasers is much lower in the visible spectral range than in the near infrared and not all wavelengths are directly accessible with diode lasers. Nonlinear frequency conversion, like second harmonic generation (SHG) is an important technology for the development of high power laser sources in the visible spectral range. Efficient SHG requires a good beam quality, a narrow spectral linewidth and a high input power [2]. High conversion efficiencies are commonly achieved with diode pumped solid state and fiber lasers [3,4], however diode laser based visible light sources have become increasingly attractive for biomedical applications [5] to fulfill the need for compact and efficient visible laser sources. Different high-brightness diode laser architectures are used for blue-green light generation [6]. Among those, tapered lasers (TPL) and tapered amplifiers (TPA) are the most promising architecture since they combine high power and acceptable beam quality in one device.
TPL and TPA devices consist of a single mode ridge waveguide (RW) followed by flared gain-guided amplifier section. Spectrally stabilized TPL are available in the visible (VIS) and near-infrared (NIR) spectral range and reach output powers up to 15 W per device [7] and can be converted to multiple watts of VIS light [8] by making use of a cascade of two SHG crystals [9,10]. Further power scaling can be achieved using an increased NIR pump laser power enabled by research on highly efficient high power diode lasers [11]. Power can also be scaled by beam combining techniques. Incoherent spectral beam combining of tapered diode lasers is a straightforward concept for this power scaling [12], however the ultimate limitation is the available output power per wavelength channel. Coherent beam combining (CBC) of light from several devices is the only way to achieve much higher powers within a single beam that retains excellent spatial and spectral beam quality [13].
We describe in this paper the CBC of three high-brightness tapered amplifiers in a simple optical setup. Furthermore, we demonstrate the importance of CBC for the development of high power visible light sources by demonstrating a significant improvement of the achievable nonlinear conversion efficiency for single pass SHG in a bulk periodically poled lithium niobate (PPLN) crystal.

Coherent beam combining
CBC is the superposition of multiple laser beams by constructive interference. It requires a proper and stable phase relationship of the sampled gain medium. Different approaches have been demonstrated: active phase locking of amplifiers seeded by a single frequency laser split into several beams or passive phase locking of emitters in an extended cavity [14]. Impressive results for CBC using a limited number of high power fiber amplifiers (P > 100W per channel) show the potential of power scaling by CBC reaching kW level powers [15,16]. CBC with diode lasers and amplifiers is still limited to lower power levels but has been demonstrated in various configurations. CBC of arrays of diode lasers in an external cavity [17][18][19] has been demonstrated but did not lead to a significant increase of the available NIR power since the combined power was limited to a few watts. CBC in MOPA configuration has been demonstrated for larger arrays of amplifiers [20,21] and led to a significant increase of the available NIR power of diode laser architectures reaching about 40 W for CBC of 47 Slab Coupled Optical Waveguide Amplifiers (SCOWA) [22].
Our approach to make CBC of diode lasers more attractive for future applications is to make use of high-brightness tapered amplifiers in order to reach similar power levels but with a reduced number of elements. Tapered laser diodes are a promising building block for CBC architectures as they can deliver high power in a close to diffraction limited beam out of one monolithic device. However, one important drawback to tapered devices is the slightly degraded beam quality at high powers with about 70% power-content in the diffraction limited central lobe while the rest of the power is distributed in side lobes. TPAs are nevertheless our preferred choice for power scaling by CBC to simplify the optical setup by using only a limited number of high-power amplifiers. At the same time as demonstrated in this study, CBC is one way to reduce the relative power content in the side lobes since CBC with non-perfect Gaussian beams favors the diffraction-limited power content leading to an improved beam quality [23]. In our previous studies, an output power of 11.5 W was reached by CBC of a monolithically integrated array of five TPAs, but it was limited by thermal effects caused by emitter to emitter heating of the laser bar [24]. For this reason, we make use of separate individual amplifiers that can be efficiently cooled in order to be able to operate them at higher currents and achieve a higher combined power per element [25]. Additionally one can develop a simplified optical setup with standard optical elements, as splitting and combining the beam is simpler.

Experimental setup
We used 6 mm long TPAs mounted p-side up on CuW-heat spreaders and C-mounts. We made use of amplifiers identical to lasers previously described in [26] but with a 2 mm long RW instead of the internal grating. Two amplifiers had a 5 µm wide single-mode RW entry section and the third amplifier a 4.5 µm wide entry section, limited by the availability of the amplifiers. Currents in RW (I rw ) and tapered section (I tp ) can be controlled independently, allowing decoupled control of the phase (via I rw ) and the power (via I tp ) in each amplifier. The amplifiers are saturated for ridge currents I rw > 300 mA. We use small variations of I rw above this limit in order to control the phase with limited impact on the optical power. The maximum power extracted from the amplifiers reaches 6.5 W at 10 A and is limited by thermal rollover due to poor thermal resistance of the c-mount packaging. The power content in the centra showing that t The CBC The seed lase double stage using non-po waveguides (R axis (FA) by cylindrical len slightly differ However, thi amplifiers (P i A 2 interfered second 50:50 each element beam block. intensities on setup [24]. Th loop is used currents of A 2 control, which The outpu described late

Combine
We measured different curre beams, respec ( Fig. 1 Measurements of combination of only the first two amplifiers (A 1+2 ) show a higher combining efficiency (ranging from 85% at 2 A to 74% at 10 A). The combining efficiency of the total setup was > 65%. The combining efficiency decreases for higher currents in the tapered section caused by the degradation of the individual beam quality of each emitter, which will be discussed in the following subsection.
Measurements of the spectrum showed that the spectral properties of the DFB seed laser (976 nm) were maintained. The side mode suppression ratio was better than 40 dB. The full width at half maximum (FWHM) was smaller than 20 pm, limited by the resolution of the optical spectrum analyzer. Fig. 2. Combined power at output and total combining efficiency as a function of the currents in the tapered section. The ridge currents were actively controlled in the range of 250 to 400 mA, the heatsink temperature was T = 20 °C. The astigmatism of the amplifiers was corrected at each operating current.

Beam quality and combining efficiency
Since the beam quality of the TPA is not diffraction limited in the slow axis (SA), one has to investigate the achievable combining efficiency together with the resulting beam quality of the combined beam. Two beams with a slightly different profile interfere at each combining element in the CBC interferometer (Fig. 1) defined by the overlap of the incident spatial amplitude and phase profiles I 1,2 (x,y) and φ 1,2 (x,y) respectively [27]. One can consider the beam of a tapered laser as a sum of the fundamental mode, corresponding to the central lobe, and emission at high angle ("side lobes"). The central lobes of each beam are very similar and interfere constructively with a high combining efficiency.
But the high-order modes exhibit a significantly higher beam mismatch and get therefore partially filtered during the CBC. Furthermore, amplified spontaneous emission is incoherent to the seed source and is therefore weakened by 50% at each beam combiner. Consequently, CBC of beams from tapered lasers leads to a clean-up of the spatial beam profile where the central lobe is maintained and the high angle non diffraction-limited emission is suppressed. The beam profile measurements shown in Figs. 3(a)-3(c) illustrate this effect. The shown beam profiles correspond to the beam waist after the focusing lens with a 1/e 2 waist diameter of about 95 µm. One can clearly see that the side lobes in SA get filtered by the two CBC steps. The power content in the central lobe gets increased from 71% for one individual amplifier to 81% for CBC of A 1+2 and 86% for CBC of A 1+2 + 3 . As a result the beam quality factor is reduced from  In order to estimate the combining efficiency of the central lobes alone, we used intensity and wavefront measurements of the individual beams at maximum power. The combining efficiency is then calculated following Eqs. (1) and (2). We applied a strong clipping criterion at 1/e 2 intensity level to the experimental data as a rough estimation for the central lobe and analyzed the overlap of the intensity and phase profiles within this area. We identified a mismatch of the intensity profile in SA caused by slightly different widths of the ridge section as mentioned earlier in section 2.1. Furthermore small differences in the collimation of each beam lead to a mismatch of the intensity profile in FA. Both effects reduce the achievable combining efficiency by 7-10% at each combining step. Furthermore, the wavefront measurements were used to evaluate combining losses linked to differences in the phase profile in between the beams. Those differences are small and had a limited impact smaller than 3% on the achievable combining efficiency of the central lobe.
Altogether, the calculated combining efficiency is η = 91% for the first CBC step (A 1+2 ) and η = 87% for the second step (A 1+2 + 3 ), resulting in an overall combining efficiency of 82% for the central lobe power content at the maximum current of 10 A. Our lower experimental combining efficiency of >65% considering the whole beam is thus clearly related to the power content in the fundamental mode of the individual amplifier beams.

Second harmonic generation
The increase of the brightness in the NIR by CBC of laser diodes can be useful for many applications requiring high power as well as good spectral and spatial beam quality. Especially for CW single-pass SHG, the conversion efficiency is strongly influenced by the focusing conditions in the nonlinear crystal, as described by Boyd and Kleinman [28]. Furthermore, the spatial quality of the pump beam directly impacts the achievable conversion efficiency [29]. We demonstrate in the following the improvement of single pass SHG efficiency resulting from the scaled brightness by coherent combining, and the subsequent increase of the visible power.

Description of the experiment
The output of the CBC-interferometer was used for single pass SHG as shown in Fig. 1. In order to test the nonlinear conversion efficiency at different levels of brightness, we did modify the CBC setup slightly in order to use the beam from either a single amplifier (A 1 ) or from the CBC of two (A 1+2 ) or three power amplifiers (A 1+2 + 3 ) as the input pump beam for the SHG. This was done by replacing the relevant combining elements with highly reflective mirrors. During the SHG experiments all three amplifiers were operated at constant current into the tapered section (I tp = 9 A) and the power used for SHG was adjusted by turning the polarization before the first polarizing beam splitter (PBS). The optical isolation between the CBC interferometer and the SHG experiment was >25 dB. After reshaping optics (L 5 : f = 50 mm and L 6 : f = 100 mm) for beam size adjustment and astigmatism correction, another halfwave plate is used to align the polarization of the NIR beam with the crystallographic Z-axis of the nonlinear crystal which is parallel to the thickness of the crystal in this case. The NIR beam was then focused with a lens (L 7 : f = 150 mm) into the 40 mm long periodically poled MgO:LiNbO 3 (PPLN). The 1/e 2 waist diameter of the focused beam inside the crystal was measured to be ~95 µm, which was experimentally verified to be the optimum focusing condition at the highest input power. The focusing conditions were identical for all pump source configurations. The PPLN crystal was mounted into a temperature-controlled closedtop oven. The temperature was measured with a temperature sensor in the oven and corresponds to an average temperature. It was optimized at each pump power level. The spectral components were separated by a dichroic mirror (DM).

Experimental results
The SHG power was measured for three source configurations: a single amplifier (A 1 ), CBC of two amplifiers (A 1+2 ) and CBC of three amplifiers (A 1+2 + 3 ). Results are shown in Fig. 4. We fitted the pump depletion approximation [30] 2 2 tanh ( ), where η is the nonlinear conversion efficiency, to the experimental values for pump powers < 6 W. The nonlinear conversion efficiencies in case A 1 , A 1+2 and A 1+2 + 3 obtained by numerical fitting are 2.6%/W, 3.7%/W and 4.5%/W, respectively. This corresponds to an increase of up to 73% enhanced by the beam clean-up and the scaled brightness. We attribute the deviation of measured SHG power from the theoretical fit for P ω ≥ 6 W to thermal dephasing due to localized heating caused by SH absorption [9]. This limited the maximum SHG output power to 2.09 W for 9.2 W of NIR input power (conversion efficiency >22%). However it is so far the highest power achieved at λ = 488 nm by single-pass SHG of a diode-laser-based system, thanks to the simultaneous increase of power and brightness provided by the coherent combining architecture.

Conclusion and perspectives
Coherent beam combining is an important approach for power scaling of diode laser systems when high spatial quality and narrow linewidth are required. In this work, we used a simple architecture and only three amplifiers allowing to combine 12.9 W in one single beam. The beam quality was significantly increased by a beam cleanup inherent to the CBC process, resulting in a beam quality factor of 2 4 1.1 2.5 M σ < × and 86% power content in the central lobe. Those results compare favorably with previous demonstrations of CBC with diode lasers [17][18][19][20][21][22][23][24], regarding both the combined power per amplifier and the simplicity of the setup. Furthermore, the multi-arm MOPA configuration used here could easily be scaled to a larger number of amplifiers, should a larger NIR power be required.
The coherently combined beam was used for single pass nonlinear frequency conversion in a PPLN crystal. As compared to direct SHG of single tapered devices, the improved beam quality of the combined beam in the infrared increases significantly the nonlinear conversion efficiency. A maximum SH power of 2.09 W at 488 nm was reached, limited by thermal dephasing of the PPLN, as confirmed using a pulsed pump beam. Further increase of the output power in the visible spectral range would firstly require the use of different nonlinear crystals in order to deal with the thermal roll-over of the conversion efficiency.
The CBC architecture demonstrated here relies only on off-the-shelf optical elements combined with state of art TPAs. As tapered amplifiers are now available at a wide range of wavelengths, from to the red to the NIR [7], our setup can easily be adapted for other applications. Besides its use for nonlinear frequency conversion towards the visible, our approach would also be effective to develop powerful pump sources for high-brightness pumping of fibre or bulk-crystal amplifiers. The output power could even be further increased by dense spectral beam combining of several similar CBC interferometers operating at slightly different wavelengths, as commonly used with single emitters and bars [32] offering a potential path to future CBC-based direct diode high brightness material processing systems.

Funding
European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 721766.