# Coherent beam combining of four semiconductor optical amplifiers for high-peak-power acousto-optic imaging

Sylvie Janicot, Qin Liu, François Figliolia, François Ramaz, Maïmouna Bocoum, Patrick Georges, Gaëlle Lucas-Leclin

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Sylvie Janicot, Qin Liu, François Figliolia, François Ramaz, Maïmouna Bocoum, et al. Coherent beam combining of four semiconductor optical amplifiers for high-peak-power acousto-optic imaging. CLEO Europe, Jun 2025, Munich (Allemagne), France. hal-05471759

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# Coherent beam combining of four semiconductor optical amplifiers for high-peak-power acousto-optic imaging

Sylvie Janicot<fnref n="1" />, Qin Liu<fnref n="1" />, François Figliolia<fnref n="2" />,

François Ramaz<fnref n="2" />, Maïmouna Bocoum<fnref n="2" />, Patrick Georges<fnref n="1" />, Gaëlle Lucas-Leclin<fnref n="1" />

Acousto-optic imaging (AOI) aims at producing in-depth images in highly scattering biological tissues, by tagging scattered photons with ultrasound (US) waves as they propagate through the sample. By selectively detecting photons traversing the insonified region, AOI generates images of the local optical absorption [1]. To approach video-rate imaging of in-vivo samples, a single-shot acquisition protocol is necessary. This requires laser sources in the 650 nm $\le \lambda \le$ 900 nm spectral range (corresponding to low absorption of haemoglobin and water) with both a high peak power and a narrow spectral linewidth (<20 MHz). The penetration depth is currently limited by the optical power available from single-frequency diode laser devices. Coherent beam combination – based on the constructive interference of multiple coherent laser beams – offers a promising solution as it enhances optical power while preserving beam quality and spectral purity.

We describe the design and evaluation of a laser architecture to address these challenges. The setup is based on the amplification of a low-power CW single-frequency laser diode (DBR) at 828 nm in four commercial tapered semiconductor optical amplifiers (TSOA). The incident power per amplifier is ~20 mW. The TSOAs operate in QCW (100 µs – 100 Hz), delivering a high peak power of approximately 5.2 W per amplifier at a driving current of 7 A. The four amplified beams interfere constructively in a 2-stage Mach-Zehnder interferometric architecture (Fig. 1) and are coherently combined into a single high-power beam. The dynamic nature of the QCW operation of the TSOAs induces a rise in the device temperature during the 100 µs - long pulse. It results in a large phase drift of about $15\pi$ and a significant focusing of the beam during the pulse. Nonetheless all four amplifiers exhibit similar behaviors, thus these dynamic evolutions do not deteriorate the maximum peak power achieved after 50 µs (Fig. 2). The maximum output peak power reaches 13.6 W, corresponding to an overall combining efficiency of 65%.

Thanks to excellent mechanical stability and good thermal isolation, the phase noise of our setup is dominated by low-frequency components ($f < 1$ Hz), requiring correction only for slow path length fluctuations. The combined power is actively stabilized by sequentially adjusting the optical path length differences between the laser beams with piezoelectric elements (PZTs) glued to folding mirrors. A photodiode (PhD) followed by a sample-and-hold circuit monitors a fraction of the combined beam's peak power, providing feedback for a servo-loop control system that maximizes average output power via a gradient-descent algorithm. The servo loop is thus able to maintain a stable output power with pulse-to-pulse peak power stability better than 1%.

<figure><img src="image_2.png" /><figcaption>Fig. 1 Scheme of the laser architecture with four amplifiers ($A_i$)</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2 Pulse profile of the coherently-combined output</figcaption></figure>

With a preliminary two-TSOA configuration in a single combining stage [2], we achieved unprecedentedly contrasted images of absorbing inclusions in a highly scattering medium (transport mean free path of 0.1 cm) in single-shot acquisition at a frame rate of 0.2 Hz. These results highlighted the critical role of peak power in improving signal-to-noise ratio for Fourier-transform AOI and validated the laser source properties for the application. The enhanced peak power and stability achieved with the four-TSOA configuration should further improve imaging quality, allowing deeper penetration into highly scattering samples and advancing the feasibility of in-vivo AOI images.

<footnote n="1">1. Université Paris-Saclay, Institut d'Optique Graduate School, CNRS, Laboratoire Charles Fabry, 91127, Palaiseau, France</footnote>

<footnote n="2">2. ESPCI Paris, PSL University, CNRS, Institut Langevin, 1 rue Jussieu, 75005 Paris, France</footnote>

<footnote>[1] L. Dutheil, M. Bocoum, M. Fink, S. Popoff, F. Ramaz, J.-M. Tualle "Fourier transform acousto-optic imaging with off-axis holographic detection," Applied Optics. 60, 7107 (2021)</footnote>

<footnote>[2] F. Figliolia, Q. Liu, S. Janicot, P. Georges, G. Lucas-Leclin, J.-P. Huignard, F. Ramaz, J.-M. Tualle, M. Bocoum "Real-time acousto-optic imaging using a high peak power long pulsed illumination," Optics Letters 49, 21, 5993-5996 (2024)</footnote>