# LED pumped alexandrite multipass amplifier designed for acousto-optic imaging

Elio Thellier, Hussein Taleb, Frederic Druon, Catherine Le Blanc, Pierre Pichon, Marc Castaing, François Balembois, Patrick Georges

► To cite this version:

Elio Thellier, Hussein Taleb, Frederic Druon, Catherine Le Blanc, Pierre Pichon, et al., LED pumped alexandrite multipass amplifier designed for acousto-optic imaging. Optics Express, 2025, 33 (19), pp.40727. 10.1364/OE.563688. hal-05269520

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Submitted on 19 Sep 2025

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# LED pumped alexandrite multipass amplifier designed for acousto-optic imaging

ELIO THELLIER,<fnref n="1" />,*, ID HUSSEIN TALEB,<fnref n="1" /> FREDERIC DRUON,<fnref n="1" /> ID CATHERINE LE BLANC,<fnref n="1" /> PIERRE PICHON,<fnref n="1" /> MARC CASTAING,<fnref n="2" /> FRANÇOIS BALEMBOIS,<fnref n="1" /> AND PATRICK GEORGES<fnref n="1" /> ID

**Abstract:** In this paper we present an alexandrite (Cr³⁺:BeAl₂O₄) multipass amplifier pumped by blue LEDs via a luminescent concentrator (LC). This emerging pump technology provides up to 2.6 kW optical pump power during 400 µs at 10 Hz in the 530-630 nm range for low cost, simple and robust implementation. We achieve a single-pass gain of 1.38, the highest ever reported in alexandrite for a semiconductor-based pump source, and a total gain of 34 after 16 passes. The amplifier is injected by a single-frequency laser operating in a quasi-continuous wave at 761 nm. It delivers µs pulses with a peak power of 14 W. This power could significantly improve penetration depth for in vivo acousto-optic imaging.

Published by Optica Publishing Group under the terms of the [Creative Commons Attribution 4.0 License](https://creativecommons.org/licenses/by/4.0/). Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

## 1. Introduction

### 1.1. Laser source requirements for acousto-optic imaging

Acousto-Optic Imaging (AOI) is an emerging multimodal technique in life sciences based on the interaction of coherent scattered light and ultrasound at a given depth. It allows to see through a scattering medium illuminated by a laser source. As the light remains coherent in time during propagation, a signature of the interaction is found in the modulation of the speckle pattern at the ultrasound frequency [1]. As ultrasound is ballistic, AOI allows the local optical properties of the medium, such as absorption or scattering, to be mapped with a spatial resolution given by the ultrasound, e.g. 100 µm to a few mm depending on the ultrasound frequency (ranging from 1 MHz to 15 MHz for conventional ultrasound scanners).

AOI requires a very specific laser source. It must be single frequency with a linewidth of less than 2 MHz to detect the frequency shifted “tagged photons” using interferometry. The laser wavelength must be in the optical therapeutic window (700-1000 nm), which offers significant transmission and many possibilities, such as oxygen blood saturation measurements. Its tunability would be a real advantage for species, metabolism and structure identification.

Due to the small number of photons passing through diffusing media, the laser source must be powerful. Today, AOI uses semiconductor lasers with tapered amplifiers [2] as illumination sources. The laser power is typically 1 W, which limits the penetration depth in biological media to less than 1 cm. To increase the penetration depth, the output power must be increased. A power of 100 W could result in an order of magnitude greater penetration depth. However, due to biological safety, the average power density must be kept below 300 mW/cm<fnref n="2" />. The laser must therefore be pulsed at a moderate average power, typically 1 W to illuminate a circular surface with a radius of around 1 cm. In the case of living tissue, the moving parts induce instabilities (speckle decorrelation) that reduce the signal-to-noise ratio. This effect requires acquisition times

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

<footnote n="2">²Alphanov, rue François Mitterand, 33400 Talence, France</footnote>

<footnote>* elio.thellier@institutoptique.fr</footnote>
of less than 100 µs [3]. In addition, the repetition rate must be high enough (typically 100 Hz) to
operate in real time.

Therefore, the ideal source for an AOI application would be a quasi-continuous wave laser emitting single-frequency 100-µs pulses in the near infrared with a peak power of 100 W and a repetition rate of 100 Hz.

1.2. *State of the art for quasi-continuous-wave high-power laser sources*

Quasi-continuous-wave (QCW) single frequency high-power laser sources emitting in the near infrared are not so common. Existing sources that approach the requirements for AOI imaging can be divided into three categories:

The first are semiconductor lasers tuned by external cavities combined with tapered amplifiers. The power reaches a few watts in continuous wave (CW) mode, limited by the damage threshold of the materials involved [4]. Recently, the coherent combination of two tapered amplifiers led to an output power of 8 W [5]. However, their tunability is limited to 20-30 nm by the emission bandwidth of the semiconductor.

The second are frequency-doubled erbium-doped fiber lasers. More power has been achieved at 780 nm (up to 20 W in CW [6]). However, the tunability is complicated for fiber amplifiers whose length and pump power are often optimized for a fixed wavelength. Moreover, considering the maximum tunability of erbium-doped fibers (typically 1520-1640 nm), the wavelength range covered by frequency doubling in the near infrared is only 60 nm in the best case.

The third are bulk transition metal lasers. In fact, Ti:sapphire (Ti³⁺:Al₂O₃) lasers have the broadest tunability in the near infrared (typically between 750 nm and 950 nm). A single-frequency Ti:sapphire oscillator has been demonstrated at the watt level in CW, over a tuning range of nearly 150 nm [7]. A quasi-continuous single-frequency Ti:sapphire oscillator pumped by a QCW Yb:YAG frequency-doubled ring laser has also been demonstrated [8]. The pulse duration is 400 µs, which is compatible with the AOI specifications. However, the maximum output power is limited to 1 W due to the available pump power in this experiment. Power scaling is possible with Yb:YAG but with a setup that could become bulky and expensive. Alexandrite (Cr³⁺:BeAl₂O₄) emitting in the range 700 nm-800 nm is another option. A flashlamp pumped tunable alexandrite have been demonstrated [9]. Alexandrite can amplify single frequency nanosecond pulses up to an energy of 5 mJ in a regenerative amplifier [10]. In this case the repetition rate is limited by the flashlamp at 4 Hz. A diode-pumped alexandrite ring laser oscillator, has been developed: 1 W output power has been achieved in CW [11] with a broad tunability between 727 nm and 792 nm.

1.3. New laser source for AOI imaging

This state of the art clearly shows that a laser source adapted for acousto-optic imaging is challenging. The aim of this work is to propose an original solution for this application. Regarding the laser medium, the need for tunability has led us to choose transition metal lasers. We choose a Master Oscillator Power Amplifier (MOPA) configuration to separate the source requirements: frequency, phase and noise control in the oscillator and power management in the amplifier. The gain of the laser medium in the amplifier is therefore a key parameter. For QCW pumping (on the scale of 100 µs for alexandrite), the gain is partly determined by the product "emission cross section × lifetime". This value is almost 2 times higher for alexandrite than for Ti:sapphire. This is the reason why we selected alexandrite for the power amplifier.

The choice of pump source is a key parameter. The broad absorption spectrum of alexandrite from 400 nm to 650 nm offers many options for pump sources: flashlamps [9,10,12], red laser diodes [11,13,14] and blue laser diodes [15,16]. Alexandrite has also been pumped by light-emitting diodes (LED) via a luminescent concentrator (LC) emitting in the yellow-orange region [17]. According to the requirements of AOI, a peak output power of 100 W means that the
pump power must typically reach the kW level. This is easily achieved with flashlamps. However, the stability of flashlamps is lower than that of semiconductor sources and the repetition rate is severely limited by the short lamp lifetime. Concerning diode-pumping of alexandrite, the highest pump reported is 10 W for a fiber-coupled blue laser diode [16] and between 200 W and 400 W for red laser diode bars [18,19]. Even if the technology of red laser diode is still progressing [20], power scaling appears to be much easier with LEDs, as the first demonstration of LED-pumped alexandrite has already been achieved with 1 kW pump power [17]. Indeed, LED pumping offers an interesting alternative, combining the low cost and robustness of flashlamps with the stability and long lifetime of semiconductors. Therefore, as in [17], we chose a Ce:YAG luminescent concentrator pumped by blue LEDs to pump the alexandrite amplifier.

Alexandrite amplifiers have been demonstrated in two different architectures: regenerative amplifiers [10,21] and geometrical multi-pass amplifiers [18]. Regenerative amplification in a cavity allows more passes than geometric amplification, but the duration of the pulse to be amplified must be less than the round-trip time of the amplifier cavity (typically in tens of ns). Therefore, geometric amplification appears to be the only solution for amplifying µs pulses. However, the gain obtained in semiconductor-pumped alexandrite amplifiers is limited: 2.13 in four passes in the case of red laser diode pumping [18] and 4 in eight passes in the case of LED pumping [17]. Assuming that our oscillator could emit a peak power of 1 W, the AOI requirements lead to a gain of 100 in the amplifier. This gain value is clearly challenging for an alexandrite whose product “emission cross section × lifetime” is more than 20 times lower than that of a Nd:YAG laser. Our strategy for achieving such a gain is twofold: first, to increase the single-pass gain of the alexandrite amplifier by increasing the pump power, and second, to increase the number of passes of the amplifier.

Therefore, we propose a new laser source for AOI imaging: a MOPA with an alexandrite amplifier pumped with the highest semiconductor-based power ever used (2.6 kW) and with a high number of passes (up to 16).

## 2. Experimental setup

### 2.1. *Laser source architecture*

The laser architecture is shown in Fig. 1. The oscillator is a single frequency laser diode followed by a tapered amplifier (Sacher Serval-Plus). The device delivers a CW signal of up to 1.7 W, with linear polarization and $M^2 = 1.3$. The signal passes through a power modulator consisting of a half-wave plate and a polarizer. A first-order diffraction acousto-optic modulator carves µs pulses in the CW signal. The pulses are synchronized with the pumping of the alexandrite amplifier.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** General layout of the laser source. The pumping head and the multipass amplifier are described in Fig. 2 and Fig. 4 respectively.</figcaption></figure>
The signal is monitored by two photodiodes: A photodiode monitoring the zero-order of the acousto-optic modulator (“photodiode in”), which allows the measurement of the incident power, and a photodiode at the output of the amplifier (“photodiode out”). The beam passes through a half-wave plate to obtain the maximum gain oriented along the b-axis of the alexandrite crystal. The waist adapter consists of two lenses (focal lengths 250 mm and 500 mm respectively) which allow to obtain a waist (radius) of 100 µm inside the alexandrite crystal. The 1 × 3 × 25 mm³ alexandrite crystal is doped with 0.22 at% Cr³⁺ ions. The b-axis is perpendicular to the 1 × 25 mm² plane and the c-axis is parallel to this plane. This orientation allows maximum pump absorption (//b and //c), since the lowest absorption is parallel to the a-axis [9]. The luminescent concentrator is glued (VBB-1 from Vitralit) to the alexandrite crystal on its 3 × 25 mm² surface (horizontal plane) leading to a transverse pumping configuration (Fig. 2). The signal beam passes through the 1 × 3 mm² faces of the alexandrite crystal, which are antireflective coated at 761 nm.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** a) Scheme of the pumping head glued to the alexandrite crystal and projections of the laser beams of the multipass amplifier in the plane of the pumping head (vertical plane). M1 is the rear mirror of the multipass amplifier. b) Emission spectrum of the blue LEDs (left pic) and fluorescence spectrum of the Ce:YAG luminescent concentrator (right pic). c) Separated experimental setup to estimate the output power launched into the alexandrite.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Different signals versus time emitted by the LED pumped alexandrite amplifier. Blue : pump signal at 530-630 nm emitted by the Ce:YAG. Orange : fluorescence signal of alexandrite. Violet : input signal at 761 nm (in case of 5 µs long pulses). Red : output amplified signal. A zoom on the input and output amplified signals is presented later in Fig. 6.</figcaption></figure>
## 2.2. Pumping head

The pumping head (Fig. 2) is composed of the gain medium (alexandrite) and its pumping system. The latter consists of a Ce:YAG luminescent concentrator of 100 × 50 × 1 mm³ in a vertical plane, pumped on its large faces by two LED panels (Fig. 2.a). The panels are each composed of 1696 LEDs (Rubix Royal blue), each LED emitting 1.6 W at 1.5 A in CW, at 440 nm. In pulsed mode (several hundred µs, repetition rate of up to 100 Hz), the current can be overdriven up to 6 A, increasing the output peak power to 4.2 W per LED. The total peak blue power is then 14.2 kW (and 89 kW electrical consumption). Compared to the previous pumping head [17], the power scaling is achieved by using a more powerful LED (4.2 W instead of 3.1 W in pulsed mode) and by increasing the number of LEDs: 3392 instead of 2240. All in all, this doubles the pumping power in the blue.

The Ce:YAG luminescent concentrator absorbs the blue pump power and re-emits it in the 530-630 nm range (Fig. 2.b). This yellow light is partially confined by total internal reflection within the concentrator and then directed by mirrors on the lateral faces towards the output. The exit face is an additional face that is cut at a 45° angle to the side faces. As explained in [22], this configuration makes it possible to collect twice as many yellow power as with a classical exit on a lateral face. At the output of the Ce:YAG (in air) we measure a peak power of 1240 W during 400 µs. The output in air is limited by total internal reflection for rays having an incidence angle larger than 33° on the output face. The UV-curing optical adhesive (refractive index 1.5) between Ce:YAG and alexandrite increases the maximum incidence angle to 55.5°. Therefore, the power launched in the alexandrite is larger. In order to estimate it experimentally, we design a cylindrical CPC (compound parabolic concentrator) inspired from spherical CPC [23]. This CPC allows to redirect the large incidence rays in the escape cone of the CPC-air interface. Theoretical calculations carried out with a ray tracing software (LightTools) shows that this cylindrical CPC is able to transmit in the air 80% of the rays crossing the Ce:YAG-glue interface. After the CPC, we measure in the air a peak power of 2225 W (Fig. 2.c). From this value, we estimate that the power launched in the CPC through the glue is 2750 W. When alexandrite crystal replace the CPC, we take into account the Fresnel reflection at the glue-alexandrite interface. This average reflection coefficient of 5.3% is calculated with all the rays (with all angles) and on both TE and TM polarizations. Thus we estimate that 2605 W are transmitted to the alexandrite crystal. As the LEDs consumed 89 kW, this equates to an electro-optical efficiency of 3% for the entire pump unit.

The absorption of the alexandrite crystal is measured in air by selecting few parallel rays from the concentrator with two pinholes. We find an absorption of 35% over the crystal thickness of 1 mm. Without pinholes the average propagation into alexandrite is 1.1 times higher and leads to an absorption of 38%. This corresponds to an absorption coefficient of 4.3 cm⁻¹ averaged over two polarization directions (b and c axis of alexandrite) and over the Ce:YAG spectrum. This value is consistent with the absorption coefficient found in [18] at 635 nm for an alexandrite with the same doping level. Once the alexandrite is bonded to the Ce:YAG, the pump rays entering in the alexandrite are no longer parallel (from 0° to 62°). In addition, some rays experience a total internal reflection in the alexandrite crystal. Therefore, the average propagation distance of the pump rays due to trapping in the alexandrite is greater than the thickness of the crystal, resulting in higher absorption. To estimate the absorption, we measure the fluorescence intensity of the alexandrite when it is glued to the Ce:YAG and when it is not. The fluorescence signal is 2.4 times higher with glue than without. It can be further increased to 2.8 when 3 faces of the alexandrite are covered with a white diffusive thermal grease. Therefore, assuming that the absorbed pump power is proportional to the fluorescence signal, we can deduce that 1320 W are absorbed in alexandrite in the best configuration. Compared to the launched power (2605 W), this represents an absorption of 51%, larger that the absorption found with only the parallel rays, as expected.
The temperature of the alexandrite crystal is heated to 85°C, which corresponds to the maximum gain of alexandrite versus temperature [24]. The pump pulse duration is chosen by monitoring the fluorescence of alexandrite (Fig. 3). Indeed, this quantity is proportional to the population of the upper state level and therefore related to the small signal gain. We observe that the fluorescence signal increases up to a pump duration of 400 µs, consistent with a lifetime of 155 µs at 85°C.

As expected for transverse pumping, we observe that the fluorescence is higher close to the pumped surface. Therefore, the closer to the concentrator, the higher the population inversion. One solution to maximize the gain is to bounce the beam onto the pumped face [25]. To enter alexandrite from the center of the input face with a bounce in the center of the crystal, we need an angle of incidence $α = 3.9°$ on the input face. This angle is well below the limit angle (28°) for total internal reflection at the alexandrite ($n = 1.7$) - glue ($n = 1.5$) interface, meaning that the signal beam can be bounced off the pumped face without loss. The small signal gain is measured in a double pass configuration after reflection from the rear mirror of the amplifier (M1). The gain reaches 1.9 with the bounce (single pass gain of 1.38) while it reaches a maximum of 1.6 without the bounce (optimizing the position of the signal beam to be very close to the pump face but without diffraction losses).

## 2.3. Multipass amplifier

The multi-pass amplifier configuration is shown in Fig. 4. Since the beam size in alexandrite is small (100 µm radius, corresponding to a Rayleigh range of 4.1 cm in air), we design a relay imaging as in [26]. This configuration ensures a large number of passes while maintaining a small beam size in the alexandrite. This is achieved with a set of highly reflective mirrors, including a concave mirror M2 (radius of curvature $R_c = 1$ m) and several plane mirrors (M1, M3, M4, M5). M2 conjugates the beam waist at M1 with a beam waist on dihedral (M4). M4 then consists in two perpendicular mirrors that translate the beam in the horizontal plane in order to change the angle of incidence on the alexandrite. M3 is a simple folded mirror used to reduce the size of the amplifier, and finally M5 is a pick-off mirror that terminates the propagation into the amplifier.

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Scheme of the multipass amplifier (horizontal plane). M1, M2, M3, M4, M5 are highly reflective mirrors at 761 nm.</figcaption></figure>

We measure the transmission of the multipass amplifier without pumping versus the number of passes (Fig. 5). We observe a decrease between the second and the fourth pass, which is more pronounced than the decrease between subsequent passes. This can be attributed to a small clipping on the M4 mirrors, as the beam is very close to the inner edges of the M4 mirrors after second pass. After the fourth pass, we measure a constant transmission of 95% between two consecutive passes.

The number of passes is limited by the size of M2 (50 mm diameter) and by the distance between two beams of consecutive passes. This distance is necessary to extract the signal in the last pass without affecting the previous passes by diffraction losses.
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Transmission of the multipass amplifier versus the pass number. Insert: beam profiles at the input and the output of the 16-pass geometry with pumping.</figcaption></figure>

After optimizing the amplifier, we obtain 16 passes with a total transmission of 58%. This losses are almost entirely due to the diffraction, the absorption of alexandrite at 761 nm being negligible.

We check the beam quality before and after the amplifier after 16 passes. The $M^2$ is 1.3 at the input and 5 at the output after amplification (insert Fig. 5). This degradation is not an issue for AOI applications since the beam travels through highly scattering media.

### 3. Experimental results

#### 3.1. *Laser characterization with 5 µs pulses*

We start the characterization of the amplifier with 5 µs seed pulses with a peak power of 430 mW. The signal pulse is synchronized to be amplified at the end of the pump pulse. The repetition rate is set to 10 Hz to limit thermal effects induced by the average pump power. The output photodiode is calibrated using a Joule meter. At the output, the photodiode records the amplified signal (Fig. 6). We observe that the amplified signal (red line) is temporally slightly modified by the amplification process compared to the unamplified signal (blue line). At the end, the peak output power is 14.7 W. This corresponds to a gain of 34, relative to the input power.

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Temporal profiles of output pulses with and without pumping.</figcaption></figure>

Next we measure the gain after 16 passes as a function of pump power by varying the current in the LEDs (Fig. 7). We compare it with the theoretical small signal gain. At the input of the alexandrite, the pump intensity reaches 14 kW/cm², corresponding to 40% of the pump saturation intensity (standardly defined as $\frac{hc}{\tau\sigma_p\lambda_p}$, whose quantities are defined below). For this reason, we have to take pump saturation of absorption into account in the gain calculation. Assuming that
the crystal is illuminated by an intensity $I_p$ (it's a flux of photons, thus in $s^{-1}.m^{-2}$), the small signal gain after 16 passes can be written :

$$G_0 = T \cdot \exp \left( 8 \cdot \ln(G_{0max}) \cdot \frac{I_p}{I_{pmax}} \cdot \frac{1 + \tau \sigma_p I_p}{1 + \tau \sigma_p I_{pmax}} \right) \quad (1)$$

where

$I_{pmax}$ is calculated for the maximum incident pump power (2605 W over a surface of $1 \times 20 \text{ mm}^2$),

$\sigma_p$, the absorption pump cross section, comes from the absorption measurement : $\sigma_p = \frac{\alpha_p}{n_t}$, where $\alpha_p$ is measured at $4.3 \text{ cm}^{-1}$ and $n_t$ is the $Cr^{+3}$ concentration $n_t=6.4 \cdot 10^{25} \text{ m}^{-3}$ for a 0.22% doped alexandrite,

$\lambda_p$ is the average pump wavelength according to Fig. 2.b (575 nm),

$T$ is the transmission of the multipass amplifier for 16 passes ($T = 58\%$),

$\tau$ is the laser transition lifetime, $\tau = 155 \text{ µs}$ at $85^{\circ}\text{C}$.

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Gain after 16 passes as a function of the pump power launched into alexandrite. Red points are experimental data. Black dash line is the theoretical small signal gain of the multipass amplifier after 16 passes following formula (1).</figcaption></figure>

Clearly, Fig. 7 shows that the gain recorded is much lower than the small signal gain. Two facts could explain this. The first is an imperfect overlap between the signal rays and the pumped volume in the alexandrite. The second is the existence of gain saturation.

To clarify this point, we record the gain for different number of passes, varying between 2 and 16 (Fig. 8). We compare it with two theoretical gains: the first is the small signal gain after n passes $G_{0n}$, taking into account the transmission of the multipass. The second is the gain $G_n$ saturated by the signal intensity in the alexandrite resulting in the superposition of n passes.

$$G_n = T_n \exp \left( \frac{n}{2} \cdot \ln(G_{0max}) \cdot \frac{1}{1 + \tau \sigma I_n} \right) \quad (2)$$

where

$T_n$ is the transmission of the multipass amplifier after n passes,

$\sigma$ is the effective emission cross section at 761 nm (taking excited state absorption into account) : $\sigma = 10^{-20} \text{ cm}^2$ at $85^{\circ}\text{C}$ [23]),

$I_n$ is the total intensity present in the multipass for n passes.

Since the signal is very long compared to the time it takes to travel from one pass to another, all the passes can be considered superimposed in time. Assuming, for the sake of simplicity, that all the beams of the multipass are also spatially superimposed, the intensities must be summed over all the passes.
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** a). Gain as a function of the number of passes. Red dots are experimental points. Dash lines come from theoretical calculations. The black dashed line is the gain after n passes assuming a double pass small signal gain of 1.9 and taking the real transmission of the multipass (data from Fig. 5). The blue dashed line is the gain also taking saturation into account. 8b) Scheme of the rays representing the different passes in the alexandrite. Horizontal plane.</figcaption></figure>

Figure 8 shows that the small signal gain $G_{0n}$ is far from the observation. Indeed, the saturated gain is closer to the experimental points. The discrepancy visible for the gain at the 16th pass could be explained by the larger angle of incidence that leads to a partial overlap and therefore produces a lower saturation.

## 3.2. Laser operation with 100 µs pulses

In biological media, the illumination pulse duration can be extended up to 100 µs without losing information about moving parts. Long pulses are useful to increase the average power and therefore the signal-to-noise ratio of the AOI. As the pump pulse duration is 400 µs, we can increase the pulse signal from 5 µs to 100 µs. The signal is synchronized to achieve a timing overlap during the last 100 µs of the pumping pulse.

We observe that the signal becomes strongly unstable. Figure 9 presents a single shot of the amplified signal for a pulse duration of 100 µs (blue curve plain line). The blue dashed-line curve presents the theoretical signal assuming a 16 pass non-saturated gain that evolves during the last 100 µs of pumping (see Fig. 3). Cleary, the signal increases much more than expected even exceeding the small signal gain. Moreover we also observe instabilities due to parasitic self-lasing at the end of the signal. It is worth to mention that no parasitic laser effect is observed when pumping without a seed signal.

Surprisingly, this effect is all the more pronounced when the input signal increases. Figure 10 shows that the output signal is stable for low input signal power (typically under 72 mW). It becomes strongly unstable for higher signal power. This is counter-intuitive since a more powerful signal should induce higher gain saturation and hence reduce the gain available for self-lasing.

Gain increase versus signal has already been observed in laser amplifiers. It is typically caused by a non-linear process of two-wave mixing [27] or degenerate four-wave mixing [28]. This is related to local gain saturation grating resulting from the interferences of the two, forward and backward, beams at the signal wavelength. Since the refractive index depends on the local population inversion, gain saturation induces an index grating that can couple waves and transfer energy from one wave to the other. Refractive index dependence with population inversion is particularly important in alexandrite [29]. In addition, four-wave mixing is enhanced in multi-pass geometries [28]. In our case, the 16 beams corresponding to the 16 passes in the amplifier are largely superimposed in the alexandrite. Since the seed laser is single frequency, interferences
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Amplified signal pulse for 100 µs duration at maximum pump power. Blue curve : signal without vibrations on M1 induced by a piezo-electric transducer. Blue dash line, expected signal assuming a non-saturated gain over 16 passes and taking the evolution of the population inversion during the signal (present after 300µs of pumping and ending at 400 µs of pumping, see Fig. 3). Red curve : signal with vibrations on M1.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Output signal after amplification (pump at maximum) for different input powers recorded on a scope with a remanence of several minutes.</figcaption></figure>

between the different beams have a high contrast and gratings are created by local saturation of the population inversion. This means that the different passes can be coupled, inducing the light to travel much more than 16 passes in the amplifier. This could explain the signal increase observed in Fig. 8 and Fig. 9, and the parasitic laser oscillations when the gain is too high.

Local gain saturation interferometric patterns and the subsequent effects can be eliminated by rapidly moving the interference fringes. This can be achieved by varying the optical path difference between the beams before and after reflection at the rear mirror M1. To validate this hypothesis, mirror M1 is mounted on a piezoelectric transducer which is driven by a 0-10 V step, synchronized with the laser signal. The red curve in Fig. 9 shows a clear impact: the additional gain disappears (with a result very closed to the one expected by the theory) and no parasitic oscillations are observed anymore. We get the same gain value as with the previous 5 µs pulses. This experiment provides a first clue of the importance of wave-mixing in our multipass amplifier. Further investigations are necessary to confirm this approach.
## 4. Conclusion

In conclusion, this work presents a high gain geometrical multipass amplifier. After 16 passes a gain of 34 is obtained for 5 µs pulses at 10 Hz operation, reaching 14 W peak power at 761 nm. This single frequency output signal is 1.75 times more powerful than last report for acousto-optical imaging [5]. This new laser source is particularly promising for improving the penetration depth and signal-to-noise ratio of AOI in scattering media, due to its high power in the therapeutic spectral window, and its potential tunability between 700 nm and 800 nm. This result has been enabled by the first LED-pumped luminescent Ce:YAG concentrator delivering a peak power of 2.6 kW in the yellow spectral region (530-630 nm) for pump pulses of 400 µs. The pump power is almost 3 times higher than in our previous setup [16]. This allows to obtain a higher single pass gain of 1.38 instead of 1.16 [16] in alexandrite. This gain is significantly higher than the one obtained with alexandrite pumped by laser diodes, that is typically of 1.2 [16].

In order to fulfill the AOI requirements completely, one has to increase the signal power up to 100 W. Our work shows that we have pushed the LED-pumped system to the limit since the alexandrite experience pump saturation absorption. In addition, it seems to be difficult to increase the number of passes without overlap problems between the signal beams and the pumped volume. Therefore, in order to reach 100 W, the addition of a second amplifier should be the best option. A peak power of 100 W over 100 µs corresponds to an energy of 10 mJ. This value is far below the stored energy in our alexandrite amplifier that can be estimated to 190 mJ in the all pumped volume (corresponding to the product absorbed pump power × lifetime). The second amplifier needs a gain of 7, that should be reached with less than 16 passes.

The second requirement to fulfill is the repetition rate (100 Hz). The repetition rate is now limited to 10 Hz although the LEDs and alexandrite cooling are ready for 100 Hz operation. The reason is related to thermal degradation of the optical adhesive between LC and alexandrite at temperature higher than 80°C. This slowly reduces the pump power launched into the alexandrite. This problem can be solved by using high temperature optical adhesives.

More generally, this high-power LED pumping technology of alexandrite opens the opportunity to revisit this “old” transition metal laser with interesting perspectives for nanosecond and femtosecond lasers amplifiers.

**Funding.** Agence Nationale de la Recherche (21-CE42-0014).

**Acknowledgment.** The authors would like to acknowledge Stephane Darbon from CEA and Loïc Meignen from LULI for providing the LED power supplies. This research project is being conducted in collaboration with acousto-optical imaging laboratory partner, the Langevin Institute.

**Disclosures.** The authors declare no conflicts of interest.

**Data availability.** The Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

## References

1. J. Gunther and S. Andersson-Engels, "Review of current methods of acousto-optical tomography for biomedical applications," *Front. Opto-Electron.* **10**(3), 211–238 (2017).

2. J.-B. Laudereau, E. B. à La Guillaume, V. Servois, et al., "Multi-modal acousto-optic/ultrasound imaging of ex vivo liver tumors at 790 nm using a sn2p2s6 wavefront adaptive holographic setup," *J. Biophotonics* **8**(5), 429–436 (2015).

3. M. M. Qureshi, J. Brake, H.-J. Jeon, et al., "In vivo study of optical speckle decorrelation time across depths in the mouse brain," *Biomed. Opt. Exp.* **8**(11), 4855–4864 (2017).

4. M. Schiemangk, K. Lampmann, A. Dinkelaker, et al., "High-power, micro-integrated diode laser modules at 767 and 780 nm for portable quantum gas experiments," *Appl. Opt.* **54**(17), 5332–5338 (2015).

5. F. Figliolia, Q. Liu, S. Janicot, et al., "Real-time acousto-optic imaging using high-peak-power long-pulsed illumination," *Opt. Lett.* **49**(21), 5993–5996 (2024).

6. J. Dong, X. Zeng, S. Cui, et al., "More than 20 W fiber-based continuous-wave single frequency laser at 780 nm," *Opt. Express* **27**(24), 35362–35367 (2019).

7. M. Tsunekane, N. Taguchi, and H. Inaba, "High-power, efficient, low-noise, continuous-wave all-solid-state Ti:sapphire laser," *Opt. Lett.* **21**(23), 1912–1914 (1996).
8. L. Cabaret and C. Drag, "Highly scannable injection seeded nanosecond Ti:sapphire ring laser," Eur. Phys. J. Appl. Phys. 51(2), 20702 (2010).

9. J. C. Walling, H. P. Jenssen, R. C. Morris, et al., "Tunable-laser performance in," Opt. Lett. 4(6), 182-183 (1979).

10. D. J. Harter and P. Bado, "Wavelength tunable alexandrite regenerative amplifier," Appl. Opt. 27(21), 4392-4395 (1988).

11. X. Sheng, G. Tawy, J. Sathian, et al., "Unidirectional single-frequency operation of a continuous-wave alexandrite ring laser with wavelength tunability," Opt. Express 26(24), 31129-31136 (2018).

12. K. Ash, J. Lord, J. Newman, et al., "Hair removal using a long-pulsed alexandrite laser," Dermatol. Clin. 17(2), 387-399 (1999).

13. R. Scheps, B. M. Gately, J. F. Myers, et al., "Alexandrite laser pumped by semiconductor lasers," Appl. Phys. Lett. 56(23), 2288-2290 (1990).

14. A. Teppitaksak, A. Minassian, G. M. Thomas, et al., "High efficiency > 26 W diode end-pumped alexandrite laser," Opt. Express 22(13), 16386-16392 (2014).

15. M. Fibrich, J. Šulc, D. Vyhlfdal, et al., "Alexandrite spectroscopic and laser characteristic investigation within a 78-400 K temperature range," Laser Phys. 27(11), 115801 (2017).

16. H. Xiao, X. Jiang, and M. J. Damzen, "Alexandrite lasers with blue-diode-pumping," Opt. Express 31(4), 5832-5842 (2023).

17. P. Pichon, A. Barbet, J.-P. Blanchot, et al., "Led-pumped alexandrite laser oscillator and amplifier," Opt. Lett. 42(20), 4191-4194 (2017).

18. A. T. Coney and M. J. Damzen, "High-energy diode-pumped alexandrite amplifier development with applications in satellite-based lidar," J. Opt. Soc. Am. B 38(1), 209-219 (2021).

19. A. Munk, S. Scheuer, M. Strotkamp, et al., "Energy-scaling of a diode-pumped alexandrite laser and prototype development for a compact general-purpose doppler lidar," Appl. Opt. 62(33), 8732-8740 (2023).

20. J. Su, F. Liu, Z. Zhu, et al., "High performance of 640 nm red laser diodes and modules for pumping source," in: High-Power Diode Laser Technology XXIII. SPIE, p. 301-310 (2025).

21. A. Hariharan, M. E. Fermann, M. L. Stock, et al., "Alexandrite-pumped alexandrite regenerative amplifier for femtosecond pulse amplification," Opt. Lett. 21(2), 128-130 (1996).

22. P. Pichon, L. Lopez, M. Nourry-Martin, et al., "Light extraction and brightness enhancement of luminescent rectangular slabs," Adv. Photonics Res. 3(7), 2100356 (2022).

23. D. K. De Boer and L. Haenen, "Extraction optics for high lumen density sources," J. Eur. Opt. Soc.-Rapid Publ. 15(1), 8 (2019).

24. U. Demirbas, A. Sennaroglu, and F. X. Kärtner, "Temperature dependence of alexandrite effective emission cross section and small signal gain over the 25-450°C range," Opt. Mater. Express 9(8), 3352-3370 (2019).

25. M. Damzen, G. Thomas, and A. Minassian, "Diode-side-pumped alexandrite slab lasers," Opt. Express 25(10), 11622-11636 (2017).

26. D. N. Papadopoulos, F. Friebel, A. Pellegrina, et al., "High repetition rate multipass amplifiers operating in the 100-mJ range," IEEE J. Sel. Top. Quantum Electron. 21(1), 464-474 (2015).

27. A. Brignon and J. P. Huignard, "Two-wave mixing in Nd:YAG by gain saturation," Opt. Lett. 18(19), 1639-1641 (1993).

28. G. J. Crofts, R. P. M. Green, and M. J. Damzen, "Investigation of multipass geometries for efficient degenerate four-wave mixing in Nd: YAG," Opt. Lett. 17(13), 920-922 (1992).

29. G. Tawy, J. Wang, and M. J. Damzen, "Pump-induced lensing effects in diode pumped Alexandrite lasers," Opt. Express 27(24), 35865-35883 (2019).