1. Introduction

Spectral beam combining or incoherent combining of fiber lasers is seen as a promising technique for laser power scaling and brightness enhancement [1]. On the other hand, multi-wavelength and Q-switched erbium-doped fiber lasers are active research areas for their potential applications in telecommunication network systems, remote sensing, laser processing range finding, or biology and medicine. The pulsed regime in such laser systems is obtained thanks to an active or a passive modulator placed in the laser cavity. In order to control the repetition rate, active modulators are preferred to passive modulators. Indeed, passive Q-switching suffers from timing jitter and the repetition rate can only be managed by changing the pumping level in the laser [2,3]. In the case of active Q-switching, the time jitter is low (depending on the type of modulator) and the repetition rate of the laser is directly managed by the active modulator operating frequency. Numerous active Q-switching techniques have been developed, the simplest ones being bulky mechanical choppers [4] which are not suitable for compact fiber laser systems and restricted to low repetition rates (few hundred of kilohertz). Most often, electro-optic modulators (EOM) or acousto-optic modulators (AOM) are used for active Q-switching. However, in addition to their expensive cost, they suffer from high insertion losses in the case of AOM [5] and from high voltage electronics command needs for EOM [6].

Recently we proposed the use of micro-electromechanical (MEMS) devices as modulators for short pulse generation in Q-switched fiber lasers [7,8]. They present a lot of benefits compared to other active modulators. Indeed, their small size ensures low actuation voltages (less than 100 V) and high operation frequencies. Moreover, thanks to their metallic surface they are achromatic and can play a dual function of modulator and end-cavity mirror. Finally, good repeatability and low cost are ensured by batch fabrication processes. By using these new type of micro-mirror modulators, modulation frequencies from Q-switch regime to mode-lock regime have been reached in fiber lasers (laser pulse duration down to ~1 ns and repetition rates up to ~5 MHz) [9]. The increase of the output repetition rate of a fiber laser will result in increasing the speed, the efficiency and the quality of a practical application process. However, the increase of the repetition rate of an actively modulated Q-switched laser results in a deterioration of the characteristics of the laser emission [8,10,11]. The pulse energy and peak power decrease with the increase of the modulation frequency. Also, for high repetition rates, the laser pulse duration increases. All these drawbacks are due to the shortening of the population inversion period. The laser pulse degradation (pulse duration, energy) can be slightly compensated by increasing the pumping level but parasitic laser oscillations may appear because of high intracavity gain. A suitable solution to increase the repetition rate or pulse energy of Q-switched laser sources without pulse characteristics degradation is the spectral combining of beams from different laser sources operating at slightly different wavelengths [12,13]. For example, Schmidt et al. combined laser beams of 25 ns-long pulses and 1.8 mJ energy from four individual fiber lasers whose wavelengths are separated by 3 nm [14]. The overall output of the combined beam reached 6.3 mJ at 10 kHz. The Q-switch operation is assumed by four acousto-optics modulators (one for each individual laser to combine) and the combining element is made up of three interference filters. The overall set-up including the AOM command electronics becomes in this case relatively complex and bulky. In this paper, we present a simple and compact setup allowing individual fiber lasers combining. It consists of individual fibers lasers (independently Q-switched by MEMS micro-mirrors) spectrally combined and temporally synchronized for obtaining an overall output with either high energy emission or with higher pulse repetition rate.

2. MEMS devices

The MEMS device presented here contains eight pairs of micro-mirrors having similar characteristics to those previously used to produce short pulses in Q-switched fiber lasers [8]. These cantilever-type structures are very efficient for Q-switching modulation thanks to the high intracavity loss difference they are inducing in the laser cavity when driving between their extreme actuating states (up- and down-actuation). Their principle of operation within a laser cavity is described on Fig. 1 and detailed in Ref. [15].

 

Fig. 1 Design and principle of operation of MEMS-type modulators for Q-switching operation.

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These mirrors are suspended metallic gold membranes (Fig. 2 ) of 75x50 µm² (1 µm-thick), anchored on one side on the Si-substrate (covered by a 1 µm-thick thermally-grown SiO₂ oxide). Their up-curved profile is obtained by using a stack of metals (Au/Cr/Au) with different types of built-in stress. The fabrication process is similar with that presented in Ref 16. The actuation voltage of the devices presented here is about 25 V.

 

Fig. 2 Scanning electron micrograph image of four pairs of fabricated micro-mirrors.

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Figure 3 shows the design of the entire MOEMS device, with the 8 pairs of micro-mirrors and the actuation voltage pads. The actuation of the individual micro-mirrors is realized by applying a voltage between their corresponding actuation pad and the conductive Si substrate (used as general ground for all devices and electrically isolated by the 1 µm-thick SiO₂ layer). We use only one membrane by pair at the same time (each micro-mirror has been doubled in the design simply for avoiding to replace the entire device in the case when only one micro-mirror is damaged). As can be seen in the focused area of Fig. 3, each micro-mirror is separated from its matching neighbors by 250 µm, corresponding to the periodicity of the V-grooves holding and aligning the laser fibers to be combined, in front of their associated MEMS device.

 

Fig. 3 Schematics of the overall MEMS device used for multiple fiber lasers Q-switching.

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3. Experimental setup

The experimental setup for multiple fiber lasers combination is depicted on Fig. 4 . To demonstrate the spectral combining principle, we used four individual fiber lasers, composed of 50 cm-long erbium-doped fiber (EDF) with a core size of 9 µm. These doped fibers are spliced at one side to a Fiber Bragg Grating (FBG, R = 50%) followed by a Wavelength Division Multiplexer (WDM) used to provide the pump power (100 mW at 976 nm for each laser). Two laser diodes (L.D. on Fig. 4) are used for core-pumping of the four fiber lasers through 2x2 couplers (not represented on Fig. 4).

 

Fig. 4 (a) Experimental setup and operating principle of the overall laser system obtained by spectral combining of four Q-switched fiber lasers; (b) fiber arrangement within V-grooves for tight alignment with the MEMS micro-mirrors.

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Finally, an 8x1 WDM (only four inputs are needed here) is used as an element combining the individual fiber laser emissions and provides the overall laser system output. The individual lasing wavelengths of each of the four fiber lasers are fixed by the corresponding FBGs (λ₁ = 1539.4 nm, λ₂ = 1542.4 nm, λ₃ = 1543.3 nm and λ₄ = 1544.2 nm), their bandwidths match the output WDM’s ones. The 8x1 WDM output is angle-cleaved in order to avoid the Fresnel reflection. At the other side, the four free outputs of the erbium-doped fibers are placed in front of the MEMS micro-mirrors. The fiber ends are handled using silicon-machined V-grooves (Fig. 4(b)) having the same pitch as the distance between the micro-mirrors (250 µm). Thus, each individual fiber is placed just in front of a deformable membrane (of 75x50 µm²) and is independently Q-switched by a single micro-mirror of the MEMS device. The principle of operation of a single Q-switched laser is the same as described in Ref. [8]. Accordingly, each micro-mirror can be actuated separately for synchronous or asynchronous modulation of the corresponding fiber lasers composing the global system.

4. Results and discussion

A typical laser emission output from the four Q-switched fiber lasers combined using the above-mentioned experimental setup is presented on Fig. 5 . On Fig. 5(a) is shown the pulse train (in red) recorded at the 8x1 WDM output, containing the four pulse trains from each separate laser. In this case, the lasers are operated in an asynchronous mode through the micro-mirrors actuation (control voltage signal waveforms equally phase-shifted between the four individual lasers). The phase shift is easily and continuously tunable by managing the individual control voltages using a waveform generator providing up to eight channel signal outputs which can be either independently controlled or generated in a master-slave configuration. The pulse duration of a single laser (operating in the Q-switching regime at 20 kHz) is less than 400 ns and the pulse energy is 25 nJ. The black curve on Fig. 5(a) represents a typical bipolar voltage signal applied to one micro-mirror. The output power of each laser depends on several parameters (the length of the doped-fiber, the splicing losses …) but the most critical one is the positioning of the fiber end in front of the active micro-mirror. This explains the unbalanced pulse peak powers (pulse amplitudes on Fig. 5(a)) which is confirmed by the corresponding spectral distribution measured at the 8x1 WDM output and represented on Fig. 5(b). Thus, using spectral combining of four fibers lasers Q-switched at 20 kHz, we obtained a laser system providing pulse trains with repetition rates of 80 kHz representing an increase of the individual repetition rates by a factor 4, without significant pulse quality degradation.

 

Fig. 5 (a) Temporal multiplexing of the four Q-switched lasers; in red: the overall laser system output, in black: typical applied voltage to the MEMS micro-mirrors for Q-switching the individual lasers; (b) associated spectral distribution of the overall emission.

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Furthermore, the concept presented above can be highly relevant for applications requiring the ability to make temporal pulse selection or to choose pulse positions in the final pulse train. As an example, we tested the two-by-two synchronization of the individual fiber lasers. The pulsed emission from the lasers emitting at λ₁ and λ₂ were temporally synchronized and so the ones from the lasers emitting at λ₃ and λ₄. However, the emission from the two pairs (λ₁ + λ₂ and λ₃ + λ₄) was phase-shifted using the corresponding micro-mirror’s actuation. As a result, the final repetition rate of the whole system is increased by a factor of two (Fig. 6(a) and 6(b)) and each pulse peak power and pulse energy is also increased by a factor of two (since each overall pulse is the sum of the individual pulses from the laser pairs λ₁ + λ₂ or λ₃ + λ₄, leading to 50 nJ per pulse).

 

Fig. 6 (a) The overall laser system output using the 2x2 synchronization of the four Q-switched lasers (red curve) and typical actuation voltage of one micro-mirror (black curve), (b) associated spectral distribution of the overall emission.

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In order to demonstrate the full capability of the combining system, on Fig. 7 are presented the experimental results corresponding to laser pulse emission obtained from two other operating modes: the perfect synchronization of the pulse trains (Fig. 7(a)) and for burst-like emission (Fig. 7(b)). In the first case, the four distinct lasers are simultaneously Q-switched (no phase shift between the micro-mirrors actuation control) leading to a pulse peak power and pulse energy increase (by a factor of four). Thus, 0.1 µJ-energy overall pulses can be obtained by incoherent combining and temporal superposition of the discrete 25 nJ pulses. The second case (Fig. 7(b)) confirms the versatility of this setup and shows that it is possible to manage precisely the relative pulsed trains’ temporal positions. The precise control of the phase delay between the individual micro-mirror’s actuation waveforms allows obtaining pulse packets with monitored time delays between the emitted pulses.

 

Fig. 7 (a) Laser pulses obtained using perfect synchronization of the individual pulse trains and (b) burst-type emission with monitored time delays between the emitted pulses.

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5. Conclusion

We have demonstrated a simple laser system setup able to spectrally combine fiber lasers in order to manage their temporal overall emission. The multiple laser combination has been realized using MEMS micro-mirrors devices which play a dual function (modulator and end-cavity mirror). The overall laser system is able to provide repetition rates or pulse energies which are multiples of the corresponding values of the individual laser sources. The proof of principle demonstrated here employed four distinct fiber lasers Q-switched at 20 kHz providing 25 nJ pulses and allowed to obtain combined laser pulse trains at 80 kHz repetition rate or higher energy per pulse (0.1 µJ), depending on the driving scheme of the discrete fiber lasers. Our approach is greatly reducing the bulkiness and the cost of the combining technique compared with alternative solutions employing acousto-optic or electro-optics modulators. The pulse duration of the system presented here (less than 400 ns), determined by the individual configuration of the laser sources, could be shortened (~10 ns pulse duration has already been reached with similar micromirrors). The repetition rate and the pulse energy could be increased and scaled up likewise. The setup can be simplified by removing the FBGs which have been inserted in order to minimize power loss in the multiplexer. In this case the right-cleave of the output of the 8x1 WDM can play the role of the output coupler. The setup presented here can be easily scaled up for combining a higher number of fiber lasers (allowing an increase of both energy per pulse and frequency repetition rates), and shows a high potential for high-speed material processing applications. Furthermore, such multi-spectral lasers with pulse shape control ability can be used for implementing tunable photonic microwaves filters or for multiplexing modulated microwave subcarriers in optical communication systems [17]. Finally, the individual MEMS-based lasers within this system could also be driven in the mode-locking regime [9], providing high repetition rates and short pulses, even at very different wavelengths. These capabilities could be relevant for applications like multi-wavelength microscopy and imaging and spectroscopy.