Novel active Q-switched fiber laser based on electrostatically actuated micro-mirror system

In this paper, active Q-switching of a double clad codoped erbium-Ytterbium fiber laser using a deformable metallic micro-mirror system is demonstrated. The electrostatically actuated micro-mirror acts both as the end laser cavity reflector and as switching/modulator element. When actuated, its shape changes from planar to a concave curvature, allowing control of the Q-factor of the laser cavity. The mirror/switching element is small, compact, highly reflective and achromatic, with a great integration potential. The laser system operates at frequencies between 20 and 200 kHz and generates short pulses (FWHM down to 300 ns) and high peak powers. ©2006 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.3540) Lasers, Q-switched; (230.4110) Modulators. References and links 1. A. Neukermans, and R. Ramaswami, “MEMS technology for optical networking applications,” IEEE Commun. Mag. 39, 62669 (2001). 2. X. M. Zhang, A. Q. Liu, D. Y. Tang and C. Lu, “Discrete wavelength tunable laser using microelectromechanical systems technology,” Appl. Phys. Lett., 84, 329-321 (2004). 3. Y.-A. Peter, H. P. Herzig, E. Rochat, R. Dändliker, C. Marxer and N. F. de Rooij, “Pulsed fiber laser using micro-electro-mechanical mirrors,” Opt. Eng., 38, 636-640 (1999). 4. W. Barnes, ‘‘Q-switched fiber lasers,’’ in Rare Earth Doped Fiber Lasers and Amplifiers, M. Digonnet, ed., (Marcel Dekker, New York, 1993) pp. 375-391. 5. D D. Zalvidea, N. A. Russo, R. Duchowicz , M. Delgado-Pinar, A. Dýez, J. L. Cruz, and M. V. Andres, “High-repetition rate acoustic-induced Q-switched all-fiber laser,” Opt. Commun. 244, 315-319 (2005). 6. A. F. El-Sherif and T. A. King, “High energy, high brightness Q-switched Tm3+-doped fibre laser using an electro-optic modulator,” Opt. Commun. 218, 337-344 (2003). 7. N. A. Russo, R. Duchowicz, J. Mora, J. L. Cruz and M. V. Andrés, Opt. Commun. 210, 361-366 (2002). 8. E. Berland, P. Blondy, P. Tristant, A. Catherinot, C. Champeaux, and D. Cros, “Dielectric materials in MEMS switches: a comparison between BST and Al2O3,” 4th Workshop on MEMS for Millimeterwave Communications, Toulouse, France (2003). 9. V. P. Jaecklin, C. Linder, N. de Rooij, J.-M. Moret, and R. Vuilleumier, ‘‘Line-addressable torsional micromirrors for light modulator arrays,’’ Sens. Actuators A 41–42, pp 324–329 (1994). 10. P. Dupriez, A. Piper, A. Malinowski, J. K. Sahu, M. Ibsen, Y. Jeong, L. M. B. Hickey, M. N. Zervas, J. Nilsson, and D. J. Richardson, “321 W average power, 1 GHz, 20 ps, 1060 nm pulsed fiber MOPA source” CLEO/IQEC 2005, paper PDP3, (2005)


Introduction
Since a few years, the association of micro-optics and micro-electro-mechanical systems (MEMS) creates a new and relatively broader area of devices, the micro-opto-electromechanical systems (MOEMS).Their properties (compact, scalable, low insertion losses, low cross-talk, polarization insensitive) along with their emerging functionalities as optical switches, micro-scanners, variable attenuators/shutters find extensive applications in telecommunications, astrophysics, biology, imaging etc. [1].Along with optical switches, tunable or high-power micro-lasers are key components in future optical networks.Within this framework MOEMS technology has been shown very promising for fabrication of miniaturized tunable [2] or pulsed fiber lasers [3] with adding merits of compactness, high speed and batch, low-cost production.
In order to generate a pulse in a conventional Q-switched fiber laser, a passive or active modulator (saturable absorbers, or acousto-optic, electro-optic or mechanical components) has to be introduced into the cavity [4].Although the conventional solutions for laser Q-switch generation are based on solid, mature technologies, most of them present inherent disadvantages that restrain their integration in miniature, compact laser systems: degradation of the beam quality, high insertion losses for the acousto-optic modulators [5], high voltages and low modulation frequencies for the electro-optics solutions [6], bulkiness for mechanical choppers, low laser power levels operation for piezoelectric Bragg gratings systems [7] or lack of control of frequency and pulse width for the passive modulators.
In this paper, we describes a fiber laser system combined with an electrostatic-actuated micro-mirror, which acts as the end cavity reflector of the lasers and as switching/modulator element.The generation of short, high-power pulses with variable repetition rate is reported.The advantages of such an element reside on its low fabrication cost and its high potential of integration in a compact micro-system.

Electrostatically actuated mirror
The fabricated micro-mirror consists of a 500 nm-thick thermal evaporated gold membrane (bridge-type) suspended over an actuation electrode placed 2.2 μm underneath the membrane and covered with a dielectric thin film (Al 2 O 3 , 200 nm thickness).The bridge is sustained at its extremities by thick metallic anchorages (made of plating gold) in order to increase its stiffness.The fabrication steps follow the general MEMS manufacture technology and are similar with those described in [8]. Figure 1 shows a SEM picture of such a mirror having dimensions of 240 x 160 μm 2 , which is the largest element of an array (partly showed in Fig. 2) containing 30 membranes with different dimensions (the smallest ones having an area of 120 x 60 μm 2 ).
When applying a pulsed modulated voltage between the membrane and the electrode, the bridge is simply moving down and up (piston-like) by combining action of the electrostatic force (between the bottom electrode and the bridge) and that of the mechanical restoring force  When actuated, the membranes act like a mirror with variable curvature, passing from planar (in the non-actuated, off state) to a curved-concave shape (in the actuated, on state).Depending on the membrane dimensions, the maximum radius of curvature may vary from 1.45 mm for the 160-μm long bridges to 4.45 mm for the longest ones (240-μm long).In the off state the membrane reflects back into the cavity the incident radiation (high Q factor cavity) while in the on-state the incident beam is deflected under an angle of ~9° outside the cavity (low Q-factor laser cavity).When the Q-factor modulation became fast enough the laser system pass in the Q-switching regime (generation of narrow, high-power pulses).Figure 3 shows a device used to measure the reflectivity variation between the off state and the on state of the mirror.This device is composed by an erbium-doped fiber laser coupled to the input port (port 1) of a circulator.The beam coming from the port 2 is then reflected by the deformable mirror.A photodiode is placed at the output of the port 3 and measures the power variations when the micro-mirror is periodically actuated.For the membrane of 80×140 µm dimensions, according to the mirror state, power fluctuation up to 45% has been measured.

Pulsed fiber laser system
Our experimental device, shown in Fig. 4, consists of a codoped Erbium-Ytterbium (Er/Yb) fiber amplifier which was cladding-side-pumped, through a multimodal pump coupler, by a laser diode emitting a power up to 5W at 920nm with a core diameter of 100µm.The Er/Ybfiber had length of ~20m corresponding to good absorption.The output coupler of the laser had a 4% reflectivity by cleaving the fiber extremity.The end of the laser was performed by placing an imaging system based on an afocal system between the other fiber amplifier end (angle cleaved) and the optical MEMS.In such configuration, the laser can be exploited to perform Q-switching.We were able to produce high-power train pulses with repetition rate which can be continuously tuned from 20 kHz to 200 kHz.For 80x 140 μm 2 area membranes the duration (FWHM) of the generated pulses was close to 300 ns for low actuation frequencies (around 30 kHz) and increased up to 800ns for higher frequencies (200 kHz).Figure 3 presents typical pulse trains for two different actuation frequencies.The average output power of the laser was from 500mW to 900mW according to the desired frequency of the pulse train, which represents pulses with peak power of several watts and pulse energy up to 25µJ.For actuation frequencies below 20 kHz the modulation is too slow to obtain Q-switching: the laser produces multiple pulses corresponding to mechanical relaxation oscillations of the mirror [9].The simplicity of this Q-switch generation technique makes it suitable for being implemented in more complex set-ups including solid-state micro-lasers, multi-wavelength fiber lasers or different families of fiber lasers (Yb-or Er-Yb-doped) actuated independently or synchronous for wavelength mixing/ tuning applications.
The main limitation of these metallic deformable mirrors is the optical losses due to the non total reflection on the gold membrane.This non reflected power induces thermal effects which can lead to the destruction of the micro element.Actually, for average power higher than 1W, a few seconds of the focused beam on the membrane is enough for its destruction.A first solution could be to increase the useful surface of the membrane.Unfortunately this increase of transverse dimension reduces the actuating time of the MOEMS.A second solution, for power scaling, could be the use of this kind of Q-switched laser as pilot laser in MOPA configuration [10].
Taking into account of the small dimensions of the Q-switch element, it could be very interesting to avoid the use of the imaging system for a more compact system.In that way, we have just positioned the cleaved output fact of the fiber in front of the gold membrane as showed in Fig. 6.As a result, in this configuration, the too small beam deflection on the membrane leads to a low discrimination between the on and off states of the micro-mirror.As a result, in spite of the difficulties to set up the fiber on the membrane without its destruction, pulses are very difficult to obtain because of high reinsertion losses.Actually, only pulses with duration of 1,4 µs for a repetition rate of 29KHz were generated (Fig. 7).

Conclusions
In summary, we reported a simple, suitable technique to produce active Q-switching in a fiber laser system.Our contribution represents a first approach for developing miniature, highpower lasers integrated with deformable micro-mirrors.The switching element is based on an electrostatic-actuated micro-mirror that is coupled with a fiber laser.The optical element present very good mechanical, electrical and optical performances (high reflectivity, achromatic, polarization insensitive) and is fabricated using a low-cost, batch, and simple, standard fabrication process.The repetition rate of the active Q-switching system can be tuned between 20 kHz and 200 kHz, with a pulse width down to 300 ns.The switch could be integrated with various types of laser amplifiers running at different wavelengths.Optimization of the laser Q-switching using such micro-mirrors will enable immediate development of applications like laser wavelength mixing or multi-laser emission synchronization.

Fig. 1 .
Fig. 1.Scanning electron micrograph (SEM) of a typical metallic micro-membrane (C) 2006 OSA 1 May 2006 / Vol.14, No. 9 / OPTICS EXPRESS 3918 given by the bridge stiffness.The dielectric assures an electrical isolation between the membrane and the landing electrode during the actuation.The parameters for the membrane fabrication (thickness, deposition rate, component dimensions, distance to the landing electrode) were optimized to obtain high-stiffness bridges (measured spring constants between 20 and 70 N/m, depending on the dimensions) with low switching times (1-3 μs) and low actuation voltages (15-35 V).The mirrors have low roughness (~ 2 nm rms as recorded by AFM) and reproducible mechanical and electrical behavior.Most of them were actuated for more than 1 billion cycles with a bi-polar waveform voltage without any sign of mechanical degradation or switching failure.

Fig. 3 .
Fig. 3. Measure of the reflectivity variation of the MOEMS between the in and off states

Fig. 4 .
Fig. 4. Set-up of the Q-switched fiber laser using a MOEMS as modulator element.

Fig. 6 .-Fig. 7 .
Fig. 6.Set-up of the Q-switched fiber laser using a MOEMS without imaging system