Actively mode-locked fiber laser usinga deformable micromirror

Marc Fabert,* Vincent Kermène, Agnès Desfarges-Berthelemot, Pierre Blondy, and Aurelian CrunteanuXLIM UMR 6172, CNRS/Université de Limoges, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France

*Corresponding author: marc.fabert@xlim.fr

Received March 23, 2011; revised May 10, 2011; accepted May 10, 2011;posted May 11, 2011 (Doc. ID 144586); published June 6, 2011

We present what we believe to be the first fiber laser system that is actively mode-locked by a deformable micro-mirror. The micromirror device is placed within the laser cavity and performs a dual function of modulator andend-cavity mirror. The mode-locked laser provides ∼1-ns-long pulses with 20nJ=pulse energy at 5MHz repetitionrates. © 2011 Optical Society of AmericaOCIS codes: 060.3510, 140.4050, 230.4110, 230.4685.

Currently, micro-optoelectromechanical devices (usuallytermed MOEMS) are extensively used as optical switches[1], optical limiters [2], or deformable mirrors [3] for abroad range of applications spanning from telecommuni-cations [1,4] to biomedical imaging and microscopy [5],projection and display systems [6], adaptative optics[7,8], and laser sources [9]. Especially in this last field,MOEMS qualities of achromaticity, polarization indepen-dence, low power consumption, and a high potentialof integration associated with low-cost fabricationthanks to batch fabrication processes place these compo-nents as very good candidates to replace conventionalmodulators as electro- or acousto-optical modulators(EOMs or AOMs) for short pulse generation [3,10,11].Metallic MOEMS have been shown to be effective ele-ments to Q-switch fiber laser systems up to 800 kHzrepetition rates with pulses as short as 8ns [11]. Becauseof the constant reduction of their size and the designimprovement of the structural materials composing themicromirrors, MEMS devices achieve increasingly highresonant frequencies, with actuation rates/modulationfrequencies of several megahertz [12]. When couplingsuch a MEMS device with a laser cavity, its high resonantfrequencies can potentially induce high-speed modula-tion of the light inside the cavity, approaching the mode-locking operation regime for the laser system.A rule of thumb for mode locking a laser cavity

consists in modulating the quality factor of the cavityat a repetition rate that corresponds to the intracavityround-trip time. If this condition is fulfilled, the lasersystem generates a pulse train with a repetition rate off ¼ c=ðn:LÞ, where c is the vacuum light velocity, n isthe refractive index, and L is the laser cavity length. Ina perfect case (i.e., all the longitudinal cavity modesare phase locked), the pulse duration is inversely propor-tional to the spectral bandwidth containing the phase-locked modes. The passive mode-locking techniquesare commonly used to obtain subpicosecond pulses inlasers. The main methods employed to passively mode-lock fiber lasers are the ones that benefit from nonlineareffects in fibers or those using intracavity saturable ab-sorbers [13–15]. The main drawbacks of these passivemethods are the cw intracavity power needed for achiev-ing mode-locked lasing when using a specific nonlineardevice. Conversely, it is rather difficult to adapt thenecessary nonlinearity in a CW laser for initiating

self-starting mode locking with a given level of intracav-ity power. Active mode-locking techniques generally giverise to longer pulses, in particular for long laser cavities(a few tens of picoseconds to a few tens of nanoseconds)[16] are preferred for applications needing the synchro-nization of the laser pulse train with an external signal orwhen several lasers must operate synchronously. Theactive implementations often consist in using AOMs [16]or EOMs [17], which are bulky and expensive elements,generally designed to work at a specific wavelength.On the other hand, thanks to their optical properties(achromaticity, polarization insensitivity) and their batchfabrication process, MEMS-type modulators are of highinterest for their introduction in compact and low-costfiber laser systems.

In this Letter, we propose, for the first time to ourknowledge, to introduce and use MOEMS componentsboth as modulators and end-cavity mirrors to achievemode-lock operation in a fiber laser system. The MOEMSdevices presented here are high-speed deformablemirrors with an operating principle similar to those pre-viously used to Q-switch fiber laser cavities [3].

As shown in Fig. 1(a), they are high-reflectivity (>80%at 1550 nm) bridge-type metallic membranes (∼0:5-μm-thick Ti/Au/Ti trilayers structure) anchored on both sidesand suspended ∼1:5 μm above a low-resistivity Si sub-strate covered by a 1-μm-thick thermally grown oxide(SiO2). The bridge-type geometry (fixed–fixed beam)was preferred to cantilever-type one since, for similardimensions, they present much higher mechanical reso-nant frequencies relevant for obtaining the mode-lockingregime. The dimensions of the fabricated micromirrorsrange from 50 μm × 50 μm to 200 μm × 100 μm. Althoughthey are not protected by specific packaging, thesemicrocomponents are highly robust, with reproducibleproperties even at high frequencies. When applying atypical actuation voltage (bipolar square-type waveformwith amplitudes from 50 to 80Vpp, peak to peak voltage)between the lower electrode (Si substrate) and the upperelectrode (the supports and the metallic membrane),the induced electrostatic force attracts the membranedown to the substrate [Figs. 1(c) and 1(d)]. When theapplied voltage is below the actuation voltage of themembrane, the micromirror recovers its initial position[Figs. 1(b) and 1(d)].

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At the upper state, the membrane reflects the opticalbeam in the incident direction, whereas, at the downstate, the optical beam is deflected out of the cavity axis(given that the incident light spot on the micromirror isfocused on the edge of the membrane close to the an-chored region). The fabricated components operate froma few hertz to several hundred kilohertz on the funda-mental resonance mode of vibration (i.e., with onlyone antinode at the center of the bridge), with actuationtimes inversely proportional to the resonant frequency inthe range of 1 μs (slightly depending on the appliedvoltage). The magnitude of the primary mechanical reso-nant frequency of the micromirror beam depends on itsgeometrical dimensions and stiffness (due to the intrinsicstress developed within the structural materials duringdeposition). For mode-locking operation, the modulationfrequency of the active element should fit the free spec-tral range of the cavity (f ¼ c=ðn:LÞ), which depends onthe cavity length, L. Thus, in our experiments describedlater, for maintaining moderate length laser cavities, theMOEMS devices were actuated very far from theirfundamental mechanical resonant frequency. Curve 2in Fig. 1(d) describes the motion of the membrane at ahigh frequency of 1:45MHz, whereas the primary reso-nant frequency is of 26 kHz. During one cycle, the centerof the membrane is maintained in contact with thesubstrate. The membrane only oscillates on specific partsbetween nodes indicated by arrows on the example ofFig. 1(d). At such high frequencies, the angular variationleads to a low modification of the quality factor of thecavity in the range of percent. The experimental lasersystem setup including the fabricated MOEMS device

is presented in Fig. 2. It consists in an erbium-doped fiberamplifier (EDFA) with a maximum gain of 30 dB insertedin a ring cavity configuration. A circulator C extracts thelight towards the electromechanical modulator through a50=50 coupler and an imaging system (L1 ¼ 6:2mm andL2 ¼ 8mm added in order to carefully adjust the spot sizeat 8 μm on the micromirror). The laser emission of theoverall system is detected from one of the two freeoutputs of the 50=50 coupler.

The first time, we voluntarily designed long laser cav-ities (of 220 and 135m, respectively) in order to reducethe necessary modulation rate close to the highest mod-ulation frequency values usually used for Q-switchingoperation (in the range of 1MHz), and we used micromir-ror devices with relatively large dimensions (200 μm×100 μm). When applying an actuation voltage to themicromirrors, at frequencies synchronous with theround-trip time of radiation circulating in the fiber laser(of 950 kHz and 1:5MHz, respectively), short pulses(60 ns) with peak powers about 200 times more thanthe cw regime appear. The frequency matching is en-sured by looking at the electrical spectrum of the laserand superimposing the modulation frequency on the cav-ity round-trip time. When this condition is fulfilled, andonly in this case, we observed a pulse regime. The mod-ulation frequency of the micromirror device can beadjusted at �1kHz to maintain the emission of the pulsetrain. To reach higher modulation frequencies (compati-ble with conventional shorter cavity lengths), we usedsmaller micromirrors (overall membrane dimensions of75 μm × 50 μm) actuated at megahertz-range frequencies.In this case, for a laser cavity length of 42m, we obtainedtrains of laser pulses with durations of about 1 ns with arepetition rate of almost 5MHz (4:96MHz) for 20W oflaser peak power [Figs. 3(a) and 3(b)]. The negativevalues on the temporal profiles are due to an artifactof the measurement system. For an extracted meanpower of 100mW (gain of the EDFA: 21 dB), the energyper pulse is 20 nJ. The duration of these laser pulses canbe considered long for a mode-locked regime but suchlong laser cavities contain a lot of longitudinal modesto phase lock and can result in delivering long pulses[16,18,19]. The pulse regime appears only when the mod-ulation frequency of the MOEMS matched the cavity freespectral range, proving the mode-lock regime. The bot-tom of the spectrum peak [Fig. 3(c)] widens significantlywhen mode locking appears.

For the bridge-type membrane, a rapid analyticalcalculation of the nth harmonic vibration mode can bedone from Eq. (1), based on [20]

Fig. 1. (Color online) (a) Profile of a metallic bridge-typemicromirror. (b), (c) Phase images using a digital holographymicroscope from LyncéeTec of a 200 μm × 100 μm micro-mirror membrane at rest (unactuated) and actuated at1:45MHz. (d) Longitudinal profiles [along the dotted lines in(b) and (c)] for the membrane at rest (curve 1) and the mem-brane actuated at 1:45MHz (curve 2, temporal superpositionof 46 profiles acquired during three actuation cycles); thearrows along curve 2 indicates the position of nonvibratingnodes.

Fig. 2. Experimental setup of the fiber laser system used todemonstrate mode-locked emission with the active MOEMS ele-ment. C, circulator; A.C., angle cleaves; L, lenses. The dashedline shows where the long fiber length was inserted in order toreduce the repetition rate of the laser cavity.

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f n ¼�nπ þ π

2

�2

2πffiffiffiffiffi12

p t

L2

ffiffiffiffiEρ

s; ð1Þ

where t and L are respectively the thickness and lengthof the bridge-type membrane, E and ρ are the Youngmodulus and the density of the structural material(i.e., Au), respectively.The 4:96MHz operating point for obtaining the mode-

lock regime of the laser system is close to the sixthharmonic vibration mode, which occurs at 4:6MHz intheory. One has to note that this simple analytical model(1) does not take into account the dimensional errorscoming from component fabrication, the membrane in-built stress, or the real shape (curved shape) and conse-quently gives only an approximate value of the harmonicfrequencies. As the mode-lock regime is reached, wesuppose that 4:96MHz is close to the sixth harmonic fre-quency of the MOEMS. However, it is to be mentionedthat, when operating the component on these high har-monic frequencies, the incident laser beam position onthe membrane must be adjusted much more preciselythan in the case of fundamental mode vibration in orderto find an appropriate operating point on the membranesurface (providing the necessary modulation contrast formode-locking operation).In conclusion, we have demonstrated that the pre-

sented MOEMS devices offer a significant and regular

modulation depth, even well above their primary reso-nant frequency and, thus, they can be used as a novel ac-tive element to mode lock a fiber laser. The resultspresented above constitute, to the best of our knowledge,the first demonstration of MOEMS implementation forhigh-modulation efficiency of fiber lasers at the mega-hertz frequency range.

Shorter mode-locked laser cavities can be obtained byadapting the size, shape, or structural materials design ofthe MOEMS devices, while pulse duration can be consid-erably reduced by managing the cavity net chromatic dis-persion and improving the modulation efficiency of themicromirrors. Mode-locked fiber laser applications inmaterial processing, medicine, or telecommunicationscould largely benefit from these new active solutionsfor short pulse generation thanks to the improvementin terms of manufacturing costs and compactness.

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Fig. 3. (a) Pulse shape and (b) pulse train of mode-lockedemission at 4.96 MHz; (c) optical spectrum from the ring lasercavity in mode-lock regime.

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