We present an Ytterbium fibre laser operating in the Q-switch regime by using a Micro- Opto- Electro- Mechanical System (MOEMS) of novel design. The cantilever-type micro-mirror is designed to generate short laser pulses with duration between 20 ns and 100 ns at repetition rates ranging from a few kilohertz up to 800 kHz. The bent profile of this new type of MOEMS ensures a high modulation rate of the laser cavity losses while keeping a high actuating frequency.
©2008 Optical Society of America
Lasers operating in the Q-Switch regime generate giant pulses by switching the Q factor of the cavity either with an active or a passive modulator. Numerous Q-switching techniques are utilized nowadays showing different advantages and drawbacks. If passive processing like saturable absorption leads to the emission of short laser pulses about nanosecond , the repetition rate is not controlled and only varies with the pumping level of the laser medium. On the contrary, the active Q-switched lasers deliver pulses with fixed repetition rate but their duration and recurrence depend on the modulator specificities. The bulky mechanical choppers  are not adapted to compact fibre laser systems and restricted to low repetition rates due to their low switching time (about 100 ms). Although they have a short switching time (<10 ns), the electro-optic elements are driven by high voltages  and the time needed to store the energy limits the modulation frequency. The acousto-optics modulators are widely used to Q-switch fibre lasers because of their interesting switching time and modulation frequency, but they have high insertion losses . Recently, an all-fibre active Q-switching has been achieved by using a magnetostrictive element to stretch and relax a fibre Bragg grating. The shortest pulses of about 180 ns were obtained at 80 kHz .
The optical MEMS devices are a new and elegant solution to the problems mentioned above [6,7]. As the gap between both electrodes of such a MOEMS device continuously growths from 1 µm to some tens of microns, they only need a few tens of volts to be actuated. Furthermore, their metallic surface ensures the achromaticity of these components. Optics can also take advantage of their low cost, batch fabrication and compactness since they can perform a dual function of modulator and end-cavity mirror. Moreover, their actuation frequency can be continuously adjustable and, due to their small dimensions, the laser system has a good integration potential. As a result, some laser sources integrating such devices have been already developed. The first micro-mirrors used to Q-switch fibre lasers were realized under the form of a tip-tilt actuated rigid plate with a maximum light deviation angle of 2.6° , leading to a poor discrimination between the on and off commutation states. Long pulse duration around 2 µs for repetition rates between 1 kHz and 30 kHz have been achieved. More recently, an erbium/ytterbium co-doped fibre associated with a bridge-type micro-mirror generated shorter pulses with duration between 300 ns and 800 ns, at higher frequencies from 30 kHz up to 200 kHz . In this case, the MOEMS is made of a fine 500-nm thick gold membrane, anchored on two opposite sides, suspended at 2.2 µm above a bottom actuation electrode covered with a dielectric layer (200-nm thick) for isolation during actuation. Although the size of the membrane may be as large as a few hundred micrometers square, the useful area of such a MOEMS is located on a very small part (some micrometers square) of the membrane close to the anchorage, where the angular discrimination between both actuating states is maximal. Such components require an imaging system to focus the intracavity beam onto the MOEMS, decreasing the interest of its small size.
Other configurations of light modulation based on MEMS technology have been investigated. For example, Y. Joeng et al. obtained repetition rates in the range of 20 kHz (for 2 µs pulses) by creating an axial stress over the waist of a fused biconical taper coupler .
Giant-pulse duration is mainly related to the cavity round trip, but also to the switching time of the modulator and to the intracavity loss difference between its both actuating states . The modulation frequency (and thus, the switching time) strongly depends on the dimensions and on the geometrical profile of the MOEMS. The laser repetition rate is limited by the optical pump level and by the switching time of the modulator.
In this paper, we present the results of an ytterbium-doped fibre laser Q-switched by a new type of MOEMS devices. They have very small dimensions and new engineered geometrical profiles (up-warded cantilever-type membranes) which allow them to be faster while keeping a high modulation rate.
2. Cantilever-type MOEMS devices
2.1. Micro-mirrors description
We previously used bridge-type devices  where only a small area of the membrane (near the membrane anchorage) was effectively useful for light redirection. Thus, we developed a new type of devices in order to improve intracavity loss difference between its both actuating states.
The new up-curved, cantilever type profile (Figs. 1 and 2) increases considerably the useful area, which could avoid the insertion of an imaging system. Moreover, this may reduce the light power density on the component surface and substantially decrease the possibility of damage.
The cantilever-type devices are metallic membranes (~1-µm thick) anchored on one side on the substrate (Si, covered by a 1-µm thick thermally-grown oxide SiO2). Their up-curved profile is obtained by using a stack of metals (Au/Cr/Au) with dissimilar types of built-in stress (tensile for chromium or compressive for gold). The thicker (~900 nm) lower gold layer with an almost zero build-in stress is used as the structural material of the membrane. The highly tensile stressed Cr layer provides the final curvature of the device and the upper thin Au layer (~100-nm thick) ensures a high reflectivity at the laser wavelength of about 80%. The thickness of the Cr layer (from 10 to 15 nm) stacked between the two gold coatings allows a precise, controlled, up-deflection of the membrane. The fabrication process is similar with that presented in . The dimensions of the fabricated micro-mirrors are between 50×50 µm2 and 150×350 µm2. Their mechanical resonant pulsation (which can be approximated in a first time as the fundamental pulsation of the membrane) is related to their dimensions with the following expression (Eq. (1)):
where h and L are the thickness and the length of the membrane respectively, E the Young’s modulus of the material and ρ is the volume density. It has to be noted that ω is independent of the membrane width. Typically, a cantilever-type MOEMS of 1.5-µm thick and 125-µm long has a mechanical resonant frequency of 260 kHz.
The deflection angle (α on Fig. 1.) of an incident beam normal to the substrate varies from 9 degrees to 15 degrees for length between 150 to 300 µm. This angle is deducted of interferometric measurements (Fig. 3).
When applying a typical actuation voltage of 20 to 40 V between the Si substrate (lower electrode) and the metallic membrane (upper electrode), the induced electrostatic force attracts the membrane down to the substrate (Fig. 1(a)). The electrical isolation is provided by the SiO2 layer which prevents shorting of the electrical actuation circuit. The membrane recovers its initial position when removing the actuation voltage (Fig 1(b)). The fabricated components are easily controllable from 1 Hz to about 200 kHz. As mentioned above, this maximum actuation frequency is limited by the primary mechanical resonant frequency of the cantilever which mainly depends on its length and stiffness. These components have been actuated for more than one billion cycles.
2.2. Optical characterization
We analysed the dynamical behaviour in reflectivity of the cantilever-type MOEMS using the setup shown in Fig. 4. A probe beam at 1550 nm was launched in a 50/50 fibre coupler (1). At one output port (3), the beam was imaged on the micro-mirror thanks to the optical system that will be used in the final fibre laser architecture. We measured the temporal evolution of the reflected beam power at the second input port of the coupler (2).
The probe beam diameter on the micro-mirror was 8 µm, when the mobile membrane of the mirror was 100-µm long and 50-µm large. The optical transmission of this system (fibre, imaging system and MOEMS) was studied at different positions of the probe beam along the MOEMS, according to the applied command voltage.
On Fig. 5 we observe the profiles of the bipolar actuation voltage applied to the MOEMS (in black) and the reflected optical signal (in red) at three different positions of the incident focused beam along the micro-mirror length. These graphs show that the reflectivity of the system previously described is highly modified by the actuation of the MOEMS. When a voltage is applied (negative or positive), the mobile membrane of the mirror stands close to the substrate (down state) and the reflected power increases because the MOEMS acts as a standard mirror utilized at normal incidence, reflecting the beam in the numerical aperture of the fibre (green rays in Fig. 4). When the voltage returns to zero, the membrane comes back to its initial up-state; the small angle of the micro-mirror with the substrate redirects the incident beam out of the fibre numerical aperture (red rays in Fig. 4). In that case, the measured reflecting power decreases followed by some modulations due to the dumping oscillations of the membrane around its resting position. As shown on Fig. 5(a), these oscillations are not optically perceptible by the system of characterization when the probe beam is focused at the mobile extremity of the MOEMS: the membrane achieves its maximal angle with the substrate (α) and the reflecting beam (deflected with an angle of 2α) cannot be launched into the numerical aperture of the fibre (e.g. 0.14 for a standard single mode fibre) whatever the oscillation amplitude around the resting position. In the following section, the MOEMS will be illuminated only on its mobile extremity to avoid any optically sensitive parasitic oscillations.
3. Experimental results
The fibre laser experimental setup including the cantilever type MOEMS is depicted Fig. 7.
The laser system consists of a 15-cm long ytterbium doped fibre (with a high rare-earth concentration of 15600 ppm weight, Liekki Yb1288-4/125), core-pumped by a 980 nm wavelength diode (400 mW maximum power) through a multiplexer (MUX) and a Fibre Bragg Grating (FBG) with a reflectivity of 15% at 1030 nm, which is the natural lasing wavelength of this short doped fibre. The maximum launched pump power is about 300 mW. Because of the short fibre length, the absorbed pump power is only of 170 mW. The laser cavity is closed on one side by the FBG and on the other side by the micro-mirror. A couple of lenses (4.5 mm and 8 mm focal lengths) images the angle-cleaved end of the doped fibre on the MOEMS. The laser beam diameter is 8 µm on the micro-mirror.
When electrically actuated, the micro-mirror switches the quality factor of the cavity generating short pulses of about 20 ns for repetition rates up to 200 kHz (Figs. 8(a) and 8(b)) with an average output power of about 80 mW. This output power is low, mainly because of the short length of the doped fibre. This is also due to the losses in the free space part of the cavity (Fresnel reflections on the optical components and partial reflection on the MOEMS). The obtained laser pulses are, to the best of our knowledge, at least 20 times shorter than any other results reported so far for fibre laser systems using optical MEMS devices. As observed in Fig. 8, the laser pulse shape is modulated with a period of 3 ns corresponding to one cavity round trip time (the laser cavity is about 31 cm long). Although the primary mechanical resonant frequency of the cantilevers is close to 200 kHz, we succeeded to obtain Q-switched laser pulses at even much higher frequencies. Indeed, the laser system can generate a pulse train at different repetition rates, continuously from some kilohertz up to 200 kHz, but it can also be operated at 400 kHz and 800 kHz which are harmonic frequencies of the primary mechanical resonant frequency of the micro-mirror. We obtained pulses of 30 ns and 100 ns durations at 400 kHz and 800 kHz respectively (Fig. 8(c) and 8(d)).
We also Q-switched the laser without using the imaging system by bringing the MOEMS as close as possible to the fibre end. We observed similar performances in terms of pulse duration and repetition rate except for the average optical power which was 10% lower. This power decrease is due to the small size (4 µm) of the fibre mode and its corresponding small Rayleigh length (12 µm) compared to the distance between the MOEMS and the fibre end (50 µm); the large size of the beam back from the micro-mirror cannot match perfectly the fibre mode, leading to higher insertion loss.
We developed a cantilever-type micro-mirror in order to generate short Q-switch pulses in a fibre laser configuration. Due to the high modulation rate provided by the micro-mirrors and to the short cavity, the laser emission consists in very short pulses with duration of about 20 ns at repetition rates up to 200 kHz. Longer pulses up to 100 ns were also obtained at higher harmonic frequencies as high as 800 kHz. To our knowledge, these results are the best reported so far for a fibre laser actively Q-switched by using a deformable micro-mirror. Further modifications of the MOEMS will, most certainly, shorten the pulse duration and increase the average output power. For example, a change of the structural material of the membrane will decrease the switching time of the MOEMS. Moreover, a larger guided mode chosen, so that its Rayleigh length is greater than the distance between the fibre end and the MOEMS, will avoid the use of the imaging system. It will reduce the insertion losses of the free space part of the setup while shortening the laser cavity.
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