We present a novel Q-switched laser source using a micro-optical-electromechanical mirror (MOEM) designed for short pulse emission. It is based on a hybrid configuration including a passively Q-switched microchip laser coupled to a fiber cavity closed by a cantilever type MOEM acting as an active modulator. This specially designed mirror with a single reflecting gold membrane is switched by low bias voltage ~50 V (peak to peak). This device emits pulses at tunable repetition rates up to 1.6 kHz, with ~564 ps duration and 3.4 kW peak power, which constitutes the shortest pulse duration ever reported with MOEMs based pulsed lasers.
©2012 Optical Society of America
Laser pulsed emission in the nanoscale range of pulse durations with tunable repetition rates and low jitter is required for a large number of applications such as accurate range finding, imaging or flow cytometry. Subnanosecond pulse generation is commonly achieved by Q-switching laser cavities. Among the existing Q-switching techniques, those based on the use of a micro-opto-electromechanical mirror (MOEM or optical MEM) coupled with compact laser sources offer several attractive features. Besides their low fabrication cost and high potential of integration, these electro-optic type elements ensure large reflectivity, polarization insensitivity and low chromaticity on a large spectral range due to their metallic surface. Furthermore, they can be switched on and off by only few tens of volts and their actuation frequency is easily tunable from few hertz up to several hundred of kilohertz thanks to their low transverse dimensions. Unfortunately the switching time of such devices remains low (> 1 µs) which forbids pulse generation in the picoseconds timescale by means of Q-switch operation.
The first pulsed laser source involving a MOEM as end mirror and active modulator was reported in 1999 by Peter et al . The authors used a Nd3+ doped fiber as gain medium and 2 µs pulses at a repetition rate of 16 kHz were obtained. Since this first demonstration of a MOEM-based Q-switched laser system, many studies have been carried out in order to shorten the pulse duration. With an Er-doped fiber laser ended by an electrostatically deformable MEM, Crunteanu et al. obtained 300 ns long laser pulses at repetition rates from 20 kHz to 120 kHz . Later, the same authors decreased the pulse duration down to 100 ns and then to 8 ns at repetition rates from 50 kHz to 800 kHz respectively, in a Q-switched Yb-doped fiber laser using a cantilever-type MOEM [3,4]. However, it seems difficult to produce significantly shorter pulses, namely in the subnanosecond and picosecond domains, with fiber lasers Q-switched by MOEM because of the long length of the laser cavity and because of the low switching time of the MOEM devices. These pulsed regimes could be reached using mode locking techniques but they require ultrafast modulators in the megahertz range which seems difficult to achieve with optical MEMs. Even if recently, MOEM components driven on superior harmonic frequencies were employed to achieve mode-lock operation in a fiber laser system, the pulse duration of such a laser system was limited to around 1 nanosecond . On the other hand, subnanosecond pulses can be produced by microchip laser sources due to their very short cavity length but, as these sources are based on passive Q-switching techniques, they are not suitable for easily tuning the repetition rate [6–8]. At last, active Q-switching could be achieved by implementing a MOEM in a microcavity, but this does not allow fast enough operation as obtained when using passive saturable absorber with subnanosecond response [9–11]. Thus microlaser sources integrating MOEM devices seem to be not suitable for picoseconds pulse generation.
In this paper, we propose a novel pulsed laser source based on a dual cavity configuration, combining a MOEM element with a fibered microchip laser. This laser configuration was designed to take advantage of both passive and active Q-switching techniques, in order to emit high peak power subnasecond pulses at tunable repetition rates. In the following, we first describe the coupled cavity, then the MOEMs devices are depicted, and finally, our experimental results are reported and discussed.
2. Laser configuration
The laser source is based on an asymmetric structure including two coupled cavities as shown in the Fig. 1 . The first cavity (cavity 1) is typically that of a microchip laser. It is composed of a 3 mm long Nd:YAG crystal as a gain medium affixed against a prismatic Cr3+:YAG crystal used as saturable absorber. The coating on the input face of the Nd:YAG crystal is anti-reflecting at the pump wavelength (808 nm) and highly reflecting at the laser operating wavelength (1064 nm). The output coupler of the laser is the opposite face of the saturable absorber with a reflection coefficient of 60% at 1064 nm. The second cavity (cavity 2) is composed of 1 m Corning HI980 fiber (cutoff wavelength of the second mode: λc = 930 ± 50 nm; mode field radius of the fundamental mode: w0 = 4.2 ± 0.3 µm) followed by a spherical lens (f = 4 mm) set to collimate the 1064 nm radiation. A tunable output coupler, consisting in an assembly made of a rotating half-wave plate and a Glan-Thomson polarizer, is used to adjust the output beam power. Light coupling from the microchip laser into the fiber is realized by a simple butt joint technique. No antireflection coatings are required for the fiber endfaces or for the components of the output coupler.
The cavity 2 is closed by a deformable end mirror able to switch the reflection coefficient of light into the cavity from low to high value, and conversely. The pump power at 808 nm is adjusted to remain below the threshold of the microchip laser in order to avoid laser emission when the reflection coefficient is low. When this reflection coefficient is switched to high value, light at 1064 nm is reflected into the cavity 2 towards cavity 1. This turns in abruptly increasing the effective reflectivity of the output mirror of the microchip laser and a pulse is emitted at 1064 nm .
3. MOEM design
As deformable end mirror, we designed and fabricated a cantilever-type MOEM consisting in a single suspended metallic membrane anchored on one side on a Si substrate covered by thermally- grown silicon oxide (SiO2). Typical scanning electron microscopy (SEM) images of the obtained device are presented in Fig. 2a , together with its 3D profiles recorded by optical interferometry (Fig. 2b). The curved micro-mirror is made of metallic stacks of Au/Cr/Au layers with dissimilar types of built-in stress . The main structural material of the beam is a 1 µm thick gold layer. The thickness of the high tensile stress Cr-layer (120 to 140 nm thick) imposes the final curved shape and the elevation of the beam while the upper thin Au-layer (~100 nm thick) allows a high reflectivity (~80% at 1064 nm) to the incident laser beam. The dimensions of this MOEM device are 100 x 200 μm2.
On Fig. 3 are reported two images showing distinct shapes of the membrane, corresponding to the upper, un-actuated position (Off state, Fig. 3a) and of the completely actuated one (On state, Fig. 3b). The longitudinal profiles corresponding to both these shapes are shown on Fig. 3c. While applying suitable actuation voltage between the Si substrate (lower electrode) and the metallic micro-mirror and anchorages (upper electrodes), the induced electrostatic forces attract the suspended membrane towards the substrate (ON state in Fig. 3). Thus, the MOEM acts as a plan mirror and it reflects light into the optical fiber (high reflection coefficient leading to pulse building and emission). When the applied voltage is decreased below the threshold actuation voltage, the membrane recovers its curved initial shape (OFF state in Fig. 3). In this case light is reflected out of the fiber (low reflection coefficient in the cavity, preventing from laser emission). Actuation signals are bi-polar sinusoidal or square-shape waveforms, in order to avoid charging effects likely to result in device failure. Their amplitude is 50 V peak to peak and frequencies can range from few kHz up to several tens of kHz. The switching time of the MOEM, depending on the actuation voltage, is about 1.5 µs.
4. Experimental results
When pumped at 808 nm by a CW diode laser in free run regime, the 6 mm long Q-switched microchip laser emits subnasecond pulses with 565 ps duration, at a repetition rate depending on the pump power, from few Hz to 10 kHz. This microchip laser was inserted in the arrangement shown in Fig. 1 and the MOEM depicted in section 3 was set and aligned in order to behave as the end mirror of the cavity 2. This setup was pumped with the 808 nm CW diode laser, emitting a 800 mW mean power, below the microchip laser threshold. The reflection coefficient of the MOEM in the cavity was zero for the OFF-state and was evaluated to be close to 80% for the ON-state. In these conditions, the pulse train shown Fig. 4a was detected by a fast 12 GHz photodiode and was displayed on a Tektronix CSA 8000 oscilloscope (Fig. 4b). The output pulse duration was ~564 ps, i.e. equal to that of pulses emitted by the microchip laser in free running. As predicted, it only depends on the microchip laser cavity length, independently from the cavity 2 length and from the switching velocity of the MOEM, whereas the repetition rate is entirely governed by the MOEM actuation frequency. Unfortunately, above 1.67 kHz, the actuation of the MOEM became erratic because of the peak power impact on the gold layer which was progressively damaged and then destroyed. Then no stable working of the device for higher repetition rates was obtained. The maximum measured mean power was 3.24 mW, corresponding to a pulse energy of 1.94 µJ for 3.4 kW of peak power. This pulse duration is the shortest ever reported in a laser Q-switched by a MOEM.
Beyond these preliminary results, the atypical working of the coupled cavities, each one including a Q-switching device, seems promising to reach shorter pulses with higher repetition rates. To this end, ultrashort microchip laser sources with few micrometers of thickness can be used . Additionally, MOEMs with larger reflecting area can be designed to decrease local power density and to prevent any irreversible damage of the MOEM .
In conclusion, we have developed a novel active-passive Q-switched laser based on a dual cavity including a passively Q-switched microchip laser with a MOEM as an end mirror and active switch. The combination of both switching techniques allows to take advantages of microchip laser in terms of pulse duration, and also those of actively switched lasers in terms of repetition rate. This laser source is able to generate short pulses with a ~564 ps duration at 1.67 kHz repetition rate and 3.4 kW of peak power. To our knowledge, it is the shortest pulse duration obtained to date with a laser using a MOEM. Shorter pulses with higher repetition rates should be obtained by modifying the microchip constitution and the MOEM design.
The authors are grateful to the French ANRT (National Association for Technical Research) for its financial support to F. El Bassri via a CIFRE convention with CILAS company.
References and links
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