Presented herein is the use of an ultrafast Si-based variable optical attenuator (VOA) as a Q-switch for rare earth-doped fiber lasers. The ultrafast VOA is based on a forward-biased p-i-n diode integrated with a ridge waveguide, which was originally designed and optimized for WDM channel power equalization in optical communication systems. By incorporating a Si-based VOA with a transient time of ~410 ns into an erbium-doped fiber-based Fabry-Perot cavity it has been shown that stable Q-switched pulses possessing a temporal width of less than ~86 ns can be readily obtained at a repetition rate of up to ~1 MHz. The laser’s peak power of ~38 W is shown to be obtainable at 20 kHz with a slope efficiency of ~21%.
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Laser Q-switching is a well-known technique used for the generation of high energy pulses that have nanosecond-order temporal widths. In particular, Q-switched rare-earth-doped fiber lasers have recently enjoyed a great deal of technical and industrial interest, since compact fiber-integrated lasers can be easily incorporated into a variety of material processing applications . Q-switching can be classified into passive and active techniques, depending on use of active modulation of the laser cavity Q-factor. In the case of passive Q-switching, a saturable absorber is incorporated into the cavity, which acts as a self-operating Q-switch. However, even if the passive Q-switching is simple and robust, it has a fundamental limitation in regards to the flexible control of the repetition rate and timing synchronization due to the fixed physical properties of the saturable absorbers . Active Q-switching uses an externally-modulated Q-switch, which allows for flexible repetition rate changes and timing synchronization [3, 4].
Commonly used active Q-switches are acousto-optic modulators  and electro-optic modulators . Using the electric pulse-driven modulators, the laser cavity can be switched between high-loss and low-loss regimes, resulting in the production of high quality of Q-switched pulses. Recently, a great many investigations have been performed in order to find active Q-switches alternatives that provide cost-effective and higher performance solutions more suitable for all-fiber integrated laser systems. The acoustically modulated fiber attenuator , the micro-optical waveguide on a micro actuating platform light modulator , the magnetostriction-based fiber Bragg grating (FBG) modulator , the electrostatically actuated micro-mirror , the microsphere resonator , the single crystal photo-elastic modulator , and the resonance optical pumping-based FBG modulator  are only a few examples. High-quality Q-switched pulses have been successfully achieved through the foregoing alternative switches in rare-earth-doped fiber lasers, but there is still a huge demand for the expansion of the technological basis for alternative Q-switches, in particular ones more suitable for simple and practical applications.
In this paper we present a new kind of Q-switch, an ultrafast Si-based variable optical attenuator (VOA) for the generation of nanosecond pulses in a rare earth-doped fiber laser. By incorporating the ultrafast Si-based VOA into an erbium-doped fiber (EDF) laser cavity it has been experimentally shown that Q-switched pulses with a temporal width of less than 86 ns can be readily obtained. The repetition rate has been shown to be tunable up to ~1 MHz.
It should be noted that VOAs have never been used as a Q-switch for nanosecond pulse generation in fiber lasers until now, due to their slow response time (typically larger than sub milliseconds) no matter whether they are mechanically or electrically controlled [12, 13]. However, the recent development of ultrafast VOAs with a response time of less than 1μs through Si photonics technology enables us to easily build high speed Q-switched fiber lasers without the need for any expensive and high electrical power consumption modulator type Q-switches. Compared to the commonly used acousto-optic modulators and electro-optic modulators, the Si VOA-based Q switch has several noticeable advantages, such as a low RF driving power (less than 200 mW), a low cost, and an extremely compact size .
Si-based photonic devices have recently been intensively investigated as potential low-cost and high performance building blocks for ultrahigh capacity optical communication systems and microprocessor optical interconnects, due to their proven capabilities in regards to electronic integration and mass production [15, 16]. Notably, the Si-based VOA used in this work, which is a commercially available component based on a silicon ridge waveguide integrated with a p-i-n diode, was originally designed and optimized for the power equalization of WDM channels in optical communication systems [14, 17]. In this work, we demonstrate one more potential use of this compact and low-cost device by applying it to the high speed Q-switching of a fiber laser.
2. Experiment setup
The schematic of our Q-switched EDF laser is shown in Fig. 1(a) . The fiber laser was constructed using a simple Fabry-Perot type cavity, in which two mirrors are defined by a fiber Bragg grating (FBG) reflector and a cleaved facet at the fiber end (4% reflection between silica and air). A 3-m-long EDF with a peak absorption of 20 dB/m at 1530 nm (Liekki, ER20-4/125) was used for the active medium, which was pumped by a 980-nm pump laser diode (LD) via a 980/1550-nm wavelength division multiplexer (WDM). The Q-switch was made using a fiberized ultrafast Si-based VOA (Kotura, UltraVOA) inserted between the FBG and the WDM [14, 17]. The reflectivity of the FBG (JDSU, P/N FBG-QT09-355-00) used in this experimental configuration was 95.5%, as shown in Fig. 1(b). The center wavelength and spectral bandwidth (full width at half maximum) of the FBG were 1550.4 nm and 0.39 nm, respectively. The total length of the laser cavity was 10 m. The laser output from the 96% output coupler defined by the Fresnel reflection from the cleaved fiber end facet was monitored using a 1-GHz digital oscilloscope and an InGaAs photodetector with a bandwidth of 150 MHz.
The key component in this Q-switched laser configuration was the ultrafast Si-based VOA, which was fabricated using a silicon p-i-n diode structure built on a silicon optical waveguide. The operating principle of this device is as follows: When a forward-biased current is applied to the p-i-n diode section, the highly concentrated free carriers absorb the photons propagating along the waveguide. This means that one can readily control the amount of photon absorption by adjusting the magnitude of the injected current. The response time is known to be less than 1 μs. This device was originally designed and optimized for optical communication systems applications, such as optical transient suppression, blocking, and analog signal modulation. The ultrafast Si-based VOA used in this experiment is commercially available. Further details on this device are fully described in  and .
3. Characterization of ultrafast Si-based VOA
In order to determine the electro-optic properties of the ultrafast Si-based VOA, we carried out a series of characterization measurements. First, we measured the optical attenuation of the device as a function of the applied forward-biased current, as shown in Fig. 2(a) . The maximum attenuation was found to be ~35 dB at a current of 65 mA, at the forward bias voltage of 5 V. Even at a current of 20 mA, a 15-dB attenuation was possible. We then measured the transient response of the device in order to determine the maximum switching speed by modulating the current with a negative square electric pulse possessing rise and fall times of ~2 ns. The measured temporal shapes of the applied electrical pulse and the corresponding optical pulse from the VOA are shown in Fig. 2(b). The variable VOA was driven at a peak current of 50 mA to provide a dynamic attenuation of ~28.6 dB. The rise and fall times, which were defined as the temporal duration between 10 and 90% of the maximum transmittance, were found to be ~410 and ~179 ns, respectively. This indicates potential for repetition rate scaling beyond 1 MHz. The insertion loss of the fiberized variable VOA was measured to be ~1.7 dB. The polarization dependent loss was measured to be less than 0.5 dB.
4. Q-switched laser output operating at 20 kHz
At first the laser was operated at a repetition rate of 20 kHz by modulating the 50 mA driving current, at a 4 V forward bias. This means that the VOA driving power was less than 200 mW. Considering the rise and fall times we decided to open the switch for a duration of 700 ns and close it for the rest of the period. In other words, the temporal duration of the applied electrical pulse was fixed to be 700 ns, resulting in a duty ratio of 1.4% at a repetition rate of 20 kHz. Figure 3(a) shows the measured oscilloscope trace of the pulse trains emitted from the laser; a close-up view is shown in Fig. 3(b). The measured pulse spacing and pulse width were measured to be 50 μs and 58 ns, respectively. Figure 4(a) shows the measured output spectrum at a pump power of 45 mW. Figure 4(b) shows the measured average output power as a function of the pump power. The laser center wavelength was ~1550.4 nm; the spectral width was observed to be ~0.35 nm. The laser threshold was ~8 mW; the maximum output average power of ~37 mW was obtained at a pump power of 160 mW. The slope efficiency was ~21%. From the average power value the maximum peak power level was estimated to be ~32W. At the pump power level of 160 mW we did not observe any damage of the VOA. According to the specification of the VOA it maximum handling optical power was ~50 mW in the continuous wave mode.
5. Operating frequency tuning of the Q-switched laser
Next, we attempted to determine the maximum operating frequency of the Q-switched laser. In order to do this we increased the modulation frequency of the driving current supplied to the variable VOA from 20 kHz to 1 MHz. The pump power and driving current were fixed at 45 mW and 50 mA, respectively. The applied electrical pulse duration was also maintained at a value of 700 ns and therefore the duty ratio was decreased accordingly. Figure 5(a) shows the measured pulse peak power variation with the increasing repetition rate; Fig. 5(b) shows the corresponding pulse width change. The output pulse width was observed to increase with the increased repetition rate from ~58 ns to ~86 ns; the output pulse peak power decrease was inversely proportional to the repetition rate.
Figure 6(a) presents the measured oscilloscope trace of the laser output at a repetition rate of 1 MHz; a close-up view is shown in Fig. 6(b). The high quality of the Q-switched pulses is clearly evident at a repetition rate of 1 MHz from these figures. As an interesting side-experiment we attempted to further increase the repetition rate faster than 1 MHz, but found that the generated pulse quality was not as stable as that found at 1 MHz. Figure 7 illustrates the measured oscilloscope traces of the applied electrical pulse, the VOA response, and the output Q-switched pulse at repetition rates of 1.5 and 2 MHz, respectively. The duty ratio of the applied electrical pulse was set to be 50% considering the pulse period to obtain appropriate gain building-up and photon emission times. It is obvious from the figure that the VOA response had a triangular temporal shape rather than a rectangular shape due to the limited rise and fall times. Consequently, the output pulses became unstable at 1.5 MHz even if the pulse width was maintained at ~86 ns. Then, significant temporal distortion of the output was observed at 2 MHz as shown in Fig. 7(b). To further confirm the reason of pulse degradation we carried out the same measurements at various duty ratios of the applied electrical pulse, but always observed the same sort of distortion at repetition rates larger than 1 MHz.
We have demonstrated that an ultrafast Si-based VOA can readily be used as a high speed and low cost Q-switch for the generation of nanosecond pulses in a rare earth-doped fiber laser. It was shown that stable Q-switched pulses with a temporal width of less than 86 ns could be readily obtained using the variable VOA with a repetition rate of up to 1 MHz. We believe that such ultrahigh speed Si-based, fiberized VOAs can be used as a highly promising alternative Q-switch for the practical implementation of high speed Q-switched fiber lasers. Further research on the enhancement of the optical power handling capacity of the devices is essentially required for their successful application to high power fiber lasers.
This research work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (MEST), Republic of Korea (No. 20110003525). This work was partly supported by the Project funded by MKE/KEIT (No. 10037371) “Ultra-Precision Green Processing Technology Using Femtosecond Fiber Laser.”
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