We present a new and simple approach for the generation of Q-switched, mode-locked pulses from a laser cavity. The approach is based on cavity loss modulation that employs a subharmonic frequency of the fundamental intermode frequency spacing. A range of experiments have been carried out using an erbium-doped fiber-based ring cavity laser in order to verify that this simple approach can readily produce high quality Q-switched, mode-locked pulses. An active tuning of the Q-switched envelope repetition rate is also shown to be easily achievable by adjusting the order of the applied subharmonic frequency.
© 2011 OSA
Ultra-short pulse lasers have drawn great technical interest in recent years due to their wide use in a variety of applications, such as high speed optical communications , biomedical imaging , and material processing . The mode-locking technique is commonly used to obtain short pulses from a laser cavity. Through the locking of the relative phases of the multiple lasing modes by modulating the loss (or gain) of the laser at a frequency of an integer multiple of the fundamental intermode frequency spacing, the independent, longitudinal modes are forced into a phase coherence. The coherent multiple lasing modes then manifest themselves into a well-defined temporal pulse form. Mode-locking techniques can be classified into two categories: passive and active. Passive mode-locking uses nonlinear optical effects, such as nonlinear amplifying loop mirror , saturable absorption  and nonlinear polarization rotation , whereas active mode-locking uses external beam modulation devices to ensure the phase locking of each mode .
Recently, a great deal of attention has been paid to the generation of bursts of ultra-short pulses as opposed to continuous pulses through the use of a special technique called “Q-switched mode-locking”. Q-switched, mode-locked pulses are known to produce a higher peak power than continuous pulses, provided that the same average optical power is used. Q-switched mode-locking is usually realized by superimposing a Q-switched envelope on top of the continuous mode-locked pulses. The simplest scheme is based on the combined use of a passive mode-locker and an active Q switch, wherein the passive mode locker produces mode-locked short pulses and an acousto-optic Q-switch adds a temporal envelope to the pulses [8–10]. The use of a single acousto-optic modulator that plays the double role of mode-locking and Q-switching, has recently been presented [11,12] as an advanced approach. Furthermore, a pure passive scheme based on a single saturable absorber has also been presented [13–15]. Using this scheme it was shown that Q-switched, mode-locked pulses could be obtained under the condition of an intracavity pulse energy being less than a critical stability level.
In this paper we present a new and simple approach for the generation of Q-switched, mode-locked pulses from a laser cavity. The approach is based on cavity loss modulation at a subharmonic frequency of the fundamental intermode frequency spacing. Through a range of experiments with an erbium-doped fiber (EDF)-based ring cavity laser, it has been verified that this simple approach can readily produce high quality Q-switched, mode-locked pulses. It is shown that the active control of the Q-switched envelope repetition rate is easily achievable by adjusting the order of the applied subharmonic frequency. It is also shown that the peak optical power of the Q-switched, mode-locked pulses can reach up to a level ~14 times larger than that found for the continuous fundamental-order mode-locked pulses generated from the same cavity.
2. Experiment setup
The schematic of our Q-switched, mode-locked EDF laser is shown in Fig. 1(a) . The fiber laser was constructed by using a simple ring cavity in which a 3-m-long EDF with a peak absorption of 20 dB/m at 1530 nm was used as the gain medium. The EDF was pumped by a 980-nm pump laser diode; the pump power was rated at 50 mW. An isolator and a polarization controller (PC) were incorporated into the cavity in order to ensure a unidirectional beam oscillation and for polarization adjustment. A 0.5-nm bandpass filter was inserted into the cavity to eliminate the background amplified spontaneous emission (ASE) noise as well as to determine the lasing wavelength. The laser output was extracted from the ring cavity by a 90:10 fiber coupler, which fed 90% of the oscillated light power back into the EDF via a silicon-based variable optical attenuator (VOA). The laser output was monitored by a 16-GHz real-time digital oscilloscope (Tektronix, DSA71604C) using a sampling rate of 100 GS/s and an InGaAs photodetector with a bandwidth of 20 GHz in order to monitor the temporal shapes of both the Q-switched envelope and the mode-locked pulses. The temporal resolution of the measurement setup used in this particular experiment was ~60 ps. Figure 1(b) shows the measured optical spectrum of the laser output. The center-wavelength was measured to be 1561.42 nm, however the 3-dB bandwidth measurement was limited by the resolution bandwidth (0.02 nm) of the optical spectrum analyzer (OSA) used in this experiment.
In order to apply the loss modulation to the laser cavity an ultrafast VOA based on a silicon p-i-n diode built on a silicon optical waveguide was used . We recently demonstrated that this simple device could be readily used for the Q-switching of an erbium-doped fiber (EDF) laser . The ultrafast Si-based VOA used in this experiment is commercially available (Kotura, UltraVOA). Further details regarding this device are fully described in .
Figure 2(a) shows the measured optical attenuation of the device as a function of the applied forward-biased current. We operated the VOA by applying a sinusoidal electrical signal with a peak-to-peak current of ~50 mA. Figure 2(b) shows the oscilloscope traces of the applied sinusoidal electrical signal and their corresponding modulated optical beam at a VOA frequency of 490 kHz. The modulated optical beam exhibited a square-like waveform rather than a sinusoidal one due to its nonlinear attenuation curve, as shown in Fig. 2(a).
3. Experimental results
We first applied the loss modulation to the cavity at the fundamental intermode spacing frequency in order to ensure that the generation of stable fundamental-order, mode-locked pulses occurred. Figure 3(a) shows the oscilloscope trace of the output pulse train emitted from the fundamental-order, mode-locked laser; a magnified view is shown in Fig. 3(b). The measured pulse spacing and pulse width were measured to be ~102 ns and ~260 ps, respectively. The temporal width measurement was carried out with the high speed real-time oscilloscope rather than an autocorrelator, since the few hundred picosecond pulse width exceeded the measurement window of the autocorrelator available in our laboratory. The side lobes seen in Fig. 3(b) are attributable to the modulated background ASE, which would be substantially reduced by using a bandpass filter with a narrower a bandwidth within the cavity.
We next monitored the temporal shape variation of the output pulses caused by changing the modulation frequency from its fundamental intermode spacing to its subharmonics. Figure 4 shows the oscilloscope traces of the output pulse trains for various orders (m) of the subharmonics, wherein the fitting curves for the Q-switched envelope are also shown. The Q-switched envelope fitting curves were obtained through the use of the following well-known equation .
In order to further confirm the existence of both the Q-switching and mode-locking effect we carried out an electrical spectrum measurement of the output pulses under a subharmonic order of 20. Figure 5(a) shows the oscilloscope trace of the output pulses; Fig. 5(b) shows their electrical spectrum. The frequency peak of the mode-locked pulses at a frequency of 9.806 MHz is clearly illustrated along with the two side frequency components of the Q-switched envelope at a frequency of 490 kHz. This electrical spectrum indicates that an amplitude modulation at a frequency of 490 kHz was imposed onto the continuous 9.806 MHz mode-locked pulse train.
Finally, we measured the variation of the output pulse characteristics, such as average optical power, Q-switched envelope width, pulse width, and main pulse peak power that occurred as we increased the subharmonic order. The main pulse is defined as the pulse that possesses the highest peak power within the Q-switched envelope. The peak power was estimated by curve-fitting of both the Q-switched envelope and the mode-locked pulses from the measured averaged optical power. The results are summarized in Fig. 6 .
The average optical power of the output pulses was observed to decrease at first when the subharmonic order was enlarged, and then reached a minimum at the subharmonic order of 10. Past the subharmonic order of 10 the average power continuously increased and was observed to saturate at the subharmonic of ~35. The saturated average optical power was equivalent to that of the fundamental-order, mode-locked pulses. As mentioned above, the erbium fiber laser seemed to be in the steady-state mode until m = 10 and then turn into the transient mode. In the steady-state mode the cavity loss modulation induces output power loss. This is evident from the average power level decrease at m=5 and 10 in Fig. 6(a). On the other hand, in the transient mode the cavity loss modulation induces Q-switching phenomenon, which is associated with transient gain build-up and photon emission process. In this case, the increase of the subharmonic order leads to larger gain build-up and photon emission times, which result in higher average output power and larger Q-switched envelope width as shown in Figs. 6(a) and 6(b).
The temporal width of the output pulses were observed to decrease with an increase in the subharmonic order, whereas that of the Q-switched envelope singularly increased, as shown in Figs. 6(b) and 6(c). A minimum pulse width of ~100 ps, which was ~2.5 times narrower than that of the fundamental-order, mode-locked pulses was observed at the subharmonic order of 45. The pulse width variation as a function of the subharmonic order can be understood as optical power-dependent soliton pulse compression, since the laser cavity has anomalous dispersion at the lasing wavelength of 1561.42 nm. As shown in Fig. 6(d) the pulse peak power suddenly decreases when the subharmonic order is changed from m=1 to 5. The suddenly lowered peak power causes temporal width broadening of the solitonic mode-locked pulses at m=5. Further increase of the subharmonic order leads to higher-order soliton pulse compression, which results in the two relative minimum widths at m=20 and 45. This explanation needs to be confirmed through a theoretical work.
The estimated peak power of the main pulse was found to increase and reached up to ~1.4 W, ~14 times greater than that found for the fundamental-order, mode-locked pulses, fully meeting our expectations, as shown in Fig. 6(d).
We have demonstrated a novel method for the generation of Q-switched, mode-locked pulses from a laser cavity based on the cavity loss modulation at a subharmonic frequency of the fundamental intermode frequency spacing. It has been shown that this approach produces high quality Q-switched, mode-locked pulses by performing a range of experiments with an EDF ring cavity laser. Since an active tuning of the Q-switched envelope repetition rate is readily possible through a simple adjustment of the order of the applied subharmonic frequency, we believe that this approach can be a powerful tool in the generation of repetition rate controllable bursts of ultra-short pulses.
This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Ministry of Education, Science, and Technology (MEST), Republic of Korea (No. 2011-0028978).
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