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Experimental investigation on a Q-switched, mode-locked fiber laser based on the combination of active mode locking and passive Q switching Junsu Lee,1 Joonhoi Koo,1 You Min Chang,1 Pulak Debnath,2 Yong-Won Song,2 and Ju Han Lee1,* 1
School of Electrical and Computer Engineering, Faculty of Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 130-743, South Korea 2 Future Convergence Research Division, Korea Institute of Science and Technology, Seoul 136-791, South Korea *Corresponding author:
[email protected] Received January 26, 2012; revised March 22, 2012; accepted March 22, 2012; posted March 22, 2012 (Doc. ID 162116); published June 1, 2012 We performed an experimental investigation on a Q-switched, mode-locked laser based on the combination of passive Q switching and active mode locking. This study was carried out using an erbium fiber ring laser incorporating an active mode locker employing a fast Si-based variable optical attenuator and a passive Q switch based on a carbon nanotube saturable absorber. It was found that stable Q-switched, mode-locked pulses can be readily generated when using a particular cavity modulation index and pump power. It was also found that our scheme can easily alter the operating regime among the Q switching, Q-switched mode locking, and mode locking by simply controlling the cavity modulation index and/or the pump power. For this particular laser configuration, the cavity modulation index and pump power operating conditions have been experimentally analyzed for three laser operating regimes. © 2012 Optical Society of America OCIS codes: 140.3510, 140.3540, 140.4050.
1. INTRODUCTION Pulsed lasers have been a hot topic, garnering a great deal of technical attention due to their wide use in a variety of applications, such as high-speed optical communications [1], biomedical imaging [2], and material processing [3]. In general, pulsed lasers are classified into two categories: Q-switched lasers and mode-locked lasers. Q-switched lasers are based on the modulation of the optical cavity’s quality factor (Q factor) [4], [5]. During pumping, the Q factor is lowered to induce a population inversion buildup without oscillation, and then it is suddenly changed back to its original high value. This triggers an extremely fast oscillation buildup and results in the emission of a bunch of photons for a short temporal duration. Q switching is known to generate high energy pulses; however, the pulse repetition rate is limited to only tens of kilohertz due to the limited photon lifetime within the cavity. Mode-locked lasers are based on the locking of the relative phases of the multiple lasing modes [6], [7]. By modulating the loss (or gain) of the laser at 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 can be accomplished through two basic schemes: passive and active. Mode locking is known to produce ultrashort pulses at high repetition rates. In addition to the traditional and conventional pulsed lasers classes, a new class, called “Q-switched, mode-locked lasers,” has been developed and tested experimentally. Q-switched, mode-locked lasers are based, as their name implies, on the 0740-3224/12/061479-07$15.00/0
combination of both Q-switching and mode-locking techniques in order to generate bursts of ultrashort pulses. The technique is known to produce higher peak power pulses compared to conventional mode locking. Q-switched mode locking is usually accomplished by superimposing a Q-switched envelope on top of continuous mode-locked pulses. The simplest scheme used for the generation of Q-switched, mode-locked pulses is based on the combination of a passive mode locker and an active Q switch, wherein the passive mode locker produces mode-locked short pulses and an active Q-switch adds a temporal envelope to the pulses [8–10]. Pure passive schemes based on a single saturable absorber have also been successfully demonstrated [11]. Recently, pure active schemes have been presented as an advanced technique; these were based on either the use of a single acousto-optic modulator that plays the double role of mode locking and Q switching [12] or on subharmonic cavity modulation [13]. Despite the recent intense technical and commercial interest in fiber lasers, a relatively small number of investigations have been done on Q-switched, mode-locked fiber lasers. In this paper we present an investigation regarding another Q-switched, mode-locked fiber laser scheme that is based on the combined use of passive Q switching and active mode locking. Unlike conventional Q-switched, mode-locked lasers, our scheme uses an active mode locker and a passive Q switch, employing a fast Si-based variable optical attenuator (VOA) and a carbon nanotube (CNT)-based saturable absorber, respectively. Using this scheme, it is demonstrated that stable Q-switched, mode-locked pulses can be readily © 2012 Optical Society of America
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generated from an erbium-doped fiber (EDF) ring laser. It is also shown that the operating regime change is readily attainable for Q switching, Q-switched mode locking, and mode locking simply by controlling the cavity modulation index or the pump power. As a matter of fact, there have been several experimental reports on Q-switched, mode-locked lasers based on the combined use of active mode locking and passive Q switching without in-depth investigation [14] [15]. A detailed experimental investigation on the scheme thus needs to be carried out in an appropriate way for a better understanding of the laser scheme.
2. EXPERIMENTAL SETUP The schematic of our Q-switched, mode-locked EDF laser is shown in Fig. 1(a). The fiber laser was constructed using a
simple ring cavity, in which a 2.3 m EDF with a peak absorption of 20 dB∕m at 1530 nm was used for the active medium. The EDF was pumped by a 980 nm pump laser diode via a 980∕1550 nm wavelength division multiplexer. An optical isolator was placed after the EDF within the cavity in order to ensure the oscillation of the directional light. A polarization controller was inserted into the cavity. The oscillated beam 0.8 0.7
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Fig. 3. (Color online) (a) Oscilloscope trace of the optical beam modulated by a sinusoidal electrical signal with a peak-to-peak voltage of 2 V at our fundamental mode-locking frequency of 12.75 MHz through the Si-based VOA. (b) Modulation index variation as a function of the peak-to-peak voltage of the applied electrical signal.
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centrifugal force to obtain only the homogeneous suspension. During the spray, the solvent in the suspension droplets needs to be evaporated as soon as they hit the substrate surface in order to minimize the stress and deformation of the CNTs. The D-shaped fiber was prepared by polishing one side of a standard single-mode fiber (SMF) while the SMF was fixed onto a V-grooved quartz block. Its insertion loss was measured to be
was then coupled into a CNT-based saturable absorber, which was prepared by spraying CNTs onto the flat face of a D-shaped fiber. Figure 1(b) shows the schematic of the CNT-based saturable absorber; the SEM image of the CNTs deposited onto the flat face of the D-shaped fiber is shown in the inset. The CNTs were dispersed in dimethylformamide without any significant agglomeration and classified by the
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(b) Fig. 5. (a) Oscilloscope traces of the laser output for the four different cavity modulation index values of 0.6%, 1%, 3.5%, and 5.5%. (b) Their corresponding electrical spectra.
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3. EXPERIMENTAL RESULTS We measured the variation of the temporal characteristics of the output pulses as the cavity modulation index was increased under a fixed pump power of 42 mW. Figure 5(a) shows the oscilloscope traces of the laser output for the four different cavity modulation index values of 0.6%, 1%, 3.5%, and 5.5% and their corresponding, measured electrical spectra are shown in Fig. 5(b). Interestingly, the operating regime of the laser was observed to change from Q-switching to Q-switched mode-locking, and then to mode-locking as we increased the cavity modulation index. The laser operated in the Q-switching regime at a repetition rate of ∼25 KHz at a cavity modulation index of 0.6%. The Q-switching was induced by the CNT-based
saturable absorber [20], [21]. When the cavity modulation index reached ∼1%, short mode-locked pulses appeared on top of the Q-switched pulses. At a cavity modulation index of 3.5%, high-quality Q-switched, mode-locked pulses were clearly observed. The Q-switched envelope then disappeared, and the laser operated in the pure mode-locking regime at a cavity modulation index of 5.5%. The operating regime change of the laser was also confirmed through the measurement of the electrical spectrum of the output pulses. As shown in Fig. 5(b), the frequency peak occurred at a mode-locking frequency of 12.75 MHz along with the many side frequency components of the Q-switched envelope at a frequency offset of 25 kHz in the case of the 1% and 3.5% cavity modulation indices. However, only the single frequency peak of the mode-locked pulses is shown at a cavity modulation index of 5.5%. Figure 6(a) shows a magnified view of the Q-switched, mode-locked pulse at the cavity modulation index of 3.5%. It is clearly evident from the figure that a Q-switched envelope appeared on top of the mode-locked pulses. In order to measure the temporal width of the mode-locked pulses, we performed an autocorrelation measurement, as shown in Fig. 6(b). The pulse width was estimated to be ∼11.63 ps after a theoretical fitting. The nonnegligible background level is attributable to the modulated ASE [13], which needs to be suppressed by spectral filtering.
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∼1.5 dB. Despite the randomized spray of the CNTs onto the D-shaped fiber, the saturable absorber exhibited nonnegligible polarization sensitivity due to the actual alignment of the CNTs along the polished surface of the fiber in the axial direction. Further details on the CNT-based saturable absorber used in this particular experiment are fully described in [16]. We measured the small-signal transmission and saturation power for the saturable absorber [17]. For this measurement, a 1.5 ps mode-locked fiber laser at an operating wavelength of 1562 nm was used. We monitored the output power as we increased the input power of the pulses, which were coupled into the CNT-based saturable absorber. The results are shown in Fig. 2. The saturation power was estimated to be ∼3 W. The small-signal transmission and nonsaturable insertion loss were ∼21% and ∼71%, respectively. The modulation depth was thus estimated to be ∼8%. An active mode locker based on a fiberized ultrafast Si-based VOA was inserted between the CNT-based saturable absorber and the output coupler. 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. The total length of the ring cavity was ∼16.21 m, which corresponds to a round-trip time of ∼78.37 ns. The frequency of the driving electrical signal coupled into the Si-VOA was therefore set at ∼12.75 MHz in order to ensure the fundamental mode-locking condition. The Si-based VOA was commercially available; it was fabricated using a silicon p-i-n diode structure built on a silicon optical waveguide. Its rise and fall times were measured to be ∼410 and ∼179 ns, respectively; these hundreds of nanosecond transient times were fast enough to weakly modulate an optical beam within the cavity at frequencies of tens of MHz [18]. Figure 3(a) shows an oscilloscope trace of the optical beam modulated by a sinusoidal electrical signal with a peak-to-peak voltage of 2 V at our fundamental mode-locking frequency of 12.75 MHz through the Si-based VOA; Fig. 3(b) shows the measured modulation index variation as a function of the peak-to-peak voltage of the applied electrical signal. The maximum modulation index was only ∼18% due to the limited transient times. Further details regarding this device are fully described in [19]. 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. Figure 4 shows the measured output spectrum at a pump power of 42 mW. The temporal characteristics of the laser output were monitored using a 16 GHz real-time oscilloscope operating at a sampling rate of 100 GS∕s and an autocorrelator.
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Next, we observed the variation of the temporal characteristics of the output pulses as we increased the pump power under a fixed cavity modulation index of 5%. Figure 7 shows the oscilloscope traces of the laser output for the four different pump power values of 15, 26, 42, and 52 mW. Interestingly, the operating regime of the laser was observed to suddenly change from Q-switched mode locking to mode locking as the pump power exceeded 46 mW. It is well known that there exists a critical energy density level in a passive mode-locking cavity using a saturable absorber, at which level the operating regime change between Q-switched mode locking and mode locking occurs [11,22]. Under the operating condition of a cavity modulation index of 5%, the pump power for satisfying the critical energy density level was found to be ∼46 mW. Figure 8(a) shows the measured temporal width and repetition rate of the Q-switching envelope versus the applied
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pump power. The temporal width of the Q-switched envelope was observed to decrease with the increase of the pump power and then turn into an increased mode at a pump power of 32 mW. The minimum temporal width was found to be ∼5.2 μs. The repetition rate of the Q-switching envelope was observed to monotonically increase as the pump power increased. It is well known that the repetition rate of passively Q-switched fiber lasers increases as the pump power is enlarged, whereas the pulse width decreases [23]. Figure 8(b) shows the average output optical power of the laser as a function of the applied pump power. The slope efficiency was estimated to be ∼1%. Figure 9 illustrates the operating regime distribution of the laser as a function of the pump power and cavity modulation index. Using the laser configuration shown in Fig. 1(a), a pump power of at least 16 mW was found to be necessary to induce the Q-switching effect through the CNT-based saturable absorber. In regards to the pump power, a cavity modulation index of larger than 2% was enough to produce Q-switched, modelocked pulses. The Q-switched, mode-locked pulses became pure fundamental mode-locked pulses when the pump power
or the cavity modulation index exceeded a certain critical level. For instance, a cavity modulation index of only 4% was large enough to generate pure mode-locked pulses under a pump power of 60 mW. The laser stability was checked by observing the temporal characteristic variation of the output pulses as a function of elapsed time for the three operating regimes. Figure 10 shows measured oscilloscope traces and average optical powers of the laser output recorded every 10 min for 1 h for three operating regimes at a fixed pump power of 42 mW. The stable operation of the laser at each operating regime is clearly evident despite a slight optical power drift due to environmental temperature change.
4. CONCLUSION We carried out an experimental investigation on a Q-switched mode-locking scheme based on the combination of passive Q switching and active mode locking. It was shown that stable Q-switched, mode-locked pulses could be readily generated under particular conditions of cavity modulation index and
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pump power. It was also shown that the other laser operating regimes, Q switching and mode locking, could also be realized by controlling the cavity modulation index or pump power. From these results, it can be concluded that the Q-switched mode-locking scheme based on the combination of passive Q-switching and active mode locking can be useful for the implementation of pulsed fiber lasers that require an operating regime change among Q-switching, Q-switched mode locking, and mode locking.
ACKNOWLEDGMENTS This work was supported by a National Research Foundation (NRF) of Korea grant funded by the Ministry of Education, Science, and Technology (MEST), Republic of Korea (no. 2011-0028978), in which the main experiment was performed by using equipment acquired by the State-of-the-Art Facility Supporting Program of the University of Seoul 2011.
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