Full length articleShort pulse generation in active Q-switched Yb-doped all fiber laser and its amplification
Introduction
In recent times, fiber lasers have been a field of intense research and development due to its diverse path breaking applications in industries and medicine owing to high beam quality, ease of delivery and high power feasibility. Pulsed Yb-doped fiber lasers with high energy and high peak power are inevitable in the fields of material processing [1], remote sensing, nonlinear frequency conversion [2], super continuum generation [3], [4] and spectroscopy due to their low quantum defect and large upper state lifetime. These vivid applications require high power lasers and one of the approach is through master oscillator power amplifier (MOPA) configuration. For the nanosecond pulse generation and its amplification through MOPA based systems, seed laser diodes (SLD) are the preferred choice [5], [6], [7], but seed diode lasers are vulnerable due to damages from back reflection of the amplified signal, heating and electronic disruption. In this regard, Q-switched fiber lasers as oscillators are rugged and compatible with all-fiber amplifier configuration. All-fiber Q-switched Yb-doped fiber lasers have been recently used as an oscillator for generation of high energy pulses [8], [9]. Although acousto-optic (AO) Q-switched fiber lasers are preferable for the generation of high energy pulses, the achievable minimum pulse duration is limited by the cavity round-trip time. As a rule of thumb, minimum pulse width achieved is always greater than or equal to the cavity round-trip time of the fiber laser in the conventional AO Q-switching process [10].There are a few reports on Er-doped fiber laser in which pulses shorter than the cavity round-trip time were generated [11], [12]. In the reported work, symmetric experimental set-up was designed such that AOM was kept at the center and gain fiber was spliced at both of its ends. Secondary pulses were generated, which were then suppressed by applying special modulation signal to AOM, which is a complex scheme [11]. In Yb-doped fiber lasers, shorter pulses of a few nanosecond duration were also generated irrespective of length of the cavity by Stimulated Brillouin Scattering (SBS) based Q-switching [13], [14], but the pulses generated using SBS Q-switching have large temporal and amplitude variations making it rather unsuitable for various applications.
A lot of work has also been reported on the study of AO Q-switched pulse shapes, multiple pulsing, multi-peak pulses and mode-locked resembling pulses [15] in the normal Q-switching regime in which the Q-switched pulses appear during the modulation signal ON-time of the AOM. In the normal Q-switching regime with AOM, the pulse duration depends on the cavity life time, pump power, and repetition rate of the modulation signal.
In this paper to the best of our knowledge, we report for the first time, generation of sub-cavity round-trip time pulses appearing after the modulation window ON-time of AOM (anomalous pulses). The studies of generation of anomalous pulses have been performed in four different experimental configurations depending upon the position of AOM and pumping configuration. In two similar setups with higher cavity loss in which the AOM is placed near the 100% FBG, the pulse duration of anomalous pulses were in the range of 50–55 ns. For other two setups with lower cavity loss obtained by placing AOM near the 10% output coupler FBG, the pulse durations were in the range from 80 to 85 ns. The anomalous pulses were more than ∼24 times shorter than the normal pulses obtained with conventional Q-switching at the same value of the pump power. Maximum average power of 12 mW with ∼55 ns pulse duration were obtained in one of the experimental setups. These anomalous pulses were amplified in the pre-amplifier to reach the average power level of more than 3.4 W. Since the amplitude of amplified spontaneous emission (ASE) was large at 3.4 W of the pre-amplifier output power, pre-amplifier output power was set at the level of ∼500 mW and amplified in the power amplifier stage to achieve ∼19 W of average output power at 80 kHz of repetition rate with slope efficiency of 29% at an input pump power of 60 W.
Section snippets
Experimental details
The schematics of four all-fiber Yb-doped Q-switched fiber laser (QYFL) setups are shown in Fig. 1(a)–(d). Generations of short pulses have been studied in four different configurations with respect to the position of the AOM, Fiber Bragg grating (FBG) mirrors, pump direction and YDF. Yb-doped double-clad fiber (YDF) of length ∼9 m with product number SM-YDF-5/130-VIII, having a core/cladding diameter of 5/125 μm and clad-pump absorption of 1.7 dB/m at 975 nm acts as the gain medium. This
Results and discussion
This section provide details of experimental results and discussion about the results. It contains three subsections, which provide results of anomalous pulse generation in different cavity configurations, delay of anomalous pulses and amplification of anomalous pulses.
A. Generation of anomalous pulses in different cavity configuration of Yb-doped fiber laser
In normal mode of Q-switching operation with AOM operated in the frequency range of 20–80 kHz, the pulse duration obtained is in the range
Conclusion
In summary, stable sub-cavity round-trip time pulses are generated in all-fiber Ytterbium-doped Q-switched fiber laser by controlling the modulation window ON-time of acousto-optic modulator (AOM). In different experimental configurations short pulses of duration in the range of 50–80 ns have been generated in cavity with round-trip time of ∼130 ns when modulation window ON-time of AOM was equal to or shorter than the pulse build-up time of the normal Q-switched pulses. The anomalous pulses
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