Actively Q-switched Fiber Laser with Smooth Pulse Based on Coupler Ring
Objective Utilizing actively Q-switched fiber lasers to generate high-energy narrow-linewidth nanosecond laser pulses is a mainstream technology.Various types of active Q-switches have been developed based on different principles.Among them,fiber-pigtailed acousto-optic modulators(AOMs)and electro-optic modulators(EOMs)have garnered wide attention owing to their nanosecond-level switching time and high extinction ratios spanning tens of decibels.However,the output of Q-switched fiber lasers based on fast switches,such as AOMs and EOMs,typically exhibits a typical multipeak structure.Researchers have proposed various methods to address this issue;however,they present certain limitations.Therefore,obtaining high-power narrow-linewidth lasers with smooth pulses from actively Q-switched fiber lasers is crucial.In this study,a novel Q-switch is designed based on the linear electro-optic effect of a β-BaB2O4(BBO)crystal with a high damage threshold.Moreover,to eliminate the multipeak phenomenon caused by the short rise time of the Q-switch,a fiber coupler is used to form a coupling ring(CR),which enables a gradual release of the initial signal energy and successfully realizes a smooth pulse output.Methods To construct an intensity modulator suitable for high-power lasers,a pair of single-mode fiber collimators(COLs)is used for spatial optical-path coupling.A high-damage-threshold BBO uniaxial crystal is placed between orthogonal polarization devices.The intensity of the transmitted laser can be modulated by applying a periodic half-wave voltage to the BBO crystal.Additionally,to obtain a smooth Q-switched pulse output,a smooth and long-rise waveform is required.The longer the circulation time within the fiber CR,the longer the rise time.However,this implies that the precision of the input pulse segmentation decreases,thereby reducing the smoothness of the rise-time waveform.Therefore,a fiber CR comprising optical coupler is inserted,with the ring length set to 0.5 m.Results and Discussions First,the performance of the constructed intensity modulator is tested(Fig.2).The measured rise time is approximately 8 ns,with an insertion loss and extinction ratio of approximately 2 dB and 20 dB,respectively,thus satisfying the requirements of a high-gain double-cladding fiber laser for Q-switching.As the pump power increases[Fig.3(a)],the repetition frequency of the pulse sequence gradually transitions from 1/4 to 1/3 and 1/2 of the modulation frequency,before eventually exhibiting a regular pulse sequence.This occurs because,after the laser pulse depletes inverted ions during cavity circulation,the gain generated in the doped fiber during the subsequent Q-switch closed period is insufficient to establish a pulse within the limited open time.By gradually increasing the pump power,the gain within a single cycle increases accordingly.When increased to a certain value,the accumulated gain within a single cycle is sufficient to support pulse establishment during the subsequent Q-switch open time,thus resulting in a pulse-sequence repetition frequency that matches the modulation frequency.The pulse-waveform details before and after the insertion of the fiber CR are shown in Fig.3(b).After the Q-switch is activated,the waveform exhibits the typical multipeak characteristics before the insertion of the CR.After inserting the fiber CR,the multipeak structure on the pulse completely disappears and the temporal waveform becomes extremely smooth.The principle by which the fiber CR smooths the temporal waveform of the pulse is explained as follows:Considering the evolution process of a rectangular pulse passing through the fiber CR(Fig.4),the analysis results summarized based on Equations(1)and(2)indicate that the original rectangular pulse,after transmission through the CR,evolves into a series of equally spaced delayed subpulses with different amplitudes,thus effectively altering the Q-switch rise time.Consequently,the temporal multipeak structure is completely eliminated,thus resulting in a smooth laser pulse.Additionally,the average power increases almost linearly with the pump power without any saturation effect[Fig.5(a)].As the damage threshold of the fiber devices and the pump power cannot be further increased to compress the pulse width,the duty cycle should be reduced to increase the gain.The width of the output pulse can be further compressed by reducing the duty cycle to increase the gain recovery time.When the duty cycle is reduced from 50％to 3％,the pulse width is compressed to 35 ns,and the pulse is extremely smooth with no multipeak structures.The laser output linewidth narrows to 0.08 nm with a signal-to-noise ratio of 60 dB[Fig.5(b)].Conclusions In this study,a highly doped large-mode-area double-cladding fiber is used as the gain medium.Simultaneously,a novel intensity modulator based on the linear electro-optic effect of a high-damage-threshold BBO crystal is designed as a Q-switch.Furthermore,to eliminate multipeaks in the output pulse caused by the rapid opening of the Q-switch,a fiber CR is introduced to allow the initial signal energy to be released gradually,thereby resulting in smooth laser pulses.The principles of pulse temporal evolution are analyzed comprehensively.The laser yields a smooth pulse output with an average power exceeding 1 W,a linewidth as narrow as 0.08 nm,and a pulse width of 35 ns.
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