By constructing an external circulating cavity to provide sufficient delay that equals a multiple of pulse repetition time, a method with a fixed experimental configuration is proposed to measure the coherence length of both single-frequency and microwave-modulated optical pulses. This method can accurately determine the number of coherent pulses as well as distinguish the coherence states: complete coherence, partial coherence and complete incoherence. In addition, all desired coherence phenomena are obtained by one-time measurement, avoiding other operations like frequent fiber-cutting or devices-reconnection in previous methods. Simulation results show that the coherence length of the dual-frequency laser is periodically extended by the reciprocal of the frequency difference, and the random jitter of pulse propagation time would result in obvious measurement errors via perturbing coherence status.
Taking all the pulse instantaneous power into consideration, we investigate the pulse interference dynamics of dissipative soliton (DS) and dissipative soliton resonance (DSR) through a discrete model based on nonlinear optical loop mirror (NOLM). We numerically and experimentally acquire that the reflection and transmission spectra of the NOLM under DS and DSR regimes show significant wavelength selectivity which results from the intensity-dependent nonlinearity. The wavelength selectivity phenomenon may be helpful for in-depth understanding on different operation dynamics of saturable absorber in passive mode-locked lasers.
We numerically present the generation of four types of pulses using nonlinear Schrödinger equations in an all-normal-dispersion dual-pump mode-locked fiber laser based on a nonlinear amplifying loop mirror (NALM). Transition state (TS) between dissipative soliton (DS) and dissipative soliton resonance (DSR), typical DSR, DS, noise-like pulse (NLP), and nonmode-locked state are investigated in a two-dimensional pump power space. Different from previous research, we first find that the pulse peak power and width vary dependently and nonmonotonously under asymmetric coupling ratios in the NALM. Under different coupler ratios and amplifier powers, the shift and distortion of simulated NALM transmittivity curve which are caused by the pump power intercoupling and the amplifier saturation are demonstrated. The ratio of incident power and the saturable power of NALM (when the ratio is >2, 1 ∼2, ∼1, <1) directly determines the pulse states (NLP, DSR, unstable, DS, or TS) in the cavity. Overall, the utilization of NALM could effectively enlarge the saturable power as well as the transmittivity, which provide a road to increase the DSR peak power.
We numerically present the generation of five types of pulse states using nonlinear Schrödinger equations in an all-normal-dispersion (ANDi) mode-locked fiber laser based on a nonlinear amplifying loop mirror (NALM). Using a two-dimensional pump power space of the two amplifiers, we investigate the pulse characteristics among typical dissipative soliton resonance (DSR), dissipative soliton (DS), transition state (TS) between DS and DSR, noise-like pulse (NLP) and unstable pulsation state. Different from previous research results, we firstly find that asymmetric coupling ratio of the NALM causes dependent and non- monotonous variation of the pulse peak power and width of DSR. The location of incident power and the saturable power of NALM (when the former is 2 times higher, higher, equal, lower than the later) determine the pulse states (NLP, DSR, unstable, DS or TS) in the cavity. The pump-power intercoupling-induced frequency-selective NALM transmission is demonstrated by the simulated pulse evolution over round-trips.
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