Abstract
1 Introduction
In recent years, low-repetition-rate ultrafast fiber lasers have found applications in a diverse range of areas, such as free-space optical communications, time–frequency transfer, biomedical surgery and unambiguous long-distance ranging[
Alternatively, simply extending the fiber length can also decrease the pulse repetition rate[
In low-repetition-rate laser oscillators assisted by lengthening the intracavity fiber with a large normal dispersion, the dissipative soliton (DS) has been extensively investigated. For example, Liu
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In this work, we solve these issues in low-repetition-rate ultrafast fiber lasers by using a phase-biased NALM and a selected filter to generate pulses at a 1.84-MHz repetition rate with suppressed ASE and Kelly sidebands. A numerical simulation is first carried out to guide the experiment on pulse build-up and Kelly sideband suppression. Experimentally, a self-started NALM mode-locked Er-fiber laser is achieved with as low as 70-mW pumping power, delivering soliton pulses with a 17-pJ energy. The lowest pump power maintaining mode-locking is 24 mW. Finally, a cascaded fiber amplifier provides a total gain of 50 dB, boosting the pulse energy to
2 Numerical simulation
Gain fiber | 2 m | 1.8 dB/m | GVD: | 40 nm | |
TOD: | |||||
PM 1550 fiber | 50 m | 0 | GVD: | / | |
TOD: | |||||
SA | 1 m | / | / | / | |
Bandpass filter | 1 m | / | / | / | Variable |
Output coupler | 1 m | / | / | / |
Table 1. Schematic configuration of the simulated laser oscillator and the related parameters.
First, we numerically build the long-cavity laser oscillator with suppressed sidebands by incorporating an intracavity bandpass filter. Commercially available software which solves the extended nonlinear Schrödinger equation by the split–step Fourier transform method is used[
Starting from quantum noise, the long-cavity laser oscillator reaches steady state after 300 round trips when the bandpass filter is absent (Figures
Figure
3 Experimental results
Guided by the numerical simulation, we experimentally construct a long-cavity Er-fiber laser with a 1.8-MHz repetition rate. The laser configuration based on the NALM mechanism is schematically shown in Figure
The self-started mode-locking benefits both from the long cavity length and the phase shifter. In the reflective NALM configuration, the desired phase difference for mode-locking between the bi-directional light has to approach 0 or
When the phase shifter is absent, mode-locking operation cannot be achieved even with the maximum available pumping power (400 mW) of the laser diode and active mechanical perturbation. With the help of a
To confirm the spectral filtering effect of the bandpass filter, we remove it from the cavity. Multiple-pulse mode-locking builds up at a 100-mW pump power, and single-pulse operation with the lowest pump power is achieved at 20 mW. Figure
Since the output pulse is too weak to be measured by the autocorrelator, a single-mode-fiber amplifier is applied to pre-amplify the output pulses (see Figure
The output pulse of the long-cavity oscillator has a spectral width of 1.48 nm, corresponding to a transform-limited (TFL) pulse width of 2.4 ps (assuming a Gaussian profile). As reported in Ref. [
To meet the requirement for high pulse energy applications, the pre-amplified pulse is further amplified by a double-clad fiber main amplifier, which is similar to the amplifier used in Ref. [
For outdoor applications, the fiber laser system (including fiber chain, electrical controller and power supply) is integrated into an aluminum box with dimensions of
4 Conclusions
In conclusion, we have demonstrated an environmentally stable Er-fiber ultrafast laser operating at a quite low repetition rate. With the assistance of a phase-biased shifter in a long nonlinear loop, the Er-fiber laser oscillator realizes self-started mode-locking at a low threshold and delivers pulses with a 17-pJ energy at a 1.84-MHz repetition rate. The longer than 100 m PM fiber, which has giant anomalous dispersion, forces the mode-locking pulse laser to operate in the soliton regime. Numerical simulation guides us to suppress the Kelly sidebands by using an intracavity bandpass filter. Furthermore, the seed pulses with eliminated ASE noise are boosted to an average power of 2.8 W, yielding a pulse energy of
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