1. Introduction

Fiber lasers have been intensively investigated these years with the emergence of rare-earth doped fiber. Pulsed fiber laser is attractive in many applications such as optical communications, biological metrology, light detection and ranging (LIDAR) system, etc. At present, some high resolution ranging systems require optical pulse with high pulse energy and sub-ns pulse width [1]. Conventional Q-switched lasers which always produce optical pulse with over tens of ns pulse width are not satisfied for those applications. Therefore, passively mode-locked fiber laser (PMFL) systems generating short pulse width (fs ~ ps) are particularly attractive [2]. However, due to the soliton collapsing mechanism [3], pulse energy of PMFL is limited to below 1 nJ. Furthermore, the repetition rate of PMFL is typically around 10 MHz ~ 100 MHz. Such repetition rate is too high for some applications such as biological metrology which requires high pulse energy with low average laser intensity. Therefore, the developing of high-energy, low repetition rate PMFL system is a challenging and attractive task.

Yb-doped fiber (YDF) has been widely used as gain medium to construct high-energy PMFL because of its wide gain bandwidth, high gain and high saturation power around 1060 nm. Previous research works show that the normal dispersion of optical fiber at 1060 nm makes it difficult to achieve stable mode-locking in YDF lasers [4]. Ilday et al. proposed similariton PMFL which used intra-cavity grating pairs to compensate the normal dispersion. Similariton lasers can generate mode-locked pulse with over 10 nJ pulse energy with around 20 MHz repetition rate by using nonlinear polarization rotation (NPR)-based mode-locking mechanism [5]. However, when the laser cavity is extended and repetition rate is reduced to around 2 MHz, the pulse energy decreased tremendously to 200 pJ [6,7]. Recently, all-normal dispersion laser with intra-cavity spectral filter has been reported. In such laser, the spectral filter is essential for achieving stable mode-locking operation [8,9]. However the requirement of dispersion compensation or spectral filter in the laser cavity always breaks the all-fiber structure of the laser.

In this paper, we demonstrate an all-fiber, high-energy, low repetition rate PMFL system. The system includes a PMFL which acts as a master oscillator and a subsequent YDF based optical amplifier. By employing ultra-long laser cavity and extremely large normal net dispersion, the PMFL can be stably mode-locked without intra-cavity dispersion compensation or spectral filter. The long cavity and large normal net dispersion are helpful to reduce the repetition rate and increase the output pulse energy. A 6.9 nJ pulse with 187 kHz repetition rate have been experimentally achieved from the master oscillator. To the best of our knowledge, it is the first time that a stable mode-locked fiber laser with high pulse energy has been achieved in such long fiber laser cavity. After amplification by 10 m Yb doped double cladding fiber (Yb-DCF), the pulse energy can be amplified to 4.3 μJ.

2. Experiment

The system consists of a PMFL and a fiber amplifier, as shown in Fig. 1. In the PMFL, a semiconductor saturable absorber mirror (SESAM) mounted on fiber connector is used to mode-lock the laser. The modulation depth of the SESAM is ΔR = 20% , the saturation fluence is φ = 120μJ/cm ², and the relaxation time constant is τ = 500fs. The SESAM is coupled into the laser cavity through a circulator which has 20 dB isolation from port 3 to port 1. Therefore, the circulator also acts as an isolator which ensures a unidirectional propagation and suppresses undesired reflections. Compared to the conventional NPR-based PMFL with long laser cavity [7,10], SESAM has the advantage that it provides invariant saturable absorption independent of the cavity length. However, the NPR-based mode-locking operation strongly depends on the polarization evolution and phase evolution of the pulse in the laser cavity. In a long laser cavity, it can be easily overdriven by the large nonlinear phase shift per-roundtrip and affected by the environment induced fiber birefringence instability. Such effects can impede stable mode-locking of the laser and lead to square-pulse generation or noise-like emission [10,11].

Laser gain is provided by single mode 4 m Yb-doped fiber (YDF) which has 7.5 μm mode diameter at 1060 nm and 250 dB/m absorption at 976 nm. A 976 nm single mode laser diode is employed to pump the laser. Hi-1060 SMF is used to extend the length of laser cavity and provide pure normal dispersion. The SMF has 6.5 μm mode diameter and -50 ps/nm/km dispersion at 1060 nm wavelength. In the experiment, we used 200 m, 500 m, 700 m and 1100 m Hi-1060 SMF to test the laser performance at different repetition rate. A polarization controller (PC) is employed to adjust the polarization state of light propagating along the laser cavity. The amplifier is constructed by 10 m Yb-doped double cladding fiber (DCF) with 7.0 μm core diameter and 210 μm clad diameter. The pump absorption in the fiber inner cladding is 0.85 dB/m. A 976 nm fiber coupled single emitter multimode pump diode with 4 W maximum output power is introduced into the Yb-DCF through a commercial fiber bundle.

 

Fig. 1. Setup of the laser system

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The laser spectrum is monitored by an optical spectrum analyzer with 0.01 nm resolution, while the pulse temporal profile is monitored by a 45 GHz photo detector and a 50 GHz high-speed sampling oscilloscope. Self-start mode-locking is observed by increasing the pump power to 118 mW, 124 mW, 127 mW and 135 mW, respectively, when the length of the SMF is 200 m, 400 m, 700 m and 1100 m. Since there is no spectral filter in the laser cavity, the central emitting wavelength of the laser is dominated by the length of YDF. The re-absorption mechanism of the YDF shifts the emission light to longer wavelength with longer YDF [12]. In our experiment, we find that the emission wavelength of the laser is also affected by the cavity length. The central wavelength intends to shift to shorter wavelength with the increasing cavity length when the YDF length is fixed, as shown in Fig. 2(a). We believe that this type of wavelength shift is due to the higher cavity loss in longer laser cavity. The central wavelength shifts from 1073 nm to 1068 nm when the length of SMF is increased from 200 m to 1100 m. The pulse spectrum has steep spectral edge which is a typical property of wave-breaking-free pulse in normal dispersion PMFL [13]. The 10 dB bandwidth of the pulse spectrum decreases from 0.91 nm to 0.63 nm and the full width half maximum (FWHM) of the pulse increases from 400 ps to 1000 ps. The experimental result shows that the pulse is extremely chirped by the strong net normal dispersion. Such pulse shall be able to be compressed by dispersion compensation [13].

When the PMFL is fundamentally mode-locked, the repetition rate of is 870 kHz, 387 kHz, 278 kHz and 187 kHz. Fig. 2(b) shows the pulse train for each repetition rate. The corresponding average power is 2.5 mW, 1.8 mW, 1.4 mW and 1.2 mW, respectively. Fig. 3(a) shows the calculated pulse energy and pulse peak power. For different repetition rate, the pulse energy is 2.9 nJ, 4.6 nJ, 5.0 nJ and 6.9 nJ respectively, while the pulse peak power is 7.2 W, 7.0 W, 7.1 W and 6.9 W respectively. The pulse energy and the pulse width increase significantly when the cavity length is increased. The experimental result is in good agreement with the theoretical prediction in [14] which indicated that the pulse energy and the pulse width can be increased by increasing the net normal dispersion.

 

Fig. 2. (a) Pulse spectrum and temporal profile (b) Pulse train for different cavity length. L is the length of the SMF

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The laser has a turn-key operation for self-start mode-locking and can keep stable for hours before intended interruption. It shows that mode-locking in all-normal dispersion PMFL with ultra-long laser cavity is robust even without intra-cavity dispersion compensation and spectral filter. However, a stable mode-locking operation can not be achieved when the cavity length is less than 50 m in our experiments. The failing to achieve stable mode-locking in short laser cavity can be caused by two factors, the first is the Q-switch instability. For mode-locked laser with ps pulse width, the pulse energy must be high enough to achieve stable mode-locking, as written in Eq. (1) [15].

where E_p is the pulse energy, E_sat,L is the saturation energy of the gain medium, E_sat,A is the saturation energy of the saturable absorber. In PMFL with all normal dispersion cavity, the pulse energy decreases with the decreasing of cavity length [14]. In short laser cavity, the pulse energy can be too low to achieve stable mode-locking [15,16]. Secondly, stable mode-locking needs to balance in the laser cavity in the both time domain and frequency domain [17]. In the time domain, the pulse broadening effect due to normal dispersion can be balanced by the pulse shortening effect of the saturable absorber. In the frequency domain, since the Yb lasers have large gain bandwidth, a spectral filter is required to balance the nonlinearity induced spectrum broadening effect [17,18]. For a strongly chirped pulse, the spectral filter can also provide saturable absorption effect with extremely low saturation energy [13]. Therefore failing to achieve stable mode-locking in all normal dispersion laser without spectral filter is reported in [8,9],

In our experiment, it reveals that the long laser cavity helps to stabilize the mode-locking. Due to the strong chirp caused by the normal dispersion of fiber, the red-shift light and blue-shift light will be located at leading edge and trailing edge of the pulse in the time domain [19]. Compared to the center part of pulse, the pulse wings have much lower intensity. As the pulse width increases with the extending of cavity length, pulse width within the laser is much larger than the recovery time of the SESAM in the long cavity, thus the SESAM can be regarded as a fast saturable absorber and response according to the pulse intensity. When the pulse is reflected by the SESAM, the red-shift and blue-shift light with low intensity will have higher loss than that of the central frequency due to the nonlinear reflectivity of the SESAM. Therefore, the SESAM performs equivalently to a spectral filter for such strongly chirped pulse and balances the spectrum broadening effect induced by fiber nonlinearity.

 

Fig. 3. Pulse peak power and pulse energy for different cavity length from (a) the master oscillator and (b) the amplified laser system.

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To further increase the pulse energy, the fiber amplifier shown in Fig. 1 is employed. Without any pulse distortion, the average power can be amplified to 1.27 W, 1.05 W, 0.85 W, and 0.8 W according to different repetition rate pulse. The pump power is 3.4 W, 3.2 W, 3.1 W and 2.8 W respectively. The pulse energy is amplified to 1.4 μJ, 2.6 μJ 3.2 μJ and 4.3 μJ respectively, as shown in Fig. 3(b), with increasing SMF length. The maximum peak power of amplified pulses can be over 4 kW. Compared with conventional Q-switched fiber laser, mode-locked laser with the low repetition rate has the advantages of all-fiber structure, self-start operation and shorter pulse width with comparable pulse energy [20].

The average power can be further increased by increasing the pump power, but the pulse will break-up due to wave-breaking effect when the pump power is further increased in our setup [19]. As wave-breaking effect is mainly caused by fiber nonlinearity, higher pulse energy can be expected by applying large-mode area YDF [21].

To test the performance of the laser system, we launched the amplified pulse to 60 m HNLF. Fig. 4 shows spectrum of the 387 kHz pulse after the HNLF. Although the HNLF is optimized to the central wavelength of 1550 nm, supercontinuum generation can be observed. Since the zero dispersion wavelength of the fiber is far from 1060 nm, only Raman shift of the pulse can be observed on the spectrum when pump power is 1.1 W. By increasing the pump power to 3.1 W, the fifth order Raman Stokes appears near the zero dispersion wavelength and generates supercotinuum spectrum with over 300 nm in term of 3 dB bandwidth. This result confirms the high peak power, high pulse energy of our laser system.

 

Fig. 4. Supercontinuum generated by the laser system

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3. Conclusion

In conclusion, we demonstrated an all-fiber structure, high pulse energy mode-locked laser system. By changing the laser cavity length, the master oscillator generated mode-locked pulse with 870 kHz, 387 kHz, 278 kHz and 187 kHz repetition rate, respectively. 6.9 nJ pulse energy was achieved directly from the master oscillator. After amplification, maximum 4.3 μJ wave-breaking free pulse has been achieved when the repetition rate was 187 kHz and the average power was only 0.8 W. Experimental results showed that extending the cavity length not only reduced the repetition rate but also increased the pulse energy. Other than using any costly high-power component, our proposed fiber laser system is able to generate high pulse energy with low average power using a simple and cost-effective all-fiber structure.