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Abstract
In this article, we report, to the best of our knowledge, the first observation of the reverse self-sweeping phenomenon in an all-polarization-maintaining bidirectional ytterbium-doped fiber laser. Conventional behaviors, including the dependence of sweeping range, sweeping rate and average pulse repetition rate on the pump power, can be observed in our fiber laser. Two couplers with ratio of 50/50 and 10/90 are respectively employed as the output coupler in fiber laser, which generates the reverse self-sweeping phenomenon for comparison.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The wavelength self-sweeping effect on fiber laser has drawn extensive attention due to its potential value of research and applications. The self-sweeping fiber lasers are distinguished from other tunable lasers in a way in which the spontaneous, periodical and stable tuning process are presented [1–4]. Since the first demonstration of self-sweeping effect in an ytterbium-doped fiber laser [5], the larger sweeping range and new spectral region have been taken as two orientations that further promote the development of this effect. Up to now, the self-sweeping regime has been observed in diversely doped fiber lasers such as ytterbium [6], bismuth [7], thulium [8], thulium-holmium [9], holmium [10], erbium [11] and neodymium [12]. Meanwhile, the largest sweeping range of 42 nm in an ytterbium-doped fiber laser was obtained based on a temperature controlled Lyot filter [13]. The improvements have heightened the practical application for optical devices testing and spectral analysis [14]. In the self-sweeping regime, there are many concomitant phenomena appear. Defining the direction of the sweeping in the way the wavelength origin to the destination, the wavelength increases in the normal self-sweeping and decreases in the reverse self-sweeping [15]. It is reported on some special multiple sweeping direction fiber lasers that the sweeping regime undergo the reverse, hybrid or stopping and normal state successively [15,16]. Latest research showed the intermediate state corresponding to the wavelength stopping indicates normal sweeping in a narrow sweeping range. The reverse sweeping course is also revealed in detail which represents the movement of sweeping range, and it is interesting that the normal sweeping is performed in this range [17].
Recently, the self-sweeping bidirectional fiber laser was realized so that the self-sweeping effect no longer necessarily relies on the linear cavity [18]. The bidirectional ring cavity as like as the linear cavity, the spatial hole-burning effect emerges due to the population inversion which causes the periodic refractive index profile to be generated along the active fiber, thereby building the dynamic gratings in the active fiber [19,20]. The bidirectional cavity was introduced into a Tm-doped fiber laser in which the reverse self-sweeping effect produced in a new wavelength region and the near-zero sweeping rate can be achieved. At present, linear cavities produce normal sweeping that laser frequency changes together with stable pulse, and the wide sweeping range can be obtained. Bidirectional ring cavities now are only reported for the generation of reverse sweeping in Tm active fiber and further studies are still absent [18].
Here we proposed an all-polarization-maintaining (PM) bidirectional ytterbium-doped fiber laser that generates reverse self-sweeping effect. Two couplers are used respectively in fiber laser to observe the output performance. As for the reverse self-sweeping regime, the sweeping range and the repetition rate change as usual. However, the sweeping rate decreases with the pump power increases. Besides, an ultra-slow self-sweeping regime is obtained by using the 10/90 coupler. At the same time, a non-continuous pulsation was observed. Further, we set forth wavelength selection process in the reverse self-sweeping regime.
2. Experimental setup
Figure 1 illustrates the experimental layout of the all-PM bidirectional ytterbium-doped fiber ring laser. A highly stable 976 nm PM laser diode (LD) with the largest output power of 650 mW was employed as the pump source. The active medium was ytterbium-doped single clad PM fiber with a length of 1.5 m (Coractive Yb 401-PM), which had a core absorption at 915 nm of 140 dB/m and the numerical aperture of 0.14. The core diameter of the active fiber and the doping concentration of Yb-ions in the core are 6 µm and 6.97×1025 ions/m-3, respectively. The PM wavelength division multiplexer (WDM) was used to connect the LD and gain fiber. The 2×2 PM broadband output coupler (operating wavelength range 1064 ± 15 nm) was placed between the WDM and the gain fiber, undertook the functions of ring cavity composition and laser output. We used 50/50 coupler and 10/90 coupler respectively in our experiments. In the whole cavity, there were no devices that force the light to work in one direction, which means the laser can operate in two directions of clockwise (CW) and counter-clockwise (CCW). The measured length of the cavity was ∼5.38 m. Note that the output port adopted the APC connector to avoid the reflected light disturbing the running of self-sweeping regime.
Fig. 1. The experimental layout of the all-polarization-maintaining self-sweeping ytterbium-doped bidirectional fiber laser. LD: laser diode; WDM: wavelength division multiplexer; Yb401: ytterbium-doped fiber; CW: clockwise; CCW: counter-clockwise.
3. Results
3.1 Results with 50/50 output coupler
The obvious feature of the reverse self-sweeping effect depicts in the spectral dynamics. The wavelength fluctuations are observed by an optical spectral analysis (OSA Yokogawa, AQ6370C) with a real resolution of 0.034 nm (@1 µm). When a 2×2 3 dB coupler was employed in the cavity, we found the threshold of the laser is about 38 mW and the reverse self-sweeping regime occurs in the pump power range between 38 to 85 mW. Figure 2 details the shift of laser frequency from the longer wavelength to the shorter wavelength at the pump power of 60 mW. The spectral train (8 s per one spectrum) in Fig. 2(a) shows the static central wavelength changes from 1060.468 nm to 1052.984 nm in a time scope of 56 seconds (0.138 nm/s). With the laser frequency change towards 1052.984 nm, we can see the competition of gain near 1060.468 nm strengthens gradually before the central wavelength goes back to the original wavelength. A clear display that laser frequency was recorded every second occurs in Fig. 2(b). The periodical movement in 7.484 nm scopes of laser frequency appear in 5 minutes at a pump power of 65 mW, and the spectral power variations are only 0.692 dB when the laser frequency changes ceaselessly. It is obvious that the linewidth is less than the resolution limit of OSA. Such narrow linewidth of the fiber laser is considered to be related to the bandwidth of dynamic gratings in the gain fiber [21,22].
Fig. 2. The output spectral performance (CW output) of the proposed fiber laser. (a) Spectra train. (b) The variation of output spectra and the power variation at 65 mW.
The intensity dynamics of the pulse signal of CW and CCW were monitored simultaneously by using of two detectors (Thorlabs DET08CFC) connected with two channels of digital storage oscilloscope (Agilent Technology DSO9104A). Figure 3(a) exhibits intensity signals of CW and CCW in the same time frame of 200 µs scope. Figure 3(b) shows the longitudinal mode beating detail, we can get a 38.3 MHz longitudinal mode spacing which corresponding to 5.4 m cavity length and well agree to our measured cavity length. The two signals look very similar in Fig. 3 in appearance. After zooming it in, one can see that the amplitude differences between CCW (black) and CW (blue) signals due to the existence of phase difference. We attribute the difference to the reflection of the dynamic gratings [19]. In the bidirectional cavity, two beams operated in CW and CCW directions are used to induce the dynamic gratings. When a grating is formed in the active fiber, the CW and CCW cavities use the grating together, then, the intensity dynamics show the microsecond pulse signals with the phase difference. Thus, this phenomenon can be considered as the typical feature of the self-sweeping bidirectional cavity. In fact, Fig. 3(b) shows the phase different is close to π. There is a slight phase difference which can be attribute be the refractive index change (RIC) in the active fiber [23–25].
Fig. 3. The measured pulse signal of the proposed fiber laser at 60 mW. (a) Pulse train. (b) The longitudinal mode beating detail in zoomed view.
The typical slope efficiency of bidirectional fiber laser is drawn in Fig. 4(a), we measured the output power in the directions of CW and CCW, respectively, by a power detector (Thorlabs PM100D) as shown in the insert picture. The total slope efficiency is about 37.38% in the self-sweeping regime with the pump power range increasing from 40 to 85 mW. One can be found in the insert picture that the slope efficiency of CCW power is obviously large than that of CW power. In our experiment, the absorption of the forward ASE (CW) is larger than the backward one (CCW) which will result in a lower intensity of the forward laser, and a bigger backward laser after the amplification in the active fiber. After the pump power exceeds 85 mW, the self-sweeping effect disappears and the extremely unstable spectrum and output power generate. At such low pump power, a laser with high slope efficiency in the self-sweeping state is obtained. From the perspective of these results, it seems to be more prone to generate self-sweeping effect for a bidirectional cavity. Further, we analyzed the average frequency, sweeping range and sweeping rate through the experimental data in different total output power. Figure 4(b) pictures the average pulse repetition rate ν increases with the total output power P as the square root function, i.e., $\nu [\textrm{kHz}] = 14.3\sqrt {P[\textrm{mW}]}$. The sweeping range increases to the maximum value and then decreases, as the total output power increases. One can see that the measured data agree well to the fitting curve [Fig. 4(c)]. Our measured results in Fig. 4(d) imply the sweeping rate is continuous decreases as the pump power increases. In Ref. [15], the sweeping rate in reverse self-sweeping regime gradually tends to ultra-slow even zero rate, it seems to indicate the transformation process of the reverse to normal sweeping. In our case, the reverse self-sweeping effect occurs in a relatively narrow range of pump power so that we cannot obtain the wavelength stopping or the normal self-sweeping. So a corresponding fitting curve can be proposed to reveal the relationship between the sweeping rate α and the total output power P in the reverse self-sweeping regime, i.e., $\alpha [\textrm{nm/s}] = 0.025\sqrt {40.8 - P[\textrm{mW}]}$.
Fig. 4. Output performance of this fiber laser in a 50/50 coupler. (a) Total output power versus pump power. (b) Average pulse repetition rate as a function of total output power. (c) Sweeping range as a function of total output power. (d) Sweeping rate as a function of total output power.
3.2 Results with 10/90 output coupler
In this part, an output coupler with a ratio of 10/90 was employed in the cavity to replace the 50/50 coupler. Figure 5 overviews the output performance of the bidirectional fiber laser in a 10% port. The self-sweeping regime occurs with the dependent pump power increasing from 24 to 36 mW. The slope efficiency is 10.38% by fitting curve [Fig. 5(a)]. The inset from Fig. 5(a) still can see that the slope efficiency of CCW power is large than that of CW power. The spectral information is shown in Fig. 5(b), the generated linewidth of laser is significantly smaller than the resolution of OSA and the OSNR is larger than 45 dB. The sweeping rate in such self-sweeping regime is very slow and decreases with total output power increases, as shown in Fig. 5(c). The initial and maximum rate is ∼108.1 pm/min and the minimum value of rate is ∼16 pm/min [Fig. 5(c)]. The sweeping range changes with total output power increases as shown in the inset of Fig. 5(c). Figure 5(d) presents the typical shift of the central wavelength (recorded per 30 seconds) in the time scope of 10 hours, which shows the self-sweeping regime operating from 1070.448 nm to 1066.026 nm with an ultra-slow sweeping rate of 16 pm/min. One can see that the coupler with a lower ratio can decrease effectively the threshold and the pump power range of self-sweeping. In the case of lower pump power, the flatter gain curve will make it easy to produce a slow sweeping rate.
Fig. 5. Output performance of this fiber laser in a 10/90 coupler. (a) Total output power–versus the pump power. (b) The spectral information at a pump power of 32 mW. (c) Sweeping rate and sweeping range (the inset) as a function of total output power. (d) The variation of laser frequency at a pump power of 32 mW.
Figure 6 shows the intensity dynamics of the ultra-slow sweeping rate. The intensity signal of 30 ms scope is sketched in Fig. 6(a) at a pump power of 32 mW, one can see that a set of pulses occurs near 1 ms. When we zoomed in this set of pulses in Fig. 6(b), we found the train of microsecond pulse with a pulse repetition rate of 28 kHz. The intensity dynamic reveals the pulse signals are not continuous in the ultra-slow self-sweeping regime. In this case, we believe the microsecond pulse signals in Fig. 6(b) represent the normal self-sweeping operation and the rest of pulse signals can be deemed the wavelength invariant state.
Fig. 6. The intensity dynamics of the ultra-slow wavelength self-sweeping regime at 32 mW. (a) The non-continuous pulse signal in a large range of 30 ms. (b). The continuous microsecond pulse train at the zoomed view in 1 ms scope.
4. Discussion
In this work, the reverse self-sweeping effect was observed in the bidirectional ytterbium-doped fiber laser. Table 1 sums up the characteristics of reverse self-sweeping effect by using two couplers. Compared the 50/50 coupler, the 10/90 coupler has lower loss, which leads to a low threshold and slope efficiency. As for the self-sweeping effect, the smaller and narrower pump range, red-shift initial wavelength and slower sweeping rate are obtained by using the lower ratio coupler. More important difference is about the pulse signals. The results in reverse self-sweeping effect [Fig. 4(b) and (d) and Fig. 6] show the sweeping rate is not follow the intensity-domain pulsations. In the experiment, when we observed the pulse repetition rate in Fig. 4(d), the non-continuous also can be observed sometimes, but the interval is very short (hundreds or dozens of microseconds). It is an analogue of the situation in Ref. [17]. But, the intensity dynamic in Fig. 6(b) shows large interval between the microsecond pulse groups (hundreds or dozens of milliseconds), which increases the sweeping time and leads to the ultra-slow self-sweeping finally. This proved that the increased interval determines the slowing down of sweeping rate. By the way, this situation as shown in Fig. 6(a) is an analogue of intensity dynamics of the externally induced dynamic population inversion grating (DPIG) mentioned in Ref. [4], which means the wavelength invariant state stands for the process of writing gratings by the CW and CCW lasing.
Table 1. Summary of fiber laser characteristics by using two couplers
The reverse self-sweeping effect indicates two wavelength selection mechanisms. The first one is the normal sweeping represented by the microsecond pulse, which puts down to the induced dynamic grating formed in active fiber. Another one is the reverse shift of laser frequency achieves through the alternation of the pulse group which represents the sweeping range. This second case is able to drive based on the gain-to-loss profile determined by the cavity parameter. Besides, the interval between pulse groups means the establishment of dynamics gratings, which leads to the mismatch between sweeping rate and pulse repetition rate. It is worth mentioning that the output powers of CW and CCW have obvious difference in our case. This is an uncertain factor of self-sweeping effect in bidirectional cavity. Under this, controlling the parameters of the CW and CCW cavities to reasonably induce the DPIG has become one direction for further experiments.
5. Conclusion
In conclusion, we have studied systematically the reverse wavelength self-sweeping operation in an all-PM bidirectional ytterbium-doped fiber laser by using 50/50 coupler and 10/90 coupler. The self-sweeping effect emerges and disappears in relatively low pump power in our bidirectional ring fiber laser. Similar to the conventional self-sweeping effect, the average pulse repetition rate and the sweeping range change as the output power increases. The difference is that the sweeping rate decreases as the output power increases, under this condition, an ultra-slow self-sweeping rate is achieved by replacing the output coupler with another one with a lower power ratio. The observed intensity dynamics of the non-continuous pulsation explained well the ultra-slow sweeping state. Besides, the results also show that the coupling ratio plays an impact on the output performance of self-sweeping fiber lasers such as the region of wavelength and the range of pump power. This means that the output coupler can be a new option to optimize the sweeping range. We believe that our studies will be instructive to the studies of the self-sweeping effect and will have promising applications for this region.
Funding
National Natural Science Foundation of China (61905193); National Key Research and Development Program of China (2017YFB0405102); Key Laboratory of Photoelectron of Education Committee Shaanxi Province, China (18JS113); Open Research Fund of State Key Laboratory of Laser-Matter Interaction (SKLLIM1812); State Key Laboratory of Transient Optics and Photonics (SKLST201805); Northwest University Innovation Fund for Postgraduate Students (YZZ17099).
Disclosures
The authors declare no conflicts of interest.
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