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Abstract
A 2.1 μm, high energy square-wave noise-like pulse (NLP) in an all-fiber Ho-doped fiber laser is proposed, which consists of an oscillator and a single-stage amplifier. In the figure-of-9 oscillator, mode-locking is achieved based on the nonlinear amplifying loop mirror, employing a long gain fiber to provide sufficient gain in 2.1 μm band and optimizing the cavity length to obtain maximum pulse energy output. With appropriate pump power and polarization state, the oscillator emits a 175.1 nJ square-wave NLP with center wavelength of 2102.2 nm and spike width of 540 fs. The 3-dB spectral width and pulse envelope width are 11.2 nm and 6.95 ns, respectively. The single-stage amplifier employs a bi-directional pump scheme. After amplification, 5.8 W NLP with a slope efficiency of 56.8% is obtained. The pulse energy of NLP is scaled to 1.52 μJ, which is the highest pulse energy of NLP at 2.1 μm to the best of our knowledge. The obtained high-energy square-wave NLP-fiber laser has great potential in mid-infrared laser generation.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The 2.1 μm square-wave pulsed fiber lasers have attracted considerable attention due to their potential application in gas detection, national defense security and industrial processing [1,2]. They were also particularly attractive light sources for mid-infrared laser generation via nonlinear frequency conversion and nonlinear spectral broadening [3,4]. For example, using a high energy square-wave pulse centered at 2.1 μm as the pump source for ZnGeP2 crystal-based mid-infrared optical parametric oscillator can achieve high nonlinear frequency conversion efficiency and excellent beam quality [4]. So far, pulsed laser generation near 2.1 µm has realized by using Tm-doped fiber (TDF) [5], Tm-Ho co-doped fiber (THDF) [6] and Ho-doped fiber (HDF) [7]. Unfortunately, due to the limited gain bandwidth of TDF at 2.1 µm band, Tm-doped amplifier is unlikely to achieve high efficiently pulse amplification [8]. Compared with THDF, HDF is capable of achieving higher lasing efficiency than THDF [9]. Therefore, the utilization of HDF is desired for efficient square-wave pulsed laser generation at 2.1 μm.
In general, square-wave pulses from mode-locking fiber lasers operate at dissipative soliton resonance (DSR) or noise-like pulse (NLP) regime. The formation mechanism of DSR pulse was first reported in 2008 by Chang et al. [10]. A significant feature of DSR is the wave-breaking-free operation, which makes it possible to increase pulse energy with the enhancement of pump power [11,12]. Therefore, DSR is capable of maintaining high pulse energy. Typically, high energy square-wave DSR can be obtained in long-cavity mode-locking fiber lasers [13]. However, the oscillator with long cavity configuration has a high intracavity loss at 2.1 µm, which restricts the pulse energy of DSR to a few nJ or several tens of nJ. To date, the highest pulse energy of square-wave DSR achieved in Ho-doped fiber laser is ∼12 nJ [14]. To further improve the square-wave pulse energy at 2.1 μm band, enabling the Ho-doped fiber laser to operate in the NLP regime is an effective solution. NLP is a typical mode-locking state in an oscillator with high pump power and nonlinearity [15]. Different from DSR, NLP is a wave packet composed of many solitons with varying amplitudes and pulse durations [16]. Because of the broad temporal width, the square-wave NLP can operate in the high energy region and allow the pulse energy to be further scaled to higher levels by an amplifier system. In recently decades, the generation and amplification of NLPs have reported in Yb-doped [17,18], Er-doped [19] and Tm-doped [20,21] fiber laser systems. Nevertheless, it is hard to boost the pulse energy of NLP to ∼μJ level by using a single-stage amplifier due to the low pulse energy of the reported oscillator. To scale the NLP energy up to ∼μJ level, Wang et al. used a three-stage Tm-doped amplifier system [21], which increases the complexity and cost of fiber laser system. In 2.1 μm band, there is only one report on the generation of square-wave NLP and the pulse energy of the obtained NLP was not given [22]. Because the formation mechanism of NLP allows to obtain high energy, exploring 2.1 μm high energy square-wave NLP in combination with the design of oscillator structure is strongly desired. Also, further realization of higher energy square-wave NLP by single-stage amplifier is conducive to practical application requiring intense pulse energy.
This paper demonstrates high energy square-wave NLP generation from the all-fiber Ho-doped oscillator and single-stage amplifier for the first time. Mode-locking is achieved by employing nonlinear amplifier loop mirror (NALM) technique. The use of figure-of-9 cavity configuration combined with in-band pump scheme facilitates high energy NLP generation. By utilizing a long gain fiber and optimizing the cavity length, stable NLP operation with center wavelength of 2102.2 nm and pulse energy of 175.1 nJ is obtained at appropriate pump power and polarization state. The 3-dB spectral width, pulse envelope width and coherent spike width are 11.2 nm, 6.95 ns and 540 fs, respectively. Amplified by a bi-directional pumped amplifier, the maximum output power of square-wave NLP reaches 5.8 W and pulse envelope broadens to 7.2 ns. The pulse energy is scaled to 1.52 μJ with a slope efficiency of 56.8%,which is, to the best of our knowledge, the highest pulse energy of NLP in 2.1 μm region.
2. Experiment structure and operation principles
Figure 1 shows the diagram of the designed 2.1 µm fiber laser system, consisting of an oscillator and a single-stage amplifier. In the oscillator, mode-locking is achieved based on the NALM mechanism. The oscillator is established in a figure-of-9 cavity configuration, which contains a NALM loop and a reflective fiber ring mirror. Compared with the figure-of-8 cavity configuration, the figure-of-9 cavity structure has the advantage of fully utilizing the power reflected back to the NALM loop by the reflective fiber ring mirror [7]. In the NALM loop, a 9.1 m HDF with a core diameter of 8 μm and a numerical aperture (NA) of 0.2 is used as gain fiber to provide sufficient gain at 2.1 μm band. Since in-band pump scheme can achieve high slope efficiency from laser [9], a 1950nm in-house built Tm-doped fiber laser (TDFL 1) with a maximum output power of 4.46 W is utilized to pump gain fiber via a 1950nm/2100 nm wavelength division multiplexer (WDM 1). The utilization of figure-of-9 cavity structure combined with in-band pump scheme favors high energy NLP generation directly from the oscillator. A segment of single-mode fiber (SMF) is employed to increase the nonlinear phase shift difference between clockwise and counterclockwise propagating light because a larger nonlinear phase shift difference reduces the input peak power required for mode-locking [23,24]. The core diameter and NA of SMF are 8.2 μm and 0.14, respectively. The NALM loop is connected with the reflectivity fiber loop mirror via a 2 × 2 fused optical coupler (OC 1) with a splitting ratio of 3:7. The use of asymmetric OC in the cavity can enhance the accumulation of nonlinear phase. The reflective fiber ring mirror is formed by splicing two ports of a 2 × 2 fused OC 2 with a splitting ratio of 1:1 together. Two polarization controllers (PC 1 and PC 2) are inserted into the NALM loop and reflective fiber ring mirror, respectively, to adjust the polarization state in the cavity. The other port of the four-port OC 1 is used as output. Note that the pigtail fibers of all passive components are SMF for minimizing splicing loss. To achieve the maximum pulse energy output, the cavity length is optimized to 52.3 m by changing the SMF length to 37 m. The dispersions of SMF and HDF are -0.1 ps2/m and -0.102 ps2/m, respectively [25]. Therefore, a net cavity dispersion of -5.25 ps2/m is yielded, indicating that the oscillator operates in the large anomalous dispersion regime.
Fig. 1. Schematic setup of the Ho-doped fiber laser system. TDFL: Tm-doped fiber laser; WDM: wavelength division multiplexer; HDF: Ho-doped fiber; OC: optical coupler; ISO: isolator; PC: polarization controller; SMF: single-mode fiber; NALM: nonlinear amplifying loop mirror
After oscillator, a 1:99 fused OC 3 is spliced to the output port of OC 1, which 1% port is used for monitoring pulse. The single-stage amplifier adopts a bi-directional pump scheme due to the high absorption of HDF [26]. The amplifier contains two 1950nm/ 2100 nm WDMs (WDM 2 and WDM 3) and 3 m HDF pumped by two in-house built TDFLs (TDFL 2 and TDFL 3) centered at 1950nm. The HDF has the same parameters as those in the oscillator. The TDFL 2 and TDFL 3 drive up to 5.8 W and 4.25 W power output, respectively. The fiber at the final output is cleaved at 8° to avoid unwanted Fresnel reflection. In order to prevent back light from the single-stage amplifier, a 10 W isolator (ISO) is placed before single-stage amplifier.
The output pulse signal is characterized with the following equipment: an optical spectrum analyzer (OSA, Yokogawa AQ6375B), a real-time 2-GHz oscilloscope (Rigol, MSO8204, OSC), a 12.5-GHz InGaAs photodetector, a radio-frequency (RF) spectrum analyzer (Rohde & Schwarz, 4 GHz), and an autocorrelator (APE-PulseCheck).
3. Experimental results and discussions
3.1 Output properties of the oscillator
In the oscillator, the continuous wave (CW) excitation threshold is about 1.9 W. Stable mode-locking pulse is obtained by increasing TDFL 1 power to 3.71 W and adjusting intracavity polarization state. Without disturbing the PCs, the mode-locking operation can be maintained to the maximum TDFL 1 power of 4.46 W. Figure 2(a) shows the evolution of spectra under different TDFL 1 powers. With the TDFL 1 power increases from 3.71 W to 4.46 W, the spectrum profile has no obvious change and the 3-dB spectral width increases from 10.8 nm to 11.2 nm. Slight broadening in the spectra is due to the enhanced self-phase modulation with increasing pump power [27]. At the TDFL 1 power of 4.46 W, the center wavelength is located at 2102.2 nm. The evolution of pulse envelopes with different TDFL 1 powers is recorded in Fig. 2(b). The pulse has square waveform and steep rising and falling edges. With the enhancement of TDFL 1 power, pulse envelope broadens from 5.4 ns to 6.95 ns continuously. During the pulse broadening process, the pulse amplitude is basically unchanged and no multi-pulse phenomenon is observed.
Fig. 2. The evolution of square-wave pulse under different TDFL 1 powers. (a) Spectra measured with a resolution of 0.05 nm. (b) Pulse waveforms. Inset: The pulse envelope width versus with TDFL 1 power.
The output characteristics of square-wave pulse at the TDFL 1 power of 4.46 W are presented in Fig. 3. Figure 3(a) depicts the pulse train captured in the OSC. The pulse interval of adjacent pulses is 261.3 ns, matching the cavity round-trip time. As shown in Fig. 3(b), the pulse train captured in the 300 μs range reveals low amplitude intensity fluctuation, demonstrating the high stability of the mode-locking pulse. Figure 3(c) illustrates the measured autocorrelation (AC) trace. The AC trace exhibits a narrow coherence spike with a width of 540 fs (assuming Gaussian fitting), implying that the achieved mode-locking pulse is different from the DSR [28]. Taking into account that the obtained mode-locking pulse has 6.95 ns square envelope and smooth broad spectrum, we can come to the conclusion that the oscillator operates in the square-wave NLP regime. The intensity ratio between the peak intensity and pedestal intensity is 1.29 which indicates the high power of the noise-like part and the pulse does not have a significantly ordered structure [29,30]. As shown in the inset of Fig. 3(c), since the pedestal of NLP is wider than the 50 ps scan range of the autocorrelator, the full double-scale structure of NLP cannot be captured by the autocorrelator used in this paper [31]. The low intensity of AC trace is due to the small average power for the autocorrelator detection. Figure 3(d) exhibits the RF spectrum around the fundamental frequency. The repetition rate of NLP operation is 3.82 MHz corresponds to the inverse of the adjacent pulse interval of 261.3 ns. The signal-to-noise ratio (SNR) is about 64.4 dB. The RF spectrum with a 100 MHz range demonstrates the stable mode-locking operation, as shown in the inset of Fig. 3(d).
Fig. 3. Output pulse characteristics at theTDFL1 power of 4.46 W. (a) Pulse train in the range of 2 μs. (b) Pulse train with the range of 300 μs. (c) AC trace of NLP measured in the range of 5 ps. Inset: AC trace measured in the 50 ps range. (d) RF spectrum at the fundamental repetition rate with 10 Hz resolution bandwidth. Inset: RF spectral distribution with 100 MHz span and 20 Hz resolution bandwidth
The output power and pulse energy versus TDFL 1 power are shown in Fig. 4. With the increase of TDFL 1 power, the output power increases linearly from 0.52 W to 0.67 W with a slope efficiency of 20.5%. Considering the repetition frequency of 3.82 MHz, the maximum pulse energy is 175.1 nJ, which is considerably higher than that in previous reports on square-wave pulse Ho-doped fiber lasers (5.8 nJ [32] and 12 nJ [14]).
In the oscillator, stable mode-locking operation is established when the intracavity gain overcomes loss. However, in 2.1 μm band, the insertion loss of passive components and the propagation loss of the laser in the silica fiber are higher than in other bands. Therefore, it is necessary to use a high power pump source and choose a long gain fiber to provide sufficient gain at 2.1 μm. Here, a 4.46 W 1950nm fiber laser is used as the pump source. The measured amplified spontaneous radiation (ASE) spectra of HDF under different lengths is shown in Fig. 5(a). It can be seen that the center peak of ASE is red-shifted from 2029.5 nm to 2089.8 nm as the HDF length increases from 3 m to 9.1 m. The 9.1 m HDF can generate higher radiation at 2.1 μm because of the re-absorption process in the gain fiber. Therefore, we select the length of HDF as 9.1 m in the oscillator.
Fig. 5. (a) The ASE spectra measured at the pump power of 1.1 W with 3 m HDF (black curve), 5.8 m HDF (red curve) and 9.1 m HDF (blue curve). (b) The output power and pulse energy as a function of cavity length
Generally, a common way to obtain high energy square-wave pulses is to decrease repetition rate by increasing cavity length. However, the increase of cavity length leads to a decrease in output power of the oscillator. The detailed reasons are as follows. As the cavity length increases, the pulse width gradually increases due to stronger nonlinear and dispersion effects [33]. In this case, the NALM-based saturation absorber will absorb more pulse power [34]. On the other hand, the increase of cavity length adds the intracavity loss due to large propagation loss at 2.1 μm (∼0.086 dB/m). As a result, the average output power of the oscillator decreases, which affects the energy of the mode-locking pulse. Thus, the cavity length needs to be optimized to achieve the maximum pulse energy output. The output characteristics of mode-locking pulse under different cavity lengths are shown in Fig. 5(b). The cavity length is controlled by altering the length of the SMF in the NALM loop. One can see that the output power gradually decreases with the increase of cavity length due to the added absorption loss of the NALM-based saturation absorber and propagation loss. As the cavity length increases from 28.9 m to 52.3 m, the pulse energy gradually increases and reaches a peak energy of 175.1 nJ, which indicates that the effect of increasing cavity length on the pulse energy is greater than the effect of decreasing output power. However, the pulse energy gradually decreases to 118.3 nJ with further increase in cavity length to 121.5 m, due to the strong influence of decreasing output power on the pulse energy. Therefore, the maximum pulse energy output from the oscillator is obtained by optimizing the cavity length to 52.3 m.
3.2 Amplifying properties of the single-stage Ho-doped amplifier
Before the NLP is injected into the single-stage amplifier, the power of NLP reduces from 0.67 W to 0.31 W due to the large insertion loss of OC 3 and ISO at 2100 nm (about 1.92 dB and 1.42 dB, respectively). In the single-stage amplifier, the NLP centered at 2102.2 nm is further amplified. The output power and pulse energy as a function of pump power are plotted in Fig. 6. The output power increases linearly with a slope efficiency of 56.8% as the total power of TDFL 2 and TDFL 3 rises from 0.85 W to 10.05 W. The maximum output power reaches 5.8 W and the corresponding pulse energy of NLP is boosted to 1.52 μJ which is the highest pulse energy acquired from NLP-based fiber lasers at 2.1 μm.
Fig. 6. The variations of (a) output power and (b) pulse energy with a total power of TDFL 2 and TDFL 3
Figure 7(a) shows the spectra at various output powers. The spectral profile does not change significantly until the output power reaches 2.3 W. As the output power further increases, the spectrum starts to broaden. This is because the pulse experiences strong nonlinear effects such as self-phase modulation (SPM), modulation instability (MI) at a high power [21]. Although the mode-locking pulse has a nanosecond envelope, it is composed of ultrashort and short soliton pulses. Therefore, the peak power of the sub-pulses within the NLP envelope is also scaled during power amplification until exceeding the nonlinear effect threshold and then spectral broadening occurs. At the maximum output power, the 3-dB spectral width of NLP broadens to 11.8 nm. The corresponding pulse waveforms evolution under different output power levels are shown in Fig. 7(b). As the output power increases, the NLP remains square profile without distortion. The envelope width increases from 6.95 ns to 7.2 ns, which is mainly due to sub-pulse splitting within the NLP envelope caused by nonlinear effects during amplification. The amplitude intensity of the pulse envelope increases significantly. Figure 7(c) records the uniform pulse sequence in the 200 μs range at the output power of 5.8 W, indicating good stability of the NLP operation.
The AC trace at the output power of 5.8 W is shown in Fig. 8(a). The width of the spike increases from 540 fs to 632 fs due to dispersion and nonlinear effects in the amplifier. The intensity ratio between the peak intensity and pedestal intensity increases from 1.29 to 1.32, indicating the increase of noise-like part after amplification. Compared with the SNR of the output pulse from oscillator, the SNR of the pulse after amplification decreases slightly from 64.4 dB to 63.2 dB, as shown in Fig. 8(b). The RF spectrum in the 100 MHz range indicates good stability of NLP operation, as shown in the inset of Fig. 8(b). Figure 9(a) exhibits the measured power stability of the single-stage amplifier in 2 hours. The standard deviation (SD) of output power is 48.1 mW, and the corresponding fluctuation is about 0.81%, further indicating stable operation of the laser system.
Fig. 7. Output characteristics after amplification. (a) Spectra measured with a resolution of 0.05 nm under different output powers. (b) Pulse waveform as a function of output power. (c) The OSC trace of NLP recorded with 200 μs range at the output power of 5.8 W.
Fig. 8. Pulse characteristics at the output power of 5.8 W. (a) The measured AC trace in the range of 4 ps. (b) RF spectrum with 10 Hz resolution bandwidth. Inset: RF spectrum over 100 MHz span with 20 Hz resolution bandwidth.
Fig. 9. (a) Output power stability in 2 hours. (b) Optical spectrum of NLP measured with a resolution of 0.5 nm at output power of 5.8 W.
In the single-stage amplifier, the output power of NLP increases linearly with the enhancement of pump power, no power saturation occurs [35]. Also, the optical spectral intensity of the signal light is 39.8 dB higher than that of the residual 1950nm pump light and the intensity of 2027.3 nm ASE is 38.3 dB below the intensity of signal light, as shown in Fig. 9(b). Thus, we believe that there is potential for further scaling of the NLP’s output power and pulse energy by improving the 1950nm pump power.
4. Conclusion
The 2.1 μm, high energy square-wave all-fiber Ho-doped fiber laser system consisting of figure-of-9 oscillator and single-stage amplifier has been proposed in this paper. This work employs NALM technique as the artificial saturable absorber and utilizes figure-of-9 cavity configuration combined with in-band pump scheme to achieve high energy NLP generation. In the oscillator, a segment of long gain fiber is used to provide gain at 2.1 μm and the cavity length is optimized through changing the SMF length in the NALM loop to obtain maximum pulse energy output. By adjusting pump power and intracavity polarization state, stable square-wave NLP with center wavelength of 2102.2 nm and 3-dB spectral width of 11.2 nm is generated. The pulse envelope width and pulse energy are 6.95 ns and 175.1 nJ, respectively. The coherent spike width is 540 fs. After a single-stage amplifier, 5.8 W NLP with a slope efficiency of 56.8% is obtained. At the maximum output power, the pulse envelope width and spike width increase to 7.2 ns and 632 fs, respectively. The pulse energy is boosted to 1.52 μJ. As far as we know, this is the highest pulse energy for 2.1 μm NLP-based fiber laser. The realized high-energy square-wave NLP fiber laser system can be applied in material processing, supercontinuum generation and so on.
Funding
State Key Laboratory of Pulsed Power Laser Technology (SKL2020ZR06).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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