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Abstract
A switchable, widely wavelength-tunable noise-like pulse (NLP) and Q-switched Er-doped fiber (EDF) laser with a linear cavity structure is proposed and experimentally demonstrated in this work. The net-normal-dispersion mode-locked NLP operation based on a semiconductor saturable mirror (SESAM) is realized in a 57 nm continuous tuning range from 1528 to 1585 nm by using a tunable filter (TF). When the pump power is 500 mW, the NLPs produce a maximum average output power of about 16 mW with a 3-dB spectral bandwidth of about 17 nm at the central wavelength of 1555 nm, while the average peak power is about 58.8 W. The measured characteristics of the output NLPs at 1555 nm are consistent with the numerical results under the condition of Δβ2, net = 0.095 ps2, and Esat = 0.77 nJ. In addition, stable Q-switched pulses with a 67 nm wavelength tuning range from 1518 to 1585 nm are obtained by adjusting the central wavelength of the filter. The maximum pulse energy reaches 231.4 nJ at the center wavelength of 1555 nm, corresponding to a peak power of about 278.8 mW. The proposed wavelength-tunable fiber laser is simple and versatile, demonstrating significant potential for numerous practical applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-energy passively mode-locked fiber lasers have attracted considerable attention due to their various applications in material processing, optical sensors, spectroscopy, and biomedical optical imaging [1–4]. Various pulse formation mechanisms have been developed and employed in mode-locked fiber lasers to boost the pulse energy. These include the conventional soliton [5], dispersion-managed soliton [6], similariton [7], and dissipative soliton [8,9]. The dissipative soliton with a highly positive chirp generated from an all-normal or large-net-normal dispersion laser cavity could provide higher pulse energy and be compressed close to the transform limit. Furthermore, the noise-like pulse (NLP) fiber laser provides another effective method to increase pulse energy further and is widely applicable in different fields, such as low spectral coherence interferometry, supercontinuum generation, and laser micromachining [10–12]. Since the first demonstration of NLP by Horowitz et al. in 1997 [13], it has been obtained in both normal and anomalous dispersion regimes with high pulse energy, broad spectrum, and low coherence [14–21]. The NLP is generally considered as a wave packet that typically consists of a series of random fine structures or sub-pulses within it. The autocorrelation trace (AC) of such pulses is characterized by a femtosecond peak located on the top of a wide temporal pedestal [13].
In general, the burst of NLPs is an inherent property of mode-locked lasers operating in strong pumping or high gain conditions [22]. Various methods have been used to promote the generation of NLPs in all normal or net-normal dispersion passively mode-locked fiber lasers. For example, NLPs were generated based on nonlinear polarization rotation (NPR) in the linear cavity [23] or ring cavity [24] by carefully adjusting the polarization controllers (PCs), where the reverse saturable absorption effect played a key role [25,26]. An offset-spliced graded-index multimode fiber with the effect of reverse saturable absorption was used in a Yb-doped fiber (YDF) laser to generate square- and chair-shaped NLPs by controlling the pump power [27]. Some real saturable absorbers (SAs), such as the WS2-based SA [28], single-wall carbon nanotube (SWCNT) [29], and tellurene-based SA [30], etc., also have reverse saturable absorption properties caused by multiphoton absorption under the condition of high input power [29,31]. NLPs were also observed in an actively mode-locked YDF laser and an actively Q-switched Er-doped fiber (EDF) laser [32,33]. In addition, numerical simulations have demonstrated that a super-Gaussian filter contributes to the generation of NLPs in normal dispersion fiber lasers [34,35]. The super-Gaussian filter has an approximately rectangular spectral shape, which ensures that all excited longitudinal modes evolving from white noise have a roughly constant height. When the pulse spectrum width extends out the edges of the filter with an increase in the pump power, the fiber laser tends to operate in the NLP regime.
A widely wavelength-tunable pulse source can provide greater flexibility in wavelength selection aiming at different objects or targets. For example, a wavelength-tunable laser with a narrow optical bandwidth was used to record absorption spectra with very high-frequency resolution [36]. In the lidar system, the laser needed to be tuned to a specific wavelength for monitoring a certain substance [37]. However, there are only a few reports on NLP fiber lasers with tunable center wavelengths. Mashiko et al. first proposed a tunable NLP mode-locked Tm-doped fiber laser with a semiconductor saturable mirror (SESAM) in the linear cavity, where a wavelength tuning range of 1895–1942 nm was obtained by utilizing the chromatic dispersion of telescope lenses [38]. NLPs with a tunable wavelength range from 1568 to 1610 nm were obtained using an Er-doped all-fiber ring laser based on NPR with net anomalous cavity dispersion, where the center wavelengths were not continuously tunable [39]. An all-fiber widely tunable NLP laser based on a fiber loop mirror fused with two segments of polarization-maintaining fibers was reported by Lin et al. recently. The central wavelength was continuously tunable in the range of 1547–1600 nm [40]. So far, all the tunable lasers mentioned above operate in the net-anomalous-dispersion regime, where the formation of NLPs is caused by the combined effect of soliton collapse and positive cavity feedback [41–43], and can be stable due to suppression of solitons localized outside an NLP and the formation of new solitons in this pulse [44]. For the net-normal-dispersion mode-locked fiber laser, the mechanism of the NLP formation, different from the previous case, is attributed to the peak power clamping effect of the laser cavity [45], or spectral filter with an appropriate super-Gaussian order and bandwidth [34,35]. Consequently, the super-Gaussian-shaped filter can play a critical role in the wavelength-tunable NLP fiber laser operating in a net-normal dispersion region, which is experimentally demonstrated in this paper.
In addition, Q-switched pulse generation can also be regarded as an effective way to obtain a high-energy pulse with a relatively long duration and a low repetition rate [46], which is highly desirable for applications in material processing, remote sensing, and medicine, etc. Some scholars have demonstrated that Q-switched and mode-locking operations can be achieved in the same fiber lasers. For example, controllable dissipative soliton and Q-switched pulses were generated based on a hybrid SA of NPR and SESAM in a normal dispersion fiber laser by slightly adjusting the PC [47]. The transition of the pulsed operation from Q-switching, Q-switched mode-locking, and continuous-wave mode-locking can be obtained by precisely adjusting the position of the carbon-nanotube-coated SA mirror in a Yb: KLuW waveguide laser [48]. Additionally, the transformation of Q-switched pulse, dissipative soliton resonance and NLP has been achieved in an all-normal dispersion Yb-doped fiber laser based on NPR by tuning the pump power combined with the orientation of the PCs [49]. However, there are few reports on Q-switched pulse and NLP generated in the same fiber laser based on a real SA, especially for the wavelength-tunable fiber laser.
In this paper, a widely wavelength-tunable NLP mode-locked EDF laser based on the linear cavity with the net-normal dispersion is first demonstrated numerically and experimentally. In Section 2, the influences of the gain saturation energy of the EDF and net-cavity dispersion on the characteristics of the NLPs are numerically analyzed in detail. The properties of output pulses are almost consistent with the experimental results under a certain condition. The experimental setup of the mode-locked EDF laser based on SESAM is presented in Section 3. In Section 4, the experimental results of the wavelength-tunable NLP EDF laser are presented, where the tunable Q-switched pulses can also be obtained in the same EDF laser by carefully adjusting the PC. Finally, conclusions are provided in Section 5.
2. Numerical simulations and results
Figure 1 shows the simulation model of the mode-locked EDF laser based on the linear cavity configuration. The mode-locked EDF laser consists of an EDF segment (LIEKKI Er110-4/125), a piece of OFS980 fiber, a piece of dispersion compensation fiber (DCF, YOFC, NDCF-G652C/250), two segments of single-mode fiber (SMF, SMF1, and SMF2), an optical coupler (OC) with 20% laser output, a wideband tunable filter (TF) (WL Photonics Inc., WLTF-WM-S-1550-100/12), and a SESAM (BATOP SAM-1550-33-2ps) as the SA. The fiber parameters used in the simulations are shown in Table 1. The shape of the spectral filter is taken to be super-Gaussian H(ω) = exp[-ω2/(2Δωf2m)], where an m of 8 is the super-Gaussian order. The parameter Δωf represents the bandwidth of the spectral filter, which corresponds to a wavelength bandwidth Δλf of 12 nm.
The evolution of pulses in the laser cavity can be described with the general nonlinear Schrödinger equation (GNLSE) with the gain coefficient g [52–54]:
The time-dependent absorption loss q(t) of SESAM can be described by the rate equation model [55]:
The numerical model is solved by using the split-step Fourier method (SSFM) and starts with white noise. The integration is then performed in a large number of cycles until the global properties of the NLP emission are displayed. The influences of the net-cavity dispersion Δβ2,net and gain saturation energy Esat of the EDF on the steady-state NLPs are first investigated, as shown in Fig. 2. The energy, Tp_FWHM, and 3-dB spectral bandwidth of the output NLP increase with Esat, where Tp_FWHM is the full width at half maximum (FWHM) duration of the pedestal after Gaussian fitting of the AC curve. For example, when Esat increases from 0.3 to 1.0 nJ, the pulse energy, TP_FWHM, and 3-dB bandwidth increase from 0.7 to 2.4 nJ, from 38.8 to 93.6 ps, and from 13 to 17 nm, respectively, where Δβ2,net = 0.22 ps2. On the other hand, Tp_FWHM can be reduced with a decrease of Δβ2,net. As a result, the spectral bandwidths become broader due to an enhanced self-phase modulation (SPM) caused by the higher peak power. Furthermore, as shown in Fig. 2, the characteristics of the output NLP corresponding to the black point (Δβ2, net = 0.095 ps2, Esat = 0.77 nJ) are almost identical to the experimental results with a pump power of 500 mW, as shown in Fig. 5. The Δβ2, net value of 0.095 ps2 is contributed to by the fiber segments and the tunable filter, which is based on free-space optical Fourier transformation combing with a pair of diffraction gratings [57].
Fig. 2. Numerical simulation results of the output NLP characteristics in the steady state: (a) Single pulse energy; (b) FWHM duration of the pedestal TP_FWHM after Gaussian fitting of the AC curve; (c) The 3-dB bandwidth of the averaged spectrum of the 300 consecutive spectral profiles as functions of cavity net dispersion Δβ2, net and EDF saturation energy Esat. The black point refers to the experiment reported here.
Figures 3(a) and 3(b) show the evolutions of the NLPs in the temporal and spectral domains at the black point of Fig. 2. It can be observed that the small NLP outside the main NLP does not withstand competition with the main NLP and disappears with the increase of round trip [44]. A stable NLP can be obtained after about 2000 round-trips for the mode-locked EDF laser. The relatively small variation range of the envelope widths in the temporal and spectral domains indicates that the pulse is in a stable state in the following round-trip simulation. The fine structures with varying intensities and durations in a pulse envelope (shown in Fig. 3(c)) and the AC trace with a double-scale structure (shown in Fig. 3(d)) are typical characteristics of the NLP, where the FWHM duration of the pedestal is 29.8 ps. The frequency chirp with a complex structure in Fig. 3(c) indicates that the NLP can hardly be compressed. The pulse spectrum of the 4000th simulation round trip is shown in Fig. 3(e), where the central wavelength of the averaged spectrum is 1555 nm and the 3-dB bandwidth is 16.7 nm.
Fig. 3. Simulation results of the NLP buildup in the net-normal dispersion EDF laser with Δβ2, net = 0.095 ps2 and Esat = 0.77 nJ: (a) Temporal domain and (b) Spectral-domain evolutions from noise; (c) Pulse envelope with frequency chirp; (d) Normalized AC trace; (e) Individual spectral intensity (blue line) at the 4000th simulation round trip on a logarithmic scale. The red line is an averaged spectrum with 300 consecutive spectral profiles.
3. Experimental setup
The wavelength-tunable mode-locked EDF laser with a SESAM as the mode locker is shown in Fig. 4. The 0.9-m-long EDF with a 110 dB/m core absorption at 1530 nm is pumped by a 976 nm laser diode with 500-mW maximum pump power through a 980/1550 nm WDM. A circulator cooperated with a TF with a tunable spectral range of 1500–1600 nm and tuning resolution of less than 0.1 nm is used as one end of the linear cavity. An SMF1 is employed to tune the total net-cavity dispersion and enhance the nonlinear effects. A PC is placed in front of the SESAM to adjust the intracavity polarization state and minimize the cavity loss. The total cavity length is about 20 m. In addition, an optical spectrum analyzer (Yokogawa Electric Corporation, AQ6375, OSA) is used to measure the output spectra. A photodetector (PD) with a 1 GHz bandwidth together with an electrical spectrum analyzer (Agilent Technologies, N9010A, ESA) and a digital sampling oscillograph (Tektronix DPO3032) are employed to monitor the stability of output pulses. The temporal width of the output pulses is obtained by a commercial optical autocorrelator (Femtochrome Research Inc, FR-103XL).
Fig. 4. Experimental setup of the wavelength-tunable EDF laser with a linear cavity structure. TF: tunable filter; Circulator: optical circulator; SMF: single-mode fiber; EDF: Erbium-doped fiber; WDM: wavelength division multiplexer; Pump: 976 nm laser diode; PC: in-line polarization controller; OFS980: pigtail fiber of the WDM; DCF: dispersion compensation fiber; OC: optical coupler; SESAM: semiconductor saturable mirror.
4. Results and discussions
4.1 Tunable NLP fiber laser
The central wavelength of the TF is first set at 1555 nm to investigate the mode-locking properties. When the pump power is over 117 mW, the stable NLPs are generated by properly adjusting the PC. Moreover, by fixing the PC’s orientation, no pulse splitting is observed as the pump power increases to the maximum of 500 mW. Due to the operation in the net-normal dispersion regime, we didn’t observe a pulse spectrum with sidebands, which is considered to be an intrinsic feature of the conventional soliton in the anomalous-dispersion regime [58,59]. Both measured and calculated pulse spectra centered at 1555 nm as well as the AC curves of the output pulse are shown in Figs. 5(a) and 5(b). The pulse spectrum without steep edges has a smooth profile with about 17-nm 3-dB bandwidth from 1548 to 1565 nm, which is nearly identical to the calculated result. However, the measured pulse spectrum exhibits a slight asymmetry because that, the actual gain spectrum of the EDF is approximated by a Lorentzian profile with a symmetric shape, while the higher-order dispersion and nonlinear effects are neglected in the simulation for a simplicity. Moreover, as shown in Fig. 5(b), the AC curve of output pulse exhibits a double-scale structure with a narrow coherent spike of 0.56 ps duration on top of the wide pedestal of 28.9 ps, which is slightly shorter than the calculated result in Fig. 3(d). Note that the spike and pedestal duration can be defined as the FWHM duration of the Gaussian fitting AC curves. A pulse train with the pulse interval of 107.6 ns in Fig. 5(c) is consistent with the RF spectrum centered at about 9.3 MHz in Fig. 5(d). A signal-to-noise ratio of about 66 dB at the fundamental frequency indicates a reasonably low noise level of the NLP.
Fig. 5. (a) Measured and calculated output spectra of the single-pulse mode-locked operation; (b) Measured and calculated AC traces (blue line) with the Gaussian fitting (red line); (c) Pulse train in the oscilloscope; (d) Radio frequency (RF) spectrum in the ESA with a span range of 20 kHz of the output pulse from the NLP mode-locked EDF laser. The pump power is 500 mW, and the center wavelength of TF is set to1555 nm.
When the PC is fixed, the output power and pulse energy of the NLP increase with pump powers, as shown in Fig. 6(a). A maximum average output power of a about 16 mW and corresponding pulse energy of about 1.7 nJ can be obtained when the pump power reaches up to 500 mW. With the pulse energy of 1.7 nJ and the pulse packet width of 28.9 ps, the average peak power of the output NLP is about 58.8 W [60]. The 3-dB spectral bandwidths of the NLPs can increase from 11 to 17 nm with the pump power due to SPM-induced spectral broadening, as shown in Fig. 6(b). In addition, the central wavelength of the output-pulse spectrum is slightly red-shifted from 1555.2 to 1556.4 nm with decreasing the pump power. The reason of this behavior is related to the reabsorption effect of the rare-earth ions in the active fibers, which can happen with decreasing the pump power or increasing the active fiber length [40,61]. Here, the EDF length in the laser cavity is fixed. When the pump power is reduced, the reabsorption effect can become strong and the reduction of gain at shorter wavelength is larger, which results in a red shift of the pulse spectrum. The output powers under three different pump powers are measured within 6 hours, as shown in Fig. 6(c). The power fluctuations from the average values are all less than 0.3 mW, indicating that the EDF laser is relatively stable for a long time in the lab.
Fig. 6. (a) Output power (blue line) and pulse energy (red line) as functions of pump power; (b) Spectral properties at different pump powers; (c) Output power measurement within 6 hours.
The wavelength tuning of the NLP mode-locked EDF laser is then realized by shifting the center wavelengths of the TF with a pump power of 500 mW, where the output spectra are shown on logarithmic and linear scales, as shown in Figs. 7(a) and 7(b). A wide wavelength-tunable range of 57 nm from 1528 to 1585 nm combined with a 3-dB bandwidth varying from 9 to 17 nm can be obtained in the net-normal-dispersion EDF laser, where the central wavelength is continuously tunable in the range of 1528–1575 nm. A tiny adjustment of the PC is needed to maintain the pulse stability at other wavelengths due to the decrease in EDF gain. As shown in Fig. 7(c), the output powers and pulse energies of the NLPs at different central wavelengths are more than 10 mW and 1.1 nJ, respectively. Furthermore, as shown in Fig. 7(d), the signal-to-noise ratios of > 40 dB also indicate that the fiber laser can operate stably over the wavelength range of 57 nm. Therefore, it has significant potential for specific applications in the C-band and L-band.
Fig. 7. (a) Output spectra of the NLPs on a logarithmic scale; (b) Normalized output spectra on a linear scale; (c) Output power (blue line) and pulse energy (red line), and (d) signal-to-noise ratio at different central wavelengths.
4.2 Tunable Q-switched pulse fiber laser
When the central wavelength of the TF is selected at 1553 nm and the pump power is set to 117 mW, the Q-switched pulse is observed in the oscilloscope by carefully adjusting the PC. Due to the pump hysteresis effect [62], the Q-switching threshold can be reduced to about 100 mW. A typical feature of Q-switching is that an increase in the pump power can decrease the pulse-to-pulse separation and increase the repetition rate [63,64]. For example, when the pump powers are 100, 300, and 500 mW, the pulse intervals obtained from oscilloscope traces are 18.6, 11.6, and 9.2 µs, respectively, as shown in Fig. 8(a). As the photon density builds up following the achievement of a net positive population inversion in EDF, the Q-switched pulse will be rapidly emitted once the energy stored in the cavity bleaches the SA into a high transmission state, which means that a higher gain can saturate the SA at a faster rate to generate the Q-switched pulse. As a result, the repetition rate increases while the pulse duration decreases [65,66]. As shown in Fig. 8(b), when the pump power increases from 100 to 500 mW, the repetition rate varies from 53.7 to 108.7 kHz, while the pulse duration decreases from 2.45 to 0.83 µs. The characteristics of the Q-switched pulses presented here are nearly identical to those of the self-Q-switched pulses reported in [67,68]. However, the physical mechanisms for the Q-switched pulse generations are different in the two cases. In the latter case, no additional SA is used in the cavity, and the physical mechanism of self-Q-switching is a strong power-dependent thermo-induced lensing within the active fiber due to excited-state absorption in erbium. In addition, it can be seen in Fig. 8(c) that the measured average output powers and corresponding pulse energies increase almost linearly with the pump powers, from 5.8 to 20.2 mW and from 109.1 to 185.6 nJ, respectively. The slope efficiency is about 3.6%. It is noteworthy that when the pulse energy reaches 184 nJ with the pump power of 450 mW, it becomes nearly unchanged with a further increase in the pump powers. However, the variations of the pulse power and energy with the pump powers of > 500 mW aren’t shown here because of the maximum available pump power. As shown in Fig. 8(d), the spectra can be broadened from 0.7 to 2.1 nm because of an increase in peak powers with the pump powers. In addition, the temporal profile of the single pulse shown in Fig. 9(a) has the FWHM duration of 0.83µs, while the RF spectrum in Fig. 9(b) shows a signal-to-noise ratio of about 60 dB with the center frequency of 108.7 KHz when the central wavelength of the TF is 1553 nm and the pump power is 500 mW.
Fig. 8. (a) Passively Q-switched pulse trains at the pump powers of 500, 300 and 100 mW, respectively. (b) Pulse duration (blue line) and repetition rate (red line), and (c) output power (blue line) and pulse energy (red line) as functions of pump power. (d) Spectral property at different pump powers.
Fig. 9. (a) Single-pulse profile in the oscilloscope; (b) RF spectrum in the ESA with a span range of 100 kHz, where the central wavelength of the TF is 1553 nm, and the pump power is set to 500 mW.
As shown in Figs. 10(a) and 10(b), when the pump power is 500 mW, the stable Q-switched pulses can be continuously tuned over a wavelength range of 1518–1585 nm by shifting the central wavelengths of the TF, where the spectra with the center wavelengths of 1545 and 1556 nm show the wider bandwidths mainly because of the relatively higher gain of the EDF in this wavelength region. For the same reason, as shown in Fig. 10(d), the average powers and energies of the output pulses have peak values near 1555 nm. Moreover, the Q-switched pulse with a center wavelength of 1535 nm exhibits another spectral peak at 1530 nm due to the ASE within the 12-nm bandwidth of the TF. However, when the center wavelengths deviate from 1535 nm, the output spectra have negligible intensities near 1530 nm because of the large losses caused by spectral filtering. For the same reason, as shown in Fig. 10(e), a low signal-to-noise ratio of 35.3 dB is observed near 1535 nm, while signal-to-noise ratios of > 45 dB are found at other wavelengths, indicating the stable state of the Q-switched pulses. Again, since a higher gain near 1555 nm will lead to a more rapid bleaching of the SA due to the faster population inversion/depletion rates [69], the repetition rates of the output pulses can reach their maximum values near 1555 nm, as shown in Fig. 10(c). However, this behavior is in contrast with the pulse duration. The Q-switched pulses with repetition rates of 83.9–111.2 kHz exhibit pulse durations of 0.75–1.27 µs, while the maximum and minimum average output powers are 25.2 and 7.7 mW at 1556 and 1520 nm, respectively, corresponding to 231.4 and 85.1 nJ of the pulse energies, and 278.8 and 67 mW of the peak powers.
Fig. 10. (a) Output spectra of Q-switched pulses on a logarithmic scale; (b) Normalized output spectra on a linear scale; (c) Pulse duration (blue line) and repetition rate (red line), and (d) output power (blue line) and pulse energy (red line) as functions of the central wavelength; (e) signal-to-noise ratio at different central wavelengths.
5. Conclusion
A switchable, widely wavelength-tunable NLP and Q-switched EDF laser with a linear cavity structure was presented and demonstrated in this work. The influences of the EDF Esat and cavity Δβ2,net on the characteristics of the output NLPs were numerically investigated. It was found that under the condition of Δβ2, net = 0.095 ps2 and Esat = 0.77 nJ, the output pulse properties, including the pulse energy, FWHM duration of the fitted AC trace, and 3-dB spectral bandwidth, were consistent with the experimental results. Experimentally, a 57-nm wavelength tuning range of 1528–1585 nm could be achieved in the mode-locked NLP operations while maintaining 3-dB spectral bandwidths in the range of 9–17 nm. Obtained signal-to-noise ratios of > 40 dB indicated that the EDF laser operated in a stable state at different center wavelengths. When the pump power reached 500 mW, the EDF laser obtained the maximum average output power of about 16 mW with the 3-dB spectral bandwidth of about 17 nm at the central wavelength of 1555 nm, while the average peak power is about 58.8 W. To the best of our knowledge, it is the first report of a net-normal-dispersion NLP EDF laser with a 57 nm tuning range based on SESAM. In addition, stable Q-switched pulses were also obtained, while the center wavelengths of the output pulses could be tuned over a wavelength range of 1518–1585 nm by using the TF. The maximum pulse energy can reach 231.4 nJ at the center wavelength of 1556 nm, while the peak power is about 278.8 mW. As far as we know, no previous reports have realized tunable NLPs and tunable Q-switched pulses in the same laser.
This simple and versatile wavelength-tunable fiber laser can meet a range of application requirements. Further improvement of the pulse energy and tunable wavelength bandwidth of the NLP will be undertaken in our future work.
Funding
National Natural Science Foundation of China (61575018).
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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