1. Introduction

Frequency-modulated continuous-wave (FMCW) Lidar based on a single-frequency fiber laser (SFFL) modulated by a sawtooth waveform is an excellent candidate for ranging static targets with high precision in remote sensing [1], automatic driving [2], and 3D imaging [3]. However, many applications require the velocity and distance information of subjects simultaneously, and the previous method unfortunately becomes invalid. Generally, this conflict can be managed via complicated modulation schemes [4,5] or advanced laser sources [6,7]. Compared with complicated modulation schemes that require sophisticated transmission systems and receiving devices [5], an advanced laser source that has two longitudinal modes makes the FMCW system have a simple structure and a low requirement on arithmetic processing [7]. A dual-frequency fiber laser (DFFL) with compact construction and a stable frequency gap is an excellent advanced laser source for FMCW Lidar. Consequently, a switchable single-dual-frequency fiber laser (SSDFFL) is eligible for the measurement of both static and dynamic targets.

During the past two decades, few methods for building SSDFFLs have been reported [8–14]. In general, an SSDFFL can be achieved by replacing some components of an SFFL. A distributed Bragg reflector (DBR) fiber laser is a classical SFFL that can be transformed into an SSDFFL by inscribing a fiber Bragg grating (FBG) in a polarization-maintaining highly doped gain fiber [8]. Although it has compact construction, the gain fiber radically increases the cost. A composite cavity fiber laser based on a circular traveling wave cavity removes this obstacle but requires loading of filters to suppress the mode hopping and longitudinal mode competition. Accordingly, researchers obtained an SSDFFL with two counterpropagating single-frequency laser beams by using a Mach-Zander interferometer, two narrow band filters, and two isolators [9]. Obviously, the stability declines and the insertion loss is enhanced due to numerous devices being inserted into the cavity. However, the FBG that has a narrow band and low insertion loss is an excellent filter. SSDFFLs consisting of two separated FBGs with different wavelengths and narrow band filters, such as a Fabry-Perot cavity, subrings and a phase-shifted FBG (PS-FBG), have been introduced [10,11]. Nevertheless, two major problems still exist. On the one hand, these SSDFFLs usually have a beat frequency over dozens of GHz, which is difficult to monitor with a photoelectric detector (PD) in practical applications. Fortunately, because the FBG is fabricated directionally, it can provide appropriate intrinsic birefringence to support two polarization-mode operation and generate a beat signal of MHz order [15]. On the other hand, the SSDFFL will be disturbed by longitudinal mode competition because the bandwidth of filters is usually several times broader than the longitudinal mode interval. A saturable absorber (SA), in which a narrow dynamic absorption grating is formed by the counterpropagating laser beams, can eliminate this disturbance [16], as only high-intensity light can pass through the SA with very low loss. In the early days, unpumped gain fibers several meters long played the role of SAs [17,18], but a high loss was also introduced. Recently, low-dimensional materials (LDMs), such as graphene [19] and MoS₂ [20], and metal-organic frameworks (MOFs) [21] and transition metal carbides and nitrides (MXenes) [22], which have fascinating nonlinear optical responses, were set as SAs to achieve SFFLs with low loss.

In this paper, we demonstrate a stable SSDFFL with a PS-FBG and Ti₂C MXene quantum dots (QDs) for the first time, which has a laser linewidth as narrow as 473.5 Hz, a relative intensity noise (RIN) as low as −101.5 dB/Hz, and an optical signal-to-noise ratio (SNR) over 80 dB when it operates as an SFFL. It has a practical beat frequency of 122 MHz when it is switched to a DFFL. Meanwhile, it has a low threshold value of 6 mW and a high slope efficiency of 2.2%. To the best of our knowledge, compared to other SSDFFLs, the SSDFFL proposed in this paper has the optimal performance in terms of laser linewidth, SNR, and threshold.

2. Experimental setup

As introduced in our previous work [23], an SFFL with high performance based on Ti₂C QDs has been achieved but with mode hopping at a certain time interval, which can be eliminated by narrow band filters. Consequently, we constructed a mode-hop-free SFFL with a PS-FBG and Ti₂C QDs. In addition, a manual polarization controller (PC) was employed to adjust the loss of differently polarized light and to switch the laser between the single-frequency and dual-frequency schemes by altering the transverse pressure that is applied on a section of intracavity fiber [24]. Figure 1 illuminates the architectural structure of this SSDFFL.

 

Fig. 1. a) Diagram of the SSDFFL with a PS-FBG and an SA, b) transmitted spectra of the PS-FBG and the ordinary FBG, and c) the saturable absorb curve and the TEM image of Ti₂C MXene QDs

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The SSDFFL is a loop cavity fiber laser pumped by a 980 nm semiconductor laser whose power is adjustable. The pump power enters a wavelength division multiplexer (WDM, 980/1550 nm) and stimulates an erbium-doped fiber (EDF), which is connected to a three-port circulator. The laser beam and residual pump light pass through Port1 to Port2 of the circulator. After Port2, an SA covered with Ti₂C QDs and a PC and an ordinary FBG (central wavelength=1548.3 nm) are situated in order. As a result, a fractional laser beam is reflected and interferes with the counterpropagating laser beam between Port2 and the FBG. Combined with the saturable absorbability of Ti₂C QDs, a dynamic grating with a narrow band is formed in the SA [25]. Next, the reflected laser is transmitted to the PS-FBG (wavelength of phase shift peak=1548.3 nm) via Port3 to select the single longitudinal mode. The spectra of the ordinary FBG and PS-FBG are shown in Fig. 1(b)). Moreover, the nonlinear optical response, or the properties of saturable absorption are proposed in Fig. 1(c)) and are studied by a balanced twin-detector system [23,26] with an ultrashort pulse laser source (pulse width=120 fs, repetition frequency=13.3 MHz). As we can see, experimental data is fitted by the function $T = 1 - \Delta T\Delta {e^{ - \frac{I}{{{I_s}}}}} - {T_{ns}}$, where T, I, $\Delta T$, ${I_s}\; $ and ${T_{ns}}$ are the transmittance, power density, modulation depth, saturation power density and nonsaturable loss, respectively. And the last three parameters are calculated to be 26%, 1.75 GW/cm² and 8.8% respectively. Figure 1(c) indicates that MXene QDs can be used as saturable absorber and the inset which is the TEM image of MXene QDs shows the size of QDs directly. Finally, an optical coupler is used to output 10% of the laser and the residual fraction back to the EDF.

Notably, mode competition occurs when the SA is removed from the cavity, as shown in Fig. 2 which is measured by a scanning Fabry-Perot interferometer (SFPI). Here, the sawtooth wave voltage which drives the piezoelectric transducers to adjust the cavity length of SFPI is plotted in yellow and the signal voltage of the laser without and with mode competition are plotted in blue and red respectively.

 

Fig. 2. SFPI spectra of the laser with the SA (blue) and without the SA (red); mode competition occurs when the SA is removed from the cavity

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3. Results and discussion

During the experiment, the pump power was set as a constant value, and the SFPI was used to monitor the output light in real time to verify the operating state. First, we achieved an SFFL by rotating the PC to a proper angle. In a typical single-frequency laser spectrum, two smooth peaks in one scanning period could be observed. We name this single-frequency state single frequency A (SFA). Then, we rotated the PC again, the power of these two peaks decreased, and two other peaks gradually grew at adjacent positions, which means that the SFFL is converted into a DFFL. By measuring the time gap, the frequency gap was estimated to be ∼122 MHz. Finally, these new peaks superseded the primary peaks, and the laser turned into an SFFL again; this state was named single frequency B (SFB). In general, this laser can be switched between three operating states, including two single-frequency states and one dual-frequency state. Figure 3(a) draws various SFPI spectra corresponding to different operating states where we can find that the beating signal of dual-frequency operating state generates from SFA and SFB. Additionally, when the operating state was fixed, the SFPI signal intensity increased as the pump power was enhanced, but the number of peaks oscillated when the pump power exceeded a certain value, which implied that the laser was a multifrequency laser. Figure 3(b) demonstrates the output power versus the pump power in the range of the non-multifrequency laser and the evolution of the SFPI spectrum in the inset. The slope efficiencies of various operating states slightly differ, caused by different intracavity losses, but the threshold values are almost equivalent, which were measured to be 6 mW. Linear fitting was performed on the experimental data, and the slope efficiencies were calculated to be 2.22%, 2.1% and 1.91% for the three operating states. Furthermore, the difference in slope efficiency leads to a distinction in the max output power. The highest output power reaches approximately 1 mW with a pump power of 50 mW under SFA conditions.

 

Fig. 3. a) Various SFPI spectra corresponding to different operating states, b) output power versus pump power for different operating states; the SFPI signal intensity increases as the pump power is enhanced

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Then, we investigated some characteristics of the two single-frequency operating states and the dual-frequency operating state. The laser linewidth is the parameter of most concern for an SFFL, which can be calculated from the −20 dB band width of the power spectra divided by $2\sqrt {99} $ that measured by a delay self-heterodyne system (DSHS) [27] and fitted with the Lorentz curve. As the bandwidths of the power spectrum were measured to be 9.47 kHz and 10.62 kHz for SFA and SFB, laser linewidths were calculated to be 473.5 Hz and 531 Hz respectively which are depicted in Fig. 4(a-b). The broadening of the laser linewidth is mainly contributed by environmental vibration noise [28], such as mechanical vibration and changes in temperature, and its influence on the laser cavity is anisotropic. In our opinion, laser beams with different polarizations will be diversely broadened owing to their dissimilar azimuths. Additionally, the RIN is another important parameter that limits the practical application of SFFLs. Because the RIN depends on the signal power [29], we measured it with different pump powers by a high-speed PD combined with an electrical frequency spectrum analyzer (EFSA, DSA815, noise floor=−135 dB/Hz). Figure 4(c) shows the RIN of SFA and SFB corresponding to a pump power of 50 mW, and the inset depicts the evolution of the RIN with changing pump power. From this graph, two relaxation oscillation peaks can be observed at approximately 20 kHz and 35 kHz, which correspond to the pump and laser signals, respectively, when the pump power is set as 50 mW. The RIN values of the pump for both SFA and SFB are almost identical, while the RIN values of the laser signal are widely divergent and measured to be −97.2 dB/Hz and −101.5 dB/Hz, respectively, which result from the distinct output powers. The RIN is restricted by the measurement system, and we believe that a more accurate RIN can be obtained by employing a system with lower floor noise. In addition, with increasing pump power, the relaxation oscillation peaks of both SFA and SFB drift to higher frequencies, and the RIN value decreases. We also note that the laser signal has a faster drift than the pump signal. As expected, the optical spectra of SFA and SFB almost overlap but slightly differ in intensity. Optical spectra of SFA and SFB are presented in Fig. 4(d), and the SNRs are 83 dB and 80 dB, respectively.

 

Fig. 4. Characteristics of SFA and SFB: measurement of a) SFA and b) SFB, and c) comparison of the RIN and d) the optical spectra

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For the dual-frequency operating state, we are more interested in the beat signal, which was monitored by the high-speed PD and EFSA. Due to the directional inscription for the fabrication of the FBG and the asymmetry of the fiber, a certain intrinsic birefringence was introduced [15,30]. As a result, a laser beam with elliptic polarization can be decomposed into two beams with perpendicular polarization which corresponds to different propagation constants in fiber. So two laser beams with perpendicular polarization modes can transmit in the laser cavity, and due to a certain length fiber between the output port and PD with a weeny birefringence [24], they can interfere with each other in the PD to generate a beat frequency with a magnitude of MHz. Figure 5 presents the beat signal, where a peak with an SNR of ∼20 dB is located at 122 MHz, which corresponds to the result of 122 MHz from the SFPI spectrum in Fig. 3(a). A beat signal with a magnitude of MHz is easy to detect and deal with in practical applications.

 

Fig. 5. Beat signal of dual-frequency operation

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In the end, for a more visual comparison, properties of SSDFFLs achieved by other researchers and the method introduced in this paper are listed in Table 1. The comparison indicates that the new laser configuration proposed in the paper not only has a practical beating frequency but also has the highest SNR, the lowest threshold, and a high-quality linewidth.

Table 1. Properties of SSDFFLs achieved with different methods

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4. Conclusion

In summary, a new SSDFFL based on a loop cavity was introduced in this paper. The laser can be switched between two SFFLs and a DFFL by adjusting a PC. Furthermore, we investigated the characteristics of the SFFLs and DFFL. The threshold value of the SSDFFL is as low as 6 mW. The laser has a better performance in the case of SFA, with a slope efficiency of 2.2%, a laser linewidth as narrow as 473.5 Hz, an RIN of −101.5 dB/Hz and an SNR reaching 83 dB. To the best of our knowledge, this SSDFFL has the optimal performance in terms of laser linewidth, SNR, and threshold for an SSDFFL. Additionally, when the laser is switched to a DFFL, a 122 MHz beat frequency can be detected by a PD, which has potential application in FMCW Lidar for simultaneous ranging and velocity measurement with a manually or electrically tunable PC which alter the transverse pressure applied on a section of intracavity fiber to adjust the polarization of the laser.