The luminescence spectra of the bismuth-doped fiber (BDF) can be located at different central wavelengths according to the excitation wavelengths and to the co-doped elements, such as aluminum, phosphorus, and germanium, ranging from 1.1 to 1.8 µm^[1,2], which can fill the special wavelength gap between conventional rare-earth ions, such as ytterbium (1.0 µm), erbium (1.5 µm), and thulium (2.0 µm). Particularly, the wavelength range at 1.3 µm is crucial for optical communication^[3]. In addition, being situated within the second biological imaging window (1100–1350 nm), 1.3 µm is well-suited for various biological imaging applications, including multiphoton microscopy^[4] and optical coherence tomography^[5], which can decrease optical scattering and absorption in deep tissues^[6]. Currently, pulse fiber lasers at this wavelength are typically achieved through Fourier domain mode locking^[7], optical parametric amplifiers^[8], second-harmonic generation^[9], soliton self-frequency shift^[10], and Raman fiber lasers^[11,12], leaving passive mode-locked bismuth-doped fiber lasers (BDFLs) largely unexplored. This is because of the technical challenges in the passive mode-locked BDFL, especially the relatively low gain coefficient of the BDF. It is also noticed that the intracavity net dispersion of the BDFL is hard to control since the zero dispersion of the single-mode fiber (SMF) is at around 1310 nm, and the effect of higher-order dispersion, such as β3, becomes comparable to second-order dispersion β2^[13]. More importantly, from the application perspective, the power scaling of ultrashort pulses at 1.3 µm through the bismuth-doped fiber amplifier (BDFA) is important^[14].

To this end, several studies have been reported on a BDFL operating at 1.3 µm. In 2013, researchers demonstrated a linear cavity BDFL at 1320 nm utilizing semiconductor saturable absorber mirrors in both anomalous and normal dispersion regimes using a chirped fiber Bragg grating (CFBG) for dispersion management^[15]. In 2017, a figure-of-eight BDFL at 1310 nm operating in a dissipative soliton regime with a large normal dispersion was demonstrated, and the pulse energy was further amplified up to 8.5 nJ by a BDFA^[16]. Additionally, a BDFL at 1320 nm mode-locked by single-walled carbon nanotubes was also operated in the dissipative soliton regime^[13].

Lasers can work in different mode-locking regimes with different laser cavity parameters, including dissipative soliton, dissipative soliton resonance (DSR), and noise-like pulse (NLP)^[17–20]. Compared with other mode-locking regimes, DSR and NLP both have wider square-wave pulses with higher pulse energies. DSR typically generates a single rectangular pulse, while square-wave NLP contains a bunch of pulses with varying pulse durations and intensities^[21–23]. In 2017, Thipparapu et al. demonstrated a ring cavity BDFL at 1340 nm operating in the NLP regime with anomalous dispersion, and the laser was amplified by a BDFA to achieve a maximum pulse energy of 2.9 nJ^[24]. Subsequently, in 2021, using a 500-m SMF inside the cavity, the same research group demonstrated a figure-of-nine BDFL at 1340 nm operating in the DSR regime with a repetition rate of 362 kHz and a maximum pulse energy of 30 nJ^[25]. In 2023, another figure-of-nine BDFL at 1310 nm also operating in the DSR regime was reported with high efficiency^[26]. Additionally, a dual-wave band DSR at 1.3/1.4 µm in a passively mode-locked BDFL was also reported^[27].

The understanding of versatile pulse dynamics in the passive mode-locked BDFL is of significant importance for generating high-quality ultrashort pulses at 1.3 µm. In this work, we present a versatile BDFL based on the nonlinear polarization rotation (NPR) technique, which operates in two regimes with different net dispersions in the same cavity, including a square-wave NLP regime at 1331 nm and a multi-pulse soliton regime at 1320 nm. A BDFA is implemented to boost its average power. We also investigate the pulse-to-pulse evolutions and the multi-pulse dynamics in these two regimes.

The experimental setups of the BDFL and BDFA are depicted in Fig. 1. Two types of BDFs were used in this work, and their key parameters are shown in Table 1. The 36-m BDF used in the BDFL was provided by the Russian Academy of Sciences, Russia, while the 200-m BDF for the BDFA was homemade by the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, China. Both BDFs are composed of bismuth, phosphorus, and silica, and the luminescence wavelengths are 1.3 µm at the O-band range. The gain coefficients of the BDFs were measured to be 0.18 and 0.11 dB/m, respectively. To this end, to provide enough gain for mode-locking and amplification, the lengths of the BDFs used in this experiment were relatively long.

The mode-locking of the BDFL was implemented with the NPR technique, wherein there were two polarization controllers (PCs) for controlling the polarization state. A Raman laser with a maximum power of 1 W at 1209 nm was used to pump the BDFL through a wavelength-division multiplexer (WDM). A polarization-dependent isolator (PD-ISO) ensured the unidirectional operation of the laser inside the cavity, and 10% intracavity power was extracted through a 90/10 optical coupler (OC).

The fiber pigtails of the OC, ISO, and WDM were all SMFs and the total length of the cavity was 41 m. The SMF has a zero-dispersion wavelength at around 1310 nm with anomalous dispersion at longer wavelengths. The BDF has a zero-dispersion wavelength at around 1333 nm with normal dispersion at shorter wavelengths^[13]. Thus, the net dispersion of the cavity depends on the mode-locked central wavelength of the laser. The 10% output port of the OC was connected to another WDM to clean up the residual pump. An ISO was placed after the WDM to protect the BDFL from backward reflection. The output from the ISO was launched into the BDFA to boost the average power.

The BDFA contained a 200-m BDF, which was simultaneously forward and backward pumped by two laser diodes (LD) through two WDMs to improve the amplification efficiency in such a long gain fiber. The central wavelengths of the two pump LDs were both 1240 nm, with a maximum pump power of 500 mW for each. An ISO was placed at the output port of the LDs to prevent optical damage from the backward light.

By adjusting the pump power and the PCs, the BDFL can achieve mode-locking in the square-wave pulse regime. The characteristics of the BDFL with a pump power of 440 mW are shown in Fig. 2. The output power of the seed signal is 4 mW. As shown in Fig. 2(a), the temporal profile of the single square-wave pulse has a pulse duration of 844 ps with a relatively flat pulse top. The inset shows the corresponding autocorrelation trace (AC) measured with an output power of 30 mW amplified by the BDFA. The AC trace has a wide pedestal and a narrow peak with 0.29 ps pulse duration in the middle, identifying that the fiber laser is mode-locked in the NLP regime. This is a typical feature of the NLP compared with the dispersion soliton resonance (DSR) regime. Although the latter also has a square-wave pulse performance in the temporal domain, the square-wave NLP contains a bunch of pulses with varying pulse durations and intensities^[21].

The optical spectra are shown in Fig. 2(b). The amplified spontaneous emission (ASE) spectrum of the 36-m BDF has a central wavelength of about 1320 nm with a full width at half-maximum (FWHM) of 70 nm under the 1209-nm pump. The square-wave NLP is mode-locked at a central wavelength of 1331 nm with an FWHM of 5 nm. The weak Kelly sidebands indicate that the BDFL operates at anomalous dispersion. The oscilloscopic trace of the pulse train illustrates that the pulse interval is about 198 ns, as shown in Fig. 2(c). The radio frequency (RF) spectrum with a resolution bandwidth (RBW) of 10 Hz and a span of 7 MHz is depicted in Fig. 2(d). The fundamental frequency is 5.05 MHz, determined by the 41-m cavity length. The signal-to-noise ratio is higher than 61 dB with weak sidebands on both sides of the fundamental frequency, indicating the existence of NLP^[20]. The inset shows the RF spectrum of higher harmonics frequency with an RBW of 1 kHz and a span of 5 GHz. The modulation bandwidth of the frequency is about 1.19 GHz, corresponding to the pulse duration of 844 ps.

Figure 3 depicts the versatile performances of the BDFL operating in the square-wave NLP pulse regime with different pump powers while keeping the PC fixed. With the increase in the pump power from 220 to 540 mW, the pulse duration (from 258 to 1110 ps), output power (from 1.3 to 4.8 mW), and pulse energy (from 0.26 to 0.95 nJ) increase, while the peak power almost remains consistent, i.e., around 900 mW, as shown in Figs. 3(a) and 3(b). The red oval indicates the mode-locked state at a pump power of 440 mW, corresponding to the results in Figs. 2 and 4(b), where the pulse duration is 844 ps, the output power is about 4 mW, the calculated pulse energy is 0.77 nJ, and the calculated peak power is 0.92 W.

The evolution of the temporal profiles of the single square-wave pulse is depicted in Fig. 3(c). The peak intensity of the pulse remains at different pump powers due to the peak power clamping effect^[28]. The corresponding optical spectra are shown in Fig. 3(d). As the pump power increases, the spectral intensity increases, and the central wavelength of the optical spectrum slightly shifts to a longer wavelength.

Then, we investigated the performance of the BDFA with the BDFL operating in the square-wave NLP regime, as shown in Fig. 4. The ASE spectrum of the 200-m BDF was measured without a signal injection, as shown in Fig. 4(a). The central wavelength is 1325 nm with an FWHM of 50 nm under the 1240 nm pump. The steep edge on the left side is caused by the transmission bandwidth of the WDM, where the shorter wavelength of the ASE spectrum is filtered.

The optical spectra before and after the BDFA are provided in Fig. 4(b). The blue curve is the optical spectrum of the seed signal with a pulse duration of 844 ps, i.e., the same as Fig. 2. The green curve is the amplified optical spectrum at the maximum total pump power of 880 mW, corresponding to the maximum output power of 102 mW. There exists a residual pump at 1209 nm, which is 20 dB lower than the signal at 1330 nm. Because of the wide pulse duration with low peak power, the amplified optical spectrum does not show obvious spectrum broadening or distortion due to the fiber nonlinearity.

Figure 4(c) shows the variation of the output power with increasing pump power under different arrangements of bidirectional pumping of the BDFA. As shown by the blue curve, the forward pumping LD1 was first turned on from zero to a maximum of 452 mW, and then the backward pumping LD2 was turned on to reach the total maximum pump power of 880 mW. At the beginning, during the forward pumping process, the average power increased slowly with a low efficiency. However, when the backward pumping LD2 was turned on, the average power rapidly increased, and the efficiency rose. Likewise, when the backward pumping LD2 was turned on first and then the forward pumping LD1, the output power of the BDFL went through a similar trend with the same maximum power.

Since the BDFL mode-locked in the NLP regime with a pulse duration tunability, we studied the variation of the maximum output power and the efficiency under different pulse durations, as shown in Fig. 4(d). The slope efficiency was calculated under the dual-pump pumping condition. The slope efficiency is higher, with a wider pulse duration, because the duty cycle of the pulse train increases. The slope efficiency can reach up to 21.3% for the pulse duration of 1110 ps, and the maximum output power is 104.6 mW, corresponding to a pulse energy of 21 nJ. Please note that the amplified power is not yet saturated, which means that the signal power can be further amplified with a higher pump power.

The BDFL can also be switched to a multi-pulse soliton regime by adjusting the PCs inside the cavity, as shown in Fig. 5. In this regime, the output power, as a function of the pump power, is shown in Fig. 5(a). When the pump power is lower than 200 mW, the laser operates with a continuous wave (CW). When the pump power increases from 205 to 620 mW, the fiber laser achieves ultrafast single-pulse mode-locking and then evolves to a multi-pulse state with the consecutive pulse number increasing. The maximum number of pulses can be up to 15. Beyond a pump power of 600 mW, the mode-locking vanishes and turns into a noisy state.

The temporal profiles of the multi-pulse evolution from 1 to 15 pulses with different pump powers are shown in Fig. 5(b). Under a pump power of 300 mW (corresponding to 4-pulse mode-locking), the AC of the signal was measured with an output power of 30 mW amplified by the BDFA, as shown in Fig. 5(c).

The corresponding optical spectra of the BDFL and that after amplification by the BDFA are shown in Fig. 5(d). Different from the NLP regime, the optical spectrum of the multi-pulse soliton regime (red curve) is mode-locked at a central wavelength of 1320 nm with an FWHM of 20 nm, which is much broader with a flat top. There is no Kelly sideband observed, indicating that the net dispersion under these circumstances is likely changed to normal dispersion due to the central wavelength difference. Then, the 4-pulse mode-locked laser was launched into the BDFA and amplified to the maximum output power of 85 mW. Different from the square-wave NLP, for the multi-pulse soliton input signal with a narrower pulse duration, the amplified optical spectrum (green curve) was significantly broadened due to the nonlinearity.

We measured the power stability of the square-wave NLP and the multi-pulse soliton mode-locking regimes. Both mode-locking regimes could maintain stability for at least 1 h with deviations of 0.49% and 0.57%, respectively, which is appropriate for applications.

To investigate the dynamics of the BDFL operating in different regimes, the temporal profiles (upper) and temporal evolutions (lower) are shown in Fig. 6, where (a),(b) and (c),(d) are two cases operating in the square-wave NLP regime and the multi-pulse soliton regime, respectively. As shown in Fig. 6(a), under the pump power of 440 mW, the square-wave NLP slightly changes due to the pulse cluster evolving inside the square-wave. When the pump power exceeds 620 mW, the single noise-like square-wave splits into multiple noise-like square waves as shown in Fig. 6(b). In the multi-pulse soliton regime under the pump power of 300 mW, the temporal separation of the 4 pulses can also be turned by changing the PCs, resulting in the consecutive 4 pulses in Fig. 6(c) and the separated 4 pulses in Fig. 6(d). The pulse-to-pulse evolution of the multi-pulse soliton regime is not that stable due to the long cavity length with a long spool of low-gain BDF^[29].

We demonstrate a BDFL mode-locked in versatile regimes using the NPR technique. The operation state of the BDFL can be changed from the square-wave NLP regime with anomalous dispersion at 1331 nm to the multi-pulse soliton regime with normal dispersion at 1320 nm. In the square-wave NLP regime, the pulse duration increases from 258 to 1110 ps with the increase of the pump power, and the larger the pulse width, the higher the slope efficiency in the bismuth fiber amplifier. The 1110-ps pulse is amplified from 0.95 to 21 nJ with a slope efficiency of 21.3 %. Multiple square-wave NLPs are also observed under higher pump power. In the multi-pulse soliton regime, the number of pulses increases to 15 when the pump power is increased. The temporal separation of the pulses can also be tuned by changing the polarization state. By analyzing the versatile mode-locking regimes of the BDFL, we believe that this BDFL can be useful for further study of 1.3 µm ultrashort pulses.