1. INTRODUCTION

Mode-locked fiber lasers (MLFLs) play a significant role in many photonic systems in biomedical, industrial, and scientific applications [1,2]. In the visible wavelength region (380–760 nm), MLFLs are fundamental building blocks of many fields including laser display, optical communication, biomedicine, microimaging, medical therapy, and scientific research [3–6]. Since the first visible passively mode-locked fiber laser at 635 nm was realized by Zou et al., ultrafast fiber lasers in the visible spectral region have gotten reported gradually based on the nonlinear mode-locking techniques of a nonlinear optical loop mirror (NOLM), nonlinear amplifying loop mirror (NALM), and nonlinear polarization rotation (NPR) [7–12]. Efforts have been taken to improve the properties such as pulse duration and output power; however, visible mode-locked fiber lasers operating in an all-fiber configuration are only confined in the red spectral band, hindering the visible ultrafast fiber lasers from stepping into diverse application scenes, where different wavelengths are required and properties such as miniaturization, compactness, and stability of an all-fiber configuration are deeply demanded. Therefore, there is a strong motivation for exploring the visible-wavelength mode-locked all-fiber lasers.

The mode-locked all-fiber lasers in the visible region based on these nonlinear mode-locking techniques heavily rely on the utilization of long silica fiber inside the cavity to accumulate nonlinearity for initiating mode-locking operation [7,11], which inevitably sacrifices the high-repetition-rate (HRR) performance. Nevertheless, a group of HRR pulses, called a burst, which itself is repeated at a much lower repetition rate, has widespread applications in the fields of precision surgery, materials processing, high-speed imaging, and LIDAR [13–20]. Especially, visible-wavelength mode-locked all-fiber lasers may show extraordinary qualities in these applications due to their unique spectral characteristics. Consequently, there is considerable interest in achieving visible-wavelength burst-mode passively mode-locked all-fiber lasers. In order to enrich the burst-mode pulse generation, different methods have already been exploited in the infrared region [21–26]. An acousto-optic or electro-optic modulator is often used as pulse picking from an HRR mode-locked laser for active burst-mode pulse generation. Although the method can directly obtain burst-mode pulses in a simple way, the use of an active optical modulator would inevitably increase the system cost. Therefore, there are strong motivations to explore the passive burst-mode pulse techniques with low cost and compactness. One of the passive techniques is to multiply additional pulses by a pulse multiplier. Although the burst-mode pulse could be generated in an all-fiber configuration, the construction of such laser systems tends to be complicated. Another promising burst-mode pulse technique is passive picking of HRR mode-locking pulse operation. It is well known that HRR mode-locking can be realized based on the dissipative four-wave-mixing (DFWM) effect in an all-fiber configuration [27,28]. The key for effectively exciting the DFWM effect is the combination of high intracavity nonlinearity and a comb filter such as a sampled fiber Bragg grating [29], a Fabry–Perot (F-P) filter [30,31], a Mach–Zehnder interferometer [32], a Lyot filter [33], a micro-ring resonator [34], and a programmable filter [35]. However, some fiber components (e.g., special fiber Bragg gratings) in the visible spectral region are immature, limiting the progress of DFWM mode-locked fiber lasers at visible wavelengths.

In this paper, we deal with the issue by combining the NOLM technique and intracavity F-P filtering assisted by the DFWM effect, and then experimentally obtain the burst-mode pulses in the passively mode-locked all-fiber lasers at 543.3 nm and 602.5 nm based on the ${\mathrm{Ho}}^{3+}$:ZBLAN fiber and ${\mathrm{Pr}}^{3+}/{\mathrm{Yb}}^{3+}$:ZBLAN fiber, respectively. Robust green and orange mode-locking in the burst-pulse status is established with inter-burst repetition rates of 3.779 MHz and 5.662 MHz, which are in accordance with the fundamental repetition rates. Besides, intra-burst repetition rates of 0.85 GHz and 0.76 GHz are simultaneously observed derived from the intracavity F-P filtering and DFWM effect. The F-P filter cavity is constructed by fiber end-facets of the gain fiber, and NOLM mode-locking and the DFWM effect are initiated thanks to the high intracavity nonlinearity accumulated by a long 460 HP fiber in the fiber loop mirror. Visible-wavelength mode-locking with a high-repetition-rate pulse and all-fiber configuration would undoubtedly pave the way for the promising prospects of ultrafast optics applications and researches in the visible spectral region.

2. EXPERIMENT SETUP AND OPERATION PRINCIPLE

The schematic diagram of our burst-mode passively mode-locked fiber laser is illustrated in Fig. 1. A figure-9 cavity is a mature scheme to demonstrate the mode-locking operation at different wavelengths [36]. Our figure-9 cavity consists of the fiber facet input mirror (the dichroic mirror), gain fiber, and fiber loop mirror made of an optical coupler (OC). The fiber loop mirror with long highly nonlinear fiber (HNLF) not only acts as an output mirror, but also is used to accumulate a nonlinear phase shift for initiating mode-locking based on the NOLM technique. Therefore, mode-locking operation is established due to the periodic saturable absorption effect of NOLM (an artificial saturable absorber), generating mode-locked inter-burst pulses with the inter-burst repetition rate ($f_{\mathrm{inter}}$) well-matched with the fundamental frequency of the figure-9 cavity, corresponding to the length of the cavity. Inside the figure-9 cavity, the gain fiber with a perpendicular fiber end-facet is connected to other silica fibers based on the standard FC/PC fiber connector and fiber adaptor, leading to an inevitable fiber-air interface. Due to the Fresnel reflection from the fiber-air interface, a weak F-P filter is directly constructed based on the gain fiber end-facets. The F-P filter combined with the high intracavity nonlinearity from the long HNLF in the NOLM would effectively excite the DFWM effect by facilitating the phase matching between longitudinal modes, helping the establishment of the DFWM effect. The DFWM effect assisted by the F-P filtering and high intracavity nonlinearity would result in the generation of phase-coherent sidebands with equal spacing, and all the sidebands have a regular phase relationship. Therefore, the HRR intra-burst pulses are generated with the intra-burst repetition rate ($f_{\mathrm{intra}}$) defined by the free spectral range of a gain-fiber-based F-P filter. The ultrashort intra-burst pulses with a higher repetition rate are contained within the single pulse profile of long inter-burst pulses with a lower repetition rate, called the burst-mode.

 

Fig. 1. Schematic diagram of the burst-mode passively mode-locked all-fiber laser. HNLF, highly nonlinear fiber; OC, optical coupler; NOLM, nonlinear optical loop mirror; DM, dichroic mirror.

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Based on the above-mentioned principle, we designed the experimental setup of a 543 nm burst-mode passively mode-locked all-fiber laser as shown in Fig. 2(a). A commercial 450 nm GaN laser diode with the output power of $\sim 2\mathrm{W}$ acts as the pump source. The output beam of the laser diode is shaped through the fast axis collimation (FAC) and the slow axis collimation (SAC), and then is coupled into the SM1950 fiber through an aspheric lens, obtaining an output power of $\sim 0.8\mathrm{W}$. The SM1950 fiber has a core diameter and numerical aperture (NA) of 8 μm and 0.2, respectively. A ${\mathrm{Ho}}^{3+}$:ZBLAN fiber (ZFG SM [2.40] 7.5/125, Le Verre Fluore, Inc.) as the gain medium has a core diameter and NA of 7.5 μm and 0.23 μm, respectively. Both the fibers at 450 nm (pump) and 543 nm (laser) wavelengths operate in multimode states. Since the two fibers share similar fiber parameters, high coupling efficiency of $\sim 95\%$ can be obtained. Then, the 450 nm pump light through a homemade fiber pigtail mirror (M) on the SM1950 fiber end-facet injects into the core of ${\mathrm{Ho}}^{3+}$: ZBLAN fiber. An ion-beam-assisted deposition system is utilized to coat the highly resistant dielectric films onto the SM1950 fiber end-facet. Its transmission spectrum in the range of $400-1000\mathrm{nm}$ is measured in Fig. 2(d). The transmittance at 450 nm is 97.7%, and the reflectivity at 543 nm is 99.3%, which can be used as a dichroic input mirror. The corresponding microscope image and photograph of the input fiber pigtail mirror (M) are exhibited in Fig. 2(b) and Fig. 2(c). A section of 11.4 cm single-clad ${\mathrm{Ho}}^{3+}$:ZBLAN fiber is employed as the gain fiber, corresponding to an $f_{\mathrm{intra}}$ of $\sim 0.85\mathrm{GHz}$. The gain fiber has the core absorption coefficient and the ${\mathrm{Ho}}^{3+}$ doping concentration of $\sim 250\text{dB/}\mathrm{m}$ at the pump wavelength and 5000 ppm (parts per million), respectively. Both of the fiber end-facets of ${\mathrm{Ho}}^{3+}$:ZBLAN fiber are handled under perpendicular polishing and connected to the SM1950 fiber based on the standard FC/PC fiber connector and fiber adaptor. Therefore, a fiber-air interface is inevitable from the fiber facets, and the Fresnel reflection emerges based on the fiber-air interface, constructing a weak F-P filter inside the cavity with a free-spectral range of $\sim 0.84\mathrm{pm}$ (854.40 MHz at 543 nm). The reflectivity is $\sim 4.5\%$ according to the core refractive index of $\sim 1.54$. The output coupler is made of the fiber loop mirror (FLM), using a 15:85 optical coupler (OC) of 630 HP fiber. The FLM based on a 15:85 OC provides a reflectivity of 51%, and extracts 49% of the intracavity power. Since a 543 nm laser operates in multimode within both SM1950 fiber and ZBLAN fiber, to realize the mode-locking operation in the multimode fibers, long 460 HP fiber is introduced in the fiber loop mirror. The 460 HP fiber not only acts as the HNLF, but also reduces the mode-locking threshold using the NOLM mode-locking technique. This fiber has a core diameter and NA of 2.5 μm and 0.13, which can ensure the single-transverse-mode operation at 543 nm and suppress the multimode operation. Besides, the extremely small-mode-field area would lead to a high fiber nonlinear coefficient, helping the establishment of the DFWM effect inside the cavity. The figure-9 cavity possesses a line length of $\sim 2.5\mathrm{m}$ and a loop length of $\sim 49\mathrm{m}$, leading to an $f_{\mathrm{inter}}$ of 3.779 MHz. To reduce the coupling loss between SM1950 fiber and 460 HP fiber, 630 HP fiber with a core diameter and NA of 3.5 μm and 0.13 can act as the transition between SM1950 fiber and 460 HP fiber. The 630 HP fiber and 460 HP fiber have similar parameters and are fused together using a commercial fusion splicer. Therefore, the coupling loss between 630 HP fiber and 460 HP fiber can be neglected. The coupling losses from SM1950 fiber to 630 HP fiber and from 630 HP fiber to SM1950 fiber are tested to be $\sim 1.3\mathrm{dB}$ and $\sim 0.3\mathrm{dB}$, respectively. Moreover, two polarization controllers (PCs) in line and loop parts help to optimize the mode-locking operation. The PC is commonly used in the traditional passively mode-locked fiber lasers using the NOLM technique based on the non-polarization-maintaining fiber system. Since the fiber gain is related to the polarization, a fiber device such as OC possesses weak polarization-dependence. The PC is utilized to optimize the laser performance. The PC in the loop part is installed on the 460 HP fiber and the PC in the line part is installed on the SM1950 fiber. For the sake of controllability and convenience, two PCs are utilized in our experiment.

 

Fig. 2. (a) Experimental setup of 543 nm burst-mode passively mode-locked all-fiber laser. (b) Microscopic image of the input fiber pigtail mirror (M). (c) Photograph of the input fiber pigtail mirror (M). (d) Transmission spectrum of the input fiber pigtail mirror (M).

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Figure 3(a) gives the experimental setup of a 602 nm burst-mode passively mode-locked all-fiber laser with the similar figure-9 cavity using the single-clad ${\mathrm{Pr}}^{3+}/{\mathrm{Yb}}^{3+}$:ZBLAN fiber (ZFG SM [0.78] 2.8/125, Le Verre Fluore, Inc.). To match with the peak absorption of the gain fiber, a 450 nm pump is replaced as the 443 nm pump. The commercial 443 nm GaN laser diode with the output power of $\sim 2\mathrm{W}$ is coupled into the 630 HP fiber with the coupling efficiency of $\sim 20\%$ with the help of FAC, SAC, and an aspheric lens. The input fiber pigtail mirror ${\mathrm{M}}_1$ and mirror ${\mathrm{M}}_2$ are made on the fiber facets of 630 HP fiber. The 630 HP fiber has a core diameter and NA of 3.5 μm and 0.13 and the ${\mathrm{Pr}}^{3+}/{\mathrm{Yb}}^{3+}$:ZBLAN fiber has a core diameter and NA of 2.8 μm and 0.23. The 630 HP fiber and Pr/Yb:ZBLAN fiber are multimode at the signal wavelength. In our experiment, the 443 nm pump from the core of 630 HP fiber can inject into the core of ${\mathrm{Pr}}^{3+}/{\mathrm{Yb}}^{3+}$: ZBLAN fiber with the coupling efficiency more than $\sim 96\%$ due to the similar fiber parameters of 630 HP fiber and ${\mathrm{Pr}}^{3+}/{\mathrm{Yb}}^{3+}$:ZBLAN fiber. The 443 nm laser passes through the input mirror (${\mathrm{M}}_1$) on the 630 HP fiber end-facet and then pumps the gain fiber. In order to suppress the generation of a red laser due to the strong gain of ${\mathrm{Pr}}^{3+}/{\mathrm{Yb}}^{3+}$:ZBLAN fiber at 635 nm, the input mirror is designed to possess a high transmittance at 635 nm. The transmission spectrum of ${\mathrm{M}}_1$ is given in Fig. 3(b), which demonstrates a high transmittance at 443 nm pump light and a high reflectivity at 602 nm. To further suppress the 635 nm laser and utilize the residual 443 nm pump light, a mirror ${\mathrm{M}}_2$ is placed after the gain fiber with reflectivity of 95.5% at 443 nm and transmittance of 97.2% at 635 nm. The transmission spectrum of ${\mathrm{M}}_2$ is depicted in Fig. 3(c). A section of 13 cm ${\mathrm{Pr}}^{3+}/{\mathrm{Yb}}^{3+}$:ZBLAN fiber is used as the gain fiber, where both the fiber end-facets are handled under plane polishing to form an F-P filter cavity based on the Fresnel reflection from the interface fiber-air (corresponding to $f_{\mathrm{intra}}$ of $\sim 0.76\mathrm{GHz}$). The single-clad gain fiber has the core absorption coefficient of 2.1 dB/cm at 443 nm with the ${\mathrm{Pr}}^{3+}$ doping concentration of 3000 ppm and the ${\mathrm{Yb}}^{3+}$ doping concentration of 20,000 ppm. Similar to the setup of the 543 nm burst-mode passively mode-locked all-fiber laser, FLM using a 10:90 OC acts as the output coupler with long 460 HP fiber as the HNLF. The FLM based on a 10:90 OC provides a reflectivity of 36%, and extracts 64% of the intracavity power. The cavity length is $\sim 34.5\mathrm{m}$, comprising a line length of $\sim 1.5\mathrm{m}$ and a loop length of $\sim 33\mathrm{m}$, resulting in an $f_{\mathrm{inter}}$ of 5.662 MHz. The PC in the loop part is installed on the 460 HP fiber and the PC in the line part is installed on the 630 HP fiber. The output optical spectrum is measured by a 350–1750 nm optical spectrum analyzer (Ando AQ-6315E). The temporal characteristics of the visible mode-locked laser are recorded by a 12.5 GHz photodetector (ET-4000F, Electro-Optics Technology, Inc.) together with a 40 GSa/s high-speed digital storage oscilloscope with a 12 GHz bandwidth (Agilent Infiniium DSO81204A) or an electrical spectrum analyzer.

 

Fig. 3. (a) Experimental setup of 602 nm burst-mode passively mode-locked all-fiber laser. (b) Transmission spectrum of the input fiber pigtail mirror (${\mathrm{M}}_1$). (c) Transmission spectrum of the fiber pigtail mirror (${\mathrm{M}}_2$).

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3. EXPERIMENTAL RESULTS

A. Burst-Mode Passively Mode-Locked All-Fiber Laser at 543 nm

In our experiment, the threshold of the continuous wave (CW) green laser is $\sim 260\mathrm{mW}$. The burst-mode mode-locking operation was observed and self-started when the pump power exceeds $\sim 310\mathrm{mW}$; after the adjustment of polarization controllers, the mode-locked pulses are optimized to be more stable and robust. The typical characteristics of a green-light mode-locked all-fiber laser under the pump power of 335 mW are illustrated in Fig. 4. Before the realization of mode-locking operation, the CW operation shows random laser wavelength components, whose typical spectrum is exhibited in Fig. 4(a). Once the mode-locked pulses are achieved, the bandwidth of the output spectrum gets noticeably broader with a fixed central wavelength of 543.3 nm. The spectrum of mode-locking operation possesses a bandwidth of 0.13 nm. Figure 4(b) gives the typical burst-mode pulse trains with an inter-burst pulse interval of 264.6 ns, which matches the cavity round-trip time, corresponding to a figure-9 cavity length. It is conspicuous that the single-pulse profile of inter-burst pulses contains the intra-burst pulses with a much higher repetition rate, whose characteristics are demonstrated below. The output radio-frequency (RF) spectrum is displayed in Fig. 4(c), with a resolution bandwidth of 10 Hz, showing a fundamental frequency and SNR of 3.779 MHz ($f_{\mathrm{inter}}$) and $\sim 65\mathrm{dB}$, respectively. The inset in Fig. 4(c) exhibits no spectral modulation in the 100-MHz-span RF spectrum, further verifying the stability of this 543 nm burst-mode passively mode-locked all-fiber laser. The average output power and inter-burst pulse energy of the green-light fiber laser as a function of pump power are depicted in Fig. 4(d). Under the maximum pump power of 420 mW, the maximum output power can reach 15 mW without any saturation, corresponding to the inter-burst pulse energy of 3.9 nJ.

 

Fig. 4. Typical mode-locking characteristics of the 543 nm all-fiber laser. (a) Spectra of CW and mode-locking. (b) Inter-burst pulse trains. (c) RF spectrum at the fundamental frequency [inset: broadband RF spectrum (100 MHz span)]. (d) Optical power and inter-burst pulse energy as a function of pump power.

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In addition to the mode-locked pulses with a fundamental repetition rate of 3.779 MHz, stable ultrashort HRR pulses within the long bursts are simultaneously observed. The characteristics of burst-mode pulses under the pump power of 335 mW are illustrated in Fig. 5. Figure 5(a) gives the single-pulse profile of an inter-burst pulse in a range of 100 ns, which exhibits a pulse duration of 22.2 ns, containing the ultrashort intra-burst pulses. As a result of long silica fibers used in the cavity, the accumulated ultrahigh normal dispersion at the visible wavelength would lead to the wide pulse duration, and the inter-burst pulses are strongly chirped. Figure 5(b) records the corresponding pulse trains of an intra-burst pulse. The pulse interval of $\sim 1.17\mathrm{ns}$ well matches the round-trip time of the F-P filter cavity formed by the 11.4 cm ${\mathrm{Ho}}^{3+}$: ZBLAN fiber. It indicates that the intra-burst pulse comes from the F-P filter and the stable and robust pulse trains further verify the assistance of the DFWM effect with the high intracavity nonlinearity. The single-pulse profile of an intra-burst pulse is recorded in Fig. 5(c) and exhibits a pulse duration of 87 ps. Similar to dissipative soliton and dissipative soliton resonance in an all-normal-dispersion passively mode-locked fiber laser, the pulse duration is the overall balance of intracavity nonlinear effects, dispersion, loss, and gain. Under the pump power of 335 mW, the green burst-mode passively mode-locked fiber laser has the inter-burst pulse energy, number of pulses with each burst, and intra-burst pulse duration of 1.5 nJ, 40, and 87 ps, respectively. So the pulse energy and peak power of the intra-burst pulses are estimated to be $\sim 0.04\mathrm{nJ}$ and $\sim 0.4\mathrm{W}$. The RF spectrum is displayed in Fig. 5(d) with a resolution bandwidth of 20 kHz in the 3-GHz-span RF spectrum. An intra-burst repetition rate of 850.05 MHz ($f_{\mathrm{intra}}$) is observed, which not only agrees with the 225 times of 3.779 MHz inter-burst repetition rate (fundamental repetition rate of the NOLM cavity), but also accords with the free spectral range of the F-P filtering in numerical simulation. The 3 dB bandwidth of the RF spectrum is measured to be $\sim 44.9\mathrm{MHz}$, corresponding to a pulse duration of 22.2 ns through the Fourier transform. It should be noticed that since the pulse repetition rate and laser wavelength are determined by the free spectral range of the comb filter and the central wavelength of the bandpass filter in the cavity, the comb spectrum with a peak-to-peak interval of $\sim 0.84\mathrm{pm}$ according to the free spectral range of the F-P cavity should have been observed in the mode-locked spectrum [Fig. 4(a)]. However, due to the limited resolution (0.05 nm) of our optical spectrum analyzer, each lasing line in the spectrum becomes unobservable.

 

Fig. 5. Typical burst-mode pulse characteristics of the 543 nm mode-locked all-fiber laser. (a) Single-pulse profile of inter-burst pulse. (b) Intra-burst pulse trains. (c) Single-pulse profile of intra-burst pulse. (d) 3 GHz span RF spectrum.

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Figure 6 shows the output characteristics of the 543 nm burst-mode passively mode-locked all-fiber laser under different pump powers. As we increase the pump power from 335 mW to 415 mW, the evolutions of the singe-pulse profile, pulse train, and spectrum are recorded. One can see that, under the increasing pump power, the pulse duration of long bursts gradually increases from 22.2 ns to 26.1 ns, and the corresponding pulse duration of the intra-burst pulse exhibits a rise from 87 ps to 158 ps as shown in Fig. 6(a) and Fig. 6(b), respectively. Meanwhile, the 3 dB bandwidth and profile of spectra get slightly broadened. It should be noticed that the mode-locked pulse from an all-normal-dispersion cavity such as a dissipative soliton and dissipative soliton resonance tends to be a chirped pulse instead of a transform limited one. In such an all-normal-dispersion cavity, the establishment of mode-locking is the overall balance of intracavity nonlinear effects, dispersion, loss, and gain. Therefore, under the increasing pump power, the intracavity nonlinear effects and chirping would broaden the spectral bandwidth and pulse duration, which is the typical feature of passively mode-locked fiber lasers in an all-normal-dispersion cavity. Besides, as we boost up the pump power, the time-bandwidth product (TBP) of intra-burst pulses rises from 11.3 to 23.7. It should be noticed that the inter-burst repetition rate of 3.779 MHz and intra-burst repetition rate of 0.85 GHz are unchanged under the increasing pump power, which is entirely different from the $Q$-switching operation.

 

Fig. 6. Characteristics of the 543 nm burst-mode passively mode-locked all-fiber laser under different pump powers. (a) Single-pulse profiles. (b) Pulse trains. (c) Optical spectra.

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B. Burst-Mode Passively Mode-Locked All-Fiber Laser at 602 nm

In the experiment, the laser operates in continuous-wave regime at 602 nm when the pump power is higher than $\sim 100\mathrm{mW}$. Stable burst-mode mode-locking operation is established after adjusting the polarization controllers while the pump power reaches $\sim 180\mathrm{mW}$. Typical characteristics of the 602 nm mode-locked all-fiber laser are depicted in Fig. 7. Figure 7(a) illustrates the comparison of spectra under the CW and mode-locking operation. The mode-locked spectrum becomes broader and possesses the bandwidth of 0.21 nm. The output radio-frequency (RF) spectrum is measured and given in Fig. 7(b). The RF spectrum with a resolution of 10 Hz shows a fundamental frequency of 5.662 MHz ($f_{\mathrm{inter}}$), which is the fundamental repetition rate of this figure-9 cavity. SNR of 64 dB and no spectral modulation in the 100-MHz-span RF spectrum exhibit the stability of our 602 nm burst-mode mode-locked all-fiber laser. Figure 7(c) records the corresponding inter-burst pulse trains. A pulse interval of 175.3 ns is in agreement with the fundamental repetition rate of the figure-9 cavity and matches well with the cavity length. The average output power and inter-burst pulse energy of the orange-light fiber laser as a function of pump power are depicted in Fig. 7(d). Under the maximum pump power of 240 mW, the maximum output power can reach 5.3 mW without any saturation, corresponding to the inter-burst pulse energy of 0.93 nJ.

 

Fig. 7. Typical mode-locking characteristics of the 602 nm all-fiber laser. (a) Spectra of CW and mode-locking. (b) RF spectrum at the fundamental frequency [inset: broadband RF spectrum (100 MHz span)]. (c) Inter-burst pulse trains. (d) Optical power and inter-burst pulse energy as a function of pump power.

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The corresponding characteristics of the burst-mode pulse in the 602 nm mode-locked all-fiber laser under the pump power of 180 mW are demonstrated in Fig. 8. The single-pulse profile of the inter-burst pulse is given in Fig. 8(a). One can see that the intra-burst pulse trains are contained within the single-pulse profile of the inter-burst pulse, which has a pulse duration of 11.6 ns. Figure 8(b) presents the corresponding pulse trains of the intra-burst pulse. The pulse interval of $\sim 1.3\mathrm{ns}$ well matches the round-trip time of the 13 cm ${\mathrm{Pr}}^{3+}/{\mathrm{Yb}}^{3+}$: ZBLAN fiber F-P cavity. Figure 8(c) exhibits the single-pulse profile of the intra-burst. Since our oscilloscope has a bandwidth and sampling rate of 12 GHz and 40 GSa/s, leading to an $\sim 80\mathrm{ps}$ minimum pulse duration that can be accurately detected, the pulse duration narrower than 80 ps in Fig. 8(c) is limited by our oscilloscope. The accurate measurement of intra-burst pulse duration at 602 nm should rely on the optical autocorrelator. However, since the green and orange wavelengths are beyond the measurement range of our optical autocorrelator, we were not able to measure the corresponding autocorrelation in this experiment. The RF spectrum is displayed in Fig. 8(d) with a resolution bandwidth of 20 kHz in the 3-GHz-span RF spectrum. An intra-burst repetition rate of 758.70 MHz ($f_{\mathrm{intra}}$) is realized, which agrees with the 134 times of 5.662 MHz inter-burst repetition rate (fundamental repetition rate of the figure-9 cavity). The 3 dB bandwidth of the RF spectrum is measured to be $\sim 86.3\mathrm{MHz}$, corresponding to a pulse duration of 11.6 ns through the Fourier transform.

 

Fig. 8. Typical burst-mode pulse characteristics of the 602 nm mode-locked all-fiber laser. (a) Single-pulse profile of inter-burst pulse. (b) Intra-burst pulse trains. (c) Single-pulse profile of intra-burst pulse. (d) 3 GHz span RF spectrum.

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Output characteristics of the 602 nm burst-mode passively mode-locked all-fiber laser under the increasing pump power are illustrated in Fig. 9. The corresponding evolution of single-pulse profile, pulse trains, and spectrum are recorded as the pump power increases from 180 mW to 206 mW. Under the increasing pump power, the pulse duration of the inter-burst pulse increases from 11.6 ns to 19.8 ns. Besides, the increase in pulse duration of the intra-burst pulse is observed when we boost up the pump power. As the pump power is increased, self-phase modulation would induce more frequency components in the mode-locked pulse, and then the strong intracavity dispersion would further stretch the pulse duration of such a chirped pulse. In the meantime, the 3 dB bandwidth shows a slight growth from 0.21 nm to 0.25 nm.

 

Fig. 9. Characteristics of the 602 nm burst-mode passively mode-locked all-fiber laser under different pump powers. (a) Single-pulse profiles. (b) Pulse trains. (c) Optical spectra.

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To evaluate the system stability of the burst-mode passively mode-locked all-fiber green and orange lasers, power stability and spectral stability are monitored during a 60 minute test. As given in Figs. 10(a) and 10(c), the power deviations based on the actual measurement data under the maximum output power of 15 mW and 5.3 mW at 543 nm and 602 nm are $\sim 2.1\%$ and $\sim 2.1\%$, respectively. Moreover, no obvious fluctuation of the center wavelength and the appearance of new components in Figs. 10(b) and 10(d) further indicate the excellent long-term stability of the burst-mode passively mode-locked all-fiber green and orange lasers. Obviously, the advantages of the all-fiber configuration and stability are beneficial to the practical applications in the future.

 

Fig. 10. Stability measurement of the burst-mode mode-locked all-fiber laser. (a) Power stability at 543 nm. (b) Spectral stability at 543 nm. (c) Power stability at 602 nm. (d) Spectral stability at 602 nm.

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4. CONCLUSION

In summary, passively mode-locked all-fiber green and orange lasers are first realized and burst-mode pulses in the visible spectral region are demonstrated from the passively mode-locked all-fiber lasers. Burst-mode pulses, a group of HRR pulses repeating at a much lower repetition rate, have widespread applications in the fields of micromachining and biomedicine. In our experiment, burst-mode pulses at green and orange wavelengths are obtained based on the ${\mathrm{Ho}}^{3+}$: ZBLAN fiber and ${\mathrm{Pr}}^{3+}/{\mathrm{Yb}}^{3+}$:ZBLAN fiber in the passively mode-locked all-fiber lasers, respectively. By the combination of the NOLM technique and DFWM effect assisted by F-P filtering under the high intracavity nonlinearity, robust mode-locking in the burst-pulse status is realized at 543/602 nm, generating inter-burst pulses of 3.779/5.662 MHz repetition rate ($f_{\mathrm{inter}}$) and intra-burst pulses of 0.85/0.76 GHz repetition rate ($f_{\mathrm{intra}}$). The former matches well with the NOLM cavity fundamental repetition rate, and the latter depends on the intracavity F-P filtering. In our figure-9 cavity, gain fibers with plane polished end-facets serve as the F-P filter cavity and a long 460 HP fiber inside the fiber loop mirror accumulates high nonlinearity, paving the way for the establishment of nonlinear fiber effects and mode-locking operation. The systems are not only an important step towards the miniaturized visible ultrafast fiber lasers, but also the demonstration of high-repetition-rate pulse generation in the visible spectral region. Through ultrafast laser power amplification and dispersion management, laser performances in terms of output power, pulse duration, and peak power may be improved, which may be applied in the fields of materials processing and biomedical science in the future.