
- Chinese Optics Letters
- Vol. 23, Issue 6, 061402 (2025)
Abstract
1. Introduction
Ultrahigh-repetition-rate pulses (UHRPs) are widely applied in spectral imaging[1], optical waveform synthesis[2], and astronomical spectrometer calibration[3]. When pulse rates approach 100 GHz, these sources become particularly important in precision measurements[4] and high-capacity optical communication[5]. Dissipative optical systems serve as an excellent platform for studying UHRPs[6,7]. Passively driven fiber ring cavities exhibit dissipative behavior due to constant external energy injection, despite lacking an internal gain component. Pulses formed within such resonators maintain stable operation even with a low-quality factor[8]. Fiber ring resonators with spectral filters function effectively as pulse shapers, enabling ultrafast pulse shaping[8]. This feature enables filter-induced instability to generate UHRPs at the sub-terahertz scale[9]. Filtering effects allow customizing repetition rates to meet specific requirements[10,11]. As more miniaturized passive drive resonators, microcavities have also gained attention for generating UHRPs[11,12]. Kippenberg et al. demonstrated temporal solitons in optical microcavities using continuous wave sweeping to achieve the detuning required for four-wave mixing (FWM)[6]. To increase UHRP power, researchers innovatively propose using pulse fields as the driving source[13,14]. This involves two key adjustments: tuning the pump frequency to the microcavity resonance to trigger modulation instability or dissipative FWM, and aligning the temporal period between the pump and microcavity for efficient operation.
Fiber lasers mode-locked using dissipative FWM[15] have been proposed as an effective approach for UHRP formation. Pasquazi et al. demonstrated a 200 GHz mode-locked laser with a microring cavity, terming the mode-locking approach as filter-driven FWM[16]. Key components for UHRP generation include multiwavelength filters and high-nonlinearity elements like microcavities[17], Fabry–Perot filters[13,18,19], Mach–Zehnder interferometers (MZIs)[20], and microring fibers[21,22]. Fiber-based comb filters provide benefits such as lower cost and reduced coupling losses between fiber and silicon/silica waveguides[23]. An adjustable MZI was employed to achieve variable repetition pulse trains in passively mode-locked fiber lasers, enabling UHRPs with GHz-range tunable repetition rates based on FWM[24]. Kbashi et al. showed that a UHRP mode-locked fiber laser using an MZI filter can serve as a sensor, offering significant advantages over other photonic strain sensing technologies[25]. In 2020, Xu et al. presented a novel 144.3 GHz pulse operation using a fiber-based comb filter[26]. This approach utilizes the nonlinear-polarization-rotation (NPR) effect, enabled by the polarization characteristics of microring devices. The technique, known as NPR-stimulated dissipative FWM, aims to boost intracavity power to enhance FWM mode-locking. The success of these UHRP schemes raises the question of whether new startup methods can effectively trigger UHRP excitation. We compared previously reported UHRP power levels shown in Table 1. The results suggest significant potential for achieving high average output power and UHRPs using the MZI filter.
Filter | Dispersion | Rate (GHz) | Pump (mW) | Output (mW) |
---|---|---|---|---|
High-nonlinear | ||||
Raman fiber laser[ | negative | 160 | — | 926 |
High-nonlinear | ||||
Fabry–Perot filter[ | — | 640 | — | 2.5 |
MZI | ||||
(Dual pump)[ | negative | 1000 | 800 | 50 |
High-nonlinear microring[ | negative | 200 | — | 15.4 |
High-nonlinear microfiber[ | negative | 144.3 | 390 | 6.21 |
Fiber loop[ | negative | 0.9 | 920 | 4.5 |
High-nonlinear microfiber[ | negative | 106.7 | 200 | — |
Fiber loop[ | normal | 280 | 2000 | 50 |
This study | normal | 275 | — | 500 |
Table 1. Comparison of Different Schemes of UHRP Fiber Lasers
In this paper, we combined pulse-driven cavities with fiber lasers to develop a novel approach for generating UHRPs. An ultrafast seed pulse was employed to initiate a pulse field in a dissipative ring cavity, increasing intracavity power and stimulating dissipative FWM. An MZI acted as the comb filter, maintaining the all-fiber structure of the dissipative ring cavity and facilitating the production of 0.275 THz UHRPs. After modifying the laser cavity structure, the output power of the UHRPs reached up to 0.5 W. We compared the power of previously reported UHRPs, as shown in Table 1. This work demonstrates the highest average power for UHRPs without high-nonlinear components in the cavity in a 1.5 µm normal dispersion fiber laser based on the dissipative FWM effect, to the best of our knowledge. Additionally, once established, the UHRPs could sustain themselves in the dissipative ring cavity even after the ultrafast seed pulse was switched off.
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2. Schematic Diagram and Experimental Setup
The schematic diagram of the UHRP fiber ring cavity is illustrated in Fig. 1. It consists of a dissipative cavity for shaping laser pulses and an external seed pulse field. The seed pulse, characterized by its ultra-short duration, was switched off once a comb spectrum was formed. The dissipative cavity incorporates a gain component, a spectral shaper, and passive fibers. The spectral shaper generates a comb-shaped modulation field in the spectral domain. The comb spacing (Fs) is on the order of hundreds of gigahertz, whereas the fundamental repetition rate (F) is in the megahertz range. Mode matching related to this setup will be addressed in the discussion section.
Figure 1.Principle of the ultrafast pulse-stimulated UHRP. T0, period of the seed pulse; F, repetition rate of the seed pulse; Fs, repetition rate of the UHRP; FF and T, repetition rate and period of the dissipative cavity.
The corresponding experimental setup is shown in Fig. 2, where an external ultrafast laser injects an auxiliary field into the cavity. An erbium-ytterbium co-doped amplifier (EYDFA) module provided gain within the dissipative cavity. A segment of dispersion-compensating fiber (DCF) with a dispersion of
Figure 2.Experimental setup of the UHRP fiber ring cavity. DCF, dispersion-compensating fiber; EYDFA, erbium-ytterbium co-doped amplifier; OC, output coupler; FROG, frequency-resolved optical gating; OSA, optical spectrum analyzer.
The seed pulse was characterized in both the spectral and time domains, as illustrated in Fig. 3. At a 10 mW seed pulse power, it exhibited a spectral width of approximately 12 nm and a span of 35 nm, as shown by the blue line in Fig. 3(a). The autocorrelation trace revealed a pulse width of 1.17 ps [white line in Fig. 3(b)], indicating its ultrafast nature, as shown in Fig. 3(b). To characterize the transmission features of the MZI, we tested the time-domain and frequency-domain characteristics at Point B. The resulting transmission curve showed a comb-like shape with a spectral spacing of 2.3 nm [Fig. 3(c)], corresponding to a frequency interval of 0.275 THz. The FROG traces indicated that the seed pulse had undergone interference. The spectral modulation induced by the MZI filtering was evident in Fig. 3(d).
Figure 3.Characteristics of the seed pulse and MZI filter. (a) Spectra of the ultrafast pulse at Point A; (b) FROG trace of the seed pulse (inset, autocorrelation trace); (c) transmission curve and (d) FROG trace of the pulse at Point B.
3. Experimental Results
We injected the seed pulse at Point A and detected the pulse information after the EYDFA and MZI at Point B [Fig. 2(b)] under different gain settings. The spectrum profile at Point B appeared to broaden when the EYDFA gain was set to 4000 mA (approximately 32.4 dBm) compared to 3500 mA (approximately 31.5 dBm), with a central wavelength of 1563.7 nm as shown in Fig. 3(c). Corresponding autocorrelation and FROG traces in Fig. 3(d) indicated that the seed pulse had undergone broadening and division due to the effects of DCF, EYDFA, and MZI filtering, which is consistent with our initial expectations. However, the spectral modulation induced by the MZI filtering was obvious.
We first set the pumping current of the EYDFA to 3500 mA (gain about 31.5 dB) and injected the seed pulse. The average power of the seed pulse was increased from 2 to 8 mW. The spectral evolution at the output port of
Figure 4.Ultrafast pulse-stimulated UHRPs. (a) The spectra at different seed pulse powers; (b) the spectrum of the UHRP when the ultrafast seed pulse is switched off (see the Visualization 1); (c) the recording of UHRP spectra every 5 min, with corresponding parts (d) detailing the FROG and the autocorrelation trace, respectively.
Figure 4(a) illustrates that, as the injected pulse power increases, sidebands around the dominant peaks become stimulated, confirming the feasibility of ultrafast pulse-stimulated UHRP. To explore more comprehensive features of UHRP comb formation and directly demonstrate ultrafast pulse stimulation, we set the EYDFA current to 4000 mA and injected a seed pulse averaging 10 mW. We recorded the spectrum at the
Figure 5.Diagram of the ultrafast pulse-stimulated UHRP. (a) Spectra when the seed pulses are ON (in blue) and OFF (in red), respectively; (b) presentation of the corresponding temporal characterization through FROG trace analysis.
4. Numerical Simulations
We simulated the formation process of the UHRP in a dissipative fiber ring cavity. The model consists of two components: The first is an external source with an ultrafast temporal envelope that serves as a seed pulse; the second is the main cavity, which includes SMFs, a gain fiber, a DCF, OCs, and a comb filter, as depicted in Fig. 6. The simulations were conducted using the fast Fourier transform method to solve the Ginzburg–Landau equation[35]:
Figure 6.Simulation model of the ultrafast pulse-stimulated UHRP fiber laser. SMF, single-mode fiber; OC, optical coupler.
The gain fiber can be approximated by the following equation:
A more detailed explanation of the parameters in equations is available in Ref. [31]. Epulse is the pulse energy and Esat is the gain saturation energy.
Figure 7.Spectra and temporal pulses at different cavity locations: (a) and (b) seed pulses; (c) and (d) pulses after the comb filter.
In the simulations, periodic boundary conditions were applied to model the cyclic nature of the optical pulses in the fiber laser cavity. A pulse wave at the end of a fiber is seamlessly connected to the pulse wave at the beginning of the next fiber, effectively simulating the closed-loop nature of the laser cavity. At the output position, reflective boundary conditions were used. Here, a 50:50 OC and a 99:1 OC were used to output and return the wave to the cavity as shown in Fig. 6. The number of spatial grid points was 212, and the time window was 100 ps. The parameters used in the simulation are shown as follows.
The lengths of
Figure 8(a) shows the simulated UHRP formation spectrum, which closely matches the experimental results in Fig. 4(b). Within the dissipative ring cavity, the pulse field undergoes amplification, division, and shaping due to the combined effects of gain, loss, dispersion, and nonlinearity. As the seed pulse passes through the gain fiber, it splits. The filter then modulates the seed pulse spectrum into a comb shape in the spectral domain, as shown in Figs. 7(c) and 8(b). This modulation creates the initial conditions for the FWM effect. As the seed pulse evolves in the cavity, the shaping effects further stabilize the spectrum, as depicted in Fig. 8(b). The dynamic operation in the time domain can be divided into three steps. In Step I, a high-peak-power seed pulse stimulates the pulse field in the cavity, leading to splitting due to self-phase modulation instability. The continuous energy supply from the gain element, coupled with nonlinear loss from the MZI, induces dissipative characteristics in the ring cavity. The pulse undergoes further division due to nonlinear effects, gain, and the influence of a comb, leading to pulse modulation (see the inset of Step I). During Step II, modulation begins to stabilize. Over time, multiple pulses evolve into stable UHRPs with a uniform spacing of 3.63 ps that corresponds to the spectral spacing in Fig. 8(a), as illustrated in Step III. Pulse evolution can also be assessed by observing energy changes across different round trips. Initially, the high peak power rapidly decreases, followed by small amplitude oscillations. This demonstrates the rapid formation of dissipative FWM and the modulation of the pulse by the dissipative cavity during laser operation, as shown in Fig. 8(d). We show the energy evolution versus round trips, where the high power quickly drops to an average level. After 100 round trips, the energy of the pulses begins oscillating. As seen in Fig. 8(c), multiple pulses are periodically and completely distributed within the cavity at the 100th round trip. This energy oscillation indicates that the multiples are adjusted by the dissipative FWM process.
Figure 8.Characteristics of the pulse evolution: (a) UHRP spectrum; (b) pulse evolution in the frequency domain; (c) pulse evolution in the time domain (Insets illustrate three steps from the split state to the UHRP state); (d) energy evolution (The inset shows energy oscillation).
5. Discussion and Conclusion
The comb spacing is on the order of hundreds of gigahertz, while the fundamental repetition rate is in the megahertz scale, as shown in Fig. 9. Generally, they should meet the following condition to form a stable mode-locking pulse[31]:
Figure 9.Relationship of the rates between MZI spacing and the cavity fundamental rate.
We highlighted two key steps in UHRP formation: the ultrafast seed pulse-driven ring cavity and the dissipative ring cavity after the seed pulse is turned off. In the first step, the ultrafast pulse serves as a pump. The EYDFA provides gains to compensate for the losses introduced by the OCs and the MZI. After the ultrafast pulse stimulates the optical field, the dissipative ring cavity generates a fiber laser. The ultrafast seed pulses serve as the initial conditions, leading to a process distinct from that of a passively mode-locked laser, which uses noise as the starting condition. We refer to this mechanism as the ultrafast pulse-stimulated FWM effect. From the comparison before and after the seed pulse is switched off in Fig. 5(a) and the Visualization 1, we observe that, when the comb teeth form, the seed pulse contributes a pedestal spectrum and acts as a perturbation due to its mismatched rate with the dissipative cavity. Since FWM is a phase-sensitive process, it fixes the phase of the mixing modes relative to each other, thus locking the modes and producing a pulse train with a period of
In conclusion, this study presents a novel method for generating UHRPs in a dissipative ring cavity. Unlike passively driven fiber ring cavities that require constant pumping and high levels of nonlinear components to sustain dissipative properties and induce the dissipative FWM effect, our method utilizes the high peak power of ultrafast pulses. This approach creates a pulse field that significantly boosts the intracavity power instantly upon injection. By integrating a gain module and a spectral shaper, our system effectively prevents pulse splitting, promotes interaction, and maintains a uniform distribution within the cavity, thus stimulating dissipative FWM mode-locking. We call this new mechanism the “ultrafast ignition-stimulated dissipative FWM effect.” We have shown through both numerical simulations and experiments that this technique consistently produces 0.275 THz UHRPs. These pulses remain stable even after the ultrafast seed pulse is switched off. Additionally, the average power of these UHRPs is 0.5 W. To our knowledge, this represents the highest power achieved in 1.5 µm normal fiber lasers with repetition rates over 200 GHz based on a spectral shaper without using high nonlinear components. Our work provides a scalable solution to address challenges in optical communication, spectral imaging, and other fields.

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