Tunable All-Fiber Femtosecond Electro-Optic Optical Frequency Comb Operating at 1.5 μm

Abstract

We propose and demonstrate a tunable femtosecond electro-optic optical frequency comb by shaping a continuous-wave seed laser in an all-fiber configuration. The seed laser, operating at 1.5 μm, is first cascade-phase-modulated and subsequently de-chirped to generate low-contrast pulses of approximately 8 ps at a repetition rate of 5.95 GHz. These pulses are then refined into clean, high-quality picosecond pulses using a Mamyshev regenerator. The generated source is further amplified using an erbium–ytterbium-doped fiber amplifier operating in a highly nonlinear regime, yielding output pulses compressed to around 470 fs. Tunable continuously across a 5.7~6 GHz range with a 1 MHz resolution, the picosecond pulses undergo nonlinear propagation in the final amplification stage, leading to output pulses that can be further compressed to a few hundred femtoseconds. By using a tunable bandpass filter, the center wavelength and spectral bandwidth can be flexibly tuned. This system eliminates the need for mode-locked cavities, simplifying conventional ultrafast electro-optic combs by relying solely on phase modulation, while delivering femtosecond pulses at multiple-gigahertz repetition rates.

1. Introduction

Ultrafast laser sources with gigahertz repetition rates have become indispensable tools in a wide range of scientific and technological applications, including high-speed optical communications [1], real-time spectroscopy [2], and precision frequency metrology [3]. Among these, electro-optic frequency combs (EOFCs) have emerged as a powerful alternative to traditional mode-locked lasers, offering tunable repetition rates, high stability [4,5,6,7], and the ability to generate ultrashort pulses without the need for complex cavity designs [8]. EOFCs are particularly attractive for applications requiring precise control over the repetition rate and phase coherence, such as in optical clock networks [9], astronomical spectrograph calibration [10], and coherent optical communication systems [11].

Traditional mode-locked lasers, while capable of generating femtosecond pulses, face significant challenges when operating at multi-gigahertz repetition rates. The short cavity lengths required for high repetition rates limit the gain medium’s efficiency and make self-starting mode locking difficult [12,13,14,15]. Additionally, the large free spectral range associated with high-repetition-rate lasers leads to increased phase noise, which can degrade performance in frequency-comb-related applications. In contrast, EOFCs, driven by radio frequency (RF) signals, provide widely tunable and accurate repetition rate setting, up or down quasi-linear chirp for convenient pulse compression, narrow comb lines, and possible stabilization of the carrier-envelope offset frequency depending on the seed laser [16,17,18,19,20].

Extensive research on EO combs, in recent years, has enriched the development of mature ultrafast laser sources [5,21,22]. Conventional EOFC systems typically rely on multiple phase modulators and intensity modulators to achieve sufficient spectral broadening and pulse shaping [23,24,25,26]. While effective, this approach increases system complexity and cost, limiting its practicality for many applications. Recent advancements have demonstrated the potential of simplified EOFC configurations using single-phase modulation, which significantly reduces the system’s complexity [27,28]. However, these systems are often affected by limited spectral bandwidth and low pulse contrast after compression, resulting in a decrease in pulse quality.

To overcome these limitations, a Mamyshev regenerator can be integrated with an EOFC [29,30,31]. The Mamyshev regenerator broadens the signal spectrum through nonlinear optical effects in optical fibers, then employs a frequency-shifted bandpass filter to eliminate the original signal and extract the reshaped new spectral components. By leveraging the frequency-domain separation characteristics between signal and noise, it simultaneously achieves signal reshaping, regeneration, and noise suppression. By eliminating unwanted continuous-wave components, the signal-to-noise ratio (SNR) is significantly enhanced, thereby generating high-quality picosecond pulses that meet the stringent requirements for pulse width [32,33]. While this method has already seen successful application in optical communication systems, its potential in the development of tunable all-fiber femtosecond lasers remains largely untapped.

In this work, we present a tunable all-fiber femtosecond EOFC operating at a 1.5 μm wavelength, a region of particular interest for optical communications due to its low loss in silica fibers. The system begins with a continuous-wave seed laser, which is phase-modulated and de-chirped to generate low-contrast pulses of approximately 8 ps at a repetition rate of 5.95 GHz. These pulses are then refined into clean, high-quality picosecond pulses using a Mamyshev regenerator, which removes temporal pedestals through self-phase modulation (SPM) and spectral filtering. The prepared source is further amplified using an erbium–ytterbium-doped fiber amplifier (EYDFA) operating in a highly nonlinear regime, enabling spectral broadening and pulse compression to the femtosecond level. Finally, by using a tunable bandpass filter, the center wavelength and spectral bandwidth of the generated broadband spectrum can be flexibly tuned.

2. Experimental Setup

The experimental setup of an all-fiber tunable electro-optic frequency comb system generating a 5.75 GHz comb based on two cascaded phase modulators is shown in Figure 1. First, a continuous-wave (CW) seed laser (wavelength: 1.5 μm; power: 10 mW/<10 kHz) undergoes phase modulation via two phase modulators (PMs) driven by a variable radio frequency (RF) source, generating an initial broadband frequency comb. Compared with using a pulsed seed laser, it has a simpler structure [34]. The modulated signal is then amplified by an RF amplifier to enhance the driving strength, ensuring a high modulation depth. Subsequently, the signal enters a pre-compression stage, where the chirp introduced by PMs is compensated using a dispersion-compensating fiber (DCF), compressing the broadened spectrum to a pulse width of 8 ps. The compressed pulses are then amplified by an erbium-doped fiber amplifier (EDFA) for initial power boosting, followed by an optical isolator (ISO) to suppress backscattering noise, and further optimized for dispersion distribution using a 50 m long highly nonlinear fiber (HNLF1); the HNLF1 group velocity dispersion is 0.3 ps²/km, and the nonlinear parameter is 10 W⁻¹ km⁻¹. To further improve pulse quality, the system incorporates a Mamyshev regenerator, which shapes the spectrum using a bandpass filter (BPF). Afterward, the pulses enter a nonlinear compression module, where an erbium–ytterbium-co-doped fiber amplifier (EYDFA) amplifies the power in a highly nonlinear regime. Here, a highly nonlinear fiber (HNLF2) further compresses the spectrum through SPM and optical wave-breaking effects, while a single-mode fiber (SMF) further optimizes the dispersion distribution and enhances the SNR. Finally, output wavelength tuning is achieved through a tunable bandpass filter (TBPF), achieving femtosecond pulses with a pulse width of approximately 470 fs; the measured pulse duration remains significantly broader than the Fourier transform-limited one, suggesting potential for further compression via phase optimization or dispersion management, enabling continuous tuning of the center wavelength and spectral bandwidth. It is worth noting that all optical components are interconnected by optical fiber fusion, and the key optical path maintains an all-fiber architecture.

3. Results and Discussion

The spectral information of the tunable electro-optic frequency comb system used in our experiment at four different stages is shown in Figure 2, reflecting the dynamic characteristics of the system during wavelength tuning and pulse compression. First, Figure 2a displays the spectrum of the EO comb after pure phase modulation, revealing resolved comb lines. Compared to traditional ultrafast electro-optic combs, the spectrum here exhibits a less smooth profile. Second, the spectrum at the pre-compression stage demonstrates improved power uniformity, as shown in Figure 2b, indicating that DCF and HNLF1 effectively compensate for the chirp introduced by phase modulation, laying the foundation for subsequent nonlinear compression. Figure 2c,d show the spectrum further broadening and presenting a smoother profile, indicating that HNLF2 and the Mamyshev regenerator effectively enhance the SNR through SPM and spectral filtering, generating high-quality picosecond pulses. However, due to the high nonlinear propagation in the final amplification stage, the accumulated nonlinear noise may induce spectral distortion or temporal jitter. Finally, the output spectrum exhibits fine-tuning capability within the range of 1554–1566 nm, suggesting slight differences in pulse compression performance at different wavelengths, which may be attributed to the wavelength-dependent effects of dispersion management and nonlinearity. At each stage of the system, the average output power values were measured using a calibrated power meter as follows: 10 mW at the seed comb, 367.3 mW after propagation using HNLF1, 25.6 mW after using the BPF, and 1.12 W after using HNLF2.

To evaluate the effectiveness of this scheme, the modulation frequency was adjusted within a range of 0.25 GHz in steps of 100 MHz, and the pulse autocorrelation traces were recorded. Figure 3a presents the autocorrelation trace after using HNLF1, exhibiting a pulse width in the order of picoseconds. Across the entire tunable range, all frequencies maintained an optimal pulse shape of approximately 8 ps. In contrast, Figure 3b shows the pulse autocorrelation trace after using HNLF2, where the pulse width was compressed to the femtosecond scale. This can be attributed to the combined effects of dispersion compensation and nonlinear interactions in HNLF, which effectively compressed the initially broadened spectrum to the femtosecond regime while ensuring pulse shape uniformity and stability.

Figure 4 illustrates the spectral characteristics of the RF signal in the tunable electro-optic frequency comb system. As illustrated in Figure 4a, the RF signal maintains a stable SNR of approximately 40 dB throughout the broadband tuning range from 5.65 to 6.05 GHz, indicating that the system can achieve stable frequency comb generation over a wide frequency range with uniform spectral distribution and no significant power attenuation or distortion. This is attributed to the efficient modulation of the PM and the gain optimization of the RF amplifier. As demonstrated in Figure 4b, the RF signal achieves an enhanced SNR of approximately 55 dB during the fine-tuning phase across the narrower frequency range of 5.91 to 5.96 GHz, as depicted in Figure 4b, demonstrating the system’s precise tuning capability within a narrow bandwidth. This high-resolution tuning characteristic is achieved through the coordinated control of the variable RF source, ensuring continuous tunability of the comb’s repetition frequency within the range of 5.65–6.05 GHz while maintaining high stability and low phase noise. The combination of broadband and high-resolution tuning provides flexible and reliable RF driving support for applications such as optical communications, precision spectroscopy, and frequency metrology.

In this paper, we also achieved the tunable characteristics of the center wavelength and bandwidth in the system. Figure 5a illustrates the wavelength-tunable spectral characteristics of the tunable electro-optic frequency comb system, with the spectrum covering the C-band communication window from 1550 to 1565 nm. The spectral uniformity and broadband coverage indicate that the system achieves efficient spectral broadening through the PM and SPM, while the dynamic filtering capability of the TBPF enables flexible adjustment of the frequency comb’s central wavelength across the entire C-band. The small power fluctuations in the spectrum demonstrate the system’s stability and high SNR over a wide wavelength range. Meanwhile, tunability of the bandwidth with a center wavelength of 1560 nm was also observed, as shown in Figure 5b. The fine spectral structure and high resolution indicate that the system achieves precise control over the frequency comb’s bandwidth through the TBPF, enabling sub-nanometer-level adjustments to the spectral width and shape. The small power fluctuations in the spectrum further demonstrate the system’s stability and high SNR within the narrowband range.

Compared with mode-locked cavities, which can also generate ultrafast lasers, the phase modulation scheme generates a frequency comb by directly driving a continuous-wave seed laser through cascaded phase modulators, eliminating the need for a short-cavity design with mode-locked cavities. This approach overcomes the issues of low gain efficiency and self-starting challenges encountered at high repetition rates. The repetition frequency can be continuously tuned within the range of 5.65–6.05 GHz, while RF signal driving ensures low phase noise. In contrast, although conventional mode-locked cavities can directly produce high-quality femtosecond pulses, their repetition rates are constrained by cavity length, exhibit limited tuning flexibility, and experience exacerbated phase noise at high repetition rates due to increased free spectral range.

4. Conclusions

In conclusion, this study demonstrates an all-fiber tunable electro-optic frequency comb system based on cascaded phase-modulation-driven architecture, generating a 5.75 GHz comb. The system begins with a CW seed laser phase-modulated by a variable RF source, producing a broadband seed comb. The subsequent stages include RF amplification for enhanced modulation depth; pre-compression via DCF to mitigate chirp-induced broadening, achieving an 8 ps pulse width; and amplification through an EDFA. Dispersion optimization is performed using HNLF1, while pulse quality is improved by a Mamyshev regenerator. Nonlinear compression is achieved through an EYDFA and HNLF2, with further dispersion and SNR optimization via SMF. The integration of the TBPF enables dynamic pulse adjustment, yielding 470 fs pulses with continuous repetition frequency tuning. Spectral evolution analysis reveals stable broadband tuning from 5.65 to 6.05 GHz with an SNR of 40 dB and precise fine-tuning from 5.91 to 5.96 GHz with an SNR of 55 dB. The system also demonstrates wavelength-tunable operation across the C-band with minimal power fluctuations, along with adjustable bandwidth control centered at 1560 nm. These results establish a robust and flexible platform for advanced applications in optical communications, precision spectroscopy, and frequency metrology, offering high stability and broad tunability.