Dual-comb acquisition speed multiplication in single short fiber

The asynchronously dual-comb generated in a single fiber laser cavity is a powerful tool for simplifying coherent measurements, as common-mode rejection eliminates the need for complex repetition rate (f_rep) locking systems, thereby significantly reducing the complexity of the measurement system. Although multiplexing techniques (such as direction, wavelength, spatial, polarization, and pulse waveform multiplexing) can generate two sets of stable pulse trains within the same fiber cavity, the limited dimensional differences in the cavity only allow for a relatively small repetition rate difference (Δf_rep, typically in the range of tens of Hz to tens of kHz). Since the acquisition speed of dual-comb measurements directly depends on Δf_rep, this performance limitation in current single-cavity fiber dual-comb systems severely restricts their potential applications in high-speed dynamic scenarios.

For a long time, single-cavity dual-combs with GHz-level repetition rates and ultra-high Δf_rep have primarily been realized in whispering-gallery-mode microcavities in the form of dissipative Kerr solitons. However, considering fiber gain and device integration, achieving higher f_rep and Δf_rep in mode-locked fiber laser systems remains challenging. In recent years, harmonic mode-locking, which utilizes the energy clamping effect of single-soliton pulses to induce pulse splitting and achieve repetition rate multiplication, has gradually demonstrated great potential for realizing ultra-GHz repetition frequencies within fiber-based architectures. If harmonic mode-locking with dual-channel multiplexing can be realized in a single fiber cavity, fiber dual-comb systems can bypass the limitations of cavity length and gain while leveraging the increased number of pulses to enhance both f_rep and Δf_rep. This would provide significant advantages in cost and efficiency compared to ultra-high-quality-factor microcavities.

Recently, a research team led by Professor Fei Xu from the College of Engineering and Applied Sciences at Nanjing University proposed a single-fiber linear-cavity laser integrated with multifunctional devices. This setup enabled flexible control of the optical intensity distribution between orthogonal polarizations by adjusting the polarization-dependent degrees of freedom inherent to the device, thereby achieving controllable polarization-multiplexed asynchronous harmonic mode-locking. The two sets of harmonic mode-locked pulses can effectively multiply the equivalent Δf_rep and generate more temporal interferograms based on the least common multiple principle. With a fundamental repetition frequency of 383 MHz, the laser achieves a repetition frequency as high as 2.3 GHz in harmonic mode-locking and an acquisition rate exceeding 244 kHz, which is two orders of magnitude higher than the conventional levels previously reported for single-cavity fiber dual-comb systems. Using a shorter laser cavity can further increase the equivalent Δf_rep to as high as 400 kHz. This orthogonally polarized GHz harmonic dual-comb laser provides a novel paradigm for high-repetition-rate dual-comb generation and offers an integrated single-fiber-cavity solution for enhancing acquisition rates in measurement applications.

The polarization-multiplexed dual-comb laser consists of a Fabry-Pérot fiber cavity, incorporating a distributed Bragg reflector (DBR), erbium-doped fiber (EDF), and the core functional component, termed the fiber-coupled dual-comb mirror (FDCM). A polarization controller (PC1) inside the cavity is used to adjust the intracavity polarization direction, while PC2 in the output optical path, in conjunction with a polarization beam splitter (PBS), separates the dual combs. The FDCM consists of two gradient-index (GRIN) lenses for collimation and focusing, a birefringent crystal for polarization multiplexing, and a commercial semiconductor saturable absorber mirror (SESAM) for mode-locking. By adjusting the polarization direction of the intracavity laser, the energy of the two polarization states split by the birefringent crystal can be properly distributed (Figure 1c), thereby simultaneously exciting two sets of mode-locked pulses within the same cavity. Additionally, fine-tuning the spacing between the SESAM and the crystal ensures that the system operates between the focal points of the o-light and e-light, providing a second degree of freedom for adjusting the intensity distribution of the two polarization states within a small range. By leveraging these two degrees of freedom, the fundamental or harmonic mode-locking of the two polarization states can be achieved simply and reliably. This study also marks the first observation of orthogonal asynchronous harmonic vector solitons in a fiber laser.

At the frequency corresponding to the least common multiple of the harmonic orders, the equivalent Δf_rep of the harmonic dual-comb is increased accordingly, producing more interferograms within the same time-domain sampling window and thus achieving a multiplied dual-comb acquisition rate.

The coherence of the generated GHz harmonic dual-comb was verified through multimode heterodyne interference of the separated dual-comb signals. After low-pass filtering, a clear interference signal was observed using a balanced photodetector. By performing a Fourier transform on the time-domain interferogram, the spectrum in the radio frequency (RF) domain exhibits a clear frequency down conversion, with a signal-to-noise ratio exceeding 20 dB. The operational stability of the harmonic dual-comb was validated by continuously scanning the RF spectrum to monitor the repetition rates of the two mode-locked pulse trains and their Δf_rep. Without any environmental control (such as temperature regulation or frequency stabilization), Δf_rep only fluctuated with a deviation of 44.42 Hz within 40 minutes.

More detailed experiments further demonstrated a f_rep of 509 MHz and a Δf_rep close to 400 kHz, achieving the highest reported performance for single-cavity fiber-based dual-comb lasers. This work provides a simple solution for improving the acquisition speed in specific dual-comb measurement scenarios, featuring a compact structure and low cost. It holds promising potential for applications in chemical composition analysis, ranging, and environmental monitoring.