In a groundbreaking development within the field of fiber optics and laser technology, researchers have unveiled a high-performance orthogonal GHz harmonic dual-comb laser system that has the potential to revolutionize measurement applications. This innovative system, proposed by a team led by Professor Fei Xu at Nanjing University’s College of Engineering and Applied Sciences, utilizes a single-fiber linear-cavity laser integrated with various functional devices. The significance of this research cannot be understated, as it addresses long-standing limitations in the dual-comb measurement technologies that have traditionally depended on complex systems and lengthy repetition rate locking mechanisms.
Harnessing the capabilities of polarization multiplexing, the researchers successfully implemented a method to flexibly control the optical intensity distribution between orthogonal polarizations. This remarkable flexibility is achieved by manipulating the polarization-dependent degrees of freedom that the integrated device possesses. The result is controllable and efficient asynchronous harmonic mode-locking, which facilitates the generation of two separate sets of harmonic mode-locked pulses. By successfully multiplying the equivalent repetition rate difference, the system generates more temporal interferograms than previously achieved, illustrating a major leap in dual-comb measurement speed.
Historically, the capability to generate dual-comb laser systems has been limited by the challenges of repetitive rate differences (Δf_rep) within fiber cavities. The asynchronous dual-comb generated from a single fiber laser cavity vastly simplifies coherent measurements. Through the rejection of common modes, the necessity for intricate and cumbersome repetition rate locking systems is eliminated. However, existing techniques such as spatial, wavelength, and pulse waveform multiplexing have led to relatively modest repetition rate differences, typically ranging from tens of Hertz to tens of kilohertz.
Transcending these limitations, the research team explored the harmonic mode-locking technique, which exploits the energy clamping effect of single-soliton pulses. This method induces pulse splitting, paving the way for repetition rate multiplication. Consequently, the researchers excelled at achieving ultra-GHz repetition frequencies within fiber-based architectures, a task that has historically posed barriers due to fiber gain and integration challenges. The implication of their findings suggests that harmonic mode-locking with dual-channel multiplexing in a single fiber cavity could serve as a breakthrough, circumventing typical limitations of cavity length while maximizing pulse generation efficiency.
The new fiber dual-comb system demonstrates a remarkable fundamental repetition frequency of 383 MHz, with potentials for reaching 2.3 GHz during harmonic mode-locking. This translates to an incredible acquisition rate that exceeds 244 kHz, vastly outpacing earlier standards in single-cavity fiber dual-comb systems. Notably, by employing a shorter laser cavity, researchers observed equivalent Δf_rep values climbing as high as 400 kHz, marking a significant advancement in dual-comb technologies that can have profound implications for high-speed measurements.
The architecture of this polarization-multiplexed dual-comb laser integrates a Fabry-Pérot fiber cavity, utilizing a distributed Bragg reflector (DBR), erbium-doped fiber (EDF), and a central feature known as the fiber-coupled dual-comb mirror (FDCM). This intricate design makes use of a polarization controller inside the cavity that allows for real-time adjustments to the polarization direction of the intracavity laser. Moreover, an additional polarization controller placed along the output optical path, coupled with a polarization beam splitter (PBS), effectively distinguishes between the dual combs produced.
The FDCM represents a critical advancement comprised of gradient-index lenses for optimal collimation and focusing, paired with a birefringent crystal that facilitates polarization multiplexing. This complex setup employs a commercial semiconductor saturable absorber mirror (SESAM) for mode-locking, setting the stage for the simultaneous excitation of two sets of mode-locked pulses within the same cavity. It is essential to recognize that the tuning of the spacing between the SESAM and the birefringent crystal is crucial, ensuring that the system reliably operates between the focal points of ordinary light and extraordinary light.
Underpinned by theoretical principles, the operational stability of the generated GHz harmonic dual-comb was verified through multimode heterodyne interference. By implementing low-pass filtering methods, a clear interference signal emerged, capturing the vitality of the dual-comb signals in action. The sensation was palpable as Fourier transforms of the time-domain signals revealed distinct frequency down conversion patterns, with a signal-to-noise ratio exceeding 20 dB.
In the semi-controlled environment of their experiments, researchers delved deeper into the operational stability and performance of this new technology. Repeated assessments confirmed an impressive repetition frequency of 509 MHz and a Δf_rep of nearly 400 kHz, thus solidifying their reputation as pioneers in pushing the boundaries of single-cavity fiber-based dual-comb lasers.
The implications of this technology stretch far and wide, heralding a future where techniques can be readily adapted for dynamic measurement scenarios such as chemical composition analysis, ranging, and environmental monitoring. The path towards more efficient and compact solutions has significant ramifications for a variety of industries and scientific inquiries, emphasizing the enduring legacy of innovation woven throughout laser technology.
In summarizing this profound accomplishment, it’s crucial to recognize that the integration of advanced polarization multiplexing and the strategic use of harmonic mode-locking opens the gateway to a new paradigm in high-repetition-rate dual-comb generation. This singular achievement stands not just as a milestone for the researchers involved but as a pivotal leap for the wider scientific community striving to harness the full potential of dual-comb measurement technologies in their entirety. By taking this innovative approach, there is heightened optimism about the future prospects and applications of fiber-based dual-comb systems.
Given the exponential growth of technological capabilities outlined by this research, the outlook for high-speed dynamics in dual-comb applications is exceptionally promising. With ongoing advancements, generations of innovative applications and uses await their emergence, all stemming from the core discoveries and breakthroughs made by this research team.
Through this innovative endeavor, researchers have cultivated not only a novel method of laser technology but also a transformative pathway for future research endeavors and applications that can significantly elevate the science of optical measurements.
Subject of Research: Not applicable
Article Title: Orthogonal GHz harmonic dual-comb generation in monolithic fiber cavity for acquisition speed multiplication
News Publication Date: 20-Feb-2025
Web References: http://dx.doi.org/10.1186/s43074-025-00161-y
References: Not available
Image Credits: Guorui Wang#, Zixuan Ding#, and Fei Xu*
Keywords
Laser technology, fiber optics, dual-comb, harmonic mode-locking, polarization multiplexing, high-speed measurement, optical intensity distribution, temporal interferograms, coherent measurements, experimental study, innovation in photonics.
