kHz Noise-Suppressed Asymmetric Dual-Cavity Bidirectional Femtosecond Fiber Laser

Abstract

We demonstrate a novel bidirectional mode-locked ultrafast fiber laser based on an asymmetric dual-cavity architecture that enables freely tunable repetition rate differentials at the kilohertz level, while maintaining inherent common-mode noise suppression through precision thermomechanical stabilization. Through cascaded amplification and nonlinear temporal compression, we obtained bidirectional pulse durations of 33.2 fs (clockwise) and 61.6 fs (counterclockwise), respectively. The developed source demonstrates exceptional capability for asynchronous optical sampling applications, particularly in enabling the compact implementation of real-time measurement systems such as terahertz time-domain spectroscopy (THz-TDS) systems.

1. Introduction

Mode-multiplexed femtosecond fiber lasers have evolved into indispensable photonic workhorses for advanced optical sampling methodologies, dual-comb spectroscopic analysis, and high-precision interferometric fiber sensing applications. Contemporary research prioritizes four interconnected development vectors: (1) bidirectional power scaling beyond 1 nJ per channel [1], (2) the fundamental characterization of soliton collision dynamics and energy transfer mechanisms [2,3,4], (3) novel saturable-absorber technologies including nonlinear polarization evolution and semiconductor saturable absorber mirrors (SESAMs) with accelerated recovery times [5,6], and (4) application-optimized system integration for field-deployable instrumentation [7,8]. Particularly significant are asynchronous bidirectional configurations demonstrating rapidly expanding implementation in terahertz time-domain spectroscopy (THz-TDS) systems requiring >0.2 THz bandwidth and multi-heterodyne detection schemes demanding sub-MHz resolution. Conventional single-cavity dual-output architectures predominantly utilize wavelength-division [9,10,11], polarization-multiplexing [12,13], or direction-dependent [14,15,16,17] mode-discrimination approaches. The WDM methodology, which exploits group velocity dispersion differentials among spectrally discrete soliton components, imposes stringent specifications on intracavity optical elements—including dispersion-compensating modules requiring <5% tolerance and gain media necessitating ultra-broadband (>80 nm) emission profiles—fundamentally constrained by material-dependent gain bandwidth limitations. Polarization-multiplexed configurations, while theoretically capable of generating repetition-rate offsets exceeding 20 kHz through orthogonally polarized eigenstates, exhibit inherent vulnerability to ambient thermal fluctuations (>50 fs/°C phase drift) and mechanical vibrations due to non-PM fiber architectures, critically compromising long-term stability in industrial environments. In contrast, directional multiplexing demonstrates compelling advantages through the coherent synchronization of identical spectral components within monolithic all-fiber frameworks. This architecture maintains backward compatibility with conventional fiber laser topologies while providing exceptional environmental tolerance and phase-stable operation, positioning it as the preferred methodology for scalable multiplexed ultrafast systems. Despite these merits, directionally multiplexed cavities employing inherent path-length asymmetries typically achieve limited repetition-rate detuning (Δf < 2 kHz), falling critically short for emerging real-time metrology applications requiring microsecond-scale temporal resolution, as exemplified by next-generation terahertz time-domain spectroscopy and ultrafast optical coherence tomography systems. Although the intracavity integration of free-space trombone delay lines enables Δf enhancement beyond 10 kHz, such hybrid configurations inevitably sacrifice vibration immunity (resonance peaks > 5 dB at 100–500 Hz) and thermal stability (>100 ppm/°C drift)—core advantages intrinsically associated with all-fiber monolithic architectures that remain indispensable for practical field deployment.

The inherent common-mode noise rejection characteristic of single-cavity asynchronous lasers originates from the correlated propagation of environmental perturbations through shared resonator components, typically achieving 30–40 dB suppression in balanced designs. However, conventional high-Δf implementations (>5 kHz) necessarily incorporate substantial path-length asymmetry, reducing common-path ratios below 60% and consequently degrading noise suppression performance—fundamentally limiting signal-to-noise ratios in precision metrology. The bidirectional laser based on carbon nanotube mode-locking proposed by S. Saito et al. suffers from a relatively small proportion of non-common cavity sections in the overall cavity length due to the insertion of excessive fiber optic components in these segments [5]. The bidirectional laser design employing a four-port circulator and dual SESAM mode-locking, as proposed by Chunmei Ouyang et al., also demonstrates inadequate control over the non-common cavity length, resulting in a pulse repetition rate difference exceeding 6 MHz [16]. Existing common-cavity (100% shared path) alternatives preserve Δf stability through passive thermal compensation techniques (e.g., athermal packaging with <1 ppm/K coefficients), yet feature immutable repetition rates (f) and fixed Δf values (typically ±5% tolerance), severely constraining operational adaptability across diverse application scenarios. Our asymmetric-cavity bidirectional laser architecture addresses these dual challenges through three synergistic innovations: (1) a maximized shared-cavity ratio (>90%), ensuring inherent common-mode noise suppression, (2) the active localized environmental control of non-shared segments via thermoelectric coolers and piezoelectric positioners, enhancing system-wide noise rejection by >28% relative to passive systems, and (3) precision frequency trimming through micrometer-resolution fiber length adjustment, enabling the deterministic acquisition of repetition-rate differences across 2–15 kHz ranges while retaining full scalability and manufacturing reproducibility. Realized within an all-PM fiber architecture utilizing Panda-type fibers with >30 dB polarization extinction ratios, this approach fundamentally eliminates polarization-mode dispersion while maintaining intrinsic immunity to environmental polarization drift—a critical advantage over conventional non-PM implementations.

We constructed a comprehensive multi-stage laser system incorporating SESAM mode-locking and cascaded amplification, generating bidirectional asynchronous pulses at 33.2 fs (clockwise) and 61.6 fs (counterclockwise) durations with broadband characteristics. Cascaded amplification in Er³⁺-doped fibers produced directional spectral broadening to 62 nm (CW) and 71 nm (CCW) bandwidths. Experimental validation demonstrated immediate utility in two critical domains: (1) real-time asynchronous optical sampling (ASOPS) pump-probe configurations, achieving 5 kHz scan rates with <1.2 ps temporal resolution [18,19,20], and (2) compact THz-TDS systems, generating 0.8 THz bandwidth with 35 dB dynamic range at 1 s integration [21,22]. In distributed fiber sensing implementations, the system resolved cm level spatial resolution over several meter standard single-mode fiber [23]. Δf control via controlled length cutting enables the optimization of the sampling rate/acquisition bandwidth compromise governed by B = f²/Δf, enhancing measurement throughput >2× versus fixed-Δf commercial systems while maintaining coherence. This integrated approach establishes a robust, environmentally insensitive platform for next-generation field-deployable ultrafast photonics requiring high-coherence bidirectional sources with tunable repetition-rate offsets, particularly benefiting portable spectroscopic analyzers and industrial process monitoring instruments operating in uncontrolled environments [7].

2. Experimental Setup

Figure 1 schematically depicts the architecture of our bidirectionally pumped mode-locked fiber laser, which incorporates four functional modules: (1) a mode-locked oscillator, (2) a pre-amplifier stage, (3) a main amplifier stage, and (4) a nonlinear pulse compressor. Notably, the entire system employs all-polarization-maintaining (PM) fiber components to ensure polarization stability and environmental robustness. The scheme employs a multi-stage architecture to achieve simultaneous spectral broadening and pulse compression for bidirectional output beams. This configuration ensures maximum tuning freedom—enabling the flexible adjustment of femtosecond-level pulse parameters (pre-chirp, pulse width, center wavelength, etc.) in both directions without altering the repetition rates or their difference. This proves particularly valuable for asynchronous sampling terahertz time-domain spectroscopy systems and similar applications.

2.1. Oscillator

The bidirectional mode-locked fiber laser system employs two counter-propagating 980 nm laser diodes (Pump1 and Pump2, DOGAIN: DG-HS02-7625B-1000PM, DOGAIN, Suzhou, China) that deliver pump beams through a polarization-maintaining (PM) wavelength-division multiplexer/coupler hybrid device (TWDM, MC:MCWTH-9856-00-P-10-F-S-P9-10-B-N, Mingchuang, Shenzhen, China) into a 30 cm erbium-doped fiber (D = −12 ps/(nm·km), IXBlue: AUT-IXF-EDF-HD-PM) gain module. The TWDM components utilize a 90:10 splitting ratio for intracavity and extra-cavity optical routing, whereas mode-locking is achieved via a bidirectional fiber-integrated semiconductor saturable absorber (SA, BATOP:SA-1550-25-2ps-0, BATOP, Jena, Germany) with a 25% modulation depth, flanked by 20 cm PM fiber pigtails (MC:MCPMFP-1550-P15-S-01-2-FA, Mingchuang, Shenzhen, China) on both ends. The net dispersion of the bidirectional cavity was calculated to be approximately −0.165 ps².

Polarization-maintaining circulators (Cir1 and Cir2, MC: MCPCIR-1550-00-P-D-S-P15-10-B-N, Mingchuang, Shenzhen, China) establish geometrically asymmetric propagation paths via precision fusion splicing. The clockwise path (L1) spans 29.00 cm of PM fiber from Port3 (Cir1) to Port1 (Cir2), whereas the counterclockwise path (L2) measures 28.98 cm from Port3 (Cir2) to Port1 (Cir1). Active fiber straightening and submicron cleaving techniques using micro displacement stages enabled a deterministic path-length difference of ΔL = L1 − L2 = 247.0 μm. This engineered asymmetry induces a repetition rate shift of Δf = 5.2 kHz, governed by the relationship Δf = cΔL/(nL²), where c is the speed of light, n = 1.468 the effective refractive index, and L = 3.16 m the total cavity length.

To mitigate environmental perturbations, the asymmetric cavity section resides within a thermoelectrically stabilized enclosure TEC-controlled (ΔT < 0.5 °C) with integrated vibration isolation (>20 dB attenuation from 10 to 500 Hz). The complete system exhibits a fundamental repetition rate of f₀ = c/(nL) = 64.6 MHz, with tunable repetition rate differences spanning 2–20 kHz through controlled adjustments of ΔL (100 μm–1 mm precision; shorter lengths cannot be achieved due to length uncertainty induced by fiber cutting). This tunability is achieved via iterative cleaving-attenuation metrology, demonstrating robust control over submillimeter fiber length variations.

2.2. Optical Amplification and Pulse Compression

The bidirectional seed pulses underwent sequential amplification and compression in three cascaded stages: pre-amplification, main amplification, and nonlinear compression (Figure 1). In the pre-amplifier stage, a forward-pumping scheme utilizing Pump3 and Pump4 energized a 1.2 m normally dispersive erbium-doped fiber (D = −12 ps/(nm·km), IXBlue: AUT-IXF-EDF-HD-PM, exail, Talence, France) to establish well-defined self-similar amplification (SSA) dynamics [24]. Self-similar amplification is employed since it maintains linear chirp while achieving spectral broadening, thereby establishing a solid foundation for subsequent power amplification.

The pre-amplified counter-propagating pulses were subsequently routed to the main amplifier stage, where a 0.6 m anomalous-dispersion erbium-doped fiber (D = +16.6 ps/(nm·km), Nufern:PM-ESF-7/125, Nufern, East Granby, CT, USA) underwent backward pumping via Pump5 through wavelength-division multiplexers (WDM3 and WDM4, MC: MCPMWDM-9855-00-P9-10-B-N, Mingchuang, Shenzhen, China). This staged amplification architecture achieves nonlinear power scaling while maintaining controlled dispersion evolution through engineered β₂(z) management governed by

dP/dz = g(P)P − αP

(1)

β₂(z) = β₂ (EDF) + β₂(SPM)(P(z))

(2)

where g(P) represents the gain saturation function, P is the optical power, z is the propagation distance, α is the attenuation (loss) coefficient, β₂(z) is the total group velocity dispersion, β₂ (EDF) is the linear dispersion of the EDF, β₂(SPM)(P(z)) is the SPM-induced equivalent dispersion, and P(z) is the local optical power (derived from Equation (1)).

The spectrally broadened pulses from the two-stage amplification process were subsequently coupled into a 2.5 m polarization-maintaining single-mode fiber (PM-SMF28, D = +17 ps/(nm·km), Mingchuang, Shenzhen, China) for tailored dispersion compensation. This configuration yielded asynchronous output pulses with durations less than 100 fs (FWHM).

3. Results and Discussion

The dual-pumped laser architecture achieves robust bidirectional mode-locking operation through independent pump power regulation at P_pump1 = 530 ± 5 mW and P_pump2 = 670 ± 7 mW. The bidirectional output channels demonstrate P_CW = 7.37 mW (clockwise) and P_CCW = 9.18 mW (counterclockwise). Direction-resolved spectral characterization using an optical spectrum analyzer (0.02 nm resolution) revealed distinct propagation-dependent spectral fingerprints (Figure 2):

The 953 ± 5 pm wavelength separation with 7.8% spectral overlap demonstrates effective direction-division multiplexing (

Table 1. Bidirectional output parameters from the oscillator.

Direction	λc	Δλ@3 dB	Average Power
CCW	1561.6267 nm	3.42 nm	9.18 mW
CW	1560.6737 nm	2.53 nm	7.37 mW

). The spectral envelopes in both directions exhibit near-Gaussian profiles, indicating that the bidirectional mode-locked solitons are of breathing type. A discernible discrepancy in spectral linewidths emerges owing to the inherent asymmetry of bidirectional mode locking. Figure 3 presents the temporal characterization results of the bidirectional emissions. The output pulses from both propagation directions were monitored using high-bandwidth photodetectors (BRF = 2 GHz) and subsequently analyzed using a microwave frequency counter (δf = 1 Hz). Both channels maintained a fundamental harmonic mode-locking operation with repetition rates of f_CCW = 64.6391 MHz (counterclockwise) and f_CW = 64.6339 MHz (clockwise) with remarkable power stability (Figure 3a), exhibiting less than 0.5% relative fluctuation over 120 min continuous operation (Figure 3b). Notably, bidirectional solitons that share a common saturable absorber exhibit no detectable self-synchronization. This absence of synchronization likely arises from the temporal asynchrony between the counter-propagating pulse trains, wherein soliton collisions occur stochastically across the entire cavity with prolonged periodicities. The lack of persistent positive feedback mechanisms fundamentally precludes the stabilization of synchronized states.

Figure 4 systematically quantifies the laser noise performance through phase-stabilized repetition rate monitoring over a 60 min continuous operation under contrasting environmental conditions. During ambient operation, the bidirectional repetition rates exhibited peak-to-peak deviations of Δf_pp = 575 Hz (CCW) and 730 Hz (CW), while maintaining a statistically significant cross-correlation (R² = 0.774) between directional outputs (Figure 4b). Notably, a slight slope variation emerges in the autocorrelation traces of bidirectional repetition rates, revealing divergent evolution directions in the long-term operation of partially shared-cavity bidirectional solitons, resulting in insufficient common-mode noise suppression capability. The subsequent implementation of active thermal control (±0.5 °C stability) and broadband vibration isolation (10–100 Hz frequency range with >40 dB attenuation) in the asymmetric cavity segments effectively suppressed the uncorrelated fluctuations (Figure 4c). Comparing autocorrelation parameters before and after environmental control, the coefficient of determination (R²) increased from 0.774 to 0.993. Bidirectional output pulses exhibited enhanced correlation, indicating significantly improved common-mode noise suppression capability with a 28.3% enhancement. Notably, environmental control did not significantly affect fluctuations in bidirectional repetition rates. Rather, it eliminated their divergent evolution by stabilizing environmental parameters in non-shared cavity sections, ultimately achieving enhanced temporal correlation across the overall repetition frequencies.

The measured repetition-rate difference Δf = 5.20 kHz confirms that our fiber cleaving technique achieves sub-100 μm level precision. This frequency detuning corresponds to an intracavity path length difference of ΔL = cΔf/(nf²) = 249 μm, where n = 1.468 denotes the group refractive index, thus establishing a new performance benchmark for repetition-rate disparity in monolithically integrated bidirectional fiber lasers [17]. Compared to prior directional multiplexing schemes, the simplified architecture of non-shared cavity sections enables superior control over minute length differences—which correspond to smaller repetition-rate differences critical for balancing sampling rate and acquisition bandwidth in high-speed asynchronous sampling systems (cf. ΔB = f²/Δf). This enhanced controllability arises because excessive components and unavoidable pigtail lengths with fusion splice losses in bidirectional cavities render precise length adjustments unfeasible [5,16]. Concurrently, the 5.2 kHz repetition-rate difference serves as a specific implementation example demonstrating the scheme’s feasibility. Theoretically, arbitrary repetition-rate differences can be achieved through the controlled length cutting of asymmetric fiber sections, though this requires a careful consideration of trade-offs with the target signal’s bandwidth [25].

The statistical characterization of 180 Δf datasets acquired through hourly measurements over continuous operation revealed significant noise suppression: peak-to-peak fluctuations were reduced by 81.5% (Δf_pp = 372 Hz to 69 Hz), equivalent to 14.2 dB suppression, confirming enhanced common-mode noise rejection through environmental regulation (Figure 5a). Comparative probability density analysis demonstrated a 68.6% reduction in distribution width, with standard deviation decreasing from σ = 68.78 Hz to 15.73 Hz (corresponding to 8.7 dB improvement) following the implementation of localized active stabilization (Figure 5b). This enhanced stability originates from the synergistic interaction between the high spatial-mode overlap (91.8% resonator sharing factor) and our Multiphysics control protocol combining thermal regulation and vibration isolation. Allan deviation analysis showed significantly lower short-term repetition jitter than long-term fluctuations, with a tenfold improvement under controlled conditions (Figure 5c). This enhanced stability is critical for high-speed asynchronous sampling systems.

The bidirectional seed pulses were amplified through a cascaded two-stage amplification process (pre-amplification, followed by main amplification) and subsequently compressed. With precisely controlled pump powers of P_pump3 = 540 mW, P_pump4 = 570 mW, and P_pump5 = 790 mW delivered via polarization-maintaining wavelength-division multiplexers (WDMs), the polarization-maintaining fiber-based compression module produced asynchronous dual-output pulses that exhibited the following key characteristics (

Table 2. Bidirectional final output parameters.

Direction	λc	Δλ@3dB	Pulse Width	Average Power
CCW	1562.1445 nm	62 nm	61.6 fs	80.3 W
CW	1597.2956 nm	71 nm	33.2 fs	92.7 mW

):

The output spectra (Figure 6a) demonstrate over 10-fold nonlinear spectral broadening in both pulse trains, facilitating compression to sub-100 fs durations through dispersion-engineered polarization-maintaining (PM) fibers. Specifically, the counterclockwise (CCW) and clockwise (CW) pulses achieved compressed durations of 61.6 fs and 33.2 fs, respectively, as validated by dispersion profile analysis (Figure 6b,c). It is observed that the presence of a residual chirp prevents full pulse compression to the Fourier-transform limit, although this non-ideal compression remains tolerable in pump–probe systems. Notably, cascaded amplification enables significant spectral broadening in both output directions (62 nm and 71 nm, respectively), exceeding the bandwidth limitations of the gain fiber. This breakthrough proves particularly valuable for dual-comb systems where such broadband sources are essential [8,26,27].

4. Conclusions

We introduce a bidirectional all-polarization-maintaining (PM) fiber laser system employing localized asymmetric cavity engineering to achieve a high repetition-rate difference of 5.2 kHz. This design incorporates a precision fiber length trimming technique that enables the continuous scaling of repetition-rate differences from a 2 kHz baseline to extended operational ranges (>15 kHz), while maintaining exceptional phase stability. Crucially, the strategic decoupling of cavity segments permits the independent environmental stabilization of non-common resonator sections, thereby preserving high common-mode noise rejection ratios. This architecture fundamentally resolves the long-standing compromise between repetition-rate flexibility and environmental stability inherent in conventional bidirectional mode-locked lasers.

The all-PM fiber configuration ensures inherent immunity to ambient perturbations, eliminating polarization drift and phase noise sources that plague conventional mode-multiplexed single-cavity designs. Through the active thermal and mechanical control of asymmetric cavity segments, we suppress differential phase noise by 28.3% compared to unstabilized configurations, achieving a fractional repetition-rate stability. Subsequent cascaded amplification stages generate broadband spectra (62 nm CCW/71 nm CW), which undergo nonlinear pulse compression to yield transform-limited outputs of 33.2 fs (clockwise) and 61.6 fs (counterclockwise) durations with <5% pulse-to-pulse timing jitter.

The asynchronous femtosecond pulse outputs demonstrate immediate utility in real-time pump-probe systems, evidenced by their application potential in terahertz time-domain spectroscopy (THz-TDS) and distributed fiber sensing with centimeter-level spatial resolution. The system’s programmable repetition-rate difference (Δf) enables the adaptive optimization of the sampling rate/acquisition bandwidth compromise (B = f²/Δf), enhancing measurement throughput by >2× compared to fixed-Δf architectures. This tunability, combined with environmental resilience, offers a viable approach for field-deployable ultrafast systems requiring high-coherence bidirectional sources.