High-contrast, fast chemical imaging by coherent Raman scattering using a self-synchronized two-colour fibre laser

The study targets a core bottleneck in translating coherent Raman scattering (CRS) microscopy—comprising CARS and SRS—into clinical settings: conventional solid-state laser sources are bulky, costly, environmentally sensitive, and require precise temporal and spatial overlap of two pulsed beams with matched pulse widths. Fibre-based approaches offer robustness and compactness but have been hampered by high intensity noise, narrow tuning, and insufficient power, which degrade image quality and speed and typically necessitate complex balanced detection. The authors aim to develop a compact, high-power, low-noise, self-synchronized two-colour fibre laser to enable high-contrast, fast CRS imaging without balanced detection, improving practicality and reliability for biomedical and potential clinical applications.

Recent two-colour fibre laser strategies for CRS include Ti:sapphire–fibre hybrids and all-fibre systems using active synchronization, parametric wavelength conversion, soliton self-frequency shift, and supercontinuum generation. Each faces limitations such as high cost, low power, and especially high intensity noise that complicates direct SRS demodulation (signals ~10⁻⁴–10⁻⁶), thus requiring balanced detection with limited robustness over wide spectral ranges. Advances in fibre optics—heavy doping enabling high power per unit length, double-clad fibres for efficient cladding pumping with MMF-coupled diodes, fibre chirped-pulse amplification (FCPA) for high-energy ultrashort pulses, and multifunctional hybrid fibre components—open pathways to high-quality ultrafast fibre sources. Building on these, the authors propose a self-synchronized two-colour fibre laser using coherent wavelength generation (CWG) via cross-phase modulation (XPM) to overcome noise and power limitations.

Laser architecture: A passively mode-locked ytterbium (Yb) fibre master laser at 1.0 µm (all-normal dispersion, NPR mode-locking) generates ~fs pulses at 80 MHz fundamental repetition rate (FRR) and ~20 mW average power. Output is split 50:50 into Stokes and pump branches; a photodiode tapping the master provides a system trigger. Stokes branch (1.0 µm): Pulses are linearly chirped in 50 m SMF, pre-amplified by core-pumped YDFA, intensity-modulated at 20 MHz via an in-line fibre-coupled intensity modulator (FIM) with >20 dB depth, and further amplified to >1 W with a cladding-pumped double-clad YDFA (DC-YDFA). Free-space pulse compression employs a grating pair and telescope (with a slit at the focal plane for ~1 nm effective spectral filtering) to yield ps pulses for Stokes excitation. Pump branch via CWG (1.5 µm): The remaining master output injects an erbium-doped fibre CWG oscillator (quasi-CW without injection) that uses NPR and XPM with chromatic dispersion to form and self-synchronize a stable ps pulse train at 1.5 µm when cavity length mismatch is <200 µm. Timing-dependent frequency shifts from XPM during optical collisions between the injected 1.0 µm pulses and circulating 1.5 µm pulses adjust group velocity (blue- or red-shift) to passively lock timing. The 1.5 µm train is amplified to ~1 W with a double-clad Er/Yb fibre amplifier (DC-EDFA) and frequency-doubled in a PPLN crystal to the visible (~789 nm typical) for optimal microscope objective transmission and Si photodiode sensitivity. Spectral tuning: Master laser coarse tuning 1010–1060 nm (~8 nm passband via intracavity filter F1) with fine tuning by slit translation (~1 nm effective bandwidth). CWG oscillator fine tuning 1540–1590 nm (<1 nm via F2). SHG from 1578 nm yields a pump at ~789 nm with 0.35 nm bandwidth (transform-limited ~2.6 ps). Performance characterization: Real-time pulse trains (20 GHz oscilloscope) show uniform intensities. Modulation depth >99% at 20 MHz; RF spectrum SNR ~67 dB at 20 MHz. Pulse widths: pump ~2.7 ps, Stokes ~3.2 ps; Stokes width constant across tuning (3.2 ± 0.06 ps, 1.8% variation). Long-term power stability (100 min): pump ~100 mW fluctuates 0.1%; Stokes ~550 mW fluctuates 0.5%. Timing jitter estimated via SFG in BBO using cross-correlation slope and SFG intensity fluctuations: ~24.3 fs (~0.8% of pulse width). Relative intensity noise (RIN) at 20 MHz: pump ~−147 dBc/Hz, Stokes ~−148 dBc/Hz; pump RIN improved by 50 dB compared to prior fibre implementations. SHG pump average power >160 mW (scalable >1 W with optimized PPLN). Microscopy and samples: Pump and Stokes are combined by DM, temporally aligned by a delay line, and launched into a custom-built laser scanning microscope. Hyperspectral CRS demonstrated across 2700–3550 cm⁻¹ (covering lipid CH₂ at 2845 cm⁻¹ and protein at 2920 cm⁻¹). SRS spectra acquired for DMSO and methanol and compared to spontaneous Raman. Multimodal imaging includes CARS, SRS, SHG, and TPEF on living human cells, mouse arterial tissues, and mouse tail, kidney, and brain sections.

- Developed a high-power, self-synchronized two-colour fibre laser enabling high-contrast, fast CRS (CARS and SRS) imaging without balanced detection. - Spectral coverage: 2700–3550 cm⁻¹ (high-wavenumber CH-stretch region), capturing lipid (2845 cm⁻¹) and protein (2920 cm⁻¹) resonances. - Modulation and detection: Fibre intensity modulation at 20 MHz with >99% modulation depth; RF SNR ~67 dB at 20 MHz; suitable for lock-in SRS. - Pulse properties: Pump ~2.7 ps; Stokes ~3.2 ps with constant width across tuning (3.2 ± 0.06 ps; 1.8% variation). - Noise and stability: RIN at 20 MHz ≈ −147 dBc/Hz (pump) and −148 dBc/Hz (Stokes); pump RIN improved by ~50 dB over previous fibre sources. Timing jitter ~24.3 fs (~0.8% of pulse width). Long-term average power fluctuations: pump 0.1%, Stokes 0.5% over 100 min. - Output powers: SHG pump >160 mW (scalable >1 W with improved PPLN efficiency). Stokes amplification >1 W before compression. - Imaging demonstrations: High-contrast SRS and CARS on living human cells and mouse arterial tissues; multimodal CRS+SHG+TPEF on mouse tail, kidney, and brain sections; hyperspectral SRS of DMSO and methanol agreeing with spontaneous Raman.

The presented fibre laser directly addresses the main barriers to practical CRS imaging by providing two synchronized, low-noise, ps pulses with sufficient power and wide spectral tunability, eliminating the need for balanced detection. Passive self-synchronization via XPM and NPR in a CWG oscillator yields robust timing lock with low jitter, while fibre-based FCPA and cladding-pumped amplifiers provide high power and stability. The constant pulse width across tuning preserves spectral fidelity for hyperspectral CRS. Demonstrations on diverse biological samples show high-contrast SRS/CARS and seamless integration with SHG and TPEF, expanding contrast mechanisms for biomedical imaging. Collectively, these results indicate that advanced fibre photonics can match many performance aspects of solid-state systems while improving robustness and system simplicity, thereby facilitating broader adoption and potential clinical translation of CRS.

A novel self-synchronized two-colour fibre laser system was developed that combines high power, low RIN, low timing jitter, wide tunability, and stable ps pulse widths, enabling high-contrast, fast CRS imaging without balanced detection. The approach leverages NPR-based mode-locking, XPM-driven coherent wavelength generation, fibre amplification, and SHG to provide a compact, robust source suitable for multimodal nonlinear microscopy. The work outlines a generalizable strategy to remodel existing lasers into synchronized two-colour sources, promoting wider deployment of CRS. Future directions include optimizing SHG efficiency to scale pump power beyond 1 W, implementing electronic feedback to further reduce RIN, and extending spectral coverage to additional Raman bands to broaden biomedical applicability.

- The laser’s intensity noise, while low enough for direct SRS detection, remains higher than that of top-tier solid-state fs lasers; contributors include pump diode noise, ASE, and environmental fluctuations. Electronic feedback could further reduce RIN. - Current SHG output after PPLN is >160 mW; although sufficient for CRS, higher powers may be desired and are contingent on further SHG optimization. - The system, as implemented, occupies ~80 × 85 cm² on an optical table and has not been size-optimized.