Noise-like pulse laser source with ultrabroadband tunability and coherence-limited sub-structure

Abstract: High brightness and low coherence laser sources with wideband tunability are essential for many full-field imaging applications aiming for high contrast and speckle free performance. However, this combination of parameters is challenging to achieve. The current solutions focus on decreasing spatial coherence or generation of time-varying speckle patterns, while suppression of temporal coherence typically compromises brightness. Here we demonstrate a wideband pulsed laser source with low temporal coherence and the absence of phase correlation between pulses as an alternative approach with simultaneous time and frequency diversity. The full gain spectrum of a Tm doped fiber laser (1650 nm 2000 nm) is operated in a tunable noise like pulse regime, which by nature is composed of countless structured elementary events with uncorrelated phases randomly varying from bunch to bunch. The measured spectral widths range from 13.8 nm to 18.8 nm, while the average output power varies between 63.3 mW and 213 mW. Numerical simulations reveal that temporal coherence decreases significantly with increasing optical gain, dropping from near unity at low gain to approximately 0.2 at high gain. The startup dynamics of the noise like pulse laser are experimentally studied using the dispersive Fourier transformation (DFT) method. Based on single shot spectra and frequency resolved optical gating traces, the coherence properties of the laser are further analyzed by calculating the mutual coherence function and cross-spectral density. The noise like pulse laser exhibits a coherence time of approximately 100 fs and an average pulse burst duration of about 40 ps in the high-gain regime.

• The paper demonstrates a noise-like pulse fiber laser that achieves ultrabroadband spectral tunability (1712–1957 nm) with a coherence-limited sub-structure of ~100 fs.
• The paper details the use of an intracavity acousto-optical tunable filter and nonlinear polarization rotation to enable efficient spectral selection and mode-locking.
• The paper shows that the source's low temporal coherence and controlled pulse dynamics support speckle-free imaging and reduce interference artifacts in biomedical applications.

Ultrawide Tunable Noise-Like Pulse Laser with Coherence-Limited Sub-Structure

Introduction

The paper investigates a pulsed fiber laser source operating in the noise-like pulse (NLP) regime, delivering ultrabroadband spectral tunability, low temporal coherence, and moderate brightness across the 1650–2000 nm range. The experimental implementation leverages a thulium-doped fiber (TDF) cavity with an intracavity acousto-optical tunable filter (AOTF), enabling direct and efficient spectral selection via RF-driven phase matching. The resulting emission characteristics circumvent traditional trade-offs between brightness and coherence in imaging applications.

Laser Architecture and Operational Characteristics

The laser cavity integrates 5.5 m of anomalous-dispersion fiber, polarization control (quarter and half waveplates), and an AOTF for tuning and spectral control. Nonlinear polarization rotation (NPR) ensures mode-locking, with two master oscillator power amplifiers pumping both ends of the TDF for sufficient gain. The AOTF simultaneously performs spectral filtering and frequency shifting, facilitating operation across the entire Tm gain spectrum. Output powers range from 63.3 mW to 213 mW with 3-dB spectral widths from 13.8 nm to 18.8 nm, supporting tunability from 1712 nm to 1957 nm. Pulse trains display stochastic amplitude fluctuations, consistent with NLP operation, whereby randomly distributed sub-pulses group into bursts (envelope duration ~40 ps) and each sub-pulse is coherence-limited (~100 fs).

A distinct phenomenon observed when operating near water absorption lines is the emergence of rare, high-energy pulses attributed to rogue wave statistics—sub-pulse ensembles occasionally avoid intra-cavity loss to generate anomalously intense bursts.

Numerical and Experimental Analysis of Regimes

Pulse dynamics and coherence evolution were elucidated via dispersive Fourier transformation (DFT) and generalized nonlinear Schrödinger equation simulations. As the intracavity gain increases, the cavity transitions through coherent soliton, drift, soliton fission, transient and chaotic (NLP) regimes. Coherence time and degree-of-coherence diminish with gain: near unity at low gain, decreasing to ~0.2 in the NLP regime. Pulse width decreases and spectral bandwidth increases with gain until the chaotic NLP regime is achieved. The NLP regime is characterized by broad spectral amplification, quasi-stationary behavior (Schell-model correlations), and strong shot-to-shot spectral fluctuations. Both DFT and frequency-resolved optical gating (FROG) measurements confirm low temporal coherence, evidenced by non-convergent retrieval and broad correlation peaks.

Coherence Characterization

Coherence properties were quantified using mutual coherence function (MCF) and cross-spectral density (CSD), with both theoretical and measured estimates validating the characterization. The coherence time in NLP regime is consistently ~100 fs, with pulse-burst durations ~40 ps, corresponding to a coherence length ~30 μm. By leveraging advanced FROG algorithms constrained by multi-shot spectra, the paper overcomes prior limitations in direct measurement of NLP coherence and provides robust correlation function estimates.

The NLP regime distributes pulse energy temporally and spectrally, resulting in low peak power but maintaining moderate average brightness. The spectral coherence is non-uniform and highly stochastic across individual shots, and the pulse structure allows for substantial suppression of interference artifacts and speckle.

Practical and Theoretical Implications

The demonstrated source is notably relevant for biomedical imaging, particularly for exploiting the third biological window (1650–1870 nm), as deeper tissue penetration and spectral flexibility can be achieved. Its low coherence but high brightness profile is ideal for speckle-free imaging modalities, reducing artifacts without sacrificing signal-to-noise ratio. In nonlinear imaging, the low peak power mitigates photodamage risk, while multi-sub-pulse bursts can, with appropriate amplification, support nonlinear excitation.

Integration of AOTF for direct tunability introduces a scalable platform—rapid, external control of emission allows for user-defined spectral bands, which is vital in wavelength scanning and spectroscopic techniques. The methodology sets a foundation for further exploration of low-coherence, high-brightness sources across broader spectral domains and longer pulse burst durations.

The theoretical characterization delineates the distinction between spectral bandwidth, coherence length, and degree-of-coherence. The findings confirm that coherence time alone does not capture pulse train correlations and highlight the necessity of second-order statistical analysis in complex pulsed regimes.

Future Directions

The results suggest opportunities in developing bright, low-coherence sources for advanced interferometric imaging, multi-contrast nonlinear microscopy, and coherent control in complex scattering media. Moderate, external amplification could extend peak power for nonlinear processes. The source architecture lends itself to extension into mid-IR and visible regimes with analogous gain media and filtering, further broadening practical utility. Quantitative analysis of rogue wave statistics and energy distribution may yield further insights into nonlinear intracavity dynamics under stochastic excitation.

Conclusion

This work presents a noise-like pulse fiber laser with ultrabroadband tunability and coherence-limited sub-structure, combining low temporal coherence (~100 fs), moderate average power (up to 213 mW), and flexible spectral control via AOTF across the Tm gain spectrum. The comprehensive experimental and theoretical analysis validates the source's simultaneous time and frequency diversity and its suppression of pulse-to-pulse phase correlation. Such sources provide a pathway to high-contrast, speckle-free, and artifact-resistant imaging modalities, with practical implications for biomedical photonics, nonlinear microscopy, and interferometric diagnostics.