Robust mode-locking in all-fiber ultrafast laser by nanocavity of two-dimensional heterostructure

Abstract

The fiber-based saturable absorber (SA) that enables mode-locking within a ring cavity serves as the core component of the ultrafast all-fiber lasers. However, the integration of SAs onto fibers with high compactness suffers from imbalanced saturable absorption properties and unstable mode-locking performance. Here, we present a robust mode-locking SA by integrating a nanocavity composed of a two-dimensional graphene heterostructure on the fiber end facet. We demonstrate a significant reduction in the saturation intensity (~65%) and improved soliton dynamic processes through precise modulation of the optical field within the heterostructure. The designed heterostructure facilitates the formation of a stable single-soliton state for robust mode-locking. A high tolerance to intracavity polarization variations is achieved in the heterostructure-SA (~85% compared to 20% for bare graphene). Our designed heterostructure-SA represents an important advancement in the development of ultracompact mode-locked all-fiber lasers, offering enhanced integrability and stability.

Fig. 1. Fiber-integrated heterostructure-SA.

a Schematic representation of the MoS₂-BN-graphene-BN-MoS₂ heterostructure embedded between optical fiber end facets. The heterostructure forms a nanocavity with a nonuniform optical field distribution. The input laser with random phases is modulated into phase-locked pulses with the interaction of the heterostructure-SA. b Optical image of the end facet of an optical fiber with the heterostructure (with an BN thickness of 240 nm) integrated. c Internal optical field intensity distribution of the heterostructure. The optical field intensity in graphene can be enhanced by ~230% with an BN thickness of 240 nm. d Optical field intensity enhancement factor (I_cavity/I_graphene) as a function of BN thickness, simulated with COMSOL. e Transmission measurements (circles) of graphene (blue) and the heterostructure (with an BN thickness of 240 nm, orange) as a function of the pump peak intensity. The solid curves represent the fitted results, with a modulation depth (α₀) of 4.2% and a saturation intensity (I_s) of 62.9 MW/cm² for bare graphene and values of 5.0% and 22.0 MW/cm² for the heterostructure

Fig. 2. Ultrafast all-fiber lasers based on the bare graphene-SA and heterostructure-SA

a Schematic of the mode-locked all-fiber laser and measurement system. The optical components include a laser diode (LD), a wavelength-division multiplexer (WDM), a dispersion compensation fiber (DCF), an erbium-doped fiber (EDF), an isolator (ISO), a saturable absorber (SA), and an optical spectrum analyzer (OSA). b Spectra of the output lasers with the bare graphene-SA and heterostructure-SA. There is an obvious nonsoliton component near the central wavelength in the spectrum of the graphene-SA, in contrast to that of the heterostructure-SA. c Output pulse trains from the all-fiber lasers. The repetition rate is 13.2 MHz for the heterostructure-SA and 14.0 MHz for the graphene-SA. The pulse train in graphene is accompanied by pulse splitting. d RF spectra measured. The signal-to-noise ratios of the heterostructure-SA and graphene-SA are 45 dB and 22 dB, respectively. e Autocorrelation traces with FWHM of ~1.20 ps for the heterostructure-SA and ~1.45 ps for the graphene-SA, fitted by Gaussian functions

Fig. 3. Real-time formation and evolution of solitons in an all-fiber laser mode-locked with the graphene-based SA.

Experimental real-time characterization of the entire buildup and evolution processes of double solitons for the graphene-SA (a) and of a single soliton for the heterostructure-SA (b). There are four evolution stages for the graphene-SA and three stages for the heterostructure-SA, as labeled in the figures. c Temporal profiles at representative roundtrips during pulse formation and pulse splitting with the graphene-SA. d Retrieved energy of individual pulses obtained with the graphene-SA. Two pulses of the same energy are formed after a violent energy fluctuation. e Temporal profiles at representative roundtrips during single-pulse formation with the heterostructure-SA. f Retrieved energy of the pulse obtained with the heterostructure-SA. A single-soliton mode-locking state remains stable during propagation (the energy fluctuation is less than 3%)

Fig. 4. Polarization-dependent mode-locking in the all-fiber laser.

a Configuration of the mode-locking state measurement system with polarization state traversal. An automatic polarization controller (APC) is used to control the intracavity polarization states. The mode-locking state and output polarization state are measured by an oscilloscope and a polarimeter, respectively. b Output polarization state distribution on the Poincaré sphere for the fiber ring cavity without an SA. c Typical output laser states on the oscilloscope under different intracavity polarization states. All states are normalized by the maximum intensity of output state 1. The mode-locking state is evaluated by integrating the fundamental peak intensity, which is marked by the blue box around time zero. The integral intensity of state 1 is normalized to 1. The typical output states 1 and 2 (integral intensity > 0.7) are defined as good single-pulse mode-locking. The output states 3, 4, and 5 represent pulse splitting, soliton rain, and a continuous-wave laser, respectively. d Output states of the graphene-SA and heterostructure-SA plotted on Poincaré spheres, color-coded by the integral intensity and the degree of polarization as the radius. Statistics on the integral intensities of the graphene-SA (e) and heterostructure-SA (f). Approximately 20% of the polarization states can maintain single-pulse mode-locking for the graphene-SA, while the value for the heterostructure-SA is approximately 85%