Quasi mode-locking of coherent feedback random fiber laser

A mode-locked random laser generating transform-limited optical pulses

Ever since the mid-1960's, locking the phases of modes enabled the generation of laser pulses of duration limited only by the uncertainty principle, opening the field of ultrafast science. In contrast to conventional lasers, mode spacing in random lasers is ill-defined because optical feedback comes from scattering centres at random positions, making it hard to use mode locking in transform limited pulse generation. Here the generation of sub-nanosecond transform-limited pulses from a mode-locked random fibre laser is reported. Rayleigh backscattering from decimetre-long sections of telecom fibre serves as laser feedback, providing narrow spectral selectivity to the Fourier limit. The laser is adjustable in pulse duration (0.34-20 ns), repetition rate (0.714-1.22 MHz) and can be temperature tuned. The high spectral-efficiency pulses are applied in distributed temperature sensing with 9.0 cm and 3.3 × 10-3 K resolution, exemplifying how the results can drive advances in the fields of spectroscopy, telecommunications, and sensing.

RF-controlled mode-locking and optoelectronic pulse compressing with distributed selectable wavelengths

Mode-locking in laser cavities has attracted great interest due to its wide range of applications in generating optical frequency combs and ultra-short pulse trains. Here, a mode-locked fiber laser with a distributed selectable wavelength feedback is proposed based on radio frequency maneuverability. The laser is capable of generating transform-limited pulses with a selectable wavelength and repetition rates by interrogating different reflectors through active modulation. Intriguing laser pulses were realized, which can have >930 times width compression ratio compared with the modulation signal and can be selectively locked to reflectors separated in centimeter scale.

Self-Induced Mode-Locking in Electrically Pumped Far-Infrared Random Lasers

Mode locking, the self-starting synchronous oscillation of electromagnetic modes in a laser cavity, is the primary way to generate ultrashort light pulses. In random lasers, without a cavity, mode-locking, the nonlinear coupling amongst low spatially coherent random modes, can be activated via optical pumping, even without the emission of short pulses. Here, by exploiting the combination of the inherently giant third-order χ⁽³⁾ nonlinearity of semiconductor heterostructure lasers and the nonlinear properties of graphene, the authors demonstrate mode-locking in surface-emitting electrically pumped random quantum cascade lasers at terahertz frequencies. This is achieved by either lithographically patterning a multilayer graphene film to define a surface random pattern of light scatterers, or by coupling on chip a saturable absorber graphene reflector. Intermode beatnote mapping unveils self-induced phase-coherence between naturally incoherent random modes. Self-mixing intermode spectroscopy reveals phase-locked random modes. This is an important milestone in the physics of disordered systems. It paves the way to the development of a new generation of miniaturized, electrically pumped mode-locked light sources, ideal for broadband spectroscopy, multicolor speckle-free imaging applications, and reservoir quantum computing.

Tailoring the spectrum and spatial mode of Yb-doped random fiber laser

In this paper, we make a comprehensive study on tailoring the spectrum and transverse mode of random fiber lasers (RFLs). By simply temperature tuning, the mode gain profile of RFL can be flexibly and precisely manipulated. The spectrum of laser output can be easily tailored in single-wavelength, dual-wavelength, and three-wavelength, respectively. Meanwhile, the operating transverse mode is also optional among LP₀₁ mode, LP₁₁ mode, and hybrid mode. The slope efficiency of 17.9% and 27.3% are obtained for LP₁₁ mode and LP₀₁ mode operation, respectively. Besides, the coherence control can be confirmed by making speckle contrast measurements. This high-efficiency RFL with the customizable spectrum and spatial mode would have unique applications in wavelength or mode division multiplexing systems, speckle-free imaging, secure communication, and information encryption.

Self-gain-modulation random distributed feedback Raman fiber laser with switchable repetition rate

We experimentally demonstrate a pulsed operation in a random fiber laser operation via self-gain-switching. The pulses have low timing jitter and high average output power. We show that repetition rate switches abruptly while varying the pump power, and introduce a simple formula for oscillation frequencies.

Demonstration of Self-Starting Nonlinear Mode Locking in Random Lasers

In ultrafast multimode lasers, mode locking is implemented by means of saturable absorbers or modulators, allowing for very short pulses. This occurs because of nonlinear interactions of modes with well equispaced frequencies. Though theory predicts that, in the absence of any device, mode locking would occur in random lasers, this has never been demonstrated so far. Through the analysis of multimode correlations we provide clear evidence for nonlinear mode coupling in random lasers. The behavior of multiresonance intensity correlations is tested against the nonlinear frequency matching condition equivalent to the one underlying phase locking in ordered ultrafast lasers. Nontrivially large correlations are clearly observed for spatially overlapping resonances that sensitively depend on the frequency matching condition to be satisfied, eventually demonstrating the occurrence of nonlinear mode-locked mode coupling. This is the first example, to our knowledge, of an experimental realization of self-starting mode locking in random lasers, allowing for many new developments in the design and use of nanostructured devices.

Few-mode random fiber laser with a switchable oscillating spatial mode

Random fiber lasers are of tremendous interest to diverse applications for optical fiber sensing, speckle-free imaging. To date, random fiber lasers with fundamental mode oscillation have been well developed. However, controllable oscillating spatial mode in random fiber lasers have not been reported yet. Here, we propose and demonstrate a few-mode random fiber laser with a switchable oscillating spatial mode based on mode injection locking. An external signal light is injected to realize the locking of transverse mode in this random fiber laser and the direct oscillations of the fundamental mode, hybrid mode, and high order mode can be realized, respectively. This random fiber laser operates in the high-order LP₁₁ mode stably with a threshold of as low as 88 mW. High efficiency and high purity cylindrical vector beams can be obtained by removing the degeneracy of the LP₁₁ mode. This work may pave a path towards random fiber lasers with controllable spatial modes for specific applications in mode division multiplexing, imaging, and laser material processing.

Mode locking of a coherent random fiber laser with selectable repetition rates

Controlling emission of light in random structures/disordered systems, e.g., implementing mode-locked pulses in a laser system with a random structures/disordered systems, is a complex task. Usually, the generation of laser pulse by mode locking needs a stable fixed-length cavity that determines a specific repetition rate of the mode-locked pulses. Here, mode-locking laser pulses with selectable repetition rates are achieved in a typical one-dimensional disordered laser by passive mode locking. The laser includes disordered reflectors to provide multiple resonant modes associated with different cavity length. The regular pulses with adjustable repetition rates can be generated and selected by a nonlinear polarization rotator and a semiconductor saturable absorber mirror. The proposed work utilizing the advantages of multiple resonances in random lasers could pave a new way for regulating emission of light in the random structures/disordered system. And it displays an effective and realistic technical route to study ultrafast pulses generation and optical soliton dynamics in random structures/disordered systems.

Electrochemical Peeling Few-Layer SnSe2 for High-Performance Ultrafast Photonics

In recent years, the photoelectric properties and nonlinear optical properties of layered metal chalcogenides (LMCs) have attracted extensive attentions. Because of lower phonon thermal conductivity, larger energy storage rate, and larger electron mobility, LMCs are widely studied in the fields of thermoelectric energy conversion, battery electrode materials, and semiconductor devices. As 2D LMCs, SnSe₂ nanosheets (Ns) are connected to each other by van der Waals force, which makes it possible to use electrochemical methods to help peel off the thin layer structure. Two-dimensional SnSe₂ has obvious adjustable band gap characteristics. Its thickness can be controlled to keep it on the desired band gap. In this article, we prepared a thin layer of SnSe₂ by electrochemical methods and detected its nonlinear optical characteristics. It shows that our prepared materials have good optical absorption characteristics; it has a modulation depth of 15% and a saturation intensity of 61 MW/cm². To investigate the nonlinear effects of SnSe₂ in short and long cavities, the Q-mode-locking phenomenon was first achieved in a fiber laser with cavity length of 6 m. After increasing the cavity length to 56 m, the pump power is adjusted to achieve an adjustable repetition frequency from MHz to GHz in turn in an Er-doped fiber laser through utilizing an SnSe₂ incorporating a tapered fiber as a saturable absorber (SA). The nonlinear optical properties of thin layer SnSe₂ are fully proven, which opens a new way for advanced photonics, optical communication, laser measurement, and other fields.

High-resolution random fiber laser acoustic emission sensor

A high resolution fiber-optic acoustic emission (AE) sensor using a random fiber laser (RFL) is proposed. The AE probe is undertaken by a random-gratings-based erbium-doped RFL. A narrow linewidth π-FBG is used as a wavelength locking and sensing element in the RFL. The random distributed feedback in RFL significantly extends the effective cavity length of the laser, thus reduces the thermal frequency noise of the laser and improves the resolution of AE signal. A narrow lasing operation with a 20 dB linewidth of ∼10.41 kHz and a frequency noise of ∼10 Hz/√Hz above 1 kHz is realized. The 3×3 coupler interrogation technique is used for signal demodulation. A high AE signal resolution of ∼280 fɛ/√Hz @ 1 kHz is obtained. To the best of our knowledge, this is the first time that RFL is used in the 3×3 coupler based AE demodulation scheme to improve the system resolution.