Spectral Failsafe System of High-Power Laser Using Dual Fiber Bragg Gratings.

Phase-modulated (PM) spectral failsafe systems are necessary to promptly terminate amplification processes following accidental seeding of a high-power laser chain with a non-PM pulse to prevent optical damage. In this work, we present a reliable spectral failsafe system that can indicate the presence or absence of sufficient PM light. This requirement is met by combining dual temperature-sensitive fiber Bragg gratings detection with high-speed RF amplitude comparisons. The failsafe trigger signal is generated when the spectral power at the peak sideband exceeds that at the center. The spectral failsafe system has the ability to distinguish between adequate and inadequate PM pulses, and it exhibits significant robustness in pulse width, TEC temperature drift, and DFB wavelength drift in experiments, making it valuable for safe high-power laser operations and providing a useful reference for other detection system designs.

Ultrafast convergent power-balance model for Raman random fiber laser with half-open cavity.

The power-relevant features of Raman random fiber laser (RRFL), such as lasing threshold, slope efficiency, and power distribution, are among the most critical parameters to characterize its operation status. In this work, focusing on the power features of the half-open cavity RRFL, an ultrafast convergent power-balance model is proposed, which highlights the physical essence of the most common RRFL type and sharply reduces the computation workload. By transforming the time-consuming serial calculation to a parallel one, the calculation efficiency can be improved by more than 100 times. Particularly, for different point-mirror reflectivities and different fiber lengths, the input-output power curves and power distribution curves calculated by the present model match nicely with those of the conventional model, as well as with the experimental data. Moreover, through the present model the relationship between point-mirror reflectivity and laser threshold is analytically derived, and the way for improving RRFL's slope efficiency is also provided with a lucid theoretical explanation.

Off-axis eight-pass neodymium glass laser amplifier with high efficiency and excellent energy stability.

A new relay-imaged off-axial eight-pass laser amplifier with several joules energy and 1 Hz repetition rate was demonstrated. The extraction efficiency and pulse-to-pulse energy stability were greatly improved. Under the single-pass small-signal gain of 3.6, a net gain of 900 and an effective extraction efficiency of 42.4% in the beam aperture were realized. Pulse-to-pulse energy stability of 0.83% (peak-valley) and 0.17% (root-mean-square) was achieved by the significant saturation of eight-pass amplification, which, to the best of our knowledge, is the best energy stability in several-joules-class amplifiers. The far-field quality was 2.52 times the diffraction limit, and the near-field modulation of the 90% beam aperture was 1.28. No parasitic oscillations or pencil beams were observed. Moreover, another key feature of the proposed amplifier was the ability to remarkably improve the pulse contrast with a unique design.

Temporal pulse precisely sculpted millijoule-level fiber laser injection system for high-power laser driver.

A fiber laser injection system used as a seeder for a high-power laser facility of inertial confinement fusion was designed to meet stringent requirements. Herein, we demonstrate the fiber laser injection system, whose output single-pulse energy reaches the millijoule class. With two-stage amplitude modulators, the system produces a pulse with a higher pulse shaping capability. In addition, amplifying the pulse with large-mode-area fiber and single polarization, large-mode-area photonic crystal fiber (PCF) ensures a good beam quality output. In this proof-of-principle experiment, the long-term stability of FM-to-AM modulation and pulse energy is demonstrated. The successful demonstration of this laser injection system holds great significance for future high-power laser drivers.

Coherent Φ-OTDR based on I/Q demodulation and homodyne detection.

We demonstrate a novel distributed acoustic sensing (DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR). Both the phase and the amplitude of the Rayleigh scattering (RS) light can be demodulated in real-time. The technique is based on I/Q demodulation and homodyne detection using a 90° optical hybrid. The theoretical analysis is given, and as a proof of the concept, the dynamic strain sensing is experimentally demonstrated, with a sensing range of 12.566 km and a spatial resolution of 10 m.

Cascaded random distributed-feedback Raman fiber laser assisted by Fresnel reflection.

We propose a new type of cascaded random distributed- feedback Raman fiber laser (RD-RFL), with the linear output at the second-order Stokes wavelength. The guideline about building such a RD-RFL with particular output power and conversion efficiency is presented based on numerical study. We also experimentally instantiate the principle, and confirm the effectiveness of the scheme on lasing generation. Moreover, the added Fresnel reflection feature maintains the simplicity and flexibility of the RD-RFL.

Role of the mirror's reflectivity in forward-pumped random fiber laser.

In this paper, we thoroughly analyze the role of the point reflector's reflectivity in the performance of forward-pumped random fiber laser, in both the long- and short-cavity cases. The results show that the power performance is sensitive to the small reflection added on the pump side of the fiber end, whereas both the power distribution and threshold tend to be stable when the reflectivity reaches a relatively high level (>0.4). Moreover, for the short cavity case (e.g. 500m), the maximum achievable 1st-oder random lasing output can even increase when the reflectivity decreases from 0.9 to 0.01, due to the different lasing power distributions with different reflectivity values. This work reveals a new and unique property of random fiber lasers and provides insights into their design for the applications such as distributed amplification and high power sources.

Third-order random lasing via Raman gain and Rayleigh feedback within a half-open cavity.

Third-order random lasing operating in 1670 nm spectral band is experimentally demonstrated for the first time to the best of our knowledge, with only 2.45 W pump threshold. The lasing cavity is formed by G.652 fiber and fiber loop mirrors (FLMs), while the former acts as the distributed reflector and the latter acts as the point reflector. The G.652 fiber and the FLMs are connected via a multi-band wavelength-division-multiplexer, which ensures each of the three Raman Stokes components generated in the long fiber is routed to one FLM and then reflected back with minimum loss. Unlike existing half-open random lasing cavities using fiber Bragg gratings, the reflection bandwidth of FLMs is wide enough to preserve the intrinsic spectral features of each lasing bands, providing a valuable platform to study the mechanism of high-order random lasing in fibers. Also, the reflection efficiency can be treated as an invariant as the pump power grows, significantly reducing the threshold of high-order random lasing. The stationary model is used to calculate the output power, and the results fit the experimental data well.

Broadband flat-amplitude multiwavelength Brillouin-Raman fiber laser with spectral reshaping by Rayleigh scattering.

In this letter, we propose a novel configuration for generating multiwavelength Brillouin-Raman fiber laser (MBRFL). The spectral reshaping effect introduced by Rayleigh scattering in a 50 km single mode fiber unifies the generated Brillouin comb in terms of both power level and linewidth. As a consequence, we are able to obtain a 40 nm flat-amplitude MBRFL with wide bandwidth from 1557 nm to 1597 nm covering >500 Stokes lines. This is, to the best of our knowledge, the widest flat-amplitude bandwidth of MBRFL with uniform Stokes combs using just a single Raman pump laser. The channel-spacing is 0.08 nm and the measured OSNR is higher than 12.5 dB. We also demonstrate that the output spectrum of the MBRFL is nearly unaffected over 14 dB range of Brillouin pumping power.