Xiang’ai Cheng

Nanosecond passively Q-switched thulium/holmium-doped fiber laser based on black phosphorus nanoplatelets

The generation of nanosecond pulses in a thulium/holmium-doped fiber laser passively Q-switched by a black phosphorus saturable absorber (BP-SA) was experimentally demonstrated. The high quality BP-SA with a modulation depth of ~24% was fabricated by depositing ~23 nm thickness of BP nanoplatelets onto a fiber ferrule. By inserting the BP-SA into a thulium/holmium-doped fiber laser cavity, stable Q-switched operation was achieved with the maximum pulse energy up to 632.4 nJ, the shortest pulse width of 731 ns and pulse repetition rates varying from 69.4 to 113.3 kHz. These results suggested that BP could be developed as an effective SA for pulsed laser operation at 2 μm.

Real-time mode decomposition for few-mode fiber based on numerical method

Today a specific attention has been paid to look into the modal characteristics of the high-power laser beam. And the instantaneous monitoring and analyzing on modal content via the mode decomposition technique will provide a novel route. We implement the first-ever experimental investigation on the real-time mode decomposition technique for few-mode laser beam based on stochastic parallel gradient descent algorithm. It will reduce the cost and the complexity of the mode decomposition system. We have succeeded to decompose the mode spectra as well as calculating the beam quality factor at about 9 Hz monitoring rate, while the high agreement between the measured and reconstructed intensity profiles in each frame indicating the high accuracy and stability during the process. By employing a fiber-squeezing-based polarization controller, the modal content under test can be time-varying automatically.

Analysis and simulation of the phenomenon of secondary spots of the TDI CCD camera irradiated by CW laser

The phenomenon of secondary spots is observed in the experiment of TDI CCD camera irradiated by CW He-Ne laser. It is considered to be related to the scattering of the slit in front of the sensor and the reflection of the window of the TDI CCD chip. Additional experiments and ray tracing simulation are performed to study the mechanism of the secondary spots. The experimental and simulated results demonstrated that the scattering of the side walls of the slit is the main source of the secondary spots. Furthermore, the operation mode of rotary scanning provides the chance of scattering incident beam to the side wall of the slit. This paper will provide a preliminary hint to the optimum design of slit in camera to reduce the effects of the secondary spots to the image quality.

Impact of high-order-mode loss on high-power fiber amplifiers

In this paper, the impact of the loss of high-order mode (HOM) on the performances of high-power fiber amplifiers (HPFA) has been numerically investigated by using the model considering transverse spatial-hole burning. The HPFA is modeled with various parameters such as pump wavelength, pumping direction, and fiber length. The fact that the output power of HOM will decrease with increasing loss of HOM is explained by the numerical analysis for the first time to the best of our knowledge. Moreover, the results show that the extracted efficiency of the amplifier will decrease dramatically with increasing HOM loss when the loss is low. The efficiency will increase smoothly and finally converge, as the growth of the HOM loss in the regime where the efficiency crosses the minimum. The evolution of the output power of HOM and the efficiency can be attributed to the interaction between the loss and the nonuniformly distributed local gain, which will be clearly illustrated by simulations. We believe the results will be beneficial to understanding the impact of the loss of HOM on the amplifier performances and providing guidelines on designing large-mode-area fiber and optimizing the HPFA aiming at effective single-mode output based on differential loss between the HOMs and the fundamental mode.

Adaptive mode control of a few-mode fiber by real-time mode decomposition

A novel approach to adaptively control the beam profile in a few-mode fiber is experimentally demonstrated. We stress the fiber through an electric-controlled polarization controller, whose driven voltage depends on the current and target modal content difference obtained with the real-time mode decomposition. We have achieved selective excitations of LP01 and LP11 modes, as well as significant improvement of the beam quality factor, which may play crucial roles for high-power fiber lasers, fiber based telecommunication systems and other fundamental researches and applications.

All-fiber thulium/holmium-doped mode-locked laser by tungsten disulfide saturable absorber

A passively mode-locked thulium/holmium-doped fiber laser (THDFL) based on tungsten disulfide (WS2) saturable absorber (SA) was demonstrated. The WS2 nanosheets were prepared by liquid phase exfoliation method and the SA was fabricated by depositing the few-layer WS2 nanosheets on the surface of a fiber taper. The modulation depth, saturable intensity, and non-saturable loss of this SA were measured to be 8.2%, 0.82 GW cm−2, and 29.4%, respectively. Based on this SA, a stable mode-locked laser operated at 1.91 µm was achieved with pulse duration of 825 fs and repetition rate of 15.49 MHz, and signal-to-noise ratio (SNR) of 67 dB. Meanwhile, by increasing the pump power and adjusting the position of polarization controller, harmonic mode-locking operations were obtained. These results showed that the WS2 nanosheet-based SA could be served as a desirable candidate for a short-pulse mode locker at 2 µm wavelength.

Modal Analysis of Fiber Laser Beam by Using Stochastic Parallel Gradient Descent Algorithm

The ability of modal decomposition based on the analysis of the near-field intensity profile using the stochastic parallel gradient descent algorithm is experimentally characterized in detail for the first time. The results show that the decomposition method proposed in this letter turns to be highly accurate and rapid. Further, this method is applied to analyze the mode behavior in dependence on the pump power in a high-power fiber amplifier with different initial mode compositions.

Thickness-dependent carrier and phonon dynamics of topological insulator Bi 2 Te 3 thin films

As a new quantum state of matter, topological insulators offer a new platform for exploring new physics, giving rise to fascinating new phenomena and new devices. Lots of novel physical properties of topological insulators have been studied extensively and are attributed to the unique electron-phonon interactions at the surface. Although electron behavior in topological insulators has been studied in detail, electron-phonon interactions at the surface of topological insulators are less understood. In this work, using optical pump-optical probe technology, we performed transient absorbance measurement on Bi2Te3 thin films to study the dynamics of its hot carrier relaxation process and coherent phonon behavior. The excitation and dynamics of phonon modes are observed with a response dependent on the thickness of the samples. The thickness-dependent characteristic time, amplitude and frequency of the damped oscillating signals are acquired by fitting the signal profiles. The results clearly indicate that the electron-hole recombination process gradually become dominant with the increasing thickness which is consistent with our theoretical calculation. In addition, a frequency modulation phenomenon on the high-frequency oscillation signals induced by coherent optical phonons is observed.

Mode instability dynamics in high-power low-numerical-aperture step-index fiber amplifier

The study on mode instability (MI) in the large-mode-area fiber is generating great interest regarding the high-power applications of fiber lasers. To the best of our knowledge, we have investigated for the first time the dynamics of the output beam from a kilowatt-level all-fiber amplifier based on the low-numerical-aperture (<0.04) step-index (SI) fiber before and after the onset of the MI, including the temporal dynamics and mode evolution. The temporal power fluctuations indicate three evolution stages apart from the onset threshold of the MI, defined as stable, transition, and chaotic regions. In addition, the mode decomposition technique is utilized to accurately observe and investigate the mode evolution and relevant modal content corresponding to the transition and chaotic regions in the SI fiber laser for the first time. According to the mode decomposition results, the reduction of the extracted power can be explained by the high bending loss of the high-order mode excited in the MI process. Finally, the difference of MI dynamics between the fiber lasers based on the SI fiber and rod-type photonic crystal fiber is discussed.

736 W Average Power All-fiber Nanosecond MOPA Based on Ultra-low NA Ytterbium Doped Fiber

We report an all-fiberized high average power pulsed fiber amplifier based on ultra-low NA step-index Yb-doped double-clad fiber. An output power of 736W with M2~1.35 is obtained, which corresponds to 64kW peak-power and 0.39mJ pulse-energy.