Weibiao Chen

High-sensitivity intravascular photoacoustic imaging of lipid–laden plaque with a collinear catheter design

A highly sensitive catheter probe is critical to catheter-based intravascular photoacoustic imaging. Here, we present a photoacoustic catheter probe design on the basis of collinear alignment of the incident optical wave and the photoacoustically generated sound wave within a miniature catheter housing for the first time. Such collinear catheter design with an outer diameter of 1.6 mm provided highly efficient overlap between optical and acoustic waves over an imaging depth of >6 mm in D2O medium. Intravascular photoacoustic imaging of lipid-laden atherosclerotic plaque and perivascular fat was demonstrated, where a lab-built 500 Hz optical parametric oscillator outputting nanosecond optical pulses at a wavelength of 1.7 μm was used for overtone excitation of C-H bonds. In addition to intravascular imaging, the presented catheter design will benefit other photoacoustic applications such as needle-based intramuscular imaging.

Experimental demonstration of phase locking of a two-dimensional fiber laser array using a self-imaging resonator

Phase locking of a two-dimensional fiber laser array is experimentally demonstrated by using a self-imaging resonator and a spatial filter. The stable beam profiles of in-phase mode and out-of-phase mode are observed by controlling the position of spatial filter. The phase locking fiber array with in-phase mode has produced 26 W coherent output. An antisymmetric eigenmode is also observed in our experiments. The phase locking is not sensitive to power variations among the pump beams and the configuration has the ability to repair a missing element

Efficient all-solid-state mid-infrared optical parametric oscillator based on resonantly pumped 1.645 μm Er:YAG laser

We first report an all-solid-state tunable mid-infrared singly resonant optical parametric oscillator based on a 1532 nm laser diode resonantly pumped, Q-switched 1.645 μm Er:YAG laser. An MgO-doped periodically poled lithium niobate was used as the nonlinear material. At the pulse repetition frequency of 2 KHz, a maximum overall average output power of 0.95 W with pump power of 2.8 W was achieved, corresponding to a conversion efficiency of 34% and a slope efficiency of 38%. The temperature tuning was performed giving signal and idler ranges of 2.67 to 2.71 μm and 4.18 to 4.31 μm, respectively.

Efficient 1645 nm continuous-wave and Q‑switched Er:YAG laser pumped by 1532 nm narrow-band laser diode

Energy transfer upconversion induced thermal effects are mainly responsible for the low efficiency of laser diode pumped Er:YAG lasers. The current work adopts Er:YAG rods with 0.25% Er(3+) doping concentration, instead of the commonly used rods with 0.5% Er(3+) doping concentration. Results show that the thermal effect is greatly alleviated. A continuous-wave output of 10.2 W is obtained using 31 W incident pump power. Optical-optical efficiency is approximately 33%. Slope efficiency, with respect to the absorbed pump power, is as high as 83%, which is close to the quantum efficiency. In a Q-switched operation, 7 mJ pulses with a pulse width of ~65 ns are obtained at 100 Hz PRF.

Electric field-induced magnetic switching in Mn:ZnO film

A large magnetic modulation, accompanied by stable bipolar resistive switching (RS) behavior, was observed in a Mn:ZnO film by applying a reversible electric field. A significant enhancement of the ferromagnetism of the film, to about five times larger than that in the initial (as-grown) state (IS), was obtained by switching the film into the low resistance state. X-ray photoelectron spectroscopy demonstrated the existence of abundant oxygen vacancies in the IS of the film. We suggest that this electric field-induced magnetic switching effect originates with the migration and redistribution of oxygen vacancies during RS. Our work indicates that electric switching is an effective and simple method to increase the ferromagnetism of diluted magnetic oxide films. This provides a promising direction for research in spintronic devices.

Eye-safe, single-frequency pulsed all-fiber laser for Doppler wind lidar

A single-frequency pulsed erbium-doped fiber (EDF) laser with master-oscillator power-amplifier configuration at 1 533 nm is developed. A short-cavity, erbium-doped phosphate glass fiber laser is utilized as a seeder laser with a linewidth of 5 kHz and power of 40 mW. The seeder laser is modulated to be a pulse laser with a repetition rate of 10 kHz and pulse duration of 500 ns. The amplifier consists of two pre-amplifiers and one main amplifier. The detailed characteristics of the spectrum and linewidth of the amplifiers are presented. A pulse energy of 116 1J and a linewidth of 1.1 MHz are obtained. This laser can be a candidate transmitter for an all-fiber Doppler wind lidar in the boundary layer.

High-energy directly diode-pumped Q-switched 1617 nm Er:YAG laser at room temperature

We describe high-energy Erbium-doped yttrium aluminum garnet (Er:YAG) lasers operating at 1617 nm, resonantly pumped using 1532 nm fiber-coupled laser diodes. A maximum continuous wave output power of 4.3 W at 1617 nm was achieved with an output coupler of 20% transmission under incident pump power of 29.7 W, resulting in an optical conversion of 14% with respect to the incident pump power. In Q-switched operation, the pulse energy of 11.8 mJ at 100 Hz pulse repetition frequency and 81 ns pulse duration was obtained. This energy is the highest pulse energy reported for a directly diode-pumped Q-switched Er:YAG laser operating at 1617 nm.

Experimental study on the 1550 nm all fiber heterodyne laser range finder

In this paper, a 1550 nm all fiber monostatic laser range finder system based on linear chirp modulation and heterodyne detection is presented. The fiber end face signal is used as a range starting indicator. The transmitted laser power is 5 mW with a laser pulse length of 131 micros and a linear chirp bandwidth of 40 MHz. The telescope with an aperture of 3 cm couples the return light into a single mode fiber. Better than 14 cm distance accuracy and 26 dB SNR can be achieved for a wood target at a distance of about 43 m by using the above system setup. Several experiments with different system parameters are conducted. The system performance is tested under variable laser pulse length, linear chirp bandwidth, local oscillator power, and background noise. Finally, an application of the linear chirp modulation heterodyne laser range finder in a spaceborne ranging system is proposed.

Tm:YLF laser-pumped periodically poled MgO-doped congruent LiNbO 3 crystal optical parametric oscillators

At room temperature, highly efficient, doubly resonant periodically poled MgO-doped congruent LiNbO3 crystal optical parametric oscillators (OPO) pumped at 1.9 μm have been realized for the first time to our knowledge. A 2.9 W, 2 kHz acousto-optic Q-switched homemade Tm:YLF laser served as a pump source. A 900 mW mid-infrared emission generated from an OPO is illustrated. The overall mid-infrared generation conversion efficiency reached 31%, whereas the slope efficiency was up to 70%.

Diode-Side-Pumped AO Q-Switched Tm, Ho:LuLiF Laser

Diode side pumped and acousto-optical (AO) Q-switched Tm, Ho, LuLiF laser at 2 μm wavelength obtained in a ring resonator is reported in this paper. The three LDAs were placed 120° apart around the laser rod of ∅4 × 20 mm dimension. A ring resonator has been used for its At free run mode, four different output loss ouplers has been tried. When the output loss is 20%, 103 mJ per pulse at 1 Hz pulse repetition frequency has been obtained with the optical to optical energy conversion efficiency of 3.17%. At the Q-switched mode, the laser pulse energy is 30 mJ with the pulse width of 417 ns. The optical to optical energy conversion efficiency is 0.93%. The slope efficiency is 6.36 and 2.90% under the two running mode, respectively.