High beam quality continuous-wave 320 nm UV laser from a walk-off compensated intracavity frequency-doubled Pr:YLF laser.

We report a high beam quality continuous-wave (CW) 320 nm ultra-violet (UV) laser. An end-pumped praseodymium-doped yttrium lithium fluoride (Pr:YLF) laser is constructed in a typical V-shaped cavity structure, while the UV output is obtained through intracavity frequency doubling using L i B 3 O 5 (LBO). We investigate the dependence of the UV output power, as well as the spot profile on the LBO length, and find that the "walk-off" within LBO severely affects both the frequency-doubling efficiency and the beam quality of the UV output. Rotated twin LBO crystals are then applied to substitute for single long LBO crystal to compensate the walk-off effect, resulting in high-power 320 nm output up to 410 mW under the absorbed pump power of 4.0 W at 444 nm, with a high beam quality of M x2=1.02 and M y2=1.04.

Ultra-short pulse burst laser around 1.98†µm obtained through an all-fiber nonlinear wavelength converter and Tm-doped fiber amplifier.

We report an all-fiber ultra-short pulse burst laser operating at around 1.98 µm that is obtained through a nonlinear wavelength converter and Tm-doped fiber amplifier. A mode-locked Er-doped fiber laser was first built and then amplified in subsequent amplifiers to an average power of 1.3 W. Ultra-short pulse burst output was achieved through a pulse multiplier and a fiber-pigtailed acousto-optic modulator. It was then injected into an all-fiber nonlinear wavelength converter constructed from P-doped fiber and Tm-doped fiber, obtaining an ultra-short pulse burst laser of 540 mW around 1.98 µm. Its average output power was then amplified to 4.33 W in a Tm-doped fiber amplifier with an intra-burst pulse repetition frequency of 0.9 GHz, a burst repetition frequency of 200 kHz, and a duty cycle of 2%, corresponding to about 200 pulses within each burst. This 1.98 µm pulse burst laser has enormous potential to be applied in bio-medical areas.

Coded-pulse-bunch-laser-based single-photon lidar for fast long-distance ranging.

Laser ranging based on a single-photon avalanche diode (SPAD), offering single-photon level high sensitivity, has been widely adopted in light detection and ranging (lidar) systems for long-distance ranging and imaging applications. Count detection through multiple pulses is commonly used when considering the existence of dark counting and strong background counting during the daytime, which improves the signal-to-noise ratio but at the expense of low detection speed. Here, we report a novel coded-pulse-bunch-laser-based single-photon lidar system, which aims to improve the ranging speed greatly and to expand the unambiguous distance to several kilometers. The schematic principle and construction of the lidar system, as well as the encoding method, are introduced. The time-of-flight (TOF) ranging information is extracted through real-time correlation between the transmitted pulse-bunch patterns and the received echo signals in a field-programmable gate array (FPGA). A daytime ranging experiment is demonstrated on a non-cooperative mountain target that is 5.4 km away. The method will be of great potential in fast three-dimension (3D) single-photon lidar imaging application for its relatively high data refreshing rate and large unambiguous distance.

Acousto-optic frequency shifted comb laser-based micro-Doppler detection for moving target identification.

We present a preliminary experimental demonstration of an acousto-optic frequency shifted (AOFS) comb laser-based micro-Doppler detection system for moving object identification. The AOFS comb laser was constructed by successively frequency shifting a single-frequency seed laser at 1063.8 nm using an acousto-optic modulator in an amplified fiber loop, which resulted in a stable pulse output with a pulse repetition rate around 150 MHz and pulse duration of about 200 ps. The AOFS comb pulse was amplified and then directed through a fiber circulator first and then an optical lens onto a moving object, which featured both linear translation and rotation. The micro-Doppler signal of the rotation was derived from the heterodyne detection of the pulse echo comb laser and the continuous-wave single-frequency local laser. It is believed that such an AOFS comb laser-based sensing system is of great potential for micro-Doppler detection of high-speed targets owing to its long-term stability and compactness.

High energy mid-infrared laser pulse output from a BaGa4Se7 crystal-based optical parametric oscillator.

We report a high energy, narrow spectral linewidth mid-infrared laser pulse output from a Nd:Y3Al5O12 laser-pumped BaGa4Se7 (BGSe) crystal-based optical parametric oscillator (OPO). Output pulse energy of 21.5 mJ was obtained at 3816 nm, with spectral width of 12 nm and pulse width of 11.4 ns, corresponding to peak power as high as 1.89 MW. A BGSe crystal with aperture size of 10mm×10mm and length of 16 mm was applied as the nonlinear crystal in a pump fed back OPO for achieving such high pulse energy output from the simple oscillator scheme.

Improved algorithm of non-line-of-sight imaging based on the Bayesian statistics.

The recovery of obscured objects is an important goal in imaging that has been approached by exploiting coherence properties, ballistic photons, and penetrating wavelengths. In this paper, a robust reconstruction non-line-of-sight (NLOS) algorithm was proposed based on the Bayesian statistics, using the temporal, spatial, and intensity information of each signal. Compared with conventional back-projection methods, this algorithm is able to handle random errors in data and to image occluded objects with a higher quality. An adjustable compensation mechanism in our method is effective in dealing with the diversity of objects with regard to reflective characteristics. Our algorithm was demonstrated to be efficient with both simulated and experimental data. In addition, we discuss the advantages over existing methods and further improvement of the proposed algorithm.

Optofluidic tunable mode-locked fiber laser using a long-period grating integrated microfluidic chip.

An optofluidic tunable mode-locked fiber laser using a microfluidic chip integrated with long-period grating (LPG) is presented. The microfluidic chip enables ultrafine adjustment of the liquid's refractive index and, thus, LPG's spectrum via tuning the mixing ratio of the microfluidic flows. With such an optofluidic spectrum-tunable filter, the central wavelength of the mode-locked laser can be tuned continuously, while the mode-locking state is steadily maintained. The mode-locked pulses are measured with a pulse duration of 0.9 ps and repetition rate of 12.14 MHz, respectively. Moreover, bound solitons with variable soliton separations are experimentally demonstrated.

Watt-level mid-infrared radiation via self-seeded difference-frequency generation from a pre-chirp managed femtosecond Yb-fiber amplifier.

We obtained over 1 W average power at ∼3550 nm wavelength via self-seeded difference-frequency generation (DFG) through a 5 cm long periodically poled MgO-doped lithium niobate crystal. The pump and signal sources are derived from the identical pre-chirp managed femtosecond Yb-fiber amplifier with sub-100-fs pulse duration and 84 MHz repetition rate for simple synchronization. This result is believed to be among the highest-average-power, femtosecond mid-infrared radiation obtained via DFG.

Tunable scalar solitons from a polarization-maintaining mode-locked fiber laser using carbon nanotube and chirped fiber Bragg grating.

Generation of tunable scalar solitons from a polarization-maintaining (PM) mode-locked fiber laser is presented. A single-walled carbon nanotube (SWCNT) absorber is used for self-started mode locking. A chirped fiber Bragg grating (CFBG) mounted on a cantilever is employed as a tunable all-fiber filter. Mode-locked solitons are obtained with typical pulse duration of ~6.94 ps and repetition rate of 28.94 MHz. Linearly polarized laser emission is characterized with degree of polarization (DOP) of ~99.5%. The wavelength of the emitted scalar soliton can be continuously tuned through adjusting the CFBG, while maintaining the polarization stability.

Compact high power mid-infrared optical parametric oscillator pumped by a gain-switched fiber laser with "figure-of-h" pulse shape.

We demonstrate a compact high power mid-infrared (MIR) optical parametric oscillator (OPO) pumped by a gain-switched linearly polarized, pulsed fiber laser. The gain-switched fiber laser was constructed with a piece of Yb doped polarization maintaining (PM) fiber, a pair of fiber Bragg gratings written into the matched passive PM fiber and 6 pigtailed pump laser diodes working at 915 nm with 30 W output peak power each. By modulating the pulse width of the pump laser diode, simple pedestal-free pulse shape or pedestal-free trailing pulse shape ("figure-of-h" as we call it) could be achieved from the gain-switched fiber laser. The laser was employed as the pump of a two-channel, periodically poled magnesium oxide lithium niobate-based OPO system. High power MIR emission was generated with average output power of 5.15 W at 3.8 µm channel and 8.54 W at 3.3 µm channel under the highest pump power of 45 W. The corresponding pump-to-idler conversion efficiency was computed to be 11.7% and 19.1%, respectively. Experimental results verify a significant improvement to signal-to-idler conversion efficiency by using "figure-of-h" pulses over simple pedestal-free pulses. Compared to the master oscillator power amplifier (MOPA) fiber laser counterpart, the presented gain switched fiber laser is more attractive in OPO pumping due to its compactness and simplicity which are beneficial to construction of OPO systems for practical MIR applications.

Broadband high-power mid-IR femtosecond pulse generation from an ytterbium-doped fiber laser pumped optical parametric amplifier.

We report on a high-power periodically poled MgO-doped lithium niobate (MgO:PPLN)-based femtosecond optical parametric amplifier (OPA), featuring a spectral seamless broadband mid-infrared (MIR) output. By modifying the initial chirp and spectrum of the mode-locked seed laser, the Yb fiber pump laser exhibits a final output power of 14 W with sub-200-fs pulse duration after power amplification and compression. When the OPA was seeded with a broadband amplified spontaneous emission (ASE) source, a damage-limited 0.6 W broadband MIR radiation was experimentally obtained under the pump power of 10.15 W at 82 MHz repetition rate, corresponding to an overall OPA conversion efficiency of 32.7%. The 3 dB bandwidth of the mid-IR idler was 291.9 nm, centering at 3.34 µm.

Temperature insensitive, high-power cascaded optical parametric oscillator based on an aperiodically poled lithium niobate crystal.

We report a novel temperature insensitive, APMgLN-based, high-power cascaded optical parametric oscillator (OPO) pumped by an Ytterbium-doped fiber laser. A monolithic APMgLN crystal was designed to compensate the phase mismatches for the nonlinear conversions from the pump to the idler and the primary signal to the idler simultaneously in a wide temperature range. Efficient parametric conversion with pump-to-idler conversion efficiency over 15% and slope efficiency higher than 20% was realized from 25 °C to 55 °C. The idler wavelength was down-shifted from 3.82 µm to 3.78 µm accordingly during the temperature rise. The highest idler power of 4.1 W at 3.8 µm under the pump power of 26.5 W was recorded which was improved by ~32% in pump-to-idler conversion efficiency when compared with the PPMgLN-based conventional OPO, in which the highest idler output power was 3.1W under the same pump and thermal condition.

Fiber laser pumped high power mid-infrared laser with picosecond pulse bunch output.

We report a novel quasi-synchronously pumped PPMgLN-based high power mid-infrared (MIR) laser with picosecond pulse bunch output. The pump laser is a linearly polarized MOPA structured all fiberized Yb fiber laser with picosecond pulse bunch output. The output from a mode-locked seed fiber laser was directed to pass through a FBG reflector via a circulator to narrow the pulse duration from 800 ps to less than 50 ps and the spectral FWHM from 9 nm to 0.15 nm. The narrowed pulses were further directed to pass through a novel pulse multiplier through which each pulse was made to become a pulse bunch composing of 13 sub-pulses with pulse to pulse time interval of 1.26 ns. The pulses were then amplified via two stage Yb fiber amplifiers to obtain a linearly polarized high average power output up to 85 W, which were then directed to pass through an isolator and to pump a PPMgLN-based optical parametric oscillator via quasi-synchronization pump scheme for ps pulse bunch MIR output. High MIR output with average power up to 4 W was obtained at 3.45 micron showing the feasibility of such pump scheme for ps pulse bunch MIR output.

High-power PPMgLN-based optical parametric oscillator pumped by a linearly polarized, semi-fiber-coupled acousto-optic Q-switched fiber master oscillator power amplifier.

We have experimentally demonstrated a periodically poled magnesium-oxide-doped lithium niobate (PPMgLN)-based, fiber-laser-pumped optical parametric oscillator (OPO) generating idler wavelength of 3.82 µm. The pump fiber laser was constructed with a linearly polarized, semi-fiber-coupled acousto-optic Q-switched fiber oscillator and a polarization-maintaining fiber amplifier with pulse duration of 190 ns at the highest output power. The OPO was specifically configured in single-pass, singly resonant linear cavity structure to avoid the damage risk of the pump fiber laser, which is always a serious issue in the fiber-laser-pumped, double-pass, singly oscillating structured OPOs. Under the highest pump power of 25 W, an idler average output power of 3.27 W with one-hour peak-to-peak instability of 5.2% was obtained. The measured M2 factors were 1.98 and 1.44 for horizontal and vertical axis, respectively. The high power stability and good beam quality demonstrated the suitability of such technology for practical application.

Large aperture PPMgLN based high-power optical parametric oscillator at 3.8 µm pumped by a nanosecond linearly polarized fiber MOPA.

We report a large aperture PPMgLN based OPO generating 21W of average output power at a slope efficiency of 45%. The OPO is pumped with the output from a polarization maintaining Ytterbium doped fiber MOPA operating at 1060nm producing 20ns pulses at a repetition rate of 100kHz and an average output power of 58W (after the isolators). A maximum of 5.5W of optical power was recorded at the idler wavelength of 3.82µm without thermal roll-off. The pulse rise/fall time plays a significant role in the OPO conversion efficiency and that further enhancements in the efficiency should be possible using pulses with faster rise and fall times.

High-power multichannel PPMgLN-based optical parametric oscillator pumped by a master oscillation power amplification-structured Q-switched fiber laser.

We experimentally demonstrated a compact fiber laser-pumped multichannel PPMgLN-based optical parametric oscillator (OPO) generating total OPO output power of 15.8, 15.2, 14.2, 12.9, and 8.8 W with idler output power of 4.7, 4.3, 4.1, 3.3, and 2.1 W at the wavelength of 3.43, 3.63, 3.72, 3.83, and 3.99 µm, respectively. The OPO was pumped by a fully fiberized polarization maintaining (PM) ytterbium-doped pulsed fiber master oscillation power amplifier (MOPA) operating at 1064 nm at a repetition rate of 65 kHz with effective pump power of 28.7 W. The MOPA system was constructed with an acousto-optic Q-switched fiber laser seed and only one stage PM fiber amplifier without any free space components, which makes the pump system compact and stable in the long-term. Comparisons on efficiencies and signal wavelength shifts between different channels showed that the idler absorption was the main factor preventing high average-power OPO operation with long idler wavelength.

Efficient parametric conversion from 1.06 to 3.8 µm by an aperiodically poled cascaded lithium niobate.

We experimentally demonstrated an efficient optical parametric oscillator (OPO) with high parametric conversion from 1.0645 to 3.8 µm. An aperiodically poled magnesium oxide doped lithium niobate wafer was designed and fabricated as the nonlinear crystal of the OPO. A linearly polarized acousto-optic Q-switched Nd:YVO4 laser was used as the pump source. High pump-to-idler conversion efficiency of 18.5% was achieved with a slope efficiency of up to 21.5%. When compared with a periodically poled channel fabricated on the same wafer, under the condition of output coupler optimized for the periodically poled lithium niobate based OPO, an improvement of slope efficiency by 28.3% from 15.2% to 19.5% and total efficiency by 12.5% from 13.6% to 15.3% under the highest pump power of 11 W was realized for the pump-to-idler conversion.

High-efficiency semi-external-cavity-structured periodically poled MgLN-based optical parametric oscillator with output power exceeding 9.2 W at 3.82 microm.

We report a high-efficiency and high-power mid-IR laser generation by a periodically poled (PP) MgLN-based optical parametric oscillator (OPO) working at room temperature, from which 9.23 W of output was obtained at 3.82 microm when pumped by a linearly polarized acousto-optic Q-switched Nd:YVO(4) laser of 48.2 W output. A semi-external-cavity structure was applied in the OPO system to avoid the conversion saturation caused by the thermal lens effect of the PPMgLN crystal and thus to maintain the high conversion efficiency and the high beam quality under the high power level. It was computed that 29.4% slope efficiency and 19.2% total efficiency were realized for the 1.063-3.82 microm conversion.

Compact dual-wavelength Nd:GdVO4 laser working at 1063 and 1065 nm.

We report a compact diode-laser pumped Nd:GdVO(4) laser with stable dual-wavelength output at 1063 nm and 1065 nm simultaneously. Two types of resonant cavity configurations were presented to support the stable dual-wavelength operation of the laser. Using a polarization beam splitter(PBS) included T-shaped cavity, we obtained a total power output over 5 W in two orthogonal polarized beam directions with 4 W in sigma polarization (1065.5 nm) and 1 W in pi polarization (1063.1 nm). By combining a half-wave-plate with the PBS in the laser cavity, a new configuration favoring one beam direction dual-wavelength output with same polarization direction was realized. A phenomenon of further line splitting was observed in both 1065 nm and 1063 nm.

Short cavity single frequency fiber laser for in-situ sensing applications over a wide temperature range.

A novel Er-doped silica fiber, with heavy Er doping, was specially developed for application to a single frequency fiber laser. Two high temperature-sustainable fiber Bragg gratings, written into Bi-Ge codoped photosensitive fiber, were chosen for the application and spliced to the specialist Er doped silica fiber to form a compact, linear cavity, fiber laser. The fiber laser retained single mode oscillation over a wide temperature range, from room temperature to 400 degrees C. The wavelength of the laser output could be tuned smoothly, without mode hopping being observed, when the temperature was changed. A narrow linewidth of less than 1 kHz was measured at the output of fiber laser and this indicates the potential of the fibre laser sensing system with extremely high sensitivity and resolution over this wide range.