Laser Doppler velocimetry (LDV) technology is highly favored in the field of speed measurement for its high accuracy and resolution. However, it has limitations in specific applications due to its ...single-point speed measurement capability. To address this issue, a method of axial multi-point laser Doppler velocimetry is proposed to expand the application range of this technology. This method is based on the dual-beam laser Doppler velocimetry mode, utilizing the diffraction characteristics of the reflected grating to achieve spatial separation of measurement points axially. To verify the feasibility of the method, a laser Doppler velocimetry system was constructed, enabling velocity measurements at three axial points within a range of 0–14 m/s, with an average measurement error of less than 4%. Measurements of water flow velocity in a water tank pipeline were conducted, and compared with a standard electronic flowmeter, resulting in an average relative error of approximately 0.77%.
A multi-gas quartz enhanced photoacoustic spectroscopy (QEPAS) sensor based on three quartz tuning forks (QTFs) with different response frequencies for trace gas detection was proposed and ...experimentally demonstrated. Three near-infrared DFB lasers are used to monitor water vapor, methane, and acetylene in the parts per million range. The sensor system was first evaluated for individual H 2 O, CH 4 , and C 2 H 2 detection, respectively. Subsequently, the sensor system was evaluated for simultaneous H 2 O, CH 4 , and C 2 H 2 detection. Finally, trace gas measurements have been assessed and minimum detection limit (MDL) of 1.3 ppmv at 1368.597 nm for H 2 O, 79 ppmv at 1653.722 nm for CH 4 , and 5 ppmv at 1532.83 nm for C 2 H 2 have been demonstrated. The continuous monitoring of H 2 O, CH 4 , and C 2 H 2 concentration levels for >3 h indicated the stability of the reported multi-gas QEPAS sensor system.
A technique for elimination of residual amplitude modulation (ERAM) in photoacoustic spectroscopy based on dual path lock-in was proposed and experimentally demonstrated. There are two lock-in ...amplifiers, one is for gas concentration demodulation and another for residual amplitude modulation (RAM) measurement by tuning the reference signal in different phases, and then a dual path lock-in technique based on subtraction is applied to RAM removal, improving the second harmonic profile significantly. In this system, the signal to noise ratio (SNR) increases about two times based on our dual path lock-in technique compared to one distributed feedback laser diode (DFB-LD). The system achieved a good linear response (R-square = 0.99887) in a concentration range from 100 ppmv to 2400 ppmv and a minimum detection limit (MDL) of 1.47 ppmv.
A tunable ring cavity terahertz parametric oscillator with fixed output direction is reported in this paper. The diamond-shaped Stokes beam resonator was designed for the adjustment of the Stokes ...beam propagation direction while the propagation direction of pump beam was adjusted by two reflection mirrors. The terahertz wave output direction can be always normal to the output surface by controlling the propagation directions of the pump and Stokes beams cooperatively during the frequency tuning in the range of 1.4–2.4 THz.
We report on a passively mode-locked Nd:YAG transparent ceramic laser with semiconductor saturable absorber mirror at an eye-safe wavelength of 1.44 μm. With the increase of incident pump power, the ...laser went through three oscillation stages: Q-switched mode locking (QML), continuous wave mode locking (CWML), QML and remained at 1443 nm single wavelength output. The maximum average output power of the CWML laser was about 842 mW. The pulse width was measured to be 20.8 ps assuming a Gauss shape.
In this letter, tin diselenide (SnSe 2 ) was successfully prepared with the liquid-phase exfoliation method and embedded into polyvinyl alcohol as a saturable absorber (SA) in a dual-wavelength ...passively mode-locked Yb-doped fiber laser. Stable mode-locked pulses with a maximum average output power of 2.55 mW and a slope efficiency of 1.3% at the fundamental repetition rate of 333 kHz were achieved. By properly adjusting polarization state, four kinds of dual-wavelength fundamental frequency mode-locked pulses with peak wavelength intervals of 0.4, 1.0, 1.5, and 1.9 nm were obtained, and the corresponding waveforms with different single-pulse shapes were investigated in our work. A maximum signal-to-noise ratio of 71.04 dB indicates that SnSe 2 is promising to serve as a SA in passively mode-locked fiber lasers.
A compact high-power 2.8-µm laser with a 2 at.%-doped Er3+:CaF2 crystal was demonstrated, which delivered a maximum output power of 2.848 W with a slope efficiency of 26.3 %. To the best of our ...knowledge, this is the highest output power among all the reported LD end-pumped Er3+-doped fluoride crystal lasers. Experimentally, the negative thermal lens effect was characterized and the thermal lens optical diopters for two axes were obtained as Mx = -0.21 m−1/W and My = -0.33 m−1/W. In addition, the visible fluorescence spectra of Er3+:CaF2 crystal with and without laser operation were measured and analyzed. With Fe2+:ZnSe as saturable absorber, a passively Q-switched 2.759-µm Er3+:CaF2 laser with an average output power of 75 mW, a peak power of 2.39 W, a single pulse energy of 1.81 µJ, and a pulse width of 755.3 ns was obtained at a repetition rate of 41.55 kHz.
Terahertz-wave parametric oscillators (TPOs) have advantages of room temperature operation, wide tunable range, narrow line-width, good coherence. They have also disadvantage of small pulse energy. ...In this paper, several factors preventing TPOs from generating high-energy THz pulses and the corresponding solutions are analyzed. A scheme to generate high-energy THz pulses by using the combination of a TPO and a Stokes-pulse-injected terahertz-wave parametric generator (spi-TPG) is proposed and demonstrated. A TPO is used as a source to generate a seed pulse for the surface-emitted spi-TPG. The time delay between the pump and Stokes pulses is adjusted to guarantee they have good temporal overlap. The pump pulses have a large pulse energy and a large beam size. The Stokes beam is enlarged to make its size be larger than the pump beam size to have a large effective interaction volume. The experimental results show that the generated THz pulse energy from the spi-TPG is 1.8 times as large as that obtained from the TPO for the same pumping pulse energy density of 0.90 J/cm(2) and the same pumping beam size of 3.0 mm. When the pumping beam sizes are 5.0 and 7.0 mm, the enhancement times are 3.7 and 7.5, respectively. The spi-TPG here is similar to a difference frequency generator; it can also be used as a Stokes pulse amplifier.
As a bridge between crystal and fiber, the single crystal fiber (SCF) can combine the advantages of the two materials and show great potential in high-power lasers. In this paper, we select a Nd:YAG ...SCF as the gain medium in a diode-end-pumped linear cavity. The Nd:YAG SCF with the dimensions of ∅ = 1 mm × 50 mm was manufactured using the micro-pulling-down technique. At an incident pump power of 153.0 W, the maximum output power of the 1064 nm laser was 72.3 W. To the best of our knowledge, this is the highest power obtained in Nd:YAG SCF lasers. In addition, using a second-type phase-matched KTP crystal, the 10 W continuous wave green laser was obtained with an intracavity double-frequency structure.
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