Yoshihito Hirano

Compact all-fiber pulsed coherent Doppler lidar system for wind sensing

A compact 1.5 microm all-fiber pulsed coherent Doppler lidar system for wind sensing, which includes the functions of variable pulse width and automatic polarization control has been developed. The system configuration is introduced and key components used in the system are explained. Theoretical performances of the system in wind sensing are estimated and compared with experimental results. The measurable range corresponding to the detection probability of >80% is approximately 1 km or more in the case of 150 m range resolution under the normal atmospheric conditions.

208-W TEM00 operation of a diode-pumped Nd:YAG rod laser.

208-W average-power TEM(00) -mode operation from a diode-pumped Nd:YAG rod laser was demonstrated. The side-pumping method of generating a high-gain aberration-free rod, the bifocusing-compensation technique with two identically pumped rods, and a suitable choice of beam spot size were employed in the design of this laser. At the maximum pump power of 1.1 kW the fundamental transverse-mode operation (M(2)<1.1) was characterized by 7.6% electrical efficiency. The extraction efficiency was almost 60% of full multimode operation. Stable operation was obtained within a pump-power range of 15% of the maximum pump power.

Development of 1.6 μm continuous-wave modulation hard-target differential absorption lidar system for CO 2 sensing

We have demonstrated the 1.6 mum cw modulation hard-target differential absorption lidar system for CO(2) sensing. In this system, ON and OFF wavelength laser lights are intensity modulated with cw signals. Received lights of the two wavelengths from the hard target are discriminated by modulation frequencies in the electrical signal domain. The optical circuit is fiber based, and this makes the system compact and reliable. It is shown that a stable CO(2) concentration measurement corresponding to a fluctuation of 4 ppm (rms) (ppm is parts per million) has been achieved in 32 s measurement intervals and the 1 km path.

Feasibility study on 1.6 μm continuous-wave modulation laser absorption spectrometer system for measurement of global CO 2 concentration from a satellite

A feasibility study is carried out on a 1.6 μm continuous-wave modulation laser absorption spectrometer system for measurement of global CO(2)concentration from a satellite. The studies are performed for wavelength selection and both systematic and random error analyses. The systematic error in the differential absorption optical depth (DAOD) is mainly caused by the temperature estimation error, surface pressure estimation error, altitude estimation error, and ON wavelength instability. The systematic errors caused by unwanted backscattering from background aerosols and dust aerosols can be reduced to less than 0.26% by using a modulation frequency of around 200 kHz, when backscatter coefficients of these unwanted backscattering have a simple profile on altitude. The influence of backscattering from cirrus clouds is much larger than that of dust aerosols. The transmission power required to reduce the random error in the DAOD to 0.26% is determined by the signal-to-noise ratio and the carrier-to-noise ratio calculations. For a satellite altitude of 400 km and receiving aperture diameter of 1 m, the required transmission power is approximately 18 W and 70 W when albedo is 0.31 and 0.08, respectively; the total measurement time in this case is 4 s, which corresponds to a horizontal resolution of 28 km.

Theory and Design of Class-J Power Amplifiers With Dynamic Load Modulation

A theory for class-J microwave amplifier operation as a function of drive level and fundamental load impedance is derived. Calculations show that, under appropriate operating conditions, it is sufficient to modulate the transistor load reactance to enable high-efficiency operation (>;70%) over a large output power dynamic range (>;10 dB) with high transistor power utilization. Such dynamic load modulation (DLM) networks are an ideal application of continuously tunable varactor technologies. Multiharmonic load-pull measurements are performed on a GaN HEMT and experimentally verify the theory of operation. A demonstrator amplifier using an SiC varactor technology is then designed and characterized by static measurements. The amplifier has a peak power of 38 dBm at 2.08 GHz and maintains efficiencies above 45% over 8 dB of power dynamic range. An analysis of the load network losses is performed to show the potential of the class-J DLM transmitter concept.

High-Current Backside-Illuminated Photodiode Array Module for Optical Analog Links

We have developed a high-current photodiode (PD) array module that consists of a beam splitter, which splits a light beam from a single-mode fiber into four beams with identical powers, a backside-illuminated InGaAs p-i-n PD array, and a two-stage Wilkinson RF power combiner. To obtain a linear PD response, the PD is equipped with a partially depleted absorber, which reduces the bias voltage. To prevent thermal failure, the backside-illuminated PD structure is used, which reduces the thermal resistivity. The beam splitter is fabricated using optical contacts at the SiO2 interfaces between a coating film and a neighboring prism to prevent degradation of the adhesive glue at the prism interfaces along the optical path; therefore, light with a power of a watt level can be used. The two-stage Wilkinson combiner combines four RF outputs of the PD array into a single-RF output through a K connector; the combiner also works as an impedance-matching circuit. Experimental results reveal that an RF power output of 29.0 dBm can be obtained at a frequency of 5 GHz, and good intermodulation distortion characteristics with a third-order intercept point of 32.5 dBm can be achieved.

Improved pump-beam distribution in a diode side-pumped solid-state laser with a highly diffuse, cross-axis beam delivery system

A new cavity for diode side-pumped solid-state lasers was designed, built, and tested. The results were efficient absorption of pumped light in the media and homogeneous pumped-beam distribution under various pump-power levels, which generated Nd:YAG active media of different radii and concentrations and shifts of the diode wavelength. Full-multimode quasi-continuous pump operation with a slope efficiency of 40.5% was obtained from a plane-plane resonator equipped with two Nd:YAG rods. At a maximum average pump power of 1.33 kW, the optical-to-optical efficiency was 31.4%. Single-transverse-mode operation (M(2) factor of less than 1.1) of 163 W was demonstrated.

Planar-waveguide quasi-phase-matched second-harmonic-generation device in Y-cut MgO-doped LiNbO3

A planar-waveguide quasi-phase matched second-harmonic generation device, which consists of a Y-cut periodically poled MgO-doped LiNbO3 core and SiO2 claddings, provides 1-W green light with 30-% conversion efficiency at room temperature.

Blue light generation in a ridge waveguide MgO:LiNbO3 crystal pumped by a fiber Bragg grating stabilized laser diode.

We report cw blue light generation by using a periodically poled MgO:LiNbO(3) crystal with a ridge waveguide pumped by a fiber Bragg grating stabilized laser diode with 0.25 nm spectrum width in the coherent collapse regime. Blue light generation of 73 mW with 27% conversion efficiency and a wide temperature tolerance of 5.7 degrees C were attained.

11-mJ, 15-Hz single-frequency diode-pumped Q-switched Er, Yb:phosphate glass laser

A single-frequency diode-pumped Q-switched Er, Yb:phosphate glass laser that oscillates at an eye-safe 1.54etam wavelength has been developed for use in coherent Doppler lidar. A maximum TEM(00)-mode Q-switched output energy of 10.9 mJ and a relatively long pulse width of 228 ns were obtained at a repetition rate of 15 Hz by use of a modified 2-m-long telescopic cavity. Frequency stability of as high as +/-1.9-MHz standard deviation and a side-mode suppression ratio of more than 30 dB were also achieved.