Kimio Asaka

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.

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.

1.5-μm eye-safe coherent lidar system for wind velocity measurement

A coherent lidar is an attractive sensor for atmospheric observation because it enables the wind velocity measurement in clear air conditions. A 1.5-um eye-safe coherent lidar is more attractive than a 2-um eye-safe coherent lidar. The wavelength of 1.5-um provides a ten times higher maximum permissible exposure for human eyes than the wavelength of 2-um. In addition, optical fiber components and devices developed for optical fiber communications are easily available. We have already reported first 1.5-um coherent lidar system for wind velocity measurement, and recently completed the full system. The system has the Er,Yb:Glass pulsed laser with the output energy 10.9-mJ and the telescope with effective aperture of 100-mm. The system provides the available measurement range of 5-km (SNR>+1-dB), the detectable wind velocity range of between -50-m/sec and +50-m/sec and the range resolution of 30-m. The velocity accuracy of 0.14-m/sec (standard deviation) is obtained by measuring velocity of a non-moving hard target.

Development of long range, real-time, and high resolution 3D imaging ladar

In the previous study, we have introduced the concept of the real-time 3D imaging LADAR (LAser Detection And Ranging) using linear array receiver. In this paper, we demonstrate a long range, high resolution, and high speed 3D imaging using the developed system. The system consists of in-house-made key components. The linear array receiver consists of the previously reported APD array and the ROIC array assembled in one package. We newly developed the transmitting optics using pupil divide method which realizes a uniform illumination on a target. By combining these devices with the one dimensional fast scanner, we realized a 256 × 256 pixels range imaging with a on-line frame rate of more than 10 Hz at a distance of more than 1 km.

All Fiber Coherent Doppler LIDAR for Wind Sensing

An 1.5 micron pulsed Coherent Doppler LIDAR system using all fiber optical components has attracted attention for remote wind sensing application because of its eye-safety, reliability and easy deployment. We report on our key technologies such as fiber based MOPA (Master Oscillator Power Amplification) transmitter, high peak power optical amplification and a real-time signal processing. Some performance results and applications are also provided.

Development of an airborne wind measurement system

Accidents to airliners caused by air turbulence have been increasing in recent years. At present there is no sure way of avoiding encounters with clear air turbulence (CAT) because conventional airborne weather radars cannot detect turbulence in clear conditions. The Japan Aerospace Exploration Agency (JAXA) is therefore developing a Doppler LIDAR (Light Detection and Ranging) which can measure wind speeds ahead of an aircraft even in clear air. Turbulence prediction in flight has already been demonstrated using experimental Doppler LIDARs, and the latest prototype aims at turbulence detection up to 5 nautical miles (9 km) ahead at jet airliner cruising altitudes. Wind velocity measurement by LIDAR is difficult at high altitude because of low aerosol particle density. Regular atmospheric observation flights are therefore being made to establish the basic specifications of a practical device. Furthermore, since air turbulence information should be displayed on the flight deck, a graphical turbulence display is also being studied. This paper describes the development of the airborne wind measurement system and presents examples of flight experiment results.

Wind sensing demonstration of more than 30km measurable range with a 1.5μm coherent Doppler lidar which has the laser amplifier using Er,Yb:glass planar waveguide

Recently, we have developed the high output power laser amplifier using Er,Yb:glass planar waveguide in order to increase the measurable range of our 1.5 μm coherent Doppler LIDAR (CDL). In this paper, we introduce this development and demonstration of a long range wind sensing using the developed system. The transmitted pulse has a peak power of 2.4 kW and a width of 580 ns (i.e. pulse energy of 1.4 mJ) with a pulse repetition frequency of 4 kHz, in addition to a nearly diffraction limited beam quality. With this laser amplifier, we demonstrate the measurable range of more than 30 km. According to our own research, this is the longest measurable range demonstration for wind sensing CDLs.

10.9-mJ single-frequency diode-pumped Q-switched Er,Yb:glass laser for a coherent Doppler lidar

A coherent Doppler lidar is a useful sensor for wind velocity detection in clear air condition. A stable single frequency and relatively long Q-switched pulse width (~200 nsec order) in eye-safe wavelength range (>1.4 μm) is required for a composed pulsed laser. For these requirements, we have developed an injection-seeded diode-pumped Q-switched Er,Yb:glass laser which oscillates at 1.54 μm. The maximum laser output energy of 10.9 mJ and the pulse width of 228 nsec were obtained. A stability of the pulsed laser output frequency was less than +/-1.9 MHz standard deviation from the seeded light frequency.

1.5-μm high-average power laser amplifier using a Er,Yb:glass planar waveguide for coherent Doppler lidar

We have developed a 1.5-μm eye-safe wavelength high average power laser amplifier using an Er,Yb:glass planar waveguide for coherent Doppler LIDAR. Large cooling surface of the planar waveguide enabled high average power pumping for Er,Yb:glass which has low thermal fracture limit. Nonlinear effects are suppressed by the large beam size which is designed by the waveguide thickness and the beam width of the planar direction. Multi-bounce optical path configuration and high-intensity pumping provide high-gain and high-efficient operation using three-level laser material. With pulsed operation, the maximum pulse energy of 1.9 mJ was achieved at the repetition rate of 4 kHz. Output average power of the amplified signal was 7.6W with the amplified gain of more than 20dB. This amplifier is suitable for coherent Doppler LIDAR to enhance the measurable range.

Semianalytic pulsed coherent laser radar equation for coaxial and apertured systems using nearest Gaussian approximation

We present a semianalytic pulsed coherent laser radar (CLR) equation for coaxial and apertured systems. It combines the conventional CLR equation, numerical Fresnel integration (NFI), and nearest Gaussian approximation, using correction factors that correspond to beam truncation. The range dependence of the signal-to-noise ratio obtained by this semianalytic equation was found to agree well with the precise NFI solution for not only the focal range, but also the near-field range. Furthermore, the optimum beam truncation condition depending on the atmospheric refractive index structure constant is shown. The derived equation is useful for precisely predicting the CLR performance simply by its semianalytic expression.