2μm solid-state lasers are the primary choice for coherent Doppler wind detection. As wind lidars, they are used for wake vortex and clear air turbulence detection providing air transport safety. In addition, 2μm lasers are one of the candidates for CO2 detection lidars. The rich CO2 absorption line around 2μm, combined with the long upper state life of time, has made Ho based 2μm lasers a viable candidate for CO2 sensing DIAL instrument. As a transmitter for a coherent wind lidar, this laser has stringent spectral line width and beam quality requirements. Among the rare-earth ions lasers operating at 2μm region, Ho doped crystals show high enough gain cross-sections for effective high energy amplification. To accommodate the efficient diode laser pumping, Tm ions were added to absorb the conventional diode laser pump energy at 780–795nm.
The operating principles of Tm:Ho lasers are well known and has been the subject of intense research in the last two decades. Basically, a 792nm quasi CW laser diode array provides the pump radiation which is absorbed by Tm ions. Consequently, Tm ions are excited from the 3H6 ground-state up to the 3H4 manifold. Tm ions in the 3H4 manifold then interact with Tm ions in the ground-state. This energy transfer usually referred as “self-quenching”, converts one ions at 3H4 excited-state into two ions at 3F4 states. Subsequently, energy transfer from the Tm 3F4 to the Ho 5I7 manifolds occurs and finally 2 micron laser radiation occurs on the Ho 5I7 → 5I8 transition.
SUMMARY AND RESULTS
An engineering packaged 2-micron laser transmitter for the wind lidar is being developed. This compact laser is configured as a master oscillator and power amplifier system. It is injection seeded with a highly stabilized seed laser operating at 2.053 μm which is very close to the oscillator operating wavelength. The line width measurement of the seed laser is done by beating two similar lasers, the result showed a line width better than 13 KHz. The oscillator output is amplified with a double pass amplifier. The target output energy is 250mJ at 10Hz pulse repetition rate.
The overall dimension of the laser is 67cm X 16.5cm X 26cm. The optical bench is populated on both sides. The seed laser and the associated fiber optics couplers and the Faraday isolators are on the back side of the bench while the oscillator and the amplifier are mounted on the front. All the optical mounts are designed to be adjustable and lockable and hardened to withstand vibrations that can occur in ground or airborne operation.
The performance of the oscillator has been evaluated at different repetition rates and at different rod coolant temperatures. The maximum Q-switched output obtained ranges from 118mJ at 10Hz to 150mJ at 2Hz. Since the laser material is not a 4-level laser, it is sensitive to heat load; and hence the output drop at higher repetition rate is not totally unexpected. Using 100mJ as a probe beam from the oscillator, the double pass amplifier produced over 300 mJ. At of 5°C with a double pass gain is slightly over 3. At a higher rod temperature of 8°C, the output energy was 275mJ. This was partially due to the reduction of the probe beam energy to 95 mJ, but the main reason is the thermal populating of the lower laser level. This shows the importance of depopulating the lower laser level for efficient operation.
The output of the amplifier is directed into the atmosphere through a large telescope after passing through a thin-film polarizer and a quarter wave plate. Since the return signal has orthogonal polarization to the output beam, the polarizer directs it into the receiver where it is mixed with the local oscillator signal.
In summary, a hardened 2μm laser, capable of producing over 300 mJ has been designed, and the performance has been verified. This diode pumped injection seeded MOPA has a transform limited line width and diffraction limited beam quality. Although it is primarily designed as a wind Doppler lidar , with minimal modification it can be used as a Differential Absorption Lidar (DIAL) instrument for CO2 sensing. In addition to being used as a field instrument, the laser will also be used as an engineering design tool for evaluating essential wind lidar parameters for a long term flight instrument.