The influence of wind-driven surface waves and air bubbles in water on the polarization characteristics of spaceborne oceanic lidar return signals is investigated through the meridian planes Monte Carlo method. The calculation results demonstrate that the depolarization ratio of lidar return signals increase with the increasing wind speed and nadir angle. However, compared with air bubbles the influence of surface waves could be neglected. The presence of air bubbles with high concentration in the upper ocean has a strong impact on the depolarization ratio. The polarization characteristics of the lidar return depend on the air bubble concentration, the bubble microstructure and the optical properties of oceanic waters. The change of the depolarization ratio is significant when the air bubble concentration exceeds 105 m-3 in open ocean water and 5×105 m-3 in coastal water.
The spectroscopic filter plays a critical role in an HSRL (high-spectral-resolution lidar) system. In this paper, a pressuretuned field-widened Michelson interferometer (PT FWMI for short) is proposed. The design of the pressure tuning and the field widening of the PT FWMI are addressed in detail, and the development of a laboratory demonstrator is described as well. The optical elements contain a solid arm made of the glass H-ZF52 with the dimension of 59.572 mm, and an air gap with the length of 32.889 mm within the sealed chamber of 1 atm. Due to the matched dimensions and refractive indices of the two arms, the experimental testing results show that the OPD variation of the developed PT FWMI is about 0.13 lambda and the RMS is less than 0.03 lambda when the divergent angle is as much as 3 degree (half angle). The filtering performance of the prototype developed is scanned with a frequency tunable laser whose bandwidth is 10MHz. Results revealed that bright to dark fringe contrast of the spectroscopic filter is approximately 33. In conclusion, this newly proposed pressure tuning design is suitable for developing stable and tunable FWMI spectroscopic filters and paves the way for designing a robust near infrared HSRL system.
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