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Narrow band optical filter for lidar

Guangning Yang

Published Jan 1, 1996• 0 citations

Abstract

A narrow-band, gate-able matched optical filter with large extinction ratio and high transmission is required for ocean lidar applications at wavelength of 532 nm. Two types of filters, the induced dichroism excited-state atomic line (IDEAL) and excited state anomalous Faraday dispersion optical filter (ESFADOF), are studied. They are operating on the 4P1/2 to 8S1/2 excited states transition line of potassium vapor and a narrow band at 532 nm. The measured critical features of the IDEAL filter are: 1.3 GHz (i.e..01A) narrow pass band, approximately 45% peak transmission, high extinction coefficient in the order of 10-5, very fast rise time (<3ns) and a 15 ns decay time determined by resonant enhanced two photon transition. The physical mechanism of the IDEAL filter operation is explained in terms of resonant enhanced two photon transition. Good agreement between experiment and the derived theoretical model is achieved. The pertinent features of the excited (ESFADOF) filter were measured to be: approximately 15% peak transmission, high extinction coefficient in the order of 10-5, very fast rise time (<3ns), and a decay time in the tens of microsecond range governed by resonant photon trapping. The physical processes contributing to the ESFADOF filter action are resonant photon trapping and Faraday anomalous dispersion. The experimental results confirmed theoretical predictions. The applications of these filters are discussed in terms of improving the lidar system performance. Specifically, the narrow bandwidth of these filters eliminates the background noise caused by stray solar radiation and thereby enhances the dynamic range of the lidar system. Furthermore the fast efficient gating of the filters minimizes reflection from the ocean surface, which is several orders of magnitude higher than the return signal from underwater targets.

DOI: 10.17918/00007879

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IEEE Photonics Technology Letters•Aug 17, 2018•8 citations

A narrow-band, gate-able matched optical filter with large extinction ratio and high transmission is required for ocean lidar applications at wavelength of 532 nm. Two types of filters, the induced dichroism excited-state atomic line (IDEAL) and excited state anomalous Faraday dispersion optical filter (ESFADOF), are studied. They are operating on the 4P1/2 to 8S1/2 excited states transition line of potassium vapor and a narrow band at 532 nm. The measured critical features of the IDEAL filter are: 1.3 GHz (i.e..01A) narrow pass band, approximately 45% peak transmission, high extinction coefficient in the order of 10-5, very fast rise time (<3ns) and a 15 ns decay time determined by resonant enhanced two photon transition. The physical mechanism of the IDEAL filter operation is explained in terms of resonant enhanced two photon transition. Good agreement between experiment and the derived theoretical model is achieved. The pertinent features of the excited (ESFADOF) filter were measured to be: approximately 15% peak transmission, high extinction coefficient in the order of 10-5, very fast rise time (<3ns), and a decay time in the tens of microsecond range governed by resonant photon trapping. The physical processes contributing to the ESFADOF filter action are resonant photon trapping and Faraday anomalous dispersion. The experimental results confirmed theoretical predictions. The applications of these filters are discussed in terms of improving the lidar system performance. Specifically, the narrow bandwidth of these filters eliminates the background noise caused by stray solar radiation and thereby enhances the dynamic range of the lidar system. Furthermore the fast efficient gating of the filters minimizes reflection from the ocean surface, which is several orders of magnitude higher than the return signal from underwater targets.

The temporal characteristics of two different narrow-band optical filters at 532 nm are reported. Both filters operate on the 4P(1/2) - 8S(1/2) atomic transition of potassium vapor, where the 4P(1/2) state is excited by a 770-nm, 10-ns laser pulse. The filters operate on the principle of circular birefringence induced by either the Faraday effect or two-photon transition, which is confirmed by experiments. The characteristic decay times of the filters are 5 and 0.015 mus, which is significant for the design of optical gating and signal processing with matched filters for ocean lidar.

In this paper the properties of an excited-state potassium Faraday anomalous dispersion optical filter in potassium vapor with a pump wavelength of 769.90 nm (i.e. 4S1/2-->4P1/2 transition) and a signal wavelength of 532 nm (i.e. 4P1/2-->8S1/2 transition) are calculated and discussed. The filter's transmissions versus the laser pump power and the external magnetic-field are obtained. The theoretical results show the transmission characteristics of the filter are relatively insensitive to the laser pump power and magnetic strength. The laser pump powers require range from 0.8 to 6 mW.cm-2. Our model predicts a peak transmission of 50% and an equivalent band-width of 4 GHz for the filter operating on the 4P1/2<--8S1/2 transition in potassium. The optical filter at 532 nm presented in this paper may be used to detect the practical and important frequency-doubled Nd:YAG laser.

The characteristics of a potassium Faraday anomalous dispersion optical filter operating on the blue and near infrared transitions are calculated. The results show that the filter can be designed to provide high transmission, very narrow pass bandwidth, and low equivalent noise bandwidth. The Faraday anomalous dispersion optical filter (FADOF) provides a narrow pass bandwidth (about GHz) optical filter for laser communications, remote sensing, and lidar. The general theoretical model for the FADOF has been established in our previous paper. In this paper, we have identified the optimum operational conditions for a potassium FADOF operating on the blue and infrared transitions. The signal transmission, bandwidth, and equivalent noise bandwidth (ENBW) are also calculated.

An induced-dichroism-excited atomic line (IDEAL) filter at 1529 nm operating on the transition of87Rb 52P3/2-42D3/2 is realized. Its peak transmittance achieves 63.8% at 80 °C by a 13.4 mW/mm2laser pumping at 780 nm and the equivalent noise bandwidth is 546 MHz. Detailed experiments show that, compared with the excited-state Faraday anomalous dispersion optical filter and excited-state Voigt atomic line filters which are realized by the same atomic vapor cell, operating on the same transition and under the same pumping intensity, the IDEAL filter achieves the same performance level without the magnetic field. This letter provides a new solution to ultra-narrow optical filtering at 1529 nm, and it is particularly useful in applications which require non-magnetic environments.