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Optics Letters

Temporal characteristics of narrow-band optical filters and their application in lidar systems

Guangning Yang, Richard I. Billmers, Peter R. Herczfeld, V.M. Contarino

Published Mar 15, 1997• 21 citations
Publisher

Abstract

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.

DOI
10.1364/ol.22.000414

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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.

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Chinese Optics Letters•May 10, 2007•8 citations
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.
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 operation of a narrow-linewidth optical f ilter based on the 4P((1/2)) ? 8S((1/2)) excited-state transition in potassium vapor is reported. The 4P((1/2)) state is excited by a circularly polarized, 769.9-nm, 10-ns pulse from a dye laser. A linearly polarized, time-sequenced, and spatially overlapped probe pulse at 532.33 nm completes the transition to the 8S((1/2)) state. The peak filter transmission is ~40% with a bandwidth of less than 4 GHz. Corroborative experimental results suggest that the rotation of probe-pulse polarization by an induced circular birefringence is the dominant mechanism behind the filter operation.
We demonstrate what is to our knowledge the first atomic optical filter that uses velocity selection to achieve a passband width that is less than the Doppler width of the filtering transition. A narrow-linewidth pump laser is used to induce circular birefringence in a narrow velocity class of atoms in a dense potassium vapor for 694-nm light resonant with the 4P(3/2)-6S(1/2) transition. The filter displays a single 170-MHz passband at a peak transmittance of 9.5%. The bandwidth is an order of magnitude lower than that of previously demonstrated atomic optical filters.
We outline what is to our knowledge the first experimental demonstration of an excited-state Faraday filter. The filter consists of potassium vapor between crossed polarizers in a dc magnetic field and operates on the 4P((1/2)) ? 8S((1/2)) transition in potassium. The 4P((1/2)) state is populated by a linearly polarized, 10-ns light pulse from a dye laser operating at 769.9 nm. Another linearly polarized, 10-nsec pulse at 532.33 nm traverses the pumped volume of the K cell and is absorbed from the 4P((1/2)) state to the 8S((1/2)) state. The transmission of the filter is approximately 3.5% at 532.33 nm with a bandwidth of less than 10 GHz.