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Quantum Electronics

Effect of intense background radiation on the sensitivity of a laser receiver with an iodine active quantum filter

Yu F Kutaev, S. K. Mankevich, O Yu Nosach, E. P. Orlov

Published Apr 30, 2002• 3 citations
Publisher

Abstract

The effect of background light on the sensitivity of a laser receiver with an iodine active quantum filter (λ=1.315 μm) was theoretically and experimentally investigated. Upon the reception of a 40-ns pulsed signal against the background of 2.5-fold attenuated radiation of a pulsed light source with a brightness temperature of 4 × 10 4 K, the sensitivity of this receiver for a signal-to-noise ratio of three and a diffraction-limited acceptance angle was experimentally shown to be equal to 20 photons, which exceeds the quantum limit by about a factor of two. This is consistent with the results of theoretical treatment and suggests that upon the detection of optical signals against the background of the solar disk, the sensitivity of this receiver should decrease by only 12%. This receiver was compared with a receiver employing a photomultiplier of the visible range. Upon the reception of optical signals with the same parameters against the background of the solar disk and an interference filter with a transmission band width of 5 nm, the sensitivity of a receiver equipped with an FEU-115 photomultiplier was shown to be equal to about 1400 photons for a signal-to-noise ratio of three.

DOI
10.1070/qe2002v032n04abeh002197

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The effect of background light on the sensitivity of a laser receiver with an iodine active quantum filter (λ=1.315 μm) was theoretically and experimentally investigated. Upon the reception of a 40-ns pulsed signal against the background of 2.5-fold attenuated radiation of a pulsed light source with a brightness temperature of 4 × 10 4 K, the sensitivity of this receiver for a signal-to-noise ratio of three and a diffraction-limited acceptance angle was experimentally shown to be equal to 20 photons, which exceeds the quantum limit by about a factor of two. This is consistent with the results of theoretical treatment and suggests that upon the detection of optical signals against the background of the solar disk, the sensitivity of this receiver should decrease by only 12%. This receiver was compared with a receiver employing a photomultiplier of the visible range. Upon the reception of optical signals with the same parameters against the background of the solar disk and an interference filter with a transmission band width of 5 nm, the sensitivity of a receiver equipped with an FEU-115 photomultiplier was shown to be equal to about 1400 photons for a signal-to-noise ratio of three.
A laser receiver (LR) with an active quantum filter (AQF) (λ =1.315 μm) based on an iodine photodissociation laser is studied. It is shown that the LR sensitivity can be brought up to the quantum detection limit by increasing the AQF gain. Taking into account the space-time statistics of spontaneous emission of the AQF, the LR sensitivity is determined as a function of the reception solid angle and the recorder response time. Under the conditions of the experiments performed, the LR sensitivity is, to a good approximation, proportional to the square root of the product of the reception solid angle and the recorder time constant. The above dependence is confirmed experimentally upon more than a hundred-fold variation of the response time. For the reception angle that was three times greater than the diffraction-limited angle, the quantum detection limit was achieved, which amounts to approximately three photons for a 40-ns pulse for the unit signal-to-noise ratio.
It is proposed to search for signals from extraterrestrial intelligence (ETI) at a wavelength of 1.315 μm of the laser 2 P 1/2 → 2 P 3/2 transition in the atomic iodine, which can be used for this purpose as the natural frequency reference. The search at this wavelength is promising because active quantum filters (AQFs) with the quantum sensitivity limit have been developed for this wavelength, which are capable of receiving laser signals, consisting of only a few photons, against the background of emission from a star under study. In addition, high-power iodine lasers emitting diffraction-limited radiation at 1.315 μm have been created, which highly developed ETI also can have. If a ETI sends in our direction a diffraction-limited 10-ns, 1-kJ laser pulse with the beam diameter of 10 m, a receiver with an AQF mounted on a ten-meter extra-atmospheric optical telescope can detect this signal at a distance of up to 300 light years, irrespective of the ETI position on the celestial sphere. The realisation of the projects for manufacturing optical telescopes of diameter 30 m will increase the research range up to 2700 light years. A weak absorption of the 1.315-μm radiation in the Earth atmosphere (the signal is attenuated by less than 20%) allows the search for ETI signals by using ground telescopes equipped with adaptive optical systems.
A proposed method of suppressing the effect of background noise in an optical communication system would exploit the transmission and reception of correlated photons at the receiver. The method would not afford any advantage in a system in which performance is limited by shot noise. However, if the performance of the system is limited by background noise (e.g., sunlight in the case of a free-space optical communication system or incoherently scattered in-band photons in the case of a fiber-optic communication system), then the proposed method could offer an advantage: the proposed method would make it possible to achieve a signal-to-noise ratio (S/N) significantly greater than that of an otherwise equivalent background- noise-limited optical communication system based on the classical transmission and reception of uncorrelated photons. The figure schematically depicts a classical optical-communication system and a system according to the proposed method. In the classical system, a modulated laser beam is transmitted along an optical path to a receiver, the optics of which include a narrow-band-pass filter that suppresses some of the background noise. A photodetector in the receiver detects the laser-beam and background photons, most or all of which are uncorrelated. In the proposed system, correlated photons would be generated at the transmitter by making a modulated laser beam pass through a nonlinear parametric down-conversion crystal. The sum of frequencies of the correlated photons in each pair would equal the frequency of the incident photon from which they were generated. As in the classical system, the correlated photons would travel along an optical path to a receiver, where they would be band-pass filtered and detected. Unlike in the classical system, the photodetector in the receiver in this system would be one that intrinsically favors the detection of pairs of correlated photons over the detection of uncorrelated photons. Even though there would be no way of knowing the precise location and time of creation of a given pair of correlated signal photons in the nonlinear down-conversion crystal, the fact that the photons are necessarily created at the same time and place makes it possible to utilize conventional geometrical imaging optics to reunite the photons in coincidence in the receiving photodetector. Because most or all of the signal photons would be correlated while most or all of the noise photons would be uncorrelated, the S/N would be correspondingly enhanced in the photodetector output. An additional advantage to be gained by use of a correlated-photon detector is that it could be capable of recovering the signal even in the presence of background light so bright that a classical uncorrelated-photon detector would be saturated. A blocked-impurity-band (BIB) photodetector that preferentially detects pairs of correlated photons over uncorrelated ones and that operates at a quantum efficiency of 88 percent is commercially available. This detector must be cooled to the temperature of liquid helium to obtain the desired low-noise performance. It is planned to use this detector in a proof-of-principle demonstration. In addition, it may be possible to develop GaN-based photodetectors that could offer the desired low-noise performance at room temperature.
When optical signals are embedded in background noise, narrowband optical filters are needed for receivers to achieve quantum limited performance. Such is typically the case with daytime optical communications and lidar. Narrowband filtered detectors. with acceptance bands of just a few picometers (pm) and high out of band rejection are well suited for these applications. The main thrust of this program has been the development of two means for narrowband detection: the Fast Atomic Line Filter/ Field Ion Detector (FALF/FID) and the Faraday magneto-optic filter. The FALF/FIDs transduce optical radiation within a narrow band directly into electrical signals. They are laser pumped filters based on the spectroscopy of atomic vapors. We designed, built and tested an infrared FALF/FID with an rise time of under 10 ns and a quantum efficiency of >25%. Faraday filters are imaging magneto-optic filters with passbands of a few pm. Faraday filters with passbands in the visable and the near infra-red were constructed and tested. Transmission spectra were measured, and found to agree closely with theoretical predictions. Both of these narrowband detection schemes must be used in conjunction with a tuned narrowband laser source. We developed a frequency locked alexandrite laser designed to match the acceptance band of a rubidium based filter. Finally, a preliminary demonstration of a laser interrogator using a tuned infrared diode laser in conjunction with a filtered receiver was demonstrated. Atomic line filter, Alkali metal vapor, Faraday filter, Active optical filter, Laser pumped filter, Magneto-optic filter, Laser communications.