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Physical Review A

Super-regenerative laser receiver: Transient dynamics of a laser with an external signal

Gautam Vemuri, Rajarshi Roy

Published Mar 1, 1989• 43 citations
Publisher

Abstract

In super-regenerative detection, a weak external signal is amplified in an oscillator that is periodically switched on and then quenched. It is shown that very weak optical signals, slightly above the noise level, can be detected effectively by a laser used as a super-regenerative receiver. The detector output for different signal levels and its operating characteristics are determined by numerical integration of stochastic differential equations that represent the growth of laser radiation from a background consisting of signal and noise.

DOI
10.1103/physreva.39.2539

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In super-regenerative detection, a weak external signal is amplified in an oscillator that is periodically switched on and then quenched. It is shown that very weak optical signals, slightly above the noise level, can be detected effectively by a laser used as a super-regenerative receiver. The detector output for different signal levels and its operating characteristics are determined by numerical integration of stochastic differential equations that represent the growth of laser radiation from a background consisting of signal and noise.
The initiation time for the intensity of an injection seeded laser is measured as a function of the externally injected signal strength. The operation of a radiation detector based on the measurement of initiation times is demonstrated, with a dynamic range of 7 orders of magnitude, bandwidth resolution of 5 MHz, and tuning range limited by the active medium of the laser employed. Such a detector, based on the Q-switched operation of an oscillator, is known as a superregenerative receiver. We report experimental measurements of the characteristics of such a receiver and explain the behavior with reference to a simple stochastic model of a laser with an injected signal. The weakest signals that can be detected are shown to be comparable to the level of spontaneous emission into the cavity mode, which corresponds to an average photon number less than unity for our system.
We have demonstrated the detection of weak optical signals via the decay of an unstable state. A Q-switched laser is an example of an unstable state, the decay of which has to be triggered either by quantum fluctuations or by an injected external signal. Measurement of the initiation time of the laser intensity allows us to determine the strength of the external signal with a dynamic range of 7 orders of magnitude. It is possible to detect signal levels that correspond to less than one photon in a selected cavity mode.
The dynamical response of a laser to frequency-shifted optical feedback is investigated both theoretically and experimentally. The ultrahigh sensitivity of a class-B laser (i.e., a laser with a cavity damping rate ${\ensuremath{\gamma}}_{c}$ higher than the population damping rate ${\ensuremath{\gamma}}_{1})$ to external light injection is demonstrated by the optical detection of weak optical feedback. Compared to a conventional optical beating, the intensity modulation induced by the coherent interaction between the laser electric field and the frequency-shifted reinjected electric field can be several orders of magnitude higher. This method permits high sensitive interferometry and hence imaging. We call this laser detection technique laser optical feedback imaging (LOFI). When the optical frequency shift is resonant with the laser relaxation frequency, the intracavity amplification of the beating is maximum and the enhancement is given by the laser damping rate ratio ${\ensuremath{\gamma}}_{c}/{\ensuremath{\gamma}}_{1}.$ This amplification is of the order of 106{10}^{6}106 for a microchip laser. We also show that without optical feedback the strong fluctuations of the laser output power are well described by the Langevin noise process. In a broad range around the laser relaxation frequency the laser quantum noise is also resonantly amplified and is then several orders of magnitude higher than the detector noise. In these conditions, the LOFI is a shot noise limited detection technique. Reflectivity as low as ${10}^{\ensuremath{-}13}$ is then easily detectable with a laser output power of a few milliwatts with a detection bandwidth of 1 kHz. Experimentally, for weak optical feedback the laser fluctuations are principally composed of the LOFI modulation signal at the shifted frequency and of the laser quantum noise amplified at the relaxation frequency. For strong optical feedback, nonlinear effects appear in the laser dynamics. In these conditions, harmonics and parametrics peaks appear in the power spectrum. The LOFI detection system is then saturated.
Detection of light is one of the most fundamental physical processes playing a very important role in both nature and technology. In nature, this process underlies vision, thus providing an opportunity to perceive the world around us. Numerous photodetectors in technology allow the quantification of the detection of optical signals, expanding the spectrum of detected radiation. The problem of detecting optical signals by optical methods themselves was first formulated apparently by Bloembergen [1]. However, this idea has not been adequately developed. In paper [2] it was shown that the signal-to-noise ratio could be improved by using nonlinear radiation detectors in which the signal transformation begins from an absorption transition. It was shown in papers [3] that photocounts appear in conventional photodetectors due to the strong Coulomb instability of a weak electron current produced in the detector by a signal being detected. This suggests that in detectors, instead of free electrons, the electrons that are bound in atoms, ions, or molecules can be used, where they are well stabilized by a strong Coulomb field of nuclei. Below a possible scheme for detecting weak optical signals by laser means is described. At present microcavities are created which can be used in the Bloembergen scheme, providing the passage from spontaneous to stimulated effect, thereby substantially improving the scheme.
This study summarizes the theory of laser-amplifier noise based on the master equation, the quantum-mechanical Langevin equation, and the rate equation. The three approaches are used differently depending on the application. Laser-amplifier noise in the direct-detection receiver is described by the master equation. Meanwhile, the quantum-mechanical Langevin equation is most effective for analyzing noise appearing in coherent optical communication systems. The rate equation approach provides a clear understanding of the intensity-noise characteristics of laser oscillators, including the amplitude squeezing.