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Physical Review A

Passage-time calculation for the detection of weak signals via the transient dynamics of a laser

S. Balle, FERDINANDO DE PASQUALE, Maxi San Miguel

Published May 1, 1990• 32 citations
Publisher

Abstract

The passage-time (PT) distribution associated with the switch on of a laser with an injected signal can be used to detect weak signals. We calculate the distribution function of such a distribution. Results for the mean PT interpolate between the limit of no injected signal and a simple logarithmic law for strong signals. It has a scaling form with a single important independent parameter. The variance of the distribution is more sensitive to the presence of the injected signal than the mean. Explicit formulas for the limit of sensitivity of the detection method and for the bandwidth resolution are given.

DOI
10.1103/physreva.41.5012

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The passage-time (PT) distribution associated with the switch on of a laser with an injected signal can be used to detect weak signals. We calculate the distribution function of such a distribution. Results for the mean PT interpolate between the limit of no injected signal and a simple logarithmic law for strong signals. It has a scaling form with a single important independent parameter. The variance of the distribution is more sensitive to the presence of the injected signal than the mean. Explicit formulas for the limit of sensitivity of the detection method and for the bandwidth resolution are given.
We calculate the passage-time statistics of a gain-switched single-mode semiconductor laser with a weak, detuned injected signal. Results are given in terms of a relevant scaling parameter. We find that a weak injected signal can be detected by measurements of the mean passage time or its variance whenever its frequency is close to the one of the semiconductor lasers in the on state. In this case, the external signal is resonant with the laser field at the time when amplification becomes possible, and hence it triggers the switch-on more efficiently. The detection bandwidth is of the order of 50 GHz for the case considered.
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.
Laser fluctuations are measured and characterized with a new technique based on the concept of first-passage-time distributions. This first-passage-time technique makes it possible to discern quantum noise in a dye laser even in the presence of pump fluctuations that are many orders of magnitude larger. We report new analytic and experimental results for the mean, variance, and skewness of the first-passage-time distributions. The analytic results agree well with numerical computations and experimental measurements. Our technique is particularly suited to the quantitative description of laser fluctuations above threshold; it complements the conventional photoelectric counting and correlation methods which are valuable for lasers below and near threshold but are difficult to implement for lasers far above threshold. The limits of validity of the theory containing a cubic nonlinearity are examined in the context of passage-time measurements. The effects of multiplicative noise and saturation on the variance of the intensity fluctuations are also discussed. A new signature of multiplicative noise is predicted.
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 process of amplification of light by a laser set above its oscillation threshold is studied. This laser has a wide bandwidth (typically 100 MHz) and is injected by a signal whose bandwidth is much narrower, which allows to separate the amplified signal from the ordinary laser spectral density. Theoretical calculations are based on simple semiclassical formulas and agree with the experimental results which are performed at 1.55 μm using single-mode semiconductor lasers. The amplification arises from a progressive channeling of energy from the wider laser line onto the narrow injected signal.