Abstract
ABSTRACT We demonstrate the use of a commercial mode-locked and Q-switched Nd:YAG laser in photon-noisereduction experiments. There are many frequency bands wherein the photodetection of such a laser isshot-noise limited. By appropriately filtering the photocurrent produced in direct detection of light froman optical parametric amplifier that is pumped with this laser, we have successfully generated twin-beam states of light. We demonstrate that such twin beams can be used to make measurements below the shot-noise limit. 2. INTRODUCTION The fundamental noise in optical measurements using lasers results from the fact that photons of light are detected at random. In a lossless system, one can ascribe the random detection of photons to the random emission of photons by the laser itself. It can be shown from the detailed laser theory' that anideal laser emits photons in a Poissonian fashion2 as shown in Fig. 1. This fact implies that there willalways be fluctuation in the number of photons that are counted in a given time interval. If = ii photonsare counted by the detector in an observation time T, then the variance in the photon count will be givenby the Poisson formula Var(N) = ii. This fluctuation in the detected photon number leads to the followingmaximum signal-to-noise ratio (SNR) that can be obtained using laser light:
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