Input power fluctuations NOISE

Fig. 7. Setup for the degenerate four wave mixing experiments. The input beam is split in three beams. The beam splitter BS3 deflects a part of one of the pump beams to a power meter, which detects laser power fluctuations. The delay line with the retro reflector R adjusts the temporal overlap of the two pump beams coming from the front side on the sample. The long delay line with retro reflector R2 is moved to probe the temporal behavior of the nonlinearity in the sample. The phase conjugated signal beam propagates from the sample back to BSj and is then deflected through a stack of attenuation filters on a second power meter. An iris in front of the power meter increases the signal to noise ratio by removing scattered light...

If the incident laser beam in Fig. 9.96a is blocked, the mean intensity (/) becomes zero. However, the measured noise power density Pn(/) does not go to zero but approaches a lower limit po that is attributed to the zero-point fluctuations of the vacuum field, which is also present in a dark room. The interferometer in Fig. 9.96a has two inputs the coherent light field and a second field, which, for a dark input part, is the vacuum field. Because the fluctuations of these two inputs are uncorrelated, their noise powers add. Increasing the input intensity Iq will increase the signal-to-noise-ratio... 

Fig. 9.101 (a) Experimental setup for the generation of bright squeezed light by second-harmonic generation in a monolithic resonator (b) spectral power density of photocurrent fluctuations for squeezed light input to a balanced homodyne detector. The upper curve (/> gives the shot-noise level [1337]... 

Similar expressions also describe parametric processes in which the input beams are of unequal frequency and intensity. If coi is a high-power beam and CO2 a low-power beam then in the mixing process the power is transferred so that the coi intensity is decreased and the CO2 intensity increased in addition to the generation of coi CO2 co ). This process also transfers the amplitude and phase fluctuations of the low-power CO2 input to the high-power output. Thus a stable low-noise high-power beam can be generated. In this mode co is referred to as the idler frequency, since it is not of particular interest. 

The major noise sources for a typical pyroelectric detector are the dielectric or Johnson noise, the amplifier current and voltage noise, and the thermal noise, caused by fluctuations in the power flow from the element to its heat sink. Each of these has an equivalent voltage generated at the amplifier input V y (given by equation (5.9)), and and Vj. respectively. These combine to give the total equivalent input noise according to the equation... 

Because the fluctuations of these two inputs are uncorrelated, their noise powers add. Increasing the input intensity Iq will increase the signal-to-noise-ratio S/p N/(v +Po), where the fundamental limitation is set by the quantum noise pg. 

In lightly damped structures the slowly varying energy can be treated as a constant over an appropriate period of oscillation, and oscillatory terms can be approximated by their time averages over one period of oscillation. Furthermore, under broadband random excitation, the relaxation time of the oscillator response is much greater than the correlation time of the excitation. Thus, it is possible to model the power input due to the excitation as a nonzero mean component plus an additional, fluctuating component with the character of white noise. 

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