An interesting feature of lasers running far above threshold is their ability to generate intensity squeezed light due to their internal dynamics. One mechanism for this is rate matching [1] in which the presence of two or more slow rates in the pumping process results in a reduction of the pump noise below the quantum noise limit (QNL). This occurs even in the presence of strong dephasing processes such as are typical in solid-state lasers.
It is possible to consider the laser as a passive device which takes in light at the pump frequency and emits it at the laser frequency. An interesting question to ask is; what is the maximum conversion efficiency from coherent input to squeezed output? In previous treatments it has been assumed that the pump is undepleted, i.e. very low conversion efficiency. In some squeezing systems low conversion efficiency is intrinsic to the production of strong squeezing [2].
Another interesting question is how signals on the pump are transferred to the laser output and whether the presence of the noise reducing mechanisms can reduce the noise penalty for amplification of signals below the quantum limit usually associated with linear/phase insensitive amplifiers [3]. A phase insensitive amplifier is one which amplifies signals and noise on the amplitude and phase quadratures equally.
We have investigated the quantum noise properties of lasers operating in the rate matching regime under conditions in which the pump beam is strongly depleted. To do this we have considered a system in which the pump beam also forms a resonant cavity mode. By varying the pump cavity decay rate we were able to change the impedance matching condition of the pump to the atom/cavity system and hence obtain total pump absorption at various pump powers.
We find that the noise characteristics of the system change markedly as the point of total pump absorption moves to higher pump powers. Rate matched squeezing only emerges at pump powers above the point of total absorption of the pump. The maximum squeezing is unaffected by the pump absorption which leads to the prediction that for a many levelled system strong squeezing can be produced with close to unit efficiency. The range of pump powers for which squeezing is present is narrowed however as the point of total pump absorption nears the matching condition.
At pump powers below the point of total pump absorption there is a region of amplified fluctuations. We find in this region that signals on the pump beam can be amplified onto the laser output with out suffering the usual noise penalty associated with phase insensitive amplifiers. As for the squeezing the amplification noise penalty is reduced further with more matched rates, approaching the ideal limit of 0 dB in the limit of very many levels.
Without the presence of competing non-linear c(2) interactions, both the output fundamental as well as the second harmonic light from the singly resonant frequency doubler are predicted to be squeezed. In particular, the second harmonic squeezing is predicted to increase with increasing fundamental pump power. The maximum attainable squeezing is theoretically calculated to be at -9.5dB (or Vshg = 1/9) [2]. However, we have found that unless explicit steps have been taken to avoid competition, the squeezing as well as the power generation of the output second harmonic will be limited by the presence of MROPO.
In a system where MROPO is present, the maximum point for squeezing will occurs at the threshold power, as shown in Fig. 1. When the system is driven above threshold, two effect will come into play: At high detection frequencies, increasing the pump power will pull the second harmonic noise towards the input field noise (normally the quantum noise level). This behaviour is again analogous to an optical limiter. At lower detection frequencies, severe noise degradation is observed. This is due to the noisy amplitude quadrature of OPO around threshold being transfered to the second harmonic.
By detuning some control over the threshold point of MROPO is possible and Fig. 2 shows two squeezing spectra with and without competitions. Good quanlitative agreement between the experimental and theoretical squeezing spectra is observed.
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Last Modified: 8 May, 1997
Jinwei.Wu@anu.edu.au