Idler wave

Figure 2.23 shows the broad spectral region covered by an OPO system, using as a pump wavelength the Q-switched radiation at 355 nm from a system based on a Nd YAG laser. Coherent radiation that is tunable from 400 nm to 2 jam can be obtained from the signal and idler waves. 

In addition, short-pulse tunable optical parametric oscillators have been realized, where the pump wavelength and the signal or idler waves can be used for... 

Fig. 9.99 Generation of squeezed states by four-wave mixing in an atomic Na beam. Both the pumped wave and signal and idler waves are resonantly enhanced by two optical resonators [1335]...

The first successful experimental realization of squeezing was reported by Slusher and coworkers [1335], who used the four-wave mixing in a Na-atomic beam as nonlinear process (Fig. 9.99). The Na atoms are pumped by a dye laser at the frequency coi = co )8, which is slightly detuned from the resonance frequency coq. In order to increase the pump power, the Na beam is placed inside an optical resonator tuned to the pump frequency co]. Because of parametric processes (Vol. 1, Sect. 6.7) of the two pump waves (coi, A l) inside the resonator, new waves at u)l are generated at this four-wave mixing process (signal and idler wave). Conservation of energy and momentum demands... 

A second resonator with a properly chosen length, such that the mode spacing is Av = 8, enhances the signal as well as the idler wave. 

The essential point is that there are definite phase relations between pump, signal and idler waves, and this fact establishes a correlation between amplitude and phase of the signal wave. This correlation is illustrated in Fig. 9.100 The four-wave mixing generates new sidebands. If the input contains the frequencies and u)l + <5, the output has the additional sideband co — 8. Thus, the amplitude of one sideband is increased at the cost of the other sideband until both sidebands have equal amplitudes. This maximizes the amplitude modulation of the output. Since the phases... 

To form an OPO, resonance is provided by a set of suitable resonator mirrors, providing resonance conditions for either, or both, the signal and idler frequencies (see Figure 4.24). Such resonators significantly increase the conversion efficiency from the pump wave into signal and idler waves. 

The gain of the signal and idler waves depends on the pump intensity and on the effective nonlinear suceptibility. Analogous to the sum- or difference-frequency generation, one can define a parametric gain coefficient per unit pathlength T = 7s//p or I /Ip... 

Fig. 5.121. Wavelengths of signal and idler waves in BBO as a function of the phasematching angle for different pump wavelengths Xp [5.295]...

In addition, short-pulse tunable optical parametric oscillators have been realized, where the pump wavelength and the signal or idler waves can be used for pump-and-probe experiments [11.128]. The wide tuning range allows more detailled investigations compared to the restricted use of fixed frequency lasers [11.129]. Another useful short-pulse source for these experiments is a three-wavelength Ti sapphire laser, where two of the wavelengths can be indepently tuned [11.130]. 

The essential point is that there are definite phase relations between pump, signal and idler waves, and this fact establishes a correlation between amplitude and phase of the signal wave. As has been shown in detail... 

This means that for each incident photon of frequency 0)2 there are two outgoing photons of the same frequency. Simultaneously with the generation of a new wave of frequency co - the incident wave of frequency (O2 is parametrically amplified. If the nonlinear medium is placed between two mirrors reflecting at the frequencies (O2 and (or) ( 3, this parametric effect may be increased. One calls such a device a parametric oscillator (see Figure 3). From this point of view, co = (o corresponds to the so-called pump wave, (jl>2 = (o to the (amplified) signal wave, and (03 = (Oi- CO2) =o)i to the idler wave. Equation [8] may be simplified to... 

Figure 3 Schematic representation of a singly-resonant optical parametric oscillator. Pump wave of frequency cop, (reflected) signal wave of frequency os, idler wave of frequency a>. The signal wave (Os becomes amplified. 6p denotes the angle of orientation of the direction of propagation with respect to the crystal optic axis. Adapted with permission from Tang CL and Cheng LK (1995) Fundamentals of Optical Parametric Processes and Oscillators. Amsterdam Harwood Academic Publishers.

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