Parametric oscillation Idler

A conceptually even simpler approach uses just one optical parametric oscillator, pumped by a dye laser or diode laser at 4f and oscillating at the two frequencies f and 3f. The signal frequency f is enforced by injection locking with light from the 3.39 pm reference laser. The pump frequency is adjusted so that the idler frequency agrees with the third harmonic of the reference laser. The seventh harmonic is then generated by simply summing idler and pump frequency. 

One of the important applications of second-order NLO materials is obtaining of tunable laser sources. Second harmonic generation or sum frequency generation systems lead to monochromatic sources. The optical parametric oscillators are based on the parametric generation of two waves with frequencies co (signal) and CO (idler). In noncentrosymmetric materials an incident photon with frequency co creates two photons satisfying the energy... 

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... 

The wavelength region below 720 nm cannot be accessed directly by the titanium-sapphire laser but is covered by another laser system, e.g., the optical parametric oscillator (OPO). The OPO system consists of a birefringent crystal, e.g., BBO (BaB204), placed within an optical cavity irradiated by an intense pump beam such as a Nd YAG laser that is operated at its third harmonic (355 nm). Through nonlinear interaction the pump beam is split into an idler beam and a signal beam, which generate a... 

Fig. 5.119a,b. Optical parametric oscillator (a) schematic diagram of experimental arrangement (b) pairs of wavelengths (Ai, X2) for idler and signal wave for collinear phase matching in LiNbOs as a function of angle 0 [5.290]... 

Fig. 5.120. Temperature tuning curves of signal and idler wavelengths for a LiNb03 optical parametric oscillator pumped by different pump wavelengths [5.292]...

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]. 

A. S. Villar, M. Martinelli, C. Fabre, P. Nussenzveig. Direct Production of Tripartite Pump-Signal-Idler Entanglement in the Above-Threshold Optical Parametric Oscillator. Physical Review Letters 2006 Oct 6 97(14) 140504(4). 

Summary Optical parametric oscillators are coherent devices similar to lasers. There are, however, important differences. While lasers can be pumped by incoherent sources, OPOs require coherent pump sources. Often diode laser-pumped solid state lasers are used. While in lasers coherent amplification can last until the inversion in the active medium has fallen below threshold, in OPO s the time dependence of the coherent output is directly coupled to that of the pump laser. Since the pump photon is split into signal and idler photon with u> = u>i, the energy of the output equals that of the input i.e. there is no energy, i.e. heat deposited in the active crystal. The spectral tuning range is by far wider than for tunable lasers. Most OPOs operate in the near infrared but can be tuned from the visible region to the far infrared. 

Fig. 2.9. Schematic sketch of an optical parametric oscillator (OPO) cavity. The OPO uses a nonlinear optical crystal to generate new frequencies (called signal and idler) from a coherent input source (pump laser)...

The Optical Parametric Oscillator (OPO) process should also be mentioned. Here a nonlinear crystal in a cavity is used to generate two new frequencies (oji and 0J2) out of a single one (a ) that is used to pump the crystal. Energy conservation requires Ui- U 2 = ct . The frequency division between the two new waves (the signal and the idler) is chosen by the phase matching condition. The parametric process can also be used in Optical Parametric Amplifiers (OPA). Parametric laser light generation was reviewed in [8.77,78]. 

Up to now, the most common laser sources used for CARS microscopy are based on Tirsapphire or Nd YVO lasers with pulse durations from tens of femtoseconds up to 10 ps. Different approaches are possible in order to generate pump and Stokes beams use of two femtosecond laser sources electronically synchronized [19], pumping of an optical parametric amplifier (OPA) to produce the Stokes beam and use of the residual pump as pump beam [18], pumping of an optical parametric oscillator (OPO) to obtain the pump beam and use the residual pump light as Stokes [20], using signal and idler beams from a synchronously pumped OPO to provide directly the two excitation beams [11, 21]. 

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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