Optical parametric oscillators/amplifiers OPOs/OPAs

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

For 2PA or ESA spectral measurements, it is necessary to use tunable laser sources where optical parametric oscillators/amplifiers (OPOs/OPAs) are extensively used for nonlinear optical measurements. An alternative approach, which overcomes the need of expensive and misalignment prone OPO/OPA sources, is the use of an intense femtosecond white-light continuum (WLC) for Z-scan measurements [71,72]. Balu et al. have developed the WLC Z-scan technique by generating a strong WLC in krypton gas, allowing for a rapid characterization of the nonlinear absorption and refraction spectra in the range of 400-800 nm [72]. 

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]. 

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