Optical parametric amplifier
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An optical parametric amplifier, abbreviated OPA, is a laser light source that emits light of variable wavelengths by an optical parametric amplification process.
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[edit] Optical parametric generation (OPG)
This light emission is based on the nonlinear optical principle. The photon of an incident laser pulse (pump) is divided into two photons, the sum energy of which is equivalent to the energy of the photon of the pump, by a nonlinear optical crystal. Ordinary light and extraordinary light are generated; the ordinary light is called the signal and the extraordinary light is called the idler. The wavelengths of the two generated laser pulses, the signal and the idler, are determined by the phase matching condition, which is changed by the angle between the incident pump laser ray and the axes of the crystal. The wavelengths of the signal and the idler lights can, therefore, be tuned by changing the phase matching condition. This process is called optical parametric generation, or OPG.
[edit] Optical parametric amplification (OPA)
After separation of the signal light from the OPG outputs, the remaining idler passes through a nonlinear optical crystal collinearly with the light of the same wavelength as the pump, and the stronger output of the same wavelength as the signal and idler is acquired as the output of the OPA. These wavelength-variable outputs are efficiently used in many spectroscopic methods. As an example of OPA, the incident pump pulse is the 800 nm (12500 cm-1) output of a Ti:sapphire laser, and the two outputs, signal and idler, are in the near-infrared region, the sum of the wavenumber of which is equal to 12500 cm-1.
[edit] Noncollinear OPA
As most nonlinear crystals are birefringent, collinear beams inside the crystals may not be collinear outside the crystal. Due to walk-off the phase fronts (wave vector) do not point in the same direction as the energy flow (poynting vector).
The phasematching angle allows one to have any gain at all ( 0th order ). In a collinear set-up the freedom do choose center wavelength allows one to have a constant gain up to first order in wavelength. Noncolinear OPAs were developed to have one more degree of freedom to have a constant gain up to second order. The optimum found was 4 degree noncollinearity, β-barium borate (BBO) as the material, pump with 400 nm, and signal around 800 nm, which leads by luck to a bandwidth 3 times as large of that of a Ti-sapphire-amplifier. The first order is mathematically equivalent to some properties of the group velocities involved, but it does not mean, that pump and signal have the same group velocity. After propagation through 1 mm BBO a short pump does not overlap with the signal anymore. So for high gain amplification in long crystals Chirped pulse amplification has to be used. Long crystal introduce such a big chirp that a compressor is needed anyway. An extreme chirp can lengthen a 20 fs seed pulse to the length of 50 ps. Then a 50 ps pulse can be used as the pump. Unchirped 50 ps pulses are available with high energy from rare earth based lasers.
The optical parametric amplifier has a wider bandwidth than a Ti-sapphire-amplifier, but the Ti-sapphire laser has a wider bandwidth than optical parametric oscillator, due to white-light generation even one octave wide. Therefore one can afford to select a subband and still generate pretty short pulses.
The higher gain per mm for BBO compared to Ti:Sa and more importantly the lower amplified spontaneous emission allows for higher overall gain. Interlacing compressors and OPA leads to tilted pulses.
[edit] Multipass OPA
Multipass can be used for
- walk off and group velocity (dispersion) compensation
- constant intensity with increasing signal power means to have an exponential rising cross section. This can be done by means of lenses, which also refocus the beams to have the beam waist in the crystal.
- reduction of OPG by increasing the pump power proportional to the signal and splitting the pump across the passes of the signal
- broadband amplification by dumping the idler and optionally individually detuning the crystals
- complete pump depletion by offseting the pump and signal in time and space at every pass and feeding one pump pulse through all passes
- high gain with BBO. Since BBO is only availalable in small dimensions.
Since the direction of the beams is fixed, multiple passes cannot be overlapped into a single small crystal like in a Ti:Sa amplifier. Unless one uses noncolinear geometry and adjusts amplified beams onto the parametric fluorescence cone produced by the pump pulse. Multipass bow type chirped pulse amplifier
[edit] Parametric amplifier (electronics)
A parametric amplifier consists of a single diode. It is named after the AC power supply (parametric=variable=nonconstant=not DC =AC). The OPA can be seen as a massive parallel and serial circuit of parametric amplifiers, which is used to achieve high gain at very high frequencies.
