Оптический параметрический усилитель: принцип работы и применение.

An optical parametric amplifier, abbreviated OPA, is a laser light source that emits light of variable wavelengths by an optical parametric amplification process. It is essentially the same as an optical parametric oscillator, but without the optical cavity (i. e., the light beams pass through the apparatus just once or twice, rather than many many times).

Optical parametric generation (OPG) (also called "optical parametric fluorescence", or "spontaneous parametric down conversion") often precedes optical parametric amplification. In optical parametric generation, the input is one light beam of frequency ωp, and the output is two light beams of lower frequencies ωs and ωi, with the requirement ωp=ωs+ωi. These two lower frequency beams are called the "signal" and "idler", respectively. This light emission is based on the nonlinear optical principle. The photon of an incident laser pulse (pump) is, by a nonlinear optical crystal, divided into two lower energy photons. The wavelengths of the signal and the idler are determined by the phase matching condition, which is changed, e. g. by temperature or, in bulk optics, by the angle between the incident pump laser ray and the optical axes of the crystal. The wavelengths of the signal and the idler photons can, therefore, be tuned by changing the phase matching condition.

The output beams in optical parametric generation are usually relatively weak and have relatively spread out direction and frequency. This problem is solved by using optical parametric amplification (OPA), also called difference frequency generation, as a second stage after the OPG. In an OPA, the input is two light beams, of frequency ωp and ωs. The OPA will make the pump beam (ωp) weaker, and amplify the signal beam (ωs), and also create a new, so called idler beam at the frequency ωi with ωp=ωs+ωi. In the OPA, the pump and idler photons usually travel collinearly through a nonlinear optical crystal. Phase matching is required for the process to work well. Because the wavelengths of an OPG+OPA system can be varied (unlike most lasers which have a fixed wavelength), they are 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.

Because most nonlinear crystals are birefringent, beams that are collinear inside a crystal may not be collinear outside of it. The phase fronts (wave vector) do not point in the same direction as the energy flow (Poynting vector) because of walk off. The phase matching angle makes possible any gain at all (0th order). In a collinear setup, the freedom to choose the center wavelength allows a constant gain up to first order in wavelength. Noncollinear OPAs were developed to have an additional degree of freedom, allowing constant gain up to second order in wavelength. The optimal parameters are 4 degrees of noncollinearity, β barium borate (BBO) as the material, a 400 nm pump wavelength, and signal around 800 nm (and can be tunable in the range 605 750 nm with sub 10 fs pulse width which allows exploring the ultrafast dynamics of large molecules) This generates 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 this does not mean that pump and signal have the same group velocity. After propagation through 1 mm BBO, a short pump pulse no longer overlaps with the signal. Therefore, chirped pulse amplification must be used in situations requiring large gain amplification in long crystals. Long crystals introduce such a large chirp that a compressor is needed anyway. An extreme chirp can lengthen a 20 fs seed pulse to 50 ps, making it suitable for use as the pump.). Therefore, a subband can be selected and fairly short pulses can still be generated. The higher gain per mm for BBO compared to Ti:Sa and, more importantly, lower amplified spontaneous emission allows for higher overall gain. Interlacing compressors and OPA leads to tilted pulses.

The idea of parametric amplification first arose at much lower frequencies: AC circuits, including radio frequency and microwave frequency (in the earliest investigations, sound waves were also studied). In these applications, typically a strong pump signal (or "local oscillator") at frequency f passes through a circuit element whose parameters are modulated by the weak "signal" wave at frequency fs (for example, the signal might modulate the capacitance of a varactor diode). The result is that some of the energy of the local oscillator gets transferred to the signal frequency fs, as well as the difference ("idler") frequency f fs. The term parametric amplifier is used because the parameters of the circuit are varied. The optical case uses the same basic principle—transferring energy from a wave at the pump frequency to waves at the signal and idler frequencies—so it took the same name.

Boichenko, V. L.; Zasavitskii, I. I.; Kosichkin, Yu. V.; Tarasevich, A. P.; Tunkin, V. G.; Shotov, A. P. (1984) "A picosecond optical parametric oscillator with amplification of the tunable semiconductor laser radiation", Soviet Journal of Quantum Electronics 11 (1): 141–143. Magnitskii, S. A.; Malakhova, V. I.; Tarasevich, A. P.; Tunkin, V. G.; Yakubovich, S. D. (1986) "Generation of bandwidth limited tunable picosecond pulses by injection locked optical parametric oscillator", Optics Letters 11 (1): 18–20.