Optical parametric oscillator: Difference between revisions

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== Introduction ==

[[Image:IR Optical Parametric Oscillator.JPG‎|thumb|300 px|right|Infrared optical parametric oscillator]]

An '''optical parametric oscillator''' ([[OPO]]) is a [[parametric oscillator]] which oscillates at optical frequencies. It converts [[second order]] [[nonlinear optics| nonlinear optical interaction]]. The sum of the output waves frequencies is equal to the input wave frequency: <math>\omega_s + \omega_i=\omega_p</math>. For historic reasons, the two output waves are called "signal" and "idler", where the wave with higher frequency is called signal. A special case is the degenerate OPO, when the output frequency is one-half the pump frequency, <math>\omega_s=\omega_i=\omega_p/2</math>.

== Overview ==

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In order to change the output wave frequencies, one can change the pump frequency or the [[Nonlinear Optics|phasematching]] properties of the nonlinear optical crystal. This latter is accomplished by changing its temperature or orientation or quasi-phasematching period (see below). For fine-tuning one can also change the optical path length of the resonator. In addition, the resonator may contain elements to suppress mode-hops of the resonating wave. This often requires active control of some element of the OPO system.

If the nonlinear optical crystal cannot be phase-matched, [[quasi-phase-matching]] (QPM) can be employed. This is accomplished by periodically changing the nonlinear optical properties of the crystal, mostly by [[periodical poling]]. With a suitable range of periods, output wavelengths from 700 &nbsp;nm to 5000 &nbsp;nm can be generated in periodically poled [[lithium niobate]] (PPLN). Common pump sources are [[Nd-YAG laser|neodymium lasers]] at 1.064 &nbsp;µm or 0.532 &nbsp;µm.

An important feature of the OPO is the coherence and the spectral width of the generated radiation.

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== Quantum properties of the generated light beams ==

[[Image:OPO Crystals.jpg‎‎|thumb|300 px|right|[[Potassium titanyl phosphate|KTP]] crystals in an OPO]]

The OPO is the physical system most widely used to generate [[squeezed coherent states]] and [[entangled]] states of light in the continuous variables regime. Many demonstrations of quantum information protocols for continuous variables were realized using OPO's .<ref>J. Jing, J. Zhang, Y. Yan, F. Zhao, C. Xie, and K. Peng, Phys. Rev. Lett. '''90''', 167903 (2003).</ref><ref>S. Koike, H. Takahashi, H. Yonezawa, N. Takei, S. L. Braunstein, T. Aoki, and A. Furusawa, Phys. Rev. Lett. '''96''', 060504 (2006).</ref><ref>N. Takei, H. Yonezawa, T. Aoki, and A. Furusawa, Phys. Rev. Lett. '''94''', 220502 (2005).</ref><ref>S. Koike, H. Takahashi, H. Yonezawa, N. Takei, S. L. Braunstein, T. Aoki, and A. Furusawa, Phys. Rev. Lett. '''96''', 060504 (2006).</ref>.

The [[Spontaneous parametric down conversion|downconversion]] process really occurs in the single photon regime: each pump photon that is annihilated inside the cavity gives rise to a pair of photons in the signal and idler intracavity modes. This leads to a quantum correlation between the intensities of signal and idler fields, so that there is squeezing in the subtraction of intensities ,<ref>A. Heidmann, R. J. Horowicz, S. Reynaud, E. Giacobino, C. Fabre, and G. Camy, Phys. Rev. Lett. '''59''', 2555 (1987).</ref>, which motivated the name "twin beams" for the downconverted fields. The highest squeezing level attained was 10.12 +/- 0.15 dB .<ref>Schnabel et al., Phys. Rev. Lett. '''100''', 033602 (2008).</ref>.

It turns out that the phases of the twin beams are quantum correlated as well, leading to [[entanglement]], theoretically predicted in 1988 .<ref>M. D. Reid and P. D. Drummond, Phys. Rev. Lett. '''60''', 2731 (1988).</ref>. Below threshold, entanglement was measured for the first time in 1992 ,<ref>Z. Y. Ou, S. F. Pereira, H. J. Kimble, and K. C. Peng, Phys. Rev. Lett. '''68''', 3663 (1992).</ref>, and in 2005 above threshold .<ref>A. S. Villar, L. S. Cruz, K. N. Cassemiro, M. Martinelli, and P. Nussenzveig, Phys. Rev. Lett. '''95''', 243603 (2005).</ref>.

Above threshold, the pump beam depletion makes it sensitive to the quantum phenomena happening inside the crystal. The first measurement of squeezing in the pump field after parametric interaction was done in 1997 .<ref>K. Kasai, J.G. Gao, and C. Fabre, Europhys. Lett. '''40''', 25 (1997).</ref>. Actually, it has been recently predicted that all three fields (pump, signal and idler) must be entangled .<ref>A. S. Villar, M. Martinelli, C Fabre, and P. Nussenzveig, Phys. Rev. Lett. '''97''', 140504 (2006).</ref>.

Not only intensity and phase of the twin beams share quantum correlations, but also do their spatial modes .<ref>M. Martinelli, N. Treps, S. Ducci, S. Gigan, A. Maître, and C. Fabre, Phys. Rev. A '''67''', 023808 (2003).</ref>. This feature could be used to enhance signal to noise ratio in image systems.

The OPO is being employed nowadays as a source of squeezed light tuned to atomic transitions, in order to study how the atoms interact with squeezed light .<ref>T. Tanimura, D. Akamatsu, Y. Yokoi, A. Furusawa, M. Kozuma, Opt. Lett. '''31''', 2344 (2006).</ref>.

== References ==

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* [[Optical parametric amplifier]]

{{DEFAULTSORT:Optical Parametric Oscillator}}

[[Category:Nonlinear optics]]

[[Category:Laser applications]]