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Optical parametric oscillator - Wikipedia, the free encyclopedia
Optical parametric oscillator - Wikipedia, the free encyclopedia
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Optical parametric oscillator
From Wikipedia, the free encyclopedia
An
optical parametric oscillator
(OPO) is a
parametric oscillator
which oscillates at optical
frequencies. It converts an input
laser
wave (called "pump") into two output waves of lower frequency
(
ω
s
,ω
i
) by means of
nonlinear optical interaction.
The sum of the output waves frequencies is equal to
the input wave frequency:
ω
s
+ ω
i
= ω
p
. For historic reasons, the two output waves are called
"signal" and "idler". A special case is the degenerate OPO, when the output frequency is one-half the
pump frequency,
ω
s
= ω
i
= ω
p
/ 2
.
Contents
1 Overview
2 Quantum properties of the generated light beams
3 References
4 External links
5 See also
Infrared optical parametric oscillator
Overview
The OPO consists essentially of an
optical resonator
and a
nonlinear optical
crystal. The optical resonator serves to resonate at least one of signal and idler
waves. In the nonlinear optical crystal, the pump, signal and idler waves overlap. The interaction between these three waves leads to amplitude gain for
signal and idler waves (parametric amplification) and a corresponding deamplification of the pump wave. The gain allows the resonating wave(s) (signal or
idler or both) to oscillate in the resonator, compensating the loss that the resonating wave(s) experience(s) at each round-trip. This loss includes the loss
due to outcoupling by one of the resonator mirrors, which provides the desired output wave. Since the (relative) loss is independent of the pump power, but
the gain is dependent on pump power, at low pump power there is insufficient gain to support oscillation. Only when the pump power reaches a particular
threshold level, oscillation occurs. Above threshold, the gain depends also on the amplitude of the resonated wave. Thus, in steady-state operation, the
amplitude of the resonated wave is determined by the condition that this gain equals the (constant) loss. The circulating amplitude increases with increasing
pump power, and so does the output power.
The photon conversion efficiency, the number of output photons per unit time in the output signal or idler wave relative to number of pump photons
incident per unit time into the OPO can be high, in the range of tens of percent. Typical threshold pump power is between tens of milliwatts to several
watts, depending on losses of the resonator, the frequencies of the interacting light, the intensity in the nonlinear material, and its nonlinearity. Output
powers of several watts can be achieved. There exist both
continuous-wave
and
pulsed
OPOs. The latter are easier to build, since the high intensity lasts
only for a tiny fraction of a second, which damages the nonlinear optical material and the mirrors less than a continuous high intensity.
In the optical parametric oscillator the initial idler and signal waves are taken from background waves, which are always present. If the idler wave is given
from the outside along with the pump beam, then the process is called
difference frequency generation
(DFG). This is a more efficient process than optical
parametric oscillation, so that also the threshold intensity is lower.
In order to change the output wave frequencies, one can change the pump frequency or the
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 nm to 5000 nm
can be generated in periodically poled
lithium niobate
(PPLN). Common pump sources are
neodymium lasers
at 1.064 µm or 0.532 µm.
An important feature of the OPO is the coherence and the spectral width of the generated radiation. When the pump power is significantly above threshold,
the two output waves are, to a very good approximation,
coherent states
(laser-like waves). The linewidth of the resonated wave is very narrow (as low as
several kHz). The nonresonated generated wave also exhibits narrow linewidth if a pump wave of narrow linewidth is employed. Narrow-linewidth OPOs
are widely used in spectroscopy.
[1]
Quantum properties of the generated light beams
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
[2]
[3]
[4]
[5]
.
The
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
http://en.wikipedia.org/wiki/Optical_parametric_oscillator
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Optical parametric oscillator - Wikipedia, the free encyclopedia
1/2/10 7:56 AM
squeezing in the subtraction of intensities
[6]
, which motivated the name "twin beams" for the
downconverted fields. The highest squeezing level attained was 10.12 +/- 0.15 dB
[7]
.
It turns out that the phases of the twin beams are quantum correlated as well, leading to
entanglement,
theoretically predicted in 1988
[8]
.
Below threshold, entanglement was measured for the first time in
1992
[9]
,
and in 2005 above threshold
[10]
.
KTP
crystals in an OPO
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
[11]
. Actually, it has been recently
predicted that all three fields (pump, signal and idler) must be entangled
[12]
.
Not only intensity and phase of the twin beams share quantum correlations, but also do their spatial modes
[13]
. 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
[14]
.
References
1.
^
F. J. Duarte
(Ed.),
Tunable Laser Applications
, 2nd Ed. (CRC, New York, 2009) Chapter 2.
2.
^
J. Jing, J. Zhang, Y. Yan, F. Zhao, C. Xie, and K. Peng, Phys. Rev. Lett.
90
, 167903 (2003).
3.
^
S. Koike, H. Takahashi, H. Yonezawa, N. Takei, S. L. Braunstein, T. Aoki, and A. Furusawa, Phys. Rev. Lett.
96
, 060504 (2006).
4.
^
N. Takei, H. Yonezawa, T. Aoki, and A. Furusawa, Phys. Rev. Lett.
94
, 220502 (2005).
5.
^
S. Koike, H. Takahashi, H. Yonezawa, N. Takei, S. L. Braunstein, T. Aoki, and A. Furusawa, Phys. Rev. Lett.
96
, 060504 (2006).
6.
^
A. Heidmann, R. J. Horowicz, S. Reynaud, E. Giacobino, C. Fabre, and G. Camy, Phys. Rev. Lett.
59
, 2555 (1987).
7.
^
Schnabel et al., Phys. Rev. Lett.
100
, 033602 (2008).
8.
^
M. D. Reid and P. D. Drummond, Phys. Rev. Lett.
60
, 2731 (1988).
9.
^
Z. Y. Ou, S. F. Pereira, H. J. Kimble, and K. C. Peng, Phys. Rev. Lett.
68
, 3663 (1992).
10.
^
A. S. Villar, L. S. Cruz, K. N. Cassemiro, M. Martinelli, and P. Nussenzveig, Phys. Rev. Lett.
95
, 243603 (2005).
11.
^
K. Kasai, J.G. Gao, and C. Fabre, Europhys. Lett.
40
, 25 (1997).
12.
^
A. S. Villar, M. Martinelli, C Fabre, and P. Nussenzveig, Phys. Rev. Lett.
97
, 140504 (2006).
13.
^
M. Martinelli, N. Treps, S. Ducci, S. Gigan, A. Maître, and C. Fabre, Phys. Rev. A
67
, 023808 (2003).
14.
^
T. Tanimura, D. Akamatsu, Y. Yokoi, A. Furusawa, M. Kozuma, Opt. Lett.
31
, 2344 (2006).
External links
Articles on OPOs
[1] (http://www.rp-photonics.com/optical_parametric_oscillators.html)
E
ncyclopedia of Laser Physics and Technology
Companies commercialising OPOs
[2] (http://www.radiantis.com)
R
adiantis
See also
Nonlinear optics
Optical parametric amplifier
Retrieved from "
http://en.wikipedia.org/wiki/Optical_parametric_oscillator"
Categories
:
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|
Nonlinear optics
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