Optical: systems and elements – Optical frequency converter – Parametric oscillator
Reexamination Certificate
1999-06-09
2001-08-28
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Parametric oscillator
C359S326000
Reexamination Certificate
active
06282014
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of optical devices, and particularly to embodiments of a cascaded optical parametric oscillator. The optical parametric oscillator includes an optical medium with multiple non-linear regions to facilitate efficient down-conversion of coherent light to longer wavelengths.
BACKGROUND OF THE INVENTION
Production of tunable coherent radiation through parametric amplification from a fixed frequency laser beam is effected through a device known as an optical parametric oscillator (OPO). The theoretical rationale and complexities associated with parametric amplification and OPOs are well known to those skilled in the art.
In a conventional OPO, the OPO receives a beam of laser radiation at a pump frequency &ohgr;
p
from a pump source. The pump frequency &ohgr;
p
is received into a resonant optical cavity, wherein pump frequency &ohgr;
p
is directed through a non-linear medium, usually a crystal, located within the resonant cavity. As a result, two lower energy signals are generated from the pump beam, which are known as the signal, at frequency us and the idler at frequency &ohgr;
i
.
The content and orientation of the crystal and the design of the resonant cavity determines the signal &ohgr;
s
and idler &ohgr;
i
frequencies. The feedback within the resonant cavity causes gain in the parametric waves, a process similar to build-up in a laser cavity. The cavity can either be singly-resonant in which end mirrors reflect only signal frequency &ohgr;
s
, or doubly-resonant in which end mirrors reflect both signal &ohgr;
s
and idler &ohgr;
i
frequencies. End mirrors of the OPO are transparent to the pump frequency &ohgr;
p
. OPOs with singly-resonant cavities are typically more stable in their output than OPOs with doubly-resonant cavities.
Due to the nature of the non-linear crystal and the conversion process, the pump frequency &ohgr;
p
is always higher than the frequency of the signal &ohgr;
s
and the idler &ohgr;
i
. The sum of the signal &ohgr;
s
and idler &ohgr;
i
frequencies is equal to the pump frequency &ohgr;
p
.
Power and energy conversion efficiency to the idler frequency &ohgr;
i
in an OPO is limited by the quantum efficiency and photon efficiency. Since idler frequency &ohgr;
i
is less than half of the pump frequency &ohgr;
p
, the quantum limit is always less than one half and significantly less so when the idler frequency wi is far from degeneracy. Furthermore, for pulsed OPOs, pump regeneration from signal &ohgr;
s
and idler &ohgr;
p
frequency reduces photon conversion efficiency due to temporally and/or spatially varying pump radiation. Nevertheless, idler output provides a useful means of generating coherent radiation in spectral regions that are difficult to access by other sources.
There are a variety of types of crystals that may be used in OPOs for various spectral regions. In particular, non-linear optical crystals capable of producing parametric output which have been developed for commercial applications, include, but are not limited to, potassium titanyl phosphate (KTP), potassium titanyl arsenate (KTA), lithium niobate (LiNbO
3
), potassium niobate (KNbO
3
), silver gallium selenide (AgGaSe
2
), and silver gallium sulfide (AgGaS
2
). When a fixed laser is used to generate tunable waves from certain crystals, an electric field may be applied to the crystal, or the crystal may be temperature or angle tuned, or a combination of electrical voltage, temperature and/or angle tuning is required to achieve phase matching.
Periodically poled LiNbO
3
(PPLN) has been shown to be particularly well-suited for OPO wavelength generation in the 1.4-4.0 &mgr;m region due to its low threshold, large non-linear coefficient, large acceptance angle, absence of walk-off, and transparency in this region (L. E. Myers, R. C. Eckardt, M. M. Fejer, R. L. Byer, W. R. Bosenberg, and J. W. Pierce, J. Opt. Soc. Am. B12, 2102-2116 (1995)). Although continuous wave OPOs utilizing PPLN have demonstrated high conversion efficiencies (W. R. Bosenberg, A. Drobshoff, J. I. Alexander, L. E. Myers, and R. L. Byer, Opt.Lett. 21, 1336-1338 (1996)), typically pulsed OPOs have not yet approached continuous wave OPO efficiencies due to factors such as back conversion of the pump wave and non-uniform pump depletion.
In a typical configuration of an OPO using a crystal or PPLN medium, the crystal or PPLN is located between the two cavity mirrors. Light is directed through the entry mirror through the crystal or PPLN medium and through the exit mirror with certain frequencies being reflected back into the cavity to again be transmitted through the crystal or PPLN medium.
Other techniques of increasing conversion efficiency in similar OPO configurations suggest the inclusion of a second crystal or PPLN medium located within the cavity, and situated between the two cavity mirrors. In these structures, an entry mirror receives the light which directs the beam through a first crystal or PPLN to be received by a second crystal or PPLN and then on to an exit mirror. Again, the exit mirror transmits certain frequencies while reflecting other frequencies back through the crystal media.
Conversion schemes using tandem and intercavity difference frequency mixing (DFM) OPOs have been proposed and analyzed (K. Koch, G. T. Moore, and E. C. Cheung, J. Opt, Soc. Am. B 12, 2268-2273 (1995); and G. T. Moore and K. Koch, IEEE J. Quantum Electron. 32, 2085-2094 (1996)) and may help mitigate some of the limitations inherent in pulsed OPOs, however, such suggested approaches fail to significantly increase conversion efficiency.
Frequency conversion schemes utilizing multiple crystals within the OPO cavity demonstrating the OPO-DFM system applying two separate PPLN crystals are discussed and analyzed in J. M. Fukumoto, H. Komine, W. H. Long et al. Advanced Solid State Lasers (1998) (Optical Society of America, Washington, D.C., 1998), post deadline paper PDP-4, where a factor of two increase in the idler conversion efficiency is demonstrated.
Reference may be had to the following patents for further information concerning the state of the technology relating to OPOs (all of the references are incorporated herein by reference):
U.S. Pat. No. 5,400,173, issued Mar. 21, 1995 entitled “Tunable Mid-Infrared Wavelength Converter Using Cascaded Parametric Oscillators” to Komine, describes an apparatus for converting a fixed wavelength pump into a plurality of spectral output beams. The first resonator is coupled to a first non-linear optical crystal for turning said pump into a first and second output beams.
U.S. Pat. No. 5,500,865, issued Mar. 19, 1996 entitled “Phased Cascading Of Multiple Non-linear Optical Elements For Frequency Conversion”, to Chakmakjian, uses two or more crystals in tandem to increase the interaction length of the non-linear optical process for improved efficiency. Additional optical components are inserted into the optical path to adjust the phase delay of the interacting waves in order to maintain coherent generation of the product radiation.
U.S. Pat. No. 4,639,923, issued Jan. 27, 1987, entitled, “Optical Parametric Oscillator Using Urea Crystal”, to Tang, et al., uses a crystal of urea as the non-linear optical medium for constructing an OPO.
U.S. Pat. No. 5,159,487, issued Oct. 27, 1992, entitled “Optical Parametric Oscillator OPO Having A Variable Line Narrowed Output”, to Geiger et al., describes an OPO that includes a pump laser for producing a pump beam; an optical resonator; an OPO crystal disposed within the optical resonator aligned with and responsive to the pump beam to produce a parametrically generated output; and a device external to the optical resonator for line narrowing the parametrically generated output.
U.S. Pat. No. 5,144,630, issued Sep. 1, 1992, entitled “Multiwavelength Solid State Laser Using Frequency Conversion Technique”, to Lin, describes an apparatus for producing multiwavelength coherent radiations ranging from deep ultraviolet to mid-infrared. The basic laser is a pulsed Nd:YAG or Nd:Y
Anderson Terry J.
Hoch, Jr. Karl J.
Lee John D.
Northrop Grumman Corporation
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