Tunable optical parametric oscillator

An optical parameter oscillator system is proposed for use in a continuous wave pump laser system having a single-frequency pump source. The system comprises a single-resonance resonator having a nonlinear medium to produce a first and second parametrically generated wave in response to the pump wave from the single-frequency pump source. The system includes means for controlling the cavity length of the resonator, means for controlling the pump frequency of the pump source and means for controlling the temperature of the nonlinear medium. The system provides for a reliable singly-resonant optical parametric oscillator capable of emitting laser light with high spectral purity and frequency stability over a wide spectral range and is resistant to mode hopping.

This application claims Paris convention priority of German patent 
applications 19706031.5 filed Feb. 7, 1997, 19718254.2 filed Apr. 30, 1997 
and 197 51 324.7 filed Nov. 19, 1997 the complete disclosures of which are 
hereby incorporated by reference. 
BACKGROUND OF THE INVENTION 
The invention concerns a compact and reliable singly-resonant optical 
parametric oscillator (SRO) capable of emitting laser light of high 
spectral purity and frequency stability over a wide spectral range. 
An optical parametric oscillator (OPO) is a nonlinear device which converts 
incident photons into photon pairs when optically excited at a power per 
unit area above a certain threshold. The threshold level is a 
characteristic of the non-linear material, the resonator, and is a 
function of wavelength. This device is usually embodied in one of two 
forms: Either a doubly-resonant oscillator (DRO) in which both the 
generated optical beams are resonated or in a singly-resonant oscillator 
mode (SRO) in which only one of the generated optical beams is in 
resonance. 
Use of optical parametric oscillators for commercial and scientific 
applications requires simultaneous achievement of several requirements. In 
particular, widely tunable laser radiation having high frequency stability 
and narrow linewidth is usable for a plurality of applications in the 
field of high-resolution spectroscopy and metrology. Continuous-wave 
operation of such laser sources is required to achieve linewidths on the 
order of one Mega Hertz or less. A plurality of continuous-wave lasers are 
available for different portions of the optical spectrum e.g. laser diodes 
in the 630-2000 nanometer range, titanium-sapphire lasers in the 710-1100 
nanometer range, dye lasers in the 400-800 nm range and color center 
lasers in the 2-3.5 .mu.m spectral regions. However, these lasers fail to 
simultaneously satisfy the following criteria: 
Large emission range (in excess of 100 nm); 
High power (in excess of 50 mW); 
Narrow linewidth (less than 1 Mega Hertz); 
Good frequency stability (drift less than 200 MHz/h); and compact size. 
In principle, nonlinear optical frequency conversion can be used to extend 
the wavelength range of lasers having the desired properties. In 
combination with solid-state lasers, such as diode-pumped Nd:YAG lasers, 
pulsed nonlinear frequency conversion has been demonstrated to be capable 
of generating light in the ultraviolet, visible and infrared spectral 
regions in compact, powerful, and reliable systems. Research on 
continuous-wave optical parametric oscillators (OPOs) driven by 
diode-pumped solid state lasers had been started in 1989 by Kozlovsky et 
al. (Optics Letters 14, 66 (1989)) using a doubly-resonant OPO (DRO) with 
both generated waves being resonantly enhanced to reduce the oscillator 
threshold. Although emission ranges of more than 200 nm in the near 
infrared region and output powers in the mW range had been demonstrated 
(Gerstenberger et al., J. Opt. Soc. Am. B 10, 1681 (1993)), the high 
susceptibility of DROs to mode-hopping and the difficult tuning behavior 
(Eckardt et al., J. Opt. Soc. Am. B 8, 646 (1991)) have caused 
continuous-wave OPOs to achieve the reputation of being non-suitable for 
high-resolution spectroscopy applications. Yang et al. (Optics Letters 18, 
971 (1993)) have shown that a singly-resonant OPO (SRO) can achieve 
mode-hop-free operation over several minutes and a continuous tuning range 
of 550 MHz has been obtained. These achievements of prior art are still 
far from the practical demands of high-resolution spectroscopy 
applications. 
In view of these disadvantages of prior art, it is the principal purpose of 
the present invention to further improve a singly-resonant oscillator of 
the above mentioned kind in such a fashion that frequency-stable and 
mode-hop-free operation with continuous frequency tuning is achieved in an 
efficient, compact, stable and widely tunable nonlinear frequency 
conversion system. 
SUMMARY OF THE INVENTION 
This purpose is achieved in accordance with the invention in an optical 
parametric oscillator system for use in a continuous-wave pumped laser 
device having a single-frequency pump source. The system comprises a 
singly-resonant cavity having a nonlinear medium for producing a first 
parametrically generated wave (signal wave) and a second parametrically 
generated wave (idler wave) in response to a pump wave from the 
single-frequency pump source, with means for controlling parameters that 
lead to changes in wavevector mismatch such as the optical path length of 
the resonator, the frequency of the pump source, and the temperature of 
the nonlinear medium. 
In accordance with the invention it has been found that certain stability 
requirements are essential to the elimination of mode-hops. In particular, 
the essential parameters of the system, i.e. typically the pump frequency, 
cavity optical path length (determined in turn by the crystal temperature 
and physical length of the cavity) must not change more than a 
predetermined amount. The allowable amount depends on the cavity design 
and dimension, the nonlinear material used and the pump and the emission 
wavelengths. To prevent mode-hops, a sufficient criterion is that the 
allowable fluctuations must be significantly less than those that would 
lead to a situation where the wavevector mismatch for oscillation with the 
frequency of the resonantly parametrically generated wave differing by one 
free spectral range of the cavity yields a larger gain. 
Viewing the change in pump angular frequency .delta..omega..sub.p, the 
change in resonator medium temperature .delta.T and the change in cavity 
length .delta.L, as independent variations leads to the following 
sufficient conditions for mode-hop free operation 
##EQU1## 
and L.sub.rt.sup.c is the round-trip of the crystal, L.sub.rt.sup.a is the 
round-trip length in air. 
These equations apply to all different kinds of SRO configurations. To 
specialize these equations to a particular case, only those equations in 
(1) are taken where the variation on the left hand side refers to an 
independent parameter, and the partial derivatives 
##EQU2## 
where they are nonzero, are calculated using the corresponding resonance 
conditions and inserted into equations (1). To illustrate this procedure, 
the case of the signal-resonant OPO with non-resonant pump 
(.omega..sub.p,L,T are the independent parameters) leads to the following: 
##EQU3## 
wherein all other partial derivatives vanish. 
These results can be generalized to include electro-optic tuning of the 
resonator optical path length. 
In applications in which it is desirable to tune the frequency of the 
signal or idler wave of the SRO over a large range, tuning of the output 
waves can be performed by changing the optical path length of the cavity 
to thereby change the resonance frequency. The frequency of the conjugate 
non-resonant wave is thereby changed indirectly through the condition of 
photon energy conservation. If the optical path length is changed by a 
substantial amount, a phase mismatch in the parametrical interaction 
causing a mode-hop will occur. In order to prevent this, the system in 
accordance with the invention includes means to change the indices of 
refraction of at least one of the waves involved in a parametric 
interaction (typically via a change in temperature applied to the 
nonlinear optical crystal, although a change in an applied electric field 
would also be possible). This change in phase mismatch is chosen to 
compensate or nearly compensate for the phase mismatch which occurs due to 
frequency tuning of the OPO output waves. In particular, a servo system 
can be employed to regulate this phase mismatch such that the emitted 
idler or signal wave power is maximized. An error signal for this 
regulation can be obtained by applying a small positive and negative 
temperature change to the crystal and comparing the emitted OPO powers. 
For frequency-stable operation of the OPO output waves, the frequency 
output of the singly-resonant OPO for the generated and emitted waves is 
determined by the optical path length of the cavity for the signal wave. 
For this reason, small changes in this length caused e.g. by mechanical 
disturbances, drifts in temperature of the nonlinear crystal to change its 
index of refraction, pressure fluctuations of the air and the like, cause 
frequency changes in the signal and for a given pump frequency, in the 
idler frequency. The goal of an active frequency stabilization system for 
SRO must therefore be to reduce the frequency changes of the signal wave 
and/or the idler wave compared to the level of the free running device. In 
accordance with the invention, the combined means for controlling the 
cavity length of the resonator, means for controlling the pump frequency 
of the pump source, and means for controlling a temperature of the 
nonlinear medium provide the necessary conditions for frequency stable 
operation and suppressed mode-hopping. 
In a preferred embodiment in accordance with the invention, the resonator 
comprises a monolithic block. This embodiment is particularly suited for 
maintaining the stability requirements mentioned above. 
In another embodiment of the invention the nonlinear medium comprises a 
quasi-phase matched crystal. This embodiment provides a particular 
nonlinear medium for generating output waves at desired wavelengths. 
In a further embodiment of the invention the cavity length controlling 
means comprise means for changing an optical path length of the resonator 
by a controlled amount. Additional means are provided for adjusting a 
phase matching efficiency of the resonator in response to a change in the 
cavity length to maximize the power conversion efficiency of the system. 
This embodiment has the advantage of permitting smooth tuning of the OPO 
frequencies over large ranges. 
In a further advantageous embodiment, the system comprises a 
frequency-stable reference and means for comparing the frequency of one of 
the first and second parametrically generated beams with the 
frequency-stable reference. Comparison with the reference permits feedback 
control to tune the system for emission with stable frequency. 
It is advantageous when the resonator has high transmission for the pump 
wave, and when an electro-optic medium is disposed within the resonator 
with means for applying an electric field to the medium, wherein the 
independent parameters comprise a pump frequency, a temperature of the 
nonlinear medium, the electric field, and a part of a round trip optical 
path length of the first parametrically generated wave external to the 
nonlinear medium. Electro-optic control of the optical path length permits 
fast tuning of the frequencies. 
In an advantageous embodiment, the resonator comprises mirrors for the 
first and second parametrically generated and the pump waves, and the 
resonator has low loss for the pump wave, with the pump wave being 
resonantly enhanced between the mirrors, with means for maximizing a 
circulating pump power through control of the pump frequency. Resonating 
the pump wave reduces the pump laser power necessary to achieve threshold. 
Locking the pump wave to the resonator is advantageous if the pump laser 
has low frequency stability. 
In an additional embodiment, the resonator comprises mirrors for the first 
and the second parametrically generated and the pump waves, the resonator 
has low loss for the pump wave and the pump wave is resonantly enhanced 
between the mirrors with means for detecting a detuning of the pump wave 
from resonance and means for controlling an optical path length of the 
resonator to maximize the circulating pump power. This embodiment is 
favourable because, in the case of a frequency-stable pump, some of the 
frequency-stability is transferred to the optical path length of the 
resonantly parametrically generated wave, leading to good frequency 
stability of both parametrically generated waves. 
In various embodiments stabilization uses a probe wave as claimed. The 
general advantage of these techniques is that they can be employed to 
generate frequency-stable output with higher frequency-stability than that 
of the pump wave or to improve the frequency stability of the output in 
case of a non-resonant pump wave. Using the harmonic of the pump as a 
probe has the advantage that, for a widely tunable device, the mirrors 
need to have low loss for a probe wave of only a single wavelength. The 
advantage of using the second harmonic of the second parametrically 
generated wave (if it is the longer wavelength one) is that its wavelength 
can fall in the range of the wavelengths of the first wave, so that there 
is no need for low resonator loss at a wavelength not already covered. 
If the probe is the polarization-rotated resonantly generated parametric 
wave, stabilization is achieved without requiring the resonator mirrors to 
have low loss at a wavelength not already covered and no additional 
nonlinear medium is necessary for frequency doubling. 
In an additional preferred embodiment, the resonator consists essentially 
of a semi-monolithic resonator comprising a quasi-phase matched 
multigrating medium for second order nonlinear optical frequency 
conversion, an external concave mirror having a mirror coating on a curved 
surface thereof, and a mirror coating on one end face of the multigrating 
medium and the end face with the mirror is flat. This embodiment has the 
advantage of being particularly simple and provides for a simple tuning of 
the resonator system and particularly stable operation. 
In a highly preferred embodiment, the resonator consists essentially of a 
Brewster angle cut resonator comprising at least one external concave 
mirror having a radius-of-curvature equal to a distance between an exit 
point out of a Brewster angle surface of the nonlinear medium and a curved 
reflecting surface of the external concave mirror, wherein the nonlinear 
medium has at least one Brewster angle surface to minimize 
Fresnel-reflection losses for waves having a polarization vector parallel 
to a plane of incidence, with waves of different wavelengths propagating 
colinearly within the nonlinear medium. This embodiment has the advantage 
of allowing for compensation of the dispersion of the different frequency 
beams exiting out of the Brewster angle cut and reflecting the beams such 
that they optimally overlap inside the nonlinear medium. A simple 
configuration is therefore achieved, wherein a focus element is provided 
within the nonlinear medium for stable resonator modes. 
In an embodiment of this particular improvement, the focussing element 
consists essentially of a curved surface having a mirror coating. 
In an alternative variation of this improvement, a curved surface having 
total internal reflection is utilized as a focussing element. This 
embodiment has the advantage of not requiring a mirror coating on the 
nonlinear medium. 
Additional improvements and advantages of the invention can be derived from 
the accompanying drawings. The features which can be extracted from the 
claims and drawings can be used, in accordance with the invention, 
individually or collectively in arbitrary combination. The drawings have 
exemplary character only and are not to be considered exhaustive 
embodiments of inventive configurations.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a pump-resonant SRO system in accordance with the invention 
comprising a pump source 1 producing a pump wave 9. The pump wave 9 is 
incident on an optical isolator system 2, passes through same and enters 
into a monolithic singly-resonant oscillator (SRO) 7. The singly-resonant 
oscillator 7 produces a signal wave 8 as well as a idler wave 10. The pump 
wave resonates in the SRO cavity. A portion of the pump wave 9 reflects 
back into isolator 2 and is sent to detector 13. An amplitude modulation 
signal due to detuning of the pump frequency from resonance is demodulated 
using a mixer 4 and a local oscillator 6 that also phase-modulates the 
pump wave 9. After filtering and amplification a correction signal 6a is 
fed into the pump source 1 for regulation of its frequency on resonance 
with the SRO 7. 
The SRO 7 of FIG. 1 is coupled to temperature control means 11 as well as 
tuning control means 12 to stabilize the temperature of the SRO and its 
optical path length to a level where mode-hops are suppressed. 
Frequency-tuning of wave 8 and 9 is achieved by changing the medium 
temperature by a controlled amount. 
FIG. 2 shows an alternative embodiment of the SRO system in accordance with 
the invention comprising a pump source 20 producing a pump wave 21. In 
this embodiment, means for controlling the frequency of the pump wave 21 
are intrinsically located within pump source 20. The pump wave 21 passes 
into a SRO system 22 comprising a first reflector 23, a second reflector 
25 and a nonlinear medium 24. The pump wave 21 enters into the nonlinear 
medium 24 to generate a signal wave 26 as well as an idler wave 27. The 
idler wave 27 is essentially transmitted through second reflector 25 to be 
externally available for further spectrographic use while signal wave 26 
passes it in part. In the embodiment of FIG. 2, stabilization and/or 
optimization of the system is effected through monitoring of the idler 
wave 28. A portion of the idler wave is reflected by mirror 28 to detector 
system 29 including signal processor means. The resulting output of the 
detector signal processor 29 is fed to a tuning control system 30. The 
detector system 29 could be a power monitor, in which case the temperature 
control changes the medium's temperature when the tuning control changes 
the length of the cavity to tune the output frequencies of waves 26, 27. 
The temperature of medium 24 is regulated to maximize the detected power. 
The detector 29 could also contain an external frequency reference such as 
a stable optical cavity, atomic ensemble or the like representing a 
constant frequency. The information concerning the detuning between 
reference and idler frequency is evaluated in tuning controller 30. The 
tuning controller 30 thereby outputs signals to temperature controller 32 
and mirror position controller 33 respectively. The mirror control system 
33 feeds back the control signal to a positioner 34 to adjust the length 
of the cavity. Such adjustments can be performed with short time constants 
for rapid response to detuning. The temperature control system 32 can 
provide for longer term, slower changes in the operating conditions of the 
system. 
A particularly preferred embodiment is shown in FIG. 3. In the embodiment 
according to FIG. 3, a pump source 40, comprising a Nd:YAG laser, outputs 
a pump wave 42 into a Faraday isolator 41. In this embodiment, means for 
controlling the frequency of the laser 40 are intrinsically located 
therein. The output from the Faraday isolator 41 is incident on dichroic 
mirror 43 and enters into a PPLN chip (periodically-poled lithium 
niobate). Translator means 47 can be used to move PPLN chip 44 from one 
grating to another. The PPLN chip 44, in response to the pump wave 42, 
generates signal and idler waves. The cavity is resonant only for the pump 
and signal waves, which are reflected from mirror 45 as well as mirror 46. 
The optical properties of the system and, in particular of the pump and 
signal waves, can be monitored in an external spectral analyzer 51. The 
pump, signal, and idler waves pass through beam spitter 50 to be incident 
on reflector 54 and into a Fabry-Perot interferometer 53 comprising an 
external detector 52. Measurement of the output characteristics and power 
of the signal and idler waves exiting as output beams 58, can be monitored 
by means of a dichroic mirror 59 directing signal and idler waves onto 
thermopile 60. The stability of the system in the embodiment of FIG. 3 is 
maintained by monitoring the reflected pump wave 42 exiting out of the 
oscillator and feeding same from the beam splitter 50 onto a detector 55. 
Means can be provided for branching off the signal wave from the pump wave 
prior to the detector 55 as schematically indicated in FIG. 3. Detector 55 
signals lock 56 which communicates with piezo 57 to stabilize the length 
of the cavity. 
In a particular configuration of the embodiment of FIG. 3 a diode pumped 
miniature Nd:YAG ring laser is used, having a single frequency output 
power of 800 mW at 1064 nm with a linewidth of 1 Kilohertz and continuous 
tunability of 10 GHz. The SRO comprises fundamental reflector elements 45, 
PPLN multigrating chip 44 and external reflector 46 and is a single cavity 
resonant system configured as a semi-monolithic linear standing wave 
resonator. The external mirror 46 is separated by 16 mm from the chip 44 
and the PPLN crystal 44 has the dimensions of 19 mm.times.11 mm.times.0.5 
mm with eight different gratings having periodicity lengths varying from 
30 to 31.2 .mu.m. One of the plane chip end faces 45 is coated with a 
broad-band dichroic mirror providing reflectivities of 92% for the pump 
(1064 nm) and average values of 99.7% for the signal (1.66-2 .mu.m) and 3% 
for the idler (2.3-3 .mu.m). An anti-reflection coating with residual 
reflectivities of 0.3%, 0.8% and 3% at the pump, signal, and idler waves 
respectively, is deposited on the other chip face. The external mirror 46 
has a 25 mm radius-of-curvature and is mounted to a piezo transducer 57. 
The TEM.sub.00 cavity mode has a waist of 29 .mu.m providing optimal 
nonlinear coupling for the given resonator geometry and crystal length. 
The pump was spatially mode matched to the fundamental resonator mode with 
an efficiency of 98%. The reflectivities of the external mirror at the 
pump, signal, and idler waves are 99.7%, 99.8% and 5% respectively on the 
curved surface, whereas the back face is uncoated. The total round-trip 
losses for the pump, signal, and idler waves are A.sub.p =10%, A.sub.s 
=2.5% and A.sub.i =99.9% respectively. The last value ensures 
singly-resonant operation. For an SRO cavity that is highly transmitting 
for the idler wave at both mirrors, an internal threshold power 
P.sub.th.sup.int =A.sub.s /2E.sub.NL =8.6 W is estimated, with a 
calculated single-path nonlinearity E.sub.NL of 1.45/kW, assuming an 
effective nonlinear coefficient d.sub.eff =15 pm/V (first order 
quasi-phase matching). A pump power enhancement of 32 is deduced from a 
measured finesse of 63 and an incoupling of 65% for the pump wave below 
threshold. 
Particularly good stabilization of the pump wave is achieved by locking the 
cavity length on resonance with the laser frequency. This is done through 
frequency modulation of the pump wave by modulating the laser crystal 
piezoelectrically with a 10 MHz signal (50 mV peak-to-peak voltage). The 
pump wave reflected from the SRO cavity is detected with a sensitive 
InGaAs photodiode to obtain an error signal through mixing the AC detector 
signal with the modulation frequency and by subsequent low-pass filtering. 
This error signal is input to the piezo to shift the external cavity 
mirror using a proportional integral servo controller. Use of the 
reflected light for stabilization is important since the transmitted pump 
wave undergoes optical limiting above threshold to cause an error signal 
which does not allow for stabilization of the reflected light at zero 
detuning. The pump wave remained stably locked for more than 50 h with 
less than 2% power fluctuations. A minimum external threshold power 
P.sub.th.sup.ext =260 mW results at a signal wavelength of 1.7 .mu.m. This 
corresponds to an internal pump wave power of 8.3 W. 
Further description of the embodiment of FIG. 3 can be found in Opt. Lett., 
volume 22, number 17, p. 1293-1295, (1997), the complete disclosure of 
which is hereby incorporated by reference. 
FIG. 4a shows another preferred embodiment in accordance with the invention 
comprising a pump source 70 generating a laser beam 71. The laser beam 71 
from the pump source 70 is fed through an optical isolator 72 and is 
incident on a second harmonic generator 73. The second harmonic generator 
73 comprising a nonlinear crystal 76, mirror 77, detector 75, servo 74 and 
piezo 78 to frequency-double the incident laser beam. The output of the 
second harmonic generator 73 is incident upon a dichroic mirror 78a and 
reflected in the form of pump wave 79 onto isolator 80. The pump wave 79 
passing through isolator 80 is incident upon dichroic mirror 81 and passed 
into a nonlinear medium 84. The nonlinear medium 84 has a reflecting 
surface 85 at one end and a Brewster surface 86 at the other end. The beam 
fractions passing out of the Brewster surface 86 are split into three 
portions corresponding to the pump wave, the idler wave and the signal 
wave and are incident upon external mirror 87. External mirror 87 has a 
radius-of-curvature equal to the distance between its reflecting surface 
and the exit point out of the Brewster surface 86 to refocus the split 
beams back into the nonlinear medium 84. The beams travel colinearly and 
coincidently within the nonlinear medium 84. Reflecting surface 85 can be 
structured to focus the beams within the medium 84. A second portion of 
the pump wave 79 is passed to detector 89 for generating a signal for 
servo 90 to control piezo 88 and the resonant length of the oscillator 
system. The output beam from the system is passed through dichroic mirror 
81 and is externally available as signal wave 82 and idler wave 83. 
An alternative embodiment of the nonlinear Brewster angle medium of FIG. 4a 
is given in FIG. 4b. In the embodiment of FIG. 4b, nonlinear medium 95 is 
fashioned with a focussing element surface 96. Internal beams 97 and 105 
are incident on Brewster surfaces 97a and 97b respectively. The idler and 
signal waves are split into two waves 98, 99, after passage through the 
first Brewster surface 97a, and are incident upon reflecting mirror 100. 
The reflecting mirror 100 has a radius-of-curvature equal to the distance 
between the output point at the external Brewster surface 97a and the 
mirror surface to refocus first 98 and second 99 external beams back into 
the nonlinear medium 95. The second portion of the beam 105 exits out of 
the nonlinear medium 95 through second Brewster surface 97b, is split into 
third and fourth beams 101, 102 and is incident upon a second concave 
mirror 103. Like mirror 100, mirror 103 has a radius-of-curvature equal to 
its separation from the exit point of the two beams 101, 102 out of the 
Brewster surface 97b to refocus the beams 101, 102 back into the nonlinear 
medium 95. 
In a particular embodiment of FIG. 4a, a miniature Nd:YAG ring laser 70 is 
used as a primary source of the system, delivering a maximum output power 
of 1.5 Watt at 1064 nm with a linewidth of 1 kHz and a frequency 
instability of about 10 MHz/h. The laser frequency is continuously tuned 
through 10 GHz by temperature control of the Nd:YAG crystal. The laser 
beam 71 is frequency-doubled in the external resonator 73 to produce a 
maximum output power of 1.1 Watt at 532 nm. The SRO is a standing wave 
monolithic cavity containing a 7.5 mm long MgO:LiNbO.sub.3 crystal 84 
(type-I phase matching). The cavity design is adapted to provide low loss 
for the p-polarized signal wave and good overlap of signal, idler, and 
pump waves within the crystal 84 over a wide tuning range. The first 
property is implemented by using a crystal cut at Brewsters angle 
(65.9.degree.) for the center signal wavelength. The transmission loss for 
the signal wave remains low over a relatively wide tuning range. The 
dispersion change of signal, idler, and pump waves is compensated by means 
of an external cavity mirror 87 placed at a distance equal to its 
radius-of-curvature of 25 mm from the exit point on the Brewster face 86. 
In this fashion, waves exiting at any angle are retroreflected to assure 
colinear propagation and good overlap of the three waves inside the 
crystal 84. This geometry requires a focussing mirror 85 at the other end 
of the crystal 84 to obtain a stable resonator mode for pump and signal. 
The crystal 84 can be configured with a 10 mm spherically polished end 
face which is dielectrically coated with average reflectivities of 92%, 
99.5%, 2% for the pump, signal, and idler waves respectively. The range 
where the reflectivity drops from 98% to 5% extends from 1040 to 1085 nm. 
The external mirror 87, mounted on a PZT 88 for cavity length locking, 
provides average reflectivities of 98% for the pump, 99% for the signal 
and 90% for the idler. A simple AR-coating is added to the Brewster face 
86 to reduce pump wave losses. SRO operation is ensured by a total 
round-trip power loss of more than 98% for the idler wave. The pump waist 
is 18 .mu.m leading to a calculated single pass nonlinearity E.sub.NL 
=1.5/kW. (An effective nonlinear coefficient d.sub.eff =4.7 pm/V has been 
assumed.) The expected internal threshold for the SRO with double-passed 
idler is P.sub.th =A.sub.S /4E.sub.NL =3.3 W for a round trip signal loss 
A.sub.s =2%. The expected external threshold is reduced to 0.15 W by the 
pump wave enhancement factor measured to be 22. Oscillation occurred at 
pump powers above 200 mW and stable operation was ensured by locking the 
cavity length on resonance with the pump frequency and a pump wave phase 
is modulated within the nonlinear crystal 84. Since the transmitted pump 
wave undergoes optical limiting, the reflected pump light is used to 
generate an appropriate error signal to lock on zero detuning of the pump 
wave. A maximum total conversion efficiency to signal plus idler of 33% is 
obtained at an input pump power of 300 mW. 
Further disclosure of this particular embodiment can be found in Appl. 
Phys. B 65, 775-777 (1997), the complete disclosure of which is hereby 
incorporated by reference. 
FIG. 5 shows a system in accordance with the invention, wherein a 
frequency-stable pump source 101 emits a pump wave which is focused by 
lens 113 into the resonator. The pump wave is essentially transmitted by 
both mirrors 109 and 108 and thus does not resonate in the resonator. 
Signal wave 111 and idler wave 112 are generated in medium 106, wherein 
the idler 112 is essentially transmitted through mirror 108. In a second 
nonlinear medium 107, the second harmonic 114 of the idler wave is 
generated and resonantly enhanced between the mirrors 108 and 109 which 
have high reflectivity for the wavelength corresponding to wave 114. The 
portion of wave 114 transmitted through mirror 109 and travelling back 
toward the pump source is reflected by dichroic mirror 102 and detected at 
detector 103. The phase modulation produced by the radiofrequency source 
115 that electro-optically modulates medium 107 is converted into 
amplitude modulation on wave 116 if wave 114 exhibits a detuning with 
respect to the cavity resonance. The amplitude modulation is converted 
into an error signal in servo system 104 which, after amplification, is 
fed to the actuator 105 which moves mirror 109 to keep the wave 114 in 
resonance. The frequency-stability of the emitted waves 111, 112 is 
thereby enhanced.