# US 8,542,435 B2 · Nested-cavity optical parametric oscillator for fast frequency tuning

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- Number: US 8,542,435 B2
- Document ID: US8542435B2
- Status: Granted
- Kind: B2
- Application number: 13/426,522
- Filed: 2012-03-21
- Published: 2012-10-18
- Granted: 2013-09-24
- Earliest claimed: 2011-03-25
- Assignees: Office National d'Etudes et de Recherches Aerospatiales (O.N.E.R.A.)
- Applicants: Myriam Raybaut; Bertrand Hardy; Antoine Godard; Michel Lefebvre
- Inventors: Myriam Raybaut; Bertrand Hardy; Antoine Godard; Michel Lefebvre
- Web page: https://patentgrep.com/patent/US8542435B2

## CPC

- G02F 1/3544: {Particular phase matching techniques}
- G02F 1/39: for parametric generation or amplification of light, infrared or ultraviolet waves
- G02F 1/3509: {Shape, e.g. shape of end face}
- G02F 1/3542: {Multipass arrangements, i.e. arrangements to make light pass multiple times through the same element, e.g. using an enhancement cavity}
- G02F 1/3548: {Quasi phase matching [QPM], e.g. using a periodic domain inverted structure}
- G02F 2203/15: involving resonance effects, e.g. resonantly enhanced interaction
- G02F: OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS

## Abstract

Nested-cavity optical parametric oscillator for fast frequency tuning including a source of pump radiation (fp), a non linear crystal situated within two resonant cavities for the signal and complementary radiations of respective optical lengths ls and lc. A doubly resonant resonator for a single pair of longitudinal signal and complementary modes is formed having a crystal of prismatic shape, the crystal (7) being moveable within the plane xz along a direction forming a non zero angle β with respect to the direction x. The value of β is fixed so that the movement of the crystal is accompanied by a change in the respective optical lengths ls and lc of the two resonant cavities in a ratio such that the double resonance condition between the longitudinal modes of each cavity is maintained.

## Claims

### Claim 1

A doubly resonant optical parametric oscillator with achromatic phase-maintaining pump reflection comprising:
a pump radiation source at a frequency fp, and comprising in the forward propagation direction of the pump radiation:
a non linear crystal having a front face and a rear face,
an achromatic mirror situated downstream of the rear face of the crystal, defining with a first mirror situated on the front face of the crystal,
a first resonant cavity for the radiation signal at the frequency fs and with a second mirror situated upstream of the front face of the crystal,
a second resonant cavity for the complementary radiation at the frequency fc, the two cavities, of respective optical lengths ls and lc, forming a doubly resonant resonator for a single pair of longitudinal signal and complementary modes, the pump radiation at the frequency fp makes through the crystal a forward path entering via the front face of the crystal and exiting via the rear face of the crystal, then, after reflection on the achromatic mirror,
a backward path entering via the rear face of the crystal, the optical indices of which are respectively ns and nc at the signal fs and complementary fc frequencies, the crystal is wedged with its rear face inclined along a non zero angle α with respect to the direction x orthogonal to the direction z of propagation of the radiations, the crystal being moveable in the plane xz,
wherein the crystal is moveable in translation in the plane xz along a direction inclined by a non zero angle b with respect to the direction x such that
tg
⁢
⁢
β
=
(
n
c
-
1
)
+
R
·
(
n
s
-
1
)
R
⁢
tg
⁢
⁢
α
,
with
R
=
(
f
s
f
c
⁢
l
c
l
s
)
,
to modify simultaneously the respective optical lengths ls and lc of the two resonant cavities in a ratio such that the double resonance for a single pair of longitudinal signal and complementary modes is maintained, and as a result the values of the frequencies fs and fc are continuously tunable over a wide range.

### Claim 2 (depends on 1)

A doubly resonant optical parametric oscillator according to claim 1, wherein the non-linear crystal comprises periodic inversion domains traversed successively by the radiations along the direction z making it possible to operate in quasi-phase matching.

### Claim 3 (depends on 2)

A doubly resonant optical parametric oscillator according to claim 2, wherein the rear face of the last periodic domain traversed before reflection on the achromatic mirror is oriented along a plane intersecting the plane xy, along a straight line parallel to the direction y, and as a result the relative phase value ΔΦ between the waves is maintained whatever the position of the crystal in the direction x.

### Claim 4 (depends on 2)

A doubly resonant optical parametric oscillator according to claim 2, wherein the periodicity of the inversion domains of the crystal varies continuously in the direction x.

### Claim 5 (depends on 4)

A doubly resonant optical parametric oscillator according to claim 4, wherein the variation in the periodicity along the direction x is such that the maximum of the parametric gain curve shifts in a synchronous manner with the double resonance ratio.

### Claim 6 (depends on 3, 4)

A doubly resonant optical parametric oscillator according to claim 3 or claim 4, wherein the rear face of the last periodic domain is polished so that it forms an angle γ with respect to the direction y in any section along a plane parallel to the plane yz, and as a result the value of the phase relative to the return (ΔΦ) between the waves may be adjusted by modification of the position along the direction y of the crystal in the optical cavity.

### Claim 7 (depends on 2)

A doubly resonant optical parametric oscillator according to claim 2, wherein the first domain is of variable optical length and as a result it is possible to compensate the optical dispersion of the crystal and the variation in the optical lengths of the cavities to obtain continuous tuning over a frequency range of more than one THz.

### Claim 8 (depends on 7)

A doubly resonant optical parametric oscillator according to claim 7, wherein the complementary wave is polarised along the crystallographic axis c of the crystal and the signal and pump waves being polarised orthogonally to this axis c oriented along the direction y, an electric field is applied along the axis c to adjust the optical length of the complementary resonant cavity in order to compensate the optical dispersion between the three radiations and the variation in the ratio R.

### Claim 9 (depends on 1)

A doubly resonant optical parametric oscillator according to claim 1, wherein the frequency of the pump laser is tunable and, as a result, it is possible to compensate the optical dispersion of the crystal and the variation in the optical lengths of the cavities, to obtain a continuous tuning over a frequency range of more than one THz.