WO2012126495A1 - Optical parametric oscillator comprising two nonlinear crystals - Google Patents

Optical parametric oscillator comprising two nonlinear crystals Download PDF

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Publication number
WO2012126495A1
WO2012126495A1 PCT/EP2011/054130 EP2011054130W WO2012126495A1 WO 2012126495 A1 WO2012126495 A1 WO 2012126495A1 EP 2011054130 W EP2011054130 W EP 2011054130W WO 2012126495 A1 WO2012126495 A1 WO 2012126495A1
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Prior art keywords
nonlinear crystal
signal
wave
parametric oscillator
optical parametric
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PCT/EP2011/054130
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French (fr)
Inventor
Ebrahim-Zadeh MAJID
Goutam Kumar SAMANTA
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Institut De Ciencies Fotoniques, Fundacio Privada
Institució Catalana De Recerca I Estudis Avançats
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Priority to PCT/EP2011/054130 priority Critical patent/WO2012126495A1/en
Publication of WO2012126495A1 publication Critical patent/WO2012126495A1/en

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    • G—PHYSICS
    • G02—OPTICS
    • 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
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/39—Non-linear optics for parametric generation or amplification of light, infrared or ultraviolet waves
    • G—PHYSICS
    • G02—OPTICS
    • 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
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/353—Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
    • G02F1/3532—Arrangements of plural nonlinear devices for generating multi-colour light beams, e.g. arrangements of SHG, SFG, OPO devices for generating RGB light beams
    • G—PHYSICS
    • G02—OPTICS
    • 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
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/355—Non-linear optics characterised by the materials used
    • G02F1/3558—Poled materials, e.g. with periodic poling; Fabrication of domain inverted structures, e.g. for quasi-phase-matching [QPM]
    • G—PHYSICS
    • G02—OPTICS
    • 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
    • G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
    • G02F2201/16—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 series; tandem
    • G—PHYSICS
    • G02—OPTICS
    • 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
    • G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
    • G02F2201/17—Multi-pass arrangements, i.e. arrangements to pass light a plurality of times through the same element, e.g. by using an enhancement cavity
    • G—PHYSICS
    • G02—OPTICS
    • 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
    • G02F2203/00—Function characteristic
    • G02F2203/05—Function characteristic wavelength dependent
    • G02F2203/055—Function characteristic wavelength dependent wavelength filtering

Definitions

  • the present invention has its application within the light sources sector and, especially, in the areas engaged in providing optical oscillators.
  • An Optical Parametric Oscillator is a light source comprising an optical resonator (also called a resonant cavity) and a nonlinear crystal pumped by a laser.
  • the nonlinear crystal receives an input laser wave (called pump signal, with a frequency w pump ) and converts said input into two output waves of lower frequency (signal wave, with frequency w si g na i; and idler wave, with frequency w id ier), by means of a nonlinear optical interaction known as parametric down-conversion.
  • a part of the pump signal is undepleted in the nonlinear crystal and also appears at the output of the crystal.
  • the pump signal In order to allow the signal wave to oscillate, the pump signal needs to exceed a certain power threshold, so the subsequent gain experienced by the signal wave is greater than the losses associated with its propagation inside the cavity. Once the gain exceeds this threshold, further increases in the pump power generate a greater gain, and thus, a greater output power. Also, for pump powers above the threshold, the signal wave and the idler wave present a high degree of correlation and coherence.
  • OPOs with a single nonlinear crystal are known, such as those disclosed in US 2007/0291801 A1 or US US'005/0243876 A1 .
  • these OPOs only have one nonlinear medium, only a single pair of signal wave and idler wave can be generated.
  • the optical power threshold required to generate said pair is directly determined by the threshold associated with a single nonlinear crystal.
  • OPOs comprising multiple nonlinear crystals
  • US 5,1 17,126 A discloses a stacked OPO which comprises two nonlinear crystals disposed coaxially in a single resonator, so the complete output of the first nonlinear crystal (signal wave, idler wave and residual pump signal) acts as a feed of the second nonlinear crystal.
  • the wavelength of the signal wave generated by each nonlinear crystal is independently tuned for example, by means of the angular orientation of the crystals.
  • each crystal can also be adjusted by different techniques, such as shortening the length of the first crystal, beveling said crystal with respect to the optical axis of the resonator, or by detuning the output coupling mirror with respect to the output of the first crystal.
  • the wavelength and power of the signal and idler wave generated by the second crystal are always limited by the output signals of the first crystal, and also, the signal waves generated by the two crystals are bound to present the same temporal regime (continuous or pulsed).
  • the optimum phase for parametric amplification namely
  • optical parametric oscillators capable of generating two or more signal waves whose wavelength, power, and temporal regime can be independently controlled, and with a self-adjusted optimum phase for the parametric amplification.
  • the current invention solves the aforementioned problems by disclosing an optical parametric oscillator (OPO) with at least two nonlinear crystals, in which the idler wave of the first nonlinear crystal is extracted from the resonant cavity before it reaches the second nonlinear crystal, avoiding any phase constraint at the input of the second crystal.
  • OPO optical parametric oscillator
  • the disclosed OPO comprises:
  • -Wavelength discrimination means that extract the idler wave generated by each crystal before it reaches the other crystal, while keeping the signal waves resonant inside the optical cavity.
  • each crystal receives a different pump signal.
  • the fraction of the pump signal which is still undepleted after feeding one crystal, is removed from the resonator by the wavelength discrimination means. This way, a greater independence between crystals is achieved, as each pump can be controlled separately.
  • a single pump is used.
  • the pump signal serves as an input of the first nonlinear crystal, while the second nonlinear crystal is fed by the remainder of said pump signal, and by the signal wave generated by the first crystal. Notice that even in the case of a single pump, the idler wave generated at one crystal is still removed from the resonator before it reaches the next crystal.
  • the wavelength discrimination means that extract the idler wave of the first crystal also keep the pump signal inside the resonator. The pump signal is not extracted from the resonator until after the output of the last crystal.
  • each crystal presents an independent pump
  • either a single laser source or multiple laser sources can be employed.
  • a single laser source is divided into two arms, each of them feeding a crystal.
  • the pump power and polarization may be controlled independently.
  • each laser source feeds one of the crystals, thus allowing to use any combination of continuous and pulsed light to pump the system, and therefore, to generate continuous signal waves, pulsed signal waves, or a combination of both. Even if both pump signals are pulsed, different pulse durations and widths can be applied to each pump.
  • the OPO also comprises tuning means that select the wavelength of each pair of idler wave and signal wave at each crystal independently of the wavelengths present at the other crystal (or crystals).
  • tuning means include temperature control, rotation control, and grating period control.
  • Figure 1 shows a diagram of an optical parametric oscillator with two nonlinear crystals and a single laser source divided into two pump signals, according to a preferred embodiment of the invention.
  • Figure 2 presents another OPO with two pump signals, in this case, from two different laser sources.
  • Figure 3 shows yet another embodiment with a single pump signal for both crystals.
  • FIG. 1 shows the architecture of an OPO according to a preferred embodiment of the present invention, in which each nonlinear crystal receives a different pump signal.
  • the OPO comprises an optical resonator 2 with two nonlinear crystals 4 and 4'.
  • the first nonlinear crystal 4 is fed by a first pump signal P and generates a first idler wave I and a first signal wave S.
  • the second nonlinear crystal 4' is fed by a second pump signal P' and generates a second idler wave and a second signal wave S'.
  • the first nonlinear crystals 4 and 4' may be birefringently-phase-matched (BPM) or quasi-phase-matched (QPM) materials.
  • the first nonlinear crystal 4 is located between two mirrors M1 and M2 with different transmission coefficients depending on the incident wavelength which therefore act as wavelength discrimination means.
  • the first mirror M1 located between a laser source 1 and the first nonlinear crystal 4, is highly transmitting for the first pump signal P, while it reflects signal waves S and S'.
  • the second mirror M2, located after the nonlinear crystal 4 is also highly transmitting for the first pump signal P and highly reflecting for the signal waves S and S', but it is also highly transmitting for the first idler wave I. Therefore, at the second mirror M2, the first pump signal P and the first idler wave I are extracted from the resonator 2 while the signal waves S and S' are maintained inside the resonator 2 and are thus able to oscillate.
  • the signal waves S and S' after being reflected from the second mirror M2, are directed towards a third mirror M3, which also reflects the signal waves S and S' and focuses them to the input of the second nonlinear crystal 4'.
  • the third mirror M3 also serves as an entry point of the second pump signal P' inside the oscillator and therefore is highly transmitting to the wavelength of said second pump signal P'.
  • the fourth mirror M4 is also highly transmitting to the wavelengths of the second pump signal P' and the second idler wave ⁇ , extracting them from the resonator 2; and highly reflecting to the wavelengths of the signal waves S and S'. Note that the architecture of the OPO ensures that both signal waves S and S' travel in the same direction inside the resonator 2.
  • the first idler wave I is separated from the first pump signal by means of a fifth mirror M5, while the second idler wave and the second pump signal P' are separated by means of a sixth mirror M6.
  • the wavelengths of the signal waves S and S' are controlled independently with no phase matching limitation between them. That is, the wavelength of the first signal wave S and the first idler wave I are tied by the phase matching condition and energy conservation condition, but the first signal wave S and the second signal wave S' can be selected independently. Consequently, the first idler wave I and the second idler wave can be selected independently. Also as the first idler wave is removed from the resonator 2 before reaching the second nonlinear crystal 4', the optimal phase for the parametric amplification is self-adjusted.
  • the wavelength of a signal wave (and thus of the associated idler wave) is controlled independently from the wavelength of the other signal wave (and thus of the associated idler wave).
  • the OPO also comprises power control means 3 to control the power (and polarization) of the pump signals P and P'.
  • the light emitted by the laser source 1 is divided by a first polarizing beam-splitter cube 5 after passing through a first half-wave-plate H1 , dividing the laser output into two arms with a variable power ratio.
  • the first arm (the one associated to the first pump signal P), also comprises a second half-wave-plate H2 to choose the polarization of the first pump signal P and a first convergent lens 6 that focalises the first pump signal P at the input of the first nonlinear crystal 4.
  • the second arm comprises a third half-wave-plate H3 and a second polarizing beam-splitter cube 5' to control the power of the second pump signal, a fourth half-wave-plate H4 to control its polarization, as well as a second lens 6' to focalize it at the input of the second nonlinear crystal 4'.
  • a third half-wave-plate H3 and a second polarizing beam-splitter cube 5' to control the power of the second pump signal
  • a fourth half-wave-plate H4 to control its polarization
  • a second lens 6' to focalize it at the input of the second nonlinear crystal 4'.
  • CARS Coherent Anti-Stokes Raman Scattering
  • FIG 2 an example of an architecture with two laser sources is presented.
  • the first laser source 1 feeds directly the first arm, while the second laser source 1 ' feeds the second arm.
  • the power and polarization control is also performed by a combination of half-wave-plates and polarizing beam-splitter-cubes as explained for the case with a single laser source.
  • each laser source is able to work in a different regime, that is, both laser sources may be continuous wave (CW), both pulsed, or one of them pulsed while the other one is continuous. Even if both of the laser sources are pulsed, it is possible to use laser sources with different pulse duration and width.
  • CW continuous wave
  • both of the laser sources are pulsed, it is possible to use laser sources with different pulse duration and width.
  • a CW pump may be applied to generate narrowband signal and idler waves at the first nonlinear crystal 4.
  • a pulsed laser source may be used for the second pump signal P', resulting in broadband pulsed signal and idler waves. Note that there is no restriction in the duration of the laser pulses (microscond, nanosecond, picosecond, femtosecond).
  • the seeding of the pulsed broadband signal and idler waves by the narrowband CW signal wave results in line-narrowing of the broadband pulsed radiation from the second nonlinear crystal 4'.
  • This scheme can be applied to achieve line narrowing and single-frequency generation of pulsed radiation seeded by CW narrowband radiation.
  • figure 3 shows another embodiment in which a single pump signal P is used.
  • the second mirror M2 is highly reflecting at the wavelength of the pump signal P, and thus, the second nonlinear crystal 4' is fed by the fraction of the pump signal P undepleted by the first nonlinear crystal 4.
  • the pump signal P leaves the resonator 2 at the fourth mirror M4.
  • first idler wave I is extracted from the resonator 2 at the second mirror M2
  • mirror M4 is highly transmitting for the second idler wave , so the optimal phase for the parametric amplification is self-adjusted at the first nonlinear crystal.
  • the bow-tie ring disclosed in the previous examples also has the advantage of having the crystals in separate arms, and refocusing the beams between successive trips through the crystals.

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

An optical parametric oscillator comprising two nonlinear crystals (4, 4') in a same resonant cavity (2), which also comprises wavelength selective means (M2, M4) to extract from the resonant cavity idler waves (I, I') generated by each nonlinear crystal (4, 4'). The disclosed oscillator is fed either with a single pump (P), or with two pumps (P, P'), being able to generate two independent signal waves (S, S') with tuneable wavelengths and power.

Description

OPTICAL PARAMETRIC OSCILLATOR COMPRISING TWO NONLINEAR
CRYSTALS
D E S C R I P T I O N
FIELD OF THE INVENTION
The present invention has its application within the light sources sector and, especially, in the areas engaged in providing optical oscillators.
BACKGROUND OF THE INVENTION - RELATED ART
An Optical Parametric Oscillator (OPO) is a light source comprising an optical resonator (also called a resonant cavity) and a nonlinear crystal pumped by a laser. The nonlinear crystal receives an input laser wave (called pump signal, with a frequency wpump) and converts said input into two output waves of lower frequency (signal wave, with frequency wsignai; and idler wave, with frequency widier), by means of a nonlinear optical interaction known as parametric down-conversion. The frequency of the input and output waves is determined by wpump= wsignai + widier. Typically a part of the pump signal is undepleted in the nonlinear crystal and also appears at the output of the crystal.
In order to allow the signal wave to oscillate, the pump signal needs to exceed a certain power threshold, so the subsequent gain experienced by the signal wave is greater than the losses associated with its propagation inside the cavity. Once the gain exceeds this threshold, further increases in the pump power generate a greater gain, and thus, a greater output power. Also, for pump powers above the threshold, the signal wave and the idler wave present a high degree of correlation and coherence.
In OPOs, wavelengths of the output waves can be varied over a wide range by altering the phase-matching condition, kpump= ksignai + kidier, of the nonlinear optical crystal, where kpump,signai,idier are the wavevectors of the pump, signal and idler, respectively. Due to this phase-matching condition, for a given pump signal, the wavelength of the signal wave can be varied by modifying the crystal temperature, crystal orientation, grating period of the crystal, or optical length of the resonator.
Many OPOs with a single nonlinear crystal are known, such as those disclosed in US 2007/0291801 A1 or US US'005/0243876 A1 . However, as these OPOs only have one nonlinear medium, only a single pair of signal wave and idler wave can be generated. Furthermore, the optical power threshold required to generate said pair is directly determined by the threshold associated with a single nonlinear crystal.
In order to generate signals waves with two or more different wavelengths in a single OPO, OPOs comprising multiple nonlinear crystals have also been disclosed. US 5,1 17,126 A discloses a stacked OPO which comprises two nonlinear crystals disposed coaxially in a single resonator, so the complete output of the first nonlinear crystal (signal wave, idler wave and residual pump signal) acts as a feed of the second nonlinear crystal. The wavelength of the signal wave generated by each nonlinear crystal is independently tuned for example, by means of the angular orientation of the crystals. The effective gain of each crystal can also be adjusted by different techniques, such as shortening the length of the first crystal, beveling said crystal with respect to the optical axis of the resonator, or by detuning the output coupling mirror with respect to the output of the first crystal.
However, in this stacked OPO, the wavelength and power of the signal and idler wave generated by the second crystal are always limited by the output signals of the first crystal, and also, the signal waves generated by the two crystals are bound to present the same temporal regime (continuous or pulsed). The optimum phase for parametric amplification, namely
Figure imgf000003_0001
Figure imgf000003_0002
is also restricted by the output signals (pump signal, signal wave, idler wave) of the first crystal.
There is thus a need of optical parametric oscillators capable of generating two or more signal waves whose wavelength, power, and temporal regime can be independently controlled, and with a self-adjusted optimum phase for the parametric amplification.
SUMMARY OF THE INVENTION
The current invention solves the aforementioned problems by disclosing an optical parametric oscillator (OPO) with at least two nonlinear crystals, in which the idler wave of the first nonlinear crystal is extracted from the resonant cavity before it reaches the second nonlinear crystal, avoiding any phase constraint at the input of the second crystal. This means that the optimum phase for parametric amplification is self-adjusted at the input of each crystal.
The disclosed OPO comprises:
-At least two nonlinear crystals in a single resonant cavity, each of which, when fed, generates a signal wave and an idler wave.
-Wavelength discrimination means that extract the idler wave generated by each crystal before it reaches the other crystal, while keeping the signal waves resonant inside the optical cavity.
By generating two independent signal waves without any constraint generated by the presence of idler waves in the input of the crystals, optimum phase for amplification is maintained in successive crystals, and independently tunable signal wave and idler wave pairs with high stability are obtained at the output of the optical cavity.
There are two preferred alternatives regarding the pumping of the two (or more) crystals:
-In a first preferred option, each crystal receives a different pump signal. The fraction of the pump signal which is still undepleted after feeding one crystal, is removed from the resonator by the wavelength discrimination means. This way, a greater independence between crystals is achieved, as each pump can be controlled separately. -In a second preferred option, a single pump is used. The pump signal serves as an input of the first nonlinear crystal, while the second nonlinear crystal is fed by the remainder of said pump signal, and by the signal wave generated by the first crystal. Notice that even in the case of a single pump, the idler wave generated at one crystal is still removed from the resonator before it reaches the next crystal. Preferably, the wavelength discrimination means that extract the idler wave of the first crystal also keep the pump signal inside the resonator. The pump signal is not extracted from the resonator until after the output of the last crystal.
In the case of the OPO in which each crystal presents an independent pump, either a single laser source or multiple laser sources can be employed. In the first scenario, a single laser source is divided into two arms, each of them feeding a crystal. In each arm, the pump power and polarization may be controlled independently.
In the case of the OPO comprising at least two different laser sources, each laser source feeds one of the crystals, thus allowing to use any combination of continuous and pulsed light to pump the system, and therefore, to generate continuous signal waves, pulsed signal waves, or a combination of both. Even if both pump signals are pulsed, different pulse durations and widths can be applied to each pump.
Preferably, the OPO also comprises tuning means that select the wavelength of each pair of idler wave and signal wave at each crystal independently of the wavelengths present at the other crystal (or crystals). Of course, at the output of a given crystal, the wavelength of the idler wave and the signal wave are tied by the phase-matching condition in that crystal. Some preferred options for the tuning means include temperature control, rotation control, and grating period control.
Therefore, with the disclosed OPO, a complete control of the wavelength, power and regime of the two (or more) output signal waves is obtained. These and other advantages will be apparent in the light of the detailed description of the invention
BRIEF DESCRIPTION OF THE DRAWINGS
For the purpose of aiding the understanding of the characteristics of the invention, according to a preferred practical embodiment thereof and in order to complement this description, the following figures are attached as an integral part thereof, having an illustrative and non-limiting character:
Figure 1 shows a diagram of an optical parametric oscillator with two nonlinear crystals and a single laser source divided into two pump signals, according to a preferred embodiment of the invention.
Figure 2 presents another OPO with two pump signals, in this case, from two different laser sources.
Figure 3 shows yet another embodiment with a single pump signal for both crystals.
DETAILED DESCRIPTION OF THE INVENTION
The matters defined in this detailed description are provided to assist in a comprehensive understanding of the invention. Accordingly, those of ordinary skill in the art will recognize that variation changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, description of well-known elements are omitted for clarity and conciseness.
Note that in this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.
Also note that when a signal is said to be "extracted" or "removed" from the resonator, it means that a significant fraction of its power is extracted, although some spurious signal may remain in the system due to physical limitations of the elements involved. This consideration is also to be applied to the expressions "maintain" or "keep" in the resonator, as well as for the terms "highly transmitting" and "highly reflecting". For example, typical values for a reflection coefficient of a highly reflecting medium are usually considered to be values between 75% and 100%, and typical values for a transmission coefficient (transmittance) of a highly transmitting medium are also usually considered to be values between 75% and 100%.
Finally, note that the invention is described according to embodiments comprising two nonlinear crystals in a ring resonator. However, OPOs with a greater number of crystals can be implemented within the scope of this invention. Also, other configurations of the resonator are possible within the scope of this invention, such as linear 2-mirror standing-wave, X-cavity, Z- cavity, or V-cavity.
Figure 1 shows the architecture of an OPO according to a preferred embodiment of the present invention, in which each nonlinear crystal receives a different pump signal. The OPO comprises an optical resonator 2 with two nonlinear crystals 4 and 4'. The first nonlinear crystal 4 is fed by a first pump signal P and generates a first idler wave I and a first signal wave S. Accordingly, the second nonlinear crystal 4' is fed by a second pump signal P' and generates a second idler wave and a second signal wave S'. The first nonlinear crystals 4 and 4' may be birefringently-phase-matched (BPM) or quasi-phase-matched (QPM) materials.
The first nonlinear crystal 4 is located between two mirrors M1 and M2 with different transmission coefficients depending on the incident wavelength which therefore act as wavelength discrimination means. The first mirror M1 , located between a laser source 1 and the first nonlinear crystal 4, is highly transmitting for the first pump signal P, while it reflects signal waves S and S'. The second mirror M2, located after the nonlinear crystal 4 is also highly transmitting for the first pump signal P and highly reflecting for the signal waves S and S', but it is also highly transmitting for the first idler wave I. Therefore, at the second mirror M2, the first pump signal P and the first idler wave I are extracted from the resonator 2 while the signal waves S and S' are maintained inside the resonator 2 and are thus able to oscillate.
In the ring resonator disclosed in figure 1 , the signal waves S and S', after being reflected from the second mirror M2, are directed towards a third mirror M3, which also reflects the signal waves S and S' and focuses them to the input of the second nonlinear crystal 4'. The third mirror M3 also serves as an entry point of the second pump signal P' inside the oscillator and therefore is highly transmitting to the wavelength of said second pump signal P'.
Finally, the fourth mirror M4 is also highly transmitting to the wavelengths of the second pump signal P' and the second idler wave Γ, extracting them from the resonator 2; and highly reflecting to the wavelengths of the signal waves S and S'. Note that the architecture of the OPO ensures that both signal waves S and S' travel in the same direction inside the resonator 2.
In this example, the first idler wave I is separated from the first pump signal by means of a fifth mirror M5, while the second idler wave and the second pump signal P' are separated by means of a sixth mirror M6.
The wavelengths of the signal waves S and S' are controlled independently with no phase matching limitation between them. That is, the wavelength of the first signal wave S and the first idler wave I are tied by the phase matching condition and energy conservation condition, but the first signal wave S and the second signal wave S' can be selected independently. Consequently, the first idler wave I and the second idler wave can be selected independently. Also as the first idler wave is removed from the resonator 2 before reaching the second nonlinear crystal 4', the optimal phase for the parametric amplification is self-adjusted.
Nevertheless, if the wavelengths of both signals are very similar, an overlap between them may occur, appearing as a single oscillating signal. However, this phenomenon can also be used advantageously, as in this case, when the two nonlinear crystals are generating signal waves with a same frequency, the total gain of the resonator 2 is increased, and thus, the minimum threshold for the signal wave oscillation is reduced accordingly, increasing the conversion efficiency and the output power.
Three possible techniques to control the emission wavelength of the crystals are:
-Modifying the temperature of the nonlinear crystal.
-Rotating the nonlinear crystal to modify the angle of incidence of the light at its input.
-Modifying the grating period of the nonlinear crystal when the nonlinear crystal is a quasi phase-matched (QPM) material.
Note that it is possible to use more than one of these techniques simultaneously to control the emission wavelength. In all the cases, the wavelength of a signal wave (and thus of the associated idler wave) is controlled independently from the wavelength of the other signal wave (and thus of the associated idler wave).
The OPO also comprises power control means 3 to control the power (and polarization) of the pump signals P and P'. The light emitted by the laser source 1 is divided by a first polarizing beam-splitter cube 5 after passing through a first half-wave-plate H1 , dividing the laser output into two arms with a variable power ratio. The first arm (the one associated to the first pump signal P), also comprises a second half-wave-plate H2 to choose the polarization of the first pump signal P and a first convergent lens 6 that focalises the first pump signal P at the input of the first nonlinear crystal 4. The second arm comprises a third half-wave-plate H3 and a second polarizing beam-splitter cube 5' to control the power of the second pump signal, a fourth half-wave-plate H4 to control its polarization, as well as a second lens 6' to focalize it at the input of the second nonlinear crystal 4'. Note that any other scheme known in the state of the art for the control of power and polarization may be used.
Also note that not only the two signal waves S and S' are available as an output of the OPO, but also idler waves I and . The following combination of pairs of independent signals are thus available:
-The two signal waves S and S'.
-The two idler waves I and .
-The first signal wave S and the second idler wave .
-The second signal wave S' and the first idler wave I.
Additionally, it is possible to obtain combinations involving three and four wavelengths, which is desirable for application requiring multiple wavelengths, such as Coherent Anti-Stokes Raman Scattering (CARS) microscopy.
It is also possible to apply the high intracavity circulating intensities of the signal waves S and S' to generate terahertz Difference Frequency Generation (DFG) and Sum Frequency Generation (SFG) at any desired wavelength using an additional nonlinear process in a third nonlinear crystal either inside or outside the resonant cavity.
In figure 2, an example of an architecture with two laser sources is presented. In this case, the first laser source 1 feeds directly the first arm, while the second laser source 1 ' feeds the second arm. The power and polarization control is also performed by a combination of half-wave-plates and polarizing beam-splitter-cubes as explained for the case with a single laser source.
The main advantage of the scheme with two different laser sources is that each laser source is able to work in a different regime, that is, both laser sources may be continuous wave (CW), both pulsed, or one of them pulsed while the other one is continuous. Even if both of the laser sources are pulsed, it is possible to use laser sources with different pulse duration and width.
As a particular example, a CW pump may be applied to generate narrowband signal and idler waves at the first nonlinear crystal 4. In the second nonlinear crystal 4', on the other hand, a pulsed laser source may be used for the second pump signal P', resulting in broadband pulsed signal and idler waves. Note that there is no restriction in the duration of the laser pulses (microscond, nanosecond, picosecond, femtosecond...). Thus, the seeding of the pulsed broadband signal and idler waves by the narrowband CW signal wave results in line-narrowing of the broadband pulsed radiation from the second nonlinear crystal 4'. This scheme can be applied to achieve line narrowing and single-frequency generation of pulsed radiation seeded by CW narrowband radiation.
Finally, figure 3 shows another embodiment in which a single pump signal P is used. In this case, the second mirror M2 is highly reflecting at the wavelength of the pump signal P, and thus, the second nonlinear crystal 4' is fed by the fraction of the pump signal P undepleted by the first nonlinear crystal 4. The pump signal P leaves the resonator 2 at the fourth mirror M4.
Still, since the first idler wave I is extracted from the resonator 2 at the second mirror M2, there are no phase constraints inhibiting parametric amplification in the second nonlinear crystal 4'. Similarly, mirror M4 is highly transmitting for the second idler wave , so the optimal phase for the parametric amplification is self-adjusted at the first nonlinear crystal.
As mentioned before, other dispositions for the optical resonator can be used, although the bow-tie ring disclosed in the previous examples also has the advantage of having the crystals in separate arms, and refocusing the beams between successive trips through the crystals.

Claims

1 . Optical parametric oscillator comprising
-a first nonlinear crystal (4) in an optical resonator (2), wherein the first nonlinear crystal generates a first idler wave (I) and a first signal wave (S); -and at least a second nonlinear crystal (4') in the optical resonator (2), wherein the at least one second nonlinear crystal (4') generates a second idler wave ( ) and a second signal wave (S');
characterized in that it further comprises:
-first wavelength discrimination means (M2) having a first wavelength dependant transmittance that extracts the first idler wave (I) from the resonator (2) and keeps the first signal wave (S) in the resonator (2);
-and second wavelength discrimination means (M4) having a second wavelength dependant transmittance that extracts the second idler wave ( ) from the resonator (2) and keeps the second signal wave (S') in the resonator (2).
2. Optical parametric oscillator according to claim 2 characterized in that the first nonlinear crystal (4) is adapted to receive a first pump signal (P) and the second nonlinear crystal (4') is adapted to receive a second pump signal (Ρ'), and in that the transmittance of the first wavelength discrimination means (M2) also extracts the first pump signal (P) from the resonator (2).
3. Optical parametric oscillator according to claim 2, characterized in that the first pump signal (P) and the second pump signal (Ρ') are two input signals from a single laser source (1 ).
4. Optical parametric oscillator according to claim 2, characterized in that the first pump signal (P) and the second pump signal (Ρ') are two input signals from two laser sources (1 , 1 ').
5. Optical parametric oscillator according to any of claims 2 to 4 characterized in that it further comprises power control means (3) adapted to control the optical power of the first pump signal (P) and the second pump signal (Ρ').
6. Optical parametric oscillator according to claim 1 characterized in that the first nonlinear crystal (4) is adapted to receive a first pump signal (P) and the second nonlinear crystal (4') is adapted to receive a part of the first pump signal (P) undepleted by the first nonlinear crystal (4).
7. Optical parametric oscillator according to claim 6, characterized in that the transmittance of the first wavelength discrimination means (M2) keeps the first pump signal (P) in the resonator (2).
8. Optical parametric oscillator according to any of the previous claims, characterized in that it further comprises first tuning means adapted to select wavelengths of emission of the first idler wave (I) and the first signal wave (S) at the first nonlinear crystal (4), and second tuning means adapted to select wavelengths of emission of the second idler wave ( ) and the second signal wave (S') at the second nonlinear crystal (4').
9. Optical parametric oscillator according to claim 8, characterized in that the first and second tuning means are temperature control means that independently control a temperature of the first nonlinear crystal (4) and a temperature of the second nonlinear crystal (4').
10. Optical parametric oscillator according to claim 8, characterized in that the first and second tuning means are rotation means that independently control an angle of the first nonlinear crystal (4) and an angle of the second nonlinear crystal (4') by rotating said nonlinear crystals.
1 1 . Optical parametric oscillator according to claim 8, characterized in that the first and second tuning means are adapted to independently control a grating period of the first nonlinear crystal (4) and a grating period of the second nonlinear crystal (4').
12. Optical parametric oscillator according to claim 8, characterized in that the first and second tuning means are adapted to independently control a subset chosen from the following parameters of the first nonlinear crystal (4) and the second nonlinear crystal (4'): temperature, angle, and grating period.
PCT/EP2011/054130 2011-03-18 2011-03-18 Optical parametric oscillator comprising two nonlinear crystals WO2012126495A1 (en)

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