WO2010061046A1

WO2010061046A1 - Optical resonator based on a grating

Info

Publication number: WO2010061046A1
Application number: PCT/FI2009/050871
Authority: WO
Inventors: Markku Vainio; Lauri Halonen
Original assignee: University Of Helsinki
Priority date: 2008-11-03
Filing date: 2009-10-29
Publication date: 2010-06-03
Also published as: FI20086044A0; FI20086044L

Abstract

The optical resonator according to the invention resonates at one frequency, either the signal or idler frequency. The optical resonator is fed with a pump wave (PB) produced with a laser source (501). The resonator comprises an optically nonlinear medium component (510), two mirrors around the medium component on its optical axis, a first (521) and a second mirror (522), and at least one mirror outside the optical axis of the medium component. The first mirror is on the incoming side of the medium component and the second mirror on the outlet side of the medium component. The medium component transforms the pump beam into a signal and idler frequency. At least one of the mirrors of the resonator is focusing at the resonant frequency and the focus is located in the medium component. One of the mirrors outside the optical axis of the medium component of the resonator is a grating (524), which is highly reflecting at the resonant frequency. The object of the grating is to produce a sufficient frequency selectivity, so that the optical resonator operates in a stable manner in one longitudinal resonator mode without mode hops. Additionally the grating makes possible the adjustment of the wavelengths of the signal and idler beams, because by changing the grating angle the wavelength resonating in the resonator can be selected. The structure according to the invention makes possible the use of a continuously operating laser device, whereby the outgoing beam (IB) is also continuous.

Description

Optical resonator based on a grating

The invention relates to an optical resonator fed from a laser source, which has an optically nonlinear medium component for transforming the pump beam coming from the source into a signal beam and an idler beam, the sum of the frequencies of which is essentially the frequency of the pump beam, of which signal and idler beam at least one is a resonating beam, the frequency of which is the resonant frequency of the optical resonator, which resonator further has a first mirror on the incoming side of the medium component on its optical axis, a second mirror on the outlet side of the medium component on its optical axis and at least one mirror outside the optical axis of said medium component in the path of the resonating beam, and the resonator is arranged to let out at least one of the signal and idler beam.

An optical resonator is a device, with which an electromagnetic wave entering it is transformed into two outgoing electromagnetic waves, which have a different wavelength than the incoming wave. The sum of these frequencies is essentially the same as the frequency of the incoming wave. The incoming wave is in practice usually from a laser source, which in this context is called a pump laser. In this description light waves and waves, the frequency of which are close to light frequencies, are called "beam". Thus the wave produced by a pump laser is called a pump beam (PB). The two waves formed in the resonator are for historical reasons called signal beam (SB) and idler beam (IB).

The formation of two beams with different frequencies in the optical resonator is based on an optically nonlinear medium component, through which the beams are led. The resonance on the other hand is based on the fact that the signal or idler beam is arranged with the aid of reflectors to pass repeatedly through the medium, so that the phases in the medium corresponding to the different times of passing through are the same. The amplitude of such a beam, i.e. a resonating beam, naturally grows compared to the initial state, where the feeding of the pump beam into the resonator is begun. At the same time the amplitude of the other beam formed in the resonator grows, because the waves formed in the nonlinear medium are tied to each other.

The energy of the signal and idler beam comes from the energy of the incoming pump beam, whereby the intensity of the pump beam in the resonator naturally 23 Dec 2009

decreases. For the resonator to even start, i.e. for a significant resonance to form, the energy the resonating beam obtains from the pump beam must be larger than the optical losses of the resonating beam in the resonator. This requires that the intensity of the incoming pump beam exceeds a certain limit, in other words the power of the pump laser exceeds a certain threshold value, which is called a threshold power.

The frequency of the pump beam produced by the pump laser is f_p and the wavelength λ_p. The frequency of the signal beam is f_s and the wavelength λ_s. The frequency of the idler beam is f, and the wavelength λj. These are referred to as the pump frequency, the signal frequency and the idler frequency. As mentioned, the frequency of the pump beam, i.e. the pump frequency f_p is the sum of the frequency of the signal beam, i.e. the signal frequency f_s and the frequency of the idler beam, i.e. the idler frequency fj. Generally optical parametric oscillators comprise at least a laser device (a pump laser) and an optical resonator, which contains an optically nonlinear medium component. The optically nonlinear medium component breaks up the pump beam into signal and idler beams. In the optically nonlinear medium component the pump beam, signal beam and idler beam are overlapping each other and these three waves interact due to the optical nonlinearity of the medium component. The interaction leads to a rise in the amplitude of the signal beam and the idler beam at the expense of the pump beam. This is called parametric amplification. The optical resonator resonates at the frequencies of the signal or idler beams or both. In some cases the resonator can be implemented so that it further resonates also at the frequency of the pump wave. The signal frequency and the idler frequency are usually of a different magnitude, whereby the beam with the higher frequency is the signal beam and respectively the one with the lower frequency is the idler beam. If for example the idler beam is taken out from the resonator, as is often done, this frequency fj = f_p - f_s. The frequency fj is typically within the infrared area, where it is difficult to produce laser radiation directly with a laser source. The practical significance of the optical resonator is precisely here. To be able to set the frequency of the usable radiation, optical resonators can be tuned.

The frequencies of the beams produced by an optical parametric oscillator further depend on which of the resonant frequencies of the resonator experience a sufficient parametric amplification to exceed the oscillating threshold. This on the other hand depends on how well the process is phase matched at the frequencies 23 Dec 2009

in question. Typically the wavelength of the idler beam is in the infrared area and precisely the idler beam is used. Optical parametric oscillators are an effective way to transform electromagnetic radiation from a laser into a longer wavelength. Additionally they provide a possibility to adjust the outgoing wavelength. Thus it is possible to achieve wavelengths, which are otherwise difficult or impossible to produce with a laser device.

Figure 1 shows a simple principle presentation of an optical resonator. A pump beam PB formed with a pump laser is fed into the optical resonator 100. The optical resonator yields either a signal beam SB or an idler beam IB or both. It is also possible to obtain a beam PB' at the frequency of the pump beam, i.e. the part of the pump beam which has remained unconsumed in the development of the signal or idler beam, i.e. in the parametric process. This outgoing beam PB' is naturally of a significantly lower power than the incoming pump beam. Generally this component is filtered out. The optical resonator can be implemented as a linear or a ring resonator. In a ring resonator the resonating beam circulates a certain closed route in one direction. In a linear resonator the resonating beam travels back and forth between at least two reflective surfaces, i.e. mirrors. The term 'linear¹ here means only that the resonating beam is reflected in its incoming direction. Figure 2 has an example of a known optical resonator, which resonates at the frequency of the signal wave. The resonator 200 is implemented as a ring resonator. It comprises an optically nonlinear medium component 210, a first mirror 221 , a second mirror 222, a third mirror 223 and a fourth mirror 224. A pump beam (PB) enters the optical resonator and an idler beam (IB) exits it. The first and the second mirror are on opposite sides of the medium component, on its optical axis. The optical axis of the medium component is the axis, along the direction of which the pump beam is meant to travel through the medium component, in order for its energy to be transferred to the signal and idler beams. The first mirror is on the incoming side of the medium component and the second mirror is on the outlet side of the medium component. The incoming side of the medium component is the side where the pump beam arrives the first time and the outlet side is the opposite side.

The first mirror 221 is arranged for example with a coating so that it lets through as large a part as possible of the radiation at the frequency of the pump beam. Thus the pump beam coming from the laser device gets into the optical resonator and 23 Dec 2009

goes into the medium component. Optically nonlinear crystal material is generally used as material for the medium component. Some commonly used materials are lithium columbate (LiNbO₃), lithium tantalate (LiTaO₃), potassium columbate (KNbO₃) and potassium titanyl phosphate (KTiOPO₄). The sides of the crystal, where the beams enter and exit, are shaped or coated with an appropriate material so that they reflect beams as little as possible. For the transformation of the pump beam into signal and idler beams to be as effective as possible, the process needs to be phase matched. The phase matching can be achieved for example based on birefringence of the crystal. In practice this means selecting the crystal direction in a manner suitable in relation to the beams. Another commonly used technique to achieve phase matching is so-called quasi-phase matching. Quasi-phase matching can be implemented by treating the crystal so that the polarity of the crystal changes in a suitable manner periodically in the travel direction of the beams (periodical poling). The second mirror 222 is arranged for example with a coating to reflect signal frequency radiation and let through pump and idler frequency radiations. The second mirror is placed so that it reflects the signal wave coming from the medium component 210 towards the third mirror 223. The third mirror is arranged for example with a coating to reflect the frequency of the signal wave. The third mirror is placed »in such a position that it reflects beams striking it towards the fourth mirror 224. The fourth mirror is arranged for example with a coating to reflect the frequency of the signal wave. The fourth mirror is placed in such a position that it reflects beams striking it towards the first mirror 221. The first mirror is arranged for example with a coating to reflect the frequency of the signal wave. The first mirror reflects the signal wave into the medium component 210, so that the reflected signal beam is in the same phase as the formed signal beam, thus amplifying it. Thus also the idler beam is amplified, because in a parametric process one photon corresponding to the idler frequency is formed for each formed signal photon, in accordance with the law of energy conservation. Figure 3 has another example of a known optical resonator, which resonates at the frequency of the signal wave. The optical resonator 300 is implemented as a linear resonator. Here the beam travels inside the optical resonator back and forth the same way. It comprises an optically nonlinear medium component 310 and two mirrors, a first mirror 321 and a second mirror 322. A pump beam (PB) enters the optical parametric oscillator and a pump beam remnant (PB¹) and an idler beam (IB) exit it. 23 Dec 2009

The first mirror 321 is arranged for example with a coating so that it lets through as large a part as possible of the radiation at the frequency of the pump beam. Thus the pump beam from the laser device gets into the optical resonator. The pump beam comes into the optically nonlinear medium component 310. In the medium component the pump beam is transformed partly to a signal and idler beam. The medium component amplifies the signal and idler beams at the expense of the pump beam. The second mirror 322 is arranged for example with a coating to reflect the frequency of the signal wave and let through pump and idler wave frequencies. The second mirror is placed so that it reflects the signal wave coming from the medium component 310 back towards the first mirror 321. The second mirror is placed so that the signal beam returning to the medium component amplifies the signal beam formed in the medium component. Thus also the formed idler beam is amplified. The signal beam, which has come through the medium component, strikes the first mirror. The first mirror is for example coated with some coating, which in addition to letting through the pump beam further reflects the signal frequency. The first mirror is placed so that the signal beam striking it is reflected into the optically nonlinear medium component, so that it amplifies the signal wave formed there.

Most optical oscillators resonate at one frequency, i.e. they have an optical resonator, which resonates either at the signal or the idler frequency, but not both. Optical resonators can also be constructed, which resonate both at the signal and idler frequency. Optical resonators can also be constructed, which further resonate at the pump frequency. In these optical parametric oscillators, which resonate at two or three frequencies, the threshold power required from the pump wave can be significantly lower than in optical parametric oscillators resonating at one frequency. Thus a pump laser with lower power can be used. These two or three frequency resonators have the problem of adjusting the signal and idler frequencies. This is due to the fact that the length of the resonator should correspond to multiple halves of the wavelength at each resonating frequency at the same time. If for example the length of the resonator or the frequency of the pump beam is changed, this resonance condition changes. The result is that the signal and idler wave frequencies experience hops, i.e. the adjustable frequency area is not continuous. For the same reason these resonators have the problem that the signal and idler frequencies hop for example when the temperature of the resonator or the medium component changes, because the variations in question change the optical length of the resonator. These frequency hops are called mode 23 Dec 2009

hops, because the beam frequencies hop from one longitudinal resonance mode of the resonator to another.

Optical resonators which produce a continuous radiation, and which resonate at one frequency, have the following problems. They have a tendency for mode hops, i.e. for the resonant frequency, usually the signal frequency, to hop from a longitudinal resonance mode of the resonator to another. In the case of an optical parametric oscillator resonating at one frequency this is usually caused by the resonant frequency of the resonator changing in relation to the frequency, at which the parametric amplification produced by the nonlinear medium component is at its largest. This change can be caused for example by mechanical vibrations or thermal expansion of the resonator. Problems can be caused also by the fact that in practice, resonator mirrors cannot be manufactured so that they only reflect one frequency. Even small residual reflections at the pump or idler frequency when the signal frequency is resonating can cause mode hopping and discontinuity of the frequency setting. Optical parametric oscillators also have a tendency for multimodeness, i.e. for several resonance modes to exceed the oscillation threshold at the same time. Thus instead of one signal frequency, several signal frequencies are obtained, and instead of one idler frequency, several idler frequencies. In order to stabilize the frequency, an optical etalon is often placed inside the optical resonator, the purpose of which etalon is to select one single oscillating frequency and prevent mode hopping between the longitudinal modes of the resonator. The etalon also makes possible the adjusting of the resonating frequency, usually the signal frequency, by turning the angle of the etalon. The adjustment area is however limited by the frequency hops between the modes of the etalon. The problem is further that optical losses caused by the etalon depend on the angle of the etalon in relation to the resonating beam. This causes variation in the output power and limits the frequency adjustment area which can be achieved with adjustment of the angle of the etalon. A problem is also the parasitic reflections caused by the surfaces of the etalon at the frequencies of the different beams, which can cause mode hopping and gaps in the adjustment area. A disadvantage with the etalon is further the complication of the device, which makes difficult the implementation of a stably operating optical parametric oscillator. Problems related to etalon in optical parametric oscillators are described widely in the publication: S. E. Bisson, K. M. Armstrong, T. J. KuIp, ja M. Hartings 23 Dec 2009

"Broadly Tunable, Mode-Hop-Tuned cw Optical Parametric Oscillator Based on Periodically Poled Lithium Niobate", Appl. Opt. 40, 6049-6055 (2001).

Most known continuously operating optical parametric oscillators require a ring resonator in order to operate stably. A problem with the ring resonator compared to the linear resonator is that the mirrors of the ring resonator are more difficult to line up in a manner required for the operation of the device. This makes the construction and use of the device difficult. Adjustment of the wavelengths produced by the optical parametric oscillator is typically slow and requires adjusting several different elements. The threshold power required by the optical resonator is remarkably easier to achieve with a pulsed pump laser than with a continuously operating pump laser. Therefore especially continuously operating optical resonators are designed so that their optical losses at the resonating wavelength are as small as possible, so the threshold power can be reached with existing pump lasers. The continuous power of the pump beam also sets larger requirements on the components of the optical resonator than the momentary powers of the pulsed pump beam. An optical parametric oscillator operating with a pulsed pump beam already to start with produces light with a broader band than one operating with a continuous pump beam, and the operation of the device does not depend as critically on its optical and mechanical implementation, so they are easier to implement than continuously operating ones.

Patent publication WO 2006/135311 presents an optical parametric oscillator, which comprises a laser light source, a nonlinear crystal, mirrors and a so-called Bragg grating. The grating is for producing a narrow-band signal. The grating is placed so that the beam coming through the nonlinear crystal strikes the grating first. The signal produced by the optical parametric oscillator according to the publication is brought out through the grating or alternatively through some mirror. The wavelength of the resonating beam is adjusted only by turning the grating or changing the distance between the grating and the mirrors. With the resonator solutions according to the publication it is impossible to achieve the parametric amplification required by the continuously operating optical parametric oscillator, because the planar mirrors and grating do not on their own make up a stable resonator, which would form a sufficiently tight focus on the medium component. A further disadvantage is the fact that the placement of the grating directly behind the medium component exposes the grating to a larger optical power (pump and idler powers in addition to the signal), whereby the damaging of the grating is more 23 Dec 2009 8

probable. In the same way the out-switching of the non-resonating outgoing beam, which usually is the idler beam, is made more difficult, since the grating can affect its travel direction or dampening depending on the wavelength.

Patent publication US 2007/0035810 presents an optical parametric oscillator, which produces an adjustable infrared light. This comprises a fibre laser source, the frequency of the pump beam formed by which can be adjusted. The pump beam can be continuous. The fine tuning of the frequency of the formed signal is done by changing the distances of the mirrors of the optical resonator. The difficulty in this solution is that in order to change the wavelength of the signal, two components must be adjusted, and in order to avoid mode hopping, etalons or similar solutions must be used.

An aim of the invention is a solution by which the drawbacks and disadvantages relating to the prior art can be considerably reduced. The aim of the invention is to implement a frequency and power stable and one-mode operating continuously radiating optical resonator, without needing an etalon or other corresponding components inside the resonator. The aim of the invention is further to simplify the structure and wavelength adjustment of a continuously operating optical resonator.

The solution according to the invention is characterized in what is presented in the independent claims. Some advantageous embodiments of the invention are presented in the dependent claims.

The optical resonator according to the invention is fed with a pump beam coming from a laser source. The optical resonator has an optically nonlinear medium component for transforming the pump beam coming from the source into a signal beam and an idler beam, the sum of the frequencies of which is essentially the frequency of the pump beam. Of the signal and idler beams at least one is a resonating beam, the frequency of which is the resonant frequency of the optical resonator. The resonator further has a first mirror on the incoming side of the medium components on its optical axis, a second mirror on the outgoing side of the medium component on its optical axis and at least one mirror outside the optical axis of said medium component in the path of the resonating beam. The resonator is arranged to let out at least one of the signal and idler beams. At least one of the mirrors is a mirror which focuses the resonating beam into the medium component. The mirror outside the optical axis of the medium component is a grating, which grating is moveable in order to tune the optical resonator. The grating is highly reflecting at the resonating frequency. The object of the grating is 23 Dec 2009

to produce a sufficient frequency selectivity, so that the optical parametric oscillator operates in a stable manner in one longitudinal resonator mode without mode hops. The grating further makes possible the adjustment of the signal and idler beam wavelengths, because by moving the grating, by changing the grating angle or the position of the grating or by otherwise moving the grating, the wavelength resonating in the resonator can be selected.

In one advantageous embodiment the pump beam is arranged to be focused so that its focus is in the medium component. This focusing is arranged with a lens arrangement, which is between the laser source and the first mirror. The location and size of the focus of the resonating beam in the medium component is adapted to the location and size of the focus of the pump beam, so that the parametric amplification in the medium component is as large as possible.

According to another advantageous embodiment two of the mirrors are focusing mirrors. According to an advantageous embodiment there is a grating outside the optical axis of the medium component. According to another advantageous embodiment there is a grating and a mirror outside the optical axis of the medium component.

According to an advantageous embodiment the mirrors of the resonator, with the exception of the first mirror, are arranged to reflect the pump beam, whereby the pump beam can be arranged to be removed from the resonator via the first mirror.

According to an advantageous embodiment at least one of the ends of the medium component is arranged to replace the mirror in the end in question of the medium component on the optical axis. According to another advantageous embodiment this replaceable mirror is the first mirror on the incoming side of the medium component.

According to an advantageous embodiment the optical resonator according to the invention is a linear resonator.

According to an advantageous embodiment the optical resonator according to the invention is a ring resonator. According to an advantageous embodiment the laser source is continuously operating. 23 Dec 2009 10

According to an advantageous embodiment the grating is a diffraction grating. According to an advantageous embodiment the grating is a Bragg grating. According to an advantageous embodiment the grating is a transversally chirped Bragg grating. In a transversally chirped Bragg grating the grating constant changes in the sideways direction, i.e. in the direction, which is perpendicular to the normal of the grating pattern. Such a grating thus reflects different frequencies depending on where on the grating tha beam strikes.

An advantage of the invention is that the optical resonator in accordance with it is one-moded, where there thus does not occur much frequency hopping of the resonating beam. This is due to said grating being very selective, dampening effectively resonances corresponding to other modes.

An advantage of the invention is further that the optical resonator in accordance with it is relatively simple. This is due to the fact that no etalon or other additional arrangements are needed to prevent multimodeness. In relation to this the optical losses of the resonator according to the invention are relatively small, which further results in that vibrating can be achieved with a reasonable threshold power also with a continuously operating pump laser.

An advantage of the invention is further that the wavelength of the resonating wave can by changing the grating angle or the position of the grating be adjusted continuously in the entire area, where the parametric amplification suffices to maintain parametric vibration. This is also due to the selective grating enabling resonance simultaneously only at one frequency, which depends on the grating angle, preventing the mode from changing. The frequency adjustment is also simple. A further advantage of the invention is that is makes possible the fine tuning of the frequency with the same element, i.e. the grating, with which also the wider frequency adjustment occurs. In the fine tuning the location of the grating is changed for example with the aid of a piezoelectric element.

A further advantage of the invention is that the optical losses produced by the grating do not significantly depend on the grating angle. Due to this the dependency of the initial power on the frequency adjustment is smaller than in known optical resonators.

A further advantage of the invention is that the grating makes possible the resonating of only one wave, selected with the grating angle, at a narrow 23 Dec 2009 11

frequency band, which prevents mode hopping. If for example the signal frequency is selected as the resonating frequency, the pump and idler beams are reflected out of the resonator. This differs from other known solutions, where the small residual reflections of the resonator's mirrors at other than the desired resonant frequency can cause competing resonances and thus mode hopping and discontinuity of the frequency adjustment.

A further advantage of the invention is that a continuously operating optical resonator in accordance with it operates in a stable manner also with a linear structure, which in known solutions is less stable than a ring structure.

An advantage of the invention is also that in it no other components are needed inside the resonator than the optically nonlinear medium component. Due to this, the optical losses can be kept small, so that the optical parametric oscillation threshold can easily be reached also with a one-mode continuously operating pump laser. This also simplifies the structure of the device and makes its implementation easier. Due to the simple structure, the frequencies and powers of the outgoing beams are easy to keep stable.

The prior art was illustrated by referring to Figures 1 , 2 and 3, and the invention is described in detail with reference to Figures 4-14, in which

Fig. 1 shows an optical resonator as a simple principle drawing, Fig. 2 shows an example of a known optical resonator,

Fig. 3 shows another example of a known optical resonator,

Fig. 4 shows as an example a flow chart of the method according to the invention for adjusting the frequency of the beam exiting from an optical parametric oscillator,

Fig. 5 shows as an example an optical resonator according to the invention, which is implemented as a linear resonator,

Fig. 6 shows an example of the spectrum of the idler beam of the optical resonator according to Fig. 5,

Fig. 7 shows a second example of an optical resonator according to the invention, 23 Dec 2009 12

Fig. 8 shows a third example of an optical resonator according to the invention,

Fig. 9 shows a fourth example of an optical resonator according to the invention, Fig. 10 shows a fifth example of an optical resonator according to the invention,

Fig. 11 shows a sixth example of an optical resonator according to the invention,

Fig. 12 shows a seventh example of an optical resonator according to the invention, Fig. 13 shows the measured frequency stability of the optical parametric oscillator according to the invention at a signal wavelength of 1611 nm and

Fig. 14 shows the measured power stability of the optical parametric oscillator according to the invention at an idler beam wavelength of 3134 nm.

Figure 4 shows as a flow chart the frequency adjustment of the optical resonator according to the invention. In stage 401 the laser source feeding the optical resonator is started. The pump beam is led into the optically nonlinear medium component of the optical resonator, whereafter the resonance is strengthened and the pump beam is transformed into a signal and idler beam. Resonance occurs either at the signal or idler beam frequency. The optical resonator has at least one mirror, which is arranged to focus the resonating beam into the medium component. One of the mirrors is structurally a grating, the position of which can be adjusted. Either the signal or idler beam is removed from the resonator. In stage 402 the frequency of the outgoing beam is measured and in stage 403 a control is done to see whether it is correct with a sufficient precision. If it is not, the angle of the grating is adjusted (stage 404), whereby the frequency of the resonating beam changes and at the same time also the frequency of the beam outgoing from the resonator. Then one returns to stage 402. When the frequency of the outgoing beam is correct with a sufficient precision, the adjustment is naturally finished (stage 405). Thus the positions and locations of the components can be stored and set again in the stored positions if it is later desirable to use the resonator to produce the same frequency. 23 Dec 2009 13

Fig. 5 shows as an example an embodiment of the optical resonator according to the invention. The optical resonator 500 is a linear resonator, where a signal beam resonates. It comprises three mirrors, a first mirror 521 , a second mirror 522 and a third mirror 523, a grating 524 and an optically nonlinear medium component 510. The first and second mirror are designed to be focusing. The first mirror is on the optical axis of the medium component on its incoming side. The second mirror is on the optical axis of the medium component on its outlet side. The pump beam (PB), which is narrow banded and linearly polarized coherent light, produced by a one-mode pump laser 501 , goes in. The pump beam residue (PB') and the idler beam (IB) exit the oscillator in this case. The first mirror 521 is arranged for example with a coating so that it lets through as large a part as possible of the radiation at the frequency of the pump beam. Thus the pump beam from the laser device gets into the nonlinear medium component. The pump beam is focused into the optically nonlinear medium component, through which it travels. This focusing can be done for example with a lens arrangement 503. The size and location of the focus of the pump beam in the medium component is arranged so that the parametric amplification is as large as possible. As the medium component 510 is typically used a crystal, which has a large optical nonlinearity and small absorption losses at the pump, signal and idler beam wavelengths. The crystal can for example be a 50 mm long MgO:PPLN crystal (periodically poled LiNbO₃, MgO- alloyed 5 % / mol). In order to minimize the optical losses and instabilities the sides of the medium component, where the beams enter and exit, are shaped or coated with an appropriate material so that they reflect beams as little as possible. In the medium component the amplitude of the signal beam and the idler beam increases and the amplitude of the pump beam correspondingly decreases.

The second mirror 522 is arranged for example with a coating to reflect the frequency of the signal beam and let through pump and idler beam frequencies. Typically the reflection factor of the power at the signal frequency is R_s >99.9%. The second mirror is designed to be focusing and it is placed so that it reflects the signal beam coming from the medium component toward the third mirror 523. The third mirror is arranged for example with a coating to reflect the frequency of the signal wave. (Typically R_s >99.9%). To avoid instabilities, the third mirror can be arranged for example with a coating to reflect the idler and pump frequencies as little as possible. The third mirror is placed in such a position that it reflects the signal beam striking it back towards the second mirror 522. The second mirror 522 reflects the signal beam coming from the third mirror through the medium component 510 toward the first mirror 521. The second mirror focuses the signal 23 Dec 2009 14

beam into the medium component so that it amplifies the signal beam forming in the medium component. The first mirror 521 is arranged for example with a coating so that it reflects as large a part as possible of the radiation at the signal beam frequency. (Typically R₅ >99.9%). The first mirror is designed to be focusing and it is placed so that it reflects the signal beam coming through the medium component toward the optical grating 524. The grating is arranged to reflect the signal wave frequency as well as possible. For example when using a diffraction grating, this can be arranged by coating the grating with an appropriate dielectric coating or metal coating, for example with gold, silver or platinum. The used coating material is selected according to the resonating wavelength, typically so that the reflection factor of the grating to the desired diffraction order, usually the first order, is over 95%. The position of the grating in relation to the beam coming to it is selected so that the signal beam coming to it is reflected as effectively as possible in its incoming direction toward the first focusing mirror. The signal beam reflected from the grating travels via the first mirror 521 into the medium component 510, so that it is focused into the medium component, so that it amplifies the signal beam forming there. When the signal beam is amplified, also the idler beam is amplified.

Because the second mirror 522 is arranged to let through pump and idler frequencies, these 'can be removed from the parametric oscillator. The pump beam (PB') and other unnecessary frequencies can be filtered out from the outgoing radiation with a filter arrangement 505. In the case according to the example, the idler beam (IB) gets through the filter arrangement.

The mirrors of the optical resonator are arranged at such a distance from each other that the signal wave resonates in it and that the signal beam has a focus in the medium component. Additionally the radii of curvature of the mirrors and their distances from each other are selected so that the size and location of the focus of the resonating beam in the medium component is as optimal as possible with regards to the optical parametric amplification. The optical parametric amplification significantly depends on the focusing of the beams involved in the parametric process, and the largest parametric amplification can usually be achieved with tightly focused beams, so that the so-called focusing parameters ξ of the beams are larger than 1 and nearly of the same size. It is advantageous for the optical parametric amplification, if the focuses of all the beams involved in the phenomenon are in the middle of the medium component in the travel direction of the beams. 23 Dec 2009 15

The focusing parameter is defined ξ = L₀Zb. L_c is the length of the crystal used as a medium component and b = 2πw₀ ²r\/λ, which is the parameter illustrating the focus length of the Gaussian beam. By Gaussian beam is meant a beam, the intensity distribution of which in the cross-direction of the beam complies with the Gaussian curve. The diameter of such a beam at the focal point is 2tv₀, which is defined as measured in the location, where the intensity of the beam has dropped to a 1/e² part of the maximum intensity in the middle of the beam. The wavelength of the beam in vacuum is λ and the refractive index of the beam in the medium component is n. When an optical resonator is in question, where only one beam resonates, the focusing parameters of the different beams are defined as follows. The focusing parameter of the pump beam essentially depends on the size and divergence of the laser beam coming out from the pump laser and on what kind of a lens arrangement is used to focus the beam into the nonlinear crystal functioning as the medium component. The focusing parameter of the resonating beam, which is most commonly the signal beam, but the idler beam can also be used, is defined by the dimensions of the resonator, i.e. by what the radii of curvature of the mirrors and the distances between the mirrors of the resonator are. The focusing parameter of the third beam involved in the parametric amplification, which is the idler beam, if the resonating beam is the signal beam, is defined based on the focusing parameters of the two other beams.

In the linear resonator shown in Fig. 5, secondary focuses are formed at the third mirror and the grating. The mirrors and the grating are arranged at such distances from each other that the location and size of the focus of the resonating beam in the medium component is adapted to the location and size of the focus of the pump beam, so that the parametric amplification is as large as possible. In practice this means dimensioning the resonator so that the focusing parameters of the beams in the crystal are ξ > 1. For example when using a 50 mm long MgO: PPLN crystal and when the pump wavelength is 1064 nm and the signal wavelength is 1611 nm, the optimal sizes of the beams in the middle of the crystal are as follows. The diameter of the pump beam is about 80 μm and the diameter of the signal beam is about 120 μm, when the intensity distributions of both beams are Gaussian. When using a gold-plated reflective grating, the optical parametric oscillator exceeds the oscillation threshold at about a 10 W continuously operating pump power, the pump wavelength being 1064 nm and the signal wavelength being 1611 nm. The output power of the idler beam at a 16 W pump power has 23 Dec 2009 16

been measured as 250 mW, the idler beam wavelength being 3134 nm. The threshold power can be lowered by using a grating, which reflects the resonating wavelength better. For example with the same 1611 nm resonating wavelength, the threshold power can using a silver coating be put at under 6 W.

In another example the focusing parameters of the resonating signal beam (SB) and pump beam (PB) are in the dimensioning of the optical resonator according to Fig. 5 selected as ξ_s = ξ_p = 2, whereby these are of the same size and larger than one. The focusing parameter of the idler beam is defined according to the other focusing parameters, whereby it also becomes essentially 2, when the others have been adapted. For the pump beam the desired focusing parameter can be implemented by selecting a lens arrangement 503, the focal length f_L of which is defined with the equation^ = πw_ow_L/λ_p , where W₀ is the desired size of the pump laser beam in the medium component 510. This can be calculated with the aid of the formulas given in the presentation of the focusing parameters, using the selected focusing parameter value ξ_p = 2. The parameter w_L is the original size of the collimated beam coming from the pump laser at the lens used for focusing and λ_p is the wavelength. The location of the focal point of the pump beam in the medium component can be implemented by selecting in a suitable manner the distance of the lens arrangement 503 in relation to the medium component. In order to adapt the focusing parameters of the signal beam, radii of curvature should firstly be selected for the first mirror 521 and the second mirror 522, with which a sufficiently large ξ_s can be achieved, still so that the physical size of the resonator remains small enough to ensure a stable one-mode operation. The stability is in addition to the mechanical stability of the resonator also affected by the frequency distance FSR of the longitudinal resonance modes. This is inversely proportional to the length of the resonator. The larger the FSR, the more unlikely mode hopping of the optical resonator is. For the stable operation it is useful to keep the length of the continuously operating optical resonator as short as possible. In the case of optical resonators operating with a pulsed pump beam this does not matter, because they do not in any case usually operate in a one-mode manner. The radius of curvature (ROC) of the first and second mirror is selected as 100 mm. With the selected radii of curvature of the mirrors, the desired focusing parameter is obtained by using the following formula.

wu =

23 Dec 2009 17

where Wi_s is the size corresponding to the desired focusing parameter of the resonating beam in the middle of the medium component. This can be calculated from the definition formula of the focusing parameter. The wavelength of the resonating beam is λ_s and f = ROC/2. The optical distance between the middle of the crystal (i.e. the focus location) and the first mirror 521 is marked with di and the distance between the second mirror 522 and the third mirror 523 with 02. As can be seen from the w_1s formula, the desired focusing parameter is possible to obtain with several combinations of di and 0₂. Of these combinations it is advantageous to select the one, which produces a resonator structure, where the size of the focus of the resonating beam is as insensitive as possible to changes in the length of the resonator.

With the numeric values presented in this example the dimensioning in question is achieved by selecting di = 56 mm and 0₂ = 262 mm. Because a symmetrical resonator is in question, the total length of the resonator is L = 2^χ(di + d₂) = 636 mm. The corresponding longitudinal resonator mode frequency distance FSR = c/2L = 236 MHz. The FSR in question is large enough to ensure a mode hop free operation in a resonator based on a grating, and at the same time the physical size of the resonator is small enough with regards to usability and mechanical stability. Said numerical values are calculated for the medium component (MgO:PPLN), the refractive index of which is n = 2, the signal wavelength being λ_s = 1611 nm. The same dimensioning also works when adjusting the resonating wavelength, because even if the size of the focus changes when the wavelength changes, the focusing parameter remains unchanged. This is because the focusing parameter ξs depends both on the wavelength and the size of the focus, in accordance with the above-mentioned formula for the focusing parameter.

In the example given in Fig. 15, measured situations have been compared, where an etalon or a grating has been placed in the optical resonator. The pump wavelength is 1064 nm and it is produced with a continuously operating laser. The frequency of the outgoing beam of the optical resonator, which in this case is the idler beam, has been adjusted by turning the etalon or the grating. The adjustment signal, i.e. the angle of the component, is in arbitrary units, and the corresponding frequencies of the idler beam are in terahertz (THz). As components have been used a diffraction grating (black balls) and 1 mm (squares) and 0.4 mm (balls) thick etalons. A wider adjustment area is achieved with the grating, because with the etalon frequency hops occur between the modes of the etalon, whereby the frequency hops after a certain angle adjustment, which depends on the thickness 23 Dec 2009 18

of the etalon, back to its original value. Thus the advantages of the grating compared to the etalon can clearly be seen.

The maximum power and thus the parametric amplification of the pump laser of optical resonators operating with a pulsed pump beam is typically very large. Thus the focusing parameters are not necessary to or even profitable to dimension as described above in order to maximize the parametric amplification. With several optical resonators operating with a pulsed pump beam, resonator solutions are used, where the focusing parameters are clearly smaller than 1 and/or the focusing parameters of the beams involved in the parametric process differ from each other remarkably. In many cases no attention needs to be paid to the dimensioning of the focusing parameters in resonators operating with a pulsed pump beam. In the case of a continuously operating optical resonator, the focusing parameters and thus also the resonator structure itself, are on the other hand more significant. If the focusing parameters have not been dimensioned to be suitable, the threshold power of the optical resonator rises so high that it cannot be reached at all with existing continuously operating and narrow-banded pump lasers or that it is unpractical with regards to the applications. In practice this means that the focusing parameters essentially implement the earlier mentioned conditions, that the location of the focuses of the resonating beam and the pump beam in the medium component are adapted so that the parametric amplification is as large as possible. Advantageously the focusing parameters are thus essentially of the same size. Advantageously the focusing parameters are larger than 1.

The frequencies of the signal and idler beams produced by the optical resonator comply with the law of energy conservation f j = f _p - f_s, but also depend on which of the frequencies are phase matched to achieve a sufficient parametric amplification. Phase matching can be arranged for example with a MgO:PPLN crystal, the poling period of which is selected in a suitable manner to achieve phase matching at desired wavelengths. The phase matching wavelengths can further be affected by changing the temperature of the crystal, which alters the optical length of the crustal and thus the length of the poling periods. For example by changing the poling period between 30.5 μm and 31.5 μm and the temperature of the crystal between 20°C-120°C, the phase matching wavelengths can for the signal beam be adjusted between 1540 nm - 1760 nm and for the idler beam between 2700 nm - 3450 nm. The wavelengths of the outgoing beams of the optical resonator can be selected in these ranges by selecting the position of the 23 Dec 2009 19

grating so that a desired signal wavelength resonates in the resonator. When the angle between the normal of the grating and the beam striking it is &\, the wavelength of the beam is λ and the grating constant is d, the angle θi between the normal of the grating and the beam of the first order reflected from the grating can in the case of a diffractive grating be obtained from the equation sin(Φ) = λ Id - sin((9i). When using the so-called Littrow configuration according to Fig. 5, the angles of the reflected and the incoming beams are the same, i.e. & = θ\. Thus the wavelength of the precisely reflected beam, i.e. the one resonating in the resonator, is defined from the equation λ = 2dsin(#i). The same equation can be used to calculate the reflecting wavelength also in the case of a Bragg grating. By changing the grating angle θ_\ one can thus select the resonating wavelength and thus the wavelengths of the outgoing beams, the signal beam and the idler beam, of the optical resonator. When using a transversally chirped Bragg grating, the grating constant of which changes in the cross-direction, the wavelength reflected by the grating can be adjusted by moving the grating component sideways. Thus the resonating beam is in the direction of the grating normal and the grating reflects a beam striking it in its incoming direction.

When keeping the poling period and temperature of the crystal constant, the frequencies of the signal and idler beams can be changed simply by turning the grating angle or otherwise moving the grating in the area, where the phase matching is sufficient to achieve a parametric amplification which exceeds the oscillation threshold. When turning the grating angle or otherwise moving the grating, the frequencies of the signal and idler beams change the same amount, but in opposite directions. For example with a 50 mm long MgOPPLN crystal, the adjustment area achievable by turning the grating angle has been measured as >400 GHz in a signal frequency environment of 171 THz (wavelength 1750 nm).

The output frequencies of an optical resonator can also be fine-tuned by altering the length of the resonator. Thus the frequencies of the signal and idler beams change in accordance with the law of energy conservation the same amount, but in opposite directions. The relative size of the frequency change is the same as the relative change in the length of the resonator. If the other elements are kept constant, the fine-tuning area corresponds to the distance between the longitudinal modes of the resonator, whereafter the frequency resonating in the resonator makes a mode hop. Typically the thus obtainable mode hop free adjustment area is less than 1 GHz, depending on the length of the resonator. A larger fine-tuning area requires the simultaneous adjustment of several elements, for example the 23 Dec 2009 20

length of the resonator and the grating angle, so that the amplification maximums of each element move at the frequency level at the same pace. This can be arranged by changing the angle and the location of the grating simultaneously. The angle and the location of the grating can be changed with a grating movement arrangement 509, which can for example be made up of piezoelectric actuators or linear motors or the like. The length of the resonator can alternatively be changed by changing the location of one or several of the resonator mirrors for example using piezoelectric actuators or linear motors or the like.

The idler frequency of the optical resonator can alternatively be fine-tuned by changing the frequency of the pump beam. If the other elements, i.e. the poling period of the crystal, the temperature of the crystal, the grating angle and the length of the resonator are kept constant, the frequency of the idler beam changes according to the law of energy conservation the same amount as the pump frequency when the signal frequency is the resonating frequency. The frequencies of the outgoing beams of the optical resonator can be adjusted over a larger area by changing, in addition to the grating angle or the grating location, also the phase matching wavelength. This can be done by changing the temperature of the MgO: PPLN crystal. For example by changing the temperature of the crystal between 20⁰C - 120°C, the signal and idler frequencies can be adjusted more than 6 THz when the poling period of the crystal is 31 μm. The adjustment area can further be increased by changing the poling period of the crystal. This can be arranged for example with a crystal, which has several different parts in parallel, whereby the poling period, through which the beams pass, can be selected by moving the crystal sideways. For example by changing the poling period of the MgO:PPLN crystal between 30.5 μm - 31.5 μm, by changing the temperature of the crystal between 20⁰C - 120⁰C and by changing the grating angle between 43.87° - 52,37° (a diffraction grating, the grating constant d = 1/900 mm) all the wavelengths between 1540 nm - 1760 nm (signal beam) and between 2700 nm - 3450 nm (idler beam) can be covered.

The grating operates in the optical resonator as a frequency selective element, which in this case reflects a beam striking it back in its travel direction only on a narrow frequency band. For example in the case of a diffraction grating the band width of the grating depends on the number of grating lines illuminated by the beam. Typically the band width 100-200 GHz of the grating is enough to produce a sufficient frequency selectivity, so that the optical parametric oscillator operates reliably in one single resonator mode. This is illustrated in Fig. 6, which shows the 23 Dec 2009 21

spectrum of an optical parametric oscillator at the idler beam frequency. The side modes have been dampened by more than 20 d B compared to the resonator mode. This is an important feature for example with regards to laser spectroscopy applications, where a narrow-banded light is often required. The grating can also be arranged to reflect or diffract a resonant beam striking it also in other directions than the incoming direction. Also in that case the frequency of the resonating beam can be changed by turning or otherwise moving the grating.

The grating also stabilizes the resonant frequency by effectively preventing mode hopping between longitudinal resonator modes. The long-term stability measured for the signal frequency shows that the relative change of the frequency is about 1.3 ppm per hour, which is a very small value. Also the output power remains very stable. Its measured long-term stability shows that the change in the output power is about 0.5 % per hour. These are shown in Figs. 13 and 14. Fig. 13 shows the measured long-term stability of the signal frequency. Due to its simple structure, the frequency of the optical parametric oscillator remains very stable. The stability of the output power is shown in Fig. 14.

In Fig. 7 is a second example of an optical resonator according to the invention. A laser device 701 produces a pump beam PB, which is focused for example with a lens arrangement 703 into the optically nonlinear medium component 710. In addition to the medium component, the resonator has a first mirror 721 and a second mirror 722 and a grating 723. The optical resonator according to Fig. 7 operates as the resonator presented in Fig. 5, except that in this case the optically nonlinear medium component has outside the optical axis only the grating 723 as a mirror. Its position can be changed with a grating movement arrangement 709.

The second mirror 722 reflects the resonating beam coming from the medium component 710 toward the grating 723. The beam is reflected from the grating back to the second mirror, which focuses the resonating beam into the medium component 710. The resonating beam, which has passed through the medium component, strikes the first mirror 721 , which reflects the resonating beam focused back into the medium component. The second mirror lets through the pump beam and for example the idler beam IB, if the resonating beam is the signal beam. The pump beam can be filtered out from this outgoing beam.

In Fig. 8 is a third example of an optical resonator according to the invention. A laser device 801 produces a pump beam PB, which is focused for example with a lens arrangement 803 into the optically nonlinear medium component 810. The 23 Dec 2009 22

example is otherwise the same as in Fig. 7, but here the end 811 toward the laser device of the optical nonlinear medium component is coated with a material, which lets through the pump beam and reflects the resonating beam, for which it thus constitutes a mirror. Additionally the end toward the laser device of the medium component is designed to be focusing.

The beam coming from the medium component 810 strikes the mirror 822, which reflects the resonating beam toward the movable grating 823. The beam is reflected from the grating back to the mirror 822, which focuses the resonating beam into the medium component 810. The resonating beam strikes the end 811 of the medium component, which is toward the laser device, which end functions as a mirror. The resonating beam is reflected focused back into the medium component. The advantage with this solution is the simplification of the optical parametric oscillator and the decreasing of optical losses, because one lossy crystal-air boundary layer is removed. In Fig. 9 is a fourth example of an optical resonator according to the invention. This example is otherwise the same as the one shown in Fig. 5, but now the initial beam IB is switched out through the planar mirror 923 corresponding to the third mirror 523 of Fig. 5.

The laser device 901 produces a pump beam PB, which gets through the first mirror 921 into the medium component focused with a lens arrangement 903. In the medium component 910 the pump beam breaks up into a signal beam and an idler beam. The second mirror 922 reflects the resonating beam and the beams meant for exiting toward the third mirror 923. The third mirror reflects the signal beam back toward the second mirror and lets through the frequencies, which are the pump frequency and the idler frequency, if the resonating frequency is the signal frequency. The frequency of the resonating beam is adjusted by turning the grating 924 with the grating movement arrangement 909. The resonating beam circulates via the grating as in Fig. 5.

In Fig. 10 is a fifth example of an optical resonator according to the invention. This example is otherwise as the one shown in Fig. 5, but now the second mirror A22 and the third mirror A23 have been arranged to reflect also at the pump beam PB frequency. For clarity only the passage of the pump beam is marked in the figure.

The laser device A01 produces a pump beam PB, which gets through the first mirror A21 into the medium component A10. In the medium component the pump 23 Dec 2009 23

beam breaks up into a signal beam and an idler beam. The remaining component of the pump beam comes out of the medium component and strikes the second mirror A22, which now also reflects the pump beam toward the third mirror A23. The third mirror reflects both the resonating and the pump beam back toward the second mirror, which reflects the resonating beam and the pump beam focused into the medium component. Because the first mirror A21 lets through the pump beam, the remaining pump beam PB' comes out of the resonator. The adjustment of the signal and idler beam frequency is done with a grating A24, as in the example in Fig. 5. With this solution the threshold power is lowered, because the unconsumed part of the pump beam is directed a second time through the medium component. The effective pump power in the medium component thus grows.

In Fig. 11 is a sixth example of an optical resonator according to the invention. The optical resonator BOO is a ring resonator. In it a pump beam PB is obtained from a laser device B01 , which pump beam is focused for example with a lens arrangement B03 into a medium component B10. The first mirror B21 lets through the pump beam. The resonating beam coming from the medium component strikes the second mirror B22, which reflects the resonating beam to the third mirror B23 and lets through the residual beam PB' and the beam IB, which is meant to be taken out. The third mirror reflects the resonating beam to the grating B24, which in turn reflects it toward the first mirror B21. The grating is made to be movable with the grating movement arrangement B09. The first mirror focuses the resonating beam coming from the grating into the medium component, where the resonating beam interacts with the beams therein. Also in the case of a ring resonator, a stable resonator solution, which forms the focus of the resonating beam into the medium component, is achieved using only one focusing mirror. By turning the grating the frequencies of the signal and idler beams can be adjusted.

In Fig. 12 is a seventh example of an optical resonator according to the invention. The optical resonator COO is a ring resonator, as in Fig. 11. The difference from the example in Fig. 11 is that the focusing mirror is not on the optical axis of the medium component.

The pump beam PB is obtained from a laser device C01 , which pump beam is focused for example with a lens arrangement C03 into the medium component C10. The first mirror C21 lets through the pump beam. The resonating beam coming from the medium component strikes the second mirror C22, which reflects the resonating beam to the third mirror C23 and lets through the outgoing beams IB, PB'. The third mirror is designed to be focusing and it is placed so that the 23 Dec 2009 24

focus of the resonating beam is inside the medium component. The third mirror reflects the resonating beam to the grating C24, which in turn reflects the resonating beam toward the first mirror, which further reflects it into the medium component. The optical resonator according to the invention can be used for example in laser spectroscopy, where precise adjustment of the wavelength and a continuous signal are needed. Typical applications for laser spectroscopy are medical and safety engineering applications, where it is attempted to determine the composition of a gaseous or liquid sample or the concentrations of molecules in it. Especially in the infrared area, wavelength larger than 2 μm, functioning narrow- banded, continuously operating devices are needed, the wavelengths of which can be adjusted precisely and quickly. Previous solutions are expensive and difficult to use.

Another use example for the invention is infrared light sources for military use. These are used for example to bluff heat-seeking missiles. Because one of those can operate at a very narrow wavelength band, a quick and wide wavelength area adjustment is required from the infrared light source.

Some advantageous embodiments according to the invention have been described above. The invention is not limited to the solutions described above, but the inventive idea can be applied in numerous ways within the scope of the claims.

Claims

23 Dec 2009 25Claims

1. An optical resonator fed with a laser source (501 ; 701 ; 801 ; 901 ; A01 ; B01 , C01), which has an optically nonlinear medium component (510; 710; 810; 910; A10; B10; C10) for transforming the pump beam (PB) coming from the source into a signal beam (SB) and an idler beam (IB), the sum of the frequencies of which is essentially the frequency of the pump beam, of which signal and idler beam one is a resonating beam, the frequency of which is the resonant frequency of the optical resonator, which resonator further has a first mirror (521 ; 721 ; 811 ; 921 ; A21 ; B21 ; C21 ) on the incoming side of the medium component on its optical axis, a second mirror (522; 722; 822; 922; A22; B22; C22) on the outlet side of the medium component on its optical axis and at least one mirror outside the optical axis of said medium component in the path of the resonating beam, and the resonator is arranged to let out at least one of the signal and idler beam, characterized in that the laser source is continuously operating and at least one of the mirrors is a mirror, which focuses the resonating beam into the medium component, and the mirror outside the optical axis of the medium component is a grating (524; 723; 823; 924; A24; B24; C24), which grating is moveable in order to tune the optical resonator.

2. The optical resonator according to claim 1 , characterized in that the grating (524; 723; 823; 924; A24; B24; C24) reflects at the resonant frequency.

3. The optical resonator according to any of the claims 1-2, characterized in that the pump beam (PB) is arranged to be focused so that its focus is in the medium component (510; 710; 810; 910; A10; B10; C10).

4. The optical resonator according to claim 3, characterized in that the pump beam (PB) is arranged to be focused with a lens arrangement (503; 703; 803; 903;

B03; C03), which is between the laser source (501 ; 701 ; 801 ; 901 ; A01 ; B01 ; C01) and the first mirror (521 ; 721 ; 811 ; 921 ; A21 ; B21 ; C21 ).

5. The optical resonator according to any of the claims 1-4, characterized in that the location and size of the focus of the resonating beam in the medium component (510; 710; 810; 910; A10; B10; C10) is adapted to the location and size of the focus of the pump beam (PB), so that the parametric amplification in the medium component is as large as possible. 23 Dec 2009 26

6. The optical resonator according to any of the claims 1-5, characterized in that two of the mirrors are focusing mirrors (521 , 522; 721 , 722; 811 , 822; 921 , 922; A21 , A22).

7. The optical resonator according to any of the claims 1-6, characterized in that outside the optical axis of the medium component there is a grating (524; 924;

A24; B24; C24) and a mirror (523; 923, A23; B23; C23).

8. The optical resonator according to any of the claims 1-7, characterized in that the mirrors of the resonator, with the exception of the first mirror, have been arranged to reflect the pump beam (PB), whereby the pump beam (PB') can be arranged to be removed from the resonator through the first mirror (A21 ).

9. The optical resonator according to any of the claims 1-8, characterized in that at least one of the ends (811) of the medium component (810) is arranged to replace the mirror on the side of the end in question on the optical axis of the medium component.

10. The optical resonator according to any of the claims 1-9, characterized in that it is a linear resonator.

11. The optical resonator according to any of the claims 1-10, characterized in that it is a ring resonator.

12. The optical resonator according to any of the claims 1-11 , characterized in that the focusing parameters of the beams involved in the parametric process are essentially of the same size.

13. The optical resonator according to any of the claims 1-12, characterized in that the focusing parameters of the beams involved in the parametric process are larger than 1.

14. The optical resonator according to any of the claims 1-13, characterized in that the grating is a Bragg grating.

15. The optical resonator according to any of the claims 1-13, characterized in that the grating is a transversallychirped Bragg grating.

16. The optical resonator according to any of the claims 1-13, characterized in that the grating is a diffraction grating.