OPTICAL PARAMETRIC OSCILLATOR

This invention relates to optical parametric oscillators (OPO) in which a non linear material is optically pumped with light energy to produce optical outputs of different wavelengths to that of the pump energy.

Such oscillators may be pumped by a pump laser (wavelength λ _p , frequency ω _p ) to emit light at two wavelengths, termed a signal wave λ _s (frequency ω,) and an idler- wave λ, (frequency o>, ) such that ω _p = ω +ω, . By convention the signal wave is defined as that wave with the higher frequency of the two generated waves. When the OPO is operated as a double resonant oscillator both the generated signal-wave and idler-wave are resonated. When the OPO is operated as a singly resonant oscillator either the generated signal-wave or the idler-wave is resonated, but not both. Both continuous wave and pulsed pump lasers have been used to provide doubly resonant parametric oscillation. However, the doubly resonant character of the resonator causes the output to be unstable both in amplitude and frequency. Singly resonant parametric oscillation has been achieved using both pulsed, and more recently continuous wave, pump lasers but the threshold pump power for the singly resonant case is much higher (as much as about two orders of magnitude) than the that of the doubly resonant case. Singly resonant oscillators produce a much more stable output and impose less stringent demands on the mirror coating design. For these reasons the singly resonant configurations is the one most frequently used.

In order to provide sufficient energy with a continuous wave beam to exceed threshold, doubly resonant optical parametric oscillators have been placed within the cavity of the laser used to generate the pump wave. In this prior art, such doubly- resonant OPO devices were proposed [see for example Kingston. Proc. IRE. 50. 472 ( 1 62) ], analysed theoretically [ Oshman & Harris, I.E.E.E. J. Quant. Elec. QE4, 491 - 502 ( 1968)] and experimentally demonstrated [ Smith & Parker. J. Appl. Phys. 41. 3401 -8 ( 1970) ]. Such arrangements suffer from tuning disadvantages.

In the prior art pulsed optical parametric oscillators have been proposed [ Kroll, Phys. Rev. 127, 1207-1 1 ( 1962)], analysed theoretically [ Falk. Yarborough & Ammann. I.E.E.E. J. Quant. Elec. QE7, 359-69 ( 1971 )] and demonstrated [see Lavi, Englander & Lallonz, Op. Lett. 21, 800-2 (1996) and references therein], but the performance characteristics and properties of continuous-wave devices of the present invention are different in kind to those of pulsed devices.

One disadvantage of the prior art is the difficulty of providing sufficient pump energy with a continuous wave pump beam so as to exceed oscillation threshold in a singly- resonant OPO placed external to the pump laser cavity. A second disadvantage of the prior art is the amplitude and frequency instability of a doubly -resonant OPO placed either internal or external to the pump laser cavity, and the difficulty of tuning and aligning the various components to optimise the oscillator output.

In order to avoid the disadvantages associated with the doubly resonant oscillator, while at the same time providing sufficient power with a continuous wave beam to exceed threshold, singly resonant oscillators have been placed within the cavity of the laser used to generate the pump wave. This arrangement is described in United States Patent No 3.628,186, but performance characteristics of embodiments have not been provided.

Such an arrangement as described suffers from a number of problems and disadvantages.

One problem of this prior art is difficulty in aligning the cavities of the singly resonant optical parametric oscillator and the pump laser due to the lack of a means which either partially or fully defines the respective cavity modes with regard to co-linearity concentricity and overlap of beam waists.

According to this invention the aforementioned problem is overcome in relation to continuous wave, singly resonant optical parametric oscillators that are arranged within the cavity of the laser used to generate the pump wave, by a mirror arrangement, or equivalent optical arrangements, that is common to both pump laser and OPO. This arrangement may facilitate the setting up and optimisation of the intracavity OPO, and further may contribute to the stability of the OPO during operation.

A second problem of this prior art is that the resonant wave, either signal or idler, within the optical parametric oscillator is not extracted as useful output. According to the present invention the aforementioned problem is overcome by providing means for extraction of the resonant wave. It is to be noted that even though the pump laser is operating multimode. it is possible for this resonant wave to display the advantage of being single mode (single frequency), with the non-resonant wave being generated in a number of modes corresponding to those of the multimode pump laser. It is therefore advantageous to be able to extract this resonant wave as useful output. A further advantage of being able to extract the resonant wave is that a lower oscillation threshold for the OPO may be obtained by resonating the wave with the lowest loss in the non-linear gain medium, whether it be the signal or the idler wave, while at the same time extracting that wave as useful output.

A third disadvantage of this prior art as stated in USA Patent No. 3,628,186 is that the pump wave power as transmitted by the optical parametric oscillator is not clamped to the threshold power. According to the present invention, means are provided to clamp the pump wave power within the cavity of the pump laser and as transmitted through the optical parametric oscillator to the threshold pump power of that oscillator. This has the advantages of reducing both detrimental thermal effects in the non-linear gain medium as well as the likelihood of optical damage by the pump wave.

A disadvantage of the prior art is that the wavelength of the pump wave is fixed and hence cannot be finely tuned in order to smoothly and continuously tune the wavelength of the non-resonant wave, particularly but not exclusively in those cases where the pump wave is single mode and hence where the non-resonant wave is also single mode (single frequency) given a single frequency resonant wave. An associated disadvantage in the prior art is that the pump wave cannot be coarsely tuned so as to provide extended but coarse (i.e. with mode hops) tuning of the resonant and non- resonant waves without the need to alter the conditions appertaining to phase matching at the non-linear gain medium. According to the present invention the aforementioned problem is overcome by providing means for supplying a pump wave of variable wavelength or frequency to the non-linear gain medium.

A further disadvantage of the prior art is that the wavelengths of the resonated and non-resonated waves (signal and idler waves) cannot be independently chosen, as would be advantageous for example in pump-and-probe spectroscopy or the provision of optical frequency standards with integral ratios between the frequencies of pump, signal and idler waves. According to the present invention the aforementioned problem is overcome by providing means for supplying a pump wave of variable frequency to the non-linear gain medium, as well as for simultaneously altering the phase matching conditions by appropriate adjustment of the non-linear gain medium itself.

The present invention provides an optical parametric oscillator (OPO) in which the non-linear gain medium is placed within the cavity (resonator) of the laser used to pump the optical parametric oscillator and where the optical parametric oscillator is operated both as a singly-resonant oscillator (SRO), in which either the generated signal- or idler-wave is resonated, but not both, and also as a continuous-wave device, with an output formed by either or both generated waves.

According to this invention an optical parametric oscillator (OPO) system comprises:

a continuous wave pump laser having a laser gain medium within a laser cavity formed between reflecting surfaces;

a singly resonant parametric oscillator having a non-linear gain material capable of generating both a signal wave and an idler wave when illuminated by a pump wave, the non linear material being arranged within the pump laser cavity between reflecting surfaces forming an oscillator cavity resonant at one of the generated waves and including means for preventing feedback of the non-resonant wave within the oscillator cavity,

means for focusing both pump and resonated wave to required waist dimensions within the non linear material to give a common coaxial and co-linear optical path to both pump and resonated wave within the non linear material, whereby simultaneous alignment of both pump and oscillator cavities is obtained with matching of said beam waists, and

means for directing one of the generated wavelengths from the oscillator to form an output signal.

Preferably the oscillator and pump laser cavities are arranged with at least one common reflecting surface which focuses both pump and resonated waves to required waist dimensions within the non linear material to give a common coaxial and co- linear optical path to both pump and resonated waves within the non linear material, whereby simultaneous alignment of both pump and oscillator cavities is obtained with matching of said beam waists. For example the non-linear material may be arranged between two focusing mirrors.

Preferably the laser gain material is tuneable so that the phase match bandwidth of the parametric oscillator may be tuned by tuning the wavelength of the pump wave without the need to alter the orientation, temperature or other relevant parameter of the non-linear gain medium itself. The tuning may be achieved by a suitable tuning element within the pump laser cavity. Alternatively, the pump laser may operate at a fixed frequency and the non linear material is tuned by variation of its temperature, angular position within the oscillator cavity, or some other parameter influencing phase matching to vary oscillator frequency.

Preferably both the laser gain material and the non-linear gain material may be simultaneously tunable, so allowing independent selection of idler wave and signal wave frequencies or wavelengths.

The non linear material may be KTP or LiNbO, or isomorphs of these materials, or periodically poled versions of these materials or their isomorphs, or LBO.

One of the reflecting surfaces forming the oscillator cavity may be partly transmitting at the resonant wave frequency, the transmission being chosen to optimise the output power available from the resonant wave.

The non-linear material may have its temperature and/or angular position varied to vary the ratio of ω to ω _r This may be additional to varying the pump laser frequency by the birefringent tuner element.

Embodiments of the invention will now be described by way of example only, and with reference to the attached figures, in which:

Figure 1 is a schematic view showing the generic principles of the invention;

Figure 2 shows a first embodiment with a parametric oscillator within the cavity of a pump laser sharing common focusing mirrors:

Figure 3 shows the dependence of both the intracavity pump-wave power and the idler- wave output power on the pump power delivered by a priming argon ion laser to the gain medium of the pump laser for the case where the pump-wave wavelength is approximately 800 nm: and where the pump power (intracavity) is clamped by the onset of oscillation in the parametric oscillator;

Figure 4 shows the dependence of the wavelength of the generated signal-wave of the optical parametric oscillator on the wavelength of the pump-wave generated by the pump laser:

Figure 5 shows the signal-wave power generated by the singly-resonant, continuous- wave optical parametric oscillator as a function of time;

Figure 6 shows the spectral content of the generated resonant-wave (signal wave in this case) as monitored by a scanning interferometer, indicating single-axial- mode (single-frequency) oscillation of the optical parametric oscillator;

Figure 7 is a schematic view showing a second embodiment of the invention in which the cavity of the pump laser is a ring (travelling wave) resonator:

Figure 8 is a schematic view showing a third embodiment of the invention in which both the cavity of the pump laser and also the cavity of the optical parametric oscillator are ring (travelling wave) resonators:

Figure 9 is a schematic view showing a fourth embodiment of the invention in which the gain medium of the pump laser is not tuneable and wide-tuning of the optical parametric oscillator is achieved by variation of one of the parameters of its non-linear gain medium;

Figure 10 shows multi parameter tuning of the singly resonant oscillator; and

Figure 1 1 shows the power output for the resonated and non-resonated waves.

Figure I shows the principles of the optical parametric oscillator. It comprising a pump laser 1 containing a parametric oscillator 2. The pump laser 1 itself comprises a laser gain medium 3 located between two mirrors 4, 5 which define a pump laser cavity 6. The laser gain medium receives input power 7, e.g. from a priming laser or other power source (not shown). The parametric oscillator 2 comprises a non-linear gain medium 8 located between two mirrors 9. 10 which form an oscillator cavity 1 1. Upon receipt of input energy 7 the laser gain medium 3 is stimulated to emit light between the two mirrors 4. 5 and in doing so passes this light through the non-linear gain medium 8 causing it to emit additional light one component of which resonates between the two oscillator mirrors 9. 10. The oscillator cavity 1 1 resonates at either a signal-wave or idler-wave (OPO field 1 ). The non resonated wave (OPO field 2) exits through either one or both of mirrors 9, 10. In addition useful output of OPO field 1 may be obtained by making either mirror 9 or 10 partially transmitting for this wavelength.

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In one embodiment of the invention mirrors 5 and 10 are the same (common) mirror. In another embodiment of the invention the laser gain medium 3 is tunable, so enabling the wavelength of the pump wave or field to be tuned. In another embodiment of the invention the phase-matching condition in the non-linear gain medium may be independently altered, which together with the ability to tune pump field allows the signal and idler waves to be independently tuned. In a further embodiment of the invention, the resonated wave (OPO field 1 ) is efficiently coupled out of the oscillator cavity 1 1 by selecting the appropriate transmission for either mirror 9 or 10.

One specific embodiment of an OPO system is shown in Figure 2 and follows the principles shown in Figure 1. The system comprises a pump laser . a parametric oscillator, and a power source.

The pump laser comprises a crystal of Ti:sapphire 3, forming a laser gain medium, located within a standing-wave cavity 6 formed by mirrors M l -6. The crystal is a standard c-cut crystal with Brewster-angied faces, and is 7.5mm long. Mirrors M2. M3 are focusing mirrors of radius of curvature 10cm. that focus the pump- wave mode to a suitable beam waist, typically about 20μm, within the crystal 3. Mirrors M l. and M4 are highly reflecting plane mirrors. Mirror M6 and M5 have a radius of curvature of 20cm and focus the pump beam to a waist of typically 30 to 50μm. A tuning element is formed by birefringent plate 15 which allows tuning of the pump laser linewidth and centre frequency by rotation of the plate 15. The pump beam within the cavity 6 is polarised with its electric vector in the plane of the figure 2.

The OPO comprises a non-linear gain medium 8 which is a potassium titanyl phosphate (KTP) crystal located within the pump laser cavity 6 and between mirror M5. M6 with the waist of the pump beam in the centre of the crystal 8. The KTP crystal 8 is 20cm long and is cut for standard type II noncritical phase matching in which the pump wave and generated signal wave are polarised with their electric vectors in the plane of the figure, and the generated idler wave is polarised with its electric vector normal to the plane of the figure. The optical faces (y-z faces, 4x4mm) of the KTP crystal 8 are cut for normal incidence of the various waves when they propagate along the x-axis of the crystal 8. and are antireflection coated; e.g. residual reflectivity ^' s typically <0.5% for pump wave and resonated signal wave, and <5% for the non resonated idler wave.

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The signal-wave is resonated within the KTP crystal 8 by means of the standing-wave cavity formed by mirrors, M6. M5. a beamsplitter 16 (which is highly reflecting for the signal wave >99% and high transmitting for the pump wave > 99%). and mirror M7. All these mirrors are of high reflectivity for the signal-wave (>99%). Additionally Mirror M7 is significantly reflective at the pump wavelength. A signal wave output ω _s may be taken from transmission through M7.

The idler-wave ω _r which is not resonated in this singly-resonant oscillator but which is generated as a single-pass wave in both directions through the KTP crystal 8, leaves the cavity 1 1 tlirough either / both mirror M6 or/and mirror M5, both of which have a significant transmission (>60%) at the idler-wave wavelength. The idler-wave may be used as an output, but will not have a single frequency if the pump laser operates at multiple frequencies.

The input power source 7 comprises an argon-ion laser 17 whose output is directed via mirrors 18, 19. 20, 21. a wave plate 22. and focusing lens 23 and mirror M2 into the Ti:sapphire crystal 3. Typically the laser 17 has an output at 514.5nm and is focused by lens 23 so as to be mode matched with the pump wave mode inside the crystal 3.. and being in a suitable polarisation state (electric vector orthogonal to the plane of the figure), which can be controlled using the wave plate 22 so as to optimise the gain experienced by the pump wave within the crystal 3.

In operation the Argon ion laser 17 optically pumps the crystal 3 to cause generation of a pump beam. The pump wave can be multimode (multi axial mode. i.e. multi frequency) while still efficiently pumping a single axial mode signal wave in the OPO. In one example the pump wave had a linewidth of the order of 20GHz. As noted above, mirrors M5. M6 focus the pump wave to a beam waist ( i.e. required energy density) within the KTP crystal 8; additionally they focus the signal wave to a beam waist to mode match with the pump wave.

An advantage of the use of common mirrors M6 and M5. is that when the optical cavity 6 of the pump-wave has been aligned and the pump laser 1 thereby brought into oscillation, it is then no longer necessary or appropriate to readjust these two mirrors in seeking to bring the cavity 1 1 of the OPO into resonance and hence the OPO into oscillation, these latter being effected solely through alignment of mirror M7. Further, by making the beamsplitter 16 to be slightly reflecting (-0.5%) at the pump-wave wavelength and with mirror M7 significantly reflecting at the pump-wave wavelength, then the correct alignment of the OPO cavity, effected through the adjustment of mirror M7, can be gauged through the enhancement in the pump-wave that occurs simultaneously with this alignment, this enhancement being as a result of the increased feedback occasioned through mirror M7 and the beamsplitter 16 reflecting back pump- wave into the pump laser cavity.

The power output characteristics of the system of Figure 2 are shown in Figure 3 in which the pump-wave (intracavity) power and the idler- wave power are displayed as a function of the optical power applied to the Ti:sapphire crystal 3 by the argon laser 17. As expected, the intracavity pump-wave power clamps at the threshold value for oscillation of the OPO.

Figure 4 shows the tuning range of the signal-wave of the OPO as a function of the pump- wave wavelength which is scanned across the gain profile of the Ti:sapphire by altering the birefringent tuner 15.

Figure 5 shows the temporal behaviour of the signal wave as a function of time, the stability of which is consistent with the OPO oscillating with only one resonant down- converted wave (i.e. singly-resonant).

Figuie 6 is the signal- wave output (resonated wave) from the OPO as monitored by a scanning interferometer, and shows the single-axial-mode (single-frequency) nature of this output despite the pump-wave being highly multi-axial-mode.

A second embodiment of the invention is shown in Figure 7 in which the cavity 6 of the pump laser is now a travelling wave cavity between mirrors 30-33 with the pump- wave constrained to be unidirectional, by the incorporation into the cavity of a suitable unidirectional device (not shown but may be e.g. a Faraday rotator and birefringent plate), but with all other features the same as in embodiment 1 above. This embodiment ensures single-axial-mode (single-frequency) oscillation of the pump laser because of the avoidance of spatial hole-burning in the Thsapphire gain medium 3. although possibly requiring the incorporation into the pump laser cavity 6 of additional line narrowing elements, not explicitly shown, in order to ensure that such single frequency oscillation is maintained. The advantage of this embodiment is that the optical parametric oscillator is now pumped by a single-frequency pump-wave, so that the non-resonated idler output (though mirror 33) is also single-frequency provided that the resonated signal wave (output through mirror 9) is or is constrained to be single frequency. However a disadvantage of this arrangement is that the parametric gain in the non-linear crystal 8 is now unidirectional because of the unidirectional nature of the pump-wave.

A third embodiment of the invention is shown in Figure 8 in which the cavity 1 1 of the optical parametric oscillator is now also a travelling wave (ring) cavity between mirror 34-37. but with all other features the same as in Figure 7 above. The advantage of this embodiment is a reduction in the parasitic loss associated with the down-converted resonated wave in the cavity of the optical parametric oscillator compared to the case where this is a standing wave cavity with only unidirectional gain, as in Figure 7 above.

ln a specific example of such an OPO. the travelling-wave Ti:sapphire laser pumps an 1 1.5mm long crystal of KTA as the nonlinear material. Single frequency operation of the Ti:sapphire laser is obtained, with single frequency outputs for both signal and idler waves simultaneously.

A fourth embodiment of the invention is shown in Figure 9 and which differs from the Figure 2 embodiment in that the gain medium 3 of the pump laser is now no longer widely tuneable. In this embodiment it is now no longer possible to tune to any significant extent the optical parametric oscillator 2 through tuning the pump-wave wavelength, and this tuning must now be effected by altering some parameter of the non-linear gain medium 8 itself, such as the temperature of the crystal or the angle of propagation of the radiation through the crystal. For such an embodiment the non linear crystal may be a periodically poled lithium niobate (LiNbO ₃ ) crystal.

A fifth embodiment of the invention is where the gain medium of the pump laser 3 in Figure 2 is tuneable, but means for wavelength tuning of the output signal and idler beams from the OPO are also available through temperature-tuning of the non-linear material 8 or grating-tuning in the case where the non-linear material is periodically- poled, such as LiNbO3 (PPLN). One significant advantage of this scheme is that it provides a multi-parameter tuning capability and allows access to a particular combination of pump, signal, and idler frequencies through independent control of pump wavelength, temperature of the non-linear material, or the period of the quasi- phase-matching grating.

In a specific example of such an OPO. the non-linear gain medium 8 is a 1 -mm-long crystal of PPLN incorporating eight gratings ranging in period from 21 to 22.4 μm. and is pumped at the intracavity focus of a tuneable Ti: sapphire laser; the laser gain medium 3 is a Ti: sapphire crystal as described in embodiment 1. By using a combination of pump-wavelength-tuning and grating-tuning at a fixed crystal temperature of 165°C, a signal (idler) wavelength range of 1.070- 1.28 μm (2.30-3.33 μm) has been accessed, as shown in Figure 10. Additional means for tuning is also available by changing the temperature of the PPLN crystal.

A sixth embodiment of the invention is where one of the OPO mirrors, for example M7 in Figure 2, is partially transmitting for the resonated field 1. This scheme allows the useful extraction of a narrow-linewidth. single-longitudinal-mode resonated beam even when the pump laser itself has broadband, multi-longitudinal-mode characteristics.

In a specific example of such an OPO. a multi-longitudinal-mode, standing-wave Ti: sapphire laser pumps an 1 1.5-mm-long crystal of KTA as the non-linear material to provide tunable outputs over 1. 1 1 - 1.22 μm (signal) and 2.38-2.92 μm (idler) through pump-tuning. By employing a partially transmitting OPO mirror (M7 in figure 2) with a transmission of 1.2% for the resonated (signal) beam, useful output powers in excess of 500 mW in a narrow linewidth signal field have been coupled out of the resonator across most of the tuning range, as shown in Figure 1 1(a). The output power simultaneously extracted in the non-resonant multimode (idler) beam is also greater than 500 mW over this range, as shown in Figure 1 1 (b).

A seventh embodiment of the invention is where the tuneable laser gain medium 3 in Figure 2 is a crystal of Ti:sapphire and the power source 17 is a diode-laser-pumped, all-solid-state green laser.

ln one specific design of such an OPO. a commercial diode-laser-pumped frequency- doubled Nd:YVO ₄ laser at 532 nm (Spectra-Physics. Millennia) is the power source for a tuneable Ti:sapphire laser pump source. By constructing an intracavity SRO using a 19-mm-long crystal of PPLN placed at the intracavity focus of the Ti:sapphire laser, signal (idler) wavelengths in the range 1.070-1.28 μm (2.30-3.33 μm) have been generated (Figure 10). and an output power of 100 mW has been produced in the non- resonant idler beam with 5.5 W of input power from the Millennia laser.

An eighth embodiment of the invention is where the laser gain medium 3 is a crystal of Nd: YAG, Nd: YLF. or Nd: YV0 , the power source 17 is a semiconductor diode laser, and the non linear material 8 is KTA. PPLN. or other periodically-poled materials including PP-KTP, PP-RTA. PP-KTA, PP-RTP and PP-CTA.

In one specific design of such an OPO, the energy source is an AlGaAs semiconductor laser operating near 800 nm, the laser gain medium is a crystal of ND:YLF providing pumping radiation at 1.047 μm, and the non-linear gain material is a 15-mm crystal of KTA. This device can provide fixed-wavelength signal (idler) radiation at 1.505 (3.440) gm. with a total output power of up to 250 mW for 7 W of input power from the semiconductor diode laser to the ND:YLF crystals.

A ninth embodiment of the invention is where the tuneable laser gain medium 3 is a vibronic crystal other than Ti:sapphire. for example CπLiSAF. CπLiCAF. CπLiSGaF, or Cπforsterite, and the energy source 17 is a semiconductor diode laser.