FIELD

[0001] This disclosure relates to optical apparatus and methods which may be used to, for example, use energy or power from first radiation at one or more first wavelengths to generate or amplify second radiation at one or more second wavelengths.

BACKGROUND

[0002] Lasers are used in a wide variety of applications. Different applications have different wavelength requirements. For example, lasers operating in the eye-safe spectral range with a wavelength around 1 .5 pm may be required for various military applications such as for ranging/target illumination with simultaneous target designation or as laser designator in tactical and eye-safe training modes. However, there are problems with generating such wavelengths at sufficiently high powers. In one approach, a Nd:YAG laser operating at around 1 pm may be used to pump an optical parametric oscillator (OPO) to covert the 1 pm to the 1 .5 pm wavelength. OPOs are complex, require active phase-matching and their conversion efficiency may be sensitive to changes in temperature.

SUMMARY

[0003] According to an aspect or embodiment there is provided apparatus configured to use energy or power from first radiation at one or more first wavelengths to amplify externally-generated second radiation at one or more second wavelengths. The apparatus may comprise an optical resonator comprising a solid-state Raman gain medium such as diamond. The optical resonator may be configured such that the first radiation and externally-generated second radiation propagate through the Raman gain medium such that the first radiation amplifies the externally-generated second radiation at one or more wavelengths corresponding to a Stokes-shift within the Raman gain medium. The first radiation may produce Stokes-shifted radiation, such as first Stokes- shifted radiation, which in turn may amplify the externally-generated second radiation at the one or more wavelengths corresponding to the Stokes-shift within the Raman gain medium.

[0004] In use, the apparatus may be used to amplify the second radiation via the stimulated Raman scattering effect. Since the second radiation is externally generated with respect to the optical resonator, for example with a seed laser or the like, parameters of the second radiation may be controlled or preserved compared with if the initial emission at the second wavelength(s) originates from within the Raman gain medium itself. For example, the seed laser may produce radiation with specified parameters such as one or more wavelengths, linewidth, beam quality, and the like which may not otherwise be achievable if the initial emission at the one or more second wavelengths originates from within the Raman gain medium. The apparatus may substantially preserve one or more of these parameters while amplifying the second radiation. The ability to substantially preserve one or more parameters of externally-generated second radiation while providing the radiation with a sufficiently high energy or power may find utility in certain applications such as remote sensing applications such as a Light Imaging, Detection, and Ranging (LIDAR), gas detection or differential LIDAR (DIAL). By using a solid-state Raman gain medium such as diamond, the conversion efficiency to amplify the second radiation may be relatively high compared with other nonlinear wavelength conversion techniques. Further, the apparatus may be less sensitive to alignment error compared with other nonlinear wavelength conversion techniques. In the case of diamond and other similar materials, the optical transparency may be relatively high across a wide wavelength range compared to some other materials, which may decrease optical losses in operation of the apparatus.

[0005] The optical resonator may be configured in such a way that it does not produce any Raman oscillation at one or more Stokes-shifted wavelengths. The optical resonator may be configured in such a way that it does not produce any Raman oscillation at one or more second Stokes-shifted wavelengths. For example, a dispersive optical element may be provided inside the optical resonator, e.g. in a cavity of the optical resonator, and may be configured to spatially separate intracavity optical paths of the first Stokes-shifted radiation and one or more second Stokes-shifted radiations. The dispersive optical element may comprise a prism, diffraction grating and/or the like.

[0006] The first radiation may be configured to generate radiation at one or more first Stokes-shifted wavelengths within the Raman gain medium. The radiation at the one or more first Stokes-shifted wavelengths may be configured to resonate with the optical resonator and amplify radiation within the Raman gain medium at one or more second Stokes-shifted wavelengths that corresponds to the one or more second wavelengths of the second radiation. The first Stokes radiation may act as a pump to amplify second radiation via the stimulated Raman scattering effect. The one or more first Stokes-shifted wavelengths may be any Stokes shift, for example, a first, second, third, fourth or higher order Stokes shift. The one or more second Stokes-shifted wavelengths may be any Stokes shift, for example, a second, third, fourth, fifth or higher order Stokes shift. [0007] The first radiation may be configured to pump the Raman gain medium to generate the radiation at the one or more first Stokes-shifted wavelengths. The radiation at the one or more first Stokes-shifted wavelengths may be configured to pump the Raman gain medium to amplify the radiation at the one or more second Stokes-shifted wavelengths.

[0008] The optical resonator may be configured such that the radiation at the one or more first Stokes-shifted wavelengths resonates within the optical resonator.

[0009] The optical resonator may be configured to permit transmission of the first radiation into the optical resonator. The optical resonator may be configured to permit transmission of the second radiation into optical resonator such that the second radiation propagates at least once through the Raman gain medium before a portion of the amplified second radiation is transmitted out of the optical resonator. The second radiation may propagate twice through the Raman gain medium, for example, by being reflected once within the optical resonator. Each pass through the Raman gain medium may amplify or further amplify the second radiation.

[0010] The optical resonator may be configured such that the second radiation passes through the Raman gain medium at least twice before the amplified second radiation exits the optical resonator.

[0011] The apparatus may be configured such that the second radiation propagates along a beam axis that is not collinear with a beam axis of the first radiation.

[0012] The externally-generated second radiation may be configured to seed amplification of the second radiation within the Raman gain medium via the effect of stimulated Raman scattering.

[0013] The apparatus may further comprise a seed laser such as a laser diode. The seed laser may be configured to generate the second radiation externally of the optical resonator and direct the second radiation into the optical resonator for amplification thereof.

[0014] The apparatus may further comprise a pump laser such as a solid-state, fibre laser or laser diode. The pump laser may be configured to generate the first radiation externally of the optical resonator and direct the first radiation into the optical resonator.

[0015] The first radiation may be one of: pulsed; and continuous wave.

[0016] According to an aspect or embodiment there is provided a remote sensing apparatus such as a Light Imaging, Detection, and Ranging (LIDAR) apparatus, gas detection apparatus or differential LIDAR (DIAL) apparatus. The remote sensing apparatus may comprise any apparatus of any aspect or embodiment described herein. [0017] According to an aspect or embodiment there is provided a method for using energy or power from first radiation at one or more first wavelengths to amplify externally- generated second radiation at one or more second wavelengths. The method may comprise propagating the first radiation and externally-generated second radiation into an optical resonator comprising a solid-state Raman gain medium such that the first radiation amplifies the externally-generated second radiation at one or more wavelengths corresponding to a Stokes shift within the Raman gain medium. The first radiation may produce the Stokes-shifted radiation, such as first Stokes-shifted radiation, which in turn may amplify the externally-generated second radiation at the one or more wavelengths corresponding to the Stokes shift within the Raman gain medium

[0018] The method may comprise using the amplified second radiation in a remote sensing application such as Light Imaging, Detection, and Ranging (LIDAR), gas detection or differential LIDAR (DIAL).

[0019] According to an aspect or embodiment there is provided apparatus configurable to produce laser radiation in different modes. The apparatus may comprise a first optical resonator comprising a laser gain medium. The first optical resonator may be configured to generate first radiation in a first mode. The first optical resonator may comprise or be coupled to a second optical resonator. The second optical resonator may comprise a solid-state Raman gain medium such as diamond. The first and second optical resonators may be configured such that the first radiation resonates within the first optical resonator and propagates through the Raman gain medium in order to generate Stokes- shifted radiation. The apparatus may comprise a third optical resonator. The third optical resonator may comprise the laser gain medium. The third optical resonator may be configured to generate second radiation in a second mode. The apparatus may comprise a mode selection element configurable to operate the apparatus in one of the first and second modes.

[0020] In use, the apparatus may be capable of producing radiation at a desired wavelength or range of wavelengths for a particular application. Certain lasers may be certified for use in certain applications, for example, eye-safe applications. By using a solid-state Raman gain medium such as diamond, the conversion efficiency to generate such wavelength(s) may be relatively high compared with other nonlinear wavelength conversion techniques. Further, the apparatus may be less sensitive to alignment error compared with other nonlinear wavelength conversion techniques. In the case of diamond and other similar materials, the optical transparency may be relatively high across a wide wavelength range compared to some other materials, which may decrease optical losses in operation of the apparatus. In addition, the ability to switch between different operating modes may permit different wavelengths to be generated for different applications without requiring separate, costly, apparatus. Examples of such applications include ranging or target illumination, target designation or remote sensing such as Light Imaging, Detection, and Ranging (LIDAR), measuring distance with radiation or illuminating objects for their designation. Thus, the apparatus may be considered to be at least dual purpose providing different operating modes depending on the application with the ability to readily and quickly switch between the different operating modes.

[0021] The first radiation may be configured to generate radiation at one or more first Stokes-shifted wavelengths within the Raman gain medium. The radiation at the one or more first Stokes-shifted wavelengths may be configured to resonate with the second optical resonator and produce radiation within the Raman gain medium at one or more second Stokes-shifted wavelengths that correspond to the one or more second wavelengths.

[0022] The first radiation may be configured to pump the Raman gain medium to generate the radiation at the one or more first Stokes-shifted wavelengths. The radiation at the one or more first Stokes-shifted wavelengths may be configured to pump the Raman gain medium to generate the radiation at the one or more second Stokes-shifted wavelengths.

[0023] The second optical resonator may be configured such that the radiation at the one or more first Stokes-shifted wavelengths resonates within the second optical resonator.

[0024] The second optical resonator may comprise a first optical coupler and a second optical coupler. In a first mode, the first optical coupler may be configured to permit transmission of the first radiation into the second optical resonator. The first optical coupler may be configured to reflect radiation at the one or more first Stokes-shifted wavelengths within the second optical resonator. The second optical coupler may be configured to reflect radiation at the one or more first Stokes-shifted wavelengths within the second optical resonator. The second optical coupler may be configured to reflect the first radiation such that the first radiation resonates within the first optical resonator, e.g. such that the first radiation remains in the first optical resonator. In another example, the second optical coupler may be configured to permit the first radiation to be transmitted therethrough such that the first radiation is transmitted out of the second optical resonator via the second optical coupler. [0025] The first and second optical couplers may be configured to reflect the second radiation. The second optical coupler may be configured to permit transmission of a portion of the second radiation out of the second optical resonator.

[0026] The first optical resonator may comprise the second optical coupler. The first optical resonator may comprise a third optical coupler configured to reflect the first radiation. The first optical coupler may be positioned optically between the second and third optical couplers.

[0027] The first optical resonator may comprise a fifth optical coupler. The first optical resonator may comprise a third optical coupler configured to reflect the first radiation. The apparatus may comprise a focusing element, which may be configured to focus the first radiation into the Raman gain medium. The focusing element may be provided between the fifth optical coupler and the first optical coupler. The focusing element may comprise a lens, mirror, or the like

[0028] The apparatus may comprise a fourth optical coupler which, when used in conjunction with the third optical coupler, may define the third optical resonator. In the second mode, at least one of the third and fourth optical couplers may be configured to transmit a portion of radiation that resonates within the third optical resonator out of the third optical resonator.

[0029] The second optical resonator may be configured to permit transmission of the first radiation into and out of the second optical resonator such that the first radiation resonates within the first optical resonator.

[0030] The second optical resonator may be configured to permit transmission of a portion of the second radiation out of the second optical resonator.

[0031] The apparatus may be configured such that the Stokes-shifted radiation is generated along a beam axis that is collinear with a beam axis of the first radiation.

[0032] The laser gain medium such as a Nd:YAIC>3 (YAP) crystal or the like may be provided in the first optical resonator. The laser gain medium may be configured to be pumped by a pump laser that is external to the first optical resonator. The apparatus may comprise the pump laser.

[0033] The mode selection element may comprise a polarisation selection element. The polarisation selection element may be configured to select a radiation polarisation that resonates in the first or third optical resonators such that in the first mode the radiation resonating within the first optical resonator has a first polarisation and in the second mode the radiation resonating within the third optical resonator has a second polarisation that is different to the first polarisation. The second polarisation may be orthogonal to the first polarisation.

[0034] The mode selection element may comprise a mirror, diffraction grating, prism, and/or electro-optic element (such as a Pockels cell or the like) configured to cause the apparatus to be operated in one of the first and second modes. The mode selection element may comprise one or more components configured to select the mode.

[0035] The mode selection element may comprise a wavelength selection element configured to select one or more radiation wavelengths that resonate in the first or third optical resonators such that in the first mode the radiation resonating within the first optical resonator has one or more first wavelengths and in the second mode the radiation resonating within the third optical resonator has one or more third wavelengths that are different to the one or more first wavelengths. Different polarisation states may exhibit different spectral gain profiles. By selecting a different polarisation state with the mode selection element, different wavelength(s) may be supported within the optical resonator(s).

[0036] The second optical resonator may be one of: intracavity of and extracavity to the first optical resonator. In the intracavity arrangement, the first radiation may resonate within the first optical resonator via the second optical resonator. In the extracavity arrangement, the first radiation may resonate with the first optical resonator and a portion of that first radiation may propagate from the first optical resonator into the second optical resonator.

[0037] According to an aspect or embodiment there is provided a ranging or target illumination apparatus, a target designation apparatus or a remote sensing apparatus such as a Light Imaging, Detection, and Ranging (LIDAR) apparatus, apparatus for measuring distance with radiation or an apparatus for illuminating objects for their designation comprising the apparatus of any aspect or embodiment described herein.

[0038] According to an aspect or embodiment there is provided a method for producing laser radiation in different modes. The method may comprise providing a first optical resonator operative to generate first radiation in a first mode. The first optical resonator may comprise or be coupled to a second optical resonator. The second optical resonator may comprise a solid-state Raman gain medium such as diamond. The first and second optical resonators may be configured such that in operation, the first radiation resonates within the first optical resonator and propagates through the Raman gain medium in order to generate Stokes-shifted radiation. The method may comprise providing a third optical resonator comprising the laser gain medium. The third optical resonator may be operative to generate second radiation in a second mode. The method may comprise selecting, with a mode selection element, a mode of operation to generate either the first or second radiation in the first or second mode, respectively.

[0039] The method may comprise using at least one of: the first radiation and the second radiation in a ranging or target illumination application, a target designation application or a remote sensing application such as a Light Imaging, Detection, and Ranging (LIDAR), measuring distance with radiation or illuminating objections for their designation.

[0040] According to an aspect or embodiment there is provided apparatus configured to use energy from first radiation at one or more first wavelengths to generate second radiation at one or more second wavelengths. The apparatus may comprise a first optical resonator configured to generate the first radiation. The first optical resonator may comprise or be coupled to a second optical resonator. The second optical resonator may comprise a solid-state Raman gain medium such as diamond. The first and second optical resonators may be configured such that the first radiation resonates within the first optical resonator and propagates through the Raman gain medium in order to generate Stokes-shifted radiation that corresponds to the one or more second wavelengths.

[0041] In use, the apparatus may be capable of producing radiation at a desired wavelength or range of wavelengths for a particular application. Certain lasers may be certified for use in certain applications, for example, eye-safe applications. By using a solid-state Raman gain medium such as diamond, the conversion efficiency to generate such wavelength(s) may be relatively high compared with other nonlinear wavelength conversion techniques. Further, the apparatus may be less sensitive to alignment error compared with other nonlinear wavelength conversion techniques. In the case of diamond and other similar materials, the optical transparency may be relatively high across a wide wavelength range compared to some other materials, which may decrease optical losses in operation of the apparatus.

[0042] The first radiation may be configured to generate radiation at one or more first Stokes-shifted wavelengths within the Raman gain medium. The radiation at the one or more first Stokes-shifted wavelengths may be configured to resonate with the second optical resonator and may produce radiation within the Raman gain medium at one or more second Stokes-shifted wavelengths that correspond to the one or more second wavelengths.

[0043] The first radiation may be configured to pump the Raman gain medium to generate the radiation at the one or more first Stokes-shifted wavelengths. The radiation at the one or more first Stokes-shifted wavelengths may be configured to pump the Raman gain medium to generate the radiation at the one or more second Stokes-shifted wavelengths.

[0044] The second optical resonator may be configured such that the radiation at the one or more first Stokes-shifted wavelengths resonates within the second optical resonator.

[0045] The second optical resonator may comprise a first optical coupler and a second optical coupler. In a first mode, the first optical coupler may be configured to permit transmission of the first radiation into the second optical resonator. The first optical coupler may be configured to reflect radiation at the one or more first Stokes-shifted wavelengths within the second optical resonator. The second optical coupler may be configured to reflect the first radiation such that the first radiation resonates within the first optical resonator, e.g. remains in the first optical resonator. In another example, the second optical coupler may be configured to permit the first radiation to be transmitted therethrough such that the first radiation is transmitted out of the second optical resonator via the second optical coupler. The second optical coupler may be configured to reflect radiation at the one or more first Stokes-shifted wavelengths within the second optical resonator.

[0046] The first and second optical couplers may be configured to reflect the second radiation. The second optical coupler may be configured to permit transmission of a portion of the second radiation out of the second optical resonator.

[0047] The first optical resonator may comprise the second optical coupler. The first optical resonator may comprise a third optical coupler configured to reflect the first radiation. The first optical coupler may be positioned optically between the second and third optical couplers.

[0048] The first optical resonator may comprise a fifth optical coupler. The first optical resonator may comprise a third optical coupler configured to reflect the first radiation. The apparatus may comprise a focusing element provided between the fifth optical coupler and the first optical coupler to focus the first radiation into the Raman gain medium.

[0049] The apparatus may comprise a fourth optical coupler which, when used in conjunction with the third optical coupler, may define a third optical resonator. In a second mode, at least one of the third and fourth optical couplers may be configured to transmit a portion of radiation that resonates within the third optical resonator out of the third optical resonator.

[0050] The apparatus may comprise a mode selection element configurable to operate the apparatus in one of the first and second modes. [0051] The second optical resonator may be configured to permit transmission of the first radiation into and out of the second optical resonator such that the first radiation resonates within the first optical resonator.

[0052] The second optical resonator may be configured to permit transmission of a portion of the second radiation out of the second optical resonator.

[0053] The apparatus may be configured such that the Stokes-shifted radiation is generated along a beam axis that is collinear with a beam axis of the first radiation.

[0054] The apparatus may comprise a laser gain medium such as a Nd:YAIC>3 (YAP) crystal provided in the first optical resonator and configured to be pumped by a pump laser that is external to the first optical resonator.

[0055] The apparatus may comprise a third optical resonator. The third optical resonator may comprise the laser gain medium and may be configured such that in a first mode the laser gain medium produces radiation that resonates within the first optical resonator and in a second mode the laser gain medium produces radiation that resonates within the third optical resonator.

[0056] The apparatus may comprise a mode selection element such as a mirror, diffraction grating, prism, electro-optic element (such as a Pockels cell or the like) and/or Q-switch device configured to cause the apparatus to be operated in one of the first and second modes.

[0057] The mode selection element may comprise a polarisation selection element configured to select a radiation polarisation that resonates in the first or third optical resonators such that in the first mode the radiation resonating within the first optical resonator has a first polarisation and in the second mode the radiation resonating within the third optical resonator has a second polarisation that is different to the first polarisation. The second polarisation may be orthogonal to the first polarisation.

[0058] The mode selection element may comprise a wavelength selection element configured to select one or more radiation wavelengths that resonate in the first or third optical resonators such that in the first mode the radiation resonating within the first optical resonator has one or more first wavelengths and in the second mode the radiation resonating within the third optical resonator has one or more third wavelengths that are different to the one or more first wavelengths.

[0059] According to an aspect or embodiment there is provided a ranging or target illumination apparatus, a target designation apparatus or a remote sensing apparatus such as a Light Imaging, Detection, and Ranging (LIDAR) apparatus, apparatus for measuring distance with radiation or an apparatus for illumination of objects for their designation comprising the apparatus of any aspect or embodiment described herein.

[0060] According to an aspect or embodiment there is provided a method for using energy from first radiation at one or more first wavelengths to generate second radiation at one or more second wavelengths. The method may comprise generating, with a first optical resonator, the first radiation. The first optical resonator may comprise or be coupled to a second optical resonator. The second optical resonator may comprise a solid-state Raman gain medium such as diamond. The method may comprise configuring at least one of the first and second optical resonators such that the first radiation resonates within the first optical resonator and propagates through the Raman gain medium in order to generate Stokes-shifted radiation that corresponds to the one or more second wavelengths.

[0061] The method may comprise using at least one of: the first radiation and the second radiation in a ranging or target illumination application, a target designation application or a remote sensing application such as a Light Imaging, Detection, and Ranging (LIDAR), measuring distance with radiation or illuminating objects for their designation.

[0062] At least one feature of any aspect or embodiment described herein may replace any corresponding feature of any aspect or embodiment described herein. At least one feature of any aspect or embodiment described herein may be combined with any other aspect or embodiment described herein.

BRIEF DESCRIPTION

[0063] Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 (a) schematically depicts an optical amplifier comprising a Raman amplifier;

Figure 1 (b) schematically depicts another optical amplifier comprising a Raman amplifier;

Figure 2 schematically depicts an example of a Raman amplification process that occurs in the Raman amplifier of Figure 1 ;

Figure 3 schematically depicts a ranging or target illumination apparatus comprising an intracavity Raman arrangement; and

Figure 4 schematically depicts a ranging or target illumination apparatus comprising an extracavity Raman arrangement. DETAILED DESCRIPTION

[0064] Figure 1 (a) schematically depicts an application of an optical amplifier 10, which in this example is embodied in a remote sensing apparatus 10. Examples of remote sensing apparatus comprise Light Imaging, Detection, and Ranging (LIDAR) apparatus, gas detection apparatus or Differential LIDAR (DIAL), and the like. The remote sensing apparatus 10 comprises apparatus in the form of a Raman amplifier 12 configured to use energy or power from first radiation 14 at a first wavelength to produce Stokes-shifted radiation, which in turn amplifies externally-generated second (seed) radiation 16 at a second wavelength that is produced externally of the Raman amplifier 12 so as to be subsequently directed into the Raman amplifier 12 for amplification thereof. The Raman amplifier 12 comprises an optical resonator 18 comprising a solid-state Raman gain medium 20 such as diamond. A Raman gain medium 20 may comprise any material with optical (Raman) gain arising from stimulated Raman scattering. Diamond is an example of a Raman gain medium with a large Raman gain coefficient (i.e. compared with some other materials).

[0065] The optical resonator 18 is configured such that the first radiation 14 and second radiation 16 propagate through the Raman gain medium 20 such that the first radiation 14 generates first Stokes-shifted radiation 26 within the Raman gain medium 20 and the frequency shift (e.g. optical frequency shift) between the first Stokes-shifted radiation 26 and second radiation 16 corresponds to the Stokes shift in the Raman gain medium 20. This way, the first Stokes-shifted radiation 26 acts as a pump in order to amplify the externally-generated second radiation 16.

[0066] In this embodiment, the remote sensing apparatus 10 comprises a pump laser 22 for providing the first radiation. The remote sensing apparatus 10 further comprises a seed laser 24 for providing the second radiation. Alternatively, one or both of: the pump laser 22 and seed laser 24 may be provided externally of the remote sensing apparatus 10.

[0067] The pump laser 22 directs the first radiation 14 into the optical resonator 18. The first radiation 14 is configured to pump the Raman gain medium 20 to generate radiation 26 at a first Stokes-shifted wavelength within the Raman gain medium 20. The radiation 26 at the first Stokes-shifted wavelength is configured to resonate with the optical resonator 18. The radiation 26 at the first Stokes-shifted wavelength is configured to pump the Raman gain medium 20 to amplify the second (seed) radiation emitting at a wavelength corresponding to the second Stokes-shifted wavelength. The Raman amplifier 12 is configured in such a way that it does not produce any Raman oscillation at the second Stokes-shifted wavelength. For example, this can be done by introducing for example a dispersive optical element (such as a prism, diffraction grating or the like) inside the resonator to separate spatially intracavity optical paths of the first Stokes- shifted radiation and one or more second Stokes-shifted radiations. In this embodiment, the seed laser 24 directs the second radiation 16 into the optical resonator 18. The second radiation 16 seeds the amplification of the radiation at the second Stokes-shifted wavelength via the effect of stimulated Raman scattering. The amplified second radiation 16’ is then coupled out of the optical resonator 18.

[0068] The optical resonator 18 comprises a first optical coupler 28 and second optical coupler 30. The Raman gain medium 20 is disposed between the first and second optical couplers 28. The first optical coupler 28 is configured to transmit the first radiation 14 into the optical resonator 18. The first optical coupler 28 is further configured to reflect the radiation 26 at the first Stokes-shifted wavelength and the second radiation 16 within the optical resonator 18. The second optical coupler 30 is configured to reflect the first radiation 14 and the radiation 26 at the first Stokes-shifted wavelength within the optical resonator 18. The second optical coupler 30 is configured such that the second radiation 16 can be transmitted into the optical resonator 18. Since it is at the same wavelength as the second radiation 16, the amplified second radiation 16’ is transmitted out of the optical resonator 18 via the second optical coupler 30. By providing an optical coupler 30 that is transparent to the second Stokes-shifted wavelength and by adjusting the intensity of the first radiation 14, the Raman amplifier 12 does not produce any Raman oscillation at the second Stokes-shifted wavelength. Additionally, Raman oscillation at the second Stokes-shifted wavelength can be suppressed, for example, by providing a dispersive element (such as a prism, diffraction grating or the like) inside the resonator 18 to spatially separate intracavity optical paths of the first Stokes-shifted radiation and one or more second Stokes-shifted radiations.

[0069] The first radiation 14 passes through the Raman gain medium 20 at least once. The second radiation 16 passes through the Raman gain medium 20 at least once (see Fig. 1 (b)) and is reflected after passing by a dichroic mirror (DM) with high transmittance at the wavelength of first radiation and high reflectivity at the wavelength of the second radiation. The second radiation 16 may also pass through the Raman gain medium 20 at least twice due to being reflected by the first optical coupler 28 or more, due to being reflected by the first optical coupler 28, as depicted by Figure 1 (a). The second radiation can propagate in the same direction as the first radiation as well as in a backward or reverse direction relative to the first radiation. The second radiation 16 that is reflected at least once by the first optical coupler 28 may be non-amplified second radiation 16 and amplified second radiation 16’. In this embodiment, the radiation 26 at the first Stokes- shifted wavelength is generated along a beam axis that is collinear with a beam axis of the first radiation 14. The second radiation 16 propagates through the Raman gain medium 20 at an angle to the beam axis of the first radiation 14 to enable the amplified second radiation 16’ to be extracted from the Raman amplifier 12. It will be appreciated that the second radiation 16 may or may not be collinear with the beam axis.

[0070] It will be appreciated that the wavelength dependent reflection and transmission characteristics of the first and second optical couplers 28, 30 may be appropriately selected for the required wavelengths. The characteristics of the Raman gain medium 20, pump laser 22 and seed laser 24 may be selected depending on the application. The first radiation 14 and the second radiation 16 may be continuous wave (CW) or pulsed. In the present embodiment, the first wavelength is around 1 pm. The second wavelength is around 1 .5 pm, which may be regarded as corresponding to an eye-safe wavelength.

[0071] Figure 1 (b) schematically depicts an application of an optical amplifier 10’, which in this example is embodied in a remote sensing apparatus 10’. The remote sensing apparatus 10’ is similar in function to the remote apparatus 10 of Figure 1 (a) albeit it has a different configuration. Features that represent like features are represented by the same reference numerals but also accompanied by an (additional) apostrophe.

[0072] In contrast to Figure 1 (a), the Raman amplifier 12’ of Figure 1 (b) is configured to receive (non-amplified) second radiation 16’ from a seed laser 24’. In contrast to Figure 1 (a), the beam of non-amplified second radiation 16’ is collinear with an optical axis of the optical resonator 18’ (i.e. the same optical axis followed by the first radiation 14). The non-amplified second radiation 16’ is transmitted through the second optical coupler 30’, through the Raman gain medium 20’ and through the first optical coupler 30’. Thus, rather than being reflected, the first optical coupler 30’ is substantially transmissive to the now- amplified second radiation 16”. Again, the Raman gain medium 20’ is pumped by the first radiation 14’ produced by the pump laser 22’ so as to generate radiation 26’ at the first Stokes-shifted wavelength, which in turn resonates within the optical resonator 18’ of the Raman amplifier 12’. A dichroic mirror DM is provided to separate the first radiation 14’ from the amplified second radiation 16”. In this example, the dichroic mirror is transmissive to the first radiation 14’ but reflective to the second radiation 16’, 16”. In contrast to the embodiment of Figure 1 (a), the second radiation 16’, 16” may be substantially transmitted once through the Raman gain medium 20’. It is possible that the second radiation 16’, 16” can propagate in the same direction as the first radiation 14’ as well as in the backward direction. The second radiation 16’, 16” may still pass through the Raman gain medium 20’ at least twice due to being reflected by the first optical coupler 28’.

[0073] Figure 2 schematically illustrates an example of a wavelength conversion process provided by the Raman amplifier 12 that uses energy or power from the first radiation 14 to amplify the second radiation 16. The first radiation 14 excites an electron of the Raman gain medium 20 from a ground state to a virtual state. The energy difference between the ground and virtual state corresponds to the first wavelength of the first radiation 14. The electron then decays to a vibrational state that is higher in energy than the ground state to emit radiation 26 at a first Stokes-shifted wavelength. This radiation 26 at the first Stokes-shifted wavelength resonates within the optical resonator 18 and causes an electron of the Raman gain medium 20 to be excited to a virtual state that is different or lower in energy than the virtual state to which the electron is excited by the first radiation 14. When the second radiation 16 is directed into the Raman amplifier 12, it causes stimulated decay of the electron to a vibrational state. This decay generates photons (radiation) at the second Stokes-shifted wavelength with the properties identical to that of the second radiation 16 These photons correspond to amplified second radiation 16’.

[0074] Raman lasers utilise the Raman effect to produce radiation at a desired wavelength. This desired wavelength is determined by the so-called Stokes shift and depends on the properties of the Raman gain medium 20. However, emission in Raman lasers originate from intracavity noise, which is then amplified in the Raman gain medium via the stimulated Raman scattering effect. This means that some parameters of Raman laser emission such as linewidth and beam quality cannot be fully controlled.

[0075] An alternative way to produce radiation at a desired wavelength in the form of the Raman amplifier 12 has been recognised with the present embodiment. Instead of the initial noise being amplified, the seed laser 24 provides second radiation 16 at the desired wavelength, linewidth and/or beam quality. The seed laser 24 may comprise a laser diode with low power emitting at the second wavelength. The second radiation 16 is amplified after propagating through the Raman gain medium 20. In this embodiment, the properties of the second radiation are preserved by the amplified Raman emission. For example, the linewidth and beam quality of the second radiation 16 may be preserved after amplification. The Raman amplifier 12 may be used to amplify narrow-linewidth low power laser emission for a range of applications including light ranging (e.g. Doppler LIDAR), and the like. [0076] The present embodiment utilises diamond as the Raman gain medium 20. Diamond is believed to have the highest Raman gain among solid-state materials. Diamond is also believed to have the highest thermal conductivity. At least these two properties make diamond attractive for use in the Raman amplifier 12 for converting radiation at a first wavelength 14 into a second wavelength 16 and/or for amplifying radiation.

[0077] Figure 3 schematically depicts an optical apparatus 1 10, which in this example is embodied in a ranging or target illumination apparatus 1 10 with multiple modes of operation (e.g. it can be switched between modes for generating radiation at different wavelengths depending on the required application). Alternatively or additionally, the apparatus 1 10 could comprise or be in the form of a target designation apparatus or a remote sensing apparatus such as a Light Imaging, Detection, and Ranging (LIDAR) apparatus.

[0078] The apparatus 1 10 comprises a laser resonator 1 12 (schematically outlined by the largest rectangular-shaped dashed line box in Figure 3) configured to use energy or power from first radiation 1 14 at a first wavelength to generate second radiation 1 16 at a second wavelength such as a visible or eye safe wavelength and/or such as a non-visible wavelength or other suitable wavelength as required for the particular application. The laser resonator 1 12 comprises a first optical resonator 1 18 (schematically outlined by the L-shaped dotted line box in Figure 3) configured to generate the first radiation 1 14. The first optical resonator 1 18 comprises a second optical resonator 1 19. The second optical resonator 1 19 comprises a solid-state Raman gain medium 120 such as diamond. The first and second optical resonators 1 18, 1 19 are configured such that the first radiation 1 14 resonates within the first optical resonator 1 18 via the second optical resonator 1 19 and propagates through the Raman gain medium 120 in order to generate Stokes-shifted radiation that corresponds to the second wavelength.

[0079] In this embodiment, the ranging or target illumination apparatus 1 10 comprises a pump laser 122 for providing pump radiation 123. The pump laser 122 is provided externally of the laser resonator 1 12 and directs the pump radiation 123 into the laser resonator 1 12. The pump radiation 123 is configured to generate the first radiation 1 14 within the first optical resonator 1 18 by pumping a laser gain medium 132 provided within the first optical resonator 1 18. The pumping of the laser gain medium 132 generates the first radiation 1 14 that resonates within the first optical resonator 1 18. In this embodiment, the laser gain medium 132 comprises a Nd:YAIC>3 (YAP) crystal although it will be appreciated that any appropriate laser gain medium 132 could be provided. [0080] The laser resonator 1 12 further comprises a third optical resonator 134 (schematically outlined by the rectangular-shaped dotted line box in Figure 3). The third optical resonator 134 comprises the laser gain medium 132. The laser resonator 1 12 is configured such that in a first mode, the laser gain medium 132 produces radiation that resonates within the first optical resonator 1 18 and in a second mode the laser gain medium 132 produces radiation that resonates within the third optical resonator 134. The radiation generated in the first mode corresponds to the first radiation 1 14.

[0081] The laser resonator 1 12 comprises a mode selection element 136 (schematically outlined by the smallest rectangular-shaped dashed line box in Figure 3). In this embodiment, the mode selection element 136 is configured to select a radiation polarisation that resonates in the first or third optical resonators 1 18, 134 such that in the first mode the radiation resonating within the first optical resonator 1 18 has a first polarisation and in the second mode the radiation resonating within the third optical resonator 134 has a second polarisation that is orthogonal to the first polarisation. In this embodiment, the mode selection element 136 comprises a polarising beam splitter 140 operative to either transmit or reflect the first radiation 1 14 in response to a polarisation selected by a Pockels cell 142 of the mode selection element 136. Alternatively or additionally, the mode selection element 136 may comprise any one of: a mirror, diffraction grating, prism, and/or any other suitable arrangement configured to cause the laser resonator 1 12 to be operated in one of the first and second modes.

[0082] In this embodiment, the mode selection element 136 may provide the functionality of a wavelength selection element. The wavelength selection element is configured to select a radiation wavelength that resonates in either the first or third optical resonators 1 18, 134. Therefore, in the first mode, the radiation that resonates within the first optical resonator 1 18 has a first wavelength. In the second mode the radiation that resonates within the third optical resonator has a third wavelength that is different to the first wavelength. Depending on the configuration of the laser resonator 1 12 and choice of laser gain medium 132, the mode selection element 136 may allow one of the wavelengths to resonate within either the first and third optical resonators 1 18, 134 depending on the selected wavelength. In the embodiment where the laser gain medium 132 comprises a Nd:YAP crystal, potential optical transition wavelengths include 1064 nm and 1079 nm. In the present embodiment, the 1079 nm wavelength resonates in the first mode (i.e. within the first optical resonator 1 18). The 1064 nm wavelength resonates in the second mode (i.e. within the third optical resonator 134). By appropriate selection of the radiation polarisation that is permitted to resonate using the Pockels cell 142, it is possible to select a laser resonator 1 12 mode of operation. In other words, it is possible to selectively switch between emission of radiation at the first wavelength and emission of radiation at the third wavelength. It will be appreciated that different optical transitions may be available with Nd:YAP or any other laser crystals so that the first radiation 1 14 may be at any available or appropriate wavelength.

[0083] Figure 3 depicts an example of an intracavity Raman configuration in which the first optical resonator 1 18 comprises a second optical resonator 1 19 comprising a Raman gain medium 120 (i.e. the second optical resonator 1 19 is internal with respect to the first optical resonator 1 18). The principle of operation of the second optical resonator 1 19 is similar to the operation of the Raman amplifier 12 of Figure 1 . However, in this embodiment, no seed laser is used. When the first mode is selected, the first radiation 1 14 resonates within the first optical resonator 1 18 via the second optical resonator 1 19. The second optical resonator 1 19 is configured to permit transmission of the first radiation 1 14 into and out of the second optical resonator 1 19 (i.e. a substantial portion of the first radiation 1 14 being reflected internally once within the second optical resonator 1 19) such that the first radiation 1 14 may resonate within the first optical resonator 1 18.

[0084] In this embodiment, the first radiation 1 14 is configured to generate radiation at a first Stokes-shifted wavelength within the Raman gain medium 120. Thus, the first radiation 1 14 pumps the Raman gain medium 120 to generate the radiation at the first Stokes-shifted wavelength. The radiation at the first Stokes-shifted wavelength is configured to resonate within the second optical resonator 1 19 and produce radiation within the Raman gain medium 120 at a second Stokes-shifted wavelength that corresponds to the second wavelength. Thus, the radiation at the first Stokes-shifted wavelength pumps the Raman gain medium 120 to generate the radiation at the second Stokes-shifted wavelength.

[0085] In this embodiment, the laser resonator 1 12 is configured such that the Stokes- shifted radiation (e.g. the second radiation 1 16) is generated along a beam axis that is collinear with a beam axis of the first radiation 1 14. A portion of the laser resonator 1 12 between the polarising beam splitter 140 and the second optical resonator 1 19 may be regarded as defining the beam axis of the first radiation 1 14 that is collinear with the beam axis of the second radiation 1 16.

[0086] In one embodiment with diamond as the Raman gain medium 120, the first Stokes-shifted wavelength is 1260 nm and the second Stokes-shifted wavelength is 1514 nm. It will be appreciated that different Stokes-shifted wavelengths may be available using diamond or any other Raman gain medium 120 so that the Stokes-shifted wavelength may be at any available or appropriate wavelength. Any order of Stokes- shifted wavelengths (e.g. 1 st, 2nd, 3rd etc) may be permitted to resonate within the second optical resonator 1 19. Although the present embodiment describes radiation at a first Stokes-shifted wavelength pumping the Raman gain medium 120 to generate radiation at a second Stokes-shifted wavelength, it will be appreciated that similar cascaded Stokes oscillations can take place to generate radiation at other Stokes-shifted wavelengths: e.g. the second Stokes-shifted wavelength may generate the third Stokes- shifted wavelength, the third Stokes-shifted wavelength may generate the fourth Stokes- shifted wavelength and so on.

[0087] In this embodiment, the second optical resonator 1 19 is configured such that the radiation at the first Stokes-shifted wavelength resonates within the second optical resonator 1 19. Similar to the Raman amplifier 12 of Figure 1 , the second optical resonator 1 19 comprises a first optical coupler 128 and a second optical coupler 130. The Raman gain medium 120 is disposed between the first and second optical couplers 128, 130. The first optical coupler 128 is configured to: permit transmission of the first radiation 1 14 into the second optical resonator 1 19; and reflect radiation at the first Stokes-shifted wavelength within the second optical resonator 1 19.

[0088] The second optical coupler 130 is configured to: reflect the first radiation 1 14 such that the first radiation 1 14 resonates within the first optical resonator 1 18; and reflect radiation at the first Stokes-shifted wavelength within the second optical resonator 1 19. The second optical coupler 130 is configured to permit transmission of a portion of the second radiation 1 16, which in this embodiment corresponds to the second Stokes- shifted wavelength, out of the second optical resonator 130.

[0089] The first optical coupler 128 is configured to reflect the second radiation 1 16. The first optical resonator 1 18 comprises the second optical coupler 130 and a third optical coupler 144 (which permits the pump radiation 123 to be transmitted into the first optical resonator 1 18 from the pump laser 122) configured to reflect the first radiation 1 14. The first optical coupler 128 is positioned optically between the second and third optical couplers 130, 144. If operating in the first mode, the first radiation 1 14 resonates within the first optical resonator 1 18. In turn, this generates the second radiation 1 16 within the second optical resonator 1 19. The second optical resonator 1 19 is configured to permit transmission of a portion of the second radiation 1 16 out of the second optical resonator 1 19.

[0090] Cascaded stimulated Raman scattering within the Raman gain medium 120 means that the first radiation 1 14 at 1079 nm excites the first Stokes-shifted wavelength in the Raman gain medium 120 (at 1260 nm). This first Stokes-shifted wavelength then serves as an excitation source (e.g. pump) to produce the second Stokes-shifted wavelength at a desired eye-safe wavelength of 1514 nm.

[0091] Due to the Raman gain medium 120 being provided within the first optical resonator 1 18, the intensity of the first radiation 1 14 may be very high (i.e. compared with the intensity of the first radiation 1 14 external to the first optical resonator 1 18). Similarly, the intensity of the first Stokes-shifted wavelength within the second optical resonator 1 19 may also be very high. These high intensities at these wavelengths may lower the threshold of the diamond Raman laser oscillation at 1514 nm in order to produce the second radiation 1 16.

[0092] The laser resonator 1 12 further comprises a fourth optical coupler 146 which, when used in conjunction with the third optical coupler 144, defines the third optical resonator 134. In the second mode, the fourth optical coupler 146 is configured to transmit a portion of radiation that resonates within the third optical resonator 134 out of the third optical resonator 134.

[0093] As discussed previously, the laser gain medium 132 may be capable of generating radiation at different wavelengths depending on the selected polarisation. In both the first and second modes, the laser gain medium 132 generates first radiation 1 14 via the same laser gain medium 132. In the first mode, the first radiation 1 14 is used to generate the second radiation 1 16. In the second mode, the first radiation 1 14 is coupled out of the fourth optical coupler 146. Different wavelengths of first radiation 1 14 are generated depending on the mode of operation. Thus, the first radiation 1 14 in this embodiment can have one of two possible wavelengths.

[0094] In this embodiment, the laser resonator 1 12 may be actively or passively Q- switched by a Q switching element 150 configured to create a giant laser pulse, which may be used for range-finding and target illumination applications.

[0095] The use of a Nd:YAP crystal for the laser gain medium 132 and diamond as the intracavity Raman gain medium 120, may allow the radiation at 1079 nm generated within the first optical resonator 1 18 to be converted into an eye-safe (e.g. around 1 .5 pm) wavelength. The Raman gain medium 120 may not require any temperature stabilisation, special orientation of the Raman gain medium 120 or have stringent alignment requirements. The ranging or target illumination apparatus 1 10 may therefore be considered to be relatively robust and adapted for use in the field (e.g. in defence applications, or the like). [0096] The laser resonator 1 12 provides the ability to switch between different wavelength operating modes to provide different wavelengths (e.g. for different applications). Some defence applications use lasers with a wavelength 1064 nm for certain purposes. Some defence applications use lasers with a wavelength of between 1 .5 and 1 .6 pm, which is regarded as being in the eye-safe spectral range. Available Nd:YAG-OPO laser systems are capable of providing radiation in both of these wavelength ranges.

[0097] The described laser resonator 1 12 design utilises a property of Nd:YAP laser crystal to emit at two wavelengths (1064 and 1079 nm) at orthogonal polarisations. The 1064 nm wavelength can be used for target designation, while the 1079 nm wavelength radiation can be used to pump an intracavity diamond Raman gain medium 120 for wavelength conversion to 1514 nm for eye-safe ranging and target illumination.

[0098] Figure 4 depicts an optical apparatus 210 that provides similar functionality to the optical apparatus 1 10 of Figure 3. In contrast to the intracavity arrangement of the optical apparatus 1 10 of Figure 3, the optical apparatus 210 comprises an extracavity Raman configuration. Many features of the optical apparatus 210 are identical to that of the optical apparatus 1 10 and are not described for brevity. Reference numerals for Figure 4 describing like or similar features are incremented by 100 compared with those depicted in relation to Figure 3. Differences between the optical apparatus 1 10, 210 are described in more detail herein.

[0099] The optical apparatus 210 comprises a first optical resonator 218 that is defined between the third optical coupler 244 and a fifth optical coupler 252 located optically between the polarising beam splitter 240 and the second optical resonator 219. Thus, when operating in the first mode, the first radiation 214 only resonates between the third and fifth optical couplers 244, 252. The fifth optical coupler 252 is reflective to the first radiation 214 while permitting transmission of a portion of the first radiation 214 into the second optical resonator 219. As such, the second optical resonator 219 is not an “intracavity” arrangement but instead is considered to be an“extracavity” arrangement with respect to the first optical resonator 218. Since the intensity of the first radiation 214 within the Raman gain medium 220 of this extracavity arrangement would be reduced compared with the intensity achievable using an intracavity arrangement, a focusing element 254 (e.g. a lens, or the like) is provided between the fifth optical coupler 252 and the first optical coupler 228 to focus the first radiation 214 into the Raman gain medium 220. The focusing element 254 therefore increases the intensity of the first radiation 214 within the Raman gain medium 220 so that the Raman effect is sufficiently large to generate the second radiation 216 in a similar manner to that described in relation to the optical apparatus 1 10. Optionally and in contrast to the optical apparatus 1 10, the second optical coupler 230 may be highly transmissive to the first radiation 214 at the first wavelength (e.g. to minimise beam reflections adversely affecting generation of the first radiation in the first mode). However, in certain embodiments beam reflections may not be a major concern and the second optical coupler 230 could optionally instead be reflective to the first radiation, e.g. to utilize pump emission twice or more, and in other embodiments, alternative strategies for preventing beam reflections may be employed.

[0100] Therefore, the optical apparatus 210 provides much the same (e.g. multiple wavelength mode) functionality to the optical apparatus 1 10 but with a slightly different implementation. By decoupling the alignment of the first and second optical resonators 218, 219, the alignment process for the optical components may be relatively simple compared with if multiple optical components are aligned within the first optical resonator 218. In addition or alternatively, the optical losses may be lower if fewer optical components are provided in the first optical resonator 218. However, it will be appreciated that the intracavity Raman arrangement described herein may or may not suffer from such problems.

[0101] In certain embodiments, the second radiation is pumped by the generated“first” Stokes-shifted radiation. However, it will be appreciated that other orders of Stokes- shifted radiation may be suitable for pumping the second radiation. For example, the 2nd, 3rd, 4th or any other order of Stokes-shifted radiation may be suitable for pumping the second radiation.

[0102] It will be appreciated that the optical couplers described herein may not have perfect reflection or transmission characteristics. Thus, while an optical coupler may have been described as being configured to reflect radiation at a certain wavelength, it is anticipated that a portion of that radiation may still be transmitted by that optical coupler. Similarly, while an optical coupler may have been described as being configured to transmit radiation at a certain wavelength, it is anticipated that a portion of that radiation may still be reflected by that optical coupler. In other words, an optical coupler configured to transmit or reflect radiation at a certain wavelength may be regarded as semi transparent or semi-reflective at that certain wavelength, respectively.

[0103] Although certain embodiments refer to using“energy” or“power” of radiation (e.g. first radiation) to amplify or generate radiation (e.g. second radiation, or the like), it will be appreciated that these terms may be understood to have the broadest possible meaning. For example, a person of ordinary skill in the art will appreciate that, where appropriate, the terms“energy” or“power” in the context of “radiation” of any type may both refer to any type of radiation source, whether continuous wave or pulsed.

[0104] Certain embodiments refer to using energy or power at a first wavelength to generate or amplify radiation at a second wavelength. Certain embodiments may refer to Stokes-shifted radiation at a certain wavelength. It will be appreciated that radiation may or may not comprise a single wavelength. For example, the radiation may comprise one or more wavelengths (i.e. one or more different wavelengths). Thus, where embodiments refer to using radiation at a certain wavelength, this may mean that the radiation may comprise one or more wavelengths. Similarly, where embodiments refer to generating or amplifying radiation at a certain wavelength, this may mean that the generated or amplified radiation may comprise one or more wavelengths. Further, where embodiments refer to radiation at a certain Stokes-shifted wavelength, this may mean that the Stokes-shifted radiation may comprise one or more wavelengths. Thus, wherever a certain wavelength is referred to in this disclosure, it will be appreciated that this may refer to one or more wavelengths. For example, where radiation comprises more than one wavelength, the radiation at those wavelengths may be near or centred about a certain wavelength. Radiation at different wavelengths may be generated by the same or one or more different radiation sources (e.g. simultaneously/at the same time or at different times).

[0105] Any material can demonstrate Raman scattering. This Raman scattering may have its own linewidth, which is usually rather narrow. In the case of the Raman amplifier described herein, more than one external wavelength can be amplified provided these wavelengths are within the Raman linewidth. For example, if there is a pump wavelength of 1064nm and a solid state material with a Stokes shift of, say 1000cm ^-1 and a full-width- half-maximum Raman linewidth of, say 2cm ^-1 , then the 1 ^st Stokes radiation could be at wavelengths between 1 190.55 and 1 190.83nm and the 2 ^nd Stokes radiation at wavelengths between 1351 .26 and 1351 .99nm. This is a small difference, but for applications like DIAL this wavelength difference may be useful. In DIAL, two very close wavelengths may be generated - one wavelength that is absorbed by a gas and another wavelength that is not absorbed by the gas. The difference in the return signal at these two wavelengths is then measured. Gases usually have narrow linewidth absorption bands. So, in principle while (as in example above) the wavelength of 1351 26nm will be absorbed by some gas, another wavelength, 1351 .99nm might not. The two different wavelengths can be provided by two external radiation sources, providing these wavelengths are within the Raman linewidth. [0106] It will be appreciated that while certain embodiments described herein refer to using/generating/amplifying radiation at the first and/or second Stokes shift, other Stokes shifts may be used/generated/amplified in different configurations of apparatus described herein. For example, a first, second, third, fourth or higher order Stokes shift may be used/generated/amplified in different configurations or implementations of the apparatus described herein. In a further example, the first radiation may be generated/produced corresponding to any Stokes shift (e.g. a first, second, third or higher order Stokes shift). In this example, energy/power from the first radiation may be used to generate/amplify second radiation corresponding to any Stokes shift (e.g. a second, third, fourth or higher order Stokes shift).

[0107] While specific embodiments of the disclosure have been described above, it will be appreciated that the disclosure may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the disclosure as described without departing from the scope of the claims set out below.