Optical Parametric Oscillator System

An optical parametric oscillator (OPO) system comprises an optical waveguide including a hollow core containing a fluid, wherein the optical waveguide is configured to receive pump light and to convert the pump light into signal light and idler light via a third order non-linear optical effect. The OPO system further comprises an optical feedback arrangement for recycling at least a portion of the signal light and/or for recycling at least a portion of the idler light in an optical cavity that includes the optical waveguide. The OPO system may be used, in particular though not exclusively, in metrology, gas and solid-state spectroscopy, laser-assisted manufacturing, semiconductor technology, biomedicine, healthcare, and scientific laboratory use.

FIELD

The present disclosure relates to an optical parametric oscillator (OPO) system based on a fluid-filled hollow-core optical waveguide and, in particular though not exclusively, to an OPO system based on a fluid-filled hollow-core optical fibre for use, in particular though not exclusively, in metrology, gas and solid-state spectroscopy, laser-assisted manufacturing, semiconductor technology, biomedicine and healthcare, and scientific laboratory use.

BACKGROUND

Tuneable light sources are required, especially in the ultraviolet region, for use in semiconductor chip metrology, biomedical and healthcare technology as well as gas and solid-state spectroscopy. Tuneable light sources may not be available for some of these technical applications and it may be necessary to use several different lasers or complex frequency conversion schemes to obtain the range of frequencies required for such technical applications. These schemes tend to be of low efficiency which increases pump power requirements and heat generation and reduces lifetime. Additionally or alternatively, these technical applications may require lamp based sources which are bulky, inefficient, sometimes dangerous, low brightness and exhibit poor spatial beam quality.

Optical parametric oscillators (OPOs) are widely-tuneable sources of coherent light. Many OPOs are based on second order non-linear optical materials (e.g. most crystals) in which pump light is converted into signal light and idler light. However, the frequency of the signal light or idler light emitted from such OPOs is limited to frequencies which are lower than a frequency of the pump light used to pump the second order non-linear optical material of the OPO since the sum of the idler and signal frequencies is equal to the pump frequency. Higher frequencies cannot be directly generated and require additional frequency conversion steps. However, such additional frequency conversion steps have a limited lifetime and low efficiency when converting to the ultraviolet spectral region.

Fibre-optic parametric oscillators (FOPOs) are also known, for example based on third order non-linear optical effects such as four-wave mixing in solid-core optical fibres. However, the tunability of the output of such known FOPOs requires a tuneable pump laser, along with a suitably designed optical fibre. This cannot be changed after manufacture, limiting the output wavelengths of such known FOPOs. Moreover, it may not be possible to generate light from such known FOPOs at frequencies in the ultraviolet or short end of the visible spectrum (violet and blue region), or in most of the mid- and far-infrared, due to the conflicting requirements between phase-matching of the pump, signal and idler waves in the optical fibre on the one hand and the transparency of the optical fibre materials on the other hand. The energy and power from known FOPOs is also limited due to the damage threshold power levels associated with the fibre materials. In addition, rapid tuning of the signal and idler frequencies generated using known FOPOs may not be possible.

SUMMARY

According to an aspect of the present disclosure there is provided an optical parametric oscillator (OPO) system, comprising:an optical waveguide including a hollow core containing a fluid, wherein the optical waveguide is configured to receive pump light and to convert the pump light into signal light and idler light via a third order non-linear optical effect; andan optical feedback arrangement for recycling at least a portion of the signal light and/or for recycling at least a portion of the idler light in an optical cavity that includes the optical waveguide.

The optical feedback arrangement may be configured to couple at least a portion of the signal light and/or at least a portion of the idler light out of the optical cavity.

The optical feedback arrangement may be configured to recycle the signal light in the optical cavity and to couple the idler light out of the optical cavity.

The optical feedback arrangement may be configured to recycle the idler light in the optical cavity and to couple the signal light out of the optical cavity.

The optical feedback arrangement may be configured to recycle the signal light in the optical cavity and to recycle the idler light in the optical cavity.

The optical waveguide may be configured to convert the pump light into the signal light and the idler light via four-wave mixing.

The signal light may include a signal frequency. The idler light may include an idler frequency.

The pump light may include a pump frequency. The sum of the signal frequency and the idler frequency may be equal to twice the pump frequency.

The pump light may include first and second pump frequencies, wherein the first and second pump frequencies are different. The sum of the signal frequency and the idler frequency may be equal to the sum of the first and second pump frequencies.

One or both of the frequencies of the signal light and the idler light may be in the vacuum UV, the deep UV, the UV, the visible or the IR regions of the electromagnetic spectrum.

The optical waveguide may comprise a hollow-core fluid-filled optical fibre.

The OPO system may be configured to control at least one of a composition, a temperature, a pressure, a distribution, a profile, and a concentration of the fluid in the hollow-core optical waveguide.

The OPO system may comprise a gas cell which is configured to control the supply of fluid to the optical waveguide. The gas cell may include a chamber which contains an end of the optical waveguide. The OPO system may comprise a fluid reservoir. The gas cell may comprise a valve for controlling the supply of fluid from the fluid reservoir to the chamber. The valve may be configured to control a pressure of the fluid in the chamber, for example, a partial pressure of the fluid in the chamber. The OPO system may comprise a fluid pump. The OPO system may be configured so that the fluid pump can pump fluid from the fluid reservoir to the chamber.

The OPO system may comprise a plurality of fluid reservoirs, each fluid reservoir containing a different fluid. The gas cell may comprise a plurality of valves for controlling the supply of the plurality of different fluids from the plurality of different fluid reservoirs to the chamber. Each valve may be configured to control a pressure of the corresponding fluid in the chamber, for example, a partial pressure of the corresponding fluid in the chamber. The OPO system may be configured so that the fluid pump can pump the plurality of different fluids from the plurality of different fluid reservoirs to the chamber. The OPO system may comprise a plurality of fluid pumps. The OPO system may be configured so that each fluid pump can pump a corresponding fluid from the corresponding fluid reservoir to the chamber.

The gas cell may be configured to control the temperature of the one or more fluids in the chamber. For example, the gas cell may include a heater and/or a cooler for controlling the temperature of the one or more fluids in the chamber.

The OPO system may comprise first and second gas cells, wherein the first gas cell is configured to control the supply of fluid to a first end of the optical waveguide and the second gas cell is configured to control the supply of fluid to a second end of the optical waveguide.

The first gas cell may include a chamber which contains the first end of the optical waveguide. The first gas cell may comprise a valve for controlling the supply of fluid from the fluid reservoir to the chamber of the first gas cell. The valve may be configured to control a pressure of the fluid in the chamber of the first gas cell, for example, a partial pressure of the fluid in the chamber of the first gas cell. The OPO system may comprise a first fluid pump. The OPO system may be configured so that the first fluid pump can pump fluid from the fluid reservoir to the chamber of the first gas cell.

The first gas cell may comprise a plurality of valves for controlling the supply of the plurality of different fluids from the plurality of different fluid reservoirs to the chamber of the first gas cell. Each valve may be configured to control a pressure of the corresponding fluid in the chamber of the first gas cell, for example, a partial pressure of the corresponding fluid in the chamber of the first gas cell. The OPO system may be configured so that the first fluid pump can pump the plurality of different fluids from the plurality of different fluid reservoirs to the chamber of the first gas cell. The OPO system may comprise a plurality of fluid pumps. The OPO system may be configured so that each fluid pump can pump a corresponding fluid from the corresponding fluid reservoir to the chamber of the first gas cell.

The first gas cell may be configured to control the temperature of the one or more fluids in the chamber of the first gas cell. For example, the first gas cell may include a heater and/or a cooler for controlling the temperature of the one or more fluids in the chamber of the first gas cell.

The second gas cell may include a chamber which contains the second end of the optical waveguide. The second gas cell may comprise a valve for controlling the supply of fluid from the fluid reservoir to the chamber of the second gas cell. The valve may be configured to control a pressure of the fluid in the chamber of the second gas cell, for example, a partial pressure of the fluid in the chamber of the second gas cell. The OPO system may comprise a second fluid pump. The OPO system may be configured so that the second fluid pump can pump fluid from the fluid reservoir to the chamber of the second gas cell.

The second gas cell may comprise a plurality of valves for controlling the supply of the plurality of different fluids from the plurality of different fluid reservoirs to the chamber of the second gas cell. Each valve may be configured to control a pressure of the corresponding fluid in the chamber of the second gas cell, for example, a partial pressure of the corresponding fluid in the chamber of the second gas cell. The OPO system may be configured so that the second fluid pump can pump the plurality of different fluids from the plurality of different fluid reservoirs to the chamber of the second gas cell. The OPO system may comprise a plurality of fluid pumps. The OPO system may be configured so that each fluid pump can pump a corresponding fluid from the corresponding fluid reservoir to the chamber of the second gas cell.

The second gas cell may be configured to control the temperature of the one or more fluids in the chamber of the second gas cell. For example, the second gas cell may include a heater and/or a cooler for controlling the temperature of the one or more fluids in the chamber of the second gas cell.

The first end of the optical waveguide may be located within a chamber of the first gas cell and the second end of the optical waveguide may be located within a chamber of the second gas cell.

The first gas cell may comprise a valve for controlling the supply of fluid to the chamber of the first gas cell. The second gas cell may comprise a valve for controlling the supply of fluid to the chamber of the second gas cell.

The fluid may comprise a liquid or a gas.

The fluid may comprise a gas or a liquid at different pressures and temperatures. For example, the fluid may comprise a monoatomic gas, a molecular gas or a liquid at different pressures and temperatures.

The optical feedback arrangement may comprise an output dichroic mirror for reflecting at least a portion of the signal light after emission of the signal light from the optical waveguide and/or for reflecting at least a portion of the idler light after emission of the idler light from the optical waveguide. The output dichroic mirror may be configured for transmitting at least a portion of the signal light after emission of the signal light from the optical waveguide and/or for transmitting at least a portion of the idler light after emission of the idler light from the optical waveguide.

An optical frequency of the pump light may be different from a lasing transition frequency or an absorption resonance frequency of the fluid.

The pump light may be continuous wave (CW).

The pump light comprises pulsed pump light in the form of a train of pump pulses.

The signal light may comprise pulsed signal light in the form of a train of signal pulses.

The idler light may comprise pulsed idler light in the form of a train of idler pulses.

The OPO system may comprise an optical pump source for generating the pump light.

The optical pump source may be coherent.

The optical pump source may comprise a pump laser.

The optical pump source may have a fixed optical frequency.

The optical pump source may have a variable optical frequency.

The optical pump source may be configured to generate pump light at different average power levels.

The optical pump source may be configured to generate CW pump light.

The optical pump source may be configured to generate pulsed pump light in the form of a train of pump pulses.

The optical pump source may be configured to vary a duration of the pump pulses.

The optical pump source may be configured to vary a repetition rate of the train of pump pulses.

The optical pump source may be located outside the optical cavity.

The optical cavity may be configured to receive the pump light from the optical pump source.

The optical feedback arrangement may comprise an input dichroic mirror for transmitting the pump light into the optical cavity and for reflecting the signal light after emission of the signal light from the optical waveguide and/or for reflecting the idler light after emission of the idler light from the optical waveguide.

The optical pump source may have an optical pump feedback arrangement defining an optical pump cavity. The optical pump cavity and the optical cavity of the OPO system may at least partially overlap. The optical waveguide may be located in both the optical pump cavity and the optical cavity of the OPO system.

The OPO system may comprise a variable optical delay arrangement for varying an optical path length of the optical cavity.

The variable optical delay arrangement may comprise two mirrors mounted on a translation stage or a retroreflector mounted on a translation stage.

The variable optical delay arrangement may comprise two or a combination of optically transmissive wedges in the optical cavity, and one or more translation stages for translating the optically transmissive wedges in a direction across the path of the signal light and/or the idler light in the optical cavity so as to vary a distance propagated by the signal light or the idler light through the wedges.

An optical path length of the optical cavity and/or a repetition period of the train of pump pulses may be configured so that the round-trip delay experienced by the recycled signal light in the optical cavity and/or the round-trip delay experienced by the recycled idler light in the optical cavity is equal to the repetition period of the train of pump pulses.

An optical path length of the optical cavity and/or a repetition period of the train of pump pulses may be configured so that the round-trip delay experienced by the recycled signal light in the optical cavity and/or the round-trip delay experienced by the recycled idler light in the optical cavity is an integer number of times the repetition period of the train of pump pulses.

An optical path length of the optical cavity and/or a repetition period of the train of pump pulses may be configured so that the repetition period of the train of pump pulses is an integer number of times the round-trip delay experienced by the recycled signal light in the optical cavity and/or the round-trip delay experienced by the recycled idler light in the optical cavity.

A carrier envelope offset and repetition rate of the pulsed pump light may be stabilised.

The OPO system may comprise a carrier envelope offset and repetition rate stabilization arrangement configured to stabilise a carrier envelope offset and repetition rate of the pulsed pump light.

The OPO system may comprise a carrier envelope offset and repetition rate stabilization arrangement configured to stabilise a carrier envelope offset and repetition rate of the pulsed signal light and/or to stabilise a carrier envelope offset and repetition rate of the pulsed idler light.

The OPO system may comprise a variable spectral filtering arrangement for spectrally filtering the signal light in the optical cavity and/or for spectrally filtering the idler light in the optical cavity.

The variable spectral filtering arrangement may be configured to transmit light having a wavelength in a variable spectral passband.

The variable spectral passband may have a tuneable centre wavelength and/or a variable spectral bandwidth.

The variable spectral filtering arrangement may comprise one or more dispersive elements for spatially dispersing the signal light in the optical cavity and/or spatially dispersing the idler light in the optical cavity and a variable aperture or slit which is moveable relative to the spatially dispersed signal light in the optical cavity and/or moveable relative to the spatially dispersed idler light in the optical cavity.

The OPO system may comprise a variable dispersion arrangement for controlling the spectral phase or chirp of the signal light in the optical cavity and/or for controlling the spectral phase or chirp of the idler light in the optical cavity.

The variable dispersion arrangement may comprise a pair of dispersive elements.

The variable dispersion arrangement may comprise a pair of prisms.

The variable dispersion arrangement may comprise a pair of gratings.

The variable dispersion arrangement may comprise a pair of grisms.

The variable dispersion arrangement may comprise a Bragg grating. For example, the variable dispersion arrangement may comprise a fibre Bragg grating system.

The variable dispersion arrangement may comprise one or more chirped mirrors.

The variable dispersion arrangement may comprise one or more optically transmissive plates or wedges and one or more translation stages for translating the one or more optically transmissive plates or wedges in a direction across the path of the signal light in the optical cavity and/or the idler light in the optical cavity so as to vary a distance propagated by the signal light and/or the idler light through the plates or wedges.

The variable dispersion arrangement may comprise a dispersive filter. The dispersive filter may be programmable. The dispersive filter may be an acousto-optic dispersive filter.

The OPO system may comprise an arrangement for varying an optical power of the signal light in the optical cavity and/or for varying an optical power of the idler light in the optical cavity.

The OPO system may comprise an arrangement for varying an average optical power of the signal light in the optical cavity and/or for varying an average optical power of the idler light in the optical cavity.

The OPO system may comprise an attenuator in the optical cavity. The attenuator may comprise at least one of a neutral density filter, a combination of a waveplate and a polarizer.

The OPO system may comprise an arrangement for varying the optical power of different pulses of signal light in the optical cavity and/or for varying the optical power of different pulses of idler light in the optical cavity. The arrangement for varying the optical power of different pulses of signal light in the optical cavity and/or for varying the optical power of different pulses of idler light in the optical cavity may comprise an acousto-optic device or an electro-optic device.

The OPO system may comprise a Bragg grating in the optical cavity. The Bragg grating may be integrated with the optical waveguide. The Bragg grating may be configured for dispersion compensation. The Bragg grating may be configured for back-reflection and/or for out-coupling of light from the optical waveguide. For example, the OPO system may comprise a fibre Bragg grating in the optical cavity. For example, the optical waveguide may comprise a fluid-filled optical fibre and the fibre Bragg grating may be integrated with the fluid-filled optical fibre.

The optical cavity may have a travelling-wave configuration.

The optical cavity may have a ring configuration.

The optical cavity may have a standing-wave configuration.

The optical cavity may have a linear configuration.

The optical cavity may have a Fabry-Perot configuration.

The optical cavity may define an optical path which includes a double-pass through the optical waveguide for each round trip around the optical cavity.

The optical cavity may have a travelling-wave configuration in which the pump light together with the recycled signal light and/or the recycled idler light is coupled into a first end of the optical waveguide. The OPO system may comprise a high-reflector located at a second end of the optical waveguide for back-reflecting the pump light, the signal light and the idler light emitted from the second end of the optical waveguide back into the second end of the optical waveguide. The back-reflected pump light, signal light and idler light may be coupled out of the first end of the optical waveguide. For example, the OPO system may comprise an optical circulator at the first end of the optical waveguide for this purpose.

The OPO system may comprise one or more focussing elements, such as one or more lenses or curved mirrors, for coupling the pump light into the optical waveguide.

The OPO system may comprise one or more focussing elements, such as one or more lenses or curved mirrors, for coupling recycled signal light into the optical waveguide and/or for coupling recycled idler light into the optical waveguide.

The OPO system may comprise one or more collimating elements, such as one or more lenses or curved mirrors, for collimating recycled signal light and/or for collimating recycled idler light.

The OPO system may comprise a plurality of optical waveguides, wherein each optical waveguide includes a hollow core containing a fluid, each optical waveguide is configured to receive pump light and to convert the pump light into signal light and idler light via a third order non-linear optical effect, and wherein the optical feedback arrangement is configured to recycle at least a portion of the signal light and/or for recycling at least a portion of the idler light in an optical cavity that includes the plurality of optical waveguides. Each optical waveguide may comprise a fluid-filled optical fibre.

The OPO system may comprise one or more optical elements, such as one or more lenses or curved mirrors, for coupling at least one of the pump light, the signal light and the idler light emitted from an output end of one of the optical waveguides to an input end of an adjacent one of the optical waveguides.

The OPO system may comprise one or more collimating elements, such as one or more lenses or curved mirrors, for collimating light output from the OPO system.

According to an aspect of the present disclosure there is provided an optical parametric oscillation (OPO) method, comprising:converting pump light into signal light and idler light in a fluid-filled hollow-core optical waveguide via a third order non-linear optical effect; andrecycling at least a portion of the signal light and/or recycling at least a portion of the idler light in an optical cavity that includes the optical waveguide.

The OPO method may comprise coupling at least a portion of the signal light and/or at least a portion of the idler light out of the optical cavity.

The OPO method may comprise converting the pump light into the signal light and the idler light in the fluid-filled hollow core optical waveguide via four-wave mixing.

The signal light may include a signal frequency. The idler light may include an idler frequency.

The pump light may include a pump frequency. The sum of the signal frequency and the idler frequency may be equal to twice the pump frequency.

The pump light may include first and second pump frequencies, wherein the first and second pump frequencies are different. The sum of the signal frequency and the idler frequency may be equal to the sum of the first and second pump frequencies.

One or both of the frequencies of the signal light and the idler light may be in the vacuum UV, the deep UV, the UV, the visible or the IR regions of the electromagnetic spectrum.

The optical waveguide may comprise a hollow-core fluid-filled optical fibre.

The OPO method may comprise controlling at least one of a composition, a temperature, a pressure, a distribution, a profile and a concentration of the fluid in the hollow-core optical waveguide.

The fluid may comprise a liquid or a gas.

The OPO method may comprise selecting an optical frequency of the pump light to be different from a lasing transition frequency or an absorption resonance frequency of the fluid.

The pump light may be CW or pulsed.

The OPO method may comprise varying an average power level of the pump light.

The OPO method may comprise generating pulsed pump light in the form of a train of pump pulses.

The OPO method may comprise varying a duration of the pump pulses.

The OPO method may comprise varying a repetition rate of the train of pump pulses.

The signal light may comprise pulsed signal light in the form of a train of signal pulses.

The idler light may comprise pulsed idler light in the form of a train of idler pulses.

The OPO method may comprise varying an optical path length of the optical cavity.

The OPO method may comprise selecting an optical path length of the optical cavity and/or a repetition period of the train of pump pulses so that the round-trip delay experienced by the recycled signal light in the optical cavity and/or the round-trip delay experienced by the recycled idler light in the optical cavity is equal to the repetition period of the train of pump pulses.

The OPO method may comprise selecting an optical path length of the optical cavity and/or a repetition period of the train of pump pulses so that the round-trip delay experienced by the recycled signal light in the optical cavity and/or the round-trip delay experienced by the recycled idler light in the optical cavity is an integer number of times the repetition period of the train of pump pulses.

The OPO method may comprise selecting an optical path length of the optical cavity and/or a repetition period of the train of pump pulses so that the repetition period of the train of pump pulses is an integer number of times the round-trip delay experienced by the recycled signal light in the optical cavity and/or the round-trip delay experienced by the recycled idler light in the optical cavity.

A carrier envelope offset and repetition rate of the pulsed pump light may be stabilised.

The OPO method may comprise stabilising a carrier envelope offset and repetition rate of the pulsed pump light.

The OPO method may comprise stabilising a carrier envelope offset and repetition rate of the pulsed signal light and/or stabilising a carrier envelope offset and repetition rate of the pulsed idler light.

The OPO method may comprise spectrally filtering the signal light in the optical cavity and/or spectrally filtering the idler light in the optical cavity.

The OPO method may comprise transmitting signal light in the optical cavity having a wavelength in a variable spectral passband and/or transmitting idler light in the optical cavity having a wavelength in a variable spectral passband.

The OPO method may comprise controlling the spectral phase or chirp of the signal light in the optical cavity and/or controlling the spectral phase or chirp of the idler light in the optical cavity.

The OPO method may comprise varying an average optical power of the signal light in the optical cavity and/or varying an average optical power of the idler light in the optical cavity.

The OPO method may comprise varying the optical power of different pulses of signal light in the optical cavity and/or for varying the optical power of different pulses of idler in the optical cavity.

The optical cavity may have a ring, or a travelling-wave, configuration.

The optical cavity may have a linear, or a standing-wave, configuration.

It should be understood that any one or more of the features of any one of the foregoing aspects of the present disclosure may be combined with any one or more of the features of any of the other foregoing aspects of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring initially toFIG.1there is shown an OPO system generally designated2including a hollow-core fluid-filled optical waveguide in the form of a hollow-core gas-filled optical fibre generally designated4. As will be described in more detail below, the hollow-core gas-filled optical fibre4is configured to convert pump light5into signal light6and idler light8via a third order non-linear optical effect in the form of four-wave mixing.

The OPO system2includes an optical pump source in the form of a pump laser10for generating the pump light5. The OPO system2further includes a dichroic input coupler12, a dichroic output coupler14and re-direction mirrors16. As will be described in more detail below, the dichroic input coupler12, the dichroic output coupler14and the re-direction mirrors16together constitute an optical feedback arrangement for recycling the signal light6emitted from the hollow-core gas-filled optical fibre4in a ring, or travelling wave, optical cavity20which includes the hollow-core gas-filled optical fibre4. Moreover, the optical feedback arrangement is also configured to couple the idler light8emitted from the hollow-core gas-filled optical fibre4out of the optical cavity20as output light26. Specifically, the dichroic output coupler14is configured to be highly reflecting at a wavelength of the signal light6and to be highly transmitting at a wavelength of the idler light8. The re-direction mirrors16are configured to be highly reflecting at a wavelength of the signal light6. The dichroic input coupler12is configured to be highly transmitting at a wavelength of the pump light5. The dichroic input coupler12is also configured to be highly reflecting at a wavelength of the signal light6.

The OPO system2includes a pump lens30for coupling pump light5from the pump laser10into an input end4aof the optical fibre4via the dichroic input coupler12. The OPO system2further includes a collimating lens32for collimating signal light6and idler light8emitted from an output end4bof the optical fibre4and a focusing lens34for focusing recycled signal light6into the input end4aof the optical fibre4.

The OPO system2includes a first gas cell40and a second gas cell50, wherein the first gas cell40is configured to control a supply of gas to the input end4aof the optical fibre4and the second gas cell50is configured to control a supply of gas to the output end4bof the optical fibre4. More specifically, the first gas cell40defines a chamber42which contains the input end4aof the optical fibre4and the first gas cell40further includes a gas inlet44having a valve46for controlling the flow of gas from a gas supply59to the chamber42of the first gas cell40. Similarly, the second gas cell50defines a chamber52which contains the output end4aof the optical fibre4and the second gas cell50includes a gas inlet54having a valve56for controlling the flow of gas from the gas supply59to the chamber52of the second gas cell50.

The first gas cell40includes a window48for transmitting the pump light5and the recycled signal light6and/or the recycled idler light8to the input end4aof the optical fibre4. Similarly, the second gas cell50includes a window58for transmitting the pump light5, the signal light6and the idler light8from the output end4bof the optical fibre4.

In use, the pump lens30couples pump light5from the pump laser10into an input end4aof the optical fibre4via the dichroic input coupler12. The OPO system2is configured to control at least one of a composition, a temperature, a pressure, a distribution, and a profile of the gas in the hollow-core gas-filled optical fibre4so as to control the generation of the signal light6and the idler light8in the hollow-core gas-filled optical fibre4. Specifically, the OPO system2is configured to control at least one of a composition, a temperature, a pressure, a distribution, and a profile of the gas in the hollow-core gas-filled optical fibre4so as to control the dispersion in the hollow-core gas-filled optical fibre4and the respective propagation constants or wavevectors of the pump light5, the signal light6and the idler light8in the hollow-core gas-filled optical fibre4for phase-matching of the four-wave mixing process and optimization of the parametric gain in the hollow-core gas-filled optical fibre4for the efficient generation of the signal light6and the idler light8. As will be understood by one of ordinary skill in the art, in general, the pump light5includes first and second pump frequencies, wherein the first and second pump frequencies are the same or different, and wherein the sum of the frequencies of the signal light6and the idler light8is equal to the sum of the first and second pump frequencies.

The signal light6, the idler light8and any remaining pump light5are emitted from the optical fibre4. Specifically, the signal light6, the idler light8and any remaining pump light5are emitted from the output end4bof the optical fibre4. The collimating lens32collimates the signal light6and idler light8, and any remaining pump light5, emitted from the output end4bof the optical fibre4. The dichroic output coupler14reflects the signal light6incident on the dichroic output coupler14and transmits the idler light8incident on the dichroic output coupler14so as to provide output light26. The signal light6reflected by the dichroic output coupler14is then recycled back into the input end4aof the optical fibre4via the re-direction mirrors16, the focusing lens34and the dichroic input coupler12.

One of ordinary skill in the art will understand that the OPO system2may be operated continuous-wave (CW). For example, the optical pump laser10may be a continuous-wave (CW) optical pump laser10.

Alternatively, the OPO system2may be operated pulsed. For example, the optical pump laser10may be a pulsed optical pump laser10. The OPO system2may include a variable optical delay arrangement generally designated60for varying an optical path length of the optical cavity20to ensure temporal overlap in the optical fibre4between a subsequent pump pulse5, a signal pulse6and an idler pulse8. The variable optical delay arrangement60includes a translation stage18and the two re-direction mirrors16mounted on the translation stage18. The translation stage18may be motor-controlled or piezo-electric. If operated pulsed, the optical path length of the optical cavity20may be selected to ensure temporal overlap in the optical fibre4between a recycled signal pulse6and a subsequent pump pulse5i.e. the OPO system2may be synchronously pumped. A synchronously-pumped pulsed OPO system will be described in more detail below with reference toFIGS.2and3.

From the foregoing description, one of ordinary skill in the art will understand that the OPO system2enables a pump laser10, at a fixed frequency, to be converted to widely-tuneable light at both a higher idler frequency and a lower signal frequency, with the signal and idler frequencies determined by the properties of the pump laser10, the structure of the optical fibre4, the properties of the gas contained in the hollow core of the optical fibre4including at least one of the composition, temperature, pressure, distribution, and profile of the gas in the hollow core of the optical fibre4, the properties of the optical elements32,14,16,34and12in the optical cavity20, and the arrangement of the optical elements32,14,16,34and12in the optical cavity20. The use of a hollow-core fluid-filled optical fibre may provide lower group velocity dispersion in the hollow-core fluid-filled optical fibre and therefore a broader phase-matching bandwidth compared with known fibre OPOs based on solid-core optical fibres. In effect, this allows the OPO system2to be tuned across a wider tuning range than known fibre OPOs based on solid-core optical fibres. The signal and idler frequencies may be located in the vacuum ultraviolet, deep ultraviolet, visible and infrared region of the electromagnetic spectrum. The signal light6and idler light8can be in spectral ranges outside the transmission band of glass-fibres and crystal materials. Both the frequency and spectral bandwidth of the light output26from the OPO system2can be tuned, for example by spectrally filtering the recycled signal light6. The frequency of the light output26from the OPO system2can be tuned rapidly in real time and the spectral bandwidth of the light output from the OPO system2can be varied rapidly in real time using a variable spectral filtering arrangement.

The pump and output optical power levels can be very high and the lifetime of the OPO system2can be highly extended relative to known fibre OPOs based on solid-core optical fibres. Conversely, the use of the hollow-core fluid-filled optical fibre4may result in the same output optical power level for a lower optical pump power level relative to known fibre OPOs based on solid-core optical fibres.

One of ordinary skill in the art will also understand that the optical frequency of the pump light5is different from a lasing transition frequency or an absorption resonance frequency of the gas in the hollow core of the optical fibre4and that the OPO system2does not require an active lasing transition to emit the signal light6and the idler light8. In other words, the gas in the hollow core of the optical fibre4does not need to exhibit a lasing transition at the frequency of operation or store any energy.

A synchronously-pumped pulsed OPO system102will now be described with reference toFIGS.2and3. The synchronously-pumped pulsed OPO system102includes features which correspond closely to the features of the OPO system2ofFIG.1, with like features of the OPO system102having the same reference numeral as the corresponding features of the OPO system2ofFIG.1incremented by “100”. Specifically, the OPO system102includes a hollow-core fluid-filled optical waveguide in the form of a hollow-core gas-filled optical fibre generally designated104. As will be described in more detail below, the hollow-core gas-filled optical fibre104is configured to convert pulsed pump light105into pulsed signal light106and pulsed idler light108via a third order non-linear optical effect in the form of four-wave mixing.

The OPO system102includes a pulsed optical pump source in the form of a pulsed pump laser110for generating the pulsed pump light105. The OPO system102further includes a dichroic input coupler112, a dichroic output coupler114and re-direction mirrors116. As will be described in more detail below, the dichroic input coupler112, the dichroic output coupler114and the re-direction mirrors116together constitute an optical feedback arrangement for recycling the pulsed signal light106emitted from the hollow-core gas-filled optical fibre104in a ring, or travelling wave, optical cavity120which includes the hollow-core gas-filled optical fibre104. Moreover, the optical feedback arrangement is also configured to couple the pulsed idler light108emitted from the hollow-core gas-filled optical fibre104out of the optical cavity120as pulsed output light126. Specifically, the dichroic output coupler114is configured to be highly reflecting at a wavelength of the signal light106and to be highly transmissive at a wavelength of the idler light108. The re-direction mirrors116are configured to be highly reflecting at the wavelength of the signal light106. The dichroic input coupler112is configured to be highly transmitting at a wavelength of the pump light105. The dichroic input coupler112is also configured to be highly reflecting at a wavelength of the signal light106.

The OPO system102includes a pump lens130for coupling pump light105from the pump laser110into the optical fibre104via the dichroic input coupler112. The OPO system102further includes a collimating lens132for collimating idler light108transmitted by the dichroic output coupler114, a further collimating lens133for collimating recycled signal light106reflected by the dichroic output coupler114, and a focusing lens134for focusing recycled signal light106back into the optical fibre104.

The OPO system102includes a first gas cell140and a second gas cell150, wherein the first gas cell140is configured to control a supply of gas from a gas supply159to an input end of the optical fibre104and the second gas cell150is configured to control a supply of gas from the gas supply159to an output end of the optical fibre104.

The OPO system102includes a variable optical delay arrangement generally designated160for varying an optical path length of the optical cavity120to ensure temporal overlap in the optical fibre104between a subsequent pump pulse105, a signal pulse106and an idler pulse108. The variable optical delay arrangement160includes a translation stage118having two of the re-direction mirrors116mounted on the translation stage118. The translation stage118may be motor-controlled or piezo-electric. As will be described in more detail below, the variable optical delay arrangement160may also be used to control the average and/or peak power of the pulsed output light126.

The OPO system102further includes a variable spectral filtering arrangement generally designated170for spectrally filtering the pulsed signal light106in the optical cavity120. The variable spectral filtering arrangement170includes a first pair of prisms172, a variable aperture or slit174mounted on a translation stage176, and a second pair of prisms178.

The OPO system102further includes an optical power control arrangement180such as a neutral density absorptive filter, a waveplate/polarizer combination, or an acousto-optic attenuator for controlling the optical power of the recycled pulsed signal light106.

The OPO system102further includes a carrier envelope offset and repetition rate stabilization arrangement generally designated190for stabilising the carrier envelope offset and repetition rate of the signal light106. The carrier envelope offset and repetition rate stabilization arrangement190includes a fast, finely-tuneable variable optical delay arrangement192which includes a pair of optically transmissive wedges and one or more piezo-electric stages for translating each of the optically transmissive wedges in a direction across the path of the pulsed signal light106in the optical cavity120so as to vary a distance propagated by the pulsed signal light106in the optical cavity120through the optically transmissive wedges to thereby vary the round-trip delay experienced by the pulsed signal light106in the optical cavity120on a sub-fs time scale. The carrier envelope offset and repetition rate stabilization arrangement190further includes a partially reflecting mirror194, a highly reflective re-direction mirror196, an f-2f detector arrangement198and a controller199.

In use, the pulsed pump laser110generates pulsed pump light105which is coupled into the hollow-core gas-filled optical fibre104via the pump focussing lens130and the dichroic input coupler112. The OPO system102is configured to control at least one of a composition, a temperature, a pressure, a distribution, and a profile of the gas in the hollow-core gas-filled optical fibre104so as to control the generation of the pulsed signal light106and the pulsed idler light108in the hollow-core gas-filled optical fibre104. Specifically, the OPO system102is configured to control at least one of a composition, a temperature, a pressure, a distribution, and a profile of the gas in the hollow-core gas-filled optical fibre104so as to control the dispersion in the hollow-core gas-filled optical fibre104and the respective propagation constants or wavevectors of the pulsed pump light105, the pulsed signal light106and the pulsed idler light108in the hollow-core gas-filled optical fibre104for phase-matching of the four-wave mixing process and gain optimization in the hollow-core gas-filled optical fibre104for the efficient generation of the pulsed signal light106and the pulsed idler light108. As will be understood by one of ordinary skill in the art, in general, the pump light105includes first and second pump frequencies, wherein the first and second pump frequencies are the same or different, and wherein the sum of the frequencies of the signal light106and the idler light108is equal to the sum of the first and second pump frequencies.

The pulsed signal light106, the pulsed idler light108and any remaining pulsed pump light105are emitted from an output end of the optical fibre104. The dichroic output coupler114reflects the pulsed signal light106incident on the dichroic output coupler114and transmits the pulsed idler light108incident on the dichroic output coupler14so as to provide pulsed output light126. The collimating lens132collimates the pulsed output light126.

The pulsed signal light106reflected by the dichroic output coupler114is then recycled back into the optical fibre104via the further collimating lens133, the variable spectral filtering arrangement generally designated170, the re-direction mirrors116, the variable optical delay arrangement160, the carrier envelope offset and repetition rate stabilization arrangement180, the focusing lens134and the dichroic input coupler112.

As shown inFIG.3, the variable optical delay arrangement160is used to control the optical path length of the optical cavity120so as to match a round-trip delay experienced by the pulsed signal light106in the optical cavity120to a repetition period of the pulsed pump light105for the temporal overlap in the optical fibre104between a pulsed signal pulse106and a subsequent pump pulse105and thereby maximise the optical power of the pulsed signal light106and/or the pulsed idler light108. Controlling the optical path length of the optical cavity120in this way maximises the optical power of the pulsed output light126.

Tuning of the pulsed output light126is accomplished using the variable spectral filtering arrangement170. Specifically, the first pair of prisms172spatially disperses the different frequencies of the pulsed signal light106in a plane of the variable aperture or slit174. The variable aperture or slit174acts as a passband filter which transmits a range of the different frequencies of the pulsed signal light106across a spectral passband which has a central frequency defined by a position of a centre of the variable aperture or slit174and which has a spectral bandwidth defined by a width of the variable aperture or slit174. The second pair of prisms178spatially recombines the spatially dispersed pulsed signal light106after transmission through the variable aperture or slit174. The selection of the frequency and the spectral bandwidth of the recycled pulsed signal light106is directly transferred to the frequency and the spectral bandwidth of the output light126.

The optical power control arrangement180is used to control the average optical power and therefore also the peak optical power of the recycled pulsed signal light106. Such control over the average optical power of the recycled pulsed signal light106in the optical cavity120provides an additional means to control different non-linear phenomena in the optical fibre104(e.g. self- and cross-phase modulation) and can be used to mitigate or control the induced shift in the spectral position of the pulsed output light126and the power of the pulsed output light126.

One of ordinary skill in the art will understand that each of the pump pulse train105, the signal pulse train106and the idler pulse train108may be represented by a corresponding optical frequency comb in the optical frequency domain. However, the frequency spacing of the frequency combs of both the pulsed pump light105and the pulsed signal light106can fluctuate as a result of several independent stochastic processes. For some technical applications, it may be desirable to generate pulsed output light126in the form of pulsed idler light108having an optical frequency comb which is stabilised, not just in the relative frequency separation between the different frequency lines of the frequency comb, but also in the absolute frequency of the different frequency lines. This requires that the optical frequency combs of the pulsed pump light105and the pulsed signal light106are locked not just in the relative frequency separation between the different frequency lines of the frequency combs, but also in the absolute frequency of the different frequency lines of the frequency combs.

A carrier envelope offset and repetition rate of the pulsed pump light105is stabilised using a conventional method. The pump laser110may include a carrier envelope offset and repetition rate stabilization arrangement (not shown) for this purpose. The carrier envelope offset and repetition rate stabilization arrangement190is then used to stabilise the carrier envelope offset and repetition rate of the pulsed signal light106i.e. the carrier envelope offset and repetition rate stabilization arrangement190is used to stabilise not just the relative frequency separation between the different frequency lines of the frequency comb of the pulsed signal light106, but also the absolute frequency of the different frequency lines of the frequency comb of the pulsed signal light106. Specifically, the partially reflecting mirror194reflects a portion of the recycled pulsed signal light106. The reflected portion of the recycled pulsed signal light106is directed by the highly reflective mirror196onto the f-2f detector arrangement198. The controller199controls the variable optical delay arrangement192so as to rapidly and finely vary the round-trip delay of the signal pulses106in the optical cavity120by translating each of the optically transmissive wedges of the further variable optical delay arrangement192in a direction across the path of the pulsed signal light106in the optical cavity120so as to vary a distance propagated by the pulsed signal light106in the optical cavity120through the optically transmissive wedges on a sub-fs time scale to maximise the temporal overlap in the optical fibre104between the recycled signal pulse106and a subsequent pump pulse105to thereby maximise the optical power of the pulsed signal light106and/or the pulsed idler light108and lock the inverse of the round-trip delay experienced by the recycled signal pulse106in the optical cavity120to the repetition rate of the stabilised pulsed pump light105.

To lock the absolute frequency comb offset for the frequency comb of the pulsed signal light106, carrier envelope offset stabilisation is used. The carrier envelope offset (i.e. the absolute frequency) of the pulsed signal light106is locked using the f-to-2f detector arrangement198. As will be understood by one of ordinary skill in the art, the f-to-2f detector arrangement198doubles the optical frequency of a portion of the pulsed signal light106and beats the frequency-doubled pulsed signal light106together with the original fundamental-frequency pulsed signal light106on a photodetector to generate an electrical beat signal and the controller199uses the electrical beat signal to finely adjust the further variable optical delay arrangement192to lock the carrier envelope offset of the pulsed signal light106(i.e. the absolute frequency of the different frequency lines of the frequency comb of the pulsed signal light106) to a desired carrier envelope offset value and thereby lock the frequency comb of the pulsed signal light106to the frequency comb of the pulsed pump light105in absolute frequency. This results in pulsed idler light108and therefore also the pulsed output light126having a frequency-stabilised optical frequency comb.

From the foregoing description of the synchronously-pumped pulsed OPO system102, one of ordinary skill in the art will understand that the pump laser110may have a fixed frequency or may be tunable. In particular, one of ordinary skill in the art will understand that the OPO system102enables a pump laser110, at a fixed frequency, to be converted to widely tuneable light at both a higher idler frequency and a lower signal frequency, with the signal and idler frequencies determined by the properties of the pump laser110, the structure of the optical fibre104, the properties of the gas contained in the hollow core of the optical fibre104including at least one of the composition, temperature, pressure, distribution, and profile of the gas in the hollow core of the optical fibre104, the properties of the optical elements114,133,170,116,160,190,134and112in the optical cavity120, and the arrangement of the optical elements114,133,170,116,160,190,134and112in the optical cavity120. The use of a hollow-core fluid-filled optical fibre104may provide lower group velocity dispersion and group velocity walk-off in the hollow-core fluid-filled optical fibre104and therefore a broader phase-matching and parametric gain bandwidth compared with known fibre OPOs based on solid-core optical fibres. In effect, this allows the OPO system2to be tuned across a wider tuning range than known fibre OPOs based on solid-core optical fibres. The signal and idler frequencies may be located in the vacuum ultraviolet, deep ultraviolet, visible and infrared region of the electromagnetic spectrum. The signal light106and idler light108can be in spectral ranges outside the transmission band of glass-fibres and crystal materials. Both the frequency and spectral bandwidth of the light output126from the OPO system102can be tuned as described above.

The pump and output power levels can be very high and the lifetime of the OPO system102can be highly extended relative to known OPOs such as fibre OPOs based on solid-core optical fibres. Conversely, the use of the hollow-core fluid-filled optical fibre104may result in the same output optical power level for a lower optical pump power level relative to known fibre OPOs based on solid-core optical fibres.

One of ordinary skill in the art will also understand that the optical frequency of the pump light105is different from a lasing transition frequency or an absorption resonance frequency of the gas in the hollow core of the optical fibre104and that the OPO system102does not require an active lasing transition to emit the signal light106and the idler light108. In other words, the gas in the hollow core of the optical fibre104does not need to exhibit a lasing transition at the frequency of operation or store any energy.

The OPO system102may be operated to generate pulsed idler light108having a stabilised optical frequency comb.

One of ordinary skill in the art will understand that various modifications are possible to the OPO systems2,102and methods described above. For example, although the fluid in the hollow-core optical fibre4,104is described as a gas, it may be possible to use a liquid in the hollow-core optical fibre4,104. The OPO system2,102may include a hollow-core fluid-filled optical waveguide of any kind.

Rather than recycling the signal light6,106and coupling the idler light8,108out of the optical cavity20,120as output light26,126, the OPO system2,102may be configured to recycle the idler light8,108and couple the signal light6,106out of the optical cavity20,120as output light26,126. A portion of the signal light6,106may be recycled and the remaining portion of the signal light6,106may be coupled out of optical cavity20,120. A portion of the idler light8,108may be recycled and the remaining portion of the idler light8,108may be coupled out of optical cavity20,120. In some embodiments, the OPO system2,102may be configured to recycle all of the signal light6,106and all of the idler light8,108i.e. without coupling any of the signal light6,106or any of the idler light8,108out of the optical cavity20,120. Such embodiments may be useful for example when performing measurements using the intra-cavity signal light6,106or the intra-cavity idler light8,108on a sample inserted in the optical cavity20,120. Where at least a portion of the idler light8,108is recycled, one of ordinary skill in the art will understand that the recycled idler light8,108may be spectrally filtered using a variable spectral filtering arrangement like variable spectral filtering arrangement170. Similarly, where at least a portion of the pulsed idler light108is recycled, one of ordinary skill in the art will understand that the repetition rate of pulsed recycled idler light108may be varied using a variable optical delay arrangement like variable optical delay arrangement160for varying an optical path length of the optical cavity120to ensure temporal overlap in the optical fibre104between a subsequent pump pulse105, a signal pulse106and an idler pulse108. Where at least a portion of the pulsed idler light108is recycled, one of ordinary skill in the art will also understand that the carrier envelope offset and repetition rate of the pulsed idler light108may be stabilised using a carrier envelope offset and repetition rate stabilization arrangement like the carrier envelope offset and repetition rate stabilization arrangement190. Rather than the optical cavity20,120having a travelling-wave configuration in the form of a ring configuration as described with reference toFIGS.1-3, the optical cavity may have a standing-wave configuration. For example, the optical cavity may have a linear configuration or a Fabry-Perot configuration.

Rather than including a variable optical delay arrangement60,160which includes a translation stage18,118and a pair of re-direction mirrors16,116mounted on the translation stage18,118, the OPO system2,102may include a variable optical delay arrangement60,160which includes a translation stage and a retroreflector mounted on the translation stage. Alternatively, the OPO system2,102may include a variable optical delay arrangement which includes two optically transmissive wedges in the optical cavity20,120, and one or more translation stages for translating the two optically transmissive wedges in a direction across the path of the signal light in the optical cavity so as to vary a distance propagated by the signal light through the wedges.

It is also possible to control the spectral phase of the pulses of signal light106inside the optical cavity120, and therefore also the phase of the pulses of idler light108output from the OPO system102by means of one or more dispersive elements inside the optical cavity120e.g. prism/grating pairs, chirped mirrors or glass plates with adjustable thickness or path, or acousto-optic programmable dispersive filters. Such a method can be used to control the chirp, duration and pulse shape of the pulses of idler light108output from the OPO system102. If signal light106is coupled out of the optical cavity120, such a method can be used to control the chirp, duration and pulse shape of the pulses of signal light106output from the OPO system102.

Rather than using a variable spectral filtering arrangement170including spatially dispersive elements such as the pair of prisms172or a pair of gratings (not shown) and a variable aperture or slit174mounted on a translation stage176, frequency selection can be achieved using linear filtering devices which simply transmit a small spectral band of the signal in the cavity. Typical examples are a tuneable spectral filter based on liquid crystals or an acousto-optic device.

Additionally or alternatively, the frequency and spectral bandwidth of the light output126from the OPO system102can be varied rapidly in real time using a more advanced scheme of frequency selection by temporally chirping the recycled pulsed signal light106and tuning the delay between a pulse of the signal light106and a subsequent pulse of the pump light105such that only a certain portion of the chirped pulse of the signal light106overlaps with the subsequent pulse of the pump light105inside the optical fibre104. The round-trip delay experienced by the signal light106in the optical cavity120thus determines which spectral components of the pulse of the signal light106get amplified. Consequently, the frequency and spectral bandwidth of the light output126from the OPO system102can be varied rapidly by controlling the variable optical delay arrangement160. This may allow the frequency and spectral bandwidth of the light output126from the OPO system102to be varied extremely rapidly, and does not depend on wavelength-specific optics.

Rather than controlling the peak power of every pulse of signal light106in the same way, a fast attenuator such as an acousto-optic or an electro-optic device (e.g. a Pockel's cell) can address different pulses of signal light106in the pulse train separately and therefore allow for more advanced configurations where the repetition rate of the pump and cavity resonant frequency (1/cavity delay) are not just equal but are proportional by an integer number. For example, an optical path length of the optical cavity120and/or a repetition period of the train of pump pulses105may be selected so that the round-trip delay experienced by the recycled signal pulses106in the optical cavity120is an integer number of times the repetition period of the train of pump pulses105. An optical path length of the optical cavity120and/or a repetition period of the train of pump pulses105may be selected so that the repetition period of the train of pump pulses105is an integer number of times the round-trip delay experienced by the recycled signal pulses106in the optical cavity120. Thus, a fast attenuator such as an acousto-optic or an electro-optic device may be used to control the average power of the pulsed signal light106and/or the pulsed idler light108.

Rather than locking the inverse of the round-trip delay experienced by the recycled signal pulse106in the optical cavity120to the repetition rate of the stabilised pulsed pump light105, the controller199of the carrier envelope offset and repetition rate stabilization arrangement190may control the variable optical delay arrangement192so that the inverse of the round-trip delay experienced by the recycled signal pulses106in the optical cavity120is locked to an external electrical reference frequency. For example, the f-to-2f detector arrangement198may include a fast photodetector for detecting a repetition rate of the recycled signal pulses106in the optical cavity120, the controller199may receive the external electrical reference frequency, and the controller199may control the variable optical delay arrangement192so as to minimise a difference between the detected repetition rate and the external electrical reference frequency.

Rather than using an f-2f technique to stabilise the carrier envelope offset of the pulsed signal light106or the pulsed idler light108, a 0-f technique or scheme may be used such as a monolithic carrier envelope phase detection technique using self-phase modulation and difference frequency generation.

Although the pump laser10,110is described as being located outside the optical cavity20,120of the OPO system2,102, one of ordinary skill in the art will understand that in some embodiments, the optical cavity of the pump laser may at least partially overlap with the optical cavity of the OPO system, wherein the optical fibre104is located in both the optical cavity of the pump laser and the optical cavity of the OPO system. In other words, the OPO system may be described as an intra-cavity OPO system.

Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives to the described embodiments in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in any embodiment, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein. In particular, one of ordinary skill in the art will understand that one or more of the features of any embodiment of the present disclosure described above with reference to the drawings may produce effects or provide advantages when used in isolation from one or more of the other features of the same embodiment of the present disclosure and that different combinations of the features are possible other than the specific combinations of the features of the embodiments of the present disclosure described above.

The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘along’, ‘side’, etc. are made with reference to in the accompanying drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an object when in an orientation as shown in the accompanying drawings.

Use of the term “comprising” when used in relation to a feature of an embodiment of the present disclosure does not exclude other features or steps. Use of the term “a” or “an” when used in relation to a feature of an embodiment of the present disclosure does not exclude the possibility that the embodiment may include a plurality of such features.

The use of reference signs in the claims should not be construed as limiting the scope of the claims.