Patent ID: 12210266

DETAILED DESCRIPTION

The same reference numbers will used for corresponding features in different embodiments.

Referring toFIG.1, an embodiment provides an optical source100comprising a supercontinuum light generation apparatus and a wideband quarter-wave retarder106. The supercontinuum light generation apparatus comprising a pump light source102and a twisted photonic crystal fibre, PCF,104.

The pump light source is arranged to provide circularly polarised pump light. The twisted PCF is arranged to receive the circularly polarised pump light from the pump light source and to convert it into circularly polarised supercontinuum light.

The wideband quarter-wave retarder is arranged to convert the circularly polarised supercontinuum light into linearly polarised supercontinuum light and has an operating bandwidth that covers at least a portion of the supercontinuum spectrum, such as a wavelength range from 400 nm to 1550 nm. The wideband quarter-wave retarder is configured to introduce a 90° phase shift in one of the two perpendicular polarization components of the received circularly polarized light, thereby bringing the two polarization components into phase, in a chosen linear polarisation state.

Whereas the polarisation filter of the standard practice implementation for generating linearly polarized supercontinuum light discards light having the unwanted linear polarisation state, the wideband quarter-wave retarder advantageously converts the component of the circularly polarised supercontinuum light having the unwanted linear polarisation state into the wanted linear polarization state. The full optical power of the generated supercontinuum light may therefore be preserved.

Referring toFIG.2, an embodiment provides an optical source200comprising a supercontinuum light generation apparatus and a wideband quarter-wave retarder202. The supercontinuum light generation apparatus comprising a pump light source102and a twisted photonic crystal fibre, PCF,104.

The wideband quarter-wave retarder is a Fresnel rhomb202optical prism. The Fresnel rhomb advantageously has an operating bandwidth that encompasses a wide bandwidth within the spectrum of the circularly polarized supercontinuum light, so that a wide band of wavelengths within the supercontinuum spectrum are converted from circular to linear polarization. The operating bandwidth of the Fresnel rhomb includes the visible wavelength spectrum (about 380 nm-700 nm). For example, the Fresnel rhomb may have an operating bandwidth of 400 nm-1550 nm.

The Fresnel rhomb is arranged such that the received circularly polarized light undergoes two total internal reflections on transmission through the Fresnel rhomb. The Fresnel rhomb is configured such that each reflection introduces a 45° phase shift in one of the two perpendicular polarization components of the received circularly polarized light. Transmission through the Fresnel rhomb therefore introduces a 90° phase shift in the selected polarization component, thereby bringing the two polarization components into phase so that the output supercontinuum light is linearly polarized at 45° to the plane of incidence and reflection of the light within the Fresnel rhomb.

The pump light source may, for example, be arranged to generate circularly polarised pump light in the Ytterbium gain band, for example around 1000 nm, such as around 1064 nm.

In an embodiment, the pump light source is arranged to provide circularly polarised pump light pulses. The pump light source may be configured to generate short optical pulses. For example, the pump light source may be configured to generate optical pulses having a full width at half maximum, FWHM, duration of less than 100 ns, less than 10 ns, less than 1 ns, less than 100 ps, less than 10 ps, less than 1 ps, less than 500 fs or less than 100 fs.

The pump light source may include a seed source and one or more optical amplifiers. The seed source may comprise an oscillator, such as a gain switched diode or a modelocked fibre laser, configured to generate seed pulses. The seed pulses output from the seed source are amplified by the one or more optical amplifiers, which may include an amplifier chain including a pre-amplifier and one or more further amplifiers. Suitable pump light sources for providing pump light having power, wavelength, pulse duration, and repetition rate characteristics suitable for supercontinuum generation, including generation of supercontinuum light spanning from below 450 nm to greater than 2000 nm, generation of supercontinuum light in which the spectral bandwidth of the pump light pulses broadens within a PCF to generate pulses having a bandwidth from 400 nm to 2.5 μm, etc., in PCFs are known per se, as described, for example in EP2081074B1.

The pump light source may be configured to generate a pulse train of pump light pulses, the pulse train may be generated with a repetition rate of greater than 10 KHz, greater than 100 KHz, greater than 1 MHz, greater than 10 MHz, greater than 20 MHz, greater than 50 MHz, or greater than 100 MHz.

The pump light source may be configured to produce pump light pulses having an average power of greater than 1 mW, greater than 10 mW, greater than 100 mW, greater than 1 W, greater than 2 W, greater than 5 W, greater than 10 W, greater than 20 W, or greater than 50 W.

In an embodiment, the twisted PCF104is a solid core twisted PCF.

The twisted PCF104includes a core region and a cladding region. The core region may be substantially circular and may for example have a core diameter of less than 10 μm, less than 5 μm, less than 4 μm, less than 3 μm, or less than 2 μm. The cladding region includes longitudinally extending features, such as air holes, disposed about the cladding region. The features have an index of refraction that is different than that of the material surrounding the features. The features facilitate light guidance via one or more mechanisms, such as, for example, index guidance or via the creation of a photonic bandgap.

As will be understood the person skilled in the art, the longitudinally extending features in the cladding region may give rise to an “effective refractive index” of the cladding region, defined for the fundamental space filling mode. The difference between the refractive index of the core region and the effective refractive index of the cladding region may be less than 10−4, or less than 10−5. The solid areas of a PCF may be formed of doped or undoped silica.

The twist rate of the twisted PCF104, which describes the helicoidal round trip of the longitudinal holes about the PCF core, may be greater than 1 rad/cm, greater than 10 rad/cm or greater than 100 rad/cm.

In an embodiment, shown inFIG.3, the optical source300further comprises an optical filter, in the form of a wavelength tunable bandpass optical filter302. The wavelength tunable bandpass optical filter302is arranged to transmit the linearly polarised supercontinuum light at wavelengths within a selected range. The pass band of the filter302is tunable across a wide bandwidth within the supercontinuum spectrum. The optical source300is therefore configurable to output linearly polarized light at selected bands of wavelengths within its operating bandwidth within the supercontinuum spectrum.

In an embodiment, the optical filter comprises a short-pass filter. The short-pass filter may, for example, be configured only to transmit light at wavelengths within the visible part of the supercontinuum spectrum, thereby avoiding all non-visible light.

In an embodiment, the optical filter comprises a long-pass filter. The long-pass filter may, for example, be configured only to transmit light at wavelengths outside the visible part of the supercontinuum spectrum, thereby avoiding all visible light.

In an embodiment, the optical filter comprises a notch filter arranged to remove light at one or more specific wavelengths within the supercontinuum.

In an embodiment, shown inFIG.4, the optical source400comprises an optical filter in the form of an acousto-optic tunable filter, AOTF402.

The AOTF is arranged to transmit the linearly polarised supercontinuum light at wavelengths within a selected range within the bandwidth of the supercontinuum light. The AOTF is tunable, so the selected range that is transmitted can be tuned across the bandwidth of the supercontinuum light, meaning that the optical source400is configurable to output linearly polarized light at a selected band of wavelengths within the supercontinuum bandwidth.

A single AOTF402may be used since the supercontinuum light output from the wideband quarter-wave retarder is linearly polarised in a single polarisation.

Corresponding embodiments and advantages apply equally to corresponding features of the supercontinuum light generation apparatus described below.

Referring toFIG.5, illustrating the components500downstream of the pump light source of an embodiment comprising a twisted photonic crystal fibre, PCF,104and a wideband quarter-wave retarder106.

The twisted PCF is arranged to receive the circularly polarised pump light502and to convert it into circularly polarised supercontinuum light.

The wideband quarter-wave retarder is arranged to convert the circularly polarised supercontinuum light into linearly polarised supercontinuum light. The wideband quarter-wave retarder is configured to introduce a 90° phase shift in one of the two perpendicular polarization components of the received circularly polarized light, thereby bringing the two polarization components into phase, in a chosen linear polarisation state.

Referring toFIG.6, illustrating the components600downstream of the pump light source of an embodiment in which the wideband quarter-wave retarder is a Fresnel rhomb202optical prism. The Fresnel rhomb advantageously has an operating bandwidth that encompasses the full spectrum of the circularly polarized supercontinuum light, so that all wavelengths within the supercontinuum are converted from circular to linear polarization.

The Fresnel rhomb is arranged such that the received circularly polarized light undergoes two total internal reflections on transmission through the Fresnel rhomb. The Fresnel rhomb is configured such that each reflection introduces a 45° phase shift in one of the two perpendicular polarization components of the received circularly polarized light. Transmission through the Fresnel rhomb therefore introduces a 90° phase shift in the selected polarization component, thereby bringing the two polarization components into phase so that the output supercontinuum light is linearly polarized at 45° to the plane of incidence and reflection of the light within the Fresnel rhomb.

In an embodiment, shown inFIG.7, the components700of the optical source additionally comprises a wavelength tunable bandpass optical filter302. The wavelength tunable bandpass optical filter302is arranged to transmit the linearly polarised supercontinuum light at wavelengths within a selected range. The pass band of the filter302is tunable across the bandwidth of the supercontinuum. The optical source300is therefore configurable to output linearly polarized light at any selected band of wavelengths within the supercontinuum bandwidth.

Referring toFIG.8, illustrating the components800downstream of the pump light source in an embodiment in which the wavelength tunable bandpass optical filter is an AOTF402. Pulses502from the pump light source are received by the twisted photonic crystal fibre, PCF,104to generate the circular polarized supercontinuum light which subsequently is converted to linear polarized supercontinuum light by the Fresnel rhomb202. The AOTF is arranged to transmit the linearly polarised supercontinuum light at wavelengths within a selected range. The AOTF is configured to transmit a narrow bandwidth within the bandwidth of the supercontinuum light. The AOTF is tunable, so the narrow bandwidth that is transmitted can be tuned across the bandwidth of the supercontinuum light, meaning that the optical source is configurable to output linearly polarized light at any selected band of wavelengths within the supercontinuum bandwidth.

Items

1. A Supercontinuum light generation apparatus comprising:a pump light source arranged to provide circularly polarised pump light;a twisted photonic crystal fibre arranged to receive circularly polarised pump light and to convert the pump light into circularly polarised supercontinuum light; anda wideband quarter-wave retarder arranged to convert the circularly polarised supercontinuum light into linearly polarised supercontinuum light.2. Supercontinuum light generation apparatus according to item 1, wherein the wideband quarter-wave retarder is a Fresnel rhomb (202).3. Supercontinuum light generation apparatus according to item 1 or item 2, wherein the twisted PCF is a solid core twisted PCF.4. Supercontinuum light generation apparatus according to item 3, further comprising a tunable bandpass optical filter arranged to transmit the linearly polarised supercontinuum light at wavelengths within a selected range.5. Supercontinuum light generation apparatus according to item 4, wherein the tunable bandpass optical filter is an acousto-optic tunable filter.