Apparatus and method for generating a photon

Abstract

An apparatus for outputting a photon, the apparatus comprising an EM source arrangement for outputting phase modulated EM radiation; the phase modulated EM radiation comprising a set of one or more properties; the arrangement comprising at least one electromagnetic, EM, source for generating EM radiation at a plurality of different wavelengths; a modulator for receiving the EM radiation; phase modulating the received EM radiation; and outputting phase modulated EM radiation; an optical fibre for receiving the phase modulated EM radiation; converting, using a third order nonlinear susceptibility of the optical fibre, at least one photon of the phase modulated EM radiation into a first new photon; and a second new photon and outputting at least the first new photon, wherein the set of one or more properties comprises a wavelength of the phase modulated EM radiation and provides for the phase modulated EM radiation to propagate along the optical fibre with at least a positive value of a second order dispersion parameter; and a negative value of a fourth order dispersion parameter.

Description

[0001] Apparatus and method for generating a photon

[0002] Field

[0003] The field of the present invention is in generating photons using nonlinear processes, in particular, but not limited to, fibre optic parametric amplifiers (FOPAs), fibre optical wavelength converters and fibre-optic parametric oscillators (FOPOs).

[0004] Background

[0005] Generating photons may be accomplished in different ways. One way is to use nonlinear material processes. Often these nonlinear processes are called nonlinear optical processes and can include effects such as second harmonic generation and four wave mixing. Such effects may be utilised in different systems such as fibre-optic parametric amplifiers wherein the gain medium, an optical fibre, is provided input ‘pump’ light at one or two wavelengths and further light of a further wavelength that may carry a data signal or a seed continuous wave to be amplified or wavelength converted. The seed wave can be generated by a FOPA itself as well without need for an external source. Typical FOPAs work by inputting the pump light into the gain fibre with an intensity sufficient to induce amplification or wavelength conversion by means of the four-wave mixing (FWM) nonlinear effect. The FWM nonlinear effect converts two photons of pump light into two new photons, preserving the conservation of energy within the process. The two new photons are commonly called the signal and idler photons.

[0006] Optical fibres support guided transverse modes. A mode of wavelength X has an associated free space wavenumber k0= 2π / λ, and a propagation constant 0 = nEffk0, wherein nEffis the effective index of the transverse mode propagating down the waveguide (in this case the waveguide is an optical fibre). Atypical requirement for parametric amplification is phase matching between the pump(s) and the newly generated photons resulting from the nonlinear processes. Phase matching may be defined by the propagation constant mismatch A0, which in turn, depends on the fibre dispersion parameters such as β2at the pump frequency, as discussed below. Typically, propagation constant mismatch A0 varies depending upon the angular frequency detuning Δωs. between the newly generated photons and the pump. As an example, when Δωs= 0, the pump angular frequency and newly generated angular frequency are the same, which typically means that that the propagation constant mismatch A0 = 0.

[0007] Dispersion of modes in optical fibres describes the effect of different spectral portions of EM modes travelling at different speeds. An EM pulse comprises a spread of wavelengths, typically centred around a central wavelength. For typical EM pulses, the shorter the pulse, the greater the spread of wavelengths (hence greater the pulse spectral bandwidth). Two types of dispersion combine together to provide a total dispersion D. The two types of dispersion making up the total dispersion D is material dispersion DMand waveguide dispersion Dw, such that D= DM+ Dw.

[0008] In more detail, material dispersion describes the wavelength dependence of the refractive indices of materials used in the waveguide to guide the EM mode. Thus, for waveguide core and cladding materials with continuous solid cross-sectional extent, the core and cladding materials are used to determine the material dispersion. When using FWM, we are assuming that the wavelengths of the pump EM radiation, any other input EM radiation and any newly generated EM radiation from a nonlinear process are far from atomic resonances and may be well approximated by the Sellmeier equation.

[0009] Waveguide dispersion describes the wavelength dependence of transverse optical modes irrespective of any material related refractive index changes with wavelength. Wavelength dispersion arises because the wavelength of the mode compared to the cross sectional waveguide dimensions determines the transverse modal amplitude profile. A shorter wavelength typically allows for the fundamental mode to be more confined within the waveguide core than a fundamental mode of a longer wavelength.

[0010] In the context of parametric amplification, dispersion describes the effect where waves (photons) with different wavelengths propagate with different velocities. Dispersion plays important role for FWM processes, because efficient FWM requires that interacting waves propagate with about the same velocity (this is called phase matching). Due to symmetry of parametric amplification in the frequency domain only even-numbered derivatives of the propagation constant with respect to angular frequencyω define the phase matching. Therefore, the second order dispersion parameter β2= d2β / dω2, also referred to as the Group Velocity Dispersion (GVD) parameter, plays an important role. The GVD parameter is typically used to determine the spread, hence ‘dispersion’ of an optical pulse propagating along a single mode fibre. As such, the dispersion parameter D (with units of ps / (km-nm)) may also be expressed as per equation 1 wherein vgis the group velocity and c is the speed of light in vacuum:

[0011] [Equ. 1 ]

[0012] When the GVD dispersion parameter β2is a negative value, equation 1 dictates that the total dispersion D is positive, which gives rise to a condition commonly referred to as Anomalous GVD. Conversely, when the GVD dispersion parameter β2is a positive value, equation 1 dictates that the total dispersion D is negative, which gives rise to a condition commonly referred to as Normal GVD. The value of GVD, for a particular waveguide typically changes with wavelength. Therefore, the GVD that pump light experiences may be at least partially dictated by the choice of wavelength of the pump light and the physical characteristics of the waveguide (for example the materials and / or cross sectional structural profile of an optical fibre).

[0013] Existing optical amplifiers, such as FOPAs, using nonlinear optical processes operate in the Anomalous GVD condition. Often, applications such as, but not limited to optical communications, require pump light to be continuous wave (CW) ratherthan pulsed.

[0014] For FOPAs with anomalous GVD, the phase matching condition is commonly only satisfied for Δωsbetween 0 and - where y is the fiber nonlinearity coefficient and P is the total pump power. This range is also called gain bandwidth. Therefore, the gain bandwidth increases as the absolute value of negative β2gets smaller. Thus, the lower the magnitude of GVD in the anomalous dispersion regime, the greater the wavelength spectrum where newly generated photons can be efficiently generated. However, in practice it is not possible to get β2very close to zero due to unavoidable fibre imperfections. Therefore, the gain bandwidth demonstrated for existing FOPAs with a CW pump has recently been limited to ~270nm. Applications such as, but not limited to optical communication systems often required wider wavelength amplification spectra.

[0015] Stimulated Brillouin scattering (SBS) is another nonlinear process occurring in optical fibres. SBS can be detrimental for operation of amplifiers relying on FWM, such as FOPAs, and SBS has to be mitigated in such cases. The paper ‘Impact of pump phase modulation on QAM signals in polarization-insensitive fiber optical parametric amplifiers’ by M. Bastamova et al., Optical fiber technology volume 84 , May 2024, describes the principles of pump phase modulation for SBS mitigation in FOPAs.

[0016] Other known references in the field of nonlinear optical processes include:

[0017] 1 . M. E. Marhic, K. K. . -Y. Wong and L. G. Kazovsky, "Wide-band tuning of the gain spectra of one-pump fiber optical parametric amplifiers," in IEEE Journal of Selected Topics in Quantum Electronics, vol. 10, no. 5, pp. 1133-1141 , Sept. -Oct. 2004,

[0018] 2. Yue Zhou, Kim K. Y. Cheung, Sigang Yang, P. C. Chui, and Kenneth K. Y. Wong, "Widely tunable picosecond optical parametric oscillator using highly nonlinear fiber," Opt. Lett. 34, 989-991 (2009), 5. Rohit Malik, Michel E. Marhic, “High-power continuous-wave operation of a fiber optical parametric oscillator in L and U bands,” Optical Fiber Technology, Volume 20, Issue 6, 2014,

[0019] 6. Y. Q. Xu, S. G. Murdoch, R. Leonhardt, and J. D. Harvey, "Raman-assisted continuous- wave tunable all-fiber optical parametric oscillator," J. Opt. Soc. Am. B 26, 1351-1356 (2009),

[0020] 7. M. Jamshidifar, A. Vedadi and M. E. Marhic, "Continuous-wave one-pump fiber optical parametric amplifier with 270 nm gain bandwidth," 200935th European Conference on Optical Communication, Vienna, Austria, 2009,

[0021] 8. Y. Q. Xu, K. F. Mak, and S. G. Murdoch, "Multiwatt level output powers from a tunable fiber optical parametric oscillator," Opt. Lett. 36, 1966-1968 (2011 ),

[0022] 9. Y. Q. Xu, S. G. Murdoch, R. Leonhardt, and J. D. Harvey, "Widely tunable photonic crystal fiber Fabry-Perot optical parametric oscillator," Opt. Lett. 33, 1351-1353 (2008),

[0023] 12. Marhic ME. Fiber Optical Parametric Amplifiers, Oscillators and Related Devices. Cambridge University Press; 2007.

[0024] Summary

[0025] According to a first aspect there is presented: an apparatus for outputting a photon; the apparatus comprising:

[0026] I) an EM source arrangement for outputting phase modulated EM radiation; the phase modulated EM radiation comprising a set of one or more properties; the arrangement comprising:

[0027] A) at least one electromagnetic, EM, source for generating EM radiation at a plurality of different wavelengths;

[0028] B) a modulator for: i) receivingthe EM radiation; ii) phase modulatingthe received EM radiation; iii) outputting phase modulated EM radiation; II) an optical fibre for:

[0029] C) receiving the phase modulated EM radiation;

[0030] D) converting, using a third order nonlinear susceptibility of the optical fibre, at least one photon of the phase modulated EM radiation into: iv) a first new photon; and, v) a second new photon;

[0031] E) outputting at least the first new photon; wherein the set of one or more properties: a) comprises a wavelength of the phase modulated EM radiation; b) provides for the phase modulated EM radiation to propagate along the optical fibre with at least: a positive value of a second order dispersion parameter; and, a negative value of a fourth order dispersion parameter.

[0032] According to a second aspect there is presented: a method for outputting a photon comprising: generating EM radiation; phase modulating the EM radiation to form phase modulated EM radiation comprising a set of one or more properties; inputtingthe phase modulated EM radiation to an optical fibre such that the phase modulated EM radiation propagates alongthe optical fibre; converting, using a third order nonlinear susceptibility of the optical fibre, at least one photon of the phase modulated EM radiation into a first new photon and a second new photon; outputting at least the first new photon, wherein the set of one or more properties: a) comprises a wavelength of the phase modulated EM radiation; b) provides for the phase modulated EM radiation to propagate alongthe optical fibre with at least: a positive value of a second order dispersion parameter; and a negative value of a fourth order dispersion parameter. The first and second aspects may be adapted accordingto any teaching herein including, but not limited to, any of the following options.

[0033] The EM source arrangement may comprise further components such as but not limited to an optical amplifier, a polarisation controller, an optical isolator, one or more optical fibres and / or integrated optic waveguides for guiding EM radiation between components of the arrangement and guiding EM radiation from the arrangement to other portions of the apparatus.

[0034] EM radiation propagating between any of the components of the apparatus may be carried by one or more waveguides such as but not limited to an optical fibre or integrated optic waveguides.

[0035] Optionally, the phase modulated EM radiation propagates alongthe optical fibre under normal dispersion.

[0036] Optionally, the apparatus may be configured to: input, into the optical fibre, a first phase modulated EM radiation comprising a first centre input wavelength wherein the first input wavelength propagates along the optical fibre with a first value of group velocity dispersion; input, into the optical fibre, a second phase modulated EM radiation comprising a second centre input wavelength that is different to the first centre input wavelength; wherein the second centre input wavelength propagates along the optical fibre with a second value of group velocity dispersion that is greater than the first value of group velocity dispersion; wherein: the respective new first photon generated for the first centre input wavelength comprises a wavelength from a first new wavelength range; and the respective new first photon generated for the second centre input wavelength comprises a wavelength from a second new wavelength range that is different to the first new wavelength range.

[0037] Optionally, the first new wavelength range is wider than the second new wavelength range.

[0038] Optionally, the first centre wavelength is not within the first new wavelength range.

[0039] Optionally, the second centre wavelength is not within the second new wavelength range.

[0040] Optionally, the first centre wavelength is within 2 nm of the gain fibre zero dispersion wavelength. Optionally, the second centre wavelength is up to 30 nm from the gain fibre zero dispersion wavelength on the normal dispersion side.

[0041] Optionally, the plurality of wavelengths of the one or more EM sources defines an input wavelength range of EM radiation; the first centre wavelength being proximal to, or at, one end of the input wavelength range; the second centre wavelength being proximal to, or at, the other end of the input wavelength range.

[0042] Optionally, the at least one EM source comprises a wavelength tuneable EM source for outputting the EM radiation at a plurality of different wavelengths.

[0043] Optionally, the plurality of different wavelengths are between 600 and 2000nm. This wavelength range may be applicable for systems using optical fibres.

[0044] Optionally, the plurality of different wavelengths are between 1300nm and1900nm. In using this band, the apparatus may be applicable for use in systems using standard silica optical fibres.

[0045] Optionally, the plurality of different wavelengths are between 1530-1620nm. In this range the apparatus may operate in the EDFA band of wavelengths.

[0046] Optionally, the EM source arrangement comprising a plurality of EM sources wherein at least:

[0047] I) a first EM source of the plurality of EM sources outputs EM radiation at a first wavelength;

[0048] II) a second EM source of the plurality of EM sources outputs EM radiation at a second wavelength; the first wavelength being different to the second wavelength.

[0049] Optionally the EM radiation output from the source arrangement is continuous wave EM radiation.

[0050] Optionally, the apparatus outputs continuous wave EM radiation comprising the first new photon.

[0051] Optionally, phase modulatingthe received EM radiation comprises using one or more of: pseudo random bit sequence (PRBS), white noise and a plurality of sine tones.

[0052] Optionally the EM source comprises a tuneable laser.

[0053] Optionally, the EM source comprises a plurality EM sources wherein each of the EM sources in the said plurality is configured to output a different wavelength of EM radiation to the other EM sources of the said plurality.

[0054] Optionally the optical fibre comprises a dispersion shifted optical fibre.

[0055] Optionally the optical fibre comprises a length between 10 and1000m. Optionally, the optical fibre converts the phase modulated EM radiation using four wave mixing.

[0056] Optionally, the optical fibre is strained.

[0057] Optionally, the optical fibre comprises Aluminium dopants.

[0058] Optionally, the optical fibre comprises temperature gradient.

[0059] Optionally, the optical fibre comprises at least a first longitudinal section and a second longitudinal section, wherein the first and second longitudinal sections are joined with an optical isolator.

[0060] Optionally, the EM radiation from the EM source propagates from the first section towards the second section.

[0061] Optionally, the optical isolator is configured to prevent EM radiation propagating from the second section into the first section.

[0062] Optionally, the optical fibre is split in several sections with optical isolators, preferably one optical isolator joining each section of the optical fibre.

[0063] Optionally, the number of sections is between one and ten sections.

[0064] Optionally, the phase modulator comprises an electro-optic phase modulator.

[0065] Optionally, the electro-optic phase modulator comprises a Lithium niobate electro- optical phase modulator.

[0066] Optionally, phase modulating the received EM radiation comprises using a plurality of sine tones.

[0067] Optionally, the plurality of sine tones are arranged with a non-integer multiplication factor between each sine tone of the plurality of sine tones and an adjacent sine tone of the plurality of sine tones.

[0068] Optionally, the non-integer multiplication factor is close to, but not equal to, 3.

[0069] Optionally, the phase modulator is configured to receive an electrical signal comprising the plurality of sine tones.

[0070] Optionally, the phase modulator is configured to receive an electrical signal comprising the plurality of sine tones.

[0071] Optionally, the electrical signal is a composite electrical signal formed from the plurality of sine tones.

[0072] Optionally, the plurality of sine tones comprises at least a first frequency and a second frequency that is different to the first frequency. Optionally, the plurality of sine tones comprises between, and including three to six sine tones.

[0073] Optionally, the plurality of sine tones comprises four sine tones.

[0074] Optionally, each of the four sine tones comprises a different frequency to any of the other sine tones.

[0075] Optionally, each of sine tones has a different amplitude to the other sine tones.

[0076] Optionally, the apparatus further comprises one or more RF amplifiers for amplify the sine tones. (These can be individual for each sine tone or amplifying all at once)

[0077] Optionally, the said one or more RF amplifiers are configured to receive an electrical signal comprising the sine tones and output an amplified RF signal for inputting to the phase modulator.

[0078] Optionally, the apparatus comprises a plurality of RF amplifiers, wherein at least one of the plurality of RF amplifiers is for amplifying a different sine tone to at least a different one of the plurality of RF amplifiers.

[0079] Optionally, each of the plurality of RF amplifiers is for amplifying a different sine tone to any of the other RF amplifiers.

[0080] Optionally, the method comprises: i) receiving, at the phase modulator, EM radiation comprising a first wavelength; ii) phase modulating the EM radiation of the first wavelength to generate one or more further wavelength of EM radiation that are different to the first wavelength. iii) outputting the one or more further wavelengths.

[0081] Optionally, the one or more further wavelengths comprises a plurality of further wavelengths wherein each of the further wavelengths is different to any of the other further wavelengths.

[0082] Optionally, the plurality of further wavelengths and the first wavelength form a set of wavelengths wherein each of the wavelengths in the set corresponds to a different frequency in a comb of frequencies.

[0083] Optionally, the comb is non-equi-spaced.

[0084] Optionally, the comb of frequencies is equi-spaced.

[0085] Optionally, the plurality of further wavelengths comprises: at least one wavelength greater than the first wavelength; and, at least one wavelength less than the first wavelength.

[0086] Optionally, the comb of frequencies comprises two first order frequencies wherein: one of the first order frequencies is adjacent to, in the comb, and greater than the frequency corresponding to the first wavelength; and the other one of the first order frequencies is adjacent to, in the comb, and less than the frequency corresponding to the first wavelength.

[0087] Optionally, the frequency spacing between adjacent frequencies in the comb is between 5 andl OO MHz.

[0088] Optionally, the first wavelength and the one or more further wavelengths distribute the energy of the EM radiation received by the EM modulator between the first wavelength and the one or more further wavelengths.

[0089] Optionally, the arrangement comprises an EM amplifier for receivingthe phase modulated EM radiation and outputting amplified phase modulated EM radiation for inputtingto the optical fibre.

[0090] Optionally, the amplified phase modulated EM radiation comprises an EM amplitude, the set of properties comprising the EM amplitude.

[0091] Optionally, the EM amplifier comprises an Erbium Doped Fibre Amplifier (EDFA).

[0092] Optionally, the EM amplifier comprises at least one of: a Bismuth doped fibre amplifier, a Ytterbium doped fibre amplifier, a Raman fibre amplifier, a FOPA.

[0093] Optionally, the EM radiation output from the EM source comprises first polarisation; and the apparatus comprises a polarisation controller for:

[0094] A) receiving the EM radiation output from the at least one EM source and,

[0095] B) outputting the EM radiation with a second polarisation different to the first polarisation.

[0096] Optionally, the polarisation controller is configured to align the polarisation of the phase modulated EM and the first new photon.

[0097] Optionally, the EM radiation from the EM source arrangement, that is input into the optical fibre, comprises a plurality of input photons; and the optical fibre is configured to convert the plurality of input photons into:

[0098] I) a plurality of the first photons

[0099] I) a plurality of the second photons. Optionally, the plurality of first photons comprises a first plurality of different wavelengths.

[0100] Optionally, the plurality of wavelengths output from the EM source arrangement comprises a first source wavelength and a second source wavelength that is different to the first source wavelength; and the optical fibre is configured to output:

[0101] A) a first plurality of the first photons using EM radiation of the first source wavelength;

[0102] B) a second plurality of the first photons using EM radiation of the second source wavelength; wherein a first wavelength range of the first plurality of the first photons is different to a second wavelength range of the second plurality of the first photons.

[0103] Optionally, the EM source may be controlled to output a subset of the plurality of wavelengths.

[0104] Optionally, the EM source may be controlled to output one of the plurality of wavelengths.

[0105] Optionally, the apparatus comprises a EM coupler for: a) receiving, along a first path, the EM radiation output from the EM source arrangement b) receiving, along a second path, further EM radiation; c) outputting, along a third path and towards the optical fibre, both the EM radiation output from the EM source arrangement and the further EM radiation.

[0106] Optionally, the first path is an optical path along at least a first optical fibre.

[0107] Optionally, the second path is a second optical path along at least a second optical fibre.

[0108] Optionally, the third path is a third optical path along at least a third optical fibre.

[0109] Optionally, the further EM radiation co propagates, with the EM radiation from the EM source arrangement, alongthe third optical fibre.

[0110] Optionally, the apparatus is configured to input both the EM radiation output from the EM source arrangement and the further EM radiation into the optical fibre.

[0111] Optionally, the further EM radiation comprises a further wavelength, the first photon comprising the further wavelength. Optionally, the apparatus is configured to amplify the further EM radiation.

[0112] Optionally, the further EM radiation comprises a data signal.

[0113] Optionally, the further EM radiation comprises a continuous wave EM radiation.

[0114] Optionally, the further EM radiation is phase-modulated. Phase modulatingthe further EM radiation may provide reduction of SBS.

[0115] Optionally, the apparatus comprises a further modulator configured to phase modulate the further EM radiation.

[0116] Optionally, the apparatus comprises: a second EM source; and a second modulator for: receiving EM radiation from the second EM source; phase modulatingthe EM radiation received from the second EM source; and outputting phase modulated EM radiation; the optical fibre comprises a first longitudinal section and a second longitudinal section; and the apparatus is configured such that: the first longitudinal section of the optical fibre receives phase modulated EM radiation from the modulator; and the second longitudinal section of the optical fibre receives phase modulated EM radiation from the second modulator, wherein the phase modulated EM radiation from the modulator is in counter phase with the phase modulated EM radiation from the second modulator.

[0117] Optionally, there is further provided a fibre optical parametric amplifier, FOPA, comprising the apparatus of the first aspect. The FOPA may include, but is not limited to any one or more of the options presented for the first aspect.

[0118] Optionally, there is further presented a fibre optical parametric oscillator, FOPO, comprising the apparatus of the first aspect. The FOPO may include, but is not limited to any one or more of the options presented for the first aspect.

[0119] Optionally, the FOPO may further comprise a cavity comprising the optical fibre.

[0120] Optionally, the cavity is a circulating loop cavity. Optionally, the cavity comprises an optical fibre-loop cavity.

[0121] Optionally, the cavity comprises a filter arrangement for: a) inputtingthe phase modulated EM radiation into the optical fibre; b) removing the phase modulated EM radiation from the cavity after being output from the fibre and before the phase modulated EM radiation can re-enter the opticalfibre.

[0122] Optionally the filter arrangement comprises:

[0123] I) a wavelength selective reflector for reflecting the phase modulated EM radiation and for transmitting the first new photon and broadband noise co-propagated with the phase modulated EM radiation;

[0124] II) a first circulator for: a. receiving the phase modulated EM radiation with co-propagating broadband noise and inputtingthe received phase modulated EM radiation into the cavity towards the wavelength selective reflector; b. receiving the phase modulated EM radiation reflected from the wavelength selective reflector and directing the received phase modulated EM radiation towards the optical fibre;

[0125] III) a second circulator for: a. receiving the first new photon and the unconverted phase modulated EM radiation output from the optical fibre; b. transmitting the first new photon and the unconverted phase modulated EM radiation towards the wavelength selective reflector; c. receiving the unconverted phase modulated EM radiation reflected from the wavelength selective reflector; d. outputting the unconverted phase modulated EM radiation reflected from the wavelength selective reflector, from the cavity. e. receiving the broadband noise from the wavelength selective reflector and outputting it from the cavity

[0126] Optionally, the wavelength selective reflector comprises a Bragg grating.

[0127] Optionally the Bragg grating is located between the first and second circulator.

[0128] Optionally, the Bragg grating is a fibre Bragg grating.

[0129] Optionally, the reflective wavelength of the wavelength selective reflector is tuneable.

[0130] Optionally, the reflective wavelength of the wavelength selective reflector is adjusted to match the pump wavelength.

[0131] Optionally, the cavity comprises an EM splitter for: I) receiving a set of EM radiation output from the optical fibre, the set of EM radiation comprising: a. unconverted phase modulated EM radiation; b. the first new photon;

[0132] II) coupling a first portion of the set of EM radiation, output from the fibre, out of the cavity;

[0133] III) transmitting a second portion of the set of EM radiation to propagate within the cavity.

[0134] Optionally, the first portion is less than the second portion.

[0135] Optionally, the first portion is larger than the second portion.

[0136] Optionally, the apparatus is configured such that propagation of the second portion of the set of EM radiation within the cavity comprises a series of circulations around the cavity and the modulator is configured to phase modulate the received EM radiation such that distortion of the second portion of the set of EM radiation incurred during a first circulation around the cavity is cancelled out by distortion of the second portion of the set of EM radiation incurred during a second subsequent circulation around the cavity.

[0137] Optionally, the length of the cavity is selected such that distortion of the second portion of the set of EM radiation incurred during a first circulation around the cavity is cancelled out by distortion of the second portion of the set of EM radiation incurred duringthe second subsequent circulation around the cavity.

[0138] Optionally, the modulator is configured to phase modulate the received EM radiation using a phase modulation waveform, the phase modulation waveform having a period which is selected such that distortion of the second portion of the set of EM radiation incurred during a first circulation around the cavity is cancelled out by distortion of the second portion of the set of EM radiation incurred duringthe second subsequent circulation around the cavity.

[0139] Optionally, the apparatus comprises: a second optical fibre, the optical fibre and the second optical fibre being configured to receive the phase modulated EM radiation from the modulator; and the modulator is configured to phase modulate the received EM radiation using a phase modulation waveform having a period.

[0140] Optionally, the modulator is configured to phase modulate the received EM radiation using a phase modulation waveform having a period which is selected so as to match the timing at which the optical fibre and the second optical fibre receive the phase modulated EM radiation from the modulator such that the phase modulated EM radiation received at the optical fibre is out of phase with the phase modulated EM radiation received at the second optical fibre by half of the period of the phase modulation waveform. Optionally, the apparatus comprises a delay line configured to control the timing at which the optical fibre and the second optical fibre receive the phase modulated EM radiation from the modulator and the delay line is configured to control the timing at which the optical fibre and the second optical fibre receive the phase modulated EM radiation such that the phase modulated EM radiation received at the optical fibre is out of phase with the phase modulated EM radiation received at the second optical fibre by half of the period of the phase modulation waveform.

[0141] Optionally, the length of the optical fibre and the length of the second optical fibre are selected such that distortion of the phase modulated EM radiation incurred in the optical fibre is equal in magnitude to distortion of the phase modulated EM radiation incurred in the second optical fibre.

[0142] Brief description of the figures

[0143] Figure 1 shows a schematic example of the present apparatus.

[0144] Figure 2 shows the relationship between pump wavelength and group velocity dispersion (GVD) for three different optical fibres;

[0145] Figure 3 shows the relationship between pump frequency detuning Acosand propagation constant mismatch Δβ under four different dispersion conditions;

[0146] Figure 4 shows an example of pump phase modulation using a single sine tone at frequency f0;

[0147] Figure 5 shows an example of pump phase modulation usingfour sine tones;

[0148] Figure 6 shows a different example of pump phase modulation usingfour sine tones;

[0149] Figure 7 shows an example of the apparatus wherein the apparatus forms part of a fibre optics parametric amplifier (FOPA);

[0150] Figure 8 shows two output power spectra generated using a FOPA resulting from two different sets of pump and probe wavelengths and a graph illustrating gain and conversion efficiency;

[0151] Figure 9 shows four ASE optical power spectra generated using four different pump wavelengths;

[0152] Figure 10 shows an example of the apparatus wherein the apparatus forms part of a fibre optics parametric oscillator (FOPO).

[0153] Figure 11 shows another example of the apparatus wherein the apparatus forms part of a fibre optics parametric oscillator (FOPO).

[0154] Figure 12 shows a further example of the apparatus wherein the apparatus forms part of a fibre optics parametric oscillator (FOPO).

[0155] Detailed description There is presented an apparatus for outputting a photon, the apparatus comprising an EM source arrangement for outputting phase modulated EM radiation; the phase modulated EM radiation comprising a set of one or more properties; the arrangement comprising at least one electromagnetic, EM, source for generating EM radiation at a plurality of different wavelengths; a modulator for receiving the EM radiation; phase modulatingthe received EM radiation; and outputting phase modulated EM radiation; an optical fibre for receiving the phase modulated EM radiation; converting, using a third order nonlinear susceptibility of the optical fibre, at least one photon of the phase modulated EM radiation into a first new photon; and a second new photon and outputting at least the first new photon, wherein the set of one or more properties comprises a wavelength of the phase modulated EM radiation and provides for the phase modulated EM radiation to propagate along the optical fibre with at least a positive value of a second order dispersion parameter; and a negative value of a fourth order dispersion parameter.

[0156] Figure 1 shows a schematic example of the present apparatus. The apparatus 100 for outputting a photon comprises an arrangement 101 for outputting phase modulated EM radiation. The apparatus 101 comprises at least one electromagnetic, EM, source 102 configured to generate EM radiation at a plurality of different wavelengths and a modulator 103 configured to receive the EM radiation; phase modulate the received EM radiation and output phase modulated EM radiation. The apparatus further comprises an optical fibre 104 configured to receive the phase modulated EM radiation; convert, using a third order nonlinear susceptibility of the optical fibre, at least one photon of the phase modulated EM radiation into a first new photon and a second new photon; and output at least the first new photon.

[0157] The apparatus may operate using different wavelengths of EM radiation, including different wavelengths of any of, but not limited to: any EM radiation entering the optical fibre and any EM radiation exiting the optical fibre. Examples of EM radiation wavelength ranges include any of: light; infrared EM radiation, C band, L band, O band, S band, E band, U band and other unnamed wavelength bands. Examples herein may refer to the EM radiation as Tight’, however it to be understood that other non-lightwavelength EM radiation may be used. The EM radiation output from the EM source(s) may be referred to as ‘pump EM radiation’, ‘pump radiation’; ‘pump light’ or ‘pump’.

[0158] The apparatus may be used as, or form at least part of, any of, but not limited to: a Fibre optical parametric amplifier (FOPA), a Fibre Optical Parametric Oscillator (FOPO), an EM source, an optical wavelength converter. Other uses of the apparatus are also envisaged.

[0159] The gain fibre is preferably a single (transverse) mode optical fibre. The gain fibre is preferably an optical fibre comprising at least one physical parameter that makes the optical fibre susceptible to nonlinear optical effects, for example in the presence of high-power pump light. Pump light, at the input to the gain fibre typically comprises a power of 30-43 dBm.

[0160] The said physical parameters of the optical fibre may be parameters of the material medium or media forming the optical fibre core and / or parameters of the material medium or media forming the optical fibre cladding peripherally surroundingthe core and / or cross sectional structural parameters of the core and / or cladding. An example of a gain fibre is a single mode optical fibre comprising dopants in the core section.

[0161] In some alterative example implementations, the optical fibre may be replaced by another type of waveguide, including but not limited to an integrated optic waveguide such as a buried channel waveguide, a rib waveguide or a ridge waveguide. Preferably the waveguide may be a photon crystal waveguide. Preferably the waveguide is a highly nonlinear waveguide comprising a substantial third order nonlinear response.

[0162] The apparatus presented herein may be configured to use a spontaneous or stimulated nonlinear parametric process for the nonlinear process to generate the first new photon. Preferably, the spontaneous nonlinear parametric process is four wave mixing (FWM) in the gain fibre. FWM uses a third-order (x(3)) nonlinearity and may be described as an elastic scattering process. The wavelengths of the pump are non-resonant with atomic energy level transitions of the gain fibre. In more detail, the wavelength of the pump, signal and idler photons are non-resonant with atomic energy level transitions of a material of the optical fibre, preferably non- resonant with atomic energy level transition of the waveguide core and cladding material of the optical fibre, hence the core and cladding materials defining the effective refractive of the EM mode.

[0163] The success of new photon generation using a nonUnear optical effect typically depends upon the at least one of, preferably multiple of: the length of the gain fibre; the above-mentioned physical parameters of the optical fibre; one or more parameters of the pump light; one or more parameters of any further EM radiation entering the gain fibre. The parameters of the EM radiation may be any of: power; polarisation: wavelength; modulation characteristics. Il is lo be understood that different versions of the apparatus may be implemented to successfully give rise to the desired nonlinear effect by choosing combinations of parameters of the input EM radiation and the parameters of the gain fibre.

[0164] In more detail, four wave mixing may annihilate two photons of the pump EM radiation to form a new signal photon and a new idler photon. The apparatus may use multiple different pump lightwavelengths in a non-degenerate FWM process wherein a photon from both different pump wavelengths are annihilated. Alternatively, and preferably, the apparatus may use a single pump lightwavelength output from an EM source in a degenerate FWM process wherein two identical wavelength photons are annihilated. As discussed elsewhere herein, phase modulation of the pump light produces instantaneous frequency (hence wavelength) changes to the pump light that may affect phase matching condition and therefore generation of new photons at the target frequency. Where multiple pump light sources are used, the pump light preferably co propagates with each other.

[0165] The apparatus may be used to generate a new single signa l / id ler photon pair (first and second new photon), however the apparatus preferably generates a plurality of signal / idler photon pairs from the corresponding annihilation of pump photon pairs. Where a plurality of signal / idler photon pairs are generated, the newly generated photons may comprise respective wavelengths determined at least from the phase matching condition, and optionally the presence of any further EM radiation input into the gain fibre that co-propagates with respect to the pump light.

[0166] The wavelengths of the first and second new photons depend upon a phase matching condition, in some example implementations of the apparatus, the only EM radiation input into the gain fibre during the process of generating the first new photon is the pump light. In this scenario, the first new photons may be generated with a range of wavelengths that satisfy the phase matching condition. Thus, the apparatus may be used as an EM source that receives pump light as an input and outputs newly generated photons with a range of wavelengths broader than the input wavelength range of the pump light.

[0167] In some other example implementations of the apparatus, the pump light is input into the gain fibre during the process of generating the first new photon, together with further EM radiation. The further EM radiation may be modulated to represent (hence carry) data, such as but not limited to telecommunication data. The further EM radiation may act as a ‘seed’ EM radiation in the gain fibre in that, if the seed wavelength is one of the phase matched wavelengths, the apparatus may preferably amplify the first EM radiation by generating first new photons at the seed wavelength. Typically, these photons are coherent and in phase with the original seed photon. In the same example, the apparatus may generate a plurality of photons at the new (idler) frequency, and these photons are coherent but in counter-phase with the original seed photon.

[0168] As described above, modes in optical fibre encounter forms of dispersion. When describing dispersion of EM modes in optical fibres it is common to relate dispersion to the second order dispersion parameter β2, as described above, however other higher order dispersion parameters may be considered. The present apparatus takes into account higher order dispersion parameters to allow for the apparatus to generate photons, using nonlinear processes, with a wider wavelength spectrum than existing systems.

[0169] In more detail, the third order dispersion parameter is provided as: p3= dβ2 / do> = d3p / do3. A typical unit of the third order dispersion coefficient is ps3 / km. The fourth order dispersion parameter is provided below as: 04= dp3 / do = d4p / do4. A typical unit of the fourth order dispersion coefficient is ps4 / km.

[0170] The apparatus is set up such that the pump light propagates with positive values of the second order dispersion parameter and negative values of fourth order dispersion parameter. In doing this, the apparatus allows for the generation of phase matched newly generated photons from the nonlinear process over a wide range of wavelengths than existing systems. In particular, by having a positive value of the second order dispersion parameter and, a negative value of a fourth order dispersion parameter, the inventors have determined that the pump light wavelengths that incur lower values of GVD can generate phase matched new photons with a wide range of non-zero value of Δωswhilst pump light wavelengths that incur higher values of GVD (β2) can also generate phase matched new photons with a wide range of non-zero value of Aw5. Furthermore, under this regime, the range of Aiusbecomes smaller the larger the values of GVD (β2) but the centre of the range of the Δωsmoves to larger values. Thus, by changing the wavelength of the pump, hence changing the GVD, the apparatus may controllably generate new photons over a wide range of wavelengths. In other words, different pump light wavelengths having different values for GVD in the optical fibre give rise to different wavelength ranges of the newly generated signal photons.

[0171] The EM radiation exiting the gain fibre comprises the first and second new photons and optionally unconverted pump light. The output EM radiation may be used in different ways or may be further operated on. For example, the output EM radiation may be input into a filter such as a wavelength selective filter, that outputs a certain range of wavelengths. Such a filter may be used to remove unconverted pump light and direct the first and / or second new photons along a desired optical path, for example along an optical fibre and / or towards and optical detector and / or towards other devices as exemplified herein.

[0172] As discussed above, inducing a nonlinear effect in a gain fibre typically requires high pump power. This in turn may give rise to Stimulated Brillouin Scattering (SBS). SBS is a phenomenon where longitudinal compression waves are established in the optical fibre that, in turn, act as temporary gratings that reflect EM radiation at wavelengths resonant with the grating. These back-reflected waves can be undesirable and limit the wavelength range of newly generated FWM photons. SBS can be detrimental for operation of amplifiers relying on FWM, such as FOPAs, and so SBS may be mitigated in such cases.

[0173] The present apparatus reduces the effects of SBS by using pump phase modulation, also known as phase dithering or pump dithering. Pump phase modulation allows to increase pump bandwidth beyond Brillouin interaction bandwidth for reduction of the Brillouin gain obtained by the input pump such as a laser, as explained in more detail below. The apparatus may phase modulate using different methods including using sine tones or using white noise.

[0174] By applying this method, the SBS is effectively suppressed without causing the pump amplitude modulation (except for a small residual pump amplitude modulation).

[0175] In more detail, pump dithering effectively mitigates SBS by broadening the pump beyond the Brillouin gain bandwidth. As discussed above, one example way of phase modulating is with sine tones. As the power of a pump phase modulated with sine tones remains constant, its power may be uniformly distributed between equally or not equally spaced discrete lines in the frequency domain. By using pump phase modulation with sinusoidal tones, the apparatus can provide control over the pump bandwidth and uniformity of the spectrum. An example of providing sine tones is applying a voltage of 0.456*Vpi (which corresponds to phase modulation amplitude of 1 .434rad) for each individual tone, however other voltages may be used.

[0176] Pump phase modulation with an amplitude of ~1 .434 rad per tone splits the pump into 3 equal power lines of zero and first orders spaced with the modulation frequency. Each additional tone splits every line produced by previous tones in three equal lines as well, i.e. producing beat frequencies. Therefore, pump phase modulation with N sine tones in the electrical frequency spectrum splits the pump into 3Nfirst order lines in optical spectrum. The high order harmonics are much weaker, so their effects can be ignored. If the frequency of the nthtone is three times the frequency of the (n-1 )thtone, then all first order lines are equally spaced with the frequency of the lowest tone, which may be termed the base tone. However, sine tones with a non-integer multiplication factor (e.g. 3.05) between them prevent first order lines from overlapping with higher order lines, resulting in flatter pump spectrum and, hence, a more efficient SBS mitigation. If the base tone frequency is largerthan the Brillouin gain bandwidth, then the pump power is split between 3Ntones which do not interact via Brillouin, and consequently the SBS threshold increases by a factor of 3N.

[0177] For the present apparatus it is preferred that the base tone frequency is between 5 MHz and 100 MHz however other frequency ranges may be used. Three tones allow the SBS threshold to be increased by a factor of up to 27, and four tones allow the SBS threshold to be increased by a factor of up to 81 . A higher number of tones can be used as well.

[0178] Pump phase modulated with N sinusoidal tones is shown by equation (2), where Am is the phase modulation amplitude, and the fmnis the modulation frequency of the nthtone. Modulation amplitude Am is generally the same for all tones and set at ~ 1 .434 rad which is the cross point of the Bessel functions J0and J1leading to equal power of the carrier and the side lobes. We define the total pump bandwidth as the bandwidth occupied by the first order lines. It is equal to twice the sum of all modulation frequencies fmn.

[0179] [Equ. 2]

[0180] The pump phase modulation can be viewed as an instantaneous pump frequency modulation f(t) upon the pump light as shown by equation 3, where fpis the central pump frequency and the portion of the equation after fPis the first derivative of the pump phase d 4>P(t) / dt divided over 2n to convert from angular to ordinary frequency.

[0181] [Equ. 3]

[0182] The inventors have further determined that pump dithering induces an instantaneous pump frequency modulation and causes gain fluctuations in FOPA degrading amplified signals. The inventors have further determined that the pump frequency modulation brought about by the phase modulation may in some circumstances have a detrimental effect on the gain spectrum when the pump light is propagating in the gain fibre under normal GVD.

[0183] To address this, the inventors have further determined that an increase of number of sine tones with reduction of spacing between them allows to greatly reduce the induced pump frequency fluctuations while improving the SBS mitigation. Further details about this further optimisation of the apparatus are described with respect to the examples below.

[0184] There is now provided more detailed discussion of the apparatus.

[0185] The present application relates to techniques for outputting EM radiation which involves generating EM radiation at an electromagnetic (EM) source configured to generate radiation at a plurality of different wavelengths, and inputting the generated EM radiation (known as a pump) into an optical (gain) fibre such as a highly nonlinear fibre (HNLF). A HNLF may be defined as a fibre having a fibre nonlinearity coefficient y in the range 5 to 30 W’1krrr1.

[0186] The present application relates to apparatus utilizing these techniques to generate output EM radiation within a desired wavelength range. The apparatus described may be used as part of a fibre optical parametric amplifier (FOPA), a fibre optical parametric oscillator (FOPO), an amplified spontaneous emission (ASE) source or other applications. As explained above, it will be apparent to the skilled person that due to the plurality of different wavelengths input to the gain fibre, dispersion occurs in the gain fibre as photons of different wavelengths travel through the gain fibre at different speeds. In other words, the refractive index of the gain fibre is perceived differently by photos of different wavelengths. It will be appreciated that, depending on the properties of the gain fibre through which the EM radiation propagates, and the EM radiation itself, the pump will experience either normal dispersion or anomalous dispersion at it propagates through the gain fibre. EM radiation experiences normal dispersion if its propagation speed (the group velocity) increases with increasing wavelength. In other words, a red photon (of larger wavelength) will propagate faster than a blue photo (of smaller wavelength). EM radiation will experience anomalous dispersion if its propagation speed decreases with increasing wavelength. In other words, a red photon (of larger wavelength) will propagate slower than a blue photo (of smaller wavelength).

[0187] As will be explained in more detail below, the present apparatus may employ certain properties of the HNLF and the input pump such that EM radiation of the pump experiences normal dispersion as it propagates through the gain fibre.

[0188] Properties of the gain fibre include dispersion parameters which include the fibre propagation constant p, group velocity dispersion (GVD) β2, which is the second derivative of the fibre propagation constant, and the higher order dispersion p4, which is the second derivative of the GVD. The group velocity dispersion β2is a characteristic of a dispersive medium and reflects how the medium affects the propagation of EM radiation travelling through it. p4depends on the fibre design and varies slowly with wavelength / frequency. Note, the apparatus operation is defined by the dispersion properties at its central wavelength (the pump wavelength in the single pump case) only for symmetry reasons.

[0189] Properties of the input pump may include the wavelength of the pump, referred to herein as the pump wavelength Ap, the power of the pump Poand whether the pump is a continuous or pulsed wave. Known amplifiers commonly utilise a pulsed pump and pulsed probe such that that produced output signal is also pulsed. As previously mentioned, some optical applications require pump light to be continuous wave rather than pulsed. The present apparatus therefore utilises a continuous wave pump.

[0190] The group velocity dispersion (GVD) (β2) strongly depends on the pump wavelength as shown by graph 200 in Figure 2. A positive β2(P2>0) indicates normal dispersion and a negative β2(P2<0) indicates anomalous dispersion.

[0191] Figure 2 includes three curves which show how the group velocity dispersion β2varies with pump wavelength Apfor three different fibres. Each fibre has a different value of nonlinearity coefficient y . It will be appreciated that p4,the second derivative of β2, describes how β2concaves in the frequency domain (figure 2 is shown in the wavelength domain).

[0192] Curve 201 represents how the group velocity dispersion β2varies with pump wavelength Apfor a standard optical fibre. A standard optical fibre may have a nonlinearity coefficient y of approximately 1 W’1km-1. Curve 202 represents the relationship between group velocity dispersion β2and pump wavelength Apfor an optical fibre with a microstructure. A microstructure fibre such as photonic crystal fibre may have a nonlinearity coefficient y greater than 30 W’1km-1and may be up to several orders of magnitude higher than this value. Curve 203 represents the relationship between group velocity dispersion β2and pump wavelength Apfor a dispersion shifted fibre, which allows to facilitate the required dispersion at the desired pump wavelength.

[0193] The gain fibre may be a silica fibre (for example, standard single mod fibre, dispersion shifted fibre, highly nonlinear fibre, etc.), a photonic crystal fibre or a silicon core fibre. The wavelength at which the β2curve crosses 0 is known as the zero dispersion wavelength. Figure 2 shows that for curve 203, β2crosses 0 at a higher pump wavelength value Apthan the other fibres 201 , 202. In other words, the zero dispersion of the HNLF is greater than the zero dispersion wavelength of the standard optical fibre and the microstructure fibre. It will be appreciated that by tuningthe pump wavelength Apit is possible to get either normal (P2>0) or anomalous (P2<0) dispersion and it is possible to vary the magnitude of the group velocity dispersion β2within the range of interest: from near-zero to substantial positive values. The near-zero values of β2allow for signal amplification close to the pump and a larger β2allows for signal amplification further from the pump thus facilitating tuneability across a broad range.

[0194] It is desirable to use a combination of a gain fibre and the pump wavelength Apsuch that the value of group velocity dispersion β2crosses zero (transitions between normal and anomalous dispersion) at or close to the pump wavelength. The present apparatus may utilise the highly nonlinear fibre 203 and the corresponding pump wavelengths in the approximate range of 1 .53-1 .54pm to achieve this condition.

[0195] In the case of the fibre optical parametric amplifier (FOPA) configured to amplify a signal, the apparatus includes a second electromagnetic source configured to generate a signal (known as a probe) which is input to the gain fibre along with the pump. An example of a FOPA is seen in figure 7 and will be described in more detail below. The second EM source is configured to generate EM radiation at a plurality of different wavelengths. The pump and the gain fibre are used to amplify the probe from the second EM source. Assuming the probe generated by the second EM source includes photons of a frequency fs, in order to amplify the probe, the apparatus is configured to increase the number of photons of frequency fswhich is emitted from the gain fibre. In other words, the EM radiation output from the gain fibre includes a higher number of photons of frequency fsthan the EM radiation input to the gain fibre. The apparatus may be configured to utilise degenerate four wave mixing in the gain fibre. In other words, two of the photons of the pump of frequency fpare converted to two new photons including at least one photon of wavelength fs. The FOPA operation also inherently performs wavelength conversion, i.e. generates photons at the frequency fi = 2fp- fs, which are called idler. In other words, whereas only photons with frequencies fsand fpwere inputted in the gain fibre, the photons with frequency fi will be generated in the gain fibre.

[0196] The signal photons that are generated (at frequency fs) may be utilised when the apparatus is part of a FOPA. In other words, in this case, the generated idler photons (at frequency fi) may be filtered out. Alternatively, when the apparatus is used as part of a wavelength converter, the idler photons are desirable (as having a different frequency / wavelength to the input signal). Thus, in this case, the generated signal photons may be filtered out.

[0197] The ratio between the signal power at the output of the gain fibre and the signal power at the input of the gain fibre is called gain. The ratio between the idler power at the output of the gain fibre and the signal power at the input of the gain fibre is called the conversion efficiency.

[0198] Atypical requirement for efficient parametric amplification is phase matching between the pump and the amplified signal (i.e. such that the amplified signal is not cancelled out by being 180 degrees phase offset from the input pump). The phase matching is defined by the propagation constant mismatch Ap, which depends on the gain fibre dispersion parameters (β2and p4) at the pump frequency Apas qualitatively demonstrated by Figure 3.

[0199] Figure 3 shows the propagation constant mismatch Ap in four possible scenarios (graphs 3a, 3b, 3c, 3d) as a function of Acos-the frequency detuning (difference) between the signal and the pump (\fs— fp|) (i.e. under four different dispersion conditions). The frequency detuning A(JOSis represented using arbitrary units. The propagation constant mismatch Ap is represented as a function of the power of the pump (pump power) Poand the fibre nonlinearity coefficient y.

[0200] When Ac0s=0, this corresponds to a scenario in which the amplified signal has the same frequency as the pump (fs— fp= 0) i.e. the input probe to be amplified has the same frequency as the pump.

[0201] These scenarios correspond to possible combinations of β2and p4being positive or negative. The bold part of each curve in Figure 3 shows the region where the phase matching condition is satisfied, and therefore parametric amplification is possible. The range of the frequency detuning Acoswhich corresponds to the bold part of each curve (where phase matching occurs) therefore corresponds to the gain bandwidth of the amplifier (the width of the frequency range in which significant gain is available from the amplifier). In other words, the gain bandwidth is the range of signal wavelengths within which it is possible to acquire the required gain for a particular pump wavelength. The tuneability range is the total range of signal wavelengths within which it is possible to acquire the required gain by adjusting the pump wavelength. In other words, the tuneability range comprises all possible gain bands for all possible pump wavelengths.

[0202] The Ap curves depend on absolute values of β2and p4rather than just their signs. The examples seen in graphs 3a to 3d use arbitrary values for β2. Each graph 3a to 3d shows two curves. Curves 301 , 303, 305 and 307 correspond to a “large” β2value and curves 302, 304, 306 and 308 correspond to a “small” β2value.

[0203] Graph 3a shows a scenario in which the group velocity dispersion β2is negative (β2<0) at the pump frequency (Acos=O) such that the pump experiences anomalous dispersion. In this example, the second derivative of the group velocity dispersion p4is also negative (p4<0) at Ac0s=0. Graph 3a shows that the Acosrange where phase matching is possible is between 0 and 1 .6 in the “large” β2case (line 301 ) and between 0 to 2.2 in the “small” β2case (line 302). This means that the gain bandwidth (the frequency range where phase matching is satisfied) increases as β2gets smaller. In other words, the pump light with a larger value of the GVD parameter β2creates new first photons in a smaller wavelength range.

[0204] It is noted that the phase matched range of Acosscales with the pump power Po(or fibre nonlinearity y), so the tuneability range and the gain bandwidth can be improved by increasingthe power of the pump Po.

[0205] In practice it may be difficult to get β2very close to 0 due to fibre imperfections. Therefore, the broadest gain bandwidth demonstrated in this case with a continuous wave pump may be approximately 270nm.

[0206] Graph 3b shows an example of conditions which may be used by the present apparatus. In this scenario, group velocity dispersion β2is positive at the pump frequency (Acos=O) such that the pump experiences normal dispersion. In this example, the second derivative of the group velocity dispersion p4is also negative (p4<0) at Ac0s=0.

[0207] In this scenario, the Acosrange where phase matching is possible is between 3.4 and 3.7 in the “large” β2case (line 303) and between 1 .8 to 2.8 in the “small” β2case (line 304). The range of frequency detuning Acoswhich corresponds to the bold part of the curve therefore decreases as the value of β2increases. Graph 3b thus illustrates that in this case the gain bandwidth is narrower than that shown in graph 3a. However, the gain bandwidth occurs at a larger value for the detuning Acos from the pump frequency. In other words, the same pump frequency may be used to achieve phase matching and thereby parametric amplification for a wide range of input probes. Thus, the pump frequency can be tuned to adjust β2and obtain phase matching (and hence amplification) across a very broad range. However, graph 3b shows that as β2increases, the gain bandwidth gets narrower. Dependence of the phase-matched range position on the value of β2also increases. In otherwords, as the gain frequency fsis tuned away from the pump fp, any existing fluctuations of β2become more significant so the impact of the β2fluctuations along the fibre becomes critical as Pancreases. In otherwords, the pump bandwidth is linked with the instantaneous pump frequency instability. As previously mentioned, the group velocity dispersion β2is a characteristic of a dispersive medium and therefore can vary along a fibre with imperfections in the medium. The fluctuations of β2may also arise from pump frequency fluctuations, because β2depends on the pump frequency fp, as discussed and seen in figure 2.

[0208] Thus, pump frequency fluctuations cause fluctuations of β2and therefore fluctuations of the detuning Acos between the pump and the amplified wave. It is therefore desirable to maintain a stable pump frequency.

[0209] Consequently, as frequency of the amplified wave fais tuned away from the pump frequency fp(i.e. where Acosgets larger), at some point these fluctuations of β2cause the frequency fato fluctuate beyond the gain bandwidth, and the gain peak collapses. This corresponds to the maximum value within the tuneability range. In otherwords, stability of the dispersion parameters along the fibre and stability of the pump frequency define the maximum tuneability range.

[0210] Equation 4 explains why the pump bandwidth should be minimised to maximise the gain tuneability range. It shows how the propagation constant mismatch depends on the signal detuning from the pump A<DSand dispersion parameters / ?2and / ?4. The instantaneous A<DSdirectly depends on the pump frequency, and / ?2and / ?4are the functions of the pump frequency. Therefore, pump frequency fluctuations amend the propagation constant mismatch and might disrupt the phase matching condition. This impact significantly increases with the signal detuningfrom the pump A<DS. Therefore, the maximum signal detuning where operation is possible depends largely on the pump frequency stability, i.e. its bandwidth.

[0211] The fibre should preferably have a group velocity dispersion β2which is as stable as possible within the pump wavelength tuning range. The main uncertainty about fibre suitability originates from requirements for p4. £4should preferably be stable as well and may be between 10-56and -10’54s4rrr1depending on other parameters (e.g. dispersion slope). The fibre can be designed / selected from existing fibres to have a particular dispersion profile β2which corresponds to the desired value of p4. As explained, the values of β2and p4can be adjusted by tuning the pump wavelength, but it is the fibre design which dictates whether the fibre has the desired values for β2and p4at the same pump wavelength. The specific design of the fibre may be adapted accordingly by selecting the appropriate distribution of dopants across the fibre cross section. In addition, it should be possible to modify the fibre β2and p4slightly by applying stress, changing fibre temperature, etc. The values of the dispersion parameters β2and p4may depend on the polarisation of EM radiation input to the fibre, known as birefringence. Therefore, the value of the gain fibre birefringence (quantified as the polarisation mode dispersion) may be as low as possible. Similarly with the conditions seen in graph 3a, the phase matched range of Acosscales with the pump power Po(or fibre nonlinearity y), so the tuneability range and the gain bandwidth can be improved by increasingthe pump power.

[0212] Graph 3c shows an example of a scenario in which the group velocity dispersion β2is negative (β2<0) at the pump frequency (Acos=O) such that the pump experiences anomalous dispersion. In this example, the second derivative of the group velocity dispersion p4is positive (p4>0) at Acos=O.

[0213] As seen in graph 3c, curves 305, 306 each have longer bold parts than the previously described examples i.e. the phase matching conditions are met across a very broad gain bandwidth. Graph 3d shows an example of a scenario in which the group velocity dispersion β2is positive (β2>0) at the pump frequency (Acos=O) such that the pump experiences normal dispersion. In this example, the second derivative of the group velocity dispersion p4is positive (p4>0) at Acos=O.

[0214] Graph 3d shows that curves 307, 308 do not include bold parts at all, illustrating that the phase matching conditions are not met. There is therefore no gain achieved for these conditions. Therefore, to achieve phase matching and therefore parametric amplification of EM radiation propagating alongthe HNLF under normal dispersion, the present apparatus utilizes a negative value of the second derivative of the group velocity dispersion (p4<0), an example of which is seen in graph 3b.

[0215] As explained, inducing a nonlinear effect in a nonlinear optical fibre typically requires high pump power. When the pump power exceeds a certain threshold (known as an SBS threshold), Stimulated Brillouin Scattering (SBS) may occur. The present apparatus looks to reduce the effects of SBS, for example by using pump phase modulation, also known as phase dithering or pump dithering. Generally, a target for SBS mitigation may be that the power scattered by the SBS is less than 1 % of the input pump power, while keeping the pump bandwidth as narrow as possible. As explained above, it can be advantageous to keep the pump bandwidth as narrow as possible as the pump bandwidth is linked with the instantaneous pump frequency instability and so achieving gain at frequencies with large detuning from the pump requires a stable pump frequency. Therefore, less pump bandwidth facilitates broader gain tuneability range.

[0216] It will be appreciated that FOPA operation is possible even with pump power being larger than the SBS threshold (i.e. the scattered power is >1 % of the input power), but this may cause a significant increase of the amplified wave noise.

[0217] As previously discussed, pump phase modulation is used to increase the SBS threshold (so that a higher pump power can be used without inducing large amounts of SBS). According to one example of the present apparatus, the apparatus is configured to use pump phase modulation to increase the SBS threshold by approximately 18dB, for example by up to 20dB, so that a high pump power can be used. As explained with respect to figure 3, the propagation constant mismatch is a function of the power of the pump (pump power) Posuch that increasingthe pump power Pocan facilitate a broad tuneability range. A pump phase modulation has been found to be an efficient way to mitigate the SBS. The pump phase modulation broadens the pump spectrum beyond the Brillouin gain bandwidth (which may be about 20-50MHz) and as such only a fraction of the total pump power contributes to the SBS at each particular frequency. This can be expressed by the Equation 4 below. In equation 4, Pthis the SBS threshold gBis the peak Brillouin gain, Lett is the effective fibre length, Aeffis the fibre effective area, K is the pump polarization factor between 1 (for polarized pump) and 2 (for depolarized pump), AvBis the Brillouin bandwidth and AvPis the pump bandwidth. Equation 5 therefore illustrates that an increase to the pump bandwidth AvPresults in an increase to the SBS threshold Pth.

[0218] However, pump phase modulation can incur a small pump frequency modulation / instability. Pump frequency modulation has been found to affect the gain spectrum shape in the anomalous dispersion regime (e.g. the case seen in graph 3a of figure 3), but it seems to have even more critical impact in case of broadband operation in the normal dispersion regime employed by this invention (e.g. the case seen in graph 3b of figure 3). It has been found that FOPA demonstrations involving the SBS mitigation by approximately 18 dB or greater has employed pump phase modulation. In some examples, SBS mitigation by pump phase modulation has incurred a pump frequency instability of greater than 40 GHz. Generally, it may be desirable that the pump frequency instability is less than a few GHz. In view of the above, it will be appreciated that for each signal frequency (fs) and value of pump power (Po), there is an optimal pump bandwidth (AvP). When the pump bandwidth is greater than the optimal pump bandwidth, the gain may be reduced and noise due to pump frequency instability may be increased. When the pump bandwidth is less than the optimal value, the gain may be reduced and the amplified wave noise due to SBS of the pump wave may be increased. The present apparatus has been developed so as to minimise the induced pump frequency modulation whilst delivering sufficient SBS mitigation.

[0219] It has been found that techniques for minimising pump frequency modulation can include modulating the pump phase with a pseudo random bit sequence (PRBS), white noise, a combination of sine tones and / or pump chirping.

[0220] Preferably, the noise spectrum is as flat as possible and for the best performance its bandwidth is as low as is sufficient for the SBS mitigation. According to an example in which the pump phase is modulated using a pseudo random bit sequence (PRBS), the PRBS may be smoothed so as to have a rise time of about half of a bit period for the best performance. This would minimise significant fluctuations of the pump frequency. According to one example, the PRBS frequency would be approximately 4GHz.

[0221] Depending on the required SBS mitigation it could be between 2 and 5GHz.

[0222] Accordingto an example in which the pump phase is modulated with white noise, a bandwidth between 1 and 5 GHz might be used.

[0223] The following examples phase modulate the pump using a combination of sine tones.

[0224] Figure 4 shows an example of pump phase modulation using a single sine tone at frequency f0. That is, the phase of a continuous wave pump is modulated at a frequency fo. In the example shown, f0is equal to 100 MHz.

[0225] Graph 400 of figure 4 shows that the optical power spectrum of the pump changes from a single line into three equal power lines 401 , 402, 403 spaced by f0and a number of low power higher order lines beside the three equal power lines.

[0226] When the signal is modulated with N tones simultaneously, the nthtone frequency is f0*3(n’1), where f0is the base tone frequency.

[0227] Generally, it may be possible to achieve sufficient SBS mitigation with 3 sine tones and base tone frequency f0s50 MHz, 4 tones and base tone frequency f0si 5 MHz or 5 tones and more reducing the base tone frequency f0by a factor of ~3 per tone added. The frequencies of the phase modulating sine tones may be optimised for each gain fibre depending on many parameters: fibre length, fibre Brillouin gain, fibre Brillouin gain bandwidth, pump power, employment of other SBS mitigation techniques, etc. Modulating the pump with multiple sine tones may involve forming a composite electrical signal from the sine tones and applying the electrical signal to the pump EM radiation. For example, the composite electrical signal may be used to drive the electrodes of a phase modulator used to impart the phase modulation on incoming EM radiation as it passes through a waveguide of the modulator. Each sine tone may be a sinusoidal electrical signal. The composite electrical signal may be formed as a linear combination of each of the multiple sine tones. For example, formingthe composite electrical signal may comprise performing a linear addition of each of the sine tones. The sine tones may be linearly added prior to being inputted to the phase modulator. The phase modulator may then apply the composite electrical signal to the pump EM radiation such that the pump is phase modulated using each of the multiple sine tones simultaneously. The combination of the multiple sine tones may be referred to as the phase modulation waveform.

[0228] Figure 5 illustrates an example of pump phase modulation usingfour sine tones (N=4) with frequencies of 100*3(n-1)MHz (f0= 100MHz). This type of phase modulation leads to 3n(i.e.81 ) first-order lines 501 spaced by f0and about equal power. These are seen in figure 5 to have spectral power of ~-20dBW spaced with f0and about equal power. The lines are not shown to be equal in power in figure 5 due to overlap between the first order lines and higher order lines

[0229] It has been found that SBS mitigation can be achieved using a non-integer factor for frequency calculation, for example: fo*3.O5(n’1)as it prevents first order lines from overlapping with higher order lines, resulting in flatter pump spectrum and, hence, a more efficient SBS mitigation. If the base tone frequency is larger than the Brillouin gain bandwidth, then the pump power is split between 3Nlines which do not interact via Brillouin scattering, and consequently the SBS threshold increases by a factor of 3N. Figure 6 shows an example of phase modulation usingfour tones (N=4) with frequencies of 100*3.05(n-1). As seen in figure 6, the first order lines 601 are of equal power.

[0230] It has also been found that reducingthe spacing between sine tones can reduce the induced pump frequency fluctuations while improvingthe SBS mitigation. For example, by using a spacing between lines (f0) which is below the Brillouin gain bandwidth (approximately 20-50 MHz depending on the fibre).

[0231] The amplitudes and phases of the sine tones used for the phase modulation may be manipulated so as to alter the shape of the resulting output spectrum, e.g. to make it more or less flat.

[0232] Accordingto one example of the present apparatus, pump phase modulation is employed with four sine tones with f0of 45 MHz. The frequencies of the four tones are determined using 45*3.05(n-1). i.e. the frequency of the four tones are 45 MHz, 137 MHz, 419 MHz, and 1277 MHz.

[0233] Preferably, each sine tone should induce phase modulation with a phase modulation amplitude of approximately 1 .4 rad in optical domain. As explained above, the optimal PM amplitude is fundamentally defined by the smallest root of the equation J0(x) = Ji (x) (the first intersection of the Bessel functions). Consequently, the sine tone amplitudes in electrical domain may be preferably 0.456*Vpi V, where Vpi is the voltage required to change phase by n. However, Vpi of optical phase modulators generally depends on the modulation frequency, so each sine tone (of a different frequency) might require a slightly different amplitude to achieve the required phase modulation amplitude of 1 .4 rad at each frequency.

[0234] These conditions correspond to a pump frequency instability of around 5 GHz while deliveringthe required SBS mitigation by 18 dB. Accordingto this example, the frequency f0is adjusted to be minimal whilst keeping the power scattered by the SBS below 1 % of the input power. On one hand, increasing f0may lead to a reduction of the power scattered by the SBS, i.e. improved the SBS mitigation. On the other hand, increasingfo can lead to reduction of gain at remote frequencies. For example, each increase of f0by 5 MHz may reduce the gain at around 1300nm by a few dB.

[0235] Accordingto another example of pump phase modulation using a combination of sine tones, five sine tones (N=5) are employed with f0of 10-15 MHz. These conditions may allowfor more uniform distribution of the pump power and consequently more efficient SBS mitigation allowingfor a reduction in the overall phase modulation bandwidth.

[0236] A higher number of tones (e.g. five or more) with smaller base tone frequency (5-15 MHz) can be used to achieve the required SBS mitigation with less overall pump bandwidth and hence the apparatus performance improvement in terms of the tunability range, the amplified wave noise, gain, etc.

[0237] It will be appreciated that there may be an optimal number of tones and an optimal spacing between each tone for each case (each combination of signal frequency and pump power) which optimises gain and tuneability while minimising amplified wave noise.

[0238] Accordingto one example, the gain fibre may comprise at least a first longitudinal section and a second longitudinal section. Each of the first and second longitudinal sections of the optical fibre may be employed with its own pump. The two pumps may be phase modulated with the same waveforms, but in counter-phase. In this exemplary arrangement, the amplified wave first passes through the first section, receives the first portion of amplification and gets distorted by the pump phase modulation. Then, the amplified wave passes through the second gain fibre section, where it gets the second portion of amplification and distortions which are opposite to those inherited from the first gain section. Therefore, the overall distortions are cancelled out rather than stacked up. The pump phase modulation waveforms may be timed so that the same signal portion (e.g. symbol) experiences opposite distortions from the gain fibre sections.

[0239] There are many other SBS mitigation techniques, which include fibre strain distribution, nonuniform dopant concentration, temperature distribution, core radius variation, employment of optical isolators, tilted fibre Bragg gratings and Al-doped fibres. These techniques may not induce the pump frequency instability but have other drawbacks such as increased insertion loss or gain fibre dispersion instability. One or more SBS mitigation techniques may be combined together and / or with pump phase modulation to achieve SBS mitigation with substantially reduced pump frequency instability and improved gain tuning range of the FOPA.

[0240] It might also be possible to reduce the requirement for SBS mitigation by using a shorter fibre length. A shorter gain fibre may result in less requirement for the SBS mitigation (hence, more stable pump frequency) and more stable fibre dispersion parameters, however, also results in less gain through the fibre. These techniques are utilised by an apparatus used as part of a fibre optics parametric amplifier (FOPA), a fibre optics parametric oscillator (FOPO), a wavelength converter, an amplified spontaneous emission (ASE) source or other applications.

[0241] It will be appreciated that in a FOPA, the signal to be amplified may also experience stimulated Brillouin scatting (SBS). Further phase modulation may therefore be applied to the input signal to minimise SBS (in addition to the phase modulation of the input pump wave). In other words, techniques described herein for phase modulation of the pump signal may be applied also to the probe signal before injection into the gain fibre. The apparatus may thus include a further modulator configured to phase modulate EM radiation input into the apparatus. However, in cases where pump phase modulation is employed in combination with other SBS mitigation techniques (as described above), it may not be necessary to employ phase modulation of the input probe signal.

[0242] Figure 7 shows an example of the apparatus wherein the apparatus forms part of a fibre optics parametric amplifier (FOPA) 700.

[0243] Figure 7 shows that the FOPA 700 contains components 701 to 714, adjacent components being joined by a thin black line, such as line 715. Each thin black line in figure 7 represents an optical fibre.

[0244] The FOPA 800 includes a first EM source 701 and a modulator 702. In this example, the EM source 701 , also known as a pump is sourced from a continuous wave tuneable laser. The tuneable laser is configured to output EM radiation in the wavelength range 1548 to 1563 nm, referred to herein as a pump. In this example, the pump from the first EM source 701 has optical power of 10 dBm. As explained above, the pump wavelength Apis adjusted such that the normal dispersion regime is achieved.

[0245] In this example, a single EM source is used. However it will be appreciated that in other examples, the EM source may comprise a plurality EM sources wherein each of the EM sources in the said plurality is configured to output a different wavelength of EM radiation to the other EM sources of the said plurality. An example of this arrangement would be a set of Distributed FeedBack (DFB) lasers.

[0246] The modulator 702 is a phase modulator, such as an electro-optic phase modulator. In this example, the phase modulator phase-modulates the pump with a combination of four sine tones at 45 MHz, 137 MHz, 419 MHz and 1277 MHz. Phase modulation is used to mitigate stimulated Brillouin scattering (SBS). As explained above, the phase modulation frequencies and amplitudes have been thoroughly optimized to provide the sufficient SBS mitigation with the minimal pump bandwidth in this experiment. As discussed previously, other examples may use different sine tones (a different number N) and / or different frequencies to achieve this effect.

[0247] The FOPA 700 further includes a pump polarisation controller 703 and an amplifier 704. The pump polarisation controller 703 is used to control polarization of the pump. This example uses single polarisation, which means that only one polarisation is amplified by the pump (the pump polarization factor is 1 ), and therefore the amplified wave must be aligned with the pump. It will be appreciated that other examples may be polarisation-insensitive to amplify dual polarisation signals or waves with arbitrary polarisations. A polarisation-insensitive operation may be achieved either by employing two orthogonally polarised pumps or a polarisation diversity architecture, whereas an amplified wave is split into two orthogonally polarised components, and each component is equally amplified by its corresponding co-polarised pump in a separate optical path. Both approaches have been successfully employed in FOPAs operating in the anomalous dispersion mode and can be employed for FOPAs operating in the normal dispersion mode as well.

[0248] The amplifier 704 is an erbium doped fire amplifier (EDFA) and in this example the pump is amplified to 41 dBm with the high power EDFA 703 (to increase the pump power Po). Depending on the length and nonlinearity of the gain fibre a suitable pump power can be in the range from 30 to 43 dBm. The amplified signal is passed into a gain fibre 705. The fibre 705 therefore receives the phase modulated EM radiation from the first EM source 701 . In this example, the fibre 705 is a 250 m long highly non-linear fibre (HNLF). As previously described, the fibre 705 has specific dispersion parameters in order to achieve the conditions needed to generate the desired amplification. In this example, the HNLF has a nonlinearity coefficient y of approximately 8 W-1km-1and the zerodispersion wavelength is approximately 1564nm. According to other examples, the HNLF may have a nonlinearity coefficient y of between 5 and 30 W-1km-1. It will be appreciated that in other examples, an EM source may be used which has the required optical power such that the amplifier 704 is not required.

[0249] The FOPA 700 includes a second EM source 706. The EM source 706, also known as a probe (i.e. signal for amplification) is sourced from a continuous tuneable laser. According to other examples, the second EM source 706 could be an SFP transmitter. In this example, the EM source 706 is configured to output EM radiation in the wavelength range 1308 to 1465nm, referred to herein as a probe. As mentioned above, the FOPA 700 may additionally include a further modulator configured to apply phase modulation to the probe signal output from the EM source 706. The further modulator may be utilised where the probe also experiences SBS. The FOPA 700 also includes a probe polarisation controller 707 and a wavelength division multiplexer 708. The probe polarisation controller 707 is used to control polarization of the probe. The wavelength division multiplexer 708 couples the probe with the pump. According to this example, the wavelength division multiplexer is a 1310 / 1550 wavelength division multiplexer. Other wavelength division multiplexers able to combine a pump and an amplified probe can also be used. The probe is also passed into the fibre 705. The second EM source 706 in this example has an optical power of -10 dBm, but other optical powers can be used as well.

[0250] The FOPA further includes equipment to monitor the power at the fibre 705. The FOPA 700 includes a 1% optical tap coupler 709 at the input to the fibre 705. The tap coupler

[0251] 709 directs 1 % of the signal towards an input power meter 710. The input power meter

[0252] 710 is used to monitor the input pump and probe powers. Also connected to the tap coupler 709 is a backscattered power meter 711 . The backscattered power meter 711 is used to monitor the power backscattered due to SBS occurring in the fibre 705.

[0253] A further 1% optical tap coupler 712 is located at the output of the fibre 705. The tap coupler 712 directs 1 % of the output signal towards an optical spectrum analyser (OSA) 713. In this example the OSA operates across 1200 to 2400nm. Finally, the FOPA 700 includes an output power meter 714 which monitors the power being output bythe fibre 705.

[0254] As explained in detail above, broadly tuneable gain is obtained when dispersion at the pump wavelength is normal (β2>0), and the fourth derivative of the propagation constant p4is negative (p4<0).

[0255] The gain spectrum resulting from the FOPA setup 700 is narrow but can be adjusted across a very broad range by adjusting the pump wavelength. For example, pump wavelength adjustment between 1548nm and 1563nm allows to obtain gain tuneable in the range of approximately 1310 to 1900nm. Within this range, the FOPA may have the ability to provide gain / conversion efficiency of up to 53 dB covering the span of 70 THz (when employing a continuous wave pump). The apparatus in this example may be adapted using components and configurations described elsewhere herein, including but not limited to any of the following: EM sources; EM radiation; waveguides; phase modulators; phase modulation characteristics; other electrical or optical components.

[0256] In contrast to known amplifiers which use nonlinear crystals as a gain medium, the use of nonlinear optical fibres has been found to be advantageous. Generally, optical fibres are much cheaper and able to operate with higher output optical power than optical crystals. Moreover, all-fibre based systems do not require free-space optical alignment as compared to optical crystals and therefore fibre-based systems may be easier to operate, more robust to vibrations, etc.

[0257] Graphs 8a and 8b of figure 8 illustrate two examples of optical power spectra for parametric amplification using the FOPA seen in figure 7. The optical power shown may be measured usingthe optical spectrum analyser 713 and output power meter 714. As above, the zero dispersion wavelength of the fibre 705 is 1564nm.

[0258] Graph 8a shows the output power spectrum for a FOPA in which the wavelength of the pump is 1548.7nm (line 801 ). The wavelength of the input probe is 1308nm. This spectrum illustrates the amplified probe 802 and its wavelength converted copy 803. In other words, peak 802 represents photons of the amplified probe signal and peak 803 represents the idler photons (of a different frequency). Thus, the first new photons (and the second new photons (the idler photons) generated have a different wavelength to the annihilated pump photons. The frequency of the amplified probe signal 802 and the frequency of the wavelength converted copy 803 are symmetric (in the frequency domain) around the pump frequency 801 .

[0259] Graph 8b shows the output power spectrum for a FOPA in which the wavelength of the pump is 1562 nm (line 804). The wavelength of the input probe is 1462.5nm. The amplified probe is peak 805 and the wavelength converted probe is peak 807. In other words, peak 805 represents photons of the amplified probe signal and peak 806 represents the idler photons. Thus, graph 8b shows an example in which the pump wavelength is closer to the zero dispersion wavelength of the fibre 705. Figures 8a and 8b illustrate that as the pump wavelength was tuned away from the HNLF zero dispersion wavelength of 1564nm, a probe further away from the pump can be amplified.

[0260] Graph 8c shows measured gain and conversion efficiency (i.e. gain of the wavelength converted copy) for several probe wavelengths. The gain efficiency is the ratio between the signal power at the output of the gain fibre and the signal power at the input of the gain fibre. The conversion efficiency is the ratio between the idler power at the output of the gain fibre and the signal power at the input of the gain fibre. Graph 8c shows the ability of FOPA in this example to produce large gain of 20-50dB across the wavelength range between 1300 and 1900nm.

[0261] As explained above, in the case of a FOPA, a probe is coupled with a pump for amplification. In contrast, in the case of a FOPO, the gain medium is to be confined within a ring resonator, so lasing occurs at the gain wavelength. Figure 9 shows the ASE spectra generated by a FOPA without a probe. These could equally be generated in a FOPO on the first recirculation and be amplified on subsequent recirculations.

[0262] Figure 9 shows four graphs (9a, 9b, 9c, 9d) which illustrate four examples of ASE optical power spectra generated without any input to the FOPA other than the pump light. It demonstrates the ability to produce narrow linewidth light tuneable across a very broad wavelength range (590nm) by employing a pump tuneable within wavelength range of only 15nm.

[0263] Graph 9a shows the FOPA output power spectrum in which the wavelength of the pump is 1548 nm (line 901 ). It shows narrow (and low) ASE peaks 902 and 903 generated at ~1310nm and ~1890nm respectively.

[0264] Graph 9b shows the FOPA output power spectrum in which the wavelength of the pump is 1555 nm (line 904). It shows narrow ASE peaks 905 and 906 generated at ~1370nm and 1800nm respectively.

[0265] Graph 9c shows the FOPA output power spectrum in which the wavelength of the pump is 1560 nm (line 907). It shows narrow ASE peaks 908 and 909 generated at ~1440nm and ~1700nm respectively.

[0266] Graph 9d shows the FOPA output power spectrum in which the wavelength of the pump is 1563 nm (line 910). It shows broader ASE peaks 911 and 912 peaking at -151 Onm and 161 Onm respectively.

[0267] Therefore, graphs 9a, 9b, 9c, 9d show that a pump which has a wavelength that is further away from the gain fibre zero-dispersion wavelength (1564nm) produces narrow ASE peaks further away from the pump, which however are decreasing in power as they get further away (e.g. 9a, 9b and 9c). Similarly, a pump which has a wavelength that is closer to the gain fibre zero-dispersion wavelength produces broad ASE peaks closer to the pump, which are however much higher power. Nevertheless, all of the pump wavelengths shown can be used to generate light within a FOPO cavity.

[0268] Figure 10 shows an example of the apparatus wherein the apparatus forms part of a fibre optical parametric oscillator (FOPO). The FOPO includes a circulating loop cavity. The start of the loop may be electromagnetically linked to the end of the loop such that EM radiation propagating round the loop enters the start of the loop after exiting the end of the loop. Figure 10 shows that the FOPO 1000 contains components 1001 to 1011 , adjacent components beingjoined by a thin black line, such as line 1013. Each thin black line in figure 10 represents an optical fibre.

[0269] The key differences between the FOPA 800 and FOPO 1000 are the presence of the resonator and the lack of a seed - it is generated in the resonator.

[0270] As shown, FOPO 1000 includes an EM source 1001 , modulator 1002, polarisation controller 1003 and amplifier 1004. Components 1001 , 1002, 1003 and 1004 are equivalent to respective components 701 , 702, 703 and 704 in the FOPA 700. As previously described, the EM source 1001 is a tuneable laser which emits a pump which is phase modulated to mitigate the SBS by the modulator 1002 and amplified by a high power EDFA 1004.

[0271] The FOPO 1000 further includes a gain fibre 1005, a first optical circulator 1006, wavelength selective reflector 1007 and a second optical circulator 1008. The FOPO 1000 also includes a power splitter 1009, a polarisation controller 1010 and an optical delay line 1011 .

[0272] Figure 10 shows that the pump which has been amplified by the amplifier 1004 is input to the first optical circulator 1006. The first optical circulator 1006 has three ports, indicated as 1 , 2 and 3 in figure 10. It will be appreciated by the skilled person that a signal input to port 1 of the optical circulator will be output at port 2, and that a signal input to port 2 will be output to port 3 of the optical circulator. The amplified pump input to port 1 of the first optical circulator 1006 is therefore output at port 2 and is incident on the wavelength selective reflector 1007. In this example, the wavelength selective reflector is a tuneable a tuneable fibre Bragg grating (FBG) 1007. The FBG 1007 is tuned such that only the input pump (at pump wavelength Ap) is reflected by the FBG 1007. Thus accordingto one particular example in which the wavelength of the pump is 1550nm, the FBG 1007 is tuned so as to only reflect incident light having a wavelength of 1550nm. Light of the pump wavelength is therefore reflected by the FBG 1007. Light of other wavelengths (e.g. broadband noise from the EDFA 1004) passes through the FBG 1007 and is input to the second optical circulator 1008 at port 2. Lightwhich enters the second optical circulator 1008 at port 2 is output at port 3 and is output from the FOPO at 1012.

[0273] Thus the pump noise is removed by passingthrough a circulator and a FBG tuned to the pump wavelength. The reflected pump is input to port 2 of the first optical circulator 1006 and is output at port 3 of the first optical circulator 1006. The reflected signal at the pump wavelength is input to the gain fibre 1005. The pump produces ASE noise on the first pass through gain fibre 1005 which is amplified it in the subsequent passes, so an output signal is produced. A part of the amplified signal generated in the gain fibre 1005 is taken from the resonator via the power splitter 1009. The part of the amplified signal output from the gain fibre 1005 which is not output from the resonator by the power splitter 1009 continues to travel around the resonator. This part of the signal is incident on the polarisation controller 1010 and the optical delay line 1011 . The polarisation controller 1010 and optical delay line 1011 match the polarisation and the phase of the amplified wave for consecutive passes. The delay line can also be used to adjust the resonator (cavity) length so that the distortions caused by the pump phase modulation on each pass appear in counter-phase with distortions caused by the pump phase modulation on the subsequent pass.

[0274] The amplified signal is then input to the second optical circulator 1008 at port 1 and is output at port 2. The residual pump (light remaining at the pump wavelength after signal amplification) is thus incident on the FBG 1007 (in the opposite direction to previously). As previously, since the FBG 1007 is tuned so as to reflect only light at the pump wavelength, light at the pump wavelength is reflected by the FBG 1007 back towards the second optical circulator 1008. The reflected light is input at port 2 and output at port 3. The residual after amplification light of the pump wavelength is thus output from the FOPO at 1012. Light of other wavelengths passes through the FBG 1007 and enters port 2 of the first optical circulator 1006, exiting the optical circulator 1006 at port 3. Thus, the generated light is combined with the pump coming from the EDFA 1004 for subsequent amplification. This generated light repeats the path previously described to recirculate around the resonator. In other words, the assembly of circulators 1006 and 1008 and the wavelength selective reflector 1007 performs the following tasks: removal of broadband noise from the pump before it being inputted in the gain fibre 1005, removal of the residual pump from the cavity, and combiningthe light generated / amplified light within a previous recirculation with the pump for its subsequent amplification.

[0275] Figure 11 shows another example of a fibre optical parametric oscillator (FOPO) 1100 comprising a cavity including a gain fibre 1103. The FOPO 1100 contains components 1101 to 1105. Adjacent components are joined by a thin black line, such as line 1106. Each thin black line in figure 11 represents an optical fibre. The FOPO includes a first EM source 1101 , a multiplexer 1102, a gain fibre 1103, a splitter 1104 and a delay line 1105.

[0276] The FOPO 1100 includes a first EM source 1101 (pump). The first EM source 1101 may be a tuneable laser which may be phase modulated or chirped (frequency modulated). The pump 1101 may be amplified by an amplifier (not shown). The amplifier may be a high power amplifier such as an Erbium or Ytterbium doped fibre amplifier. The FOPO further includes a multiplexer (MUX) 1102 and a gain fibre 1103. The pump is input to the multiplexer 1102 and to the gain fibre 1103. The multiplexer couples the pump with a signal and / or idler waves into the gain fibre. The multiplexer 1102 may contain one or more components. The multiplexer may have additional capabilities. For example, the multiplexer may be configured to perform one or more of filtering the pump, filtering the signal, filtering the idler, removing broadband noise from the pump, removing residual pump (light remaining at the pump wavelength after signal amplification) and removing waves generated in the cavity (e.g. signal and / or idler).

[0277] As previously described, the fibre 1103 has specific dispersion parameters in order to achieve the conditions needed to generate a signal of a desired wavelength, which is amplified in the cavity. FOPO 1100 includes splitter 1104 which outputs part of the signal generated in the gain fibre 1103 via output port 1107 while the other part of the signal is recirculated around the cavity at 1108 to be amplified. The splitter 1104 may be a wavelength selective splitter (WDM filter) or a non-wavelength selective splitter (e.g. a power splitter).

[0278] As described above, pump phase modulation induces instantaneous pump frequency fluctuations, which can affect the phase matching condition and therefore cause small phase and amplitude distortions on the wave amplified within the cavity. These distortions degrade the wave linewidth and noise characteristics, and are therefore undesirable. However, this degradation may be mitigated by utilising the defined and periodic nature of the pump phase modulation waveforms. Indeed, the wave generated within the cavity passes through the gain fibre on every recirculation and experiences distortion defined by the pump phase modulation waveform. Therefore, the cavity length and the phase modulation frequencies, phases and amplitudes can be chosen so that the distortion induced on even recirculations is the opposite of distortions induced on odd recirculations. This will lead to almost complete cancellation of the induced distortion and consequently improved characteristics. In other words, phase modulating the pump (the received EM radiation) may involve selecting the cavity length and a phase modulation waveform such that distortions in the output EM radiation containingthe first new photon and the second new photon are minimised.

[0279] In an example where the pump signal is phase modulated, the length of the cavity of the FOPO (e.g. FOPO 1100) may be set using an appropriate delay (controlled by delay line 1105, which may be either a device allowing for length adjustment or an appropriate length of fibre) so that the generated wave propagation time may be equal to:

[0280] Where 7" is the period of the pump modulation waveform and n is an integer. In this way, the impact of the pump dithering (pump phase modulation) may be cancelled out each time the signal circulates the cavity twice. In other words, distortions induced on a first circulation may be cancelled out by distortions introduced on a second subsequent circulation. Accordingto one example, an optimised pump modulation waveform may have a period of approximately 40ns. According to this example, the cavity length should be equal to 4*(2n+1 ) meters.

[0281] Alternatively, the period of the pump phase modulation waveform T may be adjusted (e.g. in the electrical domain) to match an existing cavity length and thus to cancel the impact of dithering without adjustingthe cavity length.

[0282] Figure 12 shows a further example of a fibre optical parametric oscillator (FOPO) 1200. The FOPO 1200 includes EM source 1201 , splitters 1202 and 1208, a delay line 1203, multiplexers 1204 and 1205, and gain fibres 1206 and 1207. These components may be of the same types as those used in FOPO 1100. The FOPO 1200 operates in the same manner as FOPO 1100 seen in figure 11 except that the oscillator includes two gain fibres 1206, 1207 and two multiplexers 1204, 1205. The same pump light (from source 1201 ) is input to both gain fibres 1206 and 1207. In other words, the pump light input to both gain fibres has the same wavelength. Where the pump signal has been modulated, the light input to the two fibres has the same modulation pattern. In other words, the pump light input to both fibres has been modulated usingthe same phase modulation waveform. The delay line 1203 may be configured to control the timing of the input of the pump signal to the two gain fibres 1206, 1207. For example, the delay line 1203 may be configured to introduce a delay between the signal being input to a first gain fibre (e.g. 1206) and a second gain fibre (e.g. 1207). As previously, the FOPO 1200 includes splitter 1208 which outputs part of the signal generated in the gain fibres while the other part of the signal is recirculated around the cavity at 1210 to be amplified.

[0283] This arrangement may be adapted so as to minimise distortion of the signal generated in the cavity. For example, the pump power (power of the pump signal provided by EM source 1201 ) and lengths of the two gain fibres may be chosen so that the signal distortion caused by pump phase modulation is induced by the same magnitude in each of the two gain fibres. Additionally, the delay between pumps introduced by delay line 1203 may be chosen to be equal to half of the period of the pump modulation waveform. In this way, the signal distortion caused by pump modulation is induced in opposite directions in each of the two gain fibres such that the distortion in one fibre cancels out the distortion in the other fibre. In other words, when the distortion caused in the two fibres is equal in magnitude and opposite in direction, the distortion in the cavity may be cancelled out. Optionally, the period of the pump phase modulation waveform may be adjusted to assist matching its period with the delay.

[0284] The FOPOs described take advantage of the narrow gain bandwidth and may be configured to generate a high power continuous narrow wave tuneable across a range hundreds of nm wide. The apparatus in these examples may be adapted using components and configurations described elsewhere herein, including but not limited to any of the following: EM sources; EM radiation; waveguides; phase modulators; phase modulation characteristics; other electrical or optical components.

[0285] The proposed FOPA / FOPO seen in figures 7 and 10 have a range of applications:

[0286] 1 ) Amplification / generation of optical signals across a very broad range, e.g. a single device can replace a combination of several sources / amplifiers each operating across a fraction of the FOPA / FOPO operation wavelength range.

[0287] 2) Amplification / generation of optical signals in the wavelength ranges lacking suitable amplifiers / sources.

[0288] 3) Ultra-fast scanning across the wavelength range i.e. sweeping speed of the pump laser can be magnified by up to 20 times. Moreover, the proposed FOPA 700 also increases the tuneability range and output power as compared to the employed pump laser.

[0289] 4) For optical communications both in fibre and free-space, a single widely tuneable FOPO / FOPA can replace a whole fleet of expensive lasers / amplifiers each operating within much narrower wavelength range.

[0290] 5) Medical applications often require high power lasers operating at particular wavelengths from a very broad range. A single widely tuneable FOPA / FOPO can provide a solution for a range of such applications at once.

[0291] The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

Claims1 . An apparatus for outputting a photon, the apparatus comprising:I) an EM source arrangement for outputting phase modulated EM radiation; the phase modulated EM radiation comprising a set of one or more properties; the arrangement comprising:A) at least one electromagnetic, EM, source for generating EM radiation at a plurality of different wavelengths;B) a modulator for: i) receivingthe EM radiation; ii) phase modulatingthe received EM radiation; iii) outputting phase modulated EM radiation;II) an optical fibre for:C) receiving the phase modulated EM radiation;D) converting, using a third order nonlinear susceptibility of the optical fibre, at least one photon of the phase modulated EM radiation into: iv) a first new photon; and, v) a second new photon;E) outputting at least the first new photon; wherein the set of one or more properties: c) comprises a wavelength of the phase modulated EM radiation; d) provides for the phase modulated EM radiation to propagate along the optical fibre with at least: a positive value of a second order dispersion parameter; and a negative value of a fourth order dispersion parameter.

2. The apparatus accordingto claim 1 , wherein the phase modulated EM radiation propagates along the optical fibre under normal dispersion.

3. The apparatus accordingto claims 1 or 2, wherein the at least one EM source comprises a wavelength tuneable EM source for outputting the EM radiation at a plurality of different wavelengths.

4. The apparatus accordingto any preceding claim, wherein the EM radiation output from the at least one EM source is continuous wave EM radiation.

5. The apparatus accordingto any preceding claim, wherein the arrangement outputs continuous wave EM radiation comprising the first new photon.

6. The apparatus accordingto any preceding claim, wherein the optical fibre comprises a dispersion shifted optical fibre.

7. The apparatus accordingto any preceding claim, wherein phase modulatingthe received EM radiation comprises using one or more of: pseudo random bit sequence (PRBS), white noise and a plurality of sine tones.

8. The apparatus according to any preceding claim, wherein phase modulatingthe received EM radiation comprises using a plurality of sine tones.

9. The apparatus according to claim 8, wherein the modulator is configured to receive an electrical signal comprisingthe plurality of sine tones.

10. The apparatus accordingto claim 9, wherein the electrical signal is a composite electrical signal formed from the plurality of sine tones.11 .The apparatus accordingto any of claims 8 to 10, wherein the plurality of sine tones comprises at least a first frequency and a second frequency that is different to the first frequency.

12. The apparatus accordingto any of claims 8 to 11 , wherein the plurality of sine tones comprises between, and including, three to six sine tones.

13. The apparatus accordingto any of claims 8 to 12, wherein the plurality of sine tones comprises four sine tones.

14. The apparatus according to claim 13, wherein each of the four sine tones comprises a different frequency to any of the other sine tones.

15. The apparatus accordingto any preceding claim, wherein the EM source arrangement comprises an EM amplifier for receiving the phase modulated EM radiation and outputting amplified phase modulated EM radiation for inputting to the nonlinear optical fibre.

16. The apparatus according to any preceding claim, wherein the apparatus comprises an EM coupler for: a) receiving, along a first path, the EM radiation output from the EM source arrangement; b) receiving, along a second path, further EM radiation; c) outputting, along a third path and towards the optical fibre, both the EM radiation output from the EM source arrangement and the further EM radiation.

17. The apparatus according to claim 16, wherein the further EM radiation comprises a further wavelength, the first new photon comprising the further wavelength.

18. The apparatus accordingto claim 16 or 17, wherein the apparatus is configured to amplify the further EM radiation.

19. The apparatus accordingto any of claims 16 to 18, wherein the further EM radiation comprises continuous wave EM radiation.

20. The apparatus accordingto any of claims 16 to 19, wherein the apparatus comprises a further modulator configured to phase modulate the further EM radiation.21 . The apparatus of any preceding claim, wherein: the apparatus comprises: a second EM source; and a second modulatorfor: receiving EM radiation from the second EM source; phase modulating the EM radiation received from the second EM source; and outputting phase modulated EM radiation; the optical fibre comprises a first longitudinal section and a second longitudinal section; and the apparatus is configured such that: the first longitudinal section of the optical fibre receives phase modulated EM radiation from the modulator; andthe second longitudinal section of the optical fibre receives phase modulated EM radiation from the second modulator, wherein the phase modulated EM radiation from the modulator is in counter phase with the phase modulated EM radiation from the second modulator.

22. The apparatus accordingto any of claims 1 to 15, wherein the apparatus comprises a cavity comprising the optical fibre.

23. The apparatus accordingto claim 22, wherein the cavity comprises a filter arrangement for: a) inputtingthe phase modulated EM radiation into the optical fibre; b) removing the phase modulated EM radiation from the cavity after being output from the fibre and before the phase modulated EM radiation can reenter the optical fibre.

24. The apparatus accordingto claim 22 or 23, wherein the cavity comprises an EM splitter for: i) receiving a set of EM radiation output from the optical fibre, the set of EM radiation comprising: a. unconverted phase modulated EM radiation; b. the first new photon; ii) coupling a first portion of the set of EM radiation, output from the fibre, out of the cavity; and iii) transmitting a second portion of the set of EM radiation to propagate within the cavity.

25. The apparatus of any of claims 22 to 24, wherein the apparatus is configured such that propagation of the second portion of the set of EM radiation within the cavity comprises a series of circulations around the cavity and the modulator is configured to phase modulate the received EM radiation such that distortion of the second portion of the set of EM radiation incurred during a first circulation around the cavity is cancelled out by distortion of the second portion of the set of EM radiation incurred during a second subsequent circulation around the cavity.

26. The apparatus of claim 25, wherein the length of the cavity is selected such that distortion of the second portion of the set of EM radiation incurred during a first circulation around the cavity is cancelled out by distortion of the second portion of theset of EM radiation incurred duringthe second subsequent circulation around the cavity.

27. The apparatus of claim 25 or 26, wherein the modulator is configured to phase modulate the received EM radiation using a phase modulation waveform, the phase modulation waveform having a period which is selected such that distortion of the second portion of the set of EM radiation incurred during a first circulation around the cavity is cancelled out by distortion of the second portion of the set of EM radiation incurred duringthe second subsequent circulation around the cavity.

28. The apparatus of any of claims 22 to 27, wherein the apparatus comprises: a second optical fibre, the optical fibre and the second optical fibre being configured to receive the phase modulated EM radiation from the modulator, and the modulator is configured to phase modulate the received EM radiation using a phase modulation waveform having a period.

29. The apparatus of claim 28, wherein the modulator is configured to phase modulate the received EM radiation using a phase modulation waveform having a period which is selected so as to match the timing at which the optical fibre and the second optical fibre receive the phase modulated EM radiation from the modulator such that the phase modulated EM radiation received at the optical fibre is out of phase with the phase modulated EM radiation received at the second optical fibre by half of the period of the phase modulation waveform.

30. The apparatus of claims 28 or 29, wherein the apparatus further comprises a delay line configured to control the timing at which the optical fibre and the second optical fibre receive the phase modulated EM radiation from the modulator, and the delay line is configured to control the timing at which the optical fibre and the second optical fibre receive the phase modulated EM radiation such that the phase modulated EM radiation received at the optical fibre is out of phase with the phase modulated EM radiation received at the second optical fibre by half of the period of the phase modulation waveform.31 . The apparatus of any of claims 28 to 30, wherein the length of the optical fibre and the length of the second optical fibre are selected such that distortion of the phase modulated EM radiation incurred in the optical fibre is equal in magnitude to distortion of the phase modulated EM radiation incurred in the second optical fibre.

32. The apparatus of any of claims 28 to 31 , wherein the power of the phase modulated EM radiation received by the optical fibre and the power of the phase modulated EM radiation received by the second optical fibre are selected such that distortion of the phase modulated EM radiation incurred in the optical fibre is equal in magnitude to distortion of the phase modulated EM radiation incurred in the second optical fibre.

33. A method for outputting a photon comprising: generating EM radiation; phase modulating the EM radiation to form phase modulated EM radiation comprising a set of one or more properties; inputtingthe phase modulated EM radiation to an optical fibre such that the phase modulated EM radiation propagates alongthe optical fibre; converting, using a third order nonlinear susceptibility of the optical fibre, at least one photon of the phase modulated EM radiation into a first new photon and a second new photon; outputting at least the first new photon, wherein the set of one or more properties: a) comprises a wavelength of the phase modulated EM radiation; b) provides for the phase modulated EM radiation to propagate alongthe optical fibre with at least: a positive value of a second order dispersion parameter; and a negative value of a fourth order dispersion parameter.

34. The method according to claim 33, wherein the method comprises: iv) receiving, at the phase modulator, EM radiation comprising a first wavelength; v) phase modulating the EM radiation of the first wavelength to generate one or more further wavelengths of EM radiation that are different to the first wavelength. vi) outputting the one or more further wavelengths.

35. The method according to claim 34, wherein the first wavelength and the one or more further wavelengths distribute the energy of the EM radiation received by the modulator between the first wavelength and the one or more further wavelengths.

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