Apparatus for use in four-wave mixing and method for constructing phase adjustment means for suppressing unwanted idlers - Patents.com

Abstract

An apparatus for use in four-wave mixing is disclosed. The apparatus comprises a nonlinear medium for receiving a plurality of co-propagating light beams along an optical path through the nonlinear medium, the plurality of light beams including at least one signal wave having a signal frequency and one or two pump waves having respective pump frequencies, the light beams generating one or more unwanted idler waves in the nonlinear medium. The apparatus further comprises phase adjusting means configured to selectively apply respective wavelength-dependent phase shifts to wavelengths of light in the nonlinear medium to adjust the phase difference between the or each pump wave and the signal wave, such that unwanted idler waves are generated along the optical path of the nonlinear medium and destructively interfere to suppress the unwanted idler waves at the output of the nonlinear medium. A method of configuring the phase adjusting means is disclosed.

Description

[Technical field]

[0001] When radiation enters a medium, the oscillating electromagnetic field interacts with the electric dipoles in the molecules of the medium, causing them to oscillate, resulting in a time-varying local electric polarization in the medium. This oscillating electric field then re-radiates an electromagnetic field, and the incident wave can be thought of as propagating through the medium through a series of such absorption and re-radiation processes. The polarization vector P induced by an electric field with an amplitude vector E can be expressed as a generalized series expansion of the form [Table 1] where ε0 is the dielectric constant of vacuum, χ is the linear susceptibility tensor of the medium, and χ2 and χ3 are the second- and third-order susceptibility tensor terms. [Background technology]

[0002] If the induced polarization has a purely linear dependence on the applied electric field, the re-radiated electric field will be identical to the incident electric field. However, if the second or higher order susceptibility terms are non-zero, such as in nonlinear media, harmonics that were not present in the incident field will start to appear in the radiated field.

[0003] Nonlinear Kerr media with a nonzero third-order susceptibility term χ (such as highly nonlinear optical fibers made of silicate glass materials with high χ values) give rise to nonlinear behavior of light propagating in the medium, including self-phase modulation (SPM), cross-phase modulation (CPM), and four-wave mixing (FWM).

[0004] Four-wave mixing is the process by which four frequencies of light waves interact through the nonlinear response of the medium to the electric field of the waves. Broadly speaking, four-wave mixing (FWM) causes the interaction between any three co-propagating waves in a Kerr medium, resulting in the generation of a fourth wave. For simplicity, if we assume that the four light waves incident on a dielectric medium are linearly polarized parallel to the x-axis, then the resultant electric field can be expressed as: [Table 2] Here, E j is the amplitude of the jth electric field, β j is the frequency ω j is the propagation constant of the wave at . The propagation constant is given by the following relation: [Table 3] where n j (ω) is the frequency-dependent refractive index and c is the speed of light in a vacuum.

[0005] Substituting the expression for the resultant electric field into the expression for the polarization vector, the second and third order expansion terms contain many terms involving products of four waves. By collecting the terms at each of the four incident frequencies, we obtain the nonlinear polarization of the material, P NL can be decomposed into components for each frequency j as follows: [Table 4] [Table 5] where f and g are functions of the electric field amplitude and the parameter θ + ,θ - is explained below.

[0006] The polarization of the material at each of the incident frequencies is P j From the equation, the self-phase modulation, cross-phase modulation, and the electric field and polarization P at the jth frequency j A parameter θ that represents the relative phase between + ,θ - It can be seen that the four-wave mixing includes terms due to the two functions θ + or θ - It is most efficient when approaches zero. [Table 6] [Table 7]

[0007] These two conditions result in different four-wave mixing phenomena. + or θ - To minimize either , both the frequency of the signal and its propagation constant must satisfy certain conditions. These conditions essentially correspond to the conservation of energy and momentum before and after the FWM interaction. The fulfillment of the latter condition is called phase matching.

[0008] θ + The condition propagation constant phase matching (i.e., β1 + β2 + β3 - β4 = 0) is difficult to satisfy, and in fact it is the second of the two FWM mechanisms typically observed in nonlinear media (i.e., θ - ).

[0009] θ - The minimization of occurs when the wave frequencies satisfy the relationship ω1+ω2=ω3+ω4 and the associated phase matching requirement is satisfied (i.e., β1+β2-β3-β4=0). This can occur in the non-degenerate case (ω1≠ω2≠ω3≠ω4) or the degenerate case (ω1=ω2≠ω3≠ω4). This FWM interaction is illustrated in Figure 1, where two input waves with frequencies ω1 and ω2 combine with a third input wave with frequency ω3 to produce an output idler wave with frequency ω4. FWM can occur with any wave combination in a nonlinear medium where the phase matching requirement is satisfied. This interaction can be exploited with a single pump wave (generated by a laser) where the input waves are the same frequency (i.e., ω1 = ω2) in the degenerate case, or with two pump waves (generated by two lasers outputting different frequencies) where the input waves are different frequencies (i.e., ω1 ≠ ω2) in the non-degenerate case, which interacts with the signal wave and generates an idler wave of a different frequency based on the signal wave. Thus, FWM can in principle be exploited for signal processing purposes such as, for example, parametric amplification, wavelength / channel conversion, phase conjugation, optical sampling, nonlinear interactions of frequency comb lines, signal regeneration, etc.

[0010] However, in addition to the necessary interactions between the pump wave or the pump and signal waves to generate the required idler wave, FWM can also give rise to various interactions with other wave combinations, leading to the generation of a large number of spurious signals and ultimately resulting in crosstalk between the channels being processed.

[0011] This is shown in Figure 2 for the degenerate case, where two pump waves with frequency ω1 combine with a signal wave of frequency ω2 to produce a 112 = 2ω1-ω2, and one pump wave with frequency ω1 combines with two signal waves with frequency ω2 to produce a frequency ω 221 = 2ω2-ω1. In the non-degenerate case where two pump wave sources with different frequencies are provided, the situation becomes even more complicated, as shown in Figure 3, which shows the desired idler wave generated based on some combination of two pump waves and one signal wave, and the unwanted idler wave based on some combination of other waves (e.g., one pump wave and two signal waves, or three pump waves). The number of combinations increases even further when higher order interactions are observed and when multiple signal waves with various frequencies propagate through the nonlinear medium.

[0012] Steps taken to increase the efficiency of generation of the wanted idler signal, such as the use of increased launch power into the nonlinear medium or improved phase matching, will generally also benefit the generation of the unwanted signal at the same time.

[0013] To mitigate the proliferation of unwanted idlers in systems with separate pumps and signals, a commonly used approach in the art is to achieve a trade-off between the generation of wanted and unwanted idlers by maximizing pump power and limiting signal power, which limits the optical signal-to-noise ratio (OSNR) of the output idler. Other proposed approaches include using special dispersion designs to isolate interacting systems, reserving bandwidth for unwanted idlers, or simply processing in multiple isolated nonlinear media. These approaches, along with the presence of unwanted and wanted idlers, limit the usefulness of four-wave mixing applied to signal processing for various applications, which limits the commercialization of signal processing based on four-wave mixing to convert signal wavelengths, multiplex fixed-wavelength transmitters, and mitigate channel contention to increase data throughput.

[0014] It is in this context that the present disclosure has been conceived. Summary of the Invention [Means for solving the problem]

[0015] Viewed from one aspect, the present disclosure provides an apparatus for use in four-wave mixing, the apparatus comprising a nonlinear medium for receiving a plurality of optical beams co-propagating along an optical path through the nonlinear medium, the plurality of optical beams including at least one signal wave having a signal frequency and one or two pump waves having respective pump frequencies, the nonlinear medium providing a plurality of individual pump waves each having a respective frequency f j ,f k ,f l Three co-propagating waves in a nonlinear medium with i =f j +f k -f l According to the frequency f i The frequency f j ,f k ,f lOnly one of the waves providing one of the frequencies f is provided by one of the pump waves, and the frequency f in the four-wave mixing interaction j ,f k ,f l When two separate waves are provided by a wave other than the pump wave, the light beam in the nonlinear medium, in use, will have individual frequencies f i.u1 ,f i.u2 ,…,f i.un The apparatus further comprises phase adjusting means configured to selectively apply individual wavelength-dependent phase shifts to wavelengths of light in the nonlinear medium at one or more locations in the optical path of the light beam through the nonlinear medium to adjust the phase difference between the or each pump wave and the signal wave such that the unwanted idler waves are generated along the optical path of the nonlinear medium and destructively interfere with each other such that the unwanted idler waves are suppressed at the output of the nonlinear medium.

[0016] In accordance with the present disclosure, an apparatus for use in four-wave mixing according to this embodiment allows for the suppression of unwanted idler waves at the output of the nonlinear medium by application of one or more wavelength-dependent phase shifts at locations throughout the nonlinear medium selected such that the unwanted idler waves are generated and destructively interfere with each other at the output, resulting in the suppression of the unwanted idler waves. This suppression of unwanted idlers is achieved without compromising the idler OSNR or bandwidth or through complex design of dispersion to separate interacting systems. Suppression of unwanted idlers according to the present disclosure is achieved simply by applying wavelength-dependent phase shifts and adjusting the phase difference between the or each pump wave and the signal wave, along with the application of simple phase filters. As shown below, once the phase shifts are determined, the phase filters can be provided by passive components such as chirped fiber Bragg gratings, which allows for high-power operation and facilitates commercial deployment in a variety of signal processing applications. As will be seen, this approach has been shown to achieve as much as 26 dB of suppression of unwanted idlers.

[0017] In the embodiment, the frequency f j ,f k ,f l Two of the waves in the four-wave mixing interaction that provide two of the frequencies f j ,f k ,f l When used, a light beam in a nonlinear medium will have individual frequencies f i.w1 ,f i.w2 ,…,f i.wn The phase adjustment means may be such that a wavelength-dependent phase shift selectively applied by the phase adjustment means is such that a desired idler wave is generated along an optical path of the nonlinear medium to constructively interfere with the desired idler wave, such that the desired idler wave is enhanced at the output of the nonlinear medium. In an embodiment, the phase adjustment means may be such that a wavelength-dependent phase shift selectively applied by the phase adjustment means is such that a ratio of the power of the desired idler wave to the power of the unwanted idler wave at the output of the nonlinear medium is maximized. In an embodiment, the unwanted idler wave may be maximally suppressed and substantially eliminated at the output of the nonlinear medium. Thus, in accordance with the present disclosure, not only are unwanted idler waves suppressed, but not only are the desired idler waves unaffected, they may even be enhanced. In particular, in accordance with the present disclosure, a ratio of the power of the desired idler wave to the power of the unwanted idler wave may be maximized.

[0018] In an embodiment, the phase adjustment means may be such that the wavelength-dependent phase shift selectively applied by the phase adjustment means is such that the dispersion of the nonlinear medium is compensated for at least in a frequency range that includes the or each pump wave, the or each signal wave, and the or each required idler wave. Due to the dispersion of the nonlinear medium, the pump and signal waves, which are of different wavelengths, experience an unknown phase shift over the length of the optical path in the nonlinear medium. In accordance with the present disclosure, a phase filter is also used to find and compensate for this relative phase shift due to the wavelength-dependent dispersion in the nonlinear medium, and can compensate the dispersion profile of the fiber to satisfy the phase matching condition, so that a further phase shift can be applied to change the relative phase between the or each pump and the or each signal to suppress the unwanted idler. The resulting phase filter thus compensates for the dispersion, suppresses the idler, and gives the phase filter a complex, non-dispersive phase profile.

[0019] In an embodiment, the phase adjustment means may be selected to apply a π radian phase shift to the or each pump wave, or the or each signal wave, at one or more of the locations where a wavelength dependent phase shift is applied to the wavelength of light in the nonlinear medium. In an embodiment, the phase adjustment means may generate matched unwanted idler waves in anti-phase at different locations along the optical path of the nonlinear medium, resulting in destructive interference and suppression of the amplitude of the unwanted idler waves output from the nonlinear medium. By applying a π radian phase shift to the pump wave or the signal wave (typically one or both pumps) at locations on the optical path in the nonlinear medium, the unwanted idler waves are generated in anti-phase. When matched to each other, these can result in overall suppression of the unwanted idler waves at the output of the nonlinear medium by destructive interference. By applying a π radian phase shift between the pump wave and the signal wave, the wanted idler waves generated by the two pump waves and the signal wave will see a 2π radian phase shift at each phase filter, meaning that they are not affected by the phase filters and are not suppressed.

[0020] In an embodiment, the phase adjustment means may selectively apply a single wavelength dependent phase shift to the wavelength of light in the nonlinear medium at a midpoint of the optical path of the nonlinear medium. In an embodiment, the phase adjustment means may select a single π radian phase shift to apply to the or each pump wave at a midpoint of the optical path of the nonlinear medium. By applying a single phase shift in this manner at a midpoint of the optical path of the nonlinear medium, unwanted idler waves can be suppressed by application of a single phase filter at a single location in the optical path.

[0021] In an embodiment, the nonlinear medium may be an optical fiber, and may be a highly nonlinear optical fiber if desired. In an embodiment, the apparatus may further comprise a single pump wave source or two non-degenerate pump wave sources configured to provide a pump wave or pump waves propagating along an optical path in the nonlinear medium. In an embodiment, the apparatus may further comprise one or more signal wave sources configured to provide a signal wave or signal waves propagating along an optical path in the nonlinear medium. In an embodiment, the apparatus may be configured as a wavelength converter in a communication network configured to convert one or more communication channel signal waves to idler waves of different frequencies for multiplexing in the communication medium and to mitigate channel contention in the communication medium. In this manner, the apparatus of the present disclosure may be applied to various fiber-based applications for signal processing, such as wavelength conversion of multiple signals from a fixed wavelength source, enabling multiplexing while avoiding signal contention, further improving the throughput of fiber-based networks.

[0022] In an embodiment, the phase adjustment means may include at least one programmable optical filter. In an embodiment, the programmable optical filter may comprise a grating and a spatial light modulator including an array of controllable elements individually programmable to apply a selected phase shift to a light beam incident thereon, the grating being configured to disperse the light beams co-propagating in the nonlinear medium across the array of controllable elements. In an embodiment, the array of controllable elements may be a reflective liquid crystal matrix on silicon element. The use of a programmable optical filter allows one to find and apply the wavelength-dependent phase shift required to suppress unwanted idlers and compensate for the dispersion of the nonlinear medium, generating a complex filter profile.

[0023] In an embodiment, the phase adjustment means may comprise only passive optical components. In an embodiment, the phase adjustment means may comprise at least one complex chirped fiber Bragg grating configured to apply a selected wavelength-dependent phase shift to at least a portion of the wavelengths of the light beam propagating in the nonlinear medium in use. Once the complex filter profile required to suppress unwanted idlers is known for the nonlinear medium and the pump and source waves (e.g., by discovery using programmable optical filters), the phase filter profile can be implemented in passive components, providing its benefits in the field using relatively inexpensive and reliable components capable of stable operation at high power.

[0024] In an embodiment, the apparatus further comprises one or more erbium-doped fiber amplifiers (EDFAs) disposed along the optical path for amplifying one or more of the waves co-propagating in the optical path, the or each erbium-doped fiber amplifier being configured to balance the amplitude of unwanted idler waves generated along the optical path, such that the amplitude of unwanted idler waves at the output of the nonlinear medium is minimized. The use of EDFAs in this manner helps to balance the interactions in various sections of the nonlinear medium after application of a wavelength-dependent phase shift.

[0025] In embodiments, the nonlinear medium along the optical path may be of a single type, or the nonlinear medium may be of different types in sections along the optical path. Although the present disclosure contemplates the application of wavelength-dependent phase shifts within a single type of nonlinear medium, the use of complex phase filters can suppress idlers occurring in sections of different nonlinear media.

[0026] Viewed from another aspect, the present disclosure provides a method of configuring a phase adjustment means for suppressing unwanted idlers at an output of a nonlinear medium in an apparatus for use in four-wave mixing according to an aspect of the present disclosure, the method comprising the steps of operating a single pump wave source or two non-degenerate pump wave sources to provide the pump wave, or pump waves, that propagate along an optical path in the nonlinear medium, operating one or more signal wave sources to provide the signal wave, or signal waves, that propagate along an optical path in the nonlinear medium, detecting the light beam at the output of the nonlinear medium with a detector and determining a signal representative of the detected power spectral density of the light beam, detecting a frequency f at the output of the nonlinear medium, the method comprising the steps of: i.u1 ,f i.u2 ,…,f i.unand determining a wavelength-dependent phase shift for a wavelength of light propagating in the nonlinear medium so as to suppress the detected power of the unwanted idler wave at frequency f. In an embodiment, determining the wavelength-dependent phase shift may include determining a wavelength-dependent phase shift such that the dispersion of the nonlinear medium is compensated within a range that includes at least the or each pump wave, the or each signal wave, and the or each desired idler wave. In an embodiment, determining a wavelength-dependent phase shift such that the dispersion of the nonlinear medium is compensated may include sweeping the wavelength of one of the signal wave sources within said range, and determining a wavelength-dependent phase shift for the swept wavelength at the swept wavelength to maximize the detected power of the wanted idler wave generated for the signal wave at the swept wavelength. In an embodiment, determining the wavelength-dependent phase shift further includes determining a wavelength-dependent phase shift for a wavelength of light propagating in the nonlinear medium at frequency f at the output of the nonlinear medium. i.u1 ,f i.u2 ,…,f i.un In an embodiment, determining a wavelength-dependent phase shift for a wavelength of light of the or each pump wave such that a detected power of an unwanted idler wave at frequency f is minimized. i.w1 ,f i.w2 ,…,f i.wn In one embodiment, the method further includes determining a wavelength-dependent phase shift for a wavelength of light propagating in the nonlinear medium so as to enhance the detected power of the wanted idler wave at the output of the nonlinear medium. In an embodiment, the method further includes determining a wavelength-dependent phase shift for a wavelength of light propagating in the nonlinear medium so as to maximize a ratio of the detected power of the wanted idler wave to the detected power of the unwanted idler wave at the output of the nonlinear medium. In this manner, a complex wavelength-dependent phase shift required to suppress the unwanted idler and compensate for dispersion can be determined.

[0027] In an embodiment, the step of determining the wavelength-dependent phase shift may include providing at least one programmable optical filter as a phase adjustment means, and adjusting the phase shift applied in the or each programmable optical filter based on the detected power to determine the wavelength-dependent phase shift. In an embodiment, the method may further include determining one or more phase masks for one or more fiber Bragg gratings to provide the determined wavelength-dependent phase shift in the optical path, fabricating the or each fiber Bragg grating with the or each determined phase mask, and providing the or each fabricated fiber Bragg grating at a location in the optical path in the nonlinear medium to provide the phase adjustment means. Using programmable optical filters in this way, the complex wavelength-dependent phase shift required to suppress unwanted idlers and compensate for dispersion can be discovered and determined, and simple passive phase filters with matching complex filter profiles can be fabricated and deployed for high power use in the field.

[0028] From the foregoing disclosure and detailed description of the following examples, it will be understood by the reader that certain features and implementations described as optional in connection with any given aspect of the disclosure above should also be understood to be disclosed in combination with other aspects of the disclosure, where applicable. Similarly, it will be understood by the reader that the attendant advantages described in connection with any given aspect of the disclosure above should also be understood to be disclosed as advantages of other aspects of the disclosure, where applicable. That is, the description of any features and advantages in connection with a particular aspect of the disclosure above should not be understood as limiting, and the disclosure of these optional features and advantages should be understood to be intended to relate to all aspects of the disclosure in combination, where such combination is applicable. [Brief description of the drawings]

[0029] Particular examples of the present disclosure will now be described with reference to the accompanying drawings.

[0030] [Figure 1] This shows the relationship between the energies of photons that interact when pump and signal waves combine in four-wave mixing. [Diagram 2] An example of the generation of wanted and unwanted idler waves for input pump and signal waves in the degenerate case is shown in the power spectrum versus frequency. [Diagram 3] An example of the generation of wanted and unwanted idler waves for two input pump waves and an input signal wave in the non-degenerate case is shown in the power spectrum versus frequency. [Figure 4] 1 illustrates an exemplary apparatus for use in degenerate four-wave mixing according to an aspect of the present disclosure. [Diagram 5] 1 illustrates another exemplary device for use in degenerate and non-degenerate four-wave mixing in accordance with an aspect of the present invention, showing a power spectrum versus frequency illustrating the generation of wanted and unwanted idler waves at various locations within the optical path of the device, as well as the suppression of unwanted idler waves. [Figure 6] In accordance with an embodiment of the present invention, a method for configuring a phase adjustment means for suppressing unwanted idlers at the output of the nonlinear medium in an apparatus such as that shown in FIGS. 4 and 5 is shown. [Figure 7] The output power spectral density of a device such as that shown in Figures 4 and 5 with three signal waves of different frequencies is shown in the nondegenerate and degenerate cases when a programmable optical filter adjusts the dispersion and suppresses unwanted idlers. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Hereinafter, the embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that the present invention is not limited to these embodiments, and all modifications and / or equivalents or alternatives to these are within the scope of the present invention. Throughout the specification and drawings, the same or similar reference numerals may be used to refer to the same or similar elements.

[0032] As used herein, the terms "having," "may have," "including," or "may include" a feature (e.g., a number, function, operation, or component, etc.) indicate the presence of that feature and do not exclude the presence of other features. Throughout the description and claims of this specification, the words "comprising" and "including" and variations thereof mean "including, but not limited to," and are not intended to exclude (and do not exclude) other components, integers, or steps. Throughout the description and claims of this specification, the singular encompasses the plural, unless the context otherwise requires. In particular, where the indefinite article is used, it should be understood that the specification contemplates the plural as well as the singular, unless the context otherwise requires.

[0033] As used herein, the terms "A or B," "at least one of A and / or B," or "one or more of A and / or B" can include all possible combinations of A and B. For example, "A or B," "at least one of A and B," or "at least one of A or B" can refer to all of: (1) including at least one A, (2) including at least one B, or (3) including at least one A and at least one B.

[0034] As used herein, the terms "first" and "second" are used to modify various components, regardless of importance, and do not limit the components. These terms are used only to distinguish one component from another. For example, references to a first component and a second component may refer to components that are different from one another, regardless of the order or importance of the components.

[0035] When an element (e.g., a first element) is referred to as being "coupled" or "connected" (physically, operationally, or communicatively) to another element (e.g., a second element), it will be understood that the element may be coupled or connected directly to the other element or through a third element. In contrast, when an element (e.g., a first element) is referred to as being "directly coupled" or "directly connected" to another element (e.g., a second element), it will be understood that there is no intervening element (e.g., a third element) between the element and the other element.

[0036] The terms used herein are provided only to describe some of the embodiments and are not intended to limit the scope of other embodiments of the present disclosure. The singular forms "a", "an" and "the" should be understood to include the plural forms unless the context clearly dictates otherwise. All terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those skilled in the art to which the embodiments of the present disclosure belong. Furthermore, terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the relevant art, and should not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0037] 4, which illustrates an exemplary apparatus 402 for use in degenerate four-wave mixing according to an embodiment of the present disclosure. The apparatus 402 includes a highly nonlinear optical fiber 408, a phase adjusting means 410, and a highly nonlinear optical fiber 414.

[0038] The highly nonlinear optical fiber 408 and the highly nonlinear optical fiber 414 together form a nonlinear medium, in use, for receiving multiple light beams co-propagating along optical paths through the nonlinear medium. The highly nonlinear optical fiber 408 and the highly nonlinear optical fiber 414 may be two equal lengths of optical fiber constructed of a material having a high third-order susceptibility term χ3, such that four-wave mixing interactions occur between the light waves of the light beams co-propagating within the fibers.

[0039] For this purpose, the frequency f S A signal wave light source 406, which provides a light source of one signal wave having a frequency f P The optical beam having a refractive index of 1.0 or less is fed into a highly nonlinear optical fiber 408.

[0040] In the first length of highly nonlinear optical fiber 408, the signal and pump waves combine to produce a wave with a frequency of 2f P -f S That is, in a four-wave mixing interaction providing two input frequencies, where two of the waves are provided by the pump wave (degenerate case), the required idler wave is generated by four-wave mixing.

[0041] In addition, in the first length of the highly nonlinear optical fiber 408, the signal wave and the pump wave combine to form a light beam with a frequency of 2f S -f P That is, in a four-wave mixing interaction that provides one of the input frequencies, only one of the waves is provided by the pump wave, and the other two input frequencies f j ,f k ,f l If the other two waves providing the pump wave are provided by waves other than the pump wave, then the unwanted idler wave will be generated by four-wave mixing (in this case the signal waves combine twice in a degenerate manner to produce the unwanted idler wave).

[0042] At a midpoint in the nonlinear medium between the first length of highly nonlinear optical fiber 408 and the second length of highly nonlinear optical fiber 414, a spectrum such as that shown in FIG. 2 results (when dispersion compensated).

[0043] To suppress unwanted idler waves, a phase adjustment means 410 is placed at the midpoint of the nonlinear medium and selectively applies individual wavelength-dependent phase shifts to wavelengths of light in the nonlinear medium using a complex phase filter. The phase adjustment means 410 can be provided by a programmable optical filter including a grating and a spatial light modulator including an array of controllable elements that are individually programmable to apply selected phase shifts to light beams incident thereon, the grating being configured to disperse the co-propagating light beams in the nonlinear medium across the array of controllable elements. The array of controllable elements may be a reflective liquid crystal matrix on silicon element. The complex filter profile that needs to be applied by the phase adjustment means 410 can be found by operation of the programmable optical filter using the method described in connection with FIG. 6. Alternatively, the phase adjustment means 410 may be provided by only passive optical components, such as a complex chirped fiber Bragg grating coupled between the first length of highly nonlinear optical fiber 408 and the second length of highly nonlinear optical fiber 414 by an optical circulator. Complex chirped fiber Bragg gratings can be profiled to provide the same complex phase filter to a light beam in a nonlinear medium as that found by programmable optical filters.

[0044] The phase adjustment means 410 is configured, in use, to apply a selected wavelength-dependent phase shift to at least a portion of the wavelengths of the optical beams co-propagating within the nonlinear medium to at least adjust the phase difference between the pump wave and the signal wave. In this embodiment, the phase adjustment means 410 is configured to selectively apply a wavelength-dependent phase shift such that dispersion within the nonlinear medium (and at the joints of the optical path) is also at least compensated for in a frequency range that includes the pump wave, the signal wave or each signal wave, and the desired idler wave or each desired idler wave and the undesired idler wave. In addition to dispersion being compensated, the phase adjustment means 410 is configured to apply a π radian phase shift to the pump wave such that the relative phase shift between the pump wave and the signal wave is π radians. In other embodiments, the relative phase shift between the pump wave and the signal wave can be achieved by applying a π radian phase shift to the frequency of the signal wave.

[0045] The light beam with the selected wavelength-dependent phase shift applied travels from the phase adjustment means 410 to a second length of highly nonlinear optical fiber 414 where the pump wave and signal wave combine by four-wave mixing to generate further wanted idler waves and further unwanted idler waves to match the power of the wanted and unwanted idler waves generated by four-wave mixing in the first length of highly nonlinear optical fiber 408.

[0046] However, at the output of the nonlinear medium, at the end of the second length of highly nonlinear optical fiber 414, the optical spectrum analyzer 412 reveals that the wanted idler waves, unaffected by the wavelength-dependent phase shift, are present in the spectrum, but the unwanted idler waves are deleted. Since momentum, and therefore phase, is conserved in four-wave mixing interactions, by applying a π radian phase shift to the pump wave at the midpoint of the optical path in the nonlinear medium using the phase adjustment means 410, the unwanted idler waves generated in the first length of highly nonlinear optical fiber 408 and the second length of highly nonlinear optical fiber 414 are in antiphase. Thus, when these unwanted idler waves match each other in power, they destructively interfere with each other, leading to an overall suppression of the unwanted idler waves at the output of the nonlinear medium. By applying a π radian phase shift between the pump and signal waves, the desired idler wave is generated by two pump waves and one signal wave and will see a 2π radian phase shift in each phase filter, which means that the desired idler waves generated in the first length of highly nonlinear optical fiber 408 and the second length of highly nonlinear optical fiber 414 are in phase and will remain unaffected by the phase filters and unsuppressed.

[0047] Reference is now made to Figure 5, which illustrates in more detail another exemplary apparatus 500 for use in degenerate and non-degenerate four-wave mixing according to aspects of the present disclosure. Unlike the embodiment of Figure 4, the apparatus of Figure 5 uses a frequency f A0 A first signal wave having frequency f B0 and a second signal wave having a phase-matched pump wave frequency f P and a first pump wave source 504 (denoted as “P” in FIG. 5 ) providing a phase-matched pump wave frequency f Qand a second pump wave source 506 (denoted as "Q" in FIG. 5) providing a first pump wave source 504. The second pump wave source 506 can be operated as needed in addition to the first pump wave source 504, such that the device operates with two non-degenerate pump wave sources. As seen in the left and center panels of FIG. 5, an exemplary initial spectrum provided to the highly nonlinear optical fiber 510 by the signal wave source 502 and the first and second pump wave sources 504 and 506 is shown in the top panel (identified with the number 1) for the non-degenerate mode of operation on the left and the degenerate mode of operation in the center. Here, the signal wave is shown as A0 and B0 at individual frequencies, and the pumps are shown as P and Q.

[0048] Before traveling to the highly nonlinear optical fiber 510, the input light beam may be amplified using an erbium-doped fiber amplifier 508. After passing through a first length of highly nonlinear optical fiber 510, the pump wave and the signal wave undergo a first stage of four-wave mixing.

[0049] The result of this initial stage of FWM is shown in the second pane of Figure 5 (identified by the number 1), with the non-degenerate mode of operation on the left and the degenerate mode in the center. With reference to the non-degenerate process, signals A0 and B0 are fed to the desired idlers Y1 and Z1 (with individual frequencies f Y1 ,f Z1 The desired non-degenerate FWM process is transformed into unwanted idlers α1, β1, ψ1, ω1 (with individual frequencies f α1 ,f β1 ,f ψ1 ,f ω1 The phases φ of these six waves can be determined using the usual rules for FWM (based on the conservation of momentum) as follows: φ Z1 =φ P +φ Q -φ A0 (1) φ Y1 =φ P +φ Q -φ B0 (2) φ α1 =2φ A0 -φ P(3) φ β1 =φ P +φ B0 -φ A0 (4) φ ω1 =2φ Z1 -φ Q (5) φ ψ1 =φ Q +φ Y1 -φ Z1 (6)

[0050] After this first stage of FWM occurs in the highly nonlinear optical fiber 510, a wavelength-dependent phase shift is applied by a complex phase filter in the form of a programmable optical filter 512. The programmable optical filter 512 is configured to apply a phase shift of π to each of the pumps P and Q (or in practice to apply +π to one pump and −π to the other). However, this complex filter can also be used to compensate for dispersion in the nonlinear medium and to compensate for dispersion that occurs at a connection between two lengths of highly nonlinear optical fiber, for example a patch cord in an optical path. The exemplary power spectrum shown in the third pane of FIG. 5 shows that after application of the complex phase filter, there is no change in the power spectrum compared to the output in the second pane after the first stage of FWM, since this is just a phase filter.

[0051] Before traveling to the highly nonlinear optical fiber 516, the optical beam output from the programmable optical filter 512 can be amplified using an Erbium-doped fiber amplifier 514, such that the idler power generated in the second stage of four-wave mixing in the second length of highly nonlinear optical fiber 516 can match the idler power generated in the first stage. The use of an EDFA in this way helps balance the interactions in the various sections of the nonlinear medium after the application of a wavelength-dependent phase shift.

[0052] The second stage of FWM generates an idler that naturally and coherently adds to the idler generated by the first stage, as shown in the power spectrum in the fourth pane. The phase of the idler generated by the second stage is: φ Z2 =(φ P +π)+(φ Q +π)-φ A0 =φ Z1 (7) φ Y2 =(φ P +π)+(φ Q +π)-φ B0 =φ Y1 (8) φ α2 =2φ A0 -(φ P +π)=φ α1 +π (9) φ β2 =(φ P +π)+φ B0 -φ A0 =φ β1 +π (10) φ ω2 =2φ Z1 -(φ Q +π)=φ ω1 +π (11) φ ψ2 =(φ Q +π)+φ Y1 -φ Z1 =φ ψ1 +π (12)

[0053] For example, looking at equations (2) and (7) above, for the phase of the required idlers Z2 and Y2 generated in the second stage of FWM, since both pumps have a π phase shift, their sum is equal to 2π, which is naturally equivalent to no phase shift, φ Y1 ,φ Y2 are equal and in phase, and φ Z1 ,φ Z2It can be seen that the same is true for . Thus, the desired idlers Z1, Z2 of signal A and Y1, Y2 of signal B occurring in both the first length of highly nonlinear optical fiber 510 and the second length of highly nonlinear optical fiber 516 interfere constructively as if no phase shift was applied at all. Therefore, the desired idler wave is reinforced at the output of the nonlinear medium.

[0054] In contrast, as shown by equations (9)-(12) above, applying a π phase shift to the pump results in an unwanted idler generated in the second stage having a π phase shift relative to the unwanted idler generated in the first stage (i.e., compared to equations (3)-(6)). For example, the phase φ of the unwanted idler α2 generated in the second stage of FWM α2 is the phase φ of the unnecessary idler α2 generated in the first stage of FWM. α1 radians with respect to the wavelength-dependent phase shift of the second length of highly nonlinear optical fiber 516. Thus, these unwanted idlers are destructively interfered with and eliminated at the output of the second length of highly nonlinear optical fiber 516, as measured by the optical spectrum analyzer 518 and shown in the bottom pane of FIG. 5. Thus, in a programmable optical filter 512 constructed in accordance with the present disclosure, the selectively applied wavelength-dependent phase shift allows the unwanted idler waves to be maximally suppressed and substantially eliminated at the output of the nonlinear medium.

[0055] This scheme works for degenerate (single pump) systems as well. The center column of Figure 5 shows the steps described above for such a degenerate system. Mathematically, the conclusion above can be derived simply by Q φ P By substituting , we can convert it to the result for the degenerate system, and the conclusion remains the same.

[0056] As can be seen, the wavelength-dependent phase shift selectively applied by the programmable optical filter 512 is such that the ratio of the power of the wanted idler wave to the power of the unwanted idler wave at the output of the nonlinear medium is maximized.

[0057] A method for constructing a programmable optical filter 512 to provide a phase adjustment means for suppressing unwanted idlers at the output of the nonlinear medium will now be described with reference to Figure 6. In particular, it relates to an experimental construction of the phase adjustment means of the apparatus shown in Figure 5, the results of which are shown in Figure 7.

[0058] In block 602, a single pump wave source or two non-degenerate pump wave sources are operated to provide a pump wave or pump waves that propagate along an optical path in the nonlinear medium. In block 604, the routine 600 operates one or more signal wave sources to provide a signal wave or signal waves that propagate along an optical path in the nonlinear medium. In the experiment based on FIG. 5, three CW lasers are multiplexed using two polarization-maintaining fused couplers to ensure that they are all co-polarized. For the degeneracy study, the second pump wave source 506 laser "Q" was deactivated. The signal was then amplified using an erbium doped fiber amplifier 508 and passed to a first length of highly nonlinear optical fiber 510. The light beam then passes to a programmable optical filter 512 for applying a wavelength-dependent phase shift, and then all of the phase-shifted light beams are amplified in a second erbium doped fiber amplifier 514 and passed through a second length of highly nonlinear optical fiber 516 to experience a second stage of FWM.

[0059] To control the programmable optical filter 512 so as to determine the wavelength-dependent phase shift, in block 606, the light beam is detected using a detector at the output of the nonlinear medium and a signal representative of the detected power spectral density of the light beam is determined.

[0060] If dispersion compensation is used, the programmable optical filter 512 is also used to block any idlers generated outside the two pumps, to simplify the bandwidth over which dispersion compensation is performed, although this is not required. A wavelength-dependent phase shift may be determined (at least within a range that includes the or each pump wave, the or each signal wave, and the or each required idler wave) such that the dispersion in the nonlinear medium is compensated for. This may include sweeping the wavelength of one of the signal wave sources and determining a wavelength-dependent phase shift of the swept wavelength to maximize the detected power of the desired idler wave generated for the signal wave at the swept wavelength.

[0061] In block 608, the power spectrum detected at the output of the second highly nonlinear optical fiber 516 is referred to to determine whether the frequency f i.u1 ,f i.u2 ,…,f i.un A wavelength-dependent phase shift is determined for the wavelengths of light propagating in the nonlinear medium so as to suppress the detected power of the unwanted idler waves of the highly nonlinear optical fiber. The wavelength-dependent phase shift can be such that the detected power of the unwanted idler waves is minimized, the detected power of the desired idler waves is enhanced, or the ratio of the detected power of the desired idler waves to the detected power of the unwanted idler waves is maximized. In this regard, the programmable optical filter 512 is tuned to minimize the unwanted idler generation by applying an appropriate π radian phase shift to the wavelength of the or each pump wave. This π radian phase shift is further applied to compensate for the dispersion of both the highly nonlinear optical fiber and the optical path between them.

[0062] To balance the unwanted idler generation between the two stages, the power of the second erbium-doped fiber amplifier 514 was adjusted so that the unwanted idler waves generated in antiphase in the two sections of highly nonlinear optical fiber were matched in amplitude, thereby minimizing the unwanted idler generation.

[0063] Once the complex profile phase filter required to suppress the unwanted idler and compensate for dispersion is determined, the bulky and expensive programmable optical filter 512 can be replaced with one or more small and inexpensive passive optical components, which can lead to field deployment. As long as the wavelength of the pump wave remains constant, the phase filter profile required to compensate for dispersion and suppress the unwanted idler light should remain stable, and by incorporating this phase profile into a properly configured passive optical component, stable idler light suppression can be provided in a four-wave mixing system for field deployment. This implementation of passive optical components can be achieved by determining one or more phase masks for one or more fiber Bragg gratings and providing the determined wavelength-dependent phase shift in the optical path. Any suitable algorithm for grating design can be used, such as the efficient inverse scattering algorithm proposed in the literature (Feced et al, "An efficient inverse scattering algorithm for the design of nonuniform fiber Bragg gratings", IEEE Journal of Quantum Electronics, Volume:35, Issue:8, Aug 1999). Once a suitable phase mask or photomask has been determined to produce a fiber Bragg grating with a desired wavelength-dependent phase-shifting filter profile, the fiber Bragg grating (or each fiber Bragg grating) is fabricated using the determined phase mask or masks, and the programmable optical filter 512 is then removed and replaced with the fabricated fiber Bragg grating to provide a means of phase adjustment.

[0064] Power spectra showing the observed experimental output of the nonlinear medium when the programmable optical filter 512 is tuned to compensate for dispersion and suppress unwanted idlers are shown in Figure 7. Results for the non-degenerate and degenerate pump systems are shown in the top and bottom parts of Figure 7, respectively. Given the similarity of the results, it is efficient to discuss both systems simultaneously.

[0065] First, a single signal was launched from the signal wave light source 502 into the nonlinear medium, and its wavelength was swept between two wavelengths from the first pump wave light source 504 and the second pump wave light source 506, and the conversion efficiency of the required idler generation was measured using the optical spectrum analyzer 518.

[0066] This measurement was repeated for four different scenarios: 1) no filter profile, 2) idlers outside the pump wavelength removed (but no phase shift applied), 3) dispersion compensation and outer idler removal (but no phase shift applied to the pump), and 4) dispersion compensation, outer idler removal, and pump phase shift applied.

[0067] Referring to the first left pane of Figure 7, it can be seen that the removal of the outer idler has little effect on the conversion efficiency compared to the case without the filter. This comparison is provided to assure the reader that no unexpected effects were introduced by removing the idler. In both cases, a low power valley is seen due to the interaction of dispersion and phase matching in FWM (most of the dispersion comes from the patch cord between the highly nonlinear optical fibers). In the first left pane of Figure 7 (labeled A-1 and A-2), it can be seen that the application of dispersion compensation increases the conversion efficiency to a fairly flat -15 to -17 dB and -16 to -18 dB for the non-degenerate and degenerate cases.

[0068] We then examined the performance in a multi-channel setup by multiplexing two additional signals alongside the original signal, giving a total of three signals in the nonlinear medium. The three signals were placed (in wavelengths) in three different scenarios, namely, Scenario 1 (denoted by B-1 and B-2 in the second pane of Figure 7), Scenario 2 (denoted by C-1 and C-2 in the third pane of Figure 7), and Scenario 3 (denoted by D-1 and D-2 in the third pane of Figure 7), to investigate a range of interactions. As the center of the spectral inversion is the same for both the degenerate and non-degenerate cases, it was possible to examine the exact same scenarios. First, considering that the solid line (marked "Without PPS" in the legend (i.e., Programmed Phase Shift)) shows the output of a system without applying a π-phase shift, the diversity of interactions represented by FWM can be easily seen. These are particularly evident in Scenario 2, where the power ratio between many of the spurious idlers and the wanted idlers can be as much as -10 dB in a non-degenerate system and as much as +2 dB in a degenerate system, indicating the potential for severe crosstalk.

[0069] The dashed line (marked "With PPS" (i.e., programmed phase shift) in the legend) shows the results after applying a pump phase shift of π radians to the pump wavelength. In this case, the unwanted idler peaks shown in the solid grey line are absent in the dashed line and are often below the ASE noise floor, indicating that the unwanted idlers are almost completely suppressed. Considering scenario 2 as an example (shown as C-1 and C-2 in the third pane), it can be seen that the unwanted idler suppression is around 18 dB for the non-degenerate system and 26 dB for the degenerate system. Overall, these results show that the techniques disclosed herein effectively correct the unwanted idler-induced crosstalk in these systems.

[0070] We have proposed and demonstrated a method to suppress spurious idler generation in FWM-based optical processing systems. Spurious idler generation is often a limiting factor for performance, forcing system designers to compromise processing bandwidth or output power and SNR. By splitting the nonlinear processing medium in two and applying a π-radian phase shift to the pump at the midpoint, spurious FWM in the latter half can be used to cancel spurious FWM in the first half. Suppression of spurious idler generation is found to be comparable, 18 dB for the non-degenerate system examined, and 26 dB for the degenerate system, reducing it to ASE levels in both cases. We believe that this method provides a practical and effective solution for spurious idler generation suppression and will prove compatible with many FWM-based processing schemes.

[0071] In the embodiments described in connection with Figures 4 and 5, the nonlinear medium is a highly nonlinear optical fiber, however, phase adjustment means may be used at various locations within any suitable nonlinear medium to suppress unwanted idlers generated by four-wave mixing.

[0072] 4 and 5, there is a single type of nonlinear medium along the optical path, but in other embodiments the nonlinear medium may be of different types at various sections along the optical path. Although the present disclosure contemplates the application of wavelength dependent phase shifts within a single type of nonlinear medium, the use of complex phase filters can suppress idlers that occur in various sections of different nonlinear media.

[0073] In the embodiment described in relation to Figures 4 and 5, the phase adjustment means is such that a single π radian wavelength-dependent phase shift is selected and applied to the wavelength of light of the or each pump wave in the nonlinear medium at a midpoint in the optical path of the nonlinear medium, although other arrangements are possible. For example, the phase adjustment means may be such that a wavelength-dependent phase shift is applied at multiple locations in the optical path of the nonlinear medium, for example by applying a π radian phase shift to the or each pump wave or the or each signal wave at each location. In this manner, the phase adjustment means may generate matching unwanted idler waves in antiphase at different locations along the optical path of the nonlinear medium to destructively interfere and suppress the amplitude of the unwanted idler waves output from the nonlinear medium.

[0074] For simplicity, the above equations and embodiments describe the operation of four-wave mixing with co-propagating, co-polarized light, but this is not a requirement and the apparatus and methods can be applied to suppress unwanted idler light that occurs when the co-propagating waves are not all co-polarized.

[0075] In accordance with the present disclosure, an apparatus for use in four-wave mixing according to this embodiment allows for the suppression of unwanted idler waves at the output of the nonlinear medium by application of one or more wavelength-dependent phase shifts at locations throughout the nonlinear medium selected such that the unwanted idler waves are generated and destructively interfere with each other at the output, resulting in the suppression of the unwanted idler waves. This suppression of unwanted idlers is achieved without compromising the idler OSNR or bandwidth or through complex design of dispersion to separate interacting systems. Suppression of unwanted idlers according to the present disclosure is achieved simply by applying wavelength-dependent phase shifts and adjusting the phase difference between the or each pump wave and the signal wave, along with the application of simple phase filters. As shown below, once the phase shifts are determined, the phase filters can be provided by passive components such as chirped fiber Bragg gratings, which allows for high-power operation and facilitates commercial deployment in a variety of signal processing applications. As will be seen, this approach has been shown to achieve as much as 26 dB of suppression of unwanted idlers.

[0076] In accordance with the present disclosure, not only are unwanted idler waves suppressed, but wanted idler waves are not only unaffected, but may even be enhanced. In particular, in accordance with the present disclosure, the ratio of wanted to unwanted idler power can be maximized.

[0077] According to the present disclosure, unwanted idler waves can be suppressed by application of a single phase filter at a single location in the optical path.

[0078] In accordance with the present disclosure, the disclosed apparatus can be applied to various fiber-based applications for signal processing, such as wavelength conversion of multiple signals from a fixed wavelength light source, enabling multiplexing while avoiding signal contention, further improving the throughput of fiber-based networks.

[0079] It should be understood that features, integers, characteristics described in connection with a particular aspect, embodiment or example of the disclosure are applicable to other aspects, embodiments or examples described herein, unless inconsistent therewith. All features disclosed herein (including the accompanying claims, abstract, drawings) and / or all steps of any method or process so disclosed may be combined in any combination, except combinations in which at least some of such features and / or steps are mutually exclusive. The disclosure extends to any novel or novel combination of features disclosed herein (including the accompanying claims, abstract, drawings) or any novel or novel combination of steps of any method or process so disclosed. In particular, any dependent claim may be combined with any of the independent claims and with any of the other dependent claims.

Claims

1. 1. An apparatus for use in four-wave mixing, comprising: A nonlinear medium for receiving, in use, a plurality of optical beams co-propagating along an optical path through the nonlinear medium, the plurality of optical beams including at least one signal wave having a signal frequency and one or two pump waves having respective pump frequencies, the nonlinear medium comprising: ｊ , f ｋ , f ｌ Three co-propagating waves in a nonlinear medium with ｉ = f ｊ +f ｋ -f ｌ According to the frequency f ｉ and generating an idler wave having Frequency f in four-wave mixing interaction ｊ , f ｋ , f ｌ Only one of the waves providing one of the frequencies f is provided by one of the pump waves, and the frequency f in the four-wave mixing interaction ｊ , f ｋ , f ｌ When the two separate waves are provided by waves other than the pump wave, in use, the light beam in the nonlinear medium will have individual frequencies f ｉ．ｕ１ , f ｉ．ｕ２ , ..., f ｉ．ｕｎ generating one or more unwanted idler waves having the apparatus further comprises phase adjusting means configured to selectively apply individual wavelength-dependent phase shifts to wavelengths of light in the nonlinear medium at one or more locations within the optical path of the light beam through the nonlinear medium to adjust the phase difference between the or each pump wave and the signal wave; A wavelength-dependent phase shift is one in which unwanted idler waves are generated along the optical path of the nonlinear medium and destructively interfere with each other, resulting in the unwanted idler waves being suppressed at the output of the nonlinear medium.

2. frequency f ｊ , f ｋ , f ｌ two of the waves in the four-wave mixing interaction that provide two of the frequencies f ｊ , f ｋ , f ｌ When used, the light beam in the nonlinear medium will have individual frequencies f ｉ．ｗ１ , f ｉ．ｗ２ , ..., f ｉ．ｗｎ generating one or more desired idler waves having 2. The apparatus of claim 1, wherein the phase adjusting means is such that the wavelength-dependent phase shift selectively applied by the phase adjusting means causes a desired idler wave to be generated along the optical path of the nonlinear medium and to constructively interfere with it, such that the desired idler wave is enhanced at the output of the nonlinear medium.

3. 3. The apparatus of claim 2, wherein the phase adjusting means is such that the wavelength-dependent phase shift selectively applied by the phase adjusting means maximizes the ratio of the power of the wanted idler wave to the power of the unwanted idler wave at the output of the nonlinear medium.

4. 10. The apparatus of claim 1, wherein unwanted idler waves are maximally suppressed and substantially eliminated at the output of the nonlinear medium.

5. 2. The apparatus of claim 1, wherein the phase adjusting means is such that the wavelength-dependent phase shift selectively applied by the phase adjusting means is such that dispersion of the nonlinear medium is compensated for in at least a frequency range that includes the or each pump wave, the or each signal wave, and the or each required idler wave.

6. 2. The apparatus of claim 1, wherein the phase adjusting means is adapted to select a π radian phase shift to be applied to the or each pump wave or the or each signal wave at one or more of the locations where a wavelength-dependent phase shift is applied to the wavelength of light in the nonlinear medium.

7. 2. The apparatus of claim 1, wherein the phase adjusting means generates matching unwanted idler waves in antiphase at different locations along the optical path of the nonlinear medium, thereby destructively interfering and suppressing the amplitude of the unwanted idler waves output from the nonlinear medium.

8. 2. The apparatus of claim 1, wherein the phase adjusting means selectively applies a single wavelength dependent phase shift to wavelengths of light within the nonlinear medium at an intermediate point in the optical path of the nonlinear medium.

9. 2. The apparatus of claim 1, wherein the phase adjustment means is adapted to select a single π radian phase shift to be applied to the or each pump wave at a midpoint in the optical path of the nonlinear medium.

10. The apparatus of claim 1 , wherein the nonlinear medium is an optical fiber.

11. 10. The apparatus of claim 1, further comprising a single pump wave source or two non-degenerate pump wave sources configured to provide the or each pump wave propagating along an optical path within the nonlinear medium.

12. 10. The apparatus of claim 1, further comprising one or more signal wave light sources configured to provide the or each signal wave propagating along an optical path within the nonlinear medium.

13. 10. The apparatus of claim 1, wherein the phase adjusting means comprises at least one programmable optical filter.

14. The programmable optical filter comprises a grating and a spatial light modulator including an array of controllable elements that are individually programmable to apply a selected phase shift to a light beam incident thereon; The apparatus of claim 1 , wherein the grating is configured to disperse a co-propagating optical beam in the nonlinear medium across the array of controllable elements.

15. 10. The device of claim 1, wherein the array of controllable elements is a reflective liquid crystal matrix on silicon element.

16. 10. The apparatus of claim 1, wherein the phase adjusting means comprises only passive optical components.

17. 10. The apparatus of claim 1, wherein the phase adjusting means comprises at least one complex chirped fiber Bragg grating configured to apply a selected wavelength-dependent phase shift to at least a portion of the wavelengths of the optical beam propagating within the nonlinear medium in use.

18. 10. The apparatus of claim 1, further comprising one or more erbium-doped fiber amplifiers disposed along the optical path to amplify one or more of the waves co-propagating in the optical path, the or each erbium-doped fiber amplifier configured to balance the amplitude of unwanted idler waves generated along the optical path, such that the amplitude of unwanted idler waves at the output of the nonlinear medium is minimized.

19. 10. The apparatus of claim 1, wherein the nonlinear medium is of a single type along the optical path, or the nonlinear medium is of different types at multiple sections along the optical path.

20. 10. The apparatus of claim 1, configured as a wavelength converter in a communications network, configured to convert one or more communications channel signal waves into idler waves of different frequencies for multiplexing into a communications medium to mitigate channel contention in the communications medium.

21. 21. A method for configuring a phase adjustment means for suppressing unwanted idler waves at the output of a nonlinear medium in an apparatus according to any one of claims 1 to 20, comprising the steps of: operating a single pump wave source or two non-degenerate pump wave sources to provide the or each pump wave propagating along an optical path within the nonlinear medium; operating one or more signal wave light sources to provide the or each signal wave propagating along an optical path within the nonlinear medium; detecting the light beam at the output of the nonlinear medium with a detector and determining a signal representative of the detected power spectral density of the light beam; At the output of the nonlinear medium, the frequency f ｉ．ｕ１ , f ｉ．ｕ２ , ..., f ｉ．ｕｎ determining a wavelength-dependent phase shift for a wavelength of light propagating in the nonlinear medium so as to suppress the detected power of the unwanted idler wave at

22. The step of determining the wavelength dependent phase shift comprises:

22. The method of claim 21, comprising determining a wavelength-dependent phase shift such that dispersion of the nonlinear medium is compensated within a range that includes at least the or each pump wave, the or each signal wave, and the or each required idler wave.

23. determining a wavelength dependent phase shift such that the dispersion of the nonlinear medium is compensated for, sweeping the wavelength of one of the signal wave light sources within said range; 23. The method of claim 22, further comprising: determining a wavelength dependent phase shift for the swept wavelength; and maximizing the detected power of a desired idler wave generated for a signal wave at the swept wavelength.

24. The step of determining the wavelength dependent phase shift further comprises: At the output of the nonlinear medium, the frequency f ｉ．ｕ１ , f ｉ．ｕ２ , ..., f ｉ．ｕｎ 22. The method of claim 21, comprising determining a wavelength-dependent phase shift for the wavelength of light of the or each pump wave such that the detected power of unwanted idler waves at

25. The step of determining the wavelength dependent phase shift further comprises: At the output of the nonlinear medium, the frequency f ｉ．ｗ１ , f ｉ．ｗ２ , ..., f ｉ．ｗｎ 22. The method of claim 21, comprising determining a wavelength dependent phase shift for a wavelength of light propagating in the nonlinear medium so as to enhance the detected power of the desired idler wave at

26. 26. The method of claim 25, further comprising determining a wavelength-dependent phase shift for a wavelength of light propagating in the nonlinear medium so as to maximize a ratio of detected power of a wanted idler wave to detected power of an unwanted idler wave at the output of the nonlinear medium.

27. The step of determining the wavelength dependent phase shift comprises: providing at least one programmable optical filter as a phase adjustment means; and adjusting the phase shift applied in the or each programmable optical filter based on the detected power to determine the wavelength-dependent phase shift.

28. determining one or more phase masks for one or more fiber Bragg gratings to provide a determined wavelength-dependent phase shift in the optical path; - fabricating the or each fiber Bragg grating using the or each determined phase mask; 28. The method of claim 27, further comprising providing the or each fabricated fiber Bragg grating at a location in the optical path within the nonlinear medium to provide the phase adjustment means.

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