WO2009000021A1

WO2009000021A1 - Dispersion engineering in highly nonlinear optical materials

Info

Publication number: WO2009000021A1
Application number: PCT/AU2008/000902
Authority: WO
Inventors: Benjamin John Eggleton; Eric Magi; Libin Fu; Mark Pelusi; Vahis Ta'eed; Michael Lamont; Hong Nguyen
Original assignee: The University Of Sydney, A Body Corporate Established Pursuant To The University Of Sydney Act 1989
Priority date: 2007-06-22
Filing date: 2008-06-20
Publication date: 2008-12-31

Abstract

This invention concerns dispersion engineering in highly nonlinear optical materials. In particular, the invention concerns a highly non-linear optical device, comprising a chalcogenide waveguide. In another aspect the invention concerns a method for fabricating such a device. In further aspects the invention is a four-wave mixing demultiplexing and supercontinuum generation device and a method of using the demultiplexing device.

Description

Title

Dispersion Engineering in Highly Nonlinear Optical Materials

Technical Field

This invention concerns dispersion engineering in highly nonlinear optical materials. In particular, the invention concerns a highly non-linear optical device, comprising a chalcogenide waveguide. In another aspect the invention concerns a method for fabricating such a device. In further aspects the invention is a four-wave mixing demultiplexing device and a method of using the demultiplexing device.

Background Art

Highly non-linear optical waveguides are of great interest for compact, low- power, all-optical non-linear devices. Non-linearity in waveguides can be enhanced either by modifications in the structure to reduce the effective area of the mode (A eg), or by using materials with higher non-linearity. Tapering is a commonly used method for reducing optical fiber dimensions and engineering the waveguide dispersion. Low- loss fiber tapers with sub-wavelength waist diameters have been fabricated from both standard single-mode fibers (SMFs)¹' ² and micro-structured fibers³ These not only serve as platforms for microphotonic device components, they also enable non-linear process such as super-continuum generation at low power thresholds ' . The reduction in the modal area and the subsequent enhancement of the non-linearity is ultimately limited by the index contrast of air and silica in the case of SMFs⁶, as well as the structural dimensions in the case of photonic crystal fibers⁷.

Materials with higher non-linearity include semiconductors such as silicon and AlGaAs⁹, and glasses such as tellurite¹⁰, lead silicate¹¹, bismuth¹² and chalcogenides¹³. Chalcogenide glasses are of particular interest for device applications based on Kerr-nonlinearity {ni), as it exhibits very high nj (100 ~1000 times greater than in silica), low two-photon absorption (TPA, β) and fast response time of less than 100 fs¹³. Chalcogenides have a good non-linear figure of merit FOM = (rc/βλ) across the telecommunications band¹⁴, making the material a suitable platform for broadband, low-power, compact all-optical signal processing devices, including all- optical 2R regenerators ' ⁵. The phrase "chalcogenide glass", as used throughout this document, refers to a glass that contains a significant proportion of one or more chalcogenide elements.

Disclosure of the Invention

The invention is a highly non-linear optical device, comprising a chalcogenide waveguide, wherein the dimensions of the waveguide are adjusted to increase signal intensity within the waveguide and modify dispersion to controllably excite nonlinear optical processes.

In particular the intensity of the light wave inside the waveguide is increased and the threshold for the nonlinear interaction is reduced by the adjustment to the dimensions.

The core of the waveguide may be fabricated from two or more of Arsenic As, Selenium Se, Sulphur S and Germanium Ge.

The adjustment of the dimensions of the waveguide may involve reducing the dimensions.

The waveguide may be cylindrical, such as a fibre. In this case the waveguide may be fabricated from Arsenic As and Sulphur S. Also in this case the diameter of the fibre may be reduced by tapering.

The waveguide may be planar, in particular rectangular. In this case the dimensions of the waveguide may be adjusted by lithography and etching techniques.

The waveguide may be left uncovered after etching, or it may be covered in a cladding material. In this event the diameter of the fibre may be reduced by etching the cladding.

The waveguides may incorporate any dopants suitable for chalcogenide waveguides.

In a further aspect the invention is a method for fabricating a highly non-linear optical device, comprising the steps of:

Fabricating a chalcogenide waveguide; Adjusting the dimensions of the waveguide to increase signal intensity within the waveguide and modify dispersion to controllably excite nonlinear optical processes.

A chalcogenide fiber waveguide may have its dimensions adjusted by tapering. Fibres made of As and any combination of one or more of S, Se and Ge may be tapered by the application of heat and tension so that the fibre stretches and reduces in cross sectional area when the glass temperature significantly exceeds the glass transition temperature.

The dimensions of waveguides made from a planar substrate of homogeneous material comprising As and any combination of one or more of S, Se and Ge can be controlled using lithography and etching techniques. Changes in the waveguide dimensions change the intensity of the light inside but also affect the dispersion of the waveguide.

The nonlinear Kerr effect modulates the phase of the guided wave. Control of this process allows control of self- and cross-phase modulation and phase matching processes including four-wave mixing. Phase modulation leads to spectral broadening while phase matching allows parametric coupling of light from one wavelength to another. Spectral broadening and parametric coupling are the basis of a number of applications of significant interest including spectrally-tailored supercontinuum generation, optical signal processing including regeneration and switching and optical signal monitoring.

A particular benefit of controlling the dispersion of a waveguide is that by creating anomalous dispersion, phase-matching processes such as four-wave mixing and supercontinuum generation can be performed. As well, the reduced transverse dimensions necessary for controlling the the dispersion will enhance the nonlinearity of the waveguide and allow nonlinear processes to occur at much lower powers. For four-wave mixing, this will allow strong coupling of energy from a pump wave at f_p to an idler wave at f_\. A particular application for this process is efficient optical multiplexing (and demultiplexing) at ultrahigh bite rates, which can be accomplished at short device length due to the enhanced nonlinearity.

In another aspect the invention is a four-wave mixing demultiplexing device operable to create a nonlinear interaction for performing the method above. The device comprises a short length of ribbed waveguide fabricated by lithography from a planar substrate of a thin film of chalcogenide glass. Wherein the combination of the chalcogenide nonlinearity and tight optical confinement provided by the ribbed form of the waveguide results in the four-wave mixing. The strong instantaneous waveguide nonlinearity and the reduction of normal dispersion makes for efficient four-wave mixing in the short waveguide.

In another aspect the invention is a method of using the device defined above for demultiplexing one data stream from a high bit rate channel in which there is more than one multiplexed data stream. The method comprising the following steps: Creating a nonlinear interaction between a synchronous clock pulse at M hertz at an optical carrier frequency f_p and the high bit rate channel at optical carrier frequency f_s to recover the data stream at M bits per second at an optical carrier frequency fj = 2 f_p - f_s. And,

"Dropping" the recovered data stream channel by use of a bandpass filter centred at fj whose pass band excludes frequencies f_p and f_s.

Another application arising out of tailored waveguide dispersion in chalcogenide is efficient phase matched processes, such as four-wave mixing, parametric gain, or amplification, and low threshold supercontinuum generation.

Other applications include low-threshold optical signal processing, such as optical regeneration, and low-threshold stimulated brillouin scattering processes, such as those applied to slow-light.

Brief Description of the Drawings

An example of the invention will now be described with reference to the accompanying drawings, in which:

Fig. 1. is a graph showing calculated γ with tapered diameter for As₂Se₃ and silica fiber tapers. Insets show calculated mode profiles for linear polarization of tapers with diameters of 0.6μm at 1550nm.

Fig. 2 (a) is a diagram illustrating a pre-taper to convert a multi-mode As₂Se₃ fiber into a single mode fiber.

Fig. 2 (b) is a diagram illustrating a further taper of the As₂Se₃ fiber down to 1.2 μm. Fig. 3 is a schematic of the experimental setup for nonlinear measurement.

Fig. 4 is a graph showing an experimentally measured power transfer function and a simulation with β= 2.5 x lO^"12 m.W^"1.

Fig. 5 is a graph showing SPM spectra under different incident power.

Fig. 6 is a graph showing dispersion as a function of taper diameter at a wavelength of 1550nm.

Fig. 7(a) is a diagram of a fibre that has been tapered from 20 μm to lμm. Fig. 7(b) is a graph showing the group velocity dispersion (combined material and waveguide dispersion) along the taper at 1550 nm.

Fig. 7 (c) is graph which shows the group velocity dispersion for the narrow end of the taper, and the wide end of the taper, across a range of different wavelengths.

Fig. 8. (a) is an illustration of an As₂S₃ planar waveguide device. Fig. 8 (b) is a scanning electron micrograph image of the device's cross- section.

Fig. 8 (c) is a numerical optical mode profile generated by Finite Element Method for physical waveguide properties. And,

Fig. 8 (d) is a schematic of FWM generating an idler wave at new frequency

Fig. 9 (a) is a diagram showing the experimental set-up for synchronized sources of 160 Gb/s signal and 10 GHz pump pulses at 1560 and 1550 nm wavelength respectively.

Fig. 9 (b) is a diagram showing the experimental set-up for 160 Gb/s all-optical

DEMUX using a 5 cm As₂S₃ waveguide. Fig. 9 (c)(i) is an optical eye diagram of input 160 Gb/s, and Fig. 9 (c)(ii) is an optical eye diagram of filtered output 10 Gb/s FWM idler signals

Fig. 10 (a) is a graph showing optical spectra at input, and Fig. 10 (b) is a graph showing optical spectra at output, of a 5 cm waveguide measured with 0.07nm resolution bandwidth Fig. 11 (a) are optical eye diagrams of 10 Gb/s signals "back to back" measured on a 80 GHz electrical sampling scope via (i) 65 GHz bandwidth PD plug- in module and (ii) 40 Gb/s PD receiver.

Fig. 11 (b) are optical eye diagrams of 160 Gb/s FWM-DEMUX signals measured on a 80 GHz electrical sampling scope via (i) 65 GHz bandwidth PD plug- in module and (ii) 40 Gb/s PD receiver

Fig. 12 is a graph showing bit-error-rate performance for all 16x10 Gb/s channels of 160 Gb/s OTDM signal after FWM-DEMUX in 5 cm As₂S₃ waveguide compared against 10 Gb/s "back-to-back" (B2B) measured at 1540 nm wavelength.

Best Modes of the Invention

Example 1. As₂Se^ Fiber Taper Design and Fabrication

This example experimentally demonstrates enhanced non-linearity in As₂Se₃ chalcogenide fiber {ru ~500 times silica) by tapering down to a sub-wavelength diameter of 1.2μm. The nonlinear propagation of picosecond pulses through the taper is measured and the results are compared with simulations of the nonlinear Schrδdinger equation. A strongly enhanced nonlinear coefficient is inferred inside the taper waist, which is 62,000 times greater than in standard SMFs, opening up new possibilities for nonlinear photonic devices with ultra low threshold energy and targeting dispersion.

Fig. 1 plots the calculated γ as a function of the taper diameter for As₂Se₃ and silica fiber tapers surrounded by air. Fig. 1 also shows calculated mode profiles for both silica and As₂Se₃ tapers having waist diameters of 0.6μm for wavelength of 1550nm. The high index (π = 2.8) of As₂Se₃ helps to strongly confine light and γ peaks at a value of 164,000 wW¹ (or 164 W^"W) where A& = 0.26 μm². This value of γ is 149,000 times stronger than in standard untapered silica (SMFS8) fibers. On the other hand for silica tapers having the same diameter of 0.6μm, the mode is expanded due to the lower index and a significant portion lies outside the taper in an evanescent field. Even when the mode is well confined in tapered silica fibers, the γ is lower than in As₂Se₃. For silica, γ peaks at 0.067 W'm^"1 when the diameter is 1.1 μm and A^ = 1.36 μm², which is inferior to tapered As₂Se₃ by a factor of 2440. The enhancement of γ originates from the 500 times larger n₂ in As₂Se₃, and the increased confinement of light associated with the higher index that results in a minimum A_e& that is ~5 times smaller.

Fig. 2 shows the schematic of the fabricated chalcogenide fiber taper used in the experiment. To taper the As₂Se₃ fiber a modified version of the standard flame brushing technique⁴ is used. Since the melting temperature of As₂Se₃ is just below

200°C, resistive heating is used to heat the fiber rather than flame. First a multi- moded chalcogenide fiber (with a core size of 7.5μm, NA: 0.19) is tapered from an outer diameter of 165 μm down to 75 μm so that the fiber core becomes single- moded. The uniform waist section of this initial taper is then cleaved, butt-coupled and secured to high numerical aperture silica fibers using UV-cured epoxy. The now single-mode waist from initial taper is further tapered to 1.2 μm diameter.

To further confirm the calculation for waist diameter⁴, the tapering process was repeated and a duplicate taper examined under a high resolution optical microscope; the waist was measured to be 1.2 μm ±0.1 μm in diameter. The length of the uniform waist is 18 mm and total length of the taper is 164 mm, as shown in Fig. 2b.

The heat brushing profile determining the shape of taper profile was constrained by the length of the stages on the taper rig. As a result, a compromise on the adiabaticity of the As₂Se₃ taper was required and the process produced tapers having tapering loss of 3dB.

Nonlinear experiments and analysis

Experimental Setup

Fig. 3 schematically shows the experimental set up 30 for the measurement of the nonlinear pulse propagation. A mode-locked fiber laser 32 feeds to a variable optical attenuator (VOA) 34; then to a PC: polarization controller 36; a tap coupler

(99:1) 38; then to the tapered fiber 40, via mode matching fiber 42; before finally entering a single mode fiber (SMFS8) 42.

Transform-limited pulses are launched into the chalcogenide taper from a mode-locked fiber laser. The transform limited pulses have full-width half-maximum pulse duration of 1.48 ps, repetition rate of 4 MHz and wavelength of 1545 nm. The pulse peak power is varied via a variable optical attenuator, with peak power of up to 5.7 W incident on the chalcogenide fiber.

The total insertion loss through the taper is 11 dB. Approximately 8 dB of this is due to butt-coupling and propagation loss whilst tapering account for a further 3 dB. The propagation loss of the untapered material is ~ ldB.m^"1. The input and output powers are monitored using optical meters. Using a finite element method, we have calculated the effective area to be A_es - 0.64 μm² and the dispersion β₂ = 59.3 ps²/km (@1545nm).

Results

To characterize the nonlinear pulse propagation, the transmitted power is measured as a function of the input power (ie power transfer function), which is plotted in Fig. 4. The average output versus input average power is not linear indicating nonlinear absorption (TPA) and this allows a value of β= 2.5 x 10^"12 m.W^"1 to be inferred, consistent with references 13, 15 & 16.

Fig. 5 shows the transmitted pulse spectra at the output of the taper at different power level. The dashed lines are simulated SPM by solving the nonlinear Schrodinger equation using split-step Fourier method¹⁷. Numerical simulations use values n₂ = l.l xlO^'13 Cm²W^{1 [13]} and β= 2.5 x 10^"12 m.W^'1. Spectral broadening due to SPM is clearly visible as the peak power is increased from 0.055 W to 5.69 W. The resultant nonlinear coefficient, γ, is 68.4 w'm^"1 from the value of ri2, which corresponds to about 62,000 times enhancement of nonlinearity in our tapered As₂Se₃ fiber compared to that of normal silica fiber (SMFS8 γ ~ 1.1 W¹IcTn^'1).

Discussion

To consider other nonlinear effects, the dispersion is calculated as a function of waist diameter (Fig. 6) at wavelength of 1550nm. For the untapered As₂Se₃ fiber, dispersion is dominated by the material dispersion (β₂ ~ 700 ps²/km at 1550 nm¹⁴' ¹⁵)whilst for tapers with waist diameters below a few microns, the waveguide dispersion dominates. It can be clearly seen for waist diameter of 1.2μm which corresponds to the current experiment, that the dispersion is still in its normal dispersion regime @ 1550 nm, thus super-continuum will not occur. This is consistent with the experimental results. However since operation is near the zero dispersion wavelength, by reducing the taper diameter further to below 1.17 μm it is expected to move the waist into anomalous dispersion regime at 1550nm and thus allow super- continuum to be produced. Similarly, since operation is near the zero dispersion wavelength, it is expected that four-wave mixing gain should occur efficiently and with a very broad bandwidth.

Conclusion In conclusion, the enhancement of the nonlinearity has been demonstrated in tapered As₂Se₃ chalcogenide fiber. The As₂Se₃ was tapered to 1.2 μm diameter with a waist length of 18 mm, and a nonlinear coefficient γ of 68.4 W'm^'1 (or, 68,400 W^" 'km^'1) was measured, which is 60 times greater than in untapered As₂Se₃ fibers and 62,000 times greater than in untapered silica fibers. The group velocity dispersion was also calculated to be zero near this taper diameter. Such strongly enhanced highly nonlinear fiber tapers, with careful optimization of the nonlinearity and dispersion, will open possibilities of nonlinear processes such as super-continuum generation and parametric processes at ultra-small power thresholds.

To be explicit about the way dispersion is engineered by tapering chalcogenide optical fiber, refer now to Fig. 7. Fig. 7(a) is a diagram of a fibre that has been tapered from 20 μm to lμm. In registration below this Fig, is a graph Fig. 7(b) showing the group velocity dispersion (combined material and waveguide dispersion) at 1550 nm along the taper. Below this is a further graph Fig. 7 (c) which shows the group velocity dispersion for the narrow end of the taper 72, and the wide end of the taper 74, along a range of different wavelengths.

The As₂Se₃ has a high positive material dispersion at 1550 nm, as is seen at the

20 μm end of the taper 74. As the taper width is reduced, negative waveguide dispersion is introduced. This negative waveguide dispersion offsets the positive material dispersion and can result in net group velocity dispersion that is negative or near zero, as is seen at the 1 μm end of the taper 72.

Example 2. Planar Waveguide

Introduction

Optical time-division multiplexing (OTDM) of 10-40 Gb/s optical signals is a means of increasing the data-transmission rate per laser wavelength beyond the modulation speed limit of opto-electronic devices to greater than 160 Gb/^s18'23. At the receiver, time-division demultiplexing (DEMUX) is used to extract a tributary channel at lower bit-rate suitable for photo-detection (PD) and bit error rate testing (BERT). Generally, highest performance DEMUX relies on ultra-fast all-optical schemes involving interaction of the optical signal with a co-propagating pump in a nonlinear medium such as optical fiber^18"21, semiconductor optical amplifier (SOA)²² or quasi-phase matched LiNbO₃ ²³. At ultra-high bit rates, optical fiber is favorable for the ultra-fast response of optical Kerr nonlinearity and its wide bandwidth. However, in contrast to the SOA and LiNbO₃ experiments utilizing device lengths of 19 and 30 mm respectively, fiber-based DEMUX ordinarily requires several kilometers length to achieve sufficient nonlinearity¹⁸, which does not lend well to a compact device or photonic integration. This has been improved by advances in highly nonlinear fiber enabling applications with a much shorter length¹⁹, such as recent report of 160 Gb/s DEMUX by four-wave mixing (FWM) in 1 meter Of BiO₂ fiber²⁰.

This example describes an all-optical DEMUX of a 160 Gb/s signal by FWM with co-propagating pump pulses in a low-loss As₂S₃ planar waveguide of just 5 cm length. Despite the waveguide's normal dispersion, the ultra-high nonlinearity in such short length allows phase matching across a wide optical bandwidth for efficient FWM of broadband signals. This is the shortest length demonstration of optical Kerr- effect based nonlinear signal processing at such high bit-rate reported to date. Such short optical waveguide offers potential to integrate optical filters² and realize other nonlinear signal processing functions such as regeneration²⁴, and wavelength conversion²⁵ on a single photonic chip.

Experiment

The 5 cm length waveguide shown in Fig. 8 is fabricated by a customized process²⁴ producing a 3.8 μm wide rib etched 1.2 μm deep in a 2.5 μm thick film of the chalcogenide, As₂S₃, deposited on silica-silicon substrate. The material nonlinearity, n₂, is more than 100 times that of silica at 3xlO^"18 m²/W and the mode effective core area of 5.7 μm² gives a nonlinearity parameter¹⁷ of γ = 2080 W^~ιkπf ' at 1550 nm. The insertion loss of the fiber-coupled waveguide is ~7.5 dB of which waveguide propagation loss constitutes -0.25 dB/cm, so the butt-coupling loss to high NA fiber is -3.1 dB per facet. The waveguide is single-mode and exhibits polarization dependant loss of -IdB between TE and TM modes.

The schematic principle of FWM is illustrated in Fig. 8 for the case of degenerate (single-pump) FWM between an intense wave at frequency f_p (denoted the pump) co-propagating with a weaker wave (signal) at frequency f_s. For a medium exhibiting nonlinearity such as optical Kerr effect (i.e. third-order electric susceptibility), the mixing of both waves generates an idler at a new frequency given by f; = 2f_p - f_s . As reported for fiber experiments¹⁸' ²⁰, applying a pulsed pump with repetition at sub-harmonic of the signal bit-rate and duration shorter than the bit- period performs selective signal channel DEMUX by generating an idler whenever pump and signal pulses coincide.

The experimental set-up is shown in Fig. 9. The 160 Gb/s OTDM signal was generated from an active mode-locked fiber laser (MLFL) emitting 1.4 ps pulses at

40 GHz repetition rate with 1.8 nm bandwidth centered at 1560 nm wavelength. An external electro-optic Mach-Zehnder modulator (MZM) encoded data on the pulses at

40 Gb/s with a 2³¹-~1 pseudo-random bit pattern, and a two-stage fiber interferometer circuit of 2⁷— 1 bit delay-length optically multiplexed (MUX) the pulses up to 160 Gb/s bit-rate. The pulse width at waveguide input was 1.9 ps corresponding to a

30% pulse duty cycle. Fig. 9(b) shows the 160 Gb/s optical eye diagram measured via a 65 GHz bandwidth PD on 80 GHz electrical sampling scope.

The pump source was a 10 GHz MLFL emitting pulses of 1.5 nm bandwidth centered at 1550 nm wavelength and was synchronized to the 160 Gb/s signal by operating from the same 40 GHz rf clock, pre-scaled to 10 GHz. Signal and pump were amplified with EDFAs then combined with a coupler and launched into the waveguide with polarization states aligned via polarization controllers (PC). The pump pulses at coupler output were 1.5 ps wide with average power set to 150 mW. Corresponding peak power coupled into waveguide (P_p) was ~4.4 W. An optical delay line (ΔT) temporally aligned the pump pulse with the desired channel of the 160 Gb/s signal to be demultiplexed. Signal power was set to 100 mW average (~0.3 W peak coupled in) for combined total of 250 mW.

Results and Discussion

The optical spectra at input and output of the waveguide are compared in Fig. 10. Both signal and pump exhibited small spectral distortion and broadening from self-phase modulation (SPM) and cross phase modulation (XPM) owing to the small nonlinear phase shift in the waveguide estimated to be -0.14 π for the pump (from Φ_SPM ⁼ PpT¹L for propagation length, L [9]). This is in contrast to XPM-based DEMUX [2], [4] requiring higher phase-shift (i.e. launch power) of ~π with adequate wavelength separation between signal and pump to accommodate their spectral broadening without overlap.

For the operating conditions described, FWM between pump and signal pulses generates a 10 Gb/s idler signal at ~1540 nm, which is extracted using two optical band-pass filters (BPF) of bandwidth 3 nm and 0.55 nm. The FWM idler power and corresponding DEMUX performance is maximized by optimizing both ΔT and polarization state of pump and co-propagating signal pulses. Efficient FWM also demands adequate phase matching between all optical waves (signal, pump and idler), expressed by the wave vector mismatch parameter, Δβ. In linear terms, the phase mismatch is defined by Δβ = β_s + βi ^~ 2β_p [9] where β_s, βj and β_p are propagation constants for signal, idler and pump respectively, defined as β = ω-n(ω)/c for effective refractive index, n, at angular frequency, ω, and speed of light in vacuum, c. When it is predominantly due to material dispersion, it can also be expressed by a Taylor series expansion as Δβ ~ β₂ Ωs² where β₂ is the second-order dispersion coefficient and Ωs the angular frequency spacing defined Ωs/ 2π = f_p - f_s.= fj - f_p ⁿ. Thus, the coherence length for maintaining phase matching as a function of the pump-signal Ωs is given by L_ooh = 2π/|Δβ| = 2π/{|β₂| Ω_s ²}¹⁷.

The β₂ for the deposited As₂S₃ films is determined from refractive index measurements and polynomial curve fitting to its wavelength differential as 469 ps²/km corresponding to a normal dispersion of — 368 ps/nm.km. In addition to the material dispersion, numerical analysis of the mode profile generated by the Finite Element Method for the physical waveguide dimensions suggests a waveguiding dispersion term of 82 ps/nm.km, which reduces total dispersion to -286 ps/nm.km (β₂ = 364 ps²/km) - similar to highly nonlinear BiO₂ fiber¹⁹' ²⁰. Despite such large value compared to standard optical fiber, the combined high nonlinearity and short device length with respect to the calculated coherence length of 28 cm (for a 10 nm wavelength separation between signal and pump) ensures efficient FWM-DEMUX of the 160 Gb/s signal.

The measured frequency conversion efficiency of the 10 Gb/s channel from

1560 to -1540 nm is calculated to be 12 % from integration of the optical spectra of the input 160 Gb/s signal and output idler and accounting for the 16 times difference in bit-rate. The theoretical conversion efficiency in case of an un-depleted pump (i.e. power of pump » signal) and ignoring propagation loss and assuming overlapping integrals for the cross-sectional mode profiles are same for all waves, is given by G₀ = (γ-P_p/g)²-sin²(g-L) where g is the parametric gain coefficient given by g² = Δβ(Δβ/4 + γP_p)¹⁷. Substituting experimental parameters gives an estimate of G₀ = 15.8% and parametric gain of G_p = G_c+1¹⁷ equal to 0.6 dB.

Numerical modeling of 160 Gb/s FWM-DEMUX by the Split-Step Fourier method¹⁷ using the same experimental parameters for both waveguide and signal (assuming Gaussian pulse shape) without noise or polarization effects generates qualitatively similar spectra as Fig. 10, with G₀ equal to 11 and 7% for propagation loss set to 0 and 0.25 dB/cm respectively.

Eye diagrams for the optically filtered 10 Gb/s output after 160 Gb/s FWM-

DEMUX are shown in Fig. 11. Clear eye openings are observed with just slightly more noise compared to the "back-to-back" (B2B) obtained for the 10 GHz MLFL tuned to 1540 nm wavelength, and data modulated at 10 Gb/s. Such B2B is representative since its bit-error rate (BER) is limited by the performance of the data

MZM and PD - and not the pulse source which is similar high quality for both

MLFLs. The B2B was also filtered by the same BPFs to ensure same signal bandwidth at the receiver. Fig. 12 reveals a small power penalty of <1 dB at BER of

10^~9 for all 16 channels of the 160 Gb/s signal thanks largely to the near instantaneous

Kerr nonlinearity response of the waveguide ensuring low crosstalk.

Conclusions A 5 cm-length low-loss planar waveguide of the chalcogenide glass, As₂S₃, has demonstrated high performance de-multiplexing of a 160 Gb/s signal by FWM, highlighting the potential for Kerr-effect based nonlinear signal processing to be realized at ultra-high bit rates in compact optical devices.

Although the invention has been described with reference to a particular example, it should be appreciated that it could be exemplified in many other forms and in combination with other features not mentioned above.

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Claims

1. A highly non-linear optical device, comprising a chalcogenide waveguide, wherein the dimensions of the waveguide have been adjusted to increase signal intensity within the waveguide and modify dispersion to controllably excite nonlinear optical processes.

2. A device according to claim 1, wherein the core of the waveguide is fabricated from two or more of Arsenic As, Selenium Se, Sulphur S and Germanium Ge.

3. A device according to claim 1 or 2, wherein the adjustment of the dimensions of the waveguide involves reducing the dimensions.

4. A device according to any preceding claim, wherein the waveguide is cylindrical, that is a fibre.

5. A device according to claim 4, wherein the waveguide is fabricated from Arsenic As and Sulphur S.

6. A device according to claim 4 or 5, wherein the diameter of the fibre is reduced by tapering, or by etching fibre cladding.

7. A device according to any one of claims 1 to 3, wherein the waveguide is a planar waveguide that is partially etched.

8. A device according to claim 7, wherein the device is a planar waveguide that is fully etched, or rectangular.

9. A device according to claim 7 or 8, wherein the dimensions of the waveguide are adjusted by lithography or etching techniques.

10. A device according to any preceding claim, wherein the waveguides incorporate any dopants suitable for chalcogenide waveguides.

11. A method for fabricating a highly non-linear optical device as claimed in any preceding claim, comprising the steps of: fabricating a chalcogenide waveguide; adjusting the dimensions of the waveguide to increase signal intensity within the waveguide and modify dispersion to controllably excite nonlinear optical processes.

12. A method according to claim 11, wherein a chalcogenide fiber waveguides has its dimensions adjusted by tapering.

13. A method according to claim 11 or 12, wherein a chalcogenide fiber waveguide made of As and S is tapered by the application of heat and tension so that the fibre stretches and reduces in cross sectional area when the glass temperature significantly exceeds the glass transition temperature.

14. A method according to claim 11, wherin the dimensions of a waveguide made from a planar substrate of homogeneous material comprising As, Se and Ge is controlled using lithography or etching techniques.

15. A four- wave mixing demultiplexing device operable to create a nonlinear interaction, the device comprising a short length of fully- or partially-etched waveguide fabricated by lithography from a planar substrate of a thin film of chalcogenide glass; wherein the combination of the chalcogenide nonlinearity and tight optical confinement provided by the high refractive index and small cross- sectional area of the waveguide results in four-wave mixing or supercontinuum generation.

16. A method of using the device claimed in claim 15 for demultiplexing one data stream from a high bit rate channel in which there is more than one multiplexed data stream, the method comprising the steps of: creating a nonlinear interaction between a synchronous clock pulse at M hertz at an optical carrier frequency f_p and the high bit rate channel at optical carrier frequency f_s to recover the data stream at M bits per second at an optical carrier frequency fj = 2 f_p - f_s;

"dropping" the recovered data stream channel by use of a bandpass filter centred at fi whose pass band excludes frequencies f_p and f_s.