Nonlinear Optical Signal Processing

Mark Pelusi Ben Eggleton
Project Manager: Mark Pelusi Science Leader: Ben Eggleton

Contributing staff: Barry Luther-Davies, Steve Madden, Duk-Yong Choi (ANU),
Students: Neil Baker, Vahid Ta’eed, Mike Lamont (Sydney), Amrita Prasad (ANU)

Project Goals and Motivation
We aim to develop innovative, compact, integrated signal processors to provide cost-effective solutions for next generation ultrahigh-bandwidth networks. Specifically, we aim to demonstrate three crucial component technologies: optical regenerators operating at ultra-high bit rates (greater than 40 Gb/s); wavelength converters for application in reconfigurable optical networks and integrated optical performance monitors for use in high speed dynamic networks for dynamic provisioning and active compensation. The CUDOS innovation and approach is based on novel miniaturized optical signal processing devices fabricated in two dimensional planar substrates of chalcogenide and lithium niobate. These devices offer performance and footprint unmatched by any other platform or technology (Figure 1) and will underpin signal processing solutions of future communication systems. The device physics is based on cross phase modulation, Four Wave Mixing and Raman scattering with dispersion engineering in strongly confined waveguides and resonant elements.

Figure 1

Figure 1. All-optical signal regenerator concept exploiting optical Kerr effecting in nonlinear waveguide (NLWG), which has bandpass optical filter (BPF) integrated in the same device.

CUDOS strategy
Our key strategy and competitive advantage is based on the use of chalcogenide glass. Optical waveguides made from this material combine high refractive index, large third order nonlinearity and good photosensitivity as well as low loss across the telecommunication wavelength band. These features enable unprecedented signal processing functionality based on the optical Kerr effect to be achieved in compact waveguide devices.

We have unique skills and facilities to undertake this work. ANU combines expertise and world-class facilities for the fabrication of novel glasses; studies of their basic physical and optical properties; film production and characterization; and film processing to create state-of-the art low loss optical waveguides with ultra-high nonlinearity (Figure 2).

Figure 2

Figure 2. Track coating and lithography tools for chalcogenide waveguide fabrication.

At Sydney, nonlinear signal processing for all-optical regeneration and optical performance monitoring are being investigated by first using commercially available fibres as preparation to implementing these functions in compact waveguide devices. The optical Bragg grating filters written into the waveguides draw on extensive in-house expertise for writing optical Bragg grating filters in optical materials. A custom laser optic system writes strong filters in chalcogenide waveguides by exploiting the photosensitivity of the refractive index.

The performance of the integrated devices in all-optical applications of optical signal wavelength conversion and regeneration is tested at Sydney using a high-speed 160 Gb/s optical communication system (Figure 3).

Figure 3aFigure 3b

Figure 3. 160 Gb/s facility and eye diagram of 160 Gb/s optical signal measured on high-speed sampling-oscilloscope for testing bit-error rate performance of nonlinear optical devices.

Recent Achievements and Highlights
Chalcogenide glass waveguides: We studied the composition of glasses to optimise nonlinearity, absorption loss, glass transition temperature and stability. In doing so, we gained valuable insight into how the Ge-As-Se based glass structure evolves. Raman spectra measurements showed that as the Ge content increases, the glass evolves from a distinct layer stack structure with low transition temperature to a more uniform 3 dimensional structure with higher transition temperature. We succeeded in adjusting the As and Ge content for AMTIR-1 glass to achieve significant enhancement of the glass nonlinearity at 1550 nm.

Wall roughness is a critical determinant of the overall waveguide loss. We developed two different approaches for reducing wall roughness, namely, changing the plasma etch chemistry and re-coating a thin film of As2S3 after etching. Optical microscopy verified that the surface roughness for either approach was reduced by a factor of two. We calculate that this will reduce the waveguide loss by a factor of three.

The attack of As2S3 films by standard photo-resist developers was mitigated by two new approaches- first, by applying a thin (~50 nm) layer of more robust glass, namely AMTIR-1, on top of As2S3 prior to processing, and secondly, using developer resistant back AR coating (BARC) as a protective 200 nm thick film below the photo-resist. After development of the photo-resist, the BARC is removed by oxygen prior to etching the chalcogenide films.

Signal processing in fibre: Nonlinear signal processing experiments with nonlinear devices based on waveguide as well as fibre platforms were carried out. We conducted a detailed numerical study of the influence of two-photon absorption (TPA) on all-optical regeneration and showed that the performance of the self-phase-modulation-based regenerator could actually benefit from TPA for improving 40 Gb/s optical signal [1], by smoothing the nonlinear power transfer function. This surprising observation contrasts with the detrimental effect of TPA on nonlinear switching.

Theoretical and experimental studies also proved that a regenerator based on self-phase modulation can achieve 3 R regeneration (re-amplification, re-shaping, and re-timing), and simultaneously improve the signal bit error rate even when placed directly before the optical receiver [2], in contrast to other power-transfer function based regenerators.

In fibre based experiments, a novel method for all-optical performance monitoring of optical signal-to-noise ratio (OSNR) was demonstrated using a nonlinear optical loop mirror [3] (Figure 4a). Experimental results (Figure 4b) showed the nonlinear power transfer function can discriminate OSNR over a dynamic range of more than 25 dB for 40 Gb/s signals with different data modulation formats such as non-return-to-zero, carrier-suppressed return-to-zero, and return-to-zero (RZ).

Figure 4a

Figure 4a. An OSNR monitor schematic using a highly nonlinear fibre (HNLF) and variable optical attenuator (VOA) connected in a nonlinear optical fibre loop mirror. For a given average input power, the output average optical power measured on the slow photodetector (PD) will depend on OSNR.

Figure 4b

Figure 4b. Measured performance of OSNR for 40 Gb/s RZ signal by measuring average output power from NOLM.

In another fibre based experiment, the all optical wavelength conversion technique (Figure 5a) was demonstrated using a 1 metre length of As2Se3 chalcogenide glass fibre whose ultra-high Kerr nonlinearity is higher than any other fibre. Bit error rate measurements at 10 Gb/s showed only 1.4 dB system penalty for wavelength conversion of the 7 ps pulses over 10 nm range around 1550 nm [4] (Figure 5b).

Figure 5a

Figure 5a. General principle of all-optical wavelength conversion by using cross phase modulation of signal pulse onto cw probe and then filtering broadened spectrum at desired output wavelength.

Figure 5b

Figure 5b. Measured 10 Gb/s bit error rate performance of wavelength conversion in 1m As2Se3 fibre with only 1.4 dB power penalty against back to back measurement at 1550 nm.

All-optical wavelength conversion of a 5.4 ps optical pulse over a 10 nm wavelength range around 1550 nm was also demonstrated in a 5 cm long As2S3 chalcogenide glass rib waveguide [5]. Frequency resolved optical gating (FROG) measurements showed good converted pulse quality, no limitation to the conversion range of the device from waveguide dispersion.

A high quality sampled Bragg grating was written into a highly photosensitive chalcogenide (As2S3) rib-waveguide using a custom built scanning Sagnac interferometer system [6] (Figure 6). The induced refractive index change of the waveguide was estimated to be over 0.03, and the corresponding waveguide grating (Figure 7) exhibited comparable strength and bandwidth to the best sampled gratings ever produced in silica optical fibre.

Figure 6

Figure 6. Schematic of custom built scanning Sagnac interferometric system for writing Bragg gratings in waveguides.

Figure 7

Figure 7. Mechanism of introducing a defect into a chalcogenide integrated waveguide grating structure to obtain a phase-shifted grating and (b) experimental transmission spectrum (solid curve) of the resulting phase-shifted grating (TM polarization) versus the spectrum obtained from modeling (dashed-dotted curve).

References
[1] Lamont MRE, Rochette M, Moss DJ, Eggleton BJ Two-Photon Absorption Effects on Self-Phase-Modulation-Based 2R Optical Regeneration IEEE PHOTONICS TECHNOLOGY LETTERS 18, 1185-1187 2006
[2] Rochette M, Blows JL, Eggleton BJ 3R optical regeneration: an all-optical solution with BER improvement OPTICS EXPRESS 14, 6414-6427 2006
[3] Adams R, Rochette M, Ng TT, Eggleton BJ All-Optical In-Band OSNR Monitoring at 40 Gb/s Using a Nonlinear Optical Loop Mirror IEEE PHOTONICS TECHNOLOGY LETTERS 18, 469-471 2006
[4] Ta'eed VG, Fu L, Pelusi M, Rochette M, Littler IC, Moss DJ, Eggleton BJ Error free all optical wavelength conversion in highly nonlinear As-Se chalcogenide glass fiber OPTICS EXPRESS 14, 10371-10376 2006
[5] Ta'eed VG, Lamont MRE, Moss DJ, Eggleton BJ, Choi DY, Madden S, Luther-Davies B All optical wavelength conversion via cross phase modulation in chalcogenide glass rib waveguides OPTICS EXPRESS 14, 11242-11247 2006
[6] Baker NJ, Lee HW, Littler IC, de Sterke CM, Eggleton BJ, Choi DY, Madden S, Luther-Davies B Sampled Bragg gratings in chalcogenide (As2S3) rib-waveguides OPTICS EXPRESS 14, 9451-9459 2006