Optical phase conjugation by an As<sub>2</sub>S<sub>3</sub> glass planar waveguide for dispersion-free transmission of WDM-DPSK signals over fiber

doi:10.1364/OE.18.026686

Optical phase conjugation by an As₂S₃ glass planar waveguide for dispersion-free transmission of WDM-DPSK signals over fiber

M.D. Pelusi, F. Luan, D.-Y. Choi, S.J. Madden, D.A.P. Bulla, B. Luther-Davies, and B.J. Eggleton »View Author Affiliations

Optics Express, Vol. 18, Issue 25, pp. 26686-26694 (2010)
http://dx.doi.org/10.1364/OE.18.026686

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▸ Article Outline
– Abstract
1. Introduction
2. Waveguide characteristics
3. Transmission experiments and results
4. Discussion
Conclusions
– References and links

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Abstract

We report the first demonstration of optical phase conjugation (OPC) transmission of phase encoded and wavelength-division multiplexed (WDM) signals by the Kerr effect in a planar structured waveguide. The phase conjugated electric field of the signal is produced by four wave mixing pumped by a CW laser during co-propagating with the signal in a highly nonlinear waveguide fabricated in As₂S₃ glass. Experiments demonstrate the capability of the device to perform dispersion-free transmission through up to 225 km of standard single mode fiber for a 3 × 40 Gb/s WDM signal, with its channels encoded as return-to-zero differential phase shift keying and spaced either 100 or 200 GHz apart. This work represents an important milestone towards demonstrating advanced signal processing of high-speed and broadband optical signals in compact planar waveguides, with the potential for monolithic optical integration.

1. Introduction

The growing demand for bandwidth in optical communication networks motivates research into advanced techniques to address the limits on systems bandwidth, energy and footprint. All-optical signal processing with nonlinear optics has been shown to enable the higher-speed applications such as switching [1

T. D. Vo, H. Hu, M. Galili, E. Palushani, J. Xu, L. K. Oxenløwe, S. J. Madden, D.-Y. Choi, D. A. P. Bulla, M. D. Pelusi, J. Schröder, B. Luther-Davies, and B. J. Eggleton, “Photonic chip based transmitter optimization and receiver demultiplexing of a 1.28 Tbit/s OTDM signal,” Opt. Express 18(16), 17252–17261 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-16-17252. [CrossRef] [PubMed]

]; wavelength conversion [2

J. Yamawaku, H. Takara, T. Ohara, K. Sato, A. Takada, and T. Morioka, “isJ. Yamawaku, H. Takara, T. Ohara, K. Sato, A. Takada, T. Morioka, O. Tadanaga, H. Miyazawa, and M. Asobe, “Simultaneous 25 GHz-spaced DWDM wavelength conversion of 1.03 Tbit∕s (103×10 Gbit∕s) signals in PPLN waveguide,” Electron. Lett. 39(15), 1144 (2003). [CrossRef]

–11

M. D. Pelusi, F. Luan, S. Madden, D.-Y. Choi, D. A. Bulla, B. Luther-Davies, and B. J. Eggleton, “Wavelength conversion of high-speed phase and intensity modulated signals using a highly nonlinear chalcogenide glass chip,” IEEE Photon. Technol. Lett. 22(1), 3–5 (2010). [CrossRef]

]; regeneration [12

S. Watanabe, “Optical signal processing using nonlinear fibers,” J. Opt. Fiber. Commun. Rep. 3(1), 1–24 (2005). [CrossRef]

–17

S. Ayotte, H. Rong, S. Xu, O. Cohen, and M. J. Paniccia, “Multichannel dispersion compensation using a silicon waveguide-based optical phase conjugator,” Opt. Lett. 32(16), 2393–2395 (2007). [CrossRef] [PubMed]

]; and performance monitoring [1

], [18

Z. Pan, C. Yub, and A. E. Willner, “Optical performance monitoring for the next generation optical communication networks,” Opt. Fiber Technol. 16(1), 20–45 (2010). [CrossRef]

]. A widely studied regeneration scheme is optical phase conjugation (OPC), which has the unique capability to all-optically compensate both chromatic dispersion and Kerr effect nonlinearities from long distance optical fiber transmission in a single operation [12

S. Watanabe, “Optical signal processing using nonlinear fibers,” J. Opt. Fiber. Commun. Rep. 3(1), 1–24 (2005). [CrossRef]

, 13

S. L. Jansen, D. van den Borne, P. M. Krummrich, S. Spälter, G.-D. Khoe, and H. de Waardt, “Long-haul DWDM transmission systems employing optical phase conjugation,” IEEE J. Sel. Top. Quant. 12(4), 505–520 (2006). [CrossRef]

]. OPC involves generating the phase conjugate of the electric field of the signal at a specific point along a fiber transmission link so that the accumulative deterministic phase distortions induced by the fiber spans before and after OPC are closely matched, and can thereby be optimally compensated. This is illustrated in Fig. 1(a) for the case of dispersion, whereby OPC reverses the sign of the linear group delay versus frequency induced by fiber dispersion. Note, OPC does not change the sign of the quadratic group delay that arises from third order dispersion due to the non-zero fiber dispersion slope, and so it fails to compensate this effect. Nevertheless, OPC can in principle eliminate the need for conventional dispersion-compensation modules (DCMs), which must otherwise be scaled to provide an equal but opposite dispersion for the entire fiber link. It can also relax the maximum launch power limit for avoiding nonlinear effects in transmission, which in turn can allow a wider spacing between optical amplifiers. OPC also has the capability to process a large number of WDM signal channels simultaneously in a single operation [2

, 13

], and with transparency to higher bit-rates [4

I. Brener, B. Mikkelsen, G. Raybon, R. Harel, K. Parameswaran, J. R. Kurz, and M. M. Fejer, “160 Gbit/s wavelength shifting and phase conjugation using periodically poled LiNbO₃ waveguide parametric converter,” Electron. Lett. 36(21), 1788–1790 (2000). [CrossRef]

, 5

H. Hu, R. Nouroozi, R. Ludwig, B. Huettl, C. Schmidt-Langhorst, H. Suche, W. Sohler, and C. Schubert, “Polarization-insensitive all-optical wavelength conversion of 320 Gb/s RZ-DQPSK signals using a Ti:PPLN waveguide,” Appl. Phys. B 101(4), 875–882 (2010), doi:. [CrossRef]

, 7

H. Furukawa, A. Nirmalathas, N. Wada, S. Shinada, H. Tsuboya, and T. Miyazaki, “Tunable all-optical wavelength conversion of 160-Gb/s RZ optical signals by cascaded SFG-DFG generation in PPLN waveguide,” IEEE Photon. Technol. Lett. 19(6), 384–386 (2007). [CrossRef]

, 11

, 14

H. Hu, R. Nouroozi, R. Ludwig, C. Schmidt-Langhorst, H. Suche, W. Sohler, and C. Schubert, “110 km transmission of 160 Gbit/s RZ-DQPSK signals by midspan polarization-insensitive optical phase conjugation in a Ti:PPLN waveguide,” Opt. Lett. 35(17), 2867–2869 (2010). [CrossRef] [PubMed]

], and more advanced data modulation formats beyond on-off keying (OOK) [5

, 6

X. Wu, W.-R. Peng, V. Arbab, J. Wang, and A. Willner, “Tunable optical wavelength conversion of OFDM signal using a periodically-poled lithium niobate waveguide,” Opt. Express 17(11), 9177–9182 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-11-9177. [CrossRef] [PubMed]

, 11

, 13

–15

P. Minzioni, V. Pusino, I. Cristiani, L. Marazzi, M. Martinelli, C. Langrock, M. M. Fejer, and V. Degiorgio, “Optical phase conjugation in phase-modulated transmission systems: experimental comparison of different nonlinearity-compensation methods,” Opt. Express 18(17), 18119–18124 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-17-18119. [CrossRef] [PubMed]

]. Its adoption can therefore avoid the system cost, complexity and bandwidth limitations of processing each channel by digital electronics, as well as the necessary demultiplexing, demodulation and photo-detection of each channel. This is important as the transmitter/receiver complexity grows to accommodate higher bit-rates of 100 Gb/s and beyond with more spectrally efficient data modulation formats [19

G. Wellbrock and T. J. Xia, “The road to 100g deployment [Commentary],” IEEE Commun. Mag. 48(3), S14–S18 (2010). [CrossRef]

Fig. 1 (a) Schematic of optical phase conjugation (OPC) of the signal at a point along an optical fiber transmission link for the purpose in this case of cancelling the accumulated dispersion of both links. (b) χ⁽³⁾ based FWM pumped by a CW laser for generating the signal phase conjugate at the wavelength, λ_i . (c) Images of (top) waveguide coupled to lensed fibers and (below) cross-section.

OPC has been demonstrated on a variety of platforms including periodically poled lithium niobate (PPLN) [2

–7

, 13

–15

]; semiconductor optical amplifiers (SOAs) [16

J. Inoue, H. Sotobayashi, W. Chujo, and H. Kawaguchi, “80 Gbit/s conventional and carrier-suppressed RZ signals transmission over 200 km standard fiber by using mid-span optical phase conjugation (invited, OECC Awarded),” IEICE Trans. on Comm. E 86-B, 1555–1561 (2003).

]; highly nonlinear optical fiber (HNLF) [12

S. Watanabe, “Optical signal processing using nonlinear fibers,” J. Opt. Fiber. Commun. Rep. 3(1), 1–24 (2005). [CrossRef]

]; and more recently, silicon chip waveguides [17

]. Generally, these schemes involve co-propagating the signal with a CW beam at different frequency to pump a nonlinear process, which produces a new (idler) electrical field, A_i , of amplitude A_i ∝ A_p ² ∙A_s*, where A_p and A_s* are the electrical field amplitude of the pump wave and phase conjugated signal, respectively. In the case of PPLN, the operation is efficiently performed by a cascaded χ⁽²⁾ process in order to produce A_i in the wavelength vicinity of A_s . One approach uses the frequency doubled wave produced from second harmonic generation (SHG) of an input CW pump to simultaneously drive difference frequency generation (DFG) with the signal [3

M. Chou, I. Brener, M. M. Fejer, E. E. Chaban, and S. B. Christman, “1.5-μm-band wavelength conversion based on cascaded second-order nonlinearity in LiNbO₃ waveguides,” IEEE Photon. Technol. Lett. 11(6), 653–655 (1999). [CrossRef]

–5

]. An alternative dual pump scheme uses the high frequency wave produced by sum-frequency generation (SFG) between the input CW pump and signal, to simultaneously undertake DFG with a second input CW pump whose frequency is in the vicinity of the first [6

], [7

]. While the single CW pump approach exhibits broadband flat conversion efficiency, the wavelength tuning flexibility of A_i is limited since the CW source wavelength must be near the specific phase matching wavelength of the device, which depends on its physical parameters, and the operating temperature. The dual pump scheme relaxes this requirement, but the conversion bandwidth is more limited.

Alternatively, wavelength flexible and broadband OPC can be performed using four-wave mixing (FWM) via the χ⁽³⁾ nonlinearity from the nonlinear gain dynamics of SOAs [16

], or the optical Kerr effect in HNLF [12

S. Watanabe, “Optical signal processing using nonlinear fibers,” J. Opt. Fiber. Commun. Rep. 3(1), 1–24 (2005). [CrossRef]

], planar silicon [8

B. G. Lee, A. Biberman, A. C. Turner-Foster, M. A. Foster, M. Lipson, A. L. Gaeta, and K. Bergman, “Demonstration of broadband wavelength conversion at 40 Gb/s in silicon waveguides,” IEEE Photon. Technol. Lett. 21(3), 182–184 (2009). [CrossRef]

, 9

W. Mathlouthi, H. Rong, and M. Paniccia, “Characterization of efficient wavelength conversion by four-wave mixing in sub-micron silicon waveguides,” Opt. Express 16(21), 16735–16745 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-21-16735. [CrossRef] [PubMed]

, 17

], or chalcogenide [11

] waveguides. In this case, A_i is generated at the wavelength λ_i ⁻¹ = 2λ_p ⁻¹ – λ_s ⁻¹ , where λ_p and λ_s are the wavelengths of the pump and signal, respectively, as shown schematically in Fig. 1(b). While HNLF has its practical advantages including an ultra-fast response and passive operation, its relatively long length can complicate integration, and make the bandwidth for phase matched FWM sensitive to the zero dispersion wavelength and its longitudinal variations [12

S. Watanabe, “Optical signal processing using nonlinear fibers,” J. Opt. Fiber. Commun. Rep. 3(1), 1–24 (2005). [CrossRef]

, 20

S. Moro, E. Myslivets, J. R. Windmiller, N. Alic, J. M. Chavez Boggio, and S. Radic, “Synthesis of equalized broadband parametric gain by localized dispersion mapping,” IEEE Photon. Technol. Lett. 20(23), 1971–1973 (2008). [CrossRef]

]. Nevertheless, broadband FWM using a short piece of HNLF spooled onto a coin sized device has been reported [21

J. M. Chavez Boggio, S. Zlatanovic, F. Gholami, J. M. Aparicio, S. Moro, K. Balch, N. Alic, and S. Radic, “Short wavelength infrared frequency conversion in ultra-compact fiber device,” Opt. Express 18(2), 439–445 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-2-439. [CrossRef] [PubMed]

]. Silicon and SOAs offer an even more compact solution and photonic integration potential, however, their performance can be impaired by free carriers, which can cause both power saturation of A_i , and signal distortion [9

]. On the other hand, chalcogenide glasses such as the widely studied As₂S₃ composition, have very low two photon absorption (TPA), and no free carrier effects [22

M. R. Lamont, C.M de Sterke, and B.J. Eggleton, “Dispersion engineering of highly nonlinear As₂S₃ waveguides for parametric gain and wavelength conversion,” Opt. Express 15, 9458–9463 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-15-9458. [CrossRef] [PubMed]

]. Thus far, the capability for OPC transmission via the Kerr nonlinearity in planar waveguides has only been evaluated for silicon devices in the case of a four channel WDM signal of low bit-rate (4 × 10 Gb/s), and OOK data format over a 320 km fiber link [17

In this paper, we report the first demonstration of optical phase conjugation (OPC) transmission of phase encoded and wavelength-division multiplexed (WDM) signals by the Kerr effect in a planar waveguide fabricated in As₂S₃ glass, and apply it to dispersion compensate a long distance optical fiber transmission link. The experiments based on a 6 cm long waveguide demonstrate dispersion-free transmission over as much as 225 km of standard single mode optical fiber (SSMF) for a 3 × 40 Gb/s WDM signal encoded as return-to-zero (RZ), differential phase shift keying (DPSK) format, and with 100 and 200 GHz channel spacing. The performance shown draws on various key features of the waveguide, namely, (i) its combined high nonlinear index, n₂ , low TPA, and reduced effective mode area (A_eff ) allowing a high nonlinear response with moderate launch powers into a short, cm-scale length device, (ii) a dispersion-shifted waveguide design for allowing broadband phase-matched FWM, (iii) improved power handling and low propagation loss, and (iv) anti-reflection coated end facets to minimize Fabry-Perot interference effects for the narrow linewidth CW pump source.

2. Waveguide characteristics

The fabrication of the 6 cm long waveguide involved deposition of a 0.85 μm thick film of As₂S₃ by thermal evaporation onto a silica-on-silicon substrate. The deposited films were formed into a series of 6 cm long, 2 μm wide ribs by creating the surrounding 0.35 μm deep trenches using standard photolithography and dry-etching [23

S. J. Madden, D.-Y. Choi, D. A. Bulla, A. V. Rode, B. Luther-Davies, V. G. Ta’eed, M. D. Pelusi, and B. J. Eggleton, “Long, low loss etched As(₂₎S(₃) chalcogenide waveguides for all-optical signal regeneration,” Opt. Express 15(22), 14414–14421 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-22-14414. [CrossRef] [PubMed]

, 24

D.-Y. Choi, S. Madden, D. A. Bulla, R. Wang, A. Rode, and B. Luther-Davies, “Submicrometer-thick low-loss As₂S₃ planar waveguides for nonlinear optical devices,” IEEE Photon. Technol. Lett. 22(7), 495–497 (2010). [CrossRef]

]. A low refractive index inorganic polymer glass (RPO Pty Ltd, IPG) was applied as an over-cladding. The high index of the As₂S₃ core (n₀ ≈2.38 at 1550 nm) led to a large index contrast with the cladding, and allowed the effective mode area, A_eff , to be reduced to ≈1.2 μm². This, combined with the high n₂ of As₂S₃ glass (≈110 times silica), enhanced the nonlinearity coefficient defined as γ = 2π·n₂/λ·A_eff at the optical wavelength, λ = 1550 nm to ≈9,900 W⁻¹km⁻¹. This is on the order of 500 times larger than for silica based HNLF [25

M. Takahashi, R. Sugizaki, J. Hiroishi, M. Tadakuma, Y. Taniguchi, and T. Yagi, “Low-loss and low-dispersion-slope highly nonlinear fibers,” J. Lightwave Technol. 23(11), 3615–3624 (2005). [CrossRef]

]. The small cross-sectional area of the rib also increased the waveguide dispersion for the TM mode due to the increased field penetration of the low index cladding. Importantly, the induced dispersion has an opposite sign to the large material dispersion of As₂S₃ glass (−364 ps/nm/km), leading to a drastic reduction in the net total dispersion to ≈28 ps/nm.km at 1550 nm [10

F. Luan, M. D. Pelusi, M. R. E. Lamont, D.-Y. Choi, S. Madden, B. Luther-Davies, and B. J. Eggleton, “Dispersion engineered As(₂₎S(₃) planar waveguides for broadband four-wave mixing based wavelength conversion of 40 Gb/s signals,” Opt. Express 17(5), 3514–3520 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-5-3514. [CrossRef] [PubMed]

], and a reversal of its sign from normal to anomalous. The dependence of dispersion on the rib dimensions has been determined by numerical modeling [22

]. Anti-reflection (AR) coatings based on SiO₂/TiO₂ were applied to the hand-cleaved end facets of the waveguide to eliminate Fabry-Perot resonances within the chip. Images of the device are shown in Fig. 1(c).

The capability of the device for broadband phase-matched FWM over a wide ranging (λ_p −λ_s ) was previously characterized by using a 40 Gb/s intensity modulated signal to pump FWM with a weak CW probe [10

]. The results demonstrated broadband wavelength conversion across the S, C and L bands of the optical communication spectrum. Tunable wavelength conversion of a single channel 40 Gb/s RZ-DPSK signal was also demonstrated with preservation of the phase encoding by pumping a FWM process with a CW laser [11

3. Transmission experiments and results

The application of the device to OPC transmission of phase encoded WDM signals was investigated with the set-up shown in Fig. 2 . The transmitter (Tx) is shown in Fig. 2(a) and generated the 3 × 40 Gb/s WDM signals by incorporating a WDM multiplexer (MUX) to combine three CW lasers positioned on the ITU grid, with a channel spacing, Δf, of either 100 or 200 GHz. All three channels were simultaneously modulated by a pair of single drive, 40 Gb/s LiNbO₃ Mach-Zehnder (MZ) modulators – one to carve each beam into a pulse train of 33% pulse duty cycle, and the other to encode a 40 Gb/s DPSK pseudo random bit sequence (PRBS) of 2³¹−1 pattern length. The PRBS phase modulation of the electric field over time, t, is ideally represented as exp(jϕ(t)) for ϕ(t) = {0, π}. The optical spectrum of the WDM signal in the case of Δf = 200 GHz is shown in Fig. 3(a) . The receiver (Rx) with the set-up shown in Fig. 2(c) comprised of an EDFA, a DPSK demodulator (DEMOD) comprised of an interferometer with free spectral range (FSR) of 43 GHz, and a bandpass optical filter (BPF) with 0.55 nm bandwidth (and a tuneable centre wavelength to extract the desired channel), and a 40 Gb/s photo-receiver (PD). It is noted that using a DEMOD with slightly higher FSR than the signal bit-rate can improve the dispersion tolerance of DPSK signals [26

Y. K. Lizé, X. Wu, M. Nazarathy, Y. Atzmon, L. Christen, S. Nuccio, M. Faucher, N. Godbout, and A. E. Willner, “Chromatic dispersion tolerance in optimized NRZ-, RZ- and CSRZ-DPSK demodulation,” Opt. Express 16(6), 4228–4236 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-6-4228. [CrossRef] [PubMed]

Fig. 2 Experimental set-ups for the (a) 3 × 40 Gb/s RZ DPSK signal transmitter (Tx) with either 100 or 200 GHz channel spacing, (b) OPC of the input WDM signal in a As₂S₃ waveguide via FWM pumped by a co-propagating CW laser at different wavelength, and (c) 225 km long link of SSMF incorporating the OPC circuit from (b) at the 105 km point.

Fig. 3 Optical spectra of (a) 3 × 40 Gb/s RZ DPSK WDM signal with 200 GHz channel spacing at input and output of the 225 km long SSMF link (including OPC) for center wavelengths of 1560.61 nm, and 1533.39 nm, respectively, and a resolution bandwidth (RBW) of 0.07 nm in both cases, and (b) input and output of As₂S₃ waveguide at the 105 km point, measured with RBW = 0.2 nm and reference power level arbitrary set to offset traces for clarity.

3.1 OPC in a 225 km link

The 3 × 40 Gb/s RZ-DPSK signal with Δf = 200 GHz was transmitted through the 225 km link by the set-up shown in Fig. 2(c). In this case, the WDM signal channels, denoted 1, 2 and 3, had centre wavelengths matched to the ITU grid at 1558.98 nm, 1560.61 nm, and 1562.16 nm, respectively. The position for OPC was designed to ensure the chromatic dispersion of Ch. 2 in the preceding SSMF would be optimally cancelled by the following link in the case of wavelength conversion by OPC to λ_i = 1533.39 nm for λ_p = 1547 nm. The transmission link of SSMF preceding the OPC had a total length of 105 km, and a launch power of 16 mW (≈5.3 mW per channel). An EDFA positioned after 63 km boosted the total power to 25 mW.

The OPC set-up is shown in Fig. 2(b), and used a CW external-cavity laser diode, which was amplified and coupled with the signal into the waveguide via lensed fibers. A WDM coupler was inserted before the waveguide as a fixed wavelength BPF to reject amplifier noise outside the 1546-1565 nm band. Another was inserted at the output to isolate the FWM idler.

The total average launch power at the input connector of the waveguide was 400 mW, and the power ratio of the CW laser to signal was 7.8 dB, which was optimized to maximize A_i . The polarization state of both the signal and CW pump were aligned using polarization controllers (PC) in order to couple into the TM mode of the waveguide. The total insertion loss for the TM mode measured from the waveguide input to output fiber connector was 13 dB, which was mainly comprised of nearly 5 dB loss per facet for coupling to SSMF. This translates to a total power within the waveguide of ≈130 mW. Figure 3(b) compares the optical spectrum at both the waveguide input and output, showing the generation of the three idler channels from between 1532 and 1535 nm. The power ratio of the combined idler to signal channels at the waveguide output was ≈−26 dB. From this, the FWM conversion efficiency, η, defined as the power ratio of the total launched input signal to output idler) is obtained by deducting the 13 dB insertion loss of the device (in dB units).

The waveguide output was transmitted through a further 120 km of SSMF with a total average launch power of 49 mW, corresponding to 16.3 mW per channel. The nonlinearity tolerance of the OPC link for such high channel powers is under investigation. An EDFA inserted after 75 km, boosted the total power to 5 mW. The optical spectrum of the WDM signal after link transmission (at the DEMOD input of the Rx) is shown in Fig. 3(a). Figure 4(b) compares the intensity eye diagram of the received signal (center channel) after DPSK demodulation to the “back to back” (B2B) case with the SSMF link (including OPC) bypassed. The bit-error rate (BER) measurements shown in Fig. 4(c) indicated a power penalty of 2-2.5 dB for all three channels at a BER of 10⁻⁹. The dispersion intolerance of the signal to transmission in SSMF, and the effectiveness of the OPC operation are highlighted in Fig. 4(a) by showing the dramatic distortion of the signal eye diagram for transmission of Ch. 3 alone through just 2 km of SSMF.

Fig. 4 Fiber transmission performance of 3 × 40 Gb/s RZ-DPSK WDM signal with 200 GHz channel spacing. Eye diagrams of (a) single channel at input and output of 2 km long SSMF (without DPSK demodulation), (b) WDM signal channels at the input and output of the 225 km long link with OPC and DPSK demodulation. (c) Bit error rate (BER) for each WDM signal channel compared to their “back to back” (B2B) case of both 225 km fiber link and OPC excluded.

3.2 OPC in a 162 km link

The OPC transmission of a 3 × 40 Gb/s RZ-DPSK WDM signal with a narrower Δf = 100 GHz was also investigated using the similar set-up shown in Fig. 5(a) . In this case, the channel wavelengths were set to 1558.16 nm, 1558.99 nm, and 1559.8 nm, denoted channels 1, 2 and 3 respectively. It is noted that the back-to-back performance for a 100 GHz channel spacing was slightly worse than for 200GHz, which required shortening the transmission link distance to 162 km in order to achieve a BER of less than 10⁻⁹ for an electrical signal from the receiver of similar amplitude. (Note, a 6dB attenuator was also attached to the PD output in this experiment, which explains the shift in the receiver sensitivity). The link consisted of two spans of SSMF. Optimum dispersion free transmission was achieved by performing OPC after the first 75 km span of SSMF, using the same OPC set-up shown in Fig. 2(b), and with λ_p = 1547 nm. The total average launch power into both SSMF spans was ≈10 mW, and the total average launch power at the input connector of the waveguide was slightly increased to ≈460 mW, with a similar ratio for the CW pump and signal as previously.

Fig. 5 OPC transmission of 3 × 40 Gb/s RZ DPSK signal with 100 GHz channel spacing in a 162 km link of SSMF (a) Experimental set-up, and signal optical spectrum at (left) transmitter output, and (right) input to DPSK demodulator in the Rx (RBW = 0.07 nm). (b) Optical spectrum at output of As₂S₃ waveguide at 75 km point of the link for performing OPC. (RBW = 0.2 nm, and arbitrary reference power level). (c) Signal eye diagrams, and Ch. 2 BER performance for OPC only, and OPC plus 162 km link transmission, compared to B2B.

The optical spectrum at both the input and output of the link are shown in Fig. 5(a). The spectrum at the As₂S₃ waveguide output is shown in Figs. 5(b), and reveals the generation of the phase conjugated signal at ≈1535 nm. In this case, the power ratio of the idler to signal at the waveguide output was ≈−29 dB due to slightly different operating conditions. The 40 Gb/s intensity eye diagrams for each channel of the WDM signal at the receiver output are shown in Fig. 5(c). The clear eye opening shows that dispersion-free transmission is achieved for all channels by the OPC operation. The B2B case for the center channel where both the link and OPC are bypassed is also shown for comparison. Figure 5(c) plots the BER performance, and shows the power penalty relative to the B2B case is 1.6 dB for OPC alone, and 2.1 dB for the combined 162 km link with OPC. The OSNR at the receiver was approximately 14 dB for the centre channel in both transmission experiments.

4. Discussion

Enabling a longer transmission reach or transmitting more WDM channels would rely upon improving the OSNR at the link output. This in turn requires boosting η for the OPC device, which can be achieved by taking advantage of the quadratic power dependence of the idler to the CW wave in the FWM process. Thus, significant gains can be expected from just reducing the ≈13 dB insertion loss of the waveguide, without further increasing the launch power. The most significant contribution to the loss is from coupling, which amounted to ≈5 dB/facet for coupling from SSMF to the waveguide using lensed fibers with a ~2.5 μm spot diameter. However, this could be drastically reduced by incorporating on-chip tapers as shown by the highly efficient coupling to SSMF of less than 1 dB loss per facet for silicon nanowires [27

T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3 μm square Si wire waveguides to singlemode fibres,” Electron. Lett. 38(25), 1669–1670 (2002). [CrossRef]

Reducing propagation losses is another route for improving η. For the sample used in our experiment, this was ≈0.5 dB/cm for the TM mode. Recent optimization of the fabrication process has yielded lower losses of 0.3 dB/cm for the latest waveguide samples of similar length and rib dimension [24

]. This was achieved by minimizing surface roughening through optimizing the thermal annealing temperature of the deposited As₂S₃ films (to avoid film evaporation at excessive temperatures, while allowing sufficient film relaxation), and developing new protective layers for the lithographic process (to prevent chemical attack of As₂S₃ during resist processing and stripping). Another option is to fabricate waveguides from more highly nonlinear chalcogenide glass compositions, such as the ternary Ge-As-Se system, whose n₂ is almost triple that of As₂S₃. Nanowire waveguides with very small A_eff have been fabricated from Ge_11.5As₂₄Se_64.5 to achieve over ten times larger γ than for the As₂S₃ chip [28

X. Gai, S. Madden, D.-Y. Choi, D. Bulla, and B. Luther-Davies, “Dispersion engineered Ge11.5As24Se64.5 nanowires with a nonlinear parameter of 136 W⁻¹m⁻¹ at 1550 nm,” Opt. Express 18(18), 18866–18874 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-18-18866. [CrossRef] [PubMed]

A lower propagation loss coefficient would also benefit the use of longer waveguides to further boost the nonlinear response [23

]. Overall, numerical modeling of OPC for a single channel RZ-DPSK signal for the same experimental parameters (including a 400 mW total launch power but just a slightly lower input CW:signal power ratio of 6.2 dB) predicts that η could be increased by 16 dB if coupling losses were reduced from 5 to 1 dB/facet, and by a further 5 dB if both also the waveguide length was increased to 14 cm, and the propagation loss reduced from 0.5 to 0.3 dB/cm. Measurements with a CW laser source have confirmed that the waveguide is capable of handling coupled average powers of at least 300 mW.

Apart from increasing η, the maximum number of channels capable of dispersion-free transmission by OPC is limited by the non-zero fiber’s dispersion slope, S, (i.e. variation of dispersion with wavelength) which increases the residual dispersion for channels spaced further from Ch. 2 (by Δλ), as Δλ⋅(L₁-L₂)⋅S where L₁ and L₂ are the link lengths proceeding and following OPC, respectively. As the dispersion of a pulse scales to the square of its bandwidth, lowering the channel bit-rate from 40 to 10 Gb/s would raise the dispersion tolerance by 16 times i.e. equivalent to a 3600 km link in our experiment (compared to 320 km for the silicon demonstration [17

]).

Another practical consideration is optical polarization. In our experiments, polarization controllers were required to ensure the states of polarization (SOP) for both the incoming signal and CW pump wave were aligned for coupling to the TM mode of the waveguide. However, polarization independent operation could be realized by adopting schemes such as a bi-directional fiber loop, as demonstrated with PPLN [5

], [13

–15

]. In that approach, the signal and CW beam are launched into a polarization beam splitter (PBS), whose output pair of orthogonally polarized and polarization maintaining fibers are connected to either end of the waveguide to form a fiber-loop. A cross splice on the TE mode output of the PBS rotates its SOP by 90 degrees so that bi-directional TM mode coupling to the waveguide is assured. Thus, by fixing the SOP of the CW pump into the PBS to 45 degrees (so that its power is split equally), polarization insensitive OPC can be achieved for arbitrary SOP of the input signal.

Conclusions

Optical phase conjugation of phase encoded WDM signals using FWM pumped by a CW laser was demonstrated for the first time with a 6 cm long highly nonlinear planar waveguide based on As₂S₃ glass. Dispersion-free transmission of a 3 × 40 Gb/s RZ-DPSK WDM signal over a standard optical fiber link as long as 225 km was achieved for all channels. This good performance stems from the high nonlinear response in a short waveguide length (enabled by the high nonlinear index and small mode area) and a dispersion shifted waveguide design ensuring broadband low dispersion. Extending the transmission reach will require improving the FWM conversion efficiency, which could be achieved by incorporating on chip taper structures to reduce the large coupling losses. Other approaches include boosting γ by fabricating waveguides in more highly nonlinear chalcogenide glasses, and with nanoscale dimension to further shrink the mode area. The results highlight the capability for phase preserving processing of broadband WDM signals by the χ⁽³⁾ nonlinearity in compact planar waveguides, which can improve the wavelength flexibility of the input signal, and the potential for monolithic integration of more complex optical circuits.

Acknowledgements

This work was supported by the Australian Research Council (ARC) through its ARC Centres of Excellence and Federation Fellowship programs.

References and links

1.	T. D. Vo, H. Hu, M. Galili, E. Palushani, J. Xu, L. K. Oxenløwe, S. J. Madden, D.-Y. Choi, D. A. P. Bulla, M. D. Pelusi, J. Schröder, B. Luther-Davies, and B. J. Eggleton, “Photonic chip based transmitter optimization and receiver demultiplexing of a 1.28 Tbit/s OTDM signal,” Opt. Express 18(16), 17252–17261 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-16-17252. [CrossRef] [PubMed]
2.	J. Yamawaku, H. Takara, T. Ohara, K. Sato, A. Takada, and T. Morioka, “isJ. Yamawaku, H. Takara, T. Ohara, K. Sato, A. Takada, T. Morioka, O. Tadanaga, H. Miyazawa, and M. Asobe, “Simultaneous 25 GHz-spaced DWDM wavelength conversion of 1.03 Tbit∕s (103×10 Gbit∕s) signals in PPLN waveguide,” Electron. Lett. 39(15), 1144 (2003). [CrossRef]
3.	M. Chou, I. Brener, M. M. Fejer, E. E. Chaban, and S. B. Christman, “1.5-μm-band wavelength conversion based on cascaded second-order nonlinearity in LiNbO₃ waveguides,” IEEE Photon. Technol. Lett. 11(6), 653–655 (1999). [CrossRef]
4.	I. Brener, B. Mikkelsen, G. Raybon, R. Harel, K. Parameswaran, J. R. Kurz, and M. M. Fejer, “160 Gbit/s wavelength shifting and phase conjugation using periodically poled LiNbO₃ waveguide parametric converter,” Electron. Lett. 36(21), 1788–1790 (2000). [CrossRef]
5.	H. Hu, R. Nouroozi, R. Ludwig, B. Huettl, C. Schmidt-Langhorst, H. Suche, W. Sohler, and C. Schubert, “Polarization-insensitive all-optical wavelength conversion of 320 Gb/s RZ-DQPSK signals using a Ti:PPLN waveguide,” Appl. Phys. B 101(4), 875–882 (2010), doi:. [CrossRef]
6.	X. Wu, W.-R. Peng, V. Arbab, J. Wang, and A. Willner, “Tunable optical wavelength conversion of OFDM signal using a periodically-poled lithium niobate waveguide,” Opt. Express 17(11), 9177–9182 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-11-9177. [CrossRef] [PubMed]
7.	H. Furukawa, A. Nirmalathas, N. Wada, S. Shinada, H. Tsuboya, and T. Miyazaki, “Tunable all-optical wavelength conversion of 160-Gb/s RZ optical signals by cascaded SFG-DFG generation in PPLN waveguide,” IEEE Photon. Technol. Lett. 19(6), 384–386 (2007). [CrossRef]
8.	B. G. Lee, A. Biberman, A. C. Turner-Foster, M. A. Foster, M. Lipson, A. L. Gaeta, and K. Bergman, “Demonstration of broadband wavelength conversion at 40 Gb/s in silicon waveguides,” IEEE Photon. Technol. Lett. 21(3), 182–184 (2009). [CrossRef]
9.	W. Mathlouthi, H. Rong, and M. Paniccia, “Characterization of efficient wavelength conversion by four-wave mixing in sub-micron silicon waveguides,” Opt. Express 16(21), 16735–16745 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-21-16735. [CrossRef] [PubMed]
10.	F. Luan, M. D. Pelusi, M. R. E. Lamont, D.-Y. Choi, S. Madden, B. Luther-Davies, and B. J. Eggleton, “Dispersion engineered As(₂₎S(₃) planar waveguides for broadband four-wave mixing based wavelength conversion of 40 Gb/s signals,” Opt. Express 17(5), 3514–3520 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-5-3514. [CrossRef] [PubMed]
11.	M. D. Pelusi, F. Luan, S. Madden, D.-Y. Choi, D. A. Bulla, B. Luther-Davies, and B. J. Eggleton, “Wavelength conversion of high-speed phase and intensity modulated signals using a highly nonlinear chalcogenide glass chip,” IEEE Photon. Technol. Lett. 22(1), 3–5 (2010). [CrossRef]
12.	S. Watanabe, “Optical signal processing using nonlinear fibers,” J. Opt. Fiber. Commun. Rep. 3(1), 1–24 (2005). [CrossRef]
13.	S. L. Jansen, D. van den Borne, P. M. Krummrich, S. Spälter, G.-D. Khoe, and H. de Waardt, “Long-haul DWDM transmission systems employing optical phase conjugation,” IEEE J. Sel. Top. Quant. 12(4), 505–520 (2006). [CrossRef]
14.	H. Hu, R. Nouroozi, R. Ludwig, C. Schmidt-Langhorst, H. Suche, W. Sohler, and C. Schubert, “110 km transmission of 160 Gbit/s RZ-DQPSK signals by midspan polarization-insensitive optical phase conjugation in a Ti:PPLN waveguide,” Opt. Lett. 35(17), 2867–2869 (2010). [CrossRef] [PubMed]
15.	P. Minzioni, V. Pusino, I. Cristiani, L. Marazzi, M. Martinelli, C. Langrock, M. M. Fejer, and V. Degiorgio, “Optical phase conjugation in phase-modulated transmission systems: experimental comparison of different nonlinearity-compensation methods,” Opt. Express 18(17), 18119–18124 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-17-18119. [CrossRef] [PubMed]
16.	J. Inoue, H. Sotobayashi, W. Chujo, and H. Kawaguchi, “80 Gbit/s conventional and carrier-suppressed RZ signals transmission over 200 km standard fiber by using mid-span optical phase conjugation (invited, OECC Awarded),” IEICE Trans. on Comm. E 86-B, 1555–1561 (2003).
17.	S. Ayotte, H. Rong, S. Xu, O. Cohen, and M. J. Paniccia, “Multichannel dispersion compensation using a silicon waveguide-based optical phase conjugator,” Opt. Lett. 32(16), 2393–2395 (2007). [CrossRef] [PubMed]
18.	Z. Pan, C. Yub, and A. E. Willner, “Optical performance monitoring for the next generation optical communication networks,” Opt. Fiber Technol. 16(1), 20–45 (2010). [CrossRef]
19.	G. Wellbrock and T. J. Xia, “The road to 100g deployment [Commentary],” IEEE Commun. Mag. 48(3), S14–S18 (2010). [CrossRef]
20.	S. Moro, E. Myslivets, J. R. Windmiller, N. Alic, J. M. Chavez Boggio, and S. Radic, “Synthesis of equalized broadband parametric gain by localized dispersion mapping,” IEEE Photon. Technol. Lett. 20(23), 1971–1973 (2008). [CrossRef]
21.	J. M. Chavez Boggio, S. Zlatanovic, F. Gholami, J. M. Aparicio, S. Moro, K. Balch, N. Alic, and S. Radic, “Short wavelength infrared frequency conversion in ultra-compact fiber device,” Opt. Express 18(2), 439–445 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-2-439. [CrossRef] [PubMed]
22.	M. R. Lamont, C.M de Sterke, and B.J. Eggleton, “Dispersion engineering of highly nonlinear As₂S₃ waveguides for parametric gain and wavelength conversion,” Opt. Express 15, 9458–9463 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-15-9458. [CrossRef] [PubMed]
23.	S. J. Madden, D.-Y. Choi, D. A. Bulla, A. V. Rode, B. Luther-Davies, V. G. Ta’eed, M. D. Pelusi, and B. J. Eggleton, “Long, low loss etched As(₂₎S(₃) chalcogenide waveguides for all-optical signal regeneration,” Opt. Express 15(22), 14414–14421 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-22-14414. [CrossRef] [PubMed]
24.	D.-Y. Choi, S. Madden, D. A. Bulla, R. Wang, A. Rode, and B. Luther-Davies, “Submicrometer-thick low-loss As₂S₃ planar waveguides for nonlinear optical devices,” IEEE Photon. Technol. Lett. 22(7), 495–497 (2010). [CrossRef]
25.	M. Takahashi, R. Sugizaki, J. Hiroishi, M. Tadakuma, Y. Taniguchi, and T. Yagi, “Low-loss and low-dispersion-slope highly nonlinear fibers,” J. Lightwave Technol. 23(11), 3615–3624 (2005). [CrossRef]
26.	Y. K. Lizé, X. Wu, M. Nazarathy, Y. Atzmon, L. Christen, S. Nuccio, M. Faucher, N. Godbout, and A. E. Willner, “Chromatic dispersion tolerance in optimized NRZ-, RZ- and CSRZ-DPSK demodulation,” Opt. Express 16(6), 4228–4236 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-6-4228. [CrossRef] [PubMed]
27.	T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3 μm square Si wire waveguides to singlemode fibres,” Electron. Lett. 38(25), 1669–1670 (2002). [CrossRef]
28.	X. Gai, S. Madden, D.-Y. Choi, D. Bulla, and B. Luther-Davies, “Dispersion engineered Ge11.5As24Se64.5 nanowires with a nonlinear parameter of 136 W⁻¹m⁻¹ at 1550 nm,” Opt. Express 18(18), 18866–18874 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-18-18866. [CrossRef] [PubMed]

OCIS Codes
(070.4340) Fourier optics and signal processing : Nonlinear optical signal processing
(070.5040) Fourier optics and signal processing : Phase conjugation
(230.4320) Optical devices : Nonlinear optical devices

ToC Category:
Chalcogenide Glass

History
Original Manuscript: November 12, 2010
Manuscript Accepted: November 22, 2010
Published: December 6, 2010

Virtual Issues
Chalcogenide Glass (2010) Optics Express

Citation
M.D. Pelusi, F. Luan, D.-Y. Choi, S.J. Madden, D.A.P. Bulla, B. Luther-Davies, and B.J. Eggleton, "Optical phase conjugation by an As₂S₃ glass planar waveguide for dispersion-free transmission of WDM-DPSK signals over fiber," Opt. Express 18, 26686-26694 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-25-26686

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References

T. D. Vo, H. Hu, M. Galili, E. Palushani, J. Xu, L. K. Oxenløwe, S. J. Madden, D.-Y. Choi, D. A. P. Bulla, M. D. Pelusi, J. Schröder, B. Luther-Davies, and B. J. Eggleton, “Photonic chip based transmitter optimization and receiver demultiplexing of a 1.28 Tbit/s OTDM signal,” Opt. Express 18(16), 17252–17261 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-16-17252 . [CrossRef] [PubMed]
J. Yamawaku, H. Takara, T. Ohara, K. Sato, A. Takada, and T. Morioka, “isJ. Yamawaku, H. Takara, T. Ohara, K. Sato, A. Takada, T. Morioka, O. Tadanaga, H. Miyazawa, and M. Asobe, “Simultaneous 25 GHz-spaced DWDM wavelength conversion of 1.03 Tbit∕s (103×10 Gbit∕s) signals in PPLN waveguide,” Electron. Lett. 39(15), 1144 (2003). [CrossRef]
M. Chou, I. Brener, M. M. Fejer, E. E. Chaban, and S. B. Christman, “1.5-μm-band wavelength conversion based on cascaded second-order nonlinearity in LiNbO3 waveguides,” IEEE Photon. Technol. Lett. 11(6), 653–655 (1999). [CrossRef]
I. Brener, B. Mikkelsen, G. Raybon, R. Harel, K. Parameswaran, J. R. Kurz, and M. M. Fejer, “160 Gbit/s wavelength shifting and phase conjugation using periodically poled LiNbO3 waveguide parametric converter,” Electron. Lett. 36(21), 1788–1790 (2000). [CrossRef]
H. Hu, R. Nouroozi, R. Ludwig, B. Huettl, C. Schmidt-Langhorst, H. Suche, W. Sohler, and C. Schubert, “Polarization-insensitive all-optical wavelength conversion of 320 Gb/s RZ-DQPSK signals using a Ti:PPLN waveguide,” Appl. Phys. B 101(4), 875–882 (2010), doi:. [CrossRef]
X. Wu, W.-R. Peng, V. Arbab, J. Wang, and A. Willner, “Tunable optical wavelength conversion of OFDM signal using a periodically-poled lithium niobate waveguide,” Opt. Express 17(11), 9177–9182 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-11-9177 . [CrossRef] [PubMed]
H. Furukawa, A. Nirmalathas, N. Wada, S. Shinada, H. Tsuboya, and T. Miyazaki, “Tunable all-optical wavelength conversion of 160-Gb/s RZ optical signals by cascaded SFG-DFG generation in PPLN waveguide,” IEEE Photon. Technol. Lett. 19(6), 384–386 (2007). [CrossRef]
B. G. Lee, A. Biberman, A. C. Turner-Foster, M. A. Foster, M. Lipson, A. L. Gaeta, and K. Bergman, “Demonstration of broadband wavelength conversion at 40 Gb/s in silicon waveguides,” IEEE Photon. Technol. Lett. 21(3), 182–184 (2009). [CrossRef]
W. Mathlouthi, H. Rong, and M. Paniccia, “Characterization of efficient wavelength conversion by four-wave mixing in sub-micron silicon waveguides,” Opt. Express 16(21), 16735–16745 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-21-16735 . [CrossRef] [PubMed]
F. Luan, M. D. Pelusi, M. R. E. Lamont, D.-Y. Choi, S. Madden, B. Luther-Davies, and B. J. Eggleton, “Dispersion engineered As(2)S(3) planar waveguides for broadband four-wave mixing based wavelength conversion of 40 Gb/s signals,” Opt. Express 17(5), 3514–3520 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-5-3514 . [CrossRef] [PubMed]
M. D. Pelusi, F. Luan, S. Madden, D.-Y. Choi, D. A. Bulla, B. Luther-Davies, and B. J. Eggleton, “Wavelength conversion of high-speed phase and intensity modulated signals using a highly nonlinear chalcogenide glass chip,” IEEE Photon. Technol. Lett. 22(1), 3–5 (2010). [CrossRef]
S. Watanabe, “Optical signal processing using nonlinear fibers,” J. Opt. Fiber. Commun. Rep. 3(1), 1–24 (2005). [CrossRef]
S. L. Jansen, D. van den Borne, P. M. Krummrich, S. Spälter, G.-D. Khoe, and H. de Waardt, “Long-haul DWDM transmission systems employing optical phase conjugation,” IEEE J. Sel. Top. Quant. 12(4), 505–520 (2006). [CrossRef]
H. Hu, R. Nouroozi, R. Ludwig, C. Schmidt-Langhorst, H. Suche, W. Sohler, and C. Schubert, “110 km transmission of 160 Gbit/s RZ-DQPSK signals by midspan polarization-insensitive optical phase conjugation in a Ti:PPLN waveguide,” Opt. Lett. 35(17), 2867–2869 (2010). [CrossRef] [PubMed]
P. Minzioni, V. Pusino, I. Cristiani, L. Marazzi, M. Martinelli, C. Langrock, M. M. Fejer, and V. Degiorgio, “Optical phase conjugation in phase-modulated transmission systems: experimental comparison of different nonlinearity-compensation methods,” Opt. Express 18(17), 18119–18124 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-17-18119 . [CrossRef] [PubMed]
J. Inoue, H. Sotobayashi, W. Chujo, and H. Kawaguchi, “80 Gbit/s conventional and carrier-suppressed RZ signals transmission over 200 km standard fiber by using mid-span optical phase conjugation (invited, OECC Awarded),” IEICE Trans. on Comm , E 86-B, 1555–1561 (2003).
S. Ayotte, H. Rong, S. Xu, O. Cohen, and M. J. Paniccia, “Multichannel dispersion compensation using a silicon waveguide-based optical phase conjugator,” Opt. Lett. 32(16), 2393–2395 (2007). [CrossRef] [PubMed]
Z. Pan, C. Yub, and A. E. Willner, “Optical performance monitoring for the next generation optical communication networks,” Opt. Fiber Technol. 16(1), 20–45 (2010). [CrossRef]
G. Wellbrock and T. J. Xia, “The road to 100g deployment [Commentary],” IEEE Commun. Mag. 48(3), S14–S18 (2010). [CrossRef]
S. Moro, E. Myslivets, J. R. Windmiller, N. Alic, J. M. Chavez Boggio, and S. Radic, “Synthesis of equalized broadband parametric gain by localized dispersion mapping,” IEEE Photon. Technol. Lett. 20(23), 1971–1973 (2008). [CrossRef]
J. M. Chavez Boggio, S. Zlatanovic, F. Gholami, J. M. Aparicio, S. Moro, K. Balch, N. Alic, and S. Radic, “Short wavelength infrared frequency conversion in ultra-compact fiber device,” Opt. Express 18(2), 439–445 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-2-439 . [CrossRef] [PubMed]
M. R. Lamont, C.M de Sterke, and B.J. Eggleton, “Dispersion engineering of highly nonlinear As2S3 waveguides for parametric gain and wavelength conversion,” Opt. Express 15, 9458–9463 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-15-9458 . [CrossRef] [PubMed]
S. J. Madden, D.-Y. Choi, D. A. Bulla, A. V. Rode, B. Luther-Davies, V. G. Ta’eed, M. D. Pelusi, and B. J. Eggleton, “Long, low loss etched As(2)S(3) chalcogenide waveguides for all-optical signal regeneration,” Opt. Express 15(22), 14414–14421 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-22-14414 . [CrossRef] [PubMed]
D.-Y. Choi, S. Madden, D. A. Bulla, R. Wang, A. Rode, and B. Luther-Davies, “Submicrometer-thick low-loss As2S3 planar waveguides for nonlinear optical devices,” IEEE Photon. Technol. Lett. 22(7), 495–497 (2010). [CrossRef]
M. Takahashi, R. Sugizaki, J. Hiroishi, M. Tadakuma, Y. Taniguchi, and T. Yagi, “Low-loss and low-dispersion-slope highly nonlinear fibers,” J. Lightwave Technol. 23(11), 3615–3624 (2005). [CrossRef]
Y. K. Lizé, X. Wu, M. Nazarathy, Y. Atzmon, L. Christen, S. Nuccio, M. Faucher, N. Godbout, and A. E. Willner, “Chromatic dispersion tolerance in optimized NRZ-, RZ- and CSRZ-DPSK demodulation,” Opt. Express 16(6), 4228–4236 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-6-4228 . [CrossRef] [PubMed]
T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3 μm square Si wire waveguides to singlemode fibres,” Electron. Lett. 38(25), 1669–1670 (2002). [CrossRef]
X. Gai, S. Madden, D.-Y. Choi, D. Bulla, and B. Luther-Davies, “Dispersion engineered Ge11.5As24Se64.5 nanowires with a nonlinear parameter of 136 W⁻¹m⁻¹ at 1550 nm,” Opt. Express 18(18), 18866–18874 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-18-18866 . [CrossRef] [PubMed]

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