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All-Optical Modulation Format Conversion and Multicasting Using Injection-Locked Laser Diodes C. W. Chow, C. S. Wong, Member, IEEE, and H. K. Tsang, Member, IEEE
Abstract—The functionality of optical modulation format conversion and wavelength conversion will be needed in next-generation wavelength routers. This paper describes a new approach for return-to-zero to nonreturn-to-zero format conversion using a birefringent fiber and an injection-locked Fabry–Pérot laser diode. This paper also describes the demonstration of an all-optical multicaster based on a similar technique. Error-free operations were confirmed in the format converted and in every channel of the 8 10-Gb/s multicast outputs. Index Terms—Modulation format conversion, multicasting, optical fiber communication, optical signal processing, wavelength conversion.
I. INTRODUCTION
O
PTICAL networks may be viewed as a three-level hierarchy consisting of backbone networks, metro networks, and access networks. Future backbone networks provide enormous bandwidth and high data rate and could be based on optical time-domain-multiplexed (OTDM) and dense-wavelengthdivision-multiplexed (DWDM) links. Access networks transport data to (and from) individual users. Metro networks play an important role by interconnecting both of them so that direct optical connections can be established. All-optical networking potentially allows high-speed optical communications to become more cost effective by the use of low-cost transparent light paths, which do not need any optical-to-electrical (O/E) and electrical-to-optical (E/O) conversions. Polarization-mode dispersion (PMD) is a limiting factor on transmission span lengths in backbone networks, particularly at bit rates above 10 Gb/s. Return-to-zero (RZ) modulation is more tolerant to PMD [1] because the energy is confined in the center of each bit slot in the RZ signal, and thus more differential group delay (DGD) is required before the energy leaks out of the bit slot to cause intersymbol interference (ISI). It has also been shown that 10-Gb/s nonlinear RZ transmission is more tolerant against nonoptimized dispersion maps compared with nonreturn-to-zero (NRZ) modulation [2]. RZ modulation
Manuscript received December 11, 2003; revised August 19, 2004. This work was supported in part by the Research Grants Council under Grants CUHK4196/03E and CUHK4192/01E. C. W. Chow and H. K. Tsang are with the Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (e-mail:
[email protected]). C. S. Wong was with the Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong. He is now with Acasia Technologies (HK), Ltd., Hong Kong (e-mail:
[email protected]). Digital Object Identifier 10.1109/JLT.2004.836808
may be employed in future OTDM networks using the bit interleaving technique. However, NRZ is more spectrally efficient and is thus better suited for the DWDM access and metro networks, which typically have lower bit rates, shorter span lengths, and more closely spaced wavelength channels. It was shown that RZ modulation is not suitable for use in DWDM systems with channel spacing lower than 100 GHz at 40 Gb/s [3]. Hence, there will likely be a requirement in future optical networks to support different modulation formats [4]. Modulation format conversion can enable the interconnection of different types of optical networks, which employ the modulation formats that are best suited to that network’s coverage, e.g., the use of RZ for higher bit rate OTDM long-haul traffic and NRZ for densely packed DWDM access networks. All-optical modulation format conversion aims to shift the switching burden into the optical domain so that compact cost-effective solutions (without needing bulky and expensive O/E and E/O conversions for each channel) can be incorporated into wavelength routers [5] for metro and access networks. The RZ-to-NRZ modulation format conversion can be used as an optical gateway from the fast OTDM networks to lower speed DWDM access networks. Fig. 1 shows the proposed architecture of a wavelength router, which is capable of modulation format conversion, wavelength conversion, and multicasting. The RZ signals ( 40 Gb/s) in future long-haul networks may be time-division demultiplexed to a lower bit rate ( 10 Gb/s) [6] [7] and then format-converted into NRZ for the local-area DWDM networks, which consist of many closely packed wavelength channels. The RZ-to-NRZ format conversion may be followed by wavelength conversion for multicasting in a wavelength-routed network and the data sent to multiple terminal nodes simultaneously according to their wavelength, as shown in Fig. 1. Different methods of all-optical modulation format conversion from RZ to NRZ include the use of a semiconductor optical amplifier (SOA)/fiber Bragg grating (FBG) [8], an SOA loop mirror [9], and a nonlinear optical loop mirror (NOLM) [10]. However, the converted NRZ suffers a low extinction ratio (ER) by the transmission slope of the grating in the SOA/FBG case. A simpler configuration (compared with the loop mirror cases) for the RZ-to-NRZ conversion was performed recently by using pulse duplicator/wavelength converter [11], [12] and repetition-rate doubler/Fabry–Pérot laser diode (FP-LD) [13]. In the authors’ previous work [13], the repetition-rate doubler consisted of two 3-dB passive fiber couplers constructed to form a Mach–Zehnder (MZ) which introduced a half-bit delay in one arm with respect to the other. The MZ arms
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Fig. 1. Proposed architecture for a wavelength router, which is capable of modulation format conversion, wavelength conversion, and multicasting. OTDD: Optical time-domain demultiplexer. MFC: Modulation format converter.
were made as short as possible to reduce any polarization deviation between the arms. Polarization deviation from the transverse-electric (TE) polarization of the FP-LD may reduce the injection-locking efficiency and contribute to a power penalty. In this paper, a new approach of RZ-to-NRZ format conversion was demonstrated by using a piece of birefringent fiber (BF) to simplify the experimental setup. The stringent requirements of time delay and polarization matching between the MZ arms of the repetition-rate doubler are not needed. The coupling imbalance between the two 3-dB fiber couplers does not exist. The result is a more stable output and reduced power penalty when compared with the authors’ previous work. The conversion from NRZ to RZ may be also needed inside the wavelength router for the reversed direction of communication. This can also be performed by using injection locking of the FP-LD, as described in previous literature [14], [15]. ApartfromtheRZ-to-NRZandNRZ-to-RZmodulationformat conversion, wavelength conversion [16] will be supported by the wavelength router. Wavelength conversion is an effective method by which to avoid wavelength congestion and light-path failure in DWDM networks. One attractive feature inside such networks is multicasting. Multicasting is the transmission of information from a source node to multiple destination nodes simultaneously (Fig.1).Applicationssuchasteleconferencingandvideodistribution require the establishment of multicast connections [17], [18]. Multicast by using an electroabsorption modulation (EAM) was described recently [19], but a high power penalty was observed in the multicast channels because of the insertion loss of the EAM. A high input power ( 10 dBm) was also required. This paper describes another approach for all-optical wavelength conversion with multicasting using a single FP-LD. A multicast operation of upto8 10Gb/swasdemonstratedwithanaveragepowerpenalty of 2.5 dB. The proposed schemes require only a single FP-LD and thus are potentially low cost. In addtion, no expensive high-speed driving circuits are needed for the dc-operated FP-LD. The paper is organized as follows. In Section II, the operation conditions, principle, and experimental results of RZ-to-NRZ modulation format conversion are given. Section III describes the multiple wavelength conversion of a single FP-LD which
can act as an all-optical multicaster. Simultaneously multicas10-Gb/s channels was demonstrated. Finally, the ting of 8 paper is concluded in Section IV. II. ALL-OPTICAL MODULATION FORMAT CONVERSION A. Operation Conditions In RZ-to-NRZ modulation format conversion using an injection-locked FP-LD, a continuous-wave (CW) probe beam is optically controlled by the RZ input pump during a broad switching window. The waveform of the format-converted NRZ signal will be determined by the shape of the switching window, which can add ripples on the output of the NRZ eye diagram. The injection-locked FP-LD in this proposed scheme not only provides wavelength conversion, but also acts as a format converter with variable switching window for smoothing off the ripples to improve the quality of the output-converted NRZ signal. By applying the measured dynamic response of the FP-LD and numerical analysis similar to that in [13], the optimum bias level of the FP-LD may be obtained to produce broadened converted pulses with acceptable timing jitter [ 3 ps root mean square (rms)]. The combination of a half-bit delayed replica of the input RZ signal with the original RZ signal is sufficient to achieve an output-converted NRZ with ER 10 dB. B. Principle and Experiment Fig. 2 shows the experimental setup for the RZ-to-NRZ modulation format conversion using an injection-locked FP-LD. The FP-LD had a central output wavelength of about 1543 nm and longitudinal mode spacing of 1.68 nm. It was biased at 17 mA . Initially, the FP-LD was injection-locked by a CW probe signal, emitted by a CW-distributed feedback laser (DFB), at with an input power of 3 a wavelength of 1551.37 nm dBm. An optical pulse train with a full-width-at-half-maximum (FWHM) pulsewidth of 25 ps and a timing jitter of 1.45 ps rms was generated from a gain-switched DFB (GS-DFB). It was enbits by an external modulator (MOD) to coded with form 10-Gb/s pseudorandom pump RZ data at a wavelength of
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Fig. 2. Experimental setup of the RZ-to-NRZ modulation format conversion. GS-DFB: Gain-switched distributed feedback laser. CW-DFB: Continuous-wave distributed feedback laser. BF: Birefringent fiber. PC: Polarization controller. EDFA: Erbium-doped fiber amplifier. MOD: Modulator. VOA: Variable optical attenuator. PD: Photo diode. BERT: Bit-error-rate test set.
1547.74 nm . An erbium-doped fiber amplifier (EDFA) amplified the signal to compensate the loss of the MOD. A tunable filter (3-B bandwidth of 1 nm) placed after the EDFA was used to remove the out-of-band amplified spontaneous emission (ASE) noise of the EDFA. The signal pulse was split into orthogonal polarizations delayed by half-bit (50 ps) in a 55-m length of BF. The input RZ pump power was 0 dBm. The orthogonal comwere coupled into the FP-LD, which ponents of the pump at had already been injection-locked by the CW probe at which was introduced via the 3-dB coupler and circulator. Since the FP-LD can act as an intensity-compensating polarizer (see Section II-C), a polarization controller (PC) was adjusted to couple equal TE-polarized (with respected to the FP-LD) power from each of the two orthogonal components of the RZ signal after the BF. If one of the longitudinal modes of the FP-LD lies within the injection-locking range of the input pump wavelength , the FP-LD becomes injection-locked to . Since the steady-state carrier density is effectively clamped above threshold, it will operate in this case with a lower carrier density than the original injection locking at . The lower carrier density results in a higher refractive index, leading to the required red shift in the longitudinal modes of the FP-LD [20] for sustained lasing at . The shift in FP modes will also help extinguish the original output from the FP-LD. Thus, the output at can be modulated by the data at . A tunable filter (3-dB bandwidth at the output port of 1 nm) selected the converted signal of the circulator. The eye diagrams and the state-of-polarization (SOP) of the converted signal were captured by a 32-GHz p-i-n photodetector (PD) connected to a digital sampling oscilloscope and a polarization analyzer (Agilent: 8509C), respectively. The optical spectrum was measured at the output port of the circulator (before the filter) by an optical spectrum analyzer at a measurement bandwidth resolution of 0.08 nm. A variable optical attenuator (VOA) varied the received optical power before being launched into a 10 Gb/s PIN PD for the bit-error rate (BER) measurements. C. Results and Discussion Fig. 3 shows (a) the 10 Gb/s back-to-back RZ pump signal and (b) the 10 Gb/s modulation format converted NRZ signal. The FP-LD bias level was adjusted to optimize the switching
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 11, NOVEMBER 2004
Fig. 3. Eye diagrams of (a) 10-Gb/s back-to-back RZ pump signal and 12 dB . (b) 10-Gb/s modulation-format-converted NRZ signal ER
(
=
)
Fig. 4. 10-Gb/s BER measurements of the back-to-back RZ pump signal and ). the converted NRZ signal (with power penalty of 1.33 dB at BER of Inset: Poincare sphere representation of the stable state-of-polarization (SOP) of the NRZ output, with degree-of-polarization DOP 95 .
(
)= %
10
window for smoothing the ripples at the output converted NRZ signal and produce an ER of 12 dB (measured in time domain). The residual ripples can be further reduced by making more replicas of the input RZ data (by deploying several pieces of BF with polarizers between them). Fig. 4 shows the 10 Gb/s BER measurements of the back-toback RZ and the converted NRZ signal. A power penalty of 1.3 , where the PIN PD measured dB was observed at BER of the average optical power. RZ can give higher receiver sensitivity for the same average power, due to the pulse energy being confined to a shorter time-slot and thus giving a higher peak power. As the internal losses of the FP-LD favors emission in the TE-polarization mode, any injected signal that is spectrally aligned with one of the FP modes will have its TE component amplified and its intensity clamped and stabilized by injection locking if the power of the TE component is above the locking threshold. The TM component is suppressed. Hence an injection-locked FP-LD can act as an intensity-compensating polarizer with its output in the TE polarization [21]. Thus a polarizer is not necessary to select the equal components of the orthogonal 20 Gb/s RZ at the output of the BF before entering the FP-LD.
CHOW et al.: ALL-OPTICAL MODULATION FORMAT CONVERSION AND MULTICASTING
Fig. 5. Dependence of the ER of the output-converted NRZ signal on FP-bias levels and the CW probe wavelength.
The SOP of the replicated 20 Gb/s RZ and the converted NRZ were measured by a polarization analyzer. The degree-of-polarization (DOP) was stabilized by injection locking from 3.4% RZ signal (measured at the output of the BF) to 95% converted NRZ signal respectively. The low DOP of the RZ was due to the fast (20 GHz) polarization fluctuations within the RZ signal. The stable SOP of the converted NRZ is represented on a Poincare sphere (pointed by an arrow) as shown in the inset of Fig. 4, and . has For the proposed modulation formation conversion to be of practical use in a wavelength router, a fast PC [22] must be inserted before the BF (Fig. 2). By adjusting the SOP of the incoming signal to align with the fast/slow axis of the BF, only wavelength conversion can be selected instead of the combined modulation and wavelength conversion for the 45 degree input. Fig. 5 shows how the ER of the output converted NRZ signal depends on FP-bias levels and the CW probe wavelength. An abrupt transition occurs on the lower bias detuning side, while a smooth transition is present on the high bias detuning. At even lower biases, there is insufficient gain for injection locking, resulting in eye-closure and degraded ER. At higher biases, the shortened carrier recovery time increase the speed (response time) of the FP-LD, making the converted pulse less broadened and introduce ripples which lowers the ER of the converted NRZ output. The wavelength detuning of the probe is asymmetric with higher ER on the higher wavelength detuning side, and is due to the asymmetric range of injection locking [20]. Therefore, the bias current of the FP-LD and the injected probe wavelength must be adjusted to optimize the quality of the converted NRZ signal. Fig. 6 shows the optical spectrum of the injection locked FP-LD for an input RZ pump at 1547.74 nm and a probe signal at 1551.37 nm. The side-mode suppression ratio (SMSR) of the pump and probe were 33.63 dB and 37.82 dB respectively. The RZ pulses are suitable for OTDM but the broad spectrum is inefficient for DWDM networks. The spectral width of the data-format converted NRZ output (14 GHz) is much reduced from that of the input RZ signal (56 GHz). With reference to Fig. 1, format conversion to NRZ is essential to achieve the narrower spectrum which allows the data to be sent over the DWDM access networks (which may have wavelength spacing of 50 GHz).
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Fig. 6. Optical spectrum of the dual-wavelength injection-locked FP-LD by the pump RZ at 1547.74 nm and probe signal at 1551.37 nm (SMSR of 33.63 and 37.82 dB and a spectral width of 56 and 14 GHz, respectively).
III. ALL-OPTICAL MULTICASTING A. Experiment The experimental setup for the all-optical multicasting is shown in Fig. 7. The back-to-back pump signal ( : 1547.38 nm) was generated from the first channel of the eight-channel pseudo random WDM source and was encoded with binary sequence by an MOD to form a 10 Gb/s NRZ data train. An EDFA amplified the signal to compensate the loss of the MOD and a tunable filter (3 dB bandwidth of 1 nm) after the EDFA was used to remove the out-of-band ASE of the EDFA. The input signal power was 0 dBm with ER of 13.5 dB. Eight CW probe signals (from : 1549.03 nm to : 1560.38 nm, 200 GHz spacing) were produced by other channels of the WDM source and a tunable laser. The CW power per channel was about 3 dBm. The eight CW signals were wavelength-multiplexed by the 200-GHz channel spacing (1.6 nm) 8-channel arrayed waveguide grating (AWG). The pump and probe signals were then combined by a 3-dB fiber coupler, before being launched into the FP-LD. The FP-LD was dc bi. It had a central wavelength of 1545.7 nm ased at 17 mA with the longitudinal mode spacing of about 1.6 nm. Initially, the probe signals were launched to injection lock the FP-LD. This provided a SMSR of about 30 dB. The data-bearing pump was then introduced into the FP-LD for injection signal locking. The pump power further decreased the carrier density inside the active layer of the FP-LD due to stimulated emission. The resulting increase in the refractive index caused a red-shift (as described in Section II-B) in the longitudinal modes of the FP-LD ( 0.16 nm). This injection-locking induced red-shift not only suppressed the optical gain of the FP-LD at the original wavelength ( 20 dB), but also extinguished the original outputs at the probe wavelengths. Thus all-optical wavelength conversion was achieved to eight output wavelengths. The longitudinal mode spacing of the FP-LD must match the channel spacing of the eight CW probe signals. As the detuning of the longitudinal mode of the FP-LD from the probe wavelength is increased, both the injection locking efficiency and output converted signal will be degraded. If the detuning is outside the injection locking range [20], there is no wavelength conversion.
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Fig. 7. Experimental setup of multicasting. CW-DFB: Continuous-wave distributed feedback laser. EDFA: Erbium-doped fiber amplifier. MOD: Modulator. AWG: Arrayed-waveguide grating. VOA: Variable optical attenuator. PD: Photo diode. BERT: Bit-error-rate test set.
Fig. 9. 10-Gb/s BER measurements of the back-to-back and eight multicast BER level, and channels (the average power penalty is 2.5 dB at 10 channel-to-channel variation is 0.4 dB). Inset: Typical eye diagram (channel 4) of the multicast channel with ER of 12 dB.
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Fig. 8. Optical spectrum of the wavelength conversion obtained at the output port of the circulator (back-to-back at 1547.38 nm ( ) and eight multicast channels from 1549.03 nm ( ) to 1560.38 nm ( ) with 200-GHz spacing (SMSR 30 dB).
The influence of ER on the BER of the output converted signal at different detuning of probe signal was previously reported [15]. The optical spectrum was measured at the output port of the circulator by a optical spectrum analyzer (resolution 0.08 nm). An EDFA amplified the output of the FP-LD, and an eight-channel arrayed-waveguide grating (AWG) removed the pump signal and out-of-band ASE and performed spatial wavelength demultiplexing of the different converted channels at the output port of the circulator. A variable optical attenuator (VOA) controlled the received optical power at a 10-Gb/s bandwidth p-i-n PD for BER measurements. B. Results and Discussion Fig. 8 shows the optical spectrum of the wavelength conversion obtained at the output port of the circulator before the EDFA. The spectrum shows the input pump wavelength at 1547.38 nm and eight multicast channels at wavelengths 1549.03 nm to 1560.38 nm with 200-GHz spacing. The SMSR of the input pump, and the converted signals was about 30 dB. The AWG filtered out the pump, other multicast channels, and the nearby FP side modes and thus increased the signal-to-noise ratio (SNR) of individual channels to about 40 dB.
BER performance of the wavelength converter was performed at 10 Gb/s and shown in Fig. 9. In this experiment, the CW probe of all eight channels were always ON, and BER measurements were performed individually one by one. The average power penalty of the 8 10-Gb/s multicast channels BER level, with channel-to-channel was about 2.5 dB at a variation by about 0.4 dB. The typical eye diagram of the multicast channel is shown in the inset of Fig. 9 and had an ER of 12 dB. The power penalty may have partially resulted from the presence of any unlocked FP mode which lowered the output ER. In this proof-of-principle demonstration, the number of channels for multicast was limited by the number of channels of the WDM source. A larger number of output channels may be possible if more channels of WDM source and AWG are available. The maximum number of channels of the wavelength conversion is determined by the gain spectrum of the FP-LD. Measurements indicated the wavelength conversion 10 dB. range to be about 32 nm for an output ER The BER performance was also investigated with different numbers of operating channels. It was observed that when one of the CW channels was switched OFF, the other seven multicast channels became “noisy” and degraded the BER. Fig. 10 shows that either increasing the optical power of the pump or increasing the temperature of the FP-LD is needed in order . The adjustment is to maintain the BER of channel 1 at needed to compensate for the blue shift in the mode spectrum when one of the injection-locking channels is switched OFF. IV. CONCLUSION Different modulation formats are appropriate to allow the optimum performance of backbone, metro, and access networks. Thus, wavelength routers for future all-optical networks should be capable of modulation format conversion and wavelength conversion to increase the network efficiency, scalability, and flexibility. Multicasting capability in wavelength routers is also desirable to support multicast applications in wavelength-routed
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multicaster exhibits desirable characteristics of stable amplitude and single polarization. REFERENCES
Fig. 10. Adjustment of input pump power or temperature of the FP-LD to maintain the BER of channel 1 at 10 , when different number of channels was (or were) switched OFF.
networks. A new approach of all-optical RZ-to-NRZ format conversion was described using a length of BF and a FP-LD. Also demonstrated in this paper was an all-optical multicaster using a single FP-LD. Error-free operations were achieved in the format conversion and every channel in the 8 10-Gb/s multicasting. In this paper, one FP-LD is used for modulation format conversion (Section II) and another FP-LD is needed for the multicaster (Section III). However, the RZ input to multicast NRZ outputs as well as the NRZ input to multicast RZ outputs could be performed by a single FP-LD. For the RZ-input-to-multicastNRZ-output case, a BF (in Fig. 2) could be put between the optical filter and 3-dB coupler in Fig. 7 to make a replica of the RZ input signal. Multiple wavelength conversions as described in Section III may then be performed by an FP-LD, and eight multicast NRZ outputs could be obtained at the output port of the circulator (Fig. 7). For the NRZ-input-to-multicast-RZ-output case, eight channels of 10-GHz optical clock pulses instead of the eight CW probe signals may be used as the WDM source, as shown in Fig. 7. After multiple wavelength conversions performed by the FP-LD, eight multicast RZ outputs will be obtained at the output port of the circulator. The eight channels of 10-GHz optical clock pulses may be generated by passing eight CW signals into a MOD, which is RF driven by a 10-GHz electrical signal. The two proposed schemes require only a single FP-LD and thus are potentially low cost, particularly as FP-LDs only required a dc bias. The wavelength conversion range is about 32 10 dB in the converted output and depends nm for an ER on the gain spectrum of the FP-LD. Although the input signal wavelength must lie within the injection-locking range of one of the modes of the FP-LD, an arbitrary input wavelength can be supported by temperature-tuning one of the FP modes to within the injection-locking range of the input signal. The format conversion scheme described in this paper is polarization dependent, but this issue may be resolved by using a fast polarization controller or using the polarization diversity scheme [14]. Since the injection locking can inherently clamp the output power and polarization when the injected optical power is above the locking threshold [21], the output from the
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C. W. Chow received the B.Eng. (First-Class Hons.) and Ph.D. degrees, both in electronic engineering, from The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, in 2001 and 2004, respectively. His Ph.D. research focused on wavelength conversion and optical labeling for wavelength-routed and packet-switched networks and applications of silicon-on-insulator waveguide and planar lightwave components.
C. S. Wong (M’00) received the B.Eng. (Hons.) degree in electronic engineering and the M.Phil. and Ph.D. degrees from The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, in 1997, 1999, and 2002, respectively. His M.Phil. research focused on terahertz generation and detection and its application of time-domain spectroscopy, and his Ph.D. research on components for future optical networks included work on all-optical wavelength conversion, time-division demultiplexing, and studies of optical nonlinearities of silicon waveguides for optical switching. In 2002, he was appointed a Postdoctoral Fellow with The Chinese University of Hong Kong. In June 2004, he joined Acasia Technologies (HK) Limited as a Member of Technical Staff.
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 11, NOVEMBER 2004
H. K. Tsang (M’90) received the B.A. (Hons.), M.A., and Ph.D. degrees from the University of Cambridge, Cambridge, U.K. He was a Visting Researcher with Bellcore, Red Bank, NJ, in 1990 and a SERC Postdoctoral Fellow at the University of Bath, Bath, U.K., before joining The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, in 1993 as a Lecturer in the Department of Electronic Engineering. He was a Director at Bookham Technology plc from 2002 to 2003. He returned to The Chinese University of Hong Kong as a Professor in summer 2003. He has published more than 120 papers in refereed journals or conference proceedings on optical modulators, all-optical wavelength and format conversion, two-photon absorption and nonlinear optics, applications of optical waveguides, and planar lightwave components.
