A fibre grating is a region of fibre whose refractive index has been permanently modified by exposing to light. Such exposure, if carried out with an interferometer or phase mask, allows a periodically varying refractive index grating or pattern to be “written” into the core of optical fibre. These “in-fibre” components are spectrally selective, which allow only specific wavelengths to be reflected or rejected, and are thus suitable for performing many tasks such as filtering and reflecting, in a highly efficient, low loss manner.

In NTRC, one of our research focus is on fabrication and applications of fibre gratings, including fibre Bragg gratings (FBG) and long-period gratings (LPG). An in-house designed Fibre Grating Fabrication System with sophisticated software for fabrication process control has been setup, allowing various types of fibre gratings to be fabricated including complex grating structures such as Moiré, sampled, chirped and phase shift gratings. For the first time, we have also demonstrated the possibility of directly writing gratings on an optical fibre coated with standard off-the-shelf coating, with the resulting reflectivity and index change found to be equivalent to that of a fibre grating fabricated with bare fibre. A US patent on this grating writing technique has since been filed.

In addition to grating fabrication, we have been studying the applications of fibre grating, particularly for communication applications. A noteworthy example is the recent work by Dr Dong Xinyong and his team on novel fibre grating-based optical fibre filters, in which they developed a bandwidth-tunable FBG filter with record tuning range (see Figure 2.2) and several LPG-based WDM filter designs (see Figure 2.3) that has potential applications in the present WDM communication systems. Another area we are pursing is the optimization of the gain profile for optical amplifiers by means of fibre gratings. Other investigations on grating-based optical devices such as filters, fibre lasers, add/drop multiplexers, and cross-connects for optical communication, as well as sensor element for optical sensing, are also being carried out.

Generalized Multiprotocol Label Switching

Current data network infrastructure consists of multiple switching technologies: the IP packet switch, ATM/FR cell switch, SDH TDM switch and the emerging wavelength switch and fibre switch. The heterogeneity of the diverse network infrastructure makes the control and management an insurmountable task. To simplify the task of performing network operations and provide efficient network resource utilization, Generalized Multiprotocol Label Switching (GMPLS) has been proposed as the unified control plane for a variety of networks capable of switching in the packet, time, wavelength, and space domains. GMPLS is an evolution of MPLS-TE, with essential enhancements to its routing and signaling protocols and the addition of a new Link Management Protocol (LMP).

In this project, we have proposed the novel model of GMPLS enabled multilayer router, shown in the Figure 2.4, and successfully developed the GMPLS prototype system based on PC/Linux platform. We then built a network testbed composed of the prototype GMPLS router to demonstrate essential aspects of GMPLS network operation, such as constraint based routing, path provisioning, integrated multilayer routing, network management etc. The implementation experience allows us to explore the insight of the GMPLS technology and related issues for the construction of next generation Internet infrastructure. New GMPLS related research issues with practical significance have been identified. These include the integrated multilayer network routing and GMPLS control plane scalability.

Channel Performance Monitoring for Terabit WDM Networks

Optical networks have an inherent property of transparency. This property makes it possible to support various data rates and coding formats simultaneously on the same network. In addition to this great flexibility, transparency also protects investments against future developments. Once deployed, an all-optical network will support a variety of future protocols and data rates without making major changes on the network as data is delivered from its source to its destination completely in optical form without undergoing any optical-to-electrical conversion. In such a transparent optical network, one of the most challenging tasks is to monitor, control and manage optical channels, since the signal quality may be affected by both linear and non-linear optical effects such as dispersion, crosstalk, amplifier noise, self-phase and cross-phase modulations. The most direct way to evaluate a digital transmission system is to measure its bit error rate (BER).

The project aims are:

(1) to propose new methods for monitoring BER of optical channels and identifying various sources of signal degradations;

(2) to develop advanced analytical models and software tools for studying various signal degradation mechanisms;

(3) to build and test an optical channel monitoring system.

So far we have developed a new BER monitoring technique based on signal’s amplitude histogram and curve fitting. To reduce the implementation cost, we have also proposed to sample the data at its sub-harmonic rate, which is much lower than the signal’s data rate. The sampled data are used to form the amplitude histograms of the signal, which is then fitted into multiple Gaussian functions by using Matlab-based programs. The fitted Gaussian functions are used to estimate the signal’s BER. Experimental studies have showed that the BER estimated by our new technique is in a good agreement with the BER measured by the bit-error-rate tester (BERT). The error range between the estimated BER and the measured BER is within an order of magnitude.

BER can determine the signal quality of optical channels but it cannot tell what causes the signal degradation. For the purpose of network management and monitoring, it is necessary to identify sources of degradations so that the network operator can locate the possible fault location or optical devices whose performances are deteriorating. It has been observed that different sources of degradation affect the shape of amplitude histograms in different manners. However, so far no effective method has been reported to identify various sources of degradations. We have developed a new method to analyze amplitude histograms of optical signals based on the X2 distance that was originally used in digital image processing. Simulation studies have shown that with this new method, three major sources of signal degradations namely, inband crosstalk, ASE noise and chromatic dispersion can be identified.

We have also proposed a new technique that can estimate the accumulated chromatic dispersion (CD) of optical channels. In this new technique, two RF signals are added to the baseband data signal as pilot tones at the transmitter and a dispersion offset such as a chirped fibre Bragg grating or a length of high-dispersive fibre is inserted just before the photodetector within the monitoring module. We can then estimate the accumulated CD based on the power ratio of two RF tones. Experimental and simulation results have shown that this method can achieve a high sensitivity and a large CD monitoring range.

Photonic Crystal Fibres Based Devices

Except for the photonic crystal fibre (PCF) filters and polarization maintaining PCF couplers which were successfully achieved, research work on all-PCF-based components and devices remains ongoing with promising experimental results published in Optics Letters. Not only the ultra-high temperature PCF sensor, and wide-passband, temperature-insensitive and compact pi-phase-shifted long-period gratings (LPGs) in PCF have been realised in collaboration with the Institute of Infocomm Research (I2R) in Singapore, but a joint developing project on PCF fabrication has also been established between NTRC and Yangtze Optical Fibre & Cable Company Ltd. (YOFC) in Wuhan of China. Preliminary results show that the first sample photonic bandgap (PBG) fibre designed by a Ph.D. student in NTRC, is soon to be completed. Several novel ideas on PCF-based devices for optical fibre communications and sensing systems such as a bandpass filter based on hollowcore silica fibre cascaded with a pair of LPGs in PCF, a PCF Mach-Zehnder interferometer with tunable wavelength by use of mechanical pressure and a helical PCF for torque sensor, have been bred and will be implemented for application.

Nonlinear fibre optics

When you dial a number on your phone or press the “send” button for your email, you actually initiate a series of light pulses on an optical fibre. The quality of the established communication link depends on both the strength and the shape of pulses when they reach the receiving end. Although optical pulses of less than 100 ps can be transmitted through several hundred kilometers without conversion to an electronic form, the signal quality is ultimately limited by nonlinear interactions between the information carrier lightwaves and the transmission fibre. NTRC’s nonlinear fibre optics group has been actively involved in theoretical and experimental investigations on the optical fibre telecommunication systems operating under nonlinear regimes. Key research areas include: fibre soliton laser, polarization mode dispersion, fibre Raman amplifier (FRA), optical buffer, wavelength converter and photonic microwave devices.

Traditionally, the light pulse propagating along the optical fibre deteriorates due to the chromatic dispersion of fibre. Therefore, dispersion compensation must be implemented to maintain highspeed data quality. However, by exploiting the nonlinear effects, fibre dispersion can be overcome and resulted in a very stable optical pulse known as soliton that is essential for the next generation optical networks and forms the major part of our research. We have realized a 72 fs ultrashort soliton pulse output from a passive mode locked fibre laser at 1550 nm wavelength, which is the shortest soliton pulse ever obtained from fibre cavity. In addition, we are also developing the ultra-high repetition rate fibre soliton lasers at 660GHz, as observed by passively mode locked technology. TeraHz pulses were observed in parametric amplifier based fibre ring laser. A multichannel 4 x 40 Gbps fibre soliton source using active mode locking technique is in the progress of developing.

With the development of large-scale optical transoceanic and terrestrial network, the signal attenuation due to inherent fibre loss must be offset. Comparing with the traditional Erbiumdoped fibre amplifier (EDFA), FRA based on the Stimulated Raman scattering in optical fibre attracted considerable attention due to its improved noise performance, simplicity and broadband feature. A novel all-optical double-pass discrete FRA has been developed in our lab with optimized gain and noise performances. Furthermore, a highly efficient wavelength conversion with 50 nm bandwidth based on Raman-enhanced parametric four-wave mixing is achieved in our doublepass configuration.

Optical buffer is a key component for future packet-switching all-optical networks. A novel dualloop optical buffer (DLOB) based on 3 x 3 collinear fibre coupler has been proposed and implemented experimentally. This optical buffer provides random read/write access and over 1 millisecond buffer time.

Polarization effects in optical fibres plays an important role in optical fibre communication systems and sensoring systems. We focus on the general theoretical description of polarization effects, measurement and real-time monitoring of polarization mode dispersion (PMD) vectors, and the distributed measurement of birefringence, polarization-dependent loss (PDL) and PMD by using Polarization Optical Time Domain Reflectometry (POTDR). The PMD & PDL emulator and PMD & PDL compensator are being developed. Furthermore, we conducted the fibre-based photonic microwave research by considering the polarization effects. A novel and simple photonic microwave notch filter using a Hi-Bi fibre is proposed to give a fixed differential group delay (DGD), together with a DGD element to provide tunable DGD. This configuration overcomes the problems of optical coherence interference and chromatic dispersion.

Organic light-emitting devices and liquid crystal devices

Transparent OLEDs fabricated based on LiF buffered Mg:Ag.

We have developed a new technique where LiF is applied to buffer a low work function cathode (Mg:Ag) to achieve improvement on electron injection, lower threshold voltage and increase in efficiency. The turn-on voltage of the device with 0.5 nm LiF as buffered layer is as low as 2.8 V. At injection current density of 20 mA/cm2, the current efficiency and power efficiency of the device is 5.56 cd/A and 1.94 lm/W respectively. Comparing the traditional device with Mg:Ag/Ag only cathode with OLEDs fabricated with LiF buffered Mg:Ag, the turn-on voltage is 1.6 V lower, the current efficiency improved by 75 %, and power efficiency is almost double.

Liquid Crystal (LC) Spiral Phase Plate

We have fabricated a “Liquid Crystal Spiral Phase Plate” which will generate doughnut beams for optical tweezers, rotational frequency shift and etc. Based on the specially designed LC cell of 18 um, where the phase plate is driven by a multi-channel analog output card and in additional with the development of a software to program the driving voltage, the topological charge of the Laguerre Gaussian beam can be changed in about one second.

High Efficiency Polarization-Insensitive Volume Diffraction Gratings

We have fabricated 2 x 2 high efficiency polarization-insensitive volume diffraction gratings based on LC-polymer composite. The highest diffraction efficiency achieved was 85.7%. The rise time and the decay time measured were 36 s and 160 s respectively at an applied electric field of 18.2 V/m. The polarization-dependant loss was 0.03 dB measured for s- and p-polarized light at the wavelength of 632.8 nm.

Ongoing Research Project