CUDOS Monash Laboratory Facilities
Over the last 5 years, the Department of Electrical and Computer Systems Engineering has invested heavily in top-end test and measurement equipment for photonics and electronics, enabling it to demonstrate its innovations in electro-photonics at data rates beyond 10 Tbit/s. In the Long-Haul Optical Transmission Systems laboratory, it has built a 1440-km optical test bed, comprising eighteen 80-km of optical fibre, with optical amplifiers between each span.
A key element of the laboratory is the ability to simulate complete optical communications systems, using VPIphotonics software, then progressively prototype the system by converting simulated waveforms to real waveforms and back again, using extremely fast digital-to-analogue (DAC) and analogue to digital converters (DACs). This means that parts of the system can be developed in software, while others are tested using hardware. This allows us to develop innovative systems using combinations of optical and electronic signal processing.
The laboratory contains the following equipment, which can be easily reconfigured to suit a different projects.
Arbitrary Waveform Generation
We are able to generate candidate waveforms for transmission over optical fibre by either using MATLAB or VPItransmissionMaker software, then converting the data files into analog electrical waveforms using Tektronix Arbitrary Waveform Generators (AWG). Our two AWGs ‘play’ a waveform at rates up to 20 billion points per second, enabling an arbitrary optical signal with a bandwidth of 14 GHz to be constructed using a ‘complex’ optical modulator.
Our expertise is in generating Orthogonal Frequency Division Multiplexed (OFDM) waveforms, which have a high spectral efficiency and very tight spectral control. We have also generated Nyquist WDM waveforms using the AWGs. The AWGs can also be used to modify the characteristics of a ‘pure’ laser source, for example, by adding phase modulation to broaden the linewidth of the source by a computer-controlled amount.
We also have a 44-Gbit/s Bit Error Ratio (BER) test set. This can be used to generate non-return to zero and QPSK signal at any rate up to 44 Gbit/s. Typically, we use this to generate all-optical OFDM signals.
1440-km Optical Test Bed
The low cost of optical fibre means that we have been able to purchase enough fibre to test most ‘long-haul’ systems realistically. We have a mixture of 30 km, 40 km and 50 km fibre spools, enabling us to test over a variety of fibre span lengths. Typically fibre spans are 80 km (50 miles). Each span has a loss in the order of 16 dB (a factor of 40), so we have purchased enough Erbium-doped fibre amplifiers (EDFAs) to compensate this loss after each span. All of the amplifiers are controlled from a single computer. This test bed allows us to investigate the degradation due to optical fibre nonlinearity at high optical powers, so test the information bandwidth of the system.
Coherent Receivers and Digitisation
We have constructed two coherent optical receivers. These are able to convert an optical signal into four electrical signals, representing the inphase (I) and quadrature (Q) components of each of the two polarisations in a fibre. These signals are then digitised using two ‘real-time’ digital sampling oscilloscopes (DSO). Each DSO can sample two channels at up to 80 billion times a second, with a bandwidth of 28 GHz, electrical. These receivers and DSOs allow us to convert a 50-GHz bandwidth optical signal in a fibre to data files representing the signal in both polarisations. Offline processing can then be used to correct for fibre dispersion polarization, chromatic) and nonlinearities, or investigate any feature of the signal using signal-processing techniques.
We used six tunable lasers as the primary source of light in our laboratory. These can be tuned to any wavelength within the C-band of optical communications. We can also modulate these sources to produce around ten lines, accurately spaced at up to 20 GHz. Each line can then be modulated with data.
We have recently acquired an Ergo mode-locked laser running a 10 Gpulses/s. Each pulse is <2 ps wide, so the laser produces a comb spectrum with around 50 lines. We normally broaden this spectrum using a highly nonlinear fibre, to produce around 250 lines. The lines can then be modulated, using one or more parallel modulators, to impose data. We have created 10 Tbit/s data streams using this technique.
We have three methods of analysing the spectrum of an optical signal: (1) a grating monochromators, which is useful for measuring the optical signal to noise ratio (OSNR) of the signals; (2) a High-Resolution Spectrophotometer, which is able to resolve spectral peaks down to 20 MHz; (3) The coherent receivers and DSOs, which are able to perform Fourier-transform analysis on waveforms with a resolution of <1 MHz.
We use four Finisar’s Waveshapers to manipulate the spectra of our optical signals. We have shown that the Waveshapers can be used to generate 10 Tbit/s test waveforms with mixes of modulation formats including OTDM, WDM, Nyquist-WDM and OFDM. We also use the waveshapers for demultiplexing optical signals, and flattening the gain spectra of EDFAs. We also have thin-film filters, Bragg gratings and ASE-blocking filters.
Although most of our work is performed using fibre-pigtailed devices, we have a precision micropositioner station on which to test optical chips and connect them into our communications systems. This is being used to test linear and nonlinear optical chips as part of OFDM systems.
We have a wide selection of fibre-pigtailed optical components, allowing us to plug together complex systems. The components include high-power optical amplifiers, nonlinear fibre, dispersion-compensating fibre, variable attenuators, splitters and couplers, polarization manipulating components, ruggedized Mach-Zehnder Interferometers (MZIs), Polarization Multiplexers, variable delays, modulators and photo-receivers.
We have a range of high-performance microwave components for driving our optical modulators and manipulating OFDM signals to add carriers and shift spectra.
The laboratory also has access to high-performance microwave test equipment including auto-calibrating two-port analysers, synthesised signal generators to 50 GHz, and calibrated power meters.