Switching-time limitation in tunable multisection lasers

The impact of the thermal transient on the frequency switching of multisection tunable lasers is studied. A simple thermal model is used to calculate the amplitude and time evolution of the frequency transient due to the thermal properties of both the laser chip and its mount. Transient time constants as large as 200 μs for the laser chip and several hundred milliseconds for the diode mount are measured, limiting the applicability of these devices to systems where the frequency is switched at low rates. A method to compensate electrically for this transient by means of a passive net work is demonstrated     

Frequency stabilisation of FDM optical signals originating from different locations

Frequency stabilisation of FDM optical signals to a comb of equally spaced frequencies has been demonstrated for optical signals originating from different locations. The result was achieved by locking each optical source to a resonance of a separate fibre Fabry-Perot cavity. The Fabry-Perot's comb of resonances was synchronised by locking all these devices to a master reference. Implementation of the frequency stabilisation circuit requires, at each location, a tunable fibre Fabry-Perot resonator, a photodetector and simple electronics. Such a simple circuit provides the means to frequency-stabilise a large number of FDM optical sources originating from different locations, as in a star network.     

Densely spaced WDM coherent optical star network

An optical WDM star network consisting of three lasers transmitting at about 234 000 GHz, spaced by 300 MHz, has been used to demonstrate dense packing of WDM signals. The three optical signals, FSK-modulated at 45Mbit/s, are multiplexed by a 4 × 4 star coupler and demultiplexed by a balanced heterodyned receiver. Receiver sensitivity is -61dBm for a BER of 10?9, or 113 photons/bit, which is 4.5dB from the shot noise limit and represents the best sensitivity yet reported for FSK modulation. The results indicate that 100 000 users in a 10km radius could be interconnected with such a system.     

An optical heterodyne mixer providing image-frequency rejection

This paper describes an optical heterodyne mixer which detects a desired signal at frequencyf_{LO} pm f_{IF}while rejecting interference from any signal at the image frequencyf_{LO} pm f_{IF}. Implementation of the mixer is relatively simple. Its performance is insensitive to fluctuations in the optical dimensions of the circuit. This mixer is particularly attractive for communication systems using wavelength division multiplex techniques.     

Densely spaced FDM coherent star network with optical signalsconfined to equally spaced frequencies

The results obtained with a fiber-optical star network using densely spaced frequency-division-multiplexing (FDM) and heterodyne detection techniques are discussed. The system consists of three optical sources transmitting around 1.28 μm, frequency-shift keying (FSK) modulated at 45 Mb/s and spaced by 300 MHz. A 4×4 optical coupler combines the three optical signals. The FDM signals, received from one of the four outputs of the coupler, are demultiplexed by a heterodyne FM receiver. The minimum received optical power needed to obtain a bit error rate (BER) of 10^-9 is -61 dBm or 113 photons/bit, which is 4.5 dB from the shot noise limit. Cochannel interference is negligible for the above channel spacing and modulation rate. The results indicate that such a system has a potential throughput of 4500 Gb/s. The results obtained with two frequency stabilization circuits used to confine these three FDM optical signals to a comb of equally spaced frequencies are also presented     

Bit error rate performance for wavelength conversion at 20 Gbit/s

Wavelength conversion at 20 Gbit/s, to longer and shorter wavelength, is achieved using cross gain-compression in a semiconductor optical amplifier. Quantitative performance is assessed from the bit error rate data at 20 Gbit/s, for the first time for any wavelength conversion scheme. For conversion of 10 nm to a shorter wavelength, the penalty is 2.6 dB     

Frequency-locked loop circuit providing large pull-in range

Presents a frequency-locking circuit that provides a larger pull-in bandwidth than that of a conventional automatic frequency control (AFC) circuit. In addition, the proposed circuit has a stable mode of operation over the full frequency response of the frequency discriminator, and locks the oscillator at exactly the centre frequency of this discriminator. The frequency offset and the unstable mode of operation presented by the conventional circuit outside its pull-in range can thus be eliminated.<>     

Optical amplification in a multichannel FSK coherent system

Reports the crosstalk degradation caused by an optical amplifier in a densely spaced four-channel heterodyne FSK system. A maximum receiver sensitivity of 250 photons/bit is obtained for an optimum input signal level. This result is 5 dB poorer than the sensitivity obtained in the absence of an optical amplifier     

A Fast-Switching Low-Loss 12-GHz Microstrip 4-PSK Path Length Modulator

A 4-PSK microstrip modulator operating at data rates of up to 800 Mbits/s at 12 GHz is described. The circuit is made of two cells in series. Each cell consists of a 3-dB branch-line hybrid coupler and two BTL p-i-n diodes. One cell provides 0° or 90° phase shifts, and the Other 0° or 180° phase shifts, so that four carrier phase values are obtained by appropriately exciting the two cells. The switching time of each cell is 200 ps. Simultaneous switching of both cells increases the switching time to a maximum value of 400 ps. The phase waveforms are nearly rectangular at the above-mentioned data rates. RF insertion loss is 1 dB ± 0.1dB for the four phase values over the 11.7-12.2 GHz frequency band.     

Wavelength conversion at 10 Gb/s using a semiconductor opticalamplifier

Data at 10 Gb/s has been translated from an input signal wavelength to another wavelength, either longer or shorter, using gain compression in a 1.5-μm semiconductor optical amplifier for wavelength conversion. To achieve operation at such high bit rates, the probe (shifted) input must be intense enough to compress the gain of the amplifier significantly. This reduces the gain recovery time of the amplifier because of probe stimulated emission. A consequence of the intense probe is an extinction ratio deduction. Using moderate input powers, wavelength conversion is achieved over a 17-nm (2-THz) range, with 0.7-3-dB power penalties