Power budget optimisation of WDM network with MSCM control

The authors have experimentally demonstrated a 28.7 dB power budget for simultaneous transmission of 2.5 Gbit/s/node payload data and 80 Mbit/s/node control channels using the multichannel subcarrier multiplexing (MSCM) signalling technique. The power budget is sufficient to support a 234 node MSCM-WDM network with 2.5 Gbit/s/node 10 km radius coverage and 0.25 dB/km fibre attenuation with 0.45 dB/km attenuation, the network can support up to 93 nodes     

Multichannel CPFSK coherent optical communications systems

A detailed theoretical analysis of multichannel coherent CPFSK communications systems is presented. The analysis accounts for the crosstalk between adjacent channels, the intersymbol interference and correlation between noise samples stemming from the limited IF bandwidth the non-Gaussian statistics of the noise at the decision gate, and the impact of the laser phase noise. It is found that the IF bandwidth needed to avoid intersymbol interference is 2.2 bit rates for a modulation index m=1; it is larger for other modulation index values. For m=1, receiver sensitivity is within 1 dB of the shot noise limit, and the electrical domain channel spacing can be as small as 2.05 bit rates with 1-dB sensitivity penalty. The foregoing conclusions are valid for a negligibly narrow linewidth; the degradation due to phase noise is shown to be modest as long as the linewidth does not exceed 1% of the bit rate if m=1. Larger linewidth can be tolerated if the modulation index is larger than unity     

Three-wave envelope solitons: possibility of controlling the speedof light in fiber

Theory predicts that three-wave envelope solitons (TWES) can he generated in dual-mode optical fibers by simultaneous injection of two copropagating optical modes. The mechanism of the three-wave interaction is the recently observed intermodal forward stimulated Brillouin scattering (FSBS). The dynamics of soliton generation depends on the duration of the injected pulses, the pump power, and the attenuation time constant. For the adiabatic modulation of injected optical waves, a new type of generation has been analyzed: an acoustical wave structure that scatters the incident pump into the Stokes wave is formed in the fiber prior to and after the soliton generation. This structure appears as a result of FSBS and serves as a TWES “launcher.” We identify this type of generation in earlier soliton experiments in stimulated Raman scattering. The TWES velocity depends on the pump power. For a typical dual-mode fiber, the speed of TWES can be adjusted over four orders of magnitude by adjusting the pump power between 0.01 and 200 mW. The duration of the soliton is <3 ms due to the acoustic attenuation. The length of the fiber can be shorter than the length of the soliton while preserving the same TWES characteristics. Both Ar⁺ and Nd:YAG lasers are suitable for TWES generation     

Impact of laser intensity noise on ASK two-port optical homodyne receivers

The letter describes initial experimental results obtained with a multiport optical homodyne receiver employing a DFB laser. The receiver performance is found to be limited by the intensity noise of the local oscillator rather than by the phase noise, even when the product of the IF linewidth and the bit duration is as large as 0.56. A relative intensity noise level of at least ? 140dB/Hz will be required for a satisfactory receiver performance with ? 15dBm local oscillator power.     

An Optical Rain Gauge Based on Forward Scattering

A novel approach to the real-time quantitative measurement of rainfall rate is proposed and investigated. The proposed method relies on the sensing of coherence degradation. suffered by a laser beam propagating through rain. A simple model explaining the system operation principle is presented and the necessary data processing algorithm is derived. Further, possible implementations of the suggested rain gauge are discussed. It is shown that the ground-based rain gauge is more easily realizable and may provide better accuracy than the satellite-assisted gauge. A 1-km ground-based rain gauge would require lasertransmitter power of the order of 3 mW; such, and much higher, powers are easily generated by modern lasers. Finally, the possible impact of multiple scatterings, absorption, aerosols/hydrometeors, and of the atmospheric turbulence on the proposed device is discussed. It is shown that with proper system design none of the aforementioned factors should impair the operation of the proposed rain gauge. Adequate system-design parameters are as follows: minimum receiver diameter is 0.1 mm; maximum receiver diameter is 16 mm; wavelength of operation is 0.63 ?m; intermediate frequency is 1 MHz; and system bandwidth is 1 kHz.     

High-repetition-rate pulsed-pump fiber OPA for amplification of communication signals

The use of a high-repetition-rate pulsed-pumped fiber optical parametric amplifier (OPA), followed by a narrow optical filter for transparent signal amplification, was proposed. Theory and simulations predict larger gain and gain bandwidth compared to a continuous-wave pump with the same average power. Experimentally, when using a pump with 0.63 W of average power in a 500-m-long highly nonlinear fiber, the gain increased from 19.7 to 29.2 dB, and the bandwidth increased when a CW pump was changed to one that is modulated by a 20-GHz cosine-squared function. Clear eye openings were demonstrated for the amplification of a 10-Gb/s NRZ signal, with a power penalty of 1.5 dB.     

Pump-to-signal transfer of low-frequency intensity modulation in fiber optical parametric amplifiers

This paper describes the theoretical and experimental investigation of the transfer of low-frequency intensity modulation (IM) from pump to signal in fiber optical parametric amplifiers (OPAs). It is first established that low-frequency IM of the pump remains unchanged over the length of the amplifier in spite of the presence of parametric gain. The pump-power dependence of the OPA gain is then used to calculate the instantaneous effect of pump IM on the signal and idler output powers. These calculations are performed for both one- and two-pump OPAs. The main predictions are that 1) the ratio /spl rho/ of the signal intensity modulation depth to that of the pump varies across the OPA gain spectrum and 2) for a 20-dB gain, /spl rho/ can exceed 10 at some wavelengths, which indicates that this effect can be detrimental. Experiments have been performed to verify these predictions. Using sinusoidal IM of the pump, the resulting amplified signal IM was measured, and the experimental results were found to be in good agreement with the theoretical predictions.     

Impact of reflections on phase-diversity optical homodyne receivers

Reflections as small as -35 dB in the local oscillator path can cause deterioration in the performance of two-branch phase-diversity optical homodyne receivers, even when optical isolators protect the laser. The impact of reflections can be alleviated through the use of balanced receivers     

DBR active optical filters: transfer function and noisecharacteristics

A theoretical model of distributed Bragg reflector (DBR) active filters is presented and verified experimentally through measurements of the transfer function and the noise spectra. The theory allows simple yet accurate evaluation of the transmission transfer function and of the noise properties of this class of filters. A conventional DBR laser device used as a filter is shown, experimentally and theoretically, to have two drawbacks: a multilobe transfer function and small gain (less than 10 dB facet-to-facet). It is shown how both problems can be overcome via reduction of two key device parameters: the grating coupling coefficient and the physical length of the active and/or phase control sections. This technique can lead to devices with attractive properties, having a gain of 35 dB, sidelobe suppression of 32 dB, and bandwidth as narrow as 1 GHz     

Optical signal processing for lightwave communications networks

A number of optical signal processing functions that might be potentially important for future lightwave communication networks are described. An optical network with a distribution capacity of 100 HDTV channels is considered along with how such a network can be implemented using the following functional subsystems: frequency converters; transmitter banks; modified (wavelength division multiplexing) WDM demultiplexers; and tunable optical receivers. Discussed are the key network-level issues: the power budget, the channel separation, and the overall rationale for selection of multiplexing techniques. A hardware implementation of the functional subsystems using three basic building blocks-tunable amplifiers/filters, phase locked loops, and comb generators-is discussed