Crosstalk penalty in a two-channel ASK heterodyne detection system with non-negligible laser line width

Crosstalk penalties as a function of channel spacing were measured in a two-channel ASK heterodyne system using 1·55?m DFB lasers. For an IF linewidth/bit rate ratio of 0.5, the channel spacing in the IF domain must be greater than 13 times the bit rate for negligible crosstalk penalty.     

Crosstalk in a two-channel coherent fibre-optic ASK system using anoptical amplifier and non-negligible linewidth lasers

Presents measurements of crosstalk penalties in a two-channel ASK system using 1.55 μm DFB lasers and a semiconductor laser amplifier. The amplifier was operated well below gain saturation. No additional receiver sensitivity penalty due to the amplifier was observed for single channel operation. However, the additional crosstalk in the amplifier required at 10% increase of the channel spacing compared to the spacing in the nonamplifier system     

The effect of crosstalk and phase noise in multichannel coherentoptical ASK systems

The authors extend the analysis of G.J. Foschini et al. (1988) to a two-channel system and to a wider class of IF filters. The authors carry out a comprehensive study of the sensitivity of an optical receiver in a two-channel ASK system, taking into account phase noise and crosstalk, including crosstalk arising from phase noise in the second channel. They compare the results with previous work in the limit of no phase noise and show how the agreement is good. They compute the penalty due to crosstalk for different linewidths and filter shapes. It is shown that the minimum channel spacing is a few bit rates more than the IF filter bandwidth, and is therefore increased significantly by significant phase noise. The detailed IF filter shape appears to have only a weak effect     

Dynamic range of an optical amplifier cascade in an ASK system withsignificant phase noise

The authors present detailed results concerning the dynamic range of an amplitude-shift keyed (ASK) heterodyne optical communication system where the transmitted signal is impaired by significant amounts of laser phase noise. Theoretical results are evaluated for a system with up to ten in-line optical amplifiers and a significant influence of the laser phase noise is found for the maximum allowable transmission span and the dynamic range. With ten amplifiers, a linewidth of 11% of the bit rate causes the span to be reduced by 12.2 dB and the dynamic range to be reduced by 5.7 dB. The results have been confirmed using two amplifiers in a field trial experiment at a bit rate of 636 Mb/s with an IF linewidth of 70 MHz     

Ultimate performance of optical DSB signal-based millimeter-wavefiber-radio system: effect of laser phase noise

It is theoretically shown that the phase noise of laser light source can be automatically eliminated by compensating the differential group delay due to the fiber dispersion between the two sidebands of the optical DSB signal. The bit error rate (BER) measurement of 60 GHz millimeter (mm)-wave subcarrier multiplexed optical double-sideband (DSB) signal transport in dispersion-compensated optical fiber link using fiber Bragg grating (FBG) mill show that the effect of the laser phase noise on the BER is as small as a few % relative to the other additive noise effect, thus realizing almost the ultimate performance     

The impact of laser phase noise on the coherent subcarriermultiplexing system

In coherent optical subcarrier multiplexing (CSCM) systems, the laser phase noise may cause signal spectrum broadening and hence, causes significant deterioration in the system performance. The impact of phase noise on the CSCM system is analyzed in terms of carrier-to-noise ratio, intermodulation distortion, and adjacent channel crosstalk. The optimal modulation index and carrier to noise ratio are also presented. Some numerical results are outlined     

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.     

Quantum phase noise and linewidth of a semiconductor laser

Quantum phase noise of a single longitudinal mode AlGaAs semiconductor laser is measured by converting it into intensity noise. A Michelson interferometer is used and a time delay is introduced. Results on spectral linewidth and noise spectrum are in good agreement with theoretical analyses based on a rate equation. Spectral linewidth of the semiconductor laser decreases from 100 MHz to 4 MHz in proportion to (I/Ith?1)?1 at the bias level of I/Ith=1.05 ? 1.4.     

Probability density function for the phase and frequency noise in a semiconductor laser

A physical definition for the phase and frequency noise of a semiconductor laser is given, which can be applied in determining the performance of coherent optical communication systems. The definition allows a filtered Poisson model to be developed for the probability density function (pdf) of the laser output phase or frequency noise, based on evaluating the cumulants of the noise processes. A condition is derived to quantify under what laser and system conditions a Gaussian pdf is a good approximation. It is shown that the phase and frequency noise can increase significantly for bit rates approaching the relaxation oscillation frequency of the laser diode.     

Effect of laser phase noise in Sagnac interferometers

A theoretical analysis of the responsivity and the noise caused by backscattering in a Sagnac interferometer used as a sensor for reciprocal measurands, such as acoustic waves, is presented. Both Rayleigh backscattering and reflections from splices are taken into account. The noise power is found to increase proportionally to the source coherence time, and a noise equivalent phase shift in the range of 0.1×10^-6 rad r.m.s./Hz^1/2 is predicted for typical fibers and diode lasers. Experimentally, a noise equivalent phase shift of 2.5×10^-7 rad r.m.s./Hz^1/2 at 10 kHz was observed, with a detector current of 3 μA     

Effect of laser linewidth on crosstalk penalty in two-channel ASKheterodyne detection system

The effect of laser linewidth on the crosstalk penalty in a two-channel ASK heterodyne system is calculated through a simple frequency domain analysis. The results show that the crosstalk penalty increases steeply when the linewidth at the IF stage exceeds one-tenth of the bit rate. The results of calculation show very good agreement with experiment by Park et al     

Theory of the phase noise and power spectrum of a single mode injection laser

Spontaneous emission alters the phase and amplitude of the laser field. The amplitude changes induce relaxation oscillations, which cause additional phase changes while restoring the field amplitude to the steady state value. It was previously shown that the additional phase changes greatly enhance the linewidth. We show here that the additional phase changes also give rise to line shape structure in the form of additional peaks separated from the main peak by multiples of the relaxation oscillation frequency. The calculated mean square phase change and power spectrum are in good agreement with published observations.     

Crosstalk in a two channel coherent fiber optic FSK system using asemiconductor laser amplifier

Bit-error-rate measurements on a two-channel optical transmission system using a low-reflectivity laser amplifier are presented. The signal lasers are directly frequency-modulated or amplitude-modulated at 1 Gb/s. The crosstalk penalties in an FSK-FSK system are very small and are mainly attributed to the residual small amplitude modulation of the lasers resulting from the direct frequency modulation. The crosstalk in an ASK-FSK system is considerably larger     

Semiconductor laser noise in an interferometer system

The noise of semiconductor laser light after passing a Michelson interferometer has been measured for gain guided as well as index guided double-heterostructure injection lasers. This noise is mainly due to the partition noise and the frequency noise of the laser emission. Unless the interferometer is perfectly balanced, the observed noise is several orders of magnitude larger than the usual intensity noise of semiconductor lasers.     

On the effect of Wiener phase noise in OFDM systems

Multicarrier modulation exhibits a significant sensitivity to the phase noise of the oscillator used for frequency down-conversion at the portable receiver. For this reason, it is important to evaluate the impact of the phase noise on the system performance. We present an accurate method to determine the error probability of an orthogonal frequency-division multiplexing (OFDM) system in the presence of phase noise. In particular, four modulation schemes are analyzed and their performances are compared     

Elimination of phase noise of pulsed laser diodes in electroopticsampling systems

The multimode operation of pulsed laser diode sources is converted into excess noise in electrooptic probing systems for electrical waveforms and prevents measurements with shot-noise-limited sensitivity. The transfer function of the sampling system is determined and analyzed in terms of the wavelength and electric field dependence. It is shown that by proper orientation of the polarizing components within the optical beam path, points of operation can by found which have both a vanishing wavelength dependence of the transfer function and maximum sensitivity to the electric fields to be measured. Excess noise sources due to wavelength fluctuations of the laser are thus eliminated, and shot-noise-limited voltage resolution is obtained. The system has been optimized to yield a voltage sensitivity of 2.3 mV/√Hz for a longitudinal LiNbO₃ probe crystal     

Performance of coherent optical CPFSK-DD with intersymbolinterference, noise correlation, and laser phase noise

An exact probability of error expression is presented for a narrow-deviation binary CPFSK coherent optical receiver utilizing differential detection (CPFSK-DD). The result is given in terms of the Marcum Q-function and takes into account the non-Gaussian noise statistics at the decision moment, intersymbol interference, noise correlation, and laser phase noise. Numerical results indicate a local oscillator shot-noise-limited receiver sensitivity of 23.4 photons/b when using a modulation index of 0.67 in combination with an IF filter having a sixth-order Butterworth magnitude response and a normalized 3-dB bandwidth of 1.09. For a given IF filter and IF linewidth, it is found that the modulation index and the IF filter bandwidth should be jointly optimized in order to achieve the best overall receiver sensitivity. When the IF linewidth times the differential delay product is 0.34 %, modulation index and the optimum normalized 3-dB bandwidth are found to be 0.72 and 1.1, respectively. By using Monte Carlo simulation, it is demonstrated that adding laser phase noise at the demodulation stage of the analysis is a valid simplifying assumption for a wide range of practical design parameters     

The effect of error control coding in multichannel FSK coherentlightwave communication systems influenced by laser phase noise

It has been shown that coherent optical fiber receivers with a two-filter structure (TFS) consisting of a wide-band IF filter and a narrow-band postdetection filter are less susceptible to the influence of phase noise. However, the expanded IF bandwidth required to achieve optimum sensitivity performance is large, particularly in multichannel FSK systems. Forward error control coding can relax the laser linewidth requirement and improve receiver sensitivity. In this paper a multichannel asynchronous FSK scheme equipped with (31, k) Reed-Solomon codes is used to verify the coding benefit. A systematic error probability analysis is developed and a stable and accurate performance evaluation procedure is provided. The sensitivity penalties due to the combined phase noise and interchannel crosstalk for both coded and uncoded systems are calculated for comparison. The results show that the performance reduction due to phase noise can be largely alleviated by choosing a proper code rate and an optimum value of the expanded IF bandwidth     

Effect of external cavity semiconductor laser phase noise on a DPSKheterodyne optical receiver

The effect of an external cavity semiconductor laser on the performance of a DPSK heterodyne optical system is evaluated. The actual shape of the frequency fluctuation spectral density of the optical source is taken into account, and a comparison with a PSK coherent heterodyne system is carried out