IF Linewidth Research Articles

Phase noise-tolerant synchronous QPSK/BPSK baseband-type intradyne receiver concept with feedforward carrier recovery

Quadrature phase-shift keying (QPSK) is attractive to increase transmission lengths and capacity, especially when it is combined with polarization division multiplex. Baseband processing at the symbol rate allows to keep the required electronic bandwidth low. So far, external cavity lasers seemed to be indispensable for such transmission systems due to linewidth requirements. We propose a feedforward carrier recovery scheme based on regenerative intradyne frequency dividers, i.e., the well-known regenerative frequency divider is extended to process baseband in-phase and quadrature (I and Q) signals. An IF linewidth tolerance of up to 0.001 times the QPSK symbol rate is predicted, 2 decades more than for an optical phase locked loop with a realistic loop delay. This means that commercially available DFB lasers shall suffice for synchronous optical QPSK/BPSK transmission.

Performance of coherent optical CPFSK-DD with intersymbol interference, 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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamic range of an optical amplifier cascade in an ASK system with significant 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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Linewidth requirements for an optical MSK heterodyne/synchronous receiver

The laser linewidth requirements for an optical MSK heterodyne/synchronous receiver are presented taking into account the effect of imperfect phase references. The performance of the receiver is evaluated in terms of the bit-error-rate, receiver sensitivity, and loop bandwidth. It is shown that an IF linewidth of less than 3.0*10/sup -4/ of the bit rate is required to avoid the power penalty of 0.5 dB with the RMS phase error of 0.11 radians. In comparison, the synchronous PSK, differential MSK, and DPSK heterodyne receivers are more tolerant to the laser phase noise than the synchronous MSK heterodyne receiver. >

Multichannel ASK system design influenced by significant laser phase noise

Results from an improved bit-error-rate analysis appropriate to heterodyne ASK receivers influenced by an interfering optical channel are reported. The analysis takes into account the effects of receiver noise and phase noise. The leading order influence of phase noise in the signal of an interfering channel is now accounted for in an exact way. The results indicate a somewhat larger phase noise influence from the signal in the interfering channel than is predicted from a stationary phase evaluation of that contribution. For no phase noise in the two channels a required electrical channel separation of three times the bit rate for 1 dB penalty due to crosstalk is found. The separation increases to 11.3 times the bit rate for an IF linewidth of 0.27 times the bit rate for the received channel and a linewidth of 0.54 for the interfering channel.

Performance of novel low intermediate frequency coherent optical DPSK receiver using baseband detection

We have studied the performance of a heterodyne DPSK receiver operating with an intermediate frequency of only half the bit rate. The system degradation owing to laser phase noise and additive receiver noise has been analysed and is compared to experimentally obtained results. The theoretical shot noise limit of our system is 24.7 photoelectron/bit for a BER of 10−9 which is only 0.9 dB from the shot noise limit of a conventional heterodyne DPSK system. The degradation of our system owing to laser phase noise is smaller than for a conventional heterodyne DPSK system because an IF linewidth of 0.8% of the bit rate introduces a 1 dB system penalty at 10−9 BER compared to 0.5% for heterodyne DPSK systems. To achieve a system performance with less than 1 dB penalty from the optimum the IF may vary within a range of up to 7.5% of the bit rate. An experimental verification of the system has been carried out with a 1 Gbit/s system.

Highly sensitive frequency discriminator for wide IF linewidth FSK demodulation

A new and highly-sensitive delay-and-multiply discriminator with an electrical sensitivity of –25 dBm is presented. The applicability of frequency discriminators in wide linewidth systems is discussed, and experiments with single-filter (SF) and discriminator demodulation in systems which are otherwise identical are compared. The receiver sensitivity is 3.4dB better, and the receiver is much more robust to the laser FM response with the discriminator than with the SF demodulator.

Coherent FM-SCM system using DFB lasers and a phase noise cancellation circuit

A multichannel coherent subcarrier multiplexing (SCM) system using standard distributed-feedback lasers is discussed. A simple phase noise canceling circuit yields excellent quality FM video transmission (SNR=56 dB), even though the 50-MHz IF linewidth exceeds the 30-MHz signal bandwidth. >

4-ary FSK coherent optical communication system

Experimental verification of the sensitivity advantage of M-ary orthogonal signalling over binary signalling is reported. A 110 Mbits 4-ary FSK system using conventional Fabry–Perot lasers with a 7% IF linewidth to data-rate ratio has achieved a sensitivity of 37 detected photons per bit at a 10−9 bit error rate. This exceeds the performance possible using ideal binary orthogonal FSK with zero linewidth lasers.

Wide-linewidth phase diversity homodyne receivers

The use of phase diversity homodyne receivers, which have excellent performance even when the laser linewidth is of the same order of magnitude as the bit rate, to construct coherent systems with semiconductor lasers and moderate bandwidth receivers is considered. Theoretical, experimental, and computer simulation results of a study of a linewidth homodyne phase-diversity receiver is presented. A 150-Mb/s system with an IF linewidth of more than 50% of the bit rate is investigated in depth and is experimentally shown to operate within 1.8 dB from its theoretical limit.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Theory for optical heterodyne narrow-deviation FSK receivers with delay demodulation

A theoretical model is presented that includes the effects of laser phase noise, receiver noise, imperfect modulation, IF bandwidth, and postdetection filtering. Detailed numerical results for 140-Mb/s and 400-Mb/s systems are presented, showing excellent agreement with independent published experimental results and strongly supporting the theoretical analysis. It is found that an IF linewidth of less than 0.25% of bit rate is required to avoid degrading the receiver sensitivity by more than 1 dB in a system with a strong local oscillator and modulation index of 0.7. A larger modulation index allows a larger linewidth to be accommodated. If the demodulation is not optimal, a narrower linewidth is necessary. >

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.

Influence of (semiconductor) laser linewidth on the error-rate floor in dual-filter optical FSK receivers

We discuss an error-rate floor in dual-filter optical FSK receivers in terms of the IF linewidth and the frequency shift between 0s and 1s, using a comprehensive model for the receiver which takes into account nonzero laser linewidth, noise correlation and signal overlap between the filters. We present calculated results for total error probability in terms of signal power, IF linewidth and frequency shift, illustrating the effect of the floor on receiver sensitivity, and we contrast it with a related effect in heterodyne ASK systems.

Error-rate floor in optical ASK heterodyne systems caused by nonzero (semiconductor) laser linewidth

Using a simple model to account for the effects of non-negligible linewidths for the transmitter and local oscillator lasers in a heterodyne ASK system, we show that there is an error probability floor which is a function of the IF bandwidth and the relative decision threshold level. We describe the physical reason for this behaviour and present some calculated results for total error probability in terms of signal power, IF linewidth and bandwidth, and threshold setting. The implications of this ‘floor’ for practical receiver design are discussed.