ASK Heterodyne Research Articles

Performance analysis of a symbol slicing majority vote combining receiver for binary optical heterodyne ASK with phase-noise-optimised decision thresholds

The performance of a binary optical heterodyne ASK receiver in the presence of photodetector shot noise and laser phase noise has been investigated. To reduce the effects of phase noise on the receiver error rate, the maximum likelihood approach is used to optimise the decision threshold on the basis of the severity of phase noise indicated by the laser linewidth. It is assumed that the receiver uses symbol slicing and majority vote decision combining. Threshold optimisation is applied to each slice (chip) of the received bit duration. It is found that the receiver performance can be significantly improved by threshold optimisation. Majority voting is also found to improve the receiver performance for a wide range of signal-to-noise ratio values.

Performance of coherent ASK lightwave systems with finite intermediate frequency

The impact of finite intermediate frequency (IF) on the performance of heterodyne ASK lightwave systems is examined and quantified in the presence of laser phase noise and shot noise. For negligible linewidths, it is shown that certain finite choices of IF (R/sub b/,3R/sub b//2,2R/sub b/,5R/sub b//2, etc.) lead to the same ideal bit-error-rate (BER) performance as infinite choices of IF. Results indicate that for negligible linewidths the worst case sensitivity penalty is 0.9 dB for proper heterodyne detection and occurs when f/sub IF/=1.25 R/sub b/. For nonnegligible linewidths (e.g., when /spl Delta//spl nu/T/spl ges/0.04) the sensitivity penalty is always less than 0.9 dB for finite choices of IF. The analysis presented does lead to a closed-form signal-to-noise ratio (SNR) expression at the decision gate of the receiver which can readily be used for BER and sensitivity penalty computations. The SNR expression provided includes all the key system parameters of interest such as system bit rate (R/sub b/), the peak IF SNR (/spl xi/), laser linewidth (/spl Delta//spl nu/), and the IF filter expansion factor (/spl alpha/). The findings of this work suggest that the number of channels in a multichannel heterodyne ASK lightwave system can be increased substantially by properly choosing a small value for the IF at the expense of a small penalty <1 dB. On the negative side, IF frequency stabilization becomes a more critical requirement in multichannel systems employing small values of IF.

On Envelope Detection of Noisy Phase Signals with Asymptotically Small Linewidths

The impact of laser's phase noise on heterodyne and homodyne linewidth communications, as well as on direct detection systems with optical amplification is governed by intractable statistics induced by the phase sample path. Here we derive analytic asymptotic bounds on the underlying distribution inherited by the squared envelope of noisy phase signals. The approach taken invokes standard results from the theory of Large-Deviations and yields straightforward, closed-form expressions. This fundamental problem has many applications in linewidth communications, such as the determination of the BER floor in heterodyne ASK systems, which is hereby illustrated. The asymptotic theory predicts an IF bandwidth increase with the square root of the linewidth-to-bit ratio for a specified floor level. A comparison with the numerical solution of the underlying Fokker-Planck equation reveals a good agreement even at significant (non-asymptotic) laser's linewidths.

The STARNET coherent WDM computer communication network: experimental transceiver employing a novel modulation format

We have designed, constructed, and investigated an experimental transceiver employing a novel combined PSK and ASK modulation format for the STARNET coherent WDM computer communication network. Using this experimental transceiver, we show that it is possible to transmit and receive 2.488 Gb/s PSK circuit-switched data and 125 Mb/s ASK packet-switched data on the same lightwave. The experimental transceiver employs a custom integrated-optic LiNbO/sub 3/ modulator with both phase and amplitude sections, a 2.488 Gb/s tunable PSK heterodyne receiver, and a 125 Mb/s ASK heterodyne receiver. Both receivers function properly with error rates less than 10/sup -9/ and a sensitivity of -32.1 dBm; the corresponding optimum ASK modulation depth is 0.57. The resulting network power budget is 26.6 dB. >

Impact of IF Fluctuations on the Performance of Heterodyne ASK Lightwave Systems

This paper examines the performance of heterodyne ASK lightwave systems in the presence of intermediate frequency (IF) errors. To this end, bit error rates (BERs) and sensitivity penalties due to IF error are calculated. Numerical simulations are used to verify theoretical calculations. The results show that the impact of IF error on system performance is more critical for small laser linewidths and/or high-spped heterodyne ASK light-wave systems.

Computationally efficient methods for bit-error rate evaluation in weakly coherent optical systems

A semianalytical method to estimate the bit-error rate performance of nonsynchronous ASK and FSK optical heterodyne systems is presented. This approach takes into account numerous system details with an accuracy similar to Monte-Carlo simulation, but with a computation time reduced by several orders of magnitude. An analytical approximation, based on a closed-form expression for the decision variable moment generating function, is also presented. The results obtained with the semianalytical method and the analytical approximation are compared to Monte-Carlo simulation results and to measurements obtained on a practical FSK system. >

Performance Evaluation of ASK Multichannel Coherent Optical Systems

AbstractIn this work we evaluate the impairments due to laser phase noise in multichannel coherent optical systems using a heterodyne ASK modulation scheme. The effects of crosstalk due to a finite value of the channel spacing are considered in a system employing a frequency division multiplexing technique. The phase noise and the frequency spacing between adjacent channels are the fundamental parameters which determine the design criteria (e.g. the bandwidth of the intermediate frequency filters) and the performance of the receiver. The accurate statistical characterization of the filtered phase noise and of the crosstalk is the key to get accurate results: to this aim the exact moments of the random processes related to the phase noise and to the crosstalk interference are used together with a Gaussian Quadrature Rule for the evaluation of the error probability.

Impact of finite intermediate frequency on the performance of heterodyne ASK lightwave receivers

The impact of finite intermediate frequency (IF) on the sensitivity of heterodyne ASK lightwave receivers is examined and quantified. It is shown that certain choices of IF (f IF =0.5R b , R b , 1.5R b , etc.) yield ideal envelope detection results. For proper heterodyne detection, the worst-case sensitivity penalty is 1.72dB which occurs when f IF =0.75R b

Optical Amplifiers in Coherent Heterodyne ASK Communication Systems with Significant Phase Noise

Article Optical Amplifiers in Coherent Heterodyne ASK Communication Systems with Significant Phase Noise was published on April 1, 1993 in the journal Journal of Optical Communications (volume 14, issue 2).

A comparison of coherent digital PPM with PCM

AbstractDigital pulse position modulation (PPM) is a preferred modulation format for the ideal photon counting channel and optical intersateilite links. Here we examine its potential for the coherent optical fibre communications channel. We present a thorough performance and optimisation analysis. Comparisons, at a wavelength of 1.5 μm, are made with shot‐noise limited coherent PCM (homodyne and heterodyne ASK, FSK and PSK) over a range of fibre band‐widths and varying PPM word size. We conclude that for moderate to high fibre bandwidths homodyne digital PPM should achieve an improvement in sensitivity of typically 5 dB over homo‐dyne PSK PCM.

Accurate performance evaluation of weakly coherent optical systems

An accurate performance evaluation approach which uses a closed-form exact analytical expression of the phase noise moments is presented. This enables one to derive a high-order Gaussian quadrature rule for the integrations needed to take into account the phase noise in the computation of error probability. A systematic comparison with results obtained through a Monte Carlo simulation shows that the approach is more accurate than previous methods. The analysis is performed on ASK and FSK heterodyne receivers with integrate-and-dump filtering, envelope detection, and optimized postdetection low-pass filtering. The feasibility of ASK and FSK heterodyne systems at bit rates comparable to the spectral line bandwidth of the laser sources is confirmed. The theory applied seems to be adequate to attack other problems, such as the evaluation of the effects of crosstalk between the FSK filters or among frequency division multiplexed channels. >

Optical FSK heterodyne detection using image rejection mixer

A new optical FSK heterodyne detection scheme is proposed and demonstrated. This scheme uses an optical image rejection mixer. The intermediate frequency bandwidth is the same as ASK heterodyne detection while equalling dual filter FSK detection receiver sensitivity.

Effect of laser linewidth on crosstalk penalty in two-channel ASK heterodyne 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.

Theoretical analysis of ASK heterodyne optical receivers with tuned front ends

Heterodyne optical receivers designed with high intermediate frequency or with large IF bandwidth suffer from frequency-squared noise spectral density components if a base-band receiver design is used. This noise component can be reduced usefully if the front-end is tuned to the IF. We present detailed calculations of noise spectral densities for an ASK receiver using square-law IF detection. An example of a 140 Mbit/s receiver with an IF filter suitable for lasers with significant phase noise is considered. The tuned receiver may be up to 6 dB more sensitive than the base band design for the range of receiver parameters considered.

Intensity noise in ASK coherent lightwave receivers

The effect of local oscillator intensity noise on the performance of two and three-branch ASK homodyne receivers and single-branch ASK heterodyne receivers is investigated and an optimum local oscillator power is found. At optimum local oscillator power, both the three-branch and heterodyne receivers are found to have a somewhat better sensitivity than the two-branch receiver. If the local oscillator power is higher than the optimum value, the three-branch receiver is significantly less sensitive to intensity noise than the other two receivers.

Evaluation of crosstalk penalty in multichannel ASK heterodyne optical communication systems

The bit error rate (BER) of a two-channel ASK heterodyne lightwave system employing envelope detection is evaluated analytically. The result is used to calculate the power penalty resulting from intrachannel crosstalk. To main a BER below 10−9, the power penalty increases from 0.2 to 1 dB when the level of crosstalk increases from −15 to −10 dB. The theory predicts a minimum channel spacing of three times the bit rate if the design criterion is to keep the crosstalk penalty below 0.1 dB. The minimum channel spacing increases to five times the bit rate for multichannel systems when crosstalk from the nearest neighbours on both sides is included.

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 1·55μm 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.

ASK multiport optical homodyne receivers

Several types of ASK multiport homodyne receivers are investigated, and the impact of the phase noise and of the shot noise on these receivers is analyzed. The simplest structure is the conventional multiport receiver with a matched filter in each branch. This structure can tolerate <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\DeltavT</tex> [ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\deltav</tex> is the laser finewidth and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</tex> is the bit duration) of several percent with a small power penalty (3.6 percent for 1-dB penalty and 5.2 percent for 2-dB penalty). Optimization of branch filters of conventional multiport receivers does not help when the linewidth (and the penalty) is small but does improve the receiver performance for larger linewidths. The most important point of the paper is the novel wide-band filter-rectifier-narrow-band filter (WIRNA) structure, proposed and investigated here for the first time for optical communication systems. It is shown that the optimized WIRNA homodyne receivers are extremely robust with respect to the phase noise: the WIRNA tolerable value of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\DeltavT</tex> is 3.6 percent for 1-dB penalty and more than 50 percent for 2-dB penalty. Thus, the WIRNA structure opens, for the first time, the possibility of constructing homodyne receivers operating at several hundred megabits per second with conventional DFB lasers without complicated external cavities. Under no-phase-noise conditions, all the multiport receivers investigated here have the same performance, which is identical to that of heterodyne ASK receivers. In addition, the optimized WIRNA receivers can tolerate tapproximately) the same laser linewidth as the heterodyne ASK receivers. Thus, the main difference between the WIRNA multiport homodyne and heterodyne receivers is that the former shifts the processing to a lower frequency range, in return for a more complicated implementation. This difference makes the WIRNA multiport homodyne receivers particularly attractive at high (say, several gigabit per second) bit rates.

Receiver Analysis for Incoherent Optical ASK Heterodyne Systems

Receiver Analysis for Incoherent Optical ASK Heterodyne Systems

Noise performance of Gbit/s tuned optical receivers

We present design criteria for minimising the influence of f2 noise in broadband heterodyne optical receivers using a serial or parallel tuning inductor. The criteria are tested experimentally in a 2GHz-bandwidth front-end design. A 4dB receiver sensitivity improvement is demonstrated for a 400 Mbit/s ASK heterodyne application, and a 1 dB/2 dB sensitivity improvement is demonstrated for a 2 Gbit/s direct-detection/homodyne application.