Optical receiver for receiving digitally transmitted data

Details Optical receiver for receiving digitally transmitted data Optical receiver for receiving digitally transmitted data

1998-12-11

2001-09-25

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200

Reexamination Certificate

active

06295152

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention is directed to an optical receiver for receiving digitally transmitted data.
When digital signals with a high bit sequence frequency (gigabit range) are transmitted, various dispersion-effects occur caused by optical fibre cables in the optical transmission system. These dispersion effects distort the data signals, thus limiting the transmission lengths. Distortions can also occur on the optical fibre due to non-linear effects, e.g. the Kerr effect. Optical receivers used in such a transmission system are required to evaluate distorted signals, permitting only low bit error rates. It is known from the publication “Decision-point steering in optical fibre communication system”, M. Sherif, IEEE Proceedings, Volume 136, No. 3, 1989, page 169 ff, that a receiver in a digital optical transmission system can be optimized by modifying decider thresholds, thus generating artificial errors. For this purpose a second decider circuit is applied parallel to the main data flow. This second signal branch is subjected to a large number of bit errors. The artificial error rate is achieved in the publication in question by modifying the decider threshold, which is changed by means of a square pulse. The results of the two deciders are compared, and from the result actuating variables are derived for an amplifier and for the receiving photodiodes. If you examine the eye diagram of the signal received, the first decider scans the eye aperture at an almost optimal point; i.e. both the decider threshold and the scanning window are roughly in the center of the open eye. The second signal path changes the decider threshold so that the threshold can already be in the noise of the eye diagram, and so artificial errors occur. By evaluating these artificial errors the whole optical receiver is readjusted to optimize the eye aperture, noise characteristics, etc.
A digital optical message transmission system is disclosed in EP 0554 736 B1 in which the receiver quality is optimized by evaluating the eye diagram. In this case an eye diagram is recorded in the receiver, and the size of the eye aperture is determined in a computer. The computer determines an optimum setting of the photodiode and the decider threshold from the data recorded. Recording an eye diagram, comparing with a stored nominal value, and adjusting various receiver activating variables does not enable the bit error rate to be directly determined and a required resultant optimization of the receiver.
SUMMARY OF THE INVENTION
The optical receiver disclosed for receiving digitally transmitted data with the characteristic features of the present invention has in contrast the advantage that bit error rates can be determined, even individual bit errors being detected. The optical receiver offers the advantage of having a pseudo-error monitor circuit for detecting bit errors as a function of threshold value and phase position, these measured data serving to adjust the parameters of the filter and the decider circuit itself. The computing unit linked to the receiver adjusts the parameters on the basis of the measured bit error data. The measures listed in the sub-claims enable advantageous developments and improvements of the optical receiver presented in the main claim. It is particularly advantageous that in the circuit disclosed two independent error signals are determined, the first error signal serving to derive high bit error rates, and the second error signal being used to detect individual bit errors.
The system has the advantage that both the phase position and the threshold value of the decider circuits are affected. In this case the adjustment is simply made by means of computing unit signals using phase shifters and phase adjusting members. In order to avoid errors of measurement through the different phase positions, the phase position is advantageously corrected by an additional scanning device. As a result the signal branches are scanned at the same times, and the results are comparable.
The optical receiver in the device disclosed enables scanning of multi-level signals to be simply compared by several deciders being connected in parallel, and their result read out via a switch activated by the computing unit. For reading out multi-level signals it is important that the phase adjustments of the various deciders are also connected to the decider via a switch, so that any phase shift present is avoided at readout time. The data derived offers the advantage of acting to optimize a filter that is constructed from a chain of input amplifiers and output: amplifiers to achieve high data rates. This makes it easy to implement a filter function of a transversal filter even at: high data rates. In this case the digital multiplier units of the filter are adapted by the computing unit. The filter of the optical receiver in the device disclosed can be cascaded over the outlets, creating a sequence of filters with which filters of even higher order can be realized.
It is particularly advantageous that the amplification factors and the weighting factors of the multiplier circuits are variable and adjustable.
It is thus possible to optimize the filter on the basis of data determined in the receiver.
The rope-ladder structure makes it easy to realize a filter function of a transversal filter even at high data rates. In this case the weighting factors of the filter's digital multiplier unit are adapted by a computing unit. The filter of the optical receiver in the device disclosed can be cascaded through the outputs to create a sequence of filters with which filters of even higher order can be realized.


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M. Sherif, “Decision-point steering in optical fibre communication system” IEEE Proceedings, vol. 136, No. 3, 1989, p. 169 ff.

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