Method and equipment for optical communication

Details Method and equipment for optical communication Method and equipment for optical communication

1998-12-28

2001-06-05

Pascal, Leslie (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C375S345000, C359S199200, C359S199200

Reexamination Certificate

active

06243183

ABSTRACT:

TECHNICAL FIELD
The invention relates to a device and a method for compensation of offset voltages during reception of a fibre optical signal.
STATE OF THE ART
Today's optical fibres have extremely small dispersion in relation to the light absorption in the fibre. This means that the optical signal in a receiver is characterized by extremely small distortion of the pulse shape. On the other hand, the signal amplitude is often extremely small. After conversion to the electrical form with the help of a light detector, e.g., a PIN-diode (Positive Intrinsic Negative), the signal amplitude is often as low as 5 mV and often superimposed on a direct current level of several volts. These low signal levels must be compared with the maximal specified offset voltage for e.g. a differential amplifier which normally is up to approximately 20 mV. In the case that the offset voltage is much higher than the signal amplitude, an extremely accurate offset compensation is required, and as the offset voltage is also temperature-dependent, an exact compensation is difficult to realize without the receiver being placed in an oven for constant temperature conditions. Such an arrangement should naturally be avoided as much as possible.
Another problem is the superimposed DC-level which the signal exhibits. It is not desirable to use an AC-coupling capacitor if an integrated manufacturing of the receiver circuit is desired, because the capacitor in general must be realized in a discrete process because capacitors with high capacities are difficult to realize in integrated designs because of their space requirements. Furthermore, even with an ideal AC-coupling there can only be achieved a zeroing of the time-average value of the signal. This average value only corresponds to a threshold level which lies in the middle between the high and low level of the signal when the transmitted signal exhibit a perfect pulse quotient of 50%, i.e., when the transmitted signal has exactly equal amounts of high symbols as low symbols in the signal. By pulse quotient in this connection is meant the number of high signals in the average in relation to the total number of symbols for a binary signal.
In the case where the pulse quotient continuously or more temporarily deviates from 50%, this threshold level will deviate from the zero level. This means that the signal after AC-coupling will show a certain displacement which, if this displacement is not compensated for, leads to reduced receiver sensitivity. For e.g. FDDI, Fibre Distributed Data Interfaces, which are ring-shaped high speed LAN/MAN-nets, a so-called 4B5B-code is used where the pulse quotient can vary between 40 and 60 percent. A pulse quotient of 40 percent means that the signal shows a DC-component of 40 percent of the signal's peak-to-peak value. This means that if a FDDI-signal is AC-coupled, the level which lies in the middle between the high and low level of the signal will be displaced 10 percent in relation to the idea level. This implies a reduction of the sensitivity of the receiver by theoretically about 1 dB.
For fibre optic signal transfer normally two level signals, i.e. binary signals, are used with some form of coding, such as e.g. 4B5B or Manchester-coding. In this way, a signal is achieved out of which the clock frequency is comparatively easy to extract.
Signal limiters which binarize the signal are normally used in fibre optic receivers, as this gives a relatively simple circuit solution for two-level signals, where the signal ideally only can attain two distinct levels. A further reason is that limiting can reduce the loss effects in the receiver, often up to 50 percent. Such a reduction of the loss effects can even itself contribute to a simpler implementation thanks to a reduced cooling requirement.
The problem of eliminating a DC-level on an incoming signal has earlier been solved for electrical signals. The American Patent Specification U.S. Pat. No. 4,966,529 solves such a problem, which might seem to be close to the above problem stated for fibre optical signals. U.S. Pat. No. 4,996,529 presents a technique intended for signal coding, such as analogue-digital converting, for the compensating of superimposed low frequency voltages. The circuit presented limits an analogue-electrical input signal and contains a feedback loop which in real time compensates for the DC-level of the input signal and adjusts the direct voltage level on the input signal to a limiter comprised in the circuit. Through sampling the limited signal with a large frequency in relation to the band width of the input signal, and accumulating the result, the limited signal is digitally integrated, whereafter it is fed back to the input of the limiter. In this way a negative feedback is achieved which implies an adjustment of the input signal of the limiter so that the output signal of the limiter attains the high signal level during an equally large amount of time as it attains the low level.
Oversampling of the signal requires that a clock signal is generated at a high frequency in relation to the signal which, because of the thereby resulting loss effects, reduces the field of use for the technique to low-frequency signals, such as speech signals.
It must, however, be obvious for the skilled person that the basic idea in the above described patent document in principle is applicable even to receivers for fibre optic signal transfer. In analogy with the above described technique, in this case an incoming signal converted to an electrical form is limited, and thereafter integrated and fed back to the limiter in such a way that the level on the fed back signal is adapted to the DC-level of the input signal so that this is compensated. In this way not only will the possible superimposed DC-voltage on the signal be compensated for, but also the offset voltage of the limiter.
This technique functions perfectly for signals where the pulse quotient is 50 percent. However, as this technique does not take account of the actual pulse quotient of the signal, the application of this technique is less suitable to optical signals where the pulse quotient permanently or somewhat more temporarily differs from 50 percent because of the not-inconsiderable reduction of the receiver sensitivity which thereby occurs. The field of use for this technique is thereby to some extent limited.
In the magazine
Electronic Design
from Jun. 12, 1995, a circuit is described with a feedback loop intended for signal transmission on twisted cable pairs with high band width. The feedback loop adjusts the DC-level on an incoming electrical three-level signal, where the signal ideally attains three different voltage values lying symmetrically around zero volts. Through the integration of the difference signal between the DC-adjusted signal and a theorectic wave form, developed from this signal, a DC-component is achieved which can be added to the signal. In this way the DC-draft which tends to occur in certain applications for wide band data communication over twinned copper wire conductors is compensated for.
The circuit measures the size of the superimposed low-frequency voltage which can occur because of asymmetric data on the copper wire network and compensates for the possible base line shifting through in real time shifting the DC-level on the incoming signal back to its original level. In order to achieve a value for the size of the superimposed low frequency voltage in the received signal, the circuit forms a theoretic wave form of how the incoming signal probably would have looked if the superimposed voltage had not been present. This idea signal is compared to the incoming signal in order to detect the size of its offset voltage and to correct the same with the help of a built-in DC-level adjuster.
DISCLOSURE OF THE INVENTION
As mentioned above, in receivers for fibre optical signals it is, because of the low signal levels, problematic to achieve an exact offset compensation which is of great weight since the offset voltage often are much greater than the signal amplitudes. A fur

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