CIRCUIT ARRANGEMENT AND METHOD FOR RECEIVING OPTICAL SIGNALS

C I R C U I T A R R A N G E M E N T A N D M E T H O D

F O R R E C E I V I N G O P T I C A L S I G N A L S

Technical field

The present invention basically relates to the technical field of optical transmission of signals. More particularly, the present invention relates to a circuit arrangement for receiving optical signals according to the preamble of claim 1 as well as to a corresponding method according to the preamble of claim 14.

Within the scope of the present invention, the term optical or light is understood not only as the range of electromagnetic radiation visible to the eye, extending in a wavelength range from about

380 nanometers to about 780 nanometers (which corresponds to a frequency of about 789 terahertz down to about 385 terahertz).

Rather, the term light or light-emitting is understood as the entire electromagnetic wavelength or frequency spectrum, including the spectrum not visible to the eye, in particular the l[nfra]R[ed] range

(wavelength range up to about 2,000 nanometers or frequency range down to about 150 terahertz), for example a wavelength of about 850 nanometers or a frequency of about 350 terahertz.

Background of the present invention

A typical optical communication system (cf. Fig. 1 : typical optical link; Fig. 2: typical optical link signalling) comprises three components:

- a light-emitting element LD, for example a laser diode, together with its driver DR, which converts the electrical data logic levels to optical power logic levels;

- a light guide GU, for example a fibre, which carries the light; and

- a light-receiving element PD, for example a photodetector, such as a photodiode, together with a transimpedance amplifier TA, which senses the light at the end of the light guide GU and converts the light back to an electrical signal. Such a system typically transmits two-level electrical data pattern V_in-data-digitai and recovers a replica two-level electrical data pattern V_out-data-digitai at the receiver side. Light (optical) power levels P-i and P₀ (cf. Fig. 2) injected into the light guide GU correspond to each electrical level V_HIGH and V_LOw- It follows that, at the receiver side, two different light power levels arrive at the input of the light-receiving element PD. These two different light power levels generate two different current levels, namely and l₀.

The current signal l_PD generated at the light-receiving element PD has to be converted to a voltage signal. The current generated by the light-receiving element PD is converted to voltage by the transimpedance amplifier TA, wherein V_out-data-anaiog = R*l_in-main, with R being the gain or transimpedance of the transimpedance amplifier TA. An integrator IN in the feedback path FP generates a control signal V_int in order to subtract the average input current coming from the light-receiving element PD. This is done in order to generate the zero crossing in the input of a limiter LI. The limiter LI acts as a comparator which generates in its output a VHIGH (VLOW) logic level for positive (negative) voltages in its input. The automatic gain control block AG controls the transimpedance amplifier gain R in order to keep the amplitude V_out-data-anaiog to a desired level (for example constant) for different l_PD current levels that might occur as input to the transimpedance amplifier TA.

In case another low speed signal is to be transmitted from the transmitter side to the receiver side, a status change in the transmitter side could be transmitted to the receiver side. This could be for example a change from EIOS (Electrical Idle Ordered Set - a type of data link layer packet) state to

EIEOS (Electric Idle Exit Ordered Set) state in the PCI Express standard.

Another optical link could be dedicated to the new signal. However, this is very costly because extra components and extra power is required. Also in some cases, dedicating a complete extra optical link might not be an option at all.

If the slow speed signal is transmitted over the same optical link, which means that the high-speed signals and the low-speed signals share the same optical components (physical medium), a third optical power level P₂ different from P-i and from P₀ is to be transmitted in order to be able to distinguish between the two signals, making a multi-level signalling necessary.

Disclosure of the present invention: object, solution, advantages

Starting from the above-explained disadvantages and technical problems as well as taking the outlined prior art into account, the object of the present invention is to further develop a circuit arrangement of the above-mentioned type and a method of the above-mentioned type in such a way that a multilevel optical link can be provided.

This object is achieved by a circuit arrangement with the features of claim 1 as well as by a method with the features of claim 14. Advantageous embodiments and expedient further developments of the present invention are characterized in the dependent claims.

According to the present invention, a circuit arrangement for receiving optical signals from at least one optical guide comprises:

- at least one light-receiving component for converting the optical signals into electrical current signals, - at least one transimpedance amplifier, being provided with the electrical current signals from the light-receiving component,

- at least one automatic gain controller for controlling the gain or transimpedance of the transimpedance amplifier, in particular in order to keep the amplitude of the output of the transimpedance amplifier to a desired, for example constant, level for different levels of the electrical current signals,

- at least one integrator in a feedback path, said integrator generating a control signal,

- at least one voltage-controlled current source, being provided with the control signal from the integrator,

- at least one limiter acting as a comparator and generating in its output a logic level for positive or negative voltages in its input,

- at least one second transimpedance amplifier arranged in parallel to the transimpedance amplifier, and

- at least one automatic offset controller for setting the voltage for the second transimpedance amplifier.

Correspondingly, the circuit arrangement works according to the following method for receiving optical signals from at least one optical guide, comprising the steps of:

- converting the optical signals into electrical current signals by means of at least one light-receiving component;

- provided the electrical current signals from the light-receiving component to at least one transimpedance amplifier;

- controlling the gain or transimpedance of the transimpedance amplifier by means of at least one automatic gain controller, in particular in order to keep the amplitude of the output of the transimpedance amplifier to a desired, for example constant, level for different levels of the electrical current signals;

- generating a control signal by means of at least one integrator in a feedback path;

- providing the control signal from the integrator to at least one voltage-controlled current source;

- generating in the output of at least one limiter a logic level for positive or negative voltages in its input; and

setting the voltage for at least one second transimpedance amplifier by means of at least one automatic offset controller, said second transimpedance amplifier being arranged in parallel to the transimpedance amplifier.

According to an advantageous embodiment of the present invention, at least one second limiter is assigned to the second transimpedance amplifier and to the automatic offset controller.

According to an expedient embodiment of the present invention, the second transimpedance amplifier

- corresponds to the transimpedance amplifier, or

- is a scaled version of the transimpedance amplifier. According to a favoured embodiment of the present invention, the automatic gain controller sets the same gain or same transimpedance for both the transimpedance amplifier and the second transimpedance amplifier by sensing the amplitude of the output of the transimpedance amplifier.

According to a preferred embodiment of the present invention, at least one peak detector circuit is provided for sensing the amplitude of the output of the transimpedance amplifier.

According to an advantageous embodiment of the present invention, the peak detector circuit is part of the automatic gain control or is shared between the automatic gain controller and the automatic offset controller.

According to an expedient embodiment of the present invention, the amplitude of the output of the transimpedance amplifier is provided to the input of the automatic offset controller.

According to a favoured embodiment of the present invention, the transimpedance amplifier is at least one multi-stage amplifier.

According to a preferred embodiment of the present invention, the second transimpedance amplifier at least one multi-stage amplifier.

According to an advantageous embodiment of the present invention, a short is arranged between the output node of the first stage of the transimpedance amplifier and the output node of the first stage of the second transimpedance amplifier.

According to an expedient embodiment of the present invention, the light-receiving component is at least one photodetector, in particular at least one photodiode.

According to a favoured embodiment of the present invention, the optical guide is at least one fibre.

According to a preferred embodiment of the present invention, the end of the optical guide, which is not assigned to the light-receiving component, is assigned to at least one light-emitting component, which is preceded by at least one driver for converting electrical data logic levels into the optical signals.

The present invention finally relates to the use of at least one circuit arrangement according to the type presented hereinbefore and/or of the method according to the type presented hereinbefore for the optical transmission of data signals and of status signals.

Brief explanation of the drawings As has already been discussed hereinbefore, there are various possibilities for configuring and further developing the teaching of the present invention in an advantageous manner. For this purpose, on the one hand reference is made to the claims subordinated to claim 1 and other the hand, further embodiments, features and advantages of the present invention are explained in detail hereinafter inter alia with reference to the exemplary embodiments illustrated by Fig. 3 to Fig. 5.

It is shown in:

Fig. 1 in a schematic diagram an example of a circuit arrangement according to the prior art operating according to the method of the prior art;

Fig. 2 in a comparative diagram an example of the prior art signalling of the circuit arrangement of Fig. 1 ;

Fig. 3 in a schematic diagram a first exemplary embodiment of a circuit arrangement according to the present invention operating according to the method of the present invention;

Fig. 4 in a comparative diagram an exemplary embodiment of the signalling of the circuit arrangement of Fig. 3; and

Fig. 5 in a schematic diagram a second exemplary embodiment of a circuit arrangement according to the present invention operating according to the method of the present invention.

The same or similar embodiments, elements or features are provided with identical reference numerals in Fig. 1 to Fig. 5.

Best mode of embodying the present invention

In order to avoid unnecessary repetitions, the following explanations regarding the embodiments, features and advantages of the present invention - unless specified otherwise - relate both

- to the first exemplary embodiment of a circuit arrangement CR shown in Fig. 3, Fig. 4 and

- to the second exemplary embodiment of a circuit arrangement CR' shown in Fig. 5.

A typical optical communication system (cf. Fig. 1 : typical optical link; Fig. 2: typical optical link signalling) has been explained above in the chapter "Background of the present invention". In order to avoid unnecessary repetitions, these explanations in the chapter "Background of the present invention" are incorporated by reference in the present chapter "Best mode of embodying the present invention" with regard to the present invention; in particular,

- the explanations in the chapter "Background of the present invention" with regard to Fig. 1 are incorporated by reference in the present chapter "Best mode of embodying the present invention" with regard to Fig. 3 and/or to Fig. 5, and

- the explanations in the chapter "Background of the present invention" with regard to Fig. 2 are incorporated by reference in the present chapter "Best mode of embodying the present invention" with regard to Fig. 4.

According to the present invention and compared to the typical optical link as shown in Fig. 1 , a signal V_in-st_atus-digitai is inputted on the transmitter side. V_in-st_atus-digitai is ^a slow signal comprising short pulses widely spaced-in-time. The transition rate of V_in-st_atus-digitai is significantly low compared to the transition rate of V_in__data__digita|. in-status-digitai, together with the high speed signal V_in-data-digitai, modulates the transmitted optical signal as shown in the signal listing in Fig. 4 (= optical link signalling according to a preferred embodiment of the present invention). The optical power P₂ is chosen to be higher than P-i , such that the received currents l₂ and fulfill the relation l₂≥ 2* .

The second transimpedance amplifier TA2 can be a copy of the transimpedance amplifier TA, or the second transimpedance amplifier TA2 can be an exactly scaled version of the transimpedance amplifier TA. The automatic gain control block AG sets the same gain or same transimpedance R for both the transimpedance amplifier TA and the second transimpedance amplifier TA2 by sensing only the Vout-data anaiog amplitude with of a peak detector circuit, which can be part of the automatic gain control AG or can be shared between the automatic gain control AG and an automatic offset control AO. The automatic offset control AO sets the voltage V₀ff_Set for the second transimpedance amplifier TA2; for example, V₀ff_Set can be R*(l-|-l₀)/2. The value of V₀ff_Set is independent of l₂, and the information about its value is extracted only from V_out-data-anaiog- For example, V₀ff_Set = R*(li-lo)/2 is achieved by setting Voffset double the amplitude. Again the amplitude V_out-data-anaiog can be measured with a peak detector circuit. The peak detector circuit can be part of the automatic offset control AO or can be shared between the automatic offset control AO and the automatic gain control AG. To close the feed-back loop for the V₀ff_Set control, V_out- status anaiog is used as the feedback signal for the automatic offset control block AO because the V_out- status anaiog average value is -V₀ff_Set- The averaging circuit is part of the automatic offset control block AO.

As to the link operation, during an initial phase, only the high speed V_in-data-digitai signal is transmitted. The Vin-status-digitai signal is kept low during this phase. Also, during this initial phase, the automatic gain control AG and the automatic offset control AO outputs settle to their final value. The time constant of these two loops is significantly lower than the time distance between the two consecutive pulses on the n-status-digitai signal. Only after this first initial phase the V_in-st_atus-digitai signal can be transmitted. When the V_in-st_atus-digitai signal is low, the V_out-data-digitai signal follows the V_in__data-digitai signal. When the V_in__status__digitai signal is high, the optical power transmitted is always P₂, independently of the value of the V_in-data-digitai signal. As a consequence, V_out-data-digitai will be high independently of the value of V_in__data-digitai- The V_out-status-digitai, as desired, goes high as well.

As to an improvement of the signal-to-noise ratio on the receiver side, in case and the second transimpedance amplifier TA2 are multistage amplifiers, in order to improve the signal-to-noise ratio, a short between the output nodes of the first stage of the first transimpedance amplifier TA and of the first stage of the second transimpedance amplifier TA2 can be provided, as depicted in Fig. 5.

Such shorting does not have any effect on the desired signal. Only the total noise power drops compared to a non-shorted version. Hence an improvement of the signal-to-noise ratio can be achieved in the receiver.

By means of the above-proposed arrangement as well as method, the slow speed signal can be reliably transmitted by sharing the same physical optical link and using multilevel signalling.

List of reference numerals

AG automatic gain controller or automatic transimpedance controller

AO automatic offset controller

CR circuit arrangement (= first embodiment; cf. Fig. 3)

CR' circuit arrangement (= second embodiment; cf. Fig. 5)

CS current source, in particular voltage-controlled current source

DR driver

FP feedback path

GND reference potential, in particular earth potential or ground potential or zero potential

GU optical guide, in particular fibre

IN integrator

l_DC input of current source CS

lin-main input of transimpedance amplifier TA

lin-rep input of second transimpedance amplifier TA2

Ip_D electrical current signal

LD light-emitting component, in particular laser diode

LI limiter, in particular first limiter

LI2 second limiter

PD light-receiving component, in particular photodetector, for example photodiode

R gain of transimpedance amplifier TA or transimpedance of transimpedance amplifier TA SH short

SI optical signal

TA transimpedance amplifier, in particular first transimpedance amplifier

TA2 second transimpedance amplifier

in-data digitai input, in particular data input, of driver DR

in-status-digitai input, in particular status input, of driver DR

Vint control signal or output of integrator IN

V₀ff_Set voltage for second transimpedance amplifier TA2

out-data anaiog output of transimpedance amplifier TA

Vout-data digitai output of limiter LI

Vout-status-anaiog output of second transimpedance amplifier TA2

Vout-status digitai output of second limiter LI2

1 S first stage of transimpedance amplifier TA

1 S2 first stage of second transimpedance amplifier TA2

2S further or second stage of transimpedance amplifier TA

2S2 further or second stage of second transimpedance amplifier TA2