FI106679B - Optical receiver - Google Patents

Description

1 106679

Optical receiver

Field of the Invention

The invention relates generally to an optical receiver. More particularly, the invention relates to a method and circuitry for use in an optical receiver to increase the dynamic range of an input amplifier.

BACKGROUND OF THE INVENTION An optical transmission system comprises an optical transmitter which converts an electrical signal to be transmitted into an optical form, (b) an optical fiber acting as a conductor of an optical signal, and (c) an optical receiver detecting and converting the transmitted optical signal.

A typical optical receiver comprises, in its input stage, a light detector 15 and a transimpedance amplifier, the input of which is coupled to a light detector. The light detector converts the optical signal it receives into an electric current that is applied to a transimpedance amplifier. The latter produces a voltage proportional to the incoming current at its output, so that a voltage proportional to the current of the light detector is obtained at the output of the amplifier. Light detector. . ·. The 20 is usually either an Avalanche Photo Diode (APD) or an optical: · '· PIN diode. The spin LED achieves better performance than the ΡΙΝΙ I diode, but is more expensive than the PIN diode and is also clearly: * V more difficult (due to the high bias voltage it requires, which must be temperature-controlled). Transimpedance amplifiers are commonly used: 25 mm. because they achieve relatively good sensitivity properties by a relatively simple structure.

One problem with optical receiver solutions is their inadequate dynamics: good sensitivity often means poor power tolerance and good power tolerance in turn means poor sensitivity. Poor dynamics diminishes the flexibility of using the receiver; For example, when switching to! ·: · shorter fiber must be fitted with an extra attenuator between the transmitter and the receiver.

V \ Because the power level of the optical signal entering the receiver can vary considerably in practice (depending on how long the fiber is used.), A transponder amplifier typically employs some type of automatic gain control (AGC), which allows the amplifier to transmit. the operating voltage is kept substantially constant when the incoming signal is greater than a preselected threshold.

When good sensitivity is sought, stray capacitances that operate at the input terminal of the amplifier are essential; even a small capacitance lowers the receiver's sensitivity. Therefore, it is also essential that the parasitics acting on the amplifier input are minimized.

Attempts have been made to extend the receiver's dynamic range by using an adjustable resistive element in front of the transimpedance amplifier.

The resistance of the element is adjusted in response to the strength of the signal to the amplifier such that at higher levels the resistance is reduced, thereby reducing the current applied to the amplifier input (as part of the current passes through the resistive element). Various variations of such a basic solution are known, and will be briefly described below.

U.S. Patent No. 5,012,202 and EP Patent Specification 433,646-B1 disclose a control circuit in which a channel transistor (FET) is used as a resistive element. In order not to decrease the sensitivity of the amplifier, the drain capacitance of the channel transistor must not be compensated by the channel transistor. · With 20 or more feedback. This type of feedback makes the circuit more or less complicated. In addition, feedback makes it difficult to design »'· * the receiver.

»· *» · “V A better alternative to a channel transistor is to use a diode of low capacitance as an adjustable resistive element. There are many different solutions based on diode.

British Patent Application 2,247,798-A discloses a diode-based solution in which a switch transistor (TR1, FIG. 1) is controlled by a voltage across a resistor (r, FIG. 1) using a voltage across the diode (D, FIG. 1) and thus resistance. For adjustment · '·. The circuit 30 would not interfere with the DC operating point of the transimpedance amplifier, it is forced to be separated by a capacitor (C2, Fig. 1) from the amplifier. However, the use of a condenser · * impeller causes an additional time constant in the feedback loop, which • complicates the design. In addition, capacitors and em.

the resistor makes the circuit more complex, which also requires * · t «·« · • · 3 106679 more space. All additional components also cause spreads at high frequencies, which reduces the sensitivity of the receiver.

U.S. Pat. No. 4,415,803 also describes a diode-based solution using a peak-hold detector circuit which controls the peak value of the output of a transimpedance amplifier. Based on this peak value, the voltage across the diode is adjusted so that part of the amplifier input current passes through the diode. The disadvantage of this solution is that at high transfer rates it is difficult to realize an accurate Peak-hold circuit. Achieving high accuracy in peak measurement is difficult and requires the use of expensive special components.

EP-A-402 044-B1 discloses a solution in which a detector current passes through a resistor (R1, Figures 4a and 4b) and causes a voltage across a diode (D0, Figures 4a and 4b). When this voltage is high enough, the diode begins to conduct, thereby acting as a dampener. The disadvantage of such a solution is e.g. the fact that the resistor causes noise, which in turn reduces the sensitivity of the receiver. Feedback also complicates implementation because it must be stable under all conditions.

Attempts have also been made to extend the dynamic range of the receiver by providing a variable impedance amplifier feedback resistance. · 20 weeks However, such a solution has the same problems as the ones described above:. the adjustment easily causes a diffuse capacitance at the input of the amplifier, and j, the adjustment has to compromise the dynamics and sensitivity * * Y.

· Brief Summary of the Invention

It is an object of the invention to overcome the drawbacks described above and to provide a solution that allows the dynamic range of the receiver to be increased so as to keep the circuit solution as simple as possible while also reducing the sensitivity of the receiver. 30 is minimal.

This object is achieved by the solution defined in the independent claims.

* · 'The idea of the invention is to perform a complete control on the input side of a transimpedance: side amplifier by adjusting the impedance connected to the input of the amplifier; Impedance of 35 soul elements directly at the receiver by optical power at I »t • · 4 106679. In other words, the idea is to perform an adjustment without a feedback field at the output of the transimpedance amplifier. Because no feedback is required, design and component requirements in this regard are reduced.

A further advantage of the solution according to the invention is that conventional economically advantageous components can be used in the coupling.

List of figures

In the following, the invention and its preferred embodiments will be described in more detail by way of example with reference to Figures 1-3 in the accompanying drawing, in which Figure 1 illustrates the solution of the invention in general, Figure 2 illustrates operation of the circuit of Figure 1; FINANCING.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 illustrates the principle of the invention in general terms by showing a front end of an optical receiver comprising. 20 trans-impedance amplifiers TA. As is known, a transimpedance amplifier is an inverting (180 degree phase shift) voltage amplifier with an internal balance | ! harness coupling resistor (not shown). The (optical) signal coming from the optical fiber OF is converted into a current in the light detector PD, which may be of any known type. In this example, it is assumed that the detector is an optical PIN diode. The anode of the PIN diode is connected to the input of the transimpedance amplifier and the cathode to the bias voltage + V1. The control unit CU according to the invention measures the optical power supplied to the receiver and accordingly adjusts the impedance of the impedance element ZU connected to the amplifier input point P1. The power measurement is done by measuring: * ·, the current through 30 diodes, which in this case is directly proportional to the average of the optical power. It is advantageous to perform power detection at the cathode of the light-emitting diode so that the sensitivity of the transimpedance amplifier is not adversely affected by the measurement.

The operation of the circuit according to the invention is illustrated in FIG. · 35 2, which shows the average optical input power and the horizontal axis as the vertical axis

»M

5 106679 is the impedance at the input (P1) of the transimpedance amplifier on the axis. At low input levels, the impedance of the impedance element ZU is high, and the impedance displayed at the input point has a (high) value (Z1) dependent on the parallel connection of the resistor R2 and the input impedance of the transimpedance amplifier.

5 As the optical input power increases above a certain threshold value P ^, the impedance of the impedance element ZU begins to decrease. The decrease is either linear (curve C1) or non-linear (curve C2), the shape of the curve depends on the properties of the impedance element used.

Fig. 3 illustrates a further embodiment of the circuit of Figs. 1 and 2, in which the impedance element consists of a series of connected PIN diodes D1 and D2 and a control unit via a differential amplifier circuit for controlling the current through the impedance element. For the sake of clarity, the diodes are high frequency diodes (RF PINs).

In the circuit of Figure 3, the anode of the light detector PD is coupled via capacitor C4 to the input of a transimpedance amplifier TA and through a resistor R2 to ground. Capacitor C4 performs a DC separation to ensure that the DC operating point of the transimpedance amplifier is not affected by the control. The capacitor is not essential to the invention,. · 20 and its need depends on the transimpedance amplifier used. Resistor R2: (high value), together, determines the output of the transimpedance amplifier. /. with the pedal at low input impedance levels of the point P1 (when no current flows through the impedance element), cf. Z1 in Figure 2.

40 0

The cathode of the light detector, in turn, is connected to ground through capacitor C1 * «'· 1 25 and through resistor R1 to bias voltage + V1. Capacitor C1 acts as a signal filtering component to provide an average voltage to the cathode of the light detector (voltage U1). The resistor R1, in turn, generates a voltage Um proportional to the optical input power, which by means of an amplifier circuit generates a control voltage (U2) · 1 ·. For 30 impedance elements.

·: 1 The light sensor side of resistor R1 is connected to the resistor. 1 · · through R7 to the first (non-inverting) input of differential amplifier A1, while the bias voltage side terminal is connected via resistor R5 * ”to the second (inverting) input of the differential amplifier. This input terminal • • 0 0 9 0 0 · · 999 9 6 106679 is further connected through ground resistor R3 and through feedback resistor R4 to the output of the amplifier.

The PIN diodes D1 and D2 form a current-controlled impedance element ZU, whose dynamic impedance control increases the dynamic range of the transimpedans-5 side amplifier. The diodes are connected in series such that the common terminal formed by the anode of the diode D1 and the cathode of the diode D2 forms an input point P1 of the transimpedance amplifier, which simultaneously forms a common terminal of resistor R2 and light detector connected via a capacitor C4 to the input of the transimpedance amplifier.

10 The anode of diode D2 is connected to the preset voltage Vi and through the condenser C3 to ground. The cathode of diode D1, in turn, is coupled to the first terminal of resistor R6 and to ground through capacitor C2. The other terminal of resistor R6 is connected to the output of a differential amplifier A1. The capacitors mentioned are not essential to the actual inventive idea; C2 and C3 form a low-impedance current path to ground, so that the diodes are parallel to each other from point P1 to ground for circuit operation. The resistor R6, in turn, acts as a limiter which limits the maximum current through the impedance element.

The circuit of Fig. 3 operates as follows. When the optical input power is. 20, the diodes D1 and D2 are in the reverse direction, and at point P1 the impedance is displayed; '] 1, the value of which is determined by the input impedance of the resistor R2 and the transimpedance amplifier. /. parallel dancing. When optical power comes into the receiver,

* the voltage Um acting over R1 is directly proportional to the photodiode PD

I · the average of the optical power received. Therefore, the voltage U1 decreases as the optical input% • \ 25 increases. The output voltage U2 of the differential amplifier A1 changes as a function of voltage U1, and when U1 is small enough (compared to the threshold set by the voltage divider formed by resistors R3 and R5), U2 is also small enough compared to the preset voltage V1 so that diodes D1 and D2 .

»'·, 30 The diodes D1 and D2 are thus impeded at low .1: 1 power levels of the optical input signal. At high power levels, the diodes are in the direction of emission and pass through a current proportional to the (average) input power. In other words, the dynamic impedance of diodes D1 and D2 (impedance element ZU) decreases and the amplitude of the input signal of the transimpedance amplifier decreases. ; 35 As a result, the dynamic range of the transimpedance amplifier increases.

a »s 1 t» 7 106679

While the invention has been described above with reference to the examples in the accompanying drawings, it is clear that the invention is not limited thereto but can be modified within the scope of the inventive idea set forth in the appended claims. In practice, the circuitry implementing the corresponding functionality may vary in many ways. Thus, the implementation of both the impedance element and the control circuit can be varied in many ways without departing from the principle described above. Some variations are briefly described below.

The impedance element may consist of one or more components and may be implemented with different types of components. However, the use of a PIN diode as an impedance element is advantageous because of its inherently low capacitance. The above implementation is advantageous in that the components of the two diodes are ready-made commercial components and the components of the said diode are commercially available. the component is easy to implement a voltage function as described above. The impedance element can be implemented using only one diode, but then it is not as easily able to supply a control current to point P1 without causing stray capacitance. The control circuit can also be implemented, for example, by a current mirror which produces a current proportional to the current through the photodiode to control the impedance of the impedance element. However, the current mirror is realized. . ·. 20 so that it has a certain threshold value that the light detector current must exceed 1 · [· before the power mirror generates power to its output. The impedance element may also consist of a multi-component network, but for the reasons described above: 2V, the fewer components are required, it is preferable. Also, the preset voltage need not have a predetermined constant value: *** 25, but can be made adjustable, e.g., base U2; basis.

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Claims (6)

8 106679 A method of expanding the dynamic range of an optical receiver, the receiver comprising a transimpedance amplifier (TA) comprising an input and an output, converting an optical power input into a light detector (PD) 5 into an electric current whose light detector output is operatively coupled to a transimpedance amplifier input ) operatively coupled to the input of the transimpedance amplifier, which according to its method receives a signal proportional to the power of the incoming optical signal 10, and the dynamic range is expanded by adjusting the impedance of the impedance element so that the impedance on the input side by directly controlling the impedance based on said signal. Method according to Claim 1, characterized in that the impedance element comprises a PIN diode through which the current passing through is controlled by a voltage (U2) dependent on the current passing through the light detector. An optical receiver comprising: v. A transimpedance amplifier (TA) comprising an input and an output,! ! - to convert the optical power of the light detector (PD) into the receiver by an electric current whose light detector output is operatively connected to a · · · · · · - an impedance amplifier (ZU) which is functionally • · ·; coupled to the input of a transimpedance amplifier, and - control means (CU) for adjusting the impedance of the impedance element, the control means comprising means for generating and coupling a control signal to the impedance element of the input optical signal, The 106679 actuators are operatively coupled between the light detector and the impedance element so that they are entirely on the input side of the transimpedance amplifier. A receiver according to claim 3, characterized in that said elements comprise a resistor (R1) for generating a voltage dependent on the input power and a differential amplifier (A1) which generates a control voltage (U2) proportional to said voltage. Receiver according to Claim 3, characterized in that the impedance element comprises at least one PIN diode. Receiver according to claim 4, characterized in that the impedance element comprises two PIN diodes which are connected in series and whose common terminal is operatively connected to the input of a transimpedance amplifier, and the first terminal of the impedance element is connected to a presetting voltage (Vi) and 15 control voltage. «<« I <11 · «« «• Ml 4 · ♦ • • • • • • • • • • • • • • • • • • • • • · · · · · · · · · · · · · • · · • · 1 · 10 106679