Optical signal receiver apparatus

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-076583, filed Mar. 19, 1999, the entire contents of which are incorporated herein by reference.

The present invention mainly relates to an optical signal receiver apparatus and, more particularly, to an optical signal receiver apparatus including, as discrete components, a photo-detector such as a photodiode and an integrated circuit having at least an amplifier.

An optical signal receiver apparatus having the function of receiving a transmitted photo-signal and converting it into an electric signal is indispensable for an optical communication system. An optical signal receiver apparatus capable of high speed communication uses a PIN photodiode as a photo-detector, converts a photoelectric current (signal current) from the PIN photodiode into a voltage signal by using a transimpedance amplifier, and outputs the signal.

In such an optical signal receiver apparatus, the PIN photodiode and transimpedance amplifier are manufactured in different processes. The former is manufactured as an element chip, and the latter is manufactured as an integrated circuit. These two components are electrically connected through bonding wires.

As the transmission rate of an optical communication system is increased for high speed communication, an improvement in high-frequency transmission characteristics is required to broaden the transmission band. If the transmission band is broadened to several GHz, bonding wires, which pose no problem in a low-frequency range, produce an adverse effect. With this adverse effect, the high-frequency transmission characteristics deteriorate. That is, bonding wires serve as inductances in a high-frequency range.

In a conventional optical signal receiver apparatus, the power source line of a photodiode as a photo-detector is much longer than the signal line of the photodiode. For this reason, if a photo-signal transmission band is as wide as several GHz, the band is limited, resulting in a decrease in gain in a high-frequency range, in particular.

It is an object of the present invention to provide an optical signal receiver apparatus which can sufficiently shorten not only the signal line of a photo-detector but also its power source line, and can properly receive a wideband photo-signal like a signal in a GHz band.

It is another object of the present invention to provide an optical signal receiver apparatus which can realize stable operation even at the time of reception of a high-level photo-signal.

According to the present invention, there is provided an optical signal receiver apparatus comprising a photo-detector configured to convert a photo-signal into an electric signal, an integrated circuit incorporating an amplifier which amplifies the electric signal, and a power source supply section which supplies a power source current to the photo-detector through the integrated circuit.

With this arrangement, for example, the signal terminal and power source terminal of the photo-detector can be directly connected to the integrated circuit with boding wires. This makes it possible to sufficiently shorten the power source line of the photo-detector as well as the signal line of the photo-detector which extends to the amplifier in the integrated circuit. Therefore, when a photo-signal in a wide band is to be received, a band is not cut off up to a high frequency. This can realize proper receiving operation while making the most of the element characteristics of the photo-detector such as a PIN photodiode.

In addition, in the present invention, the integrated circuit incorporates a current detector which is inserted in a power source supply path to the photo-detector to detect a power source current flowing into the photo-detector by utilizing the operation of supplying a power source current to the photo-detector through the integrated circuit. Various control operations can therefore be performed by using the detection results obtained by this current detector.

For example, this apparatus includes a current drain circuit which compares a predetermined reference voltage with a voltage corresponding to the power source current detected by the current detector, and drains a DC current from the input side of the amplifier when the voltage corresponding to the power source current is equal to or higher than the reference voltage. This makes it possible to prevent any unnecessary DC current from flowing into the amplifier at the time of reception of a high-level photo-signal and realize stable operation of the amplifier.

More specifically, the current detector is formed from a resistor and designed to convert a power source current flowing into the photo-detector into a voltage whose peak voltage is almost equal to twice a forward voltage drop at a p-n junction or Schottky junction and output the voltage.

The current drain circuit comprises a first level shift circuit constituted by a plurality of series-connected p-n junction elements or Schottky junction elements to obtain the reference voltage, and a second level shift circuit constituted by a plurality of series-connected p-n junction elements or Schottky junction elements to obtain a voltage corresponding to the power source current by level-shifting an output voltage from the photo-detector. The p-n junction elements or Schottky junction elements constituting the second level shift circuit are fewer than the p-n junction elements or Schottky junction elements constituting the first level shift circuit by one. With this arrangement, since the relationship in magnitude between the reference voltage and the voltage corresponding to the power source current detected by the current detector changes depending on whether a photoelectric current is output from the photo-detector, a DC current can be drained on the input side of the amplifier when a photoelectric current flows.

According to the invention, there is provided an optical signal receiver apparatus comprising: a module substrate; a photo-detector mounted on said module substrate to convert a photo-signal into an electric signal; an integrated circuit chip mounted on said module substrate to be juxtaposed with said photo-detector and incorporates an amplifier configured to amplify the electric signal; a bonding wire connecting said photo-detector to said integrated circuit chip; a power source terminal mounted on said substrate to be near said integrated circuit chip and connected to an external power source; and a bonding wire connecting said power source terminal to said integrated circuit chip to supply a power source current from said external power source to said photo-detector through said integrated circuit chip.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a block diagram showing the schematic arrangement of an optical signal receiver apparatus according to an embodiment of the present invention;

FIG. 2 is a circuit diagram showing the detailed arrangement of an integrated circuit in this embodiment;

FIG. 3 is a plan view showing an example of how the optical signal receiver apparatus according to this embodiment is mounted;

FIG. 4 is a graph showing the frequency characteristics of conversion gains of the optical signal receiver apparatus according to this embodiment and a conventional optical signal receiver apparatus;

FIG. 5 is a graph showing the relationship between the power source current flowing into a photodiode and the current drained by a current drain circuit in this embodiment; and

FIG. 6 is a block diagram showing the schematic arrangement of an optical signal receiver apparatus according to another embodiment of the present invention.

An optical signal receiver apparatus according to an embodiment of the present invention will be described with reference to FIG. 1 . This optical signal receiver apparatus is mainly comprised of a photodiode 11 as a photo-detector and an integrated circuit 12 . The photodiode 11 is an element for receiving a transmitted photo-signal and converting it into a current signal (photoelectric current). As this element, a PIN photodiode is preferably used.

The integrated circuit 12 has a transimpedance amplifier (current-to-voltage conversion amplifier) 21 for converting a photoelectric current from the photodiode 11 into a voltage signal and amplifying it, a current detector 22 , a current drain circuit 23 , and a capacitor 24 . The integrated circuit 12 operates upon receiving power from an IC power source through an IC power source terminal 31 . The integrated circuit 12 is also connected to a PD power source terminal 32 .

The anode terminal of the photodiode 11 is connected to the input terminal of the transimpedance amplifier 21 . The cathode terminal of the photodiode 11 is connected to the PD power source terminal 32 through the current detector 22 . That is, power is supplied to the photodiode 11 through the current detector 22 in the integrated circuit 12 . The capacitor 24 serves to ground the power source line between the cathode terminal of the photodiode 11 and the PD power source terminal 32 in terms of high frequencies. One terminal of the photodiode 11 is connected to the cathode terminal of the photodiode 11 , and the other terminal is connected to a ground terminal 33 .

The current detector 22 is a circuit for detecting a power source current flowing from the PD power source into the photodiode 11 . The detection result is supplied to the current drain circuit 23 . As will be described in detail later, the current drain circuit 23 compares a reference voltage with a voltage corresponding to the power source current detected by the current detector 22 , and drains a DC current from the input side of the transimpedance amplifier 21 if the voltage corresponding to the power source current is equal to or higher than the reference voltage, i.e., the photodiode 11 receives a photo-signal and outputs a photoelectric current.

The operation of the optical signal receiver apparatus of this embodiment will be described next.

The optical signal receiver apparatus receives a photo-signal transmitted through, an optical fiber (not shown). Assume that the photo-signal has been binary-modulated. The photodiode 11 receives this photo-signal and outputs a photoelectric current. The photoelectric current from the photodiode 11 is extracted from the anode terminal and converted into a voltage signal by the transimpedance amplifier 21 in the integrated circuit 12 . This voltage signal is output from a signal output terminal 34 .

The photoelectric current output from the photodiode 11 also propagates from the cathode terminal to the PD power source through the current detector 22 and PD power source terminal 32 . In this case, since the power source current from the photodiode 11 is supplied from the PD power source to the integrated circuit 12 through the PD power source terminal 32 , the photoelectric current propagating to the PD power source is also input to the integrated circuit 12 through the PD power source terminal 32 . This photoelectric current passes through the current detector 22 and is guided to the ground terminal 33 through the capacitor 24 . Since the photoelectric current output from the photodiode 11 is not influenced by detour through the power source line of the photodiode 11 , the element characteristics of the photodiode 11 can be fully utilized.

In other words, in this embodiment, the line extending from the cathode terminal of the photodiode 11 to the PD power source through the current detector 22 in the integrated circuit 12 is sufficiently short as well as the signal line from the anode terminal of the photodiode 11 to the integrated circuit 12 . The lengths of these lines are much smaller than ¼ the maximum frequency of a photo-signal in the transmission band. For this reason, the band is not limited by the signal line and power source line, and a sufficient gain is obtained up to a high-frequency range.

In addition, according to this embodiment, the current drain circuit 23 can prevent any unnecessary DC current from flowing into the transimpedance amplifier 21 . This can realize stable operation. More specifically, a photo-signal received by the optical signal receiver apparatus is a binary-modulated signal, which has no information in the amplitude direction. It is therefore only required to detect a binary change in photoelectric current. To input an excessively large photoelectric current to the transimpedance amplifier 21 when a high-level photo-signal is input to the optical signal receiver apparatus is to interfere with the realization of stable operation of the transimpedance amplifier 21 .

In this embodiment, the above problem can be solved by utilizing the operation of supplying a power source current to the photodiode 11 through the integrated circuit 12 as follows. The current detector 22 is inserted in the power source path to the photodiode 11 in the integrated circuit 12 . when the photodiode 11 outputs a photoelectric current in accordance with the detection result obtained by the current detector 22 , the current drain circuit 23 is operated to drain a DC current from the input side of the transimpedance amplifier 21 .

A practical example of the internal arrangement of the integrated circuit 12 will be described next with reference to FIG. 2 .

The correspondences with the respective components in FIG. 1 will be described. The transimpedance amplifier 21 in FIG. 1 is constituted by transistors Q 4 and Q 5 and resistors R 5 , R 6 , and R 7 . The transistor Q 4 forms an emitter-grounded amplifier (common emitter amplifier) having the resistor R 5 as a collector load. The transistor Q 5 forms an emitter-follower circuit having the resistor R 6 as an emitter load (current source). The emitter electrode of the transistor Q 5 , which is the output of the emitter-follower circuit, is negatively fed back to the base electrode of the transistor Q 4 , which is the input of the emitter-grounded amplifier, through the feedback resistor R 7 . The negative feedback amount is determined by the ratio between the resistances of the resistors R 6 and R 7 .

A current detection resistor R 4 forms the current detector 22 in FIG. 1, and is connected between the cathode terminal of the photodiode 11 and the PD power source terminal 32 . A capacitor C 1 corresponds to the high frequency ground capacitor (bypass capacitor) 24 . Transistors Q 1 , Q 2 , and Q 3 , resistors R 1 , R 2 , and R 3 , diodes D 1 , D 2 , D 3 , and D 4 , and constant current sources I 1 and I 2 constitute the current drain circuit 23 in FIG. 1 . As each of the diodes D 1 , D 2 , D 3 , and D 4 , a so-called diode-connected transistor whose collector and base electrodes are connected to each other is used. However, a general diode may be used. It is essential only that this element is a level shift element such as a p-n junction element or Schottky junction element (which can be constituted by a MOSFET and the like).

The current drain circuit 23 will be described in detail. The transistors Q 1 and Q 2 , the resistor R 3 connected between the emitter electrodes of the transistors Q 1 and Q 2 , and the constant current sources I 1 and I 2 respectively connected to the emitter electrodes of the transistors Q 1 and Q 2 constitute a voltage comparator called a linear comparator. A resistor R 0 forms an AC cutoff circuit in this case. Of the transistors Q 1 and Q 2 of the voltage comparator, the transistor Q 1 serves as a current drain element. The three diodes D 1 , D 2 , and D 3 and ground terminal 33 connected in series with each other between the PD power source terminal 32 and the ground terminal 33 constitute the first level shift circuit. This circuit level-shifts a PD power source potential Vpd by an amount corresponding to three p-n junctions, and applies the resultant potential to the base electrode of the transistor Q 1 serving as one input terminal of the voltage comparator.

The transistor Q 3 whose collector and base electrodes are respectively connected to the two terminals of the current detection resistor R 4 and the diode D 4 and resistor R 2 connected in series with each other between the emitter electrode of the transistor Q 3 and the ground terminal 33 constitute the second level shift circuit. This circuit level-shifts a voltage across the current detection resistor R 4 by an amount corresponding to two p-n junctions, and applies the resultant voltage to the base electrode of the transistor Q 2 serving as the other input terminal of the voltage comparator.

Assume that the resistance of the current detection resistor R 4 is set to convert a power source current flowing in the photodiode 11 into a voltage whose peak voltage is almost equal to twice a forward voltage drop (vd) at the p-n junction, i.e., 2Vd. In other words, a resistance is selected for the current detection resistor R 4 to produce the voltage drop 2Vd corresponding to two p-n junctions with a maximum current value estimated from the maximum intensity of a photo-signal incident on the photodiode 11 .

The operation of the current drain circuit 23 in FIG. 2 will be described next.

Of the transistors Q 1 and Q 2 constituting the voltage comparator, the transistor Q 1 has a base potential Vb 1 that is always set to Vb 1 =Vpd−3Vd. A base potential Vb 2 of the transistor Q 2 is given by Vb 2 =Vpd−2Vd because no voltage drop occurs at the current detection resistor R 4 when no photo-signal is incident on the photodiode 11 and no photoelectric current flows. Since Vb 1 <Vb 2 , and the transistors Q 1 and Q 2 are off and on, respectively, no current is drained.

Assume that a photo-signal is incident on the photodiode 11 , and a photoelectric current flows. In this case, if, for example, this photoelectric current becomes half the maximum current value, the base potential Vb 2 of the transistor Q 2 becomes Vpd−3Vd like the base potential Vb 1 of the transistor Q 1 . As a consequence, both the transistors Q 1 and Q 2 are turned on, and about half the maximum current value are drained by the transistor Q 1 .

When the photo-signal incident on the photodiode 11 reaches the maximum intensity, and the photoelectric current reaches the maximum current value, the voltage drop 2Vd occurs at the current detection resistor R 4 , and the base potential of the transistor Q 2 becomes Vb 2 =Vpd−4Vd. As a result, Vb 1 >Vb 2 , and the transistors Q 1 and Q 2 are turned on and off, respectively. Therefore, the maximum current value is drained.

This operation of the current detector 22 can prevent any photoelectric current (DC current) from flowing into the transistor Q 4 of the transimpedance amplifier 21 even at the time of reception of a high-level photo-signal. This makes it possible to realize stable operation of the transimpedance amplifier 21 .

FIG. 3 shows a practical example of how the optical signal receiver apparatus according to this embodiment is mounted. Referring to FIG. 3, the photodiode 11 , integrated circuit 12 , and signal output terminal 34 are horizontally juxtaposed with each other on a module substrate 10 . The ground terminal 33 is placed on one side (upper side in FIG. 3) of the integrated circuit 12 in the vertical direction of the module substrate 10 , and the IC power source terminal 31 and PD power source terminal 32 are placed on the other side (lower side in FIG. 3 ).

The anode terminal, i.e., the signal-side terminal, of the photodiode 11 is connected to the signal input terminal (the input terminal of the transimpedance amplifier 21 ) of the integrated circuit 12 with a is bonding wire 41 . The cathode terminal, i.e., the power-source-side terminal, of the photodiode 11 is connected to an electrode pad 35 . This electrode pad 35 is connected, with bonding wires 42 , to pads connected to the current detector 22 in the integrated circuit 12 .

In addition, the integrated circuit 12 is connected to the IC power source terminal 31 , PD power source terminal 32 , ground terminal 33 , and signal output terminal 34 with bonding wires 43 , 44 , 45 , and 46 , respectively.

A photo-signal transmitted through an optical fiber 13 is guided to the photodiode 11 through an optical system such as a mirror (not shown).

With this arrangement, since a signal component propagating to the PD power source over a photoelectric current generated by the photodiode 11 is also input to the integrated circuit 12 , the photoelectric current from the photodiode 11 can be current/voltage-converted by the transimpedance amplifier 21 and amplified without being influenced by the interconnection patterns including the bonding wires on the photodiode 11 .

The bonding wire 41 is basically the only signal line between the anode terminal of the photodiode 11 and the integrated circuit 12 , and is sufficiently short. In addition, the boding wires 42 and 44 are basically the only power source lines between the cathode terminal of the photodiode 11 and the PD power source terminal 32 . The power source lines can therefore be sufficiently shortened. Hence, the band is not limited to the signal lines and power source lines of the photodiode 11 , and sufficient gains can be obtained up to a high-frequency range.

The effects of the present invention will be described in detail below.

FIG. 4 provides a comparison between the frequency characteristics of conversion gain of the optical signal receiver apparatus according to the above embodiment of the present invention and those of the conventional optical signal receiver apparatus. In this case, “conversion gain” is the ratio of the signal output to the energy of a photo-signal incident on the photodiode. In the conventional optical signal receiver apparatus, the conversion gain abruptly decreases at a given frequency, e.g., 5 GHz, whereas in the optical signal receiver apparatus according to this embodiment of the present invention, good characteristics with a constant conversion gain can be obtained up to about twice the above frequency, i.e., up to about 10 GHz.

FIG. 5 shows the relationship between the influent current flowing into the photodiode 11 and the current drained by the current drain circuit 23 in the optical signal receiver apparatus of this embodiment. Obviously, the drained current increases in proportion to the influent current until the influent current exceeds the maximum current value (1 mA in this case). As is apparent, therefore, this embodiment can prevent any unnecessary DC current from flowing into the transimpedance amplifier 21 at the time of reception of a high-level photo-signal, thereby realizing stable operation.

An optical signal receiver apparatus according to another embodiment of the present invention will be described next with reference to FIG. 6 .

In this embodiment, a data discriminator 25 and discrimination threshold generator 26 are newly mounted in an integrated circuit 12 . The data discriminator 25 is a circuit for determining an output signal from a transimpedance amplifier 21 on the basis of threshold, discriminating between “0” and “1”, and outputting the discrimination result to a data output terminal 36 . The threshold (discrimination threshold) is supplied from the discrimination threshold generator 26 .

In this case, the discrimination threshold generator 26 is designed to change the discrimination threshold to be generated in accordance with the magnitude of a power source current from a photodiode 11 , which is detected by a current detector 22 . AS the intensity of a received photo-signal varies, the power source current flowed into the photodiode 11 changes accordingly. If, therefore, the discrimination threshold is changed to an optimal value in accordance with a change in this power source current, the data discriminator 25 can correctly discriminate between “0” and “1” regardless of changes in the intensity of the photo-signal. This makes it possible to reduce decoding errors on the subsequent stage of the data discriminator 25 .

As has been described above, according to the optical signal receiver apparatus of the present invention, not only the signal line of the photo-detector but also the power source line of the photo-detector can be sufficiently shortened, and a wideband photo-signal like a signal in a GHz band can be properly received.

In addition, according to the optical signal receiver apparatus of the present invention, a power source current in the photo-detector is detected, and a DC current is drained at the input side of the amplifier, thereby realizing stable receiving/amplifying operation even at the time of reception of a high-level photo-signal. Therefore, a wide dynamic range with respect to photo-signals can be set.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.