JP4072232B2 - Optical receiver circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical receiver circuit, and more particularly to an optical receiver circuit having an amplitude adjustment function suitable for performing signal processing with high sensitivity and a wide dynamic range with respect to changes in an input signal.
[0002]
[Prior art]
FIG. 2 shows a general configuration of an optical receiving circuit used in an optical communication system for basic transmission (the transmission distance is, for example, a maximum of several hundred km). An example of the optical signal identification operation by the optical receiver circuit having the configuration of FIG. 2 will be described with reference to FIG. The optical signal forms a light-on / off rectangular pattern corresponding to “1” and “0” for each Δt with respect to a 1-bit signal transmission amount, and is provided in the optical receiving circuit. (That is, FIG. 17 shows an optical signal for 6 bits). Inside the APD, electron and hole pairs generated by the optical signal cause avalanche multiplication, and the signal current is amplified. For this reason, even a weak optical signal can be detected. The current signal output from the APD is further converted into a voltage signal VPRE by a preamplifier. The VPRE is further amplified by an AGC amplifier (also referred to as an automatic gain control amplifier or gain adjustment amplifier) and input to the identification circuit DEC. Here, for example, as shown by a broken line in the figure, even when the intensity of the optical signal is weak, the amplitude of the output VAGC of the AGC amplifier is kept almost constant by the automatic gain control function of the AGC amplifier. Thereby, the discrimination operation in the discrimination circuit is performed more stably. In the identification circuit DEC, in synchronization with the clock signal CLK, the output signal VAGC of the AGC amplifier is “1” if it is higher than the threshold potential VTH (hereinafter referred to as VTH (DEC)). If it is low, it is judged as “0” and data is output. The clock signal CLK is generated from the output signal VAGC of the AGC amplifier by the clock extraction circuit CEXT.
[0003]
An example of the above-described AGC amplifier is described in IE Journal of Solid-State Circuits, vol. 29, No. 29, Vol. 29, No. 7 (1994), pages 815-822. 7 (1994) pp.815-822: hereinafter referred to as Document 1) (see FIG. 15). The feature of the AGC amplifier in Document 1 is that the voltage signal VPRE from the preamplifier PRE is amplified in three stages, ie, variable gain type amplifiers A1 and A2 and fixed gain type amplifier A3. First, the signal with the amplitude VA3 output from the amplifier A3 is input to the peak detection circuit PD. The peak detection circuit PD generates a DC voltage V3 based on the amplitude VA3 by a capacitor CPD attached thereto. On the other hand, the reference circuit REF independently generates a nominal value VN (the nominal value of the peak voltage: hereinafter referred to as a standard amplitude voltage). This VN is adjusted by the variable voltage source P connected to the reference circuit REF. In the gain control circuit, V3 and VN are compared, and voltages VGC1 and VGC2 for controlling the gains of the amplifiers A1 and A2 are set according to the difference, that is, the deviation of the signal amplitude from the standard amplitude voltage value. The feature of the AGC amplifier disclosed in Document 1 is that a signal with desirably uniform amplitude, in which variation in VAGC with respect to amplitude change of the input signal VPRE is suppressed, is generated due to the difference in the circuit configuration of the amplifiers A1 and A2.
[0004]
According to the AGC amplifier of Literature 1, a voltage signal VAGC having an amplitude of 500 mV is output at a speed of 13 Gb (gigabit) / s (seconds) for both input signals having an amplitude of 10 mV and 300 mV. These output signals are output from two output buffers OB, one of which is input to the identification circuit DEC of FIG. 2 and the other is input to the clock extraction circuit CEXT.
[0005]
The AGC amplifier of Document 1 is also provided with an offset control circuit OC (with an external capacitor COC) for controlling the offsets of the amplifiers A1, A2, and A3.
[0006]
Now, with respect to the output signal from the AGC amplifier, the identification circuit DEC uses a certain potential between the upper end and the lower end of the amplitude voltage of the output signal as a threshold potential VTH (DEC) from VAGC to “1”, “0”. Is determined. FIG. 16 shows a state where an optical signal “1” or “0” transmitted from an optical transmission line such as an optical fiber is converted into a voltage signal by the preamplifier PRE of the optical receiving circuit. The horizontal axis indicates the signal voltage value, and the vertical axis indicates the frequency with which a signal having a predetermined voltage value is generated. When signals of “1” and “0” are transmitted / received when the optical signal is turned on / off, for example, a distribution shown by a solid line in FIG. 16 is shown due to noise generated in the photoelectric conversion element and the preamplifier PRE. At this time, the threshold potential set by the identification circuit DEC is set to, for example, Vth (A) which is an intermediate value between the average voltage value V1 of the signal “1” and the average voltage value V0 of the signal “0”. The
[0007]
However, the “1” signal may exhibit a wider voltage distribution than the “0” signal. That is, the voltage distribution of the “1” signal has a shape (shown by a broken line) obtained by squashing the one shown by the solid line in FIG. This is particularly noticeable when an APD or erbium-doped optical fiber amplifier (EDFA) and a PIN junction type light receiving element (PIN diode) are used in combination as the light receiving element (photoelectric conversion element). For details of this phenomenon, refer to, for example, “Optical Fiber Communication Technology” (Nikkan Kogyo Shimbun). In this case, a voltage signal equal to or lower than the threshold potential Vth (A) is determined to be “0” even if the signal is “1”. The probability that such an erroneous determination is made is called an error rate. In the case of the threshold potential Vth (A), the error rate is defined as the ratio of the area of the region partitioned by the dashed line to the lower potential side with respect to the total area of the region surrounded by the voltage distribution curve of the “1” signal indicated by the broken line. The
[0008]
The error rate cannot be 0 when the voltage distribution curve for one signal (broken line) and the voltage distribution curve for zero signal (solid line) overlap. However, the threshold potential is V ₀ The error rate can be reduced by shifting it to some extent. For example, when the voltage distribution curve of 1 signal intersects the voltage distribution curve of 0 signal, the threshold potential V _{th (B)} Set up. As a result, although there is a possibility that the 0 signal is mistaken as the 1 signal, the error rate can be reduced to the area ratio between the area shown by hatching in FIG. 16 and the area surrounded by one of the voltage distribution curves. Under the threshold potential Vth (B), some of the true zero signals (those having an intensity greater than Vth (B)) are determined as one signal in the discrimination circuit, but the rate at which such misperception occurs is low. Compared to the threshold voltage Vth (A), the voltage is considerably reduced.
[0009]
Normally, the signal voltage distribution is wider than the signal magnitude (V1-V0), and the error rate cannot be completely reduced to 0 regardless of the threshold potential. Therefore, a certain error rate (for example, 10) which is very small in the optical receiving circuit. ^-12 ) And the threshold potential is set within a range where the error rate can be achieved. In this case, transmission errors that occur with a very small probability are detected and corrected by an error correction code or the like.
[0010]
FIG. 3 schematically shows the relationship between the threshold potential setting range and the optical signal intensity under a certain error rate when an APD or EDFA and a PIN diode are used in combination as a light receiving element. is there. As described above, since the “1” signal has a wider voltage distribution than the “0” signal, the threshold potential setting range is biased toward the “0” signal. Also, as the optical signal intensity decreases, the threshold potential setting range gradually decreases, and eventually a desired error rate cannot be achieved. The optical signal intensity at this time (Pmin0 in the figure) is called the minimum receiving sensitivity.
[0011]
[Problems to be solved by the invention]
In order to cover an optical signal transmitted at various distances from a short distance to a long distance with a single receiver, a wide dynamic range with respect to the received signal strength is required even in a receiver for basic transmission. That is, when the transmission distance is short, the attenuation of the signal in the optical fiber is small, so that a strong optical signal is input to the receiver. On the other hand, when the transmission distance is long, the optical signal input to the receiver becomes very weak. An optical receiver must handle both of these two types of signals.
[0012]
By the way, when a strong optical signal is input to the receiver having the configuration shown in FIG. 2, a problem due to the slew rate of the preamplifier described below occurs. FIG. 18 shows a response waveform of the receiver when a strong optical signal is input. When the intensity of the optical signal is strong, the output current of the light receiving element increases and the output amplitude of the preamplifier increases. However, if the output amplitude of the preamplifier becomes too large, as shown in the figure, the output signal changes due to the next optical signal input before the output signal reaches the steady value, and the output waveform becomes a triangular wave. A part arises. This is because there is an upper limit to the rate of change of the output VPRE of the preamplifier, and this upper limit rate (dV / dt) is called a slew rate. If the slew rate of the AGC amplifier is sufficiently larger than that of the preamplifier, the output signal of the AGC amplifier has a waveform similar to VPRE.
[0013]
In the example shown in the drawing, the first bit (the optical signal intensity 0 at the left end of FIG. 18) and the fifth bit (the second light from the right in FIG. 18), regardless of where the threshold potential VTH (DEC) is set. At least one of the bits of signal strength 1) is erroneously identified. This is because the threshold potential VTH (DEC) must be set too high to recognize the fifth bit as 1 in order to identify the first bit as 0, and This is because in order to identify the fifth bit as 1, VTH (DEC) must be set to a value that is too low to recognize the first bit as 0. In addition, since the amplitude of the output VPRE of the preamplifier increases, the voltage applied to the transistor in the output stage of the preamplifier increases, and it is necessary to use a high breakdown voltage transistor. In general, the operation speed of a high breakdown voltage transistor is slower than that of a low breakdown voltage transistor, so that there is a problem that it cannot be used for a high-speed optical receiver. Therefore, it is very difficult to receive a strong optical signal with the prior art of FIG.
[0014]
On the other hand, as an optical receiver capable of avoiding the problems due to the above slew rate, for example, IEE Journal of Solid State Circuits, Vol. 30, No. 9 (1995), pages 991-997 (IEEE Journal of Solid-State Circuits, vol. 30, No. 9 (1995), pp. 991-997: hereinafter referred to as Reference 2). FIG. 4 shows the configuration of the optical receiver circuit disclosed in Document 2. This circuit is used in a local area network (LAN) or the like, and has a short transmission distance of about several kilometers at the maximum. For this reason, a PIN photodiode PIN is used instead of the APD. Further, a limit amplifier LA is used instead of the AGC amplifier.
[0015]
In the AGC amplifier, the gain is controlled to keep the amplitude of the output signal constant, whereas in the limit amplifier (limit amplifier), the gain is set to be very large, and the output is output when the output amplitude exceeds a desired value. Clamping (suppressing to a predetermined output voltage) limits the amplitude. That is, the threshold potential VTH (LA) is controlled by the threshold potential control circuit VCNT so that it is exactly in the middle between the high potential (for example, 1) and the low potential (for example, 0) of the input signal. The input signal is compared with VTH (LA), and if it is higher than VTH (LA), a constant potential VLA (0) is output regardless of the amplitude of the input signal.
[0016]
In the example of this document, the gain of the limit amplifier LA is set to about 60 dB, and an output signal having a constant amplitude can be obtained in a wide range where the amplitude of the input signal is about several mV to 1V. In such a configuration, since the gain of the limit amplifier is very high, it is not necessary to increase the gain of the preamplifier (ratio of output voltage and input current: referred to as transimpedance). Therefore, even when a strong optical signal is input, the amplitude of the output VPRE of the preamplifier does not become as large as the above configuration, so that the waveform is not distorted as shown in FIG. For this reason, the problem resulting from a slew rate does not occur.
[0017]
However, in order to extend the transmission distance in the circuit having this configuration, there is a problem that the minimum receiving sensitivity of the receiver is deteriorated when the light receiving element is replaced with a detector combining a PIN photodiode with an APD or an EDFA and a PIN diode. . Hereinafter, this reason will be described with reference to FIG. FIG. 5 shows the setting range of the threshold voltage VTH (LA) of the limit amplifier at the preamplifier output VPRE. Since a light receiving element having an amplifying function is used, the “1” signal shows a wider voltage distribution than the “0” signal, and the threshold potential setting range is biased toward the “0” signal. The output (VLA) of the limit amplifier is obtained by extracting and amplifying a signal of VTH (LA) ± several mV from VPRE (the double line graph indicates the voltage width of this several mV). Since VTH (LA) is set to the middle of VPRE (0) and VPRE (1), it can be seen that VTH (LA) deviates from the threshold setting range in the region where the optical signal intensity is Pmin1 or less. . Therefore, the minimum reception sensitivity is Pmin1, and the minimum reception sensitivity is deteriorated as compared with the configuration of FIG.
[0018]
It should be noted that the above-described degradation of the minimum reception sensitivity does not occur in the configuration of Document 2. This is because the configuration of Document 2 uses a PIN diode as the light receiving element. Since the PIN diode has no amplification effect, the “1” signal shows the same voltage distribution as the “0” signal, and even if VTH (LA) is set to exactly the middle between VPRE (0) and VPRE (1), The problem does not occur.
[0019]
As a method of avoiding this problem, a method of adjusting the threshold potential VTH (LA) of the limit amplifier to the “0” level side instead of the center of VPRE is conceivable. However, since the intensity of the optical signal to be adjusted is weak (particularly in a range less than Pmin1) and the amplitude of VPRE is small, it is required to adjust the threshold potential VTH (LA) with high accuracy. Realization is extremely difficult. Further, since the intensity of the optical signal fluctuates due to the aging of the characteristics of the transmitter and the optical fiber, it is necessary to periodically readjust the threshold value VTH (LA). As described above, the wide dynamic range receiver circuit for LAN disclosed in Document 2 cannot be applied to the wide dynamic range receiver for long-distance transmission as it is.
[0020]
As a technique for expanding the dynamic range with respect to the reception intensity of the optical receiving circuit, a technique for improving the reference potential setting of the discrimination circuit is disclosed in Japanese Patent Laid-Open No. 5-259752, and a technique for improving the gain setting of a so-called preamplifier is disclosed in Japanese Patent Laid-Open No. Hei 8- -139526, JP-A-7-193437, JP-A-7-38342 and JP-A-5-67930, all of which were intended for application to an optical communication system with a transmission frequency of 100 MHz. . That is, these technologies are 10 ns (nanosecond: 10 ^-9 Although response characteristics to optical pulses received at intervals of seconds can be expected, for example, 100 ps (picoseconds: 10) in a 10 GHz trunk optical communication system ^-12 The optical signal received at intervals of 2 seconds) could not be practically responded.
[0021]
Further, from the technology for suppressing the deterioration of the voltage pulse width in the optical receiver circuit disclosed in Japanese Patent Application Laid-Open Nos. 9-246879 and 5-218758, no technology for solving the above-described problems of the prior art has been found. .
[0022]
An object of the present invention is to provide an optical receiver circuit having high sensitivity and a wide dynamic range.
[0023]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, in an optical receiving circuit that receives an optical signal, converts it into an electrical signal, and outputs it, a photoelectric conversion unit that converts the optical signal into a current signal, A preamplifier for converting an output current signal into a voltage signal, and an amplifier for receiving the output voltage signal of the preamplifier, wherein if the difference between the input signal and the reference potential is smaller than a predetermined voltage, linear amplification and It includes an amplifier with a limit function that performs limit amplification when the voltage is higher than a predetermined voltage, and an automatic gain amplifier that amplifies the output signal of the amplifier with limit function and outputs a signal having a constant amplitude.
[0024]
The predetermined voltage at which the amplifier with limit function starts limit amplification is preferably set to be larger than the difference between the output signal voltage and the reference potential at the minimum receiving sensitivity of the preamplifier. The output signal voltage of the preamplifier at the minimum reception sensitivity is determined by the photoelectric converter and the preamplifier, and the value depends on the error rate desired for the optical receiver circuit.
[0025]
Furthermore, it is desirable to limit the output signal amplitude of the amplifier with a limit function so as to satisfy the relationship defined by the following expression 1 within the desired dynamic range of the optical signal.
[0026]
[Expression 1]
ΔVAWL <Rs · Δt (Formula 1)
In Equation 1, ΔVAWL is the output signal amplitude of the amplifier with limit function, Δt is the time occupied by a 1-bit optical signal, and Rs is the slew rate of the amplifier with limit function.
[0027]
Further, when the photoelectric conversion unit is composed of an avalanche photodiode or an erbium-doped optical fiber amplifier and a PIN photodiode, the minimum receiving sensitivity can be improved.
[0028]
When the optical signal is weak, the optical receiver circuit of the present invention linearly amplifies the voltage signal VPRE generated therefrom by the amplifier with limit function and the AGC amplifier at the next stage, and identifies the output voltage VAGC from the AGC circuit at the subsequent stage. Input to the circuit. The identification circuit sets a threshold potential for identifying 1 and 0 of the voltage signal VAGC input thereto according to the signal amplitude of the VAGC (signal voltage difference between 1 and 0). Therefore, the discrimination circuit can set an appropriate threshold potential even for a weak optical signal by linear amplification of the voltage signal by the amplifier with limit function and the AGC amplifier. On the other hand, when the optical signal intensity is strong, the amplifier with a limit function suppresses the amplitude of the output signal VAWL, thereby preventing distortion of the output signal of the preamplifier and suppressing an increase in error rate.
[0029]
If the gist of the present invention is grasped in one limited aspect, a photoelectric conversion unit (light receiving element), a first amplifier circuit (preamplifier) that converts a current signal output from the photoelectric conversion unit into a voltage signal. , An optical receiver circuit comprising a second amplifier circuit (gain adjusting amplifier or AGC amplifier) for reducing variation in amplitude of the voltage signal output from the preamplifier, or a voltage output from the second amplifier circuit In an optical transmission terminal device or optical transmission system configured in combination with an identification circuit that performs signal processing (identification), a third amplification circuit having the following function between the first amplification circuit and the second amplification circuit It is in providing. The first function required for the third amplifier circuit is that the output signal amplitude ΔVAWL is defined by the slew rate Rs (of the third amplifier) and the time Δt occupied by the 1-bit optical signal. It is to make it smaller than Δt.
[0030]
On the other hand, the second function required of the third amplifier circuit is to linearly amplify the voltage signal input from the first amplifier circuit to the third amplifier circuit to an amplitude equal to or lower than a predetermined voltage amplitude. It is. This function is important in signal detection when the amplitude of the voltage signal input from the first amplifier circuit to the third amplifier circuit is particularly small. For this reason, the linear amplification range by the third amplifier circuit is set wider than that of the conventional limit amplifier. In general, when the linear amplification range is widened, the gain is lowered. However, since the third amplifier circuit is used in combination with the second amplifier circuit at the subsequent stage, there is no practical problem.
[0031]
According to the circuit configuration of the present invention outlined above, when the optical signal intensity is weak, the amplifier with a limit function is performing linear amplification operation. Therefore, by performing further linear amplification with the AGC amplifier at the next stage. The threshold value of the discrimination circuit can be set to the optimum point. Also, when the optical signal intensity is strong, the limit function amplifier performs a limit operation, and the output signal amplitude of the preamplifier is distorted to prevent the error rate from increasing. As a result, an optical receiver circuit with high sensitivity and wide dynamic range can be realized.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0033]
FIG. 1 is a diagram showing a basic configuration of the present invention. The prior art (reference 1) of FIG. 2 is that an amplifier AWL with a limit function and a reference potential generation circuit VRG are inserted between a preamplifier PRE and a gain adjustment amplifier (hereinafter also referred to as an AGC amplifier) AGC. Different. On the other hand, the prior art (reference 2) in FIG. 4 is a combination of a reference potential generation circuit VRG that generates a reference potential based on an input signal from the preamplifier PRE and an amplifier AWL with a limit function to which the reference potential is input. However, there is a large difference in the voltage amplification gain of the amplifier with a limit function, and the limit amplifier LA disclosed in the prior art (Document 2) does not consider linear amplification of an input signal having a weak amplitude. The point is greatly different from the amplifier AWL with limit function of the present invention. In other words, the amplifier AWL with limit function of the present invention is basically a linear amplifier, and the output amplitude is kept substantially constant at a value determined by the circuit configuration by the limit function only when the output amplitude exceeds a certain value. Work like that.
[0034]
The functions characterized by the above-described basic configuration of the optical receiver circuit of the present invention will be described with reference to FIG. FIG. 6 shows the setting range of the threshold potential for the preamplifier output VPRE.
[0035]
The reference potential Vref of the amplifier AWL with limit function is set at the exact center of the preamplifier output signals VPRE0 (0) and VPRE (1) (both are shown by broken line graphs), and the linear amplification range of AWL (in the thick diagonal line graph) The width of the sandwiched vertical axis) is much wider than the conventional limit amplifier (several hundred mV). Therefore, as shown in FIG. 6, when the optical signal intensity is the minimum reception sensitivity Pmin0, linear amplification operation is performed and VAWL is output, and, as in the conventional technique of FIG. By performing amplification, the threshold value of the identification circuit can be set to the optimum point (voltage value VPRE in the shaded portion in FIG. 6 determined with respect to the optical signal intensity). On the other hand, when the optical signal intensity is large, the output value of the VAWL is limited so that the output signal amplitude of the amplifier with a limit function does not become excessive (it is suppressed to a predetermined value regardless of the magnitude of VPRE), so the slew rate This suppresses the waveform distortion caused by the above and widens the dynamic range as a receiver.
[0036]
The functional characteristics of the present invention will be compared with the prior art of Document 2 with reference to FIG. The optical signal intensity is between Pmin1 and Pmin0 in FIG. FIG. 6A schematically shows an optical signal, a preamplifier output, and a limit amplifier output when the photoelectric conversion element of the prior art in Document 2 is replaced with an APD or EDFA and a PIN diode. The noise superimposed on the signal in the figure corresponds to the signal distribution in FIG. 16, and the photoelectric conversion unit is composed of an APD or an erbium-doped optical fiber amplifier and a PIN photodiode, so it is superimposed on the “1” signal. The noise that is generated is larger than that of the “0” signal.
[0037]
In the prior art, the threshold potential VTH (LA) of the limit amplifier is set to the middle between the average potential of the “1” signal output of the preamplifier output and the average potential of the “0” signal output. Further, since the gain of the limit amplifier is set to be very large, the linear amplification range is only several mV at most. That is, only the signal between the two thick lines shown as the linear amplification range in the figure is linearly amplified, the information held by the signal located outside the signal is ignored, and a constant potential is output. As a result, the limit amplifier output VLA has a waveform as shown in the figure. It can be seen that when the signal is “0”, it can be normally identified, but when the signal is “1”, it is buried in noise and cannot be identified.
[0038]
On the other hand, in the case of the present invention (see FIG. 22B), the reference potential is set to the same potential as the conventional VTH (LA). However, since the linear amplification range is as wide as several hundred mV, the preamplifier output VPRE is The signal is linearly amplified as it is and output from the AGC amplifier and further linearly amplified to obtain a signal (VAGC) having a desired amplitude. For this reason, the output of the AGC amplifier is a faithful reproduction of the optical signal waveform, that is, a signal that is faithfully converted into an electrical signal without losing the information of the optical signal (however, the preamplifier and the limit function) (Some noise generated by the attached amplifier and AGC amplifier is added.) Therefore, if VTH (DEC) is set slightly to the VAGC rather than the “0” signal, the signal can be normally identified.
[0039]
Next, FIG. 19 shows an example of a specific circuit configuration of the optical receiver circuit of the present invention introduced in FIG. In the circuit configuration disclosed in FIG. 19 with respect to FIG. 1, the configurations of the light receiving element APD (but not limited to the avalanche photodiode), the identification circuit DEC, and the clock signal extraction circuit are omitted.
[0040]
However, I provided in the circuit of the preamplifier PRE _IN The signal current from the light receiving element APD is input to the terminal, and the VAGC terminal provided in the circuit of the AGC amplifier is electrically connected to the voltage signal input terminal of the identification circuit DEC and the voltage signal input terminal of the clock extraction circuit CEXT, respectively. Is done. Each circuit configuration of the optical receiver circuit shown in FIG. 19 is shown in FIG. 10 for the preamplifier PRE, FIG. 7 for the amplifier AWL with a limit function, and FIG. The gain adjustment amplifier AGC will be described with reference to FIG.
[0041]
The optical receiving circuit of the present invention is not limited to the circuit configuration shown in FIG. 19, and the components of each circuit can take various modes introduced in the following first to fourth embodiments, for example. Therefore, details of each of the amplifier AWL with limit function, the preamplifier PRE, the gain adjustment amplifier AGC, and the reference potential generation circuit VRG, which are components of the optical receiver circuit of the present invention, will be described in detail with reference to the related drawings.
[0042]
<Example 1>
FIG. 7 shows an embodiment of an amplifier with a limit function (hereinafter also abbreviated as a limit amplification unit AWL) according to the present invention, which is an example using a differential amplifier composed of bipolar transistors. When the input signal input to the terminal IN in FIG. 7 is close to the reference potential Vref, it operates as a linear amplifier circuit, and its gain A can be generally expressed by the following equation 2.
[0043]
[Expression 2]
A = RC / ((2VT / IA) + RE) (Formula 2)
In Equation 2, RC is the resistance value of the load resistor RC0, RE is the resistance value of the emitter resistor RE0, VT = q / kT, and IA is the bias current of the differential amplifier (where q is the amount of charge of electrons, and k is Boltzmann constant, T represents temperature). The differential amplifier circuit shown in FIG. 7 includes a left circuit (consisting of RC0, QA0, RE0 and CE0) and a right circuit (RC1, QA1, RE1) from the ground potential (upper part of the figure) to the constant current source IA. And CE1) are required to be symmetrically configured. In practice, an error of up to 3% is allowed between the parameters (resistance value, capacitance, etc.) of each circuit element in the left and right circuits, but it is required to remove this error in circuit design. The That is, in the circuit design, the RC is set equal to RC1 and the RE is set equal to RE1. The linear amplification range VLR is generally expressed by the following equation 3.
[0044]
[Equation 3]
VLR = 2RE · IA (Formula 3)
The capacitors CE0 and CE1 are peaking capacitors, and have a function of increasing the gain at high frequencies and improving the band.
[0045]
FIG. 8 shows another embodiment of an amplifier with a limit function, and shows an example of a differential amplifier using a cascode connection. 7 differs from the embodiment of FIG. 7 only in that the cascode transistors QCS0 and QCS1 are connected to the collectors of the transistors QA0 and QA1. The gain and the linear amplification range are expressed by Equation 2 and Equation 3 as in the embodiment of FIG. By connecting the cascode transistor, the mirror capacitance of the transistor QA0 can be reduced, and the band can be extended as compared with the embodiment of FIG.
[0046]
FIG. 9 shows another embodiment of an amplifier with a limit function, and shows an example in which a transimpedance circuit is used instead of a resistor as a load circuit. Transistors QF0, QF1, resistors RF0, RF1, RC2, RC3 and a current source IF constitute a transimpedance circuit, which has a function of lowering the input impedance viewed from the collectors of the transistors QA0, QA1 and reducing the mirror capacitance. Thereby, it is possible to achieve a wider band than the embodiment of FIG. The gain A of this circuit can be generally expressed by the following equation 4.
[0047]
[Expression 4]
A = RF / ((2VT / IA) + RE) (Formula 4)
In Equation 4, RF is the resistance value of the feedback resistor RF0.
[0048]
Also in the differential amplifier circuit shown in FIG. 9, it is required to symmetrically configure the left and right circuits from the ground potential (upper part of the drawing) to the constant current source IA, similar to that of FIG. Therefore, an error of 3% at maximum is allowed in practice between the resistance values of RF0 and RF1, but it is required to make RF0 and RF1 equal in circuit design. The linear amplification range is expressed by Equation 3.
[0049]
7 to 9 show an example in which the output is directly taken out from the differential amplifier, but the output may be taken out through a buffer circuit such as an emitter follower if necessary.
[0050]
<Example 2>
FIG. 10 shows an embodiment of the preamplifier (preamplifier unit PRE) of the present invention, which is an example using a transimpedance amplifier circuit. Transimpedance Z of this circuit _T Is approximately equal to the resistance value of RF2. The reason is transimpedance Z _T Based on the following formula 5 that defines
[0051]
[Equation 5]
Z _T = (RF2 · A ₀ ) / (1 + A ₀ ... (Formula 5)
In Equation 5, A ₀ Represents the gain of the circuit comprising the transistor QP0 and the load resistor RL in FIG. 10, and RF2 represents the resistance value of RF2 in FIG. The circuit of FIG. ₀ Therefore, the following approximate expression 6 is established.
[0052]
[Formula 6]
A ₀ / (1 + A ₀ ) = 1 (Formula 6)
From the relationship of Equation 6, it is clear that the transimpedance can be approximated by the resistance value of RF2.
[0053]
On the other hand, the bandwidth of the preamplifier is governed by the pole determined by the product of the capacitance of the input section (input capacitance of the preamplifier and the capacitance of the photodiode) Cin and the input resistance Zin of the preamplifier. The pole is defined as “−2π / τ” by the time constant τ (τ = resistance × capacitance). The larger the pole (the smaller the time constant), the faster the preamplifier can respond to the received optical signal pulse. In other words, as the pole becomes larger, it becomes possible to receive an optical signal transmitted at a shorter pulse interval (higher frequency). The input resistance Zin is approximately expressed by the following equation 7.
[0054]
[Expression 7]
Zin = Z _T / AE = RF2 / AE (Expression 7)
In Equation 7, AE represents the open loop gain of the grounded emitter amplifier. Since the open loop gain can be kept large even in the high frequency region where Zin is set to be small, it is possible to increase the bandwidth of the optical signal that can be received.
[0055]
FIG. 11 shows another embodiment of the preamplifier, and shows a case where a cascode connection is used. 10 differs from the embodiment of FIG. 10 only in that a cascode transistor QCS2 is connected to the collector of the transistor QP0. By connecting the cascode transistor, the mirror capacitance of the transistor QP0 can be reduced, so that the input capacitance of the preamplifier can be reduced. Therefore, a wider band can be achieved than in the embodiment of FIG. However, since the phase margin of the feedback loop is reduced, it is necessary to design with attention to the flatness of the frequency characteristics.
[0056]
10 and 11 show an example of output via one emitter follower, the number of emitter followers may be increased as necessary.
[0057]
<Example 3>
FIG. 12 shows an example of an AGC amplifier suitable for the optical receiver circuit of the present invention. Although the case where a transimpedance circuit is used as the load circuit is shown here, the load circuit can also be configured by a resistor.
[0058]
In the circuit of FIG. 12, the output terminal VAWL0 of FIGS. 7 to 9 is connected to the input terminal IN0, and the output terminal VAWL1 of FIGS. 7 to 9 is connected to the input terminal IN1. When the potential difference between the control signals VCNT0 and VCNT1 is increased, the gain of the AGC amplifier is increased, and when the potential difference is decreased, the gain is decreased. If the gain of one stage of this circuit is insufficient, a plurality of stages may be connected in cascade. The control signals VCNT0 and VCNT1 are controlled by, for example, detecting the amplitude of VAGC and setting it by comparing with the nominal voltage VN, the peak detection circuit PD, the reference circuit REF, and the gain control circuit GC described in the above-mentioned document 1. It is better to use a technology that combines
[0059]
In this embodiment, the output is directly taken out from the AGC amplifier. However, the output may be taken out through a buffer circuit such as an emitter follower if necessary.
[0060]
<Example 4>
FIG. 13 shows an embodiment of the reference potential generation circuit (reference potential generation section VRG), which is an example constituted by a low-pass filter. The low-pass filter is composed of a resistor RLP and a capacitor CLP. Note that the capacitor CLP may be provided inside the semiconductor chip or connected to the outside. By extracting a DC component from the preamplifier output VPRE through a low-pass filter, the center potential of VPRE can be obtained.
[0061]
FIG. 14 shows another embodiment of the reference potential generating circuit. This circuit includes a circuit TH for detecting a high potential of the preamplifier output VPRE, a circuit BH for detecting a low potential, and resistors RA1 and RA2. The resistance values of the resistors RA1 and RA2 are set equal. When the preamplifier output becomes a high potential, the capacitor CTH is charged by the transistor QTH to the potential of VPRE (H) -VBE (VBE is a bias voltage between the base and the emitter of the transistor QTH). Further, when the preamplifier output becomes a low potential, the capacitor CBH is discharged to the potential of VPRE (L) + VBE by the diode DBH. If these two potentials are divided by the resistors RA1 and RA2, the center potential between the high potential and the low potential of the preamplifier output VPRE can be obtained.
[0062]
<Example 5>
In the present embodiment, an optical receiver circuit instead of the configuration of FIG. 19 is introduced. An outline of the optical receiving circuit of this embodiment is shown in FIG. In the configuration disclosed in FIG. 20 with respect to FIG. 1, the configurations of the light receiving element APD (but not limited to the avalanche photodiode), the AGC amplifier, the identification circuit DEC, and the clock signal extraction circuit are omitted. However, I provided in the circuit of the preamplifier PRE _IN A signal current from the light receiving element APD is input to the terminal, and the VAWL terminal provided in the circuit of the amplifier AWL with limit function is electrically connected to the voltage signal input terminal of the AGC amplifier.
[0063]
The optical receiver circuit shown in FIG. 20 has a signal current I generated in the photoelectric conversion element by an optical signal transmitted at 10 Gb / s (gigabit / second). _IN Is received in a wide dynamic range up to an amplitude current value of 2 mA at maximum. This optical receiver circuit receives a wide dynamic range input current signal IIN with a high transformer impedance gain, a trans-impedance amplification stage comprising a preamplifier PRE, a limit amplification stage comprising a limit amplification part AWL, and The reference potential generator VRG is configured to supply a reference voltage Vref to the limit amplifier AWL. In the present embodiment, the circuit stage described as the limit amplification stage is an abbreviation for the circuit stage indicating the function of the amplifier with limit function described above.
[0064]
The preamplifier PRE is connected to a bias voltage supply Vbias that supplies a bias voltage thereto. Each part is provided with terminals of power supply voltages VCC and VEE (where VEE <VCC), and a constant current source (indicated by a symbol enclosing an arrow in a circle) is provided on the emitter side of the transistor.
[0065]
A reference potential generator configured to include a low-pass filter having an external capacitor CEXT outputs an intermediate value of the output amplitude voltage by the transformer / impedance amplification stage as a reference voltage Vref. The output voltage VPRE of the preamplifier is applied to the base of one transistor T4 constituting a differential amplifier in the limit amplifier AWL, and the reference voltage Vref is applied to the base of the other transistor T5. Thereby, the signal current I _IN In the amplification by the trans-impedance amplification stage and the limit amplification stage for _IN When the value is small, linear amplification is performed in both amplification stages to obtain a high impedance gain, and I _IN When the value is large, the limit gain stage exhibits a limit amplification function, so that the impedance gain can be suppressed to a desired value.
[0066]
That is, as shown in FIG. 21, the limit amplification stage has a small I of about 400 μA or less predetermined by this circuit configuration. _IN Exhibits a linear amplification function and an I greater than 400 μA _IN Is configured to show a so-called limit function that outputs a value of VAWL (400 mV) determined in advance by the circuit configuration of the limit amplifier AWL regardless of the value. Thereby, in the limit amplification stage, the weak signal I _IN Can be transmitted to an AGC amplification unit (not shown) without losing _IN The distortion of the output signal waveform can be suppressed even when the dynamic range is from several μA to 2 mA. As a result, the signal identification accuracy of “1”-“0” in the identification circuit unit DEC (not shown) can be maintained and improved.
[0067]
In FIG. 20, the preamplifier PRE is composed of a grounded-emitter amplifier having a transistor T1 and a resistor R2, and a shunt feedback loop comprising a transistor T2, a diode D1, and a resistor R1. . The limit amplifier AWL is composed of a wideband differential amplifier and an output buffer. In addition, in order to improve the frequency response characteristics of both amplification units, a bias voltage supply unit Vbias is connected to the preamplification unit PRE, and a peaking network is connected to the limit amplification unit AWL.
[0068]
In the preamplifier PRE, it is necessary to stabilize the bias voltage applied to the resistor R2 in order to suppress the fluctuation of the transformer impedance with respect to the frequency response and reduce the fluctuation of the eye diagram (Eye-Diagram). Since the emitter common amplifier constituting the preamplifier PRE is susceptible to the current fluctuation of the transistor T1 and determines the frequency response characteristic of the preamplifier, in order to stabilize the bias voltage, It is necessary to prevent this from being affected by fluctuations in VCC. Since the bias voltage supplied from the bias voltage supply unit Vbias shown in FIG. 20 is determined by the ground potential and the circuit components, it is independent of the fluctuation of VCC.
[0069]
On the other hand, the limit amplifying unit AWL is required to have characteristics capable of responding to a wider frequency band than the preamplifying unit PRE, and therefore, a peaking network including resistors R3 and R4 and a capacitor CP is connected. Since the load circuit composed of the transistors T6 and T7 and the resistors R5 and R6 exhibits a low input impedance by feedback via the R5 and R6, the mirror effect of the transistor pair T4 and T5 constituting the differential input circuit is suppressed, and the limit A response in a wide frequency band of the amplification unit AWL can be realized.
[0070]
The output buffer of the limit amplifier AWL includes a two-stage emitter follower (transistor T8-T10 or T9-T11) and a damping resistor (R10, R11) in order to increase the node output speed and reduce the transformer impedance fluctuation. I have. The output signal of the preamplifier PRE is branched by the emitter-follower transistor T12 and input to the reference potential generator VRG. The reference potential generator VRG includes a resistor R14, a low-pass filter having capacitors CLP and CEXT, and an emitter-follower transistor T13. The output voltage Vref is intermediate between the output voltage VPRE of the preamplifier PRE. Indicates the value.
[0071]
<Example 6>
In the above embodiments, all bipolar transistors are used, but it goes without saying that the optical receiver circuit of the present invention can be realized by replacing them with field effect transistors such as MOSFETs and MESFETs.
[0072]
FIG. 23 shows a configuration in which all the bipolar transistors of the optical receiver circuit of the present invention shown in FIG. 19 are replaced with MOS type field effect transistors.
[0073]
In FIG. 19, the transistors QP0 and QEF of the preamplifier PRE, the transistors QA0 and QA1 of the limit amplifier (amplifier with limit function) AWL, and the transistor Q of the AGC amplifier AGC _IN (0), Q _IN (1), Q _AMP (1), Q _AMP (2), Q _AMP (3), Q _AMP (4), QF0 and QF (1) are all composed of NPN-type bipolar transistors (having an N-type collector and emitter and a P-type base). In FIG. It has been replaced with an effect transistor.
[0074]
An N-channel field effect transistor is formed by separating an N-type dose region and a drain region in a P-type semiconductor layer, and an oxide film (insulating) on the P-type semiconductor layer sandwiched between the two regions. A gate electrode is formed through a film. Although the bipolar transistor is superior in terms of signal processing speed, the field effect transistor can be easily separated from each other, so that the optical receiver circuit of the present invention is superior to the bipolar transistor when integrated on the same semiconductor substrate. Is.
[0075]
In the N-channel field effect transistor shown in FIG. 23, the potential of the P-type semiconductor layer sandwiched between the N-type source and drain is set to the low potential side (drain potential). The setting of the potential of the layer is not limited to this embodiment.
[0076]
<Example 7>
An example of an optical communication system to which the optical receiver circuit of the present invention is applied is shown in FIG.
[0077]
In order to clarify the concept of the optical communication system according to the present invention, optical fiber cables LINE A, LINE are provided for telephone stations A, B, C shown as TS (A), TS (B) and TS (C) in FIG. B, only the optical signal transmission function to the telephone station R (TS (R)) by LINE C is shown. Also, the telephone station R shows only the optical reception function from the telephone stations A, B, and C. In FIG. 24, telephones and computers indicated as USERS (A), (B), (C), or (R) indicate subscribers of respective telephone stations.
[0078]
The optical communication system according to the present invention will be described using an example in which information from USERS (A), (B), and (C) is transmitted to USERS (R). Information from each former subscriber is replaced with an optical signal having a wavelength of 1.3 to 1.6 μm at each telephone station and input to the optical fiber cable LINE A, LINE B or LINE C leading to the telephone station R. The The spectrum of the optical signal intensity (Intensity of Opt. Sig.) With respect to the time axis direction at this time is shown for each telephone station. Information transmitted from each telephone office is encoded into digital information consisting of one signal composed of optical pulses of intensity Itrs and zero signal without optical pulses.
[0079]
Time t _a0 To t _a1 Optical signal transmitted from the central office A during the time t _b0 To t _b1 Optical signal transmitted from the telephone station B during _c0 To t _c1 All the optical signals transmitted from the telephone station C during this period pass through a multiplexer (an optical coupler such as a Star Coupler) CL provided in the telephone station R or a trunk transmission line (Trunk Line) leading to the central office R, The light enters the optical fiber optically connected to the light receiving element PD provided in the optical receiving circuit of the telephone station R. The optical signal received from each telephone station is serially decoded into transmission information in accordance with the time addressed to each transmitting station in the telephone station R. A device for combining transmission information is arranged at the subsequent stage of the identification circuit DEC provided in the telephone station, and is provided in the telephone station R or in a subscriber's terminal (Terminal) according to the transmission form.
[0080]
In FIG. 24, consider the case where distances AR, BR and CR between telephone stations A, B and C and telephone station R are set to 20 km, 100 km and 500 km, respectively. At this time, the optical signal received by the telephone station R is converted into a voltage signal shown in FIG. 25A by the preamplifier PRE. The vertical axis in FIG. 25A represents the voltage value V of the output signal from the preamplifier PRE. _PRE Indicates. This output signal is the voltage amplitude V _DET When the value exceeds, the signal waveform deteriorates in the AGC amplifier at the subsequent stage, and the error rate exceeds the level that is practically acceptable. Time t from telephone station A near telephone station R _a0 To t _a1 Since the optical signal transmitted during the period is almost lost in the propagation of the optical fiber, the preamplifier PRE _DET Voltage amplitude over V _a Strong voltage pulse V _PRE Has been converted.
[0081]
On the other hand, in the present invention, the amplifier AWL with a limit function is arranged between the preamplifier PRE and the AGC amplifier, and the circuit is connected to the V amplifier. _PRE Voltage amplitude of the first potential V ₁ Output V when _AWL Voltage amplitude V ₁ (See FIG. 25B). As a result, the digital information transmitted from the central office A is almost free from the deterioration of the signal waveform at the AGC amplifier, and the voltage signal V _AGC To the identification circuit (see FIG. 25C).
[0082]
On the other hand, the threshold voltage V of the identification circuit DEC in the telephone station R _TH (DEC) is artificially set between the first potential and the zero potential. The voltage is set to, for example, half of the first potential. On the other hand, the AGC amplifier can automatically adjust the amplification gain according to the voltage signal input thereto. A practical automatic gain adjustment in the AGC amplifier is a voltage amplitude (not shown) generated by a dummy optical signal transmitted before an optical signal carrying information to be transmitted from each of the telephone stations A, B, and C. Based on. However, the automatic gain adjustment function of the AGC amplifier is not perfect. This is because the AGC amplifier has a voltage pulse input thereto having a predetermined potential (for example, the second potential V ₂ ) In the following cases, the voltage amplitude is V _TH Amplification beyond (DEC) becomes impossible.
[0083]
Time t from telephone station C, 500 km away from telephone station R _c0 To t _c1 Since the optical signal transmitted during the period is greatly attenuated due to the loss during the propagation of the optical fiber, the signal is amplified by the preamplifier PRE. ₂ Voltage amplitude less than V _c Weak voltage pulse V _PRE (See FIG. 25 (a)). In order to solve this problem, V in the identification circuit DEC _TH (DEC) must be set to vary according to the input voltage signal. However, this method has a limit in responsiveness for identifying a voltage signal supplied at a very short pulse interval.
[0084]
On the other hand, in the present invention, the voltage amplitude V is obtained by linear amplification by the amplifier AWL with limit function. _c Voltage amplitude V ₂ It can be amplified as described above (see FIG. 25B). Therefore, the amplified voltage amplitude is V _TH Even if it does not reach (DEC), it is further amplified by the AGC amplifier at the subsequent stage, so that it is sufficient to be identified by the identification circuit DEC (V _TH It is converted into a pulse having a voltage amplitude (above (DEC)) (see FIG. 25C).
[0085]
As described above, in the optical communication system according to the present invention, the linear amplifier for assisting the AGC amplifier is inserted between the preamplifier PRE and the AGC amplifier AGC, and the operation of the linear amplifier is input to the voltage. Limit according to the amplitude of the signal. That is, in the process of transmitting the voltage signal from the preamplifier to the AGC amplifier, at least the above-mentioned V _DET The voltage signal having the above voltage amplitude is suppressed to a predetermined voltage amplitude, and other voltage signals are selectively amplified. For this reason, the dynamic range of the received intensity of the optical signal can be expanded without performing the gain adjustment of the preamplifier PRE that limits the transmission speed of the voltage signal and the threshold potential adjustment of the identification circuit.
[0086]
【The invention's effect】
As described above, according to the present invention, it is possible to provide an optical receiver circuit and an optical communication system with high sensitivity and a wide dynamic range. Therefore, particularly in a trunk optical communication system that performs long-distance optical signal transmission using an optical fiber, it is possible to perform processing of received optical signals (“1”-“0” determination) with a low error rate. Also, the signal processing error rate can be significantly reduced for optical signals from optical fibers having different transmission distances.
[Brief description of the drawings]
FIG. 1 is a diagram showing a basic configuration of the present invention.
FIG. 2 is a diagram showing a conventional technique (disclosed in Document 1).
FIG. 3 is a diagram for explaining the operation of the prior art (disclosed in Document 1).
FIG. 4 is a diagram showing another conventional technique (disclosed in Document 2).
FIG. 5 is a diagram for explaining the operation of another prior art (disclosed in Document 2).
FIG. 6 is a diagram illustrating the operation of the present invention.
FIG. 7 is a diagram showing an example of an amplifier used in the present invention.
FIG. 8 is a diagram showing an example of an amplifier used in the present invention.
FIG. 9 is a diagram showing an example of an amplifier used in the present invention.
FIG. 10 is a diagram showing an example of a preamplifier used in the present invention.
FIG. 11 is a diagram showing an example of a preamplifier used in the present invention.
FIG. 12 is a diagram showing an example of an AGC amplifier used in the present invention.
FIG. 13 is a diagram showing an example of a reference potential generation circuit used in the present invention.
FIG. 14 is a diagram showing an example of a reference potential generation circuit used in the present invention.
15 is a diagram showing a circuit of an AGC amplifier disclosed in Document 1. FIG.
FIG. 16 is a diagram for explaining an error rate of “1”-“0” signal identification;
FIG. 17 is a diagram for explaining the correspondence between the optical signal input and the voltage output of the preamplifier and the AGC amplifier.
FIG. 18 is a diagram for explaining waveform distortion caused by a slew rate.
FIG. 19 is a diagram showing a preferred embodiment of an optical receiver circuit according to the present invention.
FIG. 20 is a diagram showing another preferred embodiment (Example 5) of the optical receiver circuit according to the present invention;
21 is a graph showing the relationship of the output signal amplitude with respect to the input current from the photoelectric conversion element in the circuit of FIG.
22 is an explanatory diagram comparing the operation of the prior art (disclosed in Document 2) and the present invention. FIG.
FIG. 23 is a diagram showing another desired embodiment (Example 6) of the optical receiver circuit according to the present invention;
FIG. 24 is a diagram showing an embodiment of an optical communication system to which an optical receiver circuit according to the present invention is applied.
25 is a diagram showing a change in the waveform of a voltage signal in the optical receiver circuit employed in the telephone station R of the optical communication system in FIG. 24. FIG. (A) schematically shows voltage waveforms output from the preamplifier, (b) from the amplifier with limit function, and (c) from the AGC amplifier.
[Explanation of symbols]
APD: avalanche photodiode, PRE: preamplifier, AWL: amplifier with limit function, AGC: AGC amplifier, DEC: identification circuit, CEXT: clock extraction circuit, VRG: reference potential generation circuit

Claims (6)

A photoelectric converter that converts an optical signal into a current signal; a preamplifier that converts an output current signal of the photoelectric converter into a voltage signal; and an amplifier that receives an output voltage signal of the preamplifier, the input signal; When the difference between the reference potentials is smaller than a predetermined voltage, linear amplification is performed. In an optical receiver having an optical receiving circuit that receives an optical signal, converts the optical signal into an electrical signal, and includes an automatic gain control amplifier that outputs the signal,
Within the desired dynamic range of the optical signal,
ΔVAWL <Rs · Δt
(Where ΔVAWL is the output signal amplitude of the amplifier with limit function, Δt is the time occupied by the 1-bit optical signal, and Rs is the slew rate of the amplifier with limit function) so that the output signal of the amplifier with limit function optical receiver which is characterized by limiting the amplitude.   2. The optical receiver according to claim 1, wherein the photoelectric conversion unit is composed of an avalanche photodiode.   2. The optical receiver according to claim 1, wherein the photoelectric conversion unit includes an optical fiber amplifier and a PIN photodiode. A photoelectric converter that converts an optical signal into a current signal; a preamplifier that converts an output current signal of the photoelectric converter into a voltage signal; and an amplifier that receives an output voltage signal of the preamplifier, the input signal; When the difference between the reference potentials is smaller than a predetermined voltage, linear amplification is performed. In an optical receiver having an optical receiving circuit that receives an optical signal, converts the optical signal into an electrical signal, and includes an automatic gain control amplifier that outputs the signal,
The predetermined voltage at which the amplifier with the limit function starts limit amplification is the output signal of the preamplifier circuit and the reference at the minimum reception sensitivity determined by the photoelectric conversion unit and the preamplifier circuit under a desired error rate. It is set larger than the potential difference,
Within the desired dynamic range of the optical signal,
ΔVAWL <Rs · Δt
(Where ΔVAWL is the output signal amplitude of the amplifier with limit function, Δt is the time occupied by the 1-bit optical signal, and Rs is the slew rate of the amplifier with limit function) so that the output signal of the amplifier with limit function An optical receiver characterized by limiting the amplitude.   5. The optical receiver according to claim 4, wherein the photoelectric conversion unit is composed of an avalanche photodiode. 5. The optical receiver according to claim 4, wherein the photoelectric conversion unit includes an optical fiber amplifier and a PIN photodiode.

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