JP3211492B2 - Opto-electric converters and light receiving components - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric converter and a light receiving component, which are devised to improve noise resistance.

[0002]

2. Description of the Related Art A light receiving element most frequently used in the technical field of optical communication or an optical sensor is a light receiving diode such as a PIN photodiode (hereinafter abbreviated as "PD") or an avalanche photodiode. These light receiving diodes are frequently used because they have high photoelectric conversion efficiency, low noise, and high speed response. When using such a light receiving diode, it is common to attach an amplifier for amplifying a weak photocurrent obtained by photoelectric conversion.

Here, a specific example of the prior art will be described using a PD. The combination type of the PD and the amplifier includes three types depending on the biasing method, namely, a method of connecting the anode of the PD to the input terminal of the amplifier, a method of connecting the cathode of the PD to the input terminal of the amplifier, There is a bootstrap method in which an anode and a cathode are connected to an input terminal of an amplifier.

Hereinafter, a method of connecting the anode of the PD to the input terminal of the amplifier will be described with reference to FIGS.
In the example of FIG. 8, the PD 1 alone enters the case 2. P
D1 anode lines L _a which is derived from the anode A of
Is connected to the input terminal 3a of the amplifier 3 outside the case 2, the cathode side line Lk derived from the cathode K of the PD ₁ is connected to a bias circuit (not shown) outside the case 2, and Side line L _k and ground E
Is connected to the capacitor C. Case 1 is grounded to earth E. Then, the amplifier 3 outputs an optical signal S obtained by amplifying the photocurrent.

[0005] In the example of FIG. 9, a case 2 includes a PD 1, an amplifier 3 and a capacitor C. Case 2 and capacitor C are grounded to ground E. The anode A of the PD1 is connected to the input terminal 3a of the amplifier 3 through the anode line L _a. The amplifier 3 is connected to the power line L _v
Power is supplied from a power supply (not shown) via
An optical signal S obtained by amplifying the photocurrent is output.

[0006] The configuration shown in FIG.
And the type of amplifier 3 can be selected separately, so there is versatility,
It is suitable for laboratory use or one-off custom products. The configuration shown in FIG. 9 is called Pinamp and is not high in general versatility, but is excellent in noise resistance, and is small in the influence of parasitic capacitance and has uniform performance, so that it is suitable for mass-produced products.

[0007]

In the [0006] By the way the above-mentioned prior art, there is a disadvantage that the weak to the DC-noise coming from the cathode side of the line L _k which is open to the PD1. The cathode K of PD1 as described above is
If the DC is grounded in the inside, no noise will enter from the cathode K. However, since there are the following requirements, etc., the cathode is actually opened DC and the outside is outside the case 2. Often derived. I want to monitor the average photocurrent. I want to be able to monitor the photocurrent of PD for monitoring PD deterioration and fixing alignment.

[0008] Of the terms described above, the meaning of "ground" and "monitor" will be supplementarily explained. (1) In this specification, “DC ground” means ground,
Alternatively, it is connected to a power supply line or the like equivalent to ground in an AC manner. In addition, AC grounding refers to connecting to a ground or a power line equivalent to AC ground via a large capacity capacitor. Those grounded in DC are always grounded in AC, but those grounded in AC are not necessarily grounded in DC. (2) In optical communication, AC-coupled communication is often used to increase the sensitivity of the receiver. However, with AC coupling, it is not known whether or not light is actually entering. A method of monitoring the average photocurrent is often used as a means for determining whether or not light has actually entered. (3) A PD current when no light is incident (this is referred to as a dark current) is monitored to check for PD deterioration, which is a method often used in public communication. (4) In optical communication, PD and Pinamp are optical fibers,
Alternatively, it is necessary to align and fix the optical connector and the optical receptacle. At this time, P
It is most common to monitor and adjust the D current.

[0009] The state in which the external noise N enters the cathode side line L _k, will be described with reference to FIG. 10. FIG.
As shown in ( ₁₎ , when noise N enters the cathode K from outside via the cathode side line Lk, this noise N is input to the amplifier 3 via the capacitance of the PD1. Since the amplifier 3 cannot determine whether the input signal is a photocurrent or noise sneaking through a PD capacitor, the noise N is also amplified and output together with the photocurrent. .
FIG. 10 shows an example of Pinamp, but the situation is the same in the configuration shown in FIG.

Of course, the noise N can be suppressed to some extent by grounding the cathode K in an AC manner using an AC grounding capacitor. However, even if it is suppressed by a capacitor, if the lead pin coming out of case 2 is extended by as much as 1 cm, the sneak current of about several tens μV to several mV generally occurs in many cases. It is difficult to control. In addition, in order to increase the response speed of the monitor due to system restrictions, it is often the case that the capacitance of the AC grounding capacitor must be reduced. In this case, the noise suppression of the capacitor is further reduced. Also, the first-stage amplifier for optical reception is originally a converter that converts an input photocurrent signal into a voltage signal, but if it is designed with high sensitivity, it must have a large gain A for the input voltage signal due to noise sneak. become. As a result, the deterioration of the S / N ratio is further increased.

The above situation will be described in more detail as follows. If the amplifier 3 is transimpedance type (has a feedback resistor) and (for simplicity of calculation)
Assuming that the input resistance excluding the feedback resistance is ∞, the photocurrent amplification factor is Z, the feedback resistance is Rf, the voltage gain is A, the band is fc, and P is P.
When the input capacitance of the D volume + amplifier and C _1, the Z = A · Rf / (A + 1) A + 1 = 2π · C 1 · Rf · fc. Here, if A is large, the photocurrent amplification factor, that is, the signal amplification factor, does not depend on A because Z ≒ Rf. However, since the sensitivity of the amplifier is higher as Rf is higher, it is necessary to increase A, that is, the voltage gain, as the sensitivity increases.
This voltage gain is essentially a noise amplification factor. That is, Rf
There is no problem if an amplifier having a large A and a small A can be made, but A and Rf have the above relationship, and such an amplifier cannot be made.

Next, a specific calculation example will be described. Now, the required band fc for the optical receiver is 200 MHz, the required gain for the amplifier 3 is 20 kΩ (= 10 kV / A), the input capacitance of the amplifier 3 is 0.2 pF, the input converted noise of the amplifier 3 is 20 nA, and the amplifier 3 is 1.0p PD capacity with transimpedance type
Assuming that F, the voltage gain of the amplifier 3 needs to be about 30 times. This receiver should have a sufficient S / N ratio for a signal as small as 4.8 mVp-p at the output terminal when calculated from the input converted noise, but if the cathode K has only 100 μV When the noise wraps around in the high frequency range, it is amplified 30 times and 3 mVp at the output terminal.
-P noise is generated, and the S / N ratio is greatly deteriorated (accurately, it is reduced to 1 / 7.5). As described above, P
It is difficult to suppress noise circulating from the cathode of D in the conventional example.

In the above-mentioned example, the anode A of the PD 1 is connected to the input terminal 3a of the amplifier 3, the cathode K is grounded to the earth potential for AC, and the cathode K is opened for DC. Is connected to the input terminal 3a of the amplifier 3 and the anode A is grounded to the ground potential in an AC manner and is opened in a DC manner, similarly, it is difficult to suppress the noise sneaking from the open anode side. Met.

In view of the above prior art, the present invention provides an opto-electrical converter and a light receiving device that can suppress noise even if noise enters from the DC open side of the terminals of the light receiving diode. The purpose is to provide parts.

[0015]

According to the present invention, one of the anode and the cathode of a light-receiving diode is connected to an amplifier, and the other terminal is grounded via a capacitor in an AC manner. The opto-electric converter, which is open to the dc, is provided with a resistor that works together with the capacitor to perform the function of a low-pass filter.

[0016]

The external noise is cut off by the filter composed of the capacitor and the resistor, and is not input to the light receiving diode.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings. In addition, the parts performing the same functions as those of the related art will be described with the same reference numerals.

FIG. 1 shows a first embodiment of the present invention.
It is an amp type. And in the case 2 is PD1 and amplifier 3 is provided as shown in the figure, the anode A of the PD1 is connected to the input terminal 3a of the amplifier 3 through the anode line L _a, PD1 cathode K of connected cathode line L _k is connected to the bias circuit is led to the outside of the case 2 (not shown) to. Case 2 is grounded to earth E, and further has a cathode side line L _k
The capacitor C is connected between the ground and the ground E. Therefore, the cathode K is grounded to the earth E via the capacitor C for AC, and is open for DC. The amplifier 3 with power from the power supply through the current line L _v (not shown) is supplied. The configuration up to this point is the same as in the prior art.

[0019] In addition, the present embodiment is interposed a resistor R to the cathode side line L _k in the case 2. The resistor R works together with the capacitor C to exhibit the function of a low-pass filter. The cutoff frequency of this low-pass filter is 1 / (2πCR), and noise N having a frequency higher than 1 / (2πCR) cannot enter the case 2. In this case, when the band is fc, 1 /
The effect is great if (2πCR) << fc.

The cutoff frequency of the filter is set to 1 /
Even if it is much higher than (2πCR), some noise will enter, but this is small enough and there is no problem. That is, assuming that the capacity of PD1 is Cpd and the capacity of capacitor C is C, the penetration amount is reduced to Cpd / C as compared with the conventional case. Regardless of the problem of noise this time, in order to prevent the frequency characteristics of the main signal system from deteriorating, it is usually necessary to make C larger than a certain value. This is a fundamental optical receiver design problem irrespective of the presence or absence of the resistor R of the present invention.
When C of such a value (however, a value smaller than the upper limit value obtained from the average photocurrent monitoring speed condition) is used, Cpd /
Since C << 1, there is no problem. As an example, consider the numerical value sequence described above. In this case, the lower limit of C is 30p
About F, about 100 pF in consideration of linearity. Assuming that C = 100 pF, Cpd / C = 0.01, which is reduced to one hundredth of the conventional value.

Here, considering the frequency of the noise N that has penetrated in the conventional example (FIGS. 8 and 9), the frequency characteristic is as shown in FIG. The horizontal axis is “frequency”, and the vertical axis is “the amount of noise appearing at the output terminalａPina
mp "indicates the amount of noise sneaking into the cathode terminal. As shown in FIG. 2, there are two cutoffs FH and FL for high frequency and low frequency. FH substantially coincides with the internal band (open gain band) of the amplifier 3. Usually FH >> fc. FL substantially coincides with fc.

In the embodiment shown in FIG. 1, noise having a frequency higher than 1 / (2πCR) cannot enter. The noise that actually affects the S / N ratio even if it invades is only the noise whose frequency is between fc and FH, and 1 / (2πCR) <<
The noise N cannot enter from the cathode K because of fc, or the S / N ratio does not deteriorate even if it enters.

The same effect can be obtained by using an avalanche photodiode (hereinafter abbreviated as "APD") in place of PD1 in the configuration of FIG.

FIG. 3 shows a second embodiment of the present invention.
In this embodiment, the amplifier 3 is arranged outside the case 2. APD may be used instead of PD1. Other parts are the same as in the first embodiment.

FIG. 4 shows a third embodiment of the present invention.
And using avalanche photodiode (APD) 1a as a receiving diode, the cathode-side line L _k connected to the cathode K 2 branches, one is connected to the bias circuit through a resistor R functioning as a filter,
The other is connected to a monitor terminal via a resistor R1. Note that the amplifier 3 may be housed in the case 1 together with the APD 1a to be a package type.

FIG. 5 shows a fourth embodiment of the present invention.
In this embodiment, a light emitting diode (LD) 4
The PD 1 is mounted in the same case 2 to detect the light of the light emitting diode 4 itself. In this case, PD
One anode A is connected to the LD power control circuit.

FIG. 6 shows a fifth embodiment of the present invention.
This is an application example of the first embodiment (FIG. 1). In this embodiment, discriminators 5 and 6 and a load (Rload) 7 are provided outside. In this embodiment, the capacity of one capacitor C in the first embodiment is shared by two capacitors C1 and C2. The reason why the capacitance C is divided into the two capacitances C1 and C2 is that the impedance of the cathode of the Pinamp output is increased as a function of the present invention, and the monitor circuit is more likely to be unstable. It was. When the monitor circuit is unnecessary or when the monitor circuit has sufficiently high noise resistance and does not become unstable, it is not necessary to divide the circuit into C1 and C2.

FIG. 7 shows a sixth embodiment of the present invention.
A bootstrap circuit, that is, a circuit in which an increase in the input signal to be added is partially fed back to obtain an effective input impedance higher than the input impedance. Specifically, the anode A of the PD 1 is connected to the input terminal 3a of the amplifier 3 via the amplifier 8 having a gain of 1, and the cathode K is connected to the input terminal 3a of the amplifier 3 via the capacitor C in terms of AC. It is connected.

In the embodiment shown in FIG. 7 described above, the cathode side of the light receiving element is open to DC, but the cathode of the light receiving element is connected to the input terminal of the amplifier, the anode is grounded for AC, and DC connected. The same effect can be obtained by opening the.

[0030]

According to the present invention, as described above in detail with the embodiment, a resistor is interposed on the terminal side which is DC open, and a conventional capacitor and the interposed resistor are connected. Therefore, even if external noise is generated on the open terminal side, this noise is not input to the light receiving element, and the noise resistance is improved and the reliability is improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing frequency characteristics of noise.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

FIG. 4 is a configuration diagram showing a third embodiment of the present invention.

FIG. 5 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 6 is a configuration diagram showing a fifth embodiment of the present invention.

FIG. 7 is a configuration diagram showing a sixth embodiment of the present invention.

FIG. 8 is a configuration diagram showing a conventional technique.

FIG. 9 is a configuration diagram showing a conventional technique.

FIG. 10 is a configuration diagram showing a conventional technique.

[Explanation of symbols]

1 PIN photodiode (PD) 1a avalanche photodiode (APD) 2 Case 3 amplifiers 3a input terminal 4 light emitting diodes (LD) 5, 6 discriminator 7 Load 8 amplifiers L _a anode line L _k cathode side line L _v Power Line S Optical signal E Ground C Capacitor N Noise

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 31/02-31/0392 H01L 31/10-31/119 H04B 10/00-10/30

Claims (4)

(57) [Claims] At least one of a photodiode, an amplifier, and a capacitor is housed in a grounded case, and one of an anode and a cathode of the photodiode is connected to an input terminal of the amplifier. The other terminal is connected to an external lead-out line extending from the inside of the case to the outside of the case and is connected to the ground via a capacitor. A photoelectric converter, wherein a resistor is interposed in the external lead-out line. 2. A light receiving diode, an amplifier and at least one of a capacitor are housed in a grounded case, and one of an anode and a cathode of the light receiving diode is connected to an input terminal of the amplifier. The other terminal is an opto-electrical converter connected to an external lead-out line extending from the inside of the case to the outside of the case, wherein the other terminal of the light receiving diode is connected to an input terminal of an amplifier via a capacitor. An opto-electrical converter, characterized in that a resistor acting as a low-pass filter acting together with the capacitor is interposed in the external lead-out line. 3. A first case in which a light-receiving diode, a resistor, and a capacitor are built in one grounded case, and one of an anode and a cathode of the light-receiving diode is led out of the case to the outside of the case. And the other terminal is connected to the case via the capacitor, is connected to a second line extending from the inside of the case to the outside of the case, and the resistor is connected to the second line. A light receiving component characterized by being made. 4. The light receiving component according to claim 3, wherein a light emitting element is built in the case.