JP2002289907A - Method of sorting apd element used for light-receiving module, and optical module using the apd element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the cut-off frequency of a light-receiving module for optical communication for mounting an APD element to be set within a specified range. SOLUTION: To sort the APD element in the light-receiving module for optical communication, which has the APD element for outputting a received optical signal as a electrical signal, an amplifier for amplifying the electrical signal of the output of the APD element and outputting it, and a low-pass filter connected to the output side of the amplifier, the above APD elements are sorted, so that a second difference is within a predetermined range, based on the relation between the first difference of the cut-off frequency between the first and second values out of the multiplication factor within the prescribed range and the applied bias voltage, in each of a plurality of APD elements and the second difference of the cut-off frequency between the first and second values at using each of the plural APD elements as the APD element of the light-receiving module.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for selecting an APD (avalanche photodiode) element and a light receiving module for optical communication mounting the same.
For example, a method of selecting an APD element used to set a cutoff frequency of a light receiving module for optical communication within a desired range and a cutoff frequency within a desired range by using the APD element thus selected. And a light receiving module for optical communication.

[0002]

2. Description of the Related Art In recent years, with the development of optical communication technology, a light receiving module (light receiving module) for optical communication, which is a main component in optical communication technology, is required to have performance and reliability that follow the module. An APD element is generally widely used as a light receiving element of a light receiving module due to its amplifying function and low noise characteristics.

FIG. 6 shows an example of a conventional light receiving module, in which an APD element 2 is used as a light receiving element. When the optical signal 1 enters the APD element 2, the optical signal 1
The electric signal is converted into an electric signal by the element 2, and the electric signal is amplified by the amplifier 3. An AC component of the amplified electric signal passes through the AC output capacitor 4 and is output from an output terminal 9.

Hereinafter, the evaluation of the characteristics of the APD element 2 will be described. FIG. 7 is a frequency characteristic graph of the APD element 2.
One of the characteristic evaluations of the APD element 2 is a cutoff frequency characteristic. This is because the reference frequency f0 which is a reference in the low frequency range is determined, and the output gain of the electric signal at the reference frequency f0 is more than 3%.
The frequency decreased by dB is defined as a cutoff frequency (fc3dB),
Evaluation is performed using the value of fc3 dB. That is, the degree to which the optical signal input to the APD element 2 can be followed is evaluated by the cutoff frequency. For example, if the reference frequency in the low frequency range is 300 kHz with respect to the APD element 2 and the frequency at which the electric signal output gain decreases by 3 dB at that frequency is 3 GHz, the cut-off frequency characteristic of the APD element 2 becomes That is, it is 3 GHz.

As another characteristic evaluation of the APD element 2, there is a cutoff frequency characteristic by a multiplication factor M. Where M
Will be briefly described in advance. The APD element 2 is used by applying a reverse bias voltage, and has a function of increasing an electric signal output with respect to a received light amount by increasing an applied bias voltage amount. In general, when the APD element 2 is used, a multiplication factor (M) is determined, an applied bias voltage that has not been multiplied is set as a bias voltage with a multiplication factor M = 1, and a reference voltage is set. The current value flowing through the APD element 2 at this time is called a photocurrent value Iph (M = 1). By increasing the applied bias voltage of the APD element 2, the photocurrent value Ip
The applied bias voltage when h becomes twice the photocurrent value Iph (M = 1) is referred to as an applied bias voltage with a multiplication factor M = 2. Similarly, when the photocurrent value Iph increases, the multiplication factor M
And decide. The applied bias voltage giving the multiplication factor M determined in this way has different values depending on the APD element 2. Here, FIG. 8 shows a cut-off frequency characteristic of the APD element 2 according to the multiplication factor M, wherein the horizontal axis represents the multiplication factor M and the vertical axis represents the cut-off frequency. As can be seen from FIG.
The element 2 has a different cutoff frequency depending on the multiplication factor M. As the multiplication factor M increases, the noise also increases. Therefore, the multiplication factor M generally used is about M = 3 to 15.

[0006]

In the light receiving module shown in FIG. 6, the cutoff frequency may be limited to a narrow range in order to cut off noise. For example, in a system (for example, a 2.5 Gb / s system) in which noise on the high-frequency side must be cut off and used, the cut-off frequency may need to be kept within a range from 1.3 GHz to 1.5 GHz. In such a case, it is necessary to keep the cutoff frequency of the light receiving module in FIG. 6 within the range defined by the lower limit and the upper limit. The light receiving module of FIG. 6 was useful in satisfying the lower limit.
However, in the case where the cutoff frequency is limited by both the lower limit and the upper limit, the light receiving module of FIG. 6 cannot be expected to satisfy this limit.

The present invention has been made in view of such a problem, and an object of the present invention is to enable a cut-off frequency of a light receiving module having an APD element to fall within a range defined by a lower limit and an upper limit. With the goal.

[0008]

According to the method of selecting an APD element of the present invention, an APD for outputting a received optical signal as an electric signal is provided.
The APD element in a light receiving module for optical communication, comprising: a PD element; an amplification amplifier that amplifies and outputs an electric signal output from the APD element; and a low-pass filter connected to an output side of the amplification amplifier. When selecting a plurality of APD elements, a first difference between cutoff frequencies at first and second values of a multiplication factor or an applied bias voltage in a predetermined range in each of the plurality of APD elements and the plurality of APD elements
Each of the PD elements is connected to the APD of the light receiving module.
The APD element is selected so that the second difference is within a predetermined range based on a relationship between the first and second values and a second difference between cutoff frequencies when the element is used as an element. It is configured as a feature.

A method for selecting an APD element according to the present invention includes an APD element for outputting a received optical signal as an electric signal, an amplifying amplifier for amplifying and outputting an electric signal output from the APD element, and an output of the amplifying amplifier. A low-pass filter connected to the side, and for selecting the APD element in the light receiving module for optical communication having a maximum and a predetermined range of multiplication factor or applied bias voltage in each of the plurality of APD elements. A first difference between minimum cutoff frequencies and maximum and minimum cutoffs in the predetermined range of multiplication factor or applied bias voltage when the plurality of APD elements are used as the APD elements of the respective light receiving modules. Selecting the APD element having the second difference within a predetermined range based on a relationship between the APD element and a second difference in frequency. Configured as those characterized.

A light receiving module according to the present invention comprises: an APD element for outputting a received optical signal as an electric signal;
In selecting the APD element in the light receiving module for optical communication, the amplifier includes an amplification amplifier that amplifies and outputs an electric signal output from the element and a low-pass filter connected to an output side of the amplification amplifier. In each of the APD elements, a first difference between a cutoff frequency at a first and a second value of a multiplication factor or an applied bias voltage in a predetermined range, and the plurality of APD elements, The second cutoff frequency at the first and second values when used as the APD element
And the second difference is within a predetermined range based on the relationship between the APD element and the APD element.

The light receiving module according to the present invention comprises: an APD element for outputting a received optical signal as an electric signal;
In selecting the APD element in the light receiving module for optical communication, the amplifier includes an amplification amplifier that amplifies and outputs an electric signal output from the element and a low-pass filter connected to an output side of the amplification amplifier. In each of the APD elements, a first difference between the maximum and minimum cutoff frequencies in a multiplication factor or an applied bias voltage in a predetermined range, and the plurality of APD elements were used as the APD elements of the respective light receiving modules. The APD element, wherein the second difference is within a predetermined range, based on the relationship between the predetermined range of the multiplication factor or the second difference between the maximum and minimum cutoff frequencies in the applied bias voltage. It is configured as one using.

[0012]

FIG. 1 shows a light receiving module (light receiving module) 11 for optical communication to which the present invention is applied. The light receiving module 11 includes an APD element 2 that receives the optical signal 1 and outputs it as an electric signal, a bias application terminal 7 that applies a bias voltage to the APD element 2, and an amplification amplifier 3 that amplifies an output signal of the APD element 2. A bias application terminal 8 for applying a bias voltage to the amplification amplifier 3, an AC output capacitor 4 for cutting off a DC component and passing an AC component of an output signal of the amplification amplifier, and an output from the AC output capacitor 4. And a low-pass filter (LPF) 10 including a resistor 5 and a capacitor 6 for removing unnecessary high-frequency components from the AC component, and an output terminal 9.

When the optical signal 1 enters the APD element 2, A
The optical signal 1 is converted into an electric signal by the PD element 2. After the converted electric signal is amplified by the amplification amplifier 3, an AC component is extracted by the AC output capacitor 4. Unnecessary high-frequency components are removed from the extracted AC components by the low-pass filter 10, and the required AC components are output from the output terminal 9 as output signals.

The characteristic of the low-pass filter 10 is that the filter effect in the high band is large, and the effect in the low band is small. To explain this in more detail, consider a light receiving module in which the low-pass filter 10 is removed from the light receiving module 11, for example. It is assumed that the cutoff frequency of the light receiving module is high at the multiplication factor M = 3 and low at the multiplication factor M = 12. In this case, when the low-pass filter 10 is attached to the light receiving module, the cutoff frequency is greatly reduced when M = 3 because the filter effect is large, whereas the cutoff frequency is small when M = 12 because the filter effect is small. . Therefore, by providing the low-pass filter 10, it is possible to reduce the variation of the cutoff frequency with respect to the multiplication factor M of the light receiving module.

However, the APD element 2 has a characteristic distribution in manufacturing, and if the APD element 2 has a very small variation in the cutoff frequency with respect to the multiplication factor M, conversely, the variation in the cutoff frequency with respect to the multiplication factor M will increase. Some are very large.
For this reason, even if the low-pass filter 10 is used, it may be extremely difficult to keep the cutoff frequency of the light receiving module 11 within a desired range. That is, when the APD element 2 having a very large variation with respect to the multiplication factor M is used, the low-pass filter 10 like the light receiving module 11 is used.
However, even if an attempt is made to reduce the variation of the cutoff frequency by using the method, the variation may still be large and the desired characteristics may not be satisfied. On the other hand, although the amplification amplifier 3 also has a characteristic distribution in manufacturing, the variation of the cutoff frequency with respect to the multiplication factor M is very small, so that there is little problem in setting the cutoff frequency of the light receiving module 11 within a desired range. For example, the variation of the cutoff frequency with respect to the multiplication factor M of the APD element 2 may reach several GHz, while the variation of the cutoff frequency with respect to the multiplication factor M of the amplification amplifier 3 is about several hundred MHz. Thus, the low-pass filter 1
Even if 0 is used, it is still difficult to stably set the cutoff frequency of the light receiving module 11 within a desired range.

Therefore, the present inventor has proposed a plurality of APD elements 2.
Into the light receiving module 11, and the light receiving module 1
The experiment of examining the cutoff frequency characteristic number of 1 was repeated. As a result of such an experiment, many data on the cut-off frequency characteristics of the APD element 2 and the light receiving module 11
The data of the cut-off frequency characteristic was obtained. While examining the data of the experimental results, the inventor independently learned that there is a correlation between the cutoff frequency characteristic of the light receiving module 11 and the cutoff frequency characteristic of the APD element 2. Hereinafter, examples of the present invention will be described in detail.

(Embodiment 1) FIG.
FIG. 6 shows the relationship between the cut-off frequency characteristics of the APD element 2 and the cut-off frequency characteristics of the APD element 2, and is a diagram created independently by the present inventors. The horizontal axis represents the difference between the cutoff frequency of the light receiving module 11 when the multiplication factor is M = 3 and the cutoff frequency of the same light receiving module 11 when M = 12. The vertical axis is the difference between the cutoff frequency of the APD element 2 and the multiplication factor M = 3. This is the difference between the cutoff frequency and the cutoff frequency of the APD element 2 when M = 12. FIG. 2 shows that there is a correlation between the two. That is, it can be said that the variation width of the cutoff frequency of the light receiving module 11 corresponds to the variation width of the cutoff frequency of the APD element 2. From this, by selecting an appropriate APD element 2 based on the relationship of FIG.
It can be seen that the variation range of the cutoff frequency of the light receiving module 11 can be controlled within a desired range. Therefore, it can be seen that the cutoff frequency of the light receiving module 11 can be controlled by the lower limit and the upper limit. However, in the first embodiment, the APD
In adopting the element 2, first, a predetermined multiplication factor M
Looking at cutoff frequencies at 1, M2 (for example, M1 = M6, M2 = M12), the difference between them is a certain reference value (for example, 2
GHz) or more, the variation of the cutoff frequency with respect to the multiplication factor M is too large,
Judged, not adopted. Hereinafter, the effect of the above-described knowledge will be examined by showing that the variation width of the cutoff frequency of the light receiving module 11 can be controlled by selecting an appropriate APD element 2 based on the characteristic diagram of FIG.

Now, consider a case where the cutoff frequency of the light receiving module 11 is controlled so that the lower limit is 1.32 HGz or more and the upper limit is 1.5 GHz or less. In this case, the fluctuation width of the cutoff frequency of the light receiving module 11 is 180 MHz. Therefore, according to FIG. 2, if the variation range of the cutoff frequency of the APD element 2 is selected to be 1500 MHz or less,
The fluctuation width of the cutoff frequency of the light receiving module 11 is 180 MHz
can be controlled below z. Here, the AP created in the above experiment
From the data of the cut-off frequency characteristics of the D element 2, the APD element 2 having a cut-off frequency variation of 1500 MHz or less
FIG. 3A shows the cut-off frequency characteristics of one of the APD elements 2. From FIG. 3A, this AP
The cut-off frequency of the D element 2 is 3.5 GHz with M = 3 and M = 1.
2, it can be seen that the frequency is 2.0 GHz. Therefore, the fluctuation range of the cutoff frequency is 1500 MHz, and it is clear that the above criterion is satisfied. FIG. 3B is a diagram showing a cut-off frequency characteristic based on a multiplication factor M when the output of the APD element 2 is passed through the amplification amplifier 3 having a cut-off frequency of 2 GHz. This cutoff frequency has an upper limit of about 2G as shown in FIG.
Hz. FIG. 3C shows a multiplication factor M when the output of the amplification amplifier 3 is further passed through a low-pass filter 10.
FIG. 6 is a diagram illustrating cut-off frequency characteristics of the light receiving module 11 according to a multiplication factor M of FIG. FIG.
It can be seen that the lower limit of the cutoff frequency can be set to 1.32 GHz or more even when the upper limit of the cutoff frequency is controlled at 1.5 GHz by the low-pass filter 10 as shown in FIG. That is, it can be seen that the fluctuation width of the cutoff frequency of the light receiving module 11 can be controlled to 180 MHz or less.
From the above description, it has been proved that the graph of FIG. 2 created based on the present invention is correct.

FIG. 4 shows that one of the APD elements 2 determined to be inappropriate according to the selection criteria is incorporated into the circuit of FIG.
An example in which the cutoff frequency of the light receiving module 11 is to be controlled will be described. FIG. 4A shows a cutoff frequency characteristic of the APD element 2 according to the multiplication factor M. From FIG. 4A, the cutoff frequency is 3.5 GHz when M = 6, and 1.2 G when M = 12.
Hz, the fluctuation range is 2300 MHz, and it is clear that this APD element 2 does not satisfy the selection criteria (2.0 GHz or less). FIG. 4B is a diagram showing a cutoff frequency based on the multiplication factor M when the output of the used APD element 2 is passed through the amplifier 3 for amplification. FIG.
(C) is a diagram showing a cut-off frequency characteristic when the output of the amplification amplifier 3 is further passed through the low-pass filter 10, that is, the multiplication factor M of the light receiving module 11. FIG.
As shown in (c), the lower limit of the cutoff frequency is 1.32G
Hz, and the desired characteristics cannot be obtained. That is, the variation range of the cutoff frequency of the light receiving module 11 is set to 18
It cannot be controlled below 0 MHz. When the APD element 2 that does not satisfy the relationship shown in FIG. 2 is selected,
It can be seen that the fluctuation range of the cutoff frequency of the light receiving module 11 cannot be controlled within a desired range. This can also be said to support the validity of the present invention.

In this embodiment, the multiplication factor M = 3 to 12
Although the example in the case of the range is shown, the present invention can be used not only in this range but also in a range of a desired multiplication factor M.

(Embodiment 2) In the first embodiment, in the range of a desired multiplication factor M, attention is paid to the difference between the cutoff frequency at the lower limit multiplication factor M and the cutoff frequency at the upper limit multiplication factor M. The relationship between the cut-off frequency characteristic of the module and the cut-off frequency characteristic of the APD element 2 is shown. In addition, the inventor has determined that the difference between the maximum value and the minimum value of the cutoff frequency of the APD element 2 within the range where the APD element 2 is used, that is, within the range of the bias voltage applied to the APD element 2;
It has been uniquely found that there is also a correlation between the difference between the maximum value and the minimum value of the cutoff frequency of the light receiving module 11. Hereinafter, this correlation will be described with reference to FIG. In the second embodiment, unlike the first embodiment, the APD element 2
It is not necessary to judge the appropriateness and unsuitability of the above.

FIG. 5 shows the relationship between the cut-off frequency characteristic of the APD element 2 and the cut-off frequency characteristic of the light receiving module in the range of the applied bias voltage, and is a diagram created by the present inventors. The horizontal axis is the difference between the maximum value and the minimum value of the cutoff frequency of the light receiving module 11 in the range of the applied bias voltage, and the vertical axis is the difference between the maximum value and the minimum value of the cutoff frequency of the APD element 2 in the range of the applied bias voltage. is there. FIG. 5 shows that there is a correlation between the maximum fluctuation width of the cutoff frequency of the light receiving module 11 and the maximum fluctuation width of the cutoff frequency of the APD element 2. By using this relationship, an appropriate APD element 2 can be selected and the cutoff frequency of the light receiving module 11 can be set within a desired range.

For example, consider a case where the cutoff frequency of the light receiving module is to be set to a lower limit of 1.25 GHz or more and an upper limit of 1.5 GHz or less in the range of the bias voltage applied to the APD element 2. In this case, since the fluctuation width of the cutoff frequency is 250 MHz or less, as shown in FIG.
Element 2 may be employed. By incorporating the APD element 2 thus employed in the circuit of FIG. 1, the cutoff frequency of the light receiving module 11 can be set within a desired range.

[0024]

According to the present invention, it is possible to select an APD element whose variation width of the cut-off frequency after modularization is known in advance, so that the variation range of the cut-off frequency of the light receiving module is within a desired range. In addition, the sorted APD elements can be grouped. Also,
The light receiving module having a cutoff frequency in a desired range can be manufactured stably, which is extremely economical.

[Brief description of the drawings]

FIG. 1 is a circuit used in an embodiment of the present invention.

FIG. 2 shows an APD in a range of a multiplication factor M = 3 to 12.
5 is a graph showing a correlation between a cut-off frequency characteristic of a light receiving module on which an element is mounted and a cut-off frequency characteristic of the APD element.

FIG. 3 is a diagram showing a state in which the cutoff frequency of the light receiving module is controlled within a desired range using the APD elements selected in the embodiment of the present invention.

FIG. 4 is a diagram illustrating a state in which the cut-off frequency of the light receiving module is to be controlled within a desired range using an APD element determined to be inappropriate in the embodiment of the present invention.

FIG. 5 is a graph showing a correlation between a cutoff frequency characteristic of a light receiving module on which an APD element is mounted and a cutoff frequency characteristic of the APD element within a range of an applied bias voltage.

FIG. 6 is a conventional circuit.

FIG. 7 is a diagram for explaining cut-off frequency characteristics of an APD element.

FIG. 8 is a diagram for explaining a cutoff frequency characteristic of the APD element according to a multiplication factor M;

[Explanation of symbols]

 Reference Signs List 1 optical signal input 2 APD element 3 amplification amplifier 4 AC output capacitor 5 resistor 6 capacitor 7 bias application terminal to APD element 8 bias application terminal to amplifier 9 output terminal 10 low-pass filter 11 light receiving module

Claims (4)

[Claims] 1. An APD element for outputting a received optical signal as an electric signal, an amplification amplifier for amplifying and outputting an electric signal output from the APD element, and a low-pass connected to an output side of the amplification amplifier. APD in a light receiving module for optical communication, comprising: a filter;
In selecting the elements, a first difference between a cutoff frequency at a first and a second value of a predetermined range of a gain or an applied bias voltage in each of the plurality of APD elements and the plurality of APD elements
The second difference is within a predetermined range based on a relationship between the first and second values and a second difference in cutoff frequency when each of the elements is used as the APD element of the light receiving module. APD for use in a light receiving module, wherein the APD element is selected as described in (1).
Element sorting method. 2. An APD element for outputting a received optical signal as an electric signal, an amplification amplifier for amplifying and outputting an electric signal output from the APD element, and a low-pass connected to an output side of the amplification amplifier. APD in a light receiving module for optical communication, comprising: a filter;
In selecting the elements, a first difference between a maximum and a minimum cutoff frequency in a predetermined range of a multiplication factor or an applied bias voltage in each of the plurality of APD elements, and the plurality of APD elements are each connected to the light receiving module. The second difference is a predetermined value based on the relationship between the gain and the second difference between the maximum and minimum cutoff frequencies in the predetermined range of the multiplication factor or the applied bias voltage when used as the APD element of A method for selecting an APD element for use in a light receiving module, wherein the APD element within a range is selected. 3. An APD element for outputting a received optical signal as an electric signal, an amplification amplifier for amplifying and outputting an electric signal output from the APD element, and a low-pass connected to an output side of the amplification amplifier. APD in a light receiving module for optical communication, comprising: a filter;
In selecting the elements, a first difference between a cutoff frequency at a first and a second value of a predetermined range of a gain or an applied bias voltage in each of the plurality of APD elements and the plurality of APD elements
The second difference is within a predetermined range based on a relationship between the first and second values and a second difference between cutoff frequencies when the element is used as the APD element of each of the light receiving modules. A light-receiving module using the APD element according to claim 1. 4. An APD element for outputting a received optical signal as an electric signal, an amplification amplifier for amplifying and outputting an electric signal output from the APD element, and a low pass connected to an output side of the amplification amplifier. APD in a light receiving module for optical communication, comprising: a filter;
In selecting the elements, a first difference between a maximum and a minimum cutoff frequency in a predetermined range of a multiplication factor or an applied bias voltage in each of the plurality of APD elements, and the plurality of APD elements are each connected to the light receiving module. The second difference is a predetermined value based on the relationship between the gain and the second difference between the maximum and minimum cutoff frequencies in the predetermined range of the multiplication factor or the applied bias voltage when used as the APD element of A light receiving module using the APD element within a range.