JPH09307173A - Semiconductor laser module - Google Patents

Abstract

(57) Abstract: In a semiconductor laser module that directly inputs a microwave signal as a modulation signal, power loss due to a load resistance inserted in series with a laser diode chip is suppressed to improve conversion efficiency, and a specified characteristic is achieved. It is possible to easily connect to a transmission line having impedance. SOLUTION: A characteristic impedance of a transmission line 7 connected to a modulation signal input terminal 6 is provided inside a module package that accommodates the laser diode chip 2 and includes a modulation signal input terminal 6, and a load impedance of the laser diode chip 2 is set. The impedance converter 8 is provided to convert the impedance to a value close to.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser module used in a direct modulation type optical communication system.

2. Description of the Related Art In recent years, semiconductor laser modules have been
Applications to communication fields that handle signals in the microwave frequency range, such as TV and public communication, have been actively attempted, and practical use has begun. Among these, in short-to-middle distance transmission of several hundred meters to several kilometers where the accumulation of noise and signal distortion is small, a direct intensity modulation method in which a high frequency signal is directly input to a semiconductor laser module as a modulation signal and converted into an optical signal is known. It is advantageous in terms of facility simplicity and cost. FIG. 2 shows an electrically equivalent circuit of a first configuration example of a conventional semiconductor laser module. In FIG. 2, 1 is a module package, 2 is a laser diode chip, 3 is a bonding wire, 4 is a microstrip line, 5 is a DC bias circuit, 6
Is a modulation signal input terminal, 7 is a transmission line having a specified characteristic impedance, and 10 is an input impedance matching resistor.
First, a DC drive current is applied from the DC bias circuit 5 to excite and oscillate the laser diode chip 2. Next, a high frequency modulation signal is input from the modulation signal input terminal 6 to directly modulate the laser diode chip 2. Usually, the load of the laser diode chip 2 is several Ω, and the characteristic impedance of the transmission line 7 is 50Ω. Therefore,
By inserting the load resistor 10, matching between the characteristic impedance of the transmission line 7 and the input impedance of the semiconductor laser module is realized. Further, in the second configuration example of the conventional semiconductor laser module shown in FIG. 3, the impedance converter 9 is inserted to convert the characteristic impedance into a low value, so that the characteristic impedance of the transmission line 7 and the input impedance of the semiconductor laser module are changed. Has achieved the consistency of.

However, in the above-mentioned first conventional structure, since the load resistance inserted in series is several times larger than the load of several Ω of the laser diode chip, a large amount of modulation signal power is required. Is consumed by the load resistance. For this reason, the input power efficiency converted by the laser diode chip 2 is reduced, and the modulation degree is reduced. For example, when the load of the laser diode chip is 3Ω and the load resistance is 39Ω, impedance matching is represented by the voltage standing wave ratio VSWR as shown in FIG. Further, FIG. 5 shows a graph showing the optical output level at this time as a ratio of the modulation signal input current and the optical output power. As can be seen from FIG. 4, VSWR is lowered by inserting the load resistor 10 for input impedance matching, and matching is realized. However, as can be seen from FIG. 5, since the power consumed by the resistor is large, the modulation signal power and output level that can be converted by the laser diode chip are low. Therefore, in order to obtain a desired degree of modulation, it is necessary to input a large modulation signal power, and as a result, it is necessary to newly install a pre-amplifier or to increase the gain of an existing pre-amplifier, resulting in a system complexity. This is a factor in increasing size, size, and cost. Further, in the second conventional configuration, since the characteristic impedance is converted outside the module package, it is necessary to replace the entire transmission path of the module with the converted characteristic impedance, and direct connection with the 50Ω transmission path is not possible.
Therefore, when the impedance conversion ratio is changed, it is necessary to change the characteristic impedance of the transmission line, and the flexibility is poor. In addition, it is necessary to secure a mounting space for an external impedance converter, which causes an increase in size and complexity of the system. FIG. 6 shows the impedance matching when a laser diode chip with a load of 3Ω and a 4: 1 impedance converter from 50Ω to 12.5Ω are used, and the voltage standing wave ratio VSWR is shown in FIG. 6 is a graph of frequency characteristics in which the optical output level at that time is represented by the ratio of the modulation signal input current and the optical output power. Since the characteristic impedance is converted to 12.5Ω by the impedance converter, VSWR is lowered and matching is realized. However, impedance mismatching occurs due to the electrical length of the bonding wire 3 and the microstrip line 4, and the modulation signal power and output level are lowered. Further, it is necessary to transmit signals in a plurality of bands in optical transmission of radio signals, but in this case, matching of input impedance and efficient modulation driving of the laser module become more difficult. The present invention has been made in order to solve the above conventional problems, realizes input impedance matching in a wide band, can obtain a high degree of modulation with a small amount of modulation signal power, and has a characteristic impedance transmission line. It is an object to provide a semiconductor laser module that can be directly connected.

In order to achieve this object, a semiconductor laser module of the present invention includes a laser diode chip housed in a module package having a modulation signal input terminal and a modulation signal input terminal. An impedance converter (transformer) for converting the impedance of the connected transmission line into an impedance close to the impedance of the load of the laser diode chip is provided. This transformer is preferably a wideband transformer using a ferrite core. Also,
By connecting a resistance element in series to the impedance converter, preferably on the secondary side (laser diode chip side), it is possible to suppress a change in the matching state and the drive current when the impedance of the laser diode chip changes. be able to. The resistance value of the resistance element is preferably 10Ω or less from the viewpoint of suppressing insertion loss. Furthermore, it is preferable that an LC type impedance matching circuit is provided outside the module package, and by appropriately setting the LC constant of this impedance matching circuit, the transmission efficiency becomes good in one or a plurality of frequency bands. can do.

Embodiments of the present invention will be described below with reference to the drawings. (Embodiment 1) FIG. 1 shows an equivalent circuit of a semiconductor laser module according to Embodiment 1 of the present invention. In FIG.
The same circuit elements as those in FIG. 2 used to describe the conventional example are denoted by the same reference numerals. FIG. 1 differs from FIG. 2 in the following points. That is, the broadband impedance conversion transformer 8 using the ferrite core is provided inside the module package 1. The transformer 8 is inserted between the bonding wire 3 and the microstrip line 4, and the microstrip line 4 is connected in series to the primary winding of the transformer 8 and the bonding wire 3 is connected in series to the secondary winding. Become. DC bias circuit 5 to D
When the C drive current is supplied, the laser diode chip 2
Is excited and oscillates. By inputting the high frequency modulation signal from the modulation signal input terminal 6, the laser diode chip 2 can be directly modulated. At this time,
If the conversion ratio of the impedance conversion transformer 8 is N: 1, the characteristic impedance 50Ω of the transmission line 7 is converted to 50 / NΩ by the transformer 8. As a result, the laser diode chip 2 is matched with the load of several Ω.
Further, the microstrip line 4 connected to the primary side is also impedance-converted, and the impedance of the transmission line component corresponding to this and the bonding wire 3 is sufficiently smaller than the impedance of the transformer 8. Therefore, the bonding wire 3 and the microstrip line 4 are connected. Impedance mismatching due to the electrical length component of is suppressed.
As an example, a laser diode chip with a load of 3Ω,
Impedance matching when a 4: 1 impedance converter from 50Ω to 12.5Ω is used is shown in FIG.
A graph represented by SWR is shown in FIG. Further, FIG. 9 is a graph showing the optical output level at this time as a ratio of the optical output power and the modulation signal input current. Since the characteristic impedance is converted to 12.5Ω by the impedance converter, it can be seen from FIG. 8 that VSWR is lowered in a wide frequency band and impedance matching is realized. As a result, it can be seen from FIG. 9 that the optical output level increases in a wide frequency band. As described above, according to the present embodiment, by providing the impedance converter inside the module package, the connection with the transmission line having the specified characteristic impedance becomes easy, and the impedance conversion ratio can be easily changed. Increases flexibility in device design. Further, it is possible to mitigate the influence of the electrical length component of the bonding wire and the microstrip line, which causes the impedance mismatch. In this way, it is possible to realize a semiconductor laser module that can obtain a high optical output level and a large degree of modulation even with a low power modulation signal input. The conversion ratio of the impedance converter is not limited to the above-mentioned 4: 1 shown as an example, but may be 9: 1 or 16: 1, for example. (Embodiment 2) Next, FIG. 10 shows an equivalent circuit of a semiconductor laser module according to Embodiment 2 of the present invention. This circuit differs from FIG. 1 of the first embodiment in that a resistance element (hereinafter,
A "load resistance" 11 is connected to the laser diode chip 2 side (secondary side) of the impedance converter 8. To be precise, it is connected in series between the bonding wire 3 connected to the secondary side of the impedance converter 8 and the laser diode chip 2. This load resistance 1
Reference numeral 1 has a function of suppressing a change in matching state and a change in drive current due to a change in impedance of the laser diode chip 2. That is, the laser diode chip 2
By setting the combined value of the load resistance 11 and the load resistance 11 to a value sufficiently larger than the impedance of the laser diode,
The occurrence of mismatch due to the change in impedance of the laser diode chip 2 can be suppressed. Furthermore, an increase in second-order or third-order intermodulation distortion is suppressed. The value of the load resistance 84 is 10Ω or less, preferably 5Ω or less, from the viewpoint of suppressing insertion loss. The insertion position of the load resistor is the bonding wire 3 and the laser diode chip 2.
It may be inserted between the impedance converter 8 and the bonding wire 3, for example. (Embodiment 3) Next, FIG. 11 shows an equivalent circuit of a semiconductor laser module according to Embodiment 3 of the present invention. The difference from the first embodiment shown in FIG. 1 is that an input impedance matching circuit 9 is provided outside the module package 1. In this embodiment, the input impedance matching circuit 9
By adjusting the circuit constant of, the input impedance can be matched in the desired frequency band. Further, impedance matching in a desired frequency band can be further enhanced. As a specific example, π by the LC circuit
FIG. 12 shows the frequency characteristics of the voltage standing wave ratio VSWR when the input impedance matching is performed at a frequency of 1200 MHz using the type impedance matching circuit. Further, a graph showing the optical output level at this time by the ratio of the optical output power and the modulation signal input current is shown in FIG. It can be seen from these graphs that at the desired frequency of 1200 MHz, a higher degree of impedance matching is realized and a high optical output level is obtained. The input impedance matching circuit 9 is also effective when it is desired to improve impedance matching in a plurality of frequency bands. Impedance matching can be easily achieved at a plurality of frequencies by adjusting the LC circuit constant. As an example, 900 MHz and 1
FIG. 14 shows frequency characteristics of the voltage standing wave ratio VSWR when impedance matching is performed at 500 MHz.
Further, FIG. 15 is a graph showing the optical output level at this time as a ratio of the optical output power and the modulation signal input current. It can be seen that higher impedance matching is achieved and higher optical output levels are obtained in the two desired frequency bands.
It should be noted that the input impedance matching circuit 9 provided outside the module package 1 is the CLCπ as in this embodiment.
Not only the mold circuit, but various circuit configurations can be used. Further, the plurality of frequency bands for which impedance matching should be performed are not limited to two, and may be three or more.

As described above, according to the semiconductor laser module of the present invention, by providing the impedance converter inside the module package, the input impedance matching in a wide band is realized and the signal response in the desired frequency band is achieved. A high degree of modulation can be realized with a small amount of modulation signal power. Moreover, it can be directly connected to a transmission line having a predetermined characteristic impedance.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of a semiconductor laser module according to a first embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of a semiconductor laser module according to Conventional Configuration Example 1.

FIG. 3 is an equivalent circuit diagram of a semiconductor laser module according to Conventional Configuration Example 2.

FIG. 4 is a graph illustrating the relationship between the input modulation signal frequency and VSWR in the semiconductor laser module of Conventional Configuration Example 1.

FIG. 5 is a graph illustrating the relationship between the input modulation signal frequency and the optical output power / input signal current ratio in the semiconductor laser module of Conventional Configuration Example 1.

FIG. 6 is a graph illustrating the relationship between the input modulation signal frequency and VSWR in the semiconductor laser module of Conventional Configuration Example 2;

FIG. 7 is a graph illustrating the relationship between the input modulation signal frequency and the optical output power / input signal current ratio in the semiconductor laser module of Conventional Configuration Example 2;

FIG. 8 is a graph illustrating the relationship between the input modulation signal frequency and VSWR in the semiconductor laser module according to the first embodiment of the present invention.

FIG. 9 is a graph illustrating the relationship between the input modulation signal frequency and the optical output power / input signal current ratio in the semiconductor laser module according to the first embodiment of the present invention.

FIG. 10 is an equivalent circuit diagram of the semiconductor laser module according to the second embodiment of the present invention.

FIG. 11 is an equivalent circuit diagram of the semiconductor laser module according to the third embodiment of the present invention.

FIG. 12 is a graph illustrating a relationship between an input modulation signal frequency and VSWR in the semiconductor laser module according to the third embodiment of the present invention.

FIG. 13 is a graph illustrating the relationship between the input modulation signal frequency and the optical output power / input signal current ratio in the semiconductor laser module according to the third embodiment of the present invention.

FIG. 14 is a graph showing a relationship between an input modulation signal frequency and VSWR in a modification of the semiconductor laser module according to the third embodiment of the present invention.

FIG. 15 is a diagram showing a modified example of the semiconductor laser module according to the third embodiment of the present invention, wherein the input modulation signal frequency and the optical output power /
Graph showing the relationship with the input signal current ratio

[Explanation of symbols]

 1 Module Package 2 Laser Diode Chip 3 Bonding Wire 4 Microstrip Line 5 DC Bias Circuit 6 Modulation Signal Input Terminal 7 Transmission Line 8 Impedance Converter 9 Input Impedance Matching Circuit 11 Load Resistance

Claims (7)

[Claims] 1. A characteristic impedance of a transmission line connected to the modulation signal input terminal is close to a load impedance of the laser diode chip inside a module package that houses the laser diode chip and has a modulation signal input terminal. A semiconductor laser module comprising an impedance converter for converting into impedance. 2. The semiconductor laser module according to claim 1, wherein the impedance converter is a wide band transformer using a ferrite core. 3. The semiconductor laser module according to claim 1, wherein a resistance element is connected in series to the impedance converter. 4. The semiconductor laser module according to claim 3, wherein the resistance value of the resistance element is 10Ω or less. 5. The semiconductor laser module according to claim 3, wherein the resistance element is connected to the laser diode chip side of the impedance converter. 6. The semiconductor laser module according to claim 1, further comprising an LC-type impedance matching circuit provided outside the module package. 7. The LC-type impedance matching circuit L so as to improve transmission efficiency in a plurality of frequency bands.
The semiconductor laser module according to claim 6, wherein a C constant is set.