JP2723564B2 - Semiconductor laser module - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser module having an electronic cooling unit, and more particularly to a semiconductor laser module useful as a light source for an ultra-high-speed optical communication system having a transmission speed exceeding 1 Gb / s. About.

[Conventional technology]

In general, since a semiconductor laser is a threshold device, a decrease and deterioration of an optical output with an increase in temperature are accelerated. On the other hand, a change in ambient temperature causes a change in oscillation characteristics, for example, mode hopping or center wavelength shift. For this reason, when a semiconductor laser is used as a light source for optical communication, a change in oscillation characteristics or a change in optical output causes deterioration in transmission characteristics.

Conventionally, this type of semiconductor laser module is -20 to 70 ° C
In order to obtain stable operation in a temperature range of about the same, an electronic cooling unit having a Peltier effect is housed inside the semiconductor laser module, and the semiconductor laser element and the laser light monitoring element are controlled at a constant temperature (for example, 25 ° C.). .

As such a semiconductor laser module, for example,
There is Japanese Utility Model Application No. 61-130321, the structure of which is shown in FIG.

Referring to FIG. 5, a semiconductor laser element 51, a laser light monitoring element 52, an optical fiber 53, and a thermistor 54 are arranged and mounted at predetermined positions of a block 55. An electronic cooling unit 57 having a Peltier element 56 is disposed between the bottom surface of the block 55 and the heat sink 59 of the package 58, and is closely fixed to each other. The semiconductor laser element 51 and the laser light monitoring element 52 are wire-bonded to a relay terminal 61 insulated and sealed with a low melting glass 60 in a package 58.
Wired at 62 mag.

The electronic cooling unit 57 used in the above-described semiconductor laser module generally includes an n-type and a p-type Peltier element 56 made of a bismuth tellurium compound and two printed substrates 63 and
A pair of lead wires 65 are soldered to a printed circuit board 64. The printed boards 63 and 64 are generally formed by forming a wiring pattern of a Peltier element 56 on an alumina (Al ₂ O ₃ ) ceramic substrate.

Therefore, when the block 55 is mounted on the electronic cooling unit 57, the block 55 is electrically insulated from the package 58 by the printed boards 63 and 64. Therefore, to short-circuit the block 55 to the package 58,
As shown in the figure, it is necessary to wire the block 55 and the earth relay terminal 66 fixed to the package 58 by wire bonding 62 or the like.

[Problems to be solved by the invention]

By the way, when the ground of the block 55 and the anode of the semiconductor laser element 51 are also used in the short circuit structure as described above, the block 55 becomes a virtual ground point. If this is represented by an electric equalizing circuit, it is represented as shown in FIG. In FIG. 6, the dashed line indicates the equalizing circuit of the semiconductor laser element 51, the broken line indicates the equalizing circuit in the package 58 including the block 55, and the outside of the broken line indicates the equalizing circuit including the entire package 58.

In FIG. 6, Ll2 is the parasitic inductance of the cathode lead terminal of the semiconductor laser module, Cp2 is the parasitic capacitance of the cathode lead terminal of the semiconductor laser module, LW2 is the parasitic inductance of the cathode lead terminal of the semiconductor laser module (inside the package), CSS is the parasitic capacitance of the cathode lead terminal of the semiconductor laser module (inside the package), Ch is the parasitic capacitance of the heat sink of the semiconductor laser element, LW1 is the parasitic inductance of the add terminal of the semiconductor laser module (inside the package), Ll1
Is the parasitic inductance of the anode terminal of the semiconductor laser module (outside the package), RLD is the resistance (differential resistance) component of the semiconductor laser, CLD is the capacitance component of the semiconductor laser,
Lb1 and Lb2 represent the inductances of the bonding wires of the anode and the cathode of the semiconductor laser, respectively, and Cp1 represents the parasitic capacitance parasitic on the anode terminal.

The anode of the semiconductor laser element 51 is a bonding wire
62 parasitic inductances L _b1 , L _w1 and earth relay terminal 6
It is electrically grounded through an inductance _L11 of 6. Virtual ground point of the same block 55 is grounded to the package 58 via an impedance Z _p of the electronic cooling unit 57.
Also, the impedance of the electronic cooling unit 57 can be expressed as shown in FIG. 7 (in FIG. 7, C1 is the parasitic capacitance of the hot surface substrate, R1 is the resistance value of the metallization and Peltier element, and C2 is the cooling surface substrate). Parasitic capacitance), frequency 3 ~
Capacitance C ₁ and C ₂ is dominant in the range of about 4 GHz. capacity
C ₁ or C ₂ is albeit different due to the size of the electronic cooling unit 57, printed substrate 63, 64 of 6 mm × 6 mm square approximately alumina
Has a parasitic capacitance of about 4 to 5 pF. On the other hand, a bonding wire 62 (about 1.5 mm) and a relay terminal for ground (about 6 m
m) has an inductance of about 5 to 6 nH. In the case of the conventional semiconductor laser module, the grounding of the block 55 on which the semiconductor laser element 51 is mounted, that is, the grounding of the anode of the semiconductor laser element 51, causes the parasitic capacitances C ₁ and C ₂ parasitic on the electronic cooling unit 57, the bonding wire 62, Relay terminal 6 for ground
6 is electrically grounded by a parallel circuit with a parasitic inductance that is parasitic on 6, so that the parallel resonance frequency _r of the parallel circuit of C ₁ , C ₂ and L _w1 + L _l1 described above is obtained. In this case, as shown in FIG. 4, there is a problem that the small signal frequency characteristics are significantly deteriorated.

In the case of using the above-described semiconductor laser module as an optical communication system for light source, the rise of the modulated optical output waveform for lack of signal spectrum of the laser driving signal is generated, the jitter of the falling portion becomes very large, _r Unless the transmission speed is below, there is a problem that it is not practical. For this reason, it becomes extremely difficult to apply it to an ultra-high-speed optical communication system of 1 Gb / s or more.

On the other hand, as shown in the specification of Japanese Utility Model Application No. 61-130321, when the parasitic inductance is suppressed by using a metal foil, the resonance frequency is maintained under the condition that the capacity is constant and the heat exchange efficiency of the electronic cooling unit is not reduced. The improvement effect of _r Therefore, a sufficient improvement effect cannot be obtained. in this way,
Only by improving the parasitic inductance such as wire bonding, the resonance frequency cannot be sufficiently improved, and the parasitic capacitance of the electronic cooling unit itself needs to be improved.

(Means for Solving the Problems) The present invention is characterized in that the material of the printed substrate of the electronic cooling unit of the semiconductor laser module is a silicon carbide (SiC) -based ceramic substrate containing beryllium oxide (BeO). .

(Operation)

In the present invention, silicon carbide (SiC) containing beryllium oxide (BeO) is used as the material of the printed circuit board of the electronic cooling unit.
By using C) -based ceramics substrate,
The parasitic capacitance of the electronic cooling unit is increased to reduce the impedance. This allows the parallel resonance frequency of the semiconductor laser module to be reduced without reducing the heat exchange efficiency.
_r is shifted to a low frequency region outside the range of use.

〔Example〕

Hereinafter, the present invention will be described with reference to examples. The structure of the electronic cooling unit in this embodiment is equivalent to that of the conventional electronic cooling unit.

Referring to FIG. 1 and FIG. 2, an electronic cooling unit 1 includes an n-type and a p-type Peltier element 2 and two printed substrates 3 and 4.
It is a structure assembled between them. A wiring pattern 5 for assembling the Peltier elements 2 electrically in series and thermally in parallel is printed on the printed boards 3 and 4, and is fixed together with a pair of lead wires 6 by soldering.

When a current flows from one of the lead wires 6 of the electronic cooling unit 1, a phenomenon occurs in which heat is absorbed in the upper part and heat is generated in the lower part.
When the flow is reversed, the heat absorption and heat generation sides are reversed. The electronic cooling unit built in the semiconductor laser module uses this phenomenon to cool or heat the semiconductor laser element. On the other hand, the material of the printed circuit boards 3 and 4 of the electronic cooling unit 1 is conventionally formed of alumina oxide (Al ₂ O ₃ ) -based ceramics. In this embodiment, however, a small amount of beryllium oxide (BeO) is added to this material. Silicon carbide (SiC) -based ceramics
The wiring pattern 5 is printed. The characteristics of silicon carbide (SiC) ceramics containing beryllium oxide (BeO) are that the thermal conductivity is 15 times larger and the dielectric constant is up to 12 times larger than that of alumina. In addition, the mechanical strength is as high as that of the alumina type, and the substrate is easily formed. This point is described in the literature in "Material Forum" (published by the Industrial Research Council), September 1986, No. 10, pages 21 to 24, pages 38 to 41. Such a substrate is also called “Hitaceram” (trade name). The electronic cooling unit 1, that is, the parasitic capacitance C ₁ of the printed circuit boards 3 and 4
To increase the C ₂ and is to a high dielectric material,
By using a silicon carbide ceramic containing beryllium oxide as in this embodiment, the parasitic capacitance can be reduced to about 60 pF. Here, referring to FIGS. 6 and 7, the parallel resonance which is a problem in the conventional semiconductor laser module is determined by the ground inductance (Lb1, Lw1, Ll1) and the impedance Zp on the anode side of the semiconductor laser. . In FIG. 6, since Cp1 is about 0.1 to 0.2 pF, it can be ignored. In addition, Lb1 is extremely small compared to (Lw1 + Ll1) (mostly determined by the resonance frequency due to the parasitic inductance of the lead), and thus has almost no effect even if ignored. Now, C = 1 / {(1 / C1) + (1 / C2)} When the impedance Z ₁ is approximately in parallel resonance Can be expressed as It is possible to reduce the impedance Z ₁ of the parallel resonance by increasing the parasitic capacitance As is apparent from this equation, of about 1/12 the conventional embodiment
To be improved.

It is more effective to increase the capacitance of the material. For example, if barium titanate (BaTiO ₃ ) is used for the capacitor material, the impedance Z ₁ is reduced to several hundredths. However, the thermal conductivity of this material is about 1/15 smaller than that of alumina, so the heat exchange efficiency of the electronic cooling unit is poor.

Thus, if the material of the printed circuit board is silicon carbide (SiC) ceramic containing beryllium oxide (BeO), which is a material with high thermal conductivity and high dielectric constant,
As shown in FIG. 3, the resonance point of the small signal frequency characteristic can be shifted to the low frequency region, and the notch amount (dent)
Can also be reduced. In particular, a notch generated at 1 GHz or more that is harmful to modulation characteristics in an ultra-high-speed optical communication system can be shifted to a harmless low-frequency region. In order to shift to a lower frequency side, it is sufficient to increase the parasitic capacitance by increasing the parasitic capacitance and to increase the dielectric constant by devising the mixing ratio of the materials. This can be achieved by reducing the thickness or the thickness. In this case 1Gb / s
It can be applied to the following optical communication systems.

〔The invention's effect〕

As described above, in the present invention, the resonance point of the small signal frequency characteristic can be shifted to the low frequency region by using a silicon carbide (SiC) ceramic containing beryllium oxide (BeO) as the material of the printed substrate. , The notch amount can be further reduced.

[Brief description of the drawings]

FIG. 1 is a side sectional view for explaining an electronic cooling unit of the present invention, FIG. 2 is an external perspective view of the electronic cooling unit, and FIG.
FIG. 4 is a diagram showing a small signal frequency characteristic of a semiconductor laser module using the electronic cooling unit of the present invention. FIG. 4 is a diagram showing a small signal frequency characteristic of the semiconductor laser module in the conventional electronic cooling unit. The figure shows the internal structure of an example in which an electronic cooling unit is mounted on a semiconductor laser module, FIG. 6 shows an electrical equalizing circuit of the semiconductor laser module, and FIG. 7 shows the electrical equalizing circuit of the electronic cooling unit FIG. 1 ... Electronic cooling unit, 2 ... Peltier element, 3 ... Printed board, 4 ... Printed board, 5 ... Wiring pattern, 6 ... Lead wire, 51 ... Semiconductor laser element, 52 ... Laser light monitor Element, 53… Optical fiber, 54… Thermistor, 55… Block, 56… Peltier element, 57… Electronic cooling unit, 58…
... Package, 59 ... Heat sink, 60 ... Low melting point glass, 61 ... Relay terminal, 62 ... Wire bonding, 63 ...
Printed circuit board, 64 Printed circuit board, 65 Lead wire, 66 Relay terminal for grounding

Claims (1)

(57) [Claims] A package, a semiconductor laser element supported by a block and housed in the package, a short-circuit member for short-circuiting the block to the package, and an electronic cooling unit for cooling the semiconductor laser element. The semiconductor laser module includes a first printed circuit board housed in the package and supporting the block, and a second printed circuit board housed in the package and supporting the first wiring board via a cooling element. The first and second printed circuit boards are made of silicon carbide (SiC) -based ceramics containing beryllium oxide (BeO).