JP2004296805A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter which operates a light-emitting element stably at a high temperature and reduces the electric power consumption by a cooling component that is necessary for the stable operation of the light-emitting element. <P>SOLUTION: The optical transmitter comprises a semiconductor laser (LD), a temperature detecting element connected thermally to the semiconductor laser, an electronic cooler on which the semiconductor laser is mounted, an LD driving circuit which outputs an LD driving current to the semiconductor laser, a temperature compensating circuit which controls the value of the output current from the LD driving circuit in response to a detected signal from the temperature detecting element, and a constant-current circuit which gives a constant-current to the electronic cooler. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical transmitter equipped with an electronic cooler for cooling a light emitting element.
[0002]
[Prior art]
In an optical transmitter equipped with a conventional semiconductor laser (LD), in an environment where the operating temperature changes, the element temperature of the LD is kept constant by using a Peltier element in order to stabilize the output light of the LD. .
[0003]
In such an optical transmitter, an LD is mounted on a Peltier element, and the element temperature of the LD is detected by a thermistor. Based on the temperature detected by the thermistor, temperature control of the Peltier element such as heating or cooling is performed, and the element temperature of the LD is maintained at a constant temperature of, for example, 25 ° C. At this time, the operating current of the LD is stabilized irrespective of a change in the operating temperature of the optical transmitter, so that the driving current given from the LD driving circuit to the LD may be a constant value (for example, see Patent Document 1).
[0004]
[Patent Document 1]
JP 2001-53378 A (FIG. 3, page 1-2)
[0005]
However, when the operating temperature of the optical transmitter is, for example, 70 ° C., in order to set the LD to 25 ° C., a large Peltier element is required according to the temperature difference between the operating temperature of the optical transmitter and the element temperature of the LD. It is necessary to supply power, and as a result, there is a problem that power consumption is significantly increased. For example, at this time, the power consumption of the Peltier device is about 2 W with respect to the total power consumption of 2 W of the optical transmitter excluding the Peltier device.
Further, the temperature control circuit of the Peltier element is often realized by an amplifier using an operational amplifier or a resistor, and the number of components has increased.
[0006]
On the other hand, in an environment in which the operating temperature changes without always keeping the LD temperature constant, a temperature compensation circuit for compensating the current value of the LD drive current in accordance with the LD element temperature detected by the thermistor. Is known (for example, see Patent Document 2).
[0007]
[Patent Document 2]
JP-A-2002-111120 (FIG. 1, pages 2 to 4)
[0008]
However, in such an optical transmitter, the element temperature of the LD also changes in the same manner in accordance with the change in the operating temperature. Therefore, in order to stabilize the optical output, the higher the temperature, the more the LD drive current is supplied. I needed to.
At this time, at a high temperature, the driving current is insufficient and the LD cannot be driven to obtain a desired optical output, or the frequency characteristics of the LD deteriorate when the LD is operated at high speed. For this reason, there has been a problem that the rise / fall time of the transmission light waveform of the LD is delayed, and the eye opening of the light transmission waveform is closed.
[0009]
[Problems to be solved by the invention]
In a conventional optical transmitter, when using an electronic cooler such as a Peltier element to stabilize the element temperature of an LD, which is a light emitting element, when the operating temperature of the optical transmitter increases, the power consumption supplied to the Peltier element Has a problem that it becomes significantly large.
[0010]
Further, when the current value of the drive current of the LD is adjusted according to the device temperature without keeping the device temperature of the LD constant, the drive characteristics of the LD are deteriorated at a high temperature, or the frequency characteristics of the LD are deteriorated. There is a problem that the quality of the transmitted light waveform is deteriorated.
[0011]
The present invention has been made to solve such a problem, and stably operates a light emitting element at a high temperature, and reduces power consumption of cooling components necessary for stable operation of the light emitting element. The aim is to obtain an optical transmitter.
[0012]
[Means for Solving the Problems]
An optical transmitter according to the present invention includes a light emitting element, a temperature detecting element thermally connected to the light emitting element, an electronic cooler thermally connected to the light emitting element, and outputting a drive current to the light emitting element. A temperature compensation circuit for controlling a current value of an output current of the drive circuit in accordance with a detection signal of the temperature detection element, and a constant current circuit for supplying a constant current to the electronic cooler. It is.
[0013]
Further, the constant current circuit may provide a constant current value such that a temperature detected by the temperature detecting element becomes equal to or lower than a use temperature of the housing within an operating temperature range of the housing.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiment 1 FIG.
FIG. 1 is a sectional view showing a configuration of the optical transmitter according to the first embodiment of the present invention.
In the figure, an optical transmitter 1 accommodates and fixes an optical module 2 and a substrate 3 in a metal housing 4. The optical module 2 accommodates a semiconductor laser (LD) 5, which is a light emitting element, a temperature detecting element 6, and an electronic cooler 7 inside a package 100, and holds an optical fiber 8 on a side wall of the package 100.
[0015]
The electronic cooler 7 is joined to a bottom surface inside a package 100 constituting the optical module 2 and has a substrate 9 mounted on an upper surface. The LD 5 and the temperature detecting element 6 are mounted on the upper surface of the substrate 9 and are thermally connected to each other, and are also thermally connected to the electronic cooler 6. The electronic cooler 7 is configured by sandwiching the upper and lower sides of a Peltier element with an insulating substrate, and operates so as to absorb heat generated by the LD 5 mounted on the upper surface and radiate the heat to the bottom surface of the package 100 bonded to the lower surface. . The temperature detecting element 6 is formed of a thermistor, and an output current (temperature signal) changes according to a change in the element temperature of the LD 5.
That is, the device temperature TL of the LD 5 is output. Further, a photodiode (PD) 50 is provided on the substrate 9. The PD 50 receives an optical signal emitted from the rear surface of the LD 5 and outputs a monitor current proportional to the received optical signal.
[0016]
The LD 5 is optically coupled to the optical fiber 8 via a lens fixed on the electronic cooler 7, and an optical signal emitted from the front surface of the LD 5 is input to an end face of the optical fiber 8. The package 100 has a side wall made of a metal member fitted with a feedthrough formed by enclosing a signal terminal of a conductive member with a dielectric, and transmits an electric signal inside and outside the package 100 while maintaining airtightness. .
[0017]
The substrate 3 has a plurality of electronic circuit components 10 mounted on an upper surface. The substrate 3 is electrically connected to the substrate 9 with the feedthrough of the optical module 2 and the conductor wire interposed. Thus, the electronic circuit component 10 is electrically connected to the LD 5 and the temperature detecting element 6, and an electric signal is transmitted and received between the LD 5 and the temperature detecting element 6 and the electronic circuit component 10.
[0018]
The outer bottom surface of the package 100 constituting the optical module 2 is joined to the inner bottom surface of the housing 4. In addition, a heat sink 11 is provided on the outer bottom surface of the housing 4, and thus the electronic cooler 7 joined to the bottom surface of the optical module 2 is indirectly thermally connected to the heat sink 11. An optical fiber 8 is held on a side surface of the housing 4, and transmits an optical signal input to an end surface of the optical fiber 8 to the outside of the housing 4.
[0019]
FIG. 2 is an electric circuit diagram showing a circuit configuration inside the optical transmitter 1.
In the figure, an optical transmitter 1 is provided with an LD drive circuit 20, a temperature compensation circuit 21, an APC circuit 30, and a constant current circuit 35, each of which constitutes an electronic circuit component 10. The LD drive circuit 20 receives an NRZ (Non Return Zero) code data signal (DATA) that is converted as an optical signal. The LD drive circuit 20 drives the LD 5 based on the data signal and supplies the LD 5 with a modulation current Imod for intensity-modulating the output light. The LD drive circuit 20 includes a current mirror circuit therein, detects a current signal proportional to the peak current (amplitude) of the modulation current Imod, and converts the current signal into a voltage signal to generate a modulation monitor signal. It is output as Vmon. The LD drive circuit 20 is supplied with a base voltage (not shown).
[0020]
The temperature compensation circuit 21 supplies a modulation control signal Vmod to the LD drive circuit 20 based on the temperature signal It output from the temperature detection element 6 and the modulation monitor signal Vmon. For example, in the temperature compensation circuit 21, the temperature signal It is converted into a voltage signal Vt, added to a reference voltage (constant voltage) Ve, and a temperature-compensated reference signal (temperature compensation signal) Vm (= Ve + Vt) is calculated. . Further, a difference (= Vm-Vmon) between the temperature compensation signal Vm and the modulation monitor signal Vmon is calculated, and a signal obtained by amplifying the calculated difference signal is output as the modulation control signal Vmod. The LD drive circuit 20 adjusts the modulation current Imod for adjusting the modulation amplitude of the optical output of the LD 5 according to the modulation control signal Vmod.
[0021]
The APC circuit 30 outputs a bias current Ib adjusted according to the monitor current of the PD 50 so that the intensity of the output light of the LD 5 is constant. The bias current Ib and the modulation current Imod merge and are supplied to the LD 5 as the LD drive current ILD to output an intensity-modulated optical signal.
Thus, the temperature compensating circuit 21 compensates for the modulation current Imod output from the LD driving circuit 20 according to the temperature detected by the temperature detecting element 5, and reduces the amount of the LD driving current ILD supplied to the LD 5 as a result. Compensate.
[0022]
A constant current (constant current) is supplied to the electronic cooler 7 by a constant current circuit 35. Thus, the electronic cooler 7 performs a cooling operation so as to absorb a certain amount of heat generated in the LD 5 and radiate the heat to the optical transmitter.
Note that different ground potentials are supplied to the constant current circuit 35, the LD5, and the LD drive circuit 20.
[0023]
Here, the cooling operation of the electronic cooler 7 will be described in detail.
When the working temperature of the optical transmitter 1 changes to, for example, 70 ° C., the temperature of the LD 5 rises according to the temperature change, and the element temperature rises accordingly. At this time, a large amount of current is supplied to the LD 5 by the operation of the temperature compensation circuit 21. However, when the operation limit temperature of the LD 5 for obtaining a desired optical output is 60 ° C., when the element temperature of the LD 5 becomes as high as, for example, 70 ° C. like the operating temperature of the optical transmitter, the LD to be supplied to the LD 5 is supplied. Insufficient driving current makes it impossible to obtain a desired optical output.
[0024]
However, in the first embodiment, in order to extend the operating limit temperature of the LD 5 to near the operating temperature of the optical transmitter 1, a constant current is applied to the electronic cooler 7 to reduce the LD 5 by the temperature difference (ΔTc) to be extended. By cooling, the operating limit temperature of the LD 5 is apparently extended. That is, with the constant current supplied from the constant current circuit 35, the electronic cooler 7 performs the cooling operation so as to set the element temperature TLD of the LD 5 to a temperature lower than the use temperature TTX of the optical transmitter 1 by a temperature difference ΔTc. Let it. In this embodiment, the use temperature TTX is represented by the temperature of the air surrounding the outer wall surface of the housing 4 (ambient temperature) or the temperature of the outer surface of the heat sink 11. However, when a heat sink having a high heat radiation efficiency is used for the housing 4, the temperature difference between the ambient temperature and the temperature of the internal air is small, so the surface temperature of the inner wall surface of the optical transmitter 1, the average temperature, Alternatively, the temperature of the internal air may be used.
[0025]
Assuming that the electric power released from the electronic cooler 7 for this cooling is ΔWc, the thermal resistance Rc of the electronic cooler 7 is Rc = ΔTc / ΔWc (° C./W).
On the other hand, since the housing 4 of the optical transmitter 1 is provided with the sufficiently large heat sink 11, the thermal resistance RTX with the outside air around the optical transmitter 1 is sufficiently larger than the thermal resistance Rc of the electronic cooler 7. , RTX << Rc. Thus, the temperature rise ΔTTX of the optical transmitter 1 due to the power ΔW emitted from the electronic cooler 7 is sufficiently small with respect to the temperature difference ΔTc, and has a relationship of ΔTTX << ΔTc.
[0026]
Therefore, assuming that the element temperature TLD of the LD 5 during cooling and the use temperature of the optical transmitter 1 are TTX, and that the element temperature TLD is given by a relational expression of TLD = TTX + ΔTTX−ΔTc, substantially, based on ΔTTX << ΔTc TLD = TTX−ΔTc holds. That is, the practical element temperature TLD of the laser diode LD5 is set to a temperature lower than the working temperature TTX of the optical transmitter 1 by a cooling temperature difference (ΔTc).
[0027]
Here, for example, a constant current ILD is given to the electronic cooler 7 so as to obtain a temperature difference ΔTc (= 10 ° C.) between the element temperature TLD (= 60 ° C.) of the LD 5 and the use temperature TTX of the optical transmitter 1. Assuming that the electric power ΔW = 0.5 W emitted from the electronic cooler 7 for this cooling, the thermal resistance Rc of the electronic cooler 7 becomes 20 (° C./W). At this time, for example, a heat sink 11 having a thermal resistance RTX = 2 (° C./W) (RTX << Rc) of the optical transmitter 1 (the bottom area of the housing 4 = 80 mm × 80 mm) is provided on the bottom surface of the housing 4. Then, ΔTTX = RTX × ΔW = 1 ° C., and ΔTTX << ΔTc. Further, from TTX = TLD-ΔTTX + ΔTc = 69 ° C., even when the element temperature TLD of the LD 5 is the upper limit of the operation limit temperature, the operating temperature TTX of the optical transmitter 1 can be raised to almost 70 ° C.
[0028]
Here, in order to explain that a remarkable effect can be obtained in stably operating the LD 5 according to the first embodiment, a comparison with the related art will be made below.
[0029]
Conventionally, when an optical transmitter (LD drive current compensation method) without an electronic cooler as described in Patent Document 2 is used, an optical transmitter with an LD is mounted in order to make the LD perform a desired operation at a high temperature. The upper operating temperature of the vessel had to be set to a temperature lower by about 10 ° C. than 70 ° C.
[0030]
Here, FIG. 3 is a diagram showing a temperature characteristic of an optical output with respect to an LD drive current when a conventional LD drive current compensation type optical transmitter is used. In this figure, the LD drive currents ILD (0 ° C.), ILD (25 ° C.), ILD (60 ° C.), and ILD (70 ° C.) when the LD element temperatures are 0 ° C., 25 ° C., 60 ° C., and 70 ° C. 3) shows the light output characteristics. As shown in the figure, in this type of optical transmitter, when the ambient temperature (operating temperature) = 0 ° C., 25 ° C., 60 ° C., and 70 ° C., the element temperature TLD of the LD is 0 ° C., 25 ° C., 60 ° C. , And 70 ° C.
[0031]
In this case, when the operating temperature of the optical transmitter becomes high (70 ° C. in the figure), the LD drive current becomes insufficient, and a desired optical output cannot be obtained. Thus, it is necessary to lower the operating limit temperature of the optical transmitter by about 10 ° C. from 70 ° C.
[0032]
FIG. 4 is a diagram showing the frequency band of the LD with respect to the operating temperature (ambient temperature) when a conventional LD current control type optical transmitter is used. 4 shows a frequency band for obtaining an eye opening having a large opening diameter. In the figure, when the operating temperature of the optical transmitter becomes a high temperature (for example, 70 ° C.) which is a desired operation limit temperature, in the frequency band higher than the operating frequency f0 corresponding to the maximum bit rate, the optical transmission of the LD is performed. The frequency characteristics (e.g., pass characteristics) of the waveform are degraded, resulting in insufficient band at high temperatures.
[0033]
On the other hand, FIG. 5 is a diagram illustrating a temperature characteristic of an optical output with respect to an LD drive current when the temperature difference ΔTc is set to 10 ° C. using the optical transmitter according to the first embodiment. This figure shows the optical output characteristics with respect to the LD drive currents ILD (0 ° C.), ILD (25 ° C.), and ILD (60 ° C.) when the element temperature of the LD 5 is 0 ° C., 25 ° C., and 60 ° C. . As shown in the figure, when the ambient temperature (operating temperature) of the optical transmitter 1 is 10 ° C., 35 ° C., and 70 ° C., the element temperatures TLD of the LD 5 are 0 ° C., 25 ° C., and 60 ° C.
[0034]
Therefore, even if the operating temperature of the optical transmitter 1 becomes high (70 ° C. in the drawing), the element temperature of the LD 5 becomes 60 ° C. due to the operation of the electronic cooler 7, and the shortage of the LD drive current ILD occurs. As a result, the operating limit temperature of the optical transmitter 1 can be increased by about 10 ° C.
[0035]
FIG. 6 is a diagram showing the frequency band of the LD 5 with respect to the ambient temperature (operating temperature) of the optical transmitter 1 according to the first embodiment. 3 shows a frequency band for obtaining a high eye opening. In the figure, even if the operating temperature of the optical transmitter 1 is high (for example, 70 ° C.), the LD 5 operates well, and the optical transmission of the LD 5 is performed in a frequency band higher than the operating frequency f0 corresponding to the maximum bit rate. It can be seen that the waveform satisfies predetermined frequency characteristics (for example, pass characteristics).
[0036]
As described above, according to the first embodiment, the LD 5 is cooled by using the electronic cooler 7 that operates at a constant current, so that the operating temperature limit of the operating temperature of the optical transmitter 1 on which the LD 5 is mounted is further reduced. Even if the temperature is set to a high temperature (for example, 70 ° C.), the LD 5 can be operated stably, and the waveform deterioration of the optical transmission waveform of the LD 5 can be suppressed, and a higher quality output waveform can be obtained.
[0037]
Next, according to the first embodiment, a remarkable improvement in the power consumption of the electronic cooler 7 can be obtained.
In an optical transmitter (Peltier drive current control method) as described in the conventional Patent Document 1, the current supplied to the Peltier element is set so that the detection temperature TLD of the thermistor matches the reference temperature T0 (for example, 25 ° C.). It was increased or decreased by feedback control. For this reason, for example, when the operating temperature of the optical transmitter 1 is 70 ° C., the LD is cooled by 45 ° C. with a Peltier element so that the element temperature of the LD becomes 25 ° C., while the operating temperature of the optical transmitter 1 is lowered. When the temperature reached 0 ° C., the LD was heated by a Peltier device at 25 ° C. so that the device temperature of the LD became 25 ° C.
[0038]
However, in the optical transmitter 1 according to this embodiment, the electronic cooler 7 is operated at a constant current to cool by the temperature difference ΔTc (for example, 10 ° C.) between the LD 5 and the optical transmitter 1. As a result, the power ΔWc (eg, 0.5 W) emitted from the electronic cooler is in a relation of WTX >> ΔWc with respect to the power consumption WTX (eg, 2.4 W) of the entire optical transmitter (5 times). The degree of power consumption accompanying the driving of the electronic cooler 7 is practically not found.
[0039]
FIG. 7 shows a method according to the first embodiment (the method of the present embodiment), a method based on the invention of Patent Document 1 (LD drive current compensation method), and a method based on the invention of Patent Document 2 (Peltier drive current control). 9 is a table showing an example in which the power consumption of the LD, the electronic cooler, and the entire optical transmitter is compared for each of (method).
[0040]
According to the table, the method of the present embodiment increases the power consumption of the entire optical transmitter as compared with the LD drive current compensation method that does not use the electronic cooler, but the difference is small (for example, 0.3 W). However, there is no substantial difference in power consumption. However, in the method of the present embodiment, the power consumption of the electronic cooler is significantly reduced (about 1/4) as compared with the Peltier drive current control method, and the entire optical transmitter is operated while the LD 5 is operated at a high temperature. The effect of improving power consumption is remarkably large.
[0041]
Further, according to the first embodiment, the feedback control of the temperature of the electronic cooler 7 is not performed by using an operational amplifier, an amplifier using a resistor, or the like, or a microcomputer. Thus, a cooling means for stably operating the LD 5 at a high temperature can be realized with a simple configuration in which only the constant current circuit 35 is provided. In addition, since the number of components can be reduced, a small and low-priced optical transmitter 1 can be obtained.
[0042]
In the above description, the APC circuit 30 has been described as a component different from the LD drive circuit 20. However, an LD drive circuit in which the APC circuit 30 and the LD drive circuit 20 are integrated may be configured. In this case, the LD drive circuit is controlled based on a compensation signal output from the temperature compensation circuit that changes according to the temperature. The circuit operates to supply the LD drive current (current for extinguishing the LD) to the LD.
[0043]
As described above, according to the first embodiment, the light-emitting element is cooled by using the electronic cooler that operates at a constant current, so that the operating limit temperature of the optical transmitter equipped with the light-emitting element is lower than the operating limit temperature of the light-emitting element. Even if the temperature is set to a high temperature, it is possible to suppress the waveform deterioration of the optical transmission waveform of the light emitting element and to reduce the power consumption of the cooling component required for cooling the light emitting element.
[0044]
Embodiment 2 FIG.
As another embodiment, a switch circuit may be provided for the constant current circuit 35 shown in FIG. 2 of the first embodiment, and the electronic cooler 7 may be switched. In this case, the temperature detected by the temperature detecting element 6 is compared with a predetermined threshold temperature (for example, 50 ° C.) lower than the preset operation limit temperature of the LD 5, and the detected temperature is higher than the predetermined threshold temperature. Is determined. As a result of this comparison, when the temperature detected by the temperature detecting element 6 becomes equal to or higher than a predetermined threshold value (for example, 50 ° C.), the constant current circuit 35 is turned on to start supplying a constant current (at this time). The ambient temperature of the optical transmitter 1 is less than 50 ° C.). On the other hand, when the temperature detected by the temperature detecting element 6 is lower than the predetermined threshold value (for example, when the temperature reaches 49 ° C.), the constant current circuit 35 is turned off to stop supplying the constant current.
[0045]
Thereby, when the LD 5 approaches its operating limit and near a certain high temperature (for example, 70 ° C.), the electronic cooler 7 can be driven at a constant current, and the average time during the operation of the LD 5 is considered. The duty of the power consumption of the electronic cooler 7 can be further improved.
[0046]
【The invention's effect】
As described above, according to the present invention, by using a thermoelectric cooler that operates at a constant current to cool the light emitting element, it is possible to set the operating temperature of the optical transmitter equipped with the light emitting element to a higher temperature, The power consumption of cooling components required for cooling the light emitting element can be reduced.
[Brief description of the drawings]
FIG. 1 is a sectional view showing a configuration of an optical transmitter according to a first embodiment of the present invention.
FIG. 2 is a circuit configuration diagram of the optical transmitter according to the first embodiment of the present invention.
FIG. 3 is a diagram showing a static characteristic of an LD drive current and an optical output of an optical transmitter according to a conventional technique.
FIG. 4 is a diagram showing an ambient temperature of an optical transmitter and a frequency band in which an LD operates well according to a conventional technique.
FIG. 5 is a diagram showing static characteristics of an LD drive current and an optical output of the optical transmitter according to the first embodiment of the present invention.
FIG. 6 is a diagram illustrating an ambient temperature of the optical transmitter and a frequency band in which the LD operates well according to the first embodiment of the present invention.
FIG. 7 is a diagram showing a comparison of power consumption between the first embodiment according to the present invention and a conventional technology.
[Explanation of symbols]
Reference Signs List 1 optical transmitter, 5 semiconductor laser (LD), 6 temperature detecting element, 7 electronic cooler, 11 heat sink, 20 LD drive circuit, 21 temperature compensation circuit.

Claims (6)

A light emitting element;
A temperature detecting element thermally connected to the light emitting element,
An electronic cooler thermally connected to the light emitting element,
A drive circuit that outputs a drive current to the light emitting element;
A temperature compensation circuit that controls a current value of an output current of the drive circuit according to a detection signal of the temperature detection element;
An optical transmitter comprising: a constant current circuit that supplies a constant current to the electronic cooler. The optical transmitter according to claim 1, wherein the constant current circuit gives a constant current value such that the temperature of the light emitting element becomes equal to or lower than the temperature of the housing. 2. The optical transmitter according to claim 1, wherein the constant current circuit gives a constant current value such that a temperature detected by the temperature detecting element becomes lower than a use temperature of the housing. 2. The optical transmitter according to claim 1, wherein the constant current circuit provides a constant current value such that the temperature of the light emitting element becomes equal to or lower than an operation limit temperature within a use temperature range of the housing. 2. The optical transmitter according to claim 1, wherein the constant current circuit starts supplying a constant current when the temperature detected by the temperature detecting element reaches a predetermined temperature. While mounting the light emitting element and the temperature detection element, a substrate thermally connected to the electronic cooler,
A housing that accommodates the substrate and the electronic cooler, and that is provided with a radiator that is thermally connected to the electronic cooler and radiates heat to the outside,
2. The optical transmitter according to claim 1, wherein the thermal resistance between the electronic cooler and the heat radiating portion of the housing is set to be 10 times or more larger than the thermal resistance between the light emitting element and the electronic cooler. 3.

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