JPH06216458A - Laser diode device - Google Patents

Abstract

PURPOSE:To control the temperature of a laser diode by controlling a current flowing into a thermoelectric element based on the output of a bridge circuit the output voltage of which is changed with the temperature change of the laser diode. CONSTITUTION:Two resistor type temperature detector means 2a, 2b are connected to two arbitrary arms of a bridge circuit whereby the output voltage Vo of the bridge circuit can be expressed by a linear function of the resistance values R1, R2 of two resistors 11a, 11b, and the change of the output voltage Vo corresponds to the temperature change of a V laser diode 1. Therefore, the current of a thermoelectric element 4 is controlled on the basis of the output of the bridge circuit so that the change of the output voltage Vo becomes zero, thereby controlling the temperature of the laser diode 1. Also, a constant K can be arbitrarily set by suitably setting the ratio of resistance values R1 to R2 of the two arms other than the two arms connected to the two resistor type temperature detector means 2a, 2b in the bridge circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode temperature control circuit necessary for maintaining a stable oscillation frequency of a laser diode. A laser diode whose oscillation frequency is stabilized by this technique is used as a light source for coherent systems such as optical frequency multiplex transmission.

[0002]

2. Description of the Related Art FIG. 4 shows, for example, "High stabilization of LD temperature in LD module by two thermistor method" by Kaneko et al., 1992 Autumn Meeting of The Institute of Electronics, Information and Communication Engineers, C-166.
4A is a plan view showing the internal configuration of the laser diode module, and FIG. 4B is a sectional view taken along the line AA. In the figure, 1 is a laser diode, 2
Reference numerals a and 2b are temperature detecting means whose electric resistance value changes with temperature, and 3 is a laser diode 1 and temperature detecting means 2a and 2a.
b is a chip carrier, 4 is a thermoelectric element for cooling or heating the laser diode 1, and 5 is an optical fiber whose front end is curved to collect and output front emission light of the laser diode 1, 6 is a dual in-line type airtight container provided with an electric terminal, 7 is an electric terminal connected to the laser diode 1, and 8 is a laser diode 1
And a lead wire connecting the electric terminal 7 and 9 are lead wires 8
Is an electrode for relay of.

Next, the operation will be described. FIG. 5 shows FIG.
5 (a) is an explanatory view of temperature control of the laser diode module of FIG. 5, FIG. 5 (a) is an explanatory view showing paths of heat flow to the chip carrier 3 in the AA cross section of FIG. 4 (a), and FIG. 5 (b).
Is a heat equivalent circuit of the heat flow to the chip carrier 3. Further, FIG. 6 is a diagram showing the temperature distribution of the chip carrier 3. In FIG. 5, Tp is the temperature of the dual in-line airtight container 6 and the electric terminal 7, and T _LD is the laser diode 1.
, T ₁ is the temperature detected by the temperature detecting means 2a, T _1
2 is the temperature detected by the temperature detecting means 2b, r ₁ is the thermal resistance between the electric terminal 7 and the temperature detecting means 2a, r ₂ is the thermal resistance between the temperature detecting means 2a and the temperature detecting means 2b, and r ₃ is It is a thermal resistance between the temperature detecting means 2b and the laser diode 1.

Now, consider a case where the ambient temperature becomes higher than the set temperature of the laser diode 1. Heat to the chip carrier 3 flows into the chip carrier 3 from the dual in-line type airtight container 6 via the lead wire 8 as shown in FIG. As described above, since the chip carrier 3 has a thermal resistance, a temperature distribution (temperature difference) occurs in the chip carrier 3 when there is a difference between the ambient temperature and the set temperature of the laser diode 1. Therefore, the temperatures detected by the temperature detecting means 2a and 2b provided on the chip carrier 3 are different. As a heat equivalent circuit of the heat flow to the chip carrier 3, FIG.
Considering (b), since the temperature difference occurs almost in proportion to the thermal resistance of the path of the heat flow, as shown in FIG. 6, the temperature Tp of the dual in-line type airtight container 6 and the temperature detecting means 2a.
The detected temperature T ₁ , the detected temperature T ₂ of the temperature detection means 2b, and the temperature T _LD of the laser diode 1 differ depending on the thermal resistance between them. At this time, the temperature T _LD of the laser diode 1 is expressed by the following equation with respect to the detected temperature T ₁ of the temperature detecting means 2a and the detected temperature T ₂ of the temperature detecting means 2b. T _LD = (K · T ₂ −T ₁ ) / (K−1) (1) Here, K is a constant represented by the following equation. K = (r ₂ + r ₃ ) / r ₃ (2) Therefore, from the equations (1) and (2), the temperature T _LD of the laser diode 1 is detected by the temperature T ₁ detected by the temperature detector 2a and the temperature T _LD detected by the temperature detector 2b. The temperature T ₂ enables accurate determination regardless of the value of the temperature Tp of the dual in-line type airtight container 6. Since the thermal resistances r ₂ and r ₃ are constants determined by the structure, the value of the constant K is determined by measuring these values in advance. Next, the case where the ambient temperature fluctuates is shown. Here, the temperature detection means 2a in the initial state before the ambient temperature varies, the temperature detected by 2b and T _10, T _20. Further, the laser diode 1 obtained by substituting these values into the equation (1)
The initial temperature of T _LD0 is set. Letting ΔT _{LD be} the temperature variation of the laser diode 1 when the temperature detected by the temperature detecting means 2a changes by ΔT ₁ and the temperature detected by the temperature detecting means 2b changes by ΔT ₂ due to the ambient temperature variation, ΔT _LD is T _LD0 + ΔT _LD = From {K (T ₂₀ + ΔT ₂ ) − (T ₁₀ + ΔT ₁ )} / (K−1), it can be expressed by the following equation. ΔT _LD = (K · ΔT ₂ −ΔT ₁ ) / (K−1) (3) Therefore, if the condition of K · ΔT ₂ −ΔT ₁ = 0 (4) can be satisfied, the laser diode 1 It can be seen that the temperature fluctuation ΔT _LD can be set to 0 and the initial temperature T _LD0 can be maintained. This expression (4) is a temperature stability condition expression for keeping the temperature of the laser diode stable.

[0005]

As described above, the temperature T _LD of the laser diode 1 is obtained by substituting the detected temperature T ₁ of the temperature detecting means 2a and the detected temperature T ₂ of the temperature detecting means 2b into the equation (1). Can be obtained more accurately. Further, the temperature detecting means 2
By controlling the magnitude and polarity of the current flowing through the thermoelectric element 4, the variation ΔT ₁ of the detected temperature of a and the variation ΔT ₂ of the detected temperature of the temperature detecting means 2b satisfy the equation (4). The temperature T _LD can be stabilized with high accuracy. However, in order to actually perform such control, the temperature detecting means in the form shown in formula (1) is required, and the calculating means shown in formula (4) is required. was there.

The present invention has been made in order to solve such a problem, and provides a circuit for detecting the temperature of a laser diode of a laser diode module having a built-in resistance type temperature detecting means. It is an object of the present invention to obtain a control circuit for controlling the high precision.

[0007]

According to a first aspect of the present invention, there is provided a laser diode, a chip carrier provided with the laser diode, and a thermoelectric element mounted with the chip carrier for varying the temperature of the laser diode. In the laser diode device, the resistance-type temperature detecting means provided at two predetermined positions of the chip carrier are provided as two circuits of a bridge circuit having four contact points and four circuit branches provided with resistance elements respectively. A temperature change detection circuit, which is arranged as a branch resistance element, applies a constant voltage between a pair of opposing contacts of the four contacts, and outputs a voltage difference between the other pair of opposing contacts. The temperature of the laser diode is controlled by controlling the current flowing into the thermoelectric element based on the output of the temperature change detection circuit.

According to a second aspect of the present invention, there is provided a laser diode device comprising a laser diode, a chip carrier provided with the laser diode, and a thermoelectric element for mounting the chip carrier and for varying the temperature of the laser diode. The electric resistance exponentially decreases as the temperature rises at two predetermined positions of the chip carrier N
The TC thermistor and the PTC thermistor whose electric resistance exponentially increases as the temperature rises are provided.
A C thermistor and a PTC thermistor are arranged in series as a resistance element of one circuit branch of a bridge circuit having four contact points and four circuit branches each having a resistance element, and the four contact points are arranged. A temperature change detection circuit for applying a constant voltage between one pair of opposing contact points and outputting the voltage difference between the other pair of opposing contact points as output based on the output of the temperature change detecting circuit. The temperature of the laser diode is controlled by controlling the current flowing into the thermoelectric element.

[0009]

According to the first aspect of the present invention, by connecting the two resistance type temperature detecting means to arbitrary two sides of the bridge circuit, the output voltage of the bridge circuit is the resistance of the two resistance type temperature detecting means. It is obtained by a linear function formula of the value, and the fluctuation of the output voltage corresponds to the temperature fluctuation of the laser diode. Therefore, the temperature of the laser diode can be controlled by controlling the current of the thermoelectric element so that the fluctuation of the output voltage becomes zero based on the output of the bridge circuit. Further, in this bridge circuit, the constant K can be arbitrarily set by appropriately setting the ratio of the resistance values of the two sides other than the two sides to which the two resistance-type temperature detecting means are connected, so that the constant K can be set arbitrarily. It can correspond to the laser diode module with K.

According to the invention of claim 2, an NTC thermistor whose electric resistance exponentially decreases as the temperature rises and a PTC thermistor whose electric resistance exponentially increases as the temperature rises are connected in series on the same side. The output voltage of the bridge circuit connected to is obtained by a linear function equation of the resistance values of the NTC thermistor and the PTC thermistor, and the fluctuation of the output voltage corresponds to the temperature fluctuation of the laser diode. Therefore, the temperature of the laser diode can be controlled by controlling the current of the thermoelectric element so that the fluctuation of the output voltage becomes zero based on the output of the bridge circuit. Further, since the ratio of the temperature coefficient of the NTC thermistor to that of the PTC thermistor matches the constant K, it is possible to correspond to the laser diode module having the arbitrary constant K by setting this ratio arbitrarily.

[0011]

Embodiment 1 FIG. 1 is a structural explanatory view of an embodiment of a laser diode device according to the present invention. Here, a case where the laser diode device is applied to the laser diode module shown in FIG. 11 will be described as an example. In the figure, 2a and 2b are resistance type temperature detecting means arranged on the chip carrier 3 in which the laser diode 1 of the laser diode module shown in FIG. 11 is installed, and 11a and 11b are resistors. These temperature detecting means 2a, 2b and resistor 11a,
Reference numeral 11b constitutes a bridge circuit, and temperature detecting means 2
a, a contact point between the resistor 11a and the temperature detecting means 2b, the resistor 11
The constant voltage V _I is applied between the contacts between the points b, and the voltage difference V _O between the contacts between the temperature detecting means 2a and 2b and the contacts between the resistors 11a and 11b is output. Reference numeral 12 is a circuit for detecting a minute temperature change constituted by these temperature detecting means 2a, 2b and resistors 11a, 11b, 13 is a thermoelectric element control circuit to which the output V _{O of the} circuit 12 for detecting a minute temperature change is inputted. Is a thermoelectric element connected to the thermoelectric element control circuit 13.

Next, the operation will be described. In FIG. 1, Ra is the resistance value of the temperature detecting means 2a, and if the detected temperature is T ₁ and the temperature coefficient is α _a , there is a relation of Ra = α _a · T ₁ . Rb is the resistance value of the temperature detecting means 2b, and Rb = α _{b where} T _{2 is} the temperature to be detected and α _b is the temperature coefficient.
・ There is a relationship of T ₂ . Since the resistance value of the temperature detecting means cannot take a negative value, the absolute temperature is used as the unit of temperature. Further, R ₁ is the resistance value of the resistor 11a, and R ₂ is the resistance value of the resistor 11b. At this time, the output voltage V _O of the minute temperature change detecting circuit 12 is expressed by the following equation.

[0013]

[Equation 1]

Next, a case where the ambient temperature changes will be described. Here, the temperature detection means 2a in the initial state before the ambient temperature varies, the temperature detected by 2b and T _10, T _20. Further, the initial value of the output voltage V _O obtained by substituting these values into the equation (5) is set as V _O0 . Assuming that the temperature detected by the temperature detecting means 2a changes by ΔT ₁ and the temperature detected by the temperature detecting means 2b changes by ΔT ₂ due to the change in ambient temperature, the change in the output voltage is ΔV _O , then ΔV _O is given by the following equation (5) It is represented by.

[0015]

[Equation 2]

Here, S ₁ is a constant shown below.

[0017]

[Equation 3]

The following approximation is used here.

[0019]

[Equation 4]

In this approximation, the temperature usually handled by the laser diode module is near room temperature, and T ₁ and T ₂ are about 300.
K, and the temperature fluctuations ΔT ₁ and ΔT ₂ are 1 to
Since it is a minute temperature fluctuation of about 2 degrees, it is a reasonable approximation.
Further, assuming that the temperature detecting means 2a and 2b have the same temperature coefficient, since α _a = α _b , the equation (6) is expressed by the following equation.

[0021]

[Equation 5]

On the other hand, as described above, the condition for keeping the temperature T _{LD of the} laser diode stable is to satisfy the relation of the equation (4). The temperature stability conditional expression (4) is reproduced below. K · ΔT ₂ −ΔT ₁ = 0 (10) Here, when the temperature stability condition expression (10) is compared with the expression (9) representing the output voltage fluctuation ΔV _O of the minute temperature change detection circuit 12, the expression (9) is obtained. It can be seen that the form of the expression in () of 9) is the same as the temperature stability condition expression (10). Resistor 11 here
The resistance values R ₁ and R ₂ of a and 11b are set so that the ratio becomes equal to K as shown in the following equation. K = R ₁ / R ₂ (11) By setting the output voltage fluctuation ΔV _O of the minute temperature change detecting circuit 12 set in this way to 0, the temperature stability condition expression (10) is satisfied and the temperature of the laser diode is satisfied. T _LD is kept stable. Specifically, by the thermoelectric element control circuit 13,
This is performed by switching the magnitude and polarity of the current flowing through the thermoelectric element according to the magnitude and sign of the output voltage fluctuation ΔV _O.

Although FIG. 1 shows the structure in which the temperature detecting means 2a and 2b are arranged on the two left sides of the bridge of the minute temperature change detecting circuit 12, the temperature detecting means 2a and 2b are located on the four sides of the bridge. The above-mentioned relationship can be obtained even when the two sides are arranged.

Embodiment 2 FIG. 2 is a structural explanatory view of another embodiment of the laser diode device according to the present invention. Here, a case where the laser diode device is applied to the laser diode module shown in FIG. 11 will be described as an example. In the figure, 14a, 1
4b is a thermistor which is a temperature detecting means arranged on the chip carrier 3 on which the laser diode 1 of the laser diode module shown in FIG. 11 is installed, 11a,
11b is a resistance. These thermistors 14a, 14
b and the resistors 11a and 11b form a bridge circuit, and a constant voltage V _I is applied between the contact point between the thermistor 14a and the resistor 11a and the contact point between the thermistor 14b and the resistor 11b. contact and the resistance 11a between the voltage difference V _O between the contacts between 11b is output. 12 is these thermistors 14a, 14b and resistor 1
A circuit for detecting a minute temperature change constituted by 1a and 11b, and 13 is an output V _{O of the} circuit for detecting a minute temperature change 12.
Is inputted to the thermoelectric element control circuit 4, and the thermoelectric element control circuit 4 is connected to the thermoelectric element control circuit 13.

Next, the operation will be described. In FIG. 1, R _Ta is the resistance value of the thermistor 14a, and R _Tb is the thermistor 1.
4b is the resistance value. Further, R ₁ is the resistance value of the resistor 11a, and R ₂ is the resistance value of the resistor 11b. Here, general characteristics of the thermistor will be described. For example, assuming that the temperature (absolute temperature) detected by the thermistor is T, the resistance value R _{T of the} thermistor is expressed by the following equation.

[0026]

[Equation 6]

Here, T ₀ is an arbitrary reference temperature, R ₀
Is the thermistor resistance when the temperature is T ₀ , and B is a constant. From this equation, the thermistor resistance variation ΔR _{T with} respect to the minute temperature variation ΔT is expressed by the following equation.

[0028]

[Equation 7]

Using this relationship, the output voltage fluctuation ΔV _O of the minute temperature change detecting circuit 12 due to the fluctuation of the thermistor detected temperature as shown in the first embodiment is calculated as follows.

[0030]

[Equation 8]

Here, S ₂ is a constant. On the other hand, as described above, the condition for keeping the temperature T _LD of the laser diode stable is to satisfy the relationship of the temperature stability condition expression (10). Here, the temperature stability condition expression (10) and the expression (1) representing the output voltage fluctuation ΔV _O of the minute temperature change detection circuit 12
Comparing 4), it can be seen that the form of the formula in () of formula (14) is the same as the temperature stability condition formula (10). Here, the resistance values R ₁ and R ₂ of the resistors 11a and 11b are set to K
Set to be equal to. By setting the output voltage fluctuation ΔV _O of the minute temperature change detecting circuit 12 thus set to 0, the temperature stability condition (10) is satisfied and the temperature T _{LD of the} laser diode is kept stable. Specifically, the thermoelectric element control circuit 13 switches the magnitude and polarity of the current flowing through the thermoelectric element 4 according to the magnitude and sign of the output voltage fluctuation ΔV _O. However, while the sign of the expression (9) shown in the first embodiment is +, the expression (1
Since the sign of 4) is −, the polarity when connecting the thermoelectric element 4 to the thermoelectric element control circuit 13 needs to be connected reversely as compared with the case of the first embodiment shown in FIG. .

Although FIG. 2 shows the structure in which the thermistors 14a and 14b are arranged on the two left sides of the bridge of the minute temperature change detecting circuit 12, the thermistors 14a and 14b are arranged on any two sides of the four sides of the bridge. Even if it does, the above-mentioned relationship can be obtained. Further, the thermistors 14a and 14b having the same tendency of temperature characteristics are used.

Embodiment 3 FIG. 3 is a structural explanatory view of still another embodiment of the laser diode device according to the present invention. Here, a case where the laser diode device is applied to the laser diode module shown in FIG. 11 will be described as an example. In the figure, 14
Reference numeral c is a thermistor which is one of temperature detecting means arranged at two places on the chip carrier 3 of the laser diode module shown in FIG. 11, and the electric resistance exponentially decreases as the temperature rises. The NTC thermistor 15 is another one of the temperature detecting means arranged at two places on the chip carrier 3 of the laser diode module shown in FIG. 11, and its electric resistance is exponential as the temperature rises. PTC thermistor increasing to 11
Reference characters a, 11b, and 11c are resistors. NTC thermistor 1
4c and PTC thermistor 15 are connected in series,
It constitutes one side of a four-sided bridge circuit. The other three sides of the bridge circuit are resistors 11a, 11b, 1
1c. Resistance 1 of this bridge circuit
A constant voltage V _I is applied between the contact between 1a and 11c and the contact between the PTC thermistor 15 and the resistor 11b.
Contact between TC thermistor 14c and resistor 11c and resistor 11
a, the voltage difference V _O between the contacts between 11b is output. 1
Reference numeral 2 is a circuit for detecting a minute temperature change composed of these NTC thermistor 14c, PTC thermistor 15 and resistors 11a, 11b, 11c, and 13 is a thermoelectric element control to which the output V _{O of the} circuit 12 for detecting a minute temperature change is inputted. circuit,
Reference numeral 4 is a thermoelectric element connected to the thermoelectric element control circuit 13.

Next, the operation will be described. In FIG. 3, R _Tc is the resistance value of the NTC thermistor 14c, and Rp is the PTC.
The resistance value of the thermistor 15. R ₁ is a resistor 11
a is the resistance value, R ₂ is the resistance value of the resistor 11b, and R ₃ is the resistance 1
The resistance value is 1c. Here, NTC thermistor 14c
Let T _{1 be} the temperature detected by the sensor and ΔT ₁ be the minute fluctuation thereof, and the minute fluctuation ΔR _Tc of the resistance of the NTC thermistor 14c.
Is expressed by the following equation as described in the second embodiment. ΔR _Tc = −α _Tc · ΔT ₁ (15) where α _Tc is a temperature coefficient represented by the following equation. α _Tc = B / T ₀ ² (16) On the other hand, assuming that the temperature detected by the PTC thermistor 15 is T ₂ , the PTC thermistor resistance value generally changes linearly with temperature.
The resistance value Rp of is expressed by the following equation. Rp = α _p · T ₂ (17) where α _p is a temperature coefficient. Further, from this, the minute fluctuation ΔRp of the resistance with respect to the minute fluctuation ΔT ₂ of the detected temperature is expressed by the following equation. _{ΔRp = α p · ΔT 2 ...} (18) Using these relationships, small temperature change detecting circuit 12 due to the variation of the thermistor detection temperature as shown in Example 1
When the output voltage fluctuation ΔV _O is calculated as follows.

[0035]

[Equation 9]

Here, S ₃ is a constant. On the other hand, as described above, the condition for keeping the temperature T _LD of the laser diode stable is to satisfy the relationship of the temperature stability condition expression (10). Here, the temperature stability condition expression (10) and the expression (1) representing the output voltage fluctuation ΔV _O of the minute temperature change detection circuit 12
Comparing 9), it can be seen that the form of the formula in () of formula (19) is the same as the temperature stability condition formula (10). Here, the temperature coefficient α _Tc of the thermistor 14c and the temperature coefficient α _p of the posistor 15 are set so that their ratio becomes equal to K.
By setting the output voltage fluctuation ΔV _O of the minute temperature change detecting circuit 12 thus set to 0, the temperature stability condition (10) is satisfied and the temperature T _{LD of the} laser diode is kept stable. Specifically, by the thermoelectric element control circuit 13,
This is performed by switching the magnitude and polarity of the current flowing through the thermoelectric element 4 according to the magnitude and sign of the output voltage fluctuation ΔV _O.

The above-mentioned relationship is based on N connected in series.
It can be obtained when the TC thermistor 14c and the PTC thermistor 15 are arranged on any side of the bridge of the circuit 12 for detecting a minute temperature change.

[0038]

According to the present invention, the fluctuation of the output voltage of the bridge circuit can be made to correspond to the temperature fluctuation of the laser diode. Therefore, by controlling the current flowing into the thermoelectric element based on the output of the bridge circuit, the laser can be controlled. The temperature of the diode can be controlled.

[Brief description of drawings]

FIG. 1 is a structural explanatory view of a first embodiment of a laser diode device of the present invention.

FIG. 2 is a structural explanatory view of a second embodiment of the present invention.

FIG. 3 is a structural explanatory view of a third embodiment of the present invention.

FIG. 4 is a diagram illustrating a configuration of a conventional laser diode module.

FIG. 5 is an explanatory diagram of temperature control of a conventional laser diode module.

FIG. 6 is a diagram showing a temperature distribution on a chip carrier of a conventional laser diode module.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser diode 2 Temperature detection means 3 Chip carrier 4 Thermoelectric element 5 Optical fiber 6 Dual in-line type airtight container 7 Electric terminal 8 Lead wire 9 Relay electrode 10 Arrow showing heat flow 11 Resistance 12 Micro temperature change detection circuit 13 Thermoelectric element control Circuit 14a Thermistor 14b Thermistor 14c NTC Thermistor 15 PTC Thermistor

Claims (2)

[Claims] 1. A laser diode device comprising a laser diode, a chip carrier provided with the laser diode, and a thermoelectric element for mounting the chip carrier and for varying the temperature of the laser diode. The resistance-type temperature detecting means provided at two positions are respectively arranged as resistance elements of two circuit branches of a bridge circuit having four contact points and four circuit branches each provided with a resistance element. A constant voltage is applied between a pair of opposing contacts of the contacts, and a temperature change detection circuit that outputs the voltage difference between the other pair of opposing contacts is provided. A laser diode device, wherein the temperature of the laser diode is controlled by controlling a current flowing into the thermoelectric element based on the above. 2. A laser diode device comprising: a laser diode; a chip carrier provided with the laser diode; and a thermoelectric element mounted with the chip carrier for varying the temperature of the laser diode. Of the NTC thermistor whose electric resistance exponentially decreases as the temperature rises, and the electric resistance exponentially increases of P as the temperature rises.
A TC thermistor is provided, and the above-mentioned NTC thermistor and PTC
A thermistor is arranged in series as a resistance element of one circuit branch of a bridge circuit having four contact points and four circuit branches each having a resistance element, and one of the four contacts is set. Is equipped with a temperature change detection circuit that applies a constant voltage between the opposite contacts of the pair and outputs the voltage difference between the other pair of opposite contacts, and flows into the thermoelectric element based on the output of the temperature change detection circuit. A laser diode device characterized in that the temperature of the laser diode is controlled by controlling the current to be applied.