KR20070041686A - Wavelength Temperature Tuning of Semiconductor Lasers with Variable Thermal Impedance - Google Patents

Abstract

A device is disclosed in which the temperature of a semiconductor laser (or other device) can be set to a desired value by using heat generated by the laser itself in conjunction with an adjustable thermal impedance heat sink to achieve a desired temperature rise.

Description

TEMPERATURE TUNING THE WAVELENGTH OF A SEMICONDUCTOR LASER USING A VARIABLE THERMAL IMPEDANCE

This application claims the priority of US Provisional Application 60 / 570,562 (Jan Lipson, May 14, 2004) under USC 119 (e) “apparatus for temperature tuning wavelengths of semiconductor lasers using variable thermal impedance”.

The present invention relates to temperature control of devices, mainly by manual mechanisms, which can be used to adjust their wavelength, for example by changing the temperature of a semiconductor laser.

The wavelength of the semiconductor laser has been tuned by controlling the change in temperature. Two common means of changing the temperature are thermoelectric coolers and resistive heaters. Thermoelectric coolers typically use the Peltier effect. Such a device may be heated or cooled depending on the direction of the current flowing through the Peltier element. Resistive heaters are resistors that convert current into heat.

In either case, the temperature change method consumes a lot of power. For thermoelectric coolers, the Peltier effect has limited efficiency, and the power dissipation required for cooling is usually several times the power consumed by the laser. When a thermoelectric cooler is used as a heater, it is almost as efficient as resistance. While heating may be more efficient at consuming power than cooling, there is still a need to supply large amounts of power. This is because it is generally not desirable to operate the laser at high temperatures. In order to prevent this operation, the laser is generally mounted so that the thermal impedance is small with respect to a suitable heat sink. As a result, in order to obtain a high additional temperature from a heater located near the laser, it is necessary to generate a significantly larger amount of heat than is generated by the laser. If the heater is located downstream of the laser in the heat flow, the situation is even worse, in which case the thermal impedance for the heat sink is smaller and more heat must be generated for the effect of the same temperature rise.

The additional power that must be provided for temperature tuning cannot be tolerated in situations where power consumption is critical, for example if the device is operated for a long time with a battery of a very small size.

In addition to the problem of power consumption, it can be physically inconvenient to install a heater near the laser. If cost is a serious consideration and bulky standard parts such as compact disk (CD) lasers are to be used, it may be necessary to break and open the packaging to add a heater. Accordingly, there is a need for an approach that can be easily purchased and that allows the temperature of the laser to be adjusted without compromising packaging.

US Patent No. 5,372,753 discloses a device in which the thermal impedance of the laser diode heat sink varies during the turn-on cycle of the laser. This variation is achieved by using a metal structure and an air gap to open the metal structure by its dimensions while the laser is warming up and to close it when the temperature reaches a desired value range. The closure is caused by reversible deformation occurring at temperature changes such as that obtained with a shape memory alloy. While this approach allows the laser to reach a specific temperature range, there is no way that the farthest temperature reached can vary widely or be accurately controlled. It is not possible to adjust the thermal impedance during device operation. During the manufacture of the device this value is fixed and thus the temperature range is also fixed.

U. S. Patent No. 6,243, 404 discloses a laser module, which can be tuned over the entire temperature range and different temperature ranges can be selected during manufacture. The temperature range can be selected by inserting a known thermal impedance spacer between the laser and the furthest heat sink so that the laser temperature can be raised by a known amount fixed above ambient. Next, a secondary control mechanism such as a thermoelectric cooler can adjust the laser temperature in the range near the base laser temperature. In this approach, the thermal impedance (and the underlying laser temperature) is selected at the time of device fabrication and then fixed. The temperature can then be further adjusted through changes in the thermal impedance, and this approach relies on the heating or cooling structure described above to achieve the required temperature control during device operation due to the inefficiency of the result.

Thus, there is a need to control the temperature of devices such as semiconductor lasers to consume less energy than current approaches.

The above and other limitations are addressed by the present invention, in which the laser (or other active device) prioritizes the heat generated in advance by the laser in conjunction with a heat sink whose thermal impedance can be adjusted. Discloses a device that can be temperature tuned using.

In one application, this approach is used to controllably tune the wavelength of a semiconductor laser. The heat generated by the laser itself is used as a byproduct of the laser's emission to raise the laser's temperature by an adjustable amount using variable thermal impedance. Thereby, the wavelength of the laser is adjusted to a desired value by variation of temperature and wavelength.

In one embodiment, the entire heat sink has two parts. One part is thermally coupled to the laser and the other part is properly coupled to the surroundings. The two parts can be thermally bonded or separated from each other and combined for an average time determined by the desired heat flow. Latched relays can be used to achieve the desired coupling / disconnection at low additional power consumption. If the two parts of the heat sink are not coupled to each other, the heat generated by the laser itself causes the temperature of the semiconductor laser to rise. When the desired temperature is reached, the two heat sink portions combine with each other during a time division suitable to provide an average thermal impedance, to stabilize the temperature at the desired point in conjunction with the laser heat.

As another alternative design method, the two heat sink portions are in thermal contact with each other, but their contact area is adjustable to change the overall thermal impedance. For example, the contact area can be determined by the deformable material or liquid, which can be adjusted to change the contact area.

Secondary heaters (eg, resistive heaters) may be installed in the laser assembly to shorten the time required to reach higher temperatures. Since the heater is turned on only during warming up (or when it is necessary for another case to significantly increase the temperature) and then off, this method can be done with little energy loss.

In addition, other control mechanisms may be used. In one approach, a temperature sensor is installed in the laser assembly so that the system can be operated as a closed loop control, and the controller is coupled to the temperature sensor to adjust the heat impedance of the heat sink to maintain a given temperature. As another alternative approach, wavelength sensors are used to provide direct feedback from the laser's output. The controller adjusts the heat impedance of the heat sink to maintain a given wavelength. The controller can also operate the heat sink in an open loop manner.

One advantage of this approach is that temperature control is primarily passive (ie without using an external active heater or cooler). Therefore, power consumption is low. In addition, in certain embodiments, it may be tuned over the entire temperature range, and in the example of a semiconductor laser, the semiconductor laser may be tuned over the full range of wavelengths.

Other aspects of the invention include methods corresponding to such devices and systems, and may be applied to devices other than semiconductor lasers, and for purposes other than wavelength tuning.

BRIEF DESCRIPTION OF DRAWINGS To describe the above-mentioned advantages and features of the present invention and how other advantages and features can be achieved, the more specific description of the invention briefly described above refers to specific embodiments shown in the accompanying drawings. Will be done. These drawings represent exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, for the purpose of describing the invention in more detail using the accompanying drawings.

1 shows an apparatus in which an adjustable thermal impedance is obtained by intermittent contact between different parts of a heat sink.

FIG. 2 shows a device for continuously varying thermal impedance using deformable materials.

1 shows a device in which an adjustable thermal impedance is achieved by intermittent contact between different parts of a heat sink. In this example, the heat sink is implemented in two parts, the laser heat sink 110 and the second heat sink 120. The semiconductor laser 150 is in contact with the laser heat sink 110, but is insulated from the housing 160 such that heat flow is primarily through the heat sink. The heat sink 110 may be thermally coupled to the second heat sink 120 by an actuator 170 driving the two parts together. In one design, actuator 170 is a solenoid.

1 illustrates the case where the laser heat sink 110 is deflected onto the second heat sink 120 when the actuator 170 is extended, but in another alternative approach, the position of the laser heat sink 110 is fixed. The actuator 170 is used to move the second heat sink 120 to the contact area.

In one design, the laser heat sink 110 is part of an integrated assembly that includes conductive traces 115 to set up useful circuitry. As one approach, these circuits 115 preferably include a connection point to a temperature sensor 130 that senses the temperature of the semiconductor laser 150. Using the temperature obtained from the sensor 170, the control circuit 180 can quickly reach the desired temperature and perform an appropriate operation to stabilize the temperature at that point.

The control circuit 180 performs an operation of determining when the heat sinks 110 and 120 are in contact and for how long. The temperature rise at the equilibrium state of the laser 150 is governed by the following relationship.

ΔT = ZH

Here, ΔT is the temperature rise of the laser 150 above the temperature of the second heat sink 120 (assuming that it is at a constant temperature in this example), and Z is the effective temperature from the laser 150 to the heat sink 120. Thermal impedance and H is heat exhausted by the laser 150 in equilibrium. Here, it is assumed that the thermally conductive parallel path is negligible in design, as is generally the case where heat is not conducted through the laser heat sink 110.

Two parts of the heat sink 110 and 120 have a total time t _t If connected during time t ₁ , the effective thermal impedance is approximated as follows:

Z = t _t / t ₁ Z _c

Z _c is the thermal impedance from the laser 150 to the second heat sink 120 when the actuator 170 connects the two portions 110 and 120. By adjusting the connection time t ₁ (or duty cycle t ₁ / t _t ), the desired value of Z can be obtained such that the temperature rise ΔT is the desired value.

The time t _t is preferably selected to limit the temperature variation of the laser assembly from the desired temperature for both the connected time t ₁ and the unconnected time t _t -t ₁ . During this time fluctuations occur because the thermal impedance is being adjusted from the average impedance. The laser assembly consists of a combination of materials having different masses and heat capacities. However, it can be considered to have a thermal mass that is the sum of the product of the respective mass and the appropriate heat capacity of the material. The assembly will rise in temperature for a given time by an amount proportional to the laser heating (H) and inversely proportional to the thermal mass of the entire assembly. If the temperature fluctuation is required to be lower than the set value T _v , the time t _t -t ₁ without the heat sink connected must be less than the time required for the assembly to rise in temperature by T _v .

The second criterion for setting the time t _t may result from the observation that the thermal impedance will be lower than the average near the end of the connection period because the thermal impedance is smaller than the desired average during the time interval t ₁ . However, in the preferred embodiment, if substantially all of the heat flows between the two heat sinks 110 and 120, the temperature fluctuations during the disconnection period will be greater than the fluctuations during the connection period. This is because the thermal impedance of the disconnected period is assumed to be very large by proper insulation. Thus, criteria based on fluctuations during the non-consolidation period will generally be more convincing.

Actuator 170 may be selected to be a relay-type electrical device in which a suitable current is supplied to make a connection and a second appropriate current is supplied to make a non-connection. In a preferred embodiment, the relay latches actuator 170 in a position reached due to the application of current. In such a case, the current may be stopped and no more power is needed to maintain the position of the actuator 170. When the actuator 170 needs to return to its initial position, a second current is applied to release the latch.

The relay may be based on a conventional solenoid, but is not limited thereto. Other types of actuators 170 that have basically the same mechanical function may also be used. For example, it may include a MEMS actuator, a piezoelectric actuator, a motor driven actuator, and other actuators that produce the required deflection and force. The average power consumption is preferably less than or equal to that of the laser. Since the present invention is applicable to many different power lasers, the choice of actuator depends in part on the laser. However, conventional solenoidal relays generally operate well over a wide range of laser selections and can be inexpensive.

As shown in FIG. 1, a secondary heater 155 may advantageously be employed. The secondary heater 155 may also be useful for power to which power is applied to heat the semiconductor laser 150 more quickly through the conductive traces 115 to a desired temperature. In a preferred embodiment, the heater 155 is comprised of a resistance through which a current flows. The resistive heater 155 may be used to quickly heat the laser assembly to a desired temperature in a short time compared to the time required by the laser heater alone to perform this function. After reaching the desired temperature, secondary heater 155 may be turned off. If the thermal mass of the laser assembly is not too large, the energy damage caused by using a secondary heater is small.

The heater 155 is secondary in that temperature control is primarily achieved by a heat sink having an adjustable thermal impedance. For example, the secondary heater 155 is used at initial heating (eg during startup) or as a supplement to heating by the laser (eg when the laser is moved from one operating temperature to another operating temperature). In addition, the control circuit 180 may perform an operation of applying a current to any secondary heater 155.

Advantageously, the laser heat sink 110 can be selected to be a flexible circuit with a copper ground plate. Heat is conducted through a copper layer of sufficient width and thickness with low thermal impedance over the design length, and electrical circuits can be installed using conductive traces 115 on an insulating layer that is part of a flexible circuit. This allows the temperature sensor, any secondary heater, laser bias current connector, and any monitor photodiode to be connected directly within the laser assembly.

The connection between the control circuit 180 and the laser heat sink 110 should be chosen so that its thermal impedance is large. This can be done if the traces used to install the circuit are the minimum width and thickness needed to maintain the required current. In order to make an electrical connection between the trace 115 and the control circuit 180, it is advantageous to use wires of as small dimensions as possible. In order to reduce heat flow, the laser heat sink 110 itself is preferably not attached to the control circuit 180.

In the example of FIG. 1, the temperature sensor 130 is advantageously installed on the laser assembly. The sensor can be a thermistor or semiconductor device that generates a voltage from which the temperature can be determined. Alternatively, the wavelength of the laser can be measured directly by some kind of wavelength sensor and used as feedback of the control circuit 180 so that the wavelength of the laser can be set to a predetermined target value.

Advantageously, the thermal insulator can be selected to be glass, and in a more preferred embodiment, it can be selected to be substantially glass filled with air bubbles.

An example based on a semiconductor laser which consumes 200 mW of thermal energy during normal operation and has a wavelength / temperature coefficient of 0.25 nm / ° C will be described. Consider a case where it is required to tune the laser by 5 nm, corresponding to a temperature rise of 20 ° C. The laser heat sink 110 is selected to have a thermal impedance of about 20 ° C./W when connected to the second heat sink 120. Therefore, if always connected, the laser temperature rise will be 4 ° C. To achieve this thermal impedance, a copper heat sink having a thickness of 20 mm long by 10 mm wide by 0.25 mm may be employed, but direct heat flow calculations may be sufficient to have other possible dimensions or materials. The dimensions and materials of the insulation can be selected using conventional heat flow calculations to obtain a thermal impedance of at least 120 / ° C., preferably at least 200 ° C./W. In this case, assuming that all other thermal paths can be ignored, 200 mW laser heating may cause the laser assembly to rise by at least 20 ° C. above the temperature that would be reached if the heat sink was always connected. Thus, in this example, the laser wavelength can be expected to vary by at least 5 nm between when the heat sink is always connected and when not at all.

When the actuator 170 forces the connection point so that excessive thermal impedance is not added by the limited contact area, an appropriate contact area between the two heat sinks 110 and 120 is desirable. In this example, if the second heat sink 120 is also copper, a contact area of 1 mm ² is sufficient.

If a secondary resistive heater is used in the assembly, it is desirable to design the device to provide at least as much heat as laser to have a significant impact on warm up time. In the above example, a 500 mW heater would be a suitable choice. For a current of 100 mA, the resistance will be 50 Ω.

The heat sink 110 is elastically deflected when connected to the actuator 170 and is required to return to its normal position when the contact is terminated. In this example, a deflection of about 1 mm is sufficient, and the actuator 170 may be selected to have a deflection of about 2 mm. In this case, the actuator 170 need not be in contact with the heat sink except as required, and there will be no need for heat to flow between the two heat sinks 110 and 120 if the actuator 170 is not extended. .

In a preferred embodiment, the control circuit 180 allows the temperature to be closed loop controlled. The circuit includes a device that reads the output of the temperature sensor 130 and appropriate circuitry to ensure the time required to connect the heat sinks 110 and 120. In a preferred embodiment, a microprocessor or application specific IC with computing power may be used to perform the function of setting the connection time. There are many possible control algorithms to achieve the desired temperature. One satisfactory method for this is to predict the division of time that heat sinks should be connected based on typical temperature fluctuations as a function of connection time for a sample of a suitable assembly. Individual variability of the assembly can be compensated for by the microprocessor, which will make adjustments to the division of the connected time depending on whether the temperature measured from the sensor is high or low. In order to minimize the time required, it is desirable that the heat sinks 110 and 120 be separated until the laser reaches near the set point.

In FIG. 1, the control circuit 180 also includes a device for driving the actuator 170. If a secondary heater is employed, another device for supplying current to the heater is added to the control circuit.

Rather than using the time averaging technique described above, it is possible to implement a control structure in which thermal impedance is continuously adjustable over a range. 2 shows an example of achieving this by adjusting the contact area between the laser heat sink 110 and the second heat sink 120, thereby providing an adjustable force between the two. As shown in FIG. 2, a deformable material 210 is located between the two heat sinks 110 and 120 such that a large change in the contact area can be advantageously achieved. Conveniently, this material may advantageously be selected as a solid or a liquid having a low modulus of elasticity. In the case of liquid, it is necessary to control the distance, not the force, between the laser heat sink 110 and the second heat sink 120. Advantageously, the liquid can be selected as oil or mercury. Suitable deformable solids can be obtained in the form of spheres. Selection of suitable materials may include aluminum or copper.

The particular choice of laser assembly, wavelength and wavelength range, shape elements, shape of heat sink, etc. will depend in part on the application. One class of possible applications is Raman Spectroscopy. In this application, different wavelength measurements may be useful to accurately extract the background of fluorescence and the measurement artifacts. In one embodiment, a laser wavelength of 830 nm using a semiconductor laser is used. Advantageously, the laser is installed in a TO-header type package. Advantageously, the laser can be stabilized with a precision of less than 0.2 nm, and / or finely tuned by about 1 nm, so that subtraction can be made in the spectrum before and after tuning.

Another application is optical communication. For example, the semiconductor laser may have a wavelength near 1550 nm and may advantageously be installed in a TO-header type package. In dense wavelength division multiplexing systems, the typical interchannel spacing is about 0.8 nm. The above approach can be used to control the laser with more precise precision than the channel spacing, potentially with great advantages in power consumption. In smaller packaging structures that are widely used, this is very advantageous.

Although many details are included in the detailed description, they should not be construed as limiting the scope of the invention, but as merely indicating other examples and aspects of the invention. It is to be understood that the scope of the present invention includes other embodiments which are not described above. As defined in the appended claims, various modifications that may be apparent to those skilled in the art in the construction, operation, and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the present invention. Other variations, modifications, and alterations may be made. Accordingly, the scope of the invention should be determined as that of the appended claims and their legal equivalents.

Claims (25)

In the device for tuning the wavelength of a semiconductor laser, A semiconductor laser whose wavelength of the light beam produced by the semiconductor laser is varied as a function of the temperature of the semiconductor laser; A heat sink thermally coupled to the semiconductor laser, the heat sink having an adjustable thermal impedance; And And a controller to adjust the thermal impedance of the heat sink such that the wavelength of the light beam is maintained at a given wavelength. The method of claim 1, Wherein the temperature of the semiconductor laser is determined primarily by the heat generated by the semiconductor laser and the thermal impedance of the heat sink. The method of claim 1, The semiconductor laser is not actively heated or cooled by an external device to maintain the wavelength of the light beam at a given wavelength. The method of claim 1, The heat transfer generated by the semiconductor laser is mainly generated through the heat sink. The method of claim 1, The heat sink is, A first heat sink thermally coupled to the semiconductor laser; And A second heat sink that is switchable to be thermally coupled to or separated from the first heat sink, And the controller adjusts a thermal impedance of the heat sink by adjusting a duty cycle of thermal coupling of the second heat sink to the first heat sink. The method of claim 5, And an actuator for forming and releasing mechanical contact between the first heat sink and the second heat sink to thermally couple or disconnect the first heat sink and the second heat sink to be switchable. . The method of claim 6, The actuator includes a solenoid. The method of claim 7, wherein And the solenoid is latched in a contact position and / or a noncontact position. The method of claim 1, The heat sink is, A first heat sink thermally coupled to the semiconductor laser; And A second heat sink in thermal contact with the first heat sink, Wherein the region of thermal contact is continuously adjustable over a series of regions, the controller adjusting the thermal impedance of the heat sink by adjusting the region of thermal contact. The method of claim 9, And a deformable material forming the area of thermal contact. The method of claim 10, The deformable material is a liquid. The method of claim 10, The deformable material is spherical in shape. The method of claim 1, The heat sink is, A second heat sink; And A first heat sink configured to thermally couple the semiconductor laser to the second heat sink, On the other hand, the first heat sink is thermally insulated by glass partially filled with air. The method of claim 1, The heat sink is, A first heat sink thermally coupled to the semiconductor laser; And A second heat sink that is adjustable thermally coupled to the first heat sink; And the first heat sink portion is part of an integrated assembly that also includes conductive traces for electrically connecting to the semiconductor laser. The method of claim 14, And the first heat sink portion comprises a copper layer of a flexible circuit. The method of claim 1, Wherein the wavelength of the light beam is adjustable over the entire range of wavelengths. The method of claim 16, Wherein the wavelength ranges over at least 1.0 nm. The method of claim 1, It further comprises a secondary heater, The controller activates the heater for initial heating of the semiconductor laser. The method of claim 1, It further comprises a secondary heater, And the controller activates the heater for supplemental heating of the semiconductor laser. The method of claim 19, The secondary heater includes a resistive heater. The apparatus of claim 1, wherein the controller comprises a processor. The method of claim 1, Further comprising a temperature sensor for sensing the temperature of the semiconductor laser, The controller is coupled to the temperature sensor to adjust the thermal impedance of the heat sink such that the temperature of the semiconductor laser is maintained at a temperature corresponding to a given wavelength. The method of claim 1, Further comprising a wavelength sensor for sensing the wavelength of the light beam, The controller is coupled to the wavelength sensor to adjust the thermal impedance of the heat sink such that the wavelength of the light beam is maintained at a given wavelength. In the apparatus for controlling the temperature of a semiconductor laser, Semiconductor lasers; A heat sink thermally coupled to the semiconductor laser, the heat sink having an adjustable thermal impedance; And And a controller to adjust the thermal impedance of the heat sink such that the temperature of the semiconductor laser is maintained at a given temperature. The method of claim 24, Wherein said given temperature is adjustable over a temperature range of at least 1 ° C.

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