Process for maintaining a desired temperature of a laser

Abstract: A microprocessor controlled electronic subsystem for monitoring either a thermoelectric cooler (20) or a laser diode and adjusting the thermoelectric cooler to compensate for temperature effects. The microprocessor (24), with the aid of a temperature sensor (66), monitors the current temperature of the thermoelectric cooler or laser diode and then chooses an appropriate target value to minimize the amount of time (Fig. 3) needed to drive the thermoelectric cooler or the laser to the appropriate temperature. The microprocessor contains software whose purpose is to choose the appropriate target value in order to fine tune the control response of the entire system. The system also includes the ability to pick a new target value if the environment or ambient temperature is beyond designed for values.

Description: This invention relates to a process according to the preamble of claim 1. Such a process is derivable from the prior art as shown in D. Li et al, "A scanning Temperature Control System for Laser Diodes", in Measurement Science and Technology, Vol. 4, No. 10, October 1, 1993, pp. 1111-1116. It is the primary aim of the invention to provide a method for quickly compensating for a variety of thermally induced effects. For solving this object, the present invention provides a process having the features set out in claim 1. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a linear servo controlled electronic circuit to monitor and correct for problems of laser drift; Figure 2 is a circuit diagram of a pulse width modulation electronic circuit to monitor and correct for problems of laser drift; Figure 3 is a temperature curve showing rising temperature of a thermoelectric cooler and Figure 4 is a flow chart showing a program in a microprocessor in the electronic circuit shown in Figure 1. Turning now to Figure 1, a circuit diagram of an electronic circuit to monitor and correct for problems of laser drift by monitoring temperature of a thermoelectric cooler is shown. A thermoelectric cooler 20 is used to stabilize the temperature of a laser diode (not shown). In order to more accurately stabilize the temperature the thermoelectric cooler 20 must be monitored and adjusted. A stabilizing circuit 22 is used to perform this function. A microprocessor 24 outputs a digital target value on 8-bit bus 26. The target value is then received by a digital to analog converter 28. After conversion to an analog signal the target value is passed to a thermoelectric cooler circuit 32 on line 30. The thermoelectric cooler circuit 32 has 3 operational amplifiers. Two of the operational amplifiers 34, 38 are used in the configuration to provide forward and feedback time constants, respectively, for the thermoelectric cooler circuit 32. The third, power operational amplifier 36 is used to provide the correct power output to the thermoelectric cooler 20. The three operational amplifiers 34, 36, 38 are all configured to work in inverting mode. Each operational amplifier 34, 36, 38 is also connected in parallel with a resistor 40, 42, 44 respectively. Additionally, operational amplifier 34 is also connected in parallel with a capacitor 46. Each of the three operational amplifiers 34, 36, 38 also has a resistor 48, 50, 52 respectively connected at its input. Each of the three operational amplifiers 34, 36, 38 also has a resistor 50, 54, 56 respectively connected at its output. The elements for the thermoelectric cooler circuit 32 are completed by including two capacitors 58, 60 connected to operational amplifier 58. Capacitors 58, 60 provide power supply filtering functions to the power operational amplifier 36. The capacitor 46 and resistors 40, 48 provide the forward time constant function for the thermoelectric cooler circuit 32. The capacitor 46 and resistors 40, 56 provide the feedback time constant for the thermoelectric circuit 32. An alternative embodiment, shown in Figure 2, uses a pulse width modulated electronic circuit. A thermoelectric cooler 70 is used to stabilize the temperature of a laser diode (not shown). To more accurately stabilize the temperature, the thermoelectric cooler 70, a stabilizing circuit 72 is used. In stabilizing circuit 72, a microprocessor 74 outputs a driving pulse width voltage on line 76. The driving voltage is received by buffer 78 and passed along line 80 to a switch 82. The driving voltage is then passed along line 84 to the thermo electric cooler 70. The stabilizing circuit 72 also includes a feedback path comprising a sensor 86 connected to the microprocessor 74 through line 88. The forward time constant provided by the capacitor 46 and resistors 40, 48 and the feedback time constant provided by the capacitor 46 and resistors 40, 56 in thermoelectric circuit 32 of Fig. 1, is provided in the pulse width modulation circuit of Figure 2 within the microprocessor 73. Returning to Fig. 1, the operation of the stabilizing circuit 22 may be described as follows. When the analog target value is passed to a thermoelectric cooler circuit 32 on line 30 it first encounters resistor 48. Resistor 48 is connected to the convertor 28 on one end and the parallel configuration of the operational amplifier 34 with resistor 40 and capacitor 46. Also connected to the same end as the operational amplifier 34, resistor 40, and capacitor 46 is resistor 56. Resistors 48, 40 along with capacitor 46 form an RC circuit with an RC time constant which provides a delay to compensate for the reaction time of thermoelectric cooler 20 and stabilizes the feedback portion of the thermoelectric cooler circuit 32. Operation amplifier 34 generates a time delayed signal which is then passed along line 62 to resistor 50. This function would be performed within the microprocessor 72 in the pulse width modulation circuit shown in Figure 2. Resistor 50 is connected between the parallel configuration of operational amplifier 34 with capacitor 46 and resistor 40 and the parallel configuration of operational amplifier 36 with resistor 42. Resistor 42 and resistor 50 work in concert with operational amplifier 36 for gain control. The operational amplifier 36 is then used to drive the thermoelectric cooler 20. Resistor 54 is inserted between the output of the operational amplifier 36 and the input of thermoelectric cooler 20 and is used as a current limiting resistor to avoid overdriving the thermoelectric cooler 20. This function would be performed by the switch 82 in the pulse width modulation circuit 72 shown in Figure 2. The thermoelectric cooler circuit 32 also contains a feedback circuit 64 connected to a sensor 66 which senses the temperature of the thermoelectric cooler 20. Alternatively the sensor 66 could sense the temperature of the laser (not shown), or could be replaced with optical sensor to sense the power output of the laser The sensor 66 in Figure 1 then passes the sensed information to the resistor 52 and (via an A/D converter (now shown)) to the microprocessor 24 on line 68. The resistor 52 functions along with resistors 44, 56, 48, and 40 to form an RC time constant with capacitor 46 for the operational amplifier 38. Operational amplifier 38 and its RC time constant provide the feedback circuit 64 for the stabilizing circuit 22. The sensed information on line 68 is also sent to the microprocessor 24. This information is useful to microprocessor 24 as it controls the ramp up voltage or target voltage outputted by the microprocessor 24 on the 8-bit bus 26. When the unit is first turned on, the microprocessor 24 will output an initial target voltage, higher than the actual target voltage, which represents a temperature greater than the actual target temperature. This higher voltage will drive the thermoelectric cooler 20 harder causing it to change its temperature faster than if the actual target voltage temperature were used. As the thermoelectric cooler 20 approaches the actual target voltage temperature the microprocessor 24 will output a lower target voltage which represents a temperature closer to the actual target voltage temperature. This will continue until the microprocessor 24 outputs the actual target voltage temperature and the thermoelectric cooler circuit 32 has stabilized. In the pulsewidth modulation circuit shown in Figure 2, the sensor 86 passes the sensed information directly to the microprocessor 74. The function of the RC time constant is then performed within the microprocessor 74 according to a set of training rules contained within the microprocessor 74. This effect of the stabilizing circuit, whether it is the linear servo controlled electronic circuit shown in Figure 1 or the pulse width modulated circuit shown in Figure 2, is illustrated in Figure 3. Figure 3 is a graph of the temperature of the thermoelectric cooler 20 over time. A target temperature value T3 surrounded by a range R is shown. The target temperature value T3 is the desired temperature for the thermoelectric cooler 20. If the microprocessor 24 or 74 were to output a voltage value representative of target value T3, the thermoelectric cooler circuit 32 or 72 would drive the thermoelectric cooler 20 or 70 to that target temperature value T3 along a curve C1. If however, the microprocessor 24 or 74 outputs a driving voltage which represents a temperature that is much higher than the target temperature value T3 on the 8-bit bus 26, it can cause the thermoelectric cooler circuit 32 or 72 to climb at a much faster rate, as shown by the portion of another curve C2, as it appears between an initial temperature value T 0 and temperature value T 1 . If the thermoelectric cooler 20 or 70 were allowed to stabilize at this driving voltage then it would overshoot the target temperature T 3 . Once the thermoelectric cooler 20 or 70 has reached temperature value T1, that temperature will be sensed by the sensor 66 or 86 respectively and feed back to the microprocessor 24 or 74 respectively. The microprocessor 24 or 74 respectively will then output a new value. The new value will still be greater than the voltage that represents target temperature value T3 but less than the initial value. The temperature of the thermoelectric cooler 20 or 70 will change at a slightly slower rate as represented by the portion of curve C2 between temperature value T 1 and temperature value T 2 but still at a faster rate than it would have if a voltage that represented the target temperature value T3 were used. If the thermoelectric cooler 20 or 70 were allowed to stabilize at this driving voltage then it would overshoot the target temperature T3 although by less than the initial driving voltage. When the thermoelectric cooler 20 or 70 reaches a temperature value T 2 that temperature will again be sensed by the thermoelectric cooler sensor 66 or 86 respectively and the temperature information will be sent back to the microprocessor 24 or 74 respectively. When the microprocessor 24 or 74 respectively has received this information it will again change the value output. This time the microprocessor 24 or 74 will output a value representative of the target temperature value T 3 . The temperature of the thermoelectric cooler will change at a slower rate and finally stabilize at the target temperature value T3 as shown on the portion of the curve C2 between temperature value T 2 and target temperature value T 3 . As long as the temperature of the thermoelectric cooler 20 or 70 as sensed by the thermoelectric cooler sensor 66 or 86 respectively remains within range R the system is stabilized and the microprocessor 24 or 74 respectively will not change the value output. However, should the temperature of the thermoelectric cooler 20 or 70 change, the thermoelectric cooler sensor 66 or 86 respectively will sense the changed temperature and convey a signal indicating this to the microprocessor 24 or 74 respectively. The microprocessor 24 or 74 must then respond to correct the problem. The microprocessor 24 or 74 will output a new value that either represents a warming voltage or a cooling voltage depending on whether the thermoelectric cooler 20 or 70 is below or above the target temperature T 3 . The magnitude of the output voltage will depend on how far outside the range R the temperature of the thermoelectric cooler 20 or 70 has drifted. If the temperature of the thermoelectric cooler 20 or 70 is close to the target temperature value then only a small change is necessary. However, if the temperature of the thermoelectric cooler 20 or 70 has drifted into the temperature range represented between temperature value T 1 and temperature value T 2 for instance, then the microprocessor 24 or 74 must again derive a large enough value to quickly drive the thermoelectric cooler 20 or 70 towards the target temperature value T 3 . Then as the temperature of the thermoelectric cooler 20 or 70 approaches the target temperature value T 3 , the microprocessor 24 or 74 respectively will output a voltage more representative of the target temperature value T 3 , as discussed above. In all cases, whether the stabilizing circuit 22 or 72 has been turned on for the first time or is adjusting to changes in the thermoelectric cooler 20 or 70 respectively, the output value from the microprocessor 24 or 74 respectively will always be dependent on how large the difference is between the temperature of the thermoelectric cooler 20 or 70 respectively and the target temperature value T 3 . The microprocessor 24 or 74 utilizes a look up table contained in memory to choose an appropriate output value. It should be noted that this example uses only three discrete ranges of temperatures where the microprocessor 24 or 74 evaluates and chooses an output value. However, it will be appreciated by a skilled person that this use was for explanatory purposes only and any number of ranges or a continuous function could have been used in actuality. The software with the microprocessor for monitoring and correcting temperature deviations is represented by the flow chart in Figure 4. So long as the voltage (TECMON) received from the thermoelectric cooler sensor 66 is neither less than a specified minimum voltage (TECMIN) nor greater than a specified maximum voltage (TECMAX) the status of the thermoelectric cooler 20 is reported as fine and the microprocessor does nothing. However, when the voltage received from the thermoelectric cooler sensor 66 deviates from the specified range, the microprocessor retrieves a new output value (TECVAL).

Claims: A process of maintaining a desired temperature of a laser in an apparatus comprising: a laser, a thermoelectric cooler responsive to a driving signal and operatively connected to said laser for maintaining a desired temperature of said laser, a temperature sensor being so constructed and arranged to sense a current thermoelectric cooler temperature and output a current temperature signal, and means for storing at least two temperature range values and at least two temperature target value signals where each temperature range value has an associated temperature target value signal, one of the at least two temperature range values is a desired temperature range value, the desired temperature range value's associated temperature target value signal is a desired temperature target value signal which represents the desired temperature of said laser, the other one or more temperature range values are non-desired temperature range values and the non-desired temperature range values' associated temperature target value signals are non-desired temperature target value signals which represent temperatures overshooting the desired temperature of said laser, the process comprising the steps of: (a) sensing the current thermoelectric cooler temperature with said temperature sensor and outputting the current temperature signal, (b) comparing the current temperature signal to the at least two temperature range values and determining within which temperature range the current temperature signal falls, (c) retrieving the target value signal associated with the determined temperature range value, (d) converting the retrieved target value signal into the driving signal, (e) transmitting the driving signal to said thermoelectric cooler for operating said thermoelectric cooler, and (f) repeating the process steps of (a) to (e), if the determined temperature range value does not equal the desired temperature range value, until the determined temperature range value equals the desired temperature range value so that while the determined temperature range value does not equal the desired temperature range value the thermoelectric cooler is operated by a driving signal that is converted from a non-desired temperature target value signal to shorten the time for the thermoelectric cooler to reach the desired temperature range value by using a temperature target value signal which overshoots the desired temperature of the laser in comparison with using a temperature target signal which represents the desired temperature of the laser, characterized in that process step (c) utilizes a look-up table to choose the target value signal. The process of claim 1, wherein the one or more non-desired temperature range values represent temperature ranges lower than the desired temperature range value and the non-desired temperature target value signals represent temperatures higher than the desired temperature target value signal. The process of claim 1, wherein the one or more non-desired temperature range values represent temperature ranges higher than the desired temperature range value and the non-desired temperature target value signals represent temperatures lower than the desired temperature target value signal. The process of claim 2 or 3, wherein: there are at least two non-desired temperature range values, at least two non-desired temperature target values and the highest non-desired temperature target value signal is associated with the lowest non-desired temperature range value. The process of claim 3 or 4, wherein: there are at least two non-desired temperature range values, at least two non-desired temperature target values and the lowest non-desired temperature target value signal is associated with the highest non-desired temperature range value. The process of any of claims 2 to 5, wherein: there are at least three temperature range values, at least three temperature target values, and the temperature target value signals decrease as their respective temperature range values increase. The process of any of claims 1 to 6, wherein: there are at least three temperature range values, at least three temperature target values, and the temperature target value signals increase as their respective temperature range values decrease. The process of claim 1, wherein: (a) there are at least two non-desired temperature range values, (b) there are at least two non-desired temperature target values, (c) one of the at least two non-desired temperature range values represents a temperature range higher than the desired temperature range value and its associated non-desired temperature target value signal represents a temperature lower than the desired temperature target value signal, and (d) another of the at least two non-desired temperature range values represents a temperature range lower than the desired temperature range value and its associated non-desired temperature target value signal represents a temperature higher than the desired temperature target value signal.

Inventors: CAM KRISTINE, TO TIFFANY G, YEUNG VINCENT AU, CAM, KRISTINE, TO, TIFFANY G., YEUNG, VINCENT AU
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1996
Main classification: H01S3
IPC codes: H01S5, G05D23