1. Field of the Invention
The present invention relates to a temperature compensation device and method which allows an optical communication device to have a constant temperature characteristic regardless of the change of environmental temperature, and in particular a low heat-generating temperature compensation device and method using a proportion-integration-derivation (PID) control circuit.
2. Description of the Related Art
An arrayed waveguide grating (AWG) is an optical communication device that is principally used as a multiplexer/demultiplexer (Mux/DeMux) for multiplexing or demultiplexing a multi-wavelength optical channel in an optical communication system in wavelength division multiplexing (WDM) mode. A problem with optical communication devices such as the AWG is that they are sensitive to environmental circumstances and may therefore cause performance of an optical communication system to deteriorate.
To overcome such a problem, a temperature compensation device is utilized to maintain a constant temperature characteristic in such optical communication devices regardless of change in environmental temperature. Such a temperature compensation device may be embodied principally through use of a PID control circuit.
FIG. 1 is a block diagram illustrating a conventional temperature compensation device that uses a conventional PID control circuit.
As illustrated in FIG. 1, the conventional temperature compensation device comprises a temperature comparison unit 10, a PID control unit 20, a high electric current drive unit 30, a temperature control unit 40 and a temperature sensor 50.
The temperature comparison unit 10 compares a current voltage value Vcur based on the current temperature with a reference temperature Vref to yield as a difference an error voltage value Verr. The reference voltage value Vref is set directly or through a digital-to-analog converter (DAC) fed by a universal micro controller. Vcur is set by reading a resistance value of a temperature sensor attached to an optical communication device such as an AWG and converting the resistance value into the current voltage value Vcur.
The PID control unit 20 urgesVcur into conformity with the actual present temperature by outputting a PID voltage value Vpid to the high electric current drive unit 30. The PID control unit may incorporate a P control circuit, a PI control circuit, a PD control circuit, a PID control circuit or the like in accordance with the temperature variation characteristic of the optical communication device.
The high electric current drive unit 30 receives and amplifies Vpid to derive a “high temperature” representative current Iout which it then supplies to the temperature control device 40. As the high electric current drive unit, a power operational amplifier (op-amp) is generally used.
The temperature control device 40 uses Iout to control the temperature of the optical communication device. Operationally, the polarity of Iout is determined in the PID control unit 20 based on the polarity of the error voltage value Verr. If the error voltage value exhibits a positive value, positive electric current is supplied; whereas, if the error voltage value exhibits a negative value, negative electric current is supplied. Such a temperature control device uses a heater, thermal electric cooler (TEC), or the like. The heater functions to heat the optical communication device regardless of the polarity of Iout, and the TEC functions to heat or cool the optical communication device in accordance with the polarity of Iout.
The temperature sensor 50 functions to sense a temperature and is implemented, for example, as a thermistor or a resistive thermal detector (RTD). Respective characteristics of the thermistor and the RTD differ; the resistance of the latter increases with temperature, whereas the resistance of the former decreases as temperature increases.
Conventional temperature compensation devices having the aforementioned construction have several disadvantages. The power op-amp, which is used as the high electrical drive unit, has poor thermal efficiency and thus generates too much heat. Also, it almost invariably requires a heat sink, thereby increasing material costs and the volume of entire module.
In order to overcome these problems, a low heat-generating temperature compensation device which does not require a heat sink has been implemented by using a pulse width modulation (PWD) driver and a current rectifier circuit.
FIG. 2 is a block diagram illustrating a known low heat-generating temperature compensation device.
As illustrated in FIG. 2, the conventional low heat-generating temperature compensation device has a construction which is similar to that shown in FIG. 1, but a PWM driver 31 and a current rectifier circuit 32 substitute for the power op-amp. FIG. 3 illustrates the output characteristic of the high current drive unit 32 formed from the PWM driver 31 and the current rectifier circuit 32. The analog PID control unit 20 outputs a temperature compensation signal Vpid based on the error voltage value Verr (FIG. 3a). The PWM driver 31 amplifies Vpid to yield an amplified signal Ipwm (FIG. 3b). Ipwm is passed through the current rectifier 32 and changed to a direct current (DC) signal Iout (FIG. 3c).
However, the conventional temperature compensation device shown in FIG. 2 utilizes a proportioner (P), an integrator (I), a derivator (D) in a PID control circuit to maintain the error voltage Verr at zero. The conventional device therefore requires the use of a vast number of electric components such as a high performance op-amp, a resistor (R) and a capacitor (C). Such an analog PID control circuit therefore entails increased material costs and volumetric enlargement of temperature compensation modules. Furthermore, the properties of the electric components abruptly grow worse under an extreme environment and thus the entire performance of the temperature compensation module is apt to deteriorate.
As mentioned above, the error voltage value Verr in the temperature compensation unit is generally obtained by calculating the difference between the voltage value at the current temperature Vcur and the voltage value at the reference temperature Vref using an instrument op-amp. In general, however, the polarity of Verr is dependent upon the type of temperature sensor, e.g., thermistor or RTD, used in measuring the temperature. As described above, a polarity change in Verr also changes the polarity of the electric current Iout applied to a temperature control device such as a heater or a TEC, and, consequently, changes the functioning of the temperature control device which is applied to an optical communication device. A resulting disadvantage is that the printed circuit boards (PCB) that are selected to implement the temperature comparison unit vary with the type of temperature sensor employed.