Closed loop laser diode temperature control system design for photodynamic therapy application

In cancer treatments photodynamic therapy, because of being a minimally invasive method and having no significant harmful side effect to the patient. This method begins with the administration of photosensitizer material prior to treatment in the tissue, which reacts chemically to the specific wavelength light. Dosimeter calculations determine the time required for photosensitizer the body to remain only in the tumor. At the end of this period, the tumor tissue becomes eligible for targeted treatment. In order to be able to create the thermal effect on the tissue with the laser beam, a laser with a constant wavelength must be used. When photosensitizer provides sufficient energy in the target tissue exited with specific wavelength light energy in the cell, the cancer cells lead to necrosis. In order to complete the treatment successfully, Photodynamic therapy system's laser diode must have a constant temperature and constant output power so that it can generate constant wavelengths. Since the laser diode temperature is directly related to the wavelength and the output power, control of the temperature at a single point is an essential issue.. In the study, the selected laser diode need to be stabilized at a temperature of 15 °C so that it could operate steadily at a wavelength of 635 nm which can excite the ALA- 5. The thermoelectric controller (TEC), the thermistor directly connected to the laser diode, detects the temperature of the diode. The sensed temperature value is applied to the (Dspic30f4013) microcontroller's ADC circuit via an analog circuit that will produce 5V for the maximum value and 0V for the minimum value of the thermistor. This feedback from the thermistor with PID control on the microprocessor determines the PWM on and off time ratio. Cooling control is provided instantaneously by PWM signal applied on the thermoelectric material.