Configuration of the LDT-5500B for your application is made even easier through a new front panel interface. Display modes are selectable for temperature, resistance, and current readout, easily visible with the bright green LED 4-digit display. SELECT and SET controls along with the rotary control knob allows you to quickly set temperature setpoints along with sensor calibration constants, output current limits, temperature limits, and the control loop gain. With ILX interconnect cabling and a wide variety of laser diode mounts, you can set up and control temperature within minutes.

The above figure figure shows that the threshold current and differential responsivity of a laser diode are strongly affected by the laser’s temperature. The laser threshold will increase exponentially with temperature as exp(T/T0), where T is the laser temperature and T0 is the “characteristic temperature” of the laser (typically between 60 to 150°C). T0 is a measure of the temperature sensitivity of the device with higher values implying that the device is more thermally stable. T0 is an important laser diode characteristic and is commonly extracted from multiple L-I curves. Changes in temperature also affect the bandgap of the semiconductor junction and therefore, the peak wavelength of the gain profile. This results in a linear relationship between temperature and the center wavelength of the laser diode (see above figure) with typical temperature tuning coefficients of 0.3 nm/°C. As a result, a temperature controller plays a key role in determining the laser wavelength. Please see Fundamentals of Laser Diode Control for additional information.

The 5500B’s offer user-selectable thermistor source currents of 10 µA and 100 µA allowing the instruments to measure resistance from 250Ω to 450kΩ providing control over a wide range of temperatures and applications. For a typical 10 kΩ thermistor, this corresponds to a temperature control range from -25°C to 60°C. Other temperature control ranges are possible by choosing different resistance value thermistors. For more information, refer to ILX Application Note "Selecting Thermistors for Temperature Control". In addition to compatibility with a wide range of thermistors, the LDT-5500B also accepts IC temperature sensors and with the TSC-599 sensor converter, platinum RTD sensors. Depending on the temperature sensor selected, the LDT-5500B measures the thermistor resistance, the AD590 current or the LM335 voltage, calculates the temperature for display and controls precisely to a temperature set point.

In addition to normal instrument control and operating modes, the current output of the LDT-5500B’s are bound by fully independent programmable current limits. The temperature control loop can also be bound by programmable temperature limits. Adjustment of either of the limit settings is easy and precise even with the instrument controlling temperature. Furthermore, if a temperature sensor or the TE module should open, the 5500B automatically shuts the control output off and lights up a front panel LED indicating the cause of the fault.

The 5500B’s operate in constant temperature, constant resistance (sensor), or constant current mode. Attention to detail in the design of the low noise current source, feedback loop, and sensor measurement circuits ensure accurate, low noise temperature control that won’t drift over time in any of the instrument operating modes. In constant temperature mode, the instrument will control to a setpoint temperature between -99°C and 199°C depending on the thermoelectric module performance characteristics, heat load, and temperature sensor connected to the instrument. Because these instruments are micro-processor controlled, the temperature can be displayed in °C accurately with the appropriate sensor constants entered through the front panel or USB interface. Even with two-terminal thermistors and their non-linear relationship between temperature and resistance and IC temperature sensors where a highly linear relationship exists between temperature and voltage or current, by using the appropriate equation for the selected temperature sensor and the pertinent calibration constants, residual errors of less than 0.01°C can be realized over wide temperature ranges. The 5500B’s low noise bipolar current source reduces the noise coupled to your laser diode for precise control and measurements in your application.