How to use the “software light loop” (constant power mode) on MV and HCV controllers

Some VueMetrix controllers support a constant power mode implemented entirely in firmware. This feature is found on the following controllers:

• MV
• HCV

On DPSS controllers the constant power mode is implemented in hardware, and this document does not apply.

Theory of operation

The microprocessor is not able to provide high speed regulation of the laser power. When the constant power algorithm is first engaged, the microprocessor computes an approximate value for the laser current using a simple linear model. The laser is set to this current initially. From that starting point, the light power actually produced is monitored periodically. The current is slowly adjusted as required in order to maintain the desired power. This method results in a servo bandwidth on the order of 0.1 Hz, sufficient to correct for most environmental effects.

Note that if the LIV curve contains “kinks” or other non-linear features, the initial current setting may be inaccurate.

The linear model used to describe the laser performance is based on a threshold current plus slope efficiency:

P(i) = i_th + S * (i – i_th)

where P is the laser power in watts, i the current in amps, i_th is the threshold current, and S is the slope efficiency in watts/amp. Before the constant current algorithm can operate properly, the two parameters S and i_th must be determined and entered into the controller's flash memory. WinVue provides tools to facilitate this.

Constant power mode setup procedure

WARNING: It is important to perform all of these steps before attempting to operate the controller in constant power mode. These steps need to be performed if the laser diode is changed or degrades, or if a change is made to the light power detection apparatus.

NOTE: When the controller is operating in constant power mode, the current will not exceed the maximum limit set by the “Limits” button on WinVue's main laser control window. Be sure this is set to a value that is safe for your diode.

1. Set up and calibrate the laser power input signal. Details for this procedure are in the individual product manuals. In current regulation mode the measured light power is prominently displayed in large numbers on the upper left of the Main Laser Control window. Verify that the light power reading behaves properly throughout the range of interest.

2. Open the LIV window on WinVue. Choose current limits that are appropriate to your operating conditions. Click the start button and acquire the LIV curve. This will take a few moments.

3. When the LIV acquisition is complete, WinVue will automatically calculate the threshold and slope efficiency (displayed in the lower left). Click on the button “Upload constants to controller” at the bottom of the window.

4. The constant power regulation is now ready for use.