Electronic drive circuit for multi-laser thermal printer

A multi-laser thermal printer comprising:

• a print head having a plurality of laser channels for printing a like plurality of lines of an image on a thermal print medium;
• a laser-light sensor mounted in proximity to the print head for measuring the light output of each of the laser channels;
• and
• a plurality of electronic drive circuits for driving the respective laser channels, each drive circuit comprising:
• a laser for supplying light to a respective one of the laser channels;
• power means for supplying drive current to the laser, the power means operating between a threshold current level and an enable current level, the power means having a first input for setting the threshold current to the laser, a second input for setting the enable current to the laser to produce a laser light output power level which is set to a precisely measured value, and a third input for modulating the laser light in accordance with line data input;
• current monitor feedback means for measuring the current to the laser and for supplying feedback signals to the first input of the power means to set the threshold current;
• and
• light sensor feedback means for receiving light output measurements from the laser-light sensor and for supplying feedback signals to the second input of the power means to set within a range the enable current to the laser to produce a laser light output power level at the precisely measured level, such that all of the laser channels print uniformly in spite of variations from laser to laser and changes due to ageing.

A circuit in a multi-laser thermal printer for maintaining the full-on light output power of a respective laser within closely calibrated limits during the service life of the laser, the circuit comprising:

• a semiconductor diode laser whose light output is controlled between off and full-on by drive currents;
• power drive means for supplying drive currents to the laser in accordance with control signals;
• D/A converter means for applying control signals to the power drive means, the D/A converter means having a bias level input and a line data input;
• precision adjustable bias means for applying a precisely set bias level to the bias input of the converter means;
• light sensor means for precisely measuring the light output power levels of the laser and for providing accurate data signals corresponding to the light power output levels;
• and
• feedback means for adjusting the precision bias means in accordance with data signals from the light sensor means, such that the bias of the D/A converter means is set at that value which for line data input to the converter means calling for full-on causes the power drive means to provide that full-on drive current to the laser which gives a calibrated full-on light power output of the laser as measured by the sensor means.