Datasheet
B.3 Bias-T circuit for pulsed lasers
B.3. Bias-T circuit for pulsed lasers
Since tuning of a QC laser is done by changing the temperature of the active zone,
the DC bias current can be used to control the emission wavelength of the laser via
its heating effect. The bias-T therefore allows for electrically controlled rapid scanning
of the emission wavelength.
B.3.1. Principle
Since tuning of a QC laser is done by changing the temperature of the active zone,
a DC bias current can be used to control the emission wavelength of the laser via
its heating effect. Tuning can also be achieved by changing the temperature of the
whole laser, but at a much lower speed, due to the high thermal capacity of the laser
submount and base. A bias-T circuit allows for a quick scan of the emission wave-
length because the laser emission response is much faster, as only the active zone is
heated.
The DC current is drawn from the external power supply and applied to the laser in
addition to the pulsed current. It is recommended to apply currents of up to 0.1kHz,
but several customers have used the bias-T at frequencies of up to several kHz.
Laser lifetime
Heating of the active zone will increase thermal stress of the laser, therefore the ex-
pected lifetime will decrease more rapidly compared to increasing the temperature of
the laser submount and base. If operation at only a fixed wavelength is needed, this
should be adjusted with the overall temperature control.
B.3.2. Connection
The bias-T circuit can be integrated in the TPG. The circuit included in the LDD pulser
unit is controlled by the twisted black and yellow wires of the control cable with the
CPL cable.
B.3.3. Procedure
• Since the input stage of the bias-T is a bipolar transistor, applied voltage must
be higher than about 0.6V to start bias current. The input stage has a voltage
limit of 2.6V, but the laser itself may be destroyed at lower bias-T control voltage,
therefore the maximum rating has to be agreed with Alpes Lasers SA.
• the laser should be operated initially at lowest possible temperature Since a
bias-T only allows to heat the laser, the emission wavelength can only be in-
creased (or emission wavenumber decreased), and output power will decrease
c
2012 SA 101