The most basic form of a laser driver is a current source consisting of a sensing resistor and an operational amplifier.
The op-amp measures the voltage across the sensing resistor and controls the output with a feedback loop to bring the voltage across the resistor closer to the control voltage.
Since no current flows across the negative input of the op-amp, the laser current IL is the control voltage VC divided by the sensing resistor RS.
The output stage of a typical operational amplifier is not capable of supplying more than a few tens of mA of current,
It is common to replace them with discrete transistors.
The LD driver can adjust the current as long as the LD voltage remains within a certain range.
The supply voltage VS is the sum of the detector resistor voltage VRs = RS x IL, the LD voltage VL, and the transistor voltage VT.
The transistor can be viewed as a variable resistor controlled by the op amp.
As the LD voltage rises, the op-amp tries to lower the resistance RT of the transistor to maintain a constant current.
At some point, the transistor resistance reaches a minimum value, RTmin, and the driver,
operates as if the LD were supplied with VS, which is the series of RTmin and RS.
The compline voltage is the maximum LD voltage at which the driver maintains current regulation.
This voltage is current dependent and is usually specified by the maximum operating current of the driver.
The choice of the resistor value is a trade-off between current noise and power consumption.
Modulation of laser current
Modulation can be done in at least two ways, depending on the required modulation frequency.
If the modulation frequency is smaller than the bandwidth of the feedback loop, the LD current can be modulated via the control voltage VC.
This bandwidth is typically between a few kHz and a few MHz.
If the modulation bandwidth is greater than the driver's modulation bandwidth, the LD current can be modulated via the bias tee as shown in the figure below.
The capacitor allows the AC modulation to pass through the LD while blocking the DC signal.
The inductor to isolate the driver from the AC modulation must be small enough to not add too much phase within the bandwidth of the driver
Some laser diodes have the positive (anode) or negative (cathode) side connected to the metal case of the diode.
If the metal case must be connected to ground, an anode-grounded or cathode-grounded LD driver must be used as shown in the figure below.
A driver grounded at the anode operates on a negative power supply, while a driver grounded at the cathode operates on a positive power supply.
In most cases, a floating architecture can be employed by electrically isolating the metal case of the diode from ground.
This architecture often improves power efficiency because the control electronics operate closer to ground.