Laser diodes can destroy themselves in a few nanoseconds, so testing the response and stability of a feedback-stabilized laser-diode driver can be expensive. The simulator circuit in Figure 1 shows a typical laser-diode package, which contains not only the diode, which is driven by current IL, but also a photodiode. The laser diode's front facet emits a main beam that goes to work in the outside world, and the rear facet emits a reference beam that falls on the photodiode.
|Figure 1.||A P-type-laser diode assembly includes the photodiode
Although much weaker than the main beam, the reference beam's power is directly proportional to the main beam, as is the current, IP, that produces the photodiode. Connecting the photodiode back to the laser-diode driver via a carefully designed amplifier completes a feedback loop that should hold the main beam power stable and constant. Ensuring that the laser diode never sustains a destructive overload under any conditions is the tricky part.
A laser diode exhibits a current threshold, or “knee,” below which its emission is weak and incoherent, as is photocurrent IP. Above the knee, laser action occurs, and the optical output and photocurrent rise linearly with increasing drive current.
A simulator must reflect these characteristics, and the circuit in Figure 2 comprises a basic voltage-controlled current source that presents a threshold. Based on a TO-92 or E-line-packaged PNP transistor and two resistors and packaged in a blob of epoxy, the simulator takes the place of the laser diode until circuit operation is stable. It's easy to build several modules to emulate laser diodes of various ratings.
|Figure 2.||A laser-diode simulator requires only one transistor.|
In operation, the laser driver sinks current IL and develops a voltage, VS, across R1. When VS exceeds Q1’s VBE, Q1 conducts and sources a simulated photocurrent, IP, into the feedback-control circuitry. As IL increases, IP increases linearly in proportion.
As a design example, consider an average laser diode with a threshold current (ITH) of 10 mA, an operating current at full optical output (ILMAX) of 30 mA, and a photocurrent of 100 µA at full power. R1 must then equal VBE/ITH, or 560 mV/10 mA and thus yields a value of 56 Ω for R1. Then, R2 equals
or approximately 11 kΩ. Using a value of 560 mV for VBE produces the best practical relationship between IL and IP
Inverting the transistor – that is, swapping Q1’s collector and emitter connections – produces a more abrupt conduction-threshold voltage of approximately 500 mV but decreases the slope of IP versus IL. In this example, inverting the transistor requires lowering the value of R2 to approximately 7.5 kΩ.
The inverted-transistor circuit provides a sharper threshold and thus more realistic simulation, even though it may require some experimentation with resistor values for optimal performance. You can use almost any PNP bipolar-junction transistor for Q1, such as a ZTX502, and decreasing R2 by 30% renders IP within 5% of the desired nominal value of IP.
Note that laser diodes' characteristics vary widely, even within a batch, so using preferred-value resistors for R1 and R2 makes little practical difference in performance. Laser diodes exhibit a typical forward-voltage drop of about 2 V, and the simulator circuit should thus drop no more voltage at full current. Also, the simulator circuit responds more slowly than a laser diode, but, if the feedback circuitry operates even more slowly, as it usually does, the simulator's slow response presents no problem.
A simulator for an N-type laser diode requires an NPN transistor and reversal of connections. More complex laser diodes may require more elaborate circuitry that includes current mirrors and additional connections. If adequate power-supply voltage is available for current-source compliance, you can connect an LED in series with the IL lead to provide a visual indication of circuit operation. Connecting an oscilloscope across R1 allows monitoring laser-drive and modulation currents. (In this context, “N-” and “P-type” refer not to laser-diode-device diffusions but to the polarity of the common terminal.)