Source-measure unit (SMU)
A four-quadrant precision instrument that simultaneously sources voltage or current while measuring the other quantity. The standard electrical instrument for laser diode and photodiode characterization.
A source-measure unit (SMU) is a precision instrument that operates as a programmable current/voltage source and a measurement instrument simultaneously. A single SMU can source voltage and measure current, source current and measure voltage, or operate in any of the four current/voltage quadrants (sourcing or sinking power).
Standard SMU specifications relevant to optoelectronic characterization:
| Parameter | Typical specification (Keithley 2461 class) |
|---|---|
| Current source range | 1 μA to 10 A |
| Voltage source range | 200 mV to 100 V |
| Current measurement resolution | 1 pA |
| Voltage measurement resolution | 1 μV |
| 4-wire (Kelvin) sense | Standard |
| Compliance limits | Programmable on both source modes |
| Pulse mode | Often included (1 μs – seconds) |
| Sweep modes | Linear, log, list |
For laser diode characterization, the SMU sources current and measures voltage in 4-wire Kelvin mode, eliminating voltage drops in the supply leads. The 4-wire connection uses one pair of leads to carry drive current and a second pair to sense voltage at the device terminals through a high-impedance voltmeter that draws negligible current.
Standard SMU configurations for photonic measurement:
| Use case | Configuration |
|---|---|
| Laser diode LIV | Source current, measure voltage; sweep current up |
| Photodiode IV / dark current | Source voltage, measure current; sweep bias |
| Photodiode responsivity | Source bias voltage, measure current under known illumination |
| Pulsed LIV (high-power) | Pulsed current source mode; sub-μs to ms pulse widths |
| Breakdown voltage | Source current at compliance, find breakdown |
For high-power laser characterization above 100 mW dissipated power, self-heating during CW operation biases the measurement. Pulsed SMU mode (typical: 1 μs pulses at 0.1% duty cycle) eliminates this bias — see Pulsed vs Continuous-Wave LIV Measurement.
Multi-channel SMU instruments (e.g., Keithley 2602B) provide synchronized channels for measurements requiring simultaneous bias of multiple electrodes — for instance, separately biasing the laser and EAM sections of an EML, or characterizing transistors and photodiodes simultaneously.