LC120LaserNoise

This project characterizes the noise performance of diode lasers over drive current and temperature using the LC120 Miniboard laser driver and the QL03 Photoreceiver. A 200 MHz scope is used for the data acquisition system (DAQ). The analysis includes noise spectra across varying laser drive currents and thermoelectric cooler (TEC) temperatures, producing 2D plots of integrated noise in current-temperature space. We've found that different diode lasers have regions of instability where the diode behaves erratically (mode hopping mostly).

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System Overview

The measurement setup includes:

• LC120 Miniboard: A compact, low noise, precision laser current driver with integrated TEC control.
• LP870 Nanosecond LED Pulser: A high speed LED source used to characterize the photoreceiver.
• QL03 Photoreceiver: A large area, low-noise photodetector used to sense the optical output.
• Siglent SDS1202X-E Oscilloscope: Acts as the DAQ, reading voltage traces from the QL03 output.

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Measurement Loop

The system operates by stepping through a grid of laser operating points:

1. Loop over current and temperature:
   • Set the LC120 laser current
   • Set the LC120 TEC temperature setpoint
2. Acquire waveform from the oscilloscope (connected to QL03)
3. Process noise spectrum:
   • Remove DC offset
   • Compute integrated noise
4. Store and plot:
   • Generate 2D plots of:
     • Integrated RMS noise vs. current and temperature
     • Slice plots (noise vs. temp, noise vs. current)

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Outputs

Since we're looking for regions of instability the main output are 2D color density maps showing the noise for each Current - Temperature setting.

Code Features

• Restartable runs (resumes from where data left off)
• Easily configurable CLI interface
• Supports swapping photodetectors (e.g., QL03 vs others)
• Uses PyVISA to control the scope and collect waveforms

Test Setup

Most diodes are tested using a ThorLabs LDM21 connected using the LC120 Starter Kit Adapter. The QL03 is modified to have a transimpedance of 100 (down from 1M). Reflections are prevented from getting back into the cavity using a microscope objective slightly off axis. A lens is used to spread the beam over the entire photodiode.