stabilizerpi

Laser intensity stabilization using a Raspberry Pi Pico, an acousto-optic modulator (AOM), and a photodiode. A software PID controller on the Pico reads the photodiode voltage (ADC) and drives the AOM (PWM) to hold a constant laser intensity.

Repository layout

firmware/
  CMakeLists.txt            CMake build file
  pico_sdk_import.cmake     Standard Pico SDK locator
  src/
    config.h                Pin map, PID gains, setpoint, timing
    pid.h / pid.c           PID controller (anti-windup, output clamping)
    main.c                  Stabilisation loop + USB serial telemetry

scripts/
  monitor.py                PC-side live voltage plot (matplotlib)
  requirements.txt          Python dependencies for monitor.py

Wiring

Signal	Pico pin	Notes
Photodiode output	GP26 (ADC0)	0–3.3 V analog. Use a transimpedance amplifier if the raw photodiode signal is too small or the wrong polarity.
AOM driver input	GP15 (PWM)	0–3.3 V PWM. Feed through a low-pass RC filter (e.g. 1 kΩ + 100 nF → ~1.6 kHz cutoff) to get a smooth DC level, then into the AOM driver's modulation input.
GND	GND	Common ground for Pico, photodiode circuit, and AOM driver.

The PWM frequency is ~30 kHz (125 MHz / 4096). A simple RC filter will remove the PWM ripple while preserving the ~1 kHz control bandwidth.

System diagram

flowchart LR
    LASER([Laser]) -->|beam| AOM[AOM]
    AOM -->|beam| PICK{Pickoff}
    PICK -->|main beam| EXP([To experiment])
    PICK -->|sample| PD[Photodiode]

    PD -->|voltage| PICO["Pi Pico\nADC → PID → PWM"]
    PICO --> RC["RC low-pass\nfilter"]

    RF["RF source\n80 MHz"] --> TTL["TTL switch"]
    TTL -->|RF| MIX["Mixer"]
    RC -->|"IF (DC)"| MIX
    MIX --> BPF1["Bandpass\n63–77 MHz"]
    BPF1 --> PRE["Pre-amp\n+20 dB"]
    PRE --> AMP["Amplifier\n+25 dB"]
    AMP --> BPF2["Bandpass\n63–77 MHz"]
    BPF2 -->|RF drive| AOM

    style LASER fill:#d32f2f,color:#fff
    style AOM fill:#ff9800,color:#fff
    style PD fill:#4caf50,color:#fff
    style PICO fill:#1565c0,color:#fff
    style RF fill:#7b1fa2,color:#fff
    style EXP fill:#607d8b,color:#fff

Loading

Optical loop: The laser passes through the AOM, a pickoff sends a sample to the photodiode, the Pico reads the photodiode voltage and closes the feedback loop by adjusting the AOM drive level.

RF drive chain: An 80 MHz source enters a TTL switch, then a mixer whose IF port receives the Pico's DC control voltage. The mixed signal passes through a bandpass filter (63–77 MHz), a pre-amplifier (+20 dB), a power amplifier (+25 dB), a second bandpass filter (63–77 MHz), and finally drives the AOM.

Building the firmware

You need the Raspberry Pi Pico C SDK installed and PICO_SDK_PATH set.

cd firmware
mkdir build && cd build
cmake ..
make -j$(nproc)

This produces stabilizerpi.uf2. To flash:

1. Hold the BOOTSEL button on the Pico and plug it in via USB.
2. Copy stabilizerpi.uf2 to the RPI-RP2 mass-storage drive that appears.
3. The Pico reboots and starts the stabilisation loop.

Running the live monitor

cd scripts
pip install -r requirements.txt
python monitor.py          # auto-detects serial port
python monitor.py COM5     # or specify explicitly

A two-panel plot shows the photodiode voltage (blue) and AOM drive voltage (red) in real time, with the setpoint drawn as a dashed line.

Tuning the PID

Edit the gains in firmware/src/config.h:

#define PID_KP  1.0f   // proportional
#define PID_KI  0.5f   // integral
#define PID_KD  0.01f  // derivative

Quick-start procedure (Ziegler–Nichols):

1. Set KI = 0, KD = 0.
2. Increase KP until the output oscillates steadily. Record this value as K_u and the oscillation period as T_u.
3. Set KP = 0.6 * K_u, KI = 2 * KP / T_u, KD = KP * T_u / 8.
4. Fine-tune from there. Reduce KI if there is overshoot; increase it if the steady-state error is too large.

Other parameters in config.h:

Define	Default	Purpose
SETPOINT_V	1.5 V	Target photodiode voltage
LOOP_PERIOD_US	1000 (1 kHz)	Control loop rate
SERIAL_DECIMATION	10	Print every N-th sample (100 Hz serial rate)
PID_INTEGRAL_MAX	2.0	Anti-windup clamp on the integral term

License

MIT