The Semiconductor Laser Diode

// L-I sweep for a laser diode: ESP32 ramps the driver's command voltage and reads the
// monitor photodiode, printing CSV you can paste straight into the notebook.
//
// SAFETY. Wear the goggles. Point the diode into a beam dump or a matte card, never at a
// wall you might glance along. Never run the sweep with the diode uncollimated and facing
// up. Check the diode's absolute maximum current on its datasheet and set I_MAX_MA BELOW
// it -- a laser diode has no thermal margin worth speaking of and dies in microseconds.
//
// Wiring:
//   GPIO25 (DAC1) -> driver command input (replaces RV1's wiper)
//   GPIO34 (ADC1) -> monitor photodiode across R_MON
//   ESP32 GND     -> driver GND, one point only

const int PIN_CMD = 25;      // DAC output, 8 bit, 0..3.3 V
const int PIN_MON = 34;      // ADC1, safe to use while WiFi is active

const float R_SENSE   = 10.0;   // ohm, must match the driver
const float I_MAX_MA  = 60.0;   // <-- SET FROM YOUR DIODE'S DATASHEET
const int   N_POINTS  = 40;
const int   N_AVG     = 64;     // ADC averages per point
const int   SETTLE_MS = 40;     // let the junction reach thermal steady state

float readMonitor() {
  long acc = 0;
  for (int i = 0; i < N_AVG; i++) { acc += analogRead(PIN_MON); delayMicroseconds(200); }
  return (float)acc / N_AVG;
}

void setup() {
  Serial.begin(115200);
  delay(300);
  analogReadResolution(12);
  analogSetPinAttenuation(PIN_MON, ADC_11db);

  dacWrite(PIN_CMD, 0);          // start dark, always
  delay(500);
  float dark = readMonitor();

  Serial.println("# L-I sweep");
  Serial.printf("# R_SENSE = %.1f ohm, dark level = %.1f counts\n", R_SENSE, dark);
  Serial.println("current_mA,monitor_counts,monitor_minus_dark");

  // V_cmd = I * R_SENSE. The DAC is 8 bit over 0..3.3 V.
  float v_max = (I_MAX_MA / 1000.0) * R_SENSE;
  for (int k = 0; k <= N_POINTS; k++) {
    float v_cmd = v_max * k / N_POINTS;
    int   dac   = (int)(v_cmd / 3.3 * 255.0 + 0.5);
    if (dac > 255) dac = 255;
    dacWrite(PIN_CMD, dac);
    delay(SETTLE_MS);
    float m = readMonitor();
    float i_mA = (v_cmd / R_SENSE) * 1000.0;
    Serial.printf("%.2f,%.1f,%.1f\n", i_mA, m, m - dark);
  }

  dacWrite(PIN_CMD, 0);          // and finish dark
  Serial.println("# done, diode off");
}

void loop() {}