I'm repeating this measurement, this time recording the current noise of the commercial driver before each measurement for reference. Will update tomorrow with additional measurements, plots.

Delay line frequency discriminator procedure

1. Set up the delay line frequency discriminator as before (see attachment 1)
2. Find a low noise operating point (see below) for the Rio laser(s)
3. Calibrate the delay line by tuning to the dark and bright fringes around 80 MHz and recording the DC voltage of the LF (after whatever amplification is applied)
4. Measure the current noise of the commercial driver as below [could also be done completely independent of these steps]
5. Tune the beat note frequency to the half fringe, and record the beat note spectrum.
6. Repeat for each pair of beat notes.
7. When done, turn off RF amplifier (first disconnect input, then turn off DC power) and SR560, and remember to plug in the SR560 power line.

The measurement parameters for delay line frequency discriminator are:

• Pickoff RF from 1611 to Moku (50 Ohm AC coupled spectrum analyzer) using
• RF amplifier ZHL-1A (powered by +24 V from GPS3030D) to splitter
• Level 7 mixer ZFM-3-S+ (0.04-400 MHz)
• LF from mixer through SMA-T with one end terminated at 50 Ohms, the other to SLP-1.9+ then SR560 channel A
• SR560 settings: battery powered, DC coupled, 30 kHz LP (6 dB/oct), low noise mode, G=10

Finding a 'low noise' operating point

According to the datasheet (PSOMA wiki 'documents' page), the lasers come with a recommended temperature setpoint (T_set on the datasheet). This setpoint may either lie on the upper hysteresis branch, or lie outside of the hysteresis region... but according to our datasheet, our lasers' T_set lie in the upper hysteresis branch. If we observe spectral distortions, this indicates the temperature is outside the recommended range or operating on the lower arm of the hysteresis. To achieve a low noise operating point

1. Ensure the laser is properly mounted on the PCB with drivers and control loops connected (but off)
2. activate the thermal control loop, and let the internal temperature stabilize
3. Apply the laser bias current, ramping the current at 10 mA/sec up to I_bias
4. For the delay line measurement, scan the laser current until there is a beat note near 80 MHz
   • The 1611 may be saturating. If harmonics of the main beat note are visible,
     • iteratively reduce the laser current and adjust the TEC setpoint while keeping the beat note near 80 MHz
     • Since the laser power depends more strongly on current, this will reduce the beat note power. Do so until no harmonics are visible on the spectrum analyzer.
     • Empirically, I found this to be true when the total power on the photodiode was under 500 uW

Notes about the lasers...

• If it is required to set the bias current and TEC current simultaneously, first tune the laser to several degrees below the set point in the hysteresis free region, then slowly increase the TEC temperature to the setpoint
• Ensure that the TEC control loop is restricted from entering the mode hop region, since this will put us on the lower hysteresis curve
• For the North laser (SN: 104978), T_set is 25 C, corresponding to about 9.9-10.1 kOhm. Operated at the recommended I_bias of 150 mA, I observed the next mode hop at 8.971 kOhm, so the TEC control should be kept above that level (operated at lower bias currents, the hop has been observed at lower values of TEC resistance).
• For the South laser (SN: 104987), T_set is 23 C, corresponding to about 10.8-11.04 kOhm.

 Measuring current noise

1. Turn off the laser by
   1. Ramp down then turn off LD current
   2. Turn off TEC
   3. Turn off driver box
2. Remove DB9 at the driver box, and add a DB9 breakout between the driver and the cable going to the laser.
   • The voltage across the diode is between pins 3 and 7
3. Send the voltage across pins 3 and 7 to a floating-input DC coupled SR560, and the output of the SR560 to an oscilloscope
4. Turn on the laser by
   1. Turn on driver box
   2. Turn on TEC
   3. Turn on LD current by
      1. With the current off, turn the current setpoint to 0
      2. Turn on the LD current
      3. Ramp the LD current by 10 mA/s up to the desired setpoint
5. Note the voltage across the diode at 2-3 different values of bias current near the operating point. This is the voltage to current calibration.
6. Turn off the laser as above
7. Switch the SR560 to AC coupled mode by
   1. Disconnect the BNC from the SR560 input
   2. switch the preamplifier to AC coupled mode
   3. Change the gain to G=100
   4. reconnect the BNC to SR560 input.
8. Turn on the laser as above
9. Record the output of the SR560 with a spectrum analyzer. Note the settings of the SR560, and use that and the calibration above to convert the spectrum from V/rtHz to A/rtHz of the current driver.
   • The delay line frequency discriminator calibration can then be used to convert from A/rtHz to Hz/rtHz
10. Return the system to its original state by
    1. Turn off laser as above
    2. Remove DB9 breakout and reattach DB9 cable directly to LD driver

Notes on current noise measurements:

• SR560 settings: G=100, AC coupled, 30 kHz LP with 6 db/oct, battery powered
• Updated on May 14 with S laser drive (ITC502) current noise. Measured at 123.43 mA.