Laser diode driver current noise has strong influence on laser linewidths, but the way they are related is not obvious. To quantify a laser driver current noise, the root-mean square noise amplitude is often used. Here we show that to determine the laser final linewidth, one should rather look at the noise spectrum characteristics.
Different noise spectral densities
We use a Koheron CTL200-1-B-400 digital laser controller. Thanks to the DC modulation input which provides wide band in-loop modulation we can produce high-fidelity arbitrary waveform modulation. Here we use it to emulate two current noise spectrums: a broadband white noise and a narrowband sine peak. Despite their opposite characteristics in the way power is distributed across the spectrum, amplitudes are chosen so that they both result in the same RMS noise integrated in a 10 Hz - 1 MHz band.
The modulation input is set to low. A Keysight 33600A arbitrary waveform generator is used to modulate the laser controller. The sine wave has 30 mVpp amplitude at 500 kHz resulting in an integrated noise of 5.04 µARMS. The white noise has 3 MHz bandwidth with 175 mVpp amplitude producing 5.11 µARMS of integrated noise. For comparison, the RMS noise of the laser controller CTL200-1-B-400 without modulation is 389 nARMS.
Here are the resulting spectral densities:
We measure the linewidth of a Thorlabs SFL1550P laser diode. The temperature is stabilized to a thermistor value of 9 kΩ. The linewidth is measured using a self-homodyne setup with 10 km delay.
The results for the various power spectral densities are:
Without modulation, the laser linewidth measured is 59 kHz. When a sine peak noise is applied, the linewidth increases to 69 kHz, reaching 180 kHz with a broadband noise. Indeed, a clean sine wave modulates the laser frequency but maintains a highly coherent system. Conversely, a broadband random noise scrambles the laser phase and is strongly decoherent.
To choose a driver suitable to a given narrow linewidth laser diode, one should compare drivers noise power spectrum and favor the one with as little broadband noise as possible.