The tunable lasers typically start up in the ‘telecom mode’, aka ‘dithermode’. In this mode the laser is locked to the optical filters in the cavity through a 200Hz dither signal. The frequency of the laser is moved back and forth and the response in optical power (when the laser mode gets better and worse aligned with the optical filters) is measured. Based on that feedback (about 0.005dB response) the lock is adjusted.
This locking algorithm has a fairly large (100MHz pk-pk) frequency component at 200Hz in the spectrum and some additional noise components around 16kHz in the spectrum.
Pure Photonics lasers can be moved into the whispermode, where the dither is disabled and most of the control loops are either disabled or slowed down. As a result the noise in the spectrum, especially at lower frequencies is significantly reduced.
There are several whispermodes:
Mode 2: the dither is disabled, but the electronics components that are driving the dither are not.
Mode 6: the dither and the electronics are disabled.
The mode 6 is preferred as it provides lower noise, but it does not allow for frequency modulation (if the device is so equipped).
Currently mode 2 is available on PPCL600; Mode 2 and 6 are available on PPCL700. In the next release of the firmware for PPCL700, the device automatically moves into the best whispermode (i.e. 6 if the device does not have a Frequency Modulation input).
Also in the new firmware, there is a Progressive Whispermode, which based on the locking state of the device will slow down device ADCs to get a higher resolution measurement at the expense of a lower update speed.