The Pure Photonics products are feature loaded and it may be hard to comprehend and use all the available capabilities. On this page we are highlighting some things that are ‘Good to Know’.
Register 0x33 includes some important device configuration settings. The OIF MSA has defined some, but the Pure Photonics micro-ITLAs have expanded on this.
The MCB register is part of the non-volatile memory settings and can be permanently saved with the GenCFG command.
The MCB register map for the PPCL600 is given below
The MCB register map for the PPCL700 is given below
The Pure Photonics Command Line Interface (CLI) now has the ability (since version 11) to run a user script (text file) using the Python language.
Though limited in some ways, this allows for a user to run a simple automated script with some logic to control the laser, without the need for a complete Python IDE.
Some examples can be found at and . These scripts can be run by connecting to the laser and then start the script with it.script(‘ — name of file –‘) with the file saved in the CLI software directory (or with appropriate path from there).
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.
The Pure Photonics tunable lasers should be installed upon some heatsink. As the operating power of the laser is in the range of 3.5W at max power and closer to 1.5-2W at room temperature, simple heatsink solutions (including PCB mounting with thermal vias) are acceptable. It is the user responsibility to ensure the ambient temperature complies with the device specifications.
For the device temperature, the thermistor is placed on the PCB next to the laser, not at the laser cavity (in the goldbox). So a temperature controlled heatsink with an airflow, will give a different result than putting the device in an immersive environment.
The device can be mounted upright, with a flex cable connecting the device to surrounding circuit. The device can also be installed upside down, plugged directly into an on-PCB connector. Care needs to be taken with stress relief (especially for the PPCL700).
The PPCL700 is designed to have the PCB free-hanging. So any screws should not go through the PCB and rater only go through the bottom baseplate.
The Pure Photonics laser displays some wander behavior over time (30-40MHz over 10secs). Locking it to a more stable reference (gascell, etalon, or different) allows for a stabilized solution along with the narrow linewidth and low Gaussian noise.
For PPCL700 devices with a Frequency Modulation input and Clean Measurement (ADC) input, Pure Photonics provides a firmware based on our PPCL590 (gas cell locked laser), that automatically controls the laser to lock to the error signal provided onto the ADC input.
Several device settings can be saved to permanent memory with the GenCfg command. This includes the frequency setpoint, the power, the MCB register and others. Along with e.g. the autostart option in the MCB register, the device can be configured to automatically turn-on at a target frequency, target power, when the device is powered-up.
The PPCL600 and PPCL700 have the optional configuration to automatically enable the laser upon power up. The MCB register (saved to permanent memory) is used for that.
For the PPCL700 there is an additional feature to automatically switch to the whispermode. This is also set in the MCB register.
When the laser has activated, the user can keep the laser in dithermode, providing long-term stability, at the cost of the 200Hz dither signal (about 100MHz).
The user can reduce the dither signal by 75% by using the register 0x96. The value written to this register is 2-8 and represents 25% – 100% of the original signal (in steps of 12.5%)