Precision constant optical power laser driver - Page 1
Forum section: electronics / projects

author: Cloud
author_rank: Regular Contributor
timestamp: February 28, 2018, 09:23:51 am
content: I need to have a laser at the exactly the same (max +-1%) optical power over the span of 2-3 hours. Currently I am using a simple driver which uses output from PD in laser as feedback. But I am still having problems because optical power drifts for cca 5-8% in hour and it settles in about hour and a half.What do you recommend to improve? I was having idea for keeping laser at the constant temperature...

author: CM800
author_rank: Frequent Contributor
timestamp: February 28, 2018, 09:48:30 am
content: In order to have stability, you willabsolutelyneed a stable temperature.you should use a TEC for this with a suitable controller. If you try this, you should get a lot closer to your needs.

author: ajb
author_rank: Super Contributor
timestamp: February 28, 2018, 06:28:27 pm
content: When you say that optical power drifts, do you mean that the sensitivity of your PD is changing, causing the output to drift even as the PD feedback is held constant by the driver? Or is some other part of the driver drifting so that the PD feedback is NOT held constant as it should be? What sort of power level? Is the PD integral to the diode package? What are you using to measure the actual power to determine that the output is in fact drifting?

author: LaserSteve
author_rank: Super Contributor
timestamp: March 01, 2018, 01:17:32 pm
content: Have you tried a beam splitter and external PD?Steve

author: David Hess
author_rank: Super Contributor
timestamp: March 01, 2018, 03:38:49 pm
content: Maybe the simple driver is not using the feedback photodiode correctly or well.

author: danadak
author_rank: Super Contributor
timestamp: March 02, 2018, 12:53:28 pm
content: Something like a PID controller. The below happens to be afan controller but you could mod it to handle power in diodeand manage the T issue. PSOC 5LP might be best way toapproach, it has a 20 bit delsig, 12 bit SAR, can handle diffsignals, at very low level (read current measurement off ashunt).http://www.cypress.com/documentation/application-notes/an89346-psoc-4-intelligent-fan-controllerBoard to use ($10) -http://www.cypress.com/documentation/development-kitsboards/cy8ckit-059-psoc-5lp-prototyping-kit-onboard-programmer-andRegards, Dana.

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 02, 2018, 03:47:40 pm
content: Quote from: CM800 on February 28, 2018, 09:48:30 amUsing a TEC to control or keep temperature stable will add unnecessary complexity, depending on the laser package, mechanical and thermal constraints, etc.You have not disclosed any details about the Photodiode that monitors laser optical power. If it is integral to the laser package then it’s likely your APC control loop does not track properly. External PD monitoring is problematic and can contribute to inadequate power tracking.Ideally the optical power will be captured by the PD and accurately fedback to the error circuitry in a linear fashion so proper tracking can be achieved. In the real world things sometimes don’t work as they should because there are design considerations that are easily overlooked or assumed, like self heating, thermal package drift, CTE errors, etcIf you want some additional help, please provide specifics about your design,.....ie: APC schematic, PD config, physical setup, etcWe can probably help you better after you consider PD loop error sources@grouchobyte