[Open hardware] A safe laser by-pass

Well, I remember when I started this business, a beam stop was done with a recycled block of lead and reflections stopped with carton boxes 😉 Brown boxes, black carton catches fires, of course (tell this to my undergrad-self). Not any longer, of course!

About ten years ago, I started the procurement and development of my first two-photon microscope. For the first time, I was directly responsible of laser safety and I had to take decisions about how to build a system that was safe for a user facility in a biomedical research institute. As I was coupling commercially sourced systems (Leica SP5, Chameleon Vision 2 and Pulse Select) and I was not planning much customization for the excitation path of this instrument (I heavily develop assays and detection), I opted to fully enclose the laser in lens tubes. The resulting system is safe, stable, and no more difficult to align compared to other enclosures.

I think that enclosures around the complete table might make sense in many instances, particularly when compartmentalized in sub-sections, but this is the system that worked best for me at the time. One solution I wish to share, is a bypass for the Pulse Picker we had used to develop spectrally resolved FLIM utilizing smart SPAD arrays (detectors that integrate photon counting electronics with them).

20181112_184730As I start planning replacement of this systems, I wished to share this design, in case some of you might find it useful. In the image on the left, you can see the Ti:Sapphire on the top, the pulse-picker on the right and the first enclosure by Leica used to steer the beam to their in-coupling optics (bottom-right).

In the middle, the laser bypass we utilize to direct the laser through the pulse-picker or around it.

In the image below, you see a close-up photo of the by-pass. The black box with the rectangular aluminum cover is the Leica spectral flattener used to reduce power of the Chameleon Vision at the peak wavelength. One of the few customization I needed here was simply to have a hole on a Thorlabs SM2 lens tube to accommodate this filter. This is screwed in a C4W-CC cube that can host a movable turning mirror with high reproducibility. The alignment of the microscope without the pulse-picker is done with the pair of mirrors provided by Leica. The alignment of the Pulse Picker is done with the kinematic mirrors visible on the left (M1 and M2). I placed a light-block behind them just in case one would become lose or to block the small amount of light transmitted through them. A kinematic cube is used to host ultrafast beam sampler by Newport to direct a small fraction of light to the Thorlabs PIN diode I use to feed the electronics of the pulse picker. In front of the PIN diode I have an xy-translating cage element. An empty four-way cube is used to allow the laser beam to pass from top to bottom (bypassed) or from left to right (coupled pulse picker). The aluminum block tagged as L1 is just a cover for the C4W-CC when empty.


At the output of the pulse-picker, you see the mirror image of this bypass (on the right) and the two steering mirrors by Leica (the cylindrical towers). On the far right of the picture there is the in-coupling optics by Leica, preceded by two diagnostics ports.


Below, you can see a close-up picture of this part of the coupling. Because of the layout, I needed to add one extra mirror (top left) and aiming to isolate users (placed on the top of the image) from accidental damages of the in-coupling optics, I added a light barrier.

Both diagnostics ports are based on a 4-way kinematic cube from Thorlabs hosting Newport beam samplers. The first port is used to sample the pulses after the pulse-picker and to feed our FLIM electronics. The second has two scopes. First, for course alignment of the system. I have two irises in the system that are aligned when the laser is aligned (roughly) to the in-coupling optics of Leica.

I usually remove a cover at the exit of this diagnostic port and use a fluorescent card to verify alignment, but in this picture you see the fiber coupling a spectrograph we occasionally use to diagnose faults of the laser.



The alignment is simpler that it seems. First we start with a microscope that is fully aligned without pulse-picker as per normal operations. Then, when we need the pulse picker, we insert the two turning mirrors (L1 and R1). We do this with the laser off and with the pulse-picker crystal retracted (coarse alignment) or protected by an alignment card (fine alignment). M1 and M2 are then used to align the beam with the crystal. Then we align the PIN diode and proceed with the fine alignment of the pulse-picker cavity.  Once this is done, we align the cavity with the microscope utilizing M4 and M5. For course alignment, the signals from the two diagnostics ports is very useful until some signal is picked on the microscope monitor, after which the final fine tuning of all the optics can proceed.

Be aware, alignment of Class 4 lasers can be dangerous. Therefore, do your own risk assessments and think carefully about the logistics of your system. Now that I am starting to consider the redevelopment of the system, I thought to share these notes with you, hoping that it could be of some use.