LED Fiber Illuminator

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Background

The goal for this project is to couple a multicolor LED module into an optical fiber, for the purposes of illuminating blood vessels within the human body.

Our goal was to use off-the-shelf optical hardware to create the design, in conjunction with a custom enclosure to hold everything togehter. We decided to 3D print the fixture that held the LED, heat sink, and lenses. After obtaining 3D models for the optical components (plano-convex lens, SMA fiber adapter), and creating models for the luxeon LED, heat sink, and lens, we designed an enclosure that could attach to a standard optical table and couple as much light as possible into the fiber.

Methods

Due to the relatively short time scale, we decided to use a Projet HD 3500 that was available to us to create the LED assembly.

Given the poor specifications on the functionality of the Luxeon lens, we determined that we had to test it prior to committing to printing the enclosure. In fact, it behaved as if it focuses the 7 LED spots to one ~1cm wide region - it's a bit different from a single lens functionality. This appeared to be designed to couple to a 1cm diameter fiber bundle. Given the fiber we wanted to couple to was more on the order of 0.5mm, this would lead to significant coupling inefficiency.

TODO: picture of luxeon lens on LED

To help address this issue, we added a plano-convex lens to focus the light to a smaller spot. Our preliminary results with a blue LED module allowed us to burn 3mm diameter spots in black paper with about 8W of power into the LED.

TODO: picture of lens

The final step was to mechanically attach the fiber to our assembly. To ensure flexibility for different fiber attachment mechanisms, we settled on using a 1" threaded SMA adapter commonly available on Thor Labs. This allowed us to test with fibers available in the lab.

TODO: picture of final assembly.

Conclusions

In the end, we produced a fiber-coupled LED source that we believe addresses the design criteria. We found that 3D printing the enclosure and opting for adjustable focus allowed us to compensate for the unpredictable performance of the luxeon lens assembly without resorting to redesign.

One of the interesting side-effects of our fiber coupling design is that the light output from the fiber, before being diffused, takes up different patterns depending on which LED is on. For instance, the central LED will result in a circular region being illuminated in the center of the fiber output.

TODO: show image

On the other hand, any of the perimeter LEDs will result in a ring shaped illumination region. This is largely due to different fiber coupling of those LEDs in contrast to the central LED.

References - Resources and related work

References

Software