# Tapered fiber production

## Abstract

Production of reproducible tapered fiber is a convinient tool for coupling light into integrated photonic waveguides. This is a log of educational run on December 7th, 2021, by Dr. Son.

## Material preparation
1. Teflon (or some other material safe for HF) beaker
* Dr. Son uses 50 mL one that is shaped relatively tall.
* This beaker's cross-section should have a size that can support the fiber holder.
* Beaker is supposed to be transparent to check the light better.
* VITLAB brand beaker is sold at the company inside KAIST.
2. Fiber holder made in ABS and fiber installation
* ABS or any other material safe for HF can be used.
* Holder can be in any shape that can stably hold the fiber during wet etching. Black one in the figure is the holder where fibers are taped, one by one.
* Taping fiber one by one helps handling later when they are taken out and jackets are removed.
* Out of N fibers, make a connection to a another fiber with a connector. Use splicer. Do not need to do this for all N fibers because fibers inside one beaker go through approximately the same process.


3. Fiber
* Corning's standard SMF-28 fiber works.
* However, similar products such as YOFC fiber fiber also works. This could be purchased from FiberPia in Daejeon.
* Keep in mind that the jacket material should be permeable by HF. Dr. Son recommends it.

4. 49% HF
* Any 49% HF is supposed to work, and it does not matter much which brand you purchase it from.
* Samchun 49% HF can be purchased from the company inside the KAIST.

5. Fiber stripper
* The smallest hole will be used to strip off the jacket.

## Wet bench procedure

1. Make sure beakers are in flat area. If needed, use the flat board as shown above.
2. Cut the end of the fiber by scissors so that the end of the fiber is approximately at the 10 mL mark of the beaker.
3. Pour HF 49% up to 30 mL.
4. Clean the tip of fiber with the wipe wet by IPA.
5. Dip fibers into the beaker and start timer.
6. Make sure fiber is set tup straight.
7. If you turn on the visibile laer (usually it is the fault inspector), it should look like a dot. Turn it off after checking because it may affect etching.
8. Approximatley after 1 hour, start checking the laser light by turning it on and off. It will start getting elongated.
9. Elongated shape will continue 3-5 min. Once its shape is switched back to dot-like one, stop etching
10. Rince it with DI water. Use two beakers to make sure that second beaker has negligibly small amount of HF.
11. Take out the second beaker with the immersed fiber to a desk nearby.
12. Carefully take out one fiber and strip off the jacket using the fiber stripper.
* It should be approximately 3 cm away from the etched core tip.
* For the one with the laser attached, it is easy to identify the location of the etched core tip.
* Without laser attachment, there is no method to check the location of the fiber tip. In this case, just eyeball to identify the point 5 mm into the fiber from the jacket tip. At this point, fiber tip is only made of jackets because core and claddings are etched away.
* When taking of the jacket, slide it smoothly along with the fiber.

## Inspection method
1. Beam profiling
* Right after the jacket removal, red laser can be introduced and see its beam shape by shooting it to the white wipe in the cleanroom. It may have some small irregularities due to multi-modal features, however, overall should give a circular shape.
* Bring these fibers back to lab and plug in to telecom light source. This time, single mode fibers will not suffer from multi mode interference and will give cleaner profile. Telecom beam profiler can be used to get clean Gaussian shape.
* Fiber can be easily installed as shown in the picture.


2. Optical microscope
* Basic feautures of the fiber can be seen by using optical microscope.
* 5-20X objective lenses can be used to see the tip on the order of 1um size. Any feature below can be only observable by SEM.