Week 2: Walkthrough

This week, we had an introductory walkthrough in the photonics lab with Dr. Asghari. We went over initial safety procedures including the use of goggles and sealing off entrances when the laser is active. The goggles are the main form of precaution and the importance of the goggles relies on the wavelength range they protect one’s eyes from. The Menlo Systems femtosecond fiber laser has two output ports for 780nm and 1550nm wavelengths. For this capstone’s purposes, the 1050nm wavelength is used for the OCT imaging system. We also learned that there are multiple types of fibers with angled and flatheads. All fibers used in this project are the angled variant. Cleaning these fiber optic connections is of utmost importance because even the slightest dust particle can cause component or system failure. With dust particles, there is great potential for misalignment which can degrade the signal as well as inaccuracy with reflection if the core is blocked.

In summary, the system runs as follows. The laser is projected into an input collimator in order to focus the pulses into fibers, going from 1mm to 4μm diameter. The laser is then split into two different paths, one that goes to scanning the sample and the other to two collimators separated by free-space. The free-space is used to have a delay that equals the time of the sampling laser. The sampling laser goes through a mirror galvanometer, which is an instrument that senses electric current by deflecting light beams with rotating mirrors. This instrument moves the laser across and up the sample to scan it. The laser then combines back with the laser from the free-space delay. The laser then goes through 2km of fiber dispersion. After the dispersion the laser goes through three more stages, an erbium-doped fiber amplifier, a photodetector, and a disconnecting circuit breaker before the data is readable by the oscilloscope. Finally, once the data is readable by the oscilloscope, the data can be streamed into the computer for processing. In Figure 1, a basic block diagram of the OCT system is shown.

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Figure 1: Basic Block Diagram of the OTC System

In addition to the main system, the spectrometer was introduced as an alternative instrument to display the signal. While the oscilloscope is built for speed, the spectrometer was built for sensitivity. At this point, we were encouraged to familiarize ourselves with turning on the laser and displaying it on the spectrometer as well as get acquainted with using the spectrometer.

Lastly, we had a quick overview of the code that connects with the oscilloscope and gathers data to visualize it on the computer. In the Python code houses three queues, the raw queue, frame queue, and processing queue. The Python code runs through three separate codes. One that collects the data, one that processes the data, and one that visualizes the data. The Python code also takes control of the oscilloscope, disabling all control with the psychical buttons and dials. This is crucial to allow the program to run without error. However, there is a kill program in place in case the oscilloscope is needed. Overall, we now have a general overview of the system and our next steps are to familiarize ourselves with all the instruments and understand each stage of the system on a higher level.