High-Power Pulsed Fiber Laser
In fiber lasers, the active gain medium is an optical fiber that is doped with a rare earth element. In our laser, this element is erbium. However, other elements such as ytterbium, dysprosium, and thulium can be used. The development of fiber lasers has proved an influential component in terms of both advancement and application. For example, fiber as an optical path allows for a durable, transportable, and convenient physical apparatus. In addition, fiber properties allow for a high optical quality, while also making it easy to couple light. They allow us to achieve much greater power than otherwise, up to thousands of kW.
So how does a fiber laser work? Well, fiber amplifiers form the basis of all conceptual understanding. Basically, a typical fiber laser consists of a fiber amplifier placed within a cavity. The cavity, or ring, in our case, serves the purpose of providing the necessary optical feedback. The pumping process drives the activity of the laser, while there are several ways that this can work. For ours, and basically all erbium doped fiber lasers, the pumping scheme is considered to be “three level.”
In general, fiber lasers utilize stimulated emission in order to amplify light, in keeping with the famous LASER acronym (Light Amplification by Stimulated Emission of Radiation). In the process of pumping, the erbium dopant absorbs incident photons and is excited to a higher energy state. Quickly, the erbium is then relaxed to the first excited energy state. In this process, energy is stored and used to amplify the pump signal. For our three level model, the erbium occupies the ground state after each stimulated emission event.
While many fiber lasers utilize a Fabry-Perot cavity, while ours employs a ring cavity. One advantage of this is that most of the cavity is in fiber.
Our Set Up:
Optical Bench Alignment:
Early on, aligning the optical bench was a huge area of focus for our group. The bench was in bad shape, and we quickly realized realignment was necessary. Dar and David were able to achieve a fifty percent efficiency through the bench, and we kept this number in mind. In aligning the bench, it is necessary to first achieve something close to perfect collimation for fiber ports on both sides. Each fiber port has essentially 5 degrees of freedom. There are the x and y screws, which serve to tilt the lens in such a way that the direction of the beam can be variably moved up and down, and left to right. In addition, there are three “z” screws, which move the lens either closer or further from the fiber tip. In this way, you can pass the beam between divergence, collimation, and convergence.
At first, we attempted to simply adjust these screws until we visually observed a collimated beam to the best of our ability. We used an IR card to determine the beam diameter along its path. While this improved the efficiency of the bench from when we found it, it was still very far away from where we needed to be. Theoretically, the bench should be able to achieve an efficiency of around eighty percent. Below, Plot 1 displays the loss levels for our first alignment attempt.
Soon, we stumbled upon a method that worked. We made a video of our process, so you can follow along if you would like. First, we had to make sure that we had the alignment laser pointed straight. We placed two alignment apertures on the optical bench, one as close as it could be to the fiber port in question, and one as far away as possible. Once the output through the second aperture was maximized, we were confident we had the laser pointing in the right direction. Next, we set up a beam profiler that was along this line of light. We hypothesized that collimation could be most closely achieved by minimizing the spot at some distant point. As such, the beam profiler was placed at the furthest point it could, still in line with the alignment laser. The beam profiler was a necessity, as it would be very difficult to characterize the size of a tiny spot such as the alignment laser. We observed a minimum by using the Thorlabs Beam Profiler software, then repeated the process with our second fiber port. Now, we were ready to place each fiber port back on the bench, and use a power meter to characterize our loss. Within the bench, we were able to carefully tweak the screws to fine align the bench. We achieved a maximum efficiency of 84%, a number we were thrilled with. Plot 2 displays the loss below.