Using Spectroscopy to Characterize an Optical Amplifier

Casey McKenna, Chris Corder, John Elgin and Harold Metcalf

Department of Physics and Astronomy
Stony Brook University

Optical Amplifiers are critically important for experimentation in the field of Atomic, Molecular and Optical Physics. In order to excite atoms to particular energy levels, certain beam characteristics are required. The optical amplifier increases the power of an input beam light source. This amplification process is not perfect, however. The maximum possible increase in power, or gain, is limited by an aspect of the process of laser amplification itself: amplified spontaneous emission (A.S.E.). After the gain medium within the optical amplifier is pumped with energy, stimulated emission can produce more light than the incident seed. However, the excited atoms may also spontaneously decay to the lower energy level and the light from the spontaneous emission will be amplified as well. Since this light is incoherent, the amplified light will also be incoherent, so the more of it that gets amplified, the less of the coherent light that is desired for the experiment will be amplified, and the gain will be limited. To use an optical amplifier for an experiment or for any practical application it is important to understand the characteristics of the A.S.E. and how it affects the output light.

The goal of this project is to characterize the gain curve of the A.S.E. through the method of spectroscopy. The device tested was a tapered broadband optical amplifier (B.O.A.) obtained from THORLABS. Its purpose was to amplify the seed light from a diode laser at a wavelength of 1083 nm. The type of spectrometer used was a McPherson 218 monochromator. First, the output from the B.O.A. was observed without the seed light. When light is sent through the monochromator, it is diffracted by a grating that separates the different wavelengths by angle. I scanned across the spectrum of the B.O.A. light until I found its smooth peak around a wavelength of 1000 nm. I made these measurements with a photodetector placed at the output slit of the monochromator that was connected to an oscilloscope. The B.O.A. is most efficient at amplifying the beam from a seed laser around this wavelength. I plan to retake this data in order to improve my determination of the location of the center of the gain curve so that the 1083 nm light can be optimally amplified. I will also test the effect of different conditions of the B.O.A. on the gain curve, such as a slightly different temperature.

This work is supported with funding from the Office of Naval Research.