By Dominic Siriani
My CLEO experience is off to a great start, and the theme for today seemed to be optical sources of many varieties. I began my day in a session from the Symposium on Novel Light Sources for Biomedical Applications, which focused on optical coherence tomography (OCT). All the talks revolved around Fourier-domain OCT (FD-OCT). As opposed to time-domain OCT, which creates a depth image by comparing reference and scattered beams of a low-coherence-time source in an interferometer, FD-OCT uses the Fourier transform of this information contained in the frequency domain to create a depth image without the need for scanning reference arm length in an interferometer. In FD-OCT, one can use either a broad bandwidth continuous wave source (such as a superluminescent LED) or, as was the focus of this session, a frequency swept laser.
Some important properties of these swept lasers are the sweep rate (which, along with the data acquisition electronics, can determine the imaging rate), the central wavelength (which influences the return signal by how well the scattering works), the optical bandwidth (which influences the depth resolution), and, of course, the cost. There are a number of ways in which the frequency sweeping can be done, including gratings with scanning mirrors, tunable Fabry-Perot filters, or MEMS scanning filters. Semiconductor optical amplifiers (SOAs) are the typical gain medium, as they can provide the necessary wide bandwidth, and different materials can provide the variety of central wavelengths needed for different applications. The session also included a couple of examples of impressive work in which entire OCT scanning laser sources, including SOA, wavelength scanner, and other needed optics, are contained in a single compact butterfly package.
Schematic and SEM images of micropillar whispering gallery mode laser that pumps a coupled quantum dot and microcavity resonator (Hopfmann et al., presented at CLEO, 2013).
I also had the pleasure of seeing a number of talks on single photon sources. There were too many talks on this topic for me to completely cover here, but I’ll quickly summarize a few. One talk discussed the use of amplitude modulation to filter out multi-photon events from a quantum dot and create a single photon source. Another explained how duty cycle modulation in periodically poled nonlinear optical crystals can be used to create Gaussian-shaped phase matching and realize heralded single photons with 97% spectral purity. There were also a number of presentations on the use of quantum dots for quantum optics, including one that described the use of a whispering gallery mode micropillar laser pumping other micropillar cavities coupled to a quantum dot for cavity quantum electrodynamics on a single chip.
My first day here at CLEO closed with an excellent talk by Prof. Federico Capasso on quantum cascade lasers (QCLs). His presentation primarily focused on a number of strategies to create QCLs with high power/brightness and wavelength agility. He described how distributed feedback (DFB) structures have been used to make single wavelength structures, which are then combined into arrays to make a broadband multi-wavelength source. In order to increase the output power, a tapered amplifier is included to create a master oscillator power amplifier (MOPA) configuration. As an alternative to the MOPA, a tapered laser has also been demonstrated with a distributed Bragg reflector (DBR) to provide the needed wavelength stabilization. All of these methods are working towards the goal of creating a broad bandwidth, high power and brightness, mid-IR laser that is very compact.
And that concludes my first day at CLEO. Now I need to give my brain a rest before I stuff it full of information in tomorrow’s sessions. I hope it can handle it!
Disclaimer: Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government and MIT Lincoln Laboratory.
Posted: 11 June 2013 by
Dominic Siriani
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