Photonics Research
Ying-Chih Chen
Professor of Physics
Hunter College of CUNY
My research has been focused in two areas: fiber laser arrays and the application of lasers to biomedical imaging.
Laser array
Combining the output power of a number of individual lasers in the form of laser array is a commonly used method of generating high power. When the N-elements in an array have the same phase, the combined laser beam has enhanced power (µ N2 ) and reduced beam divergence(µ 1/N ).due to constructive interference. One the other hand, if the adjacent elements have a phase difference of 180 degrees, the interference is destructive, resulting in cancellation of power. In some cases, the interaction among the elements can never be stabilized, resulting in unstable or chaotic output power.
The research in this area is currently concentrated on the techniques of phase locking of multiple fiber lasers to produce a diffractive-limited beam. Unlike conventional lasers made of bulk media, the fiber lasers typically have long cavity lengths, ranging from several meters to several tens of meters and are highly susceptible to uncontrollable thermal and mechanical perturbations. These perturbations can cause random fluctuations in the refractive indices of the individual fibers and in turn destabilize the phase locked states. The challenge is to overcome the random fluctuations and maintain a constant relative phase relation among the elements. By using a spatial filtering technique we have recently achieved stable phase locking in a two-element system operating in the continuous and in short pulse modes.1,2 We are currently working to apply this technique to phase locking in two-dimensional fiber laser arrays containing many more elements.
Photoacoustic imaging of tissues
Ultrasound is an imaging method commonly performed in pulse-echo mode, where a focused acoustic pulse is emitted by the transducer into the tissue and echoes produced wherever the pulse encounters a change in speed of ultrasound. Like all detection techniques based on waves, such as ultrasound, light, and microwave, the ability to resolve small objects is limited by diffraction. As a rule, the ultimate limitation of resolution of imaging is on the order of one wavelength. For transducers with a center frequency of 20 MHz, the resolution is on the order of 200 microns. Photoacoustic imaging, which combines optics and ultrasound to produce images, has attracted considerable interests as complementary diagnostic imaging technique. The generation of photoacoustic signal is based on thermal expansion of tissue in response to an absorbed light pulse. This rapid expansion of tissue generates a broadband acoustic pulse, which can then be detected by an ultrasound transducer. Early studies of photoacoustic imagining utilized laser pulses to illuminate a large area where the spatial resolution was still determined by the parameters of the ultrasound transducer. Recently, we have demonstrated that, by focusing the laser to a micron-sized spot which is far smaller than can be achieved with a weakly focused ultrasound transducer, the image resolution can be significantly improved by one order of magnitude compared to that obtained with pulse-echo ultrasound or by photoacoustic imaging with a non-focused light source.3 Images produced by the absorption-based contrast at a particular wavelength is independent of those imaged by pulse-echo ultrasound and may be use to highlight specific tissues of interest. For example, the laser wavelength may be tuned to the absorption band of melanin to reveal the layers of retina and choroid that have not been seen before with conventional ultrasound technique.
Potential applications of this technique include clinical examination of superficial and thin tissues containing optically absorbing pigments such as melanin or hemoglobin, including skin, mucosa, and ocular tissues (iris, retina and choroid). Another new direction is the detection of a single cell which is tagged with an light-absorbing center such as a gold nanoparticle.
1 Liping Liu, Yi Zhou, Fanting Kong, and Y. C. Chen Kotik K. Lee “Phase locking in a fiber laser array with varying path lengths”, Applied Physics Letters, 85, 4837 (2004)
2 Fanting Kong, Liping Liu, Charlotte Sanders, and Y. C. Chen, and Kotik K. Lee, “Phase locking of nanosecond pulses in a passively Q-switched two-element fiber laser array”, Applied Physics Letters, 90, 151110 (2007).
3. Fanting Kong, Y.C. Chen, Harriet O. Lloyd, Ronald H. Silverman, Hyung Kim, Jonathan M. Cannata , and K. Kirk Shung, “High-resolution photoacoustic imaging with focused laser and ultrasonic beams”. Applied Physics Letters, 94 , 033902 (2009).
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Last Updated on January 10, 2009