John B. McCoy and John T. McCoy Professor of Physics, Emeritus
Mail: Dept. of Physics, Thompson Physical Lab., Williamstown, MA 01267
- A.B. Harvard College (1975)
- Ph.D. Brown University (1985)
- AT&T Bell Laboratories: Postdoctoral Member of Technical Staff, 1984-85
- Williams College, Dept. of Physics, Chair, 1995-1996 and 1998-2000
- Williams College, College Marshal, 2007-2014
Recent honors students and summer research students
Modelocked Optical Fiber Lasers
My students and I have been investigating an optical fiber laser that uses a nonlinear optical loop mirror (NOLM) to generate picosecond pulses of light. Unlike most lasers that use mirrors to confine light to the laser cavity, our laser uses optical fiber as its cavity. A section of fiber doped with erbium serves as the gain medium. When we pump the erbium with 0.98 micron light, it lases at 1.55 microns, conveniently the same wavelength at which optical fiber is most transparent and therefore most suitable for telecommunications.
Our goals include understanding how pulses form in this laser and studying how picosecond pulses propagate in optical fibers. Dispersion tends to elongate the pulses, while self-phase modulation, a nonlinear effect, tends to compress them. Under the right circumstances, these two effects can balance each other and a special pulse shape (a “soliton”) can propagate for long distances without spreading. Solitons may prove useful for encoding data or switching data in high-speed telecommunication systems.
In practice, achieving the correct balance of dispersion and self-phase modulation is difficult, so we have experimented with fibers having different dispersion properties. Also the laser is very sensitive to the polarization of light inside the fiber. Although the optical fiber we use is designed to have no preferred polarization axes, it becomes birefringent when bent. We have developed a mathematical model accounting for polarization that helps us predict the conditions favorable for producing picosecond pulses
We built our original fiber laser in 1995. Fourteen students have written honors theses related to the fiber laser, twelve of whom have gone on to graduate programs in engineering or in physics. Optical fiber technology has improved substantially, so we recently built a new fiber laser making use of the latest components and materials. In particular we now have fibers designed for transoceanic communication that are engineered to have either positive or negative dispersion, giving us unprecedented control over the dispersion in the laser. The new fiber laser will allow us to refine our understanding of how ultrashort pulses propagate in fiber.
- S. Wu, J. Strait, R. L. Fork, and T. F. Morse, “High Power Passively Mode-locked Er-doped Fiber Laser with a Nonlinear Optical Loop Mirror,” Opt. Lett. 18, 1444 (1993).
- J. Strait, J. D. Reed (Williams ’89), A. Saunders (Williams ’90), G. C. Valley, and M. B. Klein, “Net Gain in Photorefractive InP:Fe at l = 1.32 microns Without an Applied Field,” Appl. Phys. Lett. 57, 951 (1990).
- J. Strait, J. D. Reed (Williams ’89), and N. V. Kukhtarev, “Orientational Dependence of Photorefractive Two-Beam Coupling in InP:Fe,” Opt. Lett. 15, 209 (1990).
- J. Strait and A. M. Glass, “Time-Resolved Photorefractive Four-Wave Mixing in Semiconductor Materials,” J. Opt. Soc. Am. B 3, 342 (1986).
- J. Strait and A. M. Glass,”Photorefractive Four-Wave Mixing in GaAs Using Diode Lasers Operating at 1.3 microns,” Appl. Opt. 25, 338 (1986).
- J. Strait and J. Tauc, “Light-Induced Defects in Hydrogenated Amorphous Silicon Observed by Picosecond Photoinduced Absorption,” Appl. Phys. Lett. 47, 589 (1985).
- K. M. Jones and J. Strait, editors,Optics and Spectroscopy Undergraduate Laboratory Resource Book (Optical Society of America, Washington, DC, 1993).