Lab: Bronfman B24B
A.B. Harvard University (1975)
M.S. Brown University, Physics (1981)
Ph.D. Brown University, Physics (1985)
AT&T Bell Laboratories: Postdoctoral Member of Technical
Williams College, Department of Physics, Chair, 1995-1996 and
- Williams College, College Marshal, 2007-present
Recent honors students and summer research students
The Physics of Short Pulses in Optical Fibers
For the past several years my students and I have been investigating a modelocked optical fiber laser that generates picosecond pulses of light. Unlike most lasers which use mirrors to confine light to the laser cavity, our laser uses a loop of optical fiber as its cavity. A section of fiber doped with erbium and ytterbium serves as the gain medium. We pump the gain medium with 1.06 µm light and it lases at 1.55 µm, conveniently the same wavelength at which optical fiber is most transparent and therefore most suitable for telecommunications. Our immediate goal is to understand how polarization and external feedback influence pulse formation in this laser. Our future goal is to use the laser as a light source for studying the propagation of picosecond pulses in optical fibers.
In 1993 I worked with Song Wu and Dick Fork at RPI on the first fiber laser successfully modelocked with a Nonlinear Optical Loop Mirror (NOLM). It produced 1 ps pulses, but required a moving external mirror providing feedback to the cavity in order to initiate mode-locking. It also was very sensitive to the polarization of the light in the cavity — if the polarization was detuned, the laser would produce pulses longer than 50 ps.
Todd Stievater ’95 built an NOLM fiber laser in our laboratory at Williams. Kira Maginnis ’95 constructed an autocorrelator that will enable us to measure pulse durations on the order of a picosecond. Matt DeCamp ’96 continued Todd’s work with the laser, studying its pulse trains under a variety of operating parameters. Like the RPI laser, the output of our laser depends critically on polarization and changes as the temperature of the laser varies.
To better understand the polarization effects of the fiber laser cavity, Ben Evans ’96, Aaron Kammerer ’98, and Matt Partlow (St. Lawrence U. ’97) developed an apparatus to measure the birefringence due to bending the fiber as a function of its temperature. This birefringence increases exponentially with temperature and it decreases with the time that the fiber is bent. Presently we are working on a model to explain these results. Since practically all optical fiber systems include several loops of fiber, this temperature effect has important implications for any system with polarization-dependent components, including our NOLM laser.
In the longer term, we plan to use our fiber laser to study how picosecond pulses propagate in fiber. Dispersion tends to elongate the pulses as they travel through kilometers of fiber, while self-phase modulation, a non-linear 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.
- 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).