Title: Backwards Pulse Propagation with a Negative Group Velocity in Erbium Doped Fiber
1Backwards Pulse Propagation with a Negative Group
Velocity in Erbium Doped Fiber
- George Gehring1, Aaron Schweinsberg1,
Christopher Barsi2, Natalie Kostinski3, Robert
Boyd1
1. University of Rochester, Rochester, NY 14627
USA2. Manhattan College, New York, NY 10471,
USA3. University of Michigan, Ann Arbor, MI
48109, USA
2Slow and Fast Light
- In dispersive media, pulses propagate at the
group velocity - If is made sufficiently negative, then the
group velocity becomes negative - Pulse is advanced in time
- Peak of the output pulse exits the material
before the peak of the input pulse enters
3Negative Group Velocity
- Inside the material, the peak is expected to
travel backwards, linking output and input - This raises some questions
- Why doesnt this violate causality?
- How is energy conserved?
- In what direction is energy flowing?
- Does this backwards-traveling peak really exist?
4Instructional Video
M. Ware, S. Glasgow, and J. Peatross, Optics
Express 9, 519-532 (2001)
5Coherent Population Oscillations
- Coherent Population Oscillations (CPO) utilized
to create narrow spectral hole in an absorption
or gain feature - The excited state population oscillates at the
beat frequency between pump and probe fields
Hillman, Boyd, Krasinski and Stroud, Jr.,Optics
Communications 45, No. 6, 416 (1983).
Coherent Population Oscillation effect in a ruby
crystal.
6Why EDOF?
- Erbium doped optical fiber exhibits gain or loss
dependant on optical pumping power at 980 nm - T1 10 ms
- Fiber geometry is favorable
- Tight confinement
- Large interaction lengths
Schweinsberg, Lepeshkin, Bigelow, Boyd and
Jarabo, Europhys. Lett., 73 (2), 218 (2006).
7Experimental Setup
0.5 mW
128 mW
Strong
Very Weak
- Setup for testing direction of energy flow
8Experimental Setup
- Setup for temporal resolution experiments
9Example Data Trace
- Traces taken for lengths of fiber between 0-9 m
- The fiber is physically cut in 25 cm intervals
between each data trace
G. M. Gehring, A. Schweinsberg, R. W. Boyd, et
al. Science 312, 895 (2006).
10Video Creation
- These traces are then arranged spatially and
played back simultaneously
11Video Frames
- Arrows emphasize peak positions inside and
outside the material - Peak inside the material travels backward, but
not with a constant velocity - This is a result of distortion caused by
non-uniformity of the gain curve, primarily due
to gain saturation
12Video
13Video Frames
- Effects of gain removed
- Peak position is clearly seen to travel from
right to left inside the material - Peak moves at the same speed inside and outside
the matieral - ng -4000 (vg -75 km/s)
14Video Gain Removed
15Summary
- EDOF and CPO utilized to study pulse propagation
effects in a medium with a negative group
velocity - For a pulse propagating through a medium with a
negative group velocity - Pulse peak moves backwards in the material
- Energy transport is always in the forward
direction
16Acknowledgements
- Nonlinear Optics Group
http//www.optics.rochester.edu/boyd/ - Financial support from
- DARPA/DSO Slow Light program
- NSF