Experimental Soft Condensed Matter Group Professor David A. Weitz Division of Engineering and Applied Sciences Department of Physics Harvard University
 Slow-light propagation in highly-doped Erbium fibers
We have developed a new experimental research line focused on studying slow light propagation (SLP) in solid materials at room temperature for applications in communications. Slow (fast) light is the propagation of an optical pulse at a very low (high) group velocity. We are developing experiments to generate slow and fast light in Erbium-doped fibers at 1,5 microns based on coherence population oscillations (CPO). A weak field propagates in a transparency hole created in the absorption spectrum by a strong field with a slight different frequency. The population is forced to oscillate at the beat frequency between both fields. We look for different parameters to control the propagation regime.
  • Dynamic control of group velocity in erbium doped fibers with coherent population oscillations (CPO) by using active Bragg gratings
  • Slow and fast light propagation in ultra highly-doped Erbium fibers: control of pulse distorsion
Slow light setup
This is a photo of our experimental setup inthe QUANTUM COHERENCE LABORATORY

These are our first experimental results in Erbium-doped fibers at 1,5 microns based on coherence population oscillations (CPO), i.e, a weak signal propagates in an absorption hole created by a stronger signal with a slightly different frequency. We used a diode laser at 1536 nm with 16 mW. In order to simulate both signals we periodically modulate the laser output (10%) with a frequency between 1 Hz and 10 kHz.

Temporal evolution
We show in this figure a very high delay, around 1 ms, obtained between the signal propagated through the Erbium-doped fiber and a reference one. The laser output was modulated at 100 Hz. This large time delay remains while the modulation frequency (in terms of CPO the detuning between the weak and strong fields ) has a value lower than the width of the absorption hole (given by the lifetime 10 ms of the metastable level 4I13/2 ).
Fractional delay
The fractional advancement shows above presents a large value close to 10%. In the right figure the ratio bewteen the modulation part of the signal (playing the role of the weak field) and the dc part of the signal (playing the role of the strong field) is shown versus the modulation frequency. In this figure we observe the absortion hole due to the CPO phenomenon.


Publications 

Proceedings: International Meetings

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