A groundbreaking demonstration of wide optical control of group velocity of light within a fiber may clear the way for the development of fast-access memories and optically controlled delay lines for optical computing and fiberoptic communications.
Successful experiments to achieve strongly reduced group velocities in optical fiber (slowing light or nearly stopping it) have been reported, but require special media such as cold atomic gases or electronic transitions in crystalline solids at well-defined wavelengths. Superluminal (faster than the speed of light) or even negative group velocities have also been reported. In a new technique that uses the narrowband gain or loss generated by the nonlinear optical interaction of stimulated Brillouin scattering (SBS), researchers at Ecole Polytechnique Federale de Lausanne (Lausanne, Switzerland) have achieved the first reported demonstration of wide optical control of the group velocity of light within an optical fiber-at room temperature and using off-the-shelf components.1 This active control is possible in any type of fiber at any wavelength.
In SBS, interaction of a strong pump wave (frequency = fpump) with a weaker counterpropagating probe wave (frequency = fprobe) can generate an acoustic wave if certain phase-matching conditions are met (if fpump = fprobe + νB; νB = the Brillouin shift). The acoustic wave scatters photons from the pump to the probe wave in a narrowband amplification process in which the continuous-wave (CW) pump produces a narrowband (30- to 50-MHz) gain in a spectral region around fpump - νB. Through the SBS process, the pump wave induces a change in the probe wave’s propagation constant as a function of frequency, which in turn produces a strong change in the group velocity and a corresponding delay in the pulse at the fiber output.
In conventional single-mode fibers optical time delay varies logarithmically with the net gain or loss experienced by the probe wavelength, exhibiting an approximate 1-ns delay per dB gain introduced in the probe wavelength. If the probe wavelength is tuned so that fpump = fprobe - νB, a loss (rather than a gain) in the probe wavelength is observed, leading to a negative group-index delay (and faster-than-light propagation of the pulse along the fiber).
Experimental setup
The experimental setup used a single 1552-nm distributed-feedback (DFB) laser diode to generate the pump and probe signals (see figure). The source was launched into an electro-optic modulator (EOM) to create two first-order sidebands with a frequency difference that is set exactly to the Brillouin frequency, νB, of the test fiber (around 10.8 GHz). The carrier wave was suppressed by controlling the DC (direct-current) bias voltage delivered into the EOM. The lower-frequency sideband was used as the probe pulse, while the higher-frequency sideband was used as the CW Brillouin pump wave. The time delay of the probe pulse was measured for different Brillouin gains while varying the pump amplitude.
Using an 11.8-km length of standard single-mode fiber and a probe pulse with a full-width at half-maximum of 100 ns, a maximum delay of 30 ns was obtained when the Brillouin gain was 30 dB. Alternatively, a maximum advancement of -8.4 ns was obtained when the Brillouin loss was -12 dB. Later, by cascading four identical delaying segments, an impressive delay of 152 ns was achieved using a 40-ns pulse.
Leaving before entering
Using a 2-m standard single-mode fiber and a 10-W pump power, the researchers recorded a group-index variation from +4.26 to -0.7. The negative group index means that the pulse peak exits the fiber before it enters, corresponding to a delay of -14.4 ns.
“A group velocity faster than the speed of light does not systematically break the famous principles of relativity; because the spectral transition is narrowband, all frequency components cannot experience the same group velocity and amplitude response, meaning that only smooth signals can be advanced and information (essentially broadband sharp transitions) cannot propagate at a speed faster than c,” says researcher Luc Thévenaz. “Amazing propagation conditions were achieved in extreme media these past years, but these experiments were reserved to an elite having access to the proper facilities. By realizing the same extreme propagation conditions in a standard optical fiber at room temperature with off-the-shelf elements, this novel photonic tool is available to a much broader community.”
REFERENCE
1. M. Gonzalez-Herraez et al., Appl. Phys. Lett. 87, 081113 (2005).
