All-optical switching with transverse optical patterns

We demonstrate an all-optical switch that operates at ultra-low-light levels and exhibits several features necessary for use in optical switching networks. An input switching beam, wavelength $\lambda$, with an energy density of ${10}^{-2}$ photons per optical cross section $[\sigma ={\lambda }^2∕\left(2\pi \right)]$ changes the orientation of a two-spot pattern generated via parametric instability in warm rubidium vapor. The instability is induced with less than $1\mathrm{mW}$ of total pump power and generates several $\mu \mathrm{Ws}$ of output light. The switch is cascadable: the device output is capable of driving multiple inputs, and exhibits transistor-like signal-level restoration with both saturated and intermediate response regimes. Additionally, the system requires an input power proportional to the inverse of the response time, which suggests thermal dissipation does not necessarily limit the practicality of optical logic devices.

Figure 1

(Color online) Experimental setup for transverse optical pattern generation. A cw Ti:sapphire laser serves as the light source. Optical elements are as follows: Uncoated glass plate (GP), electro-optic modulator (EOM), half-wave plates $(\lambda ∕2)$, polarizing beam splitters (PBS1, PBS2), $50∕50$ beam splitter (50:50), CCD (charge-coupled device) camera (CCD), and avalanche photodiode (APD). Beams are indicated by line type: Pump beams (solid), switch beam (dotted), and instability-generated light (dashed). The foward (backward) pump beam propagates in the cw (ccw) direction through the triangular ring cavity.

Figure 2

Instability-generated patterns and optical power as a function of pump laser frequency detuning $(\Delta =\nu -{\nu }_{F=1,F^{\prime }=1})$. (a),(b) Patterns for red and blue pump laser detunings ($\Delta =-50\mathrm{MHz}$ and $\Delta =+25\mathrm{MHz}$, respectively, indicated by arrows). (c) Orthogonally polarized output power, emitted in the forward direction, as a function of frequency. These data correspond to a single scan through the ${}^{5}S_{1∕2}(F=1)\to {}^{5}P_{3∕2}\left(F^{\prime }\right)$ transition in ${}^{87}\mathrm{Rb}$ from low to high frequency. The bold tick marks indicate the hyperfine transitions labeled by ${\mathrm{FF}}^{\prime }$, where F $\left({\mathrm{F}}^{\prime }\right)$ is the ground (excited) state quantum number. Our switching experiments are conducted with the pump laser detuned $\Delta =+30\mathrm{MHz}$ (dashed line). Pump beam power levels: $630\mu \mathrm{W}$ forward and $225\mu \mathrm{W}$ backward.

Figure 3

(Color online) Generated light is emitted along cones (blue), which are centered on the pump beams (red), and forms a transverse pattern (red spots) in the measurement plane. (a) Unperturbed system with two-spot pattern induced via weak pump-beam misalignment; (b) data showing two-spot pattern. (c) A weak switch beam (yellow) rotates the pattern; (d) data showing rotated pattern.

Figure 4

Instability thresholds. Generated optical power as a function of total pump beam power. The off-axis instability occurs with a threshold of $600\mu \mathrm{W}$ (circles) whereas the on-axis instability occurs with a threshold of $1.1\mathrm{mW}$ (squares). Data shown corresponds to a fixed pump beam power ratio of 2.7 to 1 (forward to backward).

Figure 5

Time response of the pattern. (a) While turning the switch beam on and off with successively decreasing power levels, (c), (d) two detectors simultaneously measure the spatial regions indicated on the center frames by circles which encompass bright and dark areas of the output pattern (the images have been inverted for visibility). These images are collected by CCD camera and the normalized detector output signals are shown in (b), (e) corresponding to the spatial location of the detector within the pattern. (b) The “on-state” detector and (e) The “off-state” detector.

Figure 6

Response time and number of input photons as a function of switch beam power. (a) The response time increases for decreasing switch beam power. (b) The increased response time for weak input powers indicates that a constant number of photons are required to actuate the switch regardless of the amount of input power.