If a message isn’t read, does it exist? Bishop Berkeley would say no, and a temporal cloak that creates a gap in time during the transmission of a message might prove him right.
Researchers at Purdue University have created such a cloak: It can hide about 46 percent of the time required to transmit data over a fiber optic cable, making half the transmission invisible.
This is an elaboration of a concept first demonstrated at Cornell University in 2011. However, that experiment only hid one ten-thousandth of 1 percent of a transmission at a repetition rate of 41 KHz.
“The big difference is not so much the [length of] time that the cloak exists, but rather that we can repeat the cloak at a very high rate, allowing us to hide data being transmitted at high speeds,” Joseph Lukens, a member of the Purdue team, told TechNewsWorld.
The Cornell team’s cloak hid a single event for just 50 trillionths of a second, while Purdue’s hid 12 billion such events per second.
How Light Cloaking Works
In essence, the two teams took the same approach. They fired one beam of light at another, splitting up the second beam into a variety of wavelengths that were sent through a fiber optic cable. Some wavelengths were transmitted more rapidly than others, thus creating a gap in time. The wavelengths were then transmitted through another modified cable that reversed the effect of the first, and reassembled into the beam that had been split up.
However, where the Cornell team used lasers — which consume considerable amounts of power — and modified optical cables, the Purdue researchers used commercially available equipment: optical fiber; phase control modulators; and chirped fiber Bragg gratings to reflect select wavelengths of light.
A fiber Bragg grating is one type of distributed Bragg Reflector. Such reflectors are made from alternating materials with varying refractive indices, meaning they pass light through them differently.
The boundary of each layer partially reflects an optical wave, and in certain circumstances the multiple reflections combine with constructive interference and block light within the range of wavelengths reflected from passing through.
“To achieve the same effect as 25 km of standard optical fiber, we can use a fiber Bragg grating that is only a few meters long,” Lukens said. The team used about 1 km of optical fiber to open the gap in time, and about 150 m of fiber to close the gaps, but “we could do the same thing over just a few meters if we used nothing but fiber Bragg gratings.”
The gaps in time were created by generating different wavelengths that propagate at different speeds and interfere with each other, a process known as — wait for it — “interference.” The gaps are then filled in by propagating the components through elements with the opposite properties.
Purdue’s Temporal Cloak
The technique used by the Purdue team can generate cloaks at 12.7 billion times a second because of the use of modulators. Multiple cloaks can be created simultaneously or in staggered or regular sequences. This method will let users “cloak a transmission for all time” if the stability of the process is improved, Luken said.
Communications can be hidden at will by simply turning the phase modulators on or off. The technique can be improved to the point of cloaking more than 90 percent of a transmission, and it might be possible to get closer to cloaking 100 percent in the future.
“The clear application is for security in an optical feed,” Rob Enderle, principal analyst at the Enderle Group, told TechNewsWorld. To gain access to data concealed in the feed, the recipient would need to know it was there, how to access it, and how to read it — all in real time — and the data couldn’t be recorded.
A more benign use could be to increase bandwidth, suggested Enderle, because “it’s almost like having a pocket dimension that you can put data in.”