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Graphene Sheath Modulates Fiber-Optic Transmission At 200 GHz

timothy posted about 10 months ago | from the let's-go-straight-to-x-rays dept.

Communications 18

An anonymous reader writes "Researchers in China have shown that a graphene sheath can modulate light transmission through an optical fiber at 200 GHz. The graphene, even crudely draped over the optic fiber on a microscope slide, absorbed some of the light passing through the fiber. But a preceding short-wavelength light pulse could temporarily disable the effect, enabling an all-optical infrared fiber-optic switch. Recovery was fast enough to enable modulation of transmitted light at 200 GHz using conventional fiber-optic communication wavelengths and thinned commercial telecommunications fibers. The findings could have use in telecommunications industry and future high-speed on-chip optical interconnects."

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And on the far end? (4, Interesting)

Shatrat (855151) | about 10 months ago | (#45976687)

It sounds like this is just very high speed On Off Keying, which is not only limited by modulation but also by the ability to clearly receive that signal on the far end.
This could be an alternative to coherent phase shift keying as a short range 100G+ interconnect though, where dispersion and noise aren't an issue.

Re:And on the far end? (5, Informative)

kaiser423 (828989) | about 10 months ago | (#45976845)

It's modulation depth (per the abstract) is only 38%, so it's not quite a broad-band as on-off-keying. More like a non-optimal AM modulation. Still, if you can do it at those speeds that's head and shoulders above and beyond what we have now.

Another key benefit would be the simplification of encoding/decoding hardware. Using coherent PSK, QAM and other schemes there's quite a bit of effort put into determining the channel, etc to be able to know your current place in the constellation which can be a problem in the presence of noise sources. There's the well-known algorithms, tons of proprietary ones and new research papers every day on ways to betermine estimate the transmission channel to allow for more points (bits) in a constellation. Some of the QAM's, after disruption take over 100,000 symbols to re-lock to the bit stream. But that's the price you pay to shove more data down the pipe. If this works well though, you can get rid of all of that logic, stringent requirements on the transmission media and improve immunity to noise because with a simple AM, it only takes one symbol to re-lock onto the bit stream. They're dead simple to implement and robust as can be.

I can definitely see some really big uses for this if all goes well.

Re:And on the far end? (2)

Ungrounded Lightning (62228) | about 10 months ago | (#45982091)

It's modulation depth (per the abstract) is only 38%, so it's not quite a broad-band as on-off-keying.

Once you've got that it's trivial to use beam splitters and destructive interference to change the modulation from 62% vs. 100% to 38% vs. essentially 0% amplitude.

Re:And on the far end? (1)

Ungrounded Lightning (62228) | about 10 months ago | (#45991569)

It's modulation depth (per the abstract) is only 38%, so it's not quite a broad-band as on-off-keying.

Once you've got that it's trivial to use beam splitters and destructive interference to change the modulation from 62% vs. 100% to 38% vs. essentially 0% amplitude.

Not that it matters: The receiver is AC coupled, anyhow. As long as the modulation is sufficiently deep that the signal is substantially above the noise floor, you're fine. Switching a third of the amplitude is nearly as good as switching all of the amplitude of a signal of third the amplitude. (in terms of energy, which is the square of amplitude, the switched part is more than half again the unswitched part.) If you're using stimulated-emission repeater amplification along the way you need more excitation light, but that's still a trivial cost.

Re:And on the far end? (0)

Anonymous Coward | about 10 months ago | (#45995063)

Not exactly. You want to condition the light to as close to OOK as you can get it before it enters an optical fiber, so as to send as little optical power into the fiber as the link budget permits. To do otherwise is to risk opacity damage to the fiber due to fusing (on high power, long-distance links) and shorten the lifetime of the detector.

In practice the reach of optical fibers is often dispersion limited, so more important is to approximate an optical soliton to minimise dispersion, rather than hard edged keying.

Re:And on the far end? (2)

upontheturtlesback (2605689) | about 10 months ago | (#45977211)

If this is an all-optical switch that doesn't require high-powered lasers or other difficult to achieve non-linear optics, wouldn't this have applications for all-optical computing gates, as well? Basically use something like this to construct an all-optical transistor, and have a logic circuit powered by light instead of electricity?

chops up the chip density some (3, Interesting)

swschrad (312009) | about 10 months ago | (#45977413)

you also have to dissipate the heat of conversion in the source and detector, which means large chunks of silicon in relation to the transistors. but I can see using this to cut distortion cross-chip, or up a stack of chips in Cray-ish constructions, and maintain internal speed.

there is of course the usual last line of the study document, on behalf of the lead and the graduate assistants who have several years to go in their degree yet, that "this effect needs more study."

Redundant? (0)

Anonymous Coward | about 10 months ago | (#45977581)

So, to modulate a light source, all you need is a modulated light source. Sounds redundant.

Re:Redundant? (2)

putaro (235078) | about 10 months ago | (#45982251)

Well, to modulate an electric current you need an electric current (that's how a transistor works). Amplification isn't done by increasing the voltage/current of the reference signal. You use the reference signal to modulate a higher voltage/current. Once you have a transistor-equivalent we know how to use that to do all kinds of interesting things.

Re:Redundant? (0)

Anonymous Coward | about 10 months ago | (#45995177)

Amplification isn't done by increasing the voltage/current of the reference signal.

It is if the input signal is large relative to the output signal (low gain), and you want to achieve high efficiency.

Let's say you put 25W of RF power into an amplifier, and get 50W out. You can't get better than 66.6`% efficiency (50W/(50W+25W)) unless the input power is added to the output power and both are coupled to the output port.

Since in many modulators, the output is equal power to the input (at a different frequency), the efficiency would be worse than 50% if the input power didn't end up at the output. Many modulators do in fact work this way, as the modulation is done prior to amplification to the final output power, and the low efficiency of the modulator is irrelevant at the low power of the intermediate signal.

If on the other hand, your RF is difficult to amplify, but easier to generate at high power and modulate after (say from a klystron or maser), then you absolutely need to modulate the input signal by applying attenuation or storage (in a resonator) or phas shift (in a delay line), as it's difficult, expensive or unknown how, to apply gain.

Re:Redundant? (0)

Anonymous Coward | about 10 months ago | (#45995257)

Not redundant at all. This is how a transistor (or any) pumped oscillator works. You DO need a device which produces more modulation than it takes to drive it (gain > 1), but once you have that, you just take the output of a resonant device (LC tank, crystal, cavity, transmission line, optical waveguide, pendulum, etc), apply it to a modulator (transistor, FET, valve, relay, optical switch, escapement, etc), and apply a portion of the modulated output back to the resonantor so that the resonator acquires more energy. If you don't want your resonator to explode, you do need some kind of way to limit the maximum energy, but this is often intrinsic from the damping (friction) of the oscillator and the limited power supply of the pumping device.

What isn't clear is whether this device actually has greater than unity gain (I haven't RTFA yet). Any non-linear device can be used as a modulator, but not all modulators have greater than unity gain.

still need another modulator! (0)

Anonymous Coward | about 10 months ago | (#45976819)

impressive, but I will just point out its not electrically driven. You need a second light pulse, which itself needs a modulator somewhere.

Re:still need another modulator! (1)

Shatrat (855151) | about 10 months ago | (#45976939)

It's graphene all the way down.

Re:still need another modulator! (2)

cusco (717999) | about 10 months ago | (#45977015)

Graphene. How does it work?

Re:still need another modulator! (1)

TheResilientFarter (3216187) | about 10 months ago | (#45977029)

Just read the instructions on the carton. You can pick up Graphene at any corner market - it's right next to the Flubber.

Re:still need another modulator! (1)

tanujt (1909206) | about 10 months ago | (#45977049)

Works by giving researchers high impact factor journal publications and later on tenure.

Re:still need another modulator! (1)

jellomizer (103300) | about 10 months ago | (#45978527)

Your next PC will require a pilot light.

This is a Proof-Of Concept stage report. (1)

Senior Engineer (3443705) | about 10 months ago | (#45988149)

Proof that "it works" will come after peer review and not be fully vetted til we see some solid&stable term results.
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