In February 1880, in his Washington laboratory, the American inventor Alexander Graham Bell developed a device which he himself called his greatest achievement, even greater than the telephone: the “photophone”. Bell’s idea of ââtransmitting words over great distances using light was the forerunner of a technology without which the modern Internet would be unthinkable. Today, huge amounts of data are sent incredibly quickly over fiber optic cables as pulses of light. For this, they must first be converted from electrical signals, which are used by computers and telephones, into optical signals. In Bell’s day, it was a simple, very thin mirror that transformed sound waves into modulated light. Today’s electro-optical modulators are more complicated, but they have one thing in common with their distant ancestor: at a few centimeters they are still quite large, especially compared to electronic devices which can be as small as a few micrometers.
In a founding article of the scientific journal Photonics of nature, Juerg Leuthold, professor of photonics and communication at ETH Zurich, and his colleagues are now presenting a new modulator that is 100 times smaller and can therefore be easily integrated into electronic circuits. In addition, the new modulator is considerably cheaper and faster than current models, and it consumes much less power.
The plasmon thing
For this sleight of hand, the researchers led by Leuthold and his doctoral student Christian Haffner, who contributed to the development of the modulator, use a technical trick. In order to build the smallest modulator possible, they must first focus a light beam whose intensity they want to modulate in a very small volume. The laws of optics, however, dictate that such a volume cannot be less than the wavelength of light itself. Modern telecommunications use laser light with a wavelength of one and a half micrometers, which is therefore the lower limit for the size of a modulator.
In order to overcome this limit and to make the device even smaller, the light is first transformed into so-called surface plasmon polaritons. Plasmons polaritons are a combination of electromagnetic fields and electrons that propagate along a surface of a metal strip. At the end of the tape, they are again converted to light. The advantage of this detour is that the plasmons-polaritons can be confined in a space much smaller than the light from which they originate.
Refractive index modified from the outside
In order to control the power of the light coming out of the device, and thus create the impulses necessary for the transfer of data, the researchers use the operating principle of an interferometer. For example, a laser beam can be split into two arms by a beam splitter and recombined with a beam combiner. The light waves then overlap (they âinterfereâ) and strengthen or weaken, depending on the state of their relative phase in the two arms of the interferometer. A phase change can result from a difference in refractive index, which determines the speed of the waves. If an arm contains a material whose refractive index can be changed from the outside, the relative phase of the two waves can be controlled and therefore the interferometer can be used as a light modulator.
In the modulator developed by ETH researchers, it is not light beams, but rather plasmons-polaritons that are sent through an interferometer only half a micrometer wide. By applying a voltage, the refractive index and therefore the velocity of the plasmons in an arm of the interferometer can be changed, which in turn changes their amplitude of oscillation at the output. After that, the plasmons are converted back to light, which is fed into a fiber optic cable for further transmission.
Faster communication with less energy
The modulator built by Leuthold and his colleagues has several advantages at the same time. âIt’s incredibly small and simple, and on top of that, it’s also the cheapest modulator ever built,â explains Leuthold. And it’s simple, consisting of a layer of gold on glass only 150 nanometers thick and an organic material whose refractive index changes when an electrical voltage is applied and which thus modulates the plasmons to inside the interferometer. Since such a modulator is much smaller than conventional devices, it consumes very little power – only a few thousandths of a watts at a data transmission rate of 70 gigabits per second. This is only one hundredth of the consumption of business models.
In this sense, it contributes to the protection of the environment, since the amount of energy used in the world for data transmission is considerable – after all, there are modulators in every fiber optic line. Each year, increasing amounts of data must be transmitted at an increasingly high speed, resulting in increased power consumption. A hundredfold energy savings would therefore be more than welcome. “Our modulator provides more communication with less energy”, as the ETH professor put it in a nutshell. At present, the reliability of the modulator is being tested in long-term trials, which is a crucial step in making it suitable for commercial use.
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