The Development of a Novel All-Optical Silicon Modulator

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The field of optical communications, and more specifically all-optical modulation, is becoming a field of increasing interest for many researchers and industry alike.

However, one lacking area is that of in-fibre integrated miniaturised devices that can provide a sufficient modulation depth and a low power consumption.

A team of researchers from Israel has now developed an all-optical silicon modulator with an enhanced modulation depth, that could be used as an in-fibre device for optical communication applications.

Most optical modulators nowadays rely on the use of electrical circuits to control the voltage applied on the modulator.

Due to industry demand and interest, recent research has seen a surge in all-optical modulators as a way to provide modulators that exhibit a greater modulation depth and lower power consumption. There is only one real restriction with all-optical modulators, and it is that they need to be realised on silicon to be compatible with microelectronic circuitry.

Many all-optical modulators currently use resonators, be it ring or Fabry-Perot, but the internal components are very expensive and complex to produce, especially for commercial use. There have been many other cheaper options tried, but most are not compatible with in-fibre modulators, as they have to be implemented onto a silicon-on-insulator (SOI) chip.

Cheaper Fabry-Perot modulators, made from germanium, have been tried but lack the efficiency to perform complex functions in applications such as fibre lasers and communication systems. It has been apparent for a while that a new class of silicon-based modulators needs to be found that is not only efficient, but also low cost and simplistic.

The researchers have developed the first optical modulator that is based around a simple coated silicon slab. Silicon has no optical-electrical properties so the modulation is achieved through a temporary increase in the refractive index, by using a laser pulse as a pump- also known a plasma dispersion effect.

The all-optical modulator has been found to possess a high modulation depth, low power efficiency, low cost and is compact. It also possesses a major advantage over ring resonators through its simple fabrication process. The simplicity of the device allows it be deposited onto an optical fibre, without the need for an SOI substrate.

The silicon slab is coated on both sides to improve the Q factor, i.e. the finesse, of the device. The coating has also been found to be stable to high pulse intensities, a critical factor for optical applications. The coating on the silicon slab was found to increase the Q factor from 2.5 to 30, a 12-fold increase. The coated silicon slab also allowed the researchers to reach a modulation depth of 12 dB, without the need for couplers or waveguides.

The coating that provides the extra Q factor is central to excellence of the device. The ability to increase the finesse is the reason as to why the efficiency is high enough be used as a modulator in a fibre system. The advantage of having a higher Q factor is that the energy requirements are reduced for the same modulation depth.

As a side-benefit, the lower the energy consumption, the lower the heat produced by the device, which leads to a decrease in the recovery time and a higher operational efficiency. For this device, the recovery time has found to be 600 ns and translates to a modulation rate of 1 MHz.

The production of this all-optical modulator will allow the fabrication of compact, low power consuming in-fibre integrated devices. It has also been proposed that the device could be used as a tuneable spectral filter with band-widths lower than 9 GHz FWHM. Future research aims to produce coatings which are thinner, to speed up the bulk-recombination processes even further.

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