Using a sandwiched sub-nano separator growth technique, indium arsenide / indium gallium arsenide quantum dots have been used to create a broad new band the 1.31-μm region

 

The National Institute of Information and Communications Technology (NICT) says it has developed the world's first optical source for generating light spanning a large number of wavelengths with high precision. 

The researchers used nanoscale InAs/InGaAs quantum dots grown on GaAs, in a band consisting of wavelengths in the 1.3-μm region that are not currently used for optical communication. 

What's more, the researchers say they have developed an optical transmission experiment using this light source and a photonic crystal fibre, demonstrating the possibility of using a new wavelength band for optical communication.

This “quantum dot light source technology” makes it possible to secure an optical frequency resource (about 70 THz) whose band covers about 10 times the width of the currently used 1.55-μm optical communication wavelength band.

Moreover, as quantum dots and a photonic crystal fibre have nanoscale structures, the application of nanotechnology is expected to be revolutionary for optical information communication.

Background

Current optical fibre communication systems use a 1.55-μm wavelength band of about 10 THz, where the attenuation of the optical signal and distortion of data are quite low (see Fig. 1). Although research and development for efficient use of the optical signal in this wavelength band has been in progress, these measures alone are not sufficient for bringing the ultra-high-speed and the large-capacity optical communication in the future into reality, due to lack of frequency resources.




Fig. 1: The relationship between band name and optical frequency (wavelength)

For this reason, NICT has engaged in photonics-based research in order to develop efficient optical frequency resources in the broad T and O bands which cover the 1.0–1.3 μm wavelength region.

By employing InAs/InGaAs quantum dots as an optical amplifying material acting in the 1.0–1.3 μm wavelength band, NICT has developed a quantum dot light source which is stable and has a high optical frequency.




Fig. 2 : The newly developed quantum dot light source 





(a) An optical source using high-quality InAs/InGaAs quantum dots as light-amplifying material 

(b) An example of the wavelength tunability of the quantum dot light source 


NICT says the world’s longest wavelength for optical communication (1.3 μm), has been achieved in the form of low-cost quantum dots with a large surface area on a GaAs base. By obtaining a stable high optical frequency, the scientists say they can effectively wave-tune in the T and O bands.

In order to develop the quantum dot material, which is the key to the light source, NICT has used a self-developed “subnanometer interlayer separation technique” (see Fig. 3) which controls the crystal structure on an atomic scale. 




Fig. 3 : The subnanometer interlayer separation technique





(a) A sectional diagram of the subnanometer interlayer separation technique

(b) Left: A quantum dot structure developed using the conventional technique

Right: A high-quality newly developed quantum dot structure 

(c) Newly developed quantum dot wavelength-tunable optical gain device


The interlayer separation technique originally developed by NICT has a structure where a crystal of atomic size (with a subnanometer length) is sandwiched between a quantum dot and a quantum well as shown in Fig. 3(a). 

 

In this technique, the structure of the quantum dots does not involve large aggregates leading to deterioration of the crystal properties or light amplification characteristics, in contrast to the conventional technique (Fig. 3(b), left). NICT says it has succeeded in creating a semiconductor (Fig.3(b), right) of the world’s highest quality and density, which is more than twice higher than that achieved by conventional methods.

What's more, NICT says it has succeeded in building a high-speed data transmission system (see Fig. 4) with error-free data transmission that combines this light source and the photonic crystal fibre with ultra-broadband optical propagation characteristics.




Fig. 4 : A high-speed optical propagation subsystem

The optical transmission subsystem was built using the combination of the two components in Figs. 4(a) and (b). By applying nanotechnology, the possibility of using a new optical frequency band in an optical information transmission network has been demonstrated (Fig. 4(c)).

NICT reckons its basic quantum dot and photonic crystal fibre technology could revolutionise optical communication technology. What's more, as the 1.0–1.3 μm band exhibits excellent permeation through human skin and moisture, applications in bioimaging or medical sensing are expected.

A prototype of the “quantum dot light source”, developed by NICT, has been designed by Koshin Kogaku Co. Ltd. and Sevensix, Inc. The technology transfer and development for commercialisation of the product is currently under way.

More details of this research are described in the paper " Narrow-line-width 1.31-μm wavelength tunable quantum dot laser using sandwiched sub-nano separator growth technique," by Yamamoto et al, "Optics Express, Vol. 19, Issue 26, pp. B636-B644 (2011), http://dx.doi.org/10.1364/OE.19.00B636