Thursday, 3 January 2019

Terahertz Laser for Sensing and Imaging Outperforms Its Predecessors

Terahertz Laser
Terahertz Laser is a high powered laser developed by Massachusetts Institute of Technology for chemical sensing and imaging. The Terahertz laser designed by the researchers of MIT, has three goals which includes high constant power, tight beam pattern and broad electric frequency tuning.

Terahertz Laser to be used by NASA on its 2021 mission to study our Galaxy’s origin

NASA has chosen the Terahertz laser for its 2021 mission to study more about our galaxy’s origin. With the help of a high altitude, balloon based telescope carrying photonic wire lasers, they will detect emission of gases like oxygen, nitrogen and carbon from the interstellar medium, which is the material between the galaxy’s star systems.

The Terahertz laser could also be used for skin and breast cancer imaging, detection of drugs and explosives and many other applications.

The design of the Terahertz Laser

The design of the Terahertz laser is unique which uses pairs of efficient wire lasers and sync their oscillations. On combining the output of the pairs there will be a single, high power beam with minimum beam divergence, which is the angle the beam moves away from its focus over distance. The individual paired lasers can be fine-tuned to obtain improvements in resolution and fidelity in measurements. All these give rise to less noise and higher resolution in order to achieve more cost effective and reliable chemical detection and medical imaging.

In the past, people have made a laser with high beam quality or with high continuous wave power or have done frequency tuning in lasers. But all the designs have one of the factors lacking. Ali Khalatpour, a graduate student in electrical engineering and computer science, who is also the author on the paper describing the Terahertz laser in Nature Photonics says that this is the first time where all three metrics are present in the chip-based Terahertz laser.

NASA has chosen the Terahertz laser for its 2021 mission

NASA will be using the Terahertz laser for its 2021 mission to detect gas emissions from the interstellar medium. By procuring data over a few months, they will get insight into the birth of the star and evolution. They will get more knowledge of the Milky Way and the Large Magellanic Cloud Galaxies which are close by.

NASA selected a new semiconductor based terahertz laser which is suitable for spectroscopic measurement of oxygen concentrations in terahertz radiation. It is a band of electromagnetic spectrum between microwaves and visible light.

Achieving three metrics in chip-based terahertz lasers

Researchers had developed new types of quantum cascade lasers known as photonic wire lasers. These were bidirectional and hence less powerful. This issue was sorted out by placing mirrors within the laser’s body. Since the terahertz radiation is too long, and the terahertz laser is small, it was difficult to fix it within the laser so much so most of the light travels outside the laser’s body. In the laser that NASA selected they developed a design to control how the electromagnetic wave travels along the laser and emit in one direction. They were able to obtain high efficiency and beam quality, but it did not allow for frequency tuning.

To overcome this, the researchers applied the concept of pi-bonding to the lasers, enabling close connections between the wire lasers. This coupling allowed phase locking of two or more wire lasers.
To obtain frequency tuning, they used tiny knobs to alter the current of each wire laser, which changes the refractive index. This change in refractive index together with coupled lasers, gives rise to a continuous frequency shift to the pair’s center frequency.

The researchers are building a system with a high dynamic range of more than 110 decibels which can be used for many applications including skin cancer imaging. Since skin cancer cells absorb terahertz waves more strongly than healthy cells, the terahertz lasers will be able to detect them.

With terahertz lasers having the three performance metrics, they can improve imaging capabilities and can be used in other applications.

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