STO2 landed and data secured

The STO2 first light spectrum at 1.9 THz. Credit: Delft University of Technology

https://phys.org/news/2017-01-sto2.html

The STO2 telescope with Dutch detectors on board that circled around the South Pole in December 2016 to investigate gas clouds between the stars landed safely on 30 December.

At an altitude of 39 kilometers the NASA telescope circled along with the polar vortex for a period of three weeks. During that time STO2 picked up as much radiation as possible at the frequencies of 1.4 and 1.9 THz to find ionized nitrogen (NII) and ionized carbon respectively (CII) in a part of our Milky Way. These substances indicate the process of star formation from dust and gas.

Measuring oxygen

The 4.7 THz detector that would measure neutral atomic oxygen (OI) also worked. However, something went wrong in the system for the local oscillator that had to generate the required reference signal of 4.7 THz. An electrical component needed for the communication between this local oscillator and ground control became overheated by the sun. OI reveals that a star is actually being born. This is an observation that astronomers are keen to obtain, especially if that observation is being done for the first time beyond the earth's atmosphere, as would have been possible with STO2.

STO2 project leader for SRON and TU Delft researcher Jian-Rong Gao and his team are indeed disappointed about the absence of the 4.7 THz observations but on the other hand they are extremely happy with the large quantity of data for the other two frequencies. After an initial hiccup in the orientation mechanism of the telescope, the collection of that data proceeded really well. "Once the rough data have been processed to reveal spectral lines for CII and NII then STO2 will have drastically expanded the area mapped so far for these substances."

Mission continues

STO2 was launched from Antarctica on 9 December 2016. The polar vortex also ensures that the balloon missions land again at a location that can be reached along the South Pole Traverse, a sort of Antarctic 'motorway' between the South Pole and McMurdo. When the cooling fluid for the superconducting detectors (liquid helium) had been used up and the data was safely downloaded to computers on earth, STO2 landed on the South Pole Transverse. The telescope was picked up on 10 January so that it could be brought back to McMurdo.

STO2 is an exploratory mission under the leadership of the University of Arizona for astronomy in these terahertz frequencies. On 24 January 2017, NASA will visit the University of Arizona to decide about GUSTO. This is also a balloon mission but with a longer duration (about 100 days) and with more effective instruments on board. For NII, CII and OI, GUSTO will have cameras with eight pixels that will once again be developed by SRON and Delft University of Technology.

The teams of professor Alexander Tielens (Leiden University) and professor Floris van der Tak (SRON/University of Groningen) will contribute to the scientific analysis of the observations.

Abstract-Terahertz multiheterodyne spectroscopy using laser frequency combs

Yang Yang, David Burghoff, Darren J. Hayton, Jian-Rong Gao, John L. Reno, Qing Hu

http://www.rle.mit.edu/terahertz-multiheterodyne-spectroscopy-using-laser-frequency-combs/

The terahertz region is of great importance for spectroscopy since many molecules have absorption fingerprints there. Frequency combs based on terahertz quantum cascade lasers feature broadband coverage and high output powers in a compact package, making them an attractive option for broadband spectroscopy. Here, we demonstrate the first multiheterodyne spectroscopy using two terahertz quantum cascade laser combs. Over a spectral range of 250 GHz, we achieve average signal-to-noise ratios of 34 dB using cryogenic detectors and 24 dB using room-temperature detectors, all in just 100 μs. As a proof of principle, we use these combs to measure the broadband transmission spectrum of etalon samples and show that, with proper signal processing, it is possible to extend the multiheterodyne spectroscopy to quantum cascade laser combs operating in pulsed mode. This greatly expands the range of quantum cascade lasers that could be suitable for these techniques and allows for the creation of completely solid-state terahertz laser spectrometers.

Related Links:

Terahertz multiheterodyne spectroscopy using laser frequency combs (Optica)

Speedy terahertz-based system could detect explosives (MIT News)

Professor Hu

Millimeter-wave and Terahertz Devices Group

Abstract-Terahertz multiheterodyne spectroscopy using laser frequency combs

Yang Yang, David Burghoff, Darren J. Hayton, Jian-Rong Gao, John L. Reno, and Qing Hu
https://www.osapublishing.org/optica/abstract.cfm?uri=optica-3-5-499#Abstract

The terahertz region is of great importance for spectroscopy since many molecules have absorption fingerprints there. Frequency combs based on terahertz quantum cascade lasers feature broadband coverage and high output powers in a compact package, making them an attractive option for broadband spectroscopy. Here, we demonstrate the first multiheterodyne spectroscopy using two terahertz quantum cascade laser combs. Over a spectral range of 250 GHz, we achieve average signal-to-noise ratios of 34 dB using cryogenic detectors and 24 dB using room-temperature detectors, all in just 100 μs. As a proof of principle, we use these combs to measure the broadband transmission spectrum of etalon samples and show that, with proper signal processing, it is possible to extend the multiheterodyne spectroscopy to quantum cascade laser combs operating in pulsed mode. This greatly expands the range of quantum cascade lasers that could be suitable for these techniques and allows for the creation of completely solid-state terahertz laser spectrometers.

© 2016 Optical Society of America

Full Article  |  PDF Article

Abstract-Terahertz multi-heterodyne spectroscopy using laser frequency combs

Yang Yang, David Burghoff, Darren J. Hayton, Jian-Rong Gao, John L. Reno, Qing Hu

http://www.mathpubs.com/detail/1604.01048v1/Terahertz-multi-heterodyne-spectroscopy-using-laser-frequency-combs

Frequency combs based on terahertz quantum cascade lasers feature broadband coverage and high output powers in a compact package, making them an attractive option for broadband spectroscopy. Here, we demonstrate the first multi-heterodyne spectroscopy using two terahertz quantum cascade laser combs. With just 100 μ$\mu$s of integration time, we achieve peak signal-to-noise ratios exceeding 60 dB and a spectral coverage greater than 250 GHz centered at 2.8 THz. Even with room-temperature detectors we are able to achieve peak signal-to-noise ratios of 50 dB, and as a proof-of-principle we use these combs to measure the broadband transmission spectrum of etalon samples. Finally, we show that with proper signal processing, it is possible to extend the multi-heterodyne spectroscopy to quantum cascade laser combs operating in pulsed mode, greatly expanding the range of quantum cascade lasers that could be suitable for these techniques.

Abstract-Evaluating the coherence and time-domain profile of quantum cascade laser frequency combs

Evaluating the coherence and time-domain profile of quantum cascade laser frequency combs David Burghoff, Yang Yang, Darren J. Hayton, Jian-Rong Gao, John L. Reno, and Qing Hu  »View Author Affiliations

Optics Express, Vol. 23, Issue 2, pp. 1190-1202 (2015)
http://dx.doi.org/10.1364/OE.23.001190

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-2-1190

View Full Text Article

Enhanced HTML    Acrobat PDF (1266 KB)

Recently, much attention has been focused on the generation of optical frequency combs from quantum cascade lasers. We discuss how fast detectors can be used to demonstrate the mutual coherence of such combs, and present an inequality that can be used to quantitatively evaluate their performance. We discuss several technical issues related to shifted wave interference Fourier Transform spectroscopy (SWIFTS), and show how such measurements can be used to elucidate the time-domain properties of such combs, showing that they can possess signatures of both frequency-modulation and amplitude-modulation.

© 2015 Optical Society of America

Abstract-Terahertz laser frequency combs

David Burghoff, Tsung-Yu Kao, Ningren Han, Chun Wang Ivan Chan, Xiaowei Cai, Yang Yang, Darren J. Hayton, Jian-Rong Gao, John L. Reno, Qing Hu

Nature Photonics

(2014)

doi:10.1038/nphoton.2014.85

Received

 

28 January 2014 

Accepted

 

28 March 2014 

Published online

 

11 May 2014

Article tools

• http://www.nature.com/nphoton/journal/vaop/ncurrent/abs/nphoton.2014.85.html

Terahertz light can be used to identify numerous complex molecules, but has traditionally remained unexploited due to the lack of powerful broadband sources. Pulsed lasers can be used to generate broadband radiation, but such sources are bulky and produce only microwatts of average power. Conversely, although terahertz quantum cascade lasers are compact semiconductor sources of high-power terahertz radiation, their narrowband emission makes them unsuitable for complex spectroscopy. In this work, we demonstrate frequency combs based on terahertz quantum cascade lasers, which combine the high power of lasers with the broadband capabilities of pulsed sources. By fully exploiting the quantum-mechanically broadened gain spectrum available to these lasers, we can generate 5 mW of terahertz power spread across 70 laser lines. This radiation is sufficiently powerful to be detected by Schottky-diode mixers, and will lead to compact terahertz spectrometers.

MIT develops terahertz laser the size of a comma

My note: This is translated from the Dutch, so I apologize about any errors, in the google translation
http://translate.google.com/translate?hl=en&sl=nl&u=http://www.delta.tudelft.nl/article/tiny-terahertz-laser-tamed/25971&prev=/search%3Fq%3Dmit%2Bdevelops%2Bterahertz%2Blaser%2Bsize%2Bcomma%26hl%3Den%26tbo%3Dd%26biw%3D1024%26bih%3D672%26site%3Dwebhp&sa=X&ei=R7i4UOmSG4eQ2AXM-IBw&ved=0CD4Q7gEwAg
A new MIT-produced terahertz laser, about the size of this comma, has been developed by PhD student Yuan Ren. His achievement signals a major step forward in the terahertz astronomy.

Terahertz radiation, wedged in-between far-infrared and microwave, has a growing number of applications ranging from airport security body scanners and medical imaging to the astronomy of the origin of stars and planets.

The main problem so far is the lack of suitable terahertz sources, says Dr.. Jian-Rong Gao from the Department of Quantum Nanoscience Within the Applied Sciences faculty and Ren's daily supervisor. Airport scanners Rely on microwave transmitters in the far gigahertz domain, which is a known and trusted technology, but at the expense of image resolution.

A CO2 laser can be used to generate terahertz radiation or verschillende, but it's a massive tool. And obviously not the first choice to put on board of a satellite or balloon-based astronomy mission.

Since about for years there's a new way of generating terahertz waves with a tiny structure on a chip, called a Quantum Cascade Laser. It has been produced by a team of researchers zoals Prof. Qing Hu from Massachusetts Institute of Technology in Boston and Dr.. John L. Reno from Sandia Labs in Albuquerque. And unless Dr. Gao is mistaken, we're about to hear far more from Quantum Cascade Lasers Often or QCL's in the near future.

The QCL itself is easy to overlook. The structure is a line of about a millimeter long and only 20 to 40 microns wide. Its height of about 10 microns is built up from alternating layers of two different semiconducting materials only a few nanometer high. Depositing alternating layers thesis takes about four hours.

The result is an artificial semiconductor structure All which does not exist in nature. The alternating layers of (aluminum) gallium arsenide create a structure with quantum wells, the energy levels or All which depend on the thickness of the layers. As an electron moves from one quantum well into the next generation rates and a terahertz photon, it will trigger a cascade of electrons and hence of THz photons.

So, QCLs are smart, tunable sources of terahertz radiation and tiny. What more can one ask for? Well, the output Should be high enough, the radiation Should be stable in both amplitude and frequency and it should preferably operate at room temperature.

Axis for the temperature: the QCL needs to be cooled to about minus 200 degrees Celsius (70 K) at All which point it consumes about 1 Watt and Produces 0.25 milliwatts output power, which is quite okay for terahertz astronomy. 

But cooling equipment needs All which in turn interferes with the laser's output. So, Ren, who got his MSc degree at the Purple Mountain Observatory of the Chinese Academy of Sciences had to find a way of stabilizing both amplitude and frequency of the QCL.

"This is a difficult problem," says Gao. "The laser only has one knob, so how do you control two different aspects?". The 'knob' that Gao refers to is the voltage-controlled small tuning range of the laser.

Nonetheless Ren succeeded in controlling both frequency and amplitude with a smart laboratory setup. He stabilized the laser's output by inserting a fast (up to 1 kHz) automatic diaphragm in the beam. A feedback mechanism controls the diaphragm to keep the beam at a pretty constant level.

Next was the feedback frequency. By comparing the QCL output with a fixed spectral line in methanol gas, Ren succeeded in converting frequency fluctuations into amplitude changes All which then were picked up by a detector and fed back to tune the QC laser.

By doing so, says his co-supervisor Dr. Gao, Ren has for the first time made a quantum cascade laser suitable for airborne or satellite terahertz astronomy. This is the field for All which Dr. Gao develops sensors That Are candidate for NASA space missions and balloon (see: TU develops Nasa mission detectors ). 

All set now? Not quite yet. Space engineers will have to find a way to pack Ren's laboratory set-up into the cramped confinements of the Gussto balloon-based telescope. And as far as the quantum cascade lasers are concerned: they'll have a bright future if they can be made to operate at room temperatures shall or close to it.

-> Yuan Ren, Super-heterodyne spectrometer using a terahertz quantum cascade laser, 4 December 2012, Prof. PhD supervisors. Teun Klapwijk and Prof. SC Shi, co-supervisor Dr.Jian-Rong Gao. Ren was supported by the joint PhD training program of the Academy and the Chinese Academy of Sciences (CAS).