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21 Jul 2006
CLEO/QELS' terahertz programme was a big hit with conference delegates this year, running over two days and featuring more than 50 international speakers. James Tyrrell profiles Picometrix, a firm that has successfully transferred its telecoms know-how into the terahertz sector, and looks at what is driving the market today.
Judging by the enthusiastic audience at this year's CLEO/QELS event in Long Beach, US, terahertz (THz) technology remains a hot topic for the photonics industry. Lying between microwaves and the far-infrared, non-ionizing THz radiation can safely penetrate materials such as clothing and plastics to image concealed objects and spot manufacturing flaws.
What's more, spectroscopic THz analysis can help identify explosives, detect biological agents and screen pharmaceuticals. If you couple these functions into the one device then you create a very powerful tool that has grabbed the attention of NASA and the US Department of Homeland Security, to name just two high-profile fans of the apparatus.
One of the first firms to commercialize THz technology was Picometrix, US. "We began in 1992 when THz was still very research-based and so the foundation of the company was ultrafast optoelectronic instrumentation and optical receiver components," Robin Risser, president of Picometrix and CFO of its parent company, Advanced Photonix Inc, told OLE. "However, the fast GaAs and InGaAs material that we developed for our receivers was also suitable for THz."
Distributed firstly by Newport Corporation and then through test and measurement and telecoms equipment manufacturers, the optical components were adopted by the expanding communications sector for 10 and 40 Gbps networks. As the market segment started to move and generate more revenue, Picometrix was able to put its THz plans into action.
"In 1997 we launched a full-scale major development programme on THz instrumentation," said Risser. "We did an exclusive tech-transfer of all of the know-how from Lucent Bell-Labs and licensed the technology. This was just at the time when Lucent was restructuring to focus on telecommunications."
Picometrix introduced its T-Ray 2000TM) product, a flexible research tool for both THz imaging and spectroscopy, in early 2000. At that time, applications for this promising technology were hard to explore because of the difficulty in generating a reliable THz signal. Risser and his colleagues were determined to change all of that and provide researchers with a user-friendly THz instrument.
"We wrapped our technology around the Lucent technology to harden the system and make sure that we had reliable products, which were easy to use," said Risser. "This was a big advantage to operators, because it allowed them to concentrate on the application, rather than worrying about generating a signal and re-aligning components."
Generating T-Rays
"The T-Ray 2000 is based on the photoconductive generation and detection of THz, which is a time domain method developed in the late 1980s and early 1990s," explained David Zimdars, Picometrix's manager of THz research and development. "One of the difficulties of working in the THz regime is that it is hard to build a purely electronic oscillator that will behave like a transistor and switch with any kind of power at these high speeds."
The solution is to use a femtosecond laser, which has fast optical transients. By firing high-speed laser pulses at a cleverly designed photoconductive switch, it is possible to generate THz radiation.
"We use a small 2 mm antenna [as the target] that is patterned on low-temperature grown GaAs and features a very small, biased gap," said Zimdars. "Ordinarily the resistivity is very high, for example you can have many mega-ohms across the gap."
When the laser pulse hits the switch, carriers are generated and current flows across the gap, which turns on in less than a picosecond and then turns off in less than a picosecond. It is this time-changing current flow that produces the THz emission.
"You obtain a broadband THz pulse with spectral content from 100 GHz and beyond, which suits spectroscopy and imaging applications," commented Zimdars. "In many ways this process is like an optically induced version of a spark-gap transmitter, which was the first kind of radio transmitter ever developed."
Secrets to success
At the heart of the Picometrix system is a saturated fibre-pigtailed THz module, which is permanently aligned and hermetically sealed. "Once you move from an optical table to a fibre-optic based system then you can make the unit much more rugged and portable," said Zimdars. "This has been a very important aspect of our success." Thanks to its umbilical connection, the THz head can be up to 30 m away from the main control box with the freedom to move over and around objects under challenging conditions.
"You can be in environments with high vibration, high humidity or high temperature swings and still get THz signals coming out," said Risser. "We transferred our telecoms packaging technology and fibre experience into our THz products." The firm's modular approach to design means that it can simply plug in the latest technology as it comes out of the lab. "It helps evolve the technology without making the customer's investment in the instrument obsolete," Risser added.
Stimulating the market
By targeting the research market, the hope is that new applications will emerge as scientists get to grips with the technology. "It takes a while, but I think all of the activity that you see today is basically those earlier applications beginning to open up," said Risser. "We've sold many T-Ray 2000 units to leading labs in the US and around the world."
Picometrix decided to take its T-Ray set-up a step further and migrated the instrument to a rugged 19-inch rack-mounted system in 2004. In fact, an industrial set-up was part of the company's plans from day one. "We had a customer who was paying right from the start for the development and deployment of online, real-time quality control apparatus," explained Risser, careful to abide by the terms of a non-disclosure agreement. "The client is looking to inspect up to 10,000 units/min on the factory floor."
By engaging with real customers immediately, Picometrix has been fortunate in being able to home in on the technology's key features. "Obviously, the first things that attract users to the technique are big problems that are less price sensitive, because the technology is still marching down the curve in terms of cost and the aerospace market is driving this in some respects," commented Risser. "NASA's space shuttle is a high-profile example, but in general aircraft makers are moving to composite materials and there are all types of requirements for imaging defects in manufactured products as well as gathering specific spectroscopic information from packaged goods."
Other key markets for Picometrix include pharmaceutical and defence sectors as well as homeland security. "THz scanners can not only image in high resolution with radiation that is not harmful, but they can also be used to spot items non-visually," said Risser. "Concealed weapons can be identified automatically by detecting a chemical signature or fingerprint."
Raising funds
Having stimulated the market with the T-Ray 2000 and its rack-mounted cousin the QA1000, Picometrix is now moving up a gear. "We merged with a public company called Advanced Photonix Inc in May 2005 to become a wholly owned subsidiary," said Risser. "We are ready to start ratcheting up our sales and marketing programme and we wanted access to additional growth capital that public markets could provide at a lower cost than the private markets."
Today, Picometrix is busy working on next-generation compact, high-speed systems at its corporate headquarters and manufacturing facilities in Ann Arbor, Michigan, US.
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