SANTA MONICA, CA--Quantum-cascade laser (QCL) maker Pranalytica has won a U.S. Army small-business technology transfer (STTR) contract to drastically improve the performance of QCLs. In combination with Federico Capasso’s renowned QCL group at Harvard University (Cambridge, MA), Pranalytica will concentrate on upping the power and efficiency of room-temperature QCLs operating in the 3.5 to 4.2 µm and 8 to 12 µm wavelength regions.
The military needs better QCLs to serve as light sources for infrared countermeasures, as well as for biological threat-detection systems based on light detection and ranging (LIDAR). The results of the contract should be applicable to other fields too, such as wireless communications, medical diagnostics, and remote gas sensing.
Pranalytica recently developed a novel QCL structure that will help in their STTR research; the new structure allows the company to get away from the two-photon-resonance requirement of other QCL designs, giving Pranalytica more design flexibility.
Longtime relationship
The collaboration between Pranalytica and Harvard is not a new one. Their partnership began under a program formed by the National Institute of Standards and Technology (NIST: Boulder, CO) to replace carbon dioxide (CO2) lasers with QCLs for certain uses. It strengthened when they worked together on the DARPA efficient mid-infrared laser (EMIL) and laser photoacoustic spectroscopy (L-PAS) programs.
“Harvard provides new ideas on structural design, as well as rapid analysis of new materials,” says Kumar Patel, Pranalytica’s founder and CEO. Patel, who is also professor of physics, chemistry, and electrical engineering at UCLA, is a well-known figure in the laser industry; when at Bell Labs back in 1964, he invented the CO2 laser--a development that ushered in the era of high-power lasers. (Note: On January 25th, Kumar Patel will be speaking at the Laser Focus World 2010 Lasers & Photonics Marketplace Seminar, held in San Francisco in conjunction with Photonics West.)
The ultimate goal of the STTR, says Patel, will be to extend the QCL performance that Pranalytica already demonstrated at 4.6 mm under the EMIL program to the 3.5 to 4.2 µm and 8 to 12 µm spectral windows. “Full implementation of this proposal, including Phase II, will result in commercial availability of high-power, high-efficiency QCLs emitting in these two regions,” he notes. One goal is to improve wall-plug efficiency (WPE) from a few percent to 20% or higher. In the earlier (and ongoing) EMIL program, the Phase II goal was 50% efficiency at 4.6 µm, says Patel. When the goal wasn’t reached, it was renamed Phase Ib. The aim is still for 50% WPE.
“As a result of EMIL technology, Pranalytica is already selling 4.6 µm QCLs to a number of ‘heavy-hitter’ defense customers; we will have sold 25 by this December [of 2009],” says Patel. The shorter-wavelength technology arising from the STTR will proceed from development to commercial introduction just as rapidly, Patel predicts, while the longer wavelengths will take a bit more time; the first commercial introduction of the longer-wavelength versions will be by Pranalytica; it will own the patents, although the U.S. government will have royalty-free nonexclusive rights to the patents.
“At 3.8 to 4.2 µm, we want to reach a continuous-wave (CW) output of 1 W at room temperature and a 50% WPE (this effort will also help with our EMIL Phase 1b target of 50% WPE),” explains Patel. “The longer wavelengths are somewhat more difficult. The Pranalytica nonresonant-extraction design will be applied to these longer wavelengths. The challenges in the research will be taking theory to the reality of epitaxially grown wafers. We should be able to get 1 to 2 W CW at room temperature; this is good for sensor purposes--chemicals, explosives, and so on, which have key absorptions in the 8 to 12 µm region.”
Existing longer-wavelength QCLs produce just a few hundred milliwatts at less than 5% WPE, which necessitates lots of heat extraction, according to Patel. “Higher WPE and less heat will bring about more applications, as most practical applications require air cooling,” he says. He notes that years of work on improved cooling methods done by researchers in an entirely different area--that of the computer CPU industry--will also be of great benefit to high-power longer-wavelength QCLs.
Scatter-free communications
Patel is very excited about the potential of longer-wavelength QCL technology used for free-space optical communications. Longer wavelengths are almost unaffected by fog, particulates, rain, and so on because the amount of scattering scales as the inverse of the fourth power of wavelength (Rayleigh scattering); for example, light at a 10 µm wavelength scatters thousands of times less than that at 1.5 µm. Example applications for long-wavelength QCLs in this respect include urban building-to-building communications.
Communications in the 8 to 12 µm region have been hampered up to now by the lack of directly modulated lasers. “People have been using CO2 lasers for experiments,” says Patel. “But I have to admit that, even though I invented them, they are large and power-hungry lasers. They also have to be externally modulated. The use of longer-wavelength QCLs for free-space communications will clearly take off very rapidly.”
John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.