February 16, 2007, Boulder, CO--Researchers at the National Institute of Standards and Technology (NIST) have made the results of laser frequency-comb experiments easier to visualize by directing each of the millions of different wavelengths in the comb to a unique spot in a 2-D array, which is then imaged. In contrast, frequency combs are normally depicted in one dimension--a problem for the many data points that can result. The 2-D images can be thought of as "fingerprints" of a material being spectroscopically analyzed, or of other data arising from experiments relating to optical atomic clocks, secure high-bandwidth communications, or other frequency-comb applications.
The work demonstrates a novel method for separating and identifying thousands of individual frequencies of visible light while simultaneously measuring intensity and imaging the results in real time. By providing a second dimension to the typical output of a frequency comb, the new technique efficiently packs more data into a given area without sacrificing precision, says researcher Scott Diddams. All light waves, or bristles, are displayed simultaneously, with a comb resolution as narrow as any other yet demonstrated. "This is really the first time we've seen individual elements of the stabilized comb, without interacting it with atoms or probing it with another laser, and it turns out to look more like a brush than a comb," says Diddams. "We now can see all the bristles at once with high precision."
To demonstrate the imaging technique, the researchers selected a small section of a frequency comb's spectrum (centered around 633 nm), which was passed through a filter to flexibly alter the spacing between frequencies, a technique necessary for the experimental setup that also will be useful in applications.
The light was then spatially separated twice, first vertically using a glass plate, and then horizontally with a metal grating. In combination, the two devices directed each wavelength of light in a specific and unique direction. The gridlike output was recorded by a digital camera connected to a computer. The pixels in the resulting images represent many different individual colors of light as well as the intensity of each signal. Thousands of different frequencies are shown in a single image in a pattern that repeats vertically as successive pulses of light are processed. Unique sections of demonstration images contain about 2,200 different frequencies.
The scientists demonstrated an application by making images with and without passing the laser light through iodine vapor, which absorbs some of the frequencies (bristles of the brush) in characteristic patterns, producing a fingerprint of the iodine molecules. They also altered the pulse-repetition rate of the laser to scan a wide range of optical frequencies, a technique that fully maps the absorption features of the molecules at video rates. Their research was published in the February 7, 2007 issue of Nature.
The technique will enable scientists to measure and manipulate optical frequencies in a massively parallel manner, Diddams says. The frequency brush could enable more precise control of individual frequencies than is currently possible in high-bandwidth communications, making it possible to reliably pack more channels with greater security into the same spectrum. The technique also may be useful in optical signal processing that could boost the power of surveillance, remote sensing, trace-gas detection, and high-speed computing systems. And it could enable the "ultimate" in precision control of atoms and molecules, a valuable tool in many areas of science, Diddams says.
With its high spectral resolution and intuitive user-friendly format, the new technique complements frequency-comb spectroscopy demonstrated recently at JILA, a joint venture of NIST and the University of Colorado at Boulder.
The authors of the Nature paper include a NIST guest researcher from the Council for Scientific and Industrial Research-National Metrology Laboratory and the University of the Witwatersrand, South Africa. The research was supported in part by the Defense Advanced Research Projects Agency.