Record-setting Laser May Aid Searches for Earthlike Planets

Scientists at the University of Konstanz in Germany and the National Institute of Standards and Technology (NIST) have demonstrated an ultrafast laser that offers a record combination of high speed, short pulses and high average power. The same NIST group also has shown that this type of laser, when used as a frequency comb—an ultraprecise technique for measuring different colors of light—could boost the sensitivity of astronomical tools searching for other Earthlike planets as much as 100 fold.

The dime-sized laser, to be described Thursday, May 8, at the Conference on Lasers and Electro-Optics,* emits 10 billion pulses per second, each lasting about 40 femtoseconds (quadrillionths of a second), with an average power of 650 milliwatts. For comparison, the new laser produces pulses 10 times more often than a standard NIST frequency comb while producing much shorter pulses than other lasers operating at comparable speeds. The new laser is also 100 to 1000 times more powerful than typical high-speed lasers, producing clearer signals in experiments. The laser was built by Albrecht Bartels at the Center for Applied Photonics of the University of Konstanz.

Among its applications, the new laser can be used in searches for planets orbiting distant stars. Astronomers look for slight variations in the colors of starlight over time as clues to the presence of a planet orbiting the star. The variations are due to the small wobbles induced in the star’s motion as the orbiting planet tugs it back and forth, producing minute shifts in the apparent color (frequency) of the starlight. Currently, astronomers’ instruments are calibrated with frequency standards that are limited in spectral coverage and stability.

Frequency combs could be more accurate calibration tools, helping to pinpoint even smaller variations in starlight caused by tiny Earthlike planets. Such small planets would cause color shifts equivalent to a star wobble of just a few centimeters per second. Current instruments can detect, at best, a wobble of about 1 meter per second.

Standard frequency combs have “teeth” that are too finely spaced for astronomical instruments to read. The faster laser is one approach to solving this problem. In a separate paper,** the NIST group and astronomer Steve Osterman at the University of Colorado at Boulder describe how, by bouncing the light between sets of mirrors a particular distance apart, they can eliminate periodic blocks of teeth to create a gap-toothed comb. This leaves only every 10th or 20th tooth, making an ideal ruler for astronomy.

Both approaches have advantages for astronomical planet finding and related applications. The dime-sized laser is very simple in construction and produces powerful and extremely well-defined comb teeth. On the other hand, the filtering approach can cover a broader range of wavelengths. Four or five filtering cavities in parallel would provide a high-precision comb of about 25,000 evenly spaced teeth that spans the visible to near-infrared wavelengths (400 to 1100 nanometers), NIST physicist Scott Diddams says.

Osterman says he is pursuing the possibility of testing such a frequency comb at a ground-based telescope or launching a comb on a satellite or other space mission. Other possible applications of the new laser include remote sensing of gases for medical or atmospheric studies, and on-the-fly precision control of high-speed optical communications to provide greater versatility in data and time transmissions. The application of frequency combs to planet searches is of international interest and involves a number of major institutions such as the Max-Planck Institute for Quantum Optics and Harvard Smithsonian Center for Astrophysics.

References:

* A. Bartels, D. Heinecke and S.A. Diddams. Passively mode-locked 10 GHz femtosecond Ti:sapphire laser with >1 mW of power per frequency comb mode. Post-deadline paper presented at Conference on Lasers and Electro-Optics (CLEO), San Jose, Calif., May 4-9, 2008.

** D.A. Braje, M. S. Kirchner, S. Osterman, T. Fortier and S. A. Diddams. Astronomical spectrograph calibration with broad-spectrum frequency combs. To appear in European Physics Journal D. (Posted online at arXiv:0803.0565)

Source: NIST

Related stories:

Scientists create first dense gas of ultracold 'polar' molecules
Scientists at JILA, a joint institute of the National Institute of Standards and Technology (NIST)and the University of Colorado at Boulder, have applied their expertise in ultracold atoms and lasers to produce the first high-density gas of ultracold molecules—two different atoms bonded together—that are both stable and capable of strong interactions.
An accurate speedometer for astronomy
Events on a cosmic scale are often barely discernable on Earth. This explains why astronomers are currently not able to prove directly that the universe is expanding at an ever increasing rate, nor can they search for planets that are roughly the same size as Earth and revolve around a sun-like star.
A fine-tooth comb to measure the accelerating universe
Astronomical instruments needed to answer crucial questions, such as the search for Earth-like planets or the way the Universe expands, have come a step closer with the first demonstration at the telescope of a new calibration system for precise spectrographs. The method uses a Nobel Prize-winning technology called a 'laser frequency comb', and is published in this week's issue of Science.
NIST 'Quantum Logic Clock' Rivals Mercury Ion as World's Most Accurate Clock
An atomic clock that uses an aluminum atom to apply the logic of computers to the peculiarities of the quantum world now rivals the world's most accurate clock, based on a single mercury atom. Both clocks are at least 10 times more accurate than the current U.S. time standard.
Laser light may be able to detect diseases on the breath
By blasting a person's breath with laser light, scientists from the National Institute of Standards and Technology and the University of Colorado at Boulder have shown that they can detect molecules that may be markers for diseases like asthma or cancer.
Purdue 'milestone' a step toward advanced sensors, communications
Engineers at Purdue University have shown how to finely control the spectral properties of ultrafast light pulses, a step toward creating advanced sensors, more powerful communications technologies and more precise laboratory instruments.
'Tunable' network features coordinated frequency combs
A super stable fiber-optic network that can be tuned across a range of visible and near-infrared frequencies while synchronizing the oscillations of light waves from different sources has been demonstrated at the National Institute of Standards and Technology.
Atomic clock signals may be best shared by fiber-optics
Time and frequency information can be transferred between laboratories or to other users in several ways, often using the Global Positioning System (GPS). But today's best atomic clocks are so accurate—neither gaining nor losing one second in as long as 400 million years—that more stable methods are needed. The best solution may be to use lasers to transfer data over fiber-optic cables, according to scientists at JILA, a joint institute of the National Institute of Standards and Technology and the University of Colorado at Boulder.

News discussion:

Physics news