Barbara Stumpp, Science Writer
Scientific fields such as atomic physics, chemistry, biology and even astronomy are inspired by the use of lasers and ultimately will benefit from the addition of electro-optical modulators.
The fundamental research fields of laser spectroscopy and, eventually, laser cooling were the first real driving forces behind the need for high-precision lasers. Ultracold atomic physics, which is based on laser cooling, is a vivid research field that examines phenomena such as superfluidity, where liquids move totally without friction. This concept is closely linked to superconductivity, which allows electrical conduction without resistance.
“Superfluidity and superconductivity are the same mechanisms, [the former] with neutral particles and [the latter] with electrons,” said professor Michael Köhl of the University of Bonn. “Especially with high-temperature superconductors, nobody knows yet why ceramics behave in this way and metals do not. And by means of ultracold atoms, one hopes to decipher the mechanism in order to develop superconductors that possess their property even at room temperature.”
Scientists at ETH Zurich also are busy with ultracold quantum gases. “Solids are many-body systems and can be described by so-called Hamiltonian functions. Since these equations are extremely complex and often cannot be solved, we built an easy-to-control system that behaves according to this equation and with which the Hamilton equation can be simulated,” said Dr. Tobias Donner, researcher at ETH Zurich. “Thus one can make predictions about properties that are not directly observable in the solid body, without having to solve the equation. Quantum gases provide this possibility.”
At the National Institute of Standards
and Technology in Boulder, Colo., researchers want to realize a quantum computer. For this purpose, individual ions are trapped just above a chip, cooled and quantum mechanically manipulated with ultraprecise laser fields. “The logical gate of a computer has an important function. Our quantum gates are now working 99 percent correctly. The goal is that they work properly 99.99 percent correctly; otherwise, too many errors creep in,” said NIST researcher Dr. Robert Jördens.
To prepare and precisely study the internal and external quantum mechanical states of atoms and ions, scientists need to restrain them. That’s difficult, because gaseous atoms at room temperature have average velocities on the order of 300 m/s. To cool a gas to lower temperatures, the atoms must be slowed down dramatically. With conventional refrigerators, users are limited to temperatures of about a kelvin. The invention of laser cooling enables scientists to routinely achieve temperatures on the order of microkelvins.
Detail of a typical setup at the Ultracold Atomic Physics Lab at the University of California, Berkeley, with the laser cooling/vacuum chamber at the right for Bose-Einstein Condensate (BEC). Photo courtesy of UC Berkeley.
The fundamental research field of laser cooling culminated in the long-sought quest to realize the first Bose-Einstein condensate (BEC): the coldest object in the universe, a few nanokelvins above absolute zero. The special feature of this pure quantum mechanical state: Atoms lose their individuality. These BECs are about 100,000 times larger than the “biggest” ordinary atom, even larger than a human cell, making it possible to observe quantum behavior in an otherwise unimaginable way.
However, slowing or cooling down atoms or ions can be achieved only with stringent requirements on the precision and accuracy of the laser frequency. An ordinary single-mode diode laser with a gigahertz linewidth needs to be locked to the atomic reference and narrowed down to the kilohertz level using electro-optical modulators (EOMs), for example. The most popular elements in laser cooling are alkali atoms because of their simplest-level structure, having only one valence electron. Unfortunately, the coupling of electronic and nuclear spin splits the ground into two energy levels separated by, typically, about 1 GHz. Therefore, to avoid losing the atoms in a dark state, two laser frequencies are required for efficient cooling. This additional magic frequency, referred to as a repumper, is added with a resonant electro-optic phase modulator.
The application of EOMs and lasers in astronomy is a relatively recent development. Atmospheric turbulence makes the stars flicker – and astronomers despair – because they cannot exploit the power of their telescopes. Adaptive optics (AO) fixes this.
The TMT is shown with the laser guide star at night. Photo courtesy of TMT Observatory Corporation.
The most outstanding such project is
the Thirty Meter Telescope (TMT) Observatory project. Its main mirror is 30 m in diameter, three times larger than the biggest telescopes currently in operation. During operation, its AO will enable it to observe astronomical objects with image quality and sensitivity several times better than those of the Hubble Space Telescope. “This will allow astronomers to conduct observations that are expected to bring answers to very exciting questions,” said Angel Otárola, a scientist and member of the AO team at the TMT. “The one that motivates me the most is the detection of an exoplanet of similar size to our own planet, located in a range from its host star allowing for the existence of water in its three phases.”
The telescope’s AO is composed of a wavefront sensor, a controller and correction elements. The wavefront sensor measures distortion of the incoming wavefront. The control computer reconstructs the original wavefront and necessary correction signals for the deformable mirror so that it can compensate for errors. The surface of the deformable mirror can be adjusted to rates as high as 1000 times per second, and the reflected light’s wavefronts are parallel and free of the distortion.
To determine correction values, the system needs a suitable star to measure atmospheric turbulences. These turbulences are highly variable, depending on their locale, so the star must be sufficiently bright and close to the object of astronomical observation; only then is the measured turbulence the same as that which interferes with the astronomical observation.
Since usually no suitable star exists, astronomers create one or more “guide stars” at the appropriate configuration. For this purpose, astronomers exploit a layer in the Earth’s mesosphere at an altitude of about 90 km with a relatively high proportion of sodium. To produce a guide star, a powerful laser beam is sent onto this layer in the desired sector, exciting sodium atoms to fluoresce and creating a bright yellow spot. For this, the laser must operate at a precise wavelength of about 589 nm, matching the optical transitions of the sodium atom at the D2 line.
Only a small portion of the laser light can interact with the sodium atoms because, at any given time, they can exist at one of several energy levels possible for those atoms. As a result of collisions with oxygen molecules, the sodium atoms are constantly experiencing changes in energy level.
Five artificial stars – laser guide stars – created by the the Gemini South Telescope in Chile. Photo courtesy of Gemini Observatory/AURA.
“A laser for LGS [laser guide star] adaptive optics is cleverly designed so that it will try to keep a good fraction of the atoms in that energy level that can effectively interact with the laser light and therefore create the brightest possible artificial reference star,” Otárola said. “The EOM is an important component of the clever laser design because it allows [users] to pick a portion of the laser light and shift it in frequency, enough so that it will promote some sodium atoms to the energy level where they can effectively interact with the main wavelength of the laser light.
“A trick adopted from laser cooling [is] using the repumper frequency at 1.713 GHz. To put this in perspective, when a laser uses an EOM, it is able to create a star about twice as bright as when no EOM is used.”
“The yellow guide star lasers are so powerful that the transport of the light from the laser head to the launch poses a certain problem due to nonlinear effects in optical fibers,” said Domenico Bonaccini Calia, a laser developer from the ESO (European Southern Observatory). “Qubig has developed a dedicated spectral broadener EOM and an optimized controller for high-power laser applications. Their ability to lock and track automatically the radio-frequency signal at the peak of modulation, as well as their ingenious crystal thermalization, allow for very efficient and stable operation. In our laser guide star facilities in Chile, we have successfully used their newest line of controllers and EOMs since January 2013. It is a turnkey system, and we have not had a single problem so far.”
A 20-W laser in the ESO lab in Garching, near Munich, with a spectral broadener EOM. Photo courtesy of Anton Öttl, QuBig.
Generally these EOMs are optical components that allow modulation of laser beam phase, polarization and intensity. Because these devices are directly controlled by electrical fields, they work fast – well into the gigahertz range. EOMs exploit the Pockels effect, wherein an external electric field changes the refractive indices, and consequently the birefringence, of a special class of optical crystals.
EOMs are not used only for switching lasers quickly; they also can create additional frequencies, or sidebands, in the laser spectrum. These are essential for laser frequency stabilization on atomic transitions or ultrastable optical cavities, and for laser cooling of atoms, ions and molecules. Additionally, they can be used as optical phase shifters for interferometers and optical lattices.
“Commercial devices are well-known in the telecommunications sector and operate in the infrared. But we needed EOMs for the visible range, and units on the market were expensive, had poor quality and/or did not fit,” said Michael Köhl. Although fiber modulators have tremendous bandwidth and efficiency, they suffer from high transmission losses and low optical power handling, which severely limit their range of use in atomic physics, for example.
‘Bulk’ crystals with an aperture of mostly 3 × 3 mm and 5 × 5 mm. Photo courtesy of Wolfgang Gasser.
For medium- and high-power lasers and wavelengths in the visible, UV and mid-IR free space, EOMs are indispensable. The heart of a free-space EOM is a functional bulk crystal. “There are about a dozen different crystals with ‘reasonable’ electro-optical effect,” said Dr. Anton Öttl, managing director at Qubig. “A profound knowledge of each crystal’s symmetries and properties is essential to select the right axes with respect to laser propagation, polarization and electric field.
“By utilizing the optical axes of the anisotropic crystals through appropriate cutting of the crystal, we can optimize the modulation efficiency and, for example, define whether the amplitude or the phase is to be modified.”
Free-space EOMs excel in optical transmission close to 100 percent. Special antireflection coatings can be applied on the polished end facets of the crystals, all having a common high refractive index, or Brewster-cut angles can be employed to avoid any reflections. Obviously, the linear absorption coefficient of the crystal material also plays a key role. When pushing the limits into UV and mid-IR, materials need to be carefully selected, as the transmission window of standard crystals such as lithium niobate (LN) and lithium tantalate is limited to approximately 400 nm to 3 µm. For instance, in the mid-IR, up to 10.6-µm materials such as GaAs need to be used. “For the UV down to 200 nm, we are developing special EOMs to meet the demands [of] laser cooling, ion manipulation and trends in laser developments,” Öttl said.
Detail of a typical setup of an ultracold atom lab, with a K40 (potassium-40) in the center. Photo courtesy of Enrico Vogt, Qubig.
An important characteristic of lasers is their well-defined spatial mode. For many applications, care must be taken not to deteriorate this mode by deforming the wavefront of the laser with any optical element in the light path. This imposes stringent requirements on the quality of crystals used, in terms of geometrical precision and intrinsic optical homogeneity. Outstanding quality is achieved with RTP (rubidium titanyl phosphate) crystals in the EOMs used in laser guide star systems. Additionally, this crystal is well-suited to handling high-power lasers, in contrast to materials such as LN, which suffers photorefractive damage in the visible, or KTP (potassium titanyl phosphate), which is susceptible to gray tracking.
However, it would be almost impossible to use these bulk EOMs with the required high frequencies in the megahertz and gigahertz range, as the electro-optical effect is so small and therefore requires very high voltages. “By building a high-frequency resonant circuit, we are able to boost voltage over the crystal by a factor of about 100. By sacrificing bandwidth, we achieve driver requirements well within the range of commonly available [radio-frequency] amplifiers,” said Dr. Enrico Vogt, managing director of Qubig. “We mitigate the drawback of a narrow resonance by precisely manufacturing the required frequencies and by providing [the customers themselves with] the flexibility to individually tune the resonance frequency.”
Meet the author
As a freelance technical journalist, Barabara Stumpp supports various companies in their external communications on complex products; email: firstname.lastname@example.org.