A. Nevsky

Broadly tunable single-frequency cw mid-infrared source with milliwatt-level output based on difference-frequency generation in orientation-patterned GaAs

A narrow-linewidth mid-IR source based on difference-frequency generation of an amplified 1.5 microm diode laser and a cw Tm-doped fiber laser in orientation-patterned (OP) GaAs has been developed and evaluated for spectroscopic applications. The source can be tuned to any frequency in the 7.6-8.2 microm range with an output power of 0.5 mW. The measured characteristics of the OP-GaAs sample demonstrate a high quality of the material.

The space optical clocks project: Development of high-performance transportable and breadboard optical clocks and advanced subsystems

The use of ultra-precise optical clocks in space (“master clocks”) will allow for a range of new applications in the fields of fundamental physics (tests of Einsteins theory of General Relativity, time and frequency metrology by means of the comparison of distant terrestrial clocks), geophysics (mapping of the gravitational potential of Earth), and astronomy (providing local oscillators for radio ranging and interferometry in space). Within the ELIPS-3 program of ESA, the “Space Optical Clocks” (SOC) project aims to install and to operate an optical lattice clock on the ISS towards the end of this decade, as a natural follow-on to the ACES mission, improving its performance by at least one order of magnitude. The payload is planned to include an optical lattice clock, as well as a frequency comb, a microwave link, and an optical link for comparisons of the ISS clock with ground clocks located in several countries and continents. Undertaking a necessary step towards optical clocks in space, the EU-FP7-SPACE-2010-1 project no. 263500 (SOC2) (2011–2015) aims at two “engineering confidence“, accurate transportable lattice optical clock demonstrators having relative frequency instability below 1×10−15 at 1 s integration time and relative inaccuracy below 5×10−17. This goal performance is about 2 and 1 orders better in instability and inaccuracy, respectively, than todays best transportable clocks. The devices will be based on trapped neutral ytterbium and strontium atoms. One device will be a breadboard. The two systems will be validated in laboratory environments and their performance will be established by comparison with laboratory optical clocks and primary frequency standards. In order to achieve the goals, SOC2 will develop the necessary laser systems - adapted in terms of power, linewidth, frequency stability, long-term reliability, and accuracy. Novel solutions with reduced space, power and mass requirements will be implemented. Some of the laser systems will be developed towards particularly high compactness and robustness levels. Also, the project will validate crucial laser components in relevant environments. In this paper we present the project and the results achieved during the first year.

A compact, robust, and transportable ultra-stable laser with a fractional frequency instability of 1 × 10−15

We present a compact and robust transportable ultra-stable laser system with minimum fractional frequency instability of 1 × 10(-15) at integration times between 1 and 10 s. The system was conceived as a prototype of a subsystem of a microwave-optical local oscillator to be used on the satellite mission Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST) (http://sci.esa.int/ste-quest/). It was therefore designed to be compact, to sustain accelerations occurring during rocket launch, to exhibit low vibration sensitivity, and to reach a low frequency instability. Overall dimensions of the optical system are 40 cm × 20 cm × 30 cm. The acceleration sensitivities of the optical frequency in the three directions were measured to be 1.7 × 10(-11)/g, 8.0 × 10(-11)/g, and 3.9 × 10(-10)/g, and the absolute frequency instability was determined via a three-cornered hat measurement. Two additional cavity-stabilized lasers were used for this purpose, one of which had an instability σy < 4 × 10(-16) at 1 s integration time. The design is also appropriate and useful for terrestrial applications.

Spectrally narrow, long-term stable optical frequency reference based on a Eu3+:Y2SiO5 crystal at cryogenic temperature.

Using an ultrastable continuous-wave laser at 580 nm we performed spectral hole burning of Eu(3+):Y(2)SiO(5) at a very high spectral resolution. The essential parameters determining the usefulness as a macroscopic frequency reference, linewidth, temperature sensitivity, and long-term stability, were characterized using a H-maser stabilized frequency comb. Spectral holes with a linewidth as low as 6 kHz were observed and the upper limit of the drift of the hole frequency was determined to be 5±3 mHz/s. We discuss the necessary requirements for achieving ultrahigh stability in laser frequency stabilization to these spectral holes.

Robust frequency stabilization of multiple spectroscopy lasers with large and tunable offset frequencies

We have demonstrated a compact, robust device for simultaneous absolute frequency stabilization of three diode lasers whose carrier frequencies can be chosen freely relative to the reference. A rigid ULE multicavity block is employed, and, for each laser, the sideband locking technique is applied. A small lock error, computer control of frequency offset, wide range of frequency offset, simple construction, and robust operation are the useful features of the system. One concrete application is as a stabilization unit for the cooling and trapping lasers of a neutral-atom lattice clock. The device significantly supports and improves the clocks operation. The laser with the most stringent requirements imposed by this application is stabilized to a line width of 70 Hz, and a residual frequency drift less than 0.5 Hz/s. The carrier optical frequency can be tuned over 350 MHz while in lock.

Silicon single-crystal cryogenic optical resonator

We report on the demonstration and characterization of a silicon optical resonator for laser frequency stabilization, operating in the deep cryogenic regime at temperatures as low as 1.5 K. Robust operation was achieved, with absolute frequency drift less than 20 Hz over 1 h. This stability allowed sensitive measurements of the resonator thermal expansion coefficient (α). We found that α=4.6×10(-13)  K(-1) at 1.6 K. At 16.8 K α vanishes, with a derivative equal to -6×10(-10)  K(-2). The temperature of the resonator was stabilized to a level below 10 μK for averaging times longer than 20 s. The sensitivity of the resonator frequency to a variation of the laser power was also studied. The corresponding sensitivities and the expected Brownian noise indicate that this system should enable frequency stabilization of lasers at the low-10(-17) level.

Thermal noise of whispering-gallery resonators

By direct application of the fluctuation-dissipation theorem, we numerically calculate the fundamental thermal fluctuationsofthedimensionsofcrystallineCaF2 whispering-galleryresonatorsinthecaseofstructuraldamping, and the limit that this noise imposes on the frequency stability of such resonators at both room and cryogenic temperatures. We analyze elasto-optic noise—the effect of Brownian dimensional fluctuation on frequency via the strain-dependence of the refractive index—a noise term that has so far not been considered for whisperinggallery resonators. We find that dimensional fluctuation sets a lower limit of 10 −16 to the Allan deviation for a 10-mm-radius sphere at 5 K, predominantly via induced fluctuation of the refractive index.

Silicon single-crystal cryogenic optical resonator: erratum

We correct fit formulas from a previous paper [Opt. Lett.39, 3242 (2014)10.1364/OL39.005896OPLEDP0146-9592] for the coefficient of thermal expansion αreson(T).

A transportable optical lattice clock using 171 Yb

We present first results on the spectroscopy of the ¹S₀ → ³P₀ transition at 578nm in a transportable ¹⁷¹Yb optical lattice clock. With the Yb atoms confined in a one-dimensional optical lattice, we have observed linewidths below 200 Hz, limited by saturation broadening. Currently the system is being upgraded towards full clock operation and use of more compact and robust subsystems.

Towards Neutral-atom Space Optical Clocks (SOC2): Development of high-performance transportable and breadboard optical clocks and advanced subsystems

The use of ultra-precise optical clocks in space (“master clocks”) will allow for a range of new applications covering the fields of fundamental physics (tests of Einsteins theory of General Relativity, time and frequency metrology by means of the comparison of distant terrestrial clocks), geophysics (mapping of the gravitational potential of Earth), and astronomy (providing local oscillators for radio ranging and interferometry in space). Within the ELIPS-3 program of ESA, the “Space Optical Clocks” (SOC) project aims to install and to operate an optical lattice clock on the ISS towards the end of this decade, as a natural follow-on to the ACES mission (which is based on a cesium microwave clock), improving its performance by at least one order of magnitude. The payload is planned to include an optical lattice clock, as well as a frequency comb, a microwave link, and an optical link for comparisons of the ISS clock with ground clocks located in several countries and continents. Undertaking a necessary step towards optical clocks in space, the EU-FP7-SPACE2010-1 project no. 263500 (SOC2) (2011-2015) will develop two “engineering confidence“, accurate transportable lattice optical clock demonstrators having relative frequency instability below 1×10 -15 at 1s integration time and relative inaccuracy below 5×10 -17 . This goal performance is about 2 and 1 orders better in instability and inaccuracy, respectively, than today’s best transportable clocks. The devices will be based on trapped neutral ytterbium and strontium atoms. One device will be a breadboard. The two systems will be validated in laboratory environments and their performance will be established by comparison with laboratory optical clocks and primary frequency standards. In order to achieve the goals, SOC2 will develop the necessary laser systems adapted in terms of power, linewidth, frequency stability, long-term reliability, and accuracy. Novel solutions with reduced space, power and mass requirements will be implemented. Some of the laser systems will be developed towards particularly high compactness and robustness levels. Also, the project will validate crucial laser components in relevant environments. This paper will give an overview of the project and of the results achieved during theProject ReCover aims at developing beyond state-of-the-art service capabilities to support fighting deforestation and forest degradation in the tropical region. The service capabilities mean provision of a monitoring system of forest cover, forest cover changes, and biomass including a robust accuracy assessment. This paper presents the forest monitoring concept and the first results on Recover study sites. ReCover contributes to the efforts to reduce the errors in the estimates of the terrestrial carbon balance that result from uncertain rates of tropical deforestation. It develops methods for the REDD (Reducing Emissions from Deforestation and Forest Degradation) process by developing and implementing satellite image based methods for the monitoring of tropical forests. The REDD will be a major driver for the development of more effective and more reliable procedures for the monitoring of tropical forests. Many developing countries lack human resources and funding for detailed forest inventories. This paper reports the achievements of the first year of ReCover and the results of services in Mexico, Guyana, Democratic Republic of Congo, and Fiji. Altogether 42 products were delivered to the users of Recover. The accuracy in forest and non-forest classification was from 85 % to 91 % with one exception (76 %).