Advanced technologies for satellite navigation and geodesy

Future generations of global navigation satellite systems (GNSSs) can benefit from optical technologies. Especially optical clocks could back-up or replace the currently used microwave clocks, having the potential to improve GNSS position determination enabled by their lower frequency instabilities. Furthermore, optical clock technologies—in combination with optical inter-satellite links—enable new GNSS architectures, e.g., by synchronization of distant optical frequency references within the constellation using time and frequency transfer techniques. Optical frequency references based on Doppler-free spectroscopy of molecular iodine are seen as a promising candidate for a future GNSS optical clock. Compact and ruggedized setups have been developed, showing frequency instabilities at the 10–15 level for averaging times between 1 s and 10,000 s. We introduce optical clock technologies for applications in future GNSS and present the current status of our developments of iodine-based optical frequency references.

/pdf/optical-clock-technologies-for-global-navigation-satellite-3ggffl7xqj.pdf

Optical clock technologies for global navigation satellite systems

Precision measurement tools are compulsory to reduce measurement errors or machining errors
in the processes of calibration and manufacturing. The laser interferometer is one of the most
important measurement tools invented in the 20th century. Today, it is commonly used in
ultraprecision machining and manufacturing, ultraprecision positioning control, and many
noncontact optical sensing technologies. So far, the state-of-the-art laser interferometers are the
ground-based gravitational-wave detectors, e.g. the Laser Interferometer Gravitational-wave
Observatory (LIGO). The LIGO has reached the measurement quantum limit, and some
quantum technologies with squeezed light are currently being tested in order to further
decompress the noise level. In this paper, we focus on the laser interferometry developed for
space-based gravitational-wave detection. The basic working principle and the current status of
the key technologies of intersatellite laser interferometry are introduced and discussed in detail.
The launch and operation of these large-scale, gravitational-wave detectors based on
space-based laser interferometry is proposed for the 2030s.

https://iopscience.iop.org/article/10.1088/2631-7990/ab8864/pdf

Ultraprecision intersatellite laser interferometry

We demonstrate the third harmonic generation of a 1542-nm laser using a dual-pitch periodically poled lithium niobate waveguide with a conversion efficiency of 66%/W2. The generated 514-nm light is used for saturation spectroscopy of molecular iodine and laser frequency stabilization. The achieved laser frequency stability is 1.1×10-12 at an average time of 1 s, which is approximately one order of magnitude better than the acetylene-stabilized laser at 1542 nm. Uncertainty evaluation and absolute frequency measurement are also performed. The developed frequency-stabilized laser can be used as a reliable frequency reference at the telecom wavelength for various applications including optical frequency combs and precision interferometric measurement.

Iodine-stabilized laser at telecom wavelength using dual-pitch periodically poled lithium niobate waveguide.

We present an improved technique for detection of trace impurities in iodine-filled absorption cells for laser frequency
stabilization. The results of purity investigation are compared to frequency shifts measured with a set of two iodine
stabilized Nd:YAG lasers. The setup for direct fluorescence measurement with an Argon-ion laser operating at 502 nm
wavelength is equipped with compensation for laser power and spectral instabilities.

Absolute frequency shifts of iodine cells for laser stabilization

Nowadays, wide range of space missions including space interferometry laser, fundamental physic tests, ranging measurement, distance changes between space aircraft and optical communication with each other or with ground reference, etc … require powerful lasers in a compact set-up configuration with highly intrinsic frequency stability. The couple “frequency doubled Nd:YAG laser and iodine molecular transition” in the green range of the optical domain, is one of the most promising candidate with superior frequency stability for various applications. LISA (Laser Interferometer Space Antenna), DECIGO (DECI-Hertz Interferometer Gravitational wave Observatory), STAR (Space Time Asymmetry Research), Post-GRACE (Gravity Recovery and Climate Experiment) are few examples of space missions based on ultra stable laser use. We report on our ongoing development of Nd:YAG stabilized laser using an original and reliable experimental configuration with expected frequency stability in the 10-15 range over thousands of seconds of integration time.

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10565/1056568/Nd-YAG-laser-frequency-stabilized-for-space-applications/10.1117/12.2552609.pdf

Nd: YAG LASER FREQUENCY STABILIZED FOR SPACE APPLICATIONS

We present results of investigation and comparison of spectral properties of molecular iodine transitions in the spectral region of 514.7 nm that are suitable for laser frequency stabilization and metrology of length. Eight Doppler-broadened transitions that were not studied in detail before were investigated with the help of frequency doubled Yb-doped fiber laser, and three of the most promising lines were studied in detail with prospect of using them in frequency stabilization of new laser standards. The spectral properties of hyperfine components (linewidths, signal-to-noise ratio) were compared with transitions that are well known and traditionally used for stabilization of frequency doubled Nd:YAG laser at the 532 nm region with the same molecular iodine absorption. The external frequency doubling arrangement with waveguide crystal and the Yb-doped fiber laser is also briefly described together with the observed effect of laser aging.

http://www.measurement.sk/2014/Hrabina.pdf

Comparison of Molecular Iodine Spectral Properties at 514.7 and 532 nm Wavelengths

We report on a Telecom laser diode (LD) frequency stabilization to a narrow iodine hyperfine line in the green
range, after frequency tripling process using fibered nonlinear waveguide PPLN crystals. We have generated up
to 300 mW optical power in the green range (~514 nm) from 800 mW of infrared power (~1542 nm),
corresponding to a nonlinear conversion efficiency h = P3?/P? ~ 36%. Less than 10 mW of the generated green
power are used for Doppler-free spectroscopy of 127I2 molecular iodine, and –therefore- for the frequency
stabilization purpose. The frequency tripling optical setup is very compact (< 5 l), fully fibered, and could operate
over the full C-band of the Telecom range (1530 nm – 1565 nm). Several thousands of hyperfine iodine lines may
thus be interrogated in the 510 nm – 521 nm range. We build up an optical bench used at first in free space
configuration, using the well-known modulation transfer spectroscopy technique (MTS), in order to test the
potential of this new frequency standard based on the couple “1.5 ?m laser / iodine molecule”. We have already
demonstrated a preliminary frequency stability of 4.8 x 10-14 ? -1/2 with a minimum value of 6 x 10-15 reached
after 50 s of integration time, conferred to a laser diode operating at 1542.1 nm.
We focus now our efforts to expand the frequency stability to a longer integration time in order to meet
requirements of many space experiments, such earth gravity missions, inters satellites links or space to ground
communications.
Furthermore, we investigate the potential of a new approach based on frequency modulation technique (FM),
associated to a 3rd harmonic detection of iodine lines to increase the compactness of the optical setup.

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10562/1056253/A-compact-frequency-stabilized-telecom-laser-diode-for-space-applications/10.1117/12.2296121.pdf

A compact frequency stabilized telecom laser diode for space applications

We report on a compact optical frequency standard (OFS) based on a Telecom laser diode operating at ~ 1542 nm, frequency stabilized to a narrow iodine transition located in the green range of the visible domain (~514 nm), after a highly efficient frequency tripling process. We use two cascaded waveguide Lithium Niobate nonlinear crystals for the third harmonic generation process (THG), resulting in a harmonic power of 300 mW in the green range (@3ω) using 800 mW of infrared power (@ω ). This result corresponds to an optical conversion efficiency Pω/Pω 36 % which is -to our knowledge- the best result ever reported for a third harmonic process in continuous wave regime (CW). This process uses only 20 W of total consumption power, which can be drastically reduced, knowing that less than 10% of that green power level is needed for the iodine Doppler free spectroscopy, and consequently for the frequency stabilization purpose. We have already demonstrated a frequency stability of 2.9 x10-14 τ-1/2 conferred to a laser diode operating at ~ 1542 nm, using the a1 hyperfine component of the R34 [44-0] located at ~ 514 nm. This corresponds to an amplitude spectral density of the residual frequency fluctuations < 10 Hz/√Hz. We plan to extend this approach to set up a new OFS by using a narrow linewidth fiber-laser emitting at ~1597 nm, which will be used for the phase-locking of a 1064 nm laser. Thus, this OFS, compact and mainly fibered, will perform the same role as that of a rigid optical cavity, widely used to stabilize 1064 nm lasers, involved in various terrestrial applications or space missions. The compact design of the whole setup will make it easily transportable to different sites and could be readily used as an ultra-stable frequency reference.

/pdf/compact-and-transportable-iodine-frequency-stabilized-laser-186ddk3pck.pdf

Compact and Transportable Iodine Frequency-stabilized laser

This paper reports the frequency measurement with an accuracy in the 100 kHz range of several optical transitions of atomic Sr. Measurements are performed with a frequency chain based on a femto-second laser and referenced to a hydrogen maser. They allowed the determination with a 70 kHz uncertainty of the energy of the 5s5p 3 Po state and the direct observation and frequency measurement of the doubly forbidden 1S0 rarr3 P0 transition at 698 nm. This line has a natural width of 10 - Hz and can be used for a new generation of optical frequency standards using atoms trapped in a light shift free dipole trap

Optical frequency measurements with laser cooled Sr atoms

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