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

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 report on the development of all-fiber stand-alone iodine-filled photonic microcells demonstrating record absorption contrast at room temperature. The microcell's fiber is made of inhibited coupling guiding hollow-core photonic crystal fibers. The fiber-core loading with iodine was undertaken at 10-1-10-2mbar vapor pressure using what, to the best of our knowledge, is a novel gas-manifold based on metallic vacuum parts with ceramic coated inner surfaces for corrosion resistance. The fiber is then sealed on the tips and mounted on FC/APC connectors for better integration with standard fiber components. The stand-alone microcells display Doppler lines with contrasts up to 73% in the 633 nm wavelength range, and an off-resonance insertion loss between 3 to 4 dB. Sub-Doppler spectroscopy based on saturable absorption has been carried out to resolve the hyperfine structure of the P(33)6-3 lines at room temperature with a full-width at half maximum of 24 MHz on the b4 component with the help of lock-in amplification. Also, we demonstrate distinguishable hyperfine components on the R(39)6-3 line at room temperature without any recourse to signal-to-noise ratio amplification techniques.

Fabrication and characterization of iodine photonic microcells for sub-Doppler spectroscopy applications.

Precision measurements of the molecular iodine optical transitions can be exploited for constraining a possible time variation of the fundamental constants. The sensitivities of the molecular frequencies to a variation of the proton-to-electron mass ratio and of the fine structure constant are calculated. Compact molecular iodine clocks, designed for space applications with improved stability performances, enable fractional frequency reproducibility at the 10−15 level. The comparison of the optical iodine clocks based on transitions at 532 nm, 514 nm and 502 nm with the Cs frequency standard can constrain a fractional time variation of the proton-to-electron mass ratio and of the fine structure constant at the 10−14 yr−1 level.

/pdf/towards-probing-a-variation-of-fundamental-constants-with-3sy2fqnukc.pdf

Towards Probing a Variation of Fundamental Constants with Optical Clock Transitions of 127 I 2

We report on an ongoing development of a compact and transportable iodine frequency stabilized laser setup, based on compact and fibered Telecom components with a high technological readiness level (TRL). This laser system is being planned as a compact and easily transportable ultra-stable frequency reference for ground tests of the payload of LISA mission (Laser Interferometer Space Antenna) as part of the SYRTE laboratory contribution to the French activities carried out by a consortium of several partners lead by the French Space Agency (CNES) for assembly-integration and tests (AIVT) of the payload of LISA. This frequency reference will match the LISA requirements in terms of both residual frequency and intensity noise. The target residual frequency noise for this stabilized laser is below the nominal requirement for LISA, which is currently 30 Hz/ Hz. Thus, we propose to provide a tandem of 1064 nm laser sources phase-locked to an iodine stabilized Telecom laser operating at 1596 nm. The frequency gap between the telecom domain and the green range is bridged using a frequency tripling process based on two cascaded PPLN crystals, fully fibered. The compact design of the whole setup will make it easily transportable to different sites and could be readily used for ground tests of the LISA payload.

/pdf/towards-the-development-of-a-compact-monolithic-optical-3svoir724d.pdf

Towards the development of a compact-monolithic optical assembly for Telecom laser frequency stabilization for LISA ground tests and other 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

We report on the effect of a weak external magnetic field on iodine hyperfine transitions around 514 nm, used for infrared laser frequency stabilization. We use a frequency tripled laser diode operating at 1542.1 nm, to interrogate the iodine vapor inserted into a short sealed quartz cell, surrounded by an efficient solenoid. Thus we create a controlled external magnetic fields whose orientation is longitudinal to the counter-propagating laser beams direction with a level of 7 Gauss/Ampere. The combination of the iodine cell and the solenoid is inserted into a magnetic shield that ensures a residual magnetic field with an attenuation factor of 200. We have observed a linear Zeeman Effect that affects the iodine hyperfine line width as well as its center frequency. We measured large frequency shifts, induced by the longitudinal magnetic field, at the level of 1.8x10-12/Gauss, for all 15 hyperfine components of the R 34[44-0]127 I 2 line at 514.017 nm. Thus an isolation from external magnetic fields below the mG level is required to confer a residual frequency instability at the 10-15 level.

Linear Zeeman Effect on Iodine-Based Frequency Stabilized Laser

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Compact and Transportable Iodine Frequency-stabilized laser

Linear Zeeman Effect on Iodine-Based Frequency Stabilized Laser