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Rb atomic clock with tunable eom vcsel

A novel vcsel-pumped ultra-precise (sub-microsecond per day) miniature low power rb atomic clock will be developed. Wavelength adjustment and microwave excitation matching the hyperfine splitting using electro-optic wavelength tuning and phase modulation will overcome the present limitations.

Emerging miniature atomic clocks find vast applications in numerous fields. Key applications include autonomous cars, secure and robust GNSS(GPS) navigations, marine oil exploration, transportation, telecommunication, satelite communications. electric energy distribution (power grid) and finance. Broad penetration of self-driving vehicles is expected and is already happening. To meet the challenge new technologies are demanded. One area of research addresses sensing, which allows relative positioning of the vehicles with respect to each other and with respect to the road. For example, LIDARs operating in the time-of-flight (ToF) mode, allow definition of the sizes, distances and relative motions with a high precision. However, in bad weather and in heavy traffic this technology can be severely impaired and must be combined with GPS/GNSS systems. Other techniques for precision positioning consider relative data exchange and navigation using cooperative communications between vehicles and and the road infrastructure. All of these techniques require extreme timing needed for precise positioning to be provided by an Atomic Clock, a key element to assist navigation and synchronize data. Atomic clock is based on atomic resonances to achieve unprecedented accuracy and stability, Traditional Rb atomic clocks relied on a Rb lamp for optical pumping and a separate cell in a microwave cavity for the hyperfine transition interrogation. An additional filter cell was required to select the appropriate wavelength. The arrangement performed satisfactorily, but was complex, not easily prone to be miniaturized and expensive to manufacture. Recently, the introduction of the effect of coherent population trapping (CPT) has led to a substantial reduction in power and size of commercial atomic frequency standards. The CPT effect produces a narrow transparency window in the transmission spectrum of an atomic vapor cell which can be used as the clock signal of an atomic standard. The CPT is accomplished by means of two laser radiation fields applied in a Λ scheme. A practical way to achieve the desired radiation fields is to modulate the operating current of a VCSEL laser diode with half of the frequency of the hyperfine transition. The resulting optical spectrum exhibits the two desired sidebands with a separation equal to the hyperfine transition frequency. Presently 794.9 nm and 852.1 nm transitions are used for miniature Rubidium and Cesium Clocks, pumped by vertical-cavity surface-emitting lasers (VCSELs). VCSELs are found to be the most acceptable solution as: • the devices can be made single mode at a fixed wavelength • the devices can be easily modulated to high frequencies with low power consumption • the wavelength can be fine-tuned to the necessary wavelength The problems however are: • Tuning to the precise Rb87 D1 line at 794.978…nm within a temperature window of several °C around 90°C. The spread of the wavelength across the epitaxial wafer is significant ±5nm making yield of the VCSELs very low. • As mentioned Phase modulation is obtained by directly modulating the VCSEL current to get the two sidebands. However, while direct current modulation is straightforward to apply, it both modulates the intensity of the laser output and produces a combination of amplitude modulation (AM) and phase/frequency modulation (PM/FM) on the output, which introduce unwanted frequency shifts to the clock. • VCSEL must be made polarization stable without sacrificing the performance and yield. Therefore the tasks of this project are: • A concept of electro-optically wavelength-tunable passive cavity VCSEL (EOT-VCSEL) will be explored. In the VCSEL to be developed, the electro-optic effect in the cavity section will be applied for tuning the wavelength to the exact atomic resonance, while the microwave modulation will be obtained by pure optical phase modulation, thereby eliminating the problems of producing simultaneous AM and FM/PM. This leads to a substantial improvement in the clock performance. • Furthermore, EOT-VCSEL will substantially increase the VCSEL production yield thereby reducing its cost. • A novel gain medium (tensile strained GaAlAsP barriers) and substrate orientation (211) will be applied to achieve high temperature stability of the characteristics and polarized emission. • Quantum dot (QD) technologies will be explored. Within already performed experiments growth of thin stacked InGaAs layers already with a low (~15%) InAs composition on (211)A surface results in formation of vertically coupled 3D quantum dots. The growth effects are to be explored to the benefit of the EO modulation and gain. Tuning of the cavity mode is useful for multiple devices, in particular to single QD light emitters where the cavity mode tuning allows resonance high rate outcoupling of the single photon emission from microcavity and with a narrow far field emission suitable for coupling to single mode fiber.
Acronym: 
TUNEABEAT
Project ID: 
12 674
Start date: 
01-04-2018
Project Duration: 
24months
Project costs: 
320 000.00€
Technological Area: 
Electronic measurement systems
Market Area: 
TRANSPORTATION

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