Y. Kersale

Ultra-low-noise microwave extraction from fiber-based optical frequency comb

In conclusion, we have used two FOFC based optical to microwave division frequency synthesizers referenced to a common optically source to create 11.55 GHz microwave signals with a relative frequency stability of 1.6×10-16 at 1 s. The relative phase noise spectral density at a 1 Hz offset from the 11.55 GHz carrier is measured at 111 dBrad2/Hz, limited by the readout system noise floor. Long term stability and accuracy down to 3×10-19 at 65536 s was also demonstrated from a set of 3 days continuous measurement. These results are obtained with classical double balanced mixers measurement scheme. By using a noise measurement system based on the carrier suppression method and advanced noise reduction techniques we are able to improve the results down to a phase noise spectral density at a 1 Hz of 117 dBrad2/Hz and a FFS is of 1.5×10-19 at 1000s (for a single system).

Sub-100 attoseconds stability optics-to-microwave synchronization

We use two fiber-based femtosecond frequency combs and a low-noise carrier suppression phase detection system to characterize the optical to microwave synchronization achievable with such frequency divider systems. By applying specific noise reduction strategies, a residual phase noise as low as −120 dBc/Hz at 1 Hz offset frequency from a 11.55 GHz carrier is measured. The fractional frequency instability from a single optical-to-microwave frequency divider is 1.1×10−16 at 1 s averaging down to below 2×10−19 after only 1000 s. The corresponding rms time deviation is lower than 100 attoseconds up to 1000 s averaging duration.

ELISA: A cryocooled 10 GHz oscillator with 10−15 frequency stability

This article reports the design, the breadboarding, and the validation of an ultrastable cryogenic sapphire oscillator operated in an autonomous cryocooler. The objective of this project was to demonstrate the feasibility of a frequency stability of 3x10(-15) between 1 and 1000 s for the European Space Agency deep space stations. This represents the lowest fractional frequency instability ever achieved with cryocoolers. The preliminary results presented in this paper validate the design we adopted for the sapphire resonator, the cold source, and the oscillator loop.

Maser Oscillation in a Whispering-Gallery-Mode Microwave Resonator

We report the observation of above-threshold maser oscillation in a whispering-gallery (WG)-mode resonator, whose quasitransverse-magnetic, 17th-azimuthal-order WG mode, at a frequency of approximately 12.038GHz, with a loaded Q of several hundred million, is supported on a cylinder of monocrystalline sapphire. An electron spin resonance associated with Fe3+ ions, that are substitutively included within the sapphire at an effective concentration of a few parts per billion, coincides in frequency with that of the (considerably narrower) WG mode. By applying a cw “pump” to the resonator at a frequency of approximately 31.34GHz, with no applied dc magnetic field, the WG (“signal”) mode is energized through a three-level maser scheme. Preliminary measurements demonstrate a frequency stability (Allan deviation) of a few times 10−14 for sampling intervals up to 100s.

Advanced noise reduction techniques for ultra-low phase noise optical-to-microwave division with femtosecond fiber combs

We report what we believe to be the lowest phase noise optical-to-microwave frequency division using fiber-based femtosecond optical frequency combs: a residual phase noise of -120 dBc/Hz at 1 Hz offset from an 11.55 GHz carrier frequency. Furthermore, we report a detailed investigation into the fundamental noise sources which affect the division process itself. Two frequency combs with quasi-identical configurations are referenced to a common ultrastable cavity laser source. To identify each of the limiting effects, we implement an ultra-low noise carrier-suppression measurement system, which avoids the detection and amplification noise of more conventional techniques. This technique suppresses these unwanted sources of noise to very low levels. In the Fourier frequency range of ~200 Hz to 100 kHz, a feed-forward technique based on a voltage-controlled phase shifter delivers a further noise reduction of 10 dB. For lower Fourier frequencies, optical power stabilization is implemented to reduce the relative intensity noise which causes unwanted phase noise through power-to-phase conversion in the detector. We implement and compare two possible control schemes based on an acousto-optical modulator and comb pump current. We also present wideband measurements of the relative intensity noise of the fiber comb.

A cryogenic open-cavity sapphire reference oscillator with low spurious mode density

In this paper, we describe the implementation of a microwave cryogenic sapphire oscillator (CSO) at the Laboratoire de Physique et Metrologie des Oscillateurs. In our realization we solved the problem of the spurious modes by operating the sapphire resonator in an open cavity. The CSO compared to a hydrogen maser demonstrates a frequency stability better than 3/spl times/10/sup -14/ at short term. Its long-term frequency instability of the order of 3/spl times/10/sup -12/ / day is limited by a random walk process. A first attempt to use this reference oscillator to characterize other signal sources is presented.

New-generation of cryogenic sapphire microwave oscillators for space, metrology, and scientific applications

We recently demonstrated a Cryogenic Sapphire Oscillator (CSO) presenting a short term frequency stability better than 3 × 10⁻¹⁵ for 1 s≤ τ < 1000 s and achieving 4.5 × 10⁻¹⁵ for one day integration. This CSO incorporates a pulse-tube cooler instead of a bath cryostat-thus eliminating the need for regular supplies and manual transferring of liquid helium. The advent of reliable and cryocooled CSO open the possibility to implement such an ultra-stable reference not only in metrological laboratories with liquid helium facilities but also in remote sites like base stations for space navigation, VBLI antenna sites, …

MASER OSCILLATION FROM ELECTRONIC SPIN RESONANCE IN A CRYOGENIC SAPPHIRE FREQUENCY STANDARD

We report the first observation of an Fe3+ maser oscillation at zero magnetic field inside a whispering gallery (WG) sapphire resonator. The described maser is new in that it operates at zero-field and with low ion concentration. At zero-field, the Fe3+ ion shows a 3-level structure related to the electron spin resonance (ESR). By applying a 31 GHz pump (|1/2〉 → |5/2〉), the ion operates as a maser at 12 GHz (|5/2〉 → |3/2〉). The maser effect is made possible by the high Q-factor (several 108) of the cryogenic whispering gallery resonator. Additionnaly, the sharp cavity resonance provides short term stability. Preliminary measurements indicate a frequency stability of parts in 10-14 (Allan deviation at 100 s), still limited by the instrument. The ultimate maser stability is still unknown.

Thermal stabilization of microwave sapphire resonator references

Sapphire single crystal, associated with a special mode configuration (whispering gallery mode), is an ideal material for the realisation of high Q microwave resonator. Unfortunately, the frequency sensitivity to temperature fluctuations is relatively high. With such a sensitivity it is impossible to reach high frequency stability over a long time interval without efficient temperature regulation. We implemented original thermal regulation scheme on different sapphire resonators. We report, in this paper, frequency stability measurements of temperature controlled sapphire resonator oscillator at room temperature and at liquid nitrogen temperature.

Amplification process in a high-Q cryogenic whispering gallery mode sapphire Fe3 + maser

This paper reports on the analysis of a high-Q (109) cryogenic whispering gallery mode (WGM) sapphire resonator maser oscillator based on residual impurities of the Fe3 + ion (1 ppm). A theoretical model is developed to determine the evolution of the maser output power versus the pump power and the resonator output coupling coefficient. Comparison of the theoretical model and experimental data shows a good agreement.