Artist view of the complete system.

ULiss Cryogenic Sapphire Oscillator (CSO) offers unprecedented frequency stability performances coming from the exceptional regularity of the beat of its heart: a high-purity sapphire crystal placed at low temperature in a well-controlled environment.

The sapphire crystal has the shape of a cylinder: approximately 5 cm diameter, 3 cm high. It constitutes a Whispering Gallery Microwave Resonator in which a 10 GHz signal can propagate around the cylinder making 1 billion of cycles before undergoing noticeable attenuation. Beside low dielectric losses, the cryogenic sapphire resonator presents a low sensitivity to temperature fluctuations and to mechanical vibrations. It constitutes an ultra-stable frequency reference that does not show appreciable drift before one day of integration.

The sapphire resonator is maintained at 6 Kelvin into a Closed Cycle Cryocooler specially designed to limit mechanical vibrations and thermal fluctuations. The autonomy of the whole system is thus the lifetime of the cryocooler (2 years between maintenance).

The cooled sapphire resonator is the frequency-determining element of an oscillator loop whom electrical length and circulating power are stabilized thanks to specially designed numerical electronic controls.

ULiss CSO is complemented with a low noise frequency synthesis generating useful ultra-stable signals at 10 GHz, 100 MHz and 10 MHz (standard frequency outputs). The output frequencies can be adjusted by acting on the internal Direct Digital Synthesizer enabling a relative frequency resolution of 1x10-16. A Phase Comparator can be provided to lock the CSO output signals to an external 100 MHz reference.

Whispering Gallery Microwave Resonator

The Uliss exceptional frequency performances essentially comes from our deep expertise in the Sapphire Whispering Gallery Mode Resonator technology that has been gained through research and development activities leaded in the Femto-ST Institute since more than 15 years.

Sapphire resonator.

Inside the sapphire resonator, the electromagnetic wave circulates around the inner cylindrical surface as the result of total internal reflection. The high degree of the electromagnetic field confinement leads to a quality factor essentially limited by the sapphire dielectric losses, which are very low near the liquid helium temperature. 

Temperature turnover point.

Moreover, low concentration paramagnetic ions inside the sapphire provide an efficient thermal compensation. At 6K competing paramagnetic spin and Debye expansion variations compensated them selves and the resonator temperature coefficient of frequency (TCF) is nulled at first order.

The sapphire resonator is enclosed in a gold plated cylindrical copper cavity, which has been designed to suppress all spurious resonances that can affect the CSO performances. The resonator coupling to the external circuit is adjusted through our own procedure that allows near unity coupling input factor, thus optimizing the power transfer as well as the sensitivity of the Pound Servo.

S21 parameter of the resonator.
S11 parameter of the resonator.

Closed Cycle Cryocooler

To achieve the required oscillator frequency stability, the oscillator's sapphire resonator must be maintained in a cryogenic environment that is sufficiently free of mechanical vibration and where the resonator's temperature is uniform and precisely controlled at a specific value.

Representational view of the internal of the cryocooler.

The cold source is an Oxford Instruments 4K Cryofree cryostat modified to achieve the performance required. Those were a low displacement on the experiment plate (less than two microns in three axes) with a temperature stabilization of 1 mK over 1000 s.

The two stages of the cryostat (77K shield and 4K cold plate) are thermally linked to the cryocooler stages with floppy copper heat links. The support thin-wall tubes are mounted like a hexapod to give rigidity to the system. To limit the temperature fluctuations of the cold stage, a gadolinium gallium garnet (GGG) crystal was mounted between this stage and a copper temperature stabilization block supporting the resonator.

Oscillator loop

Uliss oscillator.

The CSO uses a classical transmission oscillator circuit with the cryogenic sapphire resonator as frequency determining element. The sustaining loop is completed with two additional servo loops stabilizing the phase of the circulating signal and the power injected inside the resonator. The first servo loop is based on the Pound frequency discriminator principle; it ensures that the CSO oscillates at the resonator frequency by compensating any variation of the phase lag along the loop. It uses a phase modulation at a frequency of the order of few tens of kHz to probe the resonance.

The phase modulation is applied to the microwave signal through a first voltage-controlled-phase-shifter. A lock-in amplifier demodulates the signal reflected by the resonator to generate an error signal, which is eventually added to the dc-bias of a second voltage-controlled-phase-shifter.

The power servo loop ensures that the power injected into the resonator stays constant. A tunnel diode placed as near as possible to the resonator input enables to get a voltage proportional to the signal power. This voltage is compared to a high stability voltage reference and the resulting error signal is used to control the bias of a voltage-controlled attenuator.

Low noise frequency synthesis

Our sapphire resonator are designed to operate on the WGH15,0,0 whispering gallery mode at 9.99 GHz. The intentional 13-7 MHz frequency offset from the 10 GHz round frequency was chosen to permit to compensate for the resonator frequency uncertainty by using a low noise Direct Digital Synthesizer (DDS). 

Uliss synthesis chain.

A 2.5 GHz Dielectric Resonator Oscillator (DRO) chosen for its low phase noise is frequency multiplied by 4 and mixed with the CSO's signal. The resultant beatnote is compared to the signal coming from a DDS. The resulting error signal is used to phase lock the DRO to the CSO. Frequency dividers complete the system to generate the 100 MHz and 10 MHz frequencies from the 10 GHz signal.