Flux Flow Oscillator Linewidth Research Articles

Flux-flow Josephson oscillator as the broadband tunable terahertz source to open space

The flux-flow oscillator (FFO) based on a long Josephson junction has been implemented as a broadband tunable terahertz (THz) source to open space. For this purpose, the transmitting slot antenna has been coupled to the oscillator. Additionally, an elliptical lens with a diameter of 10 mm has been matched to the antenna, forming a narrow output beam of the THz emission. Two designs for the antenna, integrated with the oscillator and developed for operation at different frequency ranges of 0.32–0.55 THz and 0.4–0.7 THz, have been investigated. The FFO has been phase locked to an external reference oscillator by utilizing a harmonic mixer. Its linewidth in the phase-locking regime is determined by the phase noise of the reference oscillator and the number of harmonics used and has been measured to be less than 0.1 MHz. A free-running FFO linewidth from about 2 MHz to several MHz, depending on the operating point, has been obtained. Output emission to open space has been measured by a superconducting integrated spectrometer located in a separate cryostat. The FFO operation as an external source with the achieved emission power and spectral characteristics has demonstrated its applicability for different tasks and purposes where tunable THz sources are required.

Linewidth and DC properties of the flux-flow oscillator with mixed inline-overlap bias

A Flux-Flow Oscillator (FFO) is a long Josephson junction in which a unidirectional flow of magnetic flux quanta is maintained by an applied DC magnetic field and a bias current. In the overlap geometry the FFO voltage and thus its oscillation frequency is controlled independently by the (overlap) bias current and the so-called "control line" (inline) current, which creates the magnetic field at the ends of the FFO. We have studied both DC properties and the linewidth of the emitted radiation for an FFO, which can be biased in different ways; as a four current terminal object. The bias current of the mixed inline-overlap type is applied through the terminals commonly used for the "control line" current. In this configuration the bias current also contributes to the magnetic field at one of the two FFO ends. This changes the steepness of the junction's I-V characteristics and therefore the bias current dynamic resistance. The experimental FFO linewidth was found to be independent of the bias configuration and determined by the dynamic resistance of the junction with pure overlap bias current. A new method has been developed to evaluate the difference in the DC magnetic field at the ends of the junction. It may be attributed to control line redistribution caused by the impedance matching RF circuit connected to the FFO at the end where the fluxon chain annihilates and radiation is emitted. We found that the opposite end, where the fluxons enter the junction, appears to be about three times more sensitive to variations of the magnetic field than the radiating end.

Superconducting Integrated Receiver Based on Nb-AlN-NbN-Nb Circuits

The superconducting integrated receiver (SIR) comprising in one chip a superconductor-insulator-superconductor (SIS) mixer and a phase-locked superconducting flux flow oscillator (FFO) is under development for the international project TELIS. To overcome temperature constraints and extend operation frequency of the SIR we have developed and studied Nb-AlN-NbN-Nb circuits with a gap voltage Vg up to 3.7 mV and extremely low leakage currents . Based on these junctions integrated microcircuits comprising FFO and harmonic mixer have been designed, fabricated and tested; the radiation from such circuits has been measured at frequencies up to 700 GHz. Employment of NbN electrode does not result in the appearance of additional noise. For example, FFO linewidth as low as 1 MHz was measured at 600 GHz, that allows us to phase lock up to 92% of the emitted by FFO power and realize very low phase noise about 90 dBc. Preliminary results demonstrated uncorrected DSB noise temperature of the Nb-AlN-NbN SIR below 250 K at frequencies around 600 GHz.

Optimization of the Phase-Locked Flux-Flow Oscillator for the submm Integrated Receiver

The Superconducting Integrated Receiver (SIR) comprises in one chip a planar antenna integrated with an SIS mixer, a superconducting Flux Flow Oscillator (FFO) acting as Local Oscillator (LO) and a second SIS harmonic mixer (HM) for FFO phase locking. Free-running FFO linewidth well below 10 MHz is required to ensure phase-locked operation of an SIR. Comprehensive experimental study of the Nb-AlOx-Nb FFO linewidth and other main parameters has been carried out in order to achieve this goal. Essential dependence of the FFO linewidth on its width and idle region dimension has been found. It makes possible an optimization of the FFO design and selection of the best FFO parameters for practical operation of the SIR.

A superconducting phase-locked local oscillator for a submillimetre integratedreceiver

Comprehensive measurements of the flux flow oscillator (FFO) radiation linewidthare performed using an integrated harmonic SIS mixer; the FFO linewidth andspectral line profile are compared to a theory. An essential dependence of the FFOlinewidth on frequency is found; a possible explanation is proposed. The results of thenumerical solution of the perturbed sine–Gordon equation qualitatively confirm thisassumption. To optimize the FFO design, the influence of the FFO parameters on theradiation linewidth is studied. A novel FFO design at a moderate current density hasresulted in a free-running FFO linewidth of about 10 MHz in the flux flow regimeup to 712 GHz, limited only by the gap frequency of Nb. This relatively narrowfree-running linewidth (along with implementation of a wide-band phase locking loopsystem) allows continuous phase locking of the FFO in the wide frequency range of500–710 GHz. These results are the basis for the development of a 550–650 GHz integratedreceiver for the terahertz limb sounder (TELIS) intended for atmosphere study andscheduled to fly on a balloon in 2005. We report here also on the design of the secondgeneration of the phase-locked superconducting integrated receiver chip for TELIS.

Phase-locked Josephson flux flow local oscillator for sub-mm integrated receivers

The Josephson flux flow oscillator (FFO) has proven to be one of the best on-chip local oscillators for heterodyne detection in integrated sub-mm receivers based on SIS mixers. Nb–AlOx–Nb FFOs have been successfully tested from about 120 to 700 GHz (gap frequency of Nb) providing enough power to pump an SIS mixer (about 1 μW at 450 GHz). Both the frequency and the power of the FFO can be dc-tuned. Extensive measurements of the dependence of the free-running FFO linewidth on the differential resistances associated with both the bias current and the control-line current (applied magnetic field) have been performed. The FFO line is Lorentzian both in the resonant regime, on Fiske steps (FSs), and on the flux flow step (FFS). This indicates that internal wide-band noise is dominant. A phenomenological noise model can account for the FFO linewidth dependence on experimental parameters. The narrow free-running FFO linewidth achieved, in combination with the construction of a wide-band phase-locked loop (PLL) system, enable us to phase lock the FFO in the frequency range 490–712 GHz where continuous frequency tuning is possible.

Line width of Josephson flux flow oscillators

A combination of wide-band electronic tunability and moderate free-running line width makes the Josephson flux flow oscillator (FFO) a perfect on-chip local oscillator for integrated submillimeter-wave SIS receivers. The possibility of FFO phase locking at all frequencies of interest has to be proven before one initiates real FFO applications. To achieve this goal a comprehensive set of line width measurements of the FFO operating in different regimes has been performed. FFOs with tapered shape have been successfully implemented in order to avoid the superfine resonant structure with voltage spacing of about 20 nV and extremely low differential resistance, recently observed in the IVC of the standard rectangular geometry. The obtained results have been compared with existing theories and FFO models in order to understand and possibly eliminate excess noise in the FFO. The intrinsic line width increases considerably at voltages above the boundary voltage because of the abrupt increase of the internal damping due to Josephson self-coupling. The influence of FFO parameters, in particular the differential resistances associated both with the bias current and with the applied magnetic field on the radiation line width, has been studied. Possible means of decreasing the free-running FFO line width will be discussed.

Towards a phase-locked superconducting integrated receiver: prospects and limitations

Presently a Josephson flux flow oscillator (FFO) appears to be the most developed superconducting on-chip local oscillator for integrated submillimeter-wave SIS receivers. The feasibility of phase locking the FFO to an external reference oscillator at all frequencies of interest has to be proven for practical FFO implementation in radio astronomy and other spectral applications. A linewidth of a phase-locked FFO as low as 1 Hz has been measured relative to an external reference oscillator in the frequency range 270–440 GHz on steep Fiske steps in the low damping regime. The increase of the intrinsic linewidth at higher voltages due to an abrupt increase of the internal damping considerably complicates phase locking of the FFO. Comprehensive measurements of the FFO radiation linewidth have been performed using an integrated harmonic SIS mixer. Results on FFO linewidth and spectral line profile have been compared to theory in order to optimize the FFO design. The influence of FFO parameters on radiation linewidth, particularly the effect of the differential resistances associated both with the bias current and the applied magnetic field, has been studied. Two integrated receiver concepts with phase-lock loop have been developed and experimentally tested.

Superfine resonant structure on IV-curves of long Josephson junction and its influence on flux flow oscillator linewidth

The Josephson Flux Flow Oscillator (FFO) has proven to be a perfect on-chip local oscillator for integrated submm receivers; a noise temperature (DSB) below 100 K has been achieved at 500 GHz. Recently a FFO linewidth as low as 1 Hz has been measured in the frequency range 270 - 440 GHz. A new technique for both linewidth measurements and phase locking of the FFO is developed; this method employs an offchip harmonic multiplier. By measuring the frequency of the FFO radiation emission, its IV-curve (IVC) can be reconstructed with an accuracy better than 1 nV. A superfine resonant structure with a voltage spacing of about 20 nV and extremely low differential resistance has been observed in the FFO IVCs. This resonant structure modifies the performance of the FFO compared to the one expected from the "averaged IVC". The influence of this resonant structure on phase locking is discussed. Also results of FFO phase noise measurements are presented.

Phase locked 270–440 GHz local oscillator based on flux flow in long Josephson tunnel junctions

The combination of narrow linewidth and wide band tunability makes the Josephson flux flow oscillator (FFO) a perfect on-chip local oscillator for integrated sub-mm wave receivers for, e.g., spectral radio astronomy. The feasibility of phase locking the FFO to an external reference oscillator is demonstrated experimentally. A FFO linewidth as low as 1 Hz (determined by the resolution bandwidth of the spectrum analyzer) has been measured in the frequency range 270–440 GHz relative to a reference oscillator. This linewidth is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction. The results of residual FFO phase noise measurements are also presented. Finally, we propose a single-chip fully superconductive receiver with two superconductor–insulator–superconductor mixers and an integrated phase-locked loop.

Design and fabrication of Cherenkov flux-flow oscillator

The Josephson Flux-Flow Oscillator (FFO) has been used as an on chip local oscillator at frequencies up to 650 GHz. The FFO linewidth of about 1 MHz was measured in the resonant regime V<915 /spl mu/V for niobium-aluminum oxide-niobium tunnel junctions, while considerably larger values were reported at higher voltages. To overcome this fundamental linewidth broadening we propose a novel on chip Cherenkov radiation flux-flow oscillator (CRFFO). It consists of a long Josephson junction and a superconducting slow wave transmission line that modifies essentially the junction dispersion relation. Two SIS detectors are connected both to the long Josephson junction and the transmission line to evaluate available microwave power. The output power coming both from the long junction and the transmission line is estimated at different bias conditions.