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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Noise properties of high-T c superconducting transition edge bolometers with electrothermal feedback

I. A. Khrebtov, Konstantin V. Ivanov, V. G. Malyarov

Published Jun 7, 2007• 1 citation
Publisher

Abstract

Numerical and experimental modeling of the characteristics (sensitivity, constant time, noise properties) of high-Tc transition edge superconducting bolometers, operating in the various modes with an electrothermal feedback, are carried out: the mode with constant bias current, i.e. passive positive electrothermal feedback, the mode with constant bias voltage, i.e. passive negative electrothermal feedback and the mode with active electronic negative electrothermal feedback . It is shown, that in the modes with negative electrothermal feedback it is possible essentially to reduce constant time of bolometers till 5-15 of times in at some prize of the noise equivalent power on high frequencies. The estimation of influence of various noise components on a performance of the bolometers, operating with positive or negative electrothermal feedbacks, is carried out at the variation of bolometer parameters.

DOI
10.1117/12.725356

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Numerical and experimental modeling of the characteristics (sensitivity, constant time, noise properties) of high-Tc transition edge superconducting bolometers, operating in the various modes with an electrothermal feedback, are carried out: the mode with constant bias current, i.e. passive positive electrothermal feedback, the mode with constant bias voltage, i.e. passive negative electrothermal feedback and the mode with active electronic negative electrothermal feedback . It is shown, that in the modes with negative electrothermal feedback it is possible essentially to reduce constant time of bolometers till 5-15 of times in at some prize of the noise equivalent power on high frequencies. The estimation of influence of various noise components on a performance of the bolometers, operating with positive or negative electrothermal feedbacks, is carried out at the variation of bolometer parameters.
We present a theoretical analysis and experimental evaluation of a transition-edge superconducting bolometer for detecting infrared and millimeter waves. The superconducting film is voltage biased and the current is read by a superconducting quantum interference device ammeter. Strong electrothermal feedback maintains the sensor temperature within the transition, gives a current responsivity that is simply the inverse of the bias voltage, and reduces the response time by several orders of magnitude below the intrinsic time constant C/G. We evaluated a voltage-biased bolometer that operates on the Tc∼95 mK transition of a tungsten film with a thermal conductance of G∼1.2×10−9 W/K. As expected, the electrical noise equivalent power of 3.3×10−17/W√Hz is close to the thermal fluctuation noise limit and is lower than that of other technologies for these values of G and temperature. The measured time constant of 10 μs is ∼100 times faster than the intrinsic time constant.
* Abstract - In the current generation of high- T, bolometers the thermal conductance is often chosen for a short time-constant rather than for optima1 sensitivity. We describe a novel bolometer bias and readout scheme that promises to relax this constraint. Voltage bias of the superconductor results in strong negative electrothermal feedback that greatly reduces the time-constant of the bolometer. We estimate that a decrease of more than one order of magnitude in time-constant should be possible with existing high-T, thermometers. We give theoretical estimates of the performance gain with voltage bias for several bolometers that have been reported in the literature. We find cases where the sensitivity can be greatly improved (by changing the thermal conductance) while holding the time constant fixed and others where the bolometer can be made much faster while maintaining the sensitivity. Superconducting transition-edge bolometers have traditionally been operated with a constant-current bias, which results in positive electrothermal feedback (ETF).(S) The bias current must therefore be limited to avoid thermal instability. It has recently been pointed out(6) that a constant-voltage bias mode with strong negative ETF(6,7) gives several important operational advantages. The most important of these is a large reduction of the bolometer time-constant due to strong ETF. We estimate a decrease of more than one order of magnitude in time constant should be possible with existing high-Tc thermometers. In the current generation of high-Tc bolometers the thermal conductance is often chosen for a short time-constant rather than for optimal sensitivity. The time-constant decrease due to negative ETF can be used to relax this constraint. In this Daner. we give theoretical estimates of the
We present an experimental study of a composite voltage-biased superconducting bolometer (VSB). The tested VSB consists of a Ti-film superconducting thermometer (T(c) ~375 mK) on a Si substrate suspended by NbTi superconducting leads. A resistor attached to the substrate provides calibrated heat input into the bolometer. The current through the bolometer is measured with a superconducting quantum interference device ammeter. Strong negative electrothermal feedback fixes the bolometer temperature at T(c) and reduces the measured response time from 2.6 s to 13 ms. As predicted, the measured current responsivity of the bolometer is equal to the inverse of the bias voltage. A noise equivalent power of 5 x 10(-17) W/ radicalHz was measured for a thermal conductance G ~ 4.7 x 10(-10) W/K, which is consistent with the expected thermal noise. Excess noise was observed for bias conditions for which the electrothermal feedback strength was close to maximum.
Transition-edge sensor (TES) bolometers operate under strong electrothermal feedback, wherein power deposited on the bolometer is compensated by a corresponding change in electrical power dissipation. We present a comprehensive analysis of Johnson noise suppression that extends previous theoretical frameworks to the general case of AC-biased linear circuits with arbitrary parasitic impedances. Using a Thévenin-equivalent circuit formulation -- consisting of an ideal voltage source and complex series impedance external to the bolometer thermal island -- we derive analytical expressions for the noise-equivalent current in the presence of both bolometer and parasitic Johnson noise sources. Our analysis demonstrates that while the electrothermal feedback loop effectively suppresses Johnson noise originating from the bolometer resistance, it provides no suppression of Johnson noise from external series resistance. In the limit of high loop gain, the bolometer Johnson noise contribution vanishes while the parasitic contribution remains unsuppressed.
We are developing superconducting transition-edge bolometers for far-infrared and millimeter wavelengths. The bolometers described here are suspended by thin legs of silicon nitride for thermal isolation. At frequencies between 200 mHz and 10-50 Hz these devices show white noise at their thermal fluctuation limit (NEP approximately 10(-17) W/ radicalHz). At higher frequencies a broad peak appears in the noise spectrum, which we attribute to a combination of thermal fluctuations in complex thermal circuits and electrothermal feedback. Detailed noise calculations fit the noise measured in three different devices that were specifically designed to test the model. We discuss how changes in bolometer materials can shift the noise peak above the frequency range of interest for most applications.
We have developed a simple room-temperature readout for superconducting bolometers and calorimeters. Due to electro-thermal feedback, the bolometer can be noise-matched with the readout amplifier. The constructed amplifier consists of low-noise field-effect transistors, and voltage bias is actively provided by electrical feedback. According to our measurements on superconducting vacuum-bridge bolometers, the total noise is limited by the bolometer phonon and Johnson noise characterized by a critical temperature of 8.5 K. We show that, with proper sensor design, it is possible to reach the photon noise limit in video-rate submillimetre imaging applications.