## 3 Fast Electrical Modulation of Radiation Temperature of Cryogenic Source
The amplitude of these pulses had to be reduced because a noticeable parasitic interaction was found between the oscilloscope channels, affecting the detector; the electrical signal caused additional heating of the absorber. In order to reduce the electrical pulse interference penetrating into the registration system and bypass the detector, measurements were made in the bias reversal mode of the detector. The response to radiation was obtained by subtracting the signals at positive and negative bias voltages. Adding them together brings the level of interference. The results were averaged during 1024 cycles. The period between cycles was up to 5 s depending on the level of source heat, in order to ensure cooling of the source between heating pulses. We estimated the time constant of preamplifier \(\tau_{\mathrm{amp}}\) and twisted pair \(\tau_{\mathrm{RC}}\). The measured capacitance in the feed line is C = 80-100 pF, and the dynamic resistance of the detector at the working point is in the range from tens to hundreds of k\(\Omega\), so the time constant of such RC circuit is up to 10 us. Some measurements of the time constant of the whole system were carried out at room temperature using an optocoupler pair instead of a detector. A pulse from the heater generator was applied to the LED of the pair. The photodiode of the optocoupler was connected in series with a resistor R (from 1 to 100 k\(\Omega\)) to the input of the transmission line. A simulated output signal with \(R\) = 14 k\(\Omega\) is shown  
Figure 12: Calculation of changing temperature (T, red curves) and radiation power (P\({}_{\mathrm{rad}}\), black curves) at applied pulse of 10 μs (1) and 2 μs (2) and 20 and 40 V starting at t = 0. Pulse power applied to nichrome film is 0.19 and 0.75 W/cm\({}^{2}\). Graph from [12] with permission, Copyright 2018 Pleiades Publishing, Ltd.  
in Figure 13. It can be described by an exponent, and for the case shown in the figure, the time constant is 1.8 \(\upmu\)s.  
The performance of the BB source is tested by the detector response when millisecond heating pulses of moderate voltage are applied (see Figure 14). The temperature of the source is calculated by its thermal capacity and applied pulse energy. The power of radiation is calculated from the equation for a single mode SINIS detector. The detector response is proportional to the calculated radiation power, which confirms the correctness of the described procedure for estimation of the radiation power. The radiation power after applying pulses with front duration of 1 ms was estimated in the same way. The measurement results of a SINIS detector response time presented in Figure 15. Unlike the previous cases, for this measurement, the voltage pulses had higher amplitude but shorter duration. As shown in Figure 15, the pulse duration and amplitude were chosen so that the total heat power of the pulses would be close in both cases. The main feature of the presented data is the noticeable difference between the detector response and the output signal rise time of 1.8 \(\upmu\)s. Agreement with the experiment is achieved if we take into account the proper response time of the detector \(\tau_{\text{det}}\). For 10 \(\upmu\)s heating pulses,  
Figure 14: Solid line indicates detector response to source radiation when 7 V pulse was applied for 5 ms. Dashed line indicates calculated radiation power, Reprinted with permission from Ref. [12], Copyright 2018 Pleiades Publishing, Ltd.  
Figure 13: Output signal (solid line) dependence on time for voltage applied to optocoupler (dot) through 14 k\(\Omega\) resistor. Dashed line indicates exponent with \(\tau\) = 1.8 \(\upmu\)s, see also [12] (Reprinted with permission, Copyright 2018 Pleiades Publishing, Ltd.).