Comparison between Up-Conversion Detection in Glow-Discharge Detectors and the Schottky Diode for MMW/THz High-Power Single Pulse

Abstract

Generally, glow-discharge detectors (GDD), acting on miniature neon indicator lamps, and Schottky diode detectors serve as efficient, fast, and room-temperature millimeter wave (MMW)/THz detectors. Previous studies on GDD implemented a repetition of terahertz sources, and low-power radiation, and showed good results in terms of detection, responsivity, and noise-equivalent power. This paper presents a comparison between a detector based on a GDD lamp and a Schottky diode detector for the detection of a high-power single pulse. With this comparison, we touch upon two GDD detection methods, the visual light emitting from the GDD and the electrical current of the GDD detector. Results showed better response time and better sensitivity for the GDD detection method compared to with the Schottky diode.

1. Introduction

Terahertz (THz) radiation and millimeter waves (MMW) have attained widespread application in package inspection, quality control, nondestructive testing [1], medicine [2], communications [3], and spectroscopic characterization [4]. The preference to use THz and MMW is that there is no ionization hazard [5], penetration through dielectric materials is reasonably good, and the atmospheric scattering of MMW and THz radiation is relatively low [6]. This has led to the development of low-cost, fast, highly sensitive, compact, and room-temperature detectors. The most popular THz detectors are Schottky diodes, Golary cells, pyroelectric detectors, bolometers, and microbolometers [7,8]. Golary cells and bolometer detectors have a long response time, even though they have large spectral and less noise-equivalent power [9]. Schottky diodes are fast and have lower noise-equivalent power (NEP) [10]. However, they are not intended for high-power THz pulses, are much less sensitive at higher frequencies of the THz band, and require much more local oscillator (LO) power as compared with currently convenient tuneable LO sources [11]. Furthermore, these three detectors are expensive. This paper demonstrates the use and responsivity of a glow-discharge detector (GDD) in the high-power millimeter-wave and terahertz frequency radiation region. Experiment results detected a single pulse of 0.1 THz and 5 KW using GDD. The GDD lamp has productive potential for THz detection due to its fast response time, electron dynamic broad range, efficiency at room temperature, and low price [12]. The GDD’s detection mechanism is based on a slight change of current between the two electrodes of the lamp, defined by [13].

$$\Delta I=\frac{G{q}^{2}vn}{v_{i}m}\left(\frac{\tau }{{\tau }_i}\right){\eta }_0{P}_D\left(\frac{f}{f^2+{\omega }^2}\right)(1-e^{-\frac{t}{\tau }})$$

(1)

where $G=\frac{exp\left(2f_i{t}_d\right)}{2f_i{t}_d}$ is the internal signal amplification, q is electron charge, v is average electron velocity, n is electron density, $v_i$ is gas ionization potential, m is electron mass, $\tau$ is time response to create current changes, ${\tau }_i$ is time between ionization collisions of electrons with gas atoms, ${\eta }_0$ is free space impedance, $P_D$ is incident THz power density, f is electron-neutral atom elastic collision frequency, ω is electromagnetic radiation frequency, $t_d$ is average electron drift time to the anode, and $f_i$ is ionization collision frequency. Considering that the response time of the plasma inside the GDD is in the order of picoseconds, the detection mechanism is mainly dependent on visual detector response time rather than GDD circuit. The responsivity measures the electrical output per input THz power and is defined by [14]:

$$Rep=\frac{A\left(V\right)}{P\left(W\right)},$$

(2)

where A is the THz signal amplitude, and P is the THz power. The responsivity of the GDD increases with the increase in bias current. NEP is a measure of the sensitivity of an optical detector system; a small NEP means a high signal-to-noise ratio (SNR). The NEP for the GDD is as low as ${10}^{-8}\mathrm{W}/\sqrt{\mathrm{H}}$ [15], and is defined by [14]:

$$NEP=\frac{\sqrt{N}}{R}$$

(3)

where N is noise power spectral density, and R is responsivity.

2. Materials and Methods

Figure 1 shows a schematic diagram of the THz detection system. The MMW/THz source is based on the MMW electron accelerator of Ariel University [16], emitting a 0.1 THz single pulse, 5 KW power, and 10 µs width. The THz pulse was split by a power splitter. Transmitting a portion of the signal approximately 1000 times weaker than the input signal to the Schottky diode detector, i.e., for a 5 kW signal, roughly 5 W should reach the Schottky diode detector and the rest of the pulse power, transmitted by 2 mm tube to the free space. Two gold-coated parabolic mirrors were used to focus MMW/THz radiation on the GDD cross-section between electrodes. For an effective focus, it was necessary to simulate the MMW/THz radiation in the free space with portable source stuck to the tube that produced THz pulses; then, the GDD was placed in the focal point. A commercial green neon indicator lamp (from International Light Technologies, model N521) was used in these experiments as a GDD. Changes in GDD light were caused by the MMW/THz pulse incident on the GDD lamp, monitored by an optical detector and an electronic detector.

For an efficient comparison between the upconversion method of the GDD detectors and the Schottky diode detector for MMW/THz pulse detection, the MMW/THz pulse was detected by three methods: optical GDD detector, electronic GDD detector, and Schottky diode detector. The incident THz pulse increased gas ionization, which caused the current through the GDD to change. The optical detector for GDD detected the change in GDD illumination by a large-area balance detector (Thorlabs, model PDB210) that was available. The electronic detector method for the GDD consisted of a resistor, capacitor, amplifier (that could be employed to remove the noise and improve the output signal), and a 100 VDC power supply to break down plasma intergas. The used Schottky diode detector was a Millitech DXP-10 RPFW0 to utilize Schottky barrier zero-bias diodes. The maximal safe input to the detector is 16 dBm or 40 mW, explaining why a Schottky diode detector receives approximately 30 dB less than the signal power. The used oscilloscope was DSOX3104A from Keysight, with a bandwidth of 1 GHz and a max sample rate of 5 gigasamples (GSa)/s.

In order to compare the frequency response of the Schottky diode and GDD detectors, the frequency chirp in the MMW/THz pulse was analyzed (Figure 2). So, a 0.11 THz pulse similar to the MMW/THz pulse of the accelerator was generated, which was performedd by a local oscillator. Then, the MMW/THz pulse was mixed with a local oscillator signal. The difference between MMW/THz pulse and local oscillator signal was monitored by the oscilloscope.

An H-plane T junction was used in the setup in order to split the sampled signal for simultaneous power and frequency measurements (see Figure 2). A continuous wave signal from the sweep oscillator was arbitrarily chosen to be 16.667 GHz. This was then transformed by the multiplier into a local oscillator signal of 100 GHz. To measure and record the detected signal, the output signal of the mixer and the three detectors, they were directly connected to an oscilloscope.

3. Results and Discussion

Figure 3 shows the shape of the MMW pulse around 0.11 GHz, which was detected by three alternative methods. The blue graph represents the Schottky diode detector, which had a long decrease time due to the characteristic of the discharging capacitor. The red graph represents the electron method for the GDD detector. The GDD was connected to an electronic circuit that causes a limited response time of about 1 µs [17]. However, the response time of the plasma inside the GDD was in the order of picoseconds, which explains why the yellow graph, which represents the optical method for the GDD, had the shortest response time.

The optical GDD detection speed is derived largely from the speed of the photodetector. The electric GDD detection is limited in the response time, this driven by the parasitic capacitance of the GDD. Figure 3 shows that the Schottky diode detector has a longer decay time than the GDD. This means that the discharge of the parasitic capacitance of the GDD and the photodetector is shorter than the capacitance of a Schottky diode. In order to improve the recording of the falling signal of the Schottky diode detector, a 50\varOmega load was added to an oscilloscope (oscilloscope input channel had a 1M\varOmega impedance). Figure 4 shows the mixed-signal between MMW/THz pulse and local oscillator (frequency chirp of the MMW/THz pulse). This pulse contained more than one frequency, i.e., there was a change in frequency in the same pulse.

Figure 5 shows the power detection of the Schottky diode and the GDD detectors for the pulse, of which the frequency appears in Figure 4, i.e., the power detection of a pulse of which the variable frequency over time was detected by three alternative methods. This result shows that the addition of impedance to the oscilloscope significantly improved the decrease time of the Schottky diode. The frequency change response in the MMW/THz pulse for the Schottky diode and the GDD detectors shows that, due to the high bandwidth of the GDD, it was much less sensitive to frequency change in the MMW/THz pulse.

Figure 5 shows a power detection for a THz pulse that has a frequency chirp. Due to the limited bandwidth of a Schottky detector, it is sensitive to frequency chirp, i.e., although there is an intensity difference between the low and the high frequencies, the Schottky diode detects the power of the low frequency in the pulse as much as the high frequency. Figure 5 shows that the GDD detectors are less sensitive to the frequency chirp, due to the large bandwidth of the GDD detectors.

4. Conclusions

This paper has presented experiment results of high-power MMW/THz pulse detection using electronic and optical GDD detectors. These results proved that GDD green lamp type N521 can be used to detect high-power MMW/THz pulses. An MMW accelerator of Ariel University was used as a source of MMW/THz pulses. This accelerator has unique features such as high radiation power of 0.1–5 KW, radiation frequency of 95–110 GHz, and pulse duration of 1–50 μs. The NEP of the Schottky diode is lower as ${10}^{-10}\mathrm{W}/\sqrt{\mathrm{H}}$ [10], but they are much less sensitive from GDD, and it’s not appropriate to high power radiation. Results demonstrated the priority use of GDD detectors instead of Schottky diode due to the shorter response times, wide bandwidth, low cost of the GDD detectors. Results showed that the Schottky diode had a longer response time than the GDD detector did. Due to the characteristic of the discharging capacitor of the Schottky diode, it has a longer decrease time than that of the GDD. We also presented a comparison between the frequency change response of the Schottky diode and the GDD detectors. Results showed that, due to the high bandwidth of the GDD, the Schottky diode was more sensitive to the frequency change in the MMW/THz pulse. Further, the responsivity of the optical detection of the GDD was better than that in electronic detection. Optical detection is much quieter than electronic detection is. In electronic detection, the incident pulse is an electromagnetic wave, and an electrical current is an output, i.e., parasitic capacitance and inductance of the electronic circuit of the GDD limit the response time. The upconversion method monitors the optical change of the GDD; therefore there is no parasitic impedance at the output, and response time is due to the response time of the photodetector. The sensitivity of electronic GDD detection is limited by the plasma noise of the GDD. However, in the upconversion method, sensitivity is limited by photodetector noise, which is less than plasma noise. The comparison of the response time between the upconversion and the electronic method was limited because of the photodetector bandwidth. Using a faster and more sensitive photodetector can improve the upconversion method.