## 3 Experimental Set Up and Components  
The experimental set up is composed of an MMW source manufactured by Virginia Diodes Inc (VDI TX272), a quasi-optic design based on one off-axis parabolic mirror (OPM), a convex lens, a timer/counter type NI USB-6341 manufactured by national instruments (NI), a glow discharge detector N527 (of International Light Inc.), a Light-to-frequency converter (LTF) component TSL237 (of ams AG), and a near-infrared (NIR) Long-Pass Filter (LPF) with cutoff wavelength of 550 nm with an optical density 6. The experimental set up is shown in Figure 4. Careful inspection of the spectral analysis of the GDD irradiance (see Figure 3), reveals that the influence of MMW /THz radiation is noticeable in the spectral range of 800 nm to 1000 nm. Furthermore, no significant influence was noticeable at lower wavelengths [27]. Thus, a 550 nm long-pass filter was selected to cut out most of the background irradiance of the GDD that doesn't contribute to the detection of MMW/THz radiation.  
The filter is mounted in an engraved metal ring that increases the thickness of the filter. The long-pass filter (FEL0550, of Thorlabs) was placed between the GDD and the LTF as can be seen in Figure 4. The GDD neon lamp used in this set-up is N527 manufactured by International Light Technologies. The neon lamp type N527 is shown here to be very sensitive to MMW detection using the up-conversion method. The LTF converts the emitted light of the GDD to TTL levels of square wave frequency, where the square wave frequency increases proportionately with the GDD light intensity. A simple and inexpensive LTF component, TSL237 (of ams AG) was used to convert the GDD light to frequency. The TSL237 has the following parameters: 2.7-5.5 V operating voltage, 320 nm to 1050 nm spectral response range, and output square wave frequency dynamic range of 1 Hz to 1 MHz. The GDD was placed with its head toward the incident MMW radiation (seeFigure 4). Therefore, its effective detection cross-section is a circle with a diameter of about 1 mm. Propagation calculations based on the Gaussian beam approximation, show that about 0.1% of the transmitted power is received by the GDD effective cross-section [28]. Thus, the maximum MMW radiation power at the cross-section of the GDD is about 3.42 mW. A timer/counter type NI USB-6341 was used to measure the output frequency of the LTF as can be seen in Figure 4. The timer/counter was controlled and operated by a LABVIEW code. The MMW source is located at the focal point of the OPM. The MMW radiation carrier frequency is 100 GHz. This carrier wave was modulated using ON/OFF keying (OOK) at a frequency of 2 Hz (1 s with the radiation in the ON state and 1 s with the radiation in the OFF state). The maximum output power of the MMW source is 600 mW. The output power of the MMW source can be varied by using the user-controlled configuration  
Initially, the voltage across the GDD was measured through electrical detection by varying the user-controlled attenuation (UCA) input of the MMW source from 0 V to 5 V so as to ensure maximum radiation will incident on the GDD cross-section. A horn antenna is  
Figure 4: Experimental set up used for the minimum detectable signal and responsivity measurements.  
Figure 5: Variation in MMW source power according to the UCA input.  
used to couple out the MMW radiation to free space. The electrical detection characteristics of the GDD when exposed to MMW radiation are shown in Figure 6.  
The difference in LTF output with respect to the variation in the GDD bias current \(I_{DC}\) is also measured as shown in Figure 7.  
A collimated beam is obtained after the first OPM and it propagates towards the aspheric lens. The GDD and the LTF combination are located at the focal point of the second OPM. The output frequency of the LTF is measured using a computerized system as described above.