Receiver
| 1. Active Element Phonon-cooled HEB mixers are fabricated from thin NbN films on
either silicon or MgO substrates. We have fabricated devices using
UV lithography and e-beam writing. Devices fabricated with UV lithography
at UMass/Amherst are typically
1 mm long
and 4
mm wide
whereas the smaller devices,
written with e-beam at Chalmers University, are 0.4 mm long and
4 mm wide. 2. Quasi-Optical Coupling We have implemented a quasi-optical coupling scheme (Figure R1a). This method uses an elliptical silicon lens and a monolitic antenna to couple the terahertz frequencies to the device fabricated at the terminals of the antenna. A typical antenna design for linearly polarized signal is the twin-slot antenna configuration as can be seen in Figure R1b. |
|
(a) |
(b) |
Figure 1. (a) Quasi-optical coupling scheme. (b) Twin-slot antenna.
3. Mixer Block
TREND's mixer block can be seen in Figure R2. The biasing circuitry is incorporated into the mixer block.
Figure R2. Mixer block and DC/IF circuit board.
4. Cryogenics
NbN HEB devices are superconducors with typical
critical temperatures of below 10 K. The mixer block is mounted
in a cryostat operating at arround 4 K.
5. Local Oscillator
The Local Oscillator (LO) used with the TREND instrument is made of an FIR laser, which is pumped by CO2 CW laser. Figure R3 shows the TREND LO laser.
Figure R3. TREND LO laser.
6. IF Amplifier
The IF signal coming out of the HEB mixer is
amplified by a cryogenically-cooled low noise amplifier. Indium
phosphide (InP) High Electron Mobility Transistors (HEMT) have become
an excellent choice for cryogenic LNAs because of their low
noise, high frequency response, and superior cryogenic behavior.
7. Optical Path
An elaborate optical setup was implemented at the telescope in order to focus both the LO beam and the sky beam onto the HEB mixer. Different optical configurations for two different polarizations are illustrated in Figure R4.
(a)
(b)
Figure R4. Illustrations
of the optical paths for:
(a) 1.27 THz signal. (b) 1.46 THz
signal.
