Nick Rothbart

Fast 2-D and 3-D Terahertz Imaging With a Quantum-Cascade Laser and a Scanning Mirror

A terahertz imaging system based on a quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector is demonstrated. Transmission images are obtained by scanning the beam of the QCL across an object. Images with a diameter of approximately 40 mm and a signal-to-noise ratio of up to 28 dB were obtained within 1.1 s. The system was also used to obtain three-dimensional images of objects in an ellipsoidal volume with axes of approximately 40 mm by computed tomography within 87 s.

Real-time terahertz imaging through self-mixing in a quantum-cascade laser

We report on a fast self-mixing approach for real-time, coherent terahertz imaging based on a quantum-cascade laser and a scanning mirror. Due to a fast deflection of the terahertz beam, images with frame rates up to several Hz are obtained, eventually limited by the mechanical inertia of the employed scanning mirror. A phase modulation technique allows for the separation of the amplitude and phase information without the necessity of parameter fitting routines. We further demonstrate the potential for transmission imaging.

High-spectral-resolution terahertz imaging with a quantum-cascade laser

We report on a high-spectral-resolution terahertz imaging system operating with a multi-mode quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector. By tuning the frequency of the QCL, several spectra can be recorded in 1.5 s during the scan through a gas cell filled with methanol (CH3OH). These experiments yield information about the local absorption and the linewidth. Measurements with a faster frame rate of up to 3 Hz allow for the dynamic observation of CH3OH gas leaking from a terahertz-transparent tube into the evacuated cell. In addition to the relative absorption, the local pressure is mapped by exploiting the effect of pressure broadening.

Granular structure determined by terahertz scattering

Light scattering from particles reveals static and dynamical information about the particles and their correlations. Such methods are particularly powerful when the wavelength of the light is chosen similar to the sizes and distances of the particles. To apply scattering to investigate granular matter in particular —or other objects of similar submillimeter size— light of suitable wavelength in the terahertz regime needs to be chosen. By using a quantum cascade laser in a benchtop setup we determine the angle-dependent scattering of spherical particles as well as coffee powder and sugar grains. The scattering from single particles can be interpreted by form factors derived within the Mie theory. In addition, collective correlations can be extracted as static structure factors and compared to recent computer simulations.

Gas spectroscopy with 245 GHz circuits in SiGe BiCMOS and Frac-N PLL for frequency ramps

A compact gas spectroscopy system is presented, which contains a 245 GHz transmitter (TX) and a subharmonic receiver (RX) in SiGe BiCMOS technology, as well as a gas absorption cell. The local oscillators (LOs) of the RX and the TX are controlled by two external Fractional-N phase-locked loops (Frac-N PLLs) allowing fast frequency ramps with superimposed frequency shift keying (FSK) or analog frequency modulation. The sensitivity of this spectroscopic system is evaluated by measuring the high-resolution absorption spectrum of gaseous methanol (CH3OH). Spectra of CH3OH with a high signal-to-noise ratio (SNR) are shown for the range 241–242 GHz.

Gas spectroscopy system at 245 and 500 GHz using transmitters and receivers in SiGe BiCMOS

A compact gas spectroscopy system is demonstrated, which contains a transmitter (TX) and a receiver (RX) in SiGe BiCMOS, as well as a gas absorption cell. The sensitivity of this spectroscopy system is demonstrated by measuring the high-resolution 2f absorption spectrum (second harmonic detection) of gaseous methanol (CH3OH) at 238-252 GHz, and at 495-497 GHz. The 245 GHz TX consists of a 120 GHz local oscillator (LO) and a frequency doubler, and the 245 GHz RX includes a low noise amplifier (LNA), a LO, and an active subharmonic mixer. A 245 GHz TX-array increases significantly the sensitivity of the sensor system. The 500 GHz system includes a TX-array, and a subharmonic RX with a transconductance mixer. The 500 GHz TX contains a frequency quadrupler, and the RX uses a frequency doubler for the LO. The LOs of the RX and the TX are controlled by two external phase-locked loops (PLLs).

Gas Spectroscopy by Voltage-Frequency Tuning of a 245 GHz SiGe Transmitter and Receiver

We report on a terahertz/millimeter-wave gas spectroscopy sensor system based on a 245 GHz transmitter (TX) and receiver (RX) fabricated in SiGe BiCMOS technology. The frequency is tuned by applying external voltages to voltage-controlled oscillators of the TX and the RX. Exemplary 2f spectra of methanol are presented that exhibit a high SNR of up to 560 and a short acquisition time as short as 39 s, respectively.

Gas spectroscopy system with 245 GHz transmitter and receiver in SiGe BiCMOS

The implementation of an integrated mm-wave transmitter (TX) and receiver (RX) in SiGe BiCMOS or CMOS technology offers a path towards a compact and low-cost system for gas spectroscopy. Previously, we have demonstrated TXs and RXs for spectroscopy at 238 -252 GHz and 495 - 497 GHz using external phase-locked loops (PLLs) with signal generators for the reference frequency ramps. Here, we present a more compact system by using two external fractional-N PLLs allowing frequency ramps for the TX and RX, and for TX with superimposed frequency shift keying (FSK) or reference frequency modulation realized by a direct digital synthesizer (DDS) or an arbitrary waveform generator. The 1.9 m folded gas absorption cell, the vacuum pumps, as well as the TX and RX are placed on a portable breadboard with dimensions of 75 cm x 45 cm. The system performance is evaluated by high-resolution absorption spectra of gaseous methanol at 13 Pa for 241 - 242 GHz. The 2f (second harmonic) content of the absorption spectrum of the methanol was obtained by detecting the IF power of RX using a diode power sensor connected to a lock-in amplifier. The reference frequency modulation reveals a higher SNR (signal-noise-ratio) of 98 within 32 s acquisition compared to 66 for FSK. The setup allows for jumping to preselected frequency regions according to the spectral signature thus reducing the acquisition time by up to one order of magnitude.

Gas spectroscopy system with transmitters and receivers in SiGe BiCMOS for 225-273 GHz

This paper updates results of our work on gas spectroscopy based on transmitters (TXs) and receivers (RXs) in IHPs 0.13 μm SiGe BiCMOS technology. The improved performance of our system is shown by the absorption spectra of gaseous methanol in the range 241 - 242 GHz at 1.4 Pa, corresponding to an absorption line width of about 1 MHz. The signal-noise ratio (SNR) for the absorption line of methanol at 241.7 GHz is used as measure. The system includes two fractional-n phase-locked loops (PLLs), which allow frequency ramps for the TX and RX, and a superimposed frequency shift keying modulation (FSK) for the TX. Another option includes reference frequency ramps for the PLLs in integer-n mode, which are realized by a direct digital synthesizer (DDS). An SNR of 1515 is observed for the 241.7 GHz absorption line at 1.4 Pa. We extend our single band TX/RX system with the range 238 - 252 GHz to a multi-band system to cover the range 225 - 273 GHz. It is built by combining corresponding pairs of TXs and RXs of three frequency bands in this range. The multi-band operation allows parallel spectra acquisition for these bands. For the TXs and RXs appropriate frequency ramps are generated by their external fractional-n PLL devices.