2.75 THz tuning with a triple-DFB laser system at 1550 nm and InGaAs photomixers

To date, exploiting the full bandwidth of state-of-the-art InGaAs photomixers for generation and detection of continuous-wave (CW) THz radiation (typ. ~50 GHz to ~3 THz) required complex and costly external-cavity diode lasers with motorized resonator control. Distributed feedback (DFB) lasers, by contrast, are compact and inexpensive, but the tuning range per diode is limited to ~600 GHz at 1.5 μm. In this paper, we show that a combination of three DFB diodes covers the complete frequency range from 0 – 2750 GHz without any gaps. In combination with InGaAs-based photomixers for terahertz generation and detection, the system achieves a dynamic range of > 100 dB at 56 GHz, 64 dB at 1000 GHz, and 26 dB at 2500 GHz. A field-programmable gate array (FPGA)-based lock-in amplifier permits a flexible adjustment of the integration time from 0.5 ms to 600 ms. Employing an optimized “fast scan” mode, a spectrum of ~1200 GHz – the bandwidth of each subset of two lasers – and 40 MHz steps is acquired in less than one minute, still maintaining a reasonable dynamic range. To the best of our knowledge, the bandwidth of 2.75 THz presents a new record for DFB-based CW-terahertz systems

3He MRI-based assessment of posture-dependent regional ventilation gradients in rats

A recently developed method for quantitative assessment of regional lung ventilation was employed for the study of posture-dependent ventilation differences in rats. The measurement employed hyperpolarized (3)He MRI to detect the build-up of the signal intensity after increasing numbers of (3)He breaths, which allowed for computation of a regional ventilation parameter. A group of six anesthetized rats was studied in both supine and prone postures. Three-dimensional maps of the ventilation parameter were obtained with high spatial resolution (voxel volume approximately 2 mm(3)). Vertical (dorsal-ventral) gradients of the ventilation index, defined as the regional ventilation normalized by the average ventilation within the whole lung, were investigated. Variations in the regional distribution of the ventilation parameter, as well as of the ventilation index, could be detected, depending on the posture of the rats. In supine posture, ventilation was elevated in the dependent parts of the lungs, with a linear gradient of the ventilation index of -0.11 +/- 0.03 cm(-1). In prone posture, the distribution of ventilation was more uniform, with a significantly (P < 0.001) smaller gradient of the ventilation index of -0.01 +/- 0.02 cm(-1). It is concluded that the (3)He MRI-based method can detect and quantify regional ventilation gradients in animals as small as the rat and that these gradients depend on prone or supine posture of the animal