A compact, narrow-linewidth, tunable source of THz radiation has been developed for spectroscopy and other high-resolution applications. Distributed-Bragg-reflector ͑DBR͒ diode lasers at 850 nm are used to pump a low-temperature-grown GaAs photomixer. Resonant optical feedback is employed to stabilize the center frequencies and narrow the linewidths of the DBR lasers. The heterodyne linewidth full-width at half-maximum of two optically locked DBR lasers is 50 kHz on the 20 ms time scale and 2 MHz over 10 s; free-running DBR lasers have linewidths of 40 and 90 MHz on such time scales. This instrument has been used to obtain rotational spectra of acetonitrile (CH 3 CN) at 313 GHz. Detection limits of 1ϫ10 Ϫ4 Hz 1/2 ͑noise/total power͒ have been achieved, with the noise floor dominated by the detector's noise equivalent power. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00538-X͔ Recent advances in the development of low-temperaturegrown ͑LTG͒ GaAs as a THz optical heterodyne photomixer, when combined with near-infrared semiconductor lasers, promise a coherent radiation source that is continuously tunable, compact, robust, and inexpensive.1-4 Such a frequency synthesis technique would benefit many THz applications including frequency metrology of near-infrared spectral lines, air/space-borne measurements for atmospheric and astrophysical studies, and high resolution molecular spectroscopy. A handful of groups 1-4 have developed LTG GaAs based THz sources. However, these sources suffer the disadvantages of having a cumbersome size, a limited tuning range, frequency instabilities, and/or expensive pump lasers. Through the use of resonant optical feedback [5][6][7] to lock the frequency of two distributed-Bragg-reflector ͑DBR͒ diode pump lasers, we have achieved a tunable source that is inexpensive, compact, narrow linewidth, and frequency stable. Figure 1 illustrates the experimental setup. The LTG GaAs photomixer is pumped by two optically locked DBR diode lasers. We use the same geometry developed by Hollberg and co-workers.5 A small fraction of each DBR laser's output is sent to a folding mirror ͑ mirror͒ and then to a confocal Fabry-Perot cavity. The DBR lasers ''see'' optical feedback only when they are on resonance with the FabryPerot cavities. Thus, narrowing of the linewidth via optical feedback is achieved in a controlled manner. Each cavity consists of two spherical mirrors glued onto quartz and piezoelectric translator ͑PZT͒ tubes. The PZT section allows one to tune the cavity-free spectral range ͓͑FSR͒, a͒ Electronic mail: pin@gps.caltech. edu FIG. 1. The experimental setup. Each of the two DBR lasers is optically locked to an independent confocal cavity. PZT translators tune the cavity lengths and the feedback path lengths ͑-mirror positions͒. The primary beams from the DBR lasers are combined by a beamsplitter. The combined beam goes through either A for heterodyne linewidth measurements, or B for submillimeter spectroscopy.

The Broadband Radiometer (BBR) is an instrument being developed for the ESA EarthCARE satellite. The BBR instrument objective is to provide top-of-atmosphere (TOA) radiance measurements in two spectral channels, and over three along-track directions. The instrument has three fixed telescopes (one for each view) each containing a broadband detector. Each detector consists of an uncooled 30-pixel linear focal plane array (FPA) coated with gold black in order to ensure uniform spectral responsivity from 0.2 µm to 50 µm. The FPA is hybridized with a readout integrated circuit (ROIC) and a proximity electronics circuit-card assembly (CCA) packaged in an aluminum base plate with cover. This paper provides a technical description of the detector design and operation. Performance data at the FPA pixel level as well as unit-level test results on early prototypes of the detectors are also presented.

THz imaging is a very promising field rapidly growing in importance. This expanding field is at its early stage of development but already a large number of applications are foreseen. THz imaging promises to be a key technology in various fields, such as defense & security where it can be used to defeat camouflage. Based on its many years of experience in uncooled bolometers technology, INO has developed, assembled and characterized a prototype THz imager. The camera's 160 x 120 pixel array consists of pixels with a 52 µm pitch that have been optimized for the THz region. Custom camera electronics and an F/1 THz lens barrel complete the imager design. Real-time imaging at video rate of 30 frame/sec has been performed with a 3 THz quantum cascade laser set-up. THz images of numerous objectobscurant combinations are presented, proving the feasibility of video imaging in security screening applications.

No abstract

No abstract