Questions about the future:Continuous wave Terahertz vs Time Domain Terahertz

A number of recent multi-million dollar contracts with the Federal Government, (Darpa) which involves continuous wave terahertz (CWT) makes me question, if time domain terahertz (TDT) has proven to be too difficult to employ, or has reached some type of technical barrier which prevents or limits it's usefulness. I certainly, don't know the answer to this question and hence I am hoping to invoke some comment from someone who might know.

The following is a news article, (actually the latest in a series) which discusses the development of CWT.

I'm hoping Dr. Mittleman or someone else that is knowledgeable might comment on the limitations found in TDT, and why CWT appears to have gained such recent attention.
Thanks for any comments



DARPA looks to Teledyne Scientific for terahertz electronics researchARLINGTON, Va., 10 May 2009. Researchers at the U.S. Defense Advanced Research Projects Agency (DARPA) are looking to Teledyne Scientific & Imaging LLC in Thousand Oaks, Calif. to develop terahertz electronics receivers and exciters as part of the DARPA Terahertz Electronics Program.Teledyne Scientific won an $8.3 million DARPA contract to develop terahertz electronics for transceiver arrays -- specifically, receivers and exciters at carrier frequencies of 670 GHz, 850 GHz, and 1030 GHz.U.S. Department of Defense officials anticipate that terahertz integrated circuit technology will provide major advancements for military applications in advanced military imaging technology, radar technology, terahertz spectroscopy for chemical warfare detection, and military communications.Teledyne Scientific joins Science Applications International Corp. (SAIC) in San Diego, the Northrop Grumman Electronic Systems segment in Linthicum, Md., and the Northrop Grumman Space and Mission Systems segment in Redondo Beach, Calif., in performing research on the DARPA Terahertz Electronics initiative.The sub-millimeter wave (sub-MMW) frequency band between 0.3 to 3 terahertz historically has been extremely difficult because designers lack the means to generate, detect, process, and radiate radio-frequency (RF) signals in these frequencies, DARPA officials explain.Military and aerospace applications that operate at terahertz frequencies need greatly improved terahertz transmitter and receiver technologies to be effective. That is where Northrop Grumman and SAIC experts come in.For transmitters, DARPA primarily wants to develop higher power sources and amplifiers -- with acceptable wall-plug efficiency, instantaneous bandwidth, and gain -- than are available today.DARPA experts say they have an opportunity improve noise figure and phase noise characteristics in future terahertz RF applications. Signal-to-noise ratio improvements of 70 decibels or more might be possible with spectral filtering and phase coherent processing techniques such as using coherent heterodyne processing that exploits the relative phases of transmit and receive signals.A big drawback to terahertz electronics has been the lack of high-density integrated circuit technology. Achieving the level of integration necessary to enable practical terahertz systems, such as arrays, will require SAIC and Northrop Grumman to come up with methods for integrating devices into compact circuits.In charge of this project is the DARPA Microsystems Technology Office (MTO). Supervising the program will be DARPA Program Manager Mark Rosker, who can be reached by e-mail at mrosker@darpa.mil, or by phone at 571-218-4507.For more information contact the DARPA Microsystems Technology Office online at www.darpa.mil. http