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Wednesday, April 23, 2014

Precise control of optical frequency on a chip


In the 1940s, researchers learned how to precisely control the frequency of microwaves, which enabled radio transmission to transition from relatively low-fidelity amplitude modulation (AM) to high-fidelity frequency modulation (FM). This accomplishment, called microwave frequency synthesis, brought about many advanced technologies now critical to the military, such as wireless communications, radar, electronic warfare, atomic sensors and precise timing. Today, optical communications employ techniques analogous to those of pre-1940 AM radio, due to the inability to control frequency precisely at optical frequencies, which are typically 1,000 times higher than microwaves. The higher frequency of light, however, offers potential for 1,000-fold increase in available bandwidth for communications and other applications.

As both government and commercial need for bandwidth continues to grow, DARPA's new Direct On-chip Digital Optical Synthesizer program seeks to do with light waves what researchers in the 1940s achieved with radio microwaves. Currently, optical frequency synthesis is only possible in laboratories with expensive racks of equipment. If successful, the program would miniaturize optical synthesizers to fit onto microchips, opening up terahertz frequencies for wide application across military electronics systems and beyond.
"The goal of this program is to make optical frequency synthesis as ubiquitous as microwave synthesis is today," said Robert Lutwak, DARPA program manager. "There are significant challenges, but thanks to related DARPA programs POEM, Quasar, ORCHID, PULSE and E-PHI and other advanced laboratory research, technology is at the tipping point where we're ready to attempt miniaturization of optical frequency synthesis on an inexpensive, small, low-power chip."
The basic concept is to create a "gearbox" on a chip that produces laser light with a frequency that is a precise multiple of a referenced , such as is readily available within most existing DoD and consumer electronic systems. The ability to control optical frequency in a widely available microchip could enable a host of advanced applications at much lower cost, including:
  • High-bandwidth (terabit per second) 
  • Enhanced chemical spectroscopy, toxin detection and facility identification
  • Improved light detection and ranging (LiDAR)
  • High-performance atomic clocks and inertial sensors for position, navigation and timing (PNT) applications
  • High-performance optical spectrum analysis (OSA)
For example, digital optical synthesizers on a chip could increase accuracy for optical chemical sensing by six orders of magnitude while reducing cost, size and power requirements by many orders of magnitude over current capabilities. These improvements would make it possible to detect adversary chemical production facilities with high sensitivity from much farther away than is possible today.
The program envisions three phases, lasting a total of 42 months. Phase 1 would involve a demonstration of optical  synthesis in a laboratory, using low size, weight and power (SWaP) optical components. Phase 2 calls for a demonstration of an integrated electro-optical component. Phase 3 calls for successful demonstration of integrated  synthesizer and control electronics meeting all program performance and SWaP metrics.
"We're looking for multidisciplinary teams made up of experts in micro- and nano-fabrication, optics and photonics, and heterogeneous integration to bring the component technologies together," Lutwak said.

Read more at: http://phys.org/news/2014-04-precise-optical-frequency-chip.html#jCp

Tuesday, April 22, 2014

Abstract-A polarization-sensitive 4-contact detector for terahertz time-domain spectroscopy

Dmitry S. Bulgarevich, Makoto Watanabe, Mitsuharu Shiwa, Gudrun Niehues, Seizi Nishizawa, and Masahiko Tani  »View Author Affiliations

Optics Express, Vol. 22, Issue 9, pp. 10332-10340 (2014)
A light polarization angle-sensitive photoconductive detector for terahertz time-domain spectroscopy is computer-modeled, microfabricated, and tested. The experimental results show good agreement with the linear angular response for an ideal detector. The detector’s frequency, angular, and crosstalk responses are discussed in the context of theoretical and experimental considerations.
© 2014 Optical Society of America

Abstract-Rapid Scanning Terahertz Time-Domain Magnetospectroscopy with a Table-Top Repetitive Pulsed Magnet

We have performed terahertz time-domain magnetospectroscopy by combining a rapid scanning terahertz time-domain spectrometer based on the electronically coupled optical sampling method with a table-top mini-coil pulsed magnet capable of producing magnetic fields up to 30 T. We demonstrate the capability of this system by measuring coherent cyclotron resonance oscillations in a high-mobility two-dimensional electron gas in GaAs and interference-induced terahertz transmittance modifications in a magnetoplasma in lightly doped n-InSb.


By H. Hua, Y.-S. Jiang, and Y. He

The high-frequency method for the prediction of the terahertz (THz) radar cross section (RCS) of conductive targets with extremely electrically large size in free space was presented. In order to consider the scattering fields of the perfectly electric conducting (PEC) targets with extremely electrically large size in free space, the Green’s function was introduced into the conventional physical optics (PO) method which was combined with the graphical electromagnetic computing (GRECO) method and improved using the partition display algorithm. The shadow regions were eliminated quickly by displaying lists of OpenGL to rebuild the targets, and the geometry information was attained by reading the color and depth of each pixel. The THz RCS of conductive targets can be exactly calculated in free space. The RCS comparison between the partition display GRECO prediction by the self-written Visual C++ 2010 program and the simulation of FEKO software with the large element PO method proves the validity and accuracy of the proposed method. The results provide an important basis and method for the potential applications of THz radar in many fields such as military, astronomy and remote sensing.
H. Hua, Y.-S. Jiang, and Y. He, "High-frequency method for terahertz radar cross section of conductive targets in free space," Progress In Electromagnetics Research B, Vol. 59, 193-204, 2014.

Abstract-RF heating efficiency of terahertz superconducting hot-electron bolometer

Sergey Maslennikov

The system of differential equations for heat balance in a superconducting HEB and for the HEB electrical circuit is written in recurrent form and solved numerically by the Euler method. Dependence of the HEB resistance on the transport current is taken into account. RF heating efficiency, absorbed local oscillator (LO) power and conversion gain of the HEB mixer are calculated. It is shown that calculated conversion gain is in excellent agreement with experiment. It is shown that substitution of calculated RF heating efficiency and absorbed LO power to expressions for conversion gain and noise temperature given by the analytical small signal model of HEB yields excellent agreement with corresponding measured values.