Instrumentation

Focal Plane Heterodyne Array Receivers

• Motivation for Heterodyne Arrays

  • Single pixel receivers are approaching quantum limits; present limit is atmospheric for much of the millimeter and submillimeter bands.
  • Mapping speed is substantially improved by constructing focal plane arrays.
  • In addition to the N-fold decrease in mapping time, where N is the number of elements in array, time for telescope motion is considerably reduced
  • Optimal use of good weather conditions at higher frequencies
  • Mapping consistency - reduced systematic effects due to pointing offsets, relative calibration, etc.

• Desert STAR (Submillimeter Telescope Array Receiver). The University of Arizona has a collaborative agreement with FCRAO in building a 345 GHz heterodyne focal plane array receiver system for the HHT, in return for which FCRAO will receive a proportional amount of observing time on the HHT. I am leading the effort from the UMass side in the design, construction and testing of the mixer blocks. Chris Walker at the University of Arizona is the PI of this project. Neal Erickson at UMass is contributing the local oscillator chain for this array receiver system. The figures below show the atmospheric window with the the pricipal molecular lines targeted, and an example view of the array footprint on the sky.
  Desert STAR Features:
  • 7-element SIS heterodyne receiver
  • Tuning range 315 to 380 GHz
  • IF frequency 4 - 6 GHz
  • Hexagonally packed array, spacing 44" resolution 22"
  • SSB operation
  The design of the mixer elements of Desert STAR is available here. The first pixel of the array will be tested in the Fall of 1999, with first light of the entire array scheduled for the Spring of 2000.

• SEQUOIA (Second Quabbin Imaging Array) the sequel to QUARRY is a 16-pixel focal plane array that works between 85 to 115 GHz, and uses low-noise MMICs (Monolithic Microwave Integrated Chips) as the frontend. SEQUOIA is the workhorse receiver system that has been operating at FCRAO for the past couple of years. Neal Erickson and FCRAO's dedicated band of engineers and technicians designed and built SEQUOIA. My own small contribution to this project is in the design of some IF components that will enhance SEQUOIA's flexibility of use for astronomical observations.

Development of Novel Machining Techniques for submm and THz Receiver Components

Traditional metal blocks manufactured using conventional machining techniques are used in most successful receiver systems in the millimeter to submillimeter wavelength range. The components of such receiver systems include oscillators, multipliers and mixers, which mostly incorporate waveguides and their associated transitions. The waveguide is a well characterized, low-loss transmission medium, which affords easy and excellent coupling of the freespace modes to electronic circuit elements. As the desired wavelength of operation becomes smaller, so do the dimensions of waveguide and quasi-optical elements that comprise a receiver system. On the one hand, the small sizes are advantageous in the fabrication of high-resolution focal-plane imaging receiver systems. Such arrays have applications like radio astronomy, commercial and industrial applications like the extension of millimeter technology to collision avoidance radars, aircraft guidance and landing, contraband detection and personal communication systems, all of which benefit from having a large number of elements in a compact, practical sized system. However, as the sizes of components scale down, the cost of fabrication goes up. Indeed, the fractional cost of machined metal blocks containing waveguide electromagnetic elements in the overall budget of receiver systems, especially those that involve arrays, becomes prohibitively high with increasing frequency. Indeed, at frequencies above 1.5 Terahertz, machining metal waveguides using conventional techniques become exceedingly difficult, if not impossible. I have been involved in two independent efforts to bridge this gap, one that extends the possibilities of conventional machining, and the other that invokes a completely new type of technology:
1. Over the last year, working with Neal Erickson and Ron Grosslein, I helped develop a low cost technique of direct machining of precision waveguide structures on metal blocks. This novel new machine setup employs compact, precision numerically controlled (NC) stages to automate the machining process. Endmills are held in high speed pneumatic spindles (running at 70,000 rpm!), while other features are made by scraping ("broaching") in repeated small passes. We can set up two endmills and three broaches at the same time. The fabrication process is monitored under a microscope, while the ambient temperature conditions are controlled. The NC code is generated from a combination of AutoCAD drawings, commercially available CAD/CAM software and simulation software we developed for this process. The advantages of this techique are high level of accuracy (of the order of 5 microns), lower fabrication time, and automated approach that brings machining of high frequency components from an art form to science.

   Here are two views of the machine setup in our laboratory:

   Follow this link to read the paper on this machining technique that we presented at the 10th International Symposium on Space Terahertz Technology held at the University of Virginia.

   The photographs below show the full-view and details of the machining of the split blocks of a 810 GHz tripler block that was machined using this technique. This tripler has been delivered as part of a local oscillator chain for the submillimeter telescope at the South Pole, AST/RO.

   The photographs below show the full-view and details of the machining of the split blocks of one pixel of the 345 GHz heterodyne array of Desert STAR (described above) that I machined using this technique.

2. In the past decade or so, non-traditional techniques that can be collectively called ``micromachining'' are demonstrating practical means for producing a variety of submillimeter and terahertz frontend components. Micromachining as a class is derived from manufacturing tools based on batch thin and thick film fabrication techniques of the electronic industry. One such machining technique is the laser micromachining technique. While I was working at the Steward Observatory Radio Astronomy Laboratory (SORAL), I worked with Chris Walker and other collaborators in developing the use of lasers to fabricate tiny but precise structures in silicon. Go to this page hosted at the SORAL website to learn more. One example of the precision and promise offered by this technology is shown below in the details of a 810 GHz corrugated feedhorn manufactured using this laser micromachining technique.

High Frequency Multipliers

LMT/GTM Instrumentation Development

Past Instrumentation Projects