System Bits: Feb. 25

A faster Internet
While light is capable of carrying vast amounts of information, to utilize its potential, the laser light needs to be as spectrally pure—as close to a single frequency as possible. The purer the tone, the more information it can carry. For decades researchers have been trying to develop a laser that comes as close as possible to emitting just one frequency.

Today’s worldwide optical-fiber network is still powered by a laser known as the distributed-feedback semiconductor (S-DFB) laser, developed in the mid 1970s. The laser’s unusual longevity in optical communications stemmed from its then-unparalleled spectral purity. At the time, this was a direct consequence of the incorporation of a nanoscale corrugation within the multilayered structure of the laser which acted as a sort of internal filter, discriminating against spurious “noisy” waves contaminating the ideal wave frequency. Although the old S-DFB laser had a successful 40-year run in optical communications, the spectral purity, or coherence, of the laser no longer satisfies the ever-increasing demand for bandwidth.

The prime motivator for new research at Caltech was the fact that present-day laser designs have an internal architecture which is unfavorable for high spectral-purity operation because they allow a large and theoretically unavoidable optical noise to comingle with the coherent laser and thus degrade its spectral purity.

A high-coherence laser still converts current to light using the III-V material, but in a fundamental departure from the S-DFB laser, it stores the light in a layer of silicon, which does not absorb light. Spatial patterning of this silicon layer—a variant of the corrugated surface of the S-DFB laser—causes the silicon to act as a light concentrator, pulling the newly generated light away from the light-absorbing III-V material and into the near absorption-free silicon.

The new laser developed by researchers at Caltech includes a layer of silicon, which does not absorb light–a quality important for laser purity. (Source: Caltech)

This newly achieved high spectral purity—a 20 times narrower range of frequencies than possible with the S-DFB laser—could be especially important for the future of fiber-optic communications.

Single chip allows 3D images of heart, blood vessels in real time
To better guide surgeons working in the heart and potentially allow more clogged arteries to be cleared without major surgery, researchers at Georgia Tech have developed the technology for a catheter-based device that would provide forward-looking, real-time, 3D imaging from inside the heart, coronary arteries and peripheral blood vessels.

A single-chip catheter-based device that would provide forward-looking, real-time, three-dimensional imaging from inside the heart, coronary arteries and peripheral blood vessels is shown being tested. (Source: Georgia Tech)

 
The device integrates ultrasound transducers with processing electronics on a single 1.4mm silicon chip. On-chip processing of signals allows data from more than a hundred elements on the device to be transmitted using just 13 tiny cables, permitting it to easily travel through circuitous blood vessels. The forward-looking images produced by the device would provide significantly more information than existing cross-sectional ultrasound.

Researchers have developed and tested a prototype able to provide image data at 60 frames per second, and plan next to conduct animal studies that could lead to commercialization of the device.