Column: Microwave and mmWave


IMRA research During their research of alternatives to the current state-of-the-art optical atomic clocks operating at several hundreds of terahertz, IMRA researchers encountered the common issue of standing waves, also referred to as signal reflections or mismatches. Tese undesirable waves, or ripples, can attenuate power output, distort the information on the carrier and, in extreme cases, damage internal components. To alleviate the problem of standing waves


at lower microwave frequencies, engineers rely on Faraday rotation isolators – more commonly referred to simply as isolators. At their very basic level, an isolator is a two-port, input and output, component that allows EM signals to pass in one direction but absorbs them in the opposite direction. Te problem is that traditional isolators fail to deliver at higher frequencies within the THz regime. IMRA was working at hundreds of gigahertz


to interrogate molecules – which naturally rotate at a very specific frequency. A molecule will only absorb radiation when it is identical to the frequency at which it is rotating. Te


challenge for researchers is to efficiently reach frequencies within the terahertz band, where the majority of small molecules rotate. Instead of multiplying microwaves up,


which is most common, IMRA researchers used a photonic oscillator to divide lasers down using dissipative Kerr soliton (DKS) microcombs. By generating a 301.442 gigahertz wave


the researchers were able to talk to nitrous oxide (N2O) molecules in a vacuum chamber where they were isolated. However, reflected waves would have interfered with the signal, delivering sub-optimal results. Terefore, Greenberg and his team decided to insert an MMW isolator that worked within the WR 3.4 band (220 GHz - 325 GHz) into the waveguide. “We actually had a WR 3.4 band isolator


from Micro Harmonics on the shelf from past research projects,” explains Greenberg. “We plugged it in and got fantastic results.” Under a NASA awarded contract, Micro


Harmonics Corporation designed an advanced line of MMW isolators capable of optimal operation between WR-28 and WR-2.8 (26.5


GHz to 400 GHz). Te commercial off-the-shelf (COTS) components offered IMRA the high isolation, low insertion loss and small footprint it was looking for.


THz Components “High isolation was extremely important, and we were getting 30 dB of isolation with the isolators,” adds Greenberg. “We were also starved for power because our transmitter was weak, and we were really pleased with the low insertion loss the isolator provided which allowed us to get the maximum signal to noise.” For molecular spectroscopists and people


who work with clocks, tiny effects have huge implications, namely for GPS. For example, the difference between parts per million and parts per billion is the difference between having GPS and not. “If you can’t do better than a part per billion,


you can’t have GPS,” adds Greenberg. “It just doesn’t work.” Since the initial experiment, IMRA has moved


on to study other molecules in its research. “Te Micro Harmonics isolators also have


a really wide band which allows us to look at a range of molecules at different frequencies using the same spectrometer as previous experiment with the exact same isolator,” says Greenberg.


The pursuit of a molecular clock Atomic clocks all work the same way. Tere is an oscillator operating at a specific frequency and the key is to stabilize that frequency to an atomic reference, something that will not change no matter where or when you look at it. An atom, for example, is going to react the exact same way anywhere on Earth as well as in space. While the cesium atom remains the industry


standard, the world’s first atomic clock actually used a molecule (Ammonia NH3). “I love that what is old is new again,”


concludes Greenberg. “Te scientific community is now producing portable atomic clocks that use molecules. Some can outperform commercial cesium microwave clocks that most of the world relies on for their time standard. Tis is just the tip of the iceberg for what we are going to find by bridging the terahertz gap.”


For more information contact Micro Harmonics: 540.473.9983, sales@mhc1.com, or visit www.MicroHarmonics.com.


www.electronicsworld.com October 2024 27


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