Feature: RF and microwave
Figure 2: A schematic diagram of RTD injection setup and phase noise measurement
semiconductor device that, when properly biased, can generate radiation in the THz range. RTDs are like the laser diodes of the
terahertz world: T ey convert electric current directly into high-frequency radiation and, because they’re chip- scale, effi cient and fabricated with standard semiconductor processes, they’re inherently scaleable. T ey off er advantages in size, effi ciency and integration potential. However, historically they have struggled with spectral purity. T eir output linewidths, oſt en several megahertz, have limited their use in applications that require narrow frequency resolution. T ese include rotational spectroscopy, high- precision sensing, or next-generation wireless links where clean carrier signals are critical.
Injection-locked amplifi er in waveguide To overcome the RTD’s limitations in spectral purity, the IMRA team turned to a technique known as “injection locking”, which enables a lower-purity oscillator to synchronise with a higher-quality reference. In this case, that reference is a dual-wavelength Brillouin laser that’s
photomixed to generate a stable THz tone with exceptionally low phase noise. T e study emphasises that the concept
of injection locking is not new, but it is the way the IMRA researchers utilised it that makes it novel. Most previous demonstrations relied on quasi-optical free-space setups. T ose introduce signifi cant power loss, alignment sensitivity and are hard to scale. T e photomixed signal was injected
into the RTD, which re-radiated the same frequency but at signifi cantly higher power, functioning as a THz amplifi er. T is allowed the IMRA team to combine the low noise of the laser with the high output power of the RTD.
A hybrid THz circulator What made this system practical wasn’t just the injection method, but the physical implementation. T e entire signal chain – source, amplifi er and diagnostics – was built with waveguide components. T at architecture reduced insertion loss, preserved signal integrity and delivered stable, repeatable performance. Just as critically, it enabled clean isolation between components, preventing unwanted refl ections that can degrade spectral purity or disrupt the injection
Figure 3: The patent-pending hybrid circulator, HC34 by Micro Harmonics is designed for wideband millimeter wave applications
lock. IMRA was the fi rst to do this in a waveguide. T at is because in free-space set-ups, the losses are so high that it makes it nearly impossible to demonstrate amplifi cation. However, with a new THz circulator, IMRA was able to show gain of 40dB, which was unprecedented at such frequencies. Traditional Y-junction circulators
aren’t viable at most MMW and sub- THz frequencies primarily due to
www.electronicsworld.co.uk April 2026 33
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