Abstract
We present a monochromatic sub-terahertz signal generation
technique using an optical comb signal, arrayed waveguide gratings (AWGs),
and a uni-traveling carrier photodiode (UTC-PD) for spectroscopic applications.
This scheme offers random or continuous frequency tuning in the range between
100 GHz and up to 1 THz. In addition, since a RF synthesizer is employed as
a reference signal source of the photonic frequency multiplier, frequency
locking with external instruments and reliable operation are offered. Highly
coherent optical comb signal for the photonic frequency multiplication provides
a narrow linewidth and very low phase noise in the generated sub-terahertz
signal. For 125 GHz, the phase noise is approximately -92 dBc/Hz at the offset frequency of 10 kHz. This
is larger than that of the 25-GHz RF source by about 13 dB and agrees well
with the theory regarding phase noise multiplications due to frequency multiplication.
For generating monochromatic signals, unwanted spurious signals are suppressed
in the optical domain over a wide range with two AWGs, and the suppression
ratio is expected to be better than 46 dBc. Utilizing the implemented sub-terahertz
signal generator with a J-band UTC-PD module, absorption lines of N<sub>2</sub>O were measured in the frequency
range between 240 and 360 GHz and compared with theoretical calculations.
© 2008 IEEE
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