A tunable cavity-locked diode laser source for terahertzphotomixing
Matsuura, S.; Pin Chen; Blake, G.A.; Pearson, J.C.; Pickett, H.M.
Microwave Theory and Techniques, IEEE Transactions on
Volume 48, Issue 3, Mar 2000 Page(s):380 - 387
Digital Object Identifier 10.1109/22.826836
Summary:An all solid-state approach to the precise frequency synthesis and
control of widely tunable terahertz radiation by differencing
continuous-wave diode lasers at 850 nm is reported in this paper. The
difference frequency is synthesized by three fiber-coupled
external-cavity laser diodes. Two of the lasers are Pound-Drever-Hall
locked to different orders of a Fabry-Perot (FP) cavity, and the third
is offset-frequency locked to the second of the cavity-locked lasers
using a tunable microwave oscillator. The first cavity-locked laser and
the offset-locked laser produce the difference frequency, whose value is
accurately determined by the sum of an integer multiple of the free
spectral range of the FP cavity and the offset frequency. The
dual-frequency 850-nm output of the three laser system is amplified to
500 mW through two-frequency injection seeding of a single semiconductor
tapered optical amplifier. As proof of precision frequency synthesis and
control of tunability, the difference frequency is converted into a
terahertz wave by optical-heterodyne photomixing in
low-temperature-grown GaAs and used for the spectroscopy of simple
molecules. The 3-dB spectral power bandwidth of the terahertz radiation
is routinely observed to be ≲1 MHz. A simple, but highly accurate,
method of obtaining an absolute frequency calibration is proposed and an
absolute calibration of 10-7 demonstrated using the known
frequencies of carbon monoxide lines between 0.23-1.27 THz
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