We have locked the frequency of a 3 THz quantum cascade laser (QCL) to that of a far-infrared gas laser with a tunable microwave offset frequency. The locked QCL line shape is essentially Gaussian, with linewidths of 65 and 141 kHz at the -3 and -10 dB levels, respectively. The lock condition can be maintained indefinitely, without requiring temperature or bias current regulation of the QCL other than that provided by the lock error signal. The result demonstrates that a terahertz QCL can be frequency controlled with 1-part-in-10(8) accuracy, which is a factor of 100 better than that needed for a local oscillator in a heterodyne receiver for atmospheric and astronomic spectroscopy.

We have observed the 158 µm 2 P 3 2 ← 2 P 1 2 fine-structure line of 12 C II simultaneously with the F = 2 ← 1 and F = 1 ← 0 hyperfine components of this transition in 13 C II in the Orion photodissociation region near θ 1 C. The line profiles were fully resolved using a heterodyne spectrometer with 0.5 km s −1 resolution. The relative intensities of these lines give a 12 C/ 13 C isotopic ratio of R = 58 ( +6 −5 ) for the most probable 12 C II peak optical depth τ = 1.3 . The constrained range of τ ( 12 C II) between 1.0 and 1.4 corresponds to a range of 12 C/ 13 C between 52 and 61. The most probable value of 58 agrees very well with that obtained from a relationship between the isotopic ratio and galactocentric distance derived from CO measurements, but is lower than the specific value of 67±3 obtained for Orion from CO data. An isotopic ratio as low as 43, as previously suggested based on optical absorption measurements of the local interstellar medium, is excluded by the C II data at about the 2σ level.

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