The figure schematically depicts a heterodyne photonic apparatus built around three continuous-wave, near-infrared diode lasers. This apparatus generates electromagnetic radiation at an adjustable, precisely defined frequency in the terahertz range. The spectral width of the signal is less than about 1 MHz. The apparatus could serve as a prototype of tunable far-infrared sources and heterodyne up- and down-converters for fiber-optic communication systems and for testing infrared systems in general. Other potential uses lie in infrared spectroscopy for remote sensing and in research on low-energy light/matter interactions.
The three diode lasers operate at wavelengths near 850 nm. Each diode laser is packaged with a collimating lens, a resonant-cavity assembly tunable in length (and thus in frequency) by means of a piezoelectric transducer (PZT), and other optics to circularize and couple the laser output to an optical fiber. All the optical components, including the optical fibers, are polarization-maintaining.
The three diode lasers are parts of a master-oscillator subsystem that generates two laser beams at different wavelengths near 850 nm. These beams are used for two-frequency injection seeding of a single semiconductor tapered optical amplifier. The output of the amplifier is fed to a photomixer, wherein the final output terahertz signal is generated. Thus, the overall photonic system can be character ized as a photomixer pumped by a photonic master-oscillator/power-amplifier (MOPA) subsystem. Typically, the system is operated at a power-amplifier output power level of 30 mW, yielding a terahertz output power of ≈0.1 µW. At present, the output level must be limited to prevent thermal failure of the photomixer; if this limitation could be overcome, then the power amplifier could be operated at its maximum output level of 500 mW, making it possible to generate about 10 µW of terahertz output.
The frequencies of lasers 1 and 2 are controlled by locking them to different longitudinal modes of a reference cavity in the form of an ultra-low-expansion (ULE) Fabry-Perot etalon. This is accomplished by the Pound-Drever-Hall technique, which is a frequency-modulation/ optoelectronic feedback control technique for stabilizing the frequency of a laser. The frequency modulation for this purpose is effected through electro-optical modulators driven at frequencies of 120 and 80 MHz, respectively.
The difference frequency between lasers 1 and 2 is discretely selectable in increments equal to the free spectral range (FSR) of the reference cavity (increments of about 3 GHz). Laser 3 is heterodyne phase-locked to laser 2, offset in frequency by an amount established by a tunable microwave synthesizer that operates in the frequency range of 3 to 6 GHz. The difference frequency between the lasers 1 and 3 — the final output frequency in the terahertz range — is the sum of the microwave offset frequency and an integer multiple of the FSR of the reference cavity. The microwave offset frequency is locked to a reference source accurate to within a fractional error <10 —12. The accuracy of the final output frequency is determined by the accuracy of measurement of the FSR (within about ±50 kHz) and by any dc offset in the electrical portions of the lock loops. The ULE material in the reference cavity has a coefficient of thermal expansion of —2 ×10 —10 (°C) –1, resulting in frequency stability (with respect to temperature) comparable to that stability of a good quartz reference oscillator in a conventional microwave source.
This work was done by Herbert Pickett, John Pearson, Serge Dubovitsky, Pin Chen, Geoff Blake, and Shujii Matsura of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category. NPO-20636