What is a photomixer and how does it generate continuous wave terahertz radiation?
A photomixer generates continuous-wave (CW) terahertz radiation by illuminating a high-speed photodetector with two overlapping laser beams whose frequency difference equals the desired terahertz output frequency. The optical beating produces a photocurrent that oscillates at the difference frequency, which is coupled to a terahertz antenna for radiation into free space. The most common photomixer material is low-temperature-grown GaAs (LT-GaAs). The output frequency is continuously tunable by adjusting the wavelength offset between the two lasers, covering from near DC to approximately 3 THz. The primary limitation is output power, typically 0.1-10 microwatts at 1 THz. Photomixers operate at room temperature, making them attractive for compact, tunable terahertz sources.
Photomixing Technology for Tunable CW Terahertz Sources
Photomixing bridges the optical and electronic worlds by using ultrafast photodetection to convert a tunable optical frequency difference into coherent terahertz radiation.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Common Questions
Frequently Asked Questions
How much output power can I get from a photomixer at 1 THz?
A typical LT-GaAs photomixer produces 0.1-1 microwatt at 1 THz with 20-30 mW of total optical pump power. UTC-PDs produce higher power (10-100 microwatts at 300 GHz).
What lasers are used to drive a photomixer?
For LT-GaAs photomixers, tunable external-cavity diode lasers or Ti:Sapphire lasers at 780-850 nm. For InGaAs photomixers, fiber-coupled DFB lasers at 1550 nm.
Can photomixers be used for terahertz spectroscopy?
Yes. CW photomixer spectroscopy achieves frequency resolution below 1 MHz, far superior to pulsed THz-TDS systems (typical resolution 1-10 GHz).
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