Synchronized Computer Control for Pulsed Quantum Cascade Laser Frequency Scanning
Technology Professor Frank Tittel and Dr. Anatoliy Kosterev, at Rice’s Quantum Institute, have developed a tool for flexible and reproducible control of pulsed quantum cascade (QC) laser frequency scans for laser absorption spectroscopy. A pulsed mode of operation for QC lasers is used to enable their use at near-room temperature, with thermoelectric temperature control. The short (3 to 50 ns) pulses of high current (1.5 – 10 A) are applied to the laser at a repetition rate of < 1 MHz to achieve a duty cycle of ~ 1% or less. A commonly accepted technique of frequency scanning for continuous-wave diode and continuous-wave QC lasers by variation of the pump current is not applicable to the pulsed operation mode. The solution suggested consists of supplying an additional modulated sub-threshold current (tuning current, thermo-electric cooler-TC) from an independent source.
Advantages The innovation here, of digital frequency control for pulsed QC lasers, is an improvement to the existing method. Instead of using analog electronics for TC, the invention calls for synthesizing the waveform digitally and synchronizing D/A conversions to the laser pulses. This approach gives a reproducible pulse-to-pulse control of the laser frequency, adds flexibility to the laser frequency manipulation, and provides a tool for the real-time scan corrections using a digital feedback loop.
Potential Applications The synchronous digital control of pulsed QC laser frequency creates new opportunities for spectroscopic data acquisition, which include linearization of frequency scans performed with pulsed quantum cascade lasers, derivative spectroscopy to exclude low-frequency noise, and active frequency stabilization using a digital feedback loop.
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