Researchers at Rice University (Houston, TX) and the Naval Research Laboratory (Washington, DC) combined the fiber-coupled output from two diode-pumped lasers operating at 1.3 and 1.5 µm in the nonlinear crystal of silver gallium selenide (AgGaSe2) to produce room-temperature 8.7-µm output via difference-frequency generation. This continuous-wave tunable mid-infrared radiation was used to obtain the high-resolution gas-phase spectrum of sulfur dioxide (SO2). Team member Frank Tittel suggests instruments based on this process could be used to detect and measure many important gaseous species including methane, ammonia, nitrous oxide, ethylene, and benzene.
Experiment design
Because 1.3 and 1.5 µm are key telecommunication wavelengths, commercial diode-laser sources at these wavelengths are now available that have characteristics suitable for the nonlinear mixing process. In collaboration with Lew Goldberg at the Naval Research Laboratory, Konstantin Petrov, Robert Curl, and Tittel at the Rice Quantum Institute used a high-power fiber amplifier codoped with erbium and ytterbium (IRE-Polus; Moscow, Russia). An Er/Yb codoped fiber amplifier can be pumped with high-power Nd:YAG lasers and gives a greater 1.5-µm output power than possible with diode-laser-pumped Er-doped fiber amplifiers. Their device is pumped at 1.064 ?m and injection-seeded with a distributed-feedback diode laser at 1.554 µm to produce a maximum single-frequency output power of 0.5 W.
The amplifier output was passed through an isolator and combined with 35 mW of 1.319-µm output from a model 122 diode-pumped monolithic ring Nd:YAG laser (Lightwave Electronics, Mountain View, CA) through a wavelength-division multiplexer. A 4 4 10-mm AgGaSe2 crystal (Cleveland Crystals, Cleveland, OH) was used for difference-frequency mixing (with no attempt to optimize focusing). The 8.7-µm difference-frequency output was filtered from the pump wavelengths and measured with a mercury cadmium telluride (HgCdTe) detector.
Using 29-mW pump power (Nd:YAG) and 370-mW signal power (Er/Yb amplifier) incident on the crystal, the researchers measured an idler power (8.7-µm) of about 0.1 µW.1 While this value was less than expected based on calculations of the effective crystal length, the researchers determined that the active area of the detector was too small to collect all the idler output (its spot size was increased by spherical aberrations in the collimating and focusing optics). This power is, however, sufficient for spectroscopy experiments. A 10-cm-long absorption cell filled with SO2 at 5 torr was placed in the path of the idler beam. Temperature tuning of the Nd:YAG pump laser at 1.319 µm provided the frequency sweep. Lock-in detection captured the absorption signal, and band assignments were made with a HITRAN database. The frequency sweep was linear and reproducible, although tuning of the Nd:YAG pump laser allowed only a narrow tuning range.
Many molecules have characteristic “fingerprint” absorption bands in the important 8–12 µm region of the infrared spectrum. Until now, access to this region has been limited to Fourier-transform infrared spectrometers, which cover the entire infrared range; lead-salt diode lasers, which require cryogenic cooling and have limited tuning ranges; and carbon dioxide lasers, which have many discrete lines between 9 and 11 µm. The gain curve of the Er/Yb fiber indicates that useful IR powers from 1050 to 1250 cm-1 can be obtained using various injection-seeding diodes in the Er/Yb amplifier in combination with the 1.319-µm Nd:YAG pump laser.
For many high-resolution spectroscopic measurements, tunability is only required over a relatively narrow range. Using the very limited tunability of Nd:YAG, the difference-frequency mixing technique could be used to scan across about 0.5 cm-1 around 8.7 µm, which for SO2, includes five strong peaks near 1144 cm-1. Additionally, the difference-frequency generation technique allows transmission of the signal and pump beams over conventional silica optical fibers to a remote measurement site where the two beams are combined in the nonlinear crystal to generate mid-IR radiation.
To extend the IR wavelength coverage, Tittel and his associates propose to use a tunable extended-cavity diode laser to injection-seed a praeseodymium-doped fiber amplifier at 1.319 µm instead of the Nd:YAG laser. The tuning range should then extend from 900 to 1400 cm-1. In addition, this scheme should provide pump power of around 300 mW and should allow the difference-frequency output power to reach about 5 µW. The single-scan tuning range can be 10 cm-1. The researchers expect such characteristics would benefit many molecular spectroscopy and trace-gas detection experiments.
REFERENCE
1. K. P. Petrov et al., Opt. Lett., in press.