Near-IR TDLAS

Recent technological advances in the field of communications have made near-IR tundable diode lasers (TDLs) of high quality, yet relatively inexpensive. A TDL Absorption Spectrometer (TDLAS) system is currently being developed utilizing two-tone frequency modulation to measure trace atmospheric species using a long, open-path. Although the vibrational transitions in the near-IR are weak (overtones and combinations) compared to the mid-IR (fundamentals), working in the near-IR offers several advantages:

• high quality lasers and detectors
• relatively cheap
• ability to couple laser to a fiberoptic cable - allows portability
• lasers can be operated at room temperature - no need for cryogenic cold heads
• few interfering absorbing species - can sample at atmospheric pressure - no pumps

The above advantages to working in the near-IR yield compact, portable instruments well-suited for transportation to remote sites.

The Harris Group is currently working in collaboration with Unisearch Associates Inc. to develop a near-IR instrument capable of simultaneous multispecies measurement. Traditional near-IR measurements have required a separate laser (or instrument) for each desired species. We have obtained a SG-DBR widely tunable laser which is capable of accessing absorption lines of several trace gas species, namely, CO, CO₂, ammonia, acetylene, and hydrogen sulfide. Measurements of ammonia are most interesting since it's "sticky" properties don't allow simple sampling. By measuring ammonia in an open path, sampling difficulties are removed.