Multi-Harmonic Inline Reference Cell for Optical Trace Gas Sensing
US Patent 2014/0049777 A1 - Issued 20 February 2014:
https://www.google.com/patents/US20140049777?dq=2014/0049777&hl=en&sa=X&ei=KjyHU46NFdPesATwkIGAAw&ved=0CDUQ6AEwAA