Cavity ring-down spectroscopy mirror, and methods of using same

An apparatus comprising:

• a ring-down mirror cavity, wherein the ring-down mirror cavity includes a plurality of mirrors, at least one mirror of which includes:
• a mirror substrate;
• a mirror stack disposed on the mirror substrate, wherein the mirror stack includes a plurality of alternating laminates including a first film with a first index of refraction and a second film with a second index of refraction, and wherein each of the first film and the second film has a thickness of one quarter wavelength of a given light energy;
• and
• a subsequent laminate disposed on the plurality of alternating laminates, wherein the subsequent laminate includes a quarter wavelength thickness first film and a second film with a wavelength thickness in a range from greater than 1.5 quarter wavelength and less than 2 quarter wavelength.

A method comprising:

• operating a cavity ring-down spectroscopy mirror apparatus by transmitting light onto a plurality of mirrors;
• observing a first maximum reflectance and a first maximum reflectance peak in terms of wavelength;
• allowing accreted material to absorb onto at least one of the plurality of mirrors;
• and
• observing a second maximum reflectance, wherein despite Lip to 100 nanometer of accreted material on the at least one of the plurality of mirrors, the maximum reflectance peak migrates less than 500 nanometer in wavelength.