文档介绍：Application Note 7
FM Spectroscopy With
Tunable Diode Lasers
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FM Spectroscopy With Tunable Diode Lasers
Introduction
Tunable diode lasers are ideal for optical spec-
troscopy because of their narrow linewidths, large tun- GHz
I
ing ranges and stable outputs. Because they are more T
compact and rugged than traditional spectroscopic
optical sources, like Ti:Sapphire lasers, dye lasers, color-
center lasers, hollow-cathode lamps, and nonlinear sys-
tems (. optical parametric oscillators), they have
enabled spectroscopic methods to be used not only in ν
laboratory environments but also in the real world. Figure 1: A typical absorption line for rubidium, showing transmit-
ted intensity ( I ) as a function of the laser frequency ( ν).
Applications of diode-laser spectroscopy include remote T
sensing, LIDAR, laser cooling and trapping of atoms,1
Because the signal has been moved to a high frequency
frequency standards,2 length standards,3 trace gas detec-
via modulation, FMS avoids the typical limitations of
tion,4 and process They can be used to
absorption measurements such as laser-intensity fluctu-
monitor environmentally important species, such as
ations, which peak at DC and fall off roughly as 1/f,
methane, carbon dioxide, and water. In semiconductor
hence the name 1/f noise. Using this technique,
manufacturing, they can be used for closed-loop con-
absorption sensitivities can reach the part per million
trol of deposition processes, including electron-beam,
(ppm) level. For example, H S has been detected at the
sputtering, molecular-beam epitaxy (MBE), and ther- 2
ppm level in air,7 absorption of Y has been measured at
mal evaporation, resulting in a significant increase in
the ppm level,5 and methane has been detected at the
the yield of existing devices and making possible new
lev