Optical Couplers: Key Hardware for High Quality FTIR Spectroscopy

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Figure 1 Optical Coupler

Figure 2 Optical Couplers on each side of Cell

Description

Optical couplers are used to take the light coming from the source of a FTIR and route it through the sampling accessory and into the FTIR detector. This is an essential process in the effective use of the FTIR spectrometer. Therefore, optical couplers play an essential role in the proper use and maintenance of a FTIR system. Recognizing the importance of optical couplers to the overall effectiveness of the FTIR System, CIC Photonics has developed their own design for optical couplers.

The purpose of optical couplers is to direct light efficiently to the desired locations. Two essential issues must be addressed when implementing optical couplers: the couplers must efficiently transfer light to the location of interest, and they must be able to maintain this efficient transfer throughout the life span of the instrument. CIC Photonics optical coupler design has numerous features which address these key issues as well as a number of secondary issues of lesser importance.

Design

To address the issue of efficient light transfer, the CIC Photonics optical coupler design uses an off-axis parabolic mirror to transfer the light from the source to the FTIR sampling accessory (e.g. a gas cell) and from the accessory to the detector. In order to adjust for small variations in the optical path and provide the means of correcting these variations (alignment), CIC Photonics optical couplers use a set of three alignment screws on top of the off-axis mirror which imparts small changes in the mirror position. Once the optimal throughput has been obtained, the mirror is locked into place with a second set-screw.

In order to maintain the optical efficiency earned in proper alignment, CIC Photonics has implemented a system of hard-mounted optical couplers. Our couplers are made of rigid aluminum fixtures which are bolted together to eliminate undesired degrees of freedom. This means that the couplers will not rotate or slip down and out of alignment. Different optical coupler heights are developed through the use of height sleeves which hard mount to the other coupler components. This optical coupler design, developed by CIC Photonics, offers significant advantages in rigidity and in the ability to maintain the optical path in instrument installations throughout the world. The robust design of the CIC Photonics optical coupler means that traveling instruments like the IRGAS-100MT™, once aligned, stay aligned. In the end, that means the instrument is ready to run when the customer takes it out of the box!

Top View

Seen to the left is a top view of one CIC Photonics optical coupler. Note the three inner alignment screws with washers on the top of the coupler. These three screws arranged in a triangular pattern allow CIC Photonics or the end user to maximize the optical throughput of the couplers by translating small changes in the orientation of the off-axis parabolic mirror contained within the coupler. Once the optimal alignment has been obtained, the center screw is used to lock down the off-axis mirror. Note also the inlet port for purge gas. The rigid aluminum design also provides the additional benefit of being easily purged to provide cleaner background spectra free from moisture and CO2.

Mounted View

The second view, on the left, is that of an IRGAS™ system with optical couplers installed. This view shows the couplers hard-mounted to the Bomem WorkIR™ spectrometer configured with a CIC Photonics 4Runner™ gas cell. An additional advantage of the CIC Photonics couplers is that the gas cell can easily be removed from the spectrometer (release one bolt) for service such as window changes, and then easily re-installed on top of the alignment pins while the couplers maintain the original alignment.