Gases & Instrumentation - September/October 2009

Absorption Spectroscopy

MARTIN EBERMANN, NORBERT NEUMANN

The design and operation of a microspectrometer for
3-5 µm with an integrated micromachined Fabry-Perot
filter demonstrating the use of the tunable detector in
gas analyzers and spectroscopic applications

This is a t wo-part article. Part II of this article will appear in the November/December 2009 issue of Gases & Instrumentation. It will discuss the pyroelectric detector and spectrometer design and operation.

Abstract

In this paper we present basic designs, operational concepts and some application examples of a novel microspectrometer for the spectral range of 3-5µm, which is based on a pyroelectric detector with an integrated micromachined Fabry-Perot filter (FPF). We discuss the influence of different optical setups on the spectral resolution and the signal-to-noise ratio of the microspectrometer. Two basic operation modes, step-scan mode and continuous-sweep mode are demonstrated. Such a device has a large potential in the field of infrared absorption spectroscopy, particularly if multicomponent mixtures have to be analyzed.

Introduction

Infrared absorption spectroscopy is well established in gas analysis, fire and flame detection and in many other applications. A typical infrared analyzer consists of a broadband light source, a sample cell and at least one, but in most cases several, spectral measurement channels with fixed narrow bandpass filters. The filters are selected to match the characteristic absorption bands that are of interest in each specific application. This is often realized by using detectors with several spectral channels or a rotating filter wheel in front of a single channel detector.

The increasing demand for collecting more and more spectral information, reducing cross sensitivities, making

measurements faster and, last but not least, in miniaturization of such systems is challenging. The above mentioned conventional approach mainly suffers from the disadvantage that the number of channels is limited. There are several well known problems, for example, regarding homogeneous detector illumination and long-term stability. Filter wheels cannot be miniaturized and their reliability is lacking.

What in fact is needed, is a low resolving, robust and low cost microspectrometer for the mid-wave infrared range. Many approaches for microspectrometers are based on diffraction gratings1. Attempts to miniaturize the FTIR technique have also been reported2. Some of these solutions offer a relatively high spectral resolution and fast measurements, but the degree of integration and miniaturization is often not very high.

Many attempts towards the combination of a Fabry-Perot interferometer and an IR-detector have also been reported3, 4, 5 but in most cases they suffer from a low Finesse due to warping of thin membrane reflectors or low optical throughput.

Our approach, the hybrid assembly of a bulk micromachined Fabry-Perot Filter (FPF) and a pyroelectric detector, results in a very compact spectrometer module6. Existing instrument designs can be easily adapted to the new tunable detector. In this paper we present the basic design and operation concept of such a microspectrometer for the spectral range of 3-5 µm.