Integrated Two-dimensional Grating Spectral Imaging System And Its Applications

In the modern information age, it has become very important to acquire data in a fast speed, especially in the field like the optical communication field where high-speed spectral analysis is required. For quick analysis of the spectrum with high resolution, instead of using a photomultiplier tube or a photon detector to detect the photons emerging from the narrow slit of the monochromator, a CCD array detector is placed at the focal plane of spectral output of the monochromator. The improved method has been performed to measure the optical signal in the entire spectral range without mechanically scanning the wavelength window during the operation. Instead of using a single grating to diffract the light, a three-grating structure,which has different groove spacing and blaze wavelengths for each grating, was demonstrated in the design. The light diffracted by these three plane gratings will form a folded spectrum, which is imaged on the two-dimensional focal plane of the CCD detector. The effective spectral range imaged on the detector is magnified three folds. By careful design of the system, the optical spectral measurements can be completed in the 200-1000 run wavelength range with high resolution at high speed of less than 0.1 s. The nonlinear relationship between CCD pixel position and wavelength is corrected with multiple polynomial functions in the calibration procedure, which fits the data using a mathematical pattern-analysis method. The instrument can be applied for rapid spectroscopic data analyses in many types of photo-electronic experiments and routine testing.In the telecommunication field, increasing use of the density wavelength-division multiplexing technology had resulted in great advance of the techniques to make the optical thin film device achieve very high quality with a sub-nanometer bandwidth in a nearly perfect square optical gate filtering pattern and work in a wider temperature range reliably. Except of complication in the film-structure design, the key issue in the advanced film deposition process is to precisely monitor and control the optical thickness of each film layer in the high density film stack with the optical parameters being accurately specified. Instead of measuring the light intensity at fixed single wavelength, therefore, more information with the light intensity, bandwidth, and line-shape that depend on many deposition parameters such as the layer thickness, optical constants and stress and so on should be obtained by measuring the spectra of the transmittance and reflectance in situ to control the entire spectral properties of multi-layered film structure accurately. It will be necessary to develop an opticalspectral measurement system that not only will work in a wider spectral range to monitor the entire spectral change in the film deposition process, but also will have a high spectral resolution being finer than the bandwidth of the thin film filter device and as well as have a fast data acquisition and analysis speed to match with the film growth rate in the vacuum condition.In order to achieve both the high spectral resolution and fast data acquisition speed, a new type of the spectrometer working in the infrared optical communication wavelength range (1450-1650 nm) has been studied, designed and constructed. The system was made by five InGaAs infrared line-array detectors and an integrated grating consisting of five sub-gratings. Without any mechanical scanning and moving parts, the full real-time spectral image of the sample can be obtained rapidly and accurately in about 20 ms and with a spectral resolution of about 0.08 nm. By analyzing the spectral features in a fast speed with excellent data repeatability, the system will have the advantage to be used in an advanced film deposition chamber to in-situ monitor and control the spectral properties and the growth process of the multi-layered film with an integral or a fractional quarter-wavelength layer structure precisely.Those works with great effort to study and design the new optical spectrometers have been described in detail to show their useful applications in the future.