This paper presents the design, manufacture and characterization results of two optical interference filters to be used in the Lightning Imager (LI) optical head on the Meteosat Third Generation (MTG) mission. The first optical filter is a Solar Rejection Window (SRW) to limit the solar thermal radiation absorbed by the optical head while the second optical filter is a Narrow Band Filter (NBF) intended to only pass the lightning discharge emission wavelengths. Each filter has its own distinctive design considerations and manufacturing challenges. The SRW must pass wavelengths from the 760 to 780 nm spectral range and reject (block) wavelengths from the ultraviolet (UV) to the mid-wave infrared (MWIR) and up to 16.3° angle of incidence (AOI) and over a large temperature range (as when exposed to direct sunlight). The NBF is designed to pass only the oxygen emission triplet, centered around the 777.6 nm (vacuum) wavelength and rejecting other wavelengths. Considering the AOI of the light and the temperature excursion, the center wavelength (CWL) uniformity has to be better than 0.04% peak-to-valley (PV) over the 114 mm diameter clear aperture, which is a formidable challenge. We achieved a coating thickness uniformity less than ±0.01% PV, exceeding the prescribed specification. Post-deposition annealing was carried out to tune the bandpass to within pico-meters (pm) of the target CWL value while maintaining the desired CWL uniformity. To ensure that both the SRW and NBF filter meet the desired optical and physical specifications, a comprehensive series of optical and physical characterization tests, along with durability tests, were carried out on each deposition batch.
Over the past decade, tremendous strides have been made in the design, manufacture and measurement of optical thin
film filters. Driven in part from the challenging demands of fiber optic communication (telecom) filters, the
manufacture of optical coatings has advanced significantly through improved optical monitoring technologies and
algorithms; improved deposition technologies; and, very importantly, the ability to fully automate all aspects of the
coating process. This improvement in optical coating technology has since been applied to filters used in other diverse
fields ranging from bio-medical instrumentation to sensors to astronomy. In this paper, advanced optical thin film filters
will be described along with their applications, both in telecom and spectroscopic fields.
Spectroscopic ellipsometry has become one of the most powerful tools of the investigation of the optical properties of thin films. In this paper we show that it enables one to successfully detect even fine properties of high-quality thin films, such as small bulk and surface inhomogeneities. In our experiments, niobium pentoxide films were deposited by ac magnetron sputtering on to quartz substrates with a thickness approximately equal to 500 nm. A J.A. Woolam variable angel spectroscopic ellipsometer was used to measure the ellipsometric parameters of the films over a spectral range from 400 nm to 850 nm. The measured ellipsometric data obtained at the different incidence angles were then analyzed using the OptiChar characterization software. All measurements indicated the presence of a surface micro-roughness in the film, estimated to be around 1 nm. This surface micro-roughness was modeled by a surface overlayer with a 50 percent packing density. The films also possess a small positive bulk inhomogeneity on the order of about 1.5 percent. In the specific case of Nb<SUB>2</SUB>O<SUB>5</SUB> films on a quartz substrate, the ellipsometric angle (Psi) at a 65 degree angle of incidence is the most sensitive to surface and bulk inhomogeneities. The presence of inhomogeneities of both types is clearly seen at certain wavelength points.
A novel high performance thin film polarizing beam-splitter (PBS) is described. This PBS is non-absorbing and has a broad band, a wide angular field and high extinction ratios for both reflected and transmitted beam. Several high efficiency projection displays based on this novel PBS will be presented.
A novel device, the fast-scanning acousto-optic spectrophotometer, was used to measure the in situ reflectance of a thin film during deposition. The reflectance data, which was measured at different film thicknesses and over a wavelength region from 400 to 1100 nm, was used to study the refractive index profile of inhomogeneous ZrO<SUB>2</SUB> films. This data was subsequently analyzed using a homogeneous thin film model to derive an 'effective' refractive index profile.
The successful manufacture of metal/dielectric multilayer systems requires not only very close control of the thicknesses of the individual layers, but also a good knowledge of the optical constants of the materials they are made of. In the case of metal films, it is also essential to know whether any transition layers are formed at the interfaces and, if so, how their thicknesses and optical constants depend on the deposition conditions. Numerical modeling of the metal layers and their interfaces is a powerful tool for the determination of these parameters. To illustrate the method, a bandpass filer and a long-wavelength cut-off filter, both having a low reflectance for light incident on one side, were produced. Excellent agreement has been obtained between the calculated and the measured spectral transmittance and reflectance curves.
A tunable <i>Q</i>-switched thulium-doped single-mode silica fiber laser is presented for the first time. Its performance is analyzed as a function of pulse repetition rate and pumping power. The optimization of laser parameters resulted in 4 W/130 ns, 1.92-μm wavelength pulses generated at a 4-kHz repetition rate. From the measured values of small-signal attenuation and intrinsic saturation power, the thulium ion parameters at the pump and signal wavelength are derived and used in theoretical modeling of the laser. The moderate power required from the Ti:sapphire pump laser used in the experiment suggests that similar resuIts can be obtained using a laser diode pump.
Mo/Si multilayer mirrors with a high reflectance at normal incidence in the 232 - 236 angstrom spectral region have been deposited by rf magnetron sputtering for use in a XUV Ge-laser. The mirrors had a peak reflectance of 26% in this wavelength region. Characterization by TEM and XRD indicates good thickness control in the deposition process and low interface roughness, although interdiffusion is present at the interfaces. Preliminary experiments indicate that the XUV laser output intensity was increased when a multilayer mirror was added to allow a double pass through the gain medium.