Thin-film Beam Splitter Research Articles

650 GHz imaging as alignment verification for millimeter wave corneal reflectometry.

A system concept for online alignment verification of millimeter-wave, corneal reflectometry is presented. The system utilizes beam scanning to generate magnitude-only reflectivity maps of the cornea at 650 GHz and compares these images to a precomputed/measured template map to confirm/reject sufficient alignment. A system utilizing 5 off-axis parabolic mirrors, a thin film beam splitter, and 2-axis galvanometric mirror was designed, simulated, and evaluated with geometric and physical optics. Simulation results informed the construction of a demonstrator system which was tested with a reference reflector. Similarity metrics computed with the aligned template and 26 misaligned positions, distributed on a 0.5 mm x 0.5 mm x 0.5 mm mesh, demonstrated sufficient misalignment detection sensitivity in 23 out of 26 positions. The results show that positional accuracy on the order of 0.5 mm is possible using 0.462 mm wavelength radiation due to the perturbation of coupling efficiency via beam distortion and beam walk-off.

Material ejection and layer peeling-off in HfO2/SiO2 thin-film beam splitters induced by 1ω and 3ω lasers

Nanosecond pulse laser-induced damage is associated with nodular defects and nano-precursors in hafnium oxide (HfO2)/silica (SiO2) multilayer films. In this study, the dynamic evolution of material ejection and layer peeling-off in HfO2/SiO2 thin-film beam splitters used for third harmonic separation were examined by time-resolved shadowgraph technique under the fundamental frequency of 1064 nm (1ω) and third harmonics 355 nm (3ω) laser beam, respectively. The initial material ejection and subsequent peeling-off of layers were studied to clarify most thin films damage events caused by 1ω and 3ω lasers. The peeling-off of films under 1ω laser was mainly caused by thermomechanical effect. In contrast, the peeling-off of films under 3ω laser was mainly affected by laser-induced mechanical damage. The different transmission forms of layers peeled off from the films revealed the influence of mechanical strength and thermal strength on damage progression. The Taylor–Sedov theoretical model was used to analyze the propagation characteristic shock waves after nanosecond laser irradiation of films. In summary, identification of propagation forms and speed of peeling-off layers provides helpful insights into the damage mechanisms during nanosecond laser-induced damage of optical thin films.

Polarization-Insensitive Optical Receiver Module Based on Thin-Film Beam Splitters

Optical receiver modules play an important role in receiving signals from the Fiber Bragg Grating (FBG) sensors used for monitoring in civil structures, power facilities, and other sensing applications. When multiple sensors are used, more photo diodes receive optical signals; thin-film beam splitters with polarization characteristics are therefore used to reduce the size of the optical receiver module. Using a reflective semiconductor optical amplifier (R-SOA) with polarization characteristics in a sensing system with an FBG sensor, a structure that is less sensitive to polarization was developed by packaging a quad optical receiver module. A stabilization of ±4.67% was achieved using the dual optical receiver module in polarization and of ±1.20% with the quad optical receiver module in polarization.

Idea of thin-film beam splitters for two-colour beamlines.

The idea to split soft X-ray and hard X-ray beams on the same axis into different directions is presented. A thin-film beam splitter can serve this purpose. Performances of thin-film beam splitters made of carbon and aluminium were examined. The idea of thin-film beam splitters expands possibilities in the design of two-colour beamlines.

Multiple THz pulse generation with variable energy ratio and delay

Two methods for multiple high energetic THz pulse generation by two-color filamentation in air with controllable energy ratio and delay ranging from one to hundreds of ps were investigated. In the first method the laser pulse is split into two inside the optical stretcher of a CPA laser system, the resulting consecutive filaments occur in the same region and allows the study of the influence of the first plasma filament on the THz emission of the delayed filament. Based on a polarization sensitive thin film beam splitter placed in front of a 45° mirror, the second method produces multiple parallel consecutive filaments. Above a certain total pump level the THz energy delivered by multiple pulses exceeds the value given by a single filament for the same pump energy, thereby overcoming the THz emission saturation of the single filament.

Thin film beam splitter multiple short pulse generation for enhanced Ni-like Ag x-ray laser emission.

An alternative, novel multiple pulse generation scheme was implemented directly after the optical compressor output of an x-ray pump laser. The new method uses a polarization sensitive thin film beam splitter and a half-wavelength wave plate for tuning the energy ratio in the multiple short pulses. Based on this method, an extensive study was made of the running parameters for a grazing incidence pumped silver x-ray laser (XRL) pumped with a long pulse of 145mJ in 6ns at 532nm and up to 1.45J in few picoseconds at 810nm. Fivefold enhancement in the emission of the silver XRL was demonstrated using the new pump method.

An extreme ultraviolet Michelson interferometer for experiments at free-electron lasers

We present a Michelson interferometer for 13.5 nm soft x-ray radiation. It is characterized in a proof-of-principle experiment using synchrotron radiation, where the temporal coherence is measured to be 13 fs. The curvature of the thin-film beam splitter membrane is derived from the observed fringe pattern. The applicability of this Michelson interferometer at intense free-electron lasers is investigated, particularly with respect to radiation damage. This study highlights the potential role of such Michelson interferometers in solid density plasma investigations using, for instance, extreme soft x-ray free-electron lasers. A setup using the Michelson interferometer for pseudo-Nomarski-interferometry is proposed.

Dual-wavelength investigation of laser-induced damage in multilayer mirrors at 532 and 1064 nm

Thin film beam splitters with high reflectivity at 532nm and high transmittance at 1064nm were deposited via reactive electron-beam evaporation with optimized parameters. The damage performance of the samples was investigated under irradiations of 532nm laser only, 1064nm laser only, and various combined laser fluences. The damages induced by the 1064nm laser were primarily attributed to the initiators at the interface between the coatings and substrate. Under 532nm laser irradiation only, two distinctive damage pits initiated by the submicron absorptive defects located at different coating depths and correlated to interfaces were observed. The damage effect under simultaneous irradiation in multilayer films was also investigated. The respective sensitive defects of the two lasers remained the precursors for causing damage. However, the dominant damage factors in simultaneous irradiation changed with the 1064nm laser fluence, which also determined the coupling effect between the two lasers in terms of causing damage. Finally, correlative analysis methods were used to discuss the different coupling effects.

Damage performance of thin-film beam splitter for third harmonic separation under simultaneous exposure to 1 ω and 3 ω pulses

Laser damage behaviors of thin-film beam splitters for third harmonic separation were investigated under the simultaneous exposure of fundamental frequency (1 ω) and the third harmonics laser beam (3 ω). The samples were prepared by conventional e-beam evaporation, and the surface laser-induced damage thresholds (LIDT) of the samples were obtained for the irradiations of 3 ω only and 3 ω with various fluence of 1 ω. It was found that external 1 ω can raise damage probability of multilayer films; the higher the 1 ω energy density, the lower the damage threshold. Additionally, numerical simulation of damage probability was given for the simultaneous exposure to two beams.

Ultrabroadband beam splitter with matched group-delay dispersion

We present a general design strategy for a broadband thin-film beam splitter with matched group-delay dispersion. By taking the substrate dispersion into account in the coating design, any combination of input and output can show the same dispersion for transmission and reflection. As a specific implementation, an ultrabroadband 50:50 beam splitter from 600 to 1500 nm for femtosecond laser applications was designed, fabricated, and characterized.

Integrated polarization diversity front end optics

The integration of a polarization-diversity receiver front-end suitable for balanced detection has been realized by the hybrid combination of a 3-dB directional coupler and a thin-film polarization beam splitter (PBS) fabricated on silica-based waveguides. Insertion losses, excluding the intrinsic loss of the coupler, were 1 and 1.2 dB for TE- and TM-polarizations, respectively. The polarization cross talk was less than -23 dB. >

Mirau correlation microscope

We have constructed a correlation microscope based on the Mirau interferometer configuration using a thin silicon nitride film beam splitter. This microscope provides the amplitude and phase information for the reflected signal from a sample located on the microscope-object plane. The device is remarkably insensitive to vibrations and is self-correcting for spherical and chromatic range aberrations of the objective. An imaging theory for the correlation microscope has been derived, which predicts accurately both the transverse resolution at a sharp edge and the range resolution for a perfect plane reflector. The range resolution is slightly better than that for a scanning optical microscope using a lens with the same aperture.

Correlation microscope

We have constructed a correlation microscope based on the Mirau interferometer configuration using a thin silicon nitride film beam splitter, and we have developed a method to extract the amplitude and phase information of the reflected signal from a sample located at the microscope object plane. An imaging theory for the interference microscope has been derived, which predicts accurately both the transverse response at a sharp edge and the range response to a perfect plane reflector.

Division-of-wave-front thin-film beam splitter for generating binary patterns of orthogonal elliptical polarization states

A division-of-wave-front thin-film beam splitter is described that reflects monochromatic light at oblique incidence with orthogonal elliptical polarization states. It consists of a metallic substrate partially covered with a transparent thin film that inverts the ratio ρ of the complex p and s reflection coefficients at the principal angle of the metal. Any pattern of coated and uncoated areas of the substrate is imprinted upon the reflected wave front as a corresponding two-dimensional spatial binary polarization pattern. A specific design is given that uses a Au substrate at a wavelength of 632.8 nm. The effects of small errors in the film refractive index, the film thickness, and the angle of incidence are discussed. It is noted that a layer that inverts ρ at a certain (especially high) angle of incidence is an effective ρ-inverting layer at all angles.

Thin-film beam splitter that reflects light as a half-wave retarder and transmits it without change of polarization: application to a Michelson interferometer

The refractive index n1 of a transparent layer of quarter-wave optical thickness coating a transparent substrate of refractive index n2 can be chosen to produce half-wave retardation (HWR) in reflection and no change of polarization in refraction at any angle of incidence ϕ. The function n1(ϕ, n2), and the associated polarization-independent reflectance of the film-substrate system R(ϕ,n2) are determined. Such a coated surface can be used as a beam splitter with excellent characteristics (e.g., split fractions that do not depend on source polarization, a split beam whose polarization is identical to that of the incident beam and operation over a wide range of incidence angles). A concrete example of a coated Ge-slab beam splitter for 10.6-μm radiation at ϕ = 45° is given. The beam-splitter face of the slab is coated with the HWR layer, and the exit face is coated with a double layer that produces total refraction without change of polarization. Such a beam splitter is tolerant to film-thickness errors and is reasonably achromatic over a small (e.g., 10–11-μm) wavelength range. When used in a Michelson interferometer this beam splitter renders its operation totally independent of source polarization.

The problem of nonlinear phase errors introduced by misalignment of a Michelson interferometer

In Fourier transform spectroscopy, a slight angular misalignment or canting of one of the mirrors in the Michelson interferometer causes not only a reduction of the oscillatory part of the interferogram but may also lead to an asymmetry. In order to elucidate the reasons for this effect, we performed calculations for a Michelson interferometer including the effects originating from a source of finite area and from a finite aperture. Our treatment also takes into account the effect of a thin film beam-splitter in the parallel beam with a relatively large angle of incidence. The beam-splitter may be considered a dispersive medium in the sense that its efficiency varies strongly with frequency. The essential results of our calculations are that nonlinear phase errors arise under the considered conditions depending on the product of the angle of mirror misalignment and the solid angle subtended by the source from the collimator mirror. Experimentally, we studied the occurrence of asymmetric interferograms and nonlinear phase errors due to misalignment for three different types of interferometers. In the case of lamellar grating interferometer, a misalignment of one of the cam systems caused only a symmetric reduction of the oscillatory part of the interferogram. A similar symmetric reduction with no asymmetry was observed for a two-beam interferometer with the beam-splitter placed at a focus. For a Michelson interferometer with a thin film beam-splitter in the parallel beam (Polytec FIR 30), however, a mirror misalignment of a few hundredths of a degree caused asymmetries in the interferogram of as much as 30%, in satisfactory quantitative agreement with our theoretical approach.

Self-supporting thin-film beam splitter for far-infrared interferometers

Commercially available plastic films have been examined, in the spectral range up to 1200 cm−1, for use as a beam splitter in far-infrared Michelson interferometers. The materials reported include polyethylene terephthalate, polycarbonate, polyvinylchloride, polypropylene, cellulose and polyethylene.

Thin-film beam splitter for integrated optics

A simple thin-film beam splitter has been made by sputtering techniques. The device is similar in principle to beam splitters in bulk optics based on frustrated total reflection. It depends on a slow variation of guide thickness that permits the existence of an evanescent wave by which light is transferred from one side of the beam splitter to the other. The tunneling region has a variable index profile and there are no discontinuities. Fabrication techniques, theory, and experimental results are presented.

A Discussion of Nonpolarizing Angles in Thin-Film Beam Splitters for Michelson Interferometers

We have investigated the polarization properties of thin-film beam splitters for a wide range of refractive indices and angles of incidence. We have found that the polarizance is zero at certain incident angles for one or two wavelengths in each cycle of the beam splitter. For any angle other than the Brewster angle, however, the polarizance varies significantly over the remainder of the usable wavelength range and at the high angles of incidence required to achieve a nonpolarized wavelength the variation is quite large.

Study of soft mode behaviour in KDP by dispersive fourier transform spectroscopy

In recent years infrared spectroscopy has been widely used to study soft mode behaviour in ferroelectric crystals, the usual approach being to measure the power reflectivity over a wide spectral range, calculate the phase spectrum using a Kramers-Kronig analysis, and hence compute the dielectric functions. The difficulties of this approach are well known but can only be avoided by measuring the phase spectrum directly by dispersive Fourier transform spectroscopy (DFTS). However, DFTS is not yet widely used because of the difficulty of measuring the phase spectrum as a function of temperature with sufficient accuracy.We have developed techniques for measuring the amplitude and phase reflection spectra of suitable solids by DFTS at temperatures down to 77 K. Satisfactory measurements can be made in the frequency range from 5 to 500 cm-1 using instruments equipped with either dielectric thin film or wire grid beam splitters. The phase spectrum can be determined with an error of less than 1° in the region of ...