Folding Mirror Research Articles

The effect of a scanning flat fold mirror on a cosmic microwave background B-mode experiment

We investigate the possibility of using a flat-fold beam steering mirror for a cosmic microwave background B-mode experiment. An aluminium flat-fold mirror is found to add ∼0.075% polarization, which varies in a scan synchronous way. Time-domain simulations of a realistic scanning pattern are performed, and the effect on the power-spectrum illustrated, and a possible method of correction applied.

Spectral ripple effect in continuous-wave fold-type cavity ring down spectroscopy

A spectral ripple effect observed in fold-type continuous-wave cavity ring down spectroscopy is described and studied. Its period was measured as ∼0.02 cm−1 in a 49.1 cm long V-shaped glass ceramics cavity. To find its origin, various losses of high-reflectivity coating and their relations to the optical field in the V-shaped cavity, especially the optical field inside the folding mirror coating, have been discussed and analyzed according to the interference theory and the matrix calculation method. It is found that the ripple mainly comes from an interference effect at the folding mirror coating between the direct and the inverse light waves. The interference effect results in the periodic fluctuations of the scattering and absorption loss of the high-reflectivity coating, so it can be used to measure some special losses of the high-reflectivity coating and separate them from the total loss of the coating.

Application of the extended Jones matrix formalism for higher-order transverse modes to laser resonators

The extension of the Jones matrix formalism to higher-order transverse modes using N x N matrices presented in a previous paper [8] is applied to laser resonators. The resonator discussed in detail has a TEM(01)* Hermite-Gaussian mode, an axially symmetric polarizer combined with an axially symmetric phase shifter as a rear mirror and a folding mirror with conventional polarization dependent reflectivity and phase shift. The analysis reveals some useful regimes, where the output polarization is close to radial or azimuthal and the sensitivity to variations in the phase shift of the folding mirror is minimized.

Phase control of double-pass cascaded SHG/DFG wavelength conversion in Ti:PPLN channel waveguides

The efficiency of wavelength conversion by cascaded second harmonic generation / difference frequency generation (cSHG/DFG) in Ti:PPLN waveguides can be considerably improved by using a double-pass configuration. However, due to the wavelength dependent phase change by the dielectric folding mirror phase compensation is required to maintain an optimum power transfer. We experimentally investigated three different approaches and improved the wavelength conversion efficiency up to 9 dB in comparison with the single-pass configuration.

Debris mitigation techniques for petawatt-class lasers in high debris environments

Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185-1193, USA(Received 13 November 2009; published 20 April 2010)This paper addresses debris mitigation techniques for two different kinds of debris sources that arefoundinthehigh-energydensitycommunity.Thefirstdebrissourcestemsfromthelaser-targetinteractionand this debris can be mitigated by avoiding a direct line of sight to the debris source (e.g. by using asacrificial fold mirror) or by inserting a thin debris shield. Several thin film debris shields have beeninvestigated and nitrocellulose was found to be the best suited. The second debris source originates froman external high-energy density driver or experiment. In our specific case, this is the Z accelerator, aZ-pinch machine that generates 2 MJ of x rays at 300 TW. The center section of the Z accelerator is anextremely violent environment which requires the development of novel debris mitigation approaches forbacklighting with petawatt lasers. Two such approaches are presented in this paper. First, a self-closingfocusing cone. In our facility, the focused beam on target is fully enclosed inside a solid focusing cone. Inthe first debris mitigation scenario, the last part of the cone has a ‘‘flapper’’ that should seal the conewhenthe pressure wave from the Z-pinch explosion hits it. In the second scenario, an enclosed target assemblyis used, with the last part of the focusing cone connected to a ‘‘target can’’ which houses the laser target.The laser produced x rays for backlighting escape through a 3 mm diameter hole that is protected by anx-ray filter stack. Both techniques are discussed in detail and have been successfully tested on the Zaccelerator.

The presence of a vocal ligament in fetuses: a histochemical and ultrastructural study

Although it is currently believed that the vocal ligament of humans undergoes considerable development postnatally, there is no consensus as to the age at which it first emerges. In the newborn infant, the lamina propria has been described as containing a sparse collection of relatively unorganized fibres. In this study we obtained larynges from autopsy of human fetuses aged 7-9 months and used light and electron microscopy to study the collagenous and elastic system fibres in the lamina propria of the vocal fold. Collagen fibres were viewed using the Picrosirius polarization method and elastic system fibres were stained using Weigert's resorcin-fuchsin after oxidation with oxone. The histochemical and electron microscopic observations were consistent, showing collagen populations with an asymmetric distribution across different compartments of the lamina propria. In the central region, the collagen appeared as thin, weakly birefringent, greenish fibres when viewed using the Picrosirius polarization method, whereas the superficial and deep regions contained thick collagen fibres that displayed a strong red or yellow birefringence. These findings suggest that the thin fibres in the central region consist mainly of type III collagen, whereas type I collagen predominates in the superficial and deep regions, as has been reported in studies of adult vocal folds. Similarly, elastic system fibres showed a differential distribution throughout the lamina propria. Their distribution pattern was complementary to that of collagen fibres, with a much greater density of elastic fibres apparent in the central region than in the superficial and deep regions. This distribution of collagen and elastic fibres in the fetal vocal fold mirrors that classically described for the adult vocal ligament, suggesting that a vocal ligament has already begun to develop by the time of birth. The apparently high level of organization of connective tissue components in the newborn is in contrast to current hypotheses that argue that the mechanical stimuli of phonation are essential to the determination of the layered structure of the lamina propria and suggests that genetic factors may play a more significant role in the development of the vocal ligament than previously believed.

FanCam—A Near-Infrared Camera for the Fan Mountain Observatory

ABSTRACTThe Fan Mountain Near-Infrared Camera (FanCam) provides the University of Virginia’s 31 inch telescope with a near-infrared observing capability that is relatively unique among small-aperture telescopes. FanCam features an 8.7′ × 8.7′ field of view on a 1024 × 1024 Teledyne Imaging Sensors HAWAII-1 detector array. The instrument mounts at the f/15.5 focus of the 31 inch telescope. Its seeing-limited optical design, optimized for the JHK atmospheric bands, includes a field stop at the telescope focus, a doublet collimator, two eight-position filterwheels straddling a Lyot stop, and a doublet reimager. Four fold mirrors wrap the optical path onto a compact optical bench. The plate scale leads to a slightly oversampled point spread function for the typical seeing of 1.5″. The entire optical train is encased in a cryogenic dewar cooled by a closed-loop cooling system. This paper describes the optical, mechanical, cryogenic, and electronic design of the camera as well as some early results that illustrate the camera’s capabilities.

Calibration facility for airborne imaging spectrometers

A new facility designed to perform calibration measurements of airborne imaging spectrometers was established at the German Aerospace Center (DLR) in Oberpfaffenhofen. This Calibration Home Base (CHB) is optimized to characterize radiometrically, spectrally, and geometrically the APEX (Airborne Prism Experiment) imaging spectrometer, which is currently being developed under the authority of the European Space Agency (ESA). It however can be used for other optical sensors as well. Computer control of major laboratory equipment allows automation of time consuming measurements. In APEX configuration (wavelength range: 380 to 2500 nm, instantaneous field of view: 0.48 mrad, field of view: ±14 ∘) spectral measurements can be performed to a wavelength uncertainty of ±0.15 nm, geometric measurements at increments of 0.0017 mrad across track and 0.0076 mrad along track, and radiometric measurements to an uncertainty of ±3% relative to national standard. The CHB can be adapted to similar sensors (including those with thermal infrared detectors) by exchanging the monochromator’s lamp, the gratings and the filters, and by adjusting the distance between the sensor and folding mirror.

Uniform angular resolution integral imaging display with boundary folding mirrors

Uniform angular resolution integral imaging display is proposed. Conventionally, inefficient bias of the perspectives around boundaries is disregarded and it is impossible to display the field of view over full spatial resolution. However, the proposed display has four boundary folding mirrors and these mirrors fold the view volumes correctly to form uniform angular resolution within them. The distribution of perspectives and field of view in the proposed system are analyzed and the removal of this boundary effect is confirmed experimentally.

Experimental study of multipass copper vapour laser amplifiers

Repetitively pulsed multipass copper vapour amplifiers are studied experimentally. A considerable increase in the peak power of laser pulses was achieved by using a special scheme of the amplifier. It is found that the main reasons preventing an increase in the peak power during many passages of the beam are the competitive development of lasing from spontaneous seeds in a parasitic resonator formed by the fold mirrors of a multipass amplifier, a decrease in the amplification during the last passages, and an increase in the pulse width at the amplifier output.

Interference and diffraction effects in folding mirror schlieren diffraction interferometer

Experimental investigations on interference and diffraction phenomenon occurring in a folding mirror schlieren diffraction interferometer have been presented. It is shown that the well-known Lloyd mirror interferometer can easily be converted into a folding mirror schlieren diffraction interferometer. Various intermediate stages occur as the basic two-beam interference pattern in Lloyd mirror interferometer is transformed into a schlieren diffraction pattern. Study of these intermediate stages gives a good insight in understanding the transition behaviour between the fundamental phenomenon of interference and diffraction taking place as the mirror-edge approaches the Airy disk.

Efficient multi-folded Nd:YVO_4 slab amplifier

A high-efficiency multi-folded Nd:YVO(4) slab amplifier is reported. Optimizing the mode overlapping carefully, high extraction efficiency was achieved using the wedged multi-folded configuration with two planar folding mirrors. The amplifier's performance under different seed powers and different pulse repetition frequencies (PRF) is presented. The 52 W seed at the PRF of 152 kHz was amplified to 101.75 W with the pump power of 92 W, and the corresponding extraction efficiency is 54.3%. And no remarkable deterioration of the beam quality was observed.

Interferometric key readable security holograms with secrete-codes

A new method is described to create secrete-codes in the security holograms for enhancing their anti-counterfeiting characteristics. To imitate these codes is difficult as pure phase objects having complex phase distribution function are used to modulate the object beam that is recorded in conjunction with an encoded interferometric reference beam derived from a key hologram. Lloyd’s folding mirror interferometer is used to convert phase variations of the reconstructed wave-front into an intensity pattern for hologram authenticity verification. Creating the secrete-codes through an interferometric reference beam from the key hologram facilitates a multi-stage authenticity verification as well as easy repositioning of the security hologram through a specific Moire pattern generated during the verification process.

Folding mirror schlieren diffraction interferometer

We demonstrate the use of a mirror as a viewing diaphragm to generate a built-in diffracted reference beam in schlieren diffraction interferometry (SDI). The use of a mirror edge as a diffracting element instead of a conventional knife edge considerably enhances the contrast of the schlieren pattern, and it is shown to be equal to that of a phase knife edge. This increase in contrast is due to the fact that the otherwise unutilized diffracted beam in SDI is recombined in the described folding mirror geometry.

Stimulation of visible luminescence by irradiation of a novel phosphor screen with an infrared beam

A handheld gas detector is being developed that consists of a laser diode emitting light of wavelength equal to 1651 nm and an extended InGaAs detector. The IR laser beam is double-passed through a column of air via a folding mirror to a detector. The air column is in contact with the outside atmosphere so that low-relative-molecular-mass hydrocarbon gases, which diffuse into the air column, can be detected by the absorption of some of the IR light. The absorption at around 1651 nm is due to the overtones of C—H stretching vibrations and the limit of detection of the technique is at the parts per million level. One of the problems of using such a device in the field is that the operator cannot visualize the IR beam directly. Consequently, if the gas detection device fails to give a reading, the operator cannot be sure whether the laser is emitting light and the beam is correctly aligned. We describe the use of a novel phosphor screen to visualize the beam by converting the IR radiation into visible light. The properties of a number of phosphors that were developed and tested for the visualization of the beam are described. The merits of storage phosphors are compared to those of upconversion phosphors for use at this wavelength, which is longer than wavelengths previously visualized using upconversion phosphors.

Q-switched mode locking with acousto-optic modulator in a diode-pumped Nd:YVO4 laser

The Q-Switched Mode Locking (QML) regime provides the generation of relatively high peak power picosecond pulses train with energies of a few muJ each in a simple resonator. The fully modulated efficient QML regime was demonstrated in the diode-pumped Nd:YVO4 laser. The acousto-optic cell playing a double role of Q-switch and Mode Locker was located near flat output coupler. The two folding mirrors were mounted on the translation stages for matching the resonance frequency of the cavity to the radio frequency of acousto-optic modulator. The QML pulses with envelope durations of 100-150 ns and near 100% modulation depth were observed for wide range of pump powers and repetition rates. Up to 3 W of output average power, 100 muJ of the envelope energy, having approximately 5-8 mode-locked pulses were achieved.

Study on the possibilities of controlling the laser output beam properties by an intracavity deformable mirror

AbstractThe possibilities of controlling the laser beam properties by a deformable mirror introduced into the laser optical cavity were studied theoretically and experimentally. The experiments were performed under conditions of an industrial high power transverse flow cw CO2 laser operating with a stable resonator of a folded configuration. A deformable bimorph mirror of a surface profile controlled by the voltage applied to the mirror electrodes is implemented to the laser system as a back cavity mirror or as a one of the inner folding mirrors. The near-and far-field characteristics of the laser beam versus the resonator configuration controlled by the changes of the focal length of the deformable mirror are discussed in the paper. The analysis reveals that the resonator with an inner deformable mirror is much more sensitive to the mirror curvature variations than the resonator in which the deformable mirror is used as a back cavity mirror. The presented results show that dynamic and controllable changes in the resonator properties result in the controlled modification and optimisation of the laser output power and spatial parameters of the laser radiation.

직경 300 mm 광집속장치의 광학정렬

우리는 국내에서 처음으로 직경 300 mm인 광집속장치를 정렬 및 조립하였다. 이 장치는 주경, 부경 그리고 5개의 연결 거울로 구성된다. 이중에서 가장 중요한 부품인 주경은 각 구조물들과의 조립때마다 발생할 수 있는 변형을 정밀 측정하였다. 또한 사용되는 거울의 개수가 많기 때문에 광학정렬에는 체계적인 정렬 알고리즘을 도입하였다. 최종 조립결과 파면수차는 1.9 wave rms(wave =633 nm)로 예상치의 7배정도 큰 값이었다. 이것의 주원인은 조립 과정에서 발생한 연결 거울들의 변형인 것으로 확인되었다. 본 광집속장치를 정렬하면서 발생했던 문제점들에 대한 고찰은 향후 대형광학계를 만들 때 유용하게 사용될 수 있다. We assembled the optical beam director with diameter 300 mm. This consists of primary, secondary mirrors and 5 folding mirrors. Among them, the primary mirror is the most important component so that we measure any possible deformation on it at every step of assembly. Also, we developed the systematic alignment algorithm, which is essential because the number of mirrors is 7. The final wavefront error of the system is 1.9 wave rms (wave=633 nm) which is 7 times larger than we expected. The main source is the deformation of the 131ding mirrors. We expect that what we have learned from the assembly of this system would be helpful when we deal with a larger system in the future.

Design of a Superparaboloidal Solid Immersion Mirror for Optomechatronic Near-Field Recording

We designed a superparaboloidal solid immersion mirror (SP-SIM) using a tilted parabolic mirror that applied total internal reflection to an optical flying head for near-field recording. As an SP-SIM does not need an objective lens or folding mirror, the height and weight of the optical flying head can be reduced. Using theoretical ray tracing, we found the optimal parameters for rotational symmetric beam intensity on a high-numerical-aperture (NA) focus. The effective NA of the SP-SIM was 1.32 for both the sagittal and meridional focuses. A 408-nm laser source and high refractive index material, NbFD13, was used for the simulation. The minimum spot sizes at full width 1/e/sup 2/ intensity were 237 and 232 nm in the sagittal and meridional directions, respectively, at normal incidence. The size of the beam spot was kept nearly uniform within a maximum 5% deviation as the field angle changed within /spl plusmn/0.15/spl deg/. The SP-SIM measured 0.83 mm/spl times/0.95 mm/spl times/0.27 mm (W/spl times/L/spl times/H) and weighed ca. 0.6 mg.

Stray light modelling and analysis for the FY‐2 meteorological satellite

Based on the analysis of the mechanism of stray light of the FY‐2 meteorological satellite, the main component of the stray light, generated from the folding mirror direct reflections, is modelled physically. Through the analysis of the stray light outside of the Earth region as well as extracting the high‐order statistical eigenvalues of the designated areas, the stray light function matrix, A, can be calculated using the total least squares (TLS) method, which is applied to the inside of the Earth region, and the full field‐of‐view (FFOV) stray light estimation can be produced efficiently. It is shown that the mean restoration errors of the stray light outside of the Earth region of the infrared (IR), water vapour (WV) and visible (VIS) channels are less than two scaled units, while the visual images are improved greatly. The images of FY‐2B IR with stray light removed and the FY‐2B raw images are inter‐calibrated with the NOAA‐17 Advanced Very High Resolution Radiometer (AVHRR) Ch4 data. The results show that, from 180 to 300 K of the brightness temperature area, the standard deviation of the data with stray light removed is from 1.3 to 3.8 K relative to the NOAA‐17 data, which is improved by 3–10 times in calibration accuracy compared with the raw data inter‐calibration results. The model has been running for testing in the FY‐2B ground operational system and will be applied operationally to the follow‐up satellite system. With the adjustment of a few parameters, the principle of the model can be used for stray light analysis of other instruments on geostationary satellite platform.