Interferometric Control Research Articles

Control and manipulation of solitary structures in a nonlinear optical single feedback experiment

We report on the addressing and control of the lateral positions of optical spatial solitary structures in a single feedback experiment with a saturable Kerr nonlinearity. Solitary structures allow a locally self-confined switching between a dark background and a bright state. This binary character can be of use to all-optically route information in optical networks. In general the lateral positions of solitary structures are strongly influenced by mutual interactions and system inhomogeneities. For potential photonic applications these interactions must be controllable. Therefore a noninvasive interferometric control method based on spatial Fourier filtering is studied in order to position solitary structures. Numerical and experimental results show that solitary structures can be aligned to periodic grids of different scale with hexagonal or square geometries.

The inertial damping of the VIRGO superattenuator and the residual motion of the mirror

The VIRGO superattenuator (SA) is effective in suppressing seismic noise below the expected thermal noise level above 4 Hz. However, the residual mirror motion associated with the SA normal modes can saturate the interferometer control system. This motion is reduced by implementing a wideband (DC–5 Hz) multidimensional active control (the so-called inertial damping) which makes use of both accelerometers and position sensors and of a digital signal processing (DSP) system. Feedback forces are exerted by coil–magnet actuators on the top of an inverted pendulum pre-isolator stage. The residual root mean square motion of the mirror in 10 s is less than 0.1 μm.

Spherical grating monochromator with interferometer control and in-vacuum reference

Physical Science Laboratory’s new generation of spherical grating monochromators incorporates a laser interferometer to control scan angle and an in-vacuum absolute angle reference, as well as other improvements. The design accommodates up to six gratings which can be moved axially (under motor control, with encoder position readback) at any scan angle. The gratings are cooled by means of spring-loaded clamps which conduct heat to a water-cooled plate. The instruments feature hollow roller bearings on the scan axis to minimize bearing runout, and a pseudosine-bar drive for precise control of grating angle. The interferometer angle-measuring optics are mounted inside the vacuum chamber and measure the angle between the grating scan axis and the instrument’s granite base. The laser interferometer measures the grating angle with a resolution of approximately 0.02 arcsec over the entire scan range of 40°. To provide a reference for the interferometer angle measurement, we have built an in-vacuum optical reference which uses custom chrome-on-glass reticles mounted inside the vacuum chamber. Collimated light from a source outside the vacuum passes through the reticles to yield quadrature signals which precisely define an absolute reference angle for the interferometer. Repeatability of the grating angle is within a range of ±0.05 arcsec. Two of these instruments are in operation at SRRC (Taiwan) and a third instrument has been delivered to NSLS (Brookhaven).

Fine Structure of Micro-Groove for the Display System Ruled by Ruling Engine

In recent years, liquid crystal display with less power of the display consumed is very important problem. In this paper, it is denoted that the front light system for emitting the daily light with the ultra-low power consumption type is developed. It is a purpose of this paper to apply to the front light by cutting of the minute groove (optical grating), and scratching on the ITO film. In the experimental method, ultra precise cutting machine called “RULING ENGINE” is used. Micro-groove of high-density marked line is cut on aluminum thin film (evaporated), copper film (plated) and ITO film (sputtered) which are prepared on the glass substrates. Diamond cutter with the profile specially shaped in the cross section is prepared. The cross section of the diamond cutter shape is accurately transferred to the micro-groove. Marked line is succeeded in the groove of 0.4 μm pitch (2500 lines per mm) in the minimum, and of 0.2 mm pitch (5 lines per mm) in the maximum in pitch length. Using Michelson interferometer and feedback control system, the accuracy of the feed step completed 0.078 μm in pitch length. It is shown that characteristics of reflective light distribution by the groove are flat under adaptive condition in front light.

A sample scanning system with nanometric accuracy for quantitative SPM measurements

A sample scanning device operating in a working volume of 30×30×18 μm with interferometer and capacitance-based controls of displacements, is described. The xy-stage uses plane mirror linear interferometers and fast phase-meters for control of displacements of precise ball-bearing stages driven by piezo flexure actuators. The stage operates with a full range bandwidth of 200 Hz, and an estimated accuracy ( k=2) of 3 nm+1×10 −3 L, where L is the lateral displacement. A novel z-stage based on a kinematic coupling between two plates, the upper one being moved by three bimorph plates and the distance being measured by three capacitive sensor, is described. The tilt of the z-stage is kept within fractions of a microrad, leading to a full range estimated accuracy of 2 nm+2×10 −3 h, where h is the vertical displacement. The control bandwidth is of about 1 kHz, thus allowing fast and accurate step-height measurements. In order to test the device used in a scanning probe microscope, micrometric patterned surfaces made using high resolution e-beam lithography and precise metal deposition on silicon are imaged. Results of pitch measurements are discussed and compared with those obtained using optical diffractometry.

Measurement of diffusion fields of solutions using real-time phase-shift interferometer and rapid heat-transfer control system

Abstract A new measurement system for investigation of transient diffusion fields using an accurate interferometer system was developed. This system was composed of a rapid heat-transfer control system and a real-time phase-shift interferometer. The heat-transfer control system had a small test-cell in which the liquid temperature was controlled by non-equilibrium thermoelectric device. In order to investigate pure diffusion fields without double diffusive convection, micorgravity experiments using an airplane were carried out. Thermal and mass diffusion fields of aqueous solutions of NaClO 3 and Ba(NO 3 ) 2 were measured by the system. Thermal and mass diffusion fields were investigated with various temperature conditions. The effect of double diffusive convection on the measurement of diffusion fields was observed. Mass diffusion of NaClO 3 solution in various temperature gradients was measured in normal and micro-gravity conditions. The relation between crystal growth and super cooling was investigated, and no crystal growth was observed at small supersaturation ratio. The result agreed with that obtained using a small rocket experiment.

Precision stabilization of the optical frequency in a large ring laser gyroscope

Pressure-induced fractional changes of 10(-7) in the geometry of a large He-Ne ring laser gyroscope induce backscatter phase changes and thus a fractional pulling of the Sagnac frequency of ~5 x 10(-3). To counter this, the optical frequency was stabilized against an iodine-stabilized laser with a high-finesse Fabry-Perot interferometer and piezoelectric control of the ring perimeter. This scheme, although limited in principle by residual geometric asymmetry and in practice by low beam powers (10 pW), stabilized the perimeter to 2.4 nm (6 x 10(-10) or 300 kHz for the optical frequency) and the Sagnac frequency to 100 parts per million over several days.

Multichannel process monitor for real-time film thickness and rate measurements in dry etching and deposition

The versatile multichannel process monitor system, MPM- X, designed for the online control of plasma and ion beam etching and deposition is introduced. It provides a number of in situ process data like etch rate and selectivity, deposition rate, film thickness, uniformity and endpoint. This is achieved by an integration of optical emission spectroscopy (OES) and optical interferometry, where no external light source is required. By using the plasma itself as a light source for the wafer illumination, a large number of wavelengths, ranging from the deep ultra violet to the near infrared, is available for the interferometric process control. The process control capabilities of the MPM- X system are demonstrated in this paper for various etch and deposition applications.

Experimental investigation of the dynamics of a stabilized nonlinear fiber ring resonator

We investigate theoretically and experimentally the nonlinear dynamics of a synchronously pumped all-fiber passive ring cavity. Our study is based on the use of a specially designed stabilization system that allows for interferometric control of the cavity length. With this system we can achieve stable operation and we are able to perform systematic and reproducible measurements for the characterization of the fundamental nonlinear behaviors of the cavity such as optical bistability, period doubling instabilities, and dissipative modulational instabilities. Through the analysis of the output pulse spectra we show that modulational instability plays a crucial role in the dynamics of the cavity (in particular, in the period-doubling route to chaos) even with normal group-velocity dispersion. A theoretical study of modulational instability in the cavity is presented and is successfully compared with experimental results.

A VME based CCD imaging system for the VIRGO interferometer control

Large interferometric detectors, like VIRGO, are planned to detect gravitational waves, by observing the motions of the suspended mirrors of a Michelson interferometer with kilometric Fabry Perot cavity arms. Due to the large sensitivity needed, the set-up requires a real time monitoring of its mirrors positions, and the analysis of beam shape during the alignment. In this paper, we present the development and the performance of a beam imaging system which uses a CCD camera interfaced to a VME board.

A laser cavity used as an etalon for interferometric control of its own field

A quantitative relationship between the cavity parameters and the angular structure of the emission scattered in a parallel-plane active cavity filled with an arbitrary set of optical elements has been determined. The multipath interference into the active cavity is demonstrated to have properties which differ radically from similar processes in the etalon—among other things the relatively low finesse of the cavity interference rings is independent of the mirror quality and is a function of the Fresnel number only. The use of the operating laser cavity as an etalon for precise control of its own mode wave parameters is discussed.

Design of general accuracy of the dynamic measurement instrument for 3 meter leadscrew by laser interferometric control

The accuracy distribution of measuring instrument for 3-meter leadscrew by laser is studied according to error theory. In this paper, the tables of overall accuracy distribution used as foundation of design and manufacture are drafted.

A writing technique for long fiber Bragg gratings with complex reflectivity profiles

We have developed and demonstrated a novel technique to manufacture very long gratings with a nearly arbitrary reflectivity profile. The technique is based upon writing a set of consecutive subgratings with interferometric control of the relative position of each subgrating.

Digitally controlled interferometer prototype for gravitational wave detection

In this article, we describe the architecture of the 3 m suspended Michelson interferometer prototype for gravitational wave detection which is operational in Napoli. The characteristic which makes this interferometer different from the existing ones is the digital implementation of the control system, the monitoring system, the data acquisition system, and the archiving system. This architecture makes this interferometer a good test bench for the study, the development, and the test of general techniques for the automatic control of interferometers for gravitational wave detection. In particular, it is now being used for the development and the test of some subsystems of the very long baseline interferometric VIRGO antenna for gravitational wave detection. [The Virgo Project, Final Design of the Italian–French large base interferometric antenna Virgo for gravitational wave detection of which the authors are proponents and in whose construction the Authors are collaborating (INFN, Italy, and CNRS, France, 1989, 1992, 1995).]

Reliable fabrication of sub-40 nm period gratings using a nanolithography system with interferometric dynamic focus control

Electron beam nanolithography tools readily permit the fabrication of ultra small structures. However, the disadvantage of such a system is that the depth of field is very small, making it difficult to maintain the focus of the system as the sample is moved due to sample topography, tilt, and stage run out. This presents a serious limitation to nanofabrication. To overcome this problem, we have built and installed a two color, heterodyne common path interferometer on a converted JEOL 100 CXII scanning transmission electron microscope. The interferometer compensates for the very small depth of field of the system by detecting variations in the sample working distance with respect to the electron optical objective lens, and dynamically correcting the lens current. The advantage of this system is that optical access to the sample can be made in a microscope with a very small working distance or sample space. To demonstrate this system, we have fabricated 38 nm period gratings on a silicon substrate. The process was a conventional one using a single 30 nm layer of 350 k molecular weight poly methyl methacrylate resist. The grating was exposed with a linear dose of 1.6 nC/cm and developed in 3:1 IPA:MIBK for 30 s at 23 °C. Patterns were transferred by lift off of an evaporated Ni/Au film. The gratings have a 1:1 mark:space ratio and a line-width variation of 5 nm at 4σ. The stage can be moved over large areas and from chip to chip, and the lithography repeated successfully. This system therefore permits reliable state-of-the-art nanolithography.

Digital error-signal extraction technique for real-time automatic control of optical interferometers

We describe an efficient and robust method for the extraction of the longitudinal error signal for the automatic control of optical interferometers, which can also be applied when the uncontrolled optical system spans hundreds of fringes. The method is based on classic modulation techniques (phase modulation, mechanical modulation, etc.), but extends their performances by the use of the information available only at the output photodiode. We digitally implemented such a method by following modular hardware and software architectures. We then tested the whole procedure in the automatic control of a suspended Michelson interferometer, showing its feasibility and the good performances.

Balancing optical path lengths in broadband fiber interferometers

A simple model is used to show that control of dispersion in an all-fiber stellar interferometer is feasible. From the results we assume that an interferometer control system is available in which both temperature and strain are used to balance the interferometer. Within the restriction of a single polarization mode, it is shown that vacuum path errors before the coupling of starlight into the fibers as well as fiber-length differences and environmental differences between two fibers can be corrected to high order.

Real-time digital control of optical interferometers by the mechanical-modulation technique

We discuss the application of digital systems to the automatic control of dual-wave optical interferometers. We show that, if the mechanical-modulation technique is used for error-signal extraction, digital techniques can be used both for error-signal extraction and for control-signal generation. Therefore, apart from two front/end amplifiers that are necessary to match the dynamics of the detectors and actuators to the dynamics of the analog-to-digital converters and digital-to-analog converters, no other analog devices are required. In particular, the mechanical-modulation technique requires the synchronous demodulation of the photodiode output signal. Hence we need to implement a digital lock-in amplifier whose algorithm is described here. Finally, we describe one of the possible applications of this digital control procedure, such as the control of a classic Mach-Zehnder interferometer in air.

Design of an atomic force microscope with interferometric position control

Advances in the manufacture of integrated circuits, x-ray optics, magnetic read–write heads, optical data storage media, and razor blades require advances in ultraprecision metrology. Each of these industries is currently investigating the use of atomic force microscopy (AFM) to improve the precision and accuracy of their manufacturing process control. To facilitate the use of AFMs for manufacturing we have developed an AFM capable of making accurate dimensional measurements. We call this system the calibrated AFM (C-AFM). The C-AFM has been constructed as much as possible out of commercially available components. We use a flexure stage driven by piezoelectric transducers for scanning, a heterodyne interferometer to measure the X and Y displacements of the sample, a capacitance sensor to measure the Z displacement of the sample, and a commercially available AFM control system. The control system has two feedback loops which read from the X and Y interferometers, respectively, and adjust the piezoelectric voltages to keep the X-Y scan position accurate. The critical electromechanical and metrology issues involved in the construction and operation of such a system will be discussed in detail.

A VME based imaging system for the virgo project

The component test as well as the setting up of the of the large interferometer of the Virgo project requires accurate monitoring of the laser beam position and shape. Such a monitoring should work in real time and should be well integrated to the interferometer control system. In this paper we present a beam imaging system based on a CCD camera interfaced to VME and graphic analysis tools specially developed for this application.