A. Grado

High-performance modular digital lock-in amplifier

In this paper we describe an efficient and robust digital implementation of a lock‐in amplifier, based on the classic quadrature technique and on a mathematical algorithm for error signal extraction. Both the hardware and the software architecture are modular. The hardware consists of a VME‐bus (IEEE 1014) standard crate, in which commercial VME boards (the CPU, the ADC and DAC) are housed. The software is written in standard C language for portability and easy integration also within complicated software architectures. The software algorithm implementing the lock‐in amplifier can be particularized by the user on the basis of the needed performances and on the available hardware. Numerical and experimental tests on a lock‐in amplifier prototype have shown that it performs as theoretically predicted. The limit of our prototype (50 kHz maximum sampling rate for 16 bit resolution) depends only on the hardware used, and it is not the present technological limit.

Effects of misalignments and beam jitters in interferometric gravitational wave detectors

Abstract We present a calculation of the phase noise in a recycled interferometer with Fabry-Perot cavities in the arms, induced by the coupling of the geometrical fluctuations of the input laser with geometrical asymmetries of the interferometer. By comparison with the shot-noise limit planned for long-baseline interferometric gravitational wave detectors, upper limits in interferometer misalignments are established.

Effects of misalignment and beam jitter in Fabry-Perot laser stabilization

Abstract We present a first order calculation of the equivalent frequency noise in a Fabry-Perot interferometer, when used as frequency sample for laser stabilization, induced by the coupling of misalignments of the cavity with the various geometrical fluctuations of the input laser beam.

An interferometric device to measure the mechanical transfer function of the VIRGO mirrors suspensions

The design of servoloops for automating alignment and locking of long baseline gravitational wave detectors requires an accurate knowledge of the mechanical transfer function (TF) of the mirror suspensions. We present a suspended interferometric device devoted to the measurement of the TF of the multipendular seismic isolators of the VIRGO antenna.

An optical read-out system for the LISA gravitational reference sensor: present status and perspectives.

INTRODUCTION Since a few years, the LISA group in Napoli has been working to the development of an Optical Read-Out (ORO) system, based on optical levers and position sensitive detectors, for the LISA Gravitational Reference Sensor (GRS). This is intended as a more sensitive extra sensing device, in addition to capacitive readout that is the reference solution already tested on flight by the LISA-Pathfinder mission. The reliability of the proposed ORO device and the fulfillment of the sensitivity goals have been already demonstrated in bench-top measurements and tests with torsion pendulum facilities. In this paper we report on the present status of this activity, presenting the results obtained so-far and the perspectives for the future LISA mission.

A non-linear error signal extraction technique for length control of a Fabry-Perot cavity

We present a non-linear signal extraction technique which provides a signal proportional to the length of a FP cavity with an rms motion larger than one laser wavelength. This method, based on a mechanical or frequency modulation plus a suitable fringe counting technique, can be used in a closed loop configuration allowing to reduce the length variation within one resonance peak of the cavity. This is a starting point for usual linear servo-loops. The principle of operation is described. The results of a computer simulation are confirmed by an experimental test on a bench top interferometer. A possible application is the guidance into lock of a suspended FP cavity, such as those used for long baseline interferometric antennas like VIRGO or LIGO.

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

In this paper we describe the non linear error signal extraction, which is an efficient and robust technique for the automatic control of optical interferometers. It represents a global solution to the problem of the longitudinal error signal extraction also when the uncontrolled optical system spans hundreds of fringes. This technique basically uses classic modulation techniques (phase modulation, mechanical modulation, etc.), but extends their range of validity using also the information available at the output photodiode. We have digitally implemented such technique following modular hardware and software architectures. In fact, the hardware consists of commercial VME boards for A/D and D/A conversion and processing, housed in standard VME crates, while the software algorithm is written in standard C language for portability and easy integration within digital control architectures.

Fiber optic sensors for radiation dosimetry

Fiberoptic sensors are employed for the measurement of many physical quantities. 1,2 Fiberoptic sensors using glass or lead glass optical fibers have been shown in radiation detectors3,4 or dosimeters.5,6 It has been shown7-9 that the main effect of the interaction of ionizing radiation (typically, ? -rays from a Co60 source) with optical fibers is the increase in the specific optical attenuation (dB/m) of the fiber waveguide due to the radio-induced additional formation of color centers.

Fiber‐optic proton beam intensity monitor

The thermally induced change of the birefringence of a high‐birefringence fused‐silica optical fiber, when exposed to a proton beam (2 MeV, 0.5–44 nA, 5‐mm spot diameter), was measured with a fiber‐optic polarimetric sensor. A short length (5 cm) of 125‐μ‐diam, single‐mode, polarization‐maintaining optical fiber was put orthogonally to the beam in vacuo and the linear polarization of a HeNe laser beam launched into the fiber was analyzed by a polarizer and recorded by a photodetector. The response of the sensor both to a continuous and to a modulated proton beam was studied. The photodetector output, which varies with the cosine of the induced relative phase retardance between the fiber eigenmodes, provides a dc or an ac (10 Hz test modulation frequency) measurement of the beam current. This device acts as a proton beam intensity monitor, with minimum beam perturbation and a sensitivity of 0.1 nA. Its possible use also as a beam position monitor or as a wire‐scanner beam profiler is suggested.