Advanced Virgo is the project to upgrade the Virgo interferometric detector of gravitational waves, with the aim of increasing the number of observable galaxies (and thus the detection rate) by three orders of magnitude. The project is now in an advanced construction phase and the assembly and integration will be completed by the end of 2015. Advanced Virgo will be part of a network, alongside the two Advanced LIGO detectors in the US and GEO HF in Germany, with the goal of contributing to the early detection of gravitational waves and to opening a new window of observation on the universe. In this paper we describe the main features of the Advanced Virgo detector and outline the status of the construction.

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Advanced Virgo: a second-generation interferometric gravitational wave detector

The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

Modern research in atmospheric chemistry requires highly sensitive techniques for the measurement of concentrations of free radicals which determine the rate of photochemical destruction of most atmospheric pollutants. Tunable diode-laser absorption spectroscopy (TDLAS) has already been successfully used for measurements of very low concentrations of stable gases, but further improvement in its sensitivity by signal averaging has been limited by the stability of the instrument. In this paper the concept of the Allan variance is utilized to analyze the stability of an existing frequency-modulated (FM) TDLAS instrument leading to a detection limit for NO2 of 34 pptv at 6 Hz detection bandwidth. The stability of the instrument allows averaging over 60 s. Taking into account the measuring cycle consisting of the determination of the sample spectra and zero air spectra as well as gas exchange in the absorption cell, the detection limit achievable with this particular instrument was 10 pptv within 25 s under laboratory conditions. Possibilities of further improvement of the detection limit are discussed.

The limits of signal averaging in atmospheric trace-gas monitoring by tunable diode-laser absorption spectroscopy (TDLAS)

A variety of frequency-modulation methods for high-sensitivity absorption detection of gas-phase species has evolved in recent years. The distinctions among these methods are mostly semantic. The mathematical derivations for wavelength-modulation spectroscopy and one- and two-tone frequency-modulation spectroscopies are presented; a common terminology is used to permit a comprehensive comparison of predicted detection sensitivities. Applying this formalism, I compare the optimum detection sensitivities of these different methods for a typical laser system, using the same parameters. As long as residual amplitude modulation is minimized by proper adjustment of the detection phase angle, high-frequency wavelength modulation and one- and two-tone frequency-modulation methods all achieve approximately the same sensitivities. The choice among techniques is most strongly driven by the individual laser tuning characteristics, the absorption linewidth, and the detection bandwidth. It is shown that excess laser noise cannot always be excluded from consideration, even at megahertz detection frequencies. Also, detection at harmonics of the modulation or beat frequency may present certain advantages in minimizing residual amplitude-modulation noise.

Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods

Near- and mid-infrared laser-optical sensors for gas analysis

An important practical limitation to the sensitivity of frequency-modulation spectroscopy arises because of imperfect phase modulation of the laser beam. This imperfection manifests itself as residual amplitude modulation (RAM), and we present data from a careful series of experiments designed to elucidate its origin. Experimentally, we find two components to the RAM, one that depends for its intensity on the laser frequency and one that does not. We present a model that correctly accounts for the intensity and rf phase and frequency behavior of the laser-frequency-dependent component. We are unable to offer a plausible explanation for the laser-frequency-independent component.

Residual amplitude modulation in laser electro-optic phase modulation

Frequency-modulation (FM) spectroscopy permits high-resolution, high-sensitivity, easily calibrated absorption measurements of atomic and molecular species and narrow spectral features in solids. This paper reviews some important developments in laser FM spectroscopy, from its inception as a spectroscopic tool to the demonstration of quantum-limited absorption measurements, emphasizing the sensitivity limitations caused by residual amplitude modulation (RAM). Moreover, a detailed account is presented of a new double-beam, single-detector technique that efficiently suppresses the RAM and permits quantum-limited performance to be achieved in laser FM spectroscopy. We also include some recent results of the first reported FM spectroscopic investigations of the NO2 molecule.

Quantum-limited laser frequency-modulation spectroscopy

Ultrasensitive absorption spectroscopy of NO2 was performed with a tunable lead-salt diode laser (TDL) using a single-tone high-frequency modulation (FM) technique. With a detection bandwidth of 200 kHz, an optical density of 2.7 × 10−5 was detectable at SNR of 1. The detectable optical density could be further improved by reducing the detection bandwidth in agreement with the √Δf relationship, reaching 2.5 × 10−6 at a detection bandwidth of 1.56 kHz. Normalized to 1 Hz bandwidth, the demonstrated performance would then correspond to a detectable optical density of 5.9 × 10−8. This detection limit agrees well with the calculated “quantum limited” performance based on the measured laser power, modulation index, noise figure of the electronic components, and other parameters of the apparatus. These measurements and calculations show that by implementation of the FM technique, the sensitivity of the present TDL absorption spectrometers (TDLAS) can be improved by at least a factor of 10 and possibly even of 100. Such a sensitivity improvement would greatly extend the applicability of TDLAS for trace gas analysis, especially in atmospheric monitoring.

Quantum-limited FM-spectroscopy with a lead-salt diode laser

Measurement of quantum efficiencies for persistent spectral hole burning

High frequency modulation (FM) spectroscopy, (1,2,3) is a promising tool for high speed and ultrasensitive atmospheric trace gas monitoring. Based on a measured wideband noise characteristics (4) of a lead-salt diode-laser emitting in the mid IR a sensitivity improvement of 2 orders of magnitude in comparison to conventional derivative (2f) spectroscopy is expected. This improvement can be achieved by moving in modulation and detection frequency space from the 1/f noise dominated region (10 kHz) into a shot noise limited frequency domain above 100 MHz. Optical densities of 10−4 were measured on a µsec timescale with a 200 kHz detection bandwidth. Using digital averaging techniques (5) a detection limit of 2.5.10−6 has been demonstrated with the actual measurement bandwidth of 1.5 kHz. Up to this sensitivity no limitations due to etalons or laser intrinsic parameters could be observed. Further bandwidth reduction is currently being investigated and requires digital background subtraction techniques as well as etalon suppression schemes.

M. Gehrtz

Papers

Residual amplitude modulation in laser electro-optic phase modulation

Quantum-limited laser frequency-modulation spectroscopy

Quantum-limited FM-spectroscopy with a lead-salt diode laser

Measurement of quantum efficiencies for persistent spectral hole burning

High-frequency-modulation spectroscopy with a lead-salt diode laser