John L. Hall

Laser phase and frequency stabilization using an optical resonator

We describe a new and highly effective optical frequency discriminator and laser stabilization system based on signals reflected from a stable Fabry-Perot reference interferometer. High sensitivity for detection of resonance information is achieved by optical heterodyne detection with sidebands produced by rf phase modulation. Physical, optical, and electronic aspects of this discriminator/laser frequency stabilization system are considered in detail. We show that a high-speed domain exists in which the system responds to the phase (rather than frequency) change of the laser; thus with suitable design the servo loop bandwidth is not limited by the cavity response time. We report diagnostic experiments in which a dye laser and gas laser were independently locked to one stable cavity. Because of the precautions employed, the observed sub-100 Hz beat line width shows that the lasers were this stable. Applications of this system of laser stabilization include precision laser spectroscopy and interferometric gravity-wave detectors.

Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy

We consider several highly sensitive techniques commonly used in detection of atomic and molecular absorptions. Their basic operating principles and corresponding performances are summarized and compared. We then present our latest results on the ultrasensitive detection of molecular overtone transitions to illustrate the principle and application of the cavity-enhanced frequency-modulation (FM) spectroscopy. An external cavity is used to enhance the molecular response to the light field, and an FM technique is applied for shotnoise-limited signal recovery. A perfect match between the FM sideband frequency and the cavity free spectral range makes the detection process insensitive to the laser-frequency noise relative to the cavity, and, at the same time, overcomes the cavity bandwidth limit. Working with a 1.064-mm Nd:YAG laser, we obtained sub-Doppler overtone resonances of C2HD, C2H2, and CO2 molecules. A detection sensitivity of 5 3 10 213 of

Delivering the same optical frequency at two places: accurate cancellation of phase noise introduced by an optical fiber or other time-varying path.

Although a single-mode optical fiber is a convenient and efficient interface/connecting medium, it introduces phase-noise modulation, which corrupts high-precision frequency-based applications by broadening the spectrum toward the kilohertz domain. We describe a simple double-pass fiber noise measurement and control system, which is demonstrated to provide millihertz accuracy of noise cancellation.

Optical heterodyne saturation spectroscopy

We discuss a refined, hybrid rf/optical technique for studying sub‐Doppler saturated absorption/dispersion resonances with excellent precision and symmetry. Sensitivity is limited mainly by fundamental noise in the signal. Resonance profiles obtained in I2 are in remarkable agreement with theory. The method promises a new level of accuracy for laser locking to an optical resonance.

Laser photodetachment determination of the electron affinities of OH, NH2, NH, SO2, and S2

Using a fixed frequency argon ion laser we have studied the energy spectra of electrons photodetached from OH−, NH2−, NH−, SO2−, and S2−. We determined the following electron affinities: EA(OH)= 1.829−0.014+0.010 eV, EA(NH2)=0.779±0.037 eV, EA(NH)=0.38±0.03 eV, EA(SO2)=1.097±0.036 eV, and EA(S2)=1.663±0.040 eV. Additionally, the angular distribution anisotropy parameter β was measured for OH− and NH2− at 4880 A, as −0.993 ± 0.040, and 0.027±0.012, respectively, and information about negative ion vibrational constants is presented.

Interferometric real‐time display of cw dye laser wavelength with sub‐Doppler accuracy

We describe an automatic fringe‐counting interferometer with real‐time wavelength readout for cw laser sources. Sub‐Doppler absolute wavelength accuracy (∼2×10−7) is demonstrated with saturated absorption spectroscopy in neon.

Optical Frequency Synthesis Based on Mode- Locked Lasers

The synthesis of optical frequencies from the primary cesium microwave standard has traditionally been a difficult problem due to the large disparity in frequency. Recently this field has been dramatically advanced by the introduction and use of mode-locked lasers. This application of mode-locked lasers has been particularly aided by the ability to generate mode-locked spectra that span an octave. This review article describes how mode-locked lasers are used for optical frequency synthesis and gives recent results obtained using them.

Social Grading of Occupations

t-T1HIS IS the first report of work undertaken as the result of a grant | received by the London School of Economics from the Trustees of v the Nuffield Foundation. The purpose of the research is to discover the chief factors responsible for social class differences, for movement and hlndrances to movement from class to class under present conditions) and for the changes that have been taking place in the class structure of the population of England and Wales in recent years. Although the first approach to the problem must be general, because no class in the population is selfcontained and the boundary between neighbounng classes is bluxTed, interest will be centred chiefly in the middle class, for about this class much is surmised and little is known with certainty; past studies have been confined for the most part to the so-called working class. A survey of a large random sample of the population on a nation-wide scale is being camed out in order to discover possible evidence of class mobility by change of occupational grade with changing circumstances, also by comparing the grades of parents and their children and by change of grade on the part of women on mamage if the grade of occupation of the husband differs from that of the fes father. The grade of occupation first entered is largely detened by educational opportunity, and particulars as to education as well as occupation will be collected in the random sample surveyv A report on this work will be published in due course. Occupational status is of course not the only factor which contributes to the detertnination of class but, being closely linked with economic status, it obviously has a very important beanng on class. If then we are to trace changes in social class by changes in occupation, a preliminary examination of the social grading of occupations is essential.

Laser stabilization at the millihertz level

The main task of this paper is to identify a number of physical problems that must be successfully addressed to achieve stabilized laser linewidths well below 1 Hz. After presentation of the basic stability characteristics of available laser sources, we show that if any of these lasers were optimally locked to a high-finesse Fabry–Perot cavity it would be theoretically possible to obtain a laser linewidth in the millihertz domain. Problems of optical feedback, modulation waveform errors, mechanical support and isolation of the reference cavity, thermal stabilization of the environment, etc. are considered, and interim solutions are discussed. Experimentally, locking accuracy to successive cavity orders of <2 × 10−5 linewidths (±1.5 Hz) was achieved; mirror birefringence pulled the lock point approximately 10-fold more. Relative phase coherence between two independent lasers locked onto adjacent cavity orders was preserved for 8 sec, corresponding to a linewidth of each optical source of ∼50 mHz.

Simple and compact 1-Hz laser system via an improved mounting configuration of a reference cavity

We report an improved mounting configuration for a passive optical cavity used for laser frequency stabilization. The associated reduction of the vibration sensitivity of the effective cavity length has led to a simple and compact reference cavity system for laser stabilization at the level of 1 Hz linewidth.