Pierre Thomann

Simple approach to the relation between laser frequency noise and laser line shape

Frequency fluctuations of lasers cause a broadening of their line shapes. Although the relation between the frequency noise spectrum and the laser line shape has been studied extensively, no simple expression exists to evaluate the laser linewidth for frequency noise spectra that does not follow a power law. We present a simple approach to this relation with an approximate formula for evaluation of the laser linewidth that can be applied to arbitrary noise spectral densities.

Frequency noise of free-running 4.6 μm distributed feedback quantum cascade lasers near room temperature

The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 μm range and operated in cw mode near room temperature (277 K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz(2)/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.

Cold atoms in space and atomic clocks: ACES

Abstract In this article, the interest of space environment for cold atoms is outlined. After a brief review of cooling techniques and Bose–Einstein condensation, the case of atomic clocks in microgravity is discussed. The scientific objectives of the European mission ACES are presented. ACES will fly onboard the international space station in 2005–2006.

Fully stabilized optical frequency comb with sub-radian CEO phase noise from a SESAM-modelocked 1.5-µm solid-state laser

We report the first full stabilization of an optical frequency comb generated from a femtosecond diode-pumped solid-state laser (DPSSL) operating in the 1.5-μm spectral region. The stability of the comb is characterized in free-running and in phase-locked operation by measuring the noise properties of the carrier-envelope offset (CEO) beat, of the repetition rate, and of a comb line at 1558 nm. The high Q-factor of the semiconductor saturable absorber mirror (SESAM)-modelocked 1.5-µm DPSSL results in a low-noise CEO-beat, for which a tight phase lock can be much more easily realized than for a fiber comb. Using a moderate feedback bandwidth of only 5.5 kHz, we achieved a residual integrated phase noise of 0.72 rad rms for the locked CEO, which is one of the smallest values reported for a frequency comb system operating in this spectral region. The fractional frequency stability of the CEO-beat is 20‑fold better than measured in a standard self-referenced commercial fiber comb system and contributes only 10(-15) to the optical carrier frequency instability at 1 s averaging time.

Frequency discriminators for the characterization of narrow-spectrum heterodyne beat signals: application to the measurement of a sub-hertz carrier-envelope-offset beat in an optical frequency comb.

We describe a radio-frequency (RF) discriminator, or frequency-to-voltage converter, based on a voltage-controlled oscillator phase-locked to the signal under test, which has been developed to analyze the frequency noise properties of an RF signal, e.g., a heterodyne optical beat signal between two lasers or between a laser and an optical frequency comb. We present a detailed characterization of the properties of this discriminator and we compare it to three other commercially available discriminators. Owing to its large linear frequency range of 7 MHz, its bandwidth of 200 kHz and its noise floor below 0.01 Hz(2)/Hz in a significant part of the spectrum, our frequency discriminator is able to fully characterize the frequency noise of a beat signal with a linewidth ranging from a couple of megahertz down to a few hertz. As an example of application, we present measurements of the frequency noise of the carrier envelope offset beat in a low-noise optical frequency comb.

Temperature dependence of the frequency noise in a mid-IR DFB quantum cascade laser from cryogenic to room temperature

We report on the measurement of the frequency noise power spectral density in a distributed feedback quantum cascade laser over a wide temperature range, from 128 K to 303 K. As a function of the device temperature, we show that the frequency noise behavior is characterized by two different regimes separated by a steep transition at ≈200 K. While the frequency noise is nearly unchanged above 200 K, it drastically increases at lower temperature with an exponential dependence. We also show that this increase is entirely induced by current noise intrinsic to the device. In contrast to earlier publications, a single laser is used here in a wide temperature range allowing the direct assessment of the temperature dependence of the frequency noise.

Experimental validation of a simple approximation to determine the linewidth of a laser from its frequency noise spectrum

Laser frequency fluctuations can be characterized either comprehensively by the frequency noise spectrum or in a simple but incomplete manner by the laser linewidth. A formal relation exists to calculate the linewidth from the frequency noise spectrum, but it is laborious to apply in practice. We recently proposed a much simpler geometrical approximation applicable to any arbitrary frequency noise spectrum. Here we present an experimental validation of this approximation using laser sources of different spectral characteristics. For each of them, we measured both the frequency noise spectrum to calculate the approximate linewidth and the actual linewidth directly. We observe a very good agreement between the approximate and directly measured linewidths over a broad range of values (from kilohertz to megahertz) and for significantly different laser line shapes.

Performance demonstration of a compact, single optical frequency Cesium beam clock for space applications

Observatoire de Neuchatel has developed a compact optically-pumped cesium beam frequency standard in the frame of an ESA-ARTES 5 project. The simplest optical scheme, which is based on a single optical frequency for both preparation and detection processes of atoms, has been chosen to fulfill reliability constraints of space applications. With the last evolution of our laboratory demonstrator, we have measured a frequency stability of sigmay=1.14 times10-12 tau-1/2, which is compliant with the Galileo requirement and our frequency stability goal of sigmay=1.14 times10-12 tau-1/2. Present performance limitations are discussed and further improvements are proposed to possibly increase the frequency stability.

First results with a cold cesium continuous fountain resonator

We report on the design, construction, and preliminary measurements on the resonator of a continuous Cs fountain frequency standard. The construction of the resonator is described, preliminary measurements of the available atomic flux, and of the beam temperature are presented, along with the first Ramsey fringes (width /spl sime/1 Hz) obtained in this new type of fountain. We discuss theoretical aspects of the interrogation scheme with a special view on how aliasing or intermodulation effects are suppressed in a continuous fountain.

Low temperature indium-based sealing of microfabricated alkali cells for chip scale atomic clocks

In this paper, the development of a low temperature bonding process focused on indium-based technology for microfabricated alkali cells is presented. The intended application is the use of these cells in chip scale atomic clocks. The existing technology is mainly based on anodic bonding. For some applications, such as where wall coating is used instead of buffer gas, anodic bonding cannot be applied because of the relatively high temperature of the process.