Absolute frequency measurement of iodine lines with a femtosecond optical synthesizer

We have used a single laser femtosecond optical frequency synthesizer together with a widely tunable Nd:YAG laser to measure the absolute frequency of several absorption lines in molecular iodine around 532 nm. The use of two different repetition frequencies allows us to determine the number of modes used for the frequency measurement unambiguously. The lines also provide data for the determination of improved ro-vibrational constants of the iodine molecule. Received: 3 July 2001 / Published online: 19 September 2001     

The modeling and the optimization of a grating optical low-pass filter

Through the one-dimensional analysis of the diffraction of a phase grating, we calculated the optical transfer function (OTF) of a phase grating. By a 0th-order approximation of the optical transfer function, we analyzed the feasibility of using it as a spatial frequency filter and obtained the structure of a grating for the optimum-filtering characteristics. The OTF of a phase grating as an optical low-pass filter is optimal when the incoming light is equally divided into three beams. The distance of those beams must be equal to the pixel size. The theoretical development was verified through an experiment. Received: 21 May 2001 / Revised version: 16 July 2001 / Published online: 15 October 2001     

Practical optical frequency measurement system around 1.5 μm based on an acetylene-stabilized laser-locked optical frequency comb

We have developed a practical and precise frequency measurement system at 1.5 μm telecommunication band. An electro-optic-modulator based optical frequency comb is phase-locked to a dither-free acetylene-stabilized laser to realize an optical frequency comb with frequency uncertainty of 10 kHz (5 × 10⁻¹¹) and the linewidth of 15 kHz. The present frequency comb can be also used as an optical frequency reference grid defined by ITU-T (International Telecommunication Union, Telecommunication Standardization Sector). Using the present frequency measurement system, we have demonstrated the first optical frequency measurement of ¹²C¹⁶O overtone absorption lines around 1.56 μm with the uncertainty of lower than 900 kHz.     

Optical nonlinearity enhancement for an anisotropic semiconductor–insulator composite

We investigate the effect of geometric anisotropy on the optical nonlinearity enhancement for a periodic composite with a rectangular array of elliptic semiconducting cylinders in an insulating host. By using a series expression of the space-dependent electric field obtained by a simple Fourier method in a periodic composite, we calculate the frequency dependence of the effective third-order nonlinear susceptibility as a function of anisotropy. The results show that the height of the nonlinearity enhancement peak may be increased by several orders of magnitude as the aspect ratio of the ellises is decreased or the lattice edge length ratio is increased. At resonance frequency, there exists a strong anomalous dispersion. We also investigate the effect of the volume fraction of the semiconductor phase for composites with a square array of circular semiconducting cylinders. Received: 23 November 2000 / Accepted: 2 August 2001 / Published online: 17 October 2001     

Optical pumping and modulation techniques with a molecular Ramsey–Bordé interferometer

Using the example of a matter-wave interferometer with K₂ molecules, we present different methods to simplify the observed interference structures within a hyperfine pattern for improving phase measurements of the interference structure. We consider optical pumping for depletion of specific components to simplify the observed spectra. In connection with a lock-in amplifier, amplitude modulation as well as frequency modulation of laser beams in the interferometer are investigated for better detection of the interference pattern. As a follow-up treatment, digital filtering is demonstrated. Received: 22 December 2000 / Revised version: 30 April 2001 / Published online: 18 July 2001     

Proposal of a frequency-synthesis chain between the microwave and optical frequencies of the Ca intercombination line at 657 nm using diode lasers

We propose a frequency synthesis chain which can directly connect a microwave atomic clock with a visible laser. We design this chain for the frequency measurement of a visible laser locked on the intercombination transition of Ca at 657 nm. The proposed chain is based on both an optical difference frequency divider and an optical frequency comb generator, and it is designed to use nine visible and near-infrared diode lasers. We discuss the technical requirements to realize the frequency measurement accuracy level of 10^–14.     

High-precision pulsed photoacoustic spectroscopy in NO2-N2

A method for high-precision pulsed photoacoustic spectroscopy applied to a simple system for detection of NO₂ traces in nitrogen is presented. The acoustic signal from a closed cell containing NO₂/N₂ samples irradiated by a pulsed visible laser is analyzed in the frequency domain. A signal-processing method to obtain a high-resolution Fourier spectrum of the signal was developed. An accurate fitting of the resonance peaks with Lorentzian profiles gives high-precision determination of the amplitude and width of the resonance peaks. The resonance maximum is proportional to the absorbed energy; therefore, the choice of the laser wavelength, linewidth and frequency stability are critical for a precise calibration due to the fine structure of the NO₂ optical spectrum. The method also allows high-accuracy measurement of the Q of the acoustic cavity. The dependence of Q on the buffer gas pressure is characteristic of an acoustic cavity where energy losses near the walls predominate. Consequently, an important enhancement of sensitivity takes place at high N₂ pressure. Received: 1 June 2001 / Revised version: 27 July 2001 / Published online: 7 November 2001     

Frequency stabilization of a diode laser on the Cs D2 resonance line by the Zeeman effect in a vapor cell

Hyperfine optical pumping of a Cs-vapor cell through a diode laser in frequency standards requires a high purity of the laser emission spectrum, and a high stability of the emission frequency. We present here a frequency-locking scheme that is able to produce a discrimination signal without any modulation, by using the dichroism induced by a magnetic field on the atomic vapor. The larger line-width of the reference signal (Doppler-broadened) is compensated by its larger amplitude, when compared with the saturated absorption signal. As a final result, the slope is similar, and the large line-width warrants a large locking range, making robust the lock against external perturbations. This error signal is used to lock the frequency of an external cavity diode laser, with a grating reflector in Littman configuration. A fast correction band is obtained by changing the cavity length through an intra-cavity electro-optic modulator. The possible sources of instabilities of the locking point are discussed and estimated through a simulation computer program. Received: 28 November 2000 / Revised version: 21 May 2001 / Published online: 18 July 2001     

Tunable twin beams generated by a type-I LNB OPO

We present a novel scheme to separate spatially twin beams generated by a type-I lithium niobate (LNB) optical parametric oscillator near frequency degeneracy. The system is based on a holographic diffraction grating acting as a beam splitter in a balanced detector. The fast and easy temperature tuning of LNB index of refraction allows an easy control of the twin-beam wavelength distance in a range of the order of ∼100 nm. We report correlation spectra measured for different twin-beam wavelength separations (15–60 nm) with a maximum noise reduction of 3.2 dB at 3.5 MHz. The described system exhibited a pump resonance stability longer than 6 h with infrared output power fluctuations within 4% around an average value of ≃2 mW in each beam. The measured oscillation threshold pump power was lower than 31 mW. Received: 7 June 2001 / Revised version: 17 July 2001 / Published online: 10 October 2001     

Melting behaviours of nickel nanorods

We report the synthesis and measurement of an ultra-precise and extremely stable optical frequency in the telecommunications window around 1543 nm. Using a fibre-based femtosecond frequency comb we have phase-stabilised a fibre laser at 194 THz to an optical frequency standard at 344 THz, thus transferring the properties of the optical frequency standard to another spectral region. Relative to the optical frequency standard, the synthesised frequency at 194 THz is determined to within 1 mHz and its fractional frequency instability is measured to be less than 2×10^-15 at 1 s, reaching 5× 10^-18 after 8000 s. We also measured the synthesised frequency against a caesium fountain clock: here the frequency comparison itself contributes less than 4 mHz (2×10^-17) to the uncertainty. Our results confirm the suitability of fibre based frequency comb technology for precision measurements and frequency synthesis, and enable long-distance comparison of optical clocks by using optical fibres to transmit the frequency information.     

Ultrafast optical nonlinear properties of metal nanoparticles

The physical origin and the dynamics of the ultrafast optical nonlinear response of noble metal nanoparticles are analyzed around the surface plasmon resonance frequency using extension of the bulk metal electron kinetics and band structure models. The computed spectral and temporal responses are found to be in very good agreement with the measured ones in silver when taking into account the impact of electron excitation on both the interband absorption and electron optical scattering rate. A good reproduction of the strong excitation regime experimental results is also obtained in the case of gold, with a dominant contribution of the interband effect. Received: 4 July 2001 / Published online: 10 October 2001     

Single-mode hollow optical fibres for atom guiding

We present a novel design for a single-mode, hollow optical fibre, which is suitable for use as a waveguide for atomic de Broglie waves. The design, development and characterisation of such a fibre are discussed, as well as an optimised method for coupling light into the fibre. Received: 22 May 2001 / Revised version: 5 September 2001 / Published online: 29 November 2001     

Micro-humps formed in excimer laser ablation of polyimide using mask projection system

In this paper we report the results of an investigation into surface deformation caused by thermal effects during excimer laser ablation of polyimide. Obvious surface deformation around hole entrances was observed during the experiment. The surface topology and cross section of the ablated holes were analyzed using topography measurement tool and scanning electron microscopy. It was shown that a micro-hump of 17 to 150 nm in height and 1 to 3 μm in width was formed above the level of the unablated surface. The deformed surface showed rough and color-changed characteristics. An optical diffraction model was employed to explain the cause of this kind of deformation. It was found that the ablating and heating by a near- and under-threshold laser beam became a thermal effect in polyimide material ablation, which was contributed to by a diffraction effect of the optical projection system. Received: 9 October 2001 / Accepted: 17 October 2001 / Published online: 23 January 2002     

Dispersion measurement technique based on the self-seeding laser oscillation of a Fabry-Perot laser diode

A simple dispersion measurement technique has been proposed and demonstrated by using the self-seeding laser oscillation of a Fabry-Perot laser diode through an optical closed-loop path. When the multi-mode optical pulses emitted from the laser are re-injected into the laser after traversing a fiber-under-test, a single mode laser oscillation occurs through the closed-loop path due to the group velocity difference between the pulses of different wavelengths. We measured the dispersion parameter of the fiber-under-test from the modulation frequency changes required to induce single-mode laser oscillations through the optical closed-loop path. The maximum measurement error was less than 1.5% for the optical fibers as compared with a commercial instrument.     

Ionization spectrum broadening and frequency blue-shift of high-intensity femtosecond laser pulses in gas-filled capillary tubes

. We report on the experimental and theoretical study of spectrum transformation and frequency blue-shift of femtosecond laser pulses with intensities up to 2×10¹⁶ W/cm², propagating in glass capillary tubes under gas ionization. Monomode optical guiding with 45% transmission efficiency is demonstrated in a 100-μm-diameter, 20-cm length capillary. A broadening of the initial spectrum as much as several initial spectrum widths is achieved. Besides the broadening, the mean frequency of the output radiation in the spectrum experiences a blue-shift of up to several initial spectrum widths, caused by the non-stationary, non-linear process of gas ionization. Our numerical simulations, in the form of a simple one-dimensional model for the propagation of intense laser pulses in gas-filled capillaries, are in good qualitative agreement with the experimental results. These simulations show the possibility of significant compression of an output pulse in a simple compression scheme (e.g. a piece of silica glass with normal dispersion), which is very important for obtaining laser pulses with few optical cycles at the millijoule energy level. Received: 25 September 2001 / Revised version: 6 December 2001 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +7-8312/363-792, E-mail: dekart@ufp.appl.sci-nnov.ru     

Bleaching processes and their reaction rates in photo-excited chlorophyll solutions

Photo-bleaching phenomena occurring in chlorophyll-a were analysed by irradiation with UV and visible-spectrum sources. A mathematical formalism was developed to quantify the optical spatio-temporal properties of the irradiated solutions. We also measured the spatio-temporal changes of the optical response in photo-excited solutions. An empirical expression for the reaction rate was obtained and the model used to quantify and understand the experimental bleaching results. Received: 3 July 2001 / Revised version: 19 November 2001 / Published online: 7 February 2002     

Incoherent optical switching of semiconductor resonator solitons

We demonstrate experimentally the bistable nature of the bright resonator solitons in a semiconductor microresonator with mixed absorptive/defocusing nonlinearity and show that they can be written and erased by incoherent local optical injection. Received: 22 February 2001 / Published online: 27 April 2001     

Coherent anti-Stokes Raman scattering velocimetry with nearly degenerate pumps

The time evolution of the anti-Stokes signal produced from the non-linear interaction of a short Stokes pulse and two long pump pulses that are nearly degenerate in frequency has been investigated. It is shown that this approach allows us to specify the accuracy of CARS (coherent anti-Stokes Raman scattering) velocimetry and to extend the range of operation of the method. In addition, an original optical scheme capable of delivering short visible pulses with good spatial and spectral properties is reported. The optical bench has been used for the characterisation of a low-pressure laminar Mach-10 flow. Received: 24 October 2001 / Revised version: 8 January 2002 / Published online: 14 March 2002     

Full observation of single-atom dynamics in cavity QED

We report the use of broadband heterodyne spectroscopy to perform continuous measurement of the interaction energy E_int between one atom and a high-finesse optical cavity, during individual transit events of ≈250 μs duration. We achieve a fractional sensitivity ≈4×10^-4/ to variations in E_int/? within a measurement bandwidth that covers 2.5 decades of frequency (1–300 kHz). Our basic procedure is to drop cold cesium atoms into the cavity from a magnetooptic trap while monitoring the cavity’s complex optical susceptibility with a weak probe laser. The instantaneous value of the atom–cavity interaction energy, which in turn determines the coupled system’s optical susceptibility, depends on both the atomic position and (Zeeman) internal state. Measurements over a wide range of atom–cavity detunings reveal the transition from resonant to dispersive coupling, via the transfer of atom-induced signals from the amplitude to the phase of light transmitted through the cavity. By suppressing all sources of excess technical noise, we approach a measurement regime in which the broadband photocurrent may be interpreted as a classical record of conditional quantum evolution in the sense of recently developed quantum trajectory theories. Received: 2 June 1998 / Revised version: 4 December 1998 / Published online: 31 March 1999     

Determination of the dielectric function of porous silicon by high-order laser-harmonic radiation

Porous-silicon reflectance has been determined over a large energy range, from 1 eV to 16 eV, by combining a NIR/visible/UV spectrometer with a new VUV light source as laser-harmonic radiation. The porous-silicon dielectric function was deduced from reflectance measurements by Kramers–Kronig analysis. We point out that, for the first time, laser harmonics have been applied in the optical characterization of materials as a new and suitable alternative to synchrotron radiation. Received: 9 January 2001 / Accepted: 28 April 2001 / Published online: 20 June 2001