Direct observation of the second-order Stark effect of theX-B transition in molecular iodine

The second-order Stark shift of the components of the hyperfine structure of the transition¹ _g ⁺ ( = 0,j = 13, 15) ³ _ou ⁺ ( = 43,j = 12, 16) (of molecular iodine have been studied by means of saturated absorption spectroscopy in an external cell with the I₂ vapour located in an electric field. The anisotropic polarizabilities of the upper and lower levels together with the difference between the isotropic polarizabilities of the levels of the transition have been obtained.     

Study of relaxation of molecular iodine by the method of saturated absorption spectroscopy

The paper describes detailed experimental studies of the width, shape and shifts of saturated absorption resonances at the X(v=0, j=13)B(v=43, j=12) transition of molecular iodine. Use of the geometry of oppositely travelling and unidirectional waves in the experiment and study of the luminescence decay under pulsed excitation allowed estimation of the cross-sections of elastic and inelastic scattering of I₂ molecules on the upper and lower levels of transition. The observed nonlinear dependence of width and shift of the saturation resonance on iodine pressure is explained by the influence of elastic collisions with no phase mismatch.     

Second-order Zeeman effect of theX−B transition in molecular iodine

Saturated absorption spectroscopy was applied to study the line shape of the molecular iodineX( = 0,J = 13,15) B( = 43,J = 12, 16) transition ( = 514.5 nm) in a transversal magnetic field as high as 0.51 T. The Zeeman structure of several hyperfine structure (hfs) components was completely resolved and a detailed study of the second-order Zeeman shift and splitting was made. The anisotropic Magnetic Susceptibility (MS) of the molecular iodine both in theB(43) (X = 7.3 ± 1 × 10^–34 J/Oe²) and theX(0) [ = (0.6 ± 1) × 10^–34 J/Oe²] states as well as the isotropic MS difference [0 –o = (2 ± 0.2) × 10^–34 J/Oe²] was measured.     

Sensitive detection of methane with a 1.65 μm diode laser by photoacoustic and absorption spectroscopy

Received: 18 September 1997/Revised version: 11 November 1997     

Dye-laser frequency stabilization using optical resonators

We describe a study, performed using heterodyne techniques, of the frequency fluctuations of two completely independent ring dye lasers locked to independent reference cavities. Single laser linewidths of less than 750 Hz were achieved, the principal limitation being residual vibrations from the noisy laboratory environment. With future design and environmental improvements, ultranarrow linewidths are expected thus providing a useful tool for a great variety of high precision experiments.     

Short-term frequency stabilization of diode-laser-pumped Nd:YAG lasers using double-pendulum suspended cavities

We describe the improvement of short-term frequency stability of diode-laser-pumped Nd:YAG lasers. To improve the vibrational isolation of reference cavities, the reference cavities were suspended by a double pendulum with magnetic damping. The frequency noise was reduced to lower than 1 Hz/Hz at Fourier frequencies higher than 5 Hz and the minimum noise of 7 × 10^–3 Hz/Hz was recorded. The minimum root Allan variance was about 10^–14 for the sampling time of 0.01 s. Heating of the reference cavity by absorbed laser power caused the thermal drift of cavity resonance frequencies. It resulted in the laser linewidth in the range of 30–50 Hz.     

The accuracy of laser wavelength meters

The experimentally obtained accuracy and principal limitations of laser wavelength meters based on two different interferometric designs are discussed. Methods of calibrating these instruments and some experiments for testing their accuracy are presented. Finally, a comparison is given between different kinds of Lambda meters.     

A system for industrial surface monitoring utilizing laser-induced fluorescence

A system utilizing laser-induced fluorescence for surface monitoring is described. Results of laboratory and industrial measurements of surface films are reported and further applications of the technique are described.     

Remote sample characterization based on fluorescence monitoring

The possibilities and limitations of remote sample characterization using induced fluorescence are discussed. General equations for remote sensing of fluorescence are presented and discussed. The implications of different elements of a system for remote fluorescence sensing on different parameters of the equations are treated. Background light influence and signal processing are considered. Pulsed lasers, flashlamps and cw lamps as excitation sources are compared when matched to the proper detection scheme employing boxcar integration or lock-in detection. Model experiments have been performed to demonstrate different measurement strategies. Examples from remote oil-slick characterization are chosen as illustrations.     

Combined self-phase modulation and amplification of femtosecond light pulses

The influence of depletable amplification, group velocity dispersion and self-phase modulation due to Kerr-type nonlinearity on pulse shaping in femtosecond pulse amplification has been calculated. With gain depletion which is typical for the last stage of multi-stage amplifiers, spectral broadening occurs which, under certain conditions, can be utilized for pulse compression. This spectral broadening as well as a predicted spectral shift is compared with experimental results.     

Generation of subpicosecond electrical pulses by optical rectification

We describe the generation of subpicosecond electrical pulses by optical rectification of ultrashort optical pulses. The electrical pulses are generated by the second-order nonlinear response of a LiTaO(3) crystal bonded to a coplanar transmission line. A bipolar temporal waveform with a width of 875 fs was measured after a propagation distance of 175mum . This pulse width was limited by the response time of the photoconductive sampler. We observed both broadening and amplitude reduction in the temporal waveform owing to propagation.     

Picosecond spectroscopy of intermolecular interactions in dye solutions

Some new experimental data on the time development of ultrashort superfluorescence (SF) in the dye solution are presented. The correlation between temporal behaviour change and the change of the spectral and spatial intensity distribution gives direct evidence of the important role of the selffocusing phenomenon in the formation of the ultrashort SF pulse synchronized with the pumping one. The oriental relaxation of solvent molecules in the dye solution not only influences the duration of the dye SF via the selffocusing mechanism but also causes a temporal change of spontaneous emission spectrum. At least two different relaxation times were found in the fluorescence spectrum of the dye in several polar solutions.     

Intensity stabilisation of an argon laser using an electro-optic modulator —Performance and limitations

An intensity-stabilisation system for an argon laser using an electro-optic modulator is described. We achieved a degree of stabilisation of up to 40 dB (factor of 100) for intensity fluctuations below a few kHz. The performance in this frequency range did not reach that expected from the available loop gain. We have identified two serious limiting factors to the performance of our system at low frequency: geometrical fluctuations and frequency fluctuations of the laser light.     

Efficient amplification of sub-picosecond pulses of a non-CW dye laser

Efficient amplification in a dye laser amplifier is investigated theoretically and experimentally. A five-level rate equation approach is considered including rotational relaxation of the dye molecules. The effects of the pump pulse duration and of the parameters of the input pulse are discussed. The results are compared with experimental data for 0.5 ps pulses of a pulsed dye laser. Conversion efficiencies >10% are achieved for a single pass amplifier using Nd:YAG pump pulses of 2 ns while an effective fluorescence lifetime of 1.7±0.2 ns is determined for the gain medium rhodamine 6G. The triple pass amplifier stage of the laser system achieves an energy conversion of 4% with 40 J output pulses.     

Extension of frequency measurements with Schottky diodes to the 4 THz range

The 11th harmonic mixing signals between a 386 GHz oscillator and a 4.25 THz laser with a signal to noise ratio of 25 dB in 100 kHz bandwidth have been obtained with a Schottky diode.     

Generation of picosecond and subpicosecond light pulses with saturable absorbers

Single light pulses, generated by a mode-locked Nd-glass laser, were shortened with saturable absorbers of low initial transmissionT ₀. The pulse duration was reduced from 8 to 2.6 ps after a single pass through a dye cell ofT ₀=10^–7. Light pulses as short as 0.5 ps were observed after five transits through an absorberamplifier system. Detailed calculations of the stationary and the transient situation (with respect to the dye relaxation time) are presented to demonstrate optimum conditions for the pulse shortening.     

Internal amplification of subpicosecond pulses from a cw mode-locked dye laser

Subpicosecond pulses from a synchronously and passively mode-locked cw rhodamine 6G dye laser have been amplified with a gain of 10–80 in a range of 577–615 nm by a simple method in which the laser medium inside the cavity acts simultaneously as an amplifier pumped by the second harmonic of a Q-switched Nd:YAG laser. Under the good mode-locked condition, there was no appreciable pulse broadening effect due to amplification even in a very short subpicosecond regime, while imperfectly mode-locked pulses were amplified with considerable pulse broadening effect even in the picosecond regime.     

Applications of wavelength-modulation spectroscopy in resolution of pressure and modulation broadened spectra

Theoretical and experimental results for modulation absorption spectroscopy (with simultaneous amplitude and wavelength modulations) are given, with particular emphasis on the effects of pressure and modulation broadening. As expected, such broadening effects result in a loss of resolution of nearly overlapping lines, whether these are from one or more absorbing species. The effects of phase-sensitive detection at harmonics greater than the commonly used second are investigated. It is shown that when detection is performed at higher harmonics, one is able to counter the loss of resolution due to pressure and modulation broadening. Applications in measurements of congested spectra, and in the simultaneous measurement of more than one species, are discussed. Other general characteristics of signals obtained by using higher-harmonic detection are also discussed, together with corresponding applications.     

Generation of subpicosecond infrared pulses tunable between 5.2 μm and 18 μm at a repetition rate of 76 MHz

2 (2 mm thick) and GaSe (1 mm thick) crystals are applied as nonlinear media. GaSe has a larger tuning range and is more efficient in the whole spectral range than AgGaS₂. The average IR power reaches up to 2.0 mW at 8.5 μm when the GaSe crystal is used, and up to 1.3 mW at the same wavelength when AgGaS₂ is applied for the frequency conversion. Received: 17 February 1997/Revised version: 14 July 1997