Rainer Salomaa

Mode structure fluctuations in a pulsed dye laser

We have measured and analyzed high-resolution single-pulse spectra of a typical commercial dye laser. The longitudinal mode structure was resolved by using a Fizeau interferometer as the spectrum analyzer. In mode intensities we observed strong pulse-to-pulse fluctuations that are caused mainly by the variations of the spontaneous emissions to the laser modes during the starting phase of the laserpulse. Apart from the quantum noise, frequency jitter, i.e., shifts of the whole frequency comb, was also observed. The jitter appeared to be very fast, of the order of a few megahertz and hence cannot be explained by technical noise alone. A detailed knowledge of the spectral fluctuations is needed in theoretical models of noise effects in nonlinear spectroscopy. It can also be applied to novel noise reduction techniques.

Two-photon spectroscopy. III. General strong-field aspects

For pt.II see ibid., vol.9, p.1221 (1976). Discusses the steady-state response of a three-level atom in two-photon absorption. The applied laser fields can have arbitrary intensities (within the rotating-wave approximation (RWA)). A general expression is derived for the population excited on the uppermost energy level. Two special cases which allow a trivial Doppler averaging are treated in detail. For an off-resonant intermediate state the two-photon spectrum is a simple Lorentzian showing a power shift and modest cross power broadening. The same features are retained when several off-resonance intermediate levels are assumed. For very strong laser fields the Doppler shifts can be neglected in the first approximation. The corresponding spectra are analysed.

Fizeau interferometer in spectral measurements

Interferometers used for studies of spectral line profiles should produce instrumentally narrow and smooth interference fringes. We analyze in detail the fringe-formation process in a Fizeau wedge interferometer and develop a fringe-optimization method that can be used to find the optimum angle of incidence for producing the sharpest fringes in the detection plane. We show that the Fizeau fringes can be sharper than generally thought. The Fizeau interferometer can thus be used as a high-resolution spectrum analyzer that is suitable for both pulsed and cw light sources. The spectral resolution of such an analyzer can be made comparable with that of the more common Fabry–Perot interferometer. The Fizeau interferometer, however, does not require the same amount of alignment work and is thus more convenient to use with sources with variable bandwidths. Spectral measurement of pulsed lasers is an important application in which an analyzer of this kind is preferable.

Effects of finite laser bandwidths in two-photon spectroscopy

Discusses within a simple statistical model modifications of two-photon spectra caused by finite bandwidths of the light sources used for excitation. Field fluctuations are shown to affect the two-photon and step-wise contribution differently which provides a way of distinguishing them. The rate equation approximation, which ignores coherent two-photon processes, is shown to be accurate only when the frequency spread of the fields exceeds the detuning parameters and relaxation rates of the three-level system. In an inhomogeneous system satisfying the Doppler-limit criteria the step-wise contribution is efficiently cancelled away demonstrating the poor applicability of a rate description. Only when the field bandwidths are much larger than the Doppler widths are the coherence effects negligible.

Application and generation of large amplitude plasma waves by beating of two intense laser beams

Large amplitude plasma waves which can be created by beating two intense laser beams are applicable e.g., to particle acceleration, to induction of plasma currents, and to plasma heating. A central issue in these applications is the nonlinear behaviour of the plasmon. We present a theory model where nonlinear frequency shifts in the plasmon evolution and formation of electromagnetic cascades by inelastic plasmon-photon scattering are accounted. The analytical temporal solution can be constructed in terms of Bessel functions and elliptic integrals. Implications of the results to the applications and further needs to refine the model are discussed. A suggestion for broadening the light spectrum in laserfusion is made.

Dynamics of Fabry–Perot resonators with a phase-conjugate mirror

We study the dynamics of a stable Fabry–Perot resonator in which one mirror is a phase-conjugate mirror (PCM) with instantaneous response. The temporal variations in the model are caused by round-trip effects. An exact solution is given in a form of an infinite series. The step-function response is summed exactly. Numerical studies with Gaussian input pulses and PCM reflectivity variations are presented, and their behavior is explained. Approximate formulas for responses to slowly varying pulses are derived. Noise effects are simulated with a model in which the phases of the probe and the amplitude reflection coefficient of the PCM make uncorrelated jumps with Poissonian statistics. The theoretical model is aimed for optimizing experiments on phase-conjugate resonators involving transients and fluctuating fields.

Determination of hyperfine structures and Rydberg convergence limits of selected optical transitions in 93Nb using resonance ionization spectroscopy

Several two- and three-step schemes for resonance ionization of 93Nb with field ionization of Rydberg levels were investigated in the context of feasibility studies of using 93mNb/93Nb isomer ratio determination for fast neutron dosimetry. Hyperfine structures of the states 4d4(a5D)5p4D12/0, 4d35s(a5P)5P6D12/0, 4d35s(a5F)5p6F12/0, and 4d35s(a5F)6s6F1/2 of 93Nb were measured. The Rydberg spectra of the excitation schemes were measured and corresponding ionization limits estimated.

Occurrence of Split Spectra in Doppler-free Two-photon Spectroscopy

This paper discusses the origin of splitting of two-photon spectra of three-level cascade configurations having a near resonant intermediate level. A clear distinction between splitting mechanisms in homogeneously and inhomogeneously broadened systems is demonstrated. When one of the laser fields is a weak probe the velocity averaged two-photon response is primarily caused by two resonantly excited velocity groups; the absence of these implies a split spectrum as suggested by Chebotaev and collaborators. A simple derivation of the probe response is presented. When both fields have non-negligible intensities resonant excitation is inhibited and instead all level populations pulsate. Still a split spectrum can appear. A simple graph showing the regions where this occurs is given and analyzed.

Absorber-induced features in the ac response of a He-Ne laser☆

Abstract The theoretically predicted pronounced ac-response of the laser intensity to the excitation current modulation of an intracavity saturable absorber has been experimentally verified in a He-Ne laser with a Ne-absorber. Good qualitative agreement with the theory is obtained.

Backscattering of ultrashort high intensity laser pulses from solid targets at oblique incidence

Backreflection of short, intense laser pulses at oblique incidence on solid targets is explained with a model where a periodic electron density modulation acts as a diffraction grating. The pump and reflected electromagnetic waves drive through the ponderomotive force the grating, and the coupled system becomes parametrically unstable. The basic equations governing this situation are given. A linearized stability analysis is used to obtain the instability growth rate in a homogeneous plasma and the convective gain coefficients for the inhomogeneous case. The results support the feasibility of the suggested mechanism. An absolute instability is predicted to set on at a threshold intensity of about 1016 W/cm2 for a typical laser pulse with a length of 100 fs and a spot size of 30 μm. The instability is shown to saturate at a level of a few percent, because the higher harmonics in the electron density modulation make the diffraction more diffuse, thus reducing both the ponderomotive force and the backreflect...