Cw Single-mode Laser Research Articles

Generation of regular optical patterns and photonic structures by a single Gaussian beam in a photorefractive LiNbO3:Fe crystal

We report 2D micrometric scale regular optical patterns formation induced by a laser beam propagating through a Fe doped lithium niobate (LN:Fe) crystal. A single mode CW laser radiation at 632.8 nm wavelength and beam powers from 0.5 to 10 mW and LN:Fe samples with 0.03 wt% Fe concentration and thicknesses of 4 mm and 10 mm were used in the experiments. The observed phenomenon of optical patterns formation in photorefractive crystal is explained with the light-induced complex refractive index variation with a central minimum (negative lens) and symmetric maxima (positive lenses) on each side due to photovoltaic effect in the crystal. Light-induced complex lenses produce beam defocusing from the beam center and focusing on the periphery. Mutual interference of the beams leads to a regular optical pattern formation. Regular optical pattern, in its turn, leads to the formation of quasi-crystalline photonic structure in LN:Fe crystal. Experimental results are supported by numerical simulations.

Nonadiabatic effects on the positions and lifetimes of the low-lying rovibrational levels of the GK 1Σ and H 1Σ states of H2.

The term values of rovibrational levels of the GK 1Σ+g and H 1Σ+g states of H2 have been measured with absolute and relative accuracies down to 10-4 cm-1 (≈3 MHz) and 10-6 cm-1 (≈30 kHz), respectively, by measuring transitions to long-lived high-n Rydberg states using single-mode cw laser radiation and a collimated supersonic beam of cold H2 molecules. The term values are compared with those determined in earlier experimental and theoretical investigations and the comparison points at discrepancies and a need for improved theoretical treatments of nonadiabatic, relativistic and radiative corrections for these states. The lifetimes of several low-lying rovibrational levels of the GK, H and I+ 1Πg states have been determined in pump-probe experiments. These lifetimes reveal a significant dependence on the electronic, vibrational and rotational excitation and also deviate from results obtained in earlier experimental and theoretical investigations.

Precise frequency measurement and characterization of a continuous scanning single-mode laser with an optical frequency comb.

We developed a high-precision frequency-measurement system for continuously scanning single-mode cw lasers. The high precision for wide frequency range is realized by employing an optical frequency comb. The developed system operates as a strong tool for analyzing the spectral profile and scan characteristics of lasers, which is suitable for high-resolution molecular spectroscopy. We demonstrated the ability of our system with Doppler-free spectroscopy of molecular iodine.

Population of excited erbium levels in Er3+:Na0.4Y0.6F2.2 (Er:NYF) laser crystals

We have studied theoretically and experimentally the population dynamics of six radiative erbium levels in concentration series of Er3+:NYF crystals. The radiative transition probabilities were calculated by the Judd–Ofelt method; the nonradiative transition probabilities were estimated; energy transfer (ET) microparameters (for migration, selfquenching (SQ), and upconversion (UC) processes) in studied crystals were estimated theoretically using the method of quantum–mechanical model calculation. The rate equations for the six lowest energy levels of erbium in Er3+:NYF crystals, including the 3μm laser transition levels, were numerically solved for the case of excitation by nanosecond pulse at λpump=1.53μm. The theoretical and experimental results for Er3+:NYF crystals are in good agreement. Single mode CW laser action was achieved in Er:NYF crystal at 2.8μm with InGaAs LD pumping of the 4I11/2 level. Output power about 72mW with slope efficiency ∼4% was achieved in longitudinal pump scheme.

Continuous wave 2.9 μm dysprosium-doped fluoride fiber laser

Single-mode room-temperature cw laser emission on the H13/26→H15/26 2.9 μm phonon terminated electronic transition of Dy3+ is demonstrated. The maximum output power of 0.275 W was generated at a slope efficiency of 4.5% (with respect to the absorbed pump power) when a Dy3+-doped ZBLAN fiber laser was tandem pumped with the ∼1100 nm output from a diode-cladding-pumped Yb3+-doped silica fiber laser.

Development of Phase-lock System between Two Single-Mode Lasers for Optical-Optical Double Resonance Spectroscopy

The difference frequency between two single-mode CW lasers was stabilized to a microwave (MW) frequency reference by using an optical phase-lock loop (PLL). The long-term frequency jittering of the beat spectrum was much suppressed by this method. The difference frequency was able to be continuously scanned from 0.1 to 18 GHz by changing the microwave reference frequency. It was also possible to scan the two lasers synchronously keeping the difference frequency constant. This stable and flexible light source would be an ideal light source for optical-optical double resonance (OODR) spectroscopy and resonance laser Raman experiments. For the demonstration, this new system was applied to an OODR experiment using a Rubidium atom. The hyperfine structures in the D2 line were observed in the sub-Doppler condition.

UV Laser Spectroscopy Using the Velocity-Modulation Technique: New Hot Bands of the B2Σ+u–X2Σ+g System of N+2

The laser absorption spectroscopy of molecular ions using the velocity-modulation technique has been employed in the UV region. The absorption source was a cw single-mode laser beam from an extra-cavity frequency doubling unit associated with a dye laser. The high-resolution spectrum of the First Negative system B2Σ+u–X2Σ+g of N+2 has been studied. The N+2 ions were produced with an electric discharge in a flowing gas mixture of He/N2. Seven vibrational bands of the Δv = 2 sequence (v′ = 2–8) have been observed in the region between 30000 and 30600 cm−1, and five of them for the first time at high resolution (v′ = 2–4, 6, 7). A set of 770 lines have been assigned; among them are several extra – lines belonging to the A2Πu–X2Σ+g system. A standard model has been used to consider the perturbations between the rovibrational levels of the B2Σ+u and A2Πu states. An improved set of deperturbed constants and effective interaction parameters were derived for the vB = 3–vA = 14 and vB = 7–vA = 20 complexes.

EXCITATION ENERGY TRANSFER BY COLLISIONS BETWEEN EXCITED RUBIDIUM ATOMS

This paper reports the experimental observation of the excitation energy transfer to higher states by collisions between two rubidium atoms excited to a specific state by a CW single mode laser. An obvious competition has been observed between the channel of transition to the ground state by spontaneous emission, and that to higher excited states by collisions of two excited atoms. Formation rates of the high excited states with different laser power, atomic vapor temperature and laser detuning have been measured. A proposed mechanism well explained the experimental result.

Coherent Control of a Simple Quantum System

The quantum dynamics of a three-level Vee-system can be controlled by using two single-mode cw lasers. The dc component of the probability for finding a specific state in laser fields can be obtained by matching the intensities, phases and detunings of the lasers to relevant eigenvalues of the quantum system. The selectivity of control does not depend strongly on the laser intensities but the absolute amount of the product does strongly. There are generally three paths to reach such a dc component control and, at some points, two of them cross each other causing a near 100% control. The stability of the process to the fluctuation of the experiment was analyzed.

Alternative fiber-optic backreflectance sensor for measurement of the effective focal lengths of optical elements and distances

An alternative and simple fiber-optic backreflectance sensor method for accurate measurement of either effective focal lengths of optical elements and short distances using a single-mode optical fiber and cw laser is presented. The method is based on the intensity sensing of focused backreflectance laser emission and the spatial location of three specific points: the focal point of the focusing optical element and two object points. The single-mode fiber is a key optical element and serves simultaneously as a point laser source for testing, an object for projecting and a highly sensitive point receiver of the focused backreflectance emission. The experimental and analytical results demonstrate the potential of the method for locating the spatial points and determining the effective focal length with accuracies exceeding 1 μm and 0.5%, respectively, as well as for short distance measurement with submicron accuracy.

A laser displacement sensor using the self-mixing effect for modal analysis and defect detection

A low-cost, simple, noncontact displacement sensor using the self-mixing effect inside a cw single-mode laser diode has been developed for modal analysis and defect detection. The resonance frequencies of thin clamped metallic plates have been determined with a resolution of 0.2 Hz. With such targets, classic spectrum analyzers generate a measurement error up to 10% because of the weight of the accelerometer. The frequency-shift of the fundamental resonance frequency between a standard plate and the same one with a known defect can also be measured, the modal vector associated to this frequency being distorted at the place of the defect.

General analysis of instabilities in erbium-doped fiber lasers.

A detailed analysis of a general model of erbium-doped fiber laser dynamics is investigated. Depending upon the parameters the same model describes single-mode cw or self-pulsing lasers, dual-wavelength or bipolarized two-mode lasers. A summary of previous works concerning self-pulsing behavior and antiphase phenomena is presented. The basic physics responsible for the instabilities is the existence of ion clusters in heavily doped fibers. A thorough investigation of the bimode laser equations is performed. Approximate solutions for the steady states and for the second threshold are obtained. The influence of ion-pair concentration (x) on dynamical behavior is studied. In particular, it is demonstrated that optical bistability occurs between 2T- and 3T-periodic orbits versus the pumping rate in the case of intermediate pair concentrations (where T is the fundamental period of the system). The route to chaos is found to be x dependent and of codimension 2. \textcopyright{} 1996 The American Physical Society.

An alternative fiber-optic backreflectance method for measurement of distances using a continuous wave laser

We present a new and simple fiber-optic backreflectance method for precise short distance measurement using a single-mode fiber and cw laser. The method is based on the intensity sensing of focused backreflectance laser emission, and the spatial location of three specific points: the focal point of the focusing optical element and two object points. The optical fiber is used in a multifunctional regime where it serves simultaneously as a point light source, an object for projecting, and a point receiver that is highly sensitive to spatial displacements of the focused backreflectance emission. The experimental and analytical investigations conducted demonstrate the usefulness of the method for distance measurement with submicron (about 370 nm) and millimeter accuracies in the near and far distance region, respectively, for a mirror target 1 m away.

Measurement of the self-broadening rate coefficients of the cesium resonance lines

The self-broadening rate coefficients of the cesium D 1 [6 2S 1 2 − 6 2P 1 2 ] and D 2 [6 2S 1 2 − 6 2P 3 2 ] resonance lines are determined from the 6 2P 1 2 → 6 2D 3 2 , 6 2P 3 2 → 7 2D 3 2 and 6 2P 3 2 → 7 2D 5 2 transition lineshapes, which are mapped out using single-mode cw lasers. The self-broadening rates are found from Voigt function fits (including hyperfine structure) to the experimental lineshapes. Our measured D 1 and D 2 broadening coefficients, k br( D 1) = (5.7 ± 1.0) × 10 −7 cm 3 sec −1 and k br( D 2) = (6.7 ± 1.1) × 10 −7 cm 3 sec −1, are compared to previous experimental and theoretical values in the literature. The present D 2 broadening coefficient is smaller, and the D 1 broadening coefficient is larger, than theoretical values.

New room-temperature laser-diode pumped efficient quasi-cw and cw single-mode laser based on ferroelectric and ferroelastic Gd2(MoO4)3 : Nd3+ Crystal

New room-temperature laser-diode pumped efficient quasi-cw and cw single-mode laser based on ferroelectric and ferroelastic Gd2(MoO4)3 : Nd3+ Crystal

Forward-scattering ring structure and the spectra dependence on laser frequency.

We present analytical predictions based on a semiclassical theory concerning a single-mode cw laser interaction with longitudinally Zeeman tuned J=1--J=0 atomic systems. In the theoretical model, the laser frequency detuning from the inhomogeneously broadened absorber line center is considered. Equation for the interaction signal are derived for all rotations of a polarizating analyzer transmisson axis with respect to an original laser beam linear polarization. Results are presented for the forward-scattering case when the analyzer transmission axis is orthogonal to the beam polarization. Calculated results on bright and dark ring-shaped light structures are compared with experimental ones originating from a Gaussian beam interaction with a neon 1${\mathit{s}}_{4}$(J=1, g=1.464)--2${\mathit{p}}_{3}$(J=0) transition. Calculated results on forward-scattering spectra are presented, i.e., the forward-scattering light intensity as a function of the longitudinal magnetic tuning when the laser frequency is fixed to selected detunings from the absorber line center. Spectra are presented both for the case when the inhomogeneously broadened line originates from a single absorbing isotope and for the case when it originates from overlapping contributions of two isotopes. The results show that laser frequency detunings of tens of megahertz from the absorber line center(s) show up in the forward-scattering spectra. The conclusion is that reliable data on atomic parameters of a nonlinearly absorbing sample can be extracted from experimental spectra only when the sample is monoisotopic and when the laser frequency detuning from the absorber line center is known.

Efficient single-mode cw lasers based on monoclinic double potassium-(rare earth) tungstenate crystals containing Nd3+ions with semiconductor-laser pumping

Efficient (~ 20%) single-mode, cw and quasi-cw laser action of Nd3+ ions (in the 4F3/2→4I11/2 channel) has been achieved with diode-laser pumping for the first time at room temperature in monoclinic KY(WO4)2, KGd(WO4)2, and KLu(WO4)2 crystals with an ordered structure.

Zeeman quantum beat in nuclear quadrupole resonance probed by Raman heterodyne spectroscopy

The Zeeman quantum beat in nuclear quadrupole resonance of the diamond nitrogen-vacancy centre has been observed using Raman heterodyne spectroscopy — a coherent optical-RF double resonance technique. In the experiment, a magnetic field was applied and set corresponding to the level anticrossing regime, where the electron and nuclear spin states are strongly mixed and therefore the nuclear spin transitions are greatly enhanced. The 6380 Å 3A ↔3E zero-phonon transition was driven by a single mode CW laser while the mI = 0 ↔ ± 1 (mS = 0) magnetic transitions in the 3A ground state were excited by an RF pulse. Beat modulation was observed to appear as superimposed on the total RF emission signal detected by a photodiode. The beat frequency was found to be in excellent agreement with the Zeeman splitting of the mI = ± 1 (mS = 0) levels.

Polarization effects in the differential cross section of the Ba(1P1−3P2) inelastic transition induced by argon

The Ba(6s6p $^{1}$${\mathit{P}}_{1}$${\ensuremath{\rightarrow}}^{3}$${\mathit{P}}_{2}$) inelastic transition induced by argon has been investigated in a crossed beam as a function of the collision energy between 0.14 and 0.40 eV. The differential cross sections have been obtained by the Doppler technique probing the Ba(6s6p $^{3}$${\mathit{P}}_{2}$) with a single-mode cw laser. Oscillations are observed in the differential cross sections that are assigned to St\"uckelberg interferences resulting from a crossing between the $^{1}\mathrm{\ensuremath{\Pi}}_{1}$ and $^{3}\mathrm{\ensuremath{\Sigma}}_{1}$ Ba-Ar potential-energy curves. A very large polarization effect is observed by comparing the differential cross sections corresponding to a p\ensuremath{\sigma} or a p\ensuremath{\pi} preparation of the Ba${(}^{1}$${\mathit{P}}_{1}$) orbital with respect to the relative Ba-Ar velocity.

On the statistics of mode partition noise in a nearly single-mode cw laser

The probability density function of the mode partition noise of a nearly single-mode cw laser is discussed. Nearly single-mode lasers are modeled here as those with a single dominant mode and 2 n side modes with nonnegligible strengths. The drop out rate of each side mode obeys an exponential distribution. The combined probability density function of the drop out rate for 2 n side modes is derived here. This density is evaluated when side modes with various intensities are present.