Radiation fluctuations in a three-mirror system: II. Radiation noise in a three-mirror system

The mode of operation of a three-mirror system for the case where one of the cavities is filled with a gain medium, while the other contains two cells with gain and absorbing media, has been investigated. The behavior of the amplitude and phase noises of the gain and absorbing media in both arms of the three-mirror system has also been analyzed. It is shown that the presence of a gain cell narrows the absorption line and increases the resonance absorption contrast.     

On the shape of power resonances in a CO2/SF6 laser with a linear three-mirror cavity

Results of experimental and theoretical studies of the shape of power resonances of a single-mode CO₂/SF₆ laser with a linear three-mirror cavity for cases when the amplifying and the absorbing media are located in various sections are reported.     

Nonlinear resonances in the absorption spectrum of the three-level atomic V system in strong polyharmonic and weak biharmonic fields

A three-level atomic system with a strong three-mode field and a probing biharmonic field at transitions 1→2 and 1?3 (1 is a common lower level), respectively, is studied theoretically by numerical simulation and an analysis of the mathematical expressions derived for the particular case of a symmetric arrangement of the strong field components relative to the transition frequency ω₂₁. The absorption spectrum of the probing field components contains parametric supernarrow resonances against the background of resonances of the nonlinear interference effect. The ratios of the differences in the frequencies of the strong and probing fields, at which the supernarrow resonances appear, are found analytically. The results coincide with those obtained from numerical simulation.     

Probe field spectroscopy in three-level Λ systems under arbitrary collisional relaxation of low-frequency coherence

We investigate theoretically the spectrum of weak probe field absorption by three-level atoms with the Λ configuration of levels in the field of a strong electromagnetic wave acting on an adjacent transition and colliding with buffer gas atoms. Analysis is carried out for the general case of arbitrary collisional relaxation of low-frequency coherence at a transition between two lower levels. It is shown that, in the absence of collisional relaxation of low-frequency coherence, the probe field spectrum always exhibits clearly manifested anisotropy with respect to mutual orientation of wavevectors of the strong and probe radiation (even under small Doppler broadening). It is found that the probe field spectrum may acquire under certain conditions supernarrow resonances with a width proportional to the diffusion coefficient for atoms interacting with radiation. This fact may form the basis for a spectroscopic method for measuring transport frequencies of collisions between absorbing and buffer particles. A large-amplitude supernarrow resonance (with an amplitude much larger than the amplitude of the resonance near the line center), which is observed in the far wing of the absorption line, exhibits collisional narrowing (a nonlinear spectroscopic analog of the Dicke effect) at collision frequencies several orders of magnitude lower that the Doppler linewidth. Simple working equations proposed for describing the probe field spectrum are convenient for experimental data processing.     

Continuous-wave cavity-ringdown detection of stimulated Raman gain spectra

Cavity ringdown (CRD) spectroscopy, with its high sensitivity, provides a novel way to perform continuous-wave (cw) stimulated Raman gain (SRG) spectroscopy, rather than by conventional optically detected coherent Raman techniques. Tunable cw laser light at ∼1544 nm is used to probe ringdown decay from a rapidly-swept, high-finesse optical cavity containing a gas-phase sample of interest and itself located inside the cavity of a cw single-longitudinal-mode Nd:YAG ring laser operating at ∼1064.4 nm. This approach is used to measure cw SRG spectra of the ν ₁ fundamental rovibrational Raman band of methane gas at ∼2916.5 cm⁻¹. The resulting SRG-CRD resonances have ringdown times longer than in the off-resonance case, in contrast to the usual shorter ringdown times arising from absorption and other loss processes. Previously reported noise-equivalent sensitivities have been substantially improved, by using a second ringdown cavity to facilitate subtraction of infrared-absorption background signals. Moreover, by employing a ringdown cavity in the form of a ring, the SRG-pump and CRD-detected Stokes beams can co-propagate uni-directionally, which significantly reduces Doppler broadening.     

Recording of selective intracavity absorption at doubled frequencies of multimode laser radiation

We carried out measurements of the spectral distribution of wideband dye laser generation intensity within the limits and in the vicinity of an absorption line with its simultaneous recording at the frequencies of the fundamental and second harmonics of laser radiation. In the laser cavity we placed optical cuvettes with active and absorbing media, as well as selecting and nonlinear-optical elements. We show that at double frequencies the contrast of the narrow-band hole in the multimode laser generation spectrum increases. The gain in the concentration sensitivity of measurements depends in this case on the value of the absorption coefficient being determined. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., 220072, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 516–521, July–August, 1998.     

Narrowing of a two-photon resonance due to elastic collisions

The lineshape of two-photon absorption in a standing-wave field during elastic collisions is studied. It has been found that the transit broadening of the two-photon resonance on vibrational-rotational molecular transitions is sharply decreased because of an increase in the time of interaction of particles with the electromagnetic field due to elastic collisions. The effect considered allows one to obtain supernarrow optical resonances of two-photon absorption in a low-pressure gas with high intensity.     

cw dual-wavelength operation of a diode-end-pumped Nd:YVO4 laser

A dual-wavelength continuous wave (cw) diode-end-pumped Nd:YVO₄ laser that generates simultaneous laser action at the wavelengths 1064 nm and 1342 nm is demonstrated. The optimum oscillation condition for the simultaneous dual-wavelength operation in a diode-end-pumped solid-state laser has been derived. The relationship between the laser cavity and the output stability is also studied. Experimental results show that the stability of the output power at the two wavelengths could be enhanced by use of a three-mirror cavity. Received: 26 August 1999 / Revised version: 11 October 1999 / Published online: 23 February 2000     

Propagation of light through localized coupled-cavity modes in one-dimensional photonic band-gap structures

We report on the observation of a new type of propagation mechanism through evanescent coupled optical cavity modes in one-dimensional photonic crystals. The crystal is fabricated from alternating silicon-oxide/silicon-nitride pairs with silicon-oxide cavity layers. We achieved nearly full transmission throughout the guiding band of the periodic coupled cavities within the photonic band gap. The tight-binding (TB) parameter κ is determined from experimental results, and the dispersion relation, group velocity and photon lifetime corresponding to the coupled-cavity structures are analyzed within the TB approximation. The measurements are in good agreement with transfer-matrix-method simulations and predictions of the TB photon picture. Received: 21 August 2000 / Revised version: 22 August 2000 / Published online: 9 November 2000     

The effect of doping and processing conditions on the optical performance of Rh:BaTiO3

We demonstrate the advantages of using high levels of rhodium (2000–3200 ppm) to dope barium titanate for achieving finite absorption coefficients (0.36 cm^-1), high two-beam coupling gain (11.5 cm^-1), and acceptable response time (7 s) at 1.06 μm. We also report on the mass spectroscopy measurements on Rh:BaTiO₃ samples indicating a small segregation coefficient for rhodium (below 0.01) and the presence of a relatively large concentration (6000 ppm) of unintentionally added strontium. Received: 20 November 1998 / Revised version: 25 January 1999 / Published online: 12 April 1999     

Recording of a hologram by using the second-order nonlinearity

The observation of “spatial-modulation” resonances of saturated absorption in the interaction of iodine-127 vapors with a superposition of frequency-nondegenerate TEM₀₁ and TEM₁₀ transverse modes of a linear laser is reported. The modulation of the total radiation power of the probing wave, recorded at twice the beating frequency of transverse modes of the laser, is interpreted to be the result of the transformation of modulation of the spatial distribution of the field into its amplitude modulation under the action of nonlinearly absorbing iodine vapors. Resonances of saturated absorption of the weak line R(127)11-5 of iodine-127 (633 nm) in an extracavity cell are observed by the method proposed.     

Slow light and saturable absorption

Quantitative analysis of slow light experiments utilising coherent population oscillation (CPO) in a range of saturably absorbing media, including ruby and alexandrite, Er³⁺:Y₂SiO₅, bacteriorhodopsin, semiconductor quantum devices and erbium-doped optical fibres, shows that the observations may be more simply interpreted as saturable absorption phenomena. A basic two-level model of a saturable absorber displays all the effects normally associated with slow light, namely phase shift and modulation gain of the transmitted signal, hole burning in the modulation frequency spectrum and power broadening of the spectral hole, each arising from the finite response time of the non-linear absorption. Only where hole-burning in the optical spectrum is observed (using independent pump and probe beams), or pulse delays exceeding the limits set by saturable absorption are obtained, can reasonable confidence be placed in the observation of slow light in such experiments. Superluminal (“fast light”) phenomena in media with reverse saturable absorption (RSA) may be similarly explained. The article is published in the original.     

Diffraction longitudinal multidomain structures in a layered nonlinearly absorbing medium

A method for measuring the composed phase parameter ?=Ψ₁+Ψ₂?2Ф₁ (where Ψ₁ and Ψ₂ are the phases of the mirror reflectances and Ψ₁ is the phase of the mirror transmittance) of dielectric mirrors with small absorption coefficients is proposed. This method is based on the use of the properties of a laser with a three-mirror cavity, and the mirror under study is the internal mirror of this cavity. It is shown that the value of the parameter Ψ, differing slightly from π, can be measured, and the results of the measurement are independent of the character of saturation of the laser active medium. The phase parameter was also measured for mirrors with reflectances below 0.8 by using an external method in which the interferometer with the mirror under study was placed outside of the laser cavity. It turned out that the intracavity laser method for measuring Ψ has a number of advantages and ensures an appreciably higher accuracy in measuring the parameter ε=Ψ?π than do extracavity techniques.     

Second-order doppler-free spectroscopy

The observation of second-order Doppler-free optical resonances with a width of 50 Hz are reported for the first time. It was achieved due to the use of optical selection of cold particles from an absorbing gas. The experiments have been carried out by using a new laser spectrometer, supposed to obtain the saturated absorption resonances with a relative width 10^–13–10^–14. The results of experimental and theoretical studies of second-order Doppler-free effect influence on the shape of nonlinear optical resonances in transit-time conditions are considered.     

Sensitive absorption measurements in the near-infrared region using off-axis integrated-cavity-output spectroscopy

A novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of gas mixing ratios using near-infrared diode lasers and absorption-spectroscopy techniques. The instrument employs an off-axis trajectory of the laser beam through the cell to yield an effective optical path length of several kilometers without significant unwanted effects due to cavity resonances. As a result, a minimum detectable absorption of approximately 1.4×10^-5 over an effective optical path of 4.2 km was obtained in a 1.1-Hz detection bandwidth to yield a detection sensitivity of approximately 3.1×10^-11 cm^-1 Hz^-1/2. The instrument has been used for sensitive measurements of CO, CH₄, C₂H₂ and NH₃. Received: 6 May 2002 / Revised version: 31 May 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-650/965-7074, E-mail: d.baer@lgrinc.com     

Design of a nanoscale silicon laser

The recent observation of optical gain from silicon nanocrystals embedded in SiO₂ opens an opportunity to develop a nanoscale silicon-based laser. However, the challenge remains to design and develop a laser architecture using CMOS-compatible materials. In this paper we present two designs for a waveguide laser in which silicon nanocrystals embedded in SiO₂ are used as the optical gain media. One design employs a SiO₂ membrane containing encapsulated Si nanocrystals. Preliminary calculations given here show that a highly resonant laser cavity can be produced in a SiO₂ membrane using sub-wavelength structures. This photonic crystal architecture, used to guide and contain the light, can be combined with a gain medium of optically active Si nanocrystals synthesized in the SiO₂ membrane using ion implantation/thermal annealing to produce a Si-based laser. The laser cavity dimensions can be matched to the near-infrared wavelengths where optical gain has been observed from Si nanocrystals. The second design utilizes silicon nanocrystals embedded in a distributed-feedback laser cavity fabricated in SiO₂. Lasing action over a broad wavelength range centered at ∼770 nm should be possible in both of these configurations. Received: 20 December 2002 / Accepted: 7 January 2003 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-434/982-2037, E-mail: supriya@virginia.edu     

Photoproduction of η and η' mesons off nucleons close to threshold

The importance of Born terms and resonance exchange for η and η' photoproduction off both the proton and neutron within U(3) baryon chiral perturbation theory is investigated. Low-lying resonances such as the vector mesons and J ^P = 1/2⁺, 1/2^- baryon resonances are included explicitly and their contributions together with the Born terms are calculated. The coupling constants of the resonances are determined from strong and radiative decays. We obtain reasonable agreement with experimental data near threshold. Received: 17 March 2000 / Accepted: 8 September 2000     

Phase stability theory of Bloch eigenstates in active photonic lattices with coupled microlaser arrays

An generic model for the lattice dynamics of coupled microlaser arrays is employed for the lattice stability analysis. Nonlinear cross-cavity gain-coupling effects, characterizing active lattices, are included via the gain dependence on carrier depletion and cross-cavity hole burning. Passive near neighbor interactions (inter-cavity absorption and mirror reflection interference) are also included. The introduction of lattice-orthogonal modes simplifies the derivation of the coupled rate equations. The interaction phase among sites exhibits spontaneous long range “crystallization" into periodic Bloch states whereby the cavity radiation envelopes behave as laser “macro-atoms". The sign of the coupling coefficients as a function of geometry determines in- vs. out-of-phase locking and has practical implications for array design. Emphasis is placed on the stability analysis of Bloch states by including earlier omitted [1] effects of phase perturbations. The importance of the linewidth factor ι is uncovered: unconditional stability results for ι ≤1, otherwise a stability threshold exists for the coupling strength among sites. Choice of low ι gain material permits phase stability with high coupling strength, beneficial in overcoming manufacturing variations among array cavity parameters.     

High-resolution cavity leak-out absorption spectroscopy in the 10-μm region

2 laser. After excitation, the laser power is turned off for a short time and the subsequent decay of the radiation stored in the cavity is observed via detection of the light leaking out through one of the cavity mirrors. Measurement of the decay time allows one to determine the photon losses und thus to detect weakly absorbing species inside the cavity. Since the cavity is frequency-locked to the laser the decay time can be probed with a high repetition rate, basically limited by the sampling rate of the analog-to-digital converter. This approach is closely related to cavity ring-down spectroscopy with pulsed lasers, but exhibits several advantages concerning spectral resolution and detection sensitivity. As a practical example we demonstrate monitoring of trace amounts of ethylene. Using R=99.5% mirrors we achieve a detection limit of 1 ppb ethylene (integration time: 100 s) corresponding to absorption losses of 3×10^-8 /cm. Further improvement is feasible when mirrors with higher reflectivity become available. Received: 2 July 1998     

Spectral-linewidth measurement with intracavity compensation for selective losses

We propose an intracavity method of determining the spectral linewidth based on the use of a special mode of operation of a laser with a magnetized atomic absorbing medium in an anisotropic cavity when the output radiation power of the laser does not depend on the optical density of the absorbing medium. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 283–285, May–June, 2000.