Semiconfocal Cavity Research Articles

Spiking ruby revisited: self-induced periodic spiking oscillations leading to chaotic state in a Cr:Al2O3 laser with cw 532-nm pumping

This paper reexamines a 60-year-old mystery of spiking behavior in ruby lasers with a cw 532-nm pump, paying special attention to mode matching between the pump and lasing beam within a ruby crystal placed in a semi-confocal laser cavity. Periodic spiking oscillations were observed in a limited pump power regime, where spikes obeying the generic asymmetric hyperbolic function appeared at a repetition rate around 50 kHz and with a 130-150 ns width and 0.1-0.6 µJ energy depending on the pump power. The physics of the spiking behavior based on Kleinman’s mechanical approach and a plausible interpretation for the periodic spiking oscillation in terms of self-induced mode matching between the pump and laser beams through the self-induced Kerr-lens effect are addressed. The statistical nature inherent to spiking and the associated self-organized critical behavior in the quasi-periodic spiking oscillations as well as chaotic states occurring outside the periodic spiking regime are clarified from a nonlinear dynamics point of view and intriguing statistical properties inherent to chaotic oscillations in usual solid-state lasers subjected to external modulations are shown to be present in the self-induced instabilities of the ruby laser

Generation of an optical beam with two-dimensional dark focus in a fractionally-degenerate cavity

An optical beam with a dark region surrounded by light in two dimensions, formed as a coherent superposition of mainly two Hermite–Gaussian eigenmodes HG04 and HG08, has been generated in an axially end-pumped Nd:YAG laser with an astigmatic fractionally degenerate semiconfocal cavity with an intracavity obstacle (a needle). Being focused with a lens, the beam has a dark region on its axis, surrounded by high-intensity regions only in z- and y-directions while maintaining Gaussian behavior in the x direction. The influence of the number of HG0n eigenmodes and comparison of the beam with a geometric M-mode are studied numerically.

Optical gain and stimulated emission in optically pumped heavily doped Al0.74Ga0.26N:Si structures with external cavity

The pulse lasing in the broadband spectral range 420–590 nm was obtained from two squeezed together heavily Si doped Al0.74Ga0.26N/AlN/Al2O3 structures with external cavity under pump radiation with λp = 266 nm wavelength. These structures were grown by molecular beam epitaxy on (0001) oriented thick sapphire substrates. Stimulated emission at room temperature was observed with optical pumping thresholds 50 kW/cm2 in the semiconfocal cavity. The linear transverse-electric polarization dominates in the output emission. The optical gain coefficients values of (0.12–6) × 103 cm−1 for Al0.74Ga0.26N:Si films with inhomogeneous luminescence broadening were measured at 10–600 kW/cm2 pump power density radiation with 266 nm wavelength, 8 ns pulse duration and 10 Hz repetition rate.

Ultrathin Metasurface Wavelength-Selective Mirror for Millimeter/Terahertz Wave Fabry-Perot Cavities

We present an ultrathin metasurface working as both a highly reflective wavelength-selective mirror and a high transmittance output coupler for a semiconfocal Fabry-Perot cavity. The numerically optimized 2-μ m-thick metasurface achieves a peak reflectivity of R = 99.35% at ω0 = 94 GHz and peak transmittance of T ≈ 90% at the fourth harmonic (4ω0). Experimental results show that the metasurface FP cavity exhibits comparable quality factors to cavities enclosed by either a copper mirror or Bragg grating mirror. Our work highlights the potential for ultrathin metasurfaces to function as frequency selective mirrors in millimeter or terahertz wave cavities for various applications, such as nonlinear terahertz wave generation, material characterization, and imaging.

Cr2+:ZnSe active media with complex profiles of internal doping

Cr2+:ZnSe polycrystalline active elements in the form of disks with one and two inner doped layers were fabricated by using solid-state diffusion bonding (SSDB) with subsequent hot isostatic pressing. A comparison of the generation properties of active elements with the same absorption and various profiles of internal doping was made using a scheme with a semi-confocal cavity and CW thulium fiber laser pumping. All test samples showed high thermal and optical resistance at a pump power density of up to 50 kW cm−2 in room-temperature experiments that indicates the promise of using SSDB technology in high-power mid-IR lasers. The samples with two layers of internal doping demonstrated 30% higher lasing slope efficiency, than samples with one doped layer. This difference is explained by more uniform heating of the active element which was caused by increasing the volume of the sample involved in the absorption of pumping radiation.

Direct excitation of a single quantum dot with cavity-SPDC photons.

The ability to generate mode-engineered single photons to interface with disparate quantum systems is of importance for building a quantum network. Here we report on the generation of a pulsed, heralded single photon source with a sub-GHz spectral bandwidth that couples to indium arsenide quantum dots centered at 942 nm. The source is built with a type-II PPKTP down-conversion crystal embedded in a semi-confocal optical cavity and pumped with a 76 MHz repetition rate pulsed laser to emit collinear, polarization-correlated photon pairs resonant with a single quantum dot. In order to demonstrate direct coupling, we use the mode-engineered cavity-SPDC single-photon source to resonantly excite an isolated single quantum dot.

Analysis of CV mode selected resonator based on vectorial eigenvector method**Project supported by the National Natural Science Foundation of China (Grant No. 61308045) and the National Science and Technology Cooperation Project (Grant No. 2014DFR10870).

The vectorial eigenvector method was applied to compute the cylindrical vector (CV) modes of polarization-dependent resonators and multiple modes in the CV mode selected resonator were tracked. Then, the mode characteristics of CV mode selected resonator depending on the Fresnel number, geometry parameters, and polarization parameters were simulated. When the difference in reflection coefficient between radial and azimuthal polarization is greater than 0.021 for spherical semi-confocal cavity, the radially polarized TM01* mode most probably appears in the resonator due to its lowest loss, which is consistent with an azimuthal polarization-selective resonant cavity. Moreover, the appropriate phase shift factor contributes to suppress non-rotationally symmetrical modes, which promotes the selection of CV modes. The Fresnel number between 2 and 2.5 are the appropriate range of values when the value of geometry parameters g2 is 0.5, both of which decide the effective Fresnel number.

Lasing characteristic of new Russian laser ceramics

Lasing efficiency of neodymium-doped yttrium – aluminium garnet (YAG) ceramic samples, which were fabricated at the Fryazino Branch of the Kotelnikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences and the SLPG ‘Raduga’ in 2016–2017, is studied under the conditions of end- and side-pumping. The efficiency of lasers based on this ceramics was 64 % in the case of end-pumping and 68%–70% upon side-pumping. The laser level lifetimes are measured in ceramics with neodymium concentrations of 1, 2, 3, and 4 at %. The measured lifetimes well agree with the literature data. The dependences of the lasing threshold on the cavity length under the conditions of transverse mode locking and diode end-pumping are studied for different samples of laser ceramics in a semiconfocal cavity. In sum, the performed investigations showed that the quality of the first industrial samples of Russian laser ceramics are highly competitive in quality with the ceramics produced by Konoshima Chem. Corp., Ltd., which is considered by the laser community as an etalon.

Compact UV Nitrogen Laser Pumped by a Pulsed Longitudinal Inductive Discharge

The creation of a compact emitter for an inductive nitrogen laser (λ = 337.1 nm) with the active medium pumped by a pulsed inductive longitudinal discharge of the transformer type is reported for the first time. In the inductive laser head created, the lasing energy attains 0.35 mJ and the lasing pulse length is (25 ± 5) ns (FWHM) depending on the cavity Q-factor. The use of a semiconfocal cavity provides for a near-Gaussian beam profile.

95–105 GHz 352 mW All-Silicon Cavity-Coupled Pulsed Echo Rotational Spectroscopy System in 65 nm CMOS

A new CMOS-based spectroscopy instrument for in situ rotational spectroscopy of gases in space is presented. The design is based on the pulsed Fourier transform method of Balle–Flygare but operates at mm-wave frequencies as oppose to traditional microwave implementations. The core of the instrument consists of a 65 nm CMOS transceiver chipset with integrated mixers, synthesizers, and amplifiers covering the 95–105 GHz frequency band. The CMOS chips are coupled with a semiconfocal cavity which builds field strength for pumping rotational transitions. Details of the CMOS chip design as well as system level testing are discussed.

A CMOS millimeter-wave transceiver embedded in a semi-confocal Fabry-Perot cavity for molecular spectroscopy.

The extension of radio frequency complementary metal oxide semiconductor (CMOS) circuitry into millimeter wavelengths promises the extension of spectroscopic techniques in compact, power efficient systems. We are now beginning to use CMOS millimeter devices for low-mass, low-power instrumentation capable of remote or in situ detection of gas composition during space missions. We have chosen to develop a Flygare-Balle type spectrometer, with a semi-confocal Fabry-Perot cavity to amplify the pump power of a mm-wavelength CMOS transmitter that is directly coupled to the planar mirror of the cavity. We have built a pulsed transceiver system at 92-105 GHz inside a 3 cm base length cavity and demonstrated quality factor up to 4680, allowing for modes with 20 MHz bandwidth, with a sufficient cavity amplification factor for mW class transmitters. This work describes the initial gas measurements and outlines the challenges and next steps.

Highly controllable optical bistability effect in a 2 μm Tm:YAG ceramic laser at room temperature.

A highly controllable optical bistability in a Tm:YAG ceramic laser system is reported, which is attributed to the thermal-induced change in the stability of the resonator. The width of the bistable region can be tuned in a large scale from 0.8 W to 6.3 W. At nearly semi-confocal cavity configuration, a second lasing condition was observed in the bistable region with a modulated shape of the laser beam and a broadened laser spectrum. The influence of the temperature on the bistable laser operation was also discussed in detail. To our knowledge, this is the first report on the optical bistability effect in Tm:YAG ceramic lasers.

Compact 1.64 THz source based on a dual-wavelength diode end-pumped Nd:YLF laser with a nearly semiconfocal cavity

We describe a compact dual-wavelength (1.047 and 1.053 μm) diode end-pumped Q-switched Nd:YLE laser source which has a number of applications in demand. In order to achieve its dual-wavelength operation it is suggested for the first time to use essentially nonmonotonous dependences of the threshold pump powers at these wavelengths on the cavity length in the region of the cavity semiconfocal configuration under a radius of the pump beam smaller than the radius of the zero Gaussian mode. Here we demonstrate one of the most interesting applications for this laser: difference frequency generation in a GaSe crystal at a frequency of 1.64 THz. A superconducting hot-electron bolometer is used to detect the THz power generated and to measure its pulse characteristics.

Optically pumped long external cavity InGaN/GaN surface-emitting laser with injection seeding from a planar microcavity

Optically pumped InGaN/GaN quantum well vertical-external-cavity surface-emitting laser emitting at 420 nm has been realized. Lasing at external cavity lengths of up to 50 mm is demonstrated, making integration of practical sized intracavity elements possible. Spectral and beam profile measurements indicate best operation conditions in a semiconfocal cavity configuration. Lasing threshold of 20.9 W is achieved for a 49 mm long cavity with output beam quality parameter M2 not exceeding 1.1.

Laser Doppler visualisation of the velocity field

A method for the laser Doppler real-time visualisation of the velocity field of moving media was developed. The method is based on the optical frequency demodulation of the optical fields forming the image of the investigated cross section. The visualiser was constructed on the basis of a semiconfocal optical cavity. The results of the visualisation of the velocity field of a vortical flow in a cylindrical channel are presented.

A multimode model of a waveguide laser cavity with mirrors of arbitrary radius of curvature

A multimode model is developed to determine optical losses in a waveguide laser cavity of cylindrical symmetry with mirrors of arbitrary radius of curvature. Within the framework of the model, it is possible to vary the parameters of the resonator: waveguide length, distance between the waveguide and mirrors, radius of curvature of the mirrors in a wide range taking account of mutual conversion of up to 10 waveguide modes and 20 modes of free space. The model was applied successfully to determine the optimal geometry of a semiconfocal cavity.

Fourier transform microwave observation of SO (X [formula omitted], v=0–2) produced by 193 nm photodissociation of SO 2 in a pulsed supersonic jet expansion

SO (X 3 Σ ) radicals have been produced by 193 nm photodissociation of SO 2 in a pulsed supersonic free jet expansion in Ar and detected using a molecular beam–Fourier transform microwave spectrometer with a semi-confocal cavity. The performance of the spectrometer was optimized by recording the SO (X 3 Σ ) microwave spectrum near 13000 MHz. The J=1←1, N=2←1 transition of 32 SO was observed with a signal-to-noise ratio of 10 in a single photolysis laser and microwave shot. The corresponding spectrum of 34 SO was recorded with 34 S in natural abundance (4.2%).

Characteristics of mode locking in a semiconfocal cavity

It is shown that simultaneous locking of longitudinal and transverse modes is possible in a semiconfocal cavity. A laser with a high repetition rate (10–100 MHz) then emits alternately strongly and weakly diverging pulses. A laser operating in this way may be useful in various sounding and ranging applications. A laser with such locking is a wide-aperture device with a large volume of the active medium.

Phase locking of a semiconductor diode array in an external cavity

Theoretical investigations are reported of the characteristics of high-power lasers with the active medium in the form of a semiconductor diode array. The selectivity of a Talbot cavity, of a cavity with an aspherical output mirror, and of a semiconfocal cavity is studied as a function of the array parameters. Estimates are obtained of the sensitivity of the laser radiation characteristics to fluctuations of the array parameters. It is shown that the dependence of the selectivity of an aspherical cavity on these parameters is nearly the same as that of a Talbot cavity, but the fundamental mode propagates without radiation losses between the diodes and beyond the array edges. A semiconfocal cavity is more selective than a Talbot or aspherical cavity. When the number of diodes in a linear array exceeds 20, the losses of the second mode in a semiconfocal cavity grow to more than 80%. The efficiency of a laser with a semiconfocal cavity, calculated ignoring the diode efficiency, can reach 80–90%.

Graded-phase mirror resonator with a super-Gaussian output in a CW-CO/sub 2/ laser

Two graded-phase mirror (GPM) resonators have been designed to produce super-Gaussian output beam profiles of order 4 and 6. An inverse propagation method was used to calculate the appropriate shape of the GPM's. Previous experimental work with a pulsed TEA-CO/sub 2/ laser has already confirmed the high transverse mode discrimination offered by such custom resonators, as anticipated by a theoretical analysis of their diffractional properties. Here, we extend these results by presenting new experimental measurements with a CW-CO/sub 2/ laser. The main objective of this paper is to demonstrate the super-Gaussian beam shape produced by these custom resonators. This is assessed with numerical simulations involving the incoherent superposition of transverses modes. The experimental results obtained with the GPM resonators, including measurements of the propagation of the output beam profile, are compared to those of a conventional semiconfocal cavity, and show higher monomode power extraction.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>