Ordinary Laser Research Articles

Efficient generation of the first waveguide mode in the InGaAs/GaAs/InGaP heterolaser

Generation of the first excited transverse mode (TE1) in a new InGaAs/GaAs/InGaP diode heterolaser with a thin InGaP layer at the waveguide center was studied. This laser design decreases the competition of the first and fundamental modes and provides TE1 mode lasing at a threshold current comparable to that of an ordinary laser oscillating at the fundamental TE0 mode.

Multiphoton ionization of iodine atoms and CF 3I molecules by XeCl laser radiation

We report about effective ionization of iodine atoms and CF3I molecules under the action of intense XeCl laser radiation (308 nm). The only ion fragment resulting from the irradiation of the CF3I molecules is the I+ ion. We have studied the influence of the intensity, spectral composition, and polarization of the laser radiation used on the intensity of the ion signal and the shape of its time-of-flight peak. Based on the analysis of the results obtained, we have suggested the mechanism of this effect. The conclusion drawn is that the ionization of the iodine atoms by the ordinary XeCl laser with a nonselective cavity results from a three- (2 + 1)-photon REMPI process. This process is in turn due to the presence of accidental two-photon resonances between various spectral components of the laser radiation and the corresponding intermediate excited states of the iodine atom. The probability of ionization of the atoms from their ground state I(2P3/2) by the radiation of the ordinary XeCl laser is more than two orders of magnitude higher than the probability of their ionization from the metastable state I*(2P1/2). The ionization of the CF3I molecules by the XeCl laser radiation occurs as a result of a four-photon process involving the preliminary one-photon dissociation of these molecules and the subsequent (2 + 1)-photon REMPI of the resultant neutral iodine atoms.

Switchable triple-wavelength erbium-doped fiber laser using a single fiber Bragg grating in polarization-maintaining fiber

A stable and narrow wavelength spacing multiwavelength erbium-doped fiber laser is proposed and demonstrated. The laser can produce simultaneous dual- and triple-wavelength lasing oscillations with a narrow wavelength spacing of less than 0.1 nm via using a single fiber Bragg gratings written in polarization-maintaining (PM) fiber. By adjusting polarization controller, the wavelength spacing of dual-wavelength lasing oscillations can be tuned to as small as 0.032 nm. The maximum amplitude variation for every lasing wavelength is less than 0.5 dB. The room-temperature operation principle is based on the polarization hole burning and deeply saturated effect in an ordinary erbium-doped fiber ring laser (EDFRL). The laser has the advantages of simple all-fiber configuration, low cost, high stability and operating at room temperature.

Feasibility study of a conical-toroidal mirror resonator for solar-pumped thin-disk lasers

An optical resonator that is suited to a large-scale, space-based solar-pumped solid-state lasers is proposed, and it is studied by numerical simulations. The resonator consists of a conical-toroidal reflector element on which a doughnut-shaped thin-disk active medium is set, and an output coupler. Unlike the ordinary thin-disk lasers, the optical ray of the proposed resonator passes the medium radially. With this arrangement, the resonator can enjoy the benefits of the thin-disk geometry, i. e., good thermal removability and low index gradient, while getting rid of the disadvantages of them as a solar-pumped laser, low round-trip gain and poor beam quality. The output power, beam quality, thermomechanical properties, and alignment stability of the proposed resonator combined with a Nd/Cr codoped GSGG is discussed.

Stable room-temperature multi-wavelength lasing realization in ordinary erbium-doped fiber loop lasers

A suppressant effect for mode competition of multi-wavelength lasing oscillations induced by deeply saturated effect in an ordinary erbium-doped fiber ring laser (EDFRL) was observed and experimentally investigated. Results show that the effect is helpful to obtain stable multi-wavelength lasing at room temperature in the EDFRL, which offers a new and simple approach to achieve stable multi-wavelength EDF lasing. Stable two- and three- wavelength lasing oscillations were achieved based on the effect in the ordinary EDFRL for the first time to our best knowledge. The multi-wavelength lasing oscillations were so stable integrated over smaller than 1 ms that the maximum power fluctuation over more than 30 minutes of observation was less than 0.1 dB and 0.5 dB for two-wavelength lasing with a spacing of 1.28 nm and 0.76 nm, respectively.

Three-dimensional cavity cooling and trapping in an optical lattice

A robust scheme for trapping and cooling atoms is described. It combines a deep dipole-trap which localizes the atom in the center of a cavity with a laser directly exciting the atom. In that way one obtains three-dimensional cooling while the atom is dipole-trapped. In particular, we identify a cooling force along the large spatial modulations of the trap. A feature of this setup, with respect to a dipole trap alone, is that all cooling forces keep a constant amplitude if the trap depth is increased simultaneously with the intensity of the probe laser. No strong coupling is required, which makes such a technique experimentally attractive. Several analytical expressions for the cooling forces and heating rates are derived and interpreted by analogy to ordinary laser cooling.

Laser Heterodyne Measurement of Photothermal Displacement for Material Surface Characterization

AbstractSummary: Double‐heterodyne photodisplacement technique (PDT), based on the detection of the thermal expansion of a sample under irradiation by laser light has been employed to improve the resolution of photothermal displacement measurements up to orders of sub‐picometers. We have developed an ultra‐sensitive system to measure photothermal displacement of metallic and non‐metallic materials, such as silicon, due to the illumination by an ordinary laser diode. Non‐destructive characteristics of the scheme are well suited for conditions that restrict access to the sample to be measured. Careful consideration is given to eliminate the noise sources, so as to achieve the level of sensitivity that needed to detect the structural inhomogeneity of materials. Good agreement between experimental results and theoretical predictions is demonstrated.Laser double‐heterodyne detection of the photo‐thermal expansion of metallic and non‐metallic materials illuminated by a laser diode.imageLaser double‐heterodyne detection of the photo‐thermal expansion of metallic and non‐metallic materials illuminated by a laser diode.

Theoretical analysis of feedback mechanisms of two-dimensional finite-sized photonic-crystal lasers

Theoretical investigations are carried out for close-to-lasing two-dimensional finite-sized photonic crystals with active (gain) lattice points. First, laser oscillations with lower thresholds are found to occur near the photonic band edges where optical gain is enormously intensified. For several modes isolated around the band edge, the field-intensity spectra in reciprocal space and the Poynting-vector distributions in real space are investigated in detail in close-to-lasing photonic crystals. By comparing the phenomena that occur in photonic crystals with a symmetric or an asymmetric outward form, this paper clarifies the differences in the feedback mechanisms of these crystals. In a symmetric photonic crystal, laser oscillation occurs through the waves propagating along the straight passages. This feedback is basically the same as that of ordinary one-dimensional lasers, although it exhibits a complicated behavior that light waves propagating in a variety of directions interfere with each other. In an asymmetric photonic crystal, laser oscillation occurs through the waves circulating within the crystal, which could be called recurrent-photon feedback. This feedback, however, can be construed as an extension of the feedback in ordinary one-dimensional distributed-feedback lasers.

A Convenient Multicamera Self-Calibration for Virtual Environments

Virtual immersive environments or telepresence setups often consist of multiple cameras that have to be calibrated. We present a convenient method for doing this. The minimum is three cameras, but there is no upper limit. The method is fully automatic and a freely moving bright spot is the only calibration object. A set of virtual 3D points is made by waving the bright spot through the working volume. Its projections are found with subpixel precision and verified by a robust RANSAC analysis. The cameras do not have to see all points; only reasonable overlap between camera subgroups is necessary. Projective structures are computed via rank-4 factorization and the Euclidean stratification is done by imposing geometric constraints. This linear estimate initializes a postprocessing computation of nonlinear distortion, which is also fully automatic. We suggest a trick on how to use a very ordinary laser pointer as the calibration object. We show that it is possible to calibrate an immersive virtual environment with 16 cameras in less than 60 minutes reaching about 1/5 pixel reprojection error. The method has been successfully tested on numerous multicamera environments using varying numbers of cameras of varying quality.

Amplified spontaneous emission and laser emission from a high optical-gain medium of dye-doped dendrimer

We measured the amplified spontaneous emission and laser emission from high-gain media of laser-dye encapsulated dendrimers. A highly branched poly(amidoamine) (PAMAM-OH) dendrimer formed a guest–host complex with a conventional laser-dye (DCM), resulting in a high optical-gain. Of particular note was the appearance of a laser threshold, above which a super-narrowed laser spectrum was observed, although laser feedback was caused without any mirror cavity devices. The optical feedback was attributed to spatial confinement of the light due to gain guiding under optical excitation. The laser spectrum clearly indicated a resonant laser-mode with a spectrum linewidth of less than 0.1 nm. This order of spectrum narrowing is comparable to that seen in the laser emission from ordinary laser devices.

Coupled-wave theory for distributed-feedback optical parametric amplifiers and oscillators

We have derived a coupled-wave theory for optical parametric amplification and oscillation in a dielectric-modulated nonlinear optical material whose dielectric period is in resonance with the signal wave. The theory is fully consistent with the Manley–Rowe relation for nonlinear frequency conversion. A distributed-feedback optical parametric oscillator, while it retains most of the mode characteristics of a distributed-feedback laser, has the additional advantage of wavelength selectivity. Unlike a distributed-feedback laser amplifier, a distributed-feedback optical parametric amplifier, when it is seeded with an idler wave, does not have the problem of seed-signal feedback. The idler wave, which does not exist in an ordinary distributed-feedback laser, has a profound influence on the mode thresholds and resonance frequencies of a distributed-feedback optical parametric oscillator.

Large optical cavity and small vertical divergence angle semiconductor lasers

A novel coupled large optical cavity cascaded by tunnel-junction semiconductor lasers is put forward to resolve the major difficulties of ordinary laser diodes. In this structure several active regions are cascaded by tunnel junctions to couple a large optical cavity. This structure can solve the problem of catastrophic optical damage of facet and large vertical divergence caused by thin emitting area in ordinary laser diodes. The near-field facalur size reaches 1μm. Low-pressure metal-organic chemical vapor deposition is adopted to grow the novel semiconductor lasers. Slope efficiency as high as 0.80W/A per facet and vertical divergence angle of 20°and threshold current density of 277 A/cm2 are achieved from an uncoated novel laser device.

All-optical atomic clock based on coherent population trapping in ^85Rb

An all-optical microwave frequency standard based on coherent population trapping (CPT) in 85Rb is developed. The CPT resonances are detected by an ordinary edge-emitting diode laser in a simple optical setup. A buffer-gas mixture is carefully optimized to yield a narrow linewidth and a reduced temperature dependence of the resonance frequency. With the developed system we are able to measure ultranarrow optically induced hyperfine CPT resonances at <20 Hz, which is in good agreement with the linewidth calculated from experimental parameters. The frequency of an RF-signal generator has been stabilized to the CPT resonance between the two mF=0 magnetic sublevels. The relative frequency stability (square root of Allan variance) follows a slope of 3.5×10-11 τ-1/2(1 s<τ<2000 s). The best stability of 6.4×10-13 is reached at an integration time of τ=2000 s. This stability is sufficient for many high-precision applications. Frequency-shift measurements were made to evaluate the frequency dependencies on the operation parameters.

The index changes of waveguides fabricated by light irradiation in LiNbO3:Fe c rystals

The refractive index changes of waveguides fabricated by light irradiation in LiNbO3:Fe crystals were investigated by using ordinary and extraordinary laser beams at wavelengths of 632.8nm and 532nm. The experimental results show that the waveguide fabricated by ordinary light is better than that by extraordinary lig ht, as the ordinary light causes much less light-induced scattering. The illuminated time must be strictly controlled when the region with a positive index change, formed by a single thin laser beam, is used as waveguide structure. This is beacuse the waveguide properties may be influenced by the strong noise grati ng and the shift of the waveguide region after the LiNbO3:Fe crystal was irrad iated for a long time. For avoiding the influence of the noise grating, the symm etric waveguide structure with high quality can be fabricated by sheet beam illu mination employing the sandwich method at a relatively low exposure intensity. M oreover, waveguides with different index distributions and different dimensions can be fabricated by changing the width of the irradiating beam and the distance between twice irradiations.

360° shape measurement system for objects having from Lambertian to specular reflectance properties utilizing a novel rangefinder

This paper describes a 360° shape measurement system for objects with various reflectance properties. To date, ordinary laser rangefinding techniques have rarely been used to measure objects having specular reflectance. The ordinary rangefinder is also difficult to apply to objects having hybrid reflectance, i.e. those whose reflectance properties lie between Lambertian and specular. We previously proposed a rangefinder that can be applied to specular objects. The concept of that rangefinder involved the use of a light-stripe projected at a constant incident angle upon the image sensor through the use of shield masks. We propose here a new rangefinder which improves on the previous rangefinder. The main improvements are the placement of a field stop using multiple slits, automatically controlled scanning of the light source, and an automatic gain control for the image sensor. As a result, the new rangefinder allows the automatic measurement of the 360° shapes of objects exhibiting both hybrid reflectance and non-uniform reflectance. We constructed a prototype automatic 360° shape measurement system which utilizes the proposed rangefinder. The experimental results demonstrate that the system can measure the shapes of hybrid objects with almost the same level of accuracy as it can those of Lambertian objects and specular objects. The proposed rangefinder is especially valuable for industrial purposes, since it assures consistent accuracy and high reliability even for objects with unknown reflectance properties.

A novel laser flash method for measuring thermal diffusivity of molten metals

A novel laser flash method has been developed to measure the thermal diffusivity value of molten metals at 700–1800 K. The upper surface of molten metal is instantaneously irradiated by a Nd glass laser. Then, the temperature response of the same (upper) surface is measured by using an InSb infrared detector. The thermal diffusivity value can be estimated from the measured temperature response. This new laser flash method was applied to molten tin, copper and Inconel 601 at high temperature and its validity and usefulness was confirmed by showing good agreement with the values obtained by an ordinary laser flash method and with the reference values as well.

Recurrent-photon feedback in two-dimensional photonic-crystal lasers

Theoretical studies are carried out for the close-to-lasing two-dimensional finite-size photonic crystals. On the basis of the simulations on the field-intensity distributions and the photon-energy flux distributions, this paper demonstrates the occurrence of a different type of positive feedback of light, which can be called recurrent-photon feedback. This feedback mechanism can be construed as an extension of the feedback mechanism in ordinary one-dimensional distributed-feedback lasers.

Microlensed vertical-cavity surface-emitting laser for stable single fundamental mode operation

We propose and demonstrate a vertical-cavity surface-emitting laser structure that operates predominantly in the single fundamental transverse mode over a wide operation current range. In this laser structure, a microlens is integrated on top of an otherwise ordinary surface-emitting laser so that a small portion of laser output is fed back into the cavity, forcing the lasing to occur in the fundamental mode. Model calculations reproduce the observed mode selection effect.

Analytical descriptions of mode switching characteristics of diode lasers

Using perturbation analysis to solve the rate equations of semiconductor lasers, the analytical expressions for the evolution of the carrier and photon densities have been derived when the laser is switched from one oscillation mode (or wavelength) to another. The solutions show that the deviations of the carrier and photon densities are damped and oscillatory. The decay rate of these deviations depends not only on the carrier lifetime, but also on the photon lifetime, which is different from that of an ordinary diode laser experiencing switch-on processes.

New laser rangefinder for three-dimensional shape measurement of specular objects

We describe a new laser rangefinder for measurement of the shapes of specular objects. In many industrial contexts, objects with both Lambertian surfaces and specular surfaces are often manipulated. However, ordinary laser rangefinders usually cannot be used to measure specular objects. We propose a laser rangefinder for the measurement of specular objects based on a new idea in which the incident angle of a light-stripe upon the image sensor is limited through the use of a special light device. The concept of this rangefinder involves the application of triangulation to specular objects, which enables measurement of the shape of specular objects as well as Lambertian objects. We designed and built a prototype light-stripe projection rangefinder based on these principles. Experiments using the rangefinder successfully demonstrate the rangefinder's accuracy. The proposed rangefinder is a promising tool for the performance of many common industrial tasks including measurement of the shapes of polished metal and the inspection of soldered joint surfaces.