Laser-diode bars based on AlGaAsP/GaAs heterostructures emitting at a wavelength of 850 nm

Two types of laser heterostructures, i.e., those without internal mechanical stress compensation, with AlGaAs-alloy emitter and waveguide layers (type 1), and laser heterostructures with stress compensation, with AlGaAsP emitter and waveguide layers (type 2) are grown by metal-organic chemical vapor deposition (MOCVD). Laser-diode bars 5 mm wide with a fill factor of 24%, emitting at a wavelength of 850 nm are fabricated. Their power parameters in the continuous-wave (cw) and pulsed lasing modes are studied. It is shown that type-2 laser diode bars exhibit better linearity of the light-current characteristics in the cw and pulsed lasing modes in comparison with laser diode bars based on the type-1 structure.     

The temperature dependence of internal optical losses in semiconductor lasers (λ = 900–920 nm)

The temperature dependences of radiative characteristics of semiconductor lasers based on asymmetric heterostructures of the separate confinement with an extended waveguide fabricated by MOCVD epitaxy (the emission wavelength λ = 900–920 nm) are studied. It is established that the threshold concentration in the active region and waveguide layers of the laser heterostructure of the separate confinement increases in the CW lasing mode as the pumping current and temperature of the active region are increased. It is established experimentally that, in the temperature range of 20–140°C, the stimulated quantum yield remains unchanged. It is shown that the temperature delocalization of charge carriers leads to an increase in the carrier concentration in the waveguide layers of the laser heterostructure. The total increase in internal optical losses due to scattering by free charge carriers in the layers of the active region and waveguide layers of the laser heterostructure leads to a decrease in the differential quantum efficiency and to saturation of the watt-ampere characteristic of semiconductor lasers in the continuous lasing mode.     

Analysis of distributed feedback semiconductor lasers bytwo-dimensional theory

The two-dimensional theory of distributed feedback (DFB) lasers to take account of the planar waveguide structure which was presented in a previous paper (see ibid., vol.26, no.3, p.467-72, 1990) is applied to the analysis of the DFB laser threshold conditions with respect to the effects of the waveguide structure and the facet reflection. The asymmetric properties of the transverse functions of the coupled modes with respect to the Bragg frequency in the dispersion relations are found to be enhanced by the asymmetric index waveguide structure and by the asymmetric facet reflectivity. Therefore, the resulting confinement factor differences in the grating layer between the two adjacent lasing modes on both sides of the Bragg frequency give large threshold gain differences     

A rigorous boundary value solution for the lateral modes of stripe geometry injection lasers

A new mathematical model useful for analyzing lateral modes of stripe geometry lasers is presented. The oxide stripe laser is modeled as a three-layer waveguide in which the dielectric constant of the active layer varies only along the lateral direction; the dielectric constant of the surrounding passive layers is assumed to be position independent. The solution technique affords a rigorous matching of the fields of the active layer with those of the surrounding passive layers. To illustrate the model, the modes of a waveguide with parabolic dielectric variation along the lateral direction are investigated. The fields are written as a linear combination of Hermite-Gaussian (H-G) functions; heretofore, fields have been described with a single H-G function. Fundamental mode spread (spot size at halfpower) is calculated and related to the gain distribution. (Previous estimates of the lateral field spread of the fundamental mode using a single H-G function not rigorously matched at the boundaries can yield spot sizes as much as 30 percent different from results calculated from linear combinations of H-G functions.) In addition, the peak gain fields are determined at threshold for various waveguide geometries.     

Analysis of TE and TM modes in DFB lasers by two-dimensional theory

The two-dimensional theory of a distributed feedback (DFB) laser (which was previously presented and applied to the analysis of the laser threshold conditions for the transverse-electric (TE) mode in a simple three-layer waveguide structure) is developed to treat both TE and transverse-magnetic (TM) modes in a four-layer waveguide structure with a thin grating layer, which more closely reflects actual DFB laser structure. The differences between TE and TM modes for the dispersion relations and the laser threshold conditions are clarified. The effects of the waveguide structure (including grating layer thickness, refractive indexes of layers, coupling constant, and corrugation period) on the threshold gains and the gain differences between the two longitudinal modes on both sides of the Bragg frequencies are studied in detail for both TE and TM modes     

Eight-channel microdisk CW laser arrays vertically coupled to common output bus waveguides

1.6-nm spectrally spaced eight-channel semiconductor microdisk laser arrays are presented, where high-Q disk lasing modes are vertically coupled out through a common bus waveguide. The spectral channel spacing is achieved by varying the disk resonator radii from 10.6 to 10.95 /spl mu/m. Typical linewidth of 0.25 nm and side-mode suppression ratio of -20dB are observed under continuous-wave lasing operation near /spl lambda/=1.51 /spl mu/m. This is the first demonstration of integrated microresonator laser arrays.     

Circularly symmetric distributed feedback laser: coupled modetreatment of TE vector fields

The vector orientation of transverse electric (TE) fields in deriving coupled mode equations for radially outward- and inward-going modes in a circular waveguide diffraction grating is treated. The equations are derived for cylindrical waves in a system that is translationally invariant along the cylinder axis; the derivation is then extended to the waveguide geometry. The coupled mode equations are used to describe the operation of the circularly symmetric distributed feedback (DFB) laser. While predicting a similar dependence of the laser threshold gain on an azimuthal mode order to that found by a simpler, scalar-field treatment, the vector-field treatment predicts a fundamental difference in the location of the cavity resonances. The circular DFB laser is expected to lase in multiple azimuthal modes but maintain a relatively narrow overall spectral width     

Lateral mode discrimination and control in high-power single-mode diode lasers of the large-optical-cavity (LOC) type

A comprehensive study of lateral mode discrimination and control in weak-laterally-confining large-optical-cavity (LOC)-type structures is presented. The analysis centers on two types of CDH-LOC laser structures: type A, which supports only the fundamental lateral mode in both the passive and active regimes; and type B, which supports several lateral modes in the passive regime and only the fundamental mode in the active regime. The transverse confinement factorGammais peaked in the center of the structure and varies significantly across the lasing region for both device types. In the passive regime it is found that the effective-index (lateral) profile is a W-shaped waveguide for type A devices and a positive-index waveguide for type B devices. A discussion and analysis of losses in W-guides is also presented. Under carrier injection (i.e., active regime) the evolution of W-guides in CDH-LOC structures is presented as a function of increasing current density up to lasing threshold. For both type A and type B devices the effective-index profiles and corresponding lateral far-field patterns are analyzed as a function of threshold mode gain. Carrier-induced bulk-index depressions are found to be in the -0.02 to -0.04 range depending on the value of the threshold mode gain. The corresponding antiguiding parameter,R = k_{o} deltan/deltag, takes values in the -3 to -4 range, which imply values between 6 and 8 for the linewidth enhancement factor α. It is found that by controlling the threshold mode gain (i.e., changing the device length and/or its facet(s) reflectivity) devices of the same cross-sectional geometry can be made to lase either multimode (spatially) or in the fundamental mode.     

Bragg gratings in two-layer core planar silica-on-silicon waveguides and application to integrated lasers

A neodymium-doped glass waveguide laser based on a new structure employing two core layers is reported. This double-layer structure allows a photosensitive layer to be integrated with an efficient gain layer. Singlemode operation was obtained with a lasing threshold of 93 mW and an efficiency of 0.6%     

Mode structure of laser emission from ZnO Nanorods with one metal mirror

The effect of the length of ZnO nanorods (500 nm in diameter) on the mode structure of their lasing in the ultraviolet spectral region is studied by optical luminescence microscopy. It is shown that separate nanorods with a metal mirror at one of the end faces exhibit only two or three laser modes at small nanoresonator lengths (8–20 μm). The different optical losses of longitudinal and transverse waveguide modes are established for nanorods lying on a glass substrate. An increase in the optical Q factor and a decrease in the lasing thresholds can be attributed to optical reflection from the metal mirror at the end face of the nanorod.     

工作在L波段的多波长Er3 /Yb3 共掺光纤激光器

采用Er^3 ／Yb^3 共掺双包层光纤(EYDF)作为增益介质，利用一段标准的多模光纤作为空间梳状滤波器，于室温下实现了5个波长的稳定输出。通过调节偏振控制器的状态，可以实现对波长振荡个数和波长间隔的控制。腔中不加任何的稳定装置，在激光输出功率高于207mw时，仍能够得到多波长的稳定振荡，单一波长的线宽窄于0．15nm，边模抑制比大于32dB。通过优化单模光纤与多模光纤的熔接，选择更加合适的多模光纤，可实现更多波长个数的稳定的多波长输出。     

Finite time of carrier energy relaxation as a cause of optical-power limitation in semiconductor lasers

It is shown that the reason why the maximum attainable optical power in semiconductor lasers is limited is the finite time of carrier energy relaxation via scattering by nonequilibrium optical phonons in the quantum-well active region. The power and spectral characteristics of semiconductor lasers are studied experimentally at high excitation levels (up to 100 kA/cm²) in pulsed lasing mode (100 ns, 10 kHz). As the drive current increases, the maximum intensity of stimulated emission tends to a constant value (“saturates”), and the emitted power increases owing to extension of the spectrum to shorter wavelengths. The intensity saturation is due to limitation of the rate of stimulated recombination, caused by a finite time of the electron energy relaxation via scattering by polar optical phonons. It is found that the broadening of the stimulated emission spectrum is related to an increase in carrier concentration in the active region, which enhances the escape of electrons into the waveguide layers. As the drive current increases, the carrier concentration in the waveguide reaches its threshold value and there appears an effective channel of current leakage from the active region. The experiment shows that the appearance of a band of waveguide lasing correlates with a sharp drop in the differential quantum efficiency of a semiconductor laser.     

Far- and near-field investigations on the lasing modes intwo-dimensional photonic crystal slab lasers

We present experimental investigations on the lasing modes in two-dimensional (2-D) hexagonal cavities defined by photonic crystals on slab waveguide structures. The far-field emission patterns and near-field intensity distributions of the lasing modes are analyzed in polarization-resolved 2-D angular distribution measurements and spectrally resolved near-field scanning optical microscopy. The far- and near-field analyses result in identification of the various lasing modes and their subsequent classification into one- and two-dimensional modes. In the one-dimensional modes oscillating between two parallel boundaries of the cavities, longitudinal and transverse modes are identified and found to have transverse-electric polarization. In the two-dimensional modes oscillating two-dimensionally in the cavities, various modes including whispering-gallery-like modes are observed and found to exhibit various polarization states     

Continuous-wave lasing of hybrid lasers

The continuous-wave (CW) lasing of hybrid lasers, which contain a broad-band inhomogeneously broadened laser medium and a narrow-band homogeneously broadened laser medium in a single cavity, is analyzed theoretically. The interactions of the laser modes and two gain media are solved self-consistently using the coupled rate equations. The spectral, gain, and power characteristics under different gain conditions are simulated. It is shown that a small gain from a second narrowband laser medium can effectively shape the lasing spectrum and improve the spectral concentration of the hybrid laser. The total saturated gain profile of the hybrid laser is relatively smooth, due to saturation of the gain of the narrow-band medium to a smaller and below-threshold level     

AlGaAs/GaAs diode lasers (1020–1100 nm) with an asymmetric broadened single transverse mode waveguide

Approaches to the development of laser heterostructures with a broadened single-mode waveguide are studied theoretically and experimentally. It is shown that the use of n- and p-type emitters with different refractive-index values ensures lasing in the fundamental mode only, if the thickness of the waveguide layer is 2 μm. The studied semiconductor lasers fabricated using the developed heterostructure feature internal optical losses amounting to 0.6 cm^?1; the divergence in the plane perpendicular to the p-n junction is 23°. In the continuous lasing mode at room temperature, a linear power-current characteristic is obtained at an output optical power as high as 7 W.     

High efficiency 1.3 mu m superluminescent diode with absorbing tapered waveguide

A new type of superluminescent diode (SLD) with a tapered waveguide in the absorbing region is proposed. This SLD structure monitors output power, as it emits monitor light from the rear facet that is about one-tenth the output power from the front one. Device parameters have been determined by beam propagation methods. A 1.3 mu m SLD fabricated by liquid phase epitaxy (LPE) with an absorbing waveguide 200 mu m long and with a 6.5 degrees tapered angle can emit output power of 13.5 mW from the front facet at a 200 mA driving current without lasing.<>     

Self‐Assembled InAs Quantum Dots on InP(001) for Long‐Wavelength Laser Applications

Self‐assembled InAs quantum dots (QDs) embedded in an InAlGaAs matrix were grown on an InP (001) using a solid‐source molecular beam epitaxy and investigated using transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. TEM images indicated that the QD formation was strongly dependent on the growth behaviors of group III elements during the deposition of InAlGaAs barriers. We achieved a lasing operation of around 1.5 µm at room temperature from uncoated QD lasers based on the InAlGaAs‐InAlAs material system on the InP (001). The lasing wavelengths of the ridge‐waveguide QD lasers were also dependent upon the cavity lengths due mainly to the gain required for the lasing operation.     

Optical field analysis of a circular grating-coupledsurface-emitting laser with integrated focusing outcoupler

The lasing characteristics of a circular grating-coupled surface-emitting laser (GCSEL) with an integrated focusing outcoupler are investigated. Measured near and far fields suggest lasing in two near orthogonal and mutually incoherent linear elements, rather than ideal circular modes of the planar circular resonator, resulting in a degradation of the focused spot quality. Modeling of the evolution of the emitted optical field by Fresnel diffraction calculations, using a new orthogonal projection scheme to account for polarization, supports this assumption. Using the theory, we have also estimated the degradation of the focused spot quality induced by random errors in the period of the focusing outcoupler caused by the limited resolution of the electron-beam lithography system used to fabricate the device. We found this effect to be small compared to the effect of the nonideal mode characteristics     

Analysis of Cavity‐Mode Lasing Characteristics from a Resonator with Broadband Cholesteric Liquid‐Crystal Bragg Reflectors

Detailed optical lasing characteristics in liquid crystal (LC) microlasers consisting of multiple polymer cholesteric LC (PCLC) layers are presented as broadband resonators sandwiching a layer of thick gain media, dye‐containing nematic LC (NLC) or isotropic liquid, in between. Multiple lasing emission peaks due to Fabry‐Perot cavity modes are observed for both gain media, and their polarization characteristics investigated. To analyze lasing characteristics, specified eigen modes are defined, the polarization states of which are maintained before and after passing through the broadband resonator, and obtained for the present full system by using the Berreman 4 × 4 matrix method. Using these specified eigen modes, the optical density for each mode is calculated and compared with the experimental results, and shows good agreement. Finally, lasing characteristics between the resonators with NLC and isotropic gain media are compared, and the advantages of adopting dye‐doped NLC gain medium are shown for tunable red, green, blue lasing in a microlaser system with a broadband resonator.     

Performance of Hybrid Laser Diodes Consisting of Silicon Slab and InP/InGaAsP Deep‐Ridge Waveguides

The fundamental transverse mode lasing of a hybrid laser diode is a prerequisite for efficient coupling to a single‐mode silicon waveguide, which is necessary for a wavelength‐division multiplexing silicon interconnection. We investigate the lasing mode profile for a hybrid laser diode consisting of silicon slab and InP/InGaAsP deep ridge waveguides. When the thickness of the top silicon is 220 nm, the fundamental transverse mode is lasing in spite of the wide waveguide width of 3.7µm. The threshold current is 40 mA, and the maximum output power is 5 mW under CW current operation. In the case of a thick top silicon layer (1 µm), the higher modes are lasing. There is no significant difference in the thermal resistance of the two devices.