Virtual Journal of Laser

We report on continuous wave lasing characteristics of GaN vertical cavity surface emitting lasers (VCSELs). The VCSEL operates at room temperature under current injection by using highly reflective distributed Bragg reflectors (DBRs) made up of transparent
${rm ZrO}_{2}$ and
${rm SiO}_{2}$ film stacks. Together with high reflectivity and the wide stop band of the DBR, the long cavity of 6
$mu{rm m}$ allows multimode lasing oscillation with narrow mode spacing of 2.9 nm. In addition, a short cavity structure of 2
$mu{rm m}$ is fabricated and shows quasi-single mode operation. The spacing of the lasing modes shows a clear dependence of the actual cavity lengths with fairly good agreement to theory taking account of the refractive index dispersion.

We present the optical modes of a vertical-cavity surface-emitting laser resonator with a large Fresnel number in one direction, and a small Fresnel number in the other direction. The modes show properties of both the well-known Gaussian modes in lasers with a small Fresnel number and the modes of the Fourier type, which have been observed in lasers with a large Fresnel number. Also simultaneous oscillation of two modes near threshold and a higher-order mode in the direction of the small Fresnel number is observed experimentally.

The supermode behavior of lasers coherently coupled by a passive spatially filtered cavity has been experimentally measured as a function of path length error between laser cavities. A custom laser-diode pumped Nd:YAG laser is constructed containing two laser channels coupled by a spatial filter. The supermodes are seen to exhibit two different types of behavior depending on the magnitude of the relative path length error. When the path length error is small, the two supermodes present different cavity loss values and can be differentiated by gain. However, when the error exceeds a specific value, the two supermodes have identical loss and different resonant frequencies. Experiments show the two distinct regions predicted by theory, and quantitatively verify the loss, oscillation frequency, and modal power distribution of the supermodes as a function of path length error.

Due to the tremendous growth in applications for fiber laser in medical science, sensor solution, and light detection and ranging system at 1.8 to 2-
$mu{rm m}$ region, more research efforts have been directed toward developing highly efficient broadband fiber amplifiers in this range. In order to amplify this region, Thulium-Bismuth-doped fiber amplifier (TBDFA) is proposed in conjunction with 800-nm pumping. Optimal Thulium ion concentration of 4.17
$,times 10^{26}~{rm ion}/{rm m}^{3}$ and Bismuth ion concentration of 2.08
$,times 10^{26}~{rm ion}/{rm m}^{3}$ together with low phonon energy of germanate glass lead to the highest energy transfer rates. Effective energy transfer mechanism from Bismuth to Thulium in addition to the cross relaxation process between Thulium ions results in higher amplification, efficiency, and super broadband amplification in TBDFA.

Spatially and spectrally resolved near-field images of the transverse mode patterns of vertical cavity surface emitting lasers (VCSELs) are obtained by confocal microscopy with optical spectrum analyzer. A non-circular internal effective index profile of the VCSEL is inferred from the observed wavelength splitting of the degenerated modes. Using the mode intensity patterns associated with their spectral information, an elliptical parabolic profile superposed to a step-like refractive index profile is extracted by fitting the internal structural parameters of VCSEL. Wavelength splitting of the degenerated
${rm LG}_{01}$ modes is calculated from the fitting results, and is found to be in reasonable accord with our experiment results.

The unpredictability degree and bandwidth properties of chaotic signals generated by semiconductor lasers subject to dual chaotic optical injections (DCOI) are investigated numerically. The unpredictability degree is evaluated quantitatively via permutation entropy. Compared with the slave laser (SL) subject to single chaotic optical injection, both the chaotic bandwidth and the unpredictability degree can be enhanced significantly for SL with DCOI. The effects of injection strength, frequency detuning as well as feedback strength are considered. It is shown that, with the increase of injection strength, the unpredictability degree of chaotic signals generated by SL increases firstly and then decreases until saturates at a constant level. Positive frequency detuning is preferred to achieve wideband unpredictability-enhanced chaos, and higher bandwidth and unpredictability degree can be further expected by adopting two differently detuned master lasers. The physical mechanisms behind the wideband unpredictability-enhanced chaos are also revealed.

The propagation of flat top pulse is studied inside the
${rm Ce{:}BaTiO}_{3}$ crystal via degenerate two-wave mixing as a function of various polarization states of a pump-probe beam. Subluminal as well as superluminal pulse propagation is observed by tuning the crystal orientation for S-S and P-S polarization states of a pump-probe laser beam.

Two collinear laser pulses of finite spot size propagating in a plasma at an angle to density ripple wave vector are shown to produce terahertz (THz) radiation at the beat frequency. The laser pulses exert a ponderomotive force on the electrons and impart them oscillatory velocity at the difference frequency. The latter beats with the density ripple and produces a nonlinear current, whose curl is nonzero
$(nablatimesvec{J}^{NL}ne 0)$. The current drives the coherent THz radiation.

We propose and study a new definition for the effective bandwidths of the broadband chaotic signals, which sums up only those discrete spectral segments of the chaos power spectrum accounting for 80% of the total power. Compared to the definitions used conventionally, which tend to overestimate the effective bandwidths of the chaotic signals, the proposed definition measures only the bandwidths that possess significant amounts of power in the chaos spectra. With the proposed definition, the broadband chaos states can be clearly distinguished from the narrowband periodic oscillation states, based on just the values of the effective bandwidths measured. In this paper, the bandwidths of the dynamical states generated with an optically injected semiconductor laser under different definitions are studied and compared. To demonstrate the usefulness of the proposed definition in applications, such as ranging using chaos, the relations between the chaos bandwidths and the peak to sidelobe levels of the autocorrelations of the chaotic signals are also investigated.

Micro-ring lasers, with a minimum bend radius of 10
$mu{rm m}$, are presented that exhibit directional bistability and output powers up to 1.5 mW. In order to achieve devices with low threshold currents, mW output powers and directional bistability, the confilicting design constraints on the device are highlighted. Optimized adiabatic waveguide bends, bi-level evanescent couplers, and deeply etched ridge waveguide designs are presented in order to meet the stringent device design requirements. The optimized micro-ring laser devices exhibit directional extinction ratios of 25 dB and side-mode suppression ratios
${>}{rm 25}~{rm dB}$.