ABSTRACT
We show theoretically and experimentally that distributed Bragg reflector (DBR) supports a surface electromagnetic wave exhibiting evanescent decay in the air and oscillatory decay in the DBR. The wave exists in TM polarization only. The field extension in the air may reach several wavelengths of light. Once gain medium is introduced into the DBR a novel class of diode lasers, semiconductor optical amplifiers, light-emitting diodes, etc. can be developed allowing a new type of in-plane or near-field light outcoupling. To improve the wavelength stability of the laser diode, a resonant cavity structure can be coupled to the DBR, allowing a coupled state of the cavity mode and the near-field mode. A GaAlAs-based epitaxial structure of a vertical-cavity surface-emitting laser (VCSEL) having an antiwaveguiding cavity and multiple GaInAs quantum wells as an active region was grown and processed as an in-plane Fabry-Pérot resonator with cleaved facets. Windows in the top stripe contact were made to facilitate monitoring of the optical modes. Three types of the optical modes were observed in electroluminescence (EL) studies under high current densities > 1 kA/cm2. Mode A with the longest wavelength is a VCSEL-like mode emitting normal to the surface. Mode B has a shorter wavelength, emitting light at two symmetric lobes tilted with respect to the normal to the surface in the direction parallel to the stripe. Mode C has the shortest wavelength and shifts with a temperature at a rate 0.06 nm/K. Polarization studies reveal predominantly TE emission for modes A and B and purely TM for mode C in agreement with the theory. Spectral position, thermal shift and polarization of mode C confirm it to be a coupled state of the cavity mode and near-field DBR surface-trapped mode.
ABSTRACT
Oxide-confined vertical cavity surface emitting lasers (VCSELs) with anti-waveguiding AlAs-rich core presently attract a lot of attention. Anti-waveguiding cavity enables the maximum possible optical confinement of the VCSEL mode ("λ/2 design"), increases its oscillator strength and reduces dramatically the optical power accumulated in the VCSEL mesa regions outside the aperture. VCSEL designs are suggested that favor single transverse mode operation. Modeling including current-induced and absorption-induced overheating shows that the preference for the transverse fundamental mode persists up to 10 mA current at 5 µm aperture diameter. Error-free data transmission is realized up to 160 Gb/s in digital-multitone (DMT) format using single-mode anti-waveguiding VCSELs. The approach to single-mode anti-waveguiding VCSELs is extended over a broad spectral range realizing error-free high-speed data transmission at both 850 nm and 910 nm.
ABSTRACT
A method for storing optical data pulse sequences, frequency shifted with respect to the original data pulse frequency, is theoretically described and experimentally demonstrated. Data pulses are converted into long-living acoustic waves via stimulated Brillouin scattering in optical fiber by counterpropagating write pulses of one frequency, and later they are retrieved by read pulses at a different frequency giving rise to frequency-shifted stored pulses. The shifted frequency is governed by the phase-matching condition between the read pulse and the acoustic wave, which can be satisfied using birefringent fibers. The converted frequency is +/-52 GHz and is tuned by applying strain to the fiber with a slope coefficient of +/-1.8 MHz/micro epsilon, and conversion efficiency can be as high as 13% for the storage time of 8-25 ns.
ABSTRACT
A superresolving three-zone plate is applied to a Fresnel diffractive lens. It is shown that for radial incident polarization this combination produces a focal spot approaching superresolution allowed subdiffractive limit of 0.36lambda/NA for focusing. For media responsive to longitudinal field component only, our phase engineering scheme results in a focal spot size of 0.368lambda/NA. When used with a solid immersion lens, the scheme can generate the smallest focal spot available for passive optics.
ABSTRACT
Novel features in stretched-pulse and similariton mode-locked regimes of Yb-doped fiber laser with photonic bandgap fiber used for dispersion compensation are found by means of numerical simulations. We show that the mode-locked pulse may become shorter with increasing third-order dispersion. Analytical estimations explain observed behavior through resonant interaction of the main pulse with dispersive waves involving both resonant sidebands and zero-group-velocity dispersion waves. Switching between the stretched-pulse and the similariton regimes is also studied.
ABSTRACT
For pump-probe stimulated Brillouin scattering with a probe pulse of a few nanoseconds duration and with a finite DC level, the acoustic wave relaxation time varies with the pump power and the DC level. For a pump power of 1-6 mW, the acoustic wave relaxation changes from approximately 9 to 90 ns for polarization-maintaining fiber at a temperature of -40 degrees C for a 2 ns pulse width. When the pulse DC ratio of the probe varies from 10 to 20 dB, the acoustic relaxation time changes from 24 to 45 ns for single-mode fiber at 25 degrees C. This induces a power-increment spectral feature in the detected AC pump signal in the Brillouin loss spectrum of two temperature or strain sections, where both spectral components appeared at positions far from those related to the natural phonon relaxation time (approximately 10 ns) equivalent length. The theoretical calculations confirm the prolonged phonon relaxation.
ABSTRACT
The ultimate spectral and spatial resolutions of distributed sensing based on stimulated Brillouin scattering (SBS) in optical fibers is shown for several-nanosecond Stokes pulses. Precise measurements of the local Brillouin frequency, with a spectral resolution close to the natural linewidth and, simultaneously, the spatial resolution of the pulse length are provided by AC detection of the output pump in the case of a finite cw component (base) of the Stokes pulse. Simulation examples of SBS-based sensing for fibers containing sections with different Brillouin frequencies are presented, demonstrating the high resolution of the sensing.
ABSTRACT
The generation regimes of an all-fiber passively mode-locked ytterbium laser with intra-cavity photonic crystal fiber have been studied with the aim to provide recipes for obtaining chirp-free sub-picosecond pulses directly from the cavity. Small-beam area photonic-crystal fiber is used for dispersion compensation of the intra-cavity normal dispersion of b-doped and single-mode fibers as well as for spectrum expanding due to enhanced nonlinearity. Regions of the gain and fiber parameters near the generation threshold were found in both cases of normal and anomalous net intra-cavity dispersion, which provide a stable generation of ultra-short sub-picosecond pulses directly from the cavity. Laser parameters of a transition to the multi-pulsed generation regimes were also found.
ABSTRACT
Due to the resonant nature of Brillouin scattering, delay occurs while pulse is propagating in an optical fiber. This effect influences the location accuracy of distributed Brillouin sensors. The maximum delay in sensing fibers depends on length, position, pump and Stokes powers. Considering pump depletion, we have obtained integral solutions for the coupled amplitude equations under steady state conditions and then calculated the group delay. The results show that moderate pump depletion (which is the optimized sensor working range) mitigates significantly the delay, and the maximum delay induced at resonance is only a fraction of Brillouin Optical Time Domain (BOTDA) spatial resolution, which means that the use of pulse width to define the spatial resolution is valid when Brillouin slow light is considered. We have shown that uniform strain and temperature distribution in a fiber gives the maximum delay induced uncertainty.
ABSTRACT
Slow-light effect via stimulated Brillouin scattering (SBS) in single-mode optical fibers was considered for short probe pulses of nanosecond duration relevant to Gb/s data streams. Unlike recent estimations of delay versus pump based on steady-state small-signal approximation we have used numerical solution of three-wave equations describing SBS for a realistic fiber length. Both regimes of small signal and pump depletion (gain saturation) were considered. The physical origin of Stokes pulse distortion is revealed which is related to excitation of long-living acoustic field behind the pulse and prevents effective delay control by pump power increase at cw pumping. We have shown different slope of the gain-dependent delay for different pulse durations. Spectrally broadened pumping by multiple cw components, frequency-modulated pump and pulse train were studied for short pulses which allow to obtain large delay and suppress pulse distortion. In the pump-depletion regime of pumping by pulse train, both pulse delay and distortion decrease with increasing pump, and the pulse achieves advancement.
ABSTRACT
Solving the elastic wave equation exactly for a GeO2-doped silica fiber with a steplike distribution of the longitudinal and shear velocities and density, we have obtained the dispersion, attenuation, and fields of the leaky acoustic modes supported by the fiber. We have developed a model for stimulated Brillouin scattering of these modes in a pump-probe configuration and provided their Brillouin gains and frequencies for an extended range of core sizes and GeO2 doping. Parameter ranges close to cutoff of the acoustic modes and pump depletion enhance the ratio of higher-order peaks to the main peak in the Brillouin spectrum and are suitable for simultaneous strain-temperature sensing.
ABSTRACT
We show theoretically how high-order stimulated Raman scattering in the impulsive pump-probe regime can be used for generation of single mid-infrared (MIR) single-cycle pulses. The propagation of MIR probe pulses in a hollow waveguide filled with a Raman-excited gaseous medium, with a probe delay in the maximum of the molecular oscillations, results in spectral broadening covering almost 2 octaves. The spectral phases of this broadening can be compensated for by use of an output glass window with anomalous dispersion in the MIR. The spectral and temporal characteristics of the output pulses and the mechanism of pulse compression are studied by use of numerical and analytical solutions, and compression of a 70-fs input pulse at 4 microm to a single-cycle 6.5-fs output pulse is shown.
ABSTRACT
We report a 30% internal conversion efficiency for the first Stokes pulse in stimulated Raman scattering of femtosecond pulses that are dispersively stretched to 250 ps, obtained by use of an all-solid-state laser system. A transfer of the linear chirp is observed, leading to a Raman pulse duration of 190 fs after recompression. Compressed pulse energies of 80 muJ at a repetition rate of 1 kHz are obtained, with a potential for an easy increase to more than 150 muJ. The temporal and spectral characteristics of the Raman and pump pulses are calculated, and the results explain the observed transient features in the presence of chirp.
ABSTRACT
It is shown that phase-locked pulses as short as 3 fs can be generated by coherent scattering in impulsively excited Raman media without the necessity of external phase control. The underlying mechanism, temporal characteristics, spectra, phase relations, physical limitations owing to competition processes, and precompensation of dispersion by the hollow waveguide window are studied analytically and numerically without the use of the slowly varying envelope approximation and with a global approach to dispersion. Additionally, the large frequency shifts in both the Stokes and anti-Stokes directions of as much as half the carrier frequency raise the possibility of generating widely tunable ultrashort pulses.
ABSTRACT
We present a theoretical study of temporal and spectral characteristics and pulse compression in hollow waveguides, using a global approach to dispersion without application of the slowly varying envelope approximation. A novel ultrawide self-phase modulation-induced spectral-broadening regime with spectra covering almost 3 octaves is predicted for a pressure at which the group-velocity dispersion parameter is small and anomalous. Compression to subcycle pulses by an appropriate broadband modulator and pulse shortening without chirp control by a spectral filter are studied.
ABSTRACT
We study solid-state laser mode locking in the self-induced transparency regime of an intracavity absorber. Depending on the absorber dephasing, pulse energy, and resonator dispersion, we find stable coherent 2pi -pulse operation, pulse splitting, oscillatory pulse shapes, and smooth transition to incoherent saturable-absorber mode locking.
ABSTRACT
The influence of higher-order phase dispersion on the pulse generation in femtosecond Kerr-lens mode-locked lasers for small net group-velocity dispersion is numerically analyzed. Depending on the third- and the fourth-order dispersion, we obtain the formation of spectral sidebands phase matched with the principal spectrum. At a relatively large amount of fourth-order dispersion pulse splitting arises, which leads to a quasi-steady-state multipulse operation regime of the Kerr-lens mode-locked laser.
