Wavelength-dependent Gain Research Articles

Theoretical Analysis of Wavelength Conversion Based on Four-Wave Mixing in Light-Holding SOAs

We propose to use an additional injection beam of short wavelength to enhance the wavelength conversion that utilizes the four-wave-mixing (FWM) effect in a semiconductor optical amplifier (SOA). With this scheme, the assist light can increase the saturation intensity without sacrificing the gain of an SOA, and this leads to an increase in conversion efficiency. A numerical method dealing with various FWM mechanisms, such as amplified spontaneous emission (ASE) noise, longitudinal spatial hole burning, and wavelength-dependent gain spectrum, is developed to predict the static characteristics of our scheme. The carrier densities are nonuniformly distributed along the longitudinal direction of the SOA as a result of the ASE effect, which affects the measurement of the wavelength-dependent transparent current. The effects of an assist light on saturation output power and conversion efficiency are analyzed in detail. The analysis shows that using an assist light can improve both the conversion efficiency and signal-to-background-noise ratio (SBR) for SOAs of different lengths. The degree of improvement depends on the bias condition, assist light wavelength, and the device geometry. The study for the device optimization reveals that a compromise between conversion efficiency and SBR must be made to choose the device length.

Novel Infrared Quantum Dot Lasers: Theory and Reality

Semiconductor heterostructures with size quantization of carriers in all three directions, or quantum dots (QDs), open a new era in device engineering. An important breakthrough occurred when the self-organised growth approach had been refined to such an extent, that defect-free QDs made with these technique exhibited properties predicted for zero-dimensional structures. Their practical application in devices, first of all lasers, became possible. When used for active layers in injection lasers differential gain is strongly increased potentially providing improved dynamic properties. The cut-off frequencies larger than 10 GHz are presently demonstrated. The saturation material gain is as high as 15 x 10 4 cm -1 as compared to QW values of about 3 x 10 3 cm -1 , The wavelength dependence of gain is best described by Master equations of microstates. High internal (>96%) and differential (70%) efficiencies at 300 K and output power up to 4 W are realized now for QD lasers emitting in the 0.94-1.15 μm range. Transparency current density of 18 A/cm 2 has been recently demonstrated at 1.15 μm. GaAs-based QD lasers emit near 1.3 μm with J th = 70 A/cm 2 (L = 1.9 mm, 300 K) and CW output power of ∼3 W. In narrow (7 μm) stripes almost chirp-free operation and relaxation oscillations at 2.4 GHz (L = 400 μm) are manifested. Differential efficiency is 55% and internal losses are 1.5 cm -1 . 1.3 μm GaAs-based QD VCSELs (300 K, I th = 1.8 mA, η diff > 40%) are realized.

Ultraviolet-selective avalanche photodiode

An ultraviolet (UV)-selective avalanche photodiode is presented. This device has maximum responsivity at the wavelength of 380 nm. A ratio of the responsivities at 380 nm and 1 μm larger than 80 is achieved. This selectivity results from the combined effect of a potential barrier for holes and of a wavelength-dependent photomultiplication gain. In the avalanche mode, it shows a homogenous gain of 50 in the UV and 20 in the visible (VIS)/near-infrared (NIR) ranges. To our knowledge, this is the first observation of an increase of the photomultiplication gain in the UV region. A measured excess noise factor F of 2.8 (at M ph=20) maintains a detectivity limit comparable to that of conventional avalanche photodiodes. Both low-noise avalanche operation and increased UV selectivity can now be achieved simultaneously.

Dead space effect on the wavelength dependence of gain and noise in avalanche photodiodes

We extend the dead space model proposed by Hayat et al. in order to determine the wavelength-dependent multiplication mean gain and excess noise factor F(/spl lambda/) in the case of mixed electron and hole injection, as it is the case when photons are absorbed within the multiplication region. We compare the predictions of the model with measurements performed on a silicon ultraviolet-selective avalanche photodiode with submicron thick multiplication region. We show that the multiplication gain is constant in the visible and near-infrared part of the spectrum, and increases in the UV range by a factor of 1.8. Furthermore, the excess noise factor is minimal for UV radiation and increases rapidly for longer wavelengths. It appears that the extended dead space model is very adequate at predicting the gain and noise measurement results. In order to unambiguously determine the effect of the dead space, we compare the predictions of our model with those of McIntyre's local noise model. The latter qualitatively describes the wavelength dependence of the gain, but greatly overestimates the excess noise factor.

Supermirror neutron guide tube on Sirius, the high resolution powder diffractometer at KENS

A neutron guide tube was mounted on Sirius, the high-resolution powder diffractometer newly installed at Neutron Science Laboratory (KENS) at High Energy Accelerator Researh Organization (KEK), and its performance was investigated. A NiC/Ti supermirror with the critical wave number triple of that of natural nickel was employed for the guide tube coating in order to utilize shorter wavelengths (∼1 Å) required on the powder diffractometry. The wavelength-dependent intensity gain is observed, which enhances the performance of Sirius over the powder diffractometer already existing at KENS without a guide tube.

Second-order distortion improvements or degradations brought by erbium-doped fiber amplifiers in analog links using directly modulated lasers

Within lightwave analog amplitude-modulated (AM) CATV systems using directly modulated lasers, erbium-doped fiber amplifiers (EDFA's) act upon the signal distortion because of the interaction between the laser chirp and the EDFA wavelength-dependent gain. This interaction is theoretically investigated in order to predict the EDFA-induced distortion. The relevant gain tilt characteristic for analog applications and the way to measure it are described. Expected and measured distortions at the EDFA output are in excellent agreement. Fiber amplifiers are found to decrease the signal distortion level when the gain tilt is negative, i.e., for wavelength above the gain maximum. >

A self-consistent two-dimensional model of quantum-well semiconductor lasers: optimization of a GRIN-SCH SQW laser structure

A two-dimensional model for quantum-well lasers that solves, self-consistently, the semiconductor equations together with the complex scalar wave equation is described. It incorporates a position- and wavelength-dependent gain function which is derived from a quantum mechanical calculation. Such a model enables one to predict the characteristics of a quantum-well laser with a minimal number of empirical parameters. The output of the model includes light-current characteristics, the current distribution, and the optical field intensity distribution, obtained simultaneously in the calculation. Examples for modeling GRIN-SCH SQW (graded-index separate confinement heterostructure single quantum well) ridge wave guide lasers are given, and good agreement with experimental results is obtained. The model is used to optimize the geometry of a GRIN-SCH SQW laser for minimum threshold current and maximum efficiency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

New developments in resolving detail in aeromagnetic data

Stacked profiles of aeromagnetic data often fail to resolve and display detail as this can be obliterated by larger amplitude features on adjacent flight lines. Contours and images of interpolated data can also obscure detail because they are sensitive to survey levelling errors and can show erroneous distortions caused by the interpolation method. Mclntyre (1981) showed that shaded stacked profiles of the second horizontal differences of flight-line data are effective in detecting and displaying detail in aeromagnetic data. Mclntyre used a square-root function to control the dynamic range of the computed difference so that both small and large amplitude features could be displayed without being obliterated by larger amplitude features on adjacent flight-lines. The shaded profile preferentially enhances positive values. Improvements on Mclntyre's shaded stacked profiles of horizontal differences form the basis of a new method of presenting horizontal differences of aeromagnetic data: the bipole map. This is a series of stacked flight-line profiles of bar-graphs of the horizontal difference values. The heights of the bar-graphs are controlled by the amplitude of the difference values whilst polarity is depicted with colour: red for positive values and blue for negative values. The bar-graphs are centred and located precisely on the curved flight-line; the flight-line forming the centre-line of the profile. Wavelength dependent automatic gain control (AGC) filters are used to modify the dynamic range of the horizontal differences so that both small and large amplitude anomalies can be displayed. The bipole map visually enhances both negative and positive values, more precisely positions the data and does not depend upon dubious magnetic values interpolated between the flight-lines. Bipole maps of the first, third and fourth horizontal differences are effective in resolving detail in other parts of the frequency spectrum. They supplement contours and images of longer wavelength features. Magnetic models show the responses of the horizontal differences for various inclinations of the Earth's magnetic field and different widths of the magnetic source. In areas of steep inclination, the horizontal differences delineate the strike axis of thin bodies and the edges of thick bodies. Bipole maps of horizontal differences may find useful application in displaying other types of geophysical data such as VLF-EM and aeromagnetic gradiometer data. Bipole maps of higher order differences are an effective means of resolving high frequency (instrument) noise in geophysical data. For aeromagnetic data from the Mt Magnet goldfield, located in the Archean greenstone belt of Western Australia, bipole maps of the second horizontal differences revealed that an anomaly from a banded iron formation might be caused by several close-spaced banded iron bodies rather than a single body, as might first be interpreted from the contours and images of the gridded data. Bipole maps of the fourth horizontal differences revealed a series of NNE striking magnetic lineaments which are not apparent in the contours and images of the gridded data.

Wavelength dependence of gain from 248.2 to 248.4 nm in a KrF discharge laser

The small signal gain and saturation intensity of a KrF discharge laser module have been measured in the peak gain region of 248.22 to 248.40 nm. The gain has also been measured for the frequency-doubled argon-ion laser line at 248.25 nm and found to be equal to the peak KrF gain of 0.143+or-0.002 cm-1 to within the experimental accuracy. The best-fit saturation intensity for this wavelength region was found to be 2.3+or-0.3 MW cm-2.

Recombination, gain and bandwidth characteristics of 1.3-µm semiconductor laser amplifiers

The influence of the spontaneous recombination mechanism and the temperature- and wavelength-dependent material gain on the performance of 1.3-μm InGaAsP traveling-wave semiconductor laser amplifiers is analyzed. Measurements of signal gain are presented. Frequency detuning and optical bandwidth characteristics are discussed. By considering the trade-off between optical bandwidth and resonant signal gain, guidelines for the requirements to the facet reflectivity are given.

Flashlamp-pumped Cr&amp;lt;sup&amp;gt;3+&amp;lt;/sup&amp;gt;:GSAG laser

Flashlamp-pumped vibronic lasing has been demonstrated at room temperature with the garnet Cr3+: GSAG. Continuous wavelength tuning was observed from 765 to 801 nm in preliminary experiments. An energy output of 110 mJ/pulse was obtained at 784 nm with a slope efficiency of 0.12 percent. Wavelength dependent gain and loss data are presented and are discussed with reference to Cr3+: GSGG measurements.

Wavelength dependence of gain saturation in GaAs lasers

The amplification of light in GaAs lasers is calculated self-consistently. It is shown that the wavelength dependence of the gain saturation is caused by the change of the chemical potential within the excited region due to the coupling of the carrier density with the photon density via the stimulated emission process. The effect of the wavelength dependent gain saturation on the stimulated emission spectra is calculated and compared with experimental results. Comparison of different recombination models indicates that a band-to-band transition without <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</tex> -selection rules including tail states is the most appropriate description of the experimental data even for pure material.

Wavelength Dependence of Gain in Rhodamine Dye Laser Oscillation

Wavelength Dependence of Gain in Rhodamine Dye Laser Oscillation

Spontaneous and stimulated luminescence due to exciton-exciton collisions in semiconductors

Spontaneous emission and optical gain spectra have been calculated for typical radiative exciton-exciton collision processes using a simplified model interaction potential. The theoretical spontaneous emission spectra evaluated numerically are in good agreement with the P emission band of GaAs. Further, the results obtained reasonably explain the wavelength-dependent gain saturation and apparent red-shift of the stimulated emission band on increasing the excitation intensity or pumping length, observed in various semiconductors.