Presence Of Gain Research Articles

Early Results From the Cosmic Background Explorer (COBE)

AbstractThe Cosmic Background Explorer, launched November 18, 1989, has nearly completed its first full mapping of the sky with all three of its instruments: a Far Infrared Absolute Spectrophotometer (FIRAS) covering 0.1 to 10 mm, a set of Differential Microwave Radiometers (DMR) operating at 3.3, 5.7, and 9.6 mm, and a Diffuse Infrared Background Experiment (DIRBE) spanning 1 to 300 µm in ten bands. A preliminary map of the sky derived from DIRBE data is presented. Initial cosmological implications include: a limit on the Comptonization y parameter of 10−3, on the chemical potential μ parameter of 10−2, a strong limit on the existence of a hot smooth intergalactic medium, and a confirmation that the dipole anisotropy has the spectrum expected from a Doppler shift of a blackbody. There are no significant anisotropies in the microwave sky detected, other than from our own galaxy and a cosθ dipole anisotropy whose amplitude and direction agree with previous data. At shorter wavelengths, the sky spectrum and anisotropies are dominated by emission from ‘local’ sources of emission within our Galaxy and Solar System. Preliminary comparison of IRAS and DIRBE sky brightnesses toward the ecliptic poles shows the IRAS values to be significantly higher than found by DIRBE at 100 μm. We suggest the presence of gain and zero-point errors in the IRAS total brightness data. The spacecraft, instrument designs, and data reduction methods are described.

Two-dimensional periodic structures in thin-film lasers

A thin-film laser with distributed Bragg mirrors was constructed. The mirrors were of the corner-cube type and they operated in the second Bragg diffraction order. The dependences of the divergence of the radiation emitted by various two-dimensional lasers on the pumping rate were investigated. It was found that fabrication of two-dimensional structures by a holographic method followed by ion etching produced equivalent gratings in which generation of light was possible in the presence of gain.

Transient-waveguide effects and lasing time delays in electron-beam-pumped semiconductor lasers

The dynamics of electron-beam-pumped semiconductor lasers are strongly affected by transient waveguiding. We have investigated the impact of such effects on the time delay from the start of the pumping pulse to the onset of laser emission in GaAs at liquid-nitrogen temperatures. Time delays were measured as a function of pump current density for three values of the electron-beam energy (20, 30, and 40 keV). Delays up to 200 nsec were observed which are approximately two orders of magnitude larger than the minority-carrier lifetime. A theoretical model is presented for the time-dependent variations of the optical confinement to account for such long delays. The time delay to threshold is determined not by the gain buildup but by the time required to sufficiently decrease the laser-mode diffraction loss in the active layer. Initially, the presence of gain results in a negative change of the refractive index with respect to the passive material. This is opposed by a growing positive index contribution due to the rise in temperature of the pumped layer. The latter eventually dominates, thereby confining the cavity modes. Under appropriate conditions, the ensuing reduction in loss determines the onset of lasing. The importance of the waveguide effect was confirmed by measurements on devices incorporating a permanent index step at the active-passive region interface.

Review of short wavelength laser research

A review of the status of research towards achieving lasing action in the vacuum ultraviolet and soft X-ray spectral regions is presented. An analysis of the general problems likely to be encountered is accompanied by numerical results for those approaches currently considered promising. Progress on each approach is detailed and various possible methods of verifying the presence of gain with their relative merits are discussed. Research areas requiring Application of short-wavelength lasers also are given.

Bragg amplification and oscillation in periodic optical media

The presence of gain in a periodic optical medium causes a fundamental change of the propagation behavior near the Bragg regime. The usual evanescent decay turns into exponential amplification. Some implications of this behavior are discussed.

Time variations in the far-field diffraction patterns of spatial modes from electron-beam-pumped semiconductor lasers

Detailed observations of the far-field radiation patterns from electron-beam-pumped GaAs and CdSe lasers reveal angular deviations of the patterns from the laser axis at angles greater than 25°. Moreover, these angular deviations are observed to decrease monotonically as a function of time during each output pulse. A simple model for the near-field laser modes predicts such angular deviations. Furthermore, the model allows the time-varying angular deviations to be accounted for in terms of a time-varying refractive index difference between the pumped and unpumped regions of the laser cavity. Two major sources of sufficient index variations are identified, viz., heating by the electron beam and index changes related to the presence of gain. The relative magnitudes of the two effects have been determined for GaAs and CdSe. The inherent time dependence of these effects proves adequate to account for the observed ``angular tuning'' of the spatial modes. Spatial variations in the refractive index have consequences other than angular tuning of the far-field modes. In particular, it is found that the diffraction losses associated with the spatial modes are strongly dependent on the refractive index difference. The impact on laser dynamics of the consequent time-varying total cavity loss is discussed.