The laser

This article is intended as a review of the field of optical masers, or lasers as they have come to be known, summarizing both theory and practice. It starts with a theoretical section in which black body radiation theory is used to introduce the concepts of spontaneous and induced transitions. This is followed by the derivation of the Schawlow-Townes instability (start-oscillation) condition and a description of the different laser media. Other topics treated include: optical pumping, experimental techniques, output power and noise. The sections on optical resonators and communications which conclude the paper have been slightly emphasized since, perhaps to a larger extent than the other topics covered in this paper, they coincide with traditional areas of interest of microwave and communications engineers

Room-temperature-operation visible-emission semiconductor diode lasers

There were two main approaches taken to develop shorter wavelength lasers. (1) Based on (AlGa)As and liquid-phase epitaxy, significant new results were obtained: Properties of these laser diodes (power output, spectra, and beam patterns), materials considerations, laser theory, and growth problems are discussed. The design of (AlGa)As layers is discussed from the vertical point of view, and various design curves are given. Horizontal structural requirements are also discussed. Experimental results from measurements done as a function of hydrostatic pressure are correlated with other results. (2) The first heterojunction laser structures using GaAs sub l-x P sub x and In sub y Ga sub l-y P at compositions, where the lattice constants are matched, were grown using vapor-phase growth technology and are described in detail, including experimental device results. Threshold current densities from 3,000 to 5,000 A per sq cm. and emission wavelengths from 6,520 A to 6,640 A were obtained at 77 K. The limiting factor in these devices is nonradiative recombination at the heterojunctions. Life tests on facet-coated (AlGa)As CW diodes are reported

Semiconductor Ring Lasers

Miniature rib ring waveguide and pill-box laser structures as small as 12 mum in diameter were investigated. Output stripe waveguides were coupled to the rings via Y-junctions. Optical properties of the component structures were analysed using the finite difference method. The original waveguides were defined in GaAs/AlGaAs heterostructure materials by reactive ion etching through the pattern of the guide contacts with the connected bonding pads. The fabrication was then simplified by embedding the etched guides in polyimide and overcoating with a gold bonding layer. Temperature stable ohmic contacts were designed to improve the stability of the devices. Material designed with a localised gain region quantum well and an additional coupled passive guide provided single mode operation even for the broad area devices. The performance of the rib ring laser structures was comparable to that of similar size straight lasers. Standard structures defined by optical lithography in DH and QW materials lased successfully. E-beam defined devices with successively narrower ribs down to 0.4mum, and shorter ring cavities down to 12mum in diameter, increased the optical loss. This resulted in increased threshold density up to 220kA/cm2 for ring lasers made of DH material, and a lack of lasing for small structures made of QW material, where the relation is more critical. The QW devices showed evidence of both first and second quantised state operation. This type of ring laser structure is suitable for use as a light source in monolithic, integrated optics, although absorption loss due to the gold bonding layer must be eliminated to reduce threshold current of the devices

Spatial field control of broad area semiconductor lasers with integrated grating couplers

In this research, a method of designing a grating outcoupler to obtain the desired 2D-intensity profile and improved field distribution of the optical beam emitted by a grating coupled surface emitting laser is presented. The method is based on variation of the periodicity, duty cycle, and the groove tilt angle of the grating. Grating design involves numerical analysis of the optical field propagated through the grating, by applying the Rigorous Coupled Wave Approach method. Experimental evaluation of the designed grating components was done by fabrication and testing the broad area semiconductor lasers with the monolithically integrated grating outcouplers. Another grating design is presented that provides the spreading of a single optical output into multi-beams at different outcoupling angles in the emitting plane. Using field distribution presentation, an approach to provide uniform optical intensity profile from the grating outcoupler based on varying duty cycle is described. Furthermore, this work presents experimental evaluation of 1D, and 2D (8x8) square, arrays of phase-locked surface-emitters including semiconductor optical amplifiers spaced by grating outcouplers. The phase-locked multi-emitter design was based on the master oscillator power amplifier (MOPA) array approach. As the MO for the 1D array we used a monolithically integrated grating coupled laser with wavelength stabilization and a fiber coupled external laser source was used for the 2D array. Mutual coherence was experimentally evaluated by interference investigation and the obtained results were compared to numerical modeling

Optical and minority carrier confinement in lead selenide homojunction lasers.

Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science.Vita.Includes bibliographical references.Ph.D

Integrated Semiconductor Ring Lasers

The concept of a semiconductor laser with a circular resonator, its advantages and particular problems are discussed. The pillbox resonator is introduced and its operation on whispering gallery modes is illustrated using a computer model. The experimental evidence of the guiding mechanism is shown, leading to the first demonstration of continous wave operation in a semiconductor ring laser with a threshold current of 24mA. The parameters of the GaAs/AlGaAs material that are relevant for the low threshold current operation are presented and all aspects of the fabrication procedure are covered, emphasizing the processes that led to smooth sidewalls and the low loss circular cavity. A further reduction of the threshold current to 12.5mA is shown, which is owing to a coating of silicon nitride that supresses the non-radiative recombination current and reduces the scattering loss. The excess bending loss is calculated to be 3dB/360 and found to be independent of the radius between 30mum and 145mum. The influence of the Y-junction on the operation characteristic is studied and shown to cause kinks in the L-I curve; it is also held responsible for the relatively low differential quantum efficiency (0.02-0.04) of the devices. Strip-loaded guiding is demonstrated for radii between 300mum and 600mum and proposed as a solution for the problem of degradation that is caused by etching through the active layer. The integration capability of the structure is demonstrated by the succesful operation of a circuit comprising of a ring laser, a low-loss waveguide and a detector, and an optoelectronic integrated circuit featuring a ring laser and a field-effect transistor. The material parameters that are involved in performing these complicated functions are discussed and modeled numerically

The optical pumping of alkali atoms using coherent radiation from semi-conductor injection lasers and incoherent radiation from resonance lamps

An experimental study for creating population differences in the ground states of alkali atoms (Cesium 133) is presented. Studies made on GaAs-junction lasers and the achievement of population inversions among the hyperfine levels in the ground state of Cs 133 by optically pumping it with radiation from a GaAs diode laser. Laser output was used to monitor the populations in the ground state hyperfine levels as well as to perform the hyperfine pumping. A GaAs laser operated at about 77 K was used to scan the 8521 A line of Cs 133. Experiments were performed both with neon-filled and with paraflint-coated cells containing the cesium vapor. Investigations were also made for the development of the triple resonance coherent pulse technique and for the detection of microwave induced hyperfine trasistions by destroying the phase relationships produced by a radio frequency pulse. A pulsed cesium resonance lamp developed, and the lamp showed clean and reproducible switching characteristics

Quantum Electronics

Contains thirteen research projects split into three sections.U.S. Air Force - Rome Air Development Center (Contract F19628-80-C-0077)National Science Foundation (Grant PHY79-09739)Joint Services Electronics Program (Contract DAAG29-78-C-0020)Joint Services Electronics Program (Contract DAAG29-80-C-0104)U.S. Air Force Geophysics Laboratory (AFSC) (Contract F19628-79-C-0082)National Science Foundation (Grant ECS79-19475)National Science Foundation (Grant DAR80-08752)National Science Foundation (Grant ENG79-09980