The optical gain is therefore also limited to a narrow section in the middle of the active layer. Most of the major applications of semiconductor laser diodes involve the use of laser diodes in the storage and transmission of information.
LASER DIODE
DETECTOR
Thesis Outline
Calculations related to the modal gains and mode reflectances of buried heterostructure lasers are presented. The result is that basic operation is possible for buried heterostructure lasers with an active layer width of up to 8 µm.
Introduction
Other laser fabrication techniques that do not rely on opposing cleft facets of the substrate for mirrors include mature mirror lasers5, ion-milled mirror lasers6, and curved cavity lasers with mirrors on the same cleft facet or on a cleft corner7. For all of these approaches, the resulting lasers have been found to have significantly higher threshold currents and significantly lower quantum efficiencies than conventionally split-mirror lasers.
Fabrication of Lasers with Micro-cleaved Mirrors
Many other techniques for fabricating lasers that do not rely on oppositely cleaved facets of the substrate for mirrors have been previously reported. The techniques that have received the most attention are mirrors formed by etching2 and the use of scattered Bragg reflectors3• The difficulty with etched mirrors is due to the fact that any irregularities in the mirror surface must be much smaller than the wavelength of the laser light to get a high quality mirror.
- Short Cavity Lasers
One of the main application areas of micro-slit mirror lasers is for optoelectronic integrated circuits (OEICs), because the micro-slit eliminates the chip size limitation on the length of the laser cavity. Micro-division enables the elimination of one of the main areas of incompatibility, namely the limitation of chip size to the length of the laser cavity for conventionally slit-mirror lasers.
WINDOW REGION
CLEAVED /
MIRROR
ACTIVE REGION
Unfortunately, the mode reflectivity of the higher-order TE-like modes is generally higher than the mode reflectivity of the fundamental mode. Furthermore, the thickness of the optical control layer can probably be increased to approx. 1.5 µm while still maintaining fundamental mode drift in the vertical direction.
GUIDE
Effective Index Method
The use of the effective index is therefore not fully justified for this type of waveguide. These plate waveguides are composed of the layers of sections 1 and II, but are infinite in the y-direction. Many failure mechanisms of AlGaAs lasers can be attributed to the presence of the active layer on the laser mirrors.
At somewhat lower output powers, phase erosion may occur due to oxidation of the active layer2·3. Catastrophic mirror damage can be avoided by making the laser structure transparent to the light output in the vicinity of the mirrors.
Fabrication of Buried Heterostructure Window Lasers
4 shows scanning electron microscope (SEM) photographs of mesas prior to the second LPE growth showing the selective removal of the undercoat layer in the window section. 5 is a scanning electron micrograph of a cross section of a laser window section, after second growth. When n-type GaAs substrates are used, this means that the active layer is on top of the guide layer.
However, it is difficult to fabricate these lasers with the active layer on top of the guide layer, as this would require growth over the AlGaAs guide layer in the second LPE growth. Having the active layer below the optical guide layer enables the second growth layers to be grown starting from the GaAs substrate.
Properties of Buried Heterostructure Window Lasers
Typically, LOC BH lasers are grown with N-type optical guide layers to reduce electron leakage. For a double heterostructure with a given difference in aluminum content in the active and cladding layers, the leakage of electrons into the P cladding layer will be greater than the leakage of holes into the N cladding layer. If an optical guide layer with a relatively low aluminum content is incorporated in the laser structure, it is therefore desirable that it be N-type.
ISBH lasers can still have n-type optical control layers if p-type GaAs substrates are used.
WINDOW LOC BH LASER
CATASTROPHIC MIRROR DAMAGE FOR REGULAR
For stripe widths of less than 2 µm, operation of the window LOC BH lasers can be achieved in the fundamental transverse mode. Irregularities in the far field are also typically present in ordinary BH lasers due to scattering from the sidewalls of the laser8. In window LOC BH lasers, scattered light can also result from losses in the coupling of the laser light between the center and window sections.
In conclusion, LOC BH lasers with waveguides transparent to mirrors have been fabricated. Even higher output powers may be possible as power is currently limited by heating of window LOC BH lasers rather than catastrophic damage.
WINDOW LASER FAR FIELD
ACCELERATED FACET EROSION DUE TO BOILING WATER
BOILING TIME MIN
Properties of ISBH Lasers
ISBH lasers were found to have threshold currents and differential quantum efficiencies that were nearly identical to those of LOC BH lasers fabricated in our laboratory. For lasers with Al.3Ga.7As buried layers ISBH lasers operate primarily in the fundamental transverse mode for active layer widths, W2, up to 4 microns. In comparison, LOC BH lasers fabricated in our laboratory operated primarily in the fundamental mode only for active layer widths less than 2 microns.
Another important feature of ISBH lasers was that irregularities in the far-field patterns of the lasers were greatly reduced compared to LOC BH lasers that we have fabricated. Similar results have recently been reported for conventional BH lasers with high aluminum burial layers5.
FWHM
TEMPERATURE (°C)
Fabrication of Narrow Injection Buried Heterostructure Lasers
We choose the aluminum content of the active layer to be 0.12, which is higher than normal. The purpose of the lower A1.2Ga.aAs layer is to limit the amount of downward etching that occurs below the GaAs. It is a characteristic of LPE growth that growth proceeds in such a way that any irregularities in the surface that were present before growth are smoothed out.
This P layer provides a reverse biased PN junction, limiting the current to flow through the narrow top of the N GaAs epitaxial layer. The second layer is N type A1.5Ga.5As and this layer is grown until a continuous top surface is obtained.
Characteristics of Narrow Injection Buried Heterostructure Lasers
As previously mentioned, NIBH lasers can be thought of as a hybrid between narrow stripe gain-controlled lasers and BH lasers. It is therefore interesting to see whether the properties of the laser described in the previous chapter correspond more to those of narrow-stripe lasers or to those of BH lasers. Although the laser is not a single longitudinal mode, there are many fewer modes than the typical 5-10 modes found in narrow stripe gain lasers (see Chapter 7).
The NIBH structure enables the fabrication of a continuum of laser structures between the two extremes of gain-guided lasers and BH lasers. Aiki, 'Transverse mode control and reduction of threshold current in (GaAl) as buried heterostructure lasers with a buried optical guide.
Introduction
The structure to be described is able to limit the carrier injection to an extremely narrow width of the active layer. Injection stripe widths of 2 microns have been routinely obtained, and injection stripe widths as low as 0.5 microns. The main emphasis in this chapter will be simply to describe the structure and properties of these lasers.
However, the narrow-strip DCC configuration is believed to have potential applications in the fabrication of low-threshold laser structures and optically coupled laser arrays.
Fabrication of Narrow Stripe Lasers with Double Current Confinement
The undersaturated case results in melting back of the solid surface and the supersaturated case results in rapid deposition on the solid surface. 2a shows a scanning electron microscope (SEM) photograph of the mesas etched into the substrate before growth. In practice, we found that when we used n-type substrates, the IV characteristics of the devices sometimes indicated the presence of a thin blocking layer above the mesa.
At growth times shorter than 45 seconds, the blocking layer is too thin or interrupted, and at growth times longer than 3 minutes, growth has occurred on top of the interstices. For growth times within this range, the melting and growth process was found to be highly reproducible, provided that etching of the mesa into the substrate prior to growth was performed reproducibly.
Properties of Narrow Stripe DCC Lasers
The variation in the threshold currents is believed to be due to variations in the width of the injection stripe with high-threshold lasers corresponding to the devices with the narrowest injection. This is an indication of the very narrow injection that can be achieved in these lasers. The dependence of the far-field pattern on the stripe width can be understood qualitatively by examining the following simple model.
Nevertheless, most of the important features of the modes of a gain-guided laser waveguide can be understood by considering a precisely solvable model of a gain-guided dielectric flat waveguide. Amplification modes of a guided dielectric plate waveguide are solutions of the wave equation:.
FAR FIELD ANGLE
The most striking feature of these guided gain modes is the appearance of prominent sidelobes in the far-field patterns of narrowband guided gain lasers. This can be understood by the fact that with narrow strip widths, a large part of the power is spread in the cladding layers. Another interesting property of DCC narrowband lasers was that they oscillate simultaneously in a large number of longitudinal modes.
This is consistent with previous findings that the number of longitudinal modes of narrow stripe lasers increases as the stripe width decreases6. These lasers do not have as pronounced anti-conduction properties as the narrow stripe DCC lasers.
GaAs (--450A)
4 Ionization ratio for various values of the electric fields in the high and low field layers. To optimize the design of the proposed detector, it will be necessary to calculate the electron and hole energy distribution functions fe(G),fh(G) at each position as the carriers pass through the layers of the detector. However, several qualitative features of the detector design can be stated without knowing the energy distribution functions exactly.
High-field AlGaAs layers must have thicknesses and electric fields such that a significant fraction of electrons are injected into the GaAs layers with sufficient energy to produce secondary pairs. In addition, the difference between the electric fields in the high and low field regions should be as large as is practical.
