Semiconductor lasers are commonly used for low power reading operations in optical data storage systems, such as CD-ROM, audio compact disk and video disk systems. For these applications, red (620-690 nm) AlGaInP/GaAs laser diodes are favored over near infrared (780-880 nm) AlGaAs/GaAs laser diodes, because of their shorter wavelength and hence better resolution, although the latter are also sometimes used because of their lower unit price. In addition to such wavelength considerations, laser diode application often favor one particular polarization, either transverse electric (TE) or transverse magnetic (TM), over the other. Moreover, TE mode lasers typically have higher gain, higher output power and higher operating temperatures than TM mode lasers, which may be important factors in certain applications. In light of all of this, it would be advantageous to be able to selectively tailor the semiconductor laser materials to not only produce a particular emission wavelength but also to favor a preferred polarization at that selected wavelength. Unfortunately, for the AlGaInP/GaAs material system it has been found that prior laser diodes of this type operate in the TE polarization mode only for the longer 650-690 nm emission wavelengths and that only the TM polarization mode is available for the shorter 620-650 nm wavelengths. To see why this should be the case, it will be useful to consider the effects of material composition on bandgap energy and strain, and of strain on the emission polarization.
FIG. 1 shows the bandgap energies and corresponding wavelengths for unstrained In.sub.x Ga.sub.1-x P material in the range from x=0.2 to x=0.8. (The wavelengths are derived from the relationship E.sub.g =hc/.lambda., where E.sub.g is the bandgap energy, h is Planck's constant, c is the speed of light in vacuo, and .lambda. is the wavelength, and thus assumes that substantially all of emitted photon energy is contributed by the bandgap energy.) It can be seen from FIG. 1 that the alloy composition In.sub.0.5 Ga.sub.0.5 P has a bandgap of about 1.9 eV and emits light at about 650 nm. In order to produce laser light emission with a longer wavelength than 650 nm, one can decrease the bandgap of the active lasing material by changing its composition, for example by increasing the proportion of indium. Thus, In.sub.0.58 Ga.sub.0.42 P has a bandgap of about 1.8 eV and a corresponding emission wavelength of about 690 nm. Likewise, in order to produce a wavelength shorter than 650 nm, one can change the composition of the material to increase its bandgap. While this could be done by adding aluminum to the composition, as is done with quantum barriers and cladding layers in the laser diode heterostructure, in order to provide sufficient carrier confinement for room temperature CW operation, the bandgap increase for the quantum wells is normally obtained by decreasing the proportion of indium in the composition. For example, In.sub.0.42 Ga.sub.0.58 P has a bandgap of about 2.0 eV and a corresponding wavelength of about 620 nm.
FIG. 2 shows the amount of strain for various compositions of In.sub.x Ga.sub.1-x P layers on a GaAs substrate. The material is lattice-matched to the substrate for x.apprxeq.0.48. Increasing the proportion of indium results in a larger crystal lattice so that the material is under compressive strain. For example, when x.apprxeq.0.62, the alloy composition is under about 1.0% compressive strain. Likewise, decreasing the proportion of indium produces a smaller lattice size and tensile strain. Thus, when x.apprxeq.0.35, the composition experiences -1.0% compressive strain (i.e., +1.0% tensile strain). Replacing part of the gallium with aluminum has only a negligible effect on strain. For useful lifetimes, a maximum strain in the active region's quantum wells and barriers of about 2% (i.e., compositions where 0.2.ltoreq.x.ltoreq.0.8) is a practical limit for most devices. The critical limit for strain depends on the thickness of the strained layers. Thus, the cladding layers and other relatively thick layers outside of the active region require significantly less strain and may need to be substantially lattice-matched to the substrate to avoid dislocations and other lattice defects. One can see from a comparison of FIGS. 1 and 2, that compositions providing bandgaps less than 1.9 eV and corresponding emission wavelengths longer than 650 nm are under compressive strain, while compositions providing bandgaps greater than about 1.9 eV and corresponding wavelengths shorter than 650 nm are under tensile strain.
For GaInP lattice-matched to a GaAs substrate, the band-structure has "light" hole and "heavy" hole valence bands that are degenerate at k=0 (where k is the wavenumber of carriers in the lattice). There is an energy gap E.sub.g between these valence bands and the conduction band. TE polarized light is emitted when an excited electron in the conduction band recombines with a hole in the heavy hole valence band, whereas TM polarized light is emitted when an electron combines with a light hole. Because the differential gain of the TE mode is larger than that of the TM mode, a laser mode with a lattice-matched InGaP active region will generally lase in the TE mode. An In.sub.x Ga.sub.1-x P layer with x&lt;0.48 will be under tensile strain and will emit TM polarized light. However, a compressively strained layer (x&gt;0.48) will emit TE polarized light due to the dominance of the heavy hole band. Prior laser diodes emitting at the shorter 620-650 nm wavelengths have tensile strained quantum wells and thus operate in the TM mode, while those emitting at the longer 650-690 nm wavelengths are compressively strained and operate in the TE mode.
When reading stored data from optical disks, a very low random intensity noise (RIN) level from the laser illumination source is needed for a good signal-to-noise ratio at the detector. However, in such data storage systems, external reflective feedback typically occurs, in which some fraction of the emitted laser light is coupled back into the laser diode. The phase of this feedback varies in time due to vibrations in the system, other slight, but essentially random, movements of various system components, and the reading of phase-encoded data stored on some types of disks. These phase variations in the feedback cause modal instabilities in the laser cavity, resulting in random fluctuations (i.e., noise) in the laser's output power.
Although this feedback sensitivity of the laser is also dependent to some extent upon the amplitude of the optical feedback, such that the noise in the emitted output of the laser could be reduced by using an optical design that minimizes the amount of reflected light coupling back into the laser, such a design normally requires completely separate illumination and collection optics, thereby leading to a much larger and heavier (and hence mechanically slower) read head containing such optics.
Another way to reduce the feedback sensitivity of a laser diode is to modulate the laser at high speed. The high speed modulation renders the laser insensitive to feedback by destroying phase coherence between the reflected beam and the oscillating modes of the laser. Typically, the modulation rate needed for this purpose is on the order of several hundred megahertz. Unfortunately, the additional circuitry needed to bias and modulate the laser diode at high frequency considerably complicates the overall system and makes it much more expensive.
A better solution is to provide a laser diode that inherently self-pulsates, emitting a high frequency stream of successive optical pulses when driven by a DC (unmodulated) bias. Self-pulsating laser diodes have been demonstrated, as illustrated for example in U.S. Pat. Nos. 4,961,197 (Tanaka et al.); 5,003,549 (Mitsui et al.); 5,111,469 (Narui et al.); and 5,416,790 (Yodoshi et al.). Most of these prior self-pulsating lasers are of the near-infrared AlGaAs-type. Those of the AlGaInP-type are characterized by the same wavelength-polarization constraints imposed by strain as their non-pulsating counterparts.
The self-pulsating laser diodes usually achieve their rapid pulsation operation by incorporating a saturable absorbing layer into the laser structure. This absorbing layer is positioned outside of the active region, but close enough to the active region that it overlaps the propagating optical mode. The active region itself can be a single active layer (typ., .apprxeq.50 to 150 nm thick), a single quantum well (SQW) structure (typ. well .apprxeq.10 to 30 nm thick), a multiple quantum well (MQW) structure (typ. 2 to 4 wells, each .apprxeq.5 to 20 nm thick), or a superlattice structure (effective thickness .apprxeq.15 to 30 nm). The saturable absorbing layer in the neighborhood of the active region is composed of a material (such as GaInP) and has a thickness (e.g., it may form a 10 to 40 nm thick quantum well) having an effective bandgap energy that is smaller than (or at most substantially equal to) the energy corresponding to the lasing wavelength, thereby giving the absorbing layer a high intrinsic absorption coefficient at the lasing wavelength. Accordingly, when the laser diode is first energized, the absorbing layer initially absorbs the light generated in the active region quite strongly, so that the photon density in the active region is kept at a level below the lasing threshold and the density of excited electron-hole pairs in the active region is allowed to build. Carriers generated in the absorbing layer due to the absorption of light from the active region are confined there by the higher bandgap energy of adjacent cladding layer material (such as AlInP) both above and below the absorbing layer. The resulting collection of carriers in the absorbing layer causes a drop in its absorption coefficient as the absorbing layer saturates, thereby reducing the loss in the laser cavity and allowing the sudden onset of strong lasing as the photon density in the active region rises above the threshold level. The intense lasing mode rapidly depletes the carriers in the active region, so the device quickly stops lasing again. The cladding material above the absorbing layer typically has a ridge structure with lower bandgap material (such as GaAs) regrown on each side of the central ridge. Carriers confined in the absorbing layer by the cladding layer below and the ridge above are still free to diffuse laterally within the absorbing layer until they reach the areas on each side of the ridge structure, where they can fall out of the absorbing layer into the surrounding lower bandgap material. The absorbing layer can thus be quickly emptied of the accumulated carriers by means of diffusion once lasing has stopped, so as to restore the absorption coefficient to its original peak value and again become a strong absorber of light generated by the active region. Hence, the cycle starts all over again. The modulation rate attained in such self-pulsating devices is typically several hundred megahertz, but can be as high as several gigahertz, which is more than enough to reduce the laser's feedback sensitivity and obtain very low RIN emission.
It is an object of the invention to provide AlGaInP/GaAs laser diodes that operate in the TE polarization mode for 620-650 nm emission wavelengths.
It is another object of the present invention to provide such TE mode laser diodes with self-pulsating operation.