Abstract:
A linear array of multimode laser diodes in a single wafer having a ratio of approximately one emitting stripe width to each non-emitting stripe isolation region width exhibits transverse lasing in a direction perpendicular to the normal laser output at input currents above a pre-determined transverse lasing threshold. Above the transverse lasing threshold, normal lasing is rapidly quenched.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     Linear arrays of multimode laser diodes are well known in the art and generally comprise a plurality of stripes of p semiconductor material formed over additional layers of p material and n material in a wafer which is generally mounted to a heat sink and energized with a current source to induce lasing. Generally, these multistripe arrays are useful in that they increase the amount of laser output available from a semiconductor laser, and they may also be modulated by directly modulating the injection drive current, although no amplification is achieved by this direct modulation. Furthermore, there are limits to the amount of power output which can be achieved with a semiconductor multistripe laser diode device as catastrophic failure of the device occurs due to facet damage from overdriving, or in another manner due to excessive power densities in the device. Also known in the prior art is a technique for individually addressing and energizing the individual stripes of a multistripe array. This is generally achieved by forming separate electrical contacts on the individual stripes. 
     In working with multistripe laser diodes, the inventors herein have succeeded in reliably producing a phenomenon known as transverse lasing in a repeatable manner. Transverse lasing is characterized by a quenching of normal lasing along the length dimension of the stripe and the creation of a lasing effect in a perpendicular direction through the width of the array. This effect has been produced in multistripe laser arrays in which the ratio of emitting stripe width to non-emitting stripe isolation region width was substantially one-to-one. In such a wafer, there is a threshold for normal lasing such as would be ordinarily expected in a multistripe laser array. In wafers configured appropriately, a corresponding threshold current for transverse lasing has been found to be approximately twice that of the normal threshold. As this transverse threshold is reached, the normal output laser is very rapidly and effectively quenched, and transverse lasing is propagated through the wafer. It has also been found that repeatedly driving the current above this transverse lasing threshold results in no change to the power/current characteristics of the device, thereby indicating that transverse lasing does not result in damage to the wafer. Furthermore, because of the rapid switching from normal to transverse lasing at the threshold, it is anticipated that this effect can be used to provide a rather high speed communication device. In the preferred embodiment, switching rates have been achieved which indicate that frequencies as high as 50 MHz may be achieved. 
     A further feature and advantage of this transverse lasing phenomenon is the opportunity to achieve an amplification factor. This is because the injection current can be modulated in a relatively small band about the transverse lasing threshold which will cause the laser diode array to rapidly switch from normal lasing to transverse lasing at full power. Still another application for this device would utilize the individual stripe accessing technique already known in the prior art. By separately addressing the individual stripes, a high speed, multiple input, AND-gate may readily be achieved, and other logic elements could be formed as well. 
     The phenomenon of transverse lasing is generally thought to be due to parasitic oscillations in the device. Parasitic oscillation in laser diode devices has been known for some time, particularly with respect to broad area devices. However, the present invention differs from broad area lasers and other devices in which parasitic oscillation has been previously observed in that the parasitic oscillation in the present device depends upon the interaction of many otherwise normal lasers occupying the same planar waveguide and, to the inventor&#39;s knowledge, has not been achieved by others. This particular wafer arrangement makes the transverse lasing threshold for parasitic oscillation a well defined and repeatable parameter rendering it capable of useful application. 
     While some of the principal advantages and features of the present invention have been briefly described, a fuller understanding of the invention may be obtained by referring to the drawings and description of the preferred embodiment which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are perspective views of the laser diode array of the present invention; 
     FIG. 2 is a cross-sectional view detailing the various lasers of the wafer which forms the present invention; 
     FIG. 3 is a graph of optical output power versus drive current for the device of the present invention; and 
     FIG. 4 is comprised of three charts detailing the power output of the present invention with different levels of input. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The multimode laser diode array 20 of the present invention is shown in FIGS. 1A and 1B and 2 and includes a plurality of individual mesas 22 which are spaced across the top of the wafer 24. The layers comprising the wafer 24 are best shown in FIG. 2 and include an upper contact layer 26 which could be either 5000 angstroms of gold or 75 angstroms of nickel chromium and a mesa 22 which is 0.2 microns in height of gallium arsenic. Beneath the mesa 22 is a layer 28 of Al 0 .4 Ga 0 .6 As with a thickness of 1.3 microns, beneath which is another layer 30 of Al 0 .1 Ga 0 .9 As with a thickness of 0.15 microns, and beneath that is layer 32 of Al 0 .4 Ga 0 .6 As with a thickness of 3.3 microns. These three layers 28-32 form the EPI layers of the wafer. Beneath these EPI layers is the substrate 34 which is GaAs having a thickness of 75 microns, and the lower surface of the substrate 34 is coated with a contact layer 36 of  0.5 percent GeAu having a thickness of 3600 angstroms. For normal lasing, an active region 38 is energized and is shown in cross-hatch as part of layer 30 in FIG. 2. Thus, a first laser cavity is formed along the length dimension of the array 20. As shown in FIGS. 3 and 4, at input currents below a &#34;normal&#34; lasing threshold, the array lases in a normal manner along the length dimension. At input currents above the transverse lasing threshold, the array lases instead along the width dimension of the array which can thus be considered a second cavity having a lower laser loss at those higher input currents. 
     The semiconductor laser device of the present invention is grown in epitaxial layers and processed into a configuration in which the desired output is obtained efficiently along one direction at normal drive currents but is effectively quenched at higher drive currents by loss of carriers to parasitic modes propagating transverse to the normal output direction. Fabrication of such a device can be achieved in the following manner: first, a highly uniform wafer of GaAlAs double heterostructure laser diode material is grown by Metal Organic Chemical Vapor Deposition (MOCVD). Next, the highly doped cap layer is removed in strips 0.006 cm wide by several millimeters long, these strips being spaced 0.012 cm apart, center-to-center, leaving strips of undisturbed wafer material which are 0.006 cm wide. These undisturbed strips are referred to as mesas, and these mesas permit the passage of current to the active layer located near the p-n junction of the device. The strips from which the cap layer has been removed do not permit the passage of current to the epitaxial layers and these areas are normally inactive or nonemitting in a laser device. The metalization layers along both the top and bottom of the wafer are applied to provide contact surfaces, as mentioned above. The wafer is then cleaved into laser arrays 0.5 cm wide (containing 40 stripes) by a cavity length of 0.015 cm. Thus each laser array has a non-emitting region of approximately 0.013 cm. at each side thereof. A highly reflecting coating is applied to one cleaved mirror facet of the 0.5 cm dimension and a passivation coating is applied to the other 0.5 cm mirror facet to provide the Fresnel mirror reflectance of 32%. 
     Although not shown in the drawing, the devices are mounted p-side down on copper heat sinks using indium solder, as is well known in the art. Alternately, the devices can be mounted with other heat sinking techniques as is known in the art to increase the device capability for higher duty factors. In the present preferred embodiment, duty factors of approximately 5% were found to be adequately heat sinked. 
     As an alternative to the contact surfaces mentioned above, separate contacts may be applied to the individual stripes using techniques well known in the prior art, such as for example that disclosed in Botez, D. et al &#34;High-power individually Addressable Monolithic Array of Constricted Double Heterojunction LOC Lasers&#34;, Appl. Phys. Lett. 41(11), Dec. 1, 82. This individual contact technique permits individual addressing of the stripes as would be useful and desirable in high speed logic applications. 
     FIG. 3 is a graph of optical output power versus drive current for the device of the present invention, and FIG. 4 is comprised of three charts detailing the power of the present invention at different levels of input current. As shown in these figures, at input currents below a normal lasing threshold, optical power is output, thereby indicating a normal laser operation. However, at input currents above a transverse lasing threshold, normal optical power output almost immediately drops to zero, thereby indicating that normal lasing is quenched virtually instantaneously. At the same time, a laser output is experienced transverse to the normal lasing. 
     There are various changes and modifications which may be made to the invention as would be apparent to those skilled in the art. However, these changes or modifications are included in the teaching of the disclosure, and it is intended that the invention be limited only by the scope of the claims appended hereto.