The United States Government has certain rights in the invention under Contract No. DAAH01-95-C-R173 awarded by DARPA.

Cross-Reference to Related Application

This non-provisional U.S. national application, filed under 35 U.S.C. §11 1(a) claims, under 35 U.S.C. § 119(e)(1), the benefit of the filing date of provisional U.S. application no. 60/019,362, filed under 35 U.S.C. § 1 1 1(b) on June 5, 1996.

Field of the Invention

The present invention relates to a high power superluminescent semiconductor diode (SLD), and, more particularly, to a contact for a high power superluminescent semiconductor diode.

Background of the Invention

High power super luminescent diodes (SLD) have applications in fiber optic gyroscopes (FOG), optical amplifiers, external cavity lasers, tunable laser, mode-locked lasers, and in general, as light sources for applications requiring low coherence, such as low coherence interferometers (for fault detection and certain medical instrumentation and imaging). One form of SLD which has proved to be the best design is the angled stripe SLD because it exhibits the lowest spectral modulation as compared to any other design by preventing longitudinal (Fabry-Perot) modes. Referring to Fig. 1, there is shown a top view of a typical design of an angled stripe SLD 10. The SLD 10 comprises a body 12 of a semiconductor material having therein any desired structure for a semiconductor light emitting diode. The body 12 has end surfaces 14 and 16 and side surfaces 18 and 20. Unlike a laser, end surfaces 14 and 16 are made transparent by the deposition of anti-reflection coatings so as to allow the generated light to be emitted therefrom, and further minimize reflection. On the top surface 22 of the body 12 is a narrow stripe 24 of a conductive material, such as a metal. The stripe 24 extends between the end surfaces 14 and 16 and is angled with respect to the side surfaces 18 and 20. To form an angled stripe SLD having a higher power, it is necessary to increase the length of the device in order to increase the overall gain. Also, it is necessary to increase the width of the stripe in order to increase the saturation power density. However, increasing the stripe width results in a multi-mode structure (transversely) which reduces the usefulness of the device.

To prevent the excitation of multiple transverse modes, an angled stripe SLD having a stripe in the shape of a taper was designed. Referring to Fig. 2, there is shown a top view of a form of a SLD 26 having a tapered stripe. The SLD 26 comprises a body 28 of a semiconductor material having therein a structure of any well known design for a semiconductor light emitting diode. The body 28 has end surface 30 and 32 and side surfaces 34 and 36. Both end surfaces 30 and 32 are made transparent so as lo allow maximum light to be emitted therefrom and further minimize reflection. A stripe 38 of a conductive material is on the top surface 40 of the body 28 and extends between the end surfaces 30 and 32. The stripe 38 is tapered in that its sides edges 42 and 44 are not parallel, but are at an angle with respect to each other. Thus, the stripe 38 has one end 46 which is narrower than the other end 48.

It has been found that the taper shape of the stripe collimales the light and reduces the propagation angle. This, in turn, prevents the excitation of higher transverse modes, which propagate at higher angles. This structure is capable of high power and low spectral modulation. It has been demonstrated close to 30 mW output at less than 2% spectral modulation in a 840 micrometer wavelength SLD using a 500 micrometer taper or wedge tapering from 5 microns to about 55 microns in width. However, a problem with this structure is that the diameter of the emitted beam of light is relatively large. Many applications for this type of light emitting diode require the light to be coupled to a single mode fiber, requiring close mode matching between the SLD and the fiber. Thus, a small spot size is required in order to obtain high power in small spot size, while maintaining single mode-like operation.

Summary of the Invention

A semiconductor light emitting diode is formed of a body of a semiconductor material having end surfaces, side surfaces, a top surface and an active region extending between the end surfaces. A stripe of a conductive metal is on the top surface of the body and extends between the end surfaces. The width of the stripe adjacent the end surfaces of the body is less than the width of the stripe intermediate the end surfaces.

Brief Description of the Drawings

Fig. 1 is a top view of a typical angled stripe SLD;

Fig. 2 is a top view of a typical tapered stripe SLD;

Fig. 3 is a perspective view of a form of the light emitting diode of the present

invention;

Fig. 4 is a graph showing the output power vs. current characteristics of two diodes made in accordance with an embodiment of the present invention; and

Fig. 5 is a diagram showing the output wavelength spectrum of a light emitting diode made in accordance with an embodiment of the present invention.

Detailed Description

Referring now to Fig. 3, a semiconductor light emitting diode in accordance with an embodiment of the present invention is generally designated as 50. Light emitting diode 50 comprises a body 52 of a semiconductor material having end surfaces 54 and 56, side surfaces 58 and 60 and top surface 62. The body 52 may be of any well known structure for forming a light emitting diode. As shown, the body 52 comprises a substrate 64 of a highly conductive semiconductor material, such as GaAs, of one conductivity type, such as N+ type. On a surface 66 of the substrate 64 is a first clad layer 68 of a semiconductor material of the one conductivity type, such as N-type GaAlAs. On the first clad layer 68 is a thin active layer 70. The active layer 70 is preferably an undoped semiconductor material having a band gap lower than that of the clad layer 68. However, the active layer may be of any well known structure, such as a single or multi quantum well structure. On the active layer 70 is a second clad layer 72 of the same material as the first clad layer, but of opposite conductivity type, such as P- type. On the second clad layer 70 is a contact layer 73 of highly conductive semiconductor material of the same conductivity type as the second clad layer 70, such as P+ type GaAs. A termination layer 74 of a highly conductive metal, is on the bottom surface 76 of the substrate 64.

On the top surface 62 of the body 52 is a stripe 76 of a conductive metal made in accordance with an embodiment of the present invention which extends along the top surface 62 between the end surfaces 54 and 56. The stripe 76 is substantially diamond shape having truncated ends 78 and 80. This provides a stripe 76 in which the ends 78 and 80 are narrower than the midportion 82. The ends 78 and 80 are positioned adjacent the end surfaces 54 and 56 of the body 52, and the side edges 84 of the stripe 76 taper away from each other from the ends 78 and 80 to the midportion 82. The longitudinal axis of the stripe 76, shown as dash line 86, is not parallel to the side surfaces 58 and 60 of the body 52, but is at an angle with respect thereto. The angle is such as to reduce facet reflection to less than 10 ^"6 . Preferably, this angle is about 6°. Thus, the stripe 76 extends at an angle with respect to the emitting end

surface 54. A semiconductor light emitting diode in accordance with an embodiment of the present invention was made with an InGaAs (indium gallium arsenide) active layer, and with a diamond shaped stripe with the following dimensions:

Active length 1000 microns

Width at center 55 microns

Width at facet 5 microns and 20 microns (they can be identical)

Center wavelength about 8440 microns

(Can be chosen by the material composition and quantum well thickness)

Stripe angle 6 degrees quantum well active layer Fig. 4 shows the output power vs. current characteristics of two SLDs having diamond shaped stripes in accordance with an embodiment of the present invention. The diodes were randomly selected and were driven to have output powers in excess of 50 mW, Fig. 5 shows the output spectrum of a SLD having a diamond shaped stripe in accordance with an embodiment of the present invention and providing feedback. Thus, the light emitting diode 50 of an embodiment of the present invention with a diamond shaped stripe 76 can be made to provide high power single mode light emission. In addition, since the end of the stripe which is adjacent the light emitting end surface of the diode is narrow, it provides a beam of light which is relatively small in size so that it can be feed into a single mode optical fiber.

Although, for the experiment shown in Fig. 4, the diodes were driven at output powers in excess of 50 mW, they were driven in excess of 170 mW in other experiments. Also, the design of the diode of the present invention can be used to make SLDs at any of the wavelengths commonly used in semiconductor lasers. Similar results have been obtained for InGaAs SLDs operating at 980 nm and InGaAsP SLDs operating at 1550 nm.