Semiconductor laser structure with an increased catastrophic...

Details Semiconductor laser structure with an increased catastrophic... Semiconductor laser structure with an increased catastrophic...

1999-08-26

2002-04-16

Leung, Quyen (Department: 2881)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06373875

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application serial no. 88112205, filed Jul. 19, 1999, the full disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to semiconductor laser technology, and more particularly, to a semiconductor laser structure, customarily referred to as a laser diode, with an increased catastrophic optical damage (COD) level that allows the laser diode to have an increased life time of use.
2. Description of Related Art
A laser diode is a semiconductor laser device that can produce stimulated emission of coherent light at a PN junction when an electric voltage is being applied thereto. Fundamentally, a laser diode is composed of a P-type layer, an N-type layer, and an active layer sandwiched between the P-type layer and the N-type layer. Conventionally, these three layers are all formed through the MOCVD (Metal Organic Chemical-Vapor Deposition) technology. The laser diode has one end coupled to a non-output facet, which is coated with a high-reflective dielectric material, and the opposite end coupled to an output facet, which is coated with a low-reflective dielectric material. When an electric voltage is being applied to the laser diode, the induced current will be injected into the active layer, thereby causing stimulated emission of coherent light from the active layer. The produced coherent light then oscillates between the two facets to gain power and, until reaching a certain power level, penetrates through the output facet to the outside of the laser diode, resulting in a laser beam from the laser diode.
FIG. 6
is a graph, showing the output characteristic (output laser power versus injected current) of a conventional laser diode. When the output power reaches a certain level, commonly referred to as the catastraphic optical damage (COD) point, as indicated by the letter A in
FIG. 6
, it would cause melting to the facets, thus damaging the laser diode. Moreover, when the laser diode operates continuously for a long period at a high level of output power, for example at 30 mW (milliwatt), it can also easily cause damage to the facets. There exists, therefore, a need in the industry for a new semiconductor laser structure that allows the laser diode to withstand a higher COD level so as to make the laser operation more reliable and enduring.
Solutions to the foregoing problem include the use of a facet coating of high reflectivity and improvement on the window structure of the laser diode. These solutions, however, require the use of epitaxial technology and advanced coating technology to achieve, and therefore are quite complex and costly to implement.
Related patents includes U.S. Pat. No. 5,812,580 to Nabiev, Rachit F. et al; U.S. Pat. No. 5,469,457 to Nagai, Yutaka et al; U.S. Pat. No. 5,491,711 to Mand, Ranjit S. et al; and U.S. Pat. No. 4,964,135 to Shigeru Mitsui et al; to name a few.
The U.S. Pat. No. 5,812,580 to Nabiev, Rachit F. et al can increase the laser output power without causing COD through the use of a multi-layer coating of highly reflective dielectrics. One drawback to this patent, however, is that the fabrication process is quite complex and is thus quite time-consuming to implement. Moreover, it requires the use of expensive vacuum equipment to perform, which makes the fabrication quite costly to implement.
The U.S. Pat. No. 5,469,457 to Nagai, Yutaka et al can prevent the occurrence of COD by implanting impurity ions into the quantum well in the laser diode to thereby alter the window structure into a disordered state. One drawback to this patent, however, is that it require the use of epitaxial technology to achieve, and is therefore quite complex in process and costly to implement.
The U.S. Pat. No. 5,491,711 to Mand, Ranjit S. et al can increase the COD level by first coating aluminum or phosphorus on the facets and then performing a heat treatment process to cause the aluminum or phosphorus atoms to diffuse into the active layer. One drawback to this patent, however, is that it require the use of epitaxial technology to achieve, and is therefore quite complex in process and costly to implement.
The U.S. Pat. No. 4,964,135 to Shigeru Mitsui et al can increase the COD level through a first step of forming a ridge mesa over a P-type cladding layer, a second step of forming an N-type GaAs layer serving as a current-blocking structure, and a third step of forming a P-type GaAs layer serving as a contact layer. The N-type GaAs current blocking structure can help reduce the amount of injected current into the active layer, thereby increasing the COD level. One drawback to this patent, however, is that the resulted COD level is still unsatisfactory.
SUMMARY OF THE INVENTION
The invention provides a novel semiconductor laser structure. The invention has three preferred embodiments.
According to the first preferred embodiment, the semiconductor laser structure of the invention includes: (a) a semiconductor substrate of first conductive type;(b)a first cladding layer of first conductive type formed over the substrate,(c)an active layer formed over the cladding layer of first conductive type (d) a pair of facets including a first facet and a second facet disposed in parallel; (e) a second cladding layer of second conductive type formed over the substrate, the second cladding layer having a flat part and a ridge mesa over the flat part and extending from the first facet to the second facet; (f) a contact layer of second conductive type formed over the ridge mesa of the second cladding layer; (g) a first current-blocking layer formed over a first end part of the contact layer proximate to the first facet; (h) a second current-blocking layer formed over a second end part of the contact layer proximate to the second facet; and (i) a third current-blocking layer formed over the flat part of the cladding layer, the third current-blocking layer being lower in elevation than the first and second current-blocking layers over the ridge mesa.
According to the second preferred embodiment, the semiconductor laser structure of the invention includes: (a) a semiconductor substrate of first conductive type ;(b)a first cladding layer of first conductive type formed over the substrate,(c) an active layer formed over the first cladding layer of first conductive type (d) a pair of facets including a first facet and a second facet disposed in parallel; (e) a second cladding layer of second conductive type formed over the substrate, the second cladding layer having a flat part and a ridge mesa over the flat part and extending from the first facet to the second facet; (f) a contact layer of second conductive type formed over a middle portion of the ridge mesa of the cladding layer; (g) a first current-blocking layer formed over a first end part of the contact layer proximate to the first facet and neighboring the contact layer; (h) a second current-blocking layer formed over a second end part of the contact layer proximate to the second facet and neighboring the contact layer; and (i) a third current-blocking layer formed over the flat part of the second cladding layer.
According to the third preferred embodiment, the semiconductor laser structure of the invention includes: (a) a semiconductor substrate of first conductive type ; (b) a first cladding layer of first conductive type formed over the substrate, (c) an active layer formed over the first cladding layer of first conductive type (d) a pair of facets including a first facet and a second facet disposed in parallel; (e) a second cladding layer of second conductive type formed over the substrate, the second cladding layer having a flat part and a ridge mesa over the flat part and extending from the first facet to the second facet; (f) a contact layer of second conductive type formed over a middle portion of the ridge mesa of the cladding layer, the contact layer having a first end side parallel to the first facet and separated from the first facet by a first prede

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