Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-05-18
2002-09-03
Davie, James (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06445723
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to laser sources and more particular to laser diodes that have narrow emission apertures of less than one micron in width. Such laser diodes have utility as optical pickup or retrieval of optically stored data on optical recording media.
BACKGROUND OF THE INVENTION
Optical pickup heads in current optical data storage systems impose a limit on the achievable data storage density. A recognized approach to increase the data storage density is to replace the conventional head with a laser diode held in close proximity with the storage medium so that the bit size is commensurate with the extent of the diode'ss near-field emission.
Various approaches are known to produce an optically switched laser (OSL) head for optical data storage systems. Two prominent approaches to achieve a low cost OSL head include the tapered laser adopted by researchers at NTT. See the published article of H. Ukita et al., SPIE, Vol. 1499, pp. 248-261 (1991) and U.S. Pat. No. 4,860,276 to Ukita et al. Also, improved optical mode confinement can be achieved in a laser diode if an aperture or recess for output beam emission is fabricated in the front facet coating as disclosed, for example, in U.S. Pat. No. 5,625,617 to Hopkins et al. Several other approaches exist and typically include the employment of a laser in combination with a solid immersion lens (SIL) or an integrated microlens, such as disclosed in the published article of Y. Katagiri et al., SPIE, Vol. 2514, pp. 100-111 (1995), or an integrated fiber probe using the near field, such as disclosed in U.S. Pat. No. 5,288,998. However, these approaches include additional components to the OSL head structure and increase the complexity of manufacturing as well as the cost of the optical head.
The patent to Berger et al., U.S. Pat. No. 5,208,821, discloses a laser diode formed by MOCVD growth over a patterned substrate comprising dovetailed mesas for forming a “pinch-off” active region of about 2 &mgr;m to 4 &mgr;m wide, as measured relative to the device window
21
formed in SiO
2
layer
20
. Similar dovetailed structures are shown in Japanese Laid Open Application No. 1-293687, published Nov. 27, 1989 and Japanese Laid Open Application No. 2-119285, published May 7, 1990. These structures, however, are not submicron-aperture laser diodes designed for improving recording or pickup density and threshold operation in optical data storage systems.
The taper laser structure of Ukita et al. In U.S. Pat. No. 4,860,276 is integrated on a substrate with a photodetector at the back facet used to monitor the state of the laser. The taper is introduced via two etched grooves on either side of the laser stripe that converge towards the emission facet and, as such, define the lateral mode confinement at the facet. The primary drawback of this approach is the accurate pattern alignment and high resolution photolithography required to define the mask layer for performing the etching of the trenches. Additionally, the minimum aperture size that has been demonstrated is 1 &mgr;m. However, for providing enhanced density employing near field emission, a 1 &mgr;m aperture is not small enough for efficient near field emission use. The lasers with apodization in the facet coating, demonstrated by Hopkins et al. in U.S. Pat. No. 5,625,617, may be derived from standard single mode lasers. However, to achieve submicron aperture size, the facet of each laser produced requires the formation of a hole in the facet coating created by focused ion beam (FIB) etching, which does not readily lend itself to high yields and standardized reproducibility.
Buried heterostructure lasers have been fabricated in GaAs/AlGaAs based material systems, as disclosed in the articles of E. Kapon et al., “Single Quantum Wire Semiconductor Lasers”,
Applied Physics Letters
, Vol. 55(26), pp. 2715-2717 (1989); H. Narui et al., “A Submilliampere-Threshold Multiquantum-Well AlGaAs Laser Without Facet Coating Using Single-Step MOCVD”,
IEEE Journal of Quantum Electronics
, Vol. 28(1), pp. 4-8 (1992); and H. Zhao et al., “Submilliampere Threshold Current InGaAs-GaAs-AIGaAs lasers and Laser Arrays Grown on Nonplanar Substrates”,
IEEE Journal of Quantum Electronics
, Vol. 1(2), pp. 196-202 (1995). Buried heterostructure lasers have been fabricated in InP based material systems, as disclosed in the articles of K. Uomi et al., “Ultralow Threshold 2.3 &mgr;m InGaAsP/InP Compressive-Strained Multiquantum-Well Monolithic Laser Array for Parallel High-Density Optical Interconnects”,
IEEE Journal of Select Topics in Quantum Electronics
, Vol. 1(2), pp. 203-209 (1995) and T. R. Chen et al., “Strained Single Quantum Well InGaAs Lasers with a Threshold Current of 0.25 mA”,
Applied Physics Letters
, Vol. 63(19), pp. 2621-2623 (1993).
H. Zhao et al. in
IEEE Journal of Quantum Electronics
, Vol. 1(2), pp. 196-202 (1995) demonstrated that through growth of a buried heterostructure laser on a non-planar substrate, lateral active regions less than 0.5 &mgr;m can be achieved in a GaAs/AlGaAs material system leading to a lateral and vertical near field widths of 0.5 &mgr;m×0.5 &mgr;m. The Zhao et al. structure is illustrated in FIG.
1
. To achieve this type of “pinch-off” active region structure, 2 &mgr;m to 3 &mgr;m wide lines on 250 &mgr;m centers were photolithographically patterned onto the semiconductor substrate followed by a chemical etch that terminates on the (111) planes of the material. The narrow, pinch-off active region is formed because of facet dependent growth rates of the epitaxial layers grown onto the nonplanar substrate.
For high speed data links, buried heterostructure lasers have been optimized for low threshold, e.g., less than 1 mA, with high external efficiency, e.g., up to 80%, to around 2 mW output power, but have been demonstrated to, operate in a single mode to output powers as high as 40 mW to 60 mW.
Similar buried heterostructure laser diodes have been demonstrated by others, such as demonstrated by E. Kapon et al. where the structure is formed over a trough as opposed to formation over a mesa in the nonplanar substrate. However, none of these structures have been able to provide a buried heterostructure laser diode having an submicron aperture less than 0.5 &mgr;m, which is an object of this invention.
It is a further object of this invention to provide a laser diode formed on a nonplanar substrate that has a submicron emission aperture with optical emitting mode confinement at the output facet to provide for submicron beam emission, such as below about 0.45 &mgr;m wide emission aperture.
It is another object of this invention to provide a laser diode with a submicron emission aperture for utilizing a near field OSL head to extend the present limit of data density in optical recording and readout media employed in data storage and retrieval apparatus.
SUMMARY OF THE INVENTION
According to this invention, a buried heterostructure (BH) laser diode source with a narrow active region is disclosed for use in close proximity with optically-addressed data storage media for read/write functionality in a relatively high data density format. The BH laser source is formed on a pregrooved or prepatterned substrate to form mesas upon which epitaxial layers are formed to form laser source active regions that have small emission apertures at the laser source facet output. Selective removal of semiconductor cladding material and replacement of this material with lower refractive index materials provides a way of obtaining further mode-size reduction at the output facet of the laser source. Each mesa has a top surface and adjacent sidewalls such that in the growth of the epitaxial layers above the active region doped with a first conductivity type, the above active region epitaxial layers depositing on the top surface deposit as a first conductivity type and depositing on said sidewalls deposit as a second conductivity type. This growth construction provides for a naturally formed p-n junction at the laser source active region and eliminat
DeMars Scott D.
Vail Edward C.
Zhao Hanmin
Ziari Mehrdad
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Davie James
JDS Uniphase Corporation
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