Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-09-11
2003-10-21
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S097000
Reexamination Certificate
active
06636544
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to laser manufacturing and, more particularly, to vertical cavity surface-emitting lasers (VCSELs).
2. Description of the Related Art
In many applications, it is desirable to have lasers capable of producing light at several different wavelengths that are closely spaced. In particular, for optical communication applications, such as wavelength division multiplexing (WDM), many wavelengths spaced a few nanometers apart are useful. There are several possible ways to make wavelength-selectable arrays with edge-emitting semiconductor lasers. However, edge-emitting semiconductor lasers that meet specifications for telecommunication applications are typically complex and expensive to manufacture. Monolithic edge-emitting semiconductor laser arrays with large numbers of wavelengths are impractical technically and economically.
Vertical cavity surface-emitting lasers (VCSELs) provide a more cost effective solution for many applications. Vertical cavity surface-emitting lasers (VCSELs) can be made using wafer-scale processing and testing, dramatically lowering the cost in comparison to edge-emitting semiconductor lasers, for example. In a vertical cavity surface-emitting laser (VCSEL), the wavelength may be determined by the optical cavity length. The optical cavity length is the effective distance between the two generally parallel mirrors, typically distributed Bragg reflectors (DBRs), enclosing the active region of the vertical cavity surface-emitting laser (VCSEL). Since the optical cavity length is typically set by the epitaxial growth, which should be uniform across the entire wafer or workpiece, the wavelength is uniform.
Tunable vertical cavity surface-emitting lasers (VCSELs) are desired in order to provide different wavelengths on the same wafer or workpiece. One conventional approach to providing tunable vertical cavity surface-emitting lasers (VCSELs) uses a top mirror that is suspended on a micromachined cantilever. With this conventional structure, any given vertical cavity surface-emitting laser (VCSEL) can be tuned to any wavelength within the tuning range. However, this conventional approach involves a micromachined structure that is difficult to fabricate, has reliability problems and is susceptible to mechanical vibrations. A more reliable way of providing monolithically integrated vertical cavity surface-emitting laser (VCSEL) arrays is still needed.
In a vertical cavity surface-emitting laser (VCSEL), the lasing wavelength may be determined by the length of a Fabry-Perot cavity formed by two distributed Bragg reflectors (DBRs) separated by the semiconductor optical cavity active region that includes layers with optical gain. The optical gain in a vertical cavity surface-emitting laser (VCSEL) is typically provided by quantum wells. Each quantum well has a gain spectrum with a single peak wavelength, and some spectral width over which gain is present. Each distributed Bragg reflector (DBR) is composed of quarter-wave layers of alternating high and low refractive indices. The distributed Bragg reflector (DBR) reflectivity is characterized by a complex amplitude and phase spectrum. The amplitude spectrum exhibits a high reflectivity region at the center of which the reflectivity is highest. The width of the high reflectivity region is referred to as the distributed Bragg reflector (DBR) stop-band width. The phase characteristic of the distributed Bragg reflector (DBR) varies approximately linearly over the stop-band width. The lasing wavelength of a vertical cavity surface-emitting laser (VCSEL) is determined by the optical length of the semiconductor cavity and the phase characteristics of the distributed Bragg reflectors (DBRs). The gain provided by the active layer, necessary to achieve lasing (threshold gain) is determined by the roundtrip cavity loss that includes material absorption and the distributed Bragg reflector (DBR) transmission. A monolithic multiple-wavelength vertical cavity surface-emitting laser (VCSEL) array requires side-by-side fabrication of vertical cavity surface-emitting lasers (VCSELs) with varying lasing wavelengths, but otherwise uniform laser characteristics, such as threshold gain and current, and efficiency. This implies that the vertical structure of the lasers must vary from device to device on the same wafer, while the cavity losses, material gain, and the distributed Bragg reflector (DBR) transmission remain largely unchanged. The lasing wavelength variation is most commonly realized by changing the optical length of the semiconductor cavity.
One conventional method of making a monolithic multiple wavelength vertical cavity surface-emitting laser (VCSEL) array uses non-uniform growth due to a thermal gradient. The backside of a substrate is patterned prior to epitaxial growth in a molecular beam epitaxy (MBE) reactor. The resulting backside pattern produces a thermal gradient on the surface of the substrate when the wafer is heated. Because growth rate is temperature dependent, there is a variable material thickness and, hence, a variable laser wavelength along the thermal gradient. One disadvantage of this conventional approach is the fact that the arrays are limited to linear geometries. To date, it has been difficult to control the wavelengths precisely and repeatedly over large areas of the wafer.
Another conventional method is to grow a partial vertical cavity surface-emitting laser (VCSEL) structure including the lower distributed Bragg reflector (DBR), the active region, and some part of the upper distributed Bragg reflector (DBR). The wafer is masked and anodically oxidized to some controlled oxide thickness over the exposed portions. A selective etch is then used to remove the oxide. This process is repeated to create different effective cavity lengths for each laser in an array. The remainder of the vertical cavity surface-emitting laser (VCSEL) structure is regrown over the patterned wafer. However, each selective etch is sensitive to voltage and concentration variations that may affect the depth, resulting in reduced control over wavelength spacing between devices in the array.
Yet another conventional method of making a monolithic multiple wavelength vertical cavity surface-emitting laser (VCSEL) array is described, for example, in U.S. Pat. No. 6,117,699 to Lemoff et al. (“the Lemoff et al. '699 patent”), describing an array of N-wavelength vertical cavity surface-emitting lasers (VCSELs) that can be grown with wavelength control. First, as shown in
FIG. 1
, a foundation vertical cavity surface-emitting laser (VCSEL) structure
100
is grown on a gallium arsenide (GaAs) substrate
105
. The foundation vertical cavity surface-emitting laser (VCSEL) structure
100
includes a lower distributed Bragg reflector (DBR)
110
in an optical cavity
145
. The lower distributed Bragg reflector (DBR)
110
includes M pairs of layers
115
,
120
,
125
,
130
,
135
and
140
(M=6, in FIG.
1
), each member of each pair having an index of refraction differing from the other member of each pair. For example, the lower member
115
a
of the pair
115
may comprise aluminum arsenide (AlAs) and the upper member
115
b
of the pair
115
may comprise aluminum gallium arsenide (Al
x
Ga
1-x
As, where 0.15<x<1).
The optical cavity
145
also includes a first intrinsic (non-doped) layer
150
, an optical gain layer
155
and a second intrinsic (non-doped) layer
160
. The optical cavity
145
also includes N-paired semiconductor phase shift epitaxially grown layers
165
,
170
,
175
and
180
(N=4 in
FIG. 1
) of aluminum gallium arsenic (AlGaAs) and indium gallium phosphorus (InGaP), where N is the desired number of different wavelengths.
Next, a region of one of the N-paired semiconductor phase shift layers is lithographically patterned (masked and etched). For example, as shown in
FIG. 1
, a mask
185
is formed and portions
190
and
195
(shown in phantom) of the paired semiconductor phase shift epitaxially grown laye
Anselm Klaus Alexander
Baillargeon James N.
Hwang Wen-Yen
Zheng Jun
Applied Optoelectronics, Inc.
Furlong Randall C.
Ip Paul
Kinsella N. Stephan
Zahn Jeffrey
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