Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-08-09
2003-05-06
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S023000, C372S096000
Reexamination Certificate
active
06560262
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to vertical cavity surface emitting lasers (VCSELs) and, more specifically, to a process for fabricating surface emitting lasers.
2. Art Background
Laser arrays capable of both high-speed and multiple wavelength operation are important for high-capacity optical network applications based on high-density wavelength division multiplexing (HD-WDM). The laser array is formed on a single semiconductor substrate. Such arrays are referred to as monolithic arrays because they are formed on a single substrate. The emission wavelength of a laser is determined by its cavity length (which includes the length of the active region in the direction of the emission) and the refractive index of the materials in the cavity. Therefore, in an array of devices in which not all devices emit at the same wavelength, not all devices in the array have the same cavity length and/or the same material composition (refractive index). That is, some of the devices have a first cavity length and/or a first refractive index and emit at a first wavelength, some devices have a second cavity length and/or a second refractive index and emit light at a second wavelength, etc.
Two types of monolithic arrays of laser devices have been formed. In one type, light is emitted in a direction parallel to the substrate. In a second type, light is emitted in a direction perpendicular to the substrate. This second type of monolithic array is referred to as vertical cavity surface emitting lasers (VCSELs). The first type of array is somewhat easier to fabricate than a VCSEL array. However, a VCSEL array is advantageous because the individual lasers in a VCSEL array can be processed, tested, bonded, and operated in parallel.
An array of VCSEL devices has a plurality of such devices formed on a single substrate. In
FIG. 1
, three VCSEL devices,
110
,
115
, and
120
are illustrated. The three devices are formed on a single substrate
125
. Each device
110
,
115
, and
120
has a bottom mirror
130
and a top mirror
135
. The mirrors are typically Bragg mirrors made of a dielectric material. The Bragg mirrors have a high (i.e. 99.9 percent) degree of reflectivity over a broad (i.e. on the order of 30 nm to 40 nm) range of wavelengths. Each device has a cavity region
140
,
145
, and
150
. The cavity region is a composite semiconductor material, typically a III-V semiconductor material (the III and V referring to groups of elements in the Mendeleev periodic table). Each cavity region has a top confinement layer
155
and a bottom confinement layer
160
. A small portion of the top mirror
135
and bottom mirror
130
is also part of the cavity region.
The devices
110
,
115
, and
120
are depicted as having two quantum wells,
165
and
170
, each. One skilled in the art will recognize that the number of quantum wells in an active region is largely a matter of design choice. Barrier layers
175
are formed between the individual quantum wells,
165
and
170
; between the top quantum well
170
and the top separate confinement layer
155
; and between the bottom quantum well
165
and the bottom separate confinement layer
160
. The combination of quantum well layers and barrier layers is referred to herein as the device active region.
In the devices depicted in
FIG. 1
, the cavity region
140
of device
110
has a first thickness, the cavity region
145
of device
115
has a second thickness, and the cavity region of
150
of device
120
has a third thickness. Because the total thickness of the cavity region affects the wavelength of the light emitted by the laser, each device,
110
,
115
, and
120
emits light at a wavelength that is different from the emission wavelength of the other two devices. The thickness of the cavity region determines the wavelength of light emitted by the device. Furthermore, the active region of the device is the source of optical gain (which is required for laser operation) when the device is biased properly. The relationship between the optical gain spectra and the emission wavelength of the device affects device performance. For example, peak laser operation efficiency is obtained when the peak of the optical gain spectra for a device coincides with the emission wavelength of the device.
Several different techniques have been proposed for fabricating VCSEL arrays in which some devices have a cavity region with a first length and other devices have a cavity region with a length other than the first length. Maeda, M., et al., “Multigigabit/s Operation of 16-Wavelength Vertical-Cavity Surface-Emitting Laser Array,”
IEEE Photonics Technology Letters
, Vol. 3, No. 10, pp. 863-865 (1991) describes a technique in which the VCSEL array is formed by using molecular beam epitaxy to form the active layer of the VCSEL devices. In order to obtain an array of VCSEL devices in which the thickness and composition of the active region varies among the devices, the wafer is not rotated as the material for the active region is deposited. This results in compositional and thickness variations in the layer. The VCSEL devices formed from this layer emit light at a wavelength that is determined by the thickness and composition of the portion of the active layer for a particular device. However, it is difficult to precisely control the thickness and compositional variations in an active layer formed using this technique. Consequently, since the differences in the composition and thickness of the MBE layer is the result of random differences that occur in the layer during its formation, it is difficult to control the difference between the different lasing wavelengths of the devices in the array. It is also difficult to control the tuning of the gain peak and cavity modes for the devices using this technique.
Wipiejewskil, T., et al., “Vertical-Cavity Surface-Emitting Laser Diodes with Post-Growth Wavelength Adjustment,”
IEEE Photonics Technology Letters
, Vol. 7, No. 7, pp. 727-729 (1995) describes a technique in which the cavity length is adjusted after it is formed using anodic oxidation and etching. In this process, the cavity for the devices in the array is formed over an AlAs-GaAs (aluminum arsenide-gallium arsenide) Distributed Bragg Reflector (DBR). The cavity is formed by epitaxial growth of a composite semiconductor material on the DBR. The cavity layer, as formed, is uniform in composition and thickness over the surface of the substrate. The cavity layer is then selectively etched to set the laser cavity length for the individual devices in the array. The lasing mode of each device in the laser array is thereby set in this manner. However, because the composition of the quantum wells is the same for all of the devices in the array, the wavelength of the gain peak is the same for all of the devices in the array. Consequently, the gain peak cannot be controlled to match the cavity mode. Furthermore, the device is very complex to manufacture, due to the multiple mask levels required to accomplish the etching.
The relationship between the gain spectra, the emission wavelength, and the reflectivity of the DBR better understood with reference to FIG.
2
. In
FIG. 2
, the reflectivity of the DBR as a function of wavelength is illustrated by line
190
. As illustrated in
FIG. 2
, a DBR that has a reflectivity close to 1 over a wide wavelength range can be formed. Thus a single deposition step that provides a reflectivity of 1 over a broad range of wavelength can be used as the DBR for any laser device that emits light in that broad range. A wavelength emission spectrum is illustrated by line
191
. The wavelength emission spectrum is the range of wavelengths of light that are emitted by the device active region. Note that the wavelength emission spectrum (as a function of power) has peak wavelength, &lgr;
g
. This is the peak emission wavelength, i.e. that wavelength of light in the optical emission spectrum that is emitted at maximum intensity. The laser cavity of the device also has an emission spectra and is illustrated b
Alam Muhammad Ashrafal
Hybertsen Mark S.
Leung Quyen
TriQuint Technology Holding Co.
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