Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
1999-07-07
2002-05-21
Fourson, George (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C372S007000, C372S050121
Reexamination Certificate
active
06392256
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to vertical cavity surface emitting lasers (VCSELs) and photodetectors, and more particularly to the application of such optoelectronic devices where they must operate independently but where it is also desirable to have a transmitter and a receiver closely-spaced.
BACKGROUND OF THE INVENTION
There are a number of data communications applications that make use of optoelectronic sending and receiving devices (i.e. light emitters and photodetectors). For fiber optic data communication applications requiring less than 200 MBits/sec., light emitting diodes (LEDs) are the light emitters of choice because they are relatively inexpensive to manufacture. For applications requiring higher speeds, lasers are typically used as the light emitters.
Until recently, most high speed data communications applications employed edge emitting lasers in a serial (single channel) format. With the advent of Vertical Cavity Surface Emitting Lasers (VCSELs), many such applications are now implemented using VCSEL arrays that can be interfaced to ferrules carrying multiple fibers to transmit several bits of data in parallel. At the receiving end, an array of photodetectors is coupled to the multiple fibers. The ability to manufacture VCSELs in arrays (an advantage of LEDs), combined with their high speed of operation (an advantage of lasers), makes VCSELs desirable in such applications.
For high-speed serial duplex data communications applications, however, separately packaged light emitters (usually edge emitting lasers) and photodetectors are still employed. For long-haul applications (typically having distances greater than 1 kilometer), wavelength division multiplexing is often employed to transmit and receive data for a duplex channel over the same fiber. Because the primary cost of a long-haul duplex serial data channel resides in the fiber and its installation, complex beam-splitting techniques can be justified at the ends of the channel to separate the transmit and receive data streams from the single fiber.
For short-haul or “premises” applications, however, the cost of fiber and fiber installation is relatively less important than the cost of the many transmit and receive functions. Thus, it is the cost of the data transmit and receive components, and particularly the optoelectronic devices and their packaging, that drives cost considerations for short-haul applications. Typical short-haul implementations of a high-speed serial fiber optics data communications channel operating in full duplex still employ two multimode fibers, each one to connect an individually packaged transmitting light emitter to an individually packaged receiving photodetector. This is because the cost of complex beam-splitting components often cannot be justified.
FIGS.
1
(
a
) and
1
(
b
) illustrate the components comprising a typical implementation of a transmit or receive link for a short-haul high-speed duplex data communications application. FIG.
1
(
a
) illustrates a fiber assembly
12
. A round ferrule
26
houses an optical fiber
28
, which is located precisely in the center of ferrule
26
. A typical diameter for ferrule
26
is approximately 2.5 mm. Ferrule
26
comes with a latching mechanism
30
, which is used to clamp and secure the ferrule to a barrel
32
of an optical sub-assembly
10
, which is depicted in FIG.
1
(
b
). Barrel
32
houses optoelectronic device
14
typically in a TO can package
16
centrally located in the barrel as shown. Optoelectronic device
14
is typically located at an appropriate point within can
16
by a standoff
2
. Driver or amplifier circuitry is coupled to optoelectronic device
14
through leads
22
. A window
18
is provided in the top of the can package to allow transmitted light out or received light in, depending upon whether the optoelectronic device is a light emitter or a photodetector. The TO package is aligned with fiber
28
and epoxied using epoxy
24
to fix the position of the optoelectronic device with respect to the ferrule
26
and hence fiber
28
. Optical elements such as lens
20
are typically provided to focus the light for optimal optical efficiency, particularly where the light emitter is an edge emitting laser. Barrel
32
is designed to mate with latching mechanism
30
of fiber assembly
12
.
Both fiber assembly
12
and barrel
10
are precision manufactured for precise mating. Active alignment TO package
16
and optoelectronic device is ordinarily performed in the x, y and z axes. First, the optoelectronic device is precisely aligned within the package
16
. Second, the package
16
is precisely aligned within barrel
10
. Finally, optical element
20
is precisely aligned with respect to its distance from the optoelectronic device
14
to achieve proper optical operation. Because a separate package is required for both the transmit side and the receive side of the duplex data channel, a total of twelve active alignments are typically performed for each channel and each channel includes the cost of eight precision-manufactured coupling parts.
FIGS.
1
(
c
) and
1
(
d
) provide schematic illustrations of the fiber assembly
12
and optoelectronics subassembly
10
of FIGS.
1
(
a
) and
1
(
b
), respectively.
FIG. 2
illustrates a typical duplex serial data communications module
40
, which has mounted to it an optical subassembly
52
containing a light emitting device
13
disposed in a TO can package
9
having a window
17
, which is to be mated with an optical fiber assembly
46
and which is dedicated to data transmission. Module
40
also has an optical subassembly
50
mounted to it containing a photodetector
15
disposed in TO can package
11
and which is to be mated with optical fiber assembly
48
and dedicated to receiving data from a remote module not shown. Because of the differing optical requirements of the transmit and receive devices, the modules must often be mounted in a staggered fashion as shown. Moreover, the transmit devices are located at an optically appropriate point in their can packages by standoffs
4
and
6
respectively.
Because of the cost of the precision components and the large number of alignments required for implementing duplex serial modules
40
, it is highly desirable to integrate the transmit and receive optoelectronic devices (i.e. light emitter and photodetector) into one package. The integration of the two devices into a single package is not, however, an easily achieved solution. The prior art implementations as illustrated in
FIGS. 1
a-d
and
2
cannot be readily adapted to multifiber ferrules currently available for unidirectional data transmission using VCSEL arrays. These multifiber ferrules have fiber spacings which are typically about 250 microns and can be less. The diameter of the TO can package
14
commonly used in present implementations is itself 5600 microns in diameter. Thus, the standard ferrule and barrel would have to grow substantially in diameter to accommodate two fibers having the spacing dictated by the TO cans housing the optoelectronic devices.
Even if a substantially larger barrel could be created to integrate the light emitter and photodetector as commonly packaged to receive both a transmit and a receive fiber, it is not clear that the resulting package could provide the necessary separation of incoming and scattered outgoing light beams to prevent crosstalk between the transmit and receive signals (at least not without complex optics and possibly some form of isolation). Although solutions have been disclosed to stack a light emitter (typically an LED) on top of a photodetector to transmit and receive wavelength division multiplexed signals (the light emitter is transparent to the received wavelength), beam-splitting must still be employed at the opposite end.
Closely spaced VCSELs and photodetectors can suffer leakage effects which can degrade the sensitivity of and induce excess noise into the operation of the photodetector. Also, current leakage from the VCSEL to the photodetector can unacceptably alte
Scott Jeffrey W.
Wasserbauer John
Cielo Communications, Inc.
Fourson George
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