U.S. patent application number 11/084704 was filed with the patent office on 2005-09-15 for high-speed to-can optoelectronic packages.
Invention is credited to Pao, Yi-Ching, Riaziat, Majid Leonard, Tzuang, Ching-Kung.
Application Number | 20050201433 11/084704 |
Document ID | / |
Family ID | 27613408 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050201433 |
Kind Code |
A1 |
Riaziat, Majid Leonard ; et
al. |
September 15, 2005 |
High-speed TO-can optoelectronic packages
Abstract
Methods and apparatus that provide TO-can optoelectronic
packages suitable for optical telecommunications applications
involving data speed rates of up to 10 Gbps and beyond. A TO-can
optoelectronic package may comprise a TO-can cap and a TO-can
header defining an interior region of the package. An
optoelectronic component such as a laser or photodetector can be
mounted within the interior region of the TO-can. An electrical
connection may be selected for coupling the optoelectronic
component to a selected post of the TO-can package. The electrical
connection may include a transmission line formed from a conductive
element other than a bond wire having a predefined length that can
assist in reducing parasitic effects within the TO-can
optoelectronic package to accommodate high-speed data rates. The
transmission line itself is connected to the optoelectronic
component and the selected post with a plurality of bond wires.
Inventors: |
Riaziat, Majid Leonard; (San
Jose, CA) ; Tzuang, Ching-Kung; (Hsinchu, TW)
; Pao, Yi-Ching; (Los Gatos, CA) |
Correspondence
Address: |
WILSON SONSINI GOODRICH & ROSATI
650 PAGE MILL ROAD
PALO ALTO
CA
94304-1050
US
|
Family ID: |
27613408 |
Appl. No.: |
11/084704 |
Filed: |
March 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11084704 |
Mar 18, 2005 |
|
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|
10212011 |
Aug 2, 2002 |
|
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|
6920161 |
|
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|
60350610 |
Jan 18, 2002 |
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Current U.S.
Class: |
372/36 |
Current CPC
Class: |
H01L 2924/30107
20130101; H01S 5/02212 20130101; H01S 5/0683 20130101; H01S 5/06226
20130101; H01S 5/183 20130101; H01L 2224/48091 20130101; H01L
2924/3011 20130101; H01S 5/02345 20210101; H01L 2224/48091
20130101; H01L 2924/00014 20130101; H01L 2924/3011 20130101; H01L
2924/00 20130101; H01L 2924/30107 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
372/036 |
International
Class: |
H01S 003/04; G02B
006/36 |
Claims
What is claimed is:
1. A high-speed TO-can package comprising: a TO-can cap and a
TO-can header defining an interior region; a plurality of posts
connected to the TO-can header; a laser mounted within the interior
region; and an electrical connection coupling the laser and a
selected post, wherein the electrical connection includes a
transmission line formed from a conductive element other than a
bond wire having a predefined length for reducing parasitic effects
within the TO-can package to be able to accommodate high-speed data
rates, and wherein the transmission line itself is connected to the
laser and the selected post with a plurality of bond wires.
2. A high-speed TO-can package comprising: a TO-can cap and a
TO-can header defining an interior region; a plurality of posts
connected to the TO-can header; a photodetector mounted in the
interior region; and an electrical connection coupling the
photodetector and a selected post, wherein the electrical
connection includes a transmission line formed from a conductive
element other than a bond wire having a predefined length for
reducing parasitic effects within the TO-can package to be able to
accommodate high-speed data rates, and wherein the transmission
line itself is connected to the photodetector and the selected post
with a plurality of bond wires.
3. A TO-can optoelectronic package comprising: a TO-can cap and a
TO-can header defining an interior region of the package; an
optoelectronic component mounted within the interior region; a
plurality of posts for connection to a circuit board; and an
electrical connection coupling the optoelectronic component and a
selected post, wherein the electrical connection includes a
transmission line formed from a conductive element other than a
bond wire having a predefined length for reducing parasitic effects
within the TO-can optoelectronic package to be able to accommodate
high-speed data rates, and wherein the transmission line itself is
connected to the optoelectronic component and the selected post
with a plurality of bond wires.
Description
CROSS-REFERENCE
[0001] This is a continuation application which claims priority
under 35 USC .sctn. 120 to U.S. patent application Ser. No.
10/212,011 filed on Aug. 2, 2002, which claims the benefit of
priority to U.S. Provisional Application No. 60/350,610 filed on
Jan. 18, 2002, which are incorporated herein by reference in their
entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention pertains to semiconductor laser
packages, transmitters, and receivers, and more particularly to the
packaging of a plurality of optoelectronic components within TO-can
package configurations.
[0003] Lasers are used for many purposes particularly in the
telecommunications industry. Edge emitting lasers are the most
common. They are available for all major telecommunication
wavelengths and multiple types are available for various
applications. Vertical cavity surface emitting lasers (VCSELs)
generally offer a low-cost alternative and are capable of being
fabricated in larger volumes on semiconductor wafers. These lasers,
which emit light vertically from the surface of a fabricated wafer,
combine the surface emission and low production cost as with
light-emitting diodes (LEDs), and provide the necessary speed and
power for many laser applications. VCSELs operating at wavelengths
at 850 nm are often selected today as laser sources for commercial
10 Gigabit Ethernet networks and optical backplane systems.
[0004] A widely accepted package for VCSELs and edge emitting
lasers is commonly known as TO-style or TO-can packages. Many
conventional semiconductor laser apparatus utilize a metal or
composite TO-can package for optoelectronic packaging, which
includes a can and header assembly that is formed with a window or
lens. The laser emissions from a laser contained within the package
passes through the header or cap portion of the TO-can. The TO-can
design has been used to package electronic devices since the early
days of transistor technology and include characteristic features
such as these windowed tops and related fiber pigtailing. The wide
availability of relatively inexpensive TO-can parts and packaging
services makes it a very attractive package for optoelectronic
devices. Presently, relatively small TO-cans such as TO-18, TO-46
or TO-56 are used for optoelectronic packaging that accommodate
data rates as high as 2.5 and 3.3 Gbps. The TO-can package however
was not originally designed for relatively high Gbps speeds.
High-speed optoelectronic devices have primarily relied on other
more expensive laser packaging solutions such as butterfly
modules.
[0005] The parasitic reactance associated with the construction of
a TO-can package limited its operation for high-speed applications
in the past. Speeds of up to a few gigabits per second have been
achieved by implementing a "differential" drive approach well known
in the art. This approach minimizes the effect of the parasitics
associated with the grounding of the package. A virtual ground is
established that limits current flow through the physical ground.
Maintaining the balance between the differential signals is crucial
however and this task gets progressively more difficult at higher
speeds. Moreover, TO-can packages generally employ bond wires for
connecting various optoelectronic components and posts which may
further contribute to the undesirable parasitic effects often
associated with these types of packages at high data rates. The
current lack of adequate solutions for addressing these limitations
can not meet the growing demand for high-speed digital
communications services which now require optical transmission
links to operate at data rates of 10 Gbps and higher.
[0006] Early transistors were packaged in metal cans. The
"transistor outline package" or the "TO-can" was the first
transistor package standardized by the JEDEC organization in
1960's. (JEDEC: Joint Electron Devices Engineering Council was
formed in 1958.) As optoelectronic semiconductor devices such as
photodetectors became available, these components were housed in
TO-cans with optical windows. Although the TO-can was not
originally intended to be used with high-speed modulation, it has
supported increasing rates of modulation over the years. Currently,
small TO-cans such as TO-46 are widely used in optoelectronic
packaging at data rates as high as 2.5 and 3.3 Gbps. The wide
availability of inexpensive TO-can parts and packaging services
makes it a very attractive package for optoelectronic devices.
SUMMARY OF THE INVENTION
[0007] It is an object of the invention to provide high-speed
TO-can packaging for semiconductor lasers, transmitters, receivers,
and other optoelectronic components. The cost-effective
improvements described herein may further promote the continued
success of TO-can packaging in the marketplace as a desirable
choice for current high-speed applications demanding data rates of
10 Gbps and greater.
[0008] The following is a basic summary of various aspects of the
invention which may be applied separately or collectively in
achieving a TO-can package for applications requiring high-speed
data transfer rates of up to 10 Gbps and beyond:
[0009] 1. Cutting the external TO-can posts short, connecting the
posts directly to controlled impedance transmission lines, and
grounding the TO-can as well as possible.
[0010] 2. Decrease inductance below, for example, 600 picoHenries
in an electrical path from the laser or photodetector to the post.
For example, keeping bond wires inside the TO-can compartment
relatively short by incorporating transmission lines as
interconnects. Some other examples of decreasing inductance in the
electrical path include at least two parallel bond wires, a bond
ribbon, and/or multiple bond wires and at least capacitor in series
with the capacitor(s) coupled to ground. The bond wire can be less
than 500 micrometers in total length, and/or have one or more
diameters exceeding 17 microns. The transmission line can be 50
ohms or some other defined impedance for some frequency range
and/or include a coplanar waveguide.
[0011] 3. Combining interconnect transmission lines and other
necessary components such as power monitor diode(s) and/or
mirror(s) on the same submount for the active device. The
transmission line and/or other components may be mounted or formed
on the submount. Examples of forming include, for example, forming
with the semiconductor material of the submount and/or
metallization.
[0012] 4. Including signal conditioning and/or bandwidth broadening
circuitry such as filters also either on the same submount as other
TO-can components or on the PC board. For example, a lowpass filter
can be partly on the PCB and partly include at least one post of
the TO-can, and/or a coplanar waveguide bandpass filter can be on
the PCB coupled to at least one post of the TO-can.
[0013] These principles may be applied to various TO-can packages
including TO-18, TO-46 and TO-56 style packages for edge emitting
devices and vertically emitting devices, which may involve
modifications specific to such configurations as described herein.
The TO-can package can include any number posts, such as 4 posts, 5
posts, or more posts, or fewer posts. The invention may be
generally applied to various laser packages and TO-can packaging
known in the art such as those described in the following issued
patents which are incorporated by reference in their entirety: U.S.
Pat. No. 5,838,703 entitled Semiconductor Laser Package with Power
Monitoring System and Optical Element; U.S. Pat. No. 6,001,664
entitled Method for Making Closely-Spaced VCSEL and Photodetector
on A Substrate; U.S. Pat. No. 6,302,596 entitled Small Form Factor
Optoelectronic Transceivers; and U.S. Pat. No. 6,314,117 entitled
Laser Diode Package.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIGS. 1(a) and (c) illustrate the device placement and PC
board assembly of a TO-can package in a conventional configuration
that is provided herein.
[0015] FIGS. 1(b) and (d) illustrate the device placement and PC
board assembly of a TO-can package in a high-speed configuration
that is provided herein.
[0016] FIG. 2(a) is a top view illustration of a TO-can package
with posts connected with a conventional bond wire.
[0017] FIG. 2(b) is a 12 Gbps eye-diagram for the TO-can package of
FIG. 2(a).
[0018] FIG. 3(a) is a top view illustration of a TO-can package
with posts connected with a transmission line formed in accordance
with the invention.
[0019] FIG. 3(b) is a 12 Gbps eye-diagram for the TO-can package of
FIG. 3(a).
[0020] FIG. 4 is a graph comparison of the insertion loss between
the two TO-can packages of FIGS. 2 and 3.
[0021] FIG. 5 is a drawing of a VCSEL TO-can package containing
three separate mounted chips.
[0022] FIG. 6 provides a top view illustration of a VCSEL submount
consisting of transmission lines and a monitor diode within a
TO-can package.
[0023] FIGS. 7(a) and (b) illustrate filters which may be installed
between a laser component and post within a TO-can.
[0024] FIGS. 7(c) and (d) illustrate filters which may be installed
at least partly on a printed circuit board.
[0025] FIG. 8 is a perspective drawing of an edge-emitting laser
positioned on a submount which may be installed within a TO-can
package such as a TO-56 package which may be modified in accordance
with various concepts of the invention.
[0026] FIG. 9(a) shows a perspective view of part of an embodiment
of a TO-can package with an edge emitting device and a
photodetector.
[0027] FIG. 9(b) shows a two dimensional view of a vertical
submount of FIG. 9(a).
[0028] FIG. 9(c) shows a two dimensional view of a horizontal
submount of FIG. 9(a).
[0029] FIG. 10 shows a two dimensional view of part of another
embodiment of a TO-can package with an edge emitting device.
[0030] FIG. 11. Lumped element model of TO-46 consists of 10
discrete elements.
[0031] FIG. 12. The schematic diagram of a TO-46 Can and its
frequency response. Its bandwidth is approximately 3 GHz.
[0032] FIG. 13. Simulated response of a TO-46 package when two
adjacent posts connected by a bond wire, and when they are
connected by a 50-ohm transmission line. There is no external
post-inductance in either case.
[0033] FIG. 14. Simulated response of a TO-46 Can when two adjacent
posts connected by a 50-ohm transmission line. In one case, the
TO-can is touching the PC board; in the other case it is offset by
2.5 mm.
[0034] FIG. 15(a) illustrates a mirror submount.
[0035] FIG. 15(b) shows a cross section view of a TO-46 can with an
edge-emitting laser on a mirror submount.
[0036] FIG. 15(c) shows a perspective view of the TO-46 can with
the mirror submount, but without the cap.
DETAILED DESCRIPTION OF THE INVENTION
[0037] FIG. 1 (a) illustrates the conventional placement and
assembly 100 of a TO-can package 102 on a PC board 104. The TO-can
package 102 may be coupled to multiple interconnects 106 for
communication with the board. The TO-can package 102 basically
comprises a header base and a cap or cover structure that is formed
with a window through which light passes. The window may be formed
with a lens or other optical element. The TO-can type package 102
provides an enclosure that serves to protect VCSEL components and
other contents such as photodetectors or photodiode monitors. The
TO-can laser package may include a ground post and one or more
signal lines for connection to the PC board 104. Portions of the
signal lines may be metallized and formed directly on the board. A
top view illustration of the interior region of the TO-can 140 is
also provided in FIG. 1(c). The package base may be connected to a
ground post for the package and a plurality of signal lines or
posts. The TO-can 140 may include a ground post 142, and multiple
signal posts 144, 146, and 148. A VCSEL 150 may be coupled to
signal post 146 with a bond wire 152 as shown. One or more posts
may be selected for connection to various optoelectronic components
contained within the package. Posts may pass through openings
formed in the TO-can base and may be positioned with surrounding
glass feed-throughs or other dielectric material. The VCSEL may be
also positioned in the proximity of the center region of the can
base that is connected to its corresponding signal post with a bond
wire, or a ribbon wire which tends to have less inductance than
round bond wires.
[0038] FIG. 1(b) provides an embodiment of a TO-can package formed
in accordance with the concepts of the invention described herein.
The TO-can package 122 is connected and mounted to a PC board 124
to provide a TO-can package and board assembly 120. The TO-can
package 122 may be formed with a ground post 130 that is coupled to
a ground contact 126 and a transmission line 128 as described
herein. External parasitics were minimized or substantially reduced
by shortening the length of the package posts and practically
eliminating or eliminating the physical gap between the TO-can
package 122 and the board 124. The substantially shortened ground
post 130 may be positioned adjacent to the ground contact or plate
126. The ground contact or plate 126 is needed very close to the
edge of the PC board 124 to minimize the length of the ground post
130. Moreover, external parasitics were significantly reduced by
installing a transmission line substantially along a portion of the
interconnect wire. Examples of transmission lines include the
microstrip and the coplanar waveguide. Mounting the TO-can package
122 on the edge of a PC board with controlled-impedance
transmission lines extremely close to the TO-can package produces a
desired reduction in parasitics as described herein. The
transmission line 128 on the PC board may be selected for
connection to its signal post 132 in accordance with the invention
along a substantial length of the board in place of a conventional
post to further reduce external parasitics. FIG. 1(d) also includes
a top view diagram of the interior of a TO-can package 160. The
TO-can package 160 includes a ground post 162, and signal posts
164, 166, and 168. The VCSEL 170 may be coupled to the signal post
166 with a transmission line 172 and a pair of relatively shortened
bond wires on either end. In some embodiments of the invention, the
transmission line 172 is formed as a 50 ohm microstrip. A bond wire
174 may connect the transmission line 172 to the signal post 166,
and another bond wire 176 may couple the transmission line 172 to
the VCSEL 176. The installation of the transmission line 172
essentially reduces the total length of bond wire used in this
embodiment of the invention. Internal parasitics within a TO-can
package is therefore reduced significantly when selecting a
transmission line. High-speed connections traditionally made using
bond wires for lasers within TO-can packages may be replaced with
transmission line assemblies as described herein in accordance with
the invention.
[0039] Speed Limitation Comparison
[0040] A comparison between TO-can package posts connected with
bond wiring alone is provided below against those using a
transmission line and shortened bond wires as described herein.
Speed limitations of a conventional TO-can package are highlighted
below when compared against packages modified in accordance with
the invention. To begin the experimentation, the external posts of
two TO-cans packages (Can #1 and Can #2) were substantially
shortened and edge-mounted on PC boards with 50 ohm transmission
lines extending to the posts. Eliminating or reducing the offset
distance to the PC board extends the acceptable performance of the
TO-can to approximately 7 GHz. Reducing bond-wire lengths further
extends its performance to approximately 12 GHz. The inductance and
capacitance of the packaging were minimized by keeping the posts to
the TO-can as short as possible. Some embodiments include
optoelectronic transmit and receive modules in TO-cans that operate
at data rates as high as 12.5 Gbps.
[0041] In Can #1 200, two adjacent signal posts 202 and 204 were
connected with a single bond wire 208 as shown in FIG. 2(a). FIG.
2(b) provides an eye diagram 250 for Can #1 which includes a
conventional bond wire as shown in FIG. 2(a). The eye diagram 250
is provided at 12 Gbps with a measured risetime of 73 picoseconds.
In Can #2 300, a transmission line 308 is installed between two
adjacent posts 302 and 304 and connected with relatively short bond
wires to each post as shown in FIG. 3 (a). Can #2 300 may include a
plurality of signal posts 302, 304, and 306. Two bond wires 310 and
312 may couple the transmission line 308 and the signal post 302.
Additional bond wires 314 and 316 may couple the transmission line
308 and the other signal post 304. FIG. 3(b) shows an eye diagram
350 of the TO-can design illustrated in FIG. 3(a) which is modified
in accordance with the invention. The corresponding eye diagram 350
is also provided at 12 Gbps with a measured risetime of 36
picoseconds. Eye diagrams such as these shown can be generated by
an oscilloscope and is basically a plot of wave amplitude versus
time. The vertical opening of the eye indicates the margin for bit
errors due to noise, and the horizontal opening of the eye
indicates the margin for timing errors due to an imperfectly
recovered clock. Lower rise times are generally desired which are
evidenced by a larger "center eye" portion in the eye diagram. Can
#2 thus provides the more desirable results as illustrated by the
relatively larger center eye in its corresponding eye-diagram.
[0042] FIG. 4 is a graph comparison in the frequency domain of the
insertion loss for the two TO-cans of FIG. 2(a) and FIG. 3(a)
demonstrating reduced external parasitics. Again external
parasitics were reduced by shortening the TO-can package posts
thereby eliminating substantially any physical space or the gap
between the TO-can package and the adjoining PC board. As shown in
FIG. 4, the graph plots insertion loss vs. frequency for each
TO-can package. A curve 410 shows the relatively low insertion loss
of the TO-can of FIG. 3(a), and another curve 420 shows the
insertion loss of the TO-can of FIG. 2(a). The installation of a
transmission line with shorter bond wires inside the TO-can of FIG.
3(a) however extends its usable speed to approximately 12 GHz in
accordance with the concepts of the invention.
[0043] The modifications described herein provide TO-can packages
that are inherently usable at speeds up to and beyond 10 Gbps
without any substantial structural modification. A 10 Gbps VCSEL
may be installed within these widely accepted TO-can packages as
described herein and mounted on a PC board. The undesirable
parasitic effects which currently plague TO-can packages at
high-speeds are significantly reduced by approaching the problem as
provided herein from two different perspectives, external and
internal parasitic minimization. External parasitics may be
minimized by reducing the length of the posts and eliminating the
physical gap between a TO-can package and the edge of a PC board.
Internal parasitics can be reduced by providing a transmission line
attached with relatively short end bond wires inside the TO-can in
place of a single continuous bond wire.
[0044] Integrated Submounts
[0045] The VCSEL packaging provided herein may contain multiple
lasers, monitor photodiodes or other photoelectronic components.
Various components may be formed on a single chip to recognize
certain cost-saving benefits and provide a smaller footprint. For
example, it is possible to reduce the number of parts by combining
the monitor diode with the transmission line segments on the same
submount. Another distinct aspect of the disclosed invention
provides integrated submounts positioned within the interior region
of a TO-can package for integrating transmission lines and other
optoelectronic elements inside the can.
[0046] FIG. 5 is an illustration of a TO-can package provided in
accordance with the current invention. The package cap or header
502 may be formed with a ground post 504 and several signal posts
as described above. A post 506 may be dedicated for communication
with a VCSEL 505 formed on the header 502, and another post 508 may
be selected for a monitor photodiode 516. Another post 510 may be
optionally connected to the monitor 516 or VCSEL 505 particularly
when employing a differential driving technique for the laser. The
post 506 for the laser may be connected to the VCSEL 505 with a
transmission line 512 and two short bond wires 514. The
transmission line assemblies described herein demonstrate the
ability to significantly reduce the internal parasitics typically
associated with using bond wires alone within conventional TO-can
packages. These and other optoelectronic components may be
encapsulated within TO-can packages and hermetically sealed as
described herein to substantially reduce typical parasitic effects
which become more pronounced at high-speed data transfer rates.
[0047] FIG. 6 illustrates another embodiment of invention which
includes a VCSEL submount 600. The submount 600 includes a monitor
diode 620 and transmission lines for high-speed connection. The
VCSEL 610 may be mounted or sit on a ground plane 630 of the
submount 600. A transmission line 640 couples the VCSEL 610 to a
signal post 650. A bond wire 652 may couple the VCSEL 610 and the
transmission line 640. Another bond wire 654 may also couple the
signal post 650 and the transmission line 640. The VCSEL submount
may support various package contents and have complementary
dimensions relative to certain TO-can packages. The transmission
lines described herein may be used interchangeably with other
interconnects that are modified to provide additional signal
conditioning functions as may be desired.
[0048] FIG. 15 (a) illustrates another embodiment of invention
which includes a mirror submount 1500. The submount 1500 includes a
mirror 1510 and one or more transmission lines 1520 for high-speed
connection. FIG. 15 (b) shows a cross section view of a TO-46 can
1525 with an edge-emitting laser 1530 on the mirror submount 1535.
FIG. 15 (c) again shows a perspective view of the TO-46 can with
the mirror submount, but without the cap.
[0049] FIG. 7 (a) illustrates a coplanar waveguide (CPW) bandpass
filter assembly 700 which may be installed between a laser within a
TO-can package and its corresponding posts. As described above, a
waveguide 702 may be connected to the post 704 and the surface or
edge emitting laser with relatively shortened bond wires 706. The
particular geometry and dimensions of the bandpass filter may be
modified in accordance with known methods depending on desired
frequencies. The bandpass filter could allow only certain signals
between specific frequencies to pass and discriminate against
signals at other frequencies. As known by those skilled in the art,
some bandpass filters may be modified herein with amplifiers that
boost the levels of signals in the accepted frequency range. Such
amplifiers may be connected to a source of power, which may be
positioned within the TO-can package in accordance with the
invention, to provide active bandpass filters. The invention may
incorporate active, or passive bandpass filters as described above
which neither amplify nor consume power in accomplishing the
desired signal conditioning. Filters may be thus installed
separately within the TO-can package or formed monolithically, or
alternatively, a filter may be formed by lumped capacitors and bond
wires. As shown FIG. 7 (b), a series of multiple capacitors 710 may
be also arranged along the base of a TO-can header or submount 712
which lead to the laser component 714. Several bond wires 716 may
be used to establish the electrical connection between the laser
714 and its corresponding post 718. This structure is a low-pass
filter whose cutoff frequency could be higher than 10 GHz. The
elimination of a single long bond wire again reduces internal
parasitics within the TO-can package. The particular geometries and
number of the capacitors 710 connected by bond wires 716 herein for
signal conditioning may be modified by known methods as may be
required. FIG. 7(c) illustrates a coplanar waveguide bandpass
filter 720 positioned on PCB 725. TO-can 730 has one post 732
coupled to the filter 720 and another post 734 coupled to the
ground contact 736. FIG. 7(d) illustrates a filter including a
series of inductors 740 and capacitors 742 positioned on PCB 755.
TO-can 750 has one post 760 coupled to the filter and another post
762 coupled to the ground contact 766. As referenced above, many of
the improvements provided herein are applicable to both vertical
surface emitting lasers as well as edge emitting lasers. Available
TO-can packages that contain edge emitting devices can also benefit
from the inclusion of transmission lines as described herein to
reduce parasitics and improve overall performance for relatively
high-speed applications. FIG. 8 illustrates a TO-can package formed
with a vertical submount that is adapted for edge emitting devices.
A steel cap 802 and header 804 may be welded or otherwise joined
together to form a hermetically sealed interior region. The steel
cap 802 may be formed with an aperture that is covered by a glass
window 806 which may be formed with a lens and/or suitable
coatings. The header 804 may be formed with a ground post 808 for
grounding the package. A pair of laser posts 810 and 812 may also
extend through the header 804 into the interior region for
connection to a laser diode 814. The laser 814 may be mounted
vertically onto a silicon submount 816, which would otherwise be
mounted ordinarily around the central portion of a header for
surface emitting lasers. The submount 816 may further be supported
by a copper post 818 within the package interior. The first laser
post 810 may be electrically connected directly to the laser 814
with a bond wire 820. The second laser post 812 may be coupled to a
contact pad portion 822 of the laser 814 with another bond wire
815. The second laser post 812 may be utilized for known
differential driving mechanisms at certain data transfer rates.
Moreover, a portion of the second laser post 812 extending inside
the package interior may be formed as a wedge-like configuration as
illustrated. A photodiode post 824 may also extend through the
header 804 into the package interior for connection to a PIN back
facet monitor photodiode or other photodetector 826. The photodiode
826 may be connected to its respective post with a bond wire
828.
[0050] In some embodiments, the cap and the header of are
integrally formed. In other embodiments, the cap and the header are
separately formed and then joined.
[0051] FIG. 9(a) shows a perspective view a horizontal and a
vertical submount that may be fitted within the interior of a
TO-can package as shown in FIG. 8. The TO-can package may include a
vertical submount 910 and a horizontal submount 920 as shown. The
vertical submount 910 may be coupled to a post 915 and may serve as
a mount or platform to support a laser diode 925 and transmission
lines 927 and 929. Other embodiments of the vertical submount and
the horizontal submount can include one transmission line, three
transmission lines, or any plurality thereof. In one embodiment of
the invention, the transmission line may be a coplanar waveguide
(CPW). The vertical submount 910 also includes grounds 931, 933,
and 935, respectively coupled to the post 915 by bond wires 937,
939, and 941. A bond wire 943 couples the transmission line 927 to
a laser contact 945. Other embodiments, such as with a
differentially driven laser, can couple the laser to at least two
transmission lines such as transmission lines 927 and 929.
Additionally, the horizontal submount 920 may include transmission
lines 951 and 953 and a photodetector 960. One embodiment of the
photodetector is an MSM (metal semiconductor metal) photodetector.
The transmission lines 951 and 953 can be curved to change
direction and minimize discontinuities. The transmission line 951
is also coupled to the transmission line 927 by a bond wire 977,
and the transmission line 953 is coupled to the transmission line
929 by another bond wire 979. The transmission line 951 is coupled
to a post 981 by a bond wire 983, and the transmission line 953 is
coupled to a post 985 by a bond wire 987. The horizontal submount
920 also includes grounds 955, 957, and 959, respectively coupled
to the post 915 by bond wires 971, 973, and 975. A photodetector
contact 962 is again coupled by a bond wire 964 to a post 966. The
post 966 include the contact pad that the horizontal submount 920
sits on. Another post couples photodetector 960 to one of the
posts, which is not shown. FIGS. 9(b) and (c) show two dimensional
views of the vertical submount 910 and the horizontal submount 920
illustrated in FIG. 9(a), respectively, which include similar
reference numerals used therein.
[0052] FIG. 10 shows a two dimensional view of yet another
embodiment of the invention provided herein. A TO-can package 1000
includes a vertical submount 1010 coupled to a post 1015. The
vertical submount 1010 includes a laser diode 1025, and
transmission lines 1027 and 1029. A bond wire 1043 couples a laser
contact 1045 to the transmission line 1027. The laser diode 1025
includes a laser channel 1044. In one embodiment, the transmission
line is a CPW. Other embodiments of the invention which may
incorporate a differentially driven laser can include at least two
transmission lines coupled to the laser such as transmission lines
1027 and 1029. A first transmission line 1027 may be coupled to a
post 1081, and a second transmission line 1029 may be coupled to a
post 1085. A variety of bonding agents may be selected for coupling
transmission lines to the posts with known epoxy and/or bonding
materials.
[0053] In some embodiments that are differentially driven,
geometrical symmetry should be maintained the differential lines,
because unequal lengths and parasitic reactances of the signal
lines can destroy the balance of the differential signal.
[0054] In some embodiments, to avoid radiative interference between
various signal lines, RF absorber material can be placed on the PC
board or close to the board in the enclosure.
[0055] Some embodiments compensate for parasitics with additional
circuit elements. The parasitics can at least partly be "absorbed"
into a low-pass filter or an "artificial transmission line". One
embodiment includes a filter with multiple series inductors and
shunt capacitors, allowing the parasitics to provide a few of the
elements. The remaining elements may be incorporated into the
PC-board and/or the device submount. A quick way to verify this
approach is to add 0.25 pF shunt capacitors at the input and output
ports of a TO-46 package that has 2.5 to 3.0 millimeters of offset
from the board and notice a 2.0 GHz improvement in its
bandwidth.
[0056] The RF cavity formed within a TO-46 is small enough that its
lowest order resonance should not be of concern at 10 Gbps. The
first order resonance of a pillbox cavity is at the wavelength of
1.3 times its inside diameter. For a TO-46 with an inside diameter
of 4.22 mm, the first cavity resonance occurs at 55 GHz. The
inclusion of chips and submounts in the package reduces the
resonance frequency, but normally not to the extent that would be
of concern at 10 Gbps. Bandwidth limitations observed in practice
for some embodiments of the TO-46 are in the range of 3-4 GHz and
are caused largely by parasitic reactances. Some reactances are
inherent in the structure of the package and cannot be altered
without redesigning the TO-can. Other reactances are determined by
the assembly of parts inside the TO-can and by the assembly of the
TO-can on the PC board.
[0057] The simulated performance of a conventional TO-46 shows the
familiar bandwidth limitation of approximately 3 GHz or roughly 4
Gbps. This is shown in FIG. 12.
[0058] The effects of reducing the bond-wire inductance inside the
TO-can package and reducing the post inductance outside of it may
be examined. In the simulation shown in FIG. 13 the TO-can is
touching the PC board. This zero offset assembly minimizes the
external parasitics. Some embodiments have an offset distance
between the printed circuit board and the TO-can header of less
than 0.1 millimeter. In addition to this, two cases are compared:
one with a bond wire connection inside the TO-can package and the
other with a 50-ohm transmission line connection. For the bond-wire
connection, the insertion loss at 12 GHz is approaching 8 dB, while
with the 50-ohm transmission line the insertion loss is less than 2
dB. Thus, reduction of parasitics can be achieved.
[0059] FIG. 14 shows the effect of post inductance alone. The
connection inside the TO-can is a 50-ohm transmission line. In one
case, there is no post inductance and the TO-can package is
physically touching the PC board. In the other case, there is a 2.5
mm offset distance between the bottom of the TO-can (from the
bottom of the TO-can header), and the edge of the PC board.
[0060] It can be seen that post inductance plays a stronger role in
limiting the bandwidth of the TO-can package than internal
parasitics. The elimination of bond-wire parasitics and post
inductances extends the usable bandwidth to 12 GHz and the TO-can
can be used at data rates up to 16 Gbps. Some bond wire and/or
external post inductance are tolerable at 10 Gbps. In one
embodiment, the offset distance is below one millimeter and bond
wire lengths below 500 microns.
[0061] A TO-46 can be modeled as a two-port network. This was done
by connecting a two-port device to two adjacent posts of the TO-can
package. The third post was ignored. The two-port device was chosen
to be either a 50-ohm transmission line or a single bond-wire. The
lumped element model developed for this configuration is shown in
FIG. 11. The model was used to predict the RF properties of the
TO-46 package.
[0062] Based on the foregoing, various TO-can style packages are
provided which may be adapted for high-speed data transfer rates in
accordance with the present invention. While the present invention
has been described in this disclosure as set forth above, it shall
be understood that numerous modifications and substitutions can be
made without deviating from the true scope of the present invention
as would be understood by those skilled in the art. Therefore, the
present invention has been disclosed by way of illustration and not
limitation, and reference should be made to the following claims to
determine the scope of the present invention.
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