Note: Descriptions are shown in the official language in which they were submitted.
ELECTRo-OPTIC DEVICE HOUSING FOR FIBER-OPTIC APPLICATIONS
This invention relates to a novel electro-optic
device housing useful in fiber optic communication
systems.
BACKGROUND OF THE I~NTION
Commun~c~tion systems using ~odulated light
waves transmitted through opt~cal ~i~ers are ~ecoming o~
increasing commercial importance~ In these systems, t~e
moduiated output o~ a light source is coupled into an
optical ~ber, transmitted through the optical ~er to
another location and detected by a l~ght de~ec~or w~ich
converts the incident modulated light sign~l to a modulated
electrical signal corresponding to the orig~nal electr~cal
16 signal impressed upon the light source These systems
o~fex the advantages o~ large signal ~and~idth, i~munity
from electrical noise, and small size. To ~e rel~a~le~
the components of ~uch systems must be shielded against
the severe environment~l conditions o~ten found ~n
transmission and rece~ing inst~llations and in t~e`
distances ~etween 5uc~ installations~
The light source used in such systems ~s
typicall~ a semiconductor pn ~unction light em~tting diode
or in~ect~on l~ser. The light detectox used ~.n such
26 system~ is t~p~c~lly ~ p-i-n or ~valanche photodiode~
The device ~ousing includes ~ he~der on w~ich
the electro~optic device suc~ as ~ light source or
detector is mounted. ~Electx~des, required ~or supply~n~
power to the electxo-optic device ~nd transmitting
~ elQctrical signals to or ~rom the dev~cer ~re ~nserted
through the header, The open~n~s in the hQ~der t~rough
wh~ch the electrodes ~re inserted ~re sealed us~ng ~
glass to met~lt epox~ res~n or solder se~ elect~c~l
: cont~ct to t~e neader is des~.red, to ~orm a ~ermetic .:
36 se~l het~een the electrode ~nd t~e header. T~e cap w~th
a l~r~e ~re~ windo~ in ~.t, suc~ ~s th~t sh~wn ~n U, S,
P~tent 3,8a5,347 ~s~ued ~pr~l 23, 1974~ to Coll~ns et al~
~nd U. S. Patent 3,946,416 ~ssued March.23~ 1~76~ ko
- H~cskaylo, to permit tr~nsmiss-~on o~ t in~o o~ out
o~ the housing, is ~oldered ox ~e1ded to the cap to ~orm
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a hermetic seal therebetween.
~ Light sources or detectors mounted in such prior
art housings are not suitable for use in a fiber optic
communication system since the window of the package will
be typically at least 0.1 cm in thickness, and the source
or detector will be mounted some distance behind the ~nside
surface o~ the windo~, typically ~ 05 cm or more. Since
light emitted by a semiconductor light emitting diode or
injection laser diverges with a large angle, an optical
fiber must be close to the emit~ing surface to efficiently
couple the light output into the optical fiber.
Correspondingly, light emitted rom the end of the optical
fiber diverges with a large angle, so that the optical
1~ fiber must be close to the light detector in order to ob-
tain the maximum elec-crical output from the detectox.
Another problem with housings for electro-optic
devices is the assembly of the electro-optic device and
housing. The alignment of the optical fiber with the
electro-optic device must be accurate in order to obtain
maxim~n coupling in or out of the optical fiber and must
be maintained during subsequent assembly operations. There
can be no movement of the optical fiber during the final
assembly operations since slight misalignment leads to a
product with reduced efficiency.
Operations of fiber optlc communication systems
at wavelengths of about 1 miGron to about 1.2 microns are
o~ interest because of the low dispersion of the commercial-
ly available optical fibers in this wavelength region.
~30 However, in this wavelength region, thè sensitivity of
silicon light detectors is low because of the low
absorption constant of silicon. Therefore, a long path
length in the detector is required in order to maintain
maximum electrical output signals. One method of obtain
ing such a long pa'ch length is to couple lighk into the
detector parallel to the plane of the p-n junction of the
detsctor such that it propagates parallel to and in the
plane of the junction.
A further reason for having a device housing in
4~ which the optical fiber enters through the side of the
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housing is that the light sources and detectors in such
housings are o~ten mounted on electrical circuit boards
which are stacked close together, with the space allowed
for such housings of about 3/8 of an inch between the
circuit boards. It is there~ore de~irable to have the
fiber exit from the light source or detector in a direction
at right angles to the electrodes and there~ore parallel to
the circuit boards.
Therefore, it is desirable to have an electro-
optic device housing in which a fiber can be mounted and
aligned in close proximity to an electro-optic device,
which provides a long path leng~h ~or absorp~ion o~ light
in the wavelength region between 1.0 and 1.2 microns and
1~ which has ease and rellability o~ assembly.
Goell et al, U. S. Patent 4,065,203 issued
December 27, 1977, teach the use o~ an opt;cal fiber con-
tained in a support tube whioh is inserted through an
openin~ in a cap for a laser diode housing. The support
tube is glued both to the mount o~ the laser diode and
to the cap o~ the laser diode housing extending ~hrough
this housing to the règion outside the cap. ~Goell et al
teach that the cap must ~e mounted on the header a~ter
the alignment of the ~iber and the electro-optic device
2~ thus providing an opportunity for a disturbance of the
alignment of the electro-optic device and the optical
fiber.
SUMMAR~ OF THE INVENTION
An electro-optic device housiny for fiber optic
applications compri~es a header, an electro-optic device
bonded to said header, electrodes inserted through said
header, means for electrically connecting said device to
said electrodes, a sidewall bonded to said header, an
optical fiber guide tube inserted in an opening in said
~5 sidewall, bonded to said sidewall and extending radially
outward ~rom the inner surface of said sidewall, an
optical fiber inserted in said fiber guide tube and
- extending from out~ide said sidewall hrouyh said sidewall
towards said electro-optic device, and a cover pla~e bond-
ed to said sidewall which completes the enclosure of said
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electro-optic devica, said electrodes and that port.ion of
the optical fi~er interior to said sidewall.
BRIEF DE5CRIPTION OF T~E INVENTION
6 FIG. 1 shows a top view of the electro-optic
device housing of the invention.
FIG. 2 shows a cross-secti.onal view along line
2-2 of the electro-optic device housing shown in FIG. 1.
FIG. 3 shows another cross sectional view along
line 3-3 of the electro-optic device housing shown in
FIG. 1.
DETAILED DESCRIPTION OF THE INVEN~ION
The invention is a three-part, side-sntry,
electro-optic device housing including a header, side--
1~ walls and cap. In the FIGURES, the electro-optic device
housing 10 of the invention includes a header 12 through
which are inserted electrodes 14 and 16 which are
hermetically sealed to the header 12 by sealing means 18. . .
The sealing means 18 includes glass to metal seals, cured
epoxy resins or, i electrical contact o one of the
electrodes to the housing is desired, soldering or welding~
Bonded to the header 12 is a mounting block 20 whose upper
sur~ace 21 is meta~lized. An electro-optic device 22 is
bonded to the upper surface 21 of the mounting blcck 20
2~ by soldering or other means. Alternatively, the mounting
block can be removed and the electro-optiG device 22 bonded
directly to the header 12. Wires 24 and 26 make the
electrical contact between the electrodes 14 and 16 and
the electro-optic device 22.
~ ~he electro-optic device 22 can be a light source
such as a semiconductor pn junction light emitting diode
or an injection laser such as an AlGaAs double hetero-
~unctlon continuous wave injection laser. Altexnatively,
the electro-optic device can be a light detector. Suit-
~5 able light detectors include photoconductors, depletion
layer devices such as p-i-n photodiodes, or avalanche
photodiodes. Alternatively, both a light source and ~ .
detector can be combined in the same device or mounted
` separately within the device housing 10. In a~dition, an
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electrical device which can comprise an electrical circuit
or circuits coupled to the electro-optic device 22, and
~ to one or more electrodes including elec~rodes 14 and 16,
can be mounted within the electro-optic device housing 10.
5uch electrical devices can include an electro-optic device
temperature compensation circuit and sensors, a light
source dr~ver amplifier, a feedback circuit for maintaining
constant light output from a light source, a light
detector preamplifier and the like.
A sidewall 28,which can be a ring or other
geometric configuration, is bonded to the header at the
joint 30. An opening 32 is formed in the sidewall 28.
A fiber guide tube 34 is inserted into t.he opening 32 and
16 bonded to the sidewall 28. The iber guide tube 34 ax-
tends from outside the sidewall 28 to about the inner
surface of the sidewall 28. A fiber optic cable 36 which
contains an optical fiber 38 is inserted into the fiber
guide tube 34. The optical fiber 38 extends inside the
sidewall 28 towards the electro-optic device 22. A
strain relief member 40, such as heat shrinkable tubing,
is fitted over the fiber guide tube 34 and the fiber optic
cable 36 and extends a distance outward alony the fiber
optic cable 36. The optical fiber 38 is hermetically
~6 sealed to the sidewall 28 using sealing means 42. A
support block 44 can be mounted on the mounting block
20 to support the optical fiber 38 when it is aligned
with the light emitting or light sensitive surface of the
electro-optic device 22. Alternatively, the optical
~0 iber 38 can be supported by the mounting block 20~
A cap ~6 is bonded to the sidewall 28 at the
joint 48 to form a hermetic seal therebetween.
Thè fiber optic cable 36 containing the light
transmitting fiber 38 extends outward from the ring 28
forming a fiber pigtail. This fiber pigtail can be
terminated by a connector 50 suitable for interconnection
of the optical fiber of the pigtail with another fiber
optic cable containing an optical Fiber.
The optical fiber 38 transmits light emitted
by an electro-optic device out of the housing. Suitable
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light emitting devices include a semiconductor pn junction
light èmit~ing'diode or a semiconductor pn junction laser,
or any other ~odulatable light source. Altexnatively,
light emitted by an external source of light, such as an
optical fiber contained in a fiber optic cable, into the
optical fiber of the pigtail is transmitted into the
housing and detected by an electro-optic device such as a
light detector.
The utility of the electro-optic device housing
of the invention, a three part, side entry housing,
is that the position sensitive components, the electro-
optic device and the optical fiber, can be accurately
aligned and tied down before the assembly is completed.
1~ In the assembly of this housing two parts, the header and
sidewall, as well as the electro-optic device and the
optical fiber, are assembled and the optical fiber and
the electro-optic device are aligned with one another
before the third part of the housing, the cap, is joined
to the remainder o the housing. ~ince the cap does not
contact either the optical fiber or the electro-optic
de~ice, it cannot disturb the alignment of these compo-
nents.
Using the housing of the in~ention, a light
2~ detector with a high sensitivity to light with a wave-
length of 1.06 microns can be ~abricated. ~he light
dete~tor and optical fiber are aligned such that li~ht
emitted from the optical fiber towards the detector
propagates parallel to and in the p-n junction plane,
~hus resulting in a long a~50rbi~ length in the detector.
The approach is easible since the accurate alignment
o the optical fiber and the light deteator is not ~is-
~urbed during subsequent assembly steps~
Such a housing was assembled as ~ollows.
~6 A ring, with a guide tube inserted into an
opening in the ring and bonded to the ring, was welded
to a header using standard resistance wel~ing techniques.
A ceramic mounting block was soldered to ~he header~ An
optical fiber with a 100 micron diameter core, a 200 micron
outside diameter and a numerical aperture o~ 0.25 was in-
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serted through the guide tube and rested on a mounting
block~b~nded tQ the header. The optical fiber was bonded
to the inner diameter of the ring using a curable epoxy
6 resin thus forming a hermetic seal therebetween. The
optical fiber was not bonded to the mounting block but it
is preferred to bond the optical iber to the mounting
block using bonding means such as a curable epoxy resin.
The design of the p-i-n photodiode to be mounted
in the housing was optimized for 1.06 microns. The
thickness of the diode was 150 microns. The fan out of
light emitted from the optical fiber i5 accommodated by
the increasiny width of the sansitive region of the photo-
diode. The sensitive length of the diode was 2 mm and
16 the light traversed about 0.24 mm of an insensitive region
beore reaching the sensitive region of the photodiode.
The p-i-n photodiode, with its electrodes
attàched to an e~ternal bias voltage supply and signal
detection circuits was then aligned with the light beam
emitted from the optical fiber such that the maximum
`~ electrical output signal was obtained from the photodiode.
A molybdenum tab was`used as a ~him be~ween the photodiode
and the mounting block to obtain'the correct height for
the detector. The photodiode was then bonded to the mount-
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ing block using an electrically conducting cuxable epoxyresin. ~he position of the photodiode was such that the
light beam was propagated parallel to and in the pn
~unction plane o the photodiode.
The cap was then welded to the ring usin~
standard xesistance welding techniques.
' To obtain a measure of the improvement in
detectox quantum e~iciency, the 1~06 micron wavelength
light from a tungsten filament lamp tranæmitted through
a narro~ band filter was first ~ocussed on a calibra~ed
86 photodiode, impinging on the calibrated photodiode
normal to the pla~ne of the p-n junction. The calibrated
photodiode was known to have a quantum efficiency of 24%
at 1.06~microns~ The light was then ocussed onto the end
of the optical fiber external to the housing. ~he quantum
efficiency of the optical fiber plus photodiode mounted
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in the housing was 36%. If the antireflection coating
at 1.06 microns~is applied to the edge of the photodiode
it is estimated that the quantum efficiency would increase
6 to ~bout 50~. If in addition an antireflection coating
is applied to the ends of the optical fiber, ~hen it is
estimated that the quantum efficiency for the device in
the housing would increase to about 52%~
To show the effect of antireflection coatings,
a drop of a light-transmitting curable epoxy resin was
placed between and touching both the end of the optical
fiber internal to the housing and the edge of the phato-
diode. The measured quantum efficiency increased to
about 45% from 36%.
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