WO2004019093A1

WO2004019093A1 - Compression-molded three-dimensional tapered universal waveguide couplers

Info

Publication number: WO2004019093A1
Application number: PCT/US2002/026869
Authority: WO
Inventors: Suning Tang; Ray T. Chen
Original assignee: Finisar Corporation
Priority date: 2002-08-22
Filing date: 2002-08-22
Publication date: 2004-03-04
Also published as: AU2002329818A1

Abstract

The present invention is a three-dimensional tapered waveguide coupler (2, 3) capable of interconnecting electro-optical devices (4, 5) with differing optical mode profiles. The ends of the waveguide are configured to match the optical mode profiles of the electro-optical devices that the waveguide interconnects. The waveguide adiabatically transmits the fundamental mode of the photo-optic signal from the electro-optical (4, 5) device at the waveguide's input end (6) to a different electro-optical device (4, 5) at the waveguide's output end (1). A single couple can be configured with one or more waveguides, each waveguide having different optical mode profiles at either end and different optical transmission characteristics.

Description

COMPRESSION-MOLDED THREE-DIMENSIONAL TAPERED

UNIVERSAL WAVEGUI DE COUPLERS Field of the Invention

The present invention relates to waveguides, and more specifically to optical waveguide

interconnectors.

Description of the Related Art

Communications and computing systems based on electronic interconnection schemes have

nearly reached their limits of any further improvement, while demands for higher bandwidth and

speed are ever increasing for military and civilian applications. This scenario has led researchers

to seriously consider new computing architectures based on optoelectronic interconnects.

Employment of optical technologies is already a reality in the area of communications. However,

full commercial realization of optical computation and communications networks depends on the

successful transmission and control of high-speed signals among processing elements, memories and

other peripherals with minimal losses. In fact, by now, several key components and manufacturing

technologies needed for this technological revolution have attained an appreciable state of maturity.

A large variety of optoelectronic devices such as lasers, optical amplifiers, electro-optic switches,

modulators, and detectors exhibit excellent performance characteristics. Also, the transmission

characteristics achieved for optical waveguides and fibers allow optical transmission spans from

intra- wafer interconnection to global optical communications.

The compatibility and reliability aspects of optoelectronic packaging are the major hurdles

to taking full advantage of novel achievements in optoelectronic device technology. The difficulty

in packaging mismatched discrete optoelectronic components generally results in high coupling

losses. The mismatch is mainly a consequence of spatial and angular variations of individual

optoelectronic components and separations of array devices. These components, which are often fabricated on different substrates, have different mode sizes, numerical apertures and component

separations. For example, laser diodes can be made on GaAs, detectors made on silicon, and

modulators made on LiNbO₃. The dimensions of these discrete devices are pre-selected to optimize

their performance, and vary from a few microns (single-mode waveguides and vertical cavity

surface-emitting lasers) to hundreds of microns (photodetectors and multi-mode plastic fibers). The

requirements of individual element performance optimization make system integration very difficult

with comfortable alignment tolerance, and consequently integration results in high packaging costs.

In many scenarios, the packaging cost is approximately 50% of the total system cost. Achieving

efficient optical coupling among various devices with sufficient alignment tolerance is particularly

difficult when using prisms, gratings, or optical lenses.

Two-dimensional (2-D) tapered waveguides have been employed to improve optical coupling

among various optoelectronic devices. Coupling efficiency can be further improved if three-

dimensional (3-D) tapered waveguides can be fabricated and employed. However, none of the

existing optoelectronics or microelectronics fabrication methods available to optoelectronics

industrial sectors is suitable for making satisfactory 3-D tapered waveguide devices, varying from

a few microns to hundreds of microns in both horizontal and vertical directions. To grow a high-

index waveguide layer hundreds of microns in thickness is impractical using any existing waveguide

fabrication method used for inorganic materials such as GaAs, LiNbO₃ and glass. The 3-D

compression molded polymeric waveguide described herein is a solution to bridge the huge dynamic

range of different optoelectronic device-depths varying from a few microns to hundreds of microns.

The ends of the waveguide are designed to provide interfaces with fiber-optic cables. There

are three problems that have occurred at the ends, or interfaces, of the waveguide:

(1) inconveniently high coupling loss due to the optical mode mismatch, (2) strict alignment tolerance which makes the packaged system vulnerable to harsh

environments, and

(3) separation mismatch when array devices (such as laser diode arrays, smart pixel

arrays, photodetector arrays and optical fiber/waveguide arrays) are employed.

Due to the planarized nature of these devices mentioned above, the most common approach

based on microlenses is bulky and very expensive because it requires complicated tirree-dimensional

spatial and angular alignments.

Conventional optoelectronic packaging technologies have failed to provide low-loss coupling

among various optoelectronic devices, including laser diodes, optical modulators, waveguide

splinters, single-mode optic fibers, multi-mode optic fibers and optical detectors. The above

mentioned three problems are solved by the present invention.

SUMMARY

The present invention discloses an apparatus and method for interconnecting electro-optical

devices with differing optical mode profiles. The present invention is a three-dimensional tapered

waveguide with a specified index of refraction that adiabatically transmits the fundamental mode

of the photo-optic signal from an electro-optical device at the waveguide's input end to a different

electro-optical device at the waveguide's output end. The input and output ends of the three-

dimensional tapered waveguide are configured to match the optical mode profiles of the electro-

optical devices that the waveguide interconnects.

The three-dimensional tapered waveguide can be configured in an array, to interconnect

arrays of electro-optical devices with differing optical mode profiles and inter-device separation.

Depending upon system packaging requirements, the waveguide can be manufactured on a substrate

for interconnecting devices mounted on or near substrates such as printed circuit boards or silicon

substrates, or encased in a plug-type arrangement for interconnecting electro-optical devices such

as fiberoptic cables. The substrate or encasing material has a different index of refraction than the

waveguide material.

Additionally, the present invention comprises a compression-molding technique for

fabricating 3-D tapered polymeric waveguides. This embodiment of the present invention comprises

a two-piece molding tool that includes a substrate and a mold plunger where the mold plunger

provides a cavity having the shape of the desired 3-D-tapered waveguides. The present invention

provides an appropriate amount of molding material that is spin-coated or laminated onto the

substrate. The film thickness is equal to the maximum thickness of the tapered structure. The

molding process begins with the heating of the mold plunger and the polymer film. Then the two

parts of the mold are brought together under pressure that causes the polymer film, which is softened by heat, to be compression molded into the shape of the stamp of the desired 3-D-tapered

waveguides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustration of an optoelectronic interconnect module where the

three-dimensional tapered polymeric waveguide couplers are employed for interconnecting an

VCSEL array to an optic fiber array and a photodetector array to an optic fiber array.

FIG. 2 shows an illustration of an optical fiber MT-type coupler employing three-

dimensional tapered polymeric waveguides used for connecting two MT-type fiberoptic connectors

that have different mode profiles and fiber separations.

FIG. 3 shows a more detailed illustration of a polymeric waveguide coupler, which comprises

an array of 3-D tapered waveguides. Each waveguide has different guided mode profiles at the two

ends. The separation between each waveguide may also be different at the two ends in order to

match the separation optoelectronic devices under connection.

FIG. 4 shows optimized taper profiles for an adiabatic transition of waveguide mode.

FIG. 5 shows an illustration of the compression-molding technique for fabricating 3-D

tapered polymeric waveguides.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a novel universal optical coupler that comprises a unique

three-dimensional tapered polymeric waveguide with different mode-profiles at the two ends that

can match to two different optoelectronic devices respectively, for increasing the optical coupling

efficiency and the alignment tolerance. These polymeric waveguide couplers are fabricated by the

compression-molding technique. Additionally, the present invention also includes a unique

compression-molding method for manufacturing three-dimensional tapered polymeric waveguides

that comprises a compression step at the polymer phase transition state. Within this step, the 3-D

waveguide structure is transferred from the mold plunger into a polymeric film that has been

preapplied to any substrate of interest.

The most common approach for optical coupling among optoelectronic devices with different

apertures is to employ an optical lens that is often too bulky compared to the optoelectronic devices

to be coupled. Such an approach also is very expensive because it requires complicated three-

dimensional spatial and angular alignments. Another conventional approach is to employ the two-

dimensional tapered waveguide coupler that can be manufactured by standard photolithography

technique. The innovative compression-molded three-dimensional polymeric waveguide of the

present invention is an effective component for optical coupling among various optoelectronic

devices, including laser diodes, optical amplifiers, electro-optic switches, modulators, optic fibers

and photodetectors. The novel compression-molded polymeric waveguide is a solution to bridge

the huge dynamic range of different optoelectronic components. It is also applicable wherever there

is a need to interconnect optoelectronic components with different apertures, such as connecting

telecommunication (single-mode) fibers to (multi-mode) local network fibers. Compression molding is also one of the most effective methods for mass-producing high

quality low-cost optical polymeric waveguide devices. The compression molding methodology

disclosed herein adds the novel steps of compression molding a material that has been preapplied

to a substrate of interest, and performing that compression molding step when the preapplied

material is at the polymer transition stage.

The compression-molded three-dimensional universal waveguide couplers are made out of

optical polymers that can be compression-molded into optical channel waveguide format for

effective optical coupling. The 3-D tapered waveguide coupler has two different optical mode

profiles at the two ends. The two different optical mode profiles match the optical mode profiles of

two optoelectronic devices to be coupled and increase the optical coupling efficiency between the

two devices. The guided region of the 3-D waveguide coupler functions as a guided mode convertor

with a very small propagation loss. For optical coupling between two arrays of devices, a waveguide

coupler array provides both the different mode profiles at the two output ends and the required

channel separations.

The 3-D waveguide coupler can be molded directly onto any substrate of interest. The

substrate functions as a structural element, providing physical support for the waveguide during

manufacturing and operation. The substrate can also provide a convenient design solution to insure

that the optoelectronic devices to be coupled to the waveguide are properly aligned with the

waveguide, since such devices are often designed for mounting on typical electronic substrates such

as a printed circuit board or a silicon substrate. Alternatively, the 3-D waveguide coupler can be

enclosed or encased in a plug-type arrangement that provides the necessary physical support and

alignment, and is more convenient for interconnecting electro-optical cables such as fiberoptic

cables. The process of compression-molding involves fabrication of mold plungers with the desired

3-D waveguide cavity, preparation of polymer films on a substrate, and formation of waveguides.

A two-piece compression-molding tool consists of a substrate and mold plunger that provides a

cavity having the shape of the desired 3-D-tapered waveguide. An appropriate amount of molding

polymer, polymer waveguide film in this case, is spincoated or laminated onto the substrate of

interest. The film thickness is equal to the maximum thickness of the tapered structure. The

molding process begins with the heating of the mold plunger and the polymer film. The two parts

of the mold are then brought together under force. The polymer film, softened by heat, is thereby

molded into the shape of the stamp. The last step is to harden and fix the molded waveguide shape.

The fundamental differences in polymers dictate the plastic processing method to be used for the

final step in the waveguide molding. For example, thermosetting polymers are further heated under

pressure, while thermoplastics are cooled to room temperature. After the polymer is cured (fixed),

the mold plunger is removed, leaving behind are the 3-D-tapered waveguide structures on the

substrate.

FIG. 1 is an illustration of an optoelectronic interconnect module where the

three-dimensional tapered polymeric waveguide couplers are employed for interconnecting a VCSEL

array to an optic fiber array and a photodetector array to an optic fiber array. The module shown in

FIG. 1 comprises an input optical fiber array 6, 3-D tapered waveguide couplers 2 and 3,

photodetectors 4, VCSELs 5, and an output optical fiber array 1, all of which are mounted on any

substrate of interest, such as a PC board or silicon substrate with electronics system 7. The incoming

photo-optic or light signal from optical fiber array 6 is coupled to the input end of a tapered

waveguide array 3 and converted to electrical signals by an array of fast photodetectors 4 at the

input-end of the electronic system 7. From the transmission end, the electrical signals from the electronic system 7 are converted to photo-optical signals using an array of VCSELs 5 and then

routed to the output fiber array 1 by a tapered waveguide coupler 2 in conjunction with 45°

waveguide mirrors 8. Such 45° waveguide mirrors 8 can be molded simultaneously with the 3-D

tapered waveguides.

For successful coupling, it is crucial to (a) match the mode profiles at the input end of the

tapered waveguides 3 and 2 to the mode profiles of optical fibers 6 and VCSELs 5, respectively; (b)

maintain adiabatical propagation of the guided mode through the tapered waveguides 2 and 3; and

(c) match the mode profiles at the output ends of tapered waveguides 3 and 2 to the mode profiles

of the fast photodetector array 4 and the optical fiber array 1, respectively. Employing the 3-D

tapered waveguides allows alignment tolerances to be relaxed and reduces the coupling losses. The

3-D tapered waveguides provide a universal low-loss packaging scheme covering the huge dynamic

range of various optoelectronic devices, which have apertures ranging from a few micrometers to

hundreds of micrometers. The coupling technique shown in FIG. 1 is not only useful for

interconnecting optoelectronic multi-chip modules but also useful for any low-loss device-to-device

coupling.

FIG. 2 is an illustration of an optical fiber MT-type coupler 9 for connecting two MT

fiberoptic connectors 10 and 11 that have different mode profiles and fiber separations. The MT

coupler, comprising the 3-D tapered waveguides, is designed to couple standard single-mode and

multi-mode MT-type fiberoptic plugs 10 and 11 with very low insertion loss. Both the single-mode

and the multi-mode fiberoptic ribbons 12 and 13 typically have four or eight fibers in an array. A

clip 14 can be used to enhance the mechanical reliability of the connection.

FIG. 3 is a more detailed illustration of a polymeric waveguide coupler that comprises an

array of 3-D tapered waveguides. The polymeric waveguide coupler can be employed to improve the optical coupling between a single-mode fiber array and a multi-mode fiber array. Each

waveguide has different guided mode profiles at its two ends. The separation between each

waveguide may also be different at the two ends of the coupler in order to match the separation of

optoelectronic devices under connection. All 3-D tapered waveguides are compression-molded at

the same time.

FIG. 3 also illustrates the top and side view of the 3-D tapered polymeric waveguide,

revealing its basic structure. In FIG. 3, A, B and a, b are the waveguide width and height at the ends

of 3-D tapered coupler, respectively. S and s are the waveguide separation at the two ends of

coupler. And, ^ is the optical refractive index of polymer and n₂ is the refractive index of the

substrate. There is a tradeoff m determining the waveguide length L. The waveguide has to be long

enough to ensure adiabatic propagation of the guided modes. On the other hand, the length of the

waveguide should be minimized to reduce the waveguide propagation loss.

For efficient power-coupling between two different optoelectronic components facilitated

by the tapered waveguide couplers, the guided optical mode should propagate adiabatically through

the tapered region of the coupler. In other words, the waveguide must operate without any loss and

there should not be any power transfer to higher order local modes. Therefore, it is desirable to

maintain the lowest order mode in the waveguide without mode conversion to higher order mode

or to a radiation mode taking place.

In our analysis, a simplified approach based on the scalar wave equation is employed. We

assume that the tapered waveguide can be subdivided into many small waveguide segments labeled

by an index v, and defined by a refractive index distribution n^v(x,y) independent of z. With the notation φ _XJΛ and βv for the local normal modes of order m and their associated propagation

constants, respectively, the scalar wave equation for segment v reads,

Here k = 2π/λ is the free space wave number. In order to calculate the evolution of the field

along the tapered waveguide, we note that due to the completeness of the modes, the electric field

distribution within each segment can be expressed as

Based on the modes calculated from Eq. (1), the field _a ^v ₍z₎ propagating along the

structure can be described entirely in terms of the complex amplitudes that can be solved by an

iteration of two steps using a Fourier analytic approach. First, as long as both z and z+d lie within

the same segment v, propagation is described by

Second, the requirement that the electric field be continuous at the interface z^v,v+l, between

two consecutive segments v and v+1 leads to a simple relationship between the complex amplitudes

V +1 _l Λ_(z- ^Vv_'.^Vv⁺ ₊ ^, _l\₎ and i.e..

7W^X

Hence, using Eqs. (3) and (4) with the modes V \ and propagation constants _β v

obtained from Eq. (1), we can simulate the propagation of the optical field along the tapered sections.

The 3-D-tapered waveguide device is designed as an adiabatic device in which the optical

power is predominantly guided by the fundamental mode. By keeping track of the excitation

spectrum defined by the field _a^ _z\ , we can determine where deviations from adiabatic behavior

occur. The critical taper slope (—) for adiabatic operation can be given as dz

where r is the half- width of the waveguide at longitudinal position z, β₀ is the propagation constant

of the fundamental mode Φ₀, k=2π/λ is the free space wave number and n,.,^ is the cladding refractive

index. The solution to Eq. 5 has to be simulated numerically for a tapered waveguide with near-

square input and output cross sections.

FIG. 4 shows the optimized taper profiles that allow an adiabatic transition of a lowest order

guided mode propagating along the waveguide taper under certain assumptions. In FIG. 4, the tapered waveguide has a dimension of 5 μm at the small end with single-mode operation, n, and n₂

are the refractive index of polymer and substrate and λ is the optical wavelength. In the simulation,

n„ = 1.50, n₂ = 1.54, and λ = 1.3 μm are assumed.

FIG. 5 is an illustration of the compression-molding technique for fabricating 3-D tapered

polymeric waveguides. A two-piece molding tool comprising substrate 17 and mold plunger 15

provides a cavity having the shape of the desired 3-D-tapered waveguides 18. An appropriate amount

of molding material, polymer waveguide film 16 in this case, is spin-coated or laminated onto the

substrate of interest 17. The film thickness is equal to the maximum thickness of the tapered

structure. The molding process begins with the heating of the mold plunger 15 and the polymer film

16. Then the two parts of the mold are brought together under pressure. The polymer film 16,

softened by heat, is thereby molded (by the compression) into the shape of the stamp. The last step

is to harden and fix the molded waveguide shape. This final step depends on the type of polymeric

material used. For example, thermosetting polymers are further heated under pressure while the

thermoplastics are cooled to room temperature. The fundamental differences in polymers dictate the

plastic processing method to be used in the waveguide molding. After the polymer 16 is cured

(fixed), the mold plunger 15 is removed. This leaves behind ti e 3-D-tapered waveguide structures

on the substrate. The polymer materials described in this embodiment are exemplary only and non-

polymer materials may be substituted without departing from the present invention.

In summary, the present invention is a three-dimensional tapered waveguide coupler capable

of interconnecting electro-optical devices with differing optical mode profiles. The ends of the

waveguide are configured to match the optical mode profiles of the electro-optical devices that the

waveguide interconnects. The waveguide adiabatically transmits the fundamental mode of the photo- optic signal from the electro-optical device at the waveguide's input end to a different electro-optical

device at the waveguide's output end. A single coupler can be configured with one or more

waveguides, each waveguide having different optical mode profiles at either end and different optical

transmission characteristics.

parts of the mold are brought together under pressure that causes the polymer film, which is softened

by heat, to be compression molded into the shape of the stamp of the desired 3-D-tapered

waveguides.

Other embodiments of the invention will be apparent to those skilled in the art after

considering this specification or practicing the disclosed invention. The specification and examples

above are exemplary only, with the true scope of the invention being indicated by the following

claims.

Claims

CLAIMSWe claim:

1. An optical waveguide coupler for interconnection of a plurality of electro-optical devices that

transfer a photo-optic signal, comprising:

a three-dimensional tapered waveguide, having an input end and an output end and a first

index of refraction, said waveguide is configured to adiabatically transmit the fundamental mode of

the photo-optic signal from said input end to said output end in a transmission direction, said input

end of said waveguide matches the optical mode profile of the electro-optical device that transmits

the photo-optic signal at said input end, said output end of said waveguide matches the optical mode

profile of the electro-optical device that receives the photo-optic signal at said output end of said

waveguide; and

a structural member coupled to said waveguide, said structural member has a second index

of refraction that is unequal to said first index of refraction, said structural member physically

supports said waveguide and allows for proper alignment of said waveguide to the electro-optical

devices to be coupled.

2. The optical waveguide coupler of Claim 1 , further comprising a plurality of waveguides, each

waveguide within said plurality of waveguides having an input end and an output end and a first

index of refraction, each waveguide within said plurality of waveguides is configured to adiabatically

transmit the fundamental mode of the photo-optic signal from said input end to said output end in a

transmission direction, said input end of each waveguide within said plurality of waveguides matches

the optical mode profile of the electro-optical device that transmits the photo-optic signal at said input

end of each waveguide within said plurality of waveguides, said output end of each waveguide Vvithin

said plurality of waveguides matches the optical mode profile of the electro-optical device that receives the photo-optic signal at said output end of each waveguide within said plurality of

waveguides.

3. The optical waveguide coupler of Claim 1 , further comprising a port for receiving the photo-

optic signal in a direction non-parallel with said transmission direction, said port coupled to said input

end of said waveguide.

4. The optical waveguide coupler of Claim 3, further comprising a reflector coupled to said

waveguide, said reflector configured to reflect the photo-optic signal received from said port from

a direction non-parallel with said transmission direction to a direction parallel with said transmission

direction.

5. A system that interconnects a plurality of electro-optical devices together by transferring a

photo-optic signal, comprising:

waveguide; and

devices to be coupled.

6. The optical interconnection system of Claim 5, further comprising a plurality of waveguides,

each waveguide within said plurality of waveguides having an input end and an output end and a first

t^ransmission direction, said input end of each waveguide within said plurality of waveguides matches

end of each waveguide within said plurality of waveguides, said output end of each waveguide wit n

said plurality of waveguides matches the optical mode profile of the electro-optical device that

receives the photo-optic signal at said output end of each waveguide within said plurality of

waveguides.

7. The optical interconnection system of Claim 5, further comprising a port for receiving the

photo-optic signal in a direction non-parallel with said transmission direction, said port coupled to

said input end of said waveguide.

8. The optical interconnection system of Claim 7, further comprising a reflector coupled to said

direction.

9. A method of interconnecting a plurality of electro-optical devices that transfer a photo-optic

signal, comprising:

providing a three-dimensional tapered waveguide, having an input end and an output end and

a first index of refraction, said waveguide is configured to adiabatically transmit the fundamental

mode of the photo-optic signal from said input end to said output end in a transmission direction, said

input end of said waveguide matches the optical mode profile of the electro-optical device that

1« transmits the photo-optic signal at said input end, said output end of said waveguide matches the

optical mode profile of the electro-optical device that receives the photo-optic signal at said output

end of said waveguide; and

. coupling a structural member to said waveguide, said structural member has a second index

devices to be coupled.

10. The method of Claim 9, further comprising providing a plurality of waveguides, each

end of each waveguide within said plurality of waveguides, said output end of each waveguide within

waveguides.

11. The method of Claim 9, further comprising providing a port configured to receive the photo-

end of said waveguide.

12 The method of Claim 11 , further comprising providing a reflector coupled to said waveguide,

said reflector configured to reflect the photo-optic signal received from said port from a direction non-

parallel with said transmission direction to a direction parallel with said transmission direction.

13. A method that transfers a photo-optic signal through an interconnection of a plurality of

electro-optical devices, comprising:

adiabatically transmitting the fundamental mode of a photo-optic signal through a three-

dimensional tapered waveguide in a transmission direction, said waveguide having an input end and

an output end and a first index of refraction, said input end of said waveguide matches the optical

mode profile of the electro-optical device that transmits the photo-optic signal at said input end, said

output end of said waveguide matches the optical mode profile of the electro-optical device that

receives the photo-optic signal at said output end of said waveguide; and

structurally supporting and aligning said waveguide to the electro-optical devices to be

coupled with a structural member, said structural member has a second index of refraction that is

unequal to said first index of refraction.

14. The method of Claim 13 , further comprising adiabatically transmitting the fundamental mode

of a plurality of photo-optic signals through a plurality of three-dimensional tapered waveguides in

a transmission direction, each waveguide within said plurality of waveguides having an input end and

an output end and a first index of refraction, said input end of each waveguide within said plurality

of waveguides matches the optical mode profile of the electro-optical device that transmits the photo-

optic signal at said input end of each waveguide within said plurality of waveguides, said output end

of each waveguide within said plurality of waveguides matches the optical mode profile of the

electro-optical device that receives the photo-optic signal at said output end of each waveguide within

^aW^ tfality of waveguides.

15. The method of Claim 13, further comprising receiving the photo-optic signal from a direction

non-parallel with said transmission direction through a port, said port coupled to said input end of

- said waveguide.

16. The method of Claim 15, further comprising reflecting the photo-optic signal received from

said port from a direction non-parallel with said transmission direction to a direction parallel with said

transmission direction.