WO1992002856A1

WO1992002856A1 - Production of polymeric optical waveguides

Info

Publication number: WO1992002856A1
Application number: PCT/US1991/002016
Authority: WO
Inventors: David Haas; Chia-Chi Teng; Robert Norwood; Garo Khanarian
Original assignee: Hoechst Celanese Corp
Current assignee: CNA Holdings LLC
Priority date: 1990-08-03
Filing date: 1991-03-25
Publication date: 1992-02-20
Anticipated expiration: 1993-02-03

Abstract

In one embodiment this invention provides a method for producing an optical waveguide with a spatial periodic waveguiding channel structure by photobleaching and changing the index of refraction and optical nonlinearity in the exposed surface areas of a polymeric thin film which absorbs radiation energy. A preferred type of polymer in the waveguiding medium is a side chain polymer which has a recurring 4-[(2-methacroyloxyethyl)-N-methylamino]-4'-nitrostilbene type structure (I).

Description

PRODUCTION OF POLYMERIC OPTICAL WAVEGUIDES

This invention was made with Government support under Contract No. F49620-86-C-0129 awarded by the Department of Defense (DOD). The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

Optical waveguides consist of a transparent waveguiding core surrounded by

transparent materials of lower indices of

refraction. Several general methods are utilized for the fabrication of optical waveguides.

In one method optical waveguides are formed by applying a dielectric material to a transparent substrate of lower refractive index.

In another method thermoplastic polymer substrates are embossed with a metal die in a desired waveguide pattern, and subsequently filled or coated with a polymerizable higher index liquid monomer.

In another method optical waveguides are formed by selectively altering the index of refraction of a bulk transparent material. One technique involves ion bombardment in which selected regions of increased refractive index are provided by generating a molecular disorder pattern in a bulk matrix. In another technique selected regions are either photo-induced in sensitized polymeric materials such as poly(methyl) methacrylate as described in Appl. Phys. Lett., 16, 486 (1970), or electrically induced by diffusing a higher index dopant into a transparent material.

Each of the methods has inherent

disadvantages, such as insufficient index changes or high scattering losses.

Improved methods for fabricating organic optical waveguides as described in United States Patent Numbers 3,689,264; 3,809,732; 3,953,620; 4,609,252; and 4,712,854.

A particularly pertinent publication with respect to the present invention is

Electronics Letters, 26(6), 379 (March 15, 1990) by Diemeer et al. This reference describes photoinduced channel waveguide formation in nonlinear optically active side chain polymers.

There is continuing interest in the development of new and improved techniques for the fabrication of organic optical waveguides which overcome some of the inherent deficiencies of refractive index manipulations in transparent organic media.

Accordingly, it is an object of this invention to provide an improved method for the production of organic optical waveguides.

It is another object of this invention to provide a method for the production of organic optical waveguides by refractive index imaging. It is a further object of this invention to provide a method for the production of

polymeric optical waveguides which have a spatial periodic waveguiding channel structure.

Other objects and advantages of the present invention shall become apparent from the accompanying description and Examples.

DESCRIPTION OF THE INVENTION

One or more objects of the present invention are accomplished by the provision of a method for producing an optical waveguide with a two-dimensional channel structure for single mode wave transmission which comprises (1) forming a thin film optical matrix comprising an organic medium which exhibits nonlinear optical response and which has a λ_max absorption band in the range between about 250-600 nm; (2) masking an elongated channel section across the thin film matrix, and (3) applying radiation energy to the exposed thin film surface to modify the molecular configuration of the thin film organic medium and lower the refractive index of the organic medium.

In another embodiment this invention provides a method for producing an optical

waveguide with a two-dimensional channel means for single mode wave transmission and with a spatial periodic waveguiding channel structure for

quasi-phase matching of propagating wave energy, said method comprising (1) forming a thin film optical matrix comprising an organic medium which exhibits nonlinear optical response and which as a λ_max absorption band in the range between about 250-600 nm; (2) masking a longitudinal channel section across the thin film matrix;

(3) applying radiation energy to the exposed thin film surface to modify the molecular configuration of the thin film organic medium and lower the refractive index of the organic medium; (4) re-masking the longitudinal channel section in a grating pattern; and (5) applying radiation energy to the exposed channel surface to modify the molecular configuration of the thin film and change the refractive index of the exposed channel medium.

In a further embodiment this invention provides a method for producing an optical

waveguide which comprises (1) forming a

two-dimensional waveguiding channel for single mode wave transmission comprising a thin film channel of a side chain polymer medium which exhibits nonlinear optical response and has a λ_max absorption band in the range between about

250-600 nm, and which has an external electric field induced noncentrosymmetric molecular

orientation; (2) masking the thin film channel in a spatial periodic grating pattern; and

(3) applying between about 700-1500 Joules per square centimeter of radiation energy density to the exposed thin film channel surface grating pattern to modify the poled molecular

configuration of the thin film and change the refractive index of the exposed thin film channel grating pattern.

A present invention process embodiment has utility for the fabrication of the optical waveguiding component of a frequency converting module adapted for the production of a short wavelength laser beam which comprises: a. a 600-1600 nm laser generating source, in coupled combination with

b. an optical waveguide comprising a substratesupported thin film of a polymeric medium which exhibits second order nonlinear optical response, and which has a spatial periodic structure for quasi-phase matching of

propagating wave energy; wherein the coherence length ℓ_c of the periodic polymeric medium is defined by the equation:

where Δβ is the propagation constant difference which is equal to β₀(2ω₁)-2β₀(ω₁), ω₁ is the

fundamental frequency, and subscript zero denotes the zero-ordered mode in the waveguide.

A present invention process embodiment has further utility for the fabrication of the optical waveguiding component of a frequency

converting device which comprises a thin film waveguide of a polymeric medium which exhibits second order nonlinear optical response, and which has a periodic structure for quasi-phase matching of a propagating laser beam; wherein the intensity of the second harmonic generation in the optical waveguide is defined by the equation: L V v vr

/

where ω is the frequency of the incident

fundamental light beam; μ₀ and ε ₀ are the magnetic permeability and electric permittivity of free space; d_eff is the effective nonlinear optical susceptibility of the waveguide medium; n(ω) and n(2ω) are the linear refractive indices at

frequency ω and 2ω respectively; A is the area of the light beam; P(ω) is the light beam power; L is the length of the phase matching region of the waveguide medium; F is the overlap integral; and Δk is the phase mismatch between the fundamental and second harmonic light wave propagation

constants as expressed by the equation:

Δk= k(2ω)-k(ω₁)-k(ω₂)

where k(2ω) is the propagation constant for the generated second harmonic light beam; and k(ω₁) and k(ω₂) are the propagation constants of the two fundamental field modes that generate the

nonlinear polarization; and wherein the device has heat control means for temperature tuning of the waveguide medium to phase match the propagation constants of the fundamental and second harmonic light beams, so that Δk in the above represented equations approaches zero.

A present invention process embodiment has further utility for the fabrication of the optical waveguiding component of an integrated optical parameter amplifier which comprises

(1) a light signal source with a wavelength of about 0.5-4 μm, and (2) a laser beam source with a wavelength of about 0.6-1.3 μm and a power level of about 50-1000 mw, in coupled combination with (3) an optical waveguide comprising a twodimensional channel structure for single mode wave transmission, and the channel waveguiding matrix comprises a side chain polymer medium which has an external field-induced noncentrosymmetric

molecular orientation of side chains and a

nonlinear optical coefficient d of at least about 10 pm/V, and which has a spatial periodic

structure for quasi-phase matching of propagating wave energy; wherein the coherence lengthℓ_c of the waveguide periodic polymer medium is in the range of about 10-100 μm, and is defined by the equation:

where Δβ is the propagation constant difference which is equal to β₀(ω_p)-β₀(ω_s)-β₀(ω_i), ω_p is the pump frequency, ω_s is the signal frequency, ω_i is the idler frequency, and subscript zero denotes the zero-ordered mode in the waveguide medium; and wherein the output wave energy under operating conditions comprises the incident laser beam, a generated idler beam, and an amplified signal beam with a gain G of about 10-1000. The radiation energy applied in the above described invention process embodiments is of sufficient intensity and duration to accomplish a molecular transformation in the exposed organic thin film regions, and a concomitant change in the refractive index and optical nonlinearity of the exposed medium. Typically a radiation energy density between about 700-1500 Joules is applied per square centimeter of exposed thin film

surface.

The radiation applied to the exposed thin film surface area normally will have a wavelength in the range of about 200-1300 nm. The input of radiation energy preferably changes the refractive index of the modified organic by at least 0.005 up to about 0.2.

The radiation energy is applied with any conventional source of ultraviolet or visible light, such as a mercury arc lamp, argon ion laser, helium-cadmium ion laser, Nd:YAG laser, and the like.

The thin film organic medium having waveguiding properties can be in a laminated formation between two thin film electrodes. The electrodes can function to provide an external field-induced noncentrosymmetric molecular

orientation, and have further utility for the application of an electrooptic modulation effect during waveguided transmission of light energy.

Poling of a thin film waveguide medium can be accomplished conveniently by heating the medium near or above its melting point or glass transition temperature, then applying a DC

electric field (e.g., 50-150 V/μm) to the medium to align molecular dipoles in a uniaxial

orientation. The medium then is cooled while the medium is still under the influence of the applied DC electric field. In this manner a stable and permanent molecular orientation is immobilized in a rigid structure within the poled domains.

An applied external field also is useful for tuning of quasi-phase matching conditions in a waveguiding medium. Additional fine tuning of phase matching of the preparation constants of light beams can be accomplished by the provision of heat control means.

It is an important feature of the present invention that the refractive indices of the polymeric waveguiding medium and/or cladding layers on a fabricated channel waveguide can be finely adjusted during light propagation

conditions by the input of radiation energy to optimize the operation of the waveguide device.

The organic thin film waveguiding medium of an invention optical device is transparent, either liquid crystalline or amorphous in physical properties, and exhibits nonlinear optical

response. The organic medium has a higher

refractive index (e.g., 1.5) than the supporting substrate, or higher than the cladding layer

(e.g., sputtered silica) if one is composited between the organic medium and the supporting substrate. The transparent organic medium can be applied to the supporting substrate by

conventional methods, such as spin coating, spraying, Langmuir-Blodgett deposition, and the like.

The organic thin film waveguiding medium can consist of a host polymer such as poly(methyl methacrylate), and a guest organic compound which exhibits nonlinear optical response, such as

4-nitroaniline, 2-methyl-4-nitroaniline,

1-dimethylamino-4-nitronaphthalene, 2-chloro-4-nitroaniline, 4-dimethylamino-4'-nitrostilbene, 13,13-diamino-14,14-dicyanodiphenoguino-dimethane, and the like. A host polymer can be selected which also exhibits nonlinear optical response.

A present invention organic thin film waveguide medium preferably is a polymer having a comb structure of side chains which exhibit nonlinear optical response. This type of chemical structure is illustrated by thermoplastic polymers which are characterized by a recurring monomeric unit corresponding to the formula:

where P' is a polymer main chain unit, S' is a flexible spacer group having a linear chain length of between about 2-20 atoms, M' is a pendant group which exhibits second order nonlinear optical susceptibility, and where the pendant groups comprise at least about 25 weight percent of the polymer, and the polymer has a glass transition temperature or softening point above about 40°C.

Among the preferred types of side chain polymers are those characterized by a recurring monomeric unit corresponding to the formula:

where m is an integer of at least 5; p is an integer between about 2-20; X is -NR-, -O- or -S-; R is hydrogen or a C₁-C₄ alkyl; and Z is -NO₂, -CN, -CF₃, -CH=C(CN)₂, -C(CN)=C(CN)₂ or

-SO₂CF₃. Side chain polymers of interest are described in U.S. 4,694,066. Illustrative of side chain polymer species are poly[6-(4-nitrobiphenyloxy)hexyl methacrylate], poly(L-N-p-nitrophenyl-2-piperidinemethyl acrylate), and stilbene-containing polymers such as those with a recurring 4-[N-(2-methacroyl-oxyethyl)-N-methylamine]-4'-nitrostilbene monomeric unit:

The term "transparent" as employed herein refers to an organic thin film waveguide medium which is transparent or light transmitting with respect to incident fundamental and created light frequencies. In a present invention light frequency converting or parametric amplifying waveguide device, the organic thin film nonlinear optical medium is transparent to both the incident and exit light frequencies.

The term "amorphous" as employed herein refers to a transparent polymeric optical medium which does not have a preferred short range molecular order that exhibits optical anisotropy.

The term "external field" as employed herein refers to an electric, magnetic or mechanical stress field which is applied to a substrate of mobile polymer molecules, to induce dipolar alignment of the organic molecules parallel to the field.

The term "parametric" as employed herein refers to interactions in wave energy states in an optical medium in which time variations in an input signal are translated into different time variations in an output signal as determined by a nonlinearity parameter.

The following examples are further illustrative of the present invention. The optical waveguide components are presented as being typical, and various modifications in design and operation can be derived in view of the foregoing disclosure within the scope of the invention.

EXAMPLE I

This Example illustrated the

photobleaching effect of applied radiation on a transparent waveguiding thin film of side chain polymer.

A nonlinear optically active organic layer of 2.0 μm thickness is spin-coated on a series of quartz plates at 3000 rpm. The

spin-coating medium is a 20% solution of a

copolymer (50/50) of 4-[N-(2-methacroyloxyethyl)-N-methylamino]-4'-nitrostilbene/methyl

methacrylate [weight average M.W. of about

30,000]. The organic thin film is dried in a vacuum oven at 160°C for one hour. The thin film has a refractive index of 1.606.

An argon ion laser is used to apply one of three lines of radiation (457.8, 488 and

514.5 μm) to the respective thin film samples. During the radiation period the UV-VIS absorption spectrum is monitored. When a sample has been through four exposure periods of 30 minutes, the sample thin film is tested for waveguiding of 1.3 μm light, and the refractive index of the thin film is measured.

FIG. 1 is a graphic representation of a comparison of the initial UV-VIS spectrum of a thin film with that of the thin film after

photobleaching radiation for 144 minutes with 514.5 nm light and a 750 Joules per square

centimeter input of light energy ( λ_max = 440 nm for the thin film). The average thin film sample exhibits an optical density decrease of about 50 percent in the 500-580 nm range.

FIG. 2 is a graphic representation in which the normalized integrated optical density for 514.5 nm light and a radiation input of

750 Joules per square centimeter is plotted as a function of time for a thin film sample. The graph indicates that the radiation-induced

photobleaching process is linearly dependent on radiation fluence.

EXAMPLE II

This Example illustrates the fabrication of a two-dimensional channel waveguide with a spatial periodic structure in accordance with the present invention, and operation of the waveguide as a light frequency doubling device.

A silicon dioxide-coated silicon wafer is placed in a Varian electron beam vacuum

deposition apparatus, and a 200 Å film of 99.999% purity aluminum is deposited on the wafer as an electrode layer.

The aluminum electrode surface is covered with a 2.0 micron cladding layer of polymethyl methacrylate by spin-coating a 20% cyclohexanone solution at 5000 rpm for 30 seconds, and the coated wafer is baked at 160°C for

2 hours, and then cooled slowly to room

temperature over a period of 2 hours.

A nonlinear optically active waveguiding layer of 1.3 micron thickness is spin-coated on the cladding layer at 7500 rpm. The spin-coating medium is a 20% solution of a 50/50 copolymer of 4-[N-(6-methacroyloxyhexyl)-N-methylamino]-4'-nitrostilbene and methyl methacrylate in

cyclohexanone. The coated wafer is baked at 160°C in a vacuum oven as previously described.

An upper cladding layer of 2.0 micron thickness is applied by spin-coating a 20% PMMA solution in cyclohexanone at 5000 rpm for 30 seconds, and baking in the manner previously described.

A 1000 A electrode layer of gold is deposited on the upper cladding layer.

The refractive indices of the waveguiding thin films are measured at 1.34 micron and 0.67 micron, which correspond to the pump and second harmonic wavelengths under the light propagating conditions in the waveguide as

described hereinbelow. The waveguide thin film has refractive indices of 1.576 and 1.606, and the cladding layer has refractive indices of 1.485 and 1.490, respectively.

The waveguide structure is cleaved at opposite ends to provide two sharp faces to couple light in and out of the polymeric waveguiding medium. Wires are attached to the top and bottom electrodes. The waveguide is poled by placing it in a Mettler hot stage. The waveguide is heated at 1°C/minute to 90°C (T_g of copolymer), and a field of 70 V/μm is applied for

5 minutes. The waveguide is cooled at

0.2°C/minute to room temperature while the

electric field is maintained.

The top gold electrode is removed by a gold etch (Transene Corp.). The waveguide is irradiated with the 488 nm line of an Argon laser (Spectra Physics) through a periodic mask

(Photronics Inc.). The periodic mask consists of lines and spaces 6 μm wide. The mask is placed on the waveguide with a thin Kapton spacer. The waveguide is photobleached for about 30 minutes with a radiation of 500 Joules per square

centimeter. When the bleaching process has been completed, a grating pattern is visible. When a He Ne laser is incident normally on the waveguide, several diffraction orders of reflection are observed.

The following procedure is performed to determine the phase matching angle for optimizing second harmonic intensity output from the

waveguide.

A Quantel TDL 50 dye laser pumped H2 Raman cell, producing 1.34 μm light as the second Stokes lines, is focussed into the waveguide.

Interference filters are placed after the

waveguide to block the pump at 1.34 μm. A narrow pass filter at 0.67 μm is placed on a

photomultiplier tube, and a good signal to noise discrimination against background is obtained. A one half wave plate is placed before the waveguide to make the input polarization in the waveguide TM.

The waveguide is placed on a hot stage with a 0.5°C temperature control. The hot stage is mounted on a rotation stage, so that the effective periodicity of the grating can be changed. 1.34 μm light is coupled into the

waveguide and second harmonic generation is detected. Phase matching occurs near the correct periodicity of 6.9 μm. The polarization of the input and output signals is TM, indicating that the d₃₃ coefficient has been phase matched. Away from the phase matching periodicity, no second harmonic generation is observed.

A two-dimensional channel waveguiding structure is formed by the following procedure.

A light field mask containing a channel waveguide pattern is placed on the waveguide, with a Kapton filter spacer between the waveguide and the mask. In the manner previously described, an index change of about 0.007 is effected by

photobleaching the exposed regions of the pattern. The waveguide structure does not have sharply defined channel sidewalls. Each channel sidewall is in the form of a refractive index gradient zone, which minimizes optical loss from light scattering under light propagating conditions.

The waveguide is placed in an optical test apparatus, and microscope objective lenses are utilized to couple light into the channel waveguiding medium. Since the angle of the channel waveguide is near its optimum position as previously determined, only minor wavelength tuning is required to provide phase matched frequency doubling.

EXAMPLE III

This Example illustrates the fabrication of a two dimensional channel waveguide in

accordance with the present invention and

operation of the waveguide as an electrooptic modulator.

deposition apparatus, and a 200 A film of 99.999% purity aluminum is deposited on the wafer as an electrode layer.

The aluminum electrode is coated with 2.5 μm of a UV curable epoxy resin (Norland 60). The film is obtained by preparing a 20% solution of Norland 60 in trichloropropane, and spincoating the solution on the electrode at 8000 rpm for 30 seconds, and curing the formed film with a 300 W mercury lamp for 10 minutes.

A nonlinear optically active polymeric waveguide layer of 1.3 μm thickness is spin-coated on the cladding layer at 5000 rpm. The spin coating solution is a 20% solution of a 50/50 copolymer of 4-[N-(2-methacroyloxyethyl)-N-methylamino]-4'-nitrostilbene/methylmethacrylate (weight average M.W. of about 30,000). The polymeric thin film is dried in a vacuum oven at 160°C for 2 hours.

An upper cladding layer of polysiloxane is applied by spin coating a 35% solution in butanol at 8000 rpm for 30 seconds, and the 2 μm cladding layer is baked at 120°C for one hour.

A 1000 A electrode layer of gold is deposited on the upper cladding layer.

The resultant laminate structure is poled at 140°C at 100 V/μm in a Mettler hot stage by raising the temperature to the T_g of the polymeric waveguide layer, applying a DC voltage, and then cooling the structure slowly while maintaining the DC field.

The top gold electrode is removed with a gold etch (Transene Corp.). The remaining

waveguide structure is irradiated with the

514.5 μm line of an argon laser for

144 minutes to provide a 750 Joules per square centimeter input of light. The desired pattern for the channel waveguide is obtained by a dark field mask. (Photronics lab.)

A gold layer (1000 A) is deposited over the bleached area. Photoresist (AZ-1518, Hoechst) is spin-coated at 3000 rpm to provide a 1.5 μm layer. The coating is baked at 100°C for

one minute on a hot plate, then the photoresist layer is exposed in a Karl Suss model MJB3 mask aligner to give the desired electrode pattern. The patterned photoresist is developed with AZ Developer in water, and washed with deionized water. The gold is removed in all the exposed areas of the pattern, then the photoresist is removed to provide a thin film gold electrode over the channel waveguide. The waveguide structure is cleaved at opposite ends to provide two sharp faces. Light is coupled into the channel waveguide with a microscope objective. When a voltage is applied across the electrodes, the intensity of an incident laser beam is modulated.

Claims

WHAT IS CLAIMED IS:

1. A method for producing an optical waveguide with a two-dimensional channel structure for single mode wave transmission which comprises (1) forming a thin film optical matrix comprising an organic medium which exhibits nonlinear optical response and which has a λ_max absorption band in the range between about 250-600 nm; (2) masking an elongated channel section across the thin film matrix, and (3) applying radiation energy to the exposed thin film surface to modify the molecular configuration of the thin film organic medium and lower the refractive index of the organic medium.

2. A method in accordance with claim 1 wherein the thin film organic medium comprises a host polymer and a guest organic compound which exhibits nonlinear optical response and has a λ_max absorption band in the range between about

250-600 nm.

3. A method in accordance with claim 1 wherein the thin film organic medium comprises a polymer having side chains which exhibit second order nonlinear optical response and which have a λ_max absorption band in the range between about 250-600 nm.

4. A method for producing an optical waveguide with a two-dimensional channel means for single mode wave transmission and with a spatial periodic waveguiding channel structure for quasiphase matching of propagating wave energy, said method comprising (1) forming a thin film optical matrix comprising an organic medium which exhibits nonlinear optical response and which has a λ_max absorption band in the range between about

250-600 nm; (2) masking a longitudinal channel section across the thin film matrix; (3) applying radiation energy to the exposed thin film surface to modify the molecular configuration of the thin film organic medium and lower the refractive index of the organic medium; (4) re-masking the

longitudinal channel section in a grating pattern; and (5) applying radiation energy to the exposed channel surface of sufficient intensity and duration to modify the molecular configuration of the thin film and change the refractive index and optical nonlinearity of the exposed channel medium.

5. A method in accordance with claim 4 wherein the thin film organic medium in step (1) has an external field-induced noncentrosymmetric molecular orientation.

6. A method for producing an optical waveguide which comprises (1) forming a twodimensional waveguiding channel for single mode wave transmission comprising a thin film channel of a side chain polymer medium which exhibits nonlinear optical response and has a λ_max

absorption band in the range between about

250-600 nm, and which has an external electric field induced noncentrosymmetric molecular orientation; (2) masking the thin film channel in a spatial periodic grating pattern; and

(3) applying between about 700-1500 Joules per square centimeter of radiation energy to the exposed thin film channel surface grating pattern to modify the poled molecular configuration of the thin film and change the refractive index of the exposed thin film channel grating pattern.

7. A method in accordance with claim 6 wherein the waveguiding channel in step(l) is in a laminated formation between two cladding layers, each of which consists of a thin film of a side chain polymer medium which exhibits nonlinear optical response and has a lower refractive index than the waveguiding channel medium and has an external field induced noncentrosymmetric

molecular orientation.

8. A method in accordance with claim 6 wherein the waveguiding channel polymer is

characterized by a recurring monomeric unit corresponding to the formula:

where P' is a polymer main chain unit, S' is a flexible spacer group having a linear chain length of between about 1-20 atoms, M' is a pendant group which exhibits second order nonlinear optical susceptibility, and where the pendant side chains consist of at least about 25 weight percent of the polymer, and the polymer has a glass transition temperature above about 40°C.

9. A method in accordance with claim 8 wherein the recurring monomeric unit corresponds to the formula:

where m is an integer of at least 5; n is an integer between about 2-20; X is an electrondonating group; and Z is an electron-withdrawing group.

10. A method in accordance with claim 8 wherein the waveguiding channel polymer is a copolymer of 4-[N-(6-methacroyloxyhexyl)-N-methylamino]-4'-nitrostilbene and methyl

methacrylate.