Photonic and Optoelectronic Polymers - American Chemical Society

Chapter 3

Development of Polymer Optical Waveguides for Photonic Device Applications 1

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Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch003

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Toshikuni Kaino , Itaru Yokohama , Satoru Tomaru , Michiyuki Amano , and Makoto Hikita 3

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Institute for Chemical Reaction Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai-shi, Miyagi 980-77, Japan NTT Opto-electronics Laboratories, 3-1 Wakamiya, Morinosato, Atsugi-shi, Kanagawa 243-01, Japan NTT Opto-electronics Laboratories, 162 Shirakata, Tokai, Naka-gun, Ibaraki 319-11, Japan

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The development of nonlinear optical polymer waveguides directed toward practical photonic devices are discussed. Electro-optical polymer waveguides and third-Order nonlinear optical polymer waveguides have been fabricated and characterized as optical modulators and optical switches. To make the most of polymer processability, hybrid structures of waveguides are proposed since such structures are anticipated to advance optical signal processing technologies. This hybrid fabrication technique has the potential to develop waveguides for optical sampling element and compact Kerr shutter switch applications. There is a growing interest in the processing of information with optical devices for application in optical telecommunication systems. Optical signal transmission and processing require optical interconnection technology designed to overcome bottlenecks resulting from high circuit densities for high data transmission and processing rates from increased data quantities. These kinds of photonic devices have already attracted considerable attention as key elements in optical signal processing systems. For device applications, waveguide structures can be effective for controlling optical signals with low power. Polymer waveguides offer the potential to create highly complex integrated optical devices and optical interconnects on a planar substrate because their excellent optical properties can be tailored by using different types of polymers(i). Optical processing requires highly nonlinear optical (NLO) materials to 30

© 1997 American Chemical Society

In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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ΚΑΙΝΟ ET AL.

Polymer Optical Waveguides & Photonic Devices 31

operate high data transmission rate signals(2). In this case, the trade-off between large optical nonlinearity, interaction length, and processability should be considered. Because high NLO chromophore (NLO-phore) concentrations are needed to achieve large optical nonlinearity, optical loss can be increased due to the π-π* transitional absorption tail of the polymer when the NLO-phore is added into the polymer. Thus, reduction of the attenuation loss of the polymer waveguide must be balanced against optical nonlinearity enhancement Processability of the NLO polymer is also influenced by the concentration of NLO-phore; i.e., a highly functionalized polymer usually becomes difficult to process. The potential of combining an active processing function using NLO properties of materials with a passive transmission function is the most appealing prospect for waveguides made from NLO polymers. In this paper, the development of nonlinear optical polymer waveguides directed toward practical photonic devices, i.e., electro-optical polymer waveguides and third-order nonlinear optical polymer waveguides, will be discussed. Hybrid polymer waveguides for future applications will also be presented. Overview of Passive Polymer Waveguides Recently, many kinds of polymer waveguides have been studied for constructing integrated optical devices and optical interconnections. They have attracted much attention because of their potential for applications. It should be emphasized that almost all the polymer optical waveguides developed so far have an optical loss of greater than 0.1 dB/cm except for poly (deuterated and fluorinated) methylmerhacrylate (d-f- PMMA in short) core andfluorinated-polyimidecore optical waveguides, detail of which will be presented later. A silicon wafer area network using polymer integrated optics has been described^). This device is based on a combination of an active reconfiguration function with a passive transmission function. Parallel optical link for Gbyte/s data communications using polymer waveguide was also reported(4). The applicability, advantages, and limitations for creating practical optical devices was related to polymer properties and polymer waveguide fabrication processes. Photolithographic techniques are commonly used to define the polymer waveguide patterns. However, optical waveguides are fabricated from polyimide or photo-crosslinking acrylate polymer by using direct writing with lasers or electron beams for fabricating optical waveguides. Using this method, simple straight multi-mode waveguides with propagation loss of 1 dB/cm at 670 nm have been fabricated^. Polymer film layers



32

PHOTONIC AND OPTOELECTRONIC POLYMERS

are typically created by spinning or casting techniques on substrates such as silicon and SiQ glass. The process of creating a waveguide is essentially decided by the inherent properties of the selected polymer. Among the various waveguide fabrication techniques, the method using reactive ion etching (RIE) to form waveguides is an excellent example of technology which can be compared to the solvent etching process. Waveguides created with etching techniques include two types of fabrication processes: (1) an etched groove backfilled with a high-index transparent polymer ; and (2)aridgewaveguide surround by a lower-refractive-index polymer to create a buried waveguide structure. A number of research groups have used these direct methods to make waveguides on silicon, fused silica glass, or organic glass substrates. For example, polyimide is spin-coated on substrates followed by RIE to makeridgewaveguides(5). Film thickness and refractive index are usually measured by m-line spectroscopy. The refractive index at any wavelength was also determined by a single-term Sellmeier equation using the values at the wavelength of 1.3, 1.152, and 0.633 μπι which were accurately determined by the prism coupling technique. Optical loss is measured by evaluating the scattered light from a streak pattern using a video scan technique (i.e., measurement of the light intensity from the waveguide along its length) and an optical multi-channel analyzer. This method can be applied for waveguide loss from 0.1 dB/cm to about 20 dB/cm For lower losses the scattered intensity is too low, and for higher losses the streak becomes too short Reliability, reproducibility, stability, acceptable cost performance, and compatibility with other optical systems are the important goal for polymer waveguide systems. Deuterated and/or Fluorinated Polymer Waveguides Passive waveguides need highly transparent polymers with excellent processabilities. PMMA and polystyrene (PS) are typical polymers that offer low loss and processability. When discussing the transparency of a polymer, one should consider the vibrational higher harmonics of the polymer(6). Figure 1 shows the higher harmonics of the C-H vibration of PMMA In the visible wavelength region, PMMA-based polymer has the potential low loss of less than 1 dB/m. Although polymer optical waveguides have good processability and low manufacturing cost, they suffer from high optical loss in the near-IR region, 1.0-1.6 μιη. For optical components, transparency in the near-IR region, rather than in the visible region, is needed because wavelengths of 1.3 and 1.55 μπι are used in optical telecommunication. For the PMMA-based polymer, the loss in



3. ΚΑΙΝΟ ET AL.


this near-IR region will be around 1 dB/cm. This loss can be reduced by fluorination and/or deuteration of hydrogen atoms in the polymer(7). Few polymeric waveguides with a loss below 0.1 dB/cm have been reported for use at 1.3 and 1.S5 μπι. Among these are low loss deuterated and deuterated/fluorinated polymer core plastic optical fibers (POFs) which have been successfully fabricated(#). Figure 2 shows the loss spectra of a per-deuterated PMMA (P[MMA-d8]) core and afluorinatedand deuterated polystyrene (P-5F3DSt) core POFs. At around 1.3 μπι wavelength, 0.015 dB/cm loss was obtained for the latter POF. Recently, poly-perfluorinated (butenyl ether) , (CYTOP as a brand name), was also used as a core of POF and its attenuation loss of 0.06 dB/m was obtained(9). Therefore, channel waveguides composed of polymers with deuterated methacrylate and deuterated fluoro-methacrylate monomers may be promising candidates for use in optical telecommunication systems. Imamura and co-workers fabricated a single-mode waveguide by using deuterated and fluorinated PMMA (d-fPMMA for short) through a standard photo-process with RIE technique(i0). The absorption loss limits of the polymers using these monomers are estimated to be 0.03 dB/cm at 1.3 μπι and 0.40 dB/cm at 1.55 μια A polymer was fabricated for use in waveguides by copolymerization of deuterated methacrylate and deuterated fluoromethacrylate monomers using the standard radical polymerization method. To fabricate single mode waveguides, it is also important to control the refractive index of the core and cladding polymers and to generate precise core patterns. The refractive index can be controlled by the composition of the copolymer. An important factor in fabricating low-loss and stable waveguides is insuring no intermixing between core and cladding. Channel waveguides were fabricated as follows(70). First, a planar waveguide with core and buffer layers was fabricated on a substrate by spin coating. The thickness of the core layer was 8 μπι and that of the buffer layer was 15 μπι. RIE was then used to form the channel waveguide patterns by etching until the buffer layer surface was exposed. Finally, the coreridgeswere covered with a spin-coated cladding layer. The buffer and cladding layers had die same refractive index which was 0.40 % lower than that of the core. The loss value was measured by a cut-back method as a function of waveguide length, rcnfirmed using a 1.3-μΐΏ-wavelength laser diode light source. Figure 3 shows die wavelength dependence of the loss of a straight waveguide(70). The spectrum has an absorption window at 1.3 μιη. Attenuation loss of less than 0.1 dB/cm was obtained at 1.3 μπι wavelength. Hida and co-workers had fabricated several optical circuits such as a 2x2 TO switch and aringresonator using the polymer waveguide(77,i2).


5x10-2 3x10-2 2x10-2

i

5S / dB/cm



5x10-3 :

a

3x10-3

10-2 3

7xl0-

A

A

\

' A i i

/'

P-5F3DSt

2x10-3 .3 10-3

I

0.8

i,

£—I

w

I ι

1.0

Iι

Ρ (MMA-d8) !— ιI ιI > . 1.2 1.4

Wavelength / μπι Figure 2. Loss spectrum of deuterated PMMA and fluorinated/deuterated PMMA core Plastic Optical Fibers (POFs).



ΚΑΙΝΟ ET AL.

Polymer Optical Waveguides & Photonic Devices

3

1

•

ι

•

ι 2VCH

3VCD 2VCH + ÔCH 3VCH 1

0.8

Figure 3.

'

•

4VCD '

1

/ \

/ \s 1

'

'

=·

1.0

1.2 1.4 1.6 Wavelength / μπι Loss spectrum of fluorinated/deuterated PMMA waveguide.


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Sasaki and co-workers have also fabricated single-mode optical waveguides from fluorinated polyimides by using almost the same process as that for d-f-PMMA waveguides. A loss less than 0.5 dB/cm was obtained at 1.3 and 1.55 μπι wavelengths(iJ). The structure has a large anisotropy in electronic polarizability, making polyimide a candidate for a biréfringent material with large in-plane bircfringence(74).


Second order Nonlinear Polymer Waveguides Azo-dye attached poled polymers have been investigated as NLO materials, showing efficient electro-optical (EO) properties with low absorption losses(75). These poled azo-polymers have been shown to possess ΕΟ-cœfficients almost as large as that for liNb0(7, respectively (Figure 4). Each dye molecule contains a dicyanovinyl group and a nitro group as an electron acceptor, and a diethylarnino group as an electron donor. Donor-acceptor charge transfer will greatly contribute to second-order nonlinear optical molecular susceptibility, β, values. The 3RDCVXY and 3RN0 dyes contain three benzene ring connected with azo groups, and their conjugated structure is longer than that of 2RN0 . The molecular structures of the copolymers and side chain azo dyes are shown in Figure 4 along with their χ wavelength dependence. 2

2

2

(2)

The Tgs of the synthesized polymers were 80, 100 and 135 *C for 2RN0 , 3RN0 and 3RDCVXY, respectively. The maximum absorption wavelengths are 515 nm (2.41 eV) for 3RDCVXY, 500 nm (2.48 eV) for 3RN0 , and 470 nm (2.64 eV) for 2RN02. The dimemyl-substitution of the 3RDCVXY effectively increases the dye concentration in the copolymer and as a result the content of the 3RDCVXY dye is nearly twice as large as that of the 3RN0 dye. 2

2

2

2


Polymer Optical Waveguides & Photonic Devices

ΚΑΙΝΟ ET AL.

CH

CH,

3


I

R=-CH,CH,

c=o I

C = 0

0-R

O-CH,

) -0~ Ν

CH,CH,

I

Ν=Ν

Ό"

N0

2

2RN02 (DR-1) -CH CH 2

2

NO, CH,CH,

3RN02 CH

3

-CH,CH, CH=C(CN)

2

CH.

3RDCVXY

15 ο χ

ίο

3RDCVXY (23 mol %) / 3RN02 (14 mol %

• 2RN02 (14mof%)

1.0

1.2

t

ι

1.4

7

7

1.6

4

1.8

Fundamental wavelength / μπι Figure 4. Chemical structure of azo-dye attached EO-polymers and their wavelength dependent χ . (2)


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The polymer films on glass substrates were poled by parallel electrode poling. The poling direction was perpendicular to the film surface. The χ was determined using a standard procedure(20), assuming χ =χ since the film was found to be isotropic. The χ increases as the fundamental wavelength decreases which corresponds to the absorption spectrum. This is caused by the χ enhancement effect of the second harmonic resonance near the absorption band. The χ measurement revealed that die maximum χ of the 3RDCVXY copolymer readies 1.0x10* esu as shown in Figure 4, which is larger than those of 3RN0 and UNbO by 3 and 7 times, respectively. A thermal aging test shows that the χ of 3RDCVXY is stable even at 80 *C for more than 6 months. This thermal and temporal stability makes 3RDCVXY a viable substitute for current inorganic BO materials. α )

α )

ρ )

333

3 1 1

α )

α )

(2)

(2)

3


2

0 )

As discussed in a previous section, deuteration or fluorination will be needed even for NLO waveguides. It is necessary to fabricate channel waveguides with low optical loss in order to make polymeric NLO switches that can be driven with low laser power(2i). To this end, a deuterated 3RDCVXY polymer, as shown in Figure 5, was developed for BO applications(22>. The polymer is composed of transparent copolymer units whose hydrogens are deuterated. Channel waveguides were fabricated using the deuterated 3RDCVXY polymer. Figure 6 shows a schematic of the channel waveguide fabrication process which is based on RIE technique. The selection of overcoat cladding layer is important to obtain flatness of the waveguide surface. A Mach-Zehnder interferometer using the 3RDCVXY EO-polymer was fabricated(25J. The EO coefficient (r-coefficient) of the waveguide was 26 pm/V at about 70 MV/m poling voltage with a half-wave voltage of 12 V. For comparison, the IiNb0 interferometer has an r-coefficient value of around 32 pm/V. 3

Third Order Nonlinear Polymer Waveguides We have developed a novel processable third-order NLO polymer, PSTF, in which tris-azo-dye NLO-phore was incorporated into the main chain of a polyurethane with a fluorinated alkyl backbone(24j. Fluorinated alkyl units were used to reduce the polymer waveguide loss. The chemical structure of PSTF is shown in Figure 7. Since third-order NLO polymers generally have higher refractive indices than transparent polymers, the refractive index of the NLO polymers should be considered. We can apply the RIE process for PSTF polymer because the polymer has a moderate refractive index and the cladding material, UV cured fluorinated epoxy resin, has an


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ΚΑΙΝΟ ET AL.

Transparent polymer unit [P(MMA-d8)]

ÇH ÇD -(Ç-CH -)—(Ç-CD ) 3

3


2

2

C=0 I

Nonlinearity generating unit

ο C2H4> I \ 2

5

Dye content increasing unit

N=N

—

CH

1-x

c=o OCDo

\

Electron donor

Figure 5.

π-conjugation

Electron acceptor

Chemical structure of deuterated 3RDCVXY polymer. Resist 3RDCVXY §

UV Resin Si substrate

Under Cladding fabrication 02 RIE

|j

Photo-process

Core layer fabrication

υ

Dry etching

UV Resin

Q v e r

c l a d d i n g

f cation abri

Figure 6. Fabrication method of 3RDCVXY polymer core channel waveguide.


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appropriate refractive index for fabricating single-mode channel waveguides. The farfield pattern of the guided mode at a wavelength of 1.3 μπι reveals that it is a quasisingle-mode waveguide. Because of the high NLO-phore content of the PSTF, the losses of the waveguide at 1.3 and 1.55 μπι were 3.5 and 4.5 dB/cm, respectively, even though a fluorinated alkyl chain was used as part of the matrix polymer. The nonresonant χ of the PSTF polymerfilmmeasured by third harmonic generation (THG) was around 2x10 esu at 1.55 μιη. Using this wavelength, we have confirmed the nonlinear optical effect of the PSTF by detecting the self phase modulation (SPM) of the waveguide. From the result, the nonlinear refractive index, r^, of the polymer was calculated to be 2.8 xlO cnrVW(25). This value is about an order of magnitude smaller man the value obtained by THG measurement It is due to the strong two-photon absorption of the polymer at that wavelength. 0 )


11

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For an optical Kerr-switch using third-order optical nonlinearity, the switching gate power Pft for the jc-phase shift of the signal beam in the nonlinear optical waveguide is expressed as: Ρπ=3λΑ/4ΐΛ

2

(1)

where λ is the signal beam wavelength, A is the core area, and L is the medium length. To reduce the gate power, the core area should be decreased or the medium length and of the medium should be increased. To date, optical switching using the PSTF waveguide could not be performed due to strong linear and nonlinear absorption in the waveguide. The absorption limits the wavelength at which optical switching can be operated. It is important to think about the trade-off between linear and nonlinear absorption and the χ value of polymer waveguides when the wavelength for switching is selected. For chalcogenide glass fiber with n^ value of 2xl0' cm /W, peak switching power of less than 3W was obtained using 1.2 meterfiber(2

Photonic and Optoelectronic Polymers - American Chemical Society

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