K-resin based multilayer polymeric mode filter for integrated optics

Optik 122 (2011) 1719–1722

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K-resin based multilayer polymeric mode filter for integrated optics Neha Sharma ∗ , V.K. Sharma, K.N. Tripathi Department of Electronic Science, University of Delhi, South Campus, Benito Juarez Road, New Delhi 110021, India

a r t i c l e

i n f o

Article history: Received 10 May 2010 Accepted 28 October 2010

Keywords: Polymer waveguides Dispersion curves Mode filter

a b s t r a c t We demonstrate theoretically as well as experimentally a new type of four layer polymeric waveguide structure which can be used as mode filter. Various optical properties such as refractive index, birefringence and propagation constant of SAN and K-resin are presented. The thin film structure consisting of glass/K-resin/SAN/air is used to act as mode filter. Expressions for the electric field intensity spatial distribution for the structure are used to calculate the intensity profiles to support the observed behavior. The experimental values were in good agreement with the one obtained theoretically. © 2010 Elsevier GmbH. All rights reserved.

1. Introduction Mode filter is a device used to select, reject, or attenuate a certain mode or modes. An optical mode filter, that is, an optical component in which the basic mode can propagate undamped as much as possible but which in higher modes are strongly damped. The optical mode filter, is an integrated optical wave guide formed by, a higher-refracting or an absorbent material disposed near a flat wave guide core. Two buffer layers are located opposite the wave guide core, and one is formed with a thinned region to function as a filter for higher modes being transmitted through the wave guide core. In the past a number of models are suggested for designing mode filter [1–4]. Through this paper we present a new way to accomplish the task using multilayer polymeric waveguide structure. Optical polymers are versatile materials that can be readily formed into planar single mode, multimode and micro-optical waveguide structures ranging in dimensions from one micrometer to several hundred micrometers. In contrast to inorganic materials like LiNbO3 , the electrooptic polymers have advantage such as large optical nonlinear coefficients, fast response time, low dielectric constant, simple fabrication of multilayer structures [5–9]. Polymers have flexibility in the sense that they are used as spun-on layers that are compatible with many substrate materials like glass, silicon, InP, etc. Further, as material properties can be tailored for specific application polymers are an important class of materials for advanced sensor photonics [9]. In particular K-resin and SAN (styrene acrylo nitrile) have high transparency (about 90%), easy processing and high physical,

∗ Corresponding author. Tel.: +91 9810560359. E-mail address: [email protected] (N. Sharma). 0030-4026/$ – see front matter © 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2010.10.031

chemical, mechanical and thermal stability and are hard enough to be used profitably in many integrated optical devices. We have used glass/K-resin/SAN/air structure to produce a mode filter. In this paper, we are concerned with the application of polymer multilayers to provide mode filter. The optical waveguiding properties of both materials used are studied and the multilayers are formed to produce mode filter. Theoretical dispersion curves for both the structures are generated and experimental values are also shown in the theoretical plots.

2. Guide preparation and characterization 2.1. SAN and K-resin waveguides To prepare thin film waveguide, a clear solution (10%, w/v) of K-resin was made in the xylene and that of SAN in chlorobenzene. A large number of waveguides were fabricated by dip coating on clean microscopic glass slides at 35 ◦ C (SAN and K-resin) to produce good quality waveguides [10,11]. The speed of drawing is maintained such that 4–8 modes are supported by the films. The guides were dried at 65 ◦ C to evaporate the excess solvent. The observed losses for K-resin were 0.98 dB/cm and that for SAN were 2.08 dB/cm. The losses for the designed mode filter were found to be 1.381 dB/cm. 2.2. Characterization The guide parameters (refractive index and thickness) were determined by prism coupling technique. A TE/TM polarized He–Ne laser (0.6328 ␮m) was coupled into these films using a dense flint glass prism of refractive index 1.717. Two prisms, for input and output couplings were clamped to the waveguide in order to observe

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2.5

2.485

a

2.465

2.48

2.445 2.46

2.44

SAN

n2eff

n2eff

2.425 K-r esin

2.405

2.42

2.385

2.4

2.365

2.38

2.345 0

1

4

9

d1= 3.62 microns d2= 2.73 microns

16 25 36

m2

2.325 0

Fig. 1. neff 2 vs. m2 for SAN and K-resin.

1

4

9

m the m-lines. The angle at which the mode excitation occurs is related to the mode index (neff ) through



neff = np sin ˛ + sin

−1

sin  np





n2eff = n2sur − 2

m2 4t 2



25

36

2.42 2.4 2.38

where nsur is the refractive index of the waveguide material,  is the wavelength of the laser source, m is the mode order and t is the guide thickness. The plots of neff 2 vs. m2 are found to be linear and give the value nsur from intercepts. The plots for TE modes are shown in Fig. 1 for SAN and K-resin. The slope of the line is related to the guide thickness (Eq. (2)). The thickness of K-resin guides supporting four modes is 2.73 ␮m and that of SAN guides supporting eight modes is 5.18 ␮m. The refractive index of SAN and K-resin is calculated to be 1.57 and 1.5643, respectively. The depth of the guide is 6.35 ␮m and support 9 modes. The birefringence observed was 3 × 10−3 (Fig. 2).

2.36

no

16

2.44

(2)

air

36

2.46

(1)

where np is the prism index (1.717), ˛ is the prism angle (60◦ ) and  is the angle of incidence of beam with respect to the prism normal where mode excitation occurs. The index can be calculated up to an accuracy of 1 × 10−4 . The effective mode indices can be expressed as a function of the mode order according to

25

2.48

b

n2eff



16 2

2.34 2.32 0

1

4

9

m

2

Fig. 3. neff 2 vs. m2 for (a) TE modes and (b) TM modes.

X

3. Multilayer structures d1

FILM (SAN) n1 0

d2

FILM (K-resin)

n2

SUBSTRATE n 3 Fig. 2. Schematic of four layer thin film structures.

Z

This structure is also formed using dip coating of SAN on K-resin waveguide as discussed in (a). The neff 2 vs. m2 plots are shown in Fig. 3 for TE and TM modes. The slope in Fig. 3 starting from m = 2 to 7 gives the thickness of (d1 + d2 ). And the line from m = 0 to 1 gives the thickness d1 as shown in Fig. 3. Thus for both the polarizations first 2 modes are supported by SAN film and thus can be used as mode filter.

N. Sharma et al. / Optik 122 (2011) 1719–1722

1721

3.1. Theoretical dispersion equations and their solution The four-layer dispersion curves generated using the dispersion relations given by the Sun and Muller [12]. The modes can be divided in two cases: Case 1. n3 ≤ neff ≤ n2 In this region, the structure satisfies the characteristic of dispersion equation

h  1

s1

tan(h1 d1 − 10 − m1 ) +

h  2

s2

tan(h2 d2 − 23 − m2 ) = 0 (3)

where m = 0, 1, 2 . . . are given by the following condition − 2 ≤ (h1 d1 − 10 − m1 ) ≤

 2

− 2 ≤ (h1 d1 − 10 − m2 ) ≤

 2

a

and

for TE modes for TM modes

10 = tan−1

s p 1 s0 h1

i = 0, 1, 2 and 3 (layers) 23 = tan−1

s q 2 s3 h2

ˇ2 − q2 = n23 2 ˇ2 + h22 = n22 2 ˇ2 + h21 = n21 2 ˇ2 − p2 = n2o 2  = 2 

1.57

x

1.56

h 

x

1

s1

x 1.55

ˇ = neff 

Case 2. n2 ≤ neff ≤ n1 In this region, neff satisfies the dispersion equation

x

neff

si = 1 = ni 2

tan(h1 d1 − 10 − m) =

where

x

10 = tan−1

x x

ˇ2 − q23 = n23 2

1.54

1.53

s p 1

x

s0 h1

q  2

s2

 = tanh−1

coth(q2 d2 +  )

(4)

s q  3 2 s2 q3

ˇ2 − q22 = n22 2 1.52

ˇ2 + h21 = n21 2

x

8

1

3.

4.

3. 5

3. 2

9 2.

4 2.

65

1 2.

2.

8

ˇ2 − p2 = n2o 2 1.

1. 5

1.51

d1 (microns)

b

1.57

x x

1.56

x x

1.54

x

0.5

x

0.4

ELECTRIC FIELD/ ARB. UNITS

neff

1.55

x x

1.53

m is the mode order defined as the number of modes of the transverse field distribution such that m = m1 + m2. Theoretical dispersion curves for both the structures and for both the polarizations are shown in Fig. 4. These curves have been generated by solving Eqs. (3) and (or) (4) whichever is applicable by neff by an iterative method.

1.52 x

0.3 0.2

substrate

K-resin

SAN

AIR

0.1 0 -0.1 -0.2 -0.3

1.51

8

1

3.

4.

3. 5

3. 2

9

65

4 2.

2.

1 2.

2.

8 1.

1.

5

-0.4

d1 (microns) Fig. 4. Comparison between experimental and theoretical values of neff of modes (a) TE mode and (b) TM mode.

-0.5 -2

0

2

4

6

8

THICKNESS Fig. 5. Electric field distribution for TE mode for m = 3.

10 -6

x 10

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N. Sharma et al. / Optik 122 (2011) 1719–1722

difference can be used for designing. This multilayer structure may be used as an electro-optic or thermo-optic switch if dye doped polymers is used. Another important result of these four layer light guides is the relatively low propagation losses compared to a monolayer configuration. Further, examining the evanescent portions of the field distribution can lead to information on the optical properties of surrounding media and can therefore be used for sensor applications.

ELECTRIC FIELD/ ARB. UNITS

0.5 0.4 0.3 0.2

substrate

K-resin

AIR

SAN

0.1 0 -0.1

References

-0.2 -0.3 -0.4 -0.5 -2

0

2

4

6

8

THICKNESS

10 -6

x 10

Fig. 6. Electric field distribution for TM mode for m = 3.

4. Results and discussions Mode filtering action can be observed from Fig. 3. It is clear that for the first 2 modes SAN acts as the guiding layer and for both TE and TM polarizations. Thus the structure can be used as a mode filter. From Fig. 4, it can be observed that the experimental values are in good agreement with the theoretical ones. The electric field plots for both TE and TM modes are shown in Figs. 5 and 6 for m = 3 mode. The effect of the cover layer (SAN) on the propagation losses is not presented here. However, it has been shown in our previous publications that propagation losses of multilayer structure are lower than the monolayer configurations [13]. 5. Conclusion Theoretical and experimental study of four layer polymer waveguide structures has been presented to produce mode filters. The birefringence property of the two materials with large index

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