Temperature dependence of low loss polymeric AWG

Optics Communications 270 (2007) 189–194 www.elsevier.com/locate/optcom

Temperature dependence of low loss polymeric AWG Jong-Moo Lee

a,*

, Yong-Soon Baek a, Kwang-Ryong Oh a, Hyung-Jong Lee b, Yong-Seok Kim c

a

c

IT Convergence and Components Laboratory, Electronics and Telecommunications Research Institute, 161 Kajong-dong, Yusong, Taejon 305-350, Republic of Korea b ChemOptics, Inc., 104-11, Moonji-Dong, Yosong-Gu, Daejeon 350-380, Republic of Korea Advanced Material Division, Korea Research Institute of Chemical Technology 100, Jang-Dong, Yosong-Gu, Daejeon 305-600, Republic of Korea Received 18 March 2006; received in revised form 20 July 2006; accepted 25 August 2006

Abstract We investigate on the variation of loss and temperature dependence of a polymeric arrayed waveguide grating (AWG) depending on its substrate, by fabricating 16-channel polymeric AWGs with various substrate conditions. Insertion loss for a polymeric AWG on a silicon substrate is measured as low as 3.1 dB. The temperature-dependent wavelength shift for a polymeric AWG detached from the substrate is maintained within 0.1 nm from 20 to 80 C. But we observe a degradation of insertion loss and a little instability in wavelength characteristics both for the detached polymeric AWG and for a polymeric AWG on a polymer substrate. We investigate on those optical properties of the polymeric AWGs based on measured thermal expansion properties of the polymers.  2006 Elsevier B.V. All rights reserved. PACS: 42.82.E; 42.79.S Keywords: Optical waveguides; Optical communication

1. Introduction Polymeric waveguide devices have been actively studied with an attraction of economical merit based on its simple fabrication process, and are entering into industrial markets as commercial products such as thermo-optic (TO) switches and variable optical attenuators (VOA) [1–3]. Polymeric arrayed waveguide grating (AWG) [4–10] has also gained increasing attention as multiplexers/demultiplexers for an emerging passive-optical network (PON) application based on wavelength-division multiplexing (WDM) [11] or for coarse wavelength-division multiplexing (CWDM) networks [10,12]. An 8-channel all-polymeric AWG with little dependence on temperature and polarization was reported but the insertion loss was as high as 5.8 dB and 7.8 dB each for *

Corresponding author. Tel.: +82 428601618; fax: +82 428606248. E-mail address: [email protected] (J.-M. Lee).

0030-4018/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.08.037

the central and the edge channel respectively [5]. A 16channel all-polymeric AWG made of a special polymeric material with an intrinsic absorption of 0.001 dB/cm was reported with the insertion loss as low as 3 dB afterwards [6]. These results showed a bright possibility of industrialization of an all-polymeric AWG. Here, we investigate on the variation of loss and temperature dependence of polymeric AWGs depending on their substrates, by fabricating 16-channel polymeric AWGs with various substrate conditions. We fabricated polymeric AWGs each on a silicon substrate, a polyimide (PI) substrate and a polycyanurate/epoxie-copolymer (PCy/E) substrate, respectively, in addition to a polymeric AWG detached from a silicon substrate [9]. We measure insertion losses and temperature-dependent wavelength shifts (TDWS) for the AWGs and compare the data with the coefficient of thermal expansion (CTE) of the substrates measured by a thermo-mechanical analyzer (TMA). We believe these results to be useful in providing proper

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polymeric material and substrates for an industrial polymeric AWG.

Test Waveguide

0

2. Experiments and results There are many research groups developing polymer materials for optical waveguide devices but commercially available materials are very limited. We used commercial polymer materials supplied by ChemOptics, Inc., in this research. Polymeric waveguide devices are fabricated with several thin film layers of cladding and core which are formed through spin-coating and curing process. An UV curable polymer material has a merit in low temperature process and fast curing time but the homogeneity of the film and layer to layer bonding are usually inferior to a thermally curable one. In addition, UV curable polymer materials of ChemOptics have higher thermo-optic coefficient (TOC) and CTE than thermally curable one. In this regard, we used thermally curable fluorinated polyethers expecting low TOC and CTE to be beneficial for a thermally stable AWG. Experiment was performed by fabricating 1 · 16 channel AWGs with core and clad layers made of thermally curable fluorinated polyethers such as ZP2145, ZP49, ZP1010, ZP5127, ZP532, and ZP1370. The materials are selected as a pair for a proper index composition of core and clad, as an example a pair of ZP5127 and ZP49 each for core and clad respectively. The refractive index of each material is 1.484 for ZP2145, 1.49 for ZP49, 1.501 for ZP1010, 1.5127 for ZP5127, 1.532 for ZP532, and 1.547 for ZP1370. The main fabrication process of the polymeric AWG starts from spin coating of a cladding material on a silicon substrate or a polymer substrate. Then, a core material is coated and thermally cured after a thermal curing of the lower cladding at 250 C. Waveguide patterns are formed through a photo lithography process and a following dry etching process. Finally, the waveguide is finished by covering the upper cladding layer. A CTE-compensating extra layer is added for a compensation of mismatched CTE, in case of a detached polymeric AWG [9]. Each thickness of the lower cladding layer and the upper cladding layer is about 10 lm and the thickness of the core layer is varied from 4.5 to 6 lm depending on the AWG design. We used the ZP materials such as ZP 1370 as a CTE-compensating layer since it has a low CTE value. The CTE-compensating layer can be added over the upper cladding layer or under the lower cladding layer and the thickness of the CTE-compensating layer is varied from about 10 to 20 lm. Fig. 1 shows transmission spectra through a polymeric AWG made of ZP 5127 core and ZP49 core on a silicon substrate, in comparison with a test waveguide around the AWG. The cross-sectional dimension of the core is 4.5 · 4.5 lm. The insertion loss through the central channel is about 3.1 dB for transverse-electric (TE) polarization and 3.5 dB for transverse-magnetic (TM) polarization, including 1 dB extra loss compared to the test waveguide. The cross talk between different channels is about 28 dB.

Insertion loss (dB)

-10

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-40

1540

1545

1550

1555

Wavelength (nm)

Fig. 1. Transmission spectra through a polymeric AWG on a silicon substrate, in comparison with a test waveguide around the AWG. The insertion loss through the central channel is about 3.1 dB for TE polarization which includes about 1 dB extra loss compared to the test waveguide.

The loss of 2.1 dB through 4 cm test waveguide is due to an intrinsic waveguide loss of 0.4 dB/cm in addition to the coupling loss to a pair of single-mode fibers (SMF). A high NA fiber (PWG1-XP from Nufern) spliced to a normal SMF (SMF-28) was used to reduce the coupling loss. The loss of 3.1 dB is very close to the best data in a polymeric AWG with an extremely low loss polymer [6]. A degradation of mechanical and thermal property is easy to accompany in reducing the intrinsic loss of a polymer material, in my knowledge. Here, however, we show that the low loss of 3.1 dB can be achieved using a commercial polymeric material, instead of a specially synthesized low loss polymer. We made various polymeric AWGs with several designs to test the optical properties depending on its substrates. Fig. 2 shows the AWG design resulting in the spectral properties in Fig. 1. We decreased the waveguide loss by reducing the overall length of the AWG as in Fig. 2a and minimized the fiber coupling loss using the high NA fiber. In addition, we reduced the extra loss of AWG induced at the junction between the slab region and arrayed waveguide gratings by inserting low loss patterns as in Fig. 2b, similar to Ref. [13]. Sixteen band-shaped waveguides are arranged to cross the arrayed waveguides near the slab region. The width of the band-shaped waveguide is gradually decreased from 17 to 2 lm with a uniform pitch of 21 lm. The effect of low loss pattern is regarded to be more obvious in a polymeric AWG than silica AWGs, since the polymeric core pattern is deformed far less than the silica in forming the upper cladding layer. The other parameters of the AWG are shown in Table 1. Fig. 3 shows TDWS for TE polarization through a channel of a polymeric AWG, on a Si substrate with a ZP1010 core (Si + ZP1010), on a PI substrate with a ZP1370 core (PI + ZP1370), on a PI substrate with a ZP5127 core (PI + ZP5127), on a PCy/E substrate with a

J.-M. Lee et al. / Optics Communications 270 (2007) 189–194

191

Relative wavelength (nm)

4

2

0

-2 Si+ZP1010 PI+ZP1370 PI+ZP5127 PCy/E+ZP5127 Detach+ZP2145 Detach+ZP1010 Detach+ZP1370

-4

-6

-8

0

20

40

60

80

100

o

Temperature ( C) Fig. 3. Temperature-dependent wavelength shift for TE polarization through a channel of polymeric AWG: on a Si substrate with a ZP1010 core (Si + ZP1010); on a PI substrate with a ZP1370 core (PI + ZP1370); on a PI substrate with a ZP5127 core (PI + ZP5127); on a PCy/E substrate with a ZP5127 core (PCy/E + ZP5127); and detached from a Si substrate. There are two detached samples made of a ZP1010 core within ZP2145 clad, one (Detach + ZP1010) with a ZP1010 extra layer for a CTEcompensation and the other (Detach + ZP2145) without compensation. The last detached sample (Detach + ZP1370) is made of a ZP 5127 core within ZP49 clad with an extra layer of ZP1370 for a CTE-compensation.

Fig. 2. Schematic diagram of low loss polymeric AWG: (a) overall design and (b) low loss pattern.

Table 1 Design parameters for the AWG in Fig. 2 Parameters

Values

Channel spacing Number of channels Number of arrayed waveguides Focal length of slab waveguide Length difference of arrayed waveguide Pitch of adjacent channel waveguides Diffraction order

100 GHz 16 (+1) 150 5.05 mm 74.2 lm 14 lm 72

ZP5127 core (PCy/E + ZP5127), and detached from a Si substrate. There are two detached samples made of a ZP1010 core and ZP2145 clad, one (Detach + ZP1010) with a ZP1010 extra layer for a CTE-compensation and the other (Detach + ZP2145) without a compensation. The last detached sample (Detach + ZP1370) is made of a ZP 5127 core and ZP49 clad with an extra layer of ZP1370 for a CTE-compensation. We located each AWG inside a convection oven after bonding it to a pair of fiber blocks. The TDWS was normally measured during the natural cooling process of the oven after reaching a high tem-

perature such as 90 C. The cooling process was slower than 1 C/min. TDWS is 0.1 nm/C for the polymeric AWG on a Si substrate. TDWS for the detached AWG varies from +0.08 nm/C in case without a CTE-compensating layer, to 0.01 nm/C in case with a CTE-compensating layer of ZP1010, and near to zero in case with a CTE-compensating layer of ZP1370. The deviation of the wavelength is less than 0.1 nm from 20 to 80 C in case with a CTE-compensating layer of ZP1370, and it can be regarded as an athermal condition. The refractive index of ZP series material is controlled by mixing several base materials as in Fig. 4 which shows variation of the refractive index of ZP series material as the blending ratio of a couple of materials. Fig. 4a is for the mixture of ZP1335 and ZP1010, and Fig. 4b is for ZP1335 and ZP1370. Fig. 5 shows the thermal expansion ratio of the base material ZP2145, ZP1010, ZP1335, and ZP1370 measured by TMA. CTE value and glass transition temperature of each material can be obtained from Fig. 5. Glass transition temperature of each material is shown as a point of inflection in Fig. 5. The sudden change of thermal expansion at the glass transition temperature is a typical property of polymeric material. CTE value of each base material is about 120 ppm/C for ZP2145, 80 ppm/C for ZP1010, 75 ppm/C for ZP1335, and 70 ppm/C for ZP1370 at near the room temperature. The experimental result in Fig. 3 shows that TDWS of the detached polymeric AWG is compensated by ZP1370 with the CTE value of 70 ppm/C.

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a

0

1.530 o

nTE at 1550nm wavelength(250 C/2hr) o

nTM at 1550nm wavelength(250 C/2hr)

-10

1.520

Insertion Loss (dB)

Refractive indices

1.525

1.515 1.510 1.505

-40

0

20

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Contents (w%) of ZP1335M to ZP1010M

o

nTE at 1550nm wavelength(250 C/2hr)

Refractive indices

1546 1548 1550 1552 1554 1556 1558 1560 1562

Wavelength (nm) Fig. 6. Transmission spectra for TE polarization through the polymeric AWG with an extra layer of ZP1370 for a CTE-compensation before and after the detachment, in addition to the variation depending on temperature from 20 to 90 C.

1.550 o

1.545

nTM at 1550nm wavelength(250 C/2hr)

1.540 1.535 1.530 1.525 1.520 0

20

40

60

80

100

Contents (w%) of ZP1370M to ZP1335M Fig. 4. Variation of refractive index of ZP series material as the blending ratio of a couple of materials: (a) ZP1335 and ZP1010, (b) ZP1335 and ZP1370.

200

Thermal expansion (ppm)

-30

1.500 1.495

b

-20

20 29 40 50 60 68 80 90 Si

150

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TMA graph of ZP2145M TMA graph of ZP1010M TMA graph of ZP1335M TMA graph of ZP1370M

50

0

50

100

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Temperature (°C) Fig. 5. Thermal expansion of ZP series material ZP2145, ZP1010, ZP1335, and ZP1370 measured by TMA.

Fig. 6 shows transmission spectra for TE polarization through the polymeric AWG with an extra layer of ZP1370 for CTE compensation before and after the detachment, in addition to the variation depending on temperature from 20 to 90 C. The data show a degradation of

loss about 2 dB by the detachment, another 1 dB loss as the temperature increased up to 70 C, and a further degradation as the temperature increased more. The increased loss for the high temperature recovers when it is down to the room temperature. The degradation in loss seems mainly due to a mechanical deformation of the polymer film through the detachment process in addition to the thermal expansion of the film as the temperature grows. These results show that the detached AWG is a very simple way to make an athermal AWG but it has a problem of degradation in loss. Fig. 6 also shows that the TDWS of all-polymeric AWGs on a PI substrate or a PCy/E substrate. PI is a very attractive material for its mechanical and thermal stability. We expected a polymeric AWG on a PI substrate has a strong possibility for an industrial application, if the TDWS of the AWG can be compensated by the PI substrate. The PI substrate used in this experiment has a thickness of 0.5 mm and it is a commercial product of Central Glass, Co. Ltd. and CTE was informed as near 80 ppm/ C. We expected the TDWS of the AWG on the PI substrate to be around 0.01 nm/C, with the CTE. But the measured TDWS of the AWG on the PI substrate is about 0.04 nm/C at the room temperature, and it tends to decrease near to zero as the temperature approaches 100 C as in Fig. 3. For the analysis of the result, we measured the thermal expansion property of the PI substrate using a thermo-mechanical analyzer (TMA) from TA instruments, as in Fig. 7. The TMA result shows that the CTE value of the substrate is not a constant but varies a lot depending on the temperature. It is near 50 ppm/C at the room temperature and it grows to 70 ppm/C at 100 C. It explains why the TDWS of the AWG on the PI substrate is 0.04 nm/C and tends to decrease near to zero as the temperature approaches 100 C. From the TMA result, we can see that a polymeric material with TOC near 0.00007/C should be used for an athermal

J.-M. Lee et al. / Optics Communications 270 (2007) 189–194

193 170 160

3 Expansion 1st Expansion 2nd

100 1 80

CTE

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-1 -100

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Expansion ratio (%)

120

CTE (ppm/ C)

Expansion ratio (%)

Expansion

150 140

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110 100

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CTE 1st CTE 2nd

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40 350

-1 -100

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90 80 70 300

Temperature ( °C)

o

Temperature ( C) Fig. 7. Thermal expansion property of a PI substrate measured using a TMA.

50

CTE (ppm/ °C)

140

3

Fig. 8. Thermal expansion property of a PCy/E substrate measured using a TMA.

0 TE fix TE

-10 Insertion Loss (dB)

condition with the PI substrate, while it is about 0.0001/ C for ZP series material used in this experiment. CTE of a PCy/E substrate was known to be controlled by mixing a polycyanurate with a epoxie as a designed ratio [14]. So, we tried to make an AWG on a polymer substrate designed to have a CTE value near 70 ppm/C. PCy/E substrate used in this experiment was supplied by Heinrich– Hertz-Institut, and the thickness was 1.1 mm. We used such a thick substrate, to avoid a deformation of the substrate during the fabrication process. But we found it was not thick enough for the experiment. The deformation of the substrate was noticed through the curing process of the clad and core material. The reason for the deformation is the difference of thermal expansion between the substrate and the optical polymer layer. The PCy/E substrate was relatively soft compared with a PI substrate at the curing temperature of 250 C. We fabricated an AWG on the PCy/E substrate, even though there was a little deformation, and measured TDWS as in Fig. 3. The TDWS is near to zero at about 20 C but it increases up to 0.02 nm/C as the temperature grows over 80 C. The TDWS result can be compared with a thermal expansion property of the polymer substrate measured by TMA. Fig. 8 shows the TMA result of the PCy/E substrate. CTE of the PCy/E substrate is near the designed value of 70 ppm/C at about 0 C but it grows up to 90 ppm/C at about 80 C and grows further as the temperature increased. An instability of thermal expansion near 150 C may be because the PCy/E substrate is a mixture of a polycyanurate and a epoxie. The thermal expansion is measured twice with a different temperature range as in Fig. 8 and the data shows there is a little difference in CTE value for the cases. It shows TDWS may not be repeatable in case of a repeated temperature variation. Fig. 9 shows transmission spectra for TE polarization through a polymeric AWG on the PCy/E substrate, one when it is fixed on a flat mount by vacuum suction and the other when it is released from the mount after epoxy-

-20

-30

-40

1540

1550

1560

Wavelength (nm)

Fig. 9. Transmission spectra for TE polarization through a polymeric AWG on a PCy/E substrate, each when it is fixed on a flat mount by vacuum (TE fix) and when it is released from the mount (TE), respectively.

bonding to a pair of fiber blocks. The AWG in Fig. 9 is made of ZP532 core and ZP5127 clad. The loss property was best with the composition, since the deformation of the PCy/E substrate in fabrication is minimal with the composition. The matching of CTE between the substrate and the optical polymer layer was found crucial to the fabrication process with a thermally curable polymer on such a soft polymer substrate as PCy/E. The insertion loss of about 5 dB through a fixed AWG in Fig. 9 is inferior by 2 dB to the AWG on a Si substrate in Fig. 3. The extra loss is regarded coming from the influence of the deformation of the PCy/E substrate through the fabrication process, in addition to a slightly rough surface of the substrate. Fig. 9 shows further degradation of loss about 1 dB and a shift of central wavelength when the AWG is released from the mount. The reason is regarded that the all-polymeric AWG is slightly bent by the stress induced by the mismatch of CTE. These results show that the all-polymeric AWG made on a high-CTE polymeric substrate has a problem of instabilities in wavelength and

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loss due to a mechanical deformation. So, a combination of an optical polymer with a low TOC and a polymer substrate with a low CTE matching the TOC is necessary for a useful all-polymeric AWG. We do not mention about polarization-dependent wavelength shifts (PDWS) of the polymeric AWGs in detail here, since the PDWS of a polymeric AWG depends on its structure of upper clad [7,8], in addition to the substrate. The PDWS of the detached polymeric AWG was varied from 0.6 to 2 nm depending on the composition of upper clad layer and extra layer, when PDWS is defined as the difference of central wavelength of TE mode from TM mode. The PDWS of the polymer AWG on the PI substrate was around +0.6 nm and it decreased down to zero as the temperature increased up to 90 C in case of the AWG made of ZP51 core and ZP49 clad. The PDWS of the polymer AWG on the PCy/E substrate was around 2 nm. 3. Conclusion We investigated on the variation of loss and temperature dependence of a polymeric AWG depending on its substrate. We fabricated 16-channel polymeric AWGs each on a silicon substrate, a polyimide (PI) substrate and a polycyanurate/epoxie-copolymer substrate, respectively, in addition to a polymeric AWG detached from a silicon substrate. Insertion loss for a polymeric AWG on a silicon substrate is measured as low as 3.1 dB, which is remarkable considering that such a low loss polymeric AWG can be made with a commercially available polymeric material. The temperature-dependent wavelength shift for a polymeric AWG detached from the substrate is maintained within 0.1 nm from 20 to 80 C. But we observe a degradation of insertion loss about 2 dB and a little instability in wavelength characteristics both for the detached polymeric

AWG and for the polymeric AWG on a PCy/E substrate. We compared the temperature dependence of the polymeric AWGs with the thermal expansion properties of the polymer materials and substrates measured using a TMA. From the results, a combination of an optical polymer with a low TOC and a polymer substrate with a low CTE matching the TOC, is found to be necessary for a thermally stable all-polymeric AWG without a degradation in loss. References [1] Y.-O. Noh, J.-M. Kim, M.-S. Yang, H.-J. Choi, H.-J. Lee, Y.-H. Won, S.-G. Han, IEEE Photon. Technol. Lett. 16 (2004) 446. [2] Y.-O. Noh, C.-H. Lee, J.-M. Kim, W.-Y. Hwang, Y.-H. Won, H.-J. Lee, S.-G. Han, M.-C. Oh, Opt. Commun. 242 (2004) 533. [3] ChemOptics, Inc. Available from: . [4] J. Kobayashi, Y. Inoue, T. Matsuura, T. Maruno, IEICE Trans. Electron. E81-C (1998) 1020. [5] N. Keil, H. Yao, C. Zawadzki, J. Bauer, M. Bauer, C. Dreyer, J. Schneider, in: Proceedings of OFC 2001, Anaheim, Post-deadline paper PD7. [6] Renyuan Gao, Renfeng Gao, K. Takayama, A. Yeniay, A.F. Garito, in: Proceedings of the 28th European Conference on Optical Communication (ECOC), 2002, Paper 6.2.2. [7] J.-M. Lee, S. Park, M.–H. Lee, J.T. Ahn, J.J. Ju, K.H. Kim, IEEE Photon. Technol. Lett. 15 (2003) 927. [8] J.-M. Lee, S. Park, M.-S. Kim, J.T. Ahn, M.–H. Lee, Opt. Commun. 232 (2004) 139. [9] J.-M. Lee, S. Park, J.T. Ahn, M.-H. Lee, ETRI J. 26 (2004) 281. [10] J.J. Claire, C.L. Callender, C. Blanchetrie`re, J.P. Noad, S. Chen, J. Ballato, D.W. Smmith Jr., IEEE Photon. Technol. Lett. 18 (2006) 370. [11] H.D. Kim, S.-G. Kang, C.-H. Lee, IEEE Photon. Technol. Lett. 12 (2000) 1067. [12] C.R. Doerr, M. Cappuzzo, L. Gomez, E. Chen, A. Wong-Foy, C. Ho, J. Lam, K. McGreer, J. Lightwave Technol. 23 (2005) 62. [13] J. Hasegawa, K. Nara, Furukawa Rev. 26 (2004) 1. [14] M. Bauer, J. Bauer, J. Schnerider, C. Dreyer, H. Yao, N. Keil, D. Zawadzki, US Patent 6,757,469, 2004.