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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 1, JANUARY 1, 2006
Photobleached Refractive Index Tapers in Electrooptic Polymer Rib Waveguides Kevin Geary, Seong-Ku Kim, Byoung-Joon Seo, Yu-Chueh Hung, Wei Yuan, and Harold R. Fetterman, Fellow, IEEE
Abstract—Photobleached refractive index tapers in electrooptic polymer rib waveguides, which act as two-dimensional optical mode transformers, are investigated. One taper method involves a discrete step mask-shifting scheme with a fixed intensity UV light source. A second method utilizes a gray-scale mask to precisely control the intensity of UV light reaching each portion of the taper. Using a gray-scale mask, adiabatic refractive index tapers can be inscribed into electrooptic polymer waveguides in a single fabrication step with no scanning parts. Optimized taper profiles are derived and applications for their incorporation into passive-to-active waveguide transitions are described.
We propose two methods for inducing a refractive index taper via photobleaching within an EO polymer etched rib waveguide. The first involves shifting a photomask in discrete steps. The second utilizes a stationary gray-scale mask that controls the intensity of UV light reaching the device and can write adiabatic tapers in a single fabrication step with no beam scanning, as was necessary in [13].
Index Terms—Electrooptic (EO) polymer waveguides, gray-scale mask, mode transformer, passive–active integration, photobleaching, refractive index taper.
The refractive index of many polymer materials decreases as a result of UV light irradiation. In the case of the EO polymer APC/CLD1, a phenyltetraene bridged high- chromophore in an amorphous polycarbonate host, the index change is a predictable function of both exposure time and intensity [3], [14]. Therefore, a refractive index gradient can be inscribed directly into APC/CLD1 waveguides either by varying the exposure time of a fixed intensity UV source to different portions of a taper region or by varying the UV exposure intensity throughout the taper for a fixed time period. Using an 8-mW/cm UV lamp from a mask aligner, the refractive index versus exposure time of an unpoled APC/CLD1 thin film is shown in Fig. 1 at 1.55 m. For the same material, Fig. 2 depicts the index versus exposure intensity for a bleaching time of 5 h. All indexes were measured using an ellipsometer. The refractive index can be reduced to values comparable to many low-loss passive polymer materials, suggesting that in-plane passive-to-active polymer transitions incorporating photobleached tapers can achieve high performance. We first propose utilizing the time-dependent photobleaching characteristics of EO polymers to create an -stage refractive . We consider the index taper as depicted in Fig. 3 with and the index of the same index of poled APC/CLD1 material photobleached for 6 h . A mask is used to block UV light from a mask aligner so that initially only the region requiring the highest index change is exposed. After the time necessary for a desired index change, the mask is shifted so that UV light exposes both the original taper segment and the adjacent segment requiring the next highest index change. This is repeated until all taper segments are photobleached for the time necessary to achieve their desired index. At each segment interface, there is a small amount of scattered light due to the mode mismatch of the two regions. However, the total loss accumulated at each interface is less than the loss incurred had we simply created a single index discontinuity of the two extremes. Adiabatic in-plane refractive index tapers can be realized with our second photobleaching method by utilizing a gray-scale mask. Transitions between waveguides of dissimilar effective indexes are typically adiabatic in that they occur
I. INTRODUCTION
R
ECENT progress in the development of electrooptic (EO) polymers has resulted in unprecedented device performance characteristics, including low driving voltages and modulator bandwidth beyond 100 GHz [1], [2]. Unfortunately, such devices typically have high-optical losses which make large complex structures impractical. Polymer waveguide writing techniques have been developed to improve upon the transmission of etched waveguides, such as photobleaching, poling-induced writing, and stress-induced writing [3]–[5]. Total insertion loss is ultimately limited by material loss, however, which is typically 1.2–2.0 dB/cm. Therefore, transitions between low-loss passive materials and EO polymers are necessary for the realization of complex designs and integrated arrays of devices. The refractive index of most EO polymers is high (1.60–1.70) compared to silica and passive polymers, whose refractive indexes typically range 1.45–1.53. This makes a butt-couple approach for a transition between two waveguides lossy. Many groups have designed two-dimensional (2-D) tapers to reduce the mode mismatch between waveguides consisting of two unique materials [6]–[11]. This typically involves physical tapering of the waveguide dimensions and requires tight fabrication tolerances. Recently, a refractive index taper was written in a passive polymer waveguide via an electron beam to improve its mode match to fiber [12]. Similarly, an index taper using a UV beam scanned repeatedly over a passive polymer device was demonstrated [13].
Manuscript received July 7, 2005; revised September 7, 2005. The authors are with the Department of Electrical Engineering, University of California, Los Angeles, CA 90095 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2005.860044
II. PRINCIPLE
1041-1135/$20.00 © 2005 IEEE
GEARY et al.: PHOTOBLEACHED REFRACTIVE INDEX TAPERS IN ELECTROOPTIC POLYMER RIB WAVEGUIDES
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Fig. 3. Schematic diagram of three-stage photobleached in-plane refractive index taper in EO polymer inverted rib waveguide.
Fig. 1. APC/CLD1 refractive index as a function of UV exposer time for TM and TE polarizations.
(a)
(b)
Fig. 4. Fundamental TM mode shapes of a 4-m wide APC/CLD1 inverted rib waveguide in (a) photobleached and (b) unbleached regions.
device with standard microstrip electrodes since no ground plane was present; to do so would require a more complex fabrication procedure. In contrast, our proposed in-plane tapers employ a simpler fabrication procedure, are 1 mm long, and can be incorporated into EO devices with conventional driving electrodes. Fig. 2. APC/CLD1 refractive index as a function of UV exposer intensity for TM and TE polarizations after 5 h of photobleaching.
slowly for the optical field to remain in the fundamental mode as the mode itself evolves [6]. This can be accomplished by small steps in physical dimensions or by small changes in the index. Both cases result in variations in the fundamental mode shape. Adiabatic devices tend to be less wavelength and polarization dependent than their nonadiabatic alternatives, as well as have higher fabrication tolerances. Gray-scale masks darken to varying optical density (OD) levels in response to varying electron-beam dosage, resulting in a UV light transmission gradient. Current masks can achieve 500 shades of OD, with a 0.1- m horizontal resolution. In our second method, a stationary gray-scale mask is aligned above an EO polymer waveguide so that the variation of OD is in the direction of light propagation along the waveguide. The mask is designed to have high UV transmission regions (low OD) at the device edges where coupling to another waveguide takes place. Regions of low UV transmission (high OD) occur above the active regions where phase modulation is required. Properly designing a smooth variation in OD between these two extremes exposes the waveguide taper region to different UV intensities, directly resulting in an adiabatic index change according to Fig. 2. A shadow mask could be used in a similar manner. Recently, a gray-scale mask was used to fabricate 2–5-mm-long silica-to-polymer vertical transitions with 1-dB transition loss [11]. These transitions could not be incorporated into an active
III. DESIGN AND RESULTS We have conducted a design study for the proposed photobleached in-plane tapers employing a three-dimensional (3-D) vectoral simulation package, FIMMWave by Photon Design. The key taper design requirement is to move the optical field in the waveguide from the lower index, photobleached side, to the higher index, unbleached side in the shortest possible length while keeping the energy in the fundamental mode. All simulations were performed at 1.55 m and assume a 1.5-dB/cm material loss. If we consider a 4- m rib polymer waveguide made from APC/CLD1 with a refractive index of 1.61 and photobleach half the waveguide such that its index is reduced to 1.53, a 0.47-dB transition loss occurs at the interface due to the mode mismatch of the two regions, as depicted in Fig. 4. By introducing a threestage linear index taper like Fig. 3, with a total length of 300 m, the fundamental mode overlap at each interface is improved, resulting in a reduced overall loss of 0.18 dB. Here, each section change in refractive index. Total taper loss has a can be reduced further by decreasing and increasing . We have fabricated single-mode APC/CLD1 4- m rib waveguides following the procedures described in [1]. The best devices have a fiber-to-fiber insertion loss of 9–10 dB, which includes a 3-dB fiber coupling loss per interface and a waveguide loss of 1.5–1.7 dB/cm in a 2.4-cm-long device. Introduction of a three-stage linear refractive index taper into the core material of such devices at each waveguide endface would improve the overall fiber-to-fiber insertion loss by 2 dB.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 1, JANUARY 1, 2006
of Fig. 3 still results in a fiber-to-fiber insertion loss improvement of 2.1 dB. IV. CONCLUSION We have shown that photobleached refractive index tapers in EO polymer rib waveguides act as 2-D optical mode transformers. One method to achieve such a taper involves a discrete step mask-shifting scheme with a fixed intensity UV source. The second method utilizes a gray-scale mask to precisely control the intensity of UV light reaching each taper segment. Using a gray-scale mask, adiabatic index tapers can be inscribed easily into EO polymer waveguides with no requirement of scanning parts or shifts of a mask. We have experimentally characterized APC/CLD1 refractive index change versus UV exposure time and exposure intensity, and we have designed tapers for fiber-todevice transitions and in combination with in-plane butt-couple passive-to-active polymer waveguide transitions. Such tapers may form the basis for a new class of hybrid polymeric structures and complex arrays of such devices.
Fig. 5. Refractive index taper profiles.
REFERENCES
Fig. 6. Transmission of refractive index tapers depicted in Fig. 5 as a function and length of taper length. An exponential taper with steepness parameter p 480 m has 0.11-dB total loss (including 1.5-dB/cm material loss).
=1
For an adiabatic refractive index taper utilizing a gray-scale mask, we have investigated the effectiveness of both linear and exponential index profiles. The exponential profile analyzed is: , where and are normalized from zero to one, and is the steepness parameter. For each taper profile in Fig. 5, the transmission of the fundamental TM mode versus taper length is plotted in Fig. 6. The precision in the index values of each taper segment was calculated assuming 400 OD levels. Fig. 6 includes the taper losses due to mode mismatch at each of 32 taper segments and 1.5-dB/cm material loss. For longer lengths, the linear material loss dominates. An exponenand length 480 m exhibits the lowest loss tial taper with exponential ( 0.11 dB), although the linear taper and taper have only a 0.02-dB loss increase, implying that our design tolerances are favorable. The optimal taper length decreases if the material loss is 1.5 dB/cm. Fig. 6 can be used to determine the improvement in fiber coupling to a tapered waveguide. The coupling loss of singlemode fiber to a 4- m rib waveguide is 2.86 dB. Photobleaching reduces with core index , expanding the mode shape as shown the index to in Fig. 4 and improving the loss to 1.61 dB per interface. According to Fig. 6, an additional 0.11-dB loss is introduced for a 32-segment exponential taper. Subtracting this from the fixed coupling gain, a tapered device results in a 2.3-dB fiber-to-fiber insertion loss improvement. The simple three-stage linear taper
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