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A hybrid electro-optic (EO) polymer phase modulator with a 6 μm coplanar electrode gap was realized on ion exchange glass substrates. The critical alignment steps which may be required for hybrid optoelectronic devices were eliminated with a simple alignment-free fabrication technique. The low loss adiabatic transition from glass to EO polymer waveguide was enabled by gray scale patterning of novel EO polymer, AJLY. Total insertion loss of 5 dB and electrode gap of 8 μm was obtained for an optimized device design. EO polymer poling at 135 °C and 75 V/μm was demonstrated for the first time on a phosphate glass substrate and was enabled by the sol-gel buffer layer.
©2010 Optical Society of America
1. Introduction
The ever increasing demand for high speed telecommunication applications requires the development of high speed and low cost external modulators. Electro-optic (EO) polymer and LiNbO3 modulators intrinsically offer high speed operation in various applications ranging from RF photonic links and satellite communications, to fiber to the home [1–3]. However, the physical constraints i.e. index difference between microwave and optical frequencies, impedance mismatch, and microwave losses, limit the bandwidth of external modulators [2–6]. The main limiting factor for high speed LiNbO3 modulators is the large refractive index difference between microwave and optical frequencies [3]. EO polymer modulators, in addition to their higher EO coefficient, overcome this limitation owing to their low dispersion from optical to microwave. Electrode design and substrate selection are critical in order to achieve high bandwidth in EO polymer modulators. Coplanar electrode offers easier impedance matching, lower microwave losses and lower dc bias drift compared to microstrip structures and they are consequently more suitable for high speed operation [6]. However, traditionally EO polymer modulators use microstrip electrode structures that are inferior to coplanar electrodes in terms of electrical and microwave properties [7–9]. Coplanar electrodes typically have a larger electrode gap (15 μm) than microstrip electrodes due to the horizontally elongated optical mode shape of EO polymer waveguides in hybrid structures. The large electrode gap and smaller electrical and optical field overlap integral result in higher drive voltage (Vπ) for EO polymer modulators with coplanar electrodes.
Coplanar device structures on low dielectric constant glass substrates offer high bandwidth as well as low coupling and propagation losses [10]. In order to efficiently integrate passive devices with modulators, Bosc et. al. suggested using hybrid glass/EO polymer modulators [11]. Another potential advantage of hybrid ion exchange (IOX) modulators is that they can be easily integrated to rare earth ion doped IOX glass lasers [12] and amplifiers [13]. However, in-plane poling of the EO polymers on glass presents a significant challenge to obtaining low drive voltages [14,15]. The high density (e.g., 1021 cm−2) of alkali ions in IOX glass substrates makes the glass conductivity unacceptably high at elevated temperatures which results in inferior poling efficiency of the high Tg EO polymer waveguide layer. The present technologies require a lengthy, high temperature and high voltage poling of the glass itself to obtain merely a moderate poling efficiency [14].
In this study, we have developed a simple technique to fabricate hybrid glass/EO polymer modulators free of critical alignment steps. In addition to easier and potentially lower cost fabrication, this technique intrinsically provides an electrical insulation and ionic interface layer which increases the breakdown voltage by a factor of three and improves poling efficiency on glass substrates [16]. The design shown in Fig. 1(a) has IOX waveguides for SMF28 fiber coupling and for passive sections of the device. The low loss (<0.2 dB) transition to the EO polymer waveguide is achieved by patterning a physical taper with UV lithography and oxygen dry etching. The use of 1.2 μm thick, low loss sol-gel and novel fabrication technique gave excellent control over the optical mode shape and electrode gap, allowing us to achieve 5 dB total insertion loss for 8 μm electrode spacing and efficient in-device poling.
Fig. 1 a)3D illustration of a hybrid EO polymer/glass modulator. ltaper is the length of the adiabatic transition and Lactive is the length of the active section. wi is the width of the coupling region, wo is the width of the EO polymer core, and we is the width of electrode gap. The buffer layer thickness is given with hb and EO polymer film (green) thickness by hc b) The 3D surface profile of the gray scale patterned polymer physical taper (1.3 μm thick) c) The near field image of the optical mode at 1550 nm.
2. Device design
The three dimensional (3D) illustration of the fabricated device is shown in Fig. 1(a). The upper cladding layer is omitted for clarity.
Standard single mode fiber, SMF28, was used to couple light into the device. The wider IOX waveguide at the input section provides an excellent mode match to SMF28 and thus results in a low fiber coupling loss. The coupling loss of our devices was measured by the fiber-waveguide-objective method [17] and found to be 0.5 dB in good agreement with a previously reported value [18]. The output section of the device can be made symmetrical with respect to input section for more efficient fiber output coupling. The fiber coupled light (λ = 1550 nm) propagates in the low loss IOX glass waveguide before making an adiabatic transition into EO polymer waveguide The coupling region (wi = 8 μm) makes a lateral transition to the core region (wo = 2 μm). The length of this transition region was ltaper = 1 mm. The metal electrodes are shown in yellow and the EO polymer waveguide (Lactive = 1 cm) in green. The 3D surface map of the polymer physical taper is shown in Fig. 1(b). The near field image of the optical mode is shown in Fig. 1(c).
The device substrate was IOG-1 phosphate glass (nglass = 1.51) which is known to be suitable for monolithic integration of passive and active devices [19]. Several buffer layer materials were tested including SiO2, ZPU-430 and in-house sol-gel. We have obtained the best poling efficiency and lowest optical loss results with the in-house sol-gel known as 95/5 [20] and used it in our devices. The comparison of different buffer layer performances is shown in Table 1 .
Table 1. Electrical and fabrication characteristics of different buffer layers
The design of the EO polymer waveguide was critical in order to reduce the metal absorption induced losses in the active waveguide section. We have decreased the interaction of the optical mode field with the metallic electrode layer by increasing the mode confinement. We have used a commercially available computer modeling tool, FIMMWAVE, to understand the effect of EO polymer thickness, hc on the electrode induced loss. Figure 2 shows the electrode induced loss with respect to hc, for a 1 μm core width and a 5 μm electrode gap. The cross section of the modeled device geometry is shown in the inset b). The film mode matching (FMM) option was used in order to solve the modes of the structure [21].
Fig. 2 The simulated electrode induced loss due to the 100 nm gold electrode layer with respect to EO polymer thickness, hc. wo was chosen to be 1 μm and we = 5 μm. The inset a) shows the mode shape of EO polymer waveguide (refractive index nEOpol = 1.7) for hc equals 0.4, 1 and 1.6 μm. The inset b) shows the cross section of the modeled device geometry, yellow color indicates the electrode layer.
The sol-gel buffer layer had a refractive index, nb of 1.48, and thickness, hb of 1 μm in our simulations. Gold was selected as an electrode material using the properties included in the FIMMWAVE database. hc is varied between 0.4 μm and 1.6 μm in order to control the mode confinement. As seen in Fig. 2, the optimum hc was found to be close to buffer layer thickness. The inset a) shows the mode shape of the EO polymer waveguide for hc equals 0.4 μm, 1 μm and 1.6 μm. Better mode confinement for the 1 μm thick film is clearly seen. The computer modeling results show that the thickness of residual layer thickness (hc-hb), defined as the layer of polymer which is outside of the core region, has to be less than 0.3 μm to obtain high mode confinement and thus low electrode induced loss. The control of the residual layer thickness was achieved by changing the viscosity of the polymer solution and spin speed.
3. Alignment-free fabrication
Initially, the buffer layer material, 1.2 μm thick in-house sol-gel, was spin coated onto the glass substrate surface. The details of sol-gel preparation are given elsewhere [20]. Then, the sol-gel was cured at 130°C for an hour and baked at 190°C overnight in a vacuum oven to ensure stability during IOX process. To avoid sol-gel water absorption, as reported for a similar material before [22], 100 nm of chromium (Cr) was ion-beam sputtered immediately and served as an electrode layer. Next a photoresist film, S1805, was spin coated and patterned by UV lithography to serve as an etch mask. The Cr layer was etched with a commercial Cr etchant for 40 seconds, after which the sol-gel was etched by buffered oxide etchant (BOE) for 5 minutes [23]. Figure 3 shows the rest of the fabrication steps and they are as follows; a second Cr etching step was applied for an extended period of time to precisely control the gap size. The overetching rate was approximately 5 min/μm, however in general, the total overetching time is determined by factors like material adhesion, desired electrode gap size, etc. For example; to obtain a 6 μm electrode gap, the total overetching period was 10 min for wo = 2 μm.
Fig. 3 Alignment free hybrid modulator fabrication steps (light blue buffer layer material can be i.e. SiO2, ZPU 430 or sol-gel).
100 nm Ag was next deposited on both surfaces (top and bottom) of the glass substrate, so that ion exchange waveguides could be made in the glass. Exchange of Na+ and Ag+ ions results in a refractive index change (Δnmax = 0.05) that can be used for optical waveguiding. The silver film IOX process is realized at 105°C and 200 V is applied between electrodes for 2 hours [24]. After exchange of Ag+ and Na+ ions in the glass, Ag film was removed by base piranha solution and photoresist was removed by successive ultrasonic baths in acetone and isopropyl alcohol. Then, the device platforms are annealed at 230 °C for 1.5 hours to obtain low loss, single mode IOX waveguides. The 1 cm long active section was patterned on the Cr film mainly to avoid edge effects during poling and to minimize electrode induced optical losses. There was no critical alignment necessary for this patterning process. Finally, EO polymer solution was spun casted into the prepared trenches, due to the nature of the spin coating process, a residual layer with 0.4 µm thickness outside of the trenches remains. The EO polymer was dried in a 50°C vacuum oven for 16 hours.
Typically, hybrid optoelectronic devices with co-planar electrodes may require two critical alignment steps. One of them is the alignment of EO polymer waveguide to the input/output waveguide and the other is the alignment of electrodes to the EO polymer waveguide. The removal of two critical alignment steps provides ease of fabrication and superior precision compared to present technologies.
A top microscope view of the fabricated structure is shown in Fig. 4 . The picture shows the active region of the modulator platform before EO polymer and upper cladding was spin coated (we = 8 μm and wo = 2 μm). The overetching time was 15 min. The sharp, clean Cr edges demonstrate that overetching did not have any effect on the electrode quality. The microscope picture in the inset shows the alignment features etched on 1.2 μm thick sol-gel and 2 μm overetched Cr electrode layer. The 1 μm wide rectangles on top were separated by 1 μm and 2 μm wide rectangles just below them were separated by 2 μm. Less than 0.5 μm lateral etching demonstrated the anisotropic wet etching characteristics of the sol-gel. Wet etching also provided smooth side walls which helped in obtaining lower scattering losses.
Fig. 4 Top microscope view of a test sample with sol-gel buffer layer and Cr electrodes. The end facet is prepared by cleaving the glass. The electrode spacing of 8 μm was obtained by 15 min overetching. Minimum feature size on the inset is about 1 μm.
After the EO polymer was spin coated, physical tapers were patterned by standard UV lithography and dry etching. The patterning of 0.5 mm long physical tapers on EO polymers has been demonstrated by shadow masking, previously [7]. The reported transition loss was about 0.5 dB. In order to obtain negligible transition loss, a longer smooth taper and a more controlled fabrication technique is desired. We have used a gray scale lithography technique to pattern a S1813 photoresist on EO polymer. AJLY was not attacked by the photoresist unlike common host polymers APC and PMMA and thus, allowed us to spin coat photoresist on it. The pattern on photoresist was transferred to EO polymer by an oxygen RIE process in an Oxford RIE system. The RF power was kept low at 50 W and pressure was kept high at 50 mTorr to ensure smooth and controlled etching. The etch rate was about 0.2 μm/min. Both the etch rate and thickness of the EO polymer and photoresist were made to be comparable to ensure that the EO polymer physical taper is a good replicate of the photoresist taper. A key benefit of this technique is that, provided the polymer can withstand the photoresist processing, any polymer can be used with little change to the overall process. After EO polymer patterning, a 5 μm sol-gel upper cladding (n = 1.48) was spin coated and UV cured at room temperature to avoid crosslinking of the EO polymer and finally the glass substrates were cleaved to provide an optical quality end facet. The overall device length was 1.5 cm. In order to exclude the effect of the electrodes and make more accurate measurement of transition losses we have also fabricated devices without electrodes. The minimum insertion loss was found to be 4.3 dB. The AJLY propagation losses were measured with a cut-back technique on a slab waveguide and found to be 3.5 dB/cm. When the coupling and propagation losses are subtracted the transition losses are found to be less than 0.2 dB. The transition losses are decreased significantly by reducing the surface roughness (< 20 nm) and increasing the taper length (1 mm). It should be noted that patterning of physical tapers on the EO polymer has a large tolerance for misalignment (>10 μm) therefore is not considered to be a critical alignment step.
The width of the waveguide core (wo) and electrode gap size (we) was made as small as 1 μm and 6 μm, respectively, by using a sol-gel buffer layer and an alignment free fabrication technique. The total insertion loss of a device with wo = 2 μm and for we = 8 μm was measured to be 5 dB. This indicates that metal absorption loss is about 0.7 dB/cm. The higher electrode induced loss is considered to be due to the 0.4 µm thick EO polymer residual layer, which results in reduced confinement.
4. Poling and modulator performance
Before poling the fabricated devices, we have measured the r33 of AJLY with the Teng-Man method at 1340 nm wavelength [25]. The results are shown in Fig. 5 . The poling temperature was 135 °C and poling field was varied from 50 V/μm to 100 V/μm. The r33 at 1550 nm was estimated by two level model using measurements at 1340 nm [26]. Maximum r33 at 1550 nm wavelength is expected to be 34 pm/V for 100 V/μm poling field.
Fig. 5 The variation of r33 with respect to poling voltage. The poling temperature was 135 °C. The solid line is the best linear fit to the measured data. The error bars indicate the standard deviation of the three measurements at the same poling conditions. Measurements were done at 1340 nm and r33 values at 1550 nm were calculated by using two level model. The red cone represents the uncertainty in the calculation due to error in the actual measurement.
We have then poled the hybrid devices with a 6 μm electrode gap by applying 75 V/μm electrical field at 135 °C for two minutes. The effect of the poling field on the IOX waveguide can be neglected since the poling duration is short and light guiding is provided by the EO polymer in this section of the device. The sol-gel buffer layer has improved the achievable poling field by a factor of three [16]. However, the charge injection typically observed in coplanar geometry prevented higher poling fields [15]. Phase modulators were tested in a waveguide characterization setup after poling. Low frequency phase modulation was measured with an oscilloscope. The data collected by the oscilloscope is shown in Fig. 6 . The Vπ was measured as 16 V. This Vπ value corresponds to r33 and Γ (optical and electrical field overlap integral) product of 16 pm/V. According to the Teng-Man measurements the expected r33 was 25 pm/V for 75 V/μm poling field, which suggests that overlap integral was approximately 0.64. This value is in good agreement with previously reported value for coplanar electrodes with similar geometry [6].
Fig. 6 The oscilloscope output of modulation signal (red) and detected optical signal (blue). Vπ = 16 V.
5. Conclusion
We have demonstrated a novel fabrication technique for hybrid coplanar EO modulators on glass substrates. The alignment free technique brings simplicity and precise fabrication control. We have demonstrated 5 dB insertion loss for 8 μm electrode spacing. The lowest obtained Vπ value was 16 V (5.3 V for MZ device with push-pull configuration) for 1 cm active length. The gray scale patterned EO polymer AJLY enabled fabrication of 1 mm long, smooth tapers and thus allowed low transition loss. The low propagation loss and 75 V/μm poling field at 135 °C is achieved by taking advantage of excellent optical and electrical characteristics of sol-gel buffer layer. The described technique potentially allows integration of ion exchange glass waveguides with various optoelectronic devices. The high Vπ was due to low EO coefficient of the AJLY. However, several different material systems have been shown to exhibit r33 of higher than 250 pm/V and up to 380 pm/V [27], therefore it is quite possible to obtain both low Vπ (sub 1-volt for push-pull) [20] and low loss (5 dB) in coplanar electrode modulators with the availability of high r33 (160 pm/V) EO polymers which are suitable for gray scale patterning.
Acknowledgements
The authors would like to acknowledge support from the National Science Foundation MDITR Science and Technology Center under Grant # 0120967 and the National Science Foundation through CIAN NSF ERC under grant # EEC-0812072.
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