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We demonstrate a novel electro-optic polymer modulator design which shows a record low optical insertion loss of 5.7 dB at 1550 nm. The modulator consists of a high numerical aperture passive waveguide which is converted to a strip-loaded electro-optic polymer waveguide through refractive index tapers. The device is fabricated using all wet-etch techniques which results in low excess loss from roughness created during fabrication and, employs new low loss passive sol-gel materials. The fabricated device also shows a low half-wave voltage of 2.8 V.
©2009 Optical Society of America
1. Introduction
Electro-optic (EO) polymer modulators show promise in applications ranging from RF photonic links and satellite communications, to fiber to the home. Although high bandwidth [1] and low drive voltage [2] devices have been previously demonstrated, typically the insertion losses of EO polymer based devices are 10 dB or higher [3–5]. The high insertion losses are due to several factors including coupling losses, scattering losses due to fabrication, and chromophoric absorption losses. Recent work has demonstrated a 1.3 V, 1 cm long active section EO polymer modulator with an insertion loss of 7.8 dB by using a low loss (0.9 dB/cm) EO polymer (LPD80) [6]; however, the majority of EO polymers that are available have significantly higher losses, thus device designs that achieve low insertion loss using more typical EO polymers with losses of 2–3 dB/cm are highly desired.
We have previously demonstrated an EO modulator with improved coupling losses [7] utilizing photo-bleaching induced refractive index tapering. However, the total insertion loss of this device was still relatively high, 11.6 dB due to losses incurred during fabrication and high propagation losses in the passive sections. In this letter we demonstrate a new device design that reduces coupling losses and scattering losses due to fabrication, resulting in a low insertion loss of 5.7 dB and a Vπ of 2.8 V.
2. Device design and modeling
Our device design consists of a high numerical aperture passive waveguide coupled to a strip-loaded active waveguide by refractive index tapers, illustrated in Fig. 1. The passive section is a planar waveguide with a core and cladding indices of 1.539 and 1.485 respectively with a 1.1 μm thick EO polymer slab layer directly above the waveguide which, has been fully photo-bleached and has an index of 1.529 (Fig. 1(a)). In the active section of the device the high NA planar waveguide acts as strip load, creating lateral confinement in the unbleached slab layer of EO polymer which has an increased index of 1.609 (Fig. 1(b)). These two sections are joined by an index taper created with photo-bleaching which, we have previously shown to be a low loss way to convert the fundamental mode of a multimode waveguide to the mode of a single-mode waveguide [7].
Fig. 1. (a) Cross-section of the passive section waveguide (b) cross-section of a single arm of active section c) Longitudinal cross-section overlaid on a 3-D BPM simulation of the optical intensity propagation through the device for a straight channel waveguide, the white arrows indicate the direction of propagation of the optical mode.
A 3 dimensional beam propagation method (BPM) simulation was done on a straight channel waveguide in order to estimate the losses at the transition from the passive to active sections of the device. The simulation showed that a loss of less than 0.1 dB per transition can be expected from this design and can be seen in Fig. 1(c). The modes of the passive and active sections were calculated using the finite element method. From this simulation we estimate the overlap integral of the TM optical mode with the EO polymer, Γ, to be 0.7 when the EO polymer layer is 1.1 μm thick. The mode solver showed that the passive section had a second order transverse mode while the active section was single mode. This information led to designing the Mach-Zehnder Y-branches in the active section which allows the waveguide to act as quasi single-mode due to the mode filtering effects discussed in [7] and allows the modulator to achieve a high modulation extinction ratio.
The high index difference, Δn, of 0.054 between the strip-load and cladding keeps the active section waveguide lateral mode-field diameter from becoming large (Fig. 2(a)). This is advantageous as it prevents the fundamental mode from stimulating high order slab modes which would create excess losses. The high Δn also makes this design robust as it will produce a well confined mode for a large range of EO polymers refractive indices which, typically range from 1.54 – 1.8 as illustrated in Fig. 2(b).
Fig. 2. (a) Mode field diameter as a function of Δn for an EO polymer index of 1.61 and a thickness of 1.1 μm. (b) EO polymer thickness that maintains an overlap integral of 0.7 as a function of EO polymer index; the mode field diameter was between 7.4–7.1 μm.
3. Device fabrication
Organically modified sol-gels have been chosen as the passive materials due to their beneficial properties with respect to poling [8]. Two sol-gel compositions were used for this device. A 95/5 molar ratio of methacrylpropyltrimethoxysilane (MAPTMS) to zirconium(IV)-n-propoxide (ZPO) was used as the cladding in both the active and passive sections with an index of 1.485. A 30/70 molar ratio of MAPTMS to diphenyldimethoxysilane (DPDMS) was used as the core in the passive section and as the strip-load in the active section with an index of 1.539. 1.5 weight % Irgacure 369 photoinitiator was added to make the sol-gel photo-patternable. It has been previously demonstrated that the addition of DPDMS to sol-gels reduces propagation losses at 1550 nm by replacing aliphatic CH bonds with aromatic ones, reducing the strength of the molecular vibrational resonance at 1550 nm [9]. Both sol-gels were hydrolyzed with a ratio of water: methoxy of 0.37 using 0.1 N HCl. The ZPO was chelated with a 1:1 molar ratio of methacrylic acid (MAA). The EO polymer used was a guest-host material, 25 weight % AJLS102 chromophore5 doped into amorphous polycarbonate, a high glass transition temperature (Tg) host polymer.
Mach-Zehnder modulators were fabricated as follows: A 6 μm layer of the 95/5 sol-gel was spin-cast onto a Ti coated Si wafer and cured for 1 hour at 135 °C. A 1.3 μm layer of Shipley 1813 photo-resist was spin-cast onto the sol-gel layer and prebaked for 8 minutes at 110 °C. Next waveguide trenches were patterned in the resist by exposing for 23 s in a Carl Zuss MJB3 mask aligner and developing using a dark field waveguide mask to pattern 4 μm wide waveguides. 3 μm deep trenches were then wet-etched into the sol-gel at a rate of 0.1 μm per minute by immersion in a 1:6 buffered oxide etchant. The resist was then stripped and the 30/70 sol-gel core was spin-cast into the trenches, photo-cured for 30 s, and developed in a 50:50 mixture of acetone and ethanol. Due to oxygen inhibition in the free radical photo-polymerization of the sol-gel, the 30/70 MAPTMS/DPDMS sol-gel achieved planarization at the top of the trenches etched in the cladding. A cross-section of the filled trench can be seen in Fig. 3.
Fig. 3. Back illuminated optical microscope image of the cross section of a back filled sol-gel waveguide. It can be seen that the core layer planarizes with a thin overflow layer.
Next, a 1.1 μm layer of EO polymer dissolved in cyclopentanone was spin-cast and dried under vacuum at 100 °C. After drying the refractive index tapers were created in the EO polymer using a grayscale mask from Benchmark Technologies. The mask consists of two 1.5 mm long sections of linearly varying optical density connected by a wide chrome strip 1.5 cm in length which prevents the active section from becoming bleached. An exposure dose of 9 kJ/cm2 of Hg i-line radiation was applied which created refractive index tapers that can be seen in Fig. 4. In order to achieve a high extinction ration the grayscale mask was aligned such that the Y-branches occurred after the index tapering.
Finally, a 3 μm top cladding of 95/5 MAPTMS/ZPO sol-gel was spin-cast and cured for 1 hour at 135 °C. The top electrodes were 50 nm Cr deposited by electron beam evaporation and patterned using standard photolithography and commercially available Cr etchant. The fabricated devices have an active section length of 1.5 cm and total length of 2.0 cm and a total thickness of 10.1 μm. The modulators were poled near the EO polymer Tg, at 155 °C with voltages ranging from 400–600 V.
Fig. 4. The unpoled TM index of the EO polymer layer along the length of the device set over a cartoon top view of the Mach-Zehnder waveguide, a close-up view of the index taper (inset).
4. Device characterization
After poling, the devices were tested for insertion loss and Vp in a dual drive setup [3] with TM polarization at 1550 nm. The measured and simulated TM near field mode images of the passive and active sections can be seen in Fig. 5. In order to minimize insertion losses several fibers were used to measure the fiber-to-lens insertion loss. High NA fibers were chosen from Nufern which are designed to fusion splice to SMF-28 optical fiber with a splice loss of only 0.2 dB.
Fig. 5. (a) Simulated mode profile of the passive section (b) simulated mode profile of the active section (c) measured mode in the passive section (d) measured mode in the active section.
The insertion losses for various fibers can be seen in Table 1, with the best result being a fiber-to-lens insertion loss of 5.7 dB. In order to insure no stray light was incident on the detector for the loss measurement the output mode was passed through a 1 mm aperture and observed on an infrared camera before measuring the power on a 1 mm2 detector. The best measured Vπ of 2.8 V which, corresponds to an r33 of 71 pm/V can be seen in Fig. 6 and was for a poling voltage of 500 V. A modulation extinction ratio of 15 dB was measured which confirms that the modulator behaves as a quasi single-mode waveguide and also confirms that little stray light was coupled through the objective lens. No poling induced losses were observed.
Table 1. Optical Insertion Loss for various Input Fibers
Fig. 6. A 2.8 volt Vπ measurement. The top trace is the optical signal with a vertical axis of 200 mV/division and horizontal axis of 200 μs/division, the bottom trace is the electrical signal which has a vertical axis of 5 V/division and horizontal axis of 200 μs/division.
5. Conclusion
In conclusion we have demonstrated an EO polymer modulator with a fiber-to-lens insertion loss of 5.7 dB and a Vπ of 2.8 V. To the best of the authors knowledge this is the lowest reported insertion loss for a non evanescent EO polymer waveguide modulator with 1.5 cm or greater active length. The reduction in loss is attributed to the use of strip-loading to create wave guiding in the active section, the wet-etch fabrication process, the use of low loss sol-gel passive materials, gray-scale lithography to create a passive to active waveguide mode conversion and the choice of a high NA fiber to match the mode field diameter of our device. By taking advantage of recent materials advances that have resulted in higher r33 EO polymers; modulators with an insertion loss less than 6 dB and a Vπ less than 1 V are expected in the near future.
Acknowledgments
The authors would like to acknowledge support from the National Science Foundation through MDITR under Grant#-0120967 and CIAN NSF ERC under grant # EEC-0812072 and Nitto Denko Technical in Oceanside, CA.
References and links
1. D. Chen, H. R. Fetterman, A. Chen, W. H. Steier, L. R. Dalton, W. Wang, and Y. Shi, “Demonstration of 110 GHz electro-optic polymer modulators,” Appl. Phys. Lett. 70, 3335–3337 (1997). [CrossRef]
2. Y. Enami, D. Mathine, C. T. DeRose, R. A. Norwood, J. Luo, A. K.-Y. Jen, and N. Peyghambarian, “Hybrid cross-linkable polymer/sol-gel waveguide modulators with 0.65 V half wave voltage at 1550 nm,” Appl. Phys. Lett. 91, 093505 (2007). [CrossRef]
3. Y. Enami, C. T. DeRose, D. Mathine, C. Loychik, C. Greenlee, R. A. Norwood, T. D. Kim, J. Luo, Y. Tian, A. K.-Y. Jen, and N. Peyghambarian, “Hybrid polymer/sol-gel waveguide modulators with exceptionally large electro-optic coefficients,” Nat. Photonics 1, 180–185 (2007). [CrossRef]
4. Y. Enami, D. Mathine, C. T. DeRose, R. A. Norwood, J. Luo, A. K.-Y. Jen, and N. Peyghambarian, “Transversely tapered hybrid electro-optic polymer/sol-gel Mach-Zehnder waveguide modulators,” Appl. Phys. Lett. 92, 193508 (2008). [CrossRef]
5. Y. Enami, C. T. DeRose, C. Loychik, D. Mathine, R. A. Norwood, J. Luo, A. K.-Y. Jen, and N. Peyghambarian, “Low half-wave voltage and high electro-optic effect in hybrid polymer/sol-gel waveguide modulators,” Appl. Phys. Lett. 89, 143506 (2006). [CrossRef]
6. H. Chen, B. Chen, D. Huang, D. Jin, J. D. Luo, A. K.-Y. Jen, and R. Dinu, “Broadband electro-optic polymer modulators with high electro-optic activity and low poling induced optical loss,” Appl. Phys. Lett. 93, 043507 (2008). [CrossRef]
7. C. T. DeRose, D. Mathine, Y. Enami, R. A. Norwood, J. Luo, A. K.-Y. Jen, and N. Peyghambarian, “Electrooptic polymer modulator with single-mode to multimode waveguide transitions,” IEEE Photon. Technol. Lett. 20, 1051–1053 (2008). [CrossRef]
8. C. T. DeRose, Y. Enami, C. Loychik, R. A. Norwood, D. Mathine, M. Fallahi, N. Peyghambarian, J. Luo, A. Jen, M. Kathaperumal, and M. Yamamoto, “Pockel’s coefficient enhancement of poled electro-optic polymers with a hybrid organic-inorganic sol-gel cladding layer,” Appl. Phys. Lett. 89, 131102 (2006). [CrossRef]
9. M. Oubaha, P. Etienne, S. Calas, R. Sempere, J. M. Nedelec, and Y. Moreau, “Spectroscopic characterization of sol-gel organo-siloxane materials synthesized from aliphatic and aromatic alcoxysilanes,” J. Non- Cryst. Solids 351, 2122–2128 (2005). [CrossRef]
