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In this paper, a low-power 1 × 2 polymeric thermo-optic switch operating at the polymer optical fiber low-loss window of 650 nm was studied. The characteristic parameters of the switch were carefully designed and simulated. The fabrication was done by using standard semiconductor fabrication techniques such as spin-coating, photolithography, and dry etching. The device was fabricated based on poly(methyl methacrylate) (PMMA)-based materials with the Mach-Zehnder interferometer (MZI) structure. The device shows an extinction ratio of over 23.4 dB at 650 nm with a very low-power consumption of 5.3 mW. The measured switching rise time and fall time are 464.4 and 448.0 µs, respectively.
© 2014 Optical Society of America
1. Introduction
Advances in optical communication network systems over the past 1 0 years have been fueled by progress in laser and detector technology, as well as by the availability of high performance devices such as optical switches, optical modulators, optical attenuators and optical amplifiers [1–6]. Optical switches are important elements in many applications, such as optical cross connect (OXC), optical add-drop multiplexing (OADM), and optical true time delay (TTD) [7]. As the ever increasing number of channels required for optical communication systems scale up, a large number of 1 × 2 and 2 × 2 switches may be used in the implementation of these components. Therefore, it is important to develop optical switch with low power consumption, low insertion loss, and low cost, with switching time on the order of milliseconds or less [8]. From these viewpoints, polymer optical waveguides can address these issues, and the polymer functional devices such as low-power-consuming optical switches may be promising for future optical networks.
In recent years, particularly with the progress of multimedia technology, the requirement for high data rate networks at over 100 Mbits/s has been brought about, both in automobiles and in the home [9]. Though single-mode glass optical fiber (SM-GOF) has been practical in trunk lines and has already become the indispensable information transmission medium, it is mechanically weak and lacks bending ability. And worse still, it is difficult to lay down SM-GOF for very short reach networks such as building LAN [10]. At the same time, polymer optical fibers (POFs), also referred to plastic optical fibers, are increasingly being applied for data transmission over short distances, e.g. local to home Internet connections and automotive applications [11,12]. POFs have many advantages, such as low weight, immunity to electromagnetic interference, high elastic strain limits, high flexibility in bending and potential negative thermo-optic (TO) coefficients [11,13,14]. Meanwhile, the increasing use of POFs for visible optical communication system, LAN, or sensors require of devices and components that are compatible with solutions of low cost. Polymer waveguide devices have excellent compatibility with POFs optical communication system. The relevant optical waveguide devices are being under intensive research. Low cost polymer-based arrayed waveguide grating (AWG), optical amplifier, optical switch, and organic light-emitting devices (OLEDs) have been studied and reported [15–18]. Currently poly(methyl methacrylate) (PMMA) is used as the main material for the fabrication of economic POFs applications [19,20], and this material is also a popular material for fabricating optical waveguide devices [21–23].
In this paper, we have proposed the design and experimental verification of a low-power consuming, cost-effective, and fast response polymer 1 × 2 TO switch operating at 650 nm wavelength. This work is based on thermally stable P(MMA-GMA) material systems fabricated on top of a silicon substrate to facilitate effective heat flow originated from a thin-film heater. The P(MMA-GMA) material systems with low-loss window around 650 nm were used as waveguide core and cladding layers, respectively. The optical properties of the P(MMA-GMA)-based materials were characterized. Design procedures, simulation, fabrication, as well as performances of the TO switch were also described.
2. Waveguide material
In this work, cross-linkable polymer P(MMA-GMA) with chemical and physical stability properties was selected as the cladding material, which was synthesized by copolymerization of methylmethacrylate (MMA) and glycidyl methacrylate (GMA). The bisphenol-A epoxy was used as high-refractive index regulator, and the core material was formed through regulating polymeric ratio of reactive materials. Figure 1 shows the chemical structure of the core material. The refractive index of core material could be easily controlled from 1.48 to 1.55 (@1550 nm) through regulating the weight percent of bisphenol-A epoxy.
The films were prepared to measure the characteristics of thermal, optical absorption, surface roughness, and refractive index. Thermal behavior of the core material was investigated by differential scanning calorimeter (DSC) and thermo-gravimetric analysis (TGA). The measured results were shown in Fig. 2. It reveals that the glass transition temperature of the core material is about 115 °C, and no decomposition was observed at around 190 °C. The absorbance of the core material was measured by Perkin Elmer Instruments UV-visible-near-IR19 spectrophotometer over a wavelength range of 300-2000 nm. Figure 3 summarizes the results of the absorption measurements. The spectrum suggests that the P(MMA-GMA) polymer has a low absorption at 650nm, 1310 nm and 1550 nm wavelength regions. Moreover, the root-mean-square (RMS) surface roughness of the film was only about 0.452 nm in 20 × 20 µm2 scan areas, which was measured by atomic force microscopy (AFM), and the 3D AFM image is shown in the inset of Fig. 3. This indicates that the film surface is smooth enough to provide a low scattering loss for the waveguide. The refractive indices of the P(MMA-GMA)-based core and cladding were measured by an M-2000 UI variable angle incidence spectroscopic ellipsometer, and the results are shown in Fig. 4. Under 650 nm, the refractive index of core material is 1.5024, and that of the cladding material ia 1.4998.
Fig. 3 Absorption spectrum of the core material as a function of wavelength. The inset shows the surface topology of the film measured by AFM.
3. Design and simulation
The 1 × 2 polymeric TO switch is shown schematically in Fig. 5(a). It consists of a symmetric Y-junction splitter which splits light into two decoupled waveguides, a phase tuning section with 10 mm length in one of the waveguides, and a 3-dB directional coupler acting as the output combiner. Mach-Zehnder modulator with 3-dB directional coupler on the output instead of a Y-junction acts as very fast optical switch. As shown in Fig. 5(a), the applied electrical power can toggle the light output between the two output waveguides. By using a thin-film heater above one arm of the waveguides, temperature gradients can be induced with in a relative waveguide structure, accommodating the changes in refractive index profile, which, in turn, is used to realize the switching function. The designed switch is based on polymer MZI structure fabricated on a Si substrate to effectively facilitate heat flow. The cross-section view of AA’ in the TO section region is shown in Fig. 5(b). The core and upper/under cladding are all P(MMA-GMA)-based materials which were synthesized in our lab. The heating electrode is aluminum (Al) electrode. Compared to the silica/silicon, the thermal conductivity of polymer material PMMA, equal to 0.19 W/(m·K), is much smaller, so it is used as the under cladding instead of the silica to decrease the power consumption.
Fig. 5 (a) Schematic diagram and (b) cross-section view of AA’ in the TO region of the 1 × 2 polymeric TO switch; (c) The steady-state thermal distribution in the activity waveguide cross-section.
The polymer waveguide usually consists of multiple layers, principally lower cladding, core layer, and upper cladding. Their thickness, defined shape, and refractive index should be tailored carefully to provided single-mode confinement in the vertical and the lateral direction. For rib waveguides, the dimension of the rib is clearly the most important determinant in meeting a desired mode shape. In the following simulation and discussion, the parameters are chosen as follows: free-space wavelength = 650 nm, refractive index of core layer n1 = 1.5024 and that of P(MMA-GMA) under/upper cladding layer n2 = 1.4998, respectively. The relations based on the eigenvalue equations between the core thickness b and mode effective refractive indices Neff of the rib waveguide is shown in Fig. 6, where the waveguide width a = 0.8b and the rib height h = 0.6b. In order to realize single-model propagation of the mode in the rib waveguide, b = 4 µm (a = 3.2 µm, h = 2.4 µm) was chosen as the total core thickness. The optical field distribution calculated by the beam propagation method (BPM) is displayed in the inset of Fig. 6. From the inset, we can see that the optical filed is effectively confined in the core waveguide region.
Fig. 6 Relations between core thickness b and effective refractive indices Neff of the rib waveguide with a = 0.8b and h = 0.6b. The inset shows optical field distribution calculated by the beam propagation method (BPM).
When an electrical power is supplied to the thin-film heater to induce thermal energy, temperature variation changes the effective index of the polymer waveguide mode due to the TO effect, and then changes the optical path length in the heated waveguide relative to another waveguide. The induced phase shift in the heated arm will affect the phase interference in the output 3-dB directional coupler. Consequently, optical power is redistributed between the two output ports. The steady-state thermal distribution was obtained based on thermal profile change in the activity waveguide cross-section, as shown in Fig. 5(c). Depending on the phase change caused by external thermal field, two push-pull modulated output signals can be available.
4. Device fabrication, measurement and discussion
4.1 Fabrication
The 1 × 2 polymer switch was fabricated using P(MMA-GMA)-based material, commonly used for various applications. Firstly, 3 µm-thick P(MMA-GMA) layer was spin-coated on a Si substrate and baked at 120 °C for 3 h to form the under cladding. Then 4 µm-thick P(MMA-GMA) with Bis-A-epoxy layer (n = 1.5024@650 nm) was spin-coated on the under cladding to form the core layer, followed by thermal annealing at 120 °C for 3 h. Next, an Al film was evaporated on it. Then a group of 3.2 µm-wide and 2.4 µm-height rib waveguides were fabricated in the core layer by standard photolithography and inductively coupled plasma (ICP) etching technology. Finally, after the Al mask was removed off from the waveguides, P(MMA-GMA) was spin-coated to cover them to form the upper cladding. The scanning electron microscope (SEM) image of the rib waveguide without upper cladding is shown in Fig. 7. After successfully fabricating the waveguides, 100 nm-thick Al layer was deposited by thermal evaporating. Then, photoresist BP212 was spin-coated on the Al film and baked at 80 °C for 30 min. The 10 µm-width and 10 mm-long heater was patterned using conventional photolithography and completed by a wet etching process.
4.2 Switching performances
The fabricated TO switch was characterized by optical transmission measurements. The measuring process of the device was shown in Fig. 8(a). A light beam (at 650 nm) generated by a solid-state laser was directly coupled into the input port of the switch through a customized small-diameter polymer optical fiber. The output light was detected using a photodetector and measured by a power meter. Electrical power was delivered to one arm of the interferometer via the on-chip Al heater contact pads. The heated arm was on the same side of the switch as the output port labeled port 1.
Fig. 8 (a) The measuring process of the device; (b) The relative output patterns of the device without electrical power applied; (c) The relative output patterns of the device with switching power applied
For the optical measurement, the output light from the switch was firstly focused using a microscope objective lens, which images the output patterns, and captured by a charge-coupled device (CCD) camera. As shown in Fig. 8(b), when no electrical power was applied on the device, the relative patterns were displayed on a video monitor. Figure 8(c) shows the relative output patterns of the device with switching power applied. In order to measure the insertion loss of the device, the light beam was coupled directly into the optical power meter by a similar polymer optical fiber. The fiber-to-fiber insertion loss of the tested sample with 3.6 cm length was about 8.7 dB, including the coupling loss, propagation loss and excess loss. At the same time, the propagation loss of the device was also measured by cut-back method. To increase the accuracy of the measurement, several waveguides were measured at each length. And the device had a propagation loss of about 0.92 dB/cm at 650 nm wavelength. When the driving power was applied on the heater, the relative output powers from two output ports were plotted in Fig. 9. The measured power to switch from minimum to maximum output optical power on port 1 was 5.3 mW. The measured extinction ratio was 23.4 dB. The corresponding change in the temperature of the heater/polymer interface is approximately 1.63 °C. To measure the dynamic characteristics, a square-wave signal with 200 Hz was applied to the electrode with two needle-like probes, and the output power was coupled into a photodiode detector through a polymer optical fiber. The driving voltage of the switch and the detected optical response were simultaneously observed on an oscilloscope (4104B, Tektronix), as illustrated in Fig. 10. From Fig. 10, the upper trace is the square wave form of the switching voltage source, and the lower traces are the switching response from the two outputs. The light from the two ouputs are both modulated but with the modulation 180° out of phase. Figure 10(a) shows the switching response from port 1, the modulated phase is consistent with that of the applied modulation signal, and the rise time and fall time are 464.4 and 448.0 µs, respectively. However, the switching response from port 2 has the opposite modulation phase, as shown in Fig. 10(b). And the measured rise time and fall time for port 2 of the switch are 464.2 and 411.4 µs, respectively.
As a comparison, the similar TO switch, with under cladding of silica instead of polymer, was also fabricated and measured under the same condition. The TO switch with under cladding of silica has similar insertion loss and extinction ratio as that with under cladding of polymer. But the response time could be reduced to be about 260 µs. The measured switching power, however, was 12.3 mW, which was higher than that of the switch with under cladding of polymer. In order to improve the performances of the TO switch, including fast response time and low switching power, the silica/polymer hybrid and air trench waveguide structures could be corporately introduced into the design and fabrication of the TO switch devices.
In order to examine the stability of the performances of the fabricated TO switch, the temperature cycling test was introduced. For polymeric TO devices, the main effect is the coefficient of thermal expansion mismatches between the waveguide core, clads, heating electrode, and substrate, and this may affect the thermal stability of the device. Due to a larger difference of the thermal expansion coefficient between the polymer and silicon substrate, a level of stress in the polymer core layer is generally undesirable as it may result in unexpected formation of delamination and cracks during the temperature changing. However, the polymer exhibits good performance in flexibility and resilience, which could efficiently ease the effect of thermal expansion mismatch. For the experimental verification, the device underwent the temperature variation form 25 °C to 130 °C and no cracks were found through SEM characterization, and there was also no loss of adhesion observed between the heating electrode and polymer stack. The characteristics of the device were also re-performed and no decay was found, when the temperature varied from 25 °C to 80 °C for four heating cycles. These results demonstrate that the mismatch of thermal expansion has almost no effect on thermal stability of the performances of the device below 80 °C.
5. Conclusion
In order to develop polymer waveguide devices in POF visible communication system applications, a low-power 1 × 2 polymeric TO switch operating at the POF low-loss window of 650 nm was designed and fabricated. The device was fabricated based on PMMA-based materials with MZI structure. The P(MMA-GMA) material was successfully synthesized with relative low absorption loss at visible wavelengths and a glass transition temperature of 115 °C. The characteristic parameters of the switch were carefully designed and simulated, and the fabrication was done using standard semiconductor fabrication techniques. At 650 nm, measurements of the fabricated device demonstrate a low switching power of 5.3 mW, a fast response time (rise time and fall time are 464.4 and 448.0 µs, respectively), an extinction ratio of 23.4 dB, and a device insertion loss approximately equal to 8.7 dB. The reported switch with balanced output signals could be applied in optical communications and sensing systems.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos. 61177027, 61107019, 61205032, and 61261130586), and the Science and Technology Development Plan of Jilin Province (No. 20140519006JH).
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