1. Introduction

Complimentary Metal-Oxide Semiconductor (CMOS) camera sensors have gained popularity in recent years due to advances in multi-functionalization, low manufacturing costs, and low power consumption [1,2]. The need to transport real-time, high resolution digital video from such camera sources is growing for many types of applications, especially in Advanced Driver Assistance Systems (ADAS) and autonomous drive systems [3]. Considering the necessities of CAM data transport, large volumes of data must be ingested and processed under short timeframes. This means faster optical interconnects and processors with wider bandwidth properties are highly desirable for short reach link applications. However, the transmission of high volume multimedia data streams is a challenge for the limited bandwidth and low energy supplies of camera sensor networks [4]. At the same time, the chip-scale electronic–photonic systems enabled a higher bandwidth and lower power consumption compared to conventional data transport using electrical wires, especially for the short-reach optical communications systems [5–7]. A major challenge to further increase the bandwidth in optical telecommunications is to encode electronic signals onto a light wave carrier by modulating the light at high speeds. This is usually done with an electro-optic (EO) modulator [8–18]. As a result, an electro-optic modulator with a low driving voltage and a high transmission speed for uncompressed video data is required as a cost effective solution for camera sensor platforms. Considering this issue, optical-fiber interconnected CMOS platform integrated low-loss and high-speed EO modulator components with a high resolution CMOS camera has been proposed.

Among all the EO materials currently utilized in high speed EO modulators, low cost organic EO polymers offer various intrinsic advantages [5–7]. As a consequence of a low dielectric constant, the polymer based electro-optical modulators are more advantageous for broadband operation over other optimal modulators based on silicon, lithium niobate (LiNbO₃) or indium phosphide (InP) [8–13]. EO polymers can be engineered to have high EO coefficients, γ₃₃, which is advantageous for sub-volt half-wave switching voltage (V_π) [8]. For reference, silicon-organic-hybrid (SOH) EO Mach-Zehnder modulators have been demonstrated with the π-voltage−length product (V_πL) down to 0.5 V·mm [8], which is more than an order of magnitude below that of pn-depletion type silicon and InP devices [10,11] and 3 orders of magnitude lower than conventional LiNbO₃ modulators [12]. To date, compact high-speed electro-optic modulators operating over 100GHz have been developed [13]. Subsequently, EO modulators which are capable of providing OOK data rates of 100 Gbit/s have also been demonstrated [14,15]. The tremendous improvements demonstrate the unprecedented performance of SOH based EO modulators to realize high-speed energy efficient transceivers for short-reach transmission. Nevertheless, photo-stability and thermal-stability have been important milestones for the acceptance of organic polymer based EO devices in industrial applications. As a solution, the photo-stability can be addressed by packaging the device in an oxygen-free atmosphere and by designing the polymers to be less sensitive to damage by singlet oxygen [16]. While, a thermal stability tests of the EO polymer modulators at 85°C and even at 105 °C for 2000 h in our previous work have revealed high thermal resistance which meets the materials requirements for industrial optoelectronic applications including Telecordia GR468-CORE [17].

Considering the easy fabrication and integrated possibility, an inverted ridge EO polymer modulator with a 1 µm-thick EO polymer core and a V_πL of 1.44 V·cm under TM mode operation at wavelength of 1550 nm was demonstrated in this work. The optimization of the EO polymer modulator for enhanced velocity matching between the optical and RF signals to achieve a wider bandwidth operation is also discussed. The modulator showed a 3-dB modulation bandwidth of 20 GHz and clear eye diagrams at 32 Gbit/s under drive voltages of 1.5 V_pp. The measured Q factor amounted to 10.3. A data rate of 1 Gbit/s video data transmission through optical fibers was successfully demonstrated on a single CMOS chip integrated to a compact EO polymer modulator, external modulation driver and a high-vision CMOS camera sensor. This highly thermally stable, low cost, high speed operation EO polymer modulator can enable a photonic integrated camera sensor system as a reliable processing platform for real-time data processing.

2. EO modulator fabrication and measurements

The environmental stability of polymer modulators is always an important issue to be investigated. Clearly EO polymers with enhanced thermo-physical properties in terms of high glass transition temperatures (T_g) and decomposition temperatures (T_d) are preferable to assure a good temporal stability even at elevated temperatures. Compared to conventional host-guest EO polymers with a T_g around 120-145°C or lower, we developed a side-chain EO polymer where the active chromophore molecules are chemically attached to the polymer backbone to achieve a better thermal stability [17,18]. The polymer contains an adamantyl unit to enhance the thermal property and a phenyl vinylene thiophene chromophore to impart the EO activity. The synthetic procedure was described in our previous work [17]. The measured T_g of the EO polymer in this work was 162°C, which can be further raised to 194°C by increasing the mole fraction of the adamantyl units onto the polymer backbone [17]. The obtained high T_g is key to enhance the thermal stability of the polymer modulators. A standard inverted-rib structure waveguide consists of a three-layer dielectric stack which was fabricated as shown in Fig. 1(a). The cladding layers of SiO₂ are formed by spin-coating a solution of trimethoxysilyl sol-gel derivatives and heating for 120-140°C for 3 hours. The inverted waveguide structure was patterned by conventional photolithography and dry etching techniques such as a Mach-Zehnder interferometer with two 200 μm-wide split arms. By selecting a width of 4.0 μm and a depth of 1.0 μm for the trench, a single TM mode optical transmission waveguide was obtained under 1550 nm wavelength. The EO polymer was spin-coated onto an inverted patterned sol-gel layer by using cyclopentanone as the solvent, and dried at 95°C for 12 hours under vacuum to form an inverted ridge waveguide with a 1 μm-thick slab.

 

Fig. 1 (a) Schematic of the EO polymer modulator layers with the travelling wave electrode design, the polymer layers were spin-coated on to a sol-gel cladding layer. (b) The simulation result of the characteristic 3dB bandwidth model behavior and impedance of the microstrip line over the electrode width range of 10–100 μm. The characteristic impedance (at 20 μm) is matched with 50 Ω along the GSG transition direction.

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To achieve a high speed operation in the electro-optic device, a traveling-wave electrode structure which can enable the optical wave and RF wave to travel co-directionally at similar speeds is required [18]. In addition, the electrode and waveguide structures were carefully designed for optimal velocity matching between the optical and RF signals, which is proportional to the group effective indexes difference of ΔN = N_g^m- N_g^o, where N_g^m and N_g^o are the group effective refractive index of the transmission line and the optical waveguide, respectively [19]. An EO polymer with a dielectric constant of 2.5-4.0 and a refractive index of 1.6-1.7 offers the intrinsic benefit of an enhanced velocity match. The frequency dependent N_g^m of 1.37 over the range of 10-250 GHz and a waveguide dimension dependent N_g^o of 1.68 under the structure shown in Fig. 1(a) were numerically calculated using CST microwave studio (MWS). The optical waveguide is under a uniform modulation field between the electrodes, and hence, the overlap integral between the optical mode and the RF modulation field was maximized. As a result, group indices of around 0.3 have only a minor impact on the frequency response of a 8 mm long device in our experiment. Considering the 3-dB modulation bandwidth limited by a velocity mismatch was found to be 𝑓_3dB^VM = 2c/(πL| N_g^o - N_g^m |) [19], where c is the speed of light in vacuum and L is the interaction length. We can theoretically calculate the bandwidth corresponding to various modulation frequencies. The characteristic impedance of the transmission line over the electrode width range of 10–100 μm was also calculated by CST MWS. Both cases are shown in Fig. 1(b). Considering that the characteristic impedance is matched with 50 Ω along the transition direction when the width of the top electrode is 20 μm, then we can theoretically expect a high performance EO polymer modulator with the bandwidth of 85 GHz. In our case, a 2 μm-thick, 20 μm-wide, and 8 mm-long top traveling-wave gold transmission line was formed by vacuum deposition and electroplating techniques. The terminals of the traveling-wave electrode were designed by considering the literature’s geometry for the smooth transformation of the electric field of GSG-coplanar electrode to the microstrip transmission line. The detailed fabrication process can be found in previously published work [18]. A maximum poling field and temperature of 50 V/μm and 158 °C was applied to the device, the dipoles of the chromophore were aligned in the direction of the electric field, which is also parallel to the optical field of the TM mode. An in-device EO coefficient of around 100 pm/V had been tested and published in our past work [18].

The frequency response of the polymer modulator was characterized by measuring the S₂₁ parameter. The TM polarized light from a 1550 nm laser (81689A, Agilent) was coupled into the modulator by a polarization maintaining lensed fiber. The output light was collected by a single mode lensed fiber and detected by a photo detector (PDA10CS, Thorlabs). As a consequence, a low V_π•L of 1.44 V•cm was recorded in the device. A further reduction in the V_π can be anticipated in our future work. For a small signal modulation, the RF signal was generated by a vector network analyzer (VNA MS4644B, Anritsu) and fed into and out of the tapered GSG electrodes through pico-probes (40A-GSG-250-DS, GGB Industries Inc.). Each electrical transmission line in our work features an impedance close to 50 Ω and is terminated with an external 50 Ω resistor to avoid back reflections of the RF field at the end of the transmission line. The measurement system was calibrated using an automatic VNA calibrator (36585 Auto Cal, Anritsu). The measured normalized EO response of the modulator as a function of the RF frequency over 1–40 GHz is shown in Fig. 2(a). The signal intensities were normalized at 1 GHz. The measured 3 dB modulation bandwidth of the hybrid modulator was 20 GHz, and a 10 dB reduction was found in response at 40 GHz. This bandwidth is limited by the relatively low conductivity of the poorly electroplated gold electrode, which may be further enhanced by improving the electroplating quality. To demonstrate the viability of the polymer modulator for high-speed data transmission, we explored the potential of a simple non-return-to-zero (NRZ) on-off-keying (OOK) as a modulation format. The electrical NRZ drive signals were generated and operated at a data rates up to 32 Gbit/s in this experiment. The modulated light was amplified by an erbium doped fiber amplifier (EDFA), filtered using a band pass filter (BP), and detected by a high-speed photodiode (PD) connected to a real-time oscilloscope for recording eye diagrams. A clear 32 Gbit/s optical eye with a peak-to-peak drive voltage of 1.5 V_pp after equalization is depicted in Fig. 2(b). From the eye diagram we extracted the quality factor (Q-factor) of 10.3. The efficiency of our modulator and the possibility to exploit large-scale silicon photonic integration meets the need to realize compact and technologically simple high-speed short reach interconnects. All of these properties can be attributed to the EO modulators as an effective solution for real-time video data transmission under wide bandwidth.

 

Fig. 2 (a) The measured normalized EO response of the modulator as a function of RF frequency in a small signal modulation test. The 3-dB modulation bandwidth was measured as 20 GHz. (b) The measured eye diagram at 32 Gbit/s was obtained. Peak-to-peak drive voltages of 1.5 V_pp was sufficient to generate a high-quality optical signal. The measured Q factor amounted to 10.3.

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3. Video data transmission setup and measurements

A compact and cost-effective route to build electrically as well as optically controlled modulators on the well-established silicon-based complementary metal–oxide–semiconductor (CMOS) platform has been demonstrated. The technology enables real time video data transmission with a frequency of 1 GHz, which is mainly limited by the frame rate of the camera. The experimental setup and corresponding schematic figure of the transmission testing system are shown in Fig. 3(a) and 3(e). TM polarized light from a 1550 nm laser was edge coupled into the modulator by a cleaved polarization maintaining fiber. The output light was collected by a single mode fiber and transmitted and detected by a small form-factor pluggable transceiver (SFP + ) to convert optical signals to video signals. The CMOS camera (back-side) and modulation module (front-side) were compact and integrated on a 50mm x 40mm printed circuit board to collect and modulate the video data, the details are shown in Fig. 3(b)-3(d). The enlarged detail of high resolution CMOS camera is depicted in Fig. 3(b). The external modulation module consists of an external modulator driver of MAX3942 (Maxim Integrated) and the EO polymer modulator, the module is depicted in Fig. 3(c). The MAX3942 offers us a data rate up to 10.7 Gbps operation and a differential modulation voltage up to 2.0 V_pp while each of the differential outputs has an on-chip 50Ω resistor for back termination, which is sufficient to effectively drive the EO polymer modulator. The MAX3942 also includes an adjustable pulse-width control circuit to pre-compensate for asymmetrical modulator characteristics. Considering the CMOS camera sensor utilized in our testing showed a 1 GHz limitation for data collecting, a 1 GHz uncompressed video data transmission through the optical fiber using the EO polymer modulator module and modulator has been demonstrated. The final video image from the monitor shows a perfectly formed instant real time display without time delay. Based on the performance, we believe such video images can be processed by sophisticated algorithms in real time to provide feedback to many applications, especially safety control systems within autonomous driving vehicles which require high speed signal processing to make a decision on the occupant’s position. We also believe that a higher speed real time display with higher resolution video data processing under wider bandwidth can be further achieved using a better camera.

 

Fig. 3 (a). The schematic demonstration of video data transmission experimental system. (b) The CMOS camera with 1 Gbps rate integrated on the backside of CMOS chip. (c) The demonstration of modulation module consisted of the bare modulator and external modulator drive on the front side of CMOS chip. (d) The demonstration of how the on-chip modulation module and camera collect the data of a moving target object. (e) The actual demonstration of experimental system.

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4. Conclusion

In this work, we have validated a conventional inverted ridge EO polymer modulator with a low-driving voltage and large bandwidth for real-time video data processing applications. The velocity matching condition between the optical and RF signals of the EO polymer modulator is well designed to achieve a wider bandwidth operation. The modulator exhibited a π-voltage-length product of 1.44 V·cm under TM mode operation at a wavelength of 1550 nm. The EO polymer modulator exhibited an excellent thermal resistance at 85°C for 2000 h. The modulator showed a 3-dB modulation bandwidth of 20 GHz and clear eye diagrams at 32 Gbit/s under drive voltages of 1.5 V_pp. The Q factor was found to be 10.3. A high-vision CMOS camera sensor and CMOS compatible EO polymer modulator with an external on-chip modulation driver were positioned onto a CMOS chip. A data rate of 1 Gbit/s video data transmission through the optical fiber was successfully demonstrated using this modulation module without significant time delay. High thermal stability, low cost, high operating speed EO polymer modulators can facilitate a photonic integrated camera sensor system as a reliable processing platform for real-time data processing on board, especially in the ADAS and autonomous driving system.