We present performance improvement in InP travelingwave modulators (TWMs) using a periodic ion implantation on coplanar waveguide. Fabricated TWMs show the successful velocity and impedance tuning with return loss of lower than
Key words: traveling-wave modulator; benzocyclobutene (BCB); coplanar waveguide (CPW); ion implantation; microwave refractive index
INTRODUCTIONHigh-speed optical modulators are critical components in fiberoptic RF signal transmission, high-bit-rate lightwave systems, and optoelectronic signal processing. LiNbO 3 modulators have been widely used for these applications [1]. However, semiconductor modulators possess great potential for monolithic integration with active devices (such as lasers and optical amplifiers) [2,3] and passive devices [4] to create compact, robust, and functional photonic micro-systems. Figure 1 shows such a monolithic integrated InP optical arbitrary waveform generation (OAWG) chip which includes 10 amplitude and phase modulators, two arrayed waveguide gratings (AWGs), and semiconductor optical amplifiers [5]. Here, a de-mux AWG splits the wavelengths of an input short pulse into 10 different waveguides, and then, each wavelength goes through an electro-absorption-based amplitude modulator and an electro-optic-based phase modulator. Afterwards, the 10 wavelengths are multiplexed onto a single waveguide. User-specified optical waveform is generated by amplitude and phase modulation between two AWGs. In this application, high-speed amplitude and phase modulators are critical components to create truly arbitrary waveforms at an extremely broad bandwidth. Two types of modulators are commonly used on semiconductor materials: one based on a Schottky electrode configuration [6,7] and the other based on a p-i-n structure. The Schottky modulator configuration shows ultra-high speed responses but is difficult to integrate with other photonic active devices on the same semiconductor substrate. Thus, the p-i-n modulator is the typical structure of choice for photonic microsystems [2,3,8,9]. However, this structure is plagued by the thin intrinsic layer, between p-type and n-type cladding used for strong electric fields, creating a large capacitance. This large capacitance results to a low characteristic impedance (typically around 20 ⍀-30 ⍀) generating large signal reflections to common 50-⍀ terminated microwave systems. Furthermore, the microwave velocity is typically slower than the optical velocity. For long, traveling-wave modulators, this velocity mismatch becomes the bottleneck towards higher speed modulating for low voltage operation. In this article, we report a new approach to tune both the microwave velocity and characteristic impedance of TWMs, which is a step toward high performance monolithic photonic microsystems with RF devices. This is achieved by periodically implanting He ϩ ions in p-type layer, providing the designer another option to obtain desired microwave characteristics. InP/InGaAsP travelingwave Mach-Zehnder electrooptic modulators have been d...