Pamidighantam V. Ramana

Compact SOI nanowire refractive index sensor using phase shifted Bragg grating

The phase shifted vertical side wall gratings are designed and numerically simulated on a submicron SOI waveguide to obtain the performance characteristics needed for an integrated refractive index sensor. The gratings are designed to obtain narrow band width, high transmittivity and sharp line shape in the resonant transmission so that the sensor sensitivity can be improved. The proposed sensor is easy to fabricate and will provide a linear response over a wide wavelength range with a compact structure dimension which is suitable for label free biosensing applications. The detection limit of the sensor is investigated through both wavelength shift and intensity measurement method and the performance parameter is compared with other silicon based structures like Mach-Zehnder interferometer, ring resonator and surface corrugated Bragg grating.

Demonstration of Direct Coupled Optical/Electrical Circuit Board

We report the development of a low cost, simple optical/electrical circuit board (OECB) using multimode polymer waveguide on FR4 printed circuit board (PCB). The design of this OECB uses only a 45deg-ended waveguide to couple and decouple the optical signal directly between the optical devices and the waveguide. The 45deg mirror is formed using excimer laser process on a multimode waveguide with temperature stability at reflow temperature. The optical waveguide is attached to a diced channel in the FR4 PCB using adhesive to form a completely planar circuit. This allows the laser diode and the photodiode to be assembled directly above the input and output of the waveguide using precision flip chip technology, which provides good alignment accuracy. This helps to increase the mechanical reliability of the circuit and minimize assembly requirements. Most importantly, all the electronic and optoelectronic devices used are commercially available components. In the paper, we report the details of the design, simulation result, and the testing results.

Demonstration of High Frequency Data Link on FR4 Printed Circuit Board Using Optical Waveguides

We report the development of a low cost, simple optical circuit board (OECB) using large core multi mode polymer waveguide on FR4 printed circuit board (PCB). The design of this OECB uses only a 45deg-ended waveguide to couple and decouple the optical signal directly between the optical devices and the waveguide. The 45deg mirror is formed using excimer laser process on a multi mode waveguide with temperature stability at reflow temperature. The optical waveguide is attached to a diced channel in the FR4 PCB using adhesive to form a completely planar circuit. This allows the laser diode and the photodiode to be assembled directly above the input and output of the waveguide using precision flip chip technology, which provides good alignment accuracy. This helps to increase the mechanical reliability of the circuit and minimize assembly requirements. Most importantly, all the electronic and optoelectronic devices used are commercially available components, the FR4 PCB fabrication process is standard process, the component assembly process is normal reflow process and the method is amenable for manufacturing. In the present publication, we report the details of the design, assembly process, high accuracy optical device attachment, performance characterization and testing results.

Development of Optical Subassembly for Plastic Optical Fiber Transceiver in High-Speed Applications

A low-cost optical subassembly design for large core fiber transceiver is presented. The complete transceiver module is realized by assembling the low-cost optical subassembly directly on the transceiver functional printed circuit board. -7 dBm optical power output and 6 dB extinction ratio are achieved on transmitter by the vertical self-alignment and horizontal passive alignment at the transmitter. 190 m link distance is achieved at 2.5 Gb/s and 340 m link distance is achieved at 1.25 Gb/s for the plastic optic fiber (POF) transmitter with grade index plastic optical fiber. 145 m link distance is achieved at 2.5 Gb/s for the POF receiver.

Optical Coupling Methods for Cost-Effective Polymer Optical Fiber Communication

In this paper, we report cost-effective light coupling methods for polymer optical fiber (POF) communication. Here, we compare the various optical coupling schemes in detail. By optical simulations, we analyze the conventional light coupling schemes, namely the direct coupling, lens coupling, and lensed fiber coupling. The simulation studies reveal that a lensed fiber tip particularly at the receiver side improves the light coupling efficiency to a great extent. The optimized lensed POF design confers an 85% coupling efficiency. Lensed POFs are realized with two low-cost fabrication methods. The characterization of the lensed POF are carried out to evaluate the lensing properties and hence to optimize the fabrication process.

Design and Analysis of 3D Stacked Optoelectronics on Optical Printed Circuit Boards

In this study, a low-cost (with bare chips) and high (optical, electrical, and thermal) performance optoelectronic system with a data rate of 10Gbps is designed and analyzed. This system consists of a rigid printed circuit board (PCB) made of FR4 material with an optical polymer waveguide, a vertical cavity surface emitted laser (VCSEL), a driver chip, a 16:1 serializer, a photo-diode detector, a Trans-Impedance Amplifier (TIA), a 1:16 deserializer, and heat spreaders. The bare VCSEL, driver chip, and serializer chip are stacked with wire bonds and then solder jointed on one end of the optical polymer waveguide on the PCB via Cu posts. Similarly, the bare photo-diode detector, TIA chip, and deserializer chip are stacked with wire bonds and then solder jointed on the other end of the waveguide on the PCB via Cu posts. Because the devices in the 3D stacking system are made with different materials, the stresses due to the thermal expansion mismatch among various parts of the system are determined.

Broadband tunable bandpass filters using phase shifted vertical side wall grating in a submicrometer silicon-on-insulator waveguide

We propose the silicon-on-insulator (SOI) based, phase shifted vertical side wall grating as a resonant transmission filter suitable for dense wavelength division multiplexing (DWDM) communication channels with 100 GHz channel spacing. The gratings are designed and numerically simulated to obtain a minimum loss in the resonant cavity by adjusting the grating parameters so that a high transmittivity can be achieved for the resonant transmission. The resonant grating, which is designed to operate in the DWDM International Telecommunication Union (ITU) grid C band of optical communication, has a high free spectral range of 51.7 nm and a narrow band resonant transmission. The wavelength selectivity of the filter is improved through a coupled cavity configuration by applying two phase shifts to the gratings. The observed channel band width and channel isolation of the resonant transmission filter are good and in agreement with the ITU specifications.

Bi-directional optical communication at 10 Gb/s on FR4 PCB using reflow solderable SMT transceiver

We report the development of 10 Gbps bi-directional optical data communications on FR4 PCB to realize opto-electronic circuit board (OECB). The work includes the design, fabrication and high-speed performance of a 10 Gbps surface mount optical transceiver module that can be reflow soldered, developing efficient optical coupling methods using precise injection molded lenses and studying their attachment methods to the optical source and receiver, realizing efficient optical coupling between transmitter and receiver subsystems for a multi mode epoxy waveguide on the PCB and developing a test vehicle. The compact and highly integrated SMT optical transceiver module in multilayer LTCC substrate consists of optical devices like VCSEL and PIN photo diode and the electrical circuits like VCSEL driver, trans impedance amplifier (TiA) and limiting amplifier (LA). The module contains a cavity, in which the electrical, optical devices and passives are mounted using passive attachment methods, a recess to house a lens and locator holes to align the lens with the VCSEL and PIN. The lens is an injection molded epoxy lens with biconvex elements for both transmitter and receiver integrated as a single element to improve the coupling between the source and the waveguide on the PCB. The module is designed to relax the assembly tolerances of optical elements so that normal conventional electronic assembly process can be used. Solder bumps are formed on the bottom layer of the LTCC to convert the module into a cavity down SMT package. The waveguides are made of high temperature epoxy material and are attached to the PCB using adhesive material. The 45deg mirrors are formed through laser ablation process. The module is reflow soldered using conventional reflow oven. The opto-electronic circuit board (OECB) is performance evaluated and performance results are presented.

Design and Characterization of Taper Coupler for Effective Laser and Single-Mode Fiber Coupling With Large Tolerance

A new method of coupling the light from a laser diode (LD) to a single-mode fiber with large alignment tolerances and without using any coupling lens is presented. A pseudovertical tapered coupler on silicon substrate which has an input aperture of about 100 times the size of the laser waveguide cross section is designed. Results showed that the coupler relaxes the LD placement tolerance and eliminates the use of a coupling lens. The positional tolerance between the LD and taper coupler can be larger than +/- 5 mu m in the xy plane, and +/-1 deg in orientation.

Design and development of optoelectronic mixed signal system-on-package (SOP)

This paper describes the design and development of a 2.5-Gb/s optical transceiver module as a mixed signal SOP for access networks. The module development consists of concurrent design of an optoelectronic package optimizing optical, electrical, thermal, mechanical functions and optical subassembly and RFICs housed in a chip-on-board package. The optical subassembly (OSA) consists of laser and photodiode assembled on a silicon substrate. The transmit and the receiver sections are combined into a single fiber through a polymer coupler on silicon. The splitter between the transmit and receive section is realized using a polymer waveguide. The electronic ICs are assembled on a multilayer organic substrate. The package design includes optical coupling design, impedance matched transmission line design for RF signals, electrical layout design for mixed signals and thermal design for the package. The module is housed in a plastic molded nonhermetic package to achieve low cost packaging. The assembly is completed using passive alignment of optical devices and attachment of electronic devices using adhesives. In this paper, we present the details of the component design and the development of packaging process methods to achieve the design specifications, test results and process guidelines for assembly and integration.