Demonstration of a 120° hybrid based simplified coherent receiver on SOI for high speed PON applications.

We demonstrate the first simplified coherent receiver using a 120° hybrid on silicon-on-insulator (SOI) for high speed PON applications. This coherent receiver integrates an inverse taper edge coupler for the received signal, a vertical grating coupler for the local oscillator input, a polarization splitter and rotator (PSR), a 120° hybrid based on a 3×3 multimode interference (MMI) coupler, and three germanium photodetectors. We achieved 25 Gbit/s two-level pulse amplitude modulation (PAM-2) transmission over 30 km standard single mode fiber (SMF) in the C-band without any digital signal processing (DSP) (e.g., pre-emphasis, pulse shaping, equalization, nonlinearity compensation) and dispersion compensation (e.g., optical or digital) either at the transmitter or at the receiver. The requirements for frequency and phase locking of the local oscillator (LO) were avoided due to the use of intensity modulated signals. Receiver sensitivities of -23.70 dBm, -20.30 dBm, and -15.10 dBm are achieved at a bit error rate (BER) below the hard-decision forward error correction (HD-FEC) threshold (i.e., 3.8 × 10-3) in back-to-back (B2B), after 21 km and 30 km, respectively. We also demonstrate 25 Gbit/s PAM-4 transmission achieving receiver sensitivities of -15.30 dBm, -13.90 dBm, and -9.50 dBm below the HD-FEC threshold in B2B, after 10.5 km and 21 km, respectively.

Six-Degrees-of-Freedom Alignment of Two-Dimensional Array Components by Use of Off-Axis Linear Fresnel Zone Plates.

A novel six-degrees-of-freedom (six-DOFs) alignment technique for assembling two-dimensional array components is presented. The technique uses off-axis linear Fresnel zone plates on one component that are combined with alignment targets on the other. The technique is compact and sensitive to all six DOFs; it was used to package an array of microlenses with a 32 x 32 array of GaAs multiple-quantum-well modulators flip-chip bonded to a 9 mm x 9 mm complementary-metal-oxide-semiconductor chip. By use of interference fringes to control the tilt misalignment, the worst-case misalignment of the microlenses relative to the chip is calculated to be as follows: lateral = 3.0 mum, rotational = 0.023 degrees , longitudinal = 13 mum, and tilt = 0.022 degrees . We also propose alternative implementations of the technique, including one that uses on-chip photodetectors to automate this six-DOF alignment technique.

Interconnection of a two-dimensional array of vertical-cavity surface-emitting lasers to a receiver array by means of a fiber image guide.

The implementation of a 10-channel parallel optical interconnect consisting of a two-dimensional array of vertical-cavity surface-emitting lasers, a 1.35-m fiber image guide, and a metal-semiconductor-metal receiver array is described. Transmission rates of 250 Mbits/s per channel are demonstrated with an optical cross talk of less than -27 dB and a loss of -3 dB. Coupling issues associated with image guides are analyzed and discussed.

Design, implementation, and characterization of a kinematically aligned, cascaded spot- array generator for a modulator-based free-space optical interconnect.

The design and the implementation of a modular spot-array generator for a modulator-based free-space optical interconnect is presented. Two cascaded diffractive optical elements produce 4 x 8 clusters on a 1600 microm x 800 microm pitch, where each cluster is a 4 x 4 array of (1/e(2)) 13.1-microm-radius spots on a 90-microm pitch. The spot-array generator is kinematically aligned to the interconnect system such that no realignment is necessary between removal and reinsertion. Characterization results are presented.

Design, implementation, and characterization of a hybrid optical interconnect for a four-stage free-space optical backplane demonstrator.

A four-stage unidirectional ring free-space optical interconnect system was designed, analyzed, implemented, and characterized. The optical system was used within a complementary metal-oxide semiconductor-self-electro-optic-effect-device-based optical backplane demonstrator that was designed to fit into a standard VME chassis. This optical interconnect was a hybrid microlens-macrolens system, in which the microlens relays were arranged in a maximum lens-to-waist configuration to route the optical beams from the optical power supply to the transceiver arrays, while the macrolens optical relays were arranged in a telecentric configuration to route optical signal beams from stage to stage. The following aspects of the optical system design are discussed: the optical parameters for the hybrid optical system, the image mapping of the two-dimensional array of optical beams from stage to stage, the alignment tolerance of the hybrid relay system, and the power budget of the overall optical interconnect. The implementation of the optical system, including the characterization of optical components, subsystem prealignment, and final system assembly, is presented. The two-dimensional array of beams for the stage-to-stage interconnect was adjusted with a rotational error of <0.05 degrees and a lateral offset error of <3.5 mum. The measured throughput is in good agreement with the lower-bound predictions obtained in the theoretical results, with an optical power throughput of -20.2 dB from the fiber input of the optical power supply to the modulator array and -25.5 dB from the fiber input to the detector plane.

Design and operation of an in situ microchannel alignment-detection system.

The design of an alignment-detection system that uses off-axis diffractive elements and photodetectors is presented. The system was developed to detect the real-time misalignment of an array of optical beams as they pass through a microchannel relay. The design of this scheme is presented along with experimental results obtained from a prototype detection system.

Design and characterization of a microchannel optical interconnect for optical backplanes.

The design, modeling, and experimental characterization of a microchannel-based free-space optical interconnect is described. The microchannel interconnect was used to implement a representative portion of an optical backplane that was based on field-effect transistor, self-electro-optic device smart-pixel transceivers. Telecentric relays were used to form the optical interconnect, and two modes based on two different optical window clusterings were implemented. The optical system design, including the optical geometry for different degrees of clustering of windows supported by a lenslet relay and the image mapping associated with a free-space optical system, is described. A comparison of the optical beam properties at the device planes, including the spot size and power uniformity of the spot array, as well as the effects of clipping and misalignment for the different operating modes, is presented. In addition, the effects of beam clipping and misalignment for the different operating modes is presented. We show that microchannel free-space optical interconnects based on a window-clustering scheme significantly increase the connection density. A connection density of 2222 connections/cm(2) was achieved for this prototype system with 2 x 2 window clustering.

Optomechanics for a four-stage hybrid-self-electro-optic-device-based free-space optical backplane.

We present the design, fabrication, and testing of optomechanics for a free-space optical backplane mounted in a standard 6U VME backplane chassis. The optomechanics implement an optical interconnect consisting of lenslet-to-lenslet, as well as conventional lens-to-lens, links. Mechanical, optical, electrical, thermal, material, and fabrication constraints are studied. Design trade-offs that affect system scalability and ease of assembly are put forward and analyzed. Novel mounting techniques such as a thermal-loaded interference-fitted lens-mounting technique are presented and discussed. Diagnostic tools are developed to quantify the performance of the optomechanics, and experimental results are given and analyzed.

Design, implementation, and characterization of an optical power supply spot-array generator for a four-stage free-space optical backplane.

The design and implementation of a robust, scalable, and modular optical power supply spot-array generator for a modulator-based free-space optical backplane demonstrator is presented. Four arrays of 8 x 4 spots with 6.47-mum radii (at 1/e(2) points) pitched at 125 mum in the vertical direction and 250 mum in the horizontal were required to provide the light for the optical interconnect. Tight system tolerances demanded careful optical design, robust optomechanics, and effective alignment techniques. Issues such as spot-array generation, polarization, power efficiency, and power uniformity are discussed. Characterization results are presented.

In situ Interferometric alignment systems for the assembly of microchannel relay systems.

An interferometric alignment technique developed for the assembly of microchannel relay systems is described. The method uses pairs of diffractive lenslets that are arranged to form compact in situ interferometers. The relative transverse, longitudinal, and rotational alignment of the two lenslet arrays can be quantitatively determined from the resulting interference patterns. The theoretical analysis is compared with the experimental performance.

Analysis of a microchannel interconnect based on the clustering of smart-pixel-device windows.

A design analysis of a telecentric microchannel relay system developed for use with a smart-pixel-based photonic backplane is presented. The interconnect uses a clustered-window geometry in which optoelectronic device windows are grouped together about the axis of each microchannel. A Gaussian-beam propagation model is used to analyze the trade-off between window size, window density, transistor count per smart pixel, and lenslet ƒ-number for three cases of window clustering. The results of this analysis show that, with this approach, a window density of 4000 windows/cm(2) is obtained for a window size of 30 µm and a device plane separation of 25 mm. In addition, an optical power model is developed to determine the nominal power requirements of a 32 × 32 smart-pixel array as a function of window size. The power requirements are obtained assuming a complementary metal-oxide semiconductor inverter-amplifier and dual-rail multiple-quantum-well self-electro-optic-effect devices as the receiver stage of the smart pixel.

4 x 4 vertical-cavity surface-emitting laser (VCSEL) and metal-semiconductor-metal (MSM) optical backplane demonstrator system.

We describe a system demonstrator based on vertical-cavity surface-emitting lasers, metal-semiconductor-metal detectors, printed circuit board (PCB) level optoelectronic device packaging, a compact bulk optical relay, and novel barrel/PCB optomechanics. The entire system was constructed in a standard VME electrical backplane chassis and was capable of operating at >1.7 Gbit/s of aggregate data capacity. In addition to the component technologies developed, we describe operational testing and characterization of the demonstrator.