This article presents a promising approach for multi-Tb/s optical interconnects. This approach is contained in the MAUI project, which develops a parallel multiwavelength optical subassembly (PMOSA) that uses PWDM to gain the component-density advantages of two-dimensional parallel optics and the connector and cabling density advantages of CWDM. In the MAUI approach, a standard multimode 12-fiber ribbon is used with 4 wavelengths transmitted through each fiber, for a total of 48 optical channels.

In this letter, we demonstrate error-free operation of a 12-fiber /spl times/4-wavelength /spl times/5.21-Gb/s parallel-wavelength-division-multiplexed (PWDM) optical link. The 250-Gb/s transmitter and receiver assemblies each have a 5/spl times/8-mm footprint and consume a combined power of 1.5 W. To our knowledge, this is the first publication of a fully functional PWDM optical interconnect as well as the highest demonstrated bandwidth per unit area and bandwidth per unit power consumption for any multiple-channel fiber-optic interconnect. This technology is intended for short-distance high-bandwidth-density applications such as multiprocessor computer backplanes.

This paper describes a 500-Gbps parallel wavelength-division multiplexed (PWDM) optical interconnect where 48 channels of 10.42-Gbps data are transmitted over a parallel 12-fiber ribbon with 4 wavelengths per fiber. The transmitter and receiver are each chip-scale packages with a footprint of 5 mm times 8 mm and a combined power consumption of 3 W. This work is motivated by the continually increasing bandwidth needs of short-distance computer processor interconnects, which are demanding optical solutions that maximize bandwidth per unit area, power consumption, and cost

The Multiwavelength Assemblies for Ubiquitous Interconnects (MAUI) program is a collaboration between Agilent Laboratories, Palo Alto, CA, and the University of Southern California, Los Angeles, with the goal of enabling fiber-to-the-processor by developing very-high-density optical interconnects and complementary metal-oxide-semiconductor (CMOS) interface electronics. This paper focuses on the parallel wavelength-division-multiplexed optical interconnect technologies and their potential impact on computer systems.

This work presents the first fully-functional 48-channel parallel-wavelength-division-multiplexed (PWDM) transmitter, receiver and link results at a per-channel data rate of 5.21-Gb/s. This high-density PWDM optical interconnect gives an aggregate link bandwidth of a quarter terabit per second.

We demonstrate a novel technique for highly localized injection of millimeter waves in ultrafast devices that combines optical heterodyning and near-field optics. The technique relies on evanescent coupling of two interfering lasers to a submicron area of a device by means of a near-field fiber optic probe. Scanning measurements show the dc and ac photoresponses of two ultrafast device structures, namely low-temperature GaAs photoconductive switches and InP-based high electron mobility transistors. The response characteristics were rich in structures that revealed important details of device dynamics.

A novel technique, that combines optical heterodyning and near-field optics, was developed for highly localized of millimeter waves in ultrafast devices. The technique relies on evanescent coupling of the interfering laser to a small are of the device, by means of a near-field fiber optic probe. The applicability of the technique was first validated by measurements on heterojunction photo transistors up to 100GHz. Later, scanned measurements at 63GHz were performed on two ultrafast device structures, namely low temperature GaAs photoconductive switches and InP-based high electron mobility transistors. The response characteristics were rich in structures that revealed important details of the device dynamics.

The technology of optical mixing in three-terminal devices has been extended, for the first time, to submillimeter-wave frequencies. Using a new generation of 50-nm gate InP-based high electron mobility transistors (HEMT's), optically mixed signals were detected to 552 GHz with a signal-to-noise ratio of approximately 5 dB. A novel harmonic three-wave mixing scheme was used for the detection of the optically generated signals. The technique involved downconversion of the signal in the device by the second harmonic of a gate-injected millimeter-wave local oscillator. Measurements were also done at 212 and 382 GHz to estimate the absolute signal strength and conversion losses. Finally, new interesting features in the bias dependence of the optically mixed signal are reported.

Near field fiber optic probes have attracted a great deal of current interest mainly due to their ability to extend optical microscopy beyond the classical diffraction limit. Scanning optical microscopes using these probes have been yielded very high resolution in imaging and spectroscopic applications at visible and infrared wavelengths. The technology has been applied to imaging in biology, material science, surface chemistry and information storage. In this work, we explore a novel use of the near field fiber optic probes in optical heterodyne characterization of high speed devices. Because of the submicron feature size of modem electronic devices, conventional optical heterodyning only yields the response of the device as a whole. Introducing near field fiber optic probes allows one to examine spatially resolved details of device response and thereby provides a means to look more closely at the internal carrier dynamics. Our technique thus comprises an important tool for the experimental study of ultrafast devices.

A new generation of InP based 50 nm gate-length pseudomorphic high electron mobility transistors was employed in continuous wave optical mixing experiments to generate millimeter waves to extremely high frequencies. Direct radiation of optical mixed signals was demonstrated at 212 GHz. This, to our knowledge, is the highest frequency optically generated millimeter wave radiation ever reported for three terminal devices. Newly developed G-band probes and a horn antenna were used for radiation measurements. A signal to noise ratio of approximately 20 dB was obtained that indicates significant response of our devices at these frequencies. A broadband three wave detection scheme was used to further extend optical mixing to 238 and 267 GHz. Optical time domain and electrical characterization were carried out to reveal the high frequency capabilities of these devices. A photoresponse time of 6.9 psec was measured using a picosecond electrooptic sampling setup. Cut-off frequencies of 228 GHz and maximum oscillation frequencies of 124 GHz were obtained in S-parameter measurements. Specific examples of applications in communications and spectroscopy were investigated.

We demonstrate the use of near-field fiber-optic probes in optical heterodyne characterization of high-speed devices. The submicron-size optical beam obtained from the fiber-optic probe was employed to selectively excite a small local portion of the active region of the device. Optical heterodyne measurements on heterojunction bipolar transistors were conducted at 1550 nm with a difference frequency of 100 GHz. The significant response of the device with a signal-to-noise ratio of 30 dB was observed. The dc and the ac photoresponses were also measured as a function of the distance between the fiber-optic probe and the device under test. The data showed clearly distinguishable regimes of near- and far-field operation. The high-frequency near-field optical heterodyne technique as explored in this letter provides us with capabilities that can be effectively utilized in the field of optical millimeter-wave interaction in ultrafast devices.

We demonstrate the use of near-field fiber-optic probes in optical heterodyne characterization of high-speed devices. The submicrometer-size optical beam obtained from the fiber-optic probe was employed to selectively excite a tiny area of the device active region. Optical heterodyne measurements on heterojunction bipolar transistors were conducted at 1.3 mu m with a difference frequency of 60 GHz, Significant response of the device with a signal-to-noise ratio of 25 dB was observed. The de and ac photoresponse was also measured as a function of the distance between the fiber probe and the device-under-test. The data clearly showed distinguishable regimes of near- and far-field operation. The near-field high-frequency optical heterodyne technique as explored in this paper provides us with new capabilities that can be effectively utilized in the field of optical millimeter-wave interaction in ultrafast devices.

The high speed response of 50-nm gate AlInAs/GaInAs/InP pseudomorphic high electron mobility transistors (HEMTs) have been used in optical mixing experiments to generate difference frequencies to 211 GHz from two continuous wave laser beams. A 16 dB signal to noise ratio was achieved. To our knowledge, this is the highest frequency optical mixing ever obtained for three-terminal devices. A broadband three wave mixing technique was employed to detect the optically mixed signals at these high frequencies. This scheme involves the nonlinear interaction of the optically generated signal with a millimeter wave signal electrically injected at the gate. The resulting signal, downshifted to W band, was radiated into the waveguide input of an external millimeter wave receiver system. To demonstrate the wide tunability of our system a sweep of frequencies from 160-190 GHz was performed. The HEMTs exhibited a relatively flat response with signal to noise ratios of greater than 12 dB. Ultrafast response of the HEMTs as indicated by CW mixing results was also characterized in the time domain using a picosecond electro-optic sampling system. To illustrate the use of the HEMTs in optical millimeter wave systems, optically mixed signals at 97 GHz, both continuous wave and modulated, were radiated into free space using a horn antenna. Modulation was obtained by injecting a baseband signal into the gate of the HEMT. Electrical characterization of the devices yielded cut-off frequencies of 228 GHz and a maximum oscillation frequency of 124 GHz.

We report optical mixing with difference frequencies to 211 GHz in 50 nm gate pseudomorphic InP-based high electron mobility transistors (HEMTs). To our knowledge, this is the highest frequency optical mixing signal obtained in three terminal devices. To detect the signals at these frequencies, a novel three-wave-mixing configuration was employed. To demonstrate the wide tunability of this setup, a sweep of frequencies from 160 to 190 GHz was performed. The optically generated millimeter wave signals were downconverted to 97 GHz and radiated. For the radiation experiments, tunable baseband signals were also added by injection into the gate terminal of our HEMTs, thereby providing a method to transmit information.

A high-power high-speed phototransistor has been demonstrated using a traveling-wave (TW) structure with an integrated polyimide optical waveguide, In our configuration, optical power transfer is distributed along the length of the device via leaky mode coupling of light from the polyimide waveguide to the active region of the phototransistor. The TW electrode design allows for an electrically long structure while maintaining high bandwidths. Due to the increased absorption volume, the optical power handling capabilities of the TW-heterojunction phototransistors (TW-HPT's) are improved over that of conventional lumped-element HPT detectors. The experimental results show no saturation of the fundamental at 60 GHz up to 50 mA of dc photocurrent.

We have demonstrated direct radiation of continuous wave signals optically generated in ultrafast HEMTs to 212 GHz and validated the use of this radiation for spectroscopic measurements.

We present measurements on the optical frequency response of epitaxial lift-off (ELO) 1.0-/spl mu/m InP high-electron mobility transistors (HEMTs) to 140 GHz using electrooptic sampling and heterodyne techniques. Our picosecond sampling measurements established that the lift-off devices exhibited substantial optical response to 140 GHz. Heterodyne measurements made at 60 and 94 GHz later confirmed these findings. A novel three wave mixing technique was used to extend the heterodyne bandwidth to 130 GHz. In these experiments, millimeter waves were generated in our optically driven HEMT's and launched into waveguides. These lift off devices can be major additions to future millimeter wave integrated optoelectronic systems either as high frequency optical detectors or as optically driven tunable millimeter wave sources.

Optical-millimeter wave interactions in semiconductor devices have attracted much recent interest because of applications that can take advantage of both lightwave and millimeter wave techniques. Applications that can benefit from a merging of these areas include phased array radar, remote sensing, spectroscopy and communications. One technique, optical mixing, is a promising method to generate continuously tunable millimeter wave frequencies. The generated signals are of high quality in terms of stability, noise and spectral purity. In this paper, we report the extension of optical mixing techniques to difference frequencies of 211 GHz in 50-nm gate-length pseudomorphic HEMTs. To our knowledge, this is the highest frequency optical mixing to date for three terminal devices. Although two-terminal devices have been used almost exclusively in this role, three-terminal devices are becoming increasingly competitive because of their inherent gain. Moreover, the presence of a third terminal adds a degree of freedom in controlling the mixing process and allows for baseband information signals to be imposed on the optically generated carriers.

Epitaxial Lift-off (ELO) has become an increasingly important tool to integrate devices of dissimilar material systems. We explore the use of optically driven ELO InP HEMTs to generate millimeter wave radiation up to 94 GHz. Our results demonstrate the viability of using ELO techniques for integration of millimeter wave HEMT photodetectors in novel new optoelectronic systems.