Daniel N. Carothers

Demonstration of a 320×256 two-color focal plane array using InAs/InGaAs quantum dots in well detectors

We report the demonstration of a two-color infrared focal plane array based on a voltage-tunable quantum dots-in-well (DWELL) design. The active region consists of multiple layers of InAs quantum dots in an In0.15Ga0.85As quantum well. Spectral response measurements yielded a peak at 5.5μm for lower biases and at 8–10μm for higher biases. Using calibrated blackbody measurements, the midwavelength and long wavelength specific detectivity (D*) were estimated to be 7.1×1010cmHz1∕2∕W(Vb=1.0V) and 2.6×1010cmHz1∕2∕W(Vb=2.6V) at 78 K, respectively. This material was processed into a 320×256 array and integrated with an Indigo 9705 readout chip and thermal imaging was achieved at 80 K.

Demonstration of a Fourth-Order Pole-Zero Optical Filter Integrated Using CMOS Processes

We demonstrate a compact fully tunable narrowband fourth-order pole-zero optical filter that is fabricated in a silicon complementary-metal-oxide-semiconductor foundry. The filter is implemented using silicon on oxide channel waveguides and consists of a Mach-Zehnder interferometer with two ring resonator all-pass filters (APFs) on each arm. The filter architecture is based on the sum and difference of the APFs responses. The ring resonators introduce a nonlinear phase response in each arm that allows carving narrow frequency bands out of a broad spectrum. In this paper, we demonstrate a 3-dB filter bandwidth of 1.0 GHz with a stopband rejection of better than 25 dB. The filter free spectral range is 16.5 GHz. Thermooptic phase shifters are used to tune the filter. As silicon has a large thermooptic coefficient compared to silica, the demonstrated filter requires a low tuning power of less than 300 mW. In addition, this filter is compact with dimensions 25 times smaller than the same filter would be if it were made using standard silica on silicon waveguides with a 0.8% step index contrast

Process flow innovations for photonic device integration in CMOS

Multilevel thin film processing, global planarization and advanced photolithography enables the ability to integrate complimentary materials and process sequences required for high index contrast photonic components all within a single CMOS process flow. Developing high performance photonic components that can be integrated with electronic circuits at a high level of functionality in silicon CMOS is one of the basic objectives of the EPIC program sponsored by the Microsystems Technology Office (MTO) of DARPA. Our research team consisting of members from: BAE Systems, Alcatel-Lucent, Massachusetts Institute of Technology, Cornell University and Applied Wave Research reports on the latest developments of the technology to fabricate an application specific, electronic-photonic integrated circuit (AS_EPIC). Now in its second phase of the EPIC program, the team has designed, developed and integrated fourth order optical tunable filters, both silicon ring resonator and germanium electro-absorption modulators and germanium pin diode photodetectors using silicon waveguides within a full 150nm CMOS process flow for a broadband RF channelizer application. This presentation will review the latest advances of the passive and active photonic devices developed and the processes used for monolithic integration with CMOS processing. Examples include multilevel waveguides for optical interconnect and germanium epitaxy for active photonic devices such as p-i-n photodiodes and modulators.

CMOS-Compatible Si-Ring-Assisted Mach–Zehnder Interferometer With Internal Bandwidth Equalization

We demonstrate, to the best of our knowledge, the first electrooptic ring-assisted Mach-Zehnder interferometric (RAMZI) modulator in a CMOS-compatible technology. The RAMZI modulator is manufactured on a CMOS-compatible platform and entirely fabricated in a commercial CMOS foundry. We demonstrate a small-signal 3-dB bandwidth >15 GHz in a silicon-based carrier-depletion modulator with a 2-V·cm V¿L product, which is approximately two times smaller than previously reported. We achieved a 10-Gb/s eye diagram with a 2-dB extinction ratio using a 4-Vp-p drive in a modulator with a 680-¿m optic/RF interaction region. In addition, we demonstrate internal bandwidth equalization within the tunable CMOS-compatible RAMZI modulator, and discuss the optical carrier and modulation sideband response, and relaxation characteristics that lead to this behavior within resonant modulators.

Low-Loss Amorphous Silicon Channel Waveguides for Integrated Photonics

Amorphous silicon (a-Si), single-mode, channel waveguides were fabricated and measured with transmission losses as low as 6.5 dB/cm for the TE mode and 4.5 dB/cm for the TM mode. Variations in the PECVD a-Si deposition conditions yielded a-Si materials with bulk losses <1 dB/cm

Internal Bandwidth Equalization in a CMOS-Compatible Si-Ring Modulator

Bandwidth equalization using a simple complementary metal-oxide-semiconductor-compatible tunable silicon-ring modulator is shown. We demonstrate >35-GHz small signal bandwidth and use the resonator to mitigate bandwidth limitations from other measurement system components. Configuring the optical carrier to be off resonance within the ring free-spectral range allows high-frequency enhancement and low-frequency suppression of the S21 parameter to achieve system response equalization. Our results suggest that the carrier and modulation sidebands can have very different transient characteristics within the ring modulator.

Tunable Narrowband Optical Filter in CMOS

We demonstrate a compact, fully tunable, narrowband (1GHz) 4thorder pole/zero optical filter that is fabricated in a silicon complementary metal oxide semiconductor foundry.

A 32x32 pixel focal plane array ladar system using chirped amplitude modulation

The Army Research Laboratory is researching system architectures and components required to build a 32x32 pixel scannerless ladar breadboard. The 32x32 pixel architecture achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (RF) subcarrier that is linearly frequency modulated (i.e. chirped amplitude modulation). The backscattered light is focused onto an array of metal-semiconductor-metal (MSM) detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal (a product of the mixing process) whose frequency is proportional to the target range. Pixel read-out is achieved using code division multiple access techniques as opposed to the usual time-multiplexed techniques to attain high effective frame rates. The raw data is captured with analog-to-digital converters and fed into a PC to demux the pixel data, compute the target ranges, and display the imagery. Last year we demonstrated system proof-of-principle for the first time and displayed an image of a scene collected in the lab that was somewhat corrupted by pixel-to-pixel cross-talk. This year we report on system modifications that reduced pixel-to-pixel cross-talk and new hardware and display codes that enable near real-time stereo display of imagery on the ladars control computer. The results of imaging tests in the laboratory will also be presented.

PIN Germanium Photodetector Fabrication Issues and Manufacturability

We have designed and fabricated high-performance germanium vertical and lateral PIN photodetectors which can be integrated into electronic-photonic circuits and manufactured in a standard CMOS foundry. The performance of these devices will be reported in another paper. This paper describes production process issues and manufacturability. The intent of this paper is to provide solutions to fabrication process issues, to provide methods of cutting costs by simplifying fabrication processes, and to improve yield by widening the process tolerance windows. This paper discusses all the process issues encountered and provides solutions for each of them.

Advances in fully CMOS integrated photonic devices

The complete integration of photonic devices into a CMOS process flow will enable low cost photonic functionality within electronic circuits. BAE Systems, Lucent Technologies, Massachusetts Institute of Technology, Cornell University, and Applied Wave Research are participating in a high payoff research and development program for the Microsystems Technology Office (MTO) of DARPA. The goal of the program is the development of technologies and design tools necessary to fabricate an application specific, electronic-photonic integrated circuit (AS-EPIC). The first phase of the program was dedicated to photonics device designs, CMOS process flow integration, and basic electronic functionality. We will present the latest results on the performance of waveguide integrated detectors, and tunable optical filters.