Hemonth G. Rao

A next-generation optical regional access network

We describe an optical regional access network which combines electronic IP routing with intelligent networking functionality of the optical WDM layer. The optical WDM layer provides such networking functions as network logical topology reconfiguration, optical flow switching to offload traffic and bypass IP routers, wavelength routing of signals, protection switching and restoration in the optical domain, and flexible network service provisioning by reconfigurable wavelength connectivity. We discuss key enabling technologies for the WDM layer and describe their limitations. The symbiosis of electronic and optical WDM networking functions also allows support for heterogeneous format traffic and will enable efficient gigabit-per-second user access in next-generation Internet networks.

Ultra-wide-range multi-rate DPSK laser communications

We propose and demonstrate a scalable high-sensitivity approach for achieving multi-rate DPSK using a single transmitter and fixed-interferometer-receiver design. Near-theoretical real-time performance is demonstrated over static and fading channels at rates from 2.4Mbps to 2.5Gbps.

A consolidated multi-rate burst-mode DPSK transmitter using a single Mach-Zehnder modulator

We demonstrate phase modulation, pulse carving, and burst-mode windowing in a single Mach-Zehnder modulator, significantly reducing size, weight, power, and complexity of prior three-modulator designs with only 0.6-dB additional performance penalty.

A multi-rate DPSK modem for free-space laser communications

The multi-rate DPSK format, which enables efficient free-space laser communications over a wide range of data rates, is finding applications in NASA’s Laser Communications Relay Demonstration. We discuss the design and testing of an efficient and robust multi-rate DPSK modem, including aspects of the electrical, mechanical, thermal, and optical design. The modem includes an optically preamplified receiver, an 0.5-W average power transmitter, a LEON3 rad-hard microcontroller that provides the command and telemetry interface and supervisory control, and a Xilinx Virtex-5 radhard reprogrammable FPGA that both supports the high-speed data flow to and from the modem and controls the modem’s analog and digital subsystems. For additional flexibility, the transmitter and receiver can be configured to support operation with multi-rate PPM waveforms.

Multi-rate DPSK optical transceivers for free-space applications

We describe a flexible high-sensitivity laser communication transceiver design that can significantly benefit performance and cost of NASAs satellite-based Laser Communications Relay Demonstration. Optical communications using differential phase shift keying, widely deployed for use in long-haul fiber-optic networks, is well known for its superior sensitivity and link performance over on-off keying, while maintaining a relatively straightforward design. However, unlike fiber-optic links, free-space applications often require operation over a wide dynamic range of power due to variations in link distance and channel conditions, which can include rapid kHz-class fading when operating through the turbulent atmosphere. Here we discuss the implementation of a robust, near-quantum-limited multi-rate DPSK transceiver, co-located transmitter and receiver subsystems that can operate efficiently over the highly-variable free-space channel. Key performance features will be presented on the master oscillator power amplifier (MOPA) based TX, including a wavelength-stabilized master laser, high-extinction-ratio burst-mode modulator, and 0.5 W single polarization power amplifier, as well as low-noise optically preamplified DSPK receiver and built-in test capabilities.

A simple alignment technique for consolidated tri-state multi-rate burst-mode DPSK transmitters

We demonstrate an average-power-based technique for aligning data and clock signals in consolidated tri-state multi-rate burst-mode DPSK transmitters with a simple and low-SWaP implementation.

Optical networking with reconfigurable access nodes in the NGI ONRAMP test bed

We have built reconfigurable access nodes for the next generation Internet (NGI) ONRAMP test bed and used them to demonstrate basic optical networking functionality, including over a 100 km installed fiber link.

A narrow-beam undersea laser communications field demonstration

We report a demonstration of narrow-beam laser communication through the waters of Narragansett Bay in Rhode Island, USA. The transmitter and receiver were mounted on an aluminum truss and placed in the water alongside a pier operated by the Naval Undersea Warfare Center. The transmitter consisted of a real-time modulator and encoder, a 515 nm wavelength commercial laser, collimating optics, and a steering mirror. The receiver included a steering mirror, a focal plane camera, a linear-mode avalanche photo-diode (APD), a photo-multiplier tube (PMT) single photon detector, a large area imaging camera, an iris to vary the field of view, optics to split the beam between the various detectors, and field-programmable gate array (FPGA) electronics for real-time demodulation and decoding. The PMT and APD detectors were used for communications demonstrations; the imaging and focal plane cameras were used for channel characterization measurements and system alignment. Communications and characterization data were collected through a variety of conditions over the five day field experiment, including day and night, calm and high winds, and flood and ebb tide. In the experiment, the transmit power, receiver field of view, and link distance were varied. The water transmissivity and volume scattering function were measured throughout the experiment to calibrate the results. Real-time communications demonstrations with the PMT were carried out between 1 megabit-per-second (Mbps) and 8.7 Mbps at 7.8 meters, which represented between 8 and 12 beam extinction lengths. With the APD, 125 Mbps were demonstrated at 4.8 meters, representing approximately 5 extinction lengths.

Turbid-harbor demonstration of transceiver technologies for wide dynamic range undersea laser communications

Undersea laser communications represent a promising area of research with a large set of applications. Wide dynamic range receivers are necessary to operate through a range of possible water qualities and link distances. In the signal-starved regime, photon-counting photomultiplier tubes (PMTs) are a key technology for high-sensitivity communications. When more signal is available, linear avalanche photodiodes (APDs) provide an opportunity for higher-rate communication. We have designed a receiver terminal employing both kinds of detectors to show robust operation over nearly two orders of magnitude in power and data rate. An optical link including this receiver terminal was submerged in Narragansett Bay, RI to demonstrate underwater optical communication over several days. The PMT receiver demonstrated robust, error-free performance over channel rates from 1.302 Mbaud to 10.416 Mbaud for received optical power levels ranging from -84.1 dBm to -75.3 dBm. The PMT link demonstrated an error-free user rate of 8.68 Mb/s. This corresponded to nearly-ideal detector efficiency on the order of one detected photon per bit. The PMT receiver was contained entirely within the submerged enclosure and demonstrated full real-time decoding, including strong forward error correction. A low-power transmitter was used to demonstrate a link with loss equivalent to 18 extinction lengths. With moderately-powered transmitters, this distance could be extended to 22.4 extinction lengths. The PMT receiver was capable of operating at near-theoretical limits during the day and night. Its multi-rate operation demonstrated the capability of trading sensitivity for data rate efficiently. With the same low-power transmitter, the APD receiver achieved a bit error rate less than 1×10-9 at 125 Mbaud. Furthermore, it achieved an error rate correctable by forward error correction for a link with loss equivalent to 9 extinction lengths.

Electronics design of a multi-rate DPSK modem for free-space optical communications

We have designed and experimentally demonstrated a radiation-hardened modem suitable for NASA’s Laser Communications Relay Demonstration. The modem supports free-space DPSK communication over a wide range of channel rates, from 72 Mb/s up to 2.88 Gb/s. The modem transmitter electronics generate a bursty DPSK waveform, such that only one optical modulator is required. The receiver clock recovery is capable of operating over all channel rates at average optical signal levels below -70 dBm. The modem incorporates a radiation-hardened Xilinx Virtex 5 FPGA and a radiation-hardened Aeroflex UT699 CPU. The design leverages unique capabilities of each device, such as the FPGA’s multi-gigabit transceivers. The modem scrubs itself against radiation events, but does not require pervasive triple-mode redundant logic. The modem electronics include automatic stabilization functions for its optical components, and software to control its initialization and operation. The design allows the modem to be put into a low-power standby mode.