Massachusetts Institute of Technology

David O Caplan

Don M Boroson

Kerri Cahoy

Mark L Stevens

Kathleen Michelle Riesing

Tam Nguyen Thuc Nguyen

Ryan Wallace Kingsbury

Walid A Atia

Bryan Shawn Robinson

Scott A Hamilton

Marc Norvig

Hemonth G Rao

Neal W Spellmeyer

Michael Robert Watts

Zhan Su

Communication bottlenecks, particularly in the downlink direction, are a common problem for many CubeSat developers. As described herein, a CubeSat module for a CubeSat comprises an optical transmitter to transmit data to a remote terminal, a receiver to acquire an optical beacon from a remote terminal, and a fine-pointing module operably and directly coupleable to a coarse-pointing module of the CubeSat. The fine-pointing module is configured to point the optical transmitter toward the remote terminal with an accuracy range that overlaps with an accuracy range of the coarse-pointing module of the CubeSat so as to establish a communications link between the CubeSat and the remote terminal over a low-Earth-orbit (LEO) distance.

Free-space optical communication module for small satellites

An apparatus for generating a processed optical signal includes a first laser configured to emit a first optical signal in response to a first drive signal. The first optical signal has a first phase shift depending on a first integrated amplitude of the first drive signal. The apparatus also includes a spectral-temporal filter, in optical communication with the first laser, to change a first spectral profile and a first temporal profile of the first optical pulse so as to generate the processed optical signal. Replacing a conventional continuous-wave (CW) laser and external modulation with filter-based modulation can achieve the same or better performance without high-fidelity low-noise input signals.

Apparatus and methods for power efficient multi-format optical transmission

An optical receiver includes a cascade of optical filtering elements, each of which selects spectral components from incoming optical signals at a wavelengths aligned to filter passbands. The selected spectral components may be optically combined to form k pairs of intermediary signals, where k=log(M). By comparing the k pairs of intermediary signals, k bits of a digital signal representing the incident signal may be generated. The filtering elements may be configured to perform demultiplexing and demodulation simultaneously, increasing functionality and reducing excess losses. The filtering elements may also be tuned so that the optical receiver may be reconfigured to accommodate different combinations of wavelengths and modulation formats, such as wavelength division multiplexed (WDM) on off keying (OOK), M-ary orthogonal formats including frequency shift keying (FSK) and pulse position modulation (PPM), differential phase shift keying, and hybrid combinations&#x2014;providing rate and format flexibility and WDM scalability.

Apparatus and methods for reconfigurable optical receivers

The present invention provides a simple means of demodulating optical signals, e.g. wideband M-ary orthogonal. The demodulator comprises an optical processor and a comparison module. The optical processor transforms M input optical signals into 2 log(M) intermediary optical signals and the comparison module determines the logical representation of the input data based on log(M) binary comparisons of the optical power of the intermediary signals. Example embodiments may be reconfigurable to receive optical signals using M-FSK, M-PPM, M-PolSK, and hybrid M-ary orthogonal modulation formats. Example embodiments also offer small size, weight and power consumption for both free-space and fiber optic environments as well as improved receiver sensitivity and reduced electron bandwidth requirements.

Optical receiver configurable to accommodate a variety of modulation formats

A burst-mode phase shift keying (PSK) communications apparatus according to an embodiment of the present invention enables practical, power-efficient, multi-rate communications between an optical transmitter and receiver. Embodiments may operate on differential PSK (DPSK) signals. An embodiment of the apparatus includes an average power limited optical transmitter that transmits at a selectable data rate with data transmitted in bursts, the data rate being a function of a burst-on duty cycle. DPSK symbols are transmitted in bursts, and the data rate may be varied by changing the ratio of the burst-on time to the burst-off time. This approach offers a number of advantages over conventional DPSK implementations, including near-optimum photon efficiency over a wide range of data rates, simplified multi-rate transceiver implementation, and relaxed transmit laser line-width requirements at low data rates.

Method and apparatus for transmitting phase shift keyed optical signals

A burst-mode phase shift keying (PSK) communications system according to an embodiment of the present invention enables practical, power-efficient, multi-rate communications between an optical transmitter and receiver. Embodiments may operate on differential PSK (DPSK) signals. An embodiment of the system utilizes a single interferometer in the receiver with a relative path delay that is matched to the DPSK symbol rate of the link. DPSK symbols are transmitted in bursts, and the data rate may be varied by changing the ratio of the burst-on time to the burst-off time. This approach offers a number of advantages over conventional DPSK implementations, including near-optimum photon efficiency over a wide range of data rates, simplified multi-rate transceiver implementation, and relaxed transmit laser line-width requirements at low data rates.

Method and apparatus for phase shift keyed optical communications

A system includes an optical transmitter that outputs an optical signal having a substantially Gaussian waveform and an optical receiver that is optically coupled to the optical transmitter and has an impulse response essentially matching the waveform. The impulse response and waveform preferably match in the time domain. The transmitter and receiver may be average-power-limited, using, for example, an erbium-doped fiber amplifier. To achieve a high signal-to-noise ratio, the waveform may be designed to minimize jitter, sample duration, matching parasitics, and inter-symbol interference (ISI). Such a waveform may be a return-to-zero (RZ) Gaussian or Gaussian-like waveform and may be transmitted in a variety of modulation formats. Further, the system may be used in WDM or TDM systems. A method for characterizing the time domain impulse response of an optical element used in the optical receiver is provided, where the method is optionally optimized using deconvolution and/or cross-correlation techniques.

Methods of achieving optimal communications performance

A filter-based method of demodulating differentially encoded phase shift keyed (DPSK) optical signals, such as commonly used binary-DPSK (DBDPSK) and quadrature DPSK (DQPSK) signals, that can achieve optimal receiver sensitivity is described. This approach, which combines filtering and differential phase comparison, can reduce the complexity and cost of DPSK receivers by obviating delay-line interferometer-based demodulation. This can improve receiver stability and reduce size, weight, and power, while maintaining the ability to achieve optimal communications performance.

Filter-based DPSK communications system

Filter-based DPSK receiver

The present invention addresses the problem of transmitting optical signals with high extinction ratios using low-power drive signals. At present, low-power optical transmitters typically operate with modulation extinction ratios of, at best, about 10 dB. Embodiments of the present invention may achieve extinction ratios exceeding 20 dB using low-power drive signals of under 20 mW at data rates on the order of Gbits/sec. In addition, the modulation may be achieved with both low-power and low-fidelity drive waveforms, enabling conventional and often noisy high-speed, low-power electronics to generate high-extinction, high-fidelity optical waveforms.

Method and apparatus for transmitting optical signals

A polarization independent (PI) interferometer design that can be built from standard optical components is described. Based upon a Michelson interferometer, the PI interferometer uses a 50/50 splitter and Faraday Rotator Mirrors (FM's). The interferometer achieves good optical characteristics, such as high extinction ratio (ER) and low insertion loss (IL). Lack of polarization sensitivity reduces interferometer construction tolerances and cost, enhances performance and utility, and expands the scope of interferometric based devices. Such characteristics can be used to construct flexible, high performance, polarization insensitive, multi-rate, self-calibrating, optical DPSK receivers, power combiners, optical filters and interleavers, all-optical switches, and cascaded interferometers. Since polarization is not maintained in standard fiber optic networks, a PI-DPSK receiver allows for use of more sensitive DPSK communications over fiber, without need for costly polarization control hardware. Other applications of PI interferometers include optical CDMA, secure communications, optical coherence tomography (OCT), and temporal gratings with ultra-precise timing.

Reconfigurable polarization independent interferometers and methods of stabilization

An optical, multi-channel, Differential Phase Shift Keying (DPSK) receiver demodulates multiple Wavelength Division Multiplexed (WDM) channels using at least one interferometer. This distributes expense of the interferometer(s) over all channels of an optical signal, allowing for deployment of cost-effective, scalable, wideband, WDM DPSK systems. For example, for an 80 channel WDM link, the receiver uses a single interferometer instead of eighty interferometers and associated stabilization hardware, dramatically reducing size, weight, power, and cost. The receiver is architecturally compatible with existing interferometer technologies so previous development and qualification efforts can be leveraged. This allows for expedited technology insertion into existing optical communications networks, including terrestrial and space-based optical networks.

Multi-channel differentially encoded phase shift keyed receivers

An optical, multi-channel, Differential Phase Shift Keying (DPSK) receiver demodulates multiple Wavelength Division Multiplexed (WDM) channels using a single interferometer. This distributes expense of the interferometer over all channels of an optical signal, allowing for deployment of cost-effective, scalable, wideband, WDM DPSK systems. For example, for an 80 channel WDM link, the receiver uses a single interferometer instead of eighty interferometers and associated stabilization hardware, dramatically reducing size, weight, power, and cost. The receiver is architecturally compatible with existing interferometer technologies so previous development and qualification efforts can be leveraged. This allows for expedited technology insertion into existing optical communications networks, including terrestrial and space-based optical networks.

Growing community of inventors

Westford, MA, United States of America

David O Caplan