A hybrid differential phase shift keying-multipulse pulse position modulation (DPSK-MPPM) technique to enhance the receiver sensitivity of optical communication systems is presented. Both binary and quadrature formats are adopted in the proposed systems. Direct-detection DPSK schemes that are based on asymmetric Mach-Zehnder interferometers with a novel ultrafast discrete delay unit are presented to simplify the receiver implementation. Our results reveal that the proposed hybrid schemes are more energy-efficient and have higher receiver sensitivity compared with the traditional ones while improving the bandwidth-utilization efficiency. Furthermore, at an average launch power of 8 dBm and BER=10.sup.3, the hybrid DQPSK-MPPM system with a total frame length of eight time slots including two signal time slots outreaches a traditional DQPSK system by 950 km. The proposed DPSK-MPPM modulation system accommodates adjustable (or variable) bit rates, by virtue of the programmable delay integrated to the receiver system.

A system for transmitting optical differential signal includes: an optical source configured to generate two channels of optical signals having different wavelengths; an input end for a to-be-modulated electrical signal configured to input a to-be-modulated electrical signal; a unique optical modulator configured to perform optical modulation on the two channels of optical signals using the to-be-modulated electrical signal, to obtain two channels of modulated optical differential signals; an optical sending unit, configured to send the two channels of modulated optical differential signals to optical transmission media; an optical receiving unit, configured to receive the two channels of optical differential signals; an optical detector, configured to perform optical-to-electrical conversion separately on the two channels of modulated optical differential signals, to obtain two channels of electrical signals; and a comparator configured to perform a differential operation to obtain one channel of electrical signal.

A signal equalizer for compensating impairments of an optical signal received through a link of a high speed optical communications network. At least one set of compensation vectors are computed for compensating at least two distinct types of impairments. A frequency domain processor is coupled to receive respective raw multi-bit in-phase (I) and quadrature (Q) sample streams of each received polarization of the optical signal. The frequency domain processor operates to digitally process the multi-bit sample streams, using the compensation vectors, to generate multi-bit estimates of symbols modulated onto each transmitted polarization of the optical signal. The frequency domain processor exhibits respective different responses to each one of the at least two distinct types of impairments.

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.sub.2(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 combinationsproviding rate and format flexibility and WDM scalability.

In one possible implementation an in-situ property determination system includes a displacement tool configured for use in a wellbore. The displacement tool includes four or more pads symmetrically located about an axis of the displacement tool, with each pad having a contact surface configured to contact a wall of the wellbore. The four or more pads can extend from a first position proximate an outer surface of the displacement tool to a second position in contact with the wall of the wellbore such that the four or more pads deform the wellbore into an at least approximately circular cross section. The system also includes a recordation device to record force displacement information associated with extending the four or more pads from the first position to the second position.

Aspects of this technical solution can include obtaining, by a first device, a first data stream carrying a first two-bit number mapped to one or more phase-encoded optical signals, obtaining, by a second device, a second data stream carrying a second two-bit number mapped to the one or more phase-encoded optical signals, encoding, based on the one or more quadrature-phase-shift-keying phases, the first data stream, and the second data stream in a 4-phase-encoded modulation format, performing the average operation on the two transmitted optical signals, and multicasting the result of the average operation to the wavelength of the two transmitted optical channels by nonlinear wave mixing.

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.

A signal equalizer for compensating impairments of an optical signal received through a link of a high speed optical communications network. At least one set of compensation vectors are computed for compensating at least two distinct types of impairments. A frequency domain processor is coupled to receive respective raw multi-bit in-phase (I) and quadrature (Q) sample streams of each received polarization of the optical signal. The frequency domain processor operates to digitally process the multi-bit sample streams, using the compensation vectors, to generate multi-bit estimates of symbols modulated onto each transmitted polarization of the optical signal. The frequency domain processor exhibits respective different responses to each one of the at least two distinct types of impairments.

A wavelength reference device includes a broadband optical source, a repeating filter, and a wavelength-specific filter. The source, which can be a super-luminescent light-emitting diode (SLED), emits optical power. The repeating filter, which can be a Fabray-Perot etalon, filters the optical power into a repeating spectral response, and the wavelength-specific filter attenuates the optical power of at least one predefined wavelength response within the wavelength band. The repeating filter and the wavelength-specific filter output a wavelength reference signal having the repeating spectral response attenuated at the at least one predefined wavelength response. The predefined wavelength response reduces the ambiguity that can occur in the repeating frequency locations found in the repeating spectral response. In this way, an absolute wavelength reference is intrinsically provided in the wavelength reference that removes the location ambiguity caused by the repeating spectral response.