Digital coherent superposition for performance improvement of spatially multiplexed coherent optical OFDM superchannels.

We demonstrate the use of digital coherent superposition to improve the performance of space-division-multiplexed (SDM) 676-Gb/s OFDM-16QAM superchannels, achieving ~4 dB improvement in OSNR by using two SDM copies and 1075-km (14 x 76.8 km) transmission over a seven-core-fiber with an effective aggregate spectral efficiency of 23.7 b/s/Hz. We further show that the performance improvement from the coherent superposition is retained in the nonlinear transmission regime through coordinated scrambling of signal constellations at the transmitter and appropriate unscrambling at the receiver, by using a series of simple scrambling functions.

Scrambled coherent superposition for enhanced optical fiber communication in the nonlinear transmission regime.

Coherent superposition of light waves has long been used in various fields of science, and recent advances in digital coherent detection and space-division multiplexing have enabled the coherent superposition of information-carrying optical signals to achieve better communication fidelity on amplified-spontaneous-noise limited communication links. However, fiber nonlinearity introduces highly correlated distortions on identical signals and diminishes the benefit of coherent superposition in nonlinear transmission regime. Here we experimentally demonstrate that through coordinated scrambling of signal constellations at the transmitter, together with appropriate unscrambling at the receiver, the full benefit of coherent superposition is retained in the nonlinear transmission regime of a space-diversity fiber link based on an innovatively engineered multi-core fiber. This scrambled coherent superposition may provide the flexibility of trading communication capacity for performance in future optical fiber networks, and may open new possibilities in high-performance and secure optical communications.

M-ary pulse-position modulation and frequency-shift keying with additional polarization/phase modulation for high-sensitivity optical transmission.

We present a new class of optical modulation formats based on the combination of m-ary pulse-position modulation (m-PPM) or m-ary frequency-shift keying (FSK) with additional polarization and/or phase modulation, which is applied on the information carrying pulses in the case of m-PPM or on the information carrying frequency carriers in the case of m-FSK. We describe the principle and implementation of this class of optical modulation formats, and formulate their theoretical receiver sensitivities in optically pre-amplified receivers. Pilot-assisted frequency-domain equalization, similar to that used in coherent optical orthogonal frequency-division multiplexing (CO-OFDM), is used for reliable channel estimation and compensation. CO-OFDM also allows m-FSK to be implemented with high spectral efficiency. As a particular format in this class, m-PPM in combination with polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK), termed as PQ-mPPM, offers superior receiver sensitivity in optically pre-amplified receivers at bit error ratios (BERs) around the thresholds of common forward-error correction codes. Record receiver sensitivities of 3.5 photons per bit (ppb) at BER = 10(-3) and 2.7 ppb at BER = 1.5 × 10(-2) are experimentally demonstrated at 2.5 Gb/s and 6.23 Gb/s using PQ-16PPM and PQ-4PPM, respectively. We further demonstrate the transmission of a 6.23-Gb/s PQ-4PPM signal over a 370-km unrepeatered ultra-large-area-fiber span with 71.7-dB total loss budget.

Single-pass mode-locked or Q-switched pump operation of D2 gas-in-glass fiber Raman lasers operating at 1.56-microm wavelength.

Single-pass operation of a D(2) gas-in-glass fiber Raman laser operating at 1.56-microm wavelength, pumped by a Nd: YAG laser operating either Q-switched or mode-locked has been demonstrated. The group velocities in the single-mode host fiber at the pump wavelength (1.06 microm) and the Raman-shifted wavelength (1.56 microm) were matched, thereby providing long interaction lengths and concomitant efficient conversion of pump light to 1.56-microm radiation.

Synchronously pumped D(2) gas-in-glass fiber Raman laser operating at 1.56 microm.

A synchronously pumped D(2)gas-in-glass fiber Raman laser operating at 1.56 microm has been demonstrated. The active medium is molecular D(2) diffused into a 100-m-long conventional single-mode solid silica optical fiber. Synchronous pumping with a mode-locked 1.06-microm Nd:YAG laser (120-psec pulse width with 100-MHz repetition rate) produces 15-psec pulses at 1.56microm with peak output powers of 20 W.

Optical gain exceeding 35 dB at 1.56 microm due to stimulated Raman scattering by molecular D(2) in a solid silica optical fiber.

A stimulated-Raman-gain medium at 1.56-microm wavelength, consisting of molecular deuterium dissolved in low-loss silica optical fiber and pumped by 1.06-microm Nd:YAG laser light, is described. In a 100-m-long fiber with pump power of 130 W, InGaAsP injection-laser probe light at 1.56 microm was amplified by a factor of 4000 to an output power of 150 mW. The measured stimulated-Raman-gain coefficient at 1.56 microm is 7 x 10(-12) cm/W, about equal to the Raman gain of SiO(2) at the same wavelength.

Gas-in-glass-a new Raman-gain medium: molecular hydrogen in solid-silica optical fibers.

A new Raman-gain medium consisting of molecular hydrogen dissolved at high pressure in low-loss solid-silica optical fibers is described. Stimulated Raman gain at 0.88-microm wavelength was measured in a half-kilometer length of this H(2)-diffused fiber by using a krypton-laser pump and an injection-laser probe. The gain coefficient determined from these experiments at 0.88 microm is 5 x 10(-12) cm/W. The overtone absorption spectrum of high-pressure hydrogen in silica was observed.

Broadband infrared generation in liquid-bromine-core optical fibers.

A new Raman-gain medium suitable for use throughout the infrared spectral region is described. Hollow silica fibers with liquid-bromine cores are shown to have very large Raman-gain coefficients (gamma=1.7x10(-8)cm/W at 1.1 microm with a Raman shift of 312 cm(-1)). The liquid-bromine core, being a homonuclear diatomic material, has no first-order infrared absorption, and transparency between 1 and 16 microm has been verified. Broadband infrared radiationbetween 1.1 and 4 microm was generated by stimulated Raman scattering in bromine-core fibers pumped with 1.06- and 2.11-microm radiation.

High-resolution spectroscopy of matrix-isolated ReO4-molecules.

The temperature-dependent linewidth of the saturable infrared active nu(3) vibrational mode (10.8 microm) of ReO(4)(-) molecules in pure-KI host crystals has been measured between 2 and 600 K. Below 20 K the absorption line is inhomogeneously broadened and has a 0.016-cm(-1) width (FWHM). Above 30 K the transition is homogeneously broadened by elastic acoustic phonon-scattering processes. The low-temperature inhomogeneous width of ReO(4)(-) molecules in KI:RbI alloys is 2 orders of magnitude larger than for the pure KI crystal, and the absorption is coincident with the 10.8-microm P(42) CO(2) laser line at all temperatures.

Infrared hole-burning spectroscopy of matrix-isolated ReO(4)(-)molecules.

CO(2)-laser-diode-laser hole-burning spectroscopy has been used to measure the temperature-dependent homogeneous linewidth of the ReO(4)(-) infrared active internal-vibrational mode (nu(3)) in alkali halide lattices. Hole widths as small as 20 MHz (FWHM) have been observed in an inhomogeneously broadened line of width as large as 1 cm(-1). Our measurement of the homogeneous linewidth over 4 orders of magnitude indicates that elastic acoustic phonon scattering is the dominant T(2) relaxation process from 10 to 600 K.

Infrared generation by means of multiple-order stimulated Raman scattering in CCl(4)- and CBrCl(3)-filled hollow silica fibers.

Near-infrared and midinfrared radiation has been generated by means of multiple-order stimulated Raman scattering in hollow-core silica fibers filled with CBrCl(3) and CCl(4) and pumped by Nd:YAG laser radiation. Radiation between 1.2 and 2.3 microm has been generated in 300-cm-long, 12-microm core-diameter liquid-filled fibers using peak powers of 11 kW. Single-mode liquid-core waveguides were constructed by using CCl(4)-CBrCl(3) mixtures of adjustable refractive index.

Nonintrusive measurements of vapor concentrations inside sprays.

A two-wavelength absorption/scattering measurement technique is described whereby vapor concentrations inside sprays and aerosols are probed nonintrusively. The technique has been applied to measure the vapor concentration inside an n-heptane fuel spray.

Liquid surface coating for optical components used in high power laser applications.

Measured absorption coefficients of paraffin oil in the ir, visible, and uv spectral regions indicate that paraffin oil is potentially useful in these spectral regions as a protective and index-matching surface coating material for high power laser applications. Liquid coatings are unique because of their self-healing, index matching, nonporous, hydrophobic properties.