Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

Recent advances in room-temperature, near-IR and visible diode laser sources for tele-communication, high-speed computer networks, and optical data storage applications are enabling a new generation of gas-dynamic and combustion-flow sensors based on laser absorption spectroscopy. In addition to conventional species concentration and density measurements, spectroscopic techniques for temperature, velocity, pressure and mass flux have been demonstrated in laboratory, industrial and technical flows. Combined with fibreoptic distribution networks and ultrasensitive detection strategies, compact and portable sensors are now appearing for a variety of applications. In many cases, the superior spectroscopic quality of the new laser sources compared with earlier cryogenic, mid-IR devices is allowing increased sensitivity of trace species measurements, high-precision spectroscopy of major gas constituents, and stable, autonomous measurement systems. The purpose of this article is to review recent progress in this field and suggest likely directions for future research and development. The various laser-source technologies are briefly reviewed as they relate to sensor applications. Basic theory for laser absorption measurements of gas-dynamic properties is reviewed and special detection strategies for the weak near-IR and visible absorption spectra are described. Typical sensor configurations are described and compared for various application scenarios, ranging from laboratory research to automated field and airborne packages. Recent applications of gas-dynamic sensors for air flows and fluxes of trace atmospheric species are presented. Applications of gas-dynamic and combustion sensors to research and development of high-speed flows aeropropulsion engines, and combustion emissions monitoring are presented in detail, along with emerging flow control systems based on these new sensors. Finally, technology in nonlinear frequency conversion, UV laser materials, room-temperature mid-IR materials and broadly tunable multisection devices is reviewed to suggest new sensor possibilities.

https://iopscience.iop.org/article/10.1088/0957-0233/9/4/001/pdf

Diode laser absorption sensors for gas-dynamic and combustion flows.

We discuss a number of theoretical and experimental issues in quantum well lasers with emphasis on the basic behavior of the gain, the field spectrum, and the modulation dynamics It is revealed that the use of quantum well structures results in improvement of these properties and brings several new concepts to optical semiconductor devices

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Quantum well lasers--Gain, spectra, dynamics

Advanced optical modulation formats have become a key ingredient to the design of modern wavelength-division-multiplexed (WDM) optically routed networks. In this paper, we review the generation and detection of multigigabit/second intensity- and phase-modulated formats and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Modulation Formats for High-Capacity Optical Transport Networks

An overview is given of the direct modulation performance of high-speed semiconductor lasers. The high-speed response characteristics are described using a cascaded two-port model of the laser. This model separates the electrical parasitics from the intrinsic laser and enables these subsections to be considered separately. The presentation concentrates on the small-signal intensity modulation and frequency modulation responses, and the large-signal switching transients and chirping. Device-dependent limitations on high-speed performance are explored and circuit modeling techniques are briefly reviewed.

High-speed modulation of semiconductor lasers

Optical duobinary signals have been applied to dense wavelength division multiplexing (WDM) systems with high-spectral efficiency to fully utilize a limited gain bandwidth of about 35 nm (4.4 THz) for an erbium-doped fiber amplifier (EDFA). These signals are one type of partial response signals and have narrower bandwidth than conventional intensity modulation (IM) signals. Thus, the use of these signals should make possible to attain ultradense WDM systems. In this paper, characteristics of optical duobinary and IM signals in ultradense WDM systems are compared through experimental evaluations at 20 Gb/s. High-spectral efficiency of 0.6 b/s/Hz was reached in this demonstration.

Characteristics of optical duobinary signals in terabit/s capacity, high-spectral efficiency WDM systems

A new high-performance 1.3-μm InGaAsP semiconductor laser is described, in which effective current confinement into the active region has been realized. A p-n-p-n current blocking structure is made by liquid-phase epitaxy (LPE) on both sides of the active-stripe mesa which is defined by a pair of channels in the double-heterostructure wafer. The double-channel-planar-buried-heterostructure laser diodes (DC-PBH LD's) exhibit high-laser performances, such as a high differential quantum efficiency of 78-percent maximum, which results in high electrical to optical power conversion efficiency 43 percent, and high light output power of over 50 mW, as a result of the improvement in the current blocking structure. The threshold current temperature sensitivity is found experimentally to be reduced remarkably by increasing the doping concentration in the p-cladding layer. Characteristic temperature as high as 100 K has been obtained. CW operation is possible up to 130°C.

InGaAsP double-channel- planar-buried-heterostructure laser diode (DC-PBH LD) with effective current confinement

We developed compact (1.136 mm/sup 2/) eight-channel wavelength-selectable microarray distributed feedback laser diodes (DFB-LD's) with a monolithically integrated 8/spl times/1 multimode-interference (MMI) optical combiner, a semiconductor optical amplifier (SOA), and an electro-absorption (EA) modulator. By using /spl plusmn/10/spl deg/C thermal tuning, an addressable wavelength range of 15.3 nm with uniform device characteristics and 2.5-Gb/s modulation performance were successfully demonstrated.

1.55-μm wavelength-selectable microarray DFB-LD's with monolithically integrated MMI combiner, SOA, and EA-modulator

Dispersion compensation by the prechirp technique is theoretically investigated, and experimental results are discussed. An improvement of more than two times was observed for dispersion-limited transmission lengths in 5-Gb/s, 150-km and 10-Gb/s, 50-km transmission with a semiconductor electroabsorption external modulator and a normal fiber. An allowable transmission fiber dispersion of 10000 ps/nm and 2.5 Gb/s is estimated by computer simulation with this prechirp technique. >

Prechirp technique for dispersion compensation for a high-speed long-span transmission

Flat and efficient FM responses in a wide frequency range were obtained by directly modulating the injection current of a 1.3 μm phase-tunable DFB-DC-PBH laser diode. The FM response is flat between 1 kHz and 300 MHz, and the nor malised frequency deviation is 1–3 GHz/mA, which is 5–10 times larger than that of a conventional laser diode in the several hundred megahertz region.

M. Yamaguchi

Papers

Characteristics of optical duobinary signals in terabit/s capacity, high-spectral efficiency WDM systems

InGaAsP double-channel- planar-buried-heterostructure laser diode (DC-PBH LD) with effective current confinement

1.55-μm wavelength-selectable microarray DFB-LD's with monolithically integrated MMI combiner, SOA, and EA-modulator

Prechirp technique for dispersion compensation for a high-speed long-span transmission

Realisation of flat FM response by directly modulating a phase tunable DFB laser diode