C.Y. Kuo

Dispersion-induced composite second-order distortion at 1.5 mu m

Experimental and theoretical results have shown that the composite second-order (CSO) nonlinearity of 1.5- mu m AM analog laser links is inadvertently affected by the coupling of laser chirp with fiber dispersion in regular single-mode fiber. To counteract the dispersion effect, it is shown that dispersion-shifted fiber can be employed in place of the regular fiber. Other possible ways to reduce distortion are to use lasers with reduced chirp, presumably with a multiquantum-well laser structure, or lasers with well-controlled spatial hole burning. Short fiber spans (<or=5 km) for applications providing loop distribution in conjunction with fiber amplifiers are feasible without the use of countermeasures.<<ETX>>

Erbium-doped fiber amplifier second-order distortion in analog links and electronic compensation

The authors study the second-order distortion when an erbium-doped fiber amplifier (EDFA) is used to amplify the analog optical AM cable TV (CATV) multiple carrier signal from a directly modulated distributed feedback (DFB) semiconductor laser. Experimentally, it was seen that this second order distortion depends critically on the gain of the EDFA fiber amplifier. The authors attribute this distortion to the interaction between the frequency chirping of the DFB laser and the variable gain with wavelength of the amplifier. The authors describe an electronic predistorter that compensates the nonlinearity produced by the DFB-laser-EDFA combination. As a result, the high power advantage of the EDFA can be fully realized in spite of the potential for second-order distortion in the system.<<ETX>>

Fundamental second-order nonlinear distortions in analog AM CATV transport systems based on single frequency semiconductor lasers

The author analyzes the fundamental second-order nonlinearities and their composite distortions in lightwave analog AM CATV transport systems based on single frequency semiconductor lasers. The nonlinear interaction between the photons and the electrons in the laser, the nonlinear dynamics of laser spatial hole burning, and the nonlinear interaction of fiber chromatic dispersion and laser frequency chirping are strongly dependent on the CATV carrier frequencies. The author found that the overall distortions increase for higher carrier frequencies and longer fiber spans. The distortion levels, however, are within the critical requirements for current and future CATV transport systems. >

Second-order distortion and electronic compensation in analog links containing fiber amplifiers

A detailed, wide-frequency-range analysis of the second-order distortions in optically amplified subcarrier-multiplexed lightwave video transport systems is presented. It is found that the three major system distortions are from the DFB lasers intrinsic dynamic nonlinearity, the fiber amplifiers nonuniform gain profile, and the fibers chromatic dispersion. A generic electronic circuit is proposed to compensate the distortions from all three mechanisms simultaneously. A low-distortion, amplified, 1.55- mu m system in which the fiber amplifier and dispersive distortions are suppressed using the proposed circuitry is discussed. >

Fundamental nonlinear distortions in analog links with fiber amplifiers

We analyzes the second-order distortion in a modulated laser-based AM subcarrier multiplexed (AM-SCM) video transport system using fiber amplifiers for power boosting. All three of the involved system elements: the laser, the fiber amplifier, and the transmission fiber, contribute to the system distortion. The laser distortion originates from the internal photon-electron interaction, whereas the fiber amplifier and dispersive distortion are directly related to the intrinsic frequency chirping of the modulated laser. From the distortion magnitude and phase analysis, the author finds that under most situations the distortions have an electronic counterpart and can be electronically compensated. In addition, the system distortion can be minimized with a combination of critically chosen elements that have minimum or counteracting distortion mechanisms. Optically amplified AM-SCM systems can thus be engineered to perform well within the stringent distortion requirements for both 1.3- and 1.55-μm wavelength ranges.

Linewidth reduction of 1.5‐μm grating loaded external cavity semiconductor laser by geometric reconfiguration

It is found experimentally that the linewidth of a grating loaded external cavity semiconductor laser is inversely proportional to the square of the linear dimension of the overall cavity. This observed behavior is consistent with the existing theories on the linewidth of a laser with an extended passive cavity. A slope of 6.5×105 Hz cm2 is obtained from least‐squares fitting the linewidth vs 1/(cavity length)2 curve at 1 mW of power. However, the linewidth reaches a lower limit when the cavity length is extended beyond a certain limit. This result suggests that additional phase noise term should be included in the calculation of linewidths of these extended cavity semiconductor lasers. We also report a value as small as 18 kHz mW in the linewidth vs 1/(power) curve.

Compact hybrid resonant optical reflector lasers with very narrow linewidths

Hybrid resonant optical reflector lasers of less than 1 cm length are described. These external cavity devices operate with very narrow linewidth approaching the resonator‐loss‐limited minimum. Best linewidth results are 7.1 kHz at 1.3 μm and 24 kHz at 1.55 μm.

Frequency modulation responses of two‐electrode distributed feedback lasers

Frequency modulation (FM) response of a 1.3 μm two‐electrode distributed feedback laser is studied in detail. We found that by adjusting the injection current applied to the modulated section, the FM bandwidth can be increased to a value as high as 900 MHz. However, as the bandwidth increases, the FM sensitivity decreases proportionally. This behavior is described satisfactorily by below‐threshold carrier rate equation and shown to be closely related to the injected carrier lifetime.

Nearly dispersion-penalty-free transmission using blue-shifted 1.55- mu m distributed feedback lasers

Nearly dispersion-penalty-free transmission over 40 km of nondispersion-shifted single-mode fiber was demonstrated using blue-shifted 1.55- mu m distributed feedback lasers. This good system performance is attributed to the narrow and blue-shifted modulated optical spectrum which is in turn associated with the soft turn-on light-current characteristics. Transmission experiments using lasers with hard turn-on light-current characteristics give rise to broad modulated spectra and higher dispersion penalties.<<ETX>>

A field demonstration of 1.7 Gb/s coherent lightwave regenerators

Four 1.7-Gb/s frequency-shift keying (FSK) coherent regenerators that were successfully operated in a typical field environment using installed fiber cables connected between Roaring Creek and Sunbury, PA (a distance of 35 km) are discussed. Each regenerator is housed in a standard AT&T repeater shelf and consists of five separate plug-in subsystem modules including a polarization diversity receiver, a FSK transmitter, and a cold-start automatic frequency control (AFC) circuit. During one month of continuous operation, the received optical powers for 10/sup -9/ bit error rate (BER) were between -38 and -40 dBm. An error rate below 6*10/sup -15/ (less than one error per day) was achieved at received optical powers of -33 approximately -35 dBm.<<ETX>>