Tatsuya Kimura

Coherent optical fiber transmission systems

The feasibility of coherent optical fiber transmission systems, in which laser coherence is utilized to carry information, are studied from the view point of device and system consideration and expected performance. Modulation scheme comparison among optical ASK, FSK, and PSK, transmitter and receiver configurations, single-polarization transmission through fibers, problems in providing stable local oscillator waves, and wide-band photodetectors are discussed together with alternative technologies to realize high performance systems. A study on digital system impairments caused by FM quantum noise of laser oscillators, interferometric FM-AM conversion noise, IF frequency fluctuation, and optical fiber transmission turbulence show the feasibility of the systems. Repeater spacing is estimated by considering the transmitting and receiving signal levels, optical fiber loss, and fiber transmission capacity. Repeater spacing of 240 km is feasible by 400 Mbit/s PSK homodyne-detection system and 220 km by 400 Mbit/s FSK heterodyne-discrimination detection system. The regenerative repeater spacing can be expanded further by employing intermediate repeaters with direct optical signal amplification. Based on the performance of semiconductor laser amplifiers, such as traveling wave type, Fabry-Perot cavity type, and injection locked devices, it is estimated that regenerative repeater spacing of more than 104km is feasible with 50 km intermediate repeater spacing. These systems will find application in transoceanic optical fiber cable transmission as well as terrestrial long distance transmission systems.

S/N and error rate performance in AlGaAs semiconductor laser preamplifier and linear repeater systems

The applications of AlGaAs semiconductor laser preamplifier and linear repeaters in single mode optical fiber transmission systems were studied through the baseband signal-to-noise ratio and bit error rate performance measurement. Experiments were carried out with the Fabry-Perot cavity laser amplifiers whose characteristics are improved by reducing the input mirror reflectivity to 6 percent. The use of a preamplifier improves the minimum detectable power by 7.4 dB over the Si-APD direct detection level when the received signal is amplified by 30 dB before photodetection. The use of two linear repeaters increases the regenerative repeater gain by 37 dB. These experimental results are in good agreement with theoretical predictions based on the photon statistic master equation analysis.

Characteristics of a hemispherical microlens for coupling between a semiconductor laser and single-mode fiber

A hemispherical microlens is fabricated on the end of a single-mode fiber by an electric arc discharge technique. It improves coupling efficiency between InGaAsP lasers with buried heterostructure geometry and single-mode fiber. The lowest coupling loss of 2.9 dB is achieved with the optimum lens radius of 8.5 μm. This loss is 4.4 dB lower than that with a butt joint. Experimental results of coupling efficiency and alignment tolerances in coupling with different lens radii in the range of3.5-17 \mum are discussed in detail. The results are in good agreement with theoretical values derived by Gaussian beam and paraxial ray approximations.

Optical FSK heterodyne detection experiments using semiconductor laser transmitter and local oscillator

Frequency modulated semiconductor laser signals are demodulated by optical heterodyne detection using an independently temperature stabilized semiconductor laser local oscillator and a square-law detector followed by electrical frequency discrimination circuitry. Short-term and long-term beat frequency stability of the free-running laser transmitter and the local oscillator are delineated. Direct frequency modulation characteristics of the laser transmitter are studied by observing the intermediate frequency spectra. Frequency modulation-demodulation of pulse pattern signals at 100-200 Mbit/s is demonstrated. Reduction of the intermediate frequency fluctuation to less than 5 MHz is realized by employing a feedback of the RF frequency discriminator output to the local oscillator injection current.

Degree of polarization in anisotropic single-mode optical fibers: Theory

The degree of polarization for propagation waves in anisotropic single-mode fibers is formulated in terms of light source spectrum, incident polarization condition, and fiber parameters. The polarization degree deterioration is based on the incident wave split into two eigenpolarization modes inherent in the fiber. Since the two eigenpolarization modes have different group velocities from each other, the degree of polarization is degraded when both of the modes are excited. Polarization degree is preserved when only one of the eigen-polarization modes is excited. The degradation is determined by the mutual correlation function γ, between the two modes, which depends on the light source spectra, fiber polarization dispersion, and fiber length.

Chapter 3 Optical Amplification by Semiconductor Lasers

Publisher Summary This chapter discusses the design and operating characteristics of optical amplifiers that have structures similar to semiconductor lasers. Because the semiconductor lasers inherently contain light amplification processes by stimulated emission, they can be utilized as optical amplifiers besides oscillators. The chapter concentrates on the device characteristics of a linear amplifier. It describes characteristics for injection-locked laser amplifiers. Optical frequency-modulated (FM) signal amplification and FM noise reduction are also discussed in the chapter. The chapter provides a look at system applications for these amplifiers. The linear amplifier is evaluated as a preamplifier and as a linear repeater in terms of the signal-to-noise ratio. System performance features confirmed in experiments are discussed and applications of injection-locked amplifiers to intensity-modulated (IM) and FM signals are reviewed in the chapter.

High-speed optical pulse transmission at 1.29-µm wavelength using low-loss single-mode fibers

Optical-fiber transmission experiments in the 1.3-μm wavelength region are reported. GaInAsP/InP double-heterostructure semiconductor laser emitting at 1.293 μm is modulated directly in nonreturn-to-zero (NRZ) codes at digit rates tanging from 100 Mbit/s to 1.2 Gbit/s. Its output is transmitted through low-loss GeO 2 -doped single-mode silica fibers in 11-km lengths. Transmitted optical signals are detected by a high-speed Ge avalanche photodiode. Overall loss of the 11-km optical fibers, including 11 splices, is 15.5 dB at 1.3 μm. Average received optical power levels necessary for 10-9error rate are -39.9 dBm at 100 Mbit/s and -29.1 dBm at 1.2 Gbit/s. In the present system configuration, the repeater spacing is limited by loss rather than dispersion. It seems feasible that a more than 30 km repeater spacing at 100 Mbit/s and a more than 20 km even at 1.2 Gbit/s can be realized with low-loss silica fiber cables, whose loss is less than 1 dB/km. Distinctive features and problems associated with this experimental system and constituent devices are discussed.

S/N and Error Rate Performance in AlGaAs Semiconductor Laser Preamplifier and Linear Repeater Systems

The applications of AlGaAs semiconductor laser preamplifier and linear repeaters in single mode optical fiber transmission systems were studied through the baseband signal-to-noise ratio and bit error rate performance measurement. Experiments were carried out with the Fabry-Perot cavity laser amplifiers whose characteristics are improved by reducing the input mirror reflectivity to 6 percent. The use of a preamplifier improves the minimum detectable power by 7.4 dB over the Si-APD direct detection level when the received signal is amplified by 30 dB before photodetection. The use of two linear repeaters increases the regenerative repeater gain by 37 dB. These experimental results are in good agreement with theoretical predictions based on the photon statistic master equation analysis.

Existence of eigen polarization modes in anisotropic single-mode optical fibers

The existence of eigen polarization modes in a twisted single-mode fiber with an elliptical core is verified theoretically and experimentally. The eigen polarization modes are transmitted without degradation in the degree of polarization. Ellipticity and the principal axis orientation of the eigen polarizations are determined by fiber-core ellipticity and fiber twist. The ellipticity of the eigen polarization modes remains unchanged for a fixed core ellipticity and twist effect, even after long-distance transmission, whereas the orientation of the principal axis varies according to the fiber twist angle. For a sufficiently large twist rate, all the incident polarizations maintain their ellipticity of polarization.

Progress of coherent optical fibre communication systems

The recent progress of coherent optical fibre communication systems is reviewed. System constituent technologies, such as coherent optical modulation-demodulation, optical direct amplification for repeaters and single polarization fibre transmission are outlined. Several important optical device technologies, such as frequency stabilization of semiconductor lasers, AM and FM quantum noise and their reduction, and integrated opto-electronic devices, are also described. Finally, on the basis of the current state of the art in these technological areas, the expected system performance and future problems are discussed.