The optical nonlinearity in a single-mode fiber imposes a fundamental limitation on the capacity of optical frequency-division multiplexed (OFDM) systems. In particular, four-wave mixing (FWM) crosstalk may severely degrade the system performance when the fiber input powers are large and/or the channel spacing is too small. Theoretical and experimental results of the effects of FWM in OFDM systems are presented. The theoretical results demonstrate the dependence of FWM on various systems parameters. Also included is an analysis of FWM in both unidirectional and bidirectional transmission systems. The receiver sensitivity degradation from FWM crosstalk was measured in a 16-channel coherent system. A sensitivity penalty of 0.4 dB resulted when a signal power of -3 dBm/channel was transmitted through 12 km of dispersion-shifted fiber.
We discuss the design, fabrication, and performance of experimental multiwavelength laser array transmitters that have been used in the reconfigurable optical network testbed for the Optical Network Technology Consortium (ONTC). The experimental four-node multiwavelength network testbed is SONET/ATM compatible. It has employed multiwavelength DFB laser arrays with 4 nm wavelength spacing for the first time. The testbed has demonstrated that multiwavelength DFB laser arrays are indeed practical and reproducible. For the DFB laser arrays used in such a network the precise wavelength spacing in the array and the absolute wavelength control are the most challenging tasks. We have obtained wavelength accuracy better than &0.35 nm for all lasers, with some registered to better than 10.2 nm. We have also studied the array yield of our devices and used wavelength redundancy to improve the array yield. Coupling efficiencies between -2.1 to -4.5 dB for each wavelength channel have been obtained. It is achieved by using specially designed lensed fiber arrays placed on a silicon V-grooved substrate to exactly match the laser spacing. The transmitter consisted of a multichip module containing a DFB laser array, an eight-channel driver array based on GaAs IC's, and associated RF circuitry.
A theoretical and experimental study of a new, efficient technique to couple a broad-area laser, emitting a highly elliptical beam, to a single-mode fiber without the use of bulk optical components is presented. The technique involves butt coupling the laser to a wedge-shaped fiber endface. Such a n endface approximates a cylindrical lens which corrects for the phase front mismatch between the curved laser beam wavefront and the planar fiber beam. The fabrication process uses a wedge-shaped polishing tool and a simple polishing procedure. A theoretical formula for the coupling efficiency in the absence of both angular and transverse misalignments is derived. Based on the estimated mode field radii of the two-dimensional laser beam and assumed mode field radius of the fiber beam, a maximum coupling efficiency of 46% is predicted by the theory compared to the measured value of 47% (15.2-mW power coupled to the single-mode fiber) obtained by using a well-designed wedge-shaped fiber endface. For the square endface, the measured coupling efficiency was 20%. The technique was further refined by incorporating an uptapered, wedge-shaped endface to decrease the transverse misalignment sensitivity. The transverse misalignment tolerance for 3-dB reduction in maximum coupled power increases from 0.4 pm for the straight fiber wedge shape to 0.7 pm for the uptapered wedge shape. Using this technique, a single 980-nm, 30-pm stripe width, broad-area laser provided enough power to pump an erbium-doped fiber amplifier to obtain 24-dB gain. Richard S. Vodhanel (M'89) received the B.S. degree from the University of California, Irvine, in 1974, and the M.S. and Ph.D. degrees from the University of Illinois, Urbana, in 1976 and 1981, respectively, all in physics. His doctoral dissertation was on nuclear gamma ray spectroscopy.In 1980 he joined Bell Laboratories as a member of the Technical Staff, where he conducted research on the topic of single-mode fiber transmission systems. In 1984 he joined Bellcore, Red Bank, NJ, where he is presently engaged in research on coherent-optical fiber transmission systems. He has over 70 publications in physics and optical fiber communications.Dr.
Abstract-We monolithically integrate an optical front-end on InP for balanced, polarization-diversity coherent lightwave reception which is only 1 3 mm long. Low on-chip insertion loss (<4.5 dB) and balanced photoresponse (1.05:l or better) are achieved at 1.5 pm wavelength using straightforward, regrowth-free fabrication. Low capacitance photodetectors ( I 0.15 pF) are employed for high bandwidth operation.ONOLITHIC integration of optical waveguide de-M vices with other optoelectronics enhances on-chip functionality, and improves packaging cost and reliability by minimizing hybrid optical interconnections. A target application for such integration has been coherent lightwave receiver front-ends, combining photodetectors with 3 dB waveguide couplers for balanced operation [11-[6] and polarization-splitting optics for polarization-insensitive reception using diversity architectures [5], [6]. Compatibility with high 111-V materials' cost, however, requires high-yield integration processes and compact device size. The large size, typically several millimeters, of waveguide couplers with associated input-output branching is a serious obstacle to cost-effective monolithic integration. We previously reported alternative approaches for ultra-compact 3 dB couplers [7] and polarization-selective detectors [8]. Here we integrate both device types on a single InP chip to realize an ultracompact (1.3 x 0.4 mm'), balanced, polarization-diversity photodetector which is easy to fabricate (no epitaxial regrowth, minimal photolithography) and exhibits low detector capacitance (< 0.15 pF).Our device (Fig. 1) consists of two single-mode input rib waveguides for photosignal and local oscillator (LO) inputs, a multi-mode interference [9] (MMI)
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