Gigabit/s optical receiver sensitivity and zero-dispersion single-mode fiber transmission at 1.55 µm

High-speed pulse response and receiver sensitivity at 1.55 μm were measured at data rates ranging from 400 Mbits/s to 2 Gbits/s, in order to elucidate characteristics of a reach-through p+nn- Ge APD. The p+nn- Ge APD receiver provided a 2 Gbit/s received optical power level of -32.0 dBm at 1.55μm and a 10^-9error rate, which was 4 dB better than the receiving level with a p+n Ge APD. Detector performance at 1.3μm was also studied for comparison with performance at 1.55μm. Single-mode fibers, which have 0.54 dB/km loss and zero dispersion at 1.55μm, and an optical transmitter-receiver, whose repeater gain is 29.2 dB, have enabled 51.5 km fiber transmission at 2 Gbits/s. The transmission system used in this study has a data rate repeater-spacing product of 103 (Gbits/s) . km at 1.55μm. Optical pulse broadening and fiber dispersion were also studied, using 1.55 and 1.3μm dispersion-free fibers. Future repeater spacing prospects for PCM-IM single-mode fiber transmission systems are discussed based on these experimental results.     

Detailed evaluation of an attainable repeater spacing for fibre transmission at 1.3 ?m and 1.55 ?m wavelengths

An attainable repeater spacing at 1.3 ?m and 1.55 ?m wavelengths using a step-index monomode fibre cable is given, after detailed consideration of total loss as well as total dispersion of an optical fibre. The longest attainable repeater spacing at 1 Gb/s is about 100 km at 1.55 ?m wavelength using a single-mode laser diode and a monomode fibre with a relatively small core-cladding refractive index difference, e.g. about 0.2%.     

Design and performance of ultra-low-loss single-mode fiber cable in 1.5-µm wavelength region

The structural optimization of single-mode fiber for use in the 1.5-μm wavelength region is made with the aim of minimizing the total transmission-line loss over a repeater section. For example, the optimum ranges of the mode-field radius w0and effective cutoff wavelength λceare determined as 5.5 μmleq w_{0} leq 6.5 mum and 1.35μmleq lambda_{ce} leq 1.53 mum for high bit-rate transmission systems with a repeater spacing of 80 km. Based upon the design, ultra-low-loss single-mode fiber cables are fabricated. The average loss of 108 fibers in the cables is 0.19 dB/km at 1.55 μm. The total loss of a 216-km-long fiber link containing 107 splice points was 46.3 dB. Good loss stability at high temperatures as well as during the cable manufacturing processes, are achieved by the appropriate choices of coating materials and optimized fiber parameters.     

Long-span single-mode fiber transmission characteristics in long wavelength regions

Experimental and analytical results on high-speed optical pulse transmission characteristics for long-span single-mode fibers by using InGaAsP lasers, emitting at 1.1, 1.3, and 1.5 μm, as well as a Ge-APD are reported. At 1.1 μm, 400 Mbit/s transmission experiments were successfully carried out with 20 km repeater spacing. At 1.3 μm, where single-mode fiber dispersions approach zero, error rate characteristics showed that optical power penalties at 100 Mbits/s and 1.2 Gbits/s are negligible even after 30 and 23 km fiber transmission, respectively. It was confirmed that a 1.6 Gbit/s transmission system has 15 km repeater spacing. At 1.5 μm, where silica fibers have ultimately minimum loss, single-mode fiber transmission experiments were carried out at 100 Mbits/s with about 30 km repeater spacing. 400 Mbit/s transmission characteristics using 20 km fibers were also studied. Fiber bandwidths, measured by optical pulse broadenings after 20 km transmission, were 24, 140, and 37 GHz . km . nm at 1.1, 1.3, and 1.5 μm, respectively. Progress in lasers, fibers, and optical delay equalizers at 1.5μm will bring about large-capacity transmission systems having about 150 km repeater spacing. These results reveal fiber dispersion characteristics in the long wavelength region essential to high data rate single-mode fiber transmission system design.     

Gigabit/s Optical Receiver Sensitivity and Zero-Dispersion Single-Mode Fiber Transmission at 1.55 µm

High-speed pulse response and receiver sensitivity at 1.55 µm were measured at data rates ranging from 400 Mbits/s to 2 Gbits/s, in order to elucidate characteristics of a reach-through p/sup +/nn/sup -/ Ge APD. The p/sup +/nn/sup -/ Ge APD receiver provided a 2 Gbit/s received optical power level of -32.0 dBm at 1.55 µm and a 10/sup -9/ error rate, which was 4 dB better than the receiving level with a p/sup +/n Ge APD. Detector performance at 1.3 µm was also studied for comparison with performance at 1.55 um. Single-mode fibers, which have 0.54 dB/km loss and zero dispersion at 1.55 µm, and an optical transmitter-receiver, whose repeater gain is 29.2 dB, have enabled 51.5 km fiber transmission at 2 Gbits/s. The transmission system used in this study has a data rate repeater-spacing product of 103 (Gbits/s) /spl dot/ km at 1.55 µm. Optical pulse broadening and fiber dispersion were also studied, using 1.55 and 1.3 µm dispersion free fibers. Future repeater spacing prospects for PCM-IM single-mode fiber transmission systems are discussed based on these experimental results.     

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 GeO2-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.     

Measurement of mode partition in DFB lasers

Some DFB (distributed feedback) laser diodes have a satellite mode beside a main DFB mode even if FP modes are suppressed. In this paper, the mode partition noise is presented for several DFB lasers operating in multilongitudinal modes. The results show that under modulation at 140 Mbit/s, the mode partition coefficient k²of multimode DFB lasers is very small and at most 0.02 while that of FP lasers biased at the threshold level is 0.03 to 0.12. The numerical evaluation of the mode partition effect in two-mode DFB lasers suggests that a 20- dB suppression of the satellite mode power is enough to achieve a repeater spacing of over 100 km in the 280 Mbit/s fiber-optic transmission system with less than 0.1-dB power penalty.     

Characteristics of dispersion free single-mode fiber in the 1.5 µm wavelength region

Characteristics of dispersion free single-mode fibers in the wavelength regions 1.5 and 1.3 μm are compared experimentally and theoretically. We consider the influence of the refractive index profile on dispersion, the tolerance limits of structure parameters for minimum dispersion, attainable fiber bandwidth, and transmission loss including splicing and bending losses. For a fiber designed for minimum dispersion at 1.5 μm, the measured fiber loss was less than 1 dB/km and bandwidth was 250 GHz. km. nm. The achievable minimum loss estimation shows the advantage of dispersion free fibers at the 1.5 μm wavelength over dispersion free fibers at 1.3 μm.     

Single-mode fiber design for long haul transmission

By using simple yet accurate approximations for the propagation characteristics of a single-mode optical fiber, we obtain a simple model for the total loss and chromatic dispersion of single-mode fiber transmission lines as a function of the operating conditions such as splice offset, microbending loss, bends, etc. This model is then applied to typical cases of terrestrial and submarine systems and we obtain single-mode fiber designs which are stable with respect to slight operating condition changes for both 1.3 and 1.55 μm wavelengths. It appears that the same fiber can be used at 1.3 μm for both terrestrial and submarine systems, and even for 1.55 μm terrestrial systems if monochromatic sources become available at this wavelength. A general comparison between the two wavelengths is carried out and shows under which conditions the 1.55 μm wavelength is of practical interest. It is emphasized that the availability of monochromatic sources at 1.55 μm would make a major breakthrough for the repeater spacing.     

Lightwave primer

This paper presents an introduction to the principles of lightwave system engineering. The treatment is historical rather than categorical-lightwave systems are described in terms of their evolution through four generations of technology, from a first generation operating at 0.85 μm wavelength over multimode fiber to a fourth generation employing coherent techniques at 1.55 μm. Basic engineering considerations such as fiber dispersion and receiver sensitivity are introduced early, then refined as the discussion progresses toward higher-performance, more sophisticated systems. The fundamental mechanisms that limit the performance of a given technology are quantified, and a figure of merit, the product of bit rate times maximum repeater spacing, is estimated. Values of this product range from about 2 Gbits/s . km for first-generation technology to roughly 900 Gbits/s . km for coherent systems.     

Configuration of the optical transmission line using stimulated Raman scattering for signal light amplification

Expansion of a repeater spacing of optical transmission line is investigated using forward stimulated Raman scattering generated by a 1.34-μm Nd:YAG laser to amplify the signal light. Achievable repeater spacing at the 1.42-μm wavelength transmission is numerically estimated on the basis of coupled power equations and experimentally obtained Raman gain for various core-cladding index difference. A transmission distance of more than 200 km is predicted for an input signal light power of 100 μw and a pump power of 200 mW using a fiber with a relative index difference of 1 percent.     

Design considerations for the structural optimization of a single-mode fiber

Design considerations are made for the structural optimization of single-mode fibers used in high-bit-rate and long-haul transmission systems in the long-wavelength region. As the basic fiber parameters, a combination of the spot size W0and the effective cutoff wavelength λceis newly chosen, because the combination is found to suitably describe various actual index profiles which deviate from an ideal step-index profile. A procedure to specify the usable range of W0and λceis established, whereby the overall transmission-line loss in one repeater section is calculated using simple expressions for fiber intrinsic loss, excess loss in the cabling process, and splice loss, etc. The optimum values for a 400 Mbit/s transmission system operating at 1.3 μm with a repeater spacing of 20 km are obtained asW_{0} = 5.0 pm 0.5mum and 1.1 μmleqlambda_{ce}leq 1.28 mum taking into consideration the additional requirement for the possible use atlambda=1.55 mum     

445 Mbit/s transmission field experiment at 1.55-µm wavelength region over 80 km using undersea optical-fiber cable

New transmission equipment employing a 1.55-μm distributed feedback laser diode (DFB-LD) to overcome fiber dispersion has been tested at environmental conditions using 1.3-μm zero-dispersion fiber cable on the undersea section of route F-400M. The DFB-LD's dynamic spectrum characteristics, in relation to power penalty, were examined and a suitable laser prebias control level was obtained. Field experimental transmission lines operated error free for a two-month period, and applicability to 1.55-μm 445-Mbit/s systems of over 100 km was shown.     

Waveform distortion evaluation for optical CPFSK heterodyne transmission

Waveform distortion of a continuous phase FSK signal due to fiber chromatic dispersion is measured. The transmission spacing is estimated to be 100 km at 5 Gbit/s with 1.55-μm wavelength. Also the 2-Gbit/s optical CPFSK heterodyne detection transmission experiment is reported. Since there is no chirping degradation, it is possible to transmit the signal through a 200-km single-mode fiber.     

An optical bit-rate flexible transmission system with 5-Tb/s-kmcapacity employing multiple in-line erbium-doped fiber amplifiers

An optical bit-rate flexible transmission system using erbium-doped fiber amplifiers (EDFAs) is proposed, and the system design is discussed. An optical bit-rate flexible system using multiple in-line erbium-doped fiber amplifiers has produced a regenerative repeater spacing of 505 km at 10 Gb/s and 523 km at 5 Gb/s for direct-detection systems. This system proves that an optical bit-rate flexible system with a transmission capacity of 5.05 Tb/s-km can be feasibly constructed. System capacity is clarified both theoretically and experimentally. The power penalties involved are discussed. The related optical and electrical circuits proved operational above 10 Gb/s. The 523 km at 5 Gb/s and 505 km at 10 Gb/s transmission experiments successfully demonstrated that EDFAs effectively enhance a system's transmission capacity     

Laser Mode Partition Noise Evaluation for Optical Fiber Transmission

An evaluation of the influence of mode partition noise on error rate performance in a high bit rate optical fiber transmission system is presented. First, it is experimentally clarified that the intensity in each longitudinal mode of a laser diode fluctuates, although the intensity for the total mode is constant. It is also established that this fluctuation causes degradation of the error rate performance after transmission through a long optical fiber. The fluctuation is named "mode partition noise." Next a simple model for the fluctuation is proposed. The characteristics of the fluctuation are discussed on the basis of this model. Optical waveform fluctuation is found to be introduced by mode partition noise in the course of transmission through a long despersive transmission medium. This optical waveform fluctuation and its frequency spectrum are calculated. Finally, the error rate performance is evaluated and specifications required for a laser spectrum to attain a given repeater spacing are clarified.     

1.55-μm fiber-optic transmission experiments for long-spansubmarine cable system design

Results of 1.55-μm transmission experiments at 140, 280, and 565 Mb/s involving conventional and dispersion-shifted single-mode fibers along the Fabry-Perot laser diode (FP-LD) and distributed-feedback laser diode (DFB-LD) optical sources are discussed. The results show which combination of optical fiber and optical source best meet the requirement of long repeater spacing for each bit rate. The results indicate that to achieve repeater spacing more than 100 km with dispersion-shifted fibers and FP-LD optical sources will impose strict requirements on both the optical fibers and the optical sources even at 280 Mb/s. Alternatively, systems using DFB-LD optical sources will not degrade the transmission performances and will considerably loosen requirements on the fibers and optical sources. A combination of dispersion-shifted fibers and DFB-LD optical sources can further loosen the requirements on the fibers and optical sources in 560-Mb/s systems     

A universal fiber-optic (UFO) measurement system based on a near-IR fiber Raman laser

A universal fiber-optic measurement system, which is useful for measuring loss and dispersion in the1.06-1.6 mum wavelength region, is described. The source is a silica fiber Raman laser pumped by a mode-locked andQ-switched Nd:YAG laser at 1.06 μm. Subnanosecond multiple-Stokes pulses in the1.1-1.6 mum wavelength region are generated in a low-loss single-mode silica fiber. The use of this near-infrared fiber Raman laser for characterizing various transmission properties of single and multimode test fibers is demonstrated. Loss spectra, intramodal dispersion, and intermodal dispersion data are obtained in the wavelength region of minimum loss and minimum material dispersion for silica fibers.     

Long-wavelength optical fiber communication

Recent research on long-wavelength lightwave communication utilizing the wavelength region between 1.3 and 1.6 µm is reviewed with an eye toward future system development. The attraction of the long-wavelength region is the availability of the ultimately low-loss and wide-band features of the silica fiber, where minimum loss is 0.27 dB/km at a wavelength of 1.3 µm and 0.16 dB/km at 1.55 µm. The single-mode fiber has found its first significant applications in long-wavelength systems. The specific characteristics of lightwave components are discussed with focus on physical fundamentals. The practical performance of fibers and lightwave devices is surveyed. The dynamic properties of long-wavelength laser diodes are discussed in relation to fiber characteristics. The noise characteristics of long-wavelength detectors are considered for the purpose of specifying the repeater spacing. Some system studies are reviewed, for example, 1.3-µm-wavelength lightwave systems, which have demonstrated bandwidth-distance products of about 40 GHz ċ km. Various approaches to extend the capacity of long-wavelength lightwave transmission are given. In the future, the 1.5-µm wavelength system could operate at the lowest loss wavelength region extending from 1.5 to 1.65 µm. Much higher performance, for example, bandwidth-distance products of 185 GHz ċ km, achieved by further continuation of research and development on lightwave sources as well as fibers. Because of the author's familiarity with work in Japan, that work is emphasized and most frequently cited.     

Characteristics of Gbit/s optical receiver sensitivity and long-span single-mode fiber transmission at 1.3 µm

Sensitivity of a 1.3 μm Ge APD receiver was measured at data rates ranging from 100 Mbits/s to 2 Gbits/s, using a high-speed GaAs FET RZ driver, low-noise Si bipolar transistor (BIT) receiver amplifier, and a highly sensitive TD comparator. The required received optical level at a 10^-9error rate was -31.9 dBm for 2 Gbits/s with a Ge APD/Si BIT front end having a 50 Ω input impedance. A Ge APD/ GaAs FET front end, with a 500 Ω input impedance, brought about 2 dB improvement at 100 Mbits/s, as compared with a Ge APD/Si BIT (50 Ω) front end. A coupling loss of 4 dB, achieved by a hemispherical microlens tipped on a single-mode fiber, and a low fiber loss of 0.57 dB/km, including splice loss, enabled 44.3 km single-mode fiber transmission at 2 Gbits/s. The 1.3 μm transmission system has a data rate repeater-spacing product of 88.6 (Gbit/s)km. Prospects of Gbit/s receiver sensitivity and the 2 Gbit/s transmission system, with more than 50 km repeater spacing, are also discussed.