Computer simulation and noise analysis of the system performance of 1.55-µm single-frequency semiconductor lasers

A theoretical model for the noise analysis of the system performance of 1.55-μm single-frequency semiconductor lasers is presented. Computer simulations are used to analyze the role of various noise sources in a 1.7-Gbit/s transmission experiment where the data was transmitted over 69 km using a 1.56-μm distributed-feedback laser. The bit-error-rate curves generated from numerical simulations agree well with the results of the transmission experiment. The relative contributions of various noise sources in limiting the system performance are discussed and compared. In particular, we consider circuit noise, shot noise, laser intensity noise, mode-partition noise, parasitic reflections, and the frequency chirp.     

Analysis of chirp power penalty in 1.55-µm DFB-LD high-speed optical fiber transmission systems

In a 1.55-μm high-speed transmission system using a DFB-LD, the chirp effect in the LD is a factor limiting transmission span length. In order to evaluate the chirp effect, we derive expressions for the chirp power penalty for two cases: the cases of the chirp occurring in both edges of the pulse and in the whole time of the pulse. The calculations based on the results of the LD chirp measurement predict that the chirp effect will be significant above 2 Gbit/s even if the zero dispersion wavelength of fiber is shifted to the 1.55-μm band. Transmission experiments performed at 1.2 Gbit/s and 2.4 Gbit/s verify this prediction.     

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.     

Single and multimode fiber bandwidth measurements with single and multilongitudinal mode lasers operating at 0.8-, 1.3-, and 1.5-µm wavelength

The bandwidth characteristics of single and multimode optical fibers have been investigated with single and multilongitudinal mode laser sources operating at 0.8, 1.3, and 1.5 μm. It is shown that single-mode fiber with a cutoff wavelength of 1.3 μm can support 1 Gb/s transmission over at least 7.5 km with a 0.8-μm laser source.     

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.     

Fundamental mode size and bend sensitivity of graded and step-index single-mode fibers with zero-dispersion near 1.55 µm

Single-mode fibers, for near 1.55-μm operating system wavelength with triangular (alpha = 1), parabolic (alpha = 2), and step-index (alpha=infty) profiles were fabricated by the modified chemical vapor deposition (MCVD) technique. Optical transmission losses under zero-tension and with the basket weave under 10, 40, and 70 gm tensions were measured, respectively. Fundamental mode size was obtained as a function of wavelength by using the transverse offset technique. The triangular-profile fiber shows lower loss (≃0.25 dB/km at 1.55μm) under zero-tension and a larger spot size than the other fibers. However, the basket weave test showed the triangular-profile fiber incurred higher loss with tension than the other profiles.     

Gigabit single-mode fiber transmission using 1.3-µm edge-emitting LEDs for broad-band subscriber loops

This paper describes gigabit single-mode fiber transmission using 1.3-μm edge-emitting LED's for broad-band subscriber loops, focusing on a method of calculation for maximum transmission distance and 1.2-Gbit/s and 600-Mbit/s transmission experiments. Gigabit single-mode fiber transmission is necessary for subscriber loops, especially in broad-band ISDN and optical CATV systems. Edgeemitting LED's are excellent light sources because of their high power launched into the fiber compared with surface-emitting LED's, and currently lower cost and higher reliability than laser diodes. The maximum transmission distance is carefully estimated by taking into account the wavelength dependence for both chromatic dispersion and loss of the single-mode fiber, and the possibility of gigabit transmission near the dispersion free wavelength 1.3 μm, is confirmed. Encouraged by the above results, we demonstrate 1.2-Gbi,t/s 10-km and 600-Mbit/s 20-km transmission experiments using a newly developed 1.3-μm edge-emitting LED and a new driver circuit with a simple response compensation circuit. These results show the proposed calculation method and the LED response compensation circuit to be powerful tools for the realization of low-cost gigabit single-mode fiber transmission using edge-emitting LED's.     

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.     

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.     

Behavior of alkali impurities and their adverse effect on germania-doped silica fibers

Alkalies, such as sodium and potassium, were found to migrate from over-jacketing natural silica tubes into germania-doped cores forming defects leading to hydrogen-induced loss in the infrared wavelength region. These impurities were found to be removed from the same kind of natural silica tubes during the MCVD process. Results strongly point out that the absence of impurities is essential for 1.55-μm transmission systems based on germania-doped silica fiber.     

Fabrication of dispersion-free VAD single-mode fibers in the 1.5-µm wavelength region

Low-loss and low-dispersion single-mode fibers in the 1.5- μm wavelength region were fabricated by the VAD method. Causes for loss increase in these fibers were investigated. By improving uniformities in the refractive index, both in core section and along the core axis, minimum loss of 0.35 dB/km at 1.55 μm was obtained. Bending loss of the 1.5-μm optimized single-mode fiber was also discussed.     

Fluorescence and oscillation characteristics of LiNdP4O12lasers at 1.317 µm

Spectroscopic data and laser oscillation characteristics of the 1.317 μ line in lithium neodymium tetraphosphate (LNP) are reported. A stimulated emission cross section of this transition was spectroscopically determined as7.1 times 10^{-20}cm², which corresponds to 1/4.5 of that at the 1.047-μm transition. Cross-section temperature dependence, laser cavity loss, threshold versus crystal length, and threshold temperature dependence were measured experimentally. Since resonant loss was negligible at the 1.317 μm line, room temperature threshold is lower than that at 1.047-μm in the case of long crystal, and threshold temperature dependence is weaker than that at 1.047 μm. A miniaturized LNP laser, using a graded index fiber as a focusing medium, is also shown. LED pump intensity required to obtain a constant output is compared for 1.047- and 1.317-μm wavelengths. The 1.317-μm line seems to be useful as miniaturized optical sources in optical communication systems, since required intensity is around several W/cm²for the side pump, and the wavelength corresponds to the most transparent band of ultra low-loss optical fibers.     

Repeater spacing of 280 Mbit/s single-mode fiber-optic transmission system using 1.55 µm laser diode source

This paper describes an attainable repeater spacing for a high bit-rate single-mode fiber-optic transmission in the 1.55 μm wavelength region where laser mode partition noise comes to be significant. An expression for evaluating mode partition noise is given as the form involving the influence of laser spectral fluctuations under high bit-rate modulation, together with the intersymbol interference and the equalized pulse shape in tile optical receiver. After the validity of its numerical results is confirmed experimentally, the resulting evaluation of laser mode partition noise is connected to a systematic discussion on the attainable repeater spacing of a 280 Mbit/s fiber-optic transmission system operating at 1.55 μm, along with fiber loss versus dispersion tradeoffs. This discussion permits the attainable repeater spacing to be 60-70 km for the combination of a laser diode with 1.5-2.0 nm spectrum broadening and a fiber with the loss of 0.5 dB/km and the dispersion of 4-6 ps/km - nm.     

Raman amplification of 1.50-µm laser diode light in a low fiber loss region

Raman amplification utilizing the second Stokes line of 1.32-μm Nd:YAG laser light in a single-mode silica fiber was observed. Raman gains of 24 dB have been obtained for 1.50-μm laser diode light in the low-loss region. In addition, the signal-to-noise characteristics of Raman amplification using the second Stokes line was investigated. As a result, it became clear that theS/Nratio was inferior to that in 1st Stokes line by about one order of magnitude.     

Very high speed intra-office optical link experiments for 0.4- and 1.6-Gbit/s trunk lines

Very high speed optical links are studied. Applicable areas for systems using light-emitting diodes (LED'S) or laser diodes (LD's) with short transmission length are clarified. A new type differential mode inversion (DMI) decoder is proposed and it is shown that the DMI code is suitable as the line code for very high speed intra-office optical links. By using DMI code. a 400-Mbit/s information rate optical transmission experiment employing a 1.3-μm InGaAsP LED and a 0.5-km graded-index multimode optical fiber (GIF) as well as a 1.6-Gbit/s information rate optical transmission experiment using a 1.3-μm InGaAsP/InP Fabry-Perot-type LD and a 10-km single-mode optical fiber (SMF) are carried out. These results show that the feasibility of a 400-Mbit/s intra-office optical link using the LED and GIF, as well as a 1.6-Gbit/s intra-office optical link using the LD and SMF, are confirmed and this optical transmission technology has high-speed performance up to 3.2 Gbit/s.     

Practical limitations on optical amplifier performance

Practical issues are considered to determine if significant improvements in the sensitivity, of 1.55-μm optical receivers can be realized through the use of semiconductor laser preamplifiers. It is found that practical problems related to realizable values of population inversion parameter, input coupling losses, external optical filter requirements, polarization effects, and source laser stability make it unlikely that optical preamplifiers can approach their optimum performance at data rates below several Gbit/s. The most promising application of optical preamplifiers will therefore be to increase the sensitivity of receivers in future fiber optic communication systems operating at multigigabit data rates.     

Noise properties of a Raman amplifier

We present the first measurements of the noise properties of a Raman optical amplifier. Measurements on CW Raman amplified optical signals from a 1.5-μm single frequency laser in 32 km of silica fiber, pumped by a 1.45-μm color center laser, reveals that the main noise source is the shot noise generated in the receiver due to the spontaneous Raman radiation. For signal levels down to -50 dBm, no excess noise due to the Raman process could be detected.     

Design and performance of Gaussian-profile dispersion-shifted fibers manufactured by VAD process

Design and manufacturing of dispersion-shifted fibers with a Gaussian profile, which is compatible to the VAD process, are studied. From the viewpoint of stable cabling, the parametric analysis is made with emphasis on the bending loss. To make the bending sensitivity of the Gaussian core fibers comparable with the conventional 1.3- μm zero-dispersion fibers, an 0.8 percent or higher index difference is required. In the experiment, the achieved loss at the 1.55-μm dispersionless wavelength is 0.21 dB/km. The effects of a ring profile around the Gaussian core on the fiber transmission characteristics also are examined. While the cutoff wavelengths are greatly lengthened by the ring, the bending sensitivity is not so affected. It is found that the index difference of the main core still dominates the bending loss of the fiber.     

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.     

Optical fiber tapers at 1.3 µm for self-aligned beam expansion and single-mode hardware

We have experimentally demonstrated the applicability of optical fiber tapers at 1.3 μm as a simple and practical means of achieving beam expansion in a self-aligned unitary structure. These devices have a standard 8.1-μm core at one end which gradually increases in cross section to the order of 100 μm at the other end. Tapers are envisioned as basic building blocks in a multitude of single-mode optical components. Experiments performed on a batch of eight tapers verified, to experimental accuracy, that no significant amount of mode conversion or beam distortion takes places in the taper. The insertion loss of the taper was found to be under 0.1 dB. The sensitivity of the excess loss between two tapers to lateral and axial displacement is greatly reduced as compared to that between two single-mode fibers. For example, the 0.5-dB loss point of taper coupling corresponds to a 10-μm lateral displacement and a 700-μm axial displacement versus respective 1.6-μm and 36.5-μm displacements for fiber coupling. The increased sensitivity to angular displacement is within practical limits.