RF transmission over multimode fibers using VCSELs-comparing standard and high-bandwidth multimode fibers

Fiber-optic radio-frequency links have been assembled using oxide-confined vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers. Links with single and multimode VCSELs and with standard and high-bandwidth fibers have been evaluated and compared in the frequency range of 0.1-10 GHz. The best results were obtained for links with a multimode VCSEL and a high-bandwidth fiber. For a 500-m-long link, a spurious free dynamic range of 104 dB/spl middot/Hz/sup 2/3/ at 2 GHz and 100 dB/spl middot/Hz/sup 2/3/ at 5 GHz were obtained while allowing for a VCSEL-fiber misalignment of /spl plusmn/12 /spl mu/m. Corresponding numbers for the intrinsic link gain and noise figure are -29 and -33 dB, and 39 and 42 dB at 2 and 5 GHz, respectively. Inferior performance was observed for the standard fiber link due to a larger variation in modal group velocities. This paper also presents a detailed link analysis to identify performance limitations and to suggest modifications for improved performance.     

Laser diode high-power picosecond pulses

A Finnish research group from the University of Oulu has developed a new type of laser diode structure, which allows high-power picosecond optical pulses to be obtained as a result of the direct current pumping.This is a short news story only. Visit www.three-fives.com for the latest advanced semiconductor industry news.     

Group delay in multimode optical fibers

The group delay τ in multimode fibers was previously derived by Gloge and Marcatili under the assumption that τ be dependent on the propagation constant β only. This letter derives the group delay without the above assumption and clarifies when the approach used by Gloge and Marcatili fails.     

Profile inhomogeneity in multimode graded-index fibers

Profile parameters (g) of several multimode graded-index fibers have been measured. It was found that g may vary azimuthally by ±0.15 or more in fibers for which the average value is between 1.8 and 2.2     

Low-noise and high-power Pr3+-doped fluoride fiberamplifier

We have successfully developed a low-noise and high-power Pr³⁺-doped fluoride amplifier (PDFA) module with a cascade configuration. The maximum signal gain and noise figure were 40.6 dB and 5 dB, respectively. An output power of 20.1 dBm was achieved at an input signal power of 0 dBm. We confirm that this PDFA module performs well from experimental results on its use in a 40 channel AM-VSB video signal transmission and a 10 Gbit/s digital transmission     

Model for hydrogen aging in multimode fibers

In order to explain the variation of initial aging rate on the square root of the hydrogen pressure observed in two other studies, a model based on the production of hydrogen atoms by some defect site in the glass is developed. The model is then successfully tested against the detailed temporal behavior that was reported for fibers containing high levels of phosphorus as well as those containing low levels     

A compact radiator of high-power electromagnetic pulses

The design and parameters of a compact radiator of high-power electromagnetic nanosecond pulses designed for simulation of ultra-wideband exposures in electromagnetic compatibility problems are described. It is shown that the effective pulse power of the radiator is 3 GW in the forward traveling wave and 15 GW in the wave reflected from the reflector. The effective density of the continuous energy spectrum is 17 mJ/MHz at 0.3 GHz.     

Multimode fibers for compensating intermodal dispersion of graded-index multimode fibers

This paper proposes a novel type of multimode dispersion compensation fiber (MM-DCF) by which the intermodal dispersion of multimode fibers (MMFs) can be effectively compensated. A theoretical model that precisely calculates the group velocities of a target MMF and its MM-DCF and the power coupling between the two fibers is applied to confirm the proposal presented here. As a demonstration of the operation of the MM-DCF, an MMF optimized for transmission at a wavelength of 850 nm is compensated by the MM-DCF, and its overfilled launch bandwidth at a wavelength of 1300 nm is enhanced from 0.62 to 2.56GHz/spl middot/km.     

Design parameters for fluoride multimode fibers

Numerical calculations of losses due to polymer coatings and macrobending have been made for step-index multimode fluoride fibers. To minimize such losses, fiber parameters must be chosen to give a large value for V, the normalized frequency. Due to the long propagation wavelength (2.5 μm) for fluoride fiber, the parameters needed are very different from those of silica fiber. Using the criterion that ⩾90% of the modes have losses ⩽0.01 dB/km, it was found that the cladding diameters need to be rather large compared to step-index silica fiber parameters. Although the mode distributions are unknown for fluoride fiber, using a theoretical Gaussian distribution, losses were calculated for the various fiber parameters     

Synchronous amplification of subpicosecond pulses

In high-gain dye amplifiers, the effective storage time of the gain medium is only a few hundred picoseconds. Therefore, efficient amplification of ultrashort pulses places a stringent requirement on the synchronization between the pump pulse and the pulse to be amplified. We present a technique of short pulse generation using a dye laser synchronously pumped by a frequency-doubled CW mode locked Nd:YAG laser. Pulses as short as 70 fs are produced. The short optical pulses are subsequently amplified with two different synchronous amplification schemes using 100 ps pulses to establish the gain in the dye amplifier stages. Subpicosecond pulses with energies from a few hundred nanojoules at 500 Hz to a few hundred microjoules at 7 Hz can be obtained.     

Butt-coupling efficiency of VCSELs into multimode fibers

We report a detailed study on butt coupling efficiencies of vertical-cavity surface-emitting lasers (VCSELs) to standard graded index multimode silica fibers. Coupling efficiency strongly depends on active laser diameter as well as index guiding and transverse mode spectrum of the laser. For typical active laser diameters of 16-20-/spl mu/m coupling efficiencies of about 90% are obtained with weakly index guided proton-implanted vertical-cavity lasers (VCLs) whereas stronger index guided oxidized lasers show considerably lower coupling efficiencies between 75% and 55%, depending on driving currents.     

Mode conversion at splices in multimode graded-index fibers

Mode conversions occurring at a splice in multimode graded-index fibers are investigated theoretically, and their effects on impulse responses are verified experimentally. The relation describing the mode behavior at a splice in existence of a geometrical offset and fiber parameter mismatches is derived by taking a skew ray as well as meridional ray into consideration. Then the mode transfer matrix is obtained to determine the variations in mode power distribution and impulse response due to the mode conversions occuring at a splice. The measured mean delay time difference between lower and higher mode pulse responses and baseband frequency response for spliced graded-index fibers are compared with the theory obtained from the transfer functions of individual fibers and the mode transfer matrix which describes the mode conversions at a splice due to a transverse displacement and fiber parameter mismatches.     

Bragg gratings in multimode and few-mode optical fibers

Bragg gratings in optical fibers in multimode propagation are investigated experimentally and theoretically. Bragg gratings formed in optical fibers in multimode propagation show multiple reflection peaks or multiple transmission dips in the reflection or transmission spectra, respectively. For standard graded-index multimode fiber, the number of reflection peaks of a Bragg grating depends on excitation condition of propagating modes. The number of reflection peaks of a Bragg grating at around 1.55 μm is 19 for highly multimode excitation and 3-4 for lower order mode excitation. We analyze the phase-matching conditions of the propagating modes and identify half of the reflection peaks as the reflection to the same mode and the rest as the reflection to the neighboring modes. In dispersion-shifted fiber, a Bragg grating at around 0.8 μm in three-mode propagation shows three reflection peaks in the reflection spectrum. The temperature dependence of each reflection peak is similar to that of a conventional Bragg grating in single-mode fiber. Polarization dependence measured on a Bragg grating in multimode graded-index fiber is negligible. An advantage of Bragg gratings in multimode fiber (MMF) and the applications are discussed     

Generation and radiation of high-power ultrawideband nanosecond pulses

The results of investigation of the generation and radiation of high-power ultrawideband electromagnetic pulses are presented. The possibility of obtaining wave beams of radiation with orthogonal polarizations of the electric field is shown. The radiating system is a 16-element antenna array that can be separated into two subarrays operating in orthogonal polarizations and excited by pulses separated in time. Two design versions of orthogonal subarrays are studied.     

Susceptibility of lithium-niobate modulator to high-power microwave pulses

The first measurement of the susceptibility of an off-the-shelf lithiumniobate intensity modulator to damage and disruption from high-power microwave pulses is reported. The device tested survived 1 kHz repetition rate pulses at 2.5 GHz centre frequency and 40 μs width up to 200 W peak power. The results are discussed in terms of material parameters and device characteristics.     

Generation and emission of high-power ultrabroadband picosecond pulses

The generation and emission of high-power ultrabroadband electromagnetic pulses are studied. The possibility of high-voltage (about 100 kV) bipolar voltage pulses with a duration of about 200 ps is demonstrated. The electromagnetic pulses with a FWHM of less than 100 ps, an effective potential of up to 400 kV, and a repetition rate of 100 Hz are generated using the 16-element antenna array.     

Wide passband grating multiplexer for multimode fibers

A grating multiplexer in Littrow-configuration is presented, whose passband width-normalized to channel spacing-is comparable with grating demultiplexers. As shown by calculation, a wide passband is attainable by very small input fiber spacing. Using a standard-(50/125 mum)-GI-fiber as transmission fiber, a 1-dB pass-band of 30 nm with a channel spacing of 46 nm is achieved in an 8- channel device. The insertion losses for each channel are in the range 1.4-2.5 dB for 8- or 10-channel devices.     

Group velocity matching of multimode microstructure optical fibers

Group velocity matching is a key to the performance of multimode microstructure fibers. Simulations show that only partial matching is achieved using an intuitive graded-hole strategy, but that the usual velocity-matching limit can be achieved by numerically optimizing designs. These results demonstrate the perturbative multipole numerical method and are a step toward improved strategies for achieving high-performance microstructure fibers for a multimode transmission.     

Nonlinear propagation of pulses in multimode fiber with strong linear coupling

Multimode fiber has a richer spatial dimension than single-mode fiber, and is an ideal platform for studying many novel nonlinear effects. We established a strong linear coupling and short-range fiber model to understand the interactive effects of linear coupling and nonlinear effects. We find that strong linear coupling can compensate for the group delay between eigenmodes and cause energy fluctuation between modes which weakens the nonlinear effects. In high energy pulses, the interaction of linear coupling and nonlinear effects can help producing weak dispersion waves when the spectrum is broadened. Since linear coupling in a mode group is common and unavoidable, these results may provide a certain theoretical explanation for multi-mode nonlinear phenomena.     

Theory of quasi-linear pulses in optical fibers

The evolution of quasi-linear pulses in optical fibers, with strong dispersion management, is studied analytically. The nonlinear terms of the Gabitov–Turitsyn equations are analyzed asymptotically. In this paper, the polarization preserving fibers as well as the birefringent fibers are studied. Finally, the study is extended to the case of multiple channels, namely dispersion flattened fibers. The numerical simulations support the analysis.