Modal Noise Research Articles

Speckle and Modal Noise Techniques for Coupling of Light between Optical Graded Index Fibers

Distribution d'intensite de diagrammes de speckle a la surface de sortie d'une fibre multimode a gradient d'indice en fonction de l'alignement du faisceau incident sur sa face de sortie

Modal noise arising from slightly nonuniform quantum efficiency of a detector

Modal noise caused by the nonuniform quantum efficiency of a detector is analyzed. Laser partition noise is taken into account in the analysis. The signal-to-noise ratio degrades to about 30 dB in the presence of only a 2% nonuniform detector quantum efficiency. The signal-to-noise ratio dependence on laser-partition noise is clarified.

Power loss, modal noise and distortion due to microbending of optical fibers

Theoretical and experimental investigations are described for determining the transmission characteristics of a multimode fiber with microbending for coherent and partially coherent illumination. The measured values of the average excess power loss are shown to be in close agreement with the theory. Also, an estimate of the excess transient loss due to mode coupling is found to be in good agreement with previously published data. Mode-mode interference is shown to be the cause of temporal fluctuations in the microbending loss, from which expressions for modal noise and baseband/subcarrier nonlinearity are derived on a statistical basis. For a given overall loss, the results show that many uniformly distributed small amplitude microbends cause much less modal noise and distortion than a few large amplitude microbends.

Statistical properties of laser speckles produced under illumination from a multimode optical fiber

In this paper we study the statistical properties of both speckle patterns incident upon and emerging from a diffuse object under illumination of the laser light generated from a multimode optical fiber. The speckle field resulting from the modal noise at the exit face of the optical fiber is treated as a quasi-homogeneous monochromatic source that describes the quasi-stationary state within a finite region and, simultaneously, has the correlation coefficient that remains to be strictly stationary. The speckle pattern at the object plane in the Fresnel-diffraction field is first investigated by analyzing the propagation of the correlation function of the optical fields emerging from the source and found also to be quasi-homogeneous. Next, the correlation function of the optical field produced at the observation plane by scattering from a moving diffuse object under illumination of the above speckle pattern is analyzed in connection with the source parameters.

Leaky-mode loss of the second propagating mode in single-mode fibers with index well profiles

Radiation loss characteristics are calculated for the first two LP modes of a double-clad lightguide structure in which the inner cladding forms a low refractive-index well between the core and the outer cladding. The higher-order mode is defined to be effectively cut off at wavelengths where the LP(11) mode power is sufficiently attenuated to ensure negligible modal noise due to interference between LP(01) and LP(11) modes. Calculations apply for various deposited cladding thicknesses and findings can be easily extrapolated if specifications are changed for system requirements on the minimum unspliced fiber length or the extinction ratio between P(11) and P(01) mode powers, Re = 10 log P(11)/P(01). The latter is specified at points such as splice joints where modal noise is generated. Theoretical results are correlated with measurements in order to determine the effective cutoff wavelength for fibers in which leaky-mode loss mechanisms dominate. The procedure should be extremely useful for determining the shortest usable wavelength for single-mode lightwave systems.

Fiber Optic Analog--Digital Hybrid Signal Transmission Employing Frequency Modulation

Investigations are carried out on the SNR performance in fiber optic FM transmission systems which use multimode fibers and 1.3 μm laser diodes. Both laser diode noise and modal noise are quantitatively evaluated in systems with and without employing a superimposed pulse modulation scheme. Based on these investigations, parameters for analog-digital hybrid signal transmission systems employing FM are determined. The hybrid signal consists of audio, video, and 2 Mbit/s digital data. The results obtained in transmission experiments satisfied the design objectives and confirmed the technical feasibility of an analog-digital hybrid transmission employing FM. The obtained results make the fiber optic FM transmission system with a 1.3 \mum LD an attractive alternative in subscriber systems, and video transmission systems for microwave IF links and CATV trunks.

High power performance of double channel mesa laser

The fabrication and performance of 1.3 μm and 1.55 μm GaInAsP/InP double channel mesa lasers is described. These devices are relatively simple to fabricate requiring only one stage of LPE growth. Threshold currents as low as 30 mA and extremely high maximum CW output powers in excess of 100 mW have been achieved at λ = 1.3 μm. At output pwers above a few milliwatts the coherence of the laser output decreases. This property makes these devices less likely to generate modal noise to multimode optical communications systems.

Observation of modal noise in single-mode-fibre transmission systems

Modal noise has been observed in overmoded short fibre sections and transmitter pigtails resulting in performance degradation in a single-mode-fibre transmission system. The power penalty becomes negligible when the fibre is sufficiently long or is deployed with a bend to sufficiently attenuate the LP11 mode.

How modal noise in multimode fibers depends on source spectrum and fiber dispersion

The speckle contrast for multimode fibers, and thus the modal noise, is essentially given by the impulse response of the fiber and the power spectrum of the source. Theoretical and experimental results show that laser sources with a large spectrum of narrow longitudinal modes may cause high speckle contrast and important modal noise over more than 1-km length in graded-index multimode fibers.

Modal noise reduced PFM transmission by monopulse to twin-pulse conversion

Modal noise was reduced fairly by monopulse to twin-pulse conversion in pulse frequency modulation optical transmission. The pulsewidth was chosen to be less than the time in which dynamic spectral broadening was maintained. The technique is the most promising for modal noise elimination.

Video transmission characteristics in WDM star networks

In local area networks (LAN's), it is desirable to transmit not only digital data but also moving video signals. This paper proposes a system employing a wavelength-division multiplexed star network which is satisfactory for the transmission of video signals in addition to digital data, and reports the evaluation results of the video transmission characteristics of the system employing a 32 port-pair star coupler. Video transmission was realized by the pulsed analogue transmission scheme. Pulse frequency modulation (PFM) and pulsed frequency modulation (pulsed-FM) were studied, and it was found that the SNR fluctuation due to modal noise was less by the pulsed-FM than by the PFM. It was also found that the difference in the modal noise was due to the differences in the demodulation characteristics of PFM and pulsed-FM. A 32 port-pair passive star coupler was used and a maximum of a 5-km network was experimented. The results showed that a good video SNR was obtainable in the experimental star network using laser diodes. Thus the feasibility of the broad-band network, employing a multiport-pair passive star coupler by the wavelength division multiplexing scheme, was proven.

Use of superluminescent diodes in PFM optical video transmission

An 835 nm superluminescent diode with a spectral width of 8 nm is shown to reduce SNR fluctuations due to modal noise significantly in PFM optical video transmission. An SNR of 56 dB and DG/DP of 06%/0.3° are achieved over 5 km with a frequency deviation of only 400 kHz. The SNR fluctuation is less than 0.05 dB.

Modal Noise. Modal Noise in Multimode Fiber Links with Distributed Mode-Selective Losses

Summary Distributed mode-selective losses in a multimode fiber transmission link cause an accumulation of modal noise. A theory has been developed which gives an approximation of the signal-to-noise ratio at the end of the link due to the accumulated modal noise; it shows how this ratio is improved by mode-coupling in the fiber.

Measurement of higher-order mode attenuation in single-mode fibers: effective cutoff wavelength

A technique for the determination of the wavelength-dependent attenuation coefficient of the LP(11) mode in sing]emode fibers is presented. Knowledge of the magnitude of this coefficient is required, e.g., for the quantitative prediction of modal noise in an optical-fiber communication system. An effective cutoff wavelength of the LP(11) mode can be unambiguously defined as the wavelength at which the attenuation coefficient equals a specified value.

Modal noise and distortion caused by a longitudinal gap between two multimode fibers

The modal noise and distortion caused by a longitudinal gap between two multimode parabolic-index fibers are discussed and quantified by means of modal analysis. It is found that, for coherent illumination and a given loss, the modal noise and distortion due to a gap are generally as severe as or worse than those due to an axial offset between two fibers. It is also shown that, when a partially coherent source is used, the transmission characteristics of a gap can be improved by increasing the operating wavelength. Experimental results have been obtained for modal distortion and are shown to be in close agreement with the theory.

Actual modal power distributions in multimode optical fibers and their effect on modal noise

The modal power distribution, which specifies the amount of power flowing in each mode of a multimode fiber, influences the level of modal noise in optical-fiber systems. Generally, previous work has assumed that power in multimode fibers was distributed uniformly among the fiber modes. By considering the coupling of a semiconductor laser to a lensed fiber, we show that this assumption is grossly incorrect for practical excitations. Typically, half of the power is in just 3 modes, even though the fiber supports 49 modes. This leads to much lower modal noise than would be expected if the power were uniformly distributed. The dependence of the modal noise level on mode excitation is found to be much more complex than previously suggested.

Lasers. Characterization of Laser Diodes by Measuring the Speckle Contrast at the End of a Multimode Fiber

Summary A novel and simple method is described to characterize semiconductor lasers which may produce excessive modal noise in a transmission system. The method is based on an analysis of the speckle contrast at the end of a suitable length of multimode fiber. An optimal length of measuring fiber is calculated and verified experimentally. The qualitative dependence of speckle contrast on fiber length is shown. The measured speckle contrast allows to define an “effective” number of laser spectral lines accounting for the modal noise.

Modal Noise. The Influence of Modal Noise on a Digital 1.3 μm Optical Transmission System

Summary The performance increase of optical transmission systems creates new problems like optical feedback and modal noise. It is very important to overcome these problems especially in systems composed by a lot of single optical elements. In this paper we describe a new method to determine the degree of mode selectivity produced by these devices. A simple model for modal noise description together with experimental results are presented.

Evaluation of modal noise in multimode fiber-optic systems

Modal noise at fiber connecting points is studied theoretically and experimentally. Simple formulas for calculating modal-noise power, noise bandwidth, and nonlinear distortions are obtained by introducing a visibility concept. Modal noise for multimode operating LD is also analyzed. These theoretical results are applied to high-bit-rate digital systems and analog systems using direct IM. Then, it is shown that a 3-dB connection loss limits effective signal-to-noise ratio (SNR) to 22 dB in high-bit-rate digital systems, and that in analog systems, the SNR is improved more than 20 dB by using a superimposed pulse modulation with a frequency that is 100 times the baseband signal bandwidth.

Laser linewidth requirement for eliminating modal noise in pulse frequency modulation video transmission

The relation between laser linewidth and modal noise was studied in a pulse frequency modulation transmission system through employing a distributed feedback laser which shows linewidth broadening of more than 2 A. A 6-dB signal-to-noise ratio fluctuation for a 7-km transmission system, using a conventional Fabry-Perot laser, was reduced to 0.3 dB for 1.8-A linewidth. It is pointed out that more than 1-A linewidth is required to reduce the modal noise and to obtain a high-quality PFM transmission system.