Reduction Of Modal Noise Research Articles

Mitigating modal noise in multimode circular fibres by optical agitation using a galvanometer

Abstract Modal noise appears due to the non-uniform and unstable distribution of light intensity among the finite number of modes in multimode fibres. It is an important limiting factor in measuring radial velocity precisely by fibre-fed high-resolution spectrographs. The problem can become particularly severe as the fibre’s core become smaller and the number of modes that can propagate reduces. Thus, mitigating modal noise in relatively small core fibres still remains a challenge. We present here a novel technique to suppress modal noise. Two movable mirrors in the form of a galvanometer re-image the mode-pattern of an input fibre to an output fibre. The mixing of modes coupled to the output fibre can be controlled by the movement of mirrors applying two sinusoidal signals through a voltage generator. We test the technique for four multimode circular fibres: 10 and 50 $\mu$m step-index, 50 $\mu$m graded-index, and a combination of 50 $\mu$m graded-index and 5:1 tapered fibres (GI50t). We present the results of mode suppression both in terms of the direct image of the output fibre and spectrum of white light obtained with the high-resolution spectrograph. We found that the galvanometer mitigated modal noise in all the tested fibres, but was most useful for smaller core fibres. However, there is a trade-off between the modal noise reduction and light-loss. The GI50t provides the best result with about 60 per cent mitigation of modal noise at a cost of about 5 per cent output light-loss. Our solution is easy to use and can be implemented in fibre-fed spectrographs.

Reduction of modal noise due to fiber/photodetector misalignment in SSMF links based on 850 nm VCSELs through a low-cost polymer coupling structure.

A low-cost polymer-based structure is proposed to improve the coupling between a fiber end section and photodetector active surface in optical links based on standard single-mode fiber (SSMF), which employs vertical cavity surface emitting lasers operating at 850 nm, i.e., below the SSMF cutoff wavelength. Considering receivers as small-area detectors, which are generally necessary to guarantee high-speed operation but at the same time are particularly subject to power fluctuations due to modal noise (whose impact is in turn enhanced in the presence of fiber-to-photodetector misalignment), significant achievements are demonstrated by employing the presented structure. Indeed, in the presence of a misalignment of $ \pm 4 $±4 to $ \pm 6\;{\unicode{x00B5}{\rm m}} $±6µm, which is nowadays typically achievable, the relative optical power fluctuations due to modal noise reduce in the presented case more than four times (2.5% from more than 10%) with respect to the case of butt-coupling, which implies an increase of the same factor in the output signal-to-noise ratio at the receiver end.

SUPPRESSION OF FIBER MODAL NOISE INDUCED RADIAL VELOCITY ERRORS FOR BRIGHT EMISSION-LINE CALIBRATION SOURCES

Modal noise in optical fibers imposes limits on the signal to noise and velocity precision achievable with the next generation of astronomical spectrographs. This is an increasingly pressing problem for precision radial velocity (RV) spectrographs in the near-infrared (NIR) and optical that require both high stability of the observed line profiles and high signal to noise. Many of these spectrographs plan to use highly coherent emission line calibration sources like laser frequency combs and Fabry-Perot etalons to achieve precision sufficient to detect terrestrial mass planets. These high precision calibration sources often use single mode fibers or highly coherent sources. Coupling light from single mode fibers to multi-mode fibers leads to only a very low number of modes being excited, thereby exacerbating the modal noise measured by the spectrograph. We present a commercial off-the-shelf (COTS) solution that significantly mitigates modal noise at all optical and NIR wavelengths, and which can be applied to spectrograph calibration systems. Our solution uses an integrating sphere in conjunction with a diffuser that is moved rapidly using electrostrictive polymers, and is generally superior to most tested forms of mechanical fiber agitation. We demonstrate a high level of modal noise reduction with a narrow bandwidth 1550 nm laser. Our relatively inexpensive solution immediately enables spectrographs to take advantage of the innate precision of bright state-of-the art calibration sources by removing a major source of systematic noise.

Superimposition of a strong microwave signal on the laser injection current: spectral characteristics and impact on modal noise

In multimode fiber systems the strongest high-frequency modal noise generated by laser mode partitioning appears at the shorter distances where the modal dispersion is smaller than the laser coherence time. It is shown that the addition of an auxiliary microwave signal (AMS) reduces the laser mode partitioning and thus provides a reduction in the modal noise level. The optimum AMS frequency is 80-90% of the relaxation frequency, and the optimum AMS modulation index is 1.5-2.0. At these values the mode partition k-factor is reduced by a factor 3 in a 1 Gb/s system which gives rise to an estimated reduction in the detected modal noise level of 9.5 dB. The improvement in mode partitioning and modal noise reduction increases as the ratio between the relaxation frequency and the bit rate increases.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reduction of modal noise in high-speed short-distance computer data links by an electrical signal

Measured and calculated data on the tradeoffs between modal noise reduction and intermodulation penalty are presented. The authors quantify how a high modulation bandwidth and optical power can be obtained using conventional high-speed semiconductor lasers, by combining a sinusoidal auxiliary microwave signal with the normal data signal. How the characteristics of the laser diode determine the optimum values of the auxiliary carrier parameters is shown. Some results of optical data link simulations are presented. A 2.4-Gb/s transmission experiment shows an improvement of the bit error rate due to the microwave signal. >

Effect of modal noise on single mode fiber optic networks

Modal noise may arise in single mode fiber optic networks from the random coupling of the fundamental model LP 01 and the higher order mode LP 11. The higher mode LP 11 is generated in the fiber optic link from imperfect connectors. A model for the modal noise generated in a single mode fiber optic network consisting of a number of connectors or splices is presented. The effect of modal noise on the link is discussed and expression for signal to noise ratio has been obtained. Suggestions for the reduction of modal noise are given.

Modal noise reduction of laser diodes with specific thickness coating

A low facet reflectivity laser diode is able to oscillate in multilongitudinal modes, and has a high relative intensity noise (RIN) and reduces mode hopping noise. By introducing the electron-cyclotron-resonance (ECR) plasma deposition method, a good facet coating film has been obtained. We have demonstrated that a specific coated laser has very low modal noise characteristics even under direct analogue intensity modulation.

Reduction of modal noise by using reduced spot excitation

The paper describes a computational model to estimate the reduction of nonlinear distortions due to a fiber-fiber joint by properly focusing the optical beam from a single transverse-mode semiconductor laser source to the input end of the first fiber. Mode mixing effects are included in the model. Material dispersion is found to make a negligible contribution in comparison with a typical source coherence time. In a typical situation, where the fiber joint coupling efficiency eta is better than 0.7, all harmonic distortion components produced in the joint could be reduced by ~10 dB by halving the input beam spot radius from the value equal to the fiber core radius.