Selective mode injection and observation forfew-mode fiber optics
Stuart Shaklan
By etching the cladding along several millimeters of circular- or elliptical-core few-mode fiber optics, wegain access to the core and can inject extremely pure modes. The same etching technique allows one tomeasure the modal purity at the far end of the fiber. Modal purities of -26 dB have been obtained. We alsodemonstrate a holographic technique that allows all the light of each mode to be focused to anindependent spot.
Key words: Fiber optics, modes, holography.
1. Introduction
With the goal of characterizing strongly multimodefiber optics, several authors have attempted to excitepure single modes, usually with mixed results."Smaller fibers supporting fewer modes are clearlyadvantageous because of their widely separated prop-agation constants. With several modes, one maystudy such effects as index dips, differential modeattenuation, and microbending losses, although ex-trapolation to larger fibers is required for mostmultimode telecommunications applications. In Sec-tion II of this paper we describe an etching techniquethat permits side launching of extremely pure modesin fibers supporting as many as 15 modes. Thesame technique provides spatial mode separation atthe far end of the fiber, allowing modal purities to bedirectly measured.
When conventional optics is used, a two-dimen-sional detector is required for collection of all the lightescaping from the etched section. This is a disadvan-tage for applications such as faint-object astronomicalinterferometry5 and in a newly developed fiber-spliceevaluation technique.6 The problem is solved by usingthe simple holographic technique described in SectionIII. The hologram brings the light of a given mode to
The author is with the Jet Propulsion Laboratory, 4800 OakGrove Drive, M.S. 169-214, Pasadena, California 91109. Thisresearch was conducted while the author was at the Institut deRecherche en Communications Optiques et Microondes, U.R.A.Centre National de la Recherche Scientifique 356, 123 avenueAlbert-Thomas, 87060 Limoges, France.
Fig. 1. Modal separation and selection: F, 1-m length of circular-core or elliptical-core fiber; T, etched fiber (diameter 10 m,length 6 mm); W, metal wire to provide slight axial tension on thefiber; 0, index-matching paraffin oil; G, glass window in the bottomof the paraffin container; R, rings observed in the far field (arcs foran elliptical-core fiber, as seen in the lower photo); L1, laser beamused to observe rings; L, collimated laser beam used to inject asingle mode. The upper photo is the 7th mode, corresponding to thenext-to-last arc of the lower photo.
Fig. 2. Far-field modes of a step-index fiber supporting seven modes.
an independent focus, permitting detection of all thelight of that mode by a single pixel. The hologramsmay also be used in reverse for efficient selectivemode launching.
II. Selective Mode Launching
The etching technique that spatially separates themodes is described in detail elsewhere.5 In essence,the last few millimeters of fiber are etched in 40%hydrofluoric acid for 1 h. When the remainingcladding diameter is 2 p.m larger than the core, thefiber is dipped in index-matching paraffin oil. Byfrustrated total internal reflection the light that hasbeen injected into the far end escapes from the coreinto the oil. The light from each mode forms a ring inthe far field, where the ring's diameter is related tothe modal phase velocity and its width is inverselyproportional to the length of the leaky region. Obser-vation of the rings reveals whether the fiber is fullyetched. The fiber may be wiped clean and returned tothe acid for further etching if required. The techniquewas developed by Spajer et al.,7 but fundamentally itis no different from side polishing and prism cou-pling.8 Once the fiber is etched, we glue the end onto awire that provides tension to hold the 10-lim-wide
fiber straight. This process is shown schematically inFig. 1.
If the etching is circularly symmetric, and if thecore is well centered in the cladding, the ring patternmaintains the circular symmetry of the modes. For anelliptical-core fiber, circularly symmetric etching re-sults in light leakage through the long axis. In fact,we observed that light always leaks preferentially outone side, probably because of either poor axial center-ing or stresses in the cladding that affect the etchingrate. Thus, for elliptical-core fiber, arcs are observed(see Fig. 1).
The ring structure represents high angular modalselectivity that is not possible when light escapesfrom the small core. The far-field ring width may behundreds of times smaller than the speckle diameterresulting from the core diameter.
Directing a collimated laser beam along the conethat is formed by a given mode causes that mode to beexcited. This is of course exactly analogous to theexcitation of modes in a thin film.9 It is not necessaryto form the laser beam into a cone to improveselectivity; calculation of the overlap integral of anyother ring (far-field mode pattern) with the beamreveals that excellent purities are obtained even with
Fig. 3. Far-field modes of a step-index elliptical-core fiber having 2:1 major:minor axis ellipticity. Modes are shown in the order of theirappearance as the input beam angle was increased. The notation of Eyges et al. is used to identify the modes. The major axis of the fiber ishorizontal, resulting in vertically elongated mode lobes.
the collimated beam. This was first demonstrated bySpajer et al.7 for the first four fiber modes.
Modes obtained for circular-core fiber are shown inFig. 2. The C.L.T.O. (France) fiber has an 8.3-jim-diameter step-index core, and An = 0.004. At thewavelength of our argon-ion laser (488 nm), the fibersupports seven modes. Modes obtained for an 11-mode elliptical core fiber having a 2:1 major:minoraxis ratio are shown in Fig. 3. The notation of Eygeset al.O is used to identify the modes.
The symmetry and the regularity of these modepatterns indicate that the modes are in fact extremelypure. To measure modal purity properly, one needs to
etch both ends of the fiber so that any mode of Fig. 2or 3 forms a spatially independent ring when it leaksfrom the etched region. The power in the other ringsis measured to determine purity. For a circular-corefiber we achieved modal purities of -26 dB (maxi-mum optical power measured in any unexcited mode)for the first seven fiber modes. The exception to this ismode LP5 1, which was accompanied by light in LP11having 2% of the intensity of the fundamental. Thismay be because LP01 has the longest leakage regionand thus is most sensitive to imperfections in thefiber. Photographic examples of the modal puritymeasure are given in a separate paper, where we
report having used the pure modes as a probe ofintermodal coupling in long lengths of few-modefiber.
We note that, for our step-index elliptical-corefiber, the observed order of appearance of the modesis not exactly that predicted by Eyges et al."° For a 2:1ellipse, they predict the appearance of EI before E 3',
and E'I before E,,1. This is different from the ob-served order (Fig. 3) and is almost certainly the resultof the sensitivity of the mode propagation angle toslight deviations from the perfect step-index profile.
Modal injection efficiency is of the order of 1%,which can be improved by matching the beam heightto the length of the leakage region and the beamwidth to the core diameter.
IIl. Holographic Mode Collimation System
Since the leaky section of fiber is an axial line source,no standard optical system can collimate the lightfrom the rings or form an image of the rings in aplane. This is a disadvantage in applications such asbroadband and astronomical interferometry,5 in whichmode-by-mode optical path correction and single-pixel detection are called for. It also complicatesefficient mode launching, which may have nonlinearoptical applications. Also, a newly developed tech-nique for measuring the intermodal coupling matrixat a splice or mode coupler requires that all the lightof a given mode be focused on a slit.6
One simple way. to collimate the light from eachring is to make a hologram, using a collimatedreference beam. We have taken advantage of thespatial separation of the rings to go one step further.As Fig. 4 shows, we expose a holographic plate ring byring, moving the reference point source each time.The hologram is masked to pass only the light of one
ring and the overlapping reference beam. In this way,each mode is collimated in a different direction, and alens behind the hologram focuses the light of eachmode to a different point. Note that, because themodes are spatially separated, we have actually madean independent hologram for each mode. Diffractionefficiency does not suffer from the otherwise funda-mental 1/N loss from modal superposition, where Nis the number of modes. Results for a thin amplitudehologram are shown in Fig. 5.
The measured diffraction efficiency for the firstfour modes is 0.005-0.01, and the modal purity of thepoints is > 95%. Of course, we have not attempted todemonstrate extremely high diffraction efficiencies byusing thick phase holograms, but, in principle, suchefficiencies are not difficult to achieve. Efficiencies inexcess of 80% should be readily attainable.
An efficient hologram can also be used for selectivemode injection. A collimated beam masked to thewidth of one ring can be diffracted by the holograminto the desired mode. Different modes are excited bymoving a lens in the beam collimation system in thesame way as the lens of Fig. 4 was moved to changethe reference source position. It is conceivable thatlarge input powers can be injected into a selected
CONSTRUCTION
Fig. 4. Hologram for collimation of the light from each mode. Thehologram is masked to pass the light of one mode at a time. Thereference beam angle is changed for each mode. Thus each modereconstructs a spatially independent point source behind lens L2. L,Argon-ion laser, used at 488 nm; F, elliptical-core fiber supporting11 modes at 488 nm; 0, container of index-matching paraffin oil; T,etched section of the fiber; H, hologram; L,, lens that is moved tochange the reference beam (resulting in beams R, for mode 1, andR2 for beam 2, etc.).
Fig. 5. Light diffracted by the hologram. Top: just behind thehologram, the diffracted rings of the elliptical core fiber. Middle:the beams converging toward separate points. Bottom: imagesformed by the hologram.
mode for the purposes of nonlinear optical experi-ments such as intermodal cross-phase modulationstudies"i and for adjustable transverse-mode fiberlasers.
IV. Summary
The techniques described here provide a simple andeffective way to inject and select a given mode of afew-mode fiber. While we have not yet experimentedwith hard-to-come-by intermediate-sized fiber, webelieve that, based on step-index modal dispersioncurves, these techniques should be useful for the first15-17 fiber modes. It may be necessary to etch in twosections: a first section, etched for a shorter period oftime, is used for faster leaking higher-order modes;this is followed by a second, more deeply etchedsection for well-confined modes. A tapered etch (asinitially attempted by Spajer et al. 7) could perform thesame function. For elliptical-core fiber, the positionsof the azimuthal maxima and minima are fixed withrespect to the core orientation. Thus there is theadditional advantage of azimuthal selectivity, whichmay allow even more modes to be injected selectively.
For the specific application of multimode fiberoptics to a long-baseline astronomical interferometer,the holographic lens demonstrates that all the lightfrom a given mode can be focused onto a single pixel.Additionally, the hologram corrects aberrations inthe ring structure, and, more importantly, it correctssmall phase differences in the rings obtained fromdifferent fibers. Thus the single pixel can also be usedto detect aberration-free fluffed-out fringes.
M. Monerie supplied the elliptical-core fibers. Help-ful discussions with Vincent Kermene, B. Colombeau,and F. Reynaud are gratefully acknowledged. The
research of S. Shaklan was fully supported by Na-tional Science Foundation grant INT-8911104.
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