360 OPTICS LETTERS / Vol. 15, No. 7 / April 1, 1990

Mode-holding capability of dual-mode fibers

George Kotrotsios and Olivier Parriaux

Centre Suisse d'Electronique et de Microtechnique, Recherche et D6veloppement S.A., Maladibre 71, 2000 Neuchctel, Switzerland

Frangois Cochet

Cabloptic S.A., 2016 Cortaillod, Switzerland

Received November 13, 1989; accepted January 30, 1990

Standard single-mode communication fibers have been characterized with respect to their mode-preservationcapability in the dual-mode regime. For a 1300-nm single-mode fiber working at 830 nm, the equivalent hparameter for its LPoj and LP11 modes is shown to be as good as that for the best polarization-maintaining fibers,i.e., better than -20 dB-km.

The optical radiation of a source emitting in the firstwindow coupled into a 1300-nm single-mode fiber canexcite two linearly polarized modes: the fundamentalLPo1 and the first higher-order LP11 modes. As thesemodes propagate along the fiber they lose and ex-change power. The capability of these modes to keeptheir identity is an important parameter for dual-mode fiber systems in distributed'- 3 or point4 sensorsand communication applications.5' 6 The purpose ofthis Letter is to quantify this transmission character-istic of dual-mode fibers.

Light propagating in bent waveguides experiencesloss that depends on the mode order and consequentlyon the mode confinement in the waveguide. Low-order modes are better confined and therefore experi-ence lower attenuation.

In order to measure the differential mode attenua-tion at bends, a dual-mode fiber was wound on man-drels of radii varying from 3 to 7 mm. The powerattenuation of each mode is measured by means of afocusing element of semicylindrical form placed in theevanescent field on top of a laterally polished fibersection (Fig. 1). The mode spatial separation permit-ted by this configuration is measured to be better than35 dB by scanning a 75-,gm slit followed by a photode-tector in the focal plane of the module. Experimentalresults and theoretical predictions7 based on theequivalent step-index data of the fiber are plotted inFig. 2, which shows the modal power loss at bendsversus the bending radius. There is a better agree-ment between theory and experiment for the LP1,-mode loss than for the LPol-mode loss. The reason forthis is that the LP11 mode starts leaking out underlarge radii of curvature. In these conditions the ap-proximation that the modal field remains unmodifiedunder bending7 is better satisfied than in the case ofthe fundamental LPo1 mode, which leaks under a sig-nificantly shorter bending radius. A differential lossof 3.1 dB/turn is expected with a 4-mm bending radi-us; the LP 0 1 insertion loss is 0.05 dB/turn. A bentdual-mode fiber can therefore be used as a mode filter.

Two modes propagating along an optical fiber ex-

change power under the effect of the fluctuations ofthe optogeometrical characteristics of the waveguide.In polarization-maintaining fibers this coupling is de-scribed by the h parameter,8 which is the power cou-pled from the initially excited mode to the order modeper unit length. The same parameter can be definedin dual LP-mode fibers as the LP-mode-preservationcapability of the fiber.

The h parameter of high-birefringence fibers is ofthe order of -20 dB-km.8 The equivalent parameterfor dual LP-mode fibers is expected to be at least as

Polished fibersection

(a)

(b)

Fig. 1. (a) Principle of the LP-mode spatial separation andfocusing module. The focal points of the two modes differas a result of the mode-dependent leakage angle. (b) TheLP-mode spatial separation and focusing module.

0146-9592/90/070360-03$2.00/0 © 1990 Optical Society of America

April 1, 1990 / Vol. 15, No. 7 / OPTICS LETTERS 361

20

15

10

5

0

-5

-10 I

-15

-20 I0 2 4 6 a 10 12 14

Radius of Curvature (mm)is l6 20

Fig. 2. Experimental results (crosses) versus theoreticalpredictions (straight lines) of differential mode attenuationat bends. Left curve, LP 01 mode; right curve, LP,1 mode.

good, as the propagation constants of the two LPmodes are further apart than those of the polarizationmodes of a high-birefringence fiber. This means thatthe spatial period of the random microbends that maycouple the two modes is shorter (typically by a ratio ofbetween 2 and 3) in the case of LP modes than in thecase of polarization modes. Since the amplitude ofthe microbending frequency components is a decreas-ing function of the spatial frequency,9 the randomcoupling between LP modes owing to winding or ca-bling is likely to be small.

In order to measure the h parameter a pseudocut-back scheme was adopted; the experimental setup isillustrated in Fig. 3. Light coming from a 25-mmcoherence-length Philips semiconductor laser, emit-ting at 830 nm, was injected into a 5-km Cablopticfiber, single mode at 1300 nm (standard step-index

25-mm coherence-lengths.c. laser, 830 nm

telecommunication fiber; core radius 3.5 Aim, core-cladding index difference 0.0065). The end of thefiber was carefully butt-joined with the LP-mode spa-tial separation module described above. The fiber iswound on five spools of 1 km each, without connectorsor splices. LP,1-modal filtering is performed aftereach kilometer of fiber.

The h parameter is given by the average power cou-pled from the LPo, mode to the LP11 mode per unitlength or, in other words, by the slope of the straightline that fits the experimental data of Fig. 4. Themeasured mode-preservation capability of the fiber is-22.5 dB-km, with a linear regression correlation coef-ficient of 0.991. The experimental data were correct-ed with respect to differential mode attenuation. Forthe measurement of the differential mode attenuationdue to the propagation, each mode was selectivelyexcited with the help of a prism onset on a polishedfiber section, and the modal transmitted power wasmeasured at the end of the 5-km-long fiber and at 2 mfrom the input in a cutback scheme. The total powerattenuation was 11.5 dB for the LPo, mode and 13 dBfor the LP,1 mode, which means a differential modeattenuation of 0.3 dB/km.

This low value of the h parameter opens promisingperspectives for distributed fiber sensors: sensor sys-tems and detection schemes demonstrated with highlybirefringent fibers can now be applied with standardcommunication fibers at one hundredth the cost.Most detection schemes use differential mode delaytime measurement. In baseband readout systems, inwhich the location of an event is made by a radio-frequency phase measurement,' the random modecoupling sets a limit on the spatial resolution and onthe number of events that can possibly be identified.In frequency-modulation cw systems2 and white-lightinterferometric readout systems,3 the number of si-multaneously detectable events is limited and there isno error on an event's location.

For dual-mode point sensors4 that use a differential

LP-mode spatialseparation module

1 km of dual-LP mode fiber

/ LP~~~_101 >

1 kmom dual-mode fiber

1 km of dual-mode fiber

1 km of dual-mode fiber

Fig. 3. Experimental setup for the estimation of the mode-holding capability of dual-mode fibers. The arrows indicate themodal filters; s.c. laser, semiconductor laser.

362 OPTICS LETTERS / Vol. 15, No. 7 / April 1, 1990

0.04

L.3: 0.03

00o

> 0.02c1)

aDL.

~- 0.01n~

-I

2.00 3.00Distance (km)

Fig. 4. Experimental data (points) of the mode-preserva-tion capability measurements, corrected with respect to thedifferential mode attenuation of modes. The straight line isa fit to the data.

sensing scheme, the reported value of the h parameteris encouraging. In such applications the limitationswill be set mainly by the parasitic transition couplingat bends and connections along the fiber lead.

Finally, in dual-mode communication systems thatuse fibers single mode at 130 nm and sources emittingin the first window, the intermodal dispersion is avoid-ed by filtering out the LP11 power.5'6 The -22.5 dB-

km of cross-coupled power translates into pure loss(0.2 dB/km), which is superimposed onto the trans-mission loss of the fundamental mode. In addition,the insertion loss at the modal filter (-0.5 dB/filter, ifa bending filter is used) must be added to the trans-mission loss.

The financial support of a part of this research bythe Fondation Suisse pour la Recherche en Microtech-nique is gratefully acknowledged.

References

1. G. Kotrotsios, P. D6nervaud, L. Falco, and 0. Parriaux,Proc. Soc. Photo-Opt. Instrum. Eng. 1011, 109 (1988).

2. R. B. Franks, W. Torruellas, and R. C. Youngquist, Proc.Soc. Photo-Opt. Instrum. Eng. 586,84 (1985).

3. G. Kotrotsios and 0. Parriaux, in Fiber Optic Sensors, H.J. Arditty, J. P. Dakin, and R. T. Kersten, eds., Vol.44 ofSpringer Proceedings in Physics (Springer-Verlag, Ber-lin, 1989), pp. 568-574.

4. G. L. Tangonan, V. L. Jones, and D. J. Vickers, in Confer-ence on Lasers and Electro-Optics (Optical Society ofAmerica, Washington, D.C., 1989), paper TuL3.

5. R. Ries, Electron. Lett. 23, 71 (1986).6. M. Stern, W. I. Way, V. Shah, M. B. Romeiser, W. C.

Young, and J. W. Krupsky, in Conference on OpticalFiber Communication/Integrated Optics and OpticalFiber Communication (Optical Society of America,Washington, D.C., 1987), paper MD2.

7. C. Vassalo, Thgorie des guides d'ondes electromagngti-ques (Eyrolles, Paris, 1985), Chap. 6, p. 305.

8. S. C. Rashleigh, IEEE J. Lightwave Technol. LT-1, 312(1983).

9. R. Olshansky, Appl. Opt. 14, 935 (1975).