Measuring the Modal Properties of Multimode Fibres. FOToN 6th May 2004

Transcription

1 Measuring the Modal Properties of Multimode Fibres Andrew G Hallam David A Robinson FOToN 6th May 24

2 Structure of Talk Basic mode theory Measuring the mode distribution Measurement results

3 Theory: modes in a Fibre Planar waveguide: n2 TE,m modes M M n n2 n2 TM,m modes E E n n2 n>n2

4 Four types of Mode in a Fibre Circular fibre: Also has helical (skew) modes, HE n,m EH n,m (transverse magnetic dominates) (transverse electric dominates) r There are many more skew modes than meridional modes. skew (HE, EH) θ φ θ meridional (TE, TM)

5 Leaky modes r θ θ skew meridional Leaky modes exceed the local numerical aperture

6 Mode properties Each Mode has its own propagation constant (time of flight). Groups of modes have the same propagation constant (degenerate). These groups are combined to form new modes, called LP n,m modes, which are Linearly Polarised. Number of azimuthal peaks = 2n, number of radial peaks = m

7 Example: Four-fold Degeneracy of LP mode TE + HE 2 = LP TM

8 Modal Parameters of Graded Index Fibre Normalised frequency: (V-number) V 2 π a NA λ = a= radius NA = numerical aperture Total number of modes: N m 2 V 4 Number of mode groups: N g V 2 (All modes in a group have the same propagation constant) Example: a = 25um V = NA =.2 λ = 85nm N m = 376 N g =9

9 Mode Group Designation Mode group number Modes per group.. LP n,m β/k um graded fibre,.2 NA, n core =.472, n clad =.457, 85nm

10 How to measure the mode distribution...

11 Introducing the Mode Transfer Function N orm alised intensity R elative mode group All modes equally excited by Lambertian source - fully-filled fibre N orm alised intensity R elative mode group Middle-order modes preferentially excited - under-filled fibre

12 MPX- Modal Explorer

13 MPX- Modal Explorer

14 Encircled Flux - IEEE Gigabit Ethernet Standard, 62.5um Gbs 62.5um (IEEE 82.3z) : <25% of power inside 9um circle >75% of power inside 3um circle 3um 9um Gbs 5um (IEEE 82.3ae) : <3% of power inside 9um circle >86% of power inside 38um circle

15 How the MPX- works. Near-field intensity profile: I(r) MTF (m ) dm f ( r ) where: MTF(m) is the mode transfer function m is the normalised mode group number f(r) is the profile function of power-law fibre Inverting gives: MTF(m) di(r) dr r a α m= f (r) where: f ( r ) = r a α (Calzavara, M., Electronic Letts., pp , v.7, no.5, 98)

16 How the MPX- works. Mode Transfer Function Example of MTF(m): relative mode power relative mode number. Mode Power Distribution: Mode Power Distribution MPD (m) MTF (m) m relative mode power relative mode number.

17 Assumptions for the Mode Profiling Method ν Modes within a mode group carry the same power. - strong coupling generally occurs between modes in a degenerate mode group. ν Large number of modes launched (geometrical approximation) - but take care with leaky mode contribution. ν Random phases between the propagating modes. - use incoherent (wideband) source or shaking to avoid speckle noise.

18 Examples of mode distribution measurements...

19 Couplers/splitters

20 5-5 coupler Split ratio port/port2 Well-filled launch port2 port Launch Medium-filled launch.5.25 port2 port Launch Under-filled launch port2 port Launch

21 95-5 coupler 95% port (through-path) near field 5% port (cross-path) near field MTF MTF

22 Commercial LED Sources

23 Commercial LED Sources source A source B source C source D

24 Mandrel-wrap mode-filters

25 Mandrel Wrap Filters.8 Effect of mandrel wrap on output from 62.5um patchcord low NA launch, 5x2mm low NA launch, no mandrel.6 mid NA launch, 5x2mm.4.2 mid NA launch, no mandrel high NA launch, no mandrel high NA launch, 5x2mm No mode equalisation is occurring

26 Long fibre lengths

27 Fibre A: shows little mode filtering effect MTF - 2m MPD - 2m MTF - 3m MPD - 3m

28 Fibre B: shows a mode filtering effect MTF - 2m MPD - 2m MTF - 9m MPD - 9m

29 OTDR of Point Defect in Fibre B power, db increasing overfill distance, m

30 Compare with Coupled Power Ratio

31 Measurement of Coupled Power Ratio (IEC ) Light source Test patchcord Power meter Power = P dbm Light source Test patchcord Singlemode patchcord Power meter Power = P2 dbm CPR = P P2 db

32 Table of CPR values in db at 85nm. (IEC ) Fibre size Category Category 2 Category 3 Category 4 Category 5 Overfilled Greatly underfilled 5/ ,9-5,9 6-,9-5,9 62,5/ ,9 4-2,9 7-3,9-6,9 / ,9 8-25,9-7,9-9,9

33 Example of CPR Measurement of Multimode Fibre coupler Light source C oupler blue channel, LED Coupler red channel, LED normalised MTF CPR=2.8dB (category 2/3) normalised MTF CPR=27.8dB (category ) re la tive mode group re la tive mode group Dip causes error in CPR

34 Controlling the mode distribution using scramblers and filters

35 Equilibrium Mode Distribution MTF MPD.6.5 MTF4 MPD Theoretical EMD (Yamashita, JLT, v3, n3, 985) Output of noval mode controller

36 Mode Control Device - results Input Near-field Input MTF Output MTF Full launch Coupler mode controller.75 LAN tester Low NA

37 Summary Mode Profiling gives a direct, real-time, measurement of MPD, that is more meaningful than Coupled Power Ratio. Mode Profiling also measures Encircled Flux to IEEE standard. Component performance depends on mode distribution. EMD does not readily occur in long fibre lengths or with mandrel wrapping. Discussion topic: What mode distribution is optimum for measurements? (Thank you for your attention) For more details contact Andy Hallam: ahallam@halcyon-optical.co.uk