Multi-mode to Multi-mode Fiber Problem
Multi-mode to multi-mode fiber coupling is a problem that is often poorly understood by engineers used to dealing with single-mode fiber. A multi-mode fiber can be thought of as being a tight-packed fiber bundle with some finite extent. As a result, collimation is impossible in the sense that it is usually thought of for single-mode fiber collimators; the beams from any location on the source fiber may be collimated, but there is a significant beam divergence due to the extended source. This can result in a very steep IL curve.
FILE = T15XX
62.5 MM PAIR
Wavelength: 1.55000 microns
Object X coordinate: 0.00000
Object Y coordinate: 0.00000
Object Space
NA = 0.22, telecentric emitter. Fiber on surfaces 1 and 5 has a core diameter
of 62.5 microns and an NA of 0.22.
Units in mm.
# TYPE
RADIUS DISTANCE GLASS
INDEX SEMI-DIAMETER
OBJ S
Infinity 0.0000
1.000000
1 S
Infinity 3.21600 HERASIL 1.444132 0.625
STO A
-0.9890 2.50000
1.000000 0.625
3 S
Infinity 2.50000
1.000000
4 A
0.9890 3.21600 HERASIL 1.444132 0.625
IMG S
Infinity
1.000000 0.625
Aspheres (surface type
= A): conic constant = -0.48 on surfaces 2 and 4.
RESULTS: Multimode.xls. A single mode source was spliced into a 100 m spool of multi-mode fiber. The insertion loss was far less than predicted because the multi-mode fiber did not have all of the modes excited, resulting in a far smaller divergence than modeled. Typically, it takes 10-1000 m of fiber for all of the modes to be excited (1) . This same effect is often used to advantage in high-power laser systems to design beam delivery optics with smaller optics than predicted by the fiber NA.
ZEMAX: A 62.5 micron wide circular extended source (the field type was object height) was used. Dummy surfaces were used to place the stop sufficiently far from the source to make each point on the source act as a telecentric emitter (there is no telecentric option otherwise available in ZEMAX). The object space NA was set at 0.22. For the receiver, the image analysis feature was used to set an NA limit of 0.22 and an aperture was placed on the image surface that was the same size as the core. T15xx 62.5 MM pair.zip
OSLO: The pixelated source feature was used. The stop was set for object space NA=0.22 and telecentric. An aperture the same size as the core was placed on the image plane and, because there was no other way that I could find to limit the ray NA to the fiber, an aperture was placed on the convex surface of the receiving collimator that was the same size as the entrance pupil. OSLO generated a program error at 100 mm separation. T15xx 62.5 MM pair.len
OpTaliX: As of v. 5.02 the CEF module allows this calculation if only the fundamental mode is used from the source fiber. We modeled this problem as using the fundamental mode in the source fiber and the full set of modes for the receiving fiber. A step-index fiber was assumed with n_core=1.516, n_clad=1.50. Both POP and CEF were tried; both displayed some difficulties. T15xx 62.5 MM pair.otx
CODE V: Using the Grin fiber macros, the source fiber was modeled as only exciting the fundamental mode and the coupling was calculated into all of the receiving fiber modes. The agreement was excellent, because the experimental setup failed to excite all possible fiber modes. The macros are available from ORA and will be included in new releases. The macro considers the fiber size and the NA to set tp appropriate parabolic constant for the GRIN profile. ORA offers a step-index macro and the GRIN macro. The GRIN macro that was provided to us generated incorrect higher-order modes, although this would not materially affect the calculation results in this case. ORA has been notified about the error. T15xx 62.5 MM pair.seq
FRED: No data available.
ASAP: No data available. Contact BRO customer support.
1. Tom G. Brown, Handbook of Optics, 2nd ed., Michael Bass, ed. (McGraw-Hill:New York, 1978), § 10.6, p. 10.13.