Single-mode to Multi-mode Fiber Problem
This problem is designed to probe the mechanisms available for single-mode to multi-mode coupling. A priori we expect the insertion loss curve to be flat because we should be easily able to couple all of the light into the multi-mode fiber. This might be a reasonable scenario for a free-space communications setup; otherwise, no optical engineer ever gets a problem with the physics so favorable-usually the request is multi-mode-to-single-mode coupling (the technique is the same; the difference is that the losses are very high). At very large distances, some loss would be expected due to beam clipping after the Gaussian beam had diverged sufficiently. The prescription is:
FILE = T20XX SMF-28 TO T15XX 62.5 MM
Wavelength: 1.55000 micronsObject Space NA = 0.140000, telecentric emitter. Fiber Mode Field Diameter on surface 1 is 10.4 microns. Fiber on surface 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 4.39900 HERASIL
1.444132 0.625 T2000
STO A -1.3520 10.00000
1.000000 0.625
3 S Infinity 2.50000
1.000000
4 A 0.9890 3.21600 HERASIL 1.444132 0.625 T1500
IMG S Infinity 1.000000 0.625
Aspheres (surface type = A): conic constant = -0.48 on
surfaces 2 and 4.
Results: Single-mode to Multi-mode.xls
ZEMAX: A 9 micron diameter circular extended source was imaged through the optical system. A 0.14 NA was imposed in object space. A 62.5 micron aperture was placed on the image plane. In ZEMAX, the image analysis feature was used to impose the 0.22 image space NA constraint. The thickness of surface 2 was set to 0; the thickness of surface 3 was varied from 0 to 100 mm. The ray-trace model incorrectly clips the beam beyond 50 mm. The Gaussian beam easily passes through the clear aperture of the receiving lens and fits within the multi-mode fiber core, as verified by Gaussian beam calculations and by the measurements-the IL curve is constant to the precision of the measurement instrument (approximately 0.03 dB). T20XX SMF-28 TO T15XX 62.5 MM.ZIP
OSLO: Same procedure as for ZEMAX except an aperture was placed on surface 4 that was the same diameter as the clear aperture on surface 2 in order to approximate the receiving fiber NA condition. This is not quite the same as a real NA constraint, however. T20XX SMF-28 TO T15XX 62.5 MM.LEN
OpTaliX: Version 5.02 added the ability to do this calculation via CEF and POP. However,the multimode coupling remains problematic at this time. T20XX SMF-28 TO T15XX 62.5 MM.OTX.
CODE V: No data available. However, this is precisely analogous to the experimental conditions discussed in the multi-mode to multi-mode case. The SMF28 would be modeled using the step-index macro; the Gradient-index 62.5 micron fiber has enough modes that it can be treated as a step-index fiber without significantly affecting the results because of the number of receiving modes that are being integrated over. The coupling into a specific higher order mode may not be correct, but the sum should not be affected. T20XX SMF-28 TO T15XX 62.5 MM.SEQ
FRED: No data available.
ASAP: No data available. Contact BRO customer support.