Dual-fiber Collimator Problem

 

 

The dual fiber collimator problem is designed to probe short-range performance of the coupling efficiency algorithms, gradient-index glass and errors tracking sign reversal after a mirror. When the problem was selected, the assumption was that Wagner and Tomlinson-based algorithms should be able to handle this problem. The coupling efficiency errors will be due more substantially to angular misalignment of the beam and the receiving fiber than the waist location. Commercially this problem is interesting because dual-fiber collimators are used extensively in many multi-port device designs.


Text Box:

Figure 1: Dual fiber collimator in x-z plane showing that the natural stop (mirror location) is uniquely one focal length from the front principal plane.




The problem, illustrated in Figure 1, consists of light being transmitted from an off-axis SMF-28 fiber, through a TL1.80AB25-8 lens (GRADIUM® G23SF glass, 1.8 mm diameter, 1.97 mm focal length, “quarter pitch” lens with an eight degree angle on the plano surface, 1310/1550 nm DBAR coated), reflected from a mirror, and being captured by a second, off-axis SMF-28 fiber. This example uses the  0.25 pitch lens design. Other gradient index lenses from NSG, Grintech, or Corning, all of the more common radial gradient variety, could also be used as well as a homogeneous lens. Geometrically, we would expect significant cancellation of the odd aberrations and low spherical aberration because of the high-index convex lens surface. The lowest insertion loss should be reported when (1) the Gaussian beam waist is at the mirror and (2) when the mirror is placed at the telecentric stop location, i.e. the location where the beams from surface normal emitters (the off-axis fibers) naturally cross, as depicted in Figure 1. This location is one focal length in front of the front principal plane. The front principal plane is the apex of the convex surface on a TL lens and somewhere inside a radial-gradient lens. Because the principal plane is inside a radial gradient lens, the telecentric stop location is very close to the front lens facet. The fiber facets and the lens plano surface are polished at an eight degree angle in the x-y plane to suppress back reflections. The fiber centers are located at x="62.5 microns, y=0 microns; the fibers are parallel to the optical axis. The optical prescription follows:

 

FILE = MIRRORED DUAL-FIBER

 

Wavelength:  1.55000 microns

 

Object X coordinate:  0.06250

Object Y coordinate:  0.00000

 

Image X coordinate: -0.06250

Image Y coordinate:  0.00000

 

Object Space NA = 0.140000. Image Space NA = 0.140000. Beam is deviated from normal emission because of the 8 degree polish on the fiber and the lens. Fiber Mode Field Diameter on surfaces 0 and 9 is 10.4 microns. Surfaces 1, 2, 7 and 8 are angled at 8 degrees.

 

Units in mm.

 

   # TYPE        RADIUS     DISTANCE  GLASS          INDEX    SEMI-DIAMETER

 OBJ SD        Infinity   0.0000      SMF-28      1.468100    

   1>SD        Infinity      0.00900              1.000000    

   2 SDI       Infinity      3.30000  G23SFP      1.652594     0.90 GRADIUM TL1.80AB25-8 Lens

                   DELTA Z = 2.40000

   3 S          -1.4000      1.97000              1.000000     0.90

 STO SM        Infinity      0.00000  MIRROR     -1.000000     (Natural Stop Location & Gaussian Beam Waist)

   5 S         Infinity     -1.97000             -1.000000     The thickness of this surface is a pickup                                                       from surface 3

   6 SI         -1.4000     -3.30000  G23SFN     -1.710411     0.90 GRADIUM TL1.80AB25-8 Lens

                   DELTA Z = 3.70000

   7 SD        Infinity     -0.00900             -1.000000     0.90

   8 SD        Infinity      0.00000  SMF-28     -1.468100    

 IMG S         Infinity                          -1.000000

 

Decenter / Tilts :

   #              XDE        YDE        ZDE        ADE        BDE        CDE TLM TSEQ

   0           0.0000     0.0000     0.0000     0.0000     0.0000     0.0000   0 XYZABC

   1           0.0000     0.0000     0.0000    -8.0000     0.0000     0.0000   0 XYZABC

   2           0.0000     0.0000     0.0000    -8.0000     0.0000     0.0000   0 XYZABC

       GRIN    0.0000     0.0000     0.0000     0.0000     0.0000     0.0000

   7           0.0000     0.0000     0.0000    -8.0000     0.0000     0.0000   0 XYZABC

   8           0.0000     0.0000     0.0000    -8.0000     0.0000     0.0000   0 XYZABC

 

   

 

Because ZEMAX only models GRADIUM® as a surface, rather than a glass, it is not possible to put a wedge on the front surface (surface 2 in the optical prescription). Therefore that surface must either be modeled as a plano surface or the lens must be shortened and a thin wedge of N-SF8 used to put the angled surface in the system. N-SF8 happens to have the closest refractive index properties to the back side of the TL lens of any catalog glass that I have noticed. However, not only is the physical length of the lens shortened, but the delta z value on surface 2 must be increased by the center thickness of the wedge. This inability to model the wedge is not an issue with radial gradient lenses of the NSG type (GRIN9 surface).

 

As a side note, many programs generate incorrect Gaussian beam data for this problem. One issue is that the angled surfaces distort the paraxial calculations. Problems with the paraxial Gaussian beam calculations have been noted in ZEMAX, OSLO and OpTaliX.

 

RESULTS: Dual Fiber.xls

 

ZEMAX: The fiber coupling (FICL) worked perfectly. I was unable to get POP to give a reasonable result. This may be due to the inhomogeneous material, which must be handled by ray trace. Dual Fiber.zip

 

OpTaliX: CEF failed to provide meaningful results (Wagner-Tomlinson algorithm). BPM (a.k.a. POP) was also unable to produce reasonable results. The PSF-based CEF introduced in approximately v. 5.02 does better than the old algorithm.Mirrored Dual-Fiber Collimator.otx

 

OSLO: Fiber coupling is based on Wagner-Tomlinson algorithm and failed to provide meaningful results. Mirrored Dual-Fiber Collimator-tilt x.len

 

CODE V: No data available. Mirrored Dual-Fiber Collimator.seq

 

FRED: No data available.

 

ASAP: No data available. Contact BRO customer assistance.