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SELFOC® Micro Lens

SELFOC® MicroLenses are ideal for many signal transmission and focusing applications. Some typical uses are listed as follows:

 

  Laser diode-to-fiber coupling
  Fiber-to-detector coupling
  Fiber-to-fiber coupling
  Focusing and collimating

 

SELFOC® MicroLens

There are two categories of lenses, both distinguished by a different numerical aperture: SLW (wide)and SLH (high). Each NA type is appropriate for different applications and exhibits different alignment sensitivities (see Technical Charts Table 2).

While the numerical aperture is a maximum at the center of the lens aperture, the actual NA is a function of the ray parameters (height and angle) for each ray that strikes the lens’ surface. Click to the Appendix of this guide for an illustration of how the NA depends on these parameters.

SELFOC® MicroLens – Technical Charts

Table 1 Common Characteristics

ITEM SPECIFICATIONS NOTES
OPTICAL
Maximum Pitch 0.75P Lens length Z = 2πP / √A
Lens Effective Diameter approx. 60 – 70% lens diameter For low loss and low aberration
Index Gradient Constant √A Tolerance +/- 0.75% max. Within same ion exchange batch
Index Gradient Constant √A Tolerance +/- 2.5% max. Between ion exchange batches
MECHANICAL
Lens Diameter Tolerance +5 / -10 µm Excluding SLW-3.0 and SLW-4.0
Lens Diameter Tolerance +0 / -20 µm For SLW-3.0 and SLW-4.0
Lens Length (Z) Tolerance * +/- 2.5% of nominal length Adjusted according to √A variation
Lens Length (Z) Tolerance * +0 / -40 µm (vs.calculation) Machining and polishing tolerance
Minimum Lens Length 2.0 mm for non-coated lenses
2.3 mm for AR coated lenses
Facet Perpendicularity 6 mrad max.
Ellipticity 3 µm Dmax – Dmin
Glass Material Oxide Glass
OTHERS
Temperature 350 °C max. operating temp. for lens material
Humidity (Will react with non-coated lens) Store in dessicant to avoid moisture. See Storage / Handling.
Radiation (Varies by dose and wavelength) Contact NSG America for reference data

* Total lens length (Z) tolerance includes variations from both √A and machining/polishing.
** The Surface Quality Specs apply to complete, round, flat end surfaces perpendicular to the cylindrical axis of the lens. It does not apply to the flat portion of the angled end surface.
 

Table 2 Standard Lens Specifications

SML Type Features On-Axis N.A. Diameter(mm) Standard Pitch(es) Standard Wavelength (nm)
SLH “High” field of view (74°) 0.60 1.8 0.25 1310/1550
SLW “Wide” field of view (55°) 0.46 1.0 0.25 1310/1550
SLW “Wide” field of view (55°) 0.46 1.8 0.23, 0.25 1310/1550
SLW “Wide” field of view (55°) 0.46 2.0 0.25 1310/1550
SLW “Wide” field of view (55°) 0.46 3.0** 0.11 780
SLW “Wide” field of view (55°) 0.46 4.0** 0.11 780

** SLW-3.0 and SLW-4.0 lenses with 0.11 pitch are tested for wavefront quality. See section on Specialty Lenses for further details.

 

Table 3 Optical Parameters

Wavelength Type SLW SLH
N.A.(2θ) 0.46 (55°) 0.60 (74°)
Dia. (mm) 1 1.8 2 3 4 1.8
630 nm N0 1.6073 1.6354 1.6576
√A 0.608 0.339 0.304 0.207 0.154 0.430
Z (0.25P) 2.58 4.63 5.17 7.60 10.24 3.65
830 nm N0 1.5986 1.6249 1.6457
√A 0.601 0.332 0.298 0.203 0.151 0.423
Z (0.25P) 2.61 4.73 5.27 7.75 10.43 3.71
1060 nm N0 1.594 1.6194 1.6394
√A 0.599 0.329 0.296 0.201 0.149 0.419
Z (0.25P) 2.62 4.77 5.31 7.85 10.54 3.74
1310 nm N0 1.5916 1.6165 1.6360
√A 0.597 0.327 0.295 0.199 0.1483 0.418
Z (0.25P) 2.63 4.8 5.32 7.89 10.59 3.76
1550 nm N0 1.5901 1.6147 1.6340
√A 0.596 0.326 0.294 0.199 0.1478 0.417
Z (0.25P) 2.64 4.82 5.32 7.89 10.63 3.77

 

SELFOC® MicroLens – Instructions

SELFOCKey to optical parameters: (all units in millimeters unless otherwise stated)

λ Wavelength of incident light in microns (>0.55 mm)
L1 Object distance (from object point to lens’ front surface)
L2 Image distance (from lens’ back surface to image point)
N0 On-axis refractive index of SELFOC® lens
Index gradient constant (mm-1)
Z Lens length
EFL Effective focal length (from rear primary plane to rear focal plane)
BFL Back focal length (from rear lens surface to rear focal plane)
MT Transverse magnification
θ+ Maximum angle from object above axis
θ- Maximum angle from object below axis
Hm Maximum object height
Ls Distance from lens surface to aperture stop

Steps for using the SELFOC® MICROLENS Tables:

  1. If the object distance for your application is known, click the sheet tab entitled “Obj. Distance”. If the desired magnification is known, click the sheet tab entitled “Magnification”.
  2. Enter the required data in the colored data cells. As you enter numeric values, the SELFOC® lens parameters such as N0, √A, and EFL will be recalculated in the lens table.
  3. Adjust the Pitch in small increments and observe how the optical parameters are altered. Recall that 2πP=Z√A.

SELFOC® MicroLens – Coatings

Type Center Wavelength Spectrum Width Reflection
K 1450nm +/- 200nm R,0.2% (per surface)
S 830nm +/- 15nm R,0.25% (per surface)
S 630nm +/- 15nm R,0.25% (per surface)
D 830&1310nm 830 +/- 15nm
1310 +/- 30nm
R,0.5% (per suface)
H 980&1550nm +/- 30nm R,0.5% (per suface)

 

Recommended Storage and Handling of Lenses

Storage
For extended periods of time, the lenses should be stored in a “dry box” environment (40%RH or less). This entails the use of a desiccant (e.g., silica gel) or a heat source to prevent humidity from leaching the lens material. This is much more critical for non-coated lenses, since AR coatings help to protect the lens surfaces from humidity. For short term storage (less than a month), the plastic box and foam packing in which the lenses are shipped will provide adequate storage.

In addition to humidity requirement, the lenses need to have sufficient spacing to avoid potential damage such as chipping and scratching from other lenses. For this reason, Go!Foton storage boxes have built-in slots in which the lenses are placed, with surrounding packaging to hold them securely in place.

Handling
After opening the lens boxes, it is important to exercise extra care in lifting the plastic shield. Particularly with smaller lenses, it is possible that they may cling to the shield and be lost during removal. Lenses should be handled with plastic tweezers, preferably those with a tapered end. Lenses should be picked up out of their individual compartments by firmly holding each by its side surface (not the ends).

Cleaning
At times it is necessary to clean the lens surfaces due to the presence of some dust or film which may impair the image. Go!Foton generally recommends the use of ethyl alcohol as a cleaning solvent. Acetone may also be used, without harm to the lens, but it should be pure enough to no leave a residue on the lens’ surface.

 

Physics of SELFOC

The Gradient Constant

The SELFOC lens utilizes a radial index gradient. The index of refraction is highest in the center of the lens and decreases with radial distance from the axis. The following equation describes the refractive index distribution of a SELFOC lens:

Equation 1: 
N(r) = N0(1 – √A2/ 2 * r2)

This equation shows that the index falls quadratically as a function of radial distance. The resulting parabolic index distribution has a steepness that is determined by the value of the gradient constant, √A. Although the value of this parameter must be determined through indirect measurement techniques, it is a characterization of the lens’ optical performance. How rapidly rays will converge to a point for any particular wavelength depends on the gradient constant. The dependence of √A and N0, on wavelength is described by the dispersion equations listed at the end of this product guide. Note that different dispersion equations apply to different lens diameters and numerical apertures.

Lens Length & Pitch

In a SELFOC lens, rays follow sinusoidal paths until reaching the back surface of the lens. A light ray that has traversed one pitch has traversed one cycle of the sinusoidal wave that characterizes that lens. Viewed in this way, the pitch is the spatial frequency of the ray trajectory.

Equation 2:
2πP=√AZ

The above equation relates the pitch (P) to the mechanical length of the lens (Z) and the gradient constant. The figure below illustrates different ray trajectories for lenses of various pitch. Notice how an image may be formed on the back surface of the lens if the pitch is chosen appropriately.

Paraxial Optics

In contrast to the optics of homogeneous materials, gradient-index optics involve smoothly-varying ray trajectories within the GRIN media. The paraxial (first-order) behavior of these materials is modeled by assuming sinusoidal ray paths within the lens and by allowing the quadratic term in Equation 1 to vanish in the ray-tracing calculations. All of the usual paraxial quantities may be calculated with the help of the ray-trace matrices given at the end of this product guide. The formulae for common paraxial distances have also been tabulated for quick reference.

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