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

Categories: Passive Optics, SELFOC® Lenses and Collimators Tags: GRIN, Lens, Passive, SELFOC®

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.

SEFLOC® SLC-180 Collimating Lens
Features:
• Low Insertion Loss
• Improved Collimation
• Drop-In Replacement for SLW-180 SELFOC Lens

Application:
• Optical Isolators
• Collimators
• DWDM Devices

The SLC-180 SELFOC Collimating Lens is designed for use in demanding telecommunication and data communication applications. The compact size, convenient shape and high optical efficiency contribute to its world-wide recognition as a unique and economical solution to many optical design challenges.

Because of its very low insertion loss, the SLC-180 helps support the ever-increasing need for ultra low loss, high bandwidth telecommunications systems. Typical insertion loss for this lens is only 0.1dB per pair. Applications for the SLC-180 include optical isolators, collimators and DWDM devices.

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
Minimum Lens Length	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.
Radiation	(Varies by dose and wavelength)	Contact NSG America for reference data

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°)	0.6	1.8	0.25	1310/1550
SLW	“Wide” field of view (55°)	0.46	1	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	0.25	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

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	N 0	1.6073			1.6354		1.6576
	√A	0.608	0.339	0.304	0.207	0.154	0.43
	Z (0.25P)	2.58	4.63	5.17	7.6	10.24	3.65
830 nm	N 0	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	N 0	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	N 0	1.5916			1.6165		1.636
	√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	N 0	1.5901			1.6147		1.634
	√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

Angled Facet
Angling one or more of the lens facets can effectively reduce back reflection from the surface(s). This option is available for all lenses with 1.8 or 2.0 mm diameter and a length of at least 2.3 mm. There are two types of angled facets available. With the Single-Angle Option, one lens facet is tilted while the other remains perpendicular to the optical axis. With the Double-Angle Option, both facets are tilted identically such that they remain parallel to each other. Minimum order quantity is 50 pieces. Back reflections can be further minimized with the use of AR coating.

SLC-180 SELFOC Collimating Lens

Typical SLC-180 Specifications

Diameter	1.8 mm +0.005/-0.010 mm
On-Axis length (Z)	4.88 mm +0/-0.04 mm
√A	0.322 mm-1 +/-2.5%
Pitch	0.25 at 1550 nm
Angle (θ)	8º +/-0.5º
Width of flat on Angled Face (A)	0.5 mm +0.05/-0.2 mm
AR-Coating	Total lens reflectance <0.5% at 1550 nm +/-15 nm

Performance Comparision

	Diameter (mm)	√A (mm-1)	1/4 Pitch Length (mm)	Optical Insertion Loss per Pair(dB)
				Typical	Maximum
SLW-180	1.8	0.326	4.82	0.3	0.8
SLC-180	1.8	0.322	4.88	0.1	0.3

SELFOC® MicroLens – Coatings

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

SELFOC® MicroLens – Instructions

Key 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
√A 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)
M_T Transverse magnification
θ+ Maximum angle from object above axis
θ_– Maximum angle from object below axis
H_m Maximum object height
L_s Distance from lens surface to aperture stop

Steps for using the SELFOC® MICROLENS Tables:

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”.
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.
Adjust the Pitch in small increments and observe how the optical parameters are altered. Recall that 2πP=Z√A.

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 – ((√A)²/2) * 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=(√A)*Z

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.

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.

Use the SELFOC Distance Calculator:

Use the Selfoc Magnification Calculator:

SELFOC® Micro Lens