Assembly and Experimental Characterization of Fiber Collimators for
Low Loss Coupling
Ruby Raheem
Dept. of Physics, Heriot Watt University, Edinburgh, Scotland EH14 4AS, UK
ABSTRACT
The repeatability of assembled fiber collimator performance was investigated using Corning SMF28 fiber and 4.4mm
SLW GRIN lens (0.23 pitch) from NSG with a 1550nm ASE source in a laboratory environment by measuring the beam
width and optical coupling loss of collimators assembled in the laboratory. Coupling loss as low as 0.11 dB was
obtained consistently, which was a considerable improvement over the standard performance spec of 0.15 to 0.2dB on
the GRIN lens. Source fluctuation was measured to be 0.003dB. Intensity profile showed reasonable gaussian beam
behavior in the two perpendicular directions. The average coupling loss was 0.112dB with a standard deviation of
0.012dB. The average mode field diameter was 351 microns with standard deviation of 4.6 microns.
Keywords: Fiber collimators, GRIN lens, low loss coupling
1. INTRODUCTION
A variety of new micro-optic lenses have emerged in the market for free space communication devices and it is
necessary to couple light from or to these devices efficiently. The experimental study of the coupling efficiency of these
lenses can be used to compare their performance and at the same time yield information on their optical transmissivity
and beam quality. The focus of this paper is on the experimental technique used to build and optimize the collimator
performance. The ability of the micro-lens to efficiently collect and collimate the highly divergent beam from a
reference single mode fiber with minimal distortion and loss is used as a measure of its performance capability.
Imperfections in the optical material give rise to scattering and absorption of the signal, degrading the beam profile and
reducing the beam intensity. In order to make performance tests it is necessary to demonstrate capability to build
collimators with consistent and repeatable beam profile and consistently low coupling loss, using micro-lenses with
known optical characteristics, in a uniform test environment. The test environment includes the optical fiber, the light
source, the assembling station, the housing devices, the collimator aligning station, fiber splices and the power meter for
optical loss measurement. Each component in the test environment is selected such that the statistical variation in their
physical and optical parameters are significantly less than that of the test micro-lenses, thus not contributing to the lens
to lens variation in coupling loss. The micro-lens used as control and the micro-lenses whose optical performance will
be compared, have to be otherwise compatible in their physical and optical characteristics, such that these components
could be used interchangeably.
Single mode beam propagation through the fiber can be approximated by a gaussian beam profile. The refractive index
within the selected graded-index (GRIN) lens has cylindrical symmetry with index maximum at the center (axis) and
decreasing as the square of the axial separation with a parabolic index profile. A 0.23 pitch GRIN less has air gap
between the fiber (source) and lens and the beam waist. Coupling loss is sensitive to the fiber – lens separation. The
anti-reflective (AR) coating at the end face of the pigtail and GRIN lens minimized loss due to Fresnel reflection. The
coupling loss varies as a function of the transmitting and receiving collimator separation and the location of the beam
waist. For this study, the beam waist was close to the exit face of the collimator to narrow the gap between the
transmitting and receiving collimators, as would be the case with fiber optic connectors. When the separation between
two collimators increase, not all the transmitted light reach the receiver and the loss increases with separation due to the
gradual divergence of the transmitted beam.
2. EQUIPMENT AND COMPONENTS
The first step in bench marking and comparing the performance of devices require repeatability and reproducibility of
test results and a fundamental understanding of the theory and performance of the devices. The theory of GRIN lens,
thin and thick lens optics, single mode fiber and the relationship between the fiber-GRIN lens separation and beam
quality, which explain some of the principles behind this experimental work are found in many optics books [1,2] and
papers [3 -6]. Industry standards like Corning SMF28 fiber and 4.4mm SLW GRIN lens with 0.23 pitch from NSG with
a 1550nm ASE source from Agilent were selected for the study. A single spool of SMF28 fiber was used throughout
the study so as to minimize any possible batch to batch variability of the single mode fiber. The source fluctuation and
coupling loss were measured using ILX Lightwave meter. The average source fluctuation was about 0.003dB. The
beam profile was measured using Beamscan from Photon Inc. where dual slits on a rotating drum scanned the beam to
reproduce the intensity profile of the spot to within a few micron
accuracy. The mode field diameter of the collimated
o
beam was in the range of 300 to 400microns. Angle polished (8 ) pigtails purchased from AC Photonics were used to
build the collimators. The free end of the fiber was cleaved and spliced to the SMF28 reference fiber using Furukawa
fusion splicer. Ferrule for housing the pigtail were 9 ± 0.1mm long and the inner diameter was 1.808 ± 0.005mm. A
custom built base plate with 10 micron precision was used for mounting the ferrule/GRIN lens assembly and positioning
the pigtail. A microscope with ring light and camera was used to align the angle facets of the fiber and GRIN lens
within the ferrule, by aligning the back reflected images of the ring light from the two facets. The Beamscan positioned
at a predetermined fixed distance from the collimator (the distance was much greater than the dimensions of the
collimator) was used to monitor the beam width before permanently fixing the pigtail in the ferrule.
A custom-built collimator assembly unit was used to hold two collimators in position to couple light between them.
One of the two holders was mounted on a 5-axis system with large z-axis displacement capability between them.
2.1 Minimizing Error and Coupling Loss
Micro -lenses come in a variety of shapes and sizes with different NAs and wavefront errors. Choosing the right kind of
micro -lens for the right collimator is essential to minimize coupling loss. Misalignments are chiefly due to lateral,
angular and displacement errors. The critical source of error in building collimators comes from fiber to lens angular
misalignment [3,6]. In this study, the ferrule used for housing the collimator was also used for minimizing misalignment
between the fiber and the lens. With ferrule inner diameter specification of 1.808+/-0.005mm and GRIN lens diameter
specification of 1.8+/-0.005mm, the maximum
possible lateral displacement error would be 0.018mm and the maximum
o
possible angular error would be about 0.3 when about 1.5mm of the GRIN lens protruded from the ferrule. The actual
alignment error is typically a fraction of these extreme cases.
0.3
o
0.018mm
Figure 1: The inner diameter and error of the ferrule has to be such that
any tilt error between the pigtail and lens is minimised
During assembling, the major source of signal loss was found to be surface contamination. Hence cleaning the GRIN
lens surfaces and the pigtail end surface prior to mounting was critical. Acetone and lint-free paper were used to clean
the AR coated components even though acetone is not recommended for the purpose. The pigtail fiber was spliced to
9+/- 0.1mm
Fibre in
ferrule
1.8+/0.005mm
1.808+/-0.005mm
GRIN
Glass sleeve
Figure 2(a): Arrangement of the pigtail and GRIN lens in ferrule
1.5mm +/- 0.1mm
Figure 2(b): The GRIN lens is fixed first with about a third of it protruding
outside the ferrule. The pigtail is positioned in place using the beam profile
for a beam waist close to the exit face of the lens.
the reference fiber during assembling and testing. Splices with loss in the range of 0.01dB were considered acceptable
and splices with higher losses were rejected so that the splice loss contribution could be maintained at the same low
level. Cut back method was used to measure splice contribution to the total loss and subtracted to isolate the lens
contribution to coupling loss. Each grin lens was cleaned and mounted in the ferrule. Using a droplet of UV curable
epoxy, the lens was fixed in position such that about 1.5mm of the lens was outside the ferrule.
3. ASSEMBLING AND TESTING
The GRIN lens was inserted in to the ferrule and held in position vertically with about 1.5mm jutting out. UV curable
glass bonding epoxy was used sparingly to fix the GRIN lens in place in the ferrule. A droplet anywhere around the rim
would disperse evenly around the narrow space between the lens and the ferrule due to surface tension, centering the
lens in the ferrule. The UV exposure and curing was done with the ferrule and lens standing on end. Placing the ferrule
on the side could result in uneven thickness distribution of the epoxy and create misalignment. In the vertical position,
the lens would be centered inside the ferrule, minimizing both lateral and angular misalignment (Figure 2).
o
Angle (8 ) polished fiber and GRIN lens were aligned using the back reflected images of the ring light attached to the
viewing microscope (Figure:3). The GRIN lens and fiber separation within the ferrule were adjusted to provide
minimum beam width very close to the collimator, so as to enable low loss coupling of two collimators in close
proximity. Collimators were built to obtain a specific beam width at the detector positioned at a fixed distance from the
collimator. Prior tests were used to obtain the beam width needed at this particular detector position, such that the beam
waist was formed very close to the exit face of the GRIN lens. Collimators built to this specification demonstrated high
repeatability in coupling efficiency and beam profile. The output beam width of each collimator at different distances
from the beamscan was measured to obtain a profile of the beam width as a function of distance.
Microscope
Microscope used
for angle face
alignment of the
to view the
collimator
fiber and lens
Source
Beamscan
Figure 3: The collimator assembly setup. The beam width of the signal from the source is
measured using the Beamscan which is positioned at a fixed distance from the ferrule. The
horizontal microscope ring light illumination is used to align the two angled faces inside
the ferrule. The vertical microscope – camera unit is used for viewing the assembly.
3.1 Coupling Loss Measurement
To minimize alignment error during coupling loss measurement and to speed the coupling process, a parallel faced
mirror was positioned between the two collimators to parallel align the z-axis of the two mounted collimators using the
tip/tilt knobs. Once the axis of the two collimators were rendered parallel, the two collimators were centered on the zaxis using the x-y axis controls until the 1550nm signal launched through one collimator was detected when coupled to
the second collimator (Figure:4). Fine tuning to maximize the signal output was done using all five axis. This was
found to be a fast and efficient technique to avoid spurious off-axis signals. Various initial tests were conducted before
building the 10 collimators. The collimator used as reference collimator was exchanged with the test collimator to see if
there was any difference between the coupling loss as a result of the change of the reference collimator. The error
between the loss data from the two sets were negligible, indicating that the test setup was sufficiently robust and the
assembled collimators had reproducible loss performance as a function of separation.
Source
Detector
rc
z-axis
Separation
Figure 4: The collimators were first aligned for parallel z-axis and then using the x-y
axis, their z-axis were linked coupling the signal between the two collimators.
4. RESULTS AND CONCLUSION
Loss measurement was based on the optimized loss when the beam from one collimator was coupled to another similar
reference collimator and corrected for associated splice loss. Loss (in dB) and beam width (in micro ns) as a function of
the collimator separation was plotted for ten assembled collimators to show the repeatability of the collimator
performance.
Beam
Widthof
Profile
Beam Width
Profile
10 Collimators
500
480
460
Beam width
in microns
Beam Width in microns
440
420
NSG-1
NSG-2
NSG-3
NSG-4
360
NSG-5
NSG-6
340
NSG-7
NSG-8
320
NSG-9
NSG-10
400
380
300
0
10
20
30
40
50
60
Separation in mm
Collimator separation in mm
Figure 5: Beam width profile as a function of collimator separation for ten
assembled collimators
Intensity profile showed reasonable gaussian beam behavior in the two perpendicular directions. Data (Figure:5)
indicated the collimators had a consistent mode field diameter between 340 to 360um at the exit face of the collimator.
Average mode field diameter was 351microns with a standard deviation of 4.6 microns. The loss was minimum when
two collimators were separated by a millimeter or less (Figure:6). Minimum measured Loss was 0.09dB and the
maximum Loss was 0.13dB with average Loss at 0.112dB and Standard deviation at 0.0124dB.
Parameters linked to alignment errors include the uniformity and tightness of the spec in the inner diameter of the
ferrule, the surface qualities of the optical elements and the accuracy in the alignment between the collimators. By
cleaning the components and minimizing misalignment during assembly, through the choice of ferrules, vertical
positioning of GRIN lens in ferrule during curing - all contributed significantly to improving the fiber – lens alignment
and the signal through the collimator. Predefining the beam width such that the beam waist was formed close to the
collimator exit face and assembling the collimators using the beam width at a fixed distance from the collimator
(alternate approach to measuring the sub-millimeter fiber lens separation) also contributed to the improved coupling
efficiency. This approach resulted in collimators with repeatable coupling loss that was a considerable improvement
over the standard performance spec of 0.15 to 0.2dB on the SLW GRIN lens, at the time.
C o m p a r i s o n oLoss
f 1 0 NProfile
S G C o l l i of
m a 10
t o r sCollimators
built to 460um Beam Width
0.9
1
2
3
4
5
6
7
8
9
10
0.8
0.7
Loss in dB
Loss in dB
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
Separation in mm
Collimator separation in mm
Figure 6: Loss profile as a function of collimator separation for ten assembled
collimators
Acknowledgement: D.M.Trotter(retired) CorningInc.
employment at CorningInc. and released for my use.)
(This experimental work was completed in 2001during my
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4.
5.
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