Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS FOR MEASURING THE PRO~'ERTIES OF AN OPTICAL FIBER
Waveauide Fiber Measurement Apparatus
Cross-Reference to Related Applications
This application claims the benefit of U.S. Provisional Application No.
60/128,504, filed April 9, 1999 and U.S. Provisional Application No.
60/129,706,
filed April 16, 1999.
Background of the Invention
1. Field of the Invention
The present invention relates generally to an apparatus for measuring
optical properties of waveguide fiber, and particularly to an apparatus that
employs optical switching of light sources or detectors.
2. Technical Background
Waveguide fiber optical measurements have always been a costly part
of the manufacturing process. This is particularly true of multimode fiber
measurements that include bandwidth, attenuation, numerical aperture, core
diameter, and differential mode delay. Traditional optical measurement systems
have used optics benches and bulk optic components consisting of lenses and
movable mirrors to fold optical paths and to combine signals for the various
measurements. One connection to the test fiber is established using an XYZ
translation stage in front of a final lens before the detector. Translation
stages
are known to be temperature sensitive and subject to backlash in their movable
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parts. The connection at the light launch end of the fiber is made to a source
of
light appropriate for the desired measurement.
Because certain of the multimode fiber optical properties, viz., bandwidth
and attenuation, are launch sensitive, typically, measurements using more than
one launch condition are desired. In addition, measurements at more than one
wavelength are usually desired so that the launch end connection may have to
be made numerous times.
Thus, these measurement benches are notoriously slow, difficult to align
and maintain in alignment, and large in size in that they have a surface area
on
the order of a square meter. To maintain reliability, such a bench must be
periodically calibrated against a standard bench using standardized fiber.
Time
consuming and costly repeat measurements are often required.
At the present time, standard optical specifications for multimode fiber
performance criteria include measurements made using a launch condition
having a spot size and numerical aperture which excites all of the modes of
the
multimode waveguide fiber. This launch condition is called the overfilled
condition and is defined in the industry standards Fiber Optic Test Procedure
(FOTP) 54. Attenuation measurements are made using a limited or restricted
launch, referred to as Limited Phase Space Launch (LPS) and defined in FOTP
50. The LPS launch is similar to the 30 um spot size launch described below.
More recently, a demand for multimode fiber optimized for laser sources
has increased the number of different launch conditions for bandwidth
measurements. This in turn increases the number of connections at the
measurement bench that compounds the problems with such benches.
Thus there is a need for a fiber optic measurement apparatus that
provides ease of connection and alignment of the components of the apparatus
and of the fiber to be tested. Switching the launch end of the waveguide fiber
among sources having different wavelengths or launch conditions should be
fast and reliable.
The present invention meets this need for less costly, faster, and more
repeatable waveguide fiber measurements.
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Summary of the Invention
One aspect of the present invention is an apparatus for measuring
optical waveguide fiber that makes use of an N X 1 optical switch at the
launch
end of the fiber under test and a 1 X M optical switch at the detector end of
the
fiber under test. The light sources having the desired wavelengths and launch
conditions, i.e., spot size and numerical aperture, are each connected to one
of
the N ports of the N X 1 switch. The detectors are each connected to one of
the M ports of the M X1 switch. The result is, the fiber may be connected
between the two switches and remain connected while all of the desired
measurements are made.
The launch end switch is selected to preserve the launch conditions, i.e.,
the mode power distribution, of the sources. The detector end switch is
selected to preserve the mode power distribution of the light exiting the
fiber
under test. For certain of the measurements, a reference fiber is first
connected between the switches to establish, for example a baseline launch
power or launch pulse width. Thus in the bandwidth measurement, the pulse
width of a pulse passing through the fiber under test is compared to the
reference pulse width. The same comparison is made for the attenuation
measurements, except that in this measurement the power exiting the fiber
under test is compared to the launched power.
In an embodiment of the invention, the spot size or numerical aperture of
the launched light varies from one light to another source. Also certain of
the
sources are single mode lasers. In a preferred embodiment, the single mode
laser sources have a spot size in the range of about 8 ~,m to 30 ~,m.
In another embodiment either the spot size or the numerical aperture of
the launched light may be restricted so that not all modes of a multimode
fiber
carry power, that is, are excited.
A further embodiment of the measurement apparatus includes an OTDR
coupled to the switches via a 1 X 2 coupler so that a trace of reflected power
can be made of light launched into each end of the fiber. The details of the
OTDR connection are set forth below in the description of Fig. 1.
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Additional features and advantages of the invention will be set forth in
the detailed description which follows, and in part will be readily apparent
to
those skilled in the art from that description or recognized by practicing the
invention as described herein, including the detailed description which
follows,
the claims, as well as the appended drawing.
It is to be understood that both the foregoing general description and the
following detailed description are merely exemplary of the invention, and are
intended to provide an overview or framework for understanding the nature and
character of the invention as it is claimed. The accompanying drawing is
included to provide a further understanding of the invention, and is
incorporated
in and constitute a part of this specification. The drawing illustrate an
embodiment of the invention, and together with the description serve to
explain
the principles and operation of the invention.
Brief Description of the Drawings
Figure 1 is a schematic of an embodiment of the invented waveguide
fiber measurement apparatus.
Detailed Description of the Preferred embodiments
Reference will now be made in detail to the present preferred
embodiments of the invention, an example of which is illustrated in the
accompanying drawing.
An exemplary embodiment of the measurement apparatus of the present
invention is shown in Figure 1, and is designated generally throughout by
reference numeral 10.
Description
In accordance with the invention, the present invention for an apparatus
to measure waveguide fiber includes an N X 1 switch 2 for launching power into
the fiber under test. As embodied herein, and depicted in Figure 1, each of
light
sources 4 are optically coupled through 1 X 2 connector 12 to one of the N
input ports of the N X 1 switch. In the case of the OTDR, 6, a second optical
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connection is made through switch 12 to the output end of the 1 X M switch, 8.
This arrangement allows one to obtain an OTDR trace from each end of the
fiber under test. Also shown at the N input ports of switch 2 are sources 14
for
measurement of differential mode dispersion (DMD) of the fiber under test at
5 one or more wavelengths.
The fiber may be optically connected into the measurement apparatus
by means of splices 18. These may be fusion splices or any one of the
many mechanical splices known in the art. Variable attenuator 20 may be
placed in the circuit for use in cases where the launched light power is too
high
10 for the detectors 22. Overdriving the detectors is most likely to occur
when
acquiring the reference light signal mentioned above. Switch 24 is positioned
to send light power from the detector in use to data storage and analysis
means 26. Typically the analysis and storage means include an oscilloscope
and a computer having an analogue to digital interface. These analysis means,
including the computer programs used to compute bandwidth and attenuation
are known in the art (see FOTP's referenced above) and thus will not be
discussed further here.
Examples of the launch conditions or mode power distributions used in
the apparatus of Fig. 1 are as follows. A very restricted launch condition of
spot size about 9.3 ~m and numerical aperture (NA) about 0.14 may be
achieved using a standard step index single-mode fiber as optical fiber
pigtail
28 at one input port of switch 2. A plurality of restricted launches may then
be
achieved by using the standard single mode fiber in conjunction with a
multimode fiber under test and offsetting the single mode fiber core relative
to
the multimode fiber core.
Moderately restricted launch conditions can be achieved using as pigtail
28 a 50 p.m core multimode fiber wrapped about a mandrel. Five turns of such
fiber wrapped around a 5 mm diameter mandrel provided a spot size (diameter)
of 30 um and a numerical aperture of 0.13.
An over filled launch was achieved using as pigtail 28 a step index
multimode fiber having a core diameter greater than about 100 um and a
numerical aperture greater than about 0.30.
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Example
Measurements using the apparatus as embodied in Fig. 1 were carried
out. Switch 2 was a JDS, DPBT switch PN: SC1618-D2SP SN: B6B0366.
Testing was repeated using as switch 2 the respective JDS switches, 1x2
switch PN: SW12-2000311 SN: JC034991, and 1x8 switch PN: SB0108-
Z000329 SN: GB029604. Variable attenuator 20 was a JDS, PN: HA9-2046
SN: KC000660. Four different launch conditions were used to measure
bandwidth of a 62.5 micron core, 125 ~m outside diameter fiber. These were
as described above:
~ a standard overfilled defined by TIA/EIA FOTP ;
~ a moderately restricted launch condition providing a 30 um spot,
achieved using 5 turns of 50 um core fiber around a 5 mm diameter mandrel;
~ a restricted launch condition generated by offsetting the core of a
standard step index single-mode fiber by 4 um relative to a 62.5 um core
fiber;
and,
~ an very restricted launch created by using a standard step index
single-mode fiber.
Results of the test are set forth in Table 1. The percent difference of the
bandwidth measurement from that made on a reference bench are given for
each launch condition and each switch type. The percent difference in
bandwidth measurement caused by the variable attenuator is given in the last
row of Table 1. The percent differences are presented as BW850 nm/BW1300
nm. Measurements at 1300 nm wavelength were not made using the single
mode fiber (SMF) launch.
Table 1
Launch Overfilled 30 um 4 um offset SMF
DPBT 1 %/5% -19%/-8% -11 %/-6% -20%/ -
1 X2 1 %/1 % -23%/-6% -6%/-3% -21 %/
1 x8 4%/2% -48%/-15% -31 %/-15% -33%/
Variable 1 %/5% 0%/1 % -1 %/0% 1 %/ -
Attenuator
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The impact of the attenuator at the end of the system was shown to be
quite small, less than 5% in all cases. Most switches show a low percent
difference, especially in the case of the overfilled launch.
In summary, the invention provides a way of combining sources at
multiple wavelengths and with multiple launch conditions through a fiber optic
switch, thus eliminating the need for open air, bulk optical components. This
provides the means for making a multimode fiber bandwidth measurement
whereby a test fiber must undergo one connection to the test apparatus for a
complete measurement under all permutations of these conditions.
The invention also provides a means of combining multiple optical
measurements using fiber optic switch technology. Thus, through one
connection to the test apparatus, multiple measurements can be performed.
For example, an optical time domain reflectometer (OTDR) or differential mode
delay (DMD) measurement can be combined with bandwidth and attenuation
by connecting them to additional ports of the switches.
This design eliminates the optical bench and utilizes a single electronic
equipment rack for the components. One connection then provides means to
switch in the various launch conditions, various wavelengths and various
measurements without the need for open air optics. There is also a significant
boost in dynamic range of the measurement by elimination of the power that is
lost in an open air optical circuit. The dynamic range is known in the art to
be
the amount of attenuation which can be placed in a measurement path while
retaining a signal to noise ratio that allows a measurement to be made.
Dynamic range of a measurement system thus translates directly into
the length of fiber which can be measured.
It will be apparent to those skilled in the art that various modifications
and variations can be made to the present invention without departing from the
spirit and scope of the invention. Thus, it is intended that the present
invention
cover the modifications and variations of this invention provided they come
within the scope of the appended claims and their equivalents.
