Note: Descriptions are shown in the official language in which they were submitted.
~ iS~V~
1MElHOD AND APPARATUS FOR MEASURING THE INSERTION LOSS OF
2AN OPTICAL OOMPONENT
3 BY
4 Arthur H. Fitch
5Background oP Invention
6Thi9 invention relates to determining in~ertion loss of optical
7 components and re particularly to method and apparatus for measuring the
8 insertion loss o~ an optical component in a ~iber optic ~ystem.
9 In de~igning and ~abricating fiber optic systems, it is desirable
to know the insertion loss of optical components that may be used in it.
11 One method of measuring the insertion loss of a component is to launch
12 light ~rom a laser diode onto an optical fiber that is connected to a
13 radiometer for producing an indication Po of a reference level o~ radiant
14 power transmitted over the fiber. After cutting the fiber, khe severed
ends thereof are inserted into ports of an optical component ~uch as a
16 connector. The new indication Pn on the radiometer is a measure of
17 radiant power transmitted through the component. The decibel value of
18 insertion loss of the component is lO log Pn/Po. This technique oannot be
19 readily used to obtain a precision measur~ment of the los~ o~ a j~nper
fiber already containing a lossy element ~uoh as a conneotor. This is
21 because of the difficulty in matching the ooupling oonditions along t,he
22 ~umper fiber when a fiber of the same length and type i~ substituted in
23 order to obtain a true reading of the re~erence power. An object of this
24 invention is the provision o~ improved method of and apparatus for
measuring the insertlon loss of optical oomponents.
26 S mmary of Invention
27 In accordance with one a~pect of this invention, the method of
28 measurir~ the in~ertion loss o~ an optical component comprises the steps
29 o~ oonverting light ~rom a ~ource into fir~t and second light beams of the
Yame radiant pcwer, eoupling the ~irst beam to the cc~ponent, producing a
31 ~irst ~ndication Pl o~ the radiant power of light passed by the component,
32 ~nd producing a ~econd indication P2 of the radiant power of light in the
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2 second beam, the inser~ion 1099 of the component belng related to the ratio
3 of the first and second indications. In accordance with another aspect of
4 this invention, apparatus for measuring the insertion loss of an optical
5 component comprises: a light source~ an integrating light enclosure
6 receiving light from the source and converting it to diffuse light in the
7 interior thereof, and having first and second output ports providing
8 associated beams of diffusa light of the same radiant power; radiometer
9 means; first means coupling the first light beam through the component and
to the radlometer means for producing a first indication Pl of radiant
11 power passed by the componentl and second means coupling the second light
12 beam directly to the radiometer means for producing a second indication P2
13 of radiant power applied to the component, the insertion loss of the
lL component being related to the ratio of the first and second indications.
Description of Drawing
16 This invention will be more fully understood from the following
17 detailed description of preferred embodimenta thereof, together with the
18 drawing in which:
19 FIG. 1 is a block dia8ram representa~ion of apparatua embodying
this inventlon for mea~uring ~he inser~ion lo~ of a component 5.
21 ~IG. 2 i9 an enlarged ~ect:Lon view of the output port 16 taken
22 a~ong line 2---2 in FIG. 1,
23 FIG. 3 is an enlarged section vie~ of the sphere 8.
24 FIG. 4 is a block diagram representation of another embodiment.
2~ FIG. 5 is a block diagram representation of a further embodiment.
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1 _escript~on of Preferred Embod:lment~
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Referring now to F~5.S. 1-3, apparatu~ ~or measurin3 the in~ertlon
3 loss of a oomponent 5 compri es an integrat~ng Rphere 8 having an input
4 port 14 receiving light from a source 30; having a output port 16
associated with a test branch 40 $ncluding the component 5 and a first
6 radio~eter 52; and having a ~econd outpuk port 18 associated with a
7 re~erence branch 60 including a variable attenuator 65 and a second
8 radiometer 72 havir~ characteristics that are matched to those of the
9 firAt radiometer~ The element or component under test may? by way of
example, be a lossy optical connector. The source 30 is of the same type
11 that w$11 drive the co~ponent in an optical system and may, by way of
12 example, be a laser diode that is connected to and produces a light beam
13 on an cptical fiber type of pigtail 32.
14 An integrating enclosure is a device that converts input light to
diffuse light which is not incident from any particular direction.
16 Integrating spheres and enclosures are manufactured by Labsphere of New
17 London, New Hampshire. The integrating sphere 8 is o~ conventional
18 design, except that it has a pair of output ports 16 and 18 that are
19 adapted for simultaneously couplir~ beanLs of diffuse light of the same
radiant power from the sphere. The inside 3urface of the 3phere is
21 covered with a reflectance aoating of a material 9U~l as ma~nesium oxide.
22 The 3phere may be split along an axis thereof where lt i~ desirable to
23 obtain access to the interior of the ~phere. Flanges (not shown) may then
24 extend around the circumferences of the open edges of the two halves of
the ~phere, with a hinge attached to the flanges in order to facilitate
26 opening and closing the sphere. Alignment holes and pins may be located
27 along the flanges for providing precision alignment of the interior
28 ~urfaces of the ~phere parts. The two halves Or a sphere msy be secured
29 together with clamps or ~crews on the flanges to form a light-tight
enclosure.
31 ,he axes of the ports 14, 16 and 18 are eoincident with axes of
32 th2 sphere and are ~n a caomon plane that extends through the center of
33 the ~phere. The output ports 16 and 1~ are al~o oriented in the ~phere so
1 that they have a common axis A--A. The input pork 14 i~ ori~nted ~o th~t
2 the axi~ thereof i8 orthogonal to the line A--A. Thls means khat direct
3 ray~ Q~ light enterir~ the ~nput port are nok incident on elther of the
4 output ports. The exact position of the axe~ o~ the input and output
ports in the sphere are not important, although it is ~ssenkial that they
6 be such that orly reflected light in the sphere be incident on the oukput
7 ports.
8 The output port 16, for exampleJ comprises a socket 21 having
9 axially aligned openings 22 and 23 therethrough, see FIG. 3. The smaller
end of the socket ~its securely into an associated opening 24 in ~he wall
11 of the sphere prior to attachirg them together such as by brazing. The
12 end 25 of the socket is flush with the inside wall 9 of the ~phere and is
13 preferably ccated with reflective paint (see FIGS. 2 and 3). The diameter
14 of the smaller opening 23 is cnly slightly larger than that of an optical
fiber 42. The larger opening 22 is dimensioned for receiving a fiber
16 connector 26 that is adapted for releasably holding one end of khe fiber
17 42. After the fiber is threaded into a central opening in the connector
18 26, wqth the end of the fiber flush with the end 27 of the connector, a
19 spring loaded key 2B is compressed lightly against the fiber for fixing
itg posikion in the connector. Th~ ~iber connector i9 then inserted into
21 the ~ocket 21 until it contacts the shoulder on the latter for aligning
22 the ends 27 and 42A of the connector and fiber, respectively, with the
23 interior surface g of the sphere (see FIG. 3)~ The connector 26 i~ held
24 in the socket 21 by a set ~crew 29~ Fiber connectors 26 are manufactured
by Nippon Electric Company. In a similar manner, fiber connectors 15 and
26 19 secure the fiber pigtail 32 and a fiber 62 in the input port and the
27 other output pork, respectively. The optical fibers 42 and 62 are of the
28 ame type, diameter and length ~o that they have the same loss
29 characteristic3. The free ends thereof~ such 2s the end 42A in FIGo 3
30 also have the other ~ame qurface areas ~acing into the ~phere. As was
31 indicated previously, the integrating sphere 8 converts input light from
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1 the source 30 into beams of dl~u~e light of the ~ame radlank power in the
2 ~wo output fiber3 42 and 62.
3 The other ends of the output fibers 42 and 62 are connecked to
4 input ports of the component 5 and the variable optical attcnuator 65 for
illuminating these elements ~ith light of the same radiant power. The
6 output ports of these elements are connected over generally ~atched
7 optical fibers 46 and 66 to associated radiometers~ It is important that
8 care be exercised so as to obtain optimum launching of light in the
9 outputs Or the component and attenuator onto as~ociated ones of the fibers
46 and 660 Alternatively, the input ports of one or both of the
11 radiometers may be connected directly to the output port of one of the
12 elements 5 and 65.
13 In operation, the source 30 is energized ~or producing 1ight on
14 fiber 32 that is emitted into the interior of the integrating sphere 8
which causes the 1ight to be substantially perfectly diffused after one or
16 two reflections from the painted interior surfaces thereof. Diffuse light
17 that is incident on the ends of the fibers 42 and 62 in associated output
18 ports is couplsd through the component and attenuator to the matched
19 radiometers 52 and 72, respectively. The r~diometer 52 produces an
indication or measurement Pl of the radiant power o~ light transmikted
21 through the component 5. The amount of attenuation provided by the devlce
22 65 in the reference branch is then adjusted to obtain an indication P3 =
23 Pl on the radiometer 72. Since light beams of the same radiant powers are
24 applied to the input ports of the component and attenuator and elements
~5 connected in series with output ports thereof are essentially matched, the
26 decibel value of attenuation provided by the attenuator device 65 in
27 producing the indication P3 = Pl on the radiometer 72 corresponds to the
28 decibel value o~ insertion loss of the component 5 in the test branch.
29 rne constancy of the level of li~ht in the integrating ~phere 8 may be
monitored durir~ measurement of in~ertion loss by couplinB diffuse light
31 ~rom an output port 34 on the sphere which is connected through an optical
32 fiber 36 to a monitoring device 38 ~uch as a radiometer.
33
1 The radiant power in the output f~ber 42, for example, is
2 approximately equal to the product o~ the radiant power Or light in the
3 input fiber 32 and the ~atio of the area of riber 42 in the ~phere to the
4 surface area of the interior of the ~phere. Thi3 mean that the radiant
power of light on the fibers 42 and 62 is generally of a relatively low
6 level. If this radiant power of light that is coupled from the
7 integrating sphere 8 is not sufficient to obtain clear and definite
8 readings on a radiometer means, then the sensitivity of the ~ystem may be
9 improved by producing a pulsed light beam of some fixed repetition rate on
the input fiber 32 as is illustrated in the alternate embodiments in FIGS.
lt 4 and 5. In FIG. 49 the light source is pulsed at a repetition rate that
12 is set by an output o~ the control circuit 81. The radiometer means 90
13 here comprises a pair of matched phototdetectors 9l and 92 producing
14 electrical currents that vary in magnitude with the level of light from
the output of the component and in the reference branch, and a switch 95
16 for selectively coupling currents in output lines 93 and 94 of the
17 photodetectors to a lock-in amplifier l~l that is also responsive to an
18 indication of the repetition rate on line 85. The amplifier is
19 essentially tuned to the pulse repetition frequency of circuit 81 and the
light beams on fibers 42 and 75 for narrow band filtering the detected
21 signals and providirg m~re clear and definite indication~ on a meter 103
22 of the amplitudes of the radiant pawer of light passed by the component on
23 fiber 46 and in the reference beam on fiber 75~ Lock-in a~plifiers are
24 conventional and available from Princeton Applied Research Corporation of
Princeton, New Jersey.
26 In operation, modulated light b0ams of the same intensity are
27 coupled from the integrating sphere in FIG. 4. The photodetectors 9l and
28 92 detect light pulses passed by the ccmponent and in the reference beam.
29 The switch g5 is first adjusted for connecting the detected signal in the
test branch to the amplifier ~or producin~ the indication Pl of the
31 radiant pawer of light pa~sed by the component. The switch 95 is then
32 adjusted to connect the output of the reference branch to the ampllfier
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1 for producing the indicatisn P2 of thc radiant power o~ llght lncident on
2 the input pork of the component. Ihe inscrti~n 103~ of' the canpon~nt i~
3 10 log PlfP2. In FIG. 5, oontrol means 81' i8 employed to drive an
4 apertured wheel 84 for mechanically chopping li~ht frcm the aource 30 for
producing a beam of pulse~ of light o~ a fixed repetition rate on the
6 input ~iber 32.
7 Although the method and apparatus of this invention is described
8 in relation to speci~ic embodiments in FIGS. 1-5, variations and
9 modifications thereof ~ill occur to those skilled in the art. By way of
example, this novel method and apparatus are applicable to radiant energy
11 and light`in othPr than the visible electromagnetic spectrum. Thus, the
12 word light as used here means both visible radiant energy and invisible
13 radiant energy in both the low and high end of the frequency spectrum,
14 including both ultraYiolet and infrared radiation. Also, the integrating
enclosure may be cylindrical, rectangular or any other convenient shape,
16 although it preferably has a regular shape. Additionally9 the disclosed
17 method may be practiced with a single radiometer 52 by first connecting
18 the fiber 46 to the input port 50 of the radiometer and obtaining the
19 measurement Pl of the radiant power of light p~ssed by the component. The
fiber 46 is then disconnected from the radiometer and the fiber 66
21 inserted in the input port 50. The attenuation of the devlce 65 is then
22 adjusted for providing the indication P3 = Pl on the radianeter 52 and
23 thereby providing an indication of the insertion loss of the component.
24 Further, the insertion loss of the component may be dete~mined with
apparatus in FIG. 1 that does not lnclude the attenuator device 65. In
26 this structure, the other end of the fiber 62 is applied directly to the
27 input port 71 of the radiometer 72. The indication Pl of radiant power
transmitted through the component is till provided by the radiometer 52.
29 The other radiometer 72 now provides an indication P2 of the radiant power
o~ light that is appli~d to th~ input port of the camponent under test.
31 The insertion los~ of ~he component is 10 log Pl/P2. Additionally, the
32 optical ~ibers 42 and 62 may be replaced with as~ociated bundles of
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1 optical fibers Or the ~ame type that have substantially makched
2 characteriqtics and lo~s over the lengths thereof' ror coupling a higher
3 level of radiant power ~rom the ~phere 8~ A lens may be employed on the
4 output er~ of each o~ the ~iber bundles ~or focusing light frcm the
pluralitie~ of ~ibers into the input ports of the component and attenuator
6 device.
7 m e scope ~f this invention is therefore defired by the appended
8 claims, rather than the aforementioned detailed descripti~ns of preferred
9 embodiments thereof.
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