CA1165605A - Method and apparatus for measuring the insertion loss of an optical component - Google Patents

Abstract

D-23,460 METHOD AND APPARATUS FOR MEASURING THE INSERTION LOSS OF
AN OPTICAL COMPONENT
BY
Arthur H. Fitch Abstract of Disclosure An integrating sphere converts an incident light beam into first and second light beams of the same radiant power. The first light beam is coupled through an optical component to a first radiometer. The second light beam is coupled directly to a second radiometer. The reading P1 on the first radiometer is a measure of the radiant power passed by the component. The reading P2 on the second radiometer is a measure of the radiant power applied to the component. The insertion loss of the component is 10 log P1/P2. In an alternate embodiment, the second light beam is passed through a variable optical attenuator prior to application to the second radiometer. When the attenuation is adjusted to provide the same readings on the radiometers, the decibel value of attenuation corresponds to the insertion loss of the component. In another embodiment having increased sensitivity, light having a fixed pulse repetition frequency is applied to the sphere. Light pulses passed by the component and attenuator are applied to associated ones of a pair of matched photodetectors providing signals that are sequentially applied to a lock-in amplifier, which is tuned to the pulse repetition frequency, for providing clear indications of radiant power incident on the photodetectors.

Description

~ iS~V~
1MElHOD AND APPARATUS FOR MEASURING THE INSERTION LOSS OF

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 1 1~560~; D-23,ll60

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~
_.__ __ 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 ~ ~s~
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.

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 ~ 1~5 ~ r~a 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 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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Claims (20)

What is claimed is:

1. The method of measuring the insertion loss of an optical element that passes light therethrough, comprising the steps of:
converting light from a source into first and second beams of light of the same radiant power, coupling the first light beam to an input port of the element, which passes light in the first beam from an output port thereof, producing a first measurement P1 of the radiant power of light passed from the output port of the element, and producing a second measurement P2 of the radiant power of the second light beam, the insertion loss of the element being determinable from the ration of the first and second measurements.

2. The method according to claim 1 wherein said converting step comprises coupling light from the source into an integrating light enclosure which converts it to diffuse light, and coupling first and second beams of diffuse light of the same radiant power from the enclosure.

3. The method according to claim 2 wherein the first and second light beams are coupled from the integrating enclosure with associated optical fiber means of the same fiber type which have free ends of the same overall surface area incident on the interior of the integrating enclosure.

4. The method according to claim 3 wherein said second named producing step comprises the step of coupling the second light beam to the input of variable optical attenuator means which passes light in the second beam from an output port thereof, producing a third measurement P3 of the radiant power of light passed from the output port of the attenuator means, and varying the attenuation provided by the attenuator means for causing the third measurement P3 of the radiant power of light passed by the attenuator means to have the same value the first measurement Pl, the attenuation provided by the attenuator means being a direct indication of the insertion loss of the element.

5. The method according to claim 4 wherein said first named producing step comprises first connecting only light passed by the element to radiometer means for producing the first measurement Pl, and second disconnecting light passed by the element from the radiometer means, said third named producing step comprising third connecting only light passed by the attenuator means to the radiometer means for producing the third measurement P3.

6. The method according to claim 3 wherein said first named producing step comprises the steps of first connecting only light passed by the element to radiometer means for producing the first measurement, and second disconnecting light passed by the element from the radiometer means, said second named producing step comprising third connecting only the second light beam to the radiometer means for producing the second measurement.

7. The method according to claim 3 wherein light from the source has a pulse repetition frequency and further comprising the step of coupling an indication of the pulse repetition frequency of light from the source to lock-in amplifier means of the radiometer means;
said first named producing step comprising the steps of first detecting light passed by the element, and second coupling the first detected signal to the amplifier means for producing the first measurement;
said second named producing step comprising the steps of third detecting the second light beam, and fourth coupling the second detected signal to the amplifier means for producing the second measurement.

8. The method according to claim 7 wherein said second named producing step comprises the steps of:
coupling the second light beam through variable optical attenuator means.
detecting light in the second light beam that is passed by the attenuator means, coupling the detected signal corresponding to light from the attenuator means to the amplifier means for producing a third measurement P3 of the radiant power of light passed by the attenuator means, varying the attenuation of the attenuator means for causing the third measurement P3 of the radiant power of light passed by the attenuator means to have the same value as the first measurement Pl, the attenuation produced by the attenuator means being a direct indication of the insertion loss of the element.

9. The method according to claim 1 wherein said first named producing step comprises coupling light passed by the element to a first the of a pair of matched radiometers and wherein said second named producing step comprises coupling the second light beam to the other one of the pair of matched radiometers.

10. An optical beam splitter comprising:
an integrating light enclosure, an input port on said enclosure for receiving an input light beam and launching it into the interior of said enclosure for converting it to diffuse light in the latter, a pair of output ports on said enclosure that are oriented 90 as to be responsive to only diffuse light in said enclosure, said output ports having the same characteristics and the same coupling area facing into the interior of said enclosure so as to each couple diffuse light of the same radiant power from said enclosure when light is applied to said input port.

11. The beam splitter according to claim 10 wherein each of said output ports comprises fiber optic means of the same size and type and the same surface area in a hole that opens into the interior of said enclosure.

12. An optical beam splitter according to claim 10 wherein each of said output ports comprises optical fiber means having an an thereof in a hole that opers into the interior of said enclosure and that is approximately flush with the interior surface of aid enclosure so as to each couple diffuse light of the same radiant power from said enclosure.

13. Apparatus for measuring the insertion loss of an optical element that passes light therethrough comprising:
a light source, an integrating enclosure having an input port for receiving light from said source and introducing it into said enclosure which converts it to diffuse light and having first and second output ports which are adapted for coupling associated first and second beams of diffuse light of the same radiant powers from said enclosure when the latter is illuminated with light from said source, first means for coupling the first light beam to the element, radiometer means, second means for coupling light in the first beam that is passed by the element to said radiometer means for producing a first measurement Pl of the radiant power thereof, third means for coupling the second light beam to said radiometer means for producing a second measurement P2 of the radiant power thereof, the insertion loss of the element being determinable from the ratio of the first and second measurements.

14. Apparatus according to claim 13 wherein said second means couples light passed by the element to said radiometer means for producing the first measurement and is disconnected from said radiometer means prior to said third means coupling the second light beam to said radiometer means for producing the second measurement..

15. Apparatus according to claim 14 wherein said third means comprises variable optical attenuator means in series in the optical connection of the second light beam to said radiometer means, the attenuation of said attenuator being varied to cause the value of the second measurement to be equal to the value of the first measurement, the insertion loss of the element being indicated directly by the attenuation setting of said attenuator means.

16. Apparatus according to claim 13 wherein said source produces an input beam of pulses of light having a repetition frequency and wherein said radiometer means comprises:
first and second matched photodetectors responsive to light passed by the element and in the second light beam, respectively, lock-in amplifier means, and fourth means selectively operative for first coupling only the detected signal from said first photodetector to said amplifier means, which is also responsive to the pulse repetition frequency for producing the first measurement of the radiant power of light transmitted through the component, said fourth means subsequently coupling only the detected signal from said second photodetector to said amplifier means for producing the second measurement of the radiant power in the second light beam, the insertion loss 5 of the component being related to the ratio of the first and second measurements.

17. Apparatus according to claim 16 wherein said third means comprises variable optical attenuator means in series in the optical connection of the second light beam to said second photodetector, the attenuation of said attenuator means being varied for causing the value of the second measurement to be equal to the value of the first measurement, the decibel value of attenuation provided by said attenuator means being a direct indication of the insertion loss of the component.

18. Apparatus according to claim 13 wherein said first, second and third means each comprise an associated optical fiber means.

19. Apparatus according to claim 18 wherein said radiometer means comprises:
a first radiometer receiving from said second optical fiber means light transmitted through the element for producing the first measurement Pl, and a second radiometer receiving from said third optical first means the second light beam for producing the second measurement P2, the insertion loss of the element being defined by the relationship 10 log Pl/P2.

20. Apparatus according to claim 19 wherein said third optical fiber means comprises variable optical attenuator means in series in the electrical connection of the second light beam to said second radiometer, said first and second radiometers simultaneously producing associated measurements, the attenuation of said attenuator means being changed to make the value of the second measurement equal to that of the first measurement so that the decibel value of the insertion loss of the element is directly related to the attenuation of said attenuator means.