Optical loss measurements

Details Optical loss measurements Optical loss measurements

1999-06-23

2002-10-01

Font, Frank G. (Department: 2877)

Optics: measuring and testing

For optical fiber or waveguide inspection

Reexamination Certificate

active

06459478

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to optical loss measurement which are generally carried out in order to determine the optical characteristics of an optical component
BACKGROUND OF THE INVENTION
A typical optical loss measurement is the return loss measurement. The return loss (RL) of an optical component is generally defined as the ratio of the reflected optical power P
back
to the incident optical power P
in
, in units of dB
opt
. Therefore, the return loss is usually a positive number with:
RL
=−10 log (
P
back
/P
in
) in [dB
opt
]  (eq. 1).
FIG. 1
a
shows an arrangement for determining the return loss by means of a fiber optical RL-meter
10
. The RL-meter
10
comprises a source
20
(e.g. a laser source), a receiver
30
(e.g. an optical power meter), a fiber optical coupler
40
, and a connection
50
(e.g. a front panel connector) of the RL-meter
10
to a device under test (DUT)
60
.
The fiber optical coupler
40
is normally embodied by a fused fiber coupler as depicted in
FIG. 3
a
. The optical coupler
40
comprises a first fiber with a first end
41
and a second end
42
and a second fiber with a third end
43
and a forth end
44
. The first and second fibers are coupled in a way that a signal coming from one side (e.g. end
41
) is coupled to the ends (e.g. ends
42
and
43
) of the other side. The optical coupler
40
provides a strict directivity, so that the incident beam at one side is split up (e.g. in equal amounts) and provided at the ends of the opposing side, whereas only a small amount (e.g. about 10
−6 . . . −7
) of the incident beam will be reflected to the other end of the side of the incident beam.
When an optical power Ps is provided at the end
41
, an optical power M can be measured at the end
42
which substantially corresponds to the optical power Ps, with M=t
1
·Ps. When an object with a given reflectivity R is coupled to the end
44
, a returning optical power P can be measured at the end
43
, with:
P=c
1
·
M·R+c
2
·
M
  (eq. 2),
whereby c
1
and c
2
represent general factors depending on the characteristics of the fiber coupler
40
.
Before measuring the return loss of the DUT
60
, a calibration of the RL-meter
10
needs to be done, e.g. as described by Christian Hentschel, “Fiber Optics Handbook”, third Edition, March 1989, Hewlett-Packard, on page 188. As shown in
FIG. 1
b
thereof, a cable
70
is connected to the connector
50
. The return loss calibration and measuring procedure consists of three steps. In a first step, a calibration setup is performed with a connector
80
of the cable
70
open. A power meter of the receiver
30
reads a power P
1
. In a second step, the connector
80
is immersed in oil in order to avoid reflections from the end of the fiber. A measurement of the unwanted reflections from the pair connector
50
and connector
90
of the cable
70
is performed. The power meter now reads P
2
. The calibration can then be done based on the measured power values P
1
and P
2
. Finally, the DUT
60
is connected to the connector
80
and measurements of the DUT
60
can be performed in a third step (see
FIG. 1
c
).
More details about return loss measurements are also given by Dennis Derickson, Fiber Optic Test and Measurement, ISBN 0-13-534330-5, 1989, e.g. P. 387ff and P. 461ff.
Another typical optical loss measurement is the insertion loss measurement. The insertion loss (IL) of an optical component is generally defined as the ratio of the transmitted optical power P
out
to the incident optical power P
in
, in units of dB
opt
:
IL
=−10 log (
P
out
/P
in
) in [dB
opt
]  (eq. 3)
FIG. 2
a
shows a typical measurement setup for insertion loss measurements using substantially the same measurement components as for the return loss measurement in FIG.
1
. The source
20
can be coupled via the fiber coupler
40
, or directly, to the connector
50
which again couples via the cable
70
to the DUT
60
. Another end of the DUT
60
is coupled via a connector
100
to the receiver
30
. Again, before measuring a calibration of the measurement setup generally has to be performed.
FIG. 2
b
shows a calibration step for the insertion loss measurement. The connectors
80
and
100
are directly coupled together, and the receiver
30
measures the output power P
out
. For measurement purposes, the DUT
60
is inserted between the connectors
80
and
100
, as shown in
FIG. 2
a
. More details about typical insertion loss measurements are given by Christian Hentschel, “Fiber Optics Handbook”, third Edition, March 1989, Hewlett-Packard, on page 188, or in Dennis Derickson, Fiber Optic Test and Measurement, ISBN 0-13-534330-5, 1989, P. 21-22, P. 339-382 and P. 454-457.
In most applications, the calibration of the return loss measurement is performed using a specific reference cable
70
R as the cable
70
. The reference cable
70
R normally provides a defined return loss and a minimum insertion loss, e.g. due to minimized mechanical tolerances and excellent polishing, and allows a well defined calibration in a defined measurement environment. ‘Normal’ measurements of the DUT
60
are then carried out using an ‘ordinary’ so-coled customer cable
70
C as the cable
70
. It is highly recommended to use different cables for calibration and measuring, since the connector
80
(of the reference cable
70
R) might be degraded when frequently changing the DUTs
60
When using different cables
70
for calibration and measuring, however, the insertion loss of the different cables
70
might be different because of mechanical tolerances of fiber optic connectors. A change of x dB at a certain connection will result in a 2x dB measurement error in the return loss measurement and generally decrease its accuracy, since any loss change in the measurement setup will influence the detected power level at the receiver
30
. For return loss measurements, the radiation goes twice through the connection of the RL-meter
10
to the attached cable
70
, forward and reverse, thus influencing twice the loss change of the return loss measurement result.
Other inaccuracies of the return loss measurement might occur from a variation of the output power (e.g. a drift versus time or temperature) and/or of the optical spectrum of the source
20
. This leads to a variation of the insertion and return loss of the components involved in the respective measuring setup, e.g. of the connector
50
.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved loss calibration and/or measurement for optical components. The object is solved by the independent claims. Preferred embodiments are given by the dependent claims.
A first aspect of the invention concerns an improved fiber coupler as set out in claim
10
allowing to reduce an influence of reflection on the measuring results.
A second aspect of the invention concerns the calibration of a system for determining an optical loss of a device under test DUT as set out in claim
1
, and the determination of a return loss of the DUT as set out in claim
3
.
A third aspect of the invention concerns a further improved determination of the return loss of the DUT as set out in claims
5
and
7
.
A fourth aspect of the invention concerns the determination of the insertion loss of the DUT as set out in claim
8
.


REFERENCES:
patent: 4309105 (1982-01-01), Lebduska
patent: 5090802 (1992-02-01), Longhurst
patent: 5625450 (1997-04-01), Ikeno
patent: 412357 (1990-07-01), None
patent: 453816 (1991-03-01), None
patent: 636868 (1994-07-01), None
patent: 721117 (1996-07-01), None
European Search Report, EP 98 11 3175, Dec. 18, 1998.
European Search Report, EP 98 11 3175, Apr. 1, 1999.

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