Vibration noise mitigation in an interferometric system

Details Vibration noise mitigation in an interferometric system Vibration noise mitigation in an interferometric system

2002-03-14

2004-11-30

Turner, Samuel A. (Department: 2877)

Optics: measuring and testing

By light interference

Using fiber or waveguide interferometer

Reexamination Certificate

active

06825934

ABSTRACT:

TECHNICAL FIELD
The invention relates generally to obtaining measurements for optical characteristics of a device under test and more particularly to reducing the effects of vibration noise on the process of obtaining the measurements.
BACKGROUND ART
Techniques for testing or analyzing optical components are currently available. A “device” under test (DUT), such as a length of fiberoptic cable, may be tested for faults or may be analyzed to determine whether the device is suitable for use in a particular application. System components such as multiplexers, demultiplexers, cross connectors, and devices having fiber Bragg gratings may be separately tested before they are used in assembling a system.
Optical testing may be performed using a heterodyne optical network analyzer. Such analyzers are used for measuring optical characteristics of optical components. For example, the “group delay” of a component may be important in determining the suitability of the component for a particular system. Group delay is sometimes referred to as envelope delay, since it refers to the frequency-dependent delay of an envelope of frequencies. The group delay for a particular frequency is the negative of the slope of the phase curve at that frequency.
Typically, a heterodyne optical network analyzer includes two interferometers.
FIG. 1
is an example of one type of heterodyne optical network analyzer
10
. The analyzer includes two interferometers
12
and
14
connected to a tunable laser source (TLS)
16
. The TLS generates a laser light beam that is split by a coupler
18
. The TLS is continuously tuned, or swept, within a particular frequency range. By operation of the coupler
18
, a first portion of the coherent light from the TLS is directed to the DUT interferometer
12
, while a second portion is directed to the reference interferometer
14
.
The DUT interferometer
12
has a second coupler
22
that allows beam splitting between a first arm
24
and a second arm
26
. A mirror
28
is located at the end of the first arm and a DUT
20
is located near the reflective end of the second arm. The lengths of the two arms can differ, and the difference in the optical path length is represented in
FIG. 1
by L
DUT
. Since the DUT can be dispersive, the actual optical path length is a function of frequency. A detector
30
is positioned to measure the combination of the light reflected by the mirror
28
and the light reflected by the DUT
20
. Processing capability (not shown) is connected to the detector. Assuming the two arms
24
and
26
have different lengths, the light from one of the arms will be delayed by a time T
1
with respect to light from the other arm. Generally, T
1
varies as a function of frequency, since the DUT is typically dispersive. The two beam portions interfere when they recombine at the coupler
22
. By analyzing the signal that is generated at the detector
30
, the group delay and other properties of the DUT may be determined. However, in order to very precisely measure the group delay, it is necessary to obtain knowledge of the frequency tuning of the TLS
16
as a function of time. The reference interferometer
14
is used for this purpose.
The structure of the reference interferometer
14
is similar to that of the DUT interferometer
12
, but a mirror
32
takes the place of the DUT
20
. A second detector
34
receives light energy that is reflected by the combination of the mirror
32
at the end of a third arm
36
and a mirror
38
at the end of a fourth arm
40
. As in the DUT interferometer, the lengths of these two arms
36
and
40
can be different, and this difference in lengths is represented by L
REF
. The signal that is generated by the second detector
34
is also an interference signal (i.e., an intensity signal having an interference term) that is responsive to the combination of light from the two arms. However, the optical characteristics of the reference interferometer are relatively fixed and therefore predictable. Consequently, the reference interferometer can be used to measure the major variable to its operation, i.e., frequency sweep &ohgr;(t).
A concern is that vibrations to the system will diminish the precision of measurements such as group delay, group velocity, transmissivity, reflectivity, and chromatic dispersion. For example, vibrations of the second arm
26
on which the DUT
20
resides will act to change the index of refraction of the arm, which in turn acts as perturbations to the phase delay measured by processing the signal from the detector
30
. The effects of vibrations on the precision of such measurements similarly occur in transmission-type interferometers, where an interference signal is formed as the combination of two beam portions that have propagated through the two arms of an interferometer without reflection. Thus, detectors are at the ends of the arms opposite to the TLS that generates the original beam. Transmission-type interferometers, such as Mach-Zehnder interferometers, are well known in the art.
One method of addressing the vibration concern is to provide vibration isolation of the heterodyne optical network analyzer
10
. For example, the system may be supported on a platform that is specifically designed to minimize vibrations. However, additional or substitute techniques are desired. What is needed is a method and system for significantly reducing the risk that vibrations will adversely affect the performance of an interferometer.
SUMMARY OF THE INVENTION
In accordance with the invention, vibration noise within an interferometric system has reduced effects as a result of monitoring light patterns and providing corrections on the basis of the light patterns. Light propagating through a first path of the system is combined with light propagating through a second path to form at least one interference signal. Within each of the embodiments of the invention, the combination of light from the two paths is analyzed to provide a basis for the corrections.
In one embodiment, a partial reflector is added to an interferometer for analyzing a device under test (DUT), such as a fiber optic cable or the like. A source of a sweeping frequency beam is coupled to the two paths, or arms, so that beam portions are introduced to the two paths. As one possibility, the source of the beam is a tunable laser source (TLS). The DUT and the partial reflector are connected in close proximity along one of the paths. Therefore, vibrations experienced by the DUT are likely to be experienced in generally equal magnitude by the partial reflector, so that the vibration noise effects of the two components will be generally equal. Moreover, the radian frequencies at the two components will remain substantially the same as the TLS sweeps through its frequency range. With these approximations, the effects of vibration can be reduced by using techniques such as determining the phase difference between the phase of the interference signal for the DUT and the phase of the interference signal for the partial reflector. This phase difference can then be applied in known approaches to determining optical characteristics of the DUT, such as measurements of group delay, group velocity, transmissivity, reflectivity and chromatic dispersion.
The use of the partial reflector works well in reflection interferometers, i.e., interferometers in which the interference signal is formed of reflected light from the two paths. However, the same approach may be used in a transmission interferometer in which the interference signal is formed by combining light that has propagated through the two paths. The first path having the DUT may include a shunt in parallel with the DUT. Reduction of vibration noise during analysis of the DUT can be achieved if the shunt is located so that it is likely to experience the same vibrations as the DUT. When the shunt and the DUT are approximately the same length (but not exactly the same length) and experience approximately the same magnitude of vibration, the effects of the vibrations on measurements of the optical chara

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