Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2003-03-14
2004-04-20
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06724469
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates to the measurement of characteristics of optical paths and is especially, but not exclusively, applicable to the measurement of polarization-dependent characteristics of optical fibers.
2. Background Art
In optical communication systems, newly-installed optical fibers generally have low levels of, for example, polarization mode dispersion (PMD) and can handle current high bit rates. Optical fibers which have been installed for several years, however, may exhibit levels of PMD that are unacceptable for modern optical communication systems. It has been found that, in many cases, the unacceptable overall PMD is caused by a short section of the optical fiber cable. It would be desirable, therefore, to be able to determine which short sections have the worst PMD, and replace those sections only. It has been proposed to use so-called polarization-optical time domain reflectometry (P-OTDR) to locate such sections.
P-OTDR is predicated upon the fact that, although conventional optical time domain reflectometers (OTDRs) measure only the intensity of backscattered light to determine variation of attenuation along the length of a transmission path, the backscattered light also exhibits polarization dependency. P-OTDR utilizes this polarization dependency to monitor polarization dependent characteristics of the transmission path, e.g., an installed optical fiber.
The concept of P-OTDR was introduced in the early 1980s by Rogers [1], who described an OTDR sensitive to the state of polarization (SOP) of the backscattered signal. The simplest P-OTDR comprises an OTDR having a polarizer analyzer in the return path, just prior to its detector. Although initially developed as part of a fiber sensor system for monitoring spatially varying external physical parameters (temperature, strain, etc.), there has recently been heightened interest in variants of this approach to measure the distributed PMD [2-4].
U.S. Pat. No. 5,384,635 (Cohen) discloses a variation based upon synchronous detection to detect cyclic physical perturbations or vibrations of the fiber. More recently, U.S. Pat. No. 6,229,599 (Galtarossa), which is incorporated herein by reference, discloses apparatus for measuring beat length, correlation length and polarization mode dispersion at different positions along the length of the optical fiber. One limitation of Galtarossa's technique is that it derives statistics based upon wavelength and so needs a wavelength tunable source.
Each of these known techniques requires that the P-OTDR have sufficient spatial resolution to “see” the evolution of the SOP as the light propagates down the fiber. This entails the use of short pulses since, when the birefringence of the fiber is large (>1 ps/km), the backscattered beat length is short (<2.5 m) and a short P-OTDR pulse must be used (10 nsec or less). The higher the birefringence of the fiber, the shorter must be the pulse of the P-OTDR. Shorter P-OTDR pulses imply a smaller dynamic range for the instrument. Therefore, high-PMD fibers, that necessarily exhibit high birefringence, are more difficult to characterize than low-PMD fibers.
The PMD of an optical fiber depends upon both the birefringence &bgr; and the coupling length h through the following approximation:
PMD
=
β
L
L
h
where L is the length of the fiber. Generally, the coupling length, h can be defined as the distance required for a significant portion of energy in one mode (fast or slow) to be transferred to another mode. When coupling length h is short, there is a considerable amount of ‘scrambling’ between the fast and slow axes and the total PMD for the fiber increases proportionally to the square root of the fiber length ({square root over (L)}). In contrast, if coupling length h is very long, there is very little coupling between the fast and slow axes and the PMD increases linearly with distance (L).
Fibers that have very little coupling between fast and slow axes (long coupling length h) most likely will exhibit high PMD values, since PMD will accumulate more rapidly with distance. Therefore, the detection of a long coupling length h should allow the identification of most of the high PMD sections in a fiber link.
The distribution of coupling length h along the fiber may be determined by making a fully polarimetric measurement of the SOP as a function of distance. Although this can be achieved via several different P-OTDR implementations, a simple approach is to use a rotatable quarter-wave plate followed by a polarizer prior to the P-OTDR detector. The polarimetric SOP information (the four Stokes parameters S0, S1, S2 and S3) is obtained by taking four different P-OTDR traces, with an appropriate orientation for the quarter-wave plate and the polarizer of the analyzer for each trace. Each trace represents intensity of the backscatter signal against distance for the corresponding one of the settings of the analyzer. The degree of polarization (DOP) contains the critical information one needs in order to estimate coupling length h. The DOP is derived from the Stokes parameters as follows:
DOP
=
S1
2
+
S2
2
+
S3
2
S0
The degree of polarization of the light launched by the P-OTDR source can be considered as being 100% (DOP=1) to a first approximation, since the light source is a laser. The DOP of the backscattered light from a specific position along the fiber also is equal to 1.0. However, the DOP measured by the P-OTDR will diminish if the SOP of the backscattered signal against distance varies significantly within the P-OTDR resolution, i.e., if L
b
<L
p
where L
p
is the P-OTDR spatial resolution and L
b
is the beat length of the backscattered signal. The measured DOP against distance will therefore vary depending on the ratio between L
b
and L
p
. For long P-OTDR pulses (L
p
>>L
b
) a strong depolarization will occur but one can still distinguish between two situations: short and long coupling length, h.
When spatial resolution is much greater than both beat length and coupling length, i.e., L
p
>>L
b
and L
p
>>h, the orientations of the fast and slow axes change rapidly within the P-OTDR resolution. This makes the SOP of the backscattered signal along the pulse substantially random and the measured DOP collapses (however the average DOP does not reach zero since partial repolarization occurs on the way back; the DOP therefore tends toward ⅓).
When spatial resolution, i.e, the pulse length, is much greater than beat length, but much less than coupling length, i.e., L
p
>>L
b
but L
p
<<h, the orientation of the birefringence axis (the fast/slow axis on the Poincare sphere) does not change within the spatial resolution and the SOP of the backscattered signal rotates rapidly around the birefringence axis. Since the P-OTDR resolution is not sufficient to follow the rapid fluctuations of the actual backscattered SOP (i.e, as would be measured using short pulses), the “long pulse” SOP will “collapse” towards the center of the circle traced by the actual, i.e., “short pulse” SOP.
FIG. 1A
illustrates long and short pulse measurements for the case where there is a large angle between the birefringence axis BIR and the locus of the SOP for a given distance.
FIG. 1B
illustrates them for the case where the angle is small.
From the “long pulse” or measured SOP, the “long pulse” or measured DOP will be measured and will tend towards the value of the cosine of the angle between the actual SOP (using short pulses) and the birefringence axis BIR. It is therefore expected that the measured DOP (using long pulses) will be anywhere between 0 and 1.0. As long as the orientation of the birefringence axis of the fiber does not change against distance (relative to an initial point), the measured DOP value will not change. If the orientations of the slow and fast axes move, the DOP value will vary. Slow fluctuations with large amplitude of the measured DOP are therefore expected on fibers with a very long coupling leng
Adams Thomas
Exfo Electro-Optical Engineering Inc.
Nguyen Tu T.
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