Note: Descriptions are shown in the official language in which they were submitted.
CA 02314997 2000-08-03
FIELD OF INVENTION
This invention generally relates to optical fiber devices and particularly
to fiber based Mach-Zehnder interferometers (MZI) insensitive to
temperature changes.
Optical filters are very frequently used in modern optical communication
systems such as Dense Wavelength Multiplex (DWDM) systems. in these
systems, a nua~er of data channels share a single optical fiber as their
transmission media and use a unique wavelength of light as their channel
signature.
An optical waveguide Mach-Zehnder is composed of two optical
splitters/couplers. Two lengths of optical waveguides or arms connect
them to each other. When the arms of the MZI have different lengths, we
have a so-called asymmetric Mach-Zehnder interferometer (I~IZI). The
optical waveguides referred to here are optical fibers with circular cross
sections and/or planar optical waveguides with non-circular cross
sections. A MZI made of optical fibers is called an all-fiber MZI.
Asymmetric MZIs show a periodic response as a function of wavelength. The
period is a function of the length difference between the arms of the
interferometer. As the length difference increases, the oscillation in
the wavelength response decreases and, therefore, the wavelength
selectivity increases. Asymmetric MZIs, once connected to each other as
an inter-leaver, can multiplex or de-multiplex a large number of optical
signals of varying wavelengths such as the standard ITU grid wavelengths.
An optical filter inter-leaver can separate the odd and even wavelengths
from a WDHI signal consisting of several wavelengths.
One problem associated with fiber based asymmetric MZIs is their
sensitivity to temperature, and the greater the length difference, the
more severe is the problem. The problem originates mainly from a
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temperature-induced change in the optical path length of the fiber. As
the temperature changes, the refractive index and the geometrical length
of the fiber changes. Consequently, a difference in the optical path
lengths of two arms is created.
Several temperature compensation methods have been proposed to solve the
temperature sensitivity of these photonic devices. However, most of these
methods work within a limited range of temperatures and cannot be applied
easily to the asymmetric MZI cases where large differences in the optical
paths exist. Active temperature compensation of photonic devices is
typically carried out by maintaining the temperature of the fibers'
enviror~a~ent above a chosen temperature (e.g., above 60°C). This is
achieved by including a heater controller inside the package of the
device. However, the high power demands of the active temperature
compensation and its low reliability have made the search for passive
methods and on-going effort within the photonic industry.
It is the purpose of this invention to describe a novel temperature
insensitive asymmetric fiber based MZI.
Asymmetric interferometer apparatuses are known to have periodic response
as a function of wavelength. therefore, they can be used as optical
filters. It is also known that, in particular, cascaded asymmetric MZIs
can act as an optical filter with good filtering characteristics. One of
the advantages of the asymmetric MZI is the narrow channel spacing that
may be attained. In addition, the fiber-based device displays low
insertion loss and low polarization effects.
The thermal variability of a MZI with unequal fiber lengths arises from
the different optical path lengths between the two arms. The primary
cause of thermal drift is the sensitivity of the refractive index of the
silica fiber to changes in temperature. The thermal expansion of the
fiber is a smaller contributor to the observed thermal drift. In order to
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obtain narrower channel spacing by this filtering method one has to
increase the difference between the optical lengths of the interferometer
arms. Increasing the differential optical path length is achieved by
increasing the geometrical length difference between the two fibers of the
two couplers. This will in turn worsen the temperature sensitivity of the
interferometer making it imperative to reduce the thermal variability of
these filters.
The present invention proposes a fiber-based interferometer with a Mach-
Zehnder configuration in which temperature sensitivity is eliminated. The
primary challenge of temperature compensation for these devices can be
overcome by instead using unequal lengths of an insensitive fiber. The
present invention describes a MZI made of two 2x2 splitters with equal
lengths at each side fused with two unequal lengths of an insensitive
optical fiber. Alternatively one can start with a symmetric MZI and
insert a predetermined length of temperature insensitive fiber between one
of the arms of MZI to get the desired asymmetric MZI, which is insensitive
to temperature. Temperature insensitive fibers can be built by a method
that, far example, has been disclosed in the United States Patent
#5,018,827. in the named patent, an insensitive optical fiber is produced
when an optical fiber core made of a first material is enclosed within a
cladding made of a second material having a different coefficient of
thermal expansion, a~. Another method of decreasing the temperature
sensitivity of the silica fiber is to use dopants such as boron oxide.
This approach yields an inherently passive device, therefore eliminating
the need other methods of temperature compensation. By forming an
insensitive MZI, a complex for bimetallic packaging structure for passive
temperature compensation is not needed, nor is an active method necessary.
The use of expensive composite materials in the packaging of the device is
eliminated as well.
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BRIEF DESCRIPTION 0f THE FIGURES
Fig. 1 shows a fiber based Mach-Zehnder interferometer using tea~perature
insensitive fiber according to the present invention.
Fig. 2 shows another ea~odiment of a fiber based Mach-Zehnder
interferometer using temperature insensitive fiber.
In the present invention, a temperature insensitive fiber is used to make
a MZI to be used in a Di~lt~1 system and/or an interleaver. This approach
yields an inherently passive device, therefore eliminating the need for
other methods of temperature compensation.
The two general approaches to temperature compensation of fiber optic
c~ponents are active and passive control. The former is a more costly
solution and involves far greater power consumption. In addition, an
increase in the total size of the component and lower reliability over a
wide temperature range makes active control a far inferior ~thod of
temperature compensation than passive control. there are a number of ways
of passively compensating far the thermal variability of fiber-based
devices. However, as the channel spacing decreases to below lnm, a
temperature stability of less than 1 pm/'C becomes necessary.
In a recent US patent (#60$1G4,~j, a passive temperature compensating
method is presented for a fused-fiber DW~1 system. In this invention, two
dissimilar materials with different thermal expansion coefficients are
used to construct a fixture containing the DWD~i device. By using this
structure, it is possible to artificially create a negative coefficient of
thermal expansion. The DW~1 device is typically assembled on a pre-
stressed fixture. However, the device can also be built under tension and
then assembled on the relaxed bi-substrate fixture. In the former design,
the whole assembly can exert tension on, or release tension from, the
fiber. Temperature compensation is then established by adjusting the
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applied tension on the fused-fiber DWDM. It is shown that, as tension is
relieved, the thermal drift due to an increase in tea~erature is
compensated. Conversely, by increasing tension, wavelength shifts due to
a decrease in temperature are compensated. 8y using such a temperature-
compensating device, a bulky package is inevitable. In addition,
dimensional design and choice of material can be demanding requirements.
The present invention proposes an improvea~nt in the manufacturing cost
and reliability of an interleaver, with potential expendabiiity to a fli~l
system with very narrow channel spacing. The need for more
bandwidth/channels is increasing very rapidly as more information is being
transmitted through the Internet everyday.
Optical filters with sharp wavelength characteristics are vital coa~onents
of WDM technology. Interferometer devices, and in particular fiber based
interferometer devices such as the MZI, show useful filtering
characteristics, are easily expandable, and exhibit low insertion loss. A
fiber based optical MZI consists of two optical couplers or splatters with
predetermined coupling or splitting ratios connected together through two
lengths of optical fiber. In order to decrease the channel spacing, the
length difference (Sljbetween the two arms should increase. As a result,
the optical path length difference also increases, generating a higher
sensitivity within the MZI to fluctuations in its temperature.
The challenge also lies in correctly achieving the desired channel
spacing. this is accomplished by measuring the correct dl between the two
arms connecting the two couplers of the MZI. As a result of the different
optical paths between the two arms of the two couplers, a sinusoidal
wavelength response can be obtained with low polarization dependence and
low insertion loss. Using a precision reflectometer or an optical
spectrum analyzer, the difference between the two arms of the MZI can be
measured to within, ~ ~.
Shown in Fig. 1 is a IdZI made of two couplers (11 and i2j fused together
in the middle with two different lengths of an insensitive fiber, where
the fiber length 14 is longer than the fiber length 13.
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It is proposed that a MZI be made using a specialty fiber that produces a
temperature insensitive device. The United States patent #5,018,827
proposes a speciality fiber with unique characteristics. An insensitive
optical fiber can be tailored such that temperature-induced changes in its
geometrical length and in its refractive index offset each other in such a
fashion that the optical path length is, far all intents and purposes,
independent of temperature. By carefully choosing two different glasses
for the core and cladding, and by appropriately adjusting their radii, the
observed shift sensitivity of the fiber can be eliminated. The radius of
the cladding is adjusted so that the coefficient of thermal expansion of
the fiber is equal to the product of a reciprocal of the negative of the
index of refraction, n, of the first material and its rate of change with
temperature, i.e.
a=(-n) ( d )
Eq.1
This type of fiber can also be spliced with the silica fiber contained in
two couplers lI and 12.
In one ea~bodiment, the two arms of couplers 11 and 12 are cut into equal
lengths and are fusion spliced to two insensitive optical fibers with a
pre-determined ~1. By cutting the two insensitive fibers to different
lengths, an optical path length difference is produced. Using a fiber
cleaving stage equipped with a micro-positional fixture it is possible to
make a precise D1 between the two arms of an MZI. Polishing the fiber to
obtain the desired channel spacing before the arms are spliced to form the
MZI can then da the final length adjustment.
In another ea~odiment, the two arms of coupler 11 are cut as close in
length to each other as possible and are spliced to two arbitrary lengths
of the insensitive fiber. The new coupler formed is then cut to the
desired 01 and be fused to the two equal arms of coupler 12.
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Figure 2 shows another en~bodia~nt of a fiber based insensitive asymmetric
MZI made of couplers 31 and 32 in which the insensitive fiber, 33, has
been used only in one of the arms of MZI. The length of the insensitive
fiber, 33, in this case precisely equals the predetermined al. The other
arm of MZi made of conventional single made silica fiber {F1 and FZ) will
be fusion spliced together at the dashed line to form a MZI. The MZI in
Fig. 3 will be symmetric MZI if the insensitive part, 33, is taken out.
By forming an insensitive MZI, a cod ex bi~tallic packaging structure
for passive temperature compensation is not needed, nor is an active
method necessary. The use of expensive coavposite materials in the
packaging of the device is eliminated as well. An insensitive MZI of this
invention can be easily made to any desired dl. The novel design of this
invention easily provides higher 61 and thus higher channel number without
the problea~ of temperature sensitivity due to different optical path
lengths.
Z
