Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-01-07
2003-11-04
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S011000, C385S042000, C385S027000, C385S039000
Reexamination Certificate
active
06643433
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarization-combining fiber-fused optical coupler which, in the reverse mode, can also serve as a polarization beam splitter. The invention also relates to a method of manufacturing such coupler by splicing two polarization maintaining (PM) fibers to a pair of standard non-birefringent single mode (SM) fibers and forming a coupling zone near the splicing with the SM fibers by fusing and drawing the same until a desired power transfer is achieved.
2. Description of the Prior Art
Fused and tapered 2×2 couplers, formed by fusing together sections of two laterally adjacent optical fibers and by subsequently drawing these fused sections are well known in the art. They are versatile devices that can be used in many applications. Their most common functions are optical power splitting and wavelength division multiplexing (WDM). However, WDM devices can only divide or combine optical signals of different wavelengths.
A fused-tapered fiber-optic coupler, if properly designed, can also function as a polarization beam splitter (PBS). Fiber optic PBS couplers separate light power into two orthogonal polarizations. The form birefringence, which produces a difference in the propagation constants between orthogonal polarized states, causes the polarization splitting in the coupler. A fiber optic PBS coupler is used for providing polarization sensitive devices with light signals having known polarization states.
Fiber optic polarization-combining couplers combine the power of two
Fiber optic polarization-combining couplers combine the power of two orthogonal polarizations into one single output fiber. One of ordinary skill in the art will recognize that such an optical device is bidirectional and operates in a reverse fashion from what is described above with respect to the polarization beam splitter. A polarization combiner permits to introduce beams of linearly polarized light from two light sources and combine them within a single common optical fiber.
Polarization beam-splitting properties of fused taper couplers have been known since 1985, when they were disclosed by A. W. Snyder in the article entitled “Polarizing Beamsplitter from Fused-Taper Couplers”, published in Electronic Letters of Jul. 4, 1985, Vol. 21, No. 14, pp623-625.
In this regard, it should be noted that a fused-fiber coupler is made of two laterally fused SM fibers, forming a structure which has two transversely symmetrical axes. Geometrically, these axes are axes of birefringence of the coupler, namely the x-axis which passes by the two cores of the fibers and the y-axis which is perpendicular to the x-axis.
The transmission of the coupler, if it is excited by one of the input arms can be represented as follows:
P
1
=a
x
2
cos
2
(&phgr;
x
)+a
y
2
cos
2
(&phgr;
y
)
P
2
=a
x
2
sin
2
(&phgr;
x
)+a
y
2
sin
2
(&phgr;
y
)
where P
1
is the power at the output of the first arm, P
2
is the power at the output of the second arm, &phgr;
x
and &phgr;
y
are accumulated phases in the coupler according to polarizations x and y, and a
x
and a
y
are amplitudes of the signal at the coupler input according to the polarization axes.
Since the accumulated phases of &phgr;
x
and &phgr;
y
are different, a coupler will have a different response according to the polarizations at the input of the coupler. Here, one can distinguish two particular situations: (1) when the accumulated phases are in phase (equal to a multiple of 2&pgr;) and have a value which is a multiple of &pgr;, the power at the input will go to one or the other output fiber; this property of matched phases is used to make multiplexers of wavelength with little dependence on polarization; (2) in the second case, if the accumulated phases are out of phase (equal to about (2n+1) &pgr;, i.e. an odd-multiple of &pgr;) and have a value which is a multiple of &pgr;, the two polarizations will be separate and each will go into a separate fiber. An example of this latter case would be, &phgr;
x
=m&pgr; and &phgr;
y
=(m±1)&pgr;, meaning that each accumulated phase is a different multiple of &pgr;. It should be noted that in all references to a multiple of &pgr; or of 2&pgr; or the like, the values need not be an exact multiple, but could be an approximate multiple, i.e. essentially such a multiple.
Moreover, U.S. Pat. No. 5,064,267 issued Nov. 12, 1991 to Rossberg, discloses a method of manufacturing a polarization-selective coupler, in which, during the formation of the coupling region, linearly polarized light of a selected wavelength is fed into one of the input fibers, and the coupling region is drawn-out until an equal amount of light is detected from each output fiber, at which point the source of heat is turned off, thereby stopping the elongation of the coupling region. This technique is unsatisfactory because it does not properly measure the polarization state in the coupler and, in particular, it does not measure such state at the wavelength of the operation of the coupler. Also, the technique is not sufficiently precise, as it provides for feeding only one state of polarized light into only one of the input fibers, while the other input fiber remains unused during the formation of the coupler. Thus, it only achieves a partial combining of the power, and due to this, such measurement cannot guarantee the performance of the coupler.
Furthermore, in applicant's own international patent application published on May 25, 2001 under No. WO 01/37013, which is incorporated herein by reference, a method is disclosed and claimed for the fabrication of multiplexing and demultiplexing single-mode fiber couplers by elongating fused fibers by controllably pulling them longitudinally while heating them with a suitable heat source so as to create a coupling zone with an adiabatic profile, and stopping the elongation process when a match point between wavelength period and polarization phase is achieved. In this prior international application, it was found that it is possible to match the wavelength periods and the polarization phase at the same time. It has now been found that this principle can also be used to fabricate a polarization-combining optical coupler in accordance with the present invention.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to create a fiber-fused polarization-combining optical coupler adapted to combine signals originating from two polarization maintaining (PM) fibers; this coupler, in the reverse mode, can also be used as a polarization beam splitter.
Another object is to produce a polarization-combining coupler that would be operational in a broad wavelength bandwidth of at least 8 nm.
A still further object is to manufacture a polarization-combining coupler that has a small insertion loss of less than 0.3 dB.
Other objects and advantages of the present invention will be apparent from the following description thereof.
According to the invention, the novel method for manufacturing the polarization-combining coupler uses the form birefringence of the fused coupler to separate the polarizations. The coupler is manufactured from standard non-birefringent single mode (SM) fibers so as to form an adiabatic tapered profile and preferably have a polarization phase difference between &phgr;
x
and &phgr;
y
along the coupler, which is essentially an odd-multiple of &pgr;, while &phgr;
x
and &phgr;
y
have values which essentially multiples of &pgr;, these conditions prevailing at a predetermined common wavelength. Polarization maintaining (PM) fibers are spliced to the input SM fibers of the coupler, preferably as close as possible to the coupling zone. The splicing can be done either before or after the formation of the coupling zone which is produced by fusion and elongation of the SM fibers. Preferably, however, splicing is done before fusion and elongation of the coupling zone, to provide better control of the operation.
The PM fibers are known to carry optical signals w
Cottin Pierre
Dion Bruno Y.
Gonthier François
ITF Optical Technologies Inc.
Knauss Scott A
Primak George J.
Sanghavi Hemang
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