Note: Descriptions are shown in the official language in which they were submitted.
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Optical Demultiplexor
Field of the Invention
This invention relates generally wavelength division multiplexed optical
communication systems and more particularly to an optical wavelength division
demultiplexor capable of 1lti1i7ing an optical interference filter to demultiplex a plurality of
wavelengths of light.
10 Background of the Invention
In wavelength division multiplexed optical communication systems, many differentoptical wavelength carriers provide independent communication channels in a single optical
fiber. Future computation and communication systems place ever-increasing demands upon
communication link bandwidth. It is generally known that optical fibers offer much higher
bandwidth than conventional coaxial communications; furthermore a single optical channel
in a fiber waveguide uses a microscopically small fraction of the available bandwidth of the
fiber (typically a few GHz out of several tens of THz). By transmitting several channels at
different optical wavelengths into an fiber (i.e., wavelength division multiplexing, or WDM),
this bandwidth may be more efficiently utilized.
There have been many attempts to develop a compact, high resolution waveguide
demultiplexor or spectrometer for application in areas such as spectroscopy, optical networks
and optical links and more particularly optical comrnunication systerns. Such a demultiplexor
can be extremely critical in wavelength division multiplexing (WDM) links. In these links or
networks, each channel is assigned a distinct and unique wavelength for data transmission..
Thus, the optical fiber that connects channels in a WDM network carries many discrete
wavelength channels and a particular wavelength is selected before the data is received. The
data reception can be achieved by combining a wavelength demultiplexor, photodetectors and
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electronic selection circuitries. In WDM links, many wavelengths are multiplexed and
transmitted through a single optical fiber to increase the capacity of the fiber. The receiver
must demultiplex the many wavelengths and select the proper channel for reception. In these
applications, the requirements on the wavelength demultiplexor are typically: an optical
s bandwidth > 30 nm, a wavelength resolution of a few angstroms, polarization insensitivity,
compactness, low loss, low crosstalk, and a low manufacturing cost.
At present, there are many known methods of selecting particular wavelengths, however,
none are ideal for the applications outlined above. Such methods rely either on bulk optics or
o waveguide structures where the frequency selective element is either an interference grating
or a Fabry-Perot (F-P) cavity. Bulk optics are generally too large and expensive for fiber
based WDM applications. Diffraction gratings have been known for many years and produce
a high resolution spectrum where the wavelength is a function of the diffracted angle. Thus a
single grating can demultiplex many wavelengths. However, available bulk gratings have
generally been expensive and difficult to use with optical fibers; another known drawback to
these grating is their large physical size.
Techniques for multiplexing and demultiplexing between a single optical fiber comprising
the multiplexed channel and plural optical fibers comprising the plural demultiplexed
20 channels are described in various U.S. patents. For example, multiplexing/demultiplexing
with birefringent elements is disclosed in U.S. Pat. Nos. 4,744,075 and 4,745,991.
Multiplexing/demultiplexing using optical bandpass filters (such as a resonant cavity) is
disclosed in U.S. Pat. Nos. 4,707,064 and 5,111,519. Multiplexing/demultiplexing with
interference filters is disclosed in U.S. Pat. Nos. 4,474,424 and 4,630,255 and 4,735,478.
2s Multiplexing/demultiplexing using a prism is disclosed in U.S. Pat. No. 4,335,933. U.S. Pat.
No. 4,740,951 teaches a complex sequence of cascaded gratings to demultiplex plural optical
signals. U.S. Pat. Nos. 4,756,587 and 4,989,937 and 4,690,489 disclose optical coupling
between adjacent waveguides to achieve a demultiplexing function. A similar technique is
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disclosed in U.S. Pat. No. 4,900,1 18. Unfortunately, the foregoing techniques are limited by
their discrete components to a small number of wavelengths in the multiplexed channel.
Thus, there is a need for compact, manufacturable wavelength division
5 multiplexing(WDM) device for telecommunications purposes and for other applications that
is relatively easy and inexpensive to manufacture.
It is therefore an object of this invention to provide an optical demultiplexor that is
relatively inexpensive to manufacture, compact, and that utilizes few optical components.
0
It is a further object of this invention to provide an optical demultiplexor that is
capable of demultiplexing at least 4 channels l1tilizing a single optical filter.
Summary of the Invention
In accordance with the invention, there is provided, an optical demultiplexor
comprislng:
an input end and an output end;
wavelength selective means disposed between the input end and the output end, said
20 wavelength selective means having a wavelength characteristic dependent upon an angle of
incidence for transmitting light of a first predetermined wavelength and reflecting other
wavelengths of light;
an input port at the input end for launching a beam toward the wavelength selective means at
a first predetermined angle to allow a first predetermined wavelength of light to pass
therethrough; and,
reflective means, positioned to direct at least a portion of the collimated beam reflected from
the wavelength selective means, back to the wavelength selective means, at a second,
different, predetermined angle to allow a second predetermined wavelength of light to pass
through said wavelength selective means.
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In accordance with the invention there is further provided, an optical demultiplexor
comprising:
a housing having an input end and an output end, the input end having an upstanding
s endwall;
wavelength selective means disposed between the input end and the output end, said
wavelength selective means having a wavelength characteristic dependent upon an angle of
incidence for transmitting light of a first predetermined wavelength and reflecting other
wavelengths of light;
o an input port at the input end for launching a collimated beam toward the wavelength
selective means at a first predetermined angle to allow a first predetermined wavelength of
light to pass therethrough;
a first reflective means defined within the upstanding wall, positioned to direct at least a
portion of the collimated beam reflected from the wavelength selective means, back to the
5 wavelength selective means at a second, different, predetermined incident angle to allow a
second predetermined wavelength of light to pass through said wavelength selective means;
and
a second reflective means defined within the upstanding wall, positioned to direct at least a
portion of the collimated beam reflected from the wavelength selective means, back to the
20 wavelength selective means at a third, different, predetermined incident angle to allow a
third predetermined wavelength of light to pass through said wavelength selective means.
In accordance with the invention there is further provided, an optical demultiplexor
comprismg:
25 an input end and an output end;
a single wavelength selective means disposed between the input end and the output end, said
wavelength selective means having a wavelength characteristic dependent upon an angle of
incidence for transmitting light of a first predetermined wavelength and reflecting other
wavelengths of light;
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an input port at the input end for launching a beam toward the wavelength selective means at
a first predetermined angle to allow a first predetermined wavelength of light to pass
therethrough,
reflective means, positioned to direct at least a portion of the collimated beam reflected from
5 the wavelength selective means, back to the wavelength selective means at a plurality of
different predetermined angles to allow a plurality of different predetermined wavelengths of
light to pass through said wavelength selective means at a plurality of different angles; and,
a plurality of ports disposed at or near the output end to receive the different predetermined
wavelengths of light that pass through the wavelength selective means.
Advantageously, by using a single optical interference filter instead of multiple filters,
temperature stabilization circuitry becomes for more simple. In the case where multiple
filters are required for the demultiplexing of multiple channels or wavelengths, as is
described in prior art designs, different filters drift at different rates as a function of
5 temperature variation. Thus, temperature compensation for multiple filter designs is
substantially more complex.
Furthermore, by using a single filter in a reflective, folded configuration, as is
provided by this invention, a compact device having a minim~l number of costly optical
components is provided.
Brief Description of the Drawings
Exemplary embodiments of the invention will now be described in conjunction withthe drawings in which:
Fig. 1 is an illustration of a prior art optical demultiplexing arrangement ll~ili7ing a
single filter to demultiplex 4 wavelengths of light;
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Fig. 2 is an illustration of a prior art optical demultiplexing arrangement l]tili7ing four
filters to demultiplex 4 wavelengths of light;
Fig. 3 is an illustration of a optical demultiplexing device in accordance with an
embodiment of the invention; and,
Fig. 4 is an illustration of a optical demultiplexing device in accordance with an
alternative embodiment of the invention.
]o Detailed Description
Turning now to Fig. 1, a prior art demultiplexor is shown having an input beam B( l,
2, 3, 4) launched into an optical fiber 4 at an input side of a 1 to 4 wavelength
independent optical splitter 8. A first output beam B/4( 1, 2, 3, 4) having 1/4 of the
power of the input beam, is launched into an optical fiber 6 to a lens 7 positioned to focus a
beam at a predetermined angle 1 at wavelength selective means in the form of an optical
interference filter 12. The interference filter 12 has a wavelength characteristic that is
dependent upon an angle of incidence. Thus when light is directed at the filter 12 at a
predetermined incident angle the filter 12 transmits light of a predetermined wavelength and
20 reflects other wavelengths of light. At an angle of, for example, 1 light of a predetermined
wavelength 1 is transmitted and other wavelengths, 2, 3, 4 are reflected. Second, third,
and fourth other input beams B/4( 1, 2, 3, 4) each having 1/4 of the output power of the
input beam are directed through lenses 7 at the filter 12 at predetermined incident angles 2,
3, and 4 respectively. The angles are calculated such that when light is directed at the
filter at angle 2, only light of wavelength 2 is passed by the filter. When light is directed
at the filter at angles 3 or 4, only light of wavelength 3 or 4 is passed by the filter,
respectively. Although this prior art device performs a function of demultiplexing of the
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input beam B( 1, 2, 3, 4) into 4 output beams, this is done at the expense of sacrificing
more than 75% of the input signal power.
Another similar demultiplexor is shown in prior art Fig. 2, wherein an input beam is
s launched via an input fiber 4 into a splitter 8 and is split into 4 output beams each having 1/4
of the power of the input beam. Thus at best, if each of the 4 output beams are demultiplexed
with no loss, each of the demultiplexed channels will only have one quarter of the power that
was present in the input beam. The demultiplexor in this embodiment includes 4 separate
interference filters 12a, 12b, 12c, and 12d. By providing 4 separate filters, the filters and the
lo angle of incidence may be designed to demultiplex the 4 separate channels or wavelengths.
Although this may be seen as an advantage by some, it is far more difficult and costly to
provide 4 precise filters rather than the single filter as is shown in Fig. 1; and, in an eight
channel demultiplexor, 8 precision filters would be necessary utili7ing the Fig. 2
embodiment. For example a demultiplexor capable of demultiplexing 8 channels each
5 having a 1 nm bandwidth and spacing of 1.6nm, made with 8 separate filters would be
extremely difficult to manufacture and is not considered practicable. Furthermore,
temperature compensation for such a device would be highly complex and expensive to
provide.
In contrast, turning now to Fig. 3, a first embodiment of the invention illustrates a
demultiplexor having a single optical interference filter 12, and having three spaced apart
reflecting surfaces in the form of mirrors, 1 Oa, 1 Ob, and 1 Oc. Each of the mirrors are
disposed in such as manner about an input side of the filter 12 as to reflect a beam incident
upon them towards the filter 12 at a different predetermined incident angle. Thus, the mirror
1 Oa is critically positioned to reflect an incoming beam (as shown) at the filter 12 at a
predetermined angle 2. Similarly, the mirrors 1 Ob, and lOc are each precisely positioned to
reflect an incoming beam (as shown) at the filter 12 at angles 3 and 4 respectively.
Detectors 14a, 14b, 14c, and 14d, are positioned on the output side of the filter 12 to detect
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light of wavelengths 1, 2, 3, and 4, respectively. The optical interference filter can be
made quite small, thus providing a compact relatively inexpensive device. For example, in
some instances, an endface of the working surface area of the optical interference filter is
approximately between 1.5 and 10 times the size of the beam launched from the input port
5 toward the endface of the optical interference filter 12.
The operation of the demultiplexor will now be described in conjunction with Fig. 3.
An input beam B( 1, 2, 3, 4) is launched at a critical, predetermined angle 1 at the
interference filter 12. The filter 12 is designed to pass light at a wavelength 1 that is
o incident upon it at an angle of 1, and to reflect other wavelengths of light 2, 3, and 4
incident upon the filter at the angle of 1. As can be seen, light of wavelength 1 passes
through the filter and light of wavelength 2, 3, and 4 is reflected, exemplified by the
reflected beam B( 2, 3, 4). The mirror 1 Oa is positioned to reflect the beam B( 2, 3, 4)
toward the filter 12 at a critical predetermined angle 2. The filter 12 is designed to pass light
5 at a wavelength 2 that is incident upon it at an angle of 2, and to reflect other wavelengths
of light 3, and 4 incident upon the filter at the angle 2. Thus, light of wavelength 2 is
passed by the filter, and the beam B( 3, 4) is reflected from the filter 12 to the mirror lOb
that is precisely positioned to reflect the beam B( 3, 4) toward the filter at an incident angle
of 3. Light of wavelength 3 passes through the filter, and the rem~ining beam B( 4) of
20 wavelength 4 is reflected back to the mirror 1 Oc that is precisely positioned to reflect the
beam B( 4) back to the filter 12 at a predetermined critical incident angle of 3 degrees. The
beam of light of wavelength 4 then passes through the filter 12. Conveniently this
arrangement allows a single interference filter with predetermined known reflectance and
transmittance properties that change with angle of incidence to be utilized to separate or
25 demultiplex a single beam of light into a plurality of separate wavelengths of light or
channels. Detectors 1 4a through 1 4d are positioned to receive the light beams of wavelength
1 through 4 respectively. Alternatively, the detectors can be replaced by output ports for
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coupling or porting the separate beams of light to other locations. In the embodiment shown
the mirrors 1 Oa, 1 Ob, and l Oc are preferably 100% reflective, however other embodiments
may be embraced depending upon particular circumstances and requirements. For example,
by providing one or more mirrors l Oa through l Oc that allow a some small percentage of
5 light to pass therethrough, a tap is provided for tapping a small amount of light from one or
more of the beams B( 4), B( 3, 4), or B( 2, 3, 4). The embodiment described
heretofore, in accordance with the invention, may be fabricated in a number of different
ways. For example, discrete components may be used, however, relatively precisely
positioning the mirrors 1 Oa through l Oc at predetermined angles with the filter 12 may within
o small tolerances demands high precision. Alternatively, the demultiplexing device can be
fabricated in part from a glass block having at least an end wall in which to form coated
mirrored surfaces that will reflect light at predetermined angles. This embodiment will be
described in more detail in conjunction with Fig. 4.
Turning now to Fig. 4, a demultiplexor is shown that includes and is contained
within a glass block 40. An interference filter 12, approximately 2 mm x 2 mm, is housed
within the otherwise vacant interior 41 of the glass block 40; thus, the block 40 is essentially
a shell, providing a housing and providing reflective coated surfaces that serve as mirrors. At
an input end of the glass block 40, at the input side of the demultiplexor, are three polished
surfaces 44a, 44b, and 44c polished to predetermined angles 3.035 , 0.717 , and 2.735
respectively, with respect to the planar face of the filter 12. Of course, the particular angles
selected are dependent upon the location of the mirrors, their distance from the filter, and on
the design of the interference filter 12. The surfaces 44a, 44b, and 44c are coated with a
reflective coating. Of course the predetermined angles of the surfaces are selected in
accordance with the design of the interference filter 12. An input beam is provided to the
demultiplexor via a collim~ting lens 48 at an incline of approximately 4.47 . In some
instances, the input angle is tuned or varied insubstantially to compensate for angle tolerance.
At an output side of the device, four polished reflective coated surfaces 46i, 46b, 46c, and
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46d are disposed at predetermined locations to reflect 4 separated channels to 4 output ports
(not shown). It should be noted, that the glass block 40 providing a shell, could alternatively
be fabricated using a plurality of other materials. For instance, the block could be made of a
plastic material. Furthermore, the reflective surfaces 44a, 44b, and 44c may alternatively be
s in the form of discrete reflectors or partial reflectors. The number of channels or wavelengths
that can be demultiplexed should not be confined to four as shown in Fig. 3 and 4. Simulated
designs indicate, for example, that eight wavelengths can be demultiplexed lltili7ing
additional reflective surfaces in a similar configuration in accordance with the principles of
this invention.
Referring now to Fig. 5, an altemative embodiment of the invention is shown,
wherein an interference filter 52 is sandwiched between two solid glass blocks 50 and 51.
Essentially, this embodiment functions in the same manner as in the embodiment described
heretofore, of Fig. 4, however, light propagating within the device travels through glass
5 instead of air. From a manufacturing point of view, this embodiment is preferred.
Of course numerous other embodiments may be envisaged without departing from thespirit and scope of the invention.
