Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~L~CH-ZEH~DER SWITCH
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Backgrollnd of the Invention
The present invention relates to optical power
switching devices.
optical switches with switching speeds up to l
gigahertz are required for numerous applications incl~ding
local area networks, sensor arrays and communications
systems. Many forms of optical switching devices have
been developed. Typical examples are mul.iple-quant~-.-
well waveguide switches, strained-layer superlattice
directional couplers and optical fiber switches. These
devices are based on the nonlinear effect of the material
that forms them. In the case of semiconductor devices,
the required critical power for a switch is less thar. 1
mW. Until recently optical fiber switches had been
i fabricated from optical fibers having silica based cc~es.
The optical power required for these optical fiber
switches is on the order of several kilowatts since the
nonlinear coefficient of silica is extremely small.
The publication, P.L. Chu et al. "Optical Switching
in Twin-Core Erbium-Doped Fibers", Optics Letters,
February 15, 1992, Vol. 17, No. 4, pp. 255-257 reports
that it was demonstrated that erbium-doped fiber has z
nonlinear coefficient approximately 1 million times
greater than that of fused silica. However, the large
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increase in nonlinear index in erbium-doped fiber is
accompanied by a large absorption loss and a slowing of
the response time. The switch disclosed by Chu et al.
consists of a 2.26 m long piece of twin core erbium-doped
optical fiber. It is difficult to input light to and
output light from a twin core or a double core optical
fiber. Moreover, a long length of the Chu et al. Erbium-
doped fiber is required. Also, low cross-talk cannot be
achieved since the power difference in the two cores
affects the coupling mechanism. This is explained by
Caglioti et al. in "Limitations to all-optical switching
using nonlinear couplers in the presence of linear and
nonlinear absorption and saturation", Journal of the
Optical Society of America B,-vol. 5, No. 2, February,
1988, pp. 472-482. The two-core fiber requires large
power inputs or long fiber lengths that need to be
configured in such a way as to prevent environmentally-
induced phase shifts such as bend-induced phase shifts.
The publication, R.H. Pantell et al. "Analysis of
Nonlinear Optical Switching in an Erbium-30ped Fiber",
Journal of Lightwave Technology, Vol. 11, No. 9, September
1993, pp. 1416-1424 discusses switch configurations
employing both a Mach-Zehnder configuration and a two-mode
fiber configuration, each configuration utilizing an
Erbium-doped core.
Pantell et al. describe an experiment in which a 3.4
m length of two-mode fiber was utilized. A phase shift of
n required an absorbed pump power of 15.5 mW. The signal
was launched to inject approximately equal powers in the
LP~1 and ~P modes. This type of signal injection is
difficult to impliment, and the device is unstable with
respect to external vibrations and perturbatlons.
One or both fibers in the phase shift region of the
Mach-Zehnder device of Pantell et al. is ~ade of Erbium-
doped fiber, the pump power being coupled into only one of
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them. Since it is stated at page 1417 that the pump power
requirement of a two mode fiber (TMF) switch is generally
larger than for an equivalent Mach-~ehnder (MZ) switch by
a factor of 2-4, it follows that a Mach-Zehnder switch of
this type would be about 85 to 170 cm long provided that
the power remained constant. In the absence of pump power
all of the signal power appears at output port 2. When
sufficient ~ump power is applied to cause a phase
difference of ~, the signal switches to output port 3.
Pantell et al. indicate that tlle fiber core is heated due
to the generation of phonons by the pump power in the
fiber core and that the two mode fiber is advantageous
over the Mach-Zehnder interferometer since the two modes
utilize the same guiding region and therefore react
similarly to environmental changes.
The Pantell two-mode device is very sensitive to the
launch condition and any perturbations along the length of
the two mode fiber. Also, it is not directly compatible
with single-mode operation.
In order to attain compactness and ease of handling,
it would be advantageous for nonlinear switches of the
Mach-Zehnder type to be formed as a monolithic structure.
For such devices to be practical, their length should not
exceed about 15 cm.
Sl~mm~ry of the Invention
It is therefore an object of the present invention to
provide an optical switch that overcomes the heretofore
noted disadvantages of prior art switches. A further
object is to provide a compact, low power, low cross-talk
nonlinear optical switch.
Briefly, the monolithic Mach-Zehnder switch of the
present invention comprises input coupler means for
splitting an input slgnal into N equal signal components,
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where N>l. Combining means, having at least first and
second output terminals, is provided for combining the N
components. N optical waveguide paths connect the N
signal components to the combining means. At least one of
the waveguide paths contains a material having a resonant
nonlïnearity, whereby its refractive index changes when
pump power propagates through it. The input coupler means
and the combining means are free from nonlinear material.
The input coupler means, the combining means and the
1~ optical waveguide paths are in thermal contact with a
matrix glass body.
In one embodiment the switch consists of first and
second optical fibers extending longitudinally through an
elongated body of matrix glass. The body includes a phase
shift region and two spaced coupler regions at opposite
ends of the phase shift region. The diameter of the body
and the diameters of the fibers are smaller in the coupler
regions than in the phase shift region At least that
portion of the first fiber that is in the phase shift
region contains a material having a resonant nonlinearity,
whereby the refractive index of the first fiber changes
when pump power propagates through it. The fibers have
different propagation constants in the phase shift region
in the absence of pump power propagating through the first
fiber so that the first fiber subjects the light
propagating therethrough to a delay that is different from
the delay experienced by light propagating through the
second fiber.
Rrief Description of the Dr~wings
Fig. 1 is a schematic diagram of a prior art Mach-
Zehnder switch.
Fig. 2 is a plot of power output vs. wavelength for
two types of Mach-Zehnder devices.
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Fig. 3 is a plot of the pow~r (Ps) required for
switching as a function of wavelength separation between
adjacent peaks and valleys of curve of Fig. 2.
Fig. 4 is a cross-sectional view of a Mach-Zehnder
switch formed in accordance with the present invention.
Fig. 5 is a cross-sectional view taken along lines 5-
5 of Fig. 4.
Fig. 6 is a graph illustrating loss vs. launch power
for a Mach-Zehnder switch formed in accordance with the
~10 present invention.
Fig. 7 shows a planar Mach-Zehnder switch.
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Detailed Description
A Mach-Zehnder switch of the type disclosed in the
aforementioned Pantell et al. publication is schematically
; illustrated in Fig. 1. Two couplers 11 and 12 are
concatenated by waveguide paths 14 and 15. The couplers
~ are usually 3 DB couplers, whereby the signal power that
is applied to input port 2, for example, is evenly divided
between the two outputs of coupler 11. One or both of
waveguide paths 14 and 15 contains a material having a
resonant nonlinearity, whereby a refractive index change
is induced by absorption of light within a predetermined
wavelength band. The rare earth elements are particularly
suitable since they exhibit large nonlinear refractive
indices. The rare earth element erbium exhibits a very
large nonlinear index. The use of neodimium as the
nonlinear material would increase switching speed, but
more switching power would be required. There are also
other dopants with which a population inversion can be
achieved in order to provide a resonant nonlinearity.
Examples include the transition metals such as chromium
and titanium.
The light absorbed by the nonlinear material can be a
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pump or gating pulse having a wavelength different from
that of the signal. Alternatively, the signal wavelength
can be within that band of wavelengths that induces an
index change in the nonlinear material. In this case,
5 separate signal and gating pusles can be applied to one or
both input ports, or a single signal pulse can be applied
to one input port (as in the case of a power limiter), its
amplitude determining whether switching occurs, i.e. it
determines the output port at which the output signal
appears. In the present discussion it is assumed that
waveguide path 14 is the nonlinear path. Pump power is
shown as being applied to input port 1, and the signal is
shown as being applied to input port 2. If desired, both
pump and signal power could be applied to the same input
port.
In the illustrated embodiment, the characteristics of
coupler 11 are such that essentially all of the pump power
applied to input port 1 remains uncoupled whereby it
propagates only in waveguide path 14. In the absence of
pump power applied to input port 1, the signal appears at
output port 3. This is accomplished by appropriately
fixing the phase shift between the two waveguide paths 14
and 15. The pump power causes a change in refractive
index in waveguide path 14 such that when the pump is
turned on with enough power to induce a phase shift of ~,
the signal fully switches from output port 3 to output
port 4.
It is preferred that the nonlinear path exist only in
the phase shift region rather than continue into and form
part of the couplers so that the coupling characteristic
is not affected by pump power. Another important
advantage of this configuration is that it enables the use
of relatively high loss doped fibers or waveguides to
achieve nonlinearity, but since the doped fiber exists
only between the couplers, loss is minimized. If the
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nonlinear material extends through the couplers! then the
pump power should be applied to the fiber or path that
does not contain nonlinear material, the pump power being
coupled to the nonlinear fiber; this would minimize loss.
In conventional Mach-Zehnder switches of the type
disclosed in the aforementioned Pantell et al. publication
optical waveguide paths 14 and 15 are relatively long, and
problems arise as a result of the heating of the nonlinear
path 14 when pump power propagates through it. In
accordance with the present invention the heating problem
is alleviated by forming the device as a monolithic
structure whereby heat generated by the nonlinear arm of
the phase-shift region is conducted to the remaining arm
of the phase-shift region. Such a monolithic Mach-Zehnder
device can be in the form of an overclad fiber structure
or a planar circuit. However, the length of the
conven~ional device of Fig. 1 is such that it is not
suitable for such monolithic devices. For such monolithic
devices to be practical, their length should not exceed
~20 about 15 cm.
A second feature of the inven_ion results in
nonlinear switching at significantly lower power levels
(up to two orders of magnitude lower than with the
conventional design disclosed in the Pantell et al.
publication). Since there is a tradeoff between length of
nonlinear fiber and switching power, this second feature
can be empioyed to render the phase-shift region
sufficiently short that the entire device is easily
fabricated as an overclad or planar structure. That is,
the device can be shortened to an acceptable length, and
the switching power can be correspondingly ~aintained at a
relatively low level.
Output power is plotted in Fig. 2 as a function of
wavelength for two different single-stage Mach-Zehnder
devices. Curve 21 represents the output for a device in
,
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which the propagation constants of the two fibers in the
phase shift region are substantially equal. Curve 22
represents the output for a device in which the
propagation constants of the two fibers in the phase shift
region are significantly different. Whereas curve 22
includes a plurality of peaks within the wavelength range
shown, curve 21 is representative of a broadbanded
characteristic, whereby only its peak appears within the
wavelength range covered by Fig. 2.
The model discussed below shows that the amount of
power required to cause a signal to switch between the two
output ports of a Mach-Zehnder device is a function of the
wavelength separation between a peak 26 and and an
adjacent valley 2~, for example, of curve 22 of Fig. 2 and
thus, the difference between the propagation constants of
waveguide paths 14 and 15. In order to calculate the
power requirements, the model assumes that waveguide paths
14 and 15 in the phase shift region of Fig. 1 have
different effective indices. Although the model assumes
that the nonlinear material is silica, similar results
would be obtained if it were silica doped with a material
that enhanced the nonlinear property of the waveguide
path.
The normalized output power for the device of Fig. 1
(before the introduction of the gating signal) is
p = COS'(~z(n~ - nl)/~) (l)
where n7 and n, represent effective indices of propagation
in path 1 and path 2, respectively, and A is the signal
wavelength. The length z of waveguide paths 14 and 15 is
chosen so that a ~/2 phase change is introduced between
the two wavelengths of interest. If, for example, it is
assumed that a minimum is to occur at wavelength A (point
26 of Fig. 2) and a maximum is to occur at wavelsngth A2
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(point 2S of Fig. 2), z is given by
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z = [2(n2 - nl) tl/Al-l/A2)]-l (2)
S The index change needed to cause switching at Al is then
~oz/A = ~(n~ - n)z/A1 - ~(n2 - n)z/A2) (3a)
so that
~10
= A,[(n - n)z/A-. - (n~ - n)/A2J] (3b)
It is known that the nonlinear index for silica based
fibers is
n~ = 3.2xlO-I~ cm~/watt (4)
; For a single mode fiber with an effective index of 75 ~m:,
eqatio~ 4 becomes
Q0
n2 = 4.3xlO~C/watt (5
The required power (in watts) for switching is
approximately
; Ps = 1.50/(4.3/10;C) (6)
Fig 3 is a plot of the the power (Ps) required for
switching as a function of the PP Band, which is the low
power wavelength separation in nm between a peak 26 and an
adjacent valley 25 in Fig. 2. The power required to
switch the device of Fig. 1 would be about 1000 kW if the
i fibers 14 and 15 had similar propagation constants in the
I absence of pump power. The plot shows that the power
requirement for nonlinear switching is reduced by a factor
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of 100 as the propagation constants of fibers 14 and 15
become sufficiently different that the difference in
wavelength between valley 25 and peak 26 in Fig. 2
approaches 1 nm.
Similar results would be obtained for a Mach-Zehnder
device in which the lengths of the optical paths in the
phase shift region are different; this configuration is
often employed in planar devices.
An overclad Mach-Zehnder switch can be formed in
accordance with the teachings of U.S. patent No. 5,295,205
which is incorporated herein by reference. The monol~thic
structure of Figs. 4 and 5 contains concatenated overclad
couplers 41 and 42 that are joined by a phase shifting
region 44. The device is formed by inserting optical
fibers 46 and 47 into the bore 48 of a tube of matrix
glass 49. Each of the optical fibers has a core
surrounded by cladding of refractive index lower than ~r;at
of the core. In the illustrated embodiment, fiber 46 ~s a
single piece of fiber, and fiber 47 consists of secticrs
~0 47a, 47b and 47c which are fused together prior to ma~-~g
the device. Section 47a, which is located in phase sh-rt
region 44, is doped with rare earth ions, while secticns
47b and 47c do not contain rare earth ions. That port Qn
of fiber 46 that is located in the phase shift region s
designated 46a.
The difference in propagation constants ~ betwee-
the two fibers in the phase shift region 44 in the abse~ce
of pumping or switching power must be sufficient to erable
switching at low power levels as discussed above. An~;
technique for obtaining different propagation constan's
can be employed. For example, the diameter of the core of
fiber 47a can be smaller than that of fiber 46a as shc~n
in Fig. 5. The different density of dots in the cores of
fibers 46 and 47 illustrates that the core of fiber 47a
contains rare earth ions. Alternatively, the fiber cores
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could have different refractive indices, or the fiber
claddings could have different refractive indices or
diameters. Any two or more of these features can be
combined to obtain a difference in propagation constants.
; 5 Assuming the aforementioned maximum acceptable length of
15 cm and pump or switching power of less than 1 mW, then
would be equal to or greater than 0.003.
The refractive index of that portion of the matrix
glass tube adjacent the fibers is less than the lowest
refractive index of either of the fiber claddings. The
bore can be provided with funnels (not shown) at each end
to facilitate insertion of the fibers. The combination of
tube and fibers is referred to as a coupler preform.
That portion of the tube between points a and b is
j15 initially heated and collapsed onto the fibers and is at
least partially fused to them. Also, the fibers are
caused to contact one another, whereby there is good
thermal conductivity between them. This can be
accomplished by evacuating the tube bore, heating the tube
near a first end 53 to cause it to collapse at the region
of applied heat, and moving the preform relative to the
heat source to gradually extend the collapsed region
toward end 54 until the desired length of collapsed tube
is obtained. Thereafter, coupler 41 is formed near end 53
of the tube by heating a region of the tube and moving
those sections of the tube on opposite sides of the hot
zone in opposite directions to stretch the heated region.
The stretching operation is stopped after a predetermined
coupling is achieved. While stretching the tube to form
the first coupler, optical power can be coupled to an
input optical fiber, and the output signals can be
monitored to control process steps in the coupler
manufacturing process.
For best performance, couplers 41 and 42 have
substantially identical coupling characteristics over the
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wavelength band of interest. The second coupler 42 is
therefore preferably formed near tube end 54 by subjecting
the appropriate region of the tube to stretching
conditions that are identical to those used to form the
coupler 41.
A Mach-Zehnder switch was constructed in accordance
with the embodiment shown in Figs. 5 and 6. Tube 10 was
comprised of silica doped with 5 wt. ~ boron. Fiber 46
was a standard single-mode fiber having an outside
diameter of 125 ~m and a core diameter of 9 ~m. The flber
cladding was formed of silica, and the core was formed of
silica doped with a sufficient amount of germania to
provide a core-clad ~ of 0.35~. Fiber 47 consisted of a
single piece of erbium-doped fiber having an outside
diameter of 125 ~m and a core diameter of 4 ~m. The fiber
cladding was formed of silica, and the core was formed of
silica doped with 1000 ppm by weight erbium and a
sufficient amount of germania to provide a core-clad ~ of
approximately 1.0 %.
The tube was collapsed onto the fibers and stretcled
to form couplers 41 and 42 in accordance with the above-
described method. The couplers were 3dB at 1550 nm. The
overall length of the resultant device was 12.7 cm. Tne
peak to valley wavelength separation (see Fig. 2) of the
Mach-~ehnder switch was 6 nm in the absence of pump power.
A laser diode operating at 1521 nm was connected .o
input port 2 by an attenuator. This single source
functioned as the signal and also provided the power for
changing the index of the erbium-doped fiber. Fig. 6
shows the output of the device as a function input power.
Curve 61 represents the device excess loss. Curve 62
represents the insertion loss between input port 2 and
output port 4, and curve 63 represents the insertion loss
between input port 2 and output port 3. Essentially a_l
of the input appeared at output port 3 when the input
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power was low, the input switching to output port 4 as
power level increased; Fig. 6 shows that switching
occurred at an input power of less than one milliwatt.
The specific example shows that the amount of power needed
to cause a signal to switch between the two output ports 3
and 4 of Fig. 1 depends on the phase difference already
existing between the two arms 14 and 15 of the phase shift
region before the pump or gating pulse is introduced.
Fig. 7 shows that embodiment in which the Mach-
Zehnder switch is formed as a planar device. All
waveguide paths and couplers are formed in or on substrate
66. Input paths 71 and 72 are connected to phase shift
paths 69 and 70 by coupler 68. Paths 69 and 70 are
connected to output paths 73 and 74 by coupler 67~ Path
70 is longer than path 69, whereby a phase shift is
introduced between the signal components propagating
through paths 69 and 70. The phase shift can also be
induced by providing paths 69 and 70 with different
refractive indices or widths. Although either of the
paths 69 and 70 can be doped with a rare earth element,
the shading on path 69 indicates such doping in that path.
As described above, the refractive index of the doped path
changes when pump power is introduced into the appropriate
input path. This causes an input signal introduced at
input path 71 or 72 to be switched from output path 73 to
output path 74, for example.
Mach-Zehnder devices become increasingly more
sensitive to temperature as the wavelength separation
between the peaks of the power output vs. wavelength curve
3~0 becomes smaller. However, suitable overclad devices of
the type shown in Fig. 4 having a peak separation as small
as 3.5 nm ha~e been made, and devices devices having a
; peak separation of about 1 nm are possible. This is
, possible because the fibers in the phase shift region of
the overclad structure are buried in the matrix glass.
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Thus, heat generated in the nonlinear fiber can conduc- to
the other fiber. Similarly, planar Mach-Zehnders are
stablized with respect temperature because heat can
conduct from one path to the other through the substra~e.
Whereas Mach-Zehnder switches having two optical
paths have been illustrated, it is thought that devices
having arrays of more than two paths could be formed. In
a three path device, for example, one path in the phase
shift region would be free from rare earth ions, the
second path would have some rare earth ions, and the t:-ird
path would have twice the amount of rare earth ions as the
second path. Each of the paths in the phase shift reg:on
would delay the signal a different amount, the first path
providing the least delay and the third path providlng the
most delay. A method of making a N path Mach-Zehr.der
device (N>2) is disclosed in U.S. patent No. 5,351,325.
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