AU697911B2

AU697911B2 - Mach-zehnder switch

Info

Publication number: AU697911B2
Application number: AU59299/96A
Authority: AU
Inventors: William J. Miller; Daniel A. Nolan
Original assignee: Corning Inc
Current assignee: Corning Inc
Priority date: 1995-06-09
Filing date: 1996-05-23
Publication date: 1998-10-22
Anticipated expiration: 2016-05-23
Also published as: WO1996042027A1; EP0873532A1; JPH11507741A; CA2221749A1; EP0873532A4; AU5929996A

Description

MACH-ZEHNDER SWITCH

Background of the Invention

The present invention relates to optical power switching devices. Optical switches with switching speeds up to 1 gigahertz are required for numerous applications including local area networks, sensor arrays and communications systems. Many forms of optical switching devices have been developed. Typical examples are multiple-quantur.- 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 than 1 . Until recently optical fiber switches had been fabricated from optical fibers having silica based ceres. 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 a nonlinear coefficient approximately 1 million times greater than that of fused silica. However, the large 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-Doped 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 π required an absorbed pump power of 15.5 m . The signal was launched to inject approximately equal powers in the LP₀₁ and LP_:: modes. This type of signal injection is difficult to impliment, and the device is unstable with respect to external vibrations and perturbations.

One or both fibers in the phase shift region of the Mach-Zehnder device of Pantell et al. is made of Erbiu - doped fiber, the pump power being coupled into only one of the . 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-Zehnder (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 pump power is applied to cause a phase difference of π, the signal switches to output port 3. Pantell et al. indicate that the 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.

Summary of the Invention

It is therefore an obj ect of the present invention to provide an optical switch that overcomes the heretofore noted disadvantages of prior art switches . A further obj ect 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 signal into N equal signal components . where N>1. 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 nonlinearity, 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 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.

Brief Description of the Drawings

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. Fig. 3 is a plot of the power (P_s) 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 present invention.

Fig. 7 shows a planar Mach-Zehnder switch.

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 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, 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 π, 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 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 conventional 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 about 15 cm.

A second feature of the invention 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 employed 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 maintained 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 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 25, 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 = C0S^:(πz(n_: - n_x)/λ) (1)

where n₂ and n represent effective indices of propagation in path 1 and path 2, respectively, and λ is the signal wavelength. The length z of waveguide paths 14 and 15 is chosen so that a π/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 λ_: (point 26 of Fig. 2) and a maximum is to occur at wavelsngth λ2 (point 25 of Fig. 2), z is given by

z = [2(n₂ - n_1# (l/λ_ϊ-l/λj)]^"1 (2)

5 The index change needed to cause switching at X, is then

»

πδz/λ = π(n_; - n) z/λ_α - π(n₂ - n)z/λ2) (3a)

so that

10 δ = λ,[(n_: - n)z/λ, - (n_: - n)/λ2)] (3b)

It is known that the nonlinear index for silica based fibers is

15 n₂ = 3.2xl0^"16 cirr/watt (4)

For a single mode fiber with an effective index of 75 μπ , eqation 4 becomes

20 n₂ = 4.3xl0^"lc/watt (5)

The required power (in watts) for switching is approximately

25

P_s = 1.5δ/(4.3/10^:o) (6)

Fig. 3 is a plot of the the power (P_s) required for switching as a function of the PP Band, which is the low

30 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 fibers 14 and 15 had similar propagation constants in the absence of pump power. The plot shows that the power

35 requirement for nonlinear switching is reduced by a factor 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 monolithic 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 that of the core. In the illustrated embodiment, fiber 46 is a single piece of fiber, and fiber 47 consists of sections 47a, 47b and 47c which are fused together prior to making the device. Section 47a, which is located in phase shift region 44, is doped with rare earth ions, while sections 47b and 47c do not contain rare earth ions. That portion of fiber 46 that is located in the phase shift region is designated 46a.

The difference in propagation constants Δβ between the two fibers in the phase shift region 44 in the absence of pumping or switching power must be sufficient to enable switching at low power levels as discussed above. Any technique for obtaining different propagation constants can be employed. For example, the diameter of the core of fiber 47a can be smaller than that of fiber 46a as shewn 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 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. Assuming the aforementioned maximum acceptable length of 15 cm and pump or switching power of less than 1 mW, then Δβ 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 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 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 μm and a core diameter of 9 urn. The fiber cladding was formed of silica, and the core was formed of silica doped with a sufficient amount of germania to provide a core-clad Δ of 0.35%. Fiber 47 consisted of a single piece of erbium-doped fiber having an outside diameter of 125 μm and a core diameter of 4 μm. The fiber cladding was formed of silica, and the core was formed of silica doped with 1000 pp by weight erbium and a sufficient amount of germania to provide a core-clad Δ of approximately 1.0 %. The tube was collapsed onto the fibers and stretched 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. The peak to valley wavelength separation (see Fig. 2) of the Mach-Zehnder switch was 6 nm in the absence of pump power. A laser diode operating at 1521 nm was connected to 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 all of the input appeared at output port 3 when the input 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 5 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-

10 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

15 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

20 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

25 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

30 becomes smaller. However, suitable overclad devices of the type shown in Fig. 4 having a peak separation as small as 3.5 nm have been made, and devices devices having a , peak separation of about 1 n are possible. This is possible because the fibers in the phase shift region of

35 the overclad structure are buried in the matrix glass. Thus, heat generated in the nonlinear fiber can conduct 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 substrate. 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 third path would have twice the amount of rare earth ions as the second path. Each of the paths in the phase shift region would delay the signal a different amount, the first path providing the least delay and the third path providing the most delay. A method of making a N path Mach-Zehnder device (N>2) is disclosed in U.S. patent No. 5,351,325.

Claims

I claim:

1. A monolithic Mach-Zehnder switch comprising input coupler means for splitting an input signal into N equal signal components, where N>1, combining means for combining said N components, said combining means having at least first and second output terminals,

N optical waveguide paths connecting said N signal components to said combining means, at least one of said waveguide paths containing a material having a resonant nonlinearity, whereby the refractive index of the path changes when pump power propagates through it, said input coupler means and said combining means being free from said material, and a matrix glass body, said input coupler means, said combining means and said optical waveguide paths being in thermal contact with said body.

2. A monolithic Mach-Zehnder switch in accordance with claim 1 wherein there is a difference Δβ between the propagation constants of said waveguide paths such that each of said N waveguide paths subjects the light propagating therethrough to a delay that is different from the delay experienced by light propagating through each of the other waveguide paths when no pump power is propagating through said at least one waveguide path.

3. A monolithic Mach-Zehnder switch in accordance with claim 2 wherein Δβ is equal to or greater than 0.003.

4. A monolithic Mach-Zehnder switch in accordance with claim 1 wherein the length of said matrix glass body is no greater than 15 cm.

5. A monolithic Mach-Zehnder switch in accordance with claim 1 wherein said paths are optical fibers, and wherein said fibers, said input coupler means and said combining means are surrounded by an elongated body of said matrix glass .

6. A monolithic Mach-Zehnder switch in accordance with claim 5 wherein said input coupler means and said combining means are regions in said body wherein the diameter of said body and the diameters of said fibers are smaller than the diameters thereof in said phase shift region.

7. A monolithic Mach-Zehnder switch in accordance with claim 1 wherein said matrix glass body comprises a planar sustrate, said paths, said input coupler means and said combining means being located at the surface of said substrate.

8. A monolithic Mach-Zehnder switch in accordance with claim 1 wherein said material having a resonant nonlinearity is a rare earth.

9. A monolithic Mach-Zehnder switch comprising an elongated body of matrix glass, first and second optical fibers extending longitudinally through said body, a phase shift region in said body, two spaced coupler regions in said body at opposite ends of said phase shift region, the diameter of said body and the diameters of said fibers being smaller in said coupler regions than in said phase shift region, at least that portion of said first fiber that is in said phase shift region containing a material having a resonant nonlinearity, whereby the refractive index of said first fiber changes when pump power propagates through it, said fibers having different propagation constants in said phase shift region in the absence of pump power propagating through said first fiber so that said first fiber subjects the light propagating therethrough to a delay that is different from the delay experienced by light propagating through said second fiber.

10. A monolithic Mach-Zehnder switch in accordance with claim 9 wherein said material having a resonant nonlinearity is a rare earth.