Compact wavelength routing device having substantially flat passband

[0001] This application claims benefit of U.S. provisional patent application serial No. 60/307,906, filed Jul. 26, 2001, which is incorporated herein by reference in its entirety.

[0002] 1. Field of the Invention

[0003] The present invention generally relates to optical switching components and, more particularly, to a compact wavelength routing device having a substantially flat passband.

[0004] 2. Description of the Related Art

[0005] Optical wavelength division multiplexed (WDM) systems are becoming increasingly important. As the spectral efficiency of such WDM systems increases, wavelength routing devices that multiplex and demultiplex optical wavelengths require maximal passband width. A wider passband in the optical multiplexer/demultiplexer reduces the need for accurate control of the transmitted wavelengths and causes less signal distortion.

[0006] Presently, optical multiplexers and demultiplexers are realized in integrated form by using an imaging arrangement of waveguides having a constant path-length difference. Such devices are typically referred to as waveguide grating routers (WGRs). Attempts to increase or flatten the passband of WGRs have generally resulted in either lossy or large devices.

[0007] An optical apparatus comprising a first wavelength routing device having a first plurality of waveguide arms and a 180 optical coupler, and a second wavelength routing device coupled to the first wavelength routing device, the second wavelength routing device having a second plurality of waveguide arms. Each of the second plurality of waveguide arms has a substantially opposite curvature relative to each of the first plurality of waveguide arms. In one embodiment, the first wavelength routing device is a Mach-Zehnder interferometer (MZI) having first and second waveguide arms, and the second wavelength routing devices is a waveguide grating router (WGR).

[0008] The present invention can be used to provide an optical multiplexer/demultiplexer having a substantially flat passband. In addition, the present invention is compact and stackable given the opposite curvature of the waveguide arms between the first wavelength routing device and the second wavelength routing device.

[0009] So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.

[0010] It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0011] FIG. 1 depicts one embodiment of an optical multiplexer/demultiplexer device in accordance with the present invention;

[0012] FIG. 2 depicts an alternative embodiment of the optical multiplexer/demultiplexer in accordance with the present invention;

[0013] FIG. 3 depicts a block diagram showing an exemplary embodiment of an add/drop device employing the multiplexer/demultiplexer of the present invention;

[0014] FIG. 4 depicts an exemplary embodiment of a 56-wavelength (or channel) multiplexer/demultiplexer device constructed in accordance with the present invention;

[0015] FIG. 5 graphically illustrates the fiber-to-fiber transmissivities for transverse-electrically polarized light for all 56 channels of the device of FIG. 4; and

[0016] FIG. 6 depicts the details of the edge and center channels illustrated in FIG. 5.

[0017] The present invention is a compact wavelength routing device having a substantially flat passband. The present invention is useful for implementing optical multiplexers and demultiplexers in wavelength division multiplexed (WDM) optical transmission systems. Those skilled in the art, however, will appreciate that the present invention has broad applications beyond the optical transmission systems described herein, and can be used in any system that requires multiplexing and demultiplexing of wavelengths of light.

[0018] FIG. 1 depicts one embodiment of an optical multiplexer/demultiplexer device 100 in accordance with the present invention. The optical multiplexer/demultiplexer 100 can be fabricated in a silica planar lightwave circuit. The optical multiplexer/demultiplexer 100 comprises an input waveguide 112, a first wavelength routing device 102, a second wavelength routing device 104, and a plurality of output waveguides 114. The input waveguide 112 is coupled to the first wavelength routing device 102, which generally comprises a plurality of waveguide arms 108 (e.g., waveguide arms 108₁ and 108₂). The output waveguides 114 are coupled to the second wavelength routing device 104, which also comprises a plurality of waveguide arms 110. The waveguide arms 110 of the second wavelength routing device 104 have a substantially opposite curvature relative the waveguide arms 108₁ and 108₂ of the first wavelength routing device 102. The flip in the sign of curvature between the first wavelength routing device 102 and the second wavelength routing device 104 allows for a compact and stackable device.

[0019] As shown in FIG. 1, the first wavelength routing device 102 illustratively comprises an optical coupler 116, waveguide arms 108₁ and 108₂, a 180 optical coupler 106, and a 90 optical coupler 122. In the present example, the waveguide arms 108₁ and 108₂ form a Mach-Zehnder Interferometer (MZI). The input waveguide 112 is coupled to the waveguide arms 108₁ and 108₂ via optical coupler 116. The optical coupler 116 divides an optical signal propagating in the input waveguide 112 among the waveguide arms 108₁ and 108₂. The waveguide arms 108₁ and 108₂ each have different physical lengths so that a phase difference arises between optical signals of equal wavelength as the signals traverse the waveguide arms 108₁ and 108₂. An optical phase shifter 138 is optionally disposed in one of the waveguide arms 108₁ and 108₂, in order to wavelength-align the resulting MZI with the second wavelength routing device 104. A wavelength misalignment typically results in a passband tilt. The optical phase shifter 138 comprises a thermooptic phase shifter, and electrooptic phase shifter, or like type optical phase shifters known in the art.

[0020] The optical coupler 116 comprises a Y-branch, a directional coupler, or any other type of 50/50 optical power splitter known in the art. The waveguide arms 108₁ and 108₂ are coupled to waveguide portions 128₁ and 128₂, respectively, of the 180 optical coupler 106. The waveguide portions 128₁ and 128₂ have a length and spacing selected to affect a waveguide crossing. That is, the output of waveguide portion 128₁ is the optical signal that traversed waveguide arm 108₂, and the output of waveguide portion 128₂ is the optical signal that traversed waveguide arm 108₁. The waveguide portions 128₁ and 128₂ are coupled to respective waveguide portions 130₁ and 130₂ of the 90 optical coupler 122. The 180 optical coupler 106 allows for the flip in the sign of curvature between waveguide arms 108 of the first wavelength routing device 102 and waveguide arms 110 of the second wavelength routing device 104.

[0021] The second wavelength routing device 104 illustratively comprises a WGR. The WGR 104 comprises a first free space region 118, a second free space region 120, and the plurality of waveguide arms 110. The waveguide arms 110 are configured to provide a constant amount of path length difference from arm to arm. In addition, each of the waveguide arms 110 has a curvature substantially opposite from that of the waveguide arms 108 of the first wavelength routing device 102. The 90 optical coupler 122 is coupled to the first free space region 118. In particular, waveguide portions 130₁ and 130₂ of the 90 optical coupler 122 are coupled to an input circle 124 of the first free space region 118. The waveguide portions 130₁ and 130₂ have a length and spacing to affect a substantially 50/50 optical power split therebetween. The 90 optical coupler 122 comprises a directional coupler, for example. In practice, little distinction is made between waveguide arms 108₁ and 108₂, waveguide portions 128₁ and 128₂, and waveguide portions 130₁ and 130₂, since these waveguides can be fabricated as integral, continuous waveguides.

[0022] The waveguide arms 110 are coupled between the output circuit 132 of the first free space region 118 and the input circle 134 of the second free space region 120. The output waveguides 114 are coupled to the output circle 136 of the second free space region 120. The first wavelength routing device 102 and the second wavelength routing device 104 are designed such that across each passband, the spot movement versus wavelength at the interface (e.g., the interface of the input circle 124 and the 90 optical coupler 122) is synchronized. That is, if there are N wavelength channels in the free spectral range of the second wavelength routing device 104, then the path-length difference between arms 108₁ and 108₂ is substantially N times the path length difference between adjacent arms in the plurality of waveguide arms 110.

[0023] In this illustrative embodiment, an optical phase shift 126 is formed in one waveguide path between the 180 optical coupler 106 and the 90 optical coupler 122 (i.e., between waveguide portions 128₁ and 130₁ or waveguide portions 128₂ and 130₂). The optical phase shift 126 comprises a path length difference between the waveguide paths. If the optical couplers 106 and 122 are ideal and wavelength-independent, then the phase shift between couplers 106 and 122 is not necessary. Since the optical couplers 106 and 122 are typically not ideal, a phase shift can be used to minimize the passband shape and chromatic dispersion change with optical coupler variation. The optimum phase shift to be produced by phase shift 126 can be determined as follows: Suppose each coupler 106 and 122 has an error in its relative eigenmode phase shift of /90 percent. Assuming <<1, then the action of the net coupler is given by: $\frac{1}{\sqrt{2}}\left[\begin{array}{cc}1-& j+j\\ j+j& 1-\end{array}\right]\left[\begin{array}{cc}1& 0\\ 0& {}^j\end{array}\right]\left[\begin{array}{cc}-2& j\\ j& -2\end{array}\right]\left[\begin{array}{cc}1& 0\\ 0& {}^j\end{array}\right]$

[0024] where a phase shift is added in front of one of the paths to undo the second phase shift produced by the couplers 106 and 122. For phase shifts 0, 120, and 180, the action of the net coupler (couplers 106 and 122 and the phase shift 126) is: $\frac{1}{\sqrt{2}}\left[\begin{array}{cc}-1-3& j-3j\\ j-3j& -1-3\end{array}\right],\frac{-1+\sqrt{3}}{2\sqrt{2}}\left[\begin{array}{cc}-1+\sqrt{3}j& j+\sqrt{3}\\ j-\sqrt{3}& -1-\sqrt{3}j\end{array}\right],\mathrm{and}\frac{-1}{\sqrt{2}}\left[\begin{array}{cc}-1+& j+j\\ j+j& -1+\end{array}\right]$

[0025] respectively, where j denotes a complex operator.

[0026] A phase shift of 0 gives the largest error in amplitude, since this results in a very long optical coupler. A phase shift of 120 also gives a large error in amplitude, but a zero error in magnitude. A phase shift of 120 is a preferred choice for the lens-coupled embodiment of the invention described below with respect to FIG. 2, since the error in phase from this coupler can be corrected post-fabrication by a second phase shifter. Finally, a phase shift of 180 results in the least error in amplitude and is thus the best choice for the present embodiment of the invention, where the 90 optical coupler 122 is directly coupled to the first free space region 118 of the second wavelength routing device 104.

[0027] The present invention thus provides a compact and stackable optical multiplexer/demultiplexer 100 having a substantially flat passband. Because the sign of curvature is flipped between the first and second wavelength routing devices 102 and 104, the present invention is stackable (i.e., multiple multiplexer/demultiplexer devices can be formed on a substrate). In addition, the present invention allows the second wavelength routing device 104 to have approximately 2N waveguide arms 110, where N is the number of wavelength channels in the free spectral range of the second wavelength routing device 104. Since the focal spot velocities versus wavelength change are matched at the interface between the first and second wavelength routing devices 102 and 104, the transmissivity spectrum of the optical multiplexer/demultiplexer 100 is substantially flat, substantially chromatic dispersion free, and substantially lossless across each passband.

[0028] FIG. 2 depicts an alternative embodiment of the optical multiplexer/demultiplexer 200 of the present invention. Elements of FIG. 2 that are similar to those depicted in FIG. 1 are designated with identical reference numerals and operate substantially as described above. In accordance with the second embodiment, the first wavelength routing device 102 is coupled to the second wavelength routing device 104 with two uncoupled waveguides 202₁ and 202₂ of substantially equal length. In particular, the two uncoupled waveguides 202₁ and 202₂ are coupled to the first free space region 118. A plurality of unconnected waveguides 206 are also coupled to the first free space region 118, where the waveguides 206 are adjacent to the uncoupled waveguides 202₁ and 202₂.

[0029] In addition, an optical phase shifter 204 is optionally disposed in one of the uncoupled waveguides 202₁ and 202₂. The optical phase shifter 204 is used to correct any phase error not compensated for by the phase shift 126. As described above, the phase shift produced by the optical phase shift 126 can be set to 120, with any phase errors being corrected post-fabrication by the optical phase shifter 204. The optical phase shifter 204 comprises a thermooptic phase shifter, an electrooptic phase shifter, or like type optical phase shifters known in the art.

[0030] Although the various embodiments of the present invention have been described as performing a demultiplexing function, those skilled in the art will understand that the present invention can also perform the reciprocal function of multiplexing. As such, the various elements described above that were modified by the terms input and output are merely illustrative of the demultiplexing function. If performing a multiplexing function, the terms modified by input would be modified by output and vice versa. For example, the input waveguide 112 would become the output waveguide of a multiplexer.

[0031] FIG. 3 depicts a block diagram showing an exemplary embodiment of an add/drop device 300 in accordance with the present invention. The device 300 is coupled to a wavelength division multiplexed (WDM) network (not shown) via a circulator 302. The device 300 illustratively comprises an optical multiplexer/demultiplexer 304 and a switch 306 coupled to each demultiplexed port of the multiplexer/demultiplexer 304. In accordance with the present invention, the multiplexer/demultiplexer 304 is implemented as described above with respect to FIGS. 1 and 2. The switches 306 can be implemented as 12 Mach-Zehnder switches. Each Mach-Zehnder switch comprises two couplers connected by waveguides containing phase shifters.

[0032] Input channels pass through the circulator 302 and are demultiplexed, as a function of wavelength, by the multiplexer/demultiplexer 304. Each switch 306 routes a demultiplexed signal either to a mirror 308 or to an add/drop port 310. Signals routed to a mirror 308 are reflected back through a respective switch 306, back through the multiplexer/demultiplexer 304, and back out to the WDM network via the through port of the circulator 302.

[0033] The add/drop port 310 acts as a bidirectional port to add and drop data. An external circulator 312 coupled to the add/drop port 310 can provide separate add and drop ports 314 and 316, respectively. The add/drop device 300 is illustrative of the type of network element in which the present invention can be used. Those skilled in the art will appreciate that the optical multiplexer/demultiplexer device of the present invention can be used in other types of network elements in an optical transmission system, for example, transmitter and receiver network elements.

[0034] FIG. 4 depicts an exemplary embodiment of a 56-wavelength (or channel) multiplexer/demultiplexer device 400 in accordance with the present invention. The exemplary 56-wavelength device 400 has 100-GHz channel spacing and is formed on a substrate 401 as described above using silica waveguides. The device 400 comprises an input waveguide 402, a MZI 404, a WGR 406, and 56 output waveguides 408. The length of the device 400 is approximately 9.6 cm. The normalized index step is 0.65%, the substrate 401 is silicon, and a thermooptic phase shifter is used in one arm of the MZI 404 in order to wavelength-align the MZI 404 with the WGR 406. As described above, a misalignment between the MZI 404 and the WGR 406 results in a passband tilt.

[0035] In the present example, the WGR 406 comprises 195 waveguide arms, and the number of channels in the free spectral range of the WGR 406, N, is 76. The fiber-to-fiber transmissivities for transverse-electrically polarized light for all 56 channels are shown in FIG. 5, where axis 502 represents the transmissivity in dB, and axis 504 represents the wavelength in nm. FIG. 6 depicts the details of the edge channels 606 and 610, as well as a center channel 608, shown in FIG. 5, where axes 602 represent the transmissivity in dB, and axes 604 represent the wavelength in nm. All 56 channels have less than 4 dB fiber-to-fiber loss. All 56 channels have a 1-dB bandwidth greater than 70 GHz and a 20 dB bandwidth less than 140 GHz. Considering a bandwidth of 25 GHz around each channel center, the total adjacent crosstalk is 19 dB, and the total non-adjacent crosstalk is 22 dB.

[0036] While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.