Electro-optically tunable optical filter

The present invention provides a method and apparatus for filtering an optical signal. The method includes receiving at least one input optical signal, forming first and second optical signals using the at least one input optical signal, and modifying at least one portion of the first optical signal using a plurality of non-waveguiding electro-optic phase adjusters. The method also includes forming an output optical signal by combining the first optical signal, including the at least one modified portion of the first optical signal, with the second optical signal.

This invention relates generally to an optical transmission system, and, more particularly, to an electro-optically tunable optical filter for use in an optical transmission system.

Photonics, the use of light to store, transmit, and/or process information, is rapidly penetrating the market for commodity and high technology products. For example, optics is the transmission medium of choice for many metropolitan and local-area networks. To sustain bandwidth, and to allow different components of the optical transmission network to work together, optical transmission networks typically use sophisticated optical filters that may dynamically equalize the power on the wavelength, or frequency, channels of the networks. Exemplary optical filters that dynamically equalize power on a broad spectral feature basis include Mach-Zehnder filters, acousto-optic filters, holograms, and micro-mechanically driven mirrors. Exemplary optical filters that may dynamically equalize the power on a channel-by-channel basis include demultiplexers, arrays of programmable attenuators, multiplexers, and the like.

Optical filters may include one or more waveguides for transmitting light, as well as one or more elements that may adjust the phase of the light propagating in the waveguides. In traditional phase adjustable waveguides, a Joule heater is deployed proximate the waveguides and used to vary the temperature of the optical waveguide. The effective refractive index of the optics waveguide depends on the temperature of the waveguide, so varying the temperature changes the optical path length of the waveguide and thereby varies the phase of the light traveling in the optical waveguide. Thermo-optic phase adjustment is used in optical attenuators, spectrally selective filters, interferometers, and the like. For example, Doerr (U.S. Pat. No. 6,212,315) describes a channel power equalizer that uses thermo-optic phase adjustment in a plurality of phase shifters.

However, traditional methods of changing the phase of light propagating in a waveguide, including thermo-optic phase adjustment, may not be well-suited for spectral filtering applications. The sensitivity of temperature-dependent phase controllers may be limited by the relatively small thermo-optic coefficient of silica. Although other materials may exhibit larger thermo-optic coefficients, these may be difficult to form into low-loss single mode waveguides. Furthermore, thermo-optic methods of phase control may not respond fast enough to be integrated tightly with other electronic devices in the optical transmission network.

Furthermore, optical filters are often formed on a semiconductor substrate, and thermal crosstalk between multiple temperature-dependent phase controllers formed on the same semiconductor substrate may reduce the accuracy, finesse, and control of the temperature-dependent phase controllers. Consequently, fewer temperature-dependent phase controllers may be included on a single semiconductor substrate. Thermal crosstalk may also reduce the range of phase expression of the temperature-dependent phase controller. Although the reduction in the range of phase expression may be, at least in part, compensated for by increasing the range of temperatures applied to the phase controllers, increasing the temperature range typically results in a corresponding increase in power consumption of the device. Furthermore, the polarization independence of orthonormal modes may be reduced by thermal crosstalk.

In one aspect of the present invention, a method is provided for filtering an optical signal. The method includes receiving at least one input optical signal, forming first and second optical signals using the at least one input optical signal, and modifying at least one portion of the first optical signal using a plurality of non-waveguiding electro-optic phase adjusters. The method also includes forming an output optical signal by combining the first optical signal, including the at least one modified portion of the first optical signal, with the second optical signal.

In another aspect of the instant invention, an apparatus is provided. The apparatus includes an optical demultiplexer, a plurality of non-waveguiding electro-optic phase adjusters optically coupled to the optical demultiplexer, and an optical multiplexer optically coupled to the plurality of electro-optic phase adjusters.

In yet another aspect of the instant invention, an electro-optically tunable optical filter is provided. The electro-optically tunable optical filter includes a first optical transmission medium, a second optical transmission medium, and a first optical coupler for coupling portions of the first and second optical transmission media. The electro-optically tunable optical filter also includes an optical demultiplexer coupled to the second optical transmission medium, a plurality of non-waveguiding electro-optic phase adjusters optically coupled to the optical demultiplexer, and an optical multiplexer optically coupled to the plurality of non-waveguiding electro-optic phase adjusters. The electro-optically tunable optical filter further includes a third optical transmission medium optically coupled to the optical multiplexer and a second optical coupler for coupling portions of the second and the third optical transmission media.

FIG. 1Aconceptually illustrates a first exemplary embodiment of a dynamically and chromatically variable transmissivity apparatus, such as a dynamic gain flattening filter100. Although the following description will be presented in the context of the embodiments of the dynamic gain flattening filters100shown inFIGS. 1A and 1B, the present invention is not so limited. In alternative embodiments, the variable transmissivity apparatus100may be one of variety of optical elements known to those of ordinary skill in the art. For example, the variable transmissivity apparatus100may be a channel equalizer for controlling channel powers in wavelength-division multiplexed systems, a Mach-Zehnder filter, a Michelson interferometer, and the like.

The first exemplary embodiment of the dynamic gain flattening filter100includes first and second optical transmission media101,102. In one embodiment, the first and second optical transmission media101,102are waveguides. Although not necessary for the practice of the present invention, a first optical signal may enter the dynamic gain flattening filter100through a first port105in a non-reciprocal device110. In one embodiment, the non-reciprocal device110is a circulator101that may be formed using materials having a high Verdet constant, as will be appreciated by those of ordinary skill in the art.

The non-reciprocal device110may be optically coupled to the waveguide101so that the first optical signal may be transmitted to the waveguide101and then enter the dynamic gain flattening filter100through a first port115in a first optical coupler120. However, in alternative embodiments, the first optical signal may enter the dynamic gain flattening filter100without passing through the non-reciprocal device110. Although not necessary for the practice of the present invention, a second optical signal propagating along the waveguide102may enter the dynamic gain flattening filter100through a second port125in the first optical coupler120. In one embodiment, the first optical signal and, if present, the second optical signal, are wavelength division multiplexed optical signals.

The first optical coupler120may split and/or combine the first and second optical signals to form two signal components that are transmitted to upper and lower arms125,130of the waveguides101,102, respectively. For example, if no second optical signal is provided to the dynamic gain flattening filter100via the waveguide102, the first optical coupler120splits the first signal into the two signal components √{square root over (R)} and j√{square root over (1−R)}, where R is a splitting ratio of the first optical coupler120. The two signal components, √{square root over (R)} and j√{square root over (1−R)}, are transmitted to the upper and lower arms125,130, respectively. In one embodiment, at least a portion of the upper and lower arms125,130are waveguides. For example, the upper arm125may be waveguide. For another example, a first portion133(1-2) of the lower arm130may be a waveguide.

The first portion133(1) of the lower arm130is optically coupled to an optical demultiplexer135. In one embodiment, the optical demultiplexer135receives the signal component j√{square root over (1−R)} from the lower arm125and splits the signal component j√{square root over (1−R)} into portions corresponding to a plurality of selected frequency and/or wavelength bands. For example, the signal component j√{square root over (1−R)} may have a bandwidth of 60 nm and be demultiplexed into 60 portions having a bandwidth of 1 nm. However, in alternative embodiments, a variety of devices well known to those of ordinary skill in the art may be used to split the signal component j√{square root over (1−R)} into portions corresponding to the plurality of selected frequency and/or wavelength bands. These devices may include, but are not limited to, optical splitters, prisms, gratings, and the like.

The optical demultiplexer135provides the portions of the signal component j√{square root over (1−R)} to a corresponding plurality of electro-optic phase adjusters140, which are optically coupled to the optical demultiplexer135. As will be appreciated by persons of ordinary skill in the art, the number of electro-optic phase adjusters140is a matter of design choice. Thus, although three electro-optic phase adjusters140are shown inFIG. 1, alternative embodiments of the present invention may include more or fewer electro-optic phase adjusters140.

In the first exemplary embodiment of the dynamic gain flattening filter100shown inFIG. 1A, the plurality of electro-optic phase adjusters140are optically coupled to a mirror145. The upper arm125is also optically coupled to the mirror145. Although not necessary for the practice of the present invention, a wave plate150may be deployed adjacent the mirror145such that the two signal components √{square root over (R)} and j√{square root over (1−R)} propagating in the upper arm125and the electro-optic phase adjusters140, respectively, pass through the wave plate150before reflecting from the mirror145. For example, a quarter-wave plate150may be deployed between the mirror and the upper arm125and electro-optic phase adjusters140. Incorporating the quarter-wave plate150may reduce, or null, birefringence in the portions of the signal component j√{square root over (1−R)}.

The optical path length of the upper and lower arms125,130may, in one embodiment, be approximately equal. For example, the optical path length of the upper arm125and lower arm130, including the optical demultiplexer135, the electro-optic phase adjusters140, and the wave plate150, may be equal to within about a few wavelengths of the first and, if present, the second optical signal. As will be discussed in detail below, the effective optical path length of the electro-optic phase adjusters140, and consequently the optical path length of portions of the lower arm130, may be controlled, or tuned, to modify the portions of the signal component j√{square root over (1−R)}. In one embodiment, the effective optical path length of one or more of the electro-optic phase adjusters140may be varied so that one or more relative phase differences between the portions of the signal component j√{square root over (1−R)} are introduced. For example, a phase difference of π/4 may be introduced between two of the portions of the signal component j√{square root over (1−R)}.

After the two signal components √{square root over (R)} and j√{square root over (1−R)} reflect from the mirror145, they are transmitted back along approximately the same optical path to the first optical coupler120. Consequently, the one or more relative phase differences between the portions of the signal component j√{square root over (1−R)} introduced by the electro-optic phase adjusters140may be approximately doubled. For example, if one of the electro-optic phase adjusters140introduces a phase difference of about π/4 between two of the portions of the signal component j√{square root over (1−R)} during a single pass, then a total phase difference of about π/2 may be introduced between the two of the portions of the signal component j√{square root over (1−R)}.

In the first exemplary embodiment, the optical demultiplexer135may also function as an optical multiplexer for the reflected portions of the signal component j√{square root over (1−R)}. For example, the optical demultiplexer135may combine to reflected portions of the signal component j√{square root over (1−R)} to form the modified signal component j√{square root over (1−R)}. The first optical coupler120may combine and/or split the signal component √{square root over (R)} and the modified signal component j√{square root over (1−R)} to form an output signal. For example, the signal component √{square root over (R)} and the modified signal component j√{square root over (1−R)} may interfere destructively and/or constructively to form a filtered output signal. In one embodiment, the filtered output signal may be provided to the non-reciprocal device110and may then exit the dynamic gain flattening filter via a second port155. However, as discussed above, the non-reciprocal device110is optional and may be omitted in alternative embodiments of the present invention.

FIG. 1Bshows a second exemplary embodiment of the dynamic gain flattening filter100. In the second exemplary embodiment of the dynamic gain flattening filter100, the plurality of electro-optic phase adjusters140are optically coupled to an optical multiplexer160. Portions of the signal component j√{square root over (1−R)}, including any modified portions, may be provided to the optical multiplexer160. In one embodiment, the optical multiplexer160may combine the portions to form a modified signal component j√{square root over (1−R)}.

In the second exemplary embodiment of the dynamic gain flattening filter100, the signal component √{square root over (R)} and the modified signal component j√{square root over (1−R)} propagating in the upper and lower arms125,130, respectively, are provided to a second optical coupler165, which may split and/or combine the signal component √{square root over (R)} and the modified signal component j√{square root over (1−R)}. For example, the signal component √{square root over (R)} and the modified signal component j√{square root over (1−R)} may interfere destructively and/or constructively to form a filtered output signal. In one embodiment, the first and second optical couplers120,165have the same splitting ratio, R, although this is not necessary for the practice of the present invention. Furthermore, the second optical coupler165may be omitted in various alternative embodiments of the present invention.

The optical path length of the upper and lower arms125,130may, in one embodiment, be approximately equal. For example, the optical path length of the upper arm125and lower arm130, including the optical demultiplexer135, the electro-optic phase adjusters140, and the optical multiplexer145, may be equal to within about a few wavelengths of the first and, if present, the second optical signal. As will be discussed in detail below, the effective optical path length of the electro-optic phase adjusters140, and consequently the optical path length of portions of the lower arm130, may be controlled, or tuned, to modify the portions of the signal component j√{square root over (1−R)}. In one embodiment, the effective optical path length of one or more of the electro-optic phase adjusters140may be varied so that one or more relative phase differences between the portions of the signal component j√{square root over (1−R)} are introduced. For example, a phase difference of π/4 may be introduced between two of the portions of the signal component j√{square root over (1−R)}.

In either the first or the second exemplary embodiments shown inFIGS. 1A and 1B, respectively, one or more components of the dynamic gain flattening filter100may be formed on a single planar waveguide platform (not shown). For example, the optical demultiplexer135, the plurality of electro-optic phase adjusters140, and the mirror150or the optical multiplexer160may be formed on the planar waveguide platform. In various alternative embodiments, the planar waveguide platform may be formed of a polymer, silica-on-silicon, a semiconductor, or like materials.

At least in part because of the fast response time of the plurality of electro-optic phase adjusters140, the two embodiments of the dynamic gain flattening filter100shown inFIGS. 1A and 1Bmay be integrated tightly with other electronic devices. Furthermore, the number of electro-optic phase adjusters140that may be formed on a single platform may be increased because thermal crosstalk between multiple electro-optic phase adjusters140may be reduced relative to, e.g., a plurality of thermo-optic phase adjusters. The electro-optic phase adjusters140may also have an increased range of phase expression and/or reduced power consumption compared to, e.g., a plurality of thermo-optic phase adjusters.

FIG. 2conceptually illustrates the plurality of electro-optic phase adjusters140, in accordance with one embodiment of the present invention. As discussed above, in one embodiment, the signal component j√{square root over (1−R)} is provided to the optical demultiplexer135via the first portion133(1) of the lower arm130. In the illustrated embodiment, the optical demultiplexer135is optically coupled to a plurality of optical transmission media, such as waveguides200, which may be deployed proximate a corresponding plurality of slots210. In one embodiment, an end of the waveguide200may be deployed proximate the slot210so that the waveguide200is optically coupled to the slot210and may provide portions of the signal component j√{square root over (1−R)} to the slot210. For example, each of the waveguides200may provide a portion of the signal component j√{square root over (1−R)} having a wavelength, or a frequency, approximately within a selected wavelength, or frequency, band to the corresponding one of the plurality of slots210.

An electro-optically active phase adjusting element220may be positioned in at least a portion of the slot210. In one embodiment, the electro-optically active phase adjusting element220may be an electro-optically active material such as a liquid crystal, a polymer-dispersed liquid crystal, a birefringent material, and the like, which may be located in the slot210. However, any desirable type of electro-optically active phase adjusting element220may be used. For example, in one alternative embodiment, the electro-optically active phase adjusting element220may be a silicon substrate having an opening that is filled with an electro-optically active material. In this alternative embodiment, the electro-optically active phase adjusting element220may be formed separately and subsequently inserted into the electro-optic phase adjusters140.

One or more electrodes230are deployed proximate the slot210. In the illustrated embodiment, two electrodes230are deployed near the slot and above at least a portion (drawn in ghosted lines) of the waveguide200. However, the present invention is not so limited. In alternative embodiments, more or fewer electrodes230may be deployed proximate the slot210. Furthermore, in other alternative embodiments, at least a portion of the electrodes230may be deployed within the slot210.

The electrodes230are coupled to a control unit240via lines250. In various alternative embodiments, the lines250may be wires, conductive traces, and the like. The control unit240may provide selected signals, such as voltages and/or currents, to the electrodes230. As will be appreciated by those of ordinary skill in the art, the signals provided by the control unit240may be used to vary the optical path length of the electro-optically active phase adjusting element220. For example, applying a voltage to one or more of the electrodes230may create an electric field, and at least a portion of the electric field may penetrate into the slot210. Varying the strength of the signal, e.g. the voltage, may change the amplitude and/or orientation of the electric field, which may change the optical path length of the electro-optically active phase adjusting element220.

A phase of one or more of the portions of the signal component j√{square root over (1−R)} may be modified when the portions of the signal component j√{square root over (1−R)} propagate through the electro-optically active phase adjusting element220. In one embodiment, a relative phase difference may be introduced between the portions of the signal component j√{square root over (1−R)} by providing different signals to the electrodes230deployed proximate the slots210corresponding to the appropriate portions of the signal component j√{square root over (1−R)}. For example, a relative phase difference may be introduced between two portions of the signal component j√{square root over (1−R)} by varying the strength of the signal provided to the corresponding slots210such that the optical path length of the slot210corresponding to a first portion of the signal component j√{square root over (1−R)} differs from the optical path length of the slot210corresponding to a second portion of the signal component j√{square root over (1−R)} by approximately one quarter of a wavelength of the signal component j√{square root over (1−R)}.

Another plurality of optical transmission media, such as waveguides260, may be deployed proximate the slot210. In one embodiment, an end of the waveguide260may be deployed proximate the slot210so that the waveguide260is optically coupled to the slot210and may receive the portions of the signal component j√{square root over (1−R)} from the slot210. In one embodiment, a portion (drawn in ghosted lines) of the waveguide260may be positioned beneath one or more of the electrodes230. In the first exemplary embodiment of the dynamic gain flattening filter100, shown inFIG. 1A, the waveguides260may be optically coupled to the mirror145and/or the wave plate150. Alternatively, in the second exemplary embodiment of the dynamic gain flattening filter100, shown inFIG. 1B, the waveguides260may be optically coupled to the multiplexer160, which may, as discussed above, split and/or combine the portions of the signal component j√{square root over (1−R)}.

The slot210and the electro-optically active phase adjusting element220are, in one embodiment, non-waveguiding. Thus, although waveguiding elements, such as the waveguides200,260, may be included in the plurality of electro-optic phase adjusters140, the electro-optic phase adjusters140are referred to hereinafter as “non-waveguiding” electro-optic phase adjusters140.

FIG. 3conceptually illustrates a perspective view of one embodiment of the electro-optic phase adjuster140. In the illustrated embodiment, one or more waveguide portions305(1-2) are formed within a dielectric layer, commonly referred to in the art as a cladding layer310, which is formed above a semiconductor substrate320, such as silicon. It should be appreciated that the configuration of the electro-optic phase adjuster140is exemplary in nature, and that in alternative embodiments, the electro-optic phase adjuster140may include other components not shown inFIG. 3.

The waveguide portions305(1-2) shown in the illustrated embodiment are formed of material having a refractive index that is larger than a refractive index of the cladding layer310. For example, the waveguide portions305(1-2) may be formed of un-doped silica having a refractive index of about 1.4557 and the cladding layer310may be formed of doped or un-doped silica having a refractive index of about 1.445. In other embodiments, the waveguide portions305(1-2) and the cladding layer310may be formed of any desirable materials. In one embodiment, the cladding layer310may include an under cladding layer (not shown) formed, at least in part, in a region315beneath the waveguide portions305(1-2) and an upper cladding layer (not shown) formed, at least in part, in a region320above the waveguide portions305(1-2). In one embodiment, the upper cladding layer and the under cladding layer do not have the same refractive index. For example, the upper cladding layer may have a refractive index of about 1.4448 and the under cladding layer may have a refractive index of about 1.4451.

A slot330is incised in the cladding layer310so that the waveguide portions305(1-2) terminate proximate the slot330. However, in alternative embodiments, the waveguide portions305(1-2) may not terminate proximate the slot330. For example, a part of the waveguide portions305(1-2) may be proximate the slot330even though the waveguide portions305(1-2) terminate at a location spaced from the slot330. In one embodiment, the slot330is incised so that an evanescent field amplitude due to the signals propagating in the waveguide portions305(1-2) at transverse edges350(1-2) of the slot330is less than −40 dB of the peak value. However, the precise location of the slot330and the desired evanescent field amplitude at the transverse edges350(1-2) are matters of design choice. Furthermore, although the slot330is depicted as rectangular inFIG. 3, the geometry of the slot330is a matter of design choice, taking on any of a variety of geometric cross sectional configurations and even varying in cross sectional configuration along its length.

FIG. 4illustrates one embodiment of an exemplary method of filtering an optical signal using, for example, the dynamic gain flattening filter100shown inFIGS. 1A and 1B. The illustrated embodiment of the method includes receiving (at400) at least one input optical signal. First and second optical signals are then formed (at410) using the at least one input optical signal. For example, the optical coupler110shown inFIGS. 1A and 1Bmay form the two signal components √{square root over (R)} and j√{square root over (1−R)} using the input optical signal. As discussed in detail above, at least one portion of the first optical signal may be modified (at420) using a plurality of electro-optic phase adjusters, such as the electro-optic phase adjusters140shown inFIGS. 1A and 1B. An output optical signal may then be formed (at430) by combining the first optical signal, including the at least one modified portion of the first optical signal, with the second optical signal.

By using one or more embodiments of the dynamic gain flattening filter100including electro-optic phase adjusters140, as discussed in detail above, the accuracy, finesse, and control of the dynamic gain flattening filter100may be increased relative to, e.g., thermo-optic phase controllers. For example, a larger number of electro-optic phase adjusters140may be included in a dynamic gain flattening filter100that is formed on a single semiconductor substrate. The range of phase expression of the electro-optic phase adjusters140may also be increased without necessarily requiring a corresponding increase in power consumption of the device. The polarization independence of orthonormal modes of signals propagating in the dynamic gain flattening filter100may also be improved.

Furthermore, the future development of adaptive filter components such as the variable transmissivity apparatus100is, at least in part, likely to be driven by the increasing sophistication of signaling paradigms adopted for use in access and metropolitan networks, as well as transmission backbones. It is anticipated that the current invention, perhaps in conjunction with other developments, foreseen and unforeseen, may permit a much greater range of these applications to be addressed. In particular, the greater finesse and lower power requirements may facilitate the adoption of this approach in highly functional assemblies in access and metropolitan networks, as well as transmission backbones.