Patent Description:
WDM schemes support multiple channels through a light-carrying medium, such as an optical waveguide or an optical fiber. WDM schemes are typically distinguished by the spacing between wavelengths. For example, a "normal" WDM system supports <NUM> channels spaced apart by <NUM> nanometers (nm), a coarse WDM (CWDM) system supports up to eighteen (<NUM>) channels that are spaced apart by <NUM>, and a dense WDM (DWDM) system supports up to eighty (<NUM>) channels that are spaced apart by <NUM>. Due to the wavelength spacing, a CWDM system tends to be more tolerant than a DWDM system and does not require high-precision controlled laser sources. As a result, a CWDM system tends to be less expensive and consumes less power.

<CIT> is directed to a grating type triplexer. The output end of a first grating filter is connected with the input end of a second grating filter sequentially through a first single-mode connection optical waveguide, a first mode filter and a second single-mode connection optical waveguide; after sequentially passing through a third single-mode connection optical waveguide, a curved waveguide and a second mode filter, the output end of the second grating filter is connected with the input end of a third grating filter; the download end of the third grating filter is a third output waveguide; an input waveguide is used as an input port of two channel signals and an output port of one channel signal, a first output waveguide is used as an input port, a second output waveguide is used as an output port, and a third output waveguide is used as an output port; and the first and second mode filters are curved waveguides.

In one embodiment, an optical apparatus comprising an input port configured to receive an optical signal comprising a plurality of wavelengths, and a plurality of output ports. Each output port is configured to output a respective wavelength of the plurality of wavelengths. The optical apparatus further comprises a first plurality of two-mode Bragg gratings in a cascaded arrangement. Each grating of the first plurality of two-mode Bragg gratings is configured to reflect a respective wavelength of the plurality of wavelengths toward a respective output port of the plurality of output ports, and transmit any remaining wavelengths of the plurality of wavelengths. The optical apparatus further comprising: a second plurality of two-mode Bragg gratings, wherein each grating of the second plurality of two-mode Bragg gratings is configured to: receive the respective wavelength reflected by a respective grating of the first plurality of two-mode Bragg gratings; and reflect the respective wavelength toward the respective output port of the plurality of output ports.

To achieve a WDM-based optical transceiver module with a small size, optical multiplexing and demultiplexing (mux/demux) functionality may be implemented in (or integrated with) a photonic integrated circuit (IC) of the optical transceiver module. Low optical losses with the optical mux/demux are preferable to support a lower-power optical communication system. Further, optical mux/demux having flat-top passbands are beneficial to eliminate the temperature control of the laser and will reduce the total power consumption of the optical communication system.

According to embodiments described herein, an optical apparatus comprises an input port configured to receive an optical signal comprising a plurality of wavelengths, and a plurality of output ports, wherein each output port is configured to output a respective wavelength of the plurality of wavelengths. The optical apparatus further comprises a plurality of two-mode Bragg gratings in a cascaded arrangement. Each grating is configured to reflect a respective wavelength of the plurality of wavelengths toward a respective output port, and transmit any remaining wavelengths of the plurality of wavelengths. In some embodiments, the two-mode Bragg gratings are formed in optical waveguides of a silicon photonic chip. The two-mode Bragg gratings may have sidewall corrugation shapes, such as a rectangle shape, a sine shape, or a cosine shape.

Beneficially, using the cascaded arrangement of two-mode Bragg gratings provides the multiplexer and/or the demultiplexer with a relatively flat-top passband, and silicon nitride or silicon oxynitride-based two-mode Bragg gratings can be used to eliminate the temperature control on the laser source and/or to reduce the power consumption of the optical apparatus. Further, the two-mode Bragg gratings may be capable of achieving very low insertion loss, such that the multiplexer and/or the demultiplexer has a low insertion loss, e.g., less than <NUM>-<NUM> decibels (dB). Further, the two-mode Bragg gratings may have much wider passbands and greater fabrication tolerances.

<FIG> is a diagram <NUM> of an exemplary optical apparatus, according to one or more embodiments. In some embodiments, the optical apparatus represents an optical transceiver module integrated into a silicon photonic chip. Other implementations of the optical apparatus are also contemplated.

The optical apparatus comprises a plurality of transmitters <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M (generically, a transmitter <NUM>) that provide optical signals via a respective plurality of optical links <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M (generically, an optical link <NUM>) to a multiplexer <NUM>. In some embodiments, each transmitter <NUM> comprises a laser source generating a respective optical signal (e.g., an unmodulated continuous wave (CW) optical signal) having a respective wavelength. The wavelengths of the optical signals may be selected according to a predefined multiplexing scheme, such as WDM, DWDM, or CWDM. Each transmitter <NUM> may further comprise an optical modulator configured to modulate the respective optical signal, and may further comprise circuitry for further processing of the respective optical signal. In some embodiments, the optical links <NUM> are optical waveguides formed in a silicon photonic chip. In other embodiments, the optical links <NUM> are optical fibers.

The multiplexer <NUM> combines the several optical signals into a multiplexed optical signal that is output onto an optical link <NUM>. In some embodiments, the multiplexer <NUM> comprises a CWDM multiplexer, although implementations using other WDM schemes are also contemplated. In some embodiments, the optical link <NUM> is an optical waveguide formed in the silicon photonic chip. In other embodiments, the optical link <NUM> is an optical fiber.

A demultiplexer <NUM> is communicatively coupled with the multiplexer <NUM> via the optical link <NUM>. The demultiplexer <NUM> demultiplexes the multiplexed optical signal transmitted by the optical link <NUM> into a plurality of optical signals. In some embodiments, the demultiplexer <NUM> comprises a CWDM demultiplexer, although other implementations are also contemplated. The plurality of optical signals is provided from the demultiplexer <NUM> via a respective plurality of optical links <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-N (generically, an optical link <NUM>) to a plurality of receivers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-N (generically, a receiver <NUM>). In some embodiments, the optical links <NUM> are optical waveguides formed in the silicon photonic chip. In other embodiments, the optical links <NUM> are optical fibers. In some embodiments, each receiver <NUM> comprises an optical demodulator to demodulate the respective optical signal, and may further comprise circuitry for further processing of the respective optical signal.

In some embodiments, and as will be discussed in greater detail, the multiplexer <NUM> and/or the demultiplexer <NUM> comprises two-mode Bragg gratings in a cascaded arrangement. Beneficially, using the cascaded arrangement of two-mode Bragg gratings provides the multiplexer <NUM> and/or the demultiplexer <NUM> with a relatively flat-top passband, and may be used to eliminate the temperature control on the laser source of the transmitters <NUM> and/or to reduce the power consumption of the optical apparatus. Further, the two-mode Bragg gratings may be capable of achieving very low insertion loss, such that the multiplexer <NUM> and/or the demultiplexer <NUM> has an insertion loss of less than <NUM>-<NUM> dB.

<FIG> are diagrams <NUM>, <NUM> of exemplary silicon-on-insulator (SOI) based optical waveguides, according to one or more embodiments. The features of the diagrams <NUM>, <NUM> may be used in conjunction with other embodiments. For example, the multiplexer <NUM> and/or demultiplexer <NUM> of <FIG> may be implemented in a silicon photonic chip using the SOI structures illustrated in the diagrams <NUM>, <NUM>.

In some embodiments, a silicon substrate <NUM> comprises a bulk silicon (Si) substrate in which one or more features or materials for active optical device(s) to be produced (e.g., a laser, detector, modulator, absorber) are pre-processed. The thickness of the silicon substrate <NUM> may vary depending on the specific application. For example, the silicon substrate <NUM> may be the thickness of a typical semiconductor wafer (e.g., <NUM>-<NUM> microns), or may be thinned and mounted on another substrate.

The diagrams <NUM>, <NUM> each depict the silicon substrate <NUM>, an insulator layer <NUM> disposed above the silicon substrate <NUM>, and an optical waveguide <NUM> formed in a waveguide layer <NUM> disposed above the insulator layer <NUM>. In some embodiments, the insulator layer <NUM> comprises a buried oxide (BOX) layer formed of silicon dioxide. The thickness of the insulator layer <NUM> may vary depending on the desired application. In some embodiments, the thickness of the insulator layer <NUM> may range from less than one micron to tens of microns. In some embodiments, the waveguide layer <NUM> is formed of elemental Si (e.g., monocrystalline or polycrystalline Si). In other embodiments, the waveguide layer <NUM> may be formed of other suitable semiconductor materials, such as silicon nitride or silicon oxynitride deposited on the insulator layer <NUM>. The thickness of the waveguide layer <NUM> may range from less than <NUM> to greater than a micron. More specifically, the waveguide layer <NUM> may be between <NUM>-<NUM> thick.

In the diagram <NUM>, the optical waveguide <NUM> is formed as a ridge waveguide comprising a ridge <NUM> projecting from a base <NUM>. The ridge waveguide generally confines a propagating optical signal within a portion of the waveguide layer <NUM>. In some embodiments, the waveguide layer <NUM> has a thickness between <NUM>-<NUM> microns. In some embodiments, the width of the ridge <NUM> (as shown, in the left-right direction) is between <NUM>-<NUM> microns. With such dimensioning, the diameter of the optical mode may be <NUM>-<NUM> microns.

As mentioned above, grating patterns may be etched along the sidewalls of the optical waveguide <NUM> to form the two-mode Bragg gratings of the multiplexer <NUM> and/or demultiplexer <NUM>. <FIG> is a diagram <NUM> illustrating exemplary implementations of a two-mode Bragg grating with different sidewall corrugation shapes, according to one or more embodiments. The features of the diagram <NUM> may be used in conjunction with other embodiments. For example, the sidewall gratings may be used by the two-mode Bragg grating to transmit one wavelength and reflect another wavelength of light propagating through the optical waveguide <NUM>.

The diagram <NUM> depicts a mode multiplexer <NUM> and a two-mode Bragg grating <NUM>. A first arm <NUM> of the mode multiplexer <NUM> propagates a fundamental mode <NUM> (e.g., a fundamental transverse electric (TE) mode), which is propagated to the two-mode Bragg grating <NUM>. The two-mode Bragg grating <NUM> comprises sidewalls <NUM>-<NUM>, <NUM>-<NUM> that have a grating pattern with a corrugation period Λ<NUM> and a depth d<NUM>. Although the corrugation shapes of the sidewalls <NUM>-<NUM>, <NUM>-<NUM> are shown as being a rectangle shape, alternate shapes such as a sine shape (as in grating pattern <NUM>), a cosine shape, etc. are also contemplated.

The grating pattern may be formed, e.g., by deep etching into an edge of an optical waveguide to create the periodic grating pattern along the length of the optical waveguide. As shown, the two-mode Bragg grating <NUM> transmits a first mode (e.g., the fundamental mode <NUM>) and reflects a second mode (e.g., a second-order mode <NUM>) to the mode multiplexer <NUM>. A second arm <NUM> of the mode multiplexer <NUM> propagates the second-order mode. Other implementations of mode multiplexers <NUM> are also contemplated, such as asymmetric Y-junction mode multiplexers.

In some embodiments, the grating patterns are formed with a silicon nitride or silicon oxynitride material. For example, the silicon nitride or silicon oxynitride material may be deposited above a silicon oxide layer and the optical waveguide is formed using a dry etching process. Both silicon nitride and silicon oxynitride have thermo-optic coefficients smaller than that of elemental silicon, which results in the two-mode Bragg gratings (and the associated optical apparatus) being less sensitive to temperature variations during operation. In some cases, the lower temperature sensitivity means that no thermal tuning of the optical apparatus is required during operation.

The grating patterns may have any suitable alternate implementation. For example, one or more grating patterns may be formed using a buried grating layer. Further, in cases where the length of a two-mode Bragg grating is sufficiently long (e.g., implemented within an optical fiber), the sidewall gratings may be spaced apart from each other (e.g., at different positions along the length of the first Bragg grating).

<FIG> and <FIG> are diagrams <NUM>, <NUM> of exemplary implementations of a demultiplexer <NUM>, <NUM> with a cascaded arrangement of two-mode Bragg gratings, according to one or more embodiments. The features illustrated in the diagrams <NUM>, <NUM> may be used in conjunction with other embodiments. For example, mode multiplexers and two-mode Bragg gratings included in the demultiplexers <NUM>, <NUM> may be configured as shown in <FIG>.

In the diagram <NUM>, the demultiplexer <NUM> comprises an input port <NUM> and a plurality of two-mode Bragg gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> (which are also referred to herein as "gratings" or "Bragg gratings") in a cascaded arrangement (which may alternately be referred to as a "serial" arrangement). Each grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> reflects a respective wavelength, and transmits any remaining wavelengths. For example, the grating <NUM>-<NUM> reflects a first wavelength via a drop port, and transmits at least one wavelength via an output port to gratings <NUM>-<NUM>, <NUM>-<NUM> that are downstream of the grating <NUM>-<NUM>.

The gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> may have any suitable filter responses for separating the respective wavelength for reflecting. In some embodiments, the gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> are bandpass filters, which may have non-overlapping or partially overlapping passbands. For examples, the gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> may have partially overlapping passbands with a center wavelength and an upper roll-off wavelength selected such that a range of the respective wavelength reflected by the grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> is entirely included between the center wavelength and the upper roll-off wavelength. In other embodiments, the gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> are low-pass filters and may have successively greater roll-off wavelengths.

In some embodiments, the demultiplexer <NUM> further comprises a plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each mode multiplexer of the plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> receives the wavelength reflected by a respective grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each mode multiplexer converts the mode of the reflected wavelength (e.g., a first-order TE mode) into a fundamental TE mode. The plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> may have any suitable implementation, e.g., using on-resonance and off-resonance switching rings. Each mode multiplexer <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> has an output that is coupled with a respective output port <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> of a plurality of output ports <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> of the demultiplexer <NUM>. In other embodiments, the plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> may be omitted, such that the gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> provide the reflected wavelengths (e.g., as a first order or higher mode) directly to the output ports <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>.

Thus, responsive to receiving an optical signal <NUM> comprising a plurality of wavelengths λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM> at the input port <NUM>, the grating <NUM>-<NUM> reflects the wavelength λ<NUM> and transmits the remaining wavelengths λ<NUM>, λ<NUM>, λ<NUM>. The mode multiplexer <NUM>-<NUM> receives the wavelength λ<NUM> and provides the wavelength λ<NUM> (with the mode converted to a fundamental mode) to the output port <NUM>-<NUM> as an optical signal <NUM>-<NUM>. The grating <NUM>-<NUM> receives the wavelengths λ<NUM>, λ<NUM>, λ<NUM>, reflects the wavelength λ<NUM>, and transmits the remaining wavelengths λ<NUM>, λ<NUM>. The mode multiplexer <NUM>-<NUM> receives the wavelength λ<NUM> and provides the wavelength λ<NUM> (with the mode converted to a fundamental mode) to the output port <NUM>-<NUM> as an optical signal <NUM>-<NUM>.

The grating <NUM>-<NUM> receives the wavelengths λ<NUM>, λ<NUM>, reflects the wavelength λ<NUM>, and transmits the remaining wavelength λ<NUM>. The mode multiplexer <NUM>-<NUM> receives the wavelength λ<NUM> and provides the wavelength λ<NUM> (with the mode converted to a fundamental mode) to the output port <NUM>-<NUM> as an optical signal <NUM>-<NUM>. The remaining wavelength λ<NUM> is provided from the grating <NUM>-<NUM> to the output port <NUM>-<NUM> as an optical signal <NUM>-<NUM>.

In the demultiplexer <NUM>, the grating <NUM>-<NUM> represents a "last" grating in the cascaded arrangement of the gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Here, the grating <NUM>-<NUM> reflects a "second-to-last" wavelength (i.e., the wavelength λ<NUM>) of the plurality of wavelengths λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM> toward the output port <NUM>-<NUM>, and transmits a "last" wavelength (i.e., the wavelength λ<NUM>) to the output port <NUM>-<NUM>.

In the diagram <NUM>, the demultiplexer <NUM> comprises the input port <NUM>, the plurality of output ports <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and a cascaded arrangement of the gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> and a grating <NUM>-<NUM>. The operation of the demultiplexer <NUM> is generally similar to that of the demultiplexer <NUM>. However, the grating <NUM>-<NUM> receives the wavelength λ<NUM> from the grating <NUM>-<NUM>, and reflects the wavelength λ<NUM>. The demultiplexer <NUM> further comprises a mode multiplexer <NUM>-<NUM> that receives the wavelength λ<NUM> and provides the wavelength λ<NUM> (with the mode converted to a fundamental mode) to the output port <NUM>-<NUM> as the optical signal <NUM>-<NUM>. In another embodiment, the mode multiplexer <NUM>-<NUM> may be omitted.

In some embodiments, the output of the grating <NUM>-<NUM> (e.g., a transmit port) is coupled with an optical absorber <NUM>. In some embodiments, the optical absorber <NUM> comprises a heavily-doped silicon waveguide. Beneficially, the optical absorber <NUM> mitigates reflections of optical signals, which can further improve the signal-to-noise ratio (SNR) of the optical signal <NUM>-<NUM>.

In the demultiplexer <NUM>, the grating <NUM>-<NUM> represents a "last" grating in the cascaded arrangement of the gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Here, the grating <NUM>-<NUM> reflects a "last" wavelength (i.e., the wavelength λ<NUM>) of the plurality of wavelengths λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM> toward the output port <NUM>-<NUM>.

<FIG> and <FIG> are diagrams <NUM>, <NUM> of exemplary implementations of a demultiplexer <NUM>, <NUM> with mitigated crosstalk.

The features illustrated in the diagrams <NUM>, <NUM> may be used in conjunction with other examples. For example, mode multiplexers and two-mode Bragg gratings included in the demultiplexers <NUM>, <NUM> may be configured as shown in <FIG>.

In the diagram <NUM>, the demultiplexer <NUM> comprises the input port <NUM>, the plurality of output ports <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, a cascaded arrangement of the gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and the plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. The operation of the demultiplexer <NUM> is generally similar to that of the demultiplexer <NUM>, discussed above.

The demultiplexer <NUM> further comprises a second plurality of two-mode Bragg gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> receives a wavelength reflected by a respective grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and reflects the wavelength toward a respective output port <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. The demultiplexer <NUM> further comprises a plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each mode multiplexer of the plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> receives the wavelength reflected by a respective grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each mode multiplexer <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> has an output that is coupled with a respective output port <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. The demultiplexer <NUM> further comprises a plurality of optical absorbers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> has an output coupled with a respective optical absorber <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, each of which may be configured similarly to the optical absorber <NUM>.

In the diagram <NUM>, the demultiplexer <NUM> comprises the input port <NUM>, the plurality of output ports <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, a cascaded arrangement of the gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> and the plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. The operation of the demultiplexer <NUM> is generally similar to that of the demultiplexer <NUM>, discussed above.

The demultiplexer <NUM> further comprises a second plurality of two-mode Bragg gratings <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> receives a wavelength reflected by a respective grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> and reflects the wavelength toward a respective output port <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. The demultiplexer <NUM> further comprises a plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each mode multiplexer of the plurality of mode multiplexers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> receives the wavelength reflected by a respective grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each mode multiplexer <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> has an output that is coupled with a respective output port <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. The demultiplexer <NUM> further comprises a plurality of optical absorbers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each grating <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> has an output coupled with a respective optical absorber <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>.

Although the combination of a mode multiplexer <NUM> with a two-mode Bragg grating <NUM>, as shown in <FIG>, is used in the implementations of the demultiplexer <NUM>, <NUM>, <NUM>, <NUM> to perform a demultiplexing function, it will be noted that the combination of the mode multiplexer <NUM> with the two-mode Bragg grating <NUM> may be used to perform a multiplexing function. As a result, the combination of the mode multiplexer <NUM> with the two-mode Bragg grating <NUM> may be used in implementations of a multiplexer comprising a plurality of two-mode Bragg gratings in a cascaded arrangement.

<FIG> are graphs <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> illustrating operation of the two-mode Bragg gratings as bandpass filters, according to one or more embodiments. The features illustrated in the graphs <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> may be used in conjunction with other embodiments. For example, the cascaded arrangement in any of the demultiplexers <NUM>, <NUM>, <NUM>, <NUM> may have gratings configured as bandpass filters. As discussed above, the gratings may have non-overlapping or partially overlapping passbands.

In the graph <NUM>-<NUM>, the first grating in the cascaded arrangement receives an optical signal comprising a plurality of signal components <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> at a respective plurality of wavelengths λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM>. A filter response <NUM>-<NUM> of the first grating includes a first passband <NUM>-<NUM>, such that the signal component <NUM>-<NUM> (at the wavelength λ<NUM>) is reflected by the first grating. The remaining wavelengths λ<NUM>, λ<NUM>, λ<NUM> (represented as a group <NUM>-<NUM> of the signal components <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>) are transmitted by the first grating to a second grating in the cascaded arrangement.

In the graph <NUM>-<NUM>, the second grating receives the signal components <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> at the respective wavelengths λ<NUM>, λ<NUM>, λ<NUM>. A filter response <NUM>-<NUM> of the second grating includes a second passband <NUM>-<NUM>, such that the signal component <NUM>-<NUM> (at the wavelength λ<NUM>) is reflected by the second grating. The remaining wavelengths λ<NUM>, λ<NUM> (represented as a group <NUM>-<NUM> of the signal components <NUM>-<NUM>, <NUM>-<NUM>) are transmitted by the second grating to a third grating in the cascaded arrangement.

In the graph <NUM>-<NUM>, the third grating receives the signal components <NUM>-<NUM>, <NUM>-<NUM> at the respective wavelengths λ<NUM>, λ<NUM>. A filter response <NUM>-<NUM> of the third grating includes a third passband <NUM>-<NUM>, such that the signal component <NUM>-<NUM> (at the wavelength λ<NUM>) is reflected by the third grating. The remaining wavelength λ<NUM> (represented as a group <NUM>-<NUM> of the signal component <NUM>-<NUM>) is transmitted by the third grating.

The signal component <NUM>-<NUM> (at the wavelength λ<NUM>) is illustrated in the graph <NUM>-<NUM>. In some embodiments, the signal component <NUM>-<NUM> is transmitted by the third grating to an output port. In other embodiments, the signal components <NUM>-<NUM> is reflected by a fourth grating toward the output port. Although the graphs <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> show one sequence of filtering the signal components <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> using the cascaded arrangement, other embodiments may have alternate sequences of filtering the signal components <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>.

<FIG> are graphs <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> illustrating operation of the two-mode Bragg gratings as low-pass filters, according to one or more embodiments. The features illustrated in the graphs <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> may be used in conjunction with other embodiments. For example, the cascaded arrangement in any of the demultiplexers <NUM>, <NUM>, <NUM>, <NUM> may have gratings configured as low-pass filters.

In the graph <NUM>-<NUM>, the first grating in the cascaded arrangement receives the optical signal comprising the plurality of signal components <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. A filter response <NUM>-<NUM> of the first grating includes a first passband <NUM>-<NUM>, such that the signal component <NUM>-<NUM> (at the wavelength λ<NUM>) is reflected by the first grating. The remaining wavelengths λ<NUM>, λ<NUM>, λ<NUM> are transmitted by the first grating to a second grating in the cascaded arrangement.

In the graph <NUM>-<NUM>, the second grating receives the signal components <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. A filter response <NUM>-<NUM> of the second grating includes a second passband <NUM>-<NUM>, such that the signal component <NUM>-<NUM> (at the wavelength λ<NUM>) is reflected by the second grating. The remaining wavelengths λ<NUM>, λ<NUM> are transmitted by the second grating to a third grating in the cascaded arrangement.

In the graph <NUM>-<NUM>, the third grating receives the signal components <NUM>-<NUM>, <NUM>-<NUM>. A filter response <NUM>-<NUM> of the third grating includes a third passband <NUM>-<NUM>, such that the signal component <NUM>-<NUM> (at the wavelength λ<NUM>) is reflected by the third grating. The remaining wavelength λ<NUM> is transmitted by the third grating.

The signal component <NUM>-<NUM> (at the wavelength λ<NUM>) is illustrated in the graph <NUM>-<NUM>. In some embodiments, the signal component <NUM>-<NUM> is transmitted by the third grating to an output port. In other embodiments, the signal components <NUM>-<NUM> is reflected by a fourth grating toward the output port.

As shown, the passbands <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> are all partially overlapping with each other. However, other embodiments may include different combinations of passbands, which may include some passbands that are non-overlapping. For example, the cascaded arrangement may include a combination of one or more gratings configured as low-pass filters and one or more gratings configured as bandpass filters. Further, gratings configured as high-pass filters are also contemplated, whether used in isolation or in combination with other types of filters.

<FIG> is a graph <NUM> illustrating operation of the two-mode Bragg gratings as bandpass filters having partially overlapping passbands, according to one or more embodiments. The features illustrated in the graph <NUM> may be used in conjunction with other embodiments. For example, the cascaded arrangement in any of the demultiplexers <NUM>, <NUM>, <NUM>, <NUM> may have gratings configured as bandpass filters.

The graph <NUM> illustrates the filter responses <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> for the respective gratings. The filter responses <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> include the partially overlapping passbands <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>. Each passband <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> has a respective center wavelength λC0, λC1, λC2, λC3 and a respective upper roll-off wavelength λR0, λR1, λR2, λR3. The center wavelengths λC0, λC1, λC2, λC3 and the upper roll-off wavelengths λR0, λR1, λR2, λR3 are selected such that ranges <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> surrounding the respective wavelengths λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM> are entirely included between the center wavelengths λC0, λC1, λC2, λC3 and the upper roll-off wavelengths λR0, λR1, λR2, λR3. Stated another way, a first grating is designed such that a range <NUM>-<NUM> surrounding a first wavelength λ<NUM> is entirely included between the center wavelength λC0 and the upper roll-off wavelength λR0, a second grating is designed such that a range <NUM>-<NUM> surrounding a second wavelength λ<NUM> is entirely included between the center wavelength λC1 and the upper roll-off wavelength λR1, and so forth. By accommodating the ranges <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> in this manner, the partially overlapping passbands <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> may be spaced closer together to have a greater amount of overlap while maintaining suitable selectivity of the gratings (i.e., to reflect one wavelength but not an adjacent wavelength) in the cascaded arrangement.

In one non-limiting example of a CWDM scheme, four (<NUM>) lanes are defined such that the wavelength λ<NUM> = <NUM>, the wavelength λ<NUM> = <NUM>, the wavelength λ<NUM> = <NUM>, and the wavelength λ<NUM> = <NUM>. Each of the ranges <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> is ± <NUM> of the respective wavelength λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM>, such that the range <NUM>-<NUM> is <NUM> to <NUM> (corresponding to a total range of <NUM>), the range <NUM>-<NUM> is <NUM> to <NUM>, the range <NUM>-<NUM> is <NUM> to <NUM>, and the range <NUM>-<NUM> is <NUM> to <NUM>.

Assume that the center wavelength λC0 = <NUM>, the center wavelength λC1 = <NUM>, the center wavelength λC2 = <NUM>, and the center wavelength λC3 = <NUM> (corresponding to a channel spacing of <NUM>). As each of the gratings has a passband of <NUM>, the upper roll-off wavelength λR0 = <NUM>, the upper roll-off wavelength λR1 = <NUM>, the upper roll-off wavelength λR2 = <NUM>, and the upper roll-off wavelength λR3 = <NUM>.

In this way, the range <NUM>-<NUM> (<NUM> to <NUM>) is entirely included between the center wavelength λC0 (<NUM>) and the upper roll-off wavelength λR0 (<NUM>) for the first grating, the range <NUM>-<NUM> (<NUM> to <NUM>) is entirely included between the center wavelength λC1 (<NUM>) and the upper roll-off wavelength λR1 (<NUM>) for the second grating, and so forth.

Beneficially, by configuring the gratings to provide the passbands <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> (<FIG>, <FIG>) and the passbands <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> (<FIG>) as relatively wide and flat-top passbands with a steep edge spectrum response, the demultiplexer tends to have greater tolerance for fabrication variations, material layer thickness variations, and/or temperature variations. Using a silicon nitride or silicon oxynitride material for the gratings further increases the tolerance for these variations.

While the above example is discussed in terms of partially overlapping passbands <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> for bandpass filters, similar techniques may be used to space the passbands of low-pass filters closer together while maintaining a suitable selectivity. For example, the cut-off wavelengths of the low-pass filters may be selected such that a minimum margin (e.g., <NUM>) exists between the cut-off wavelength for a grating and the range surrounding the particular wavelength of the optical signal to be reflected by the grating.

Further, while the demultiplexers <NUM>, <NUM>, <NUM>, <NUM> have been depicted as a <NUM>-to-<NUM> (<NUM>:<NUM>) demultiplexer having three (<NUM>) or four (<NUM>) two-mode Bragg gratings in a cascaded arrangement, other configurations of the demultiplexers <NUM>, <NUM>, <NUM>, <NUM> are also contemplated. For example, the demultiplexer <NUM> may include a larger or smaller number of Bragg gratings in the cascaded arrangement, different filter responses for the gratings, and so forth.

<FIG> illustrates a method <NUM> of demultiplexing using a cascaded arrangement of two-mode Bragg gratings, according to one or more embodiments. The method <NUM> may be used in conjunction with other embodiments, e.g., performed using any of the demultiplexers <NUM>, <NUM>, <NUM>, <NUM> described above.

The method <NUM> begins at block <NUM>, where the demultiplexer receives, at an input port, an optical signal comprising a plurality of wavelengths. At block <NUM>, a respective wavelength is reflected using a two-mode Bragg grating of a cascaded arrangement. At block <NUM>, the mode of the reflected wavelength is converted into a fundamental mode. In some embodiments, the conversion is performed using a mode multiplexer arranged at a drop port of the two-mode Bragg grating.

At block <NUM>, any remaining wavelengths are transmitted using the two-mode Bragg grating. The method <NUM> returns from block <NUM> to block <NUM> for each of the two-mode Bragg gratings of the cascaded arrangement. In some embodiments, a last wavelength is reflected by a last grating of the cascaded arrangement. In other embodiments, a last grating reflects a second-to-last wavelength and transmits the last wavelength. At block <NUM>, individual wavelengths are output at respective output ports. The method <NUM> ends following completion of block <NUM>.

In the preceding, reference is made to embodiments presented in this disclosure. However, the scope of the present disclosure is not limited to specific described embodiments. Instead, any combination of the described features and elements, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the preceding aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s).

Aspects of the present disclosure are described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments presented in this disclosure.

The flowchart and block diagrams in the Figures illustrate the architecture, functionality and operation of possible implementations of systems, methods and computer program products according to various embodiments.

Claim 1:
An optical apparatus (<NUM>, <NUM>) comprising:
an input port (<NUM>) configured to receive an optical signal comprising a plurality of wavelengths;
a plurality of output ports (<NUM>), wherein each output port is configured to output a respective wavelength of the plurality of wavelengths; and
a first plurality of two-mode Bragg gratings (<NUM>) in a cascaded arrangement,
wherein each grating of the first plurality of two-mode Bragg gratings (<NUM>) is configured to:
reflect a respective wavelength of the plurality of wavelengths toward a respective output port of the plurality of output ports (<NUM>); and
transmit any remaining wavelengths of the plurality of wavelengths,
the optical apparatus further comprising:
a second plurality of two-mode Bragg gratings (<NUM>), wherein each grating of the second plurality of two-mode Bragg gratings (<NUM>) is configured to:
receive the respective wavelength reflected by a respective grating of the first plurality of two-mode Bragg gratings (<NUM>); and
reflect the respective wavelength toward the respective output port of the plurality of output ports (<NUM>).