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 laser sources. As a result, a CWDM system tends to be less expensive and consumes less power.

<CIT> describes, according to its abstract, an integrated optical device having a first and a second integrated waveguide; a section of the first waveguide and a section of the second waveguide arranged so as to be in optical coupling relationship. A first and a second modulated refractive index structures are respectively formed along the first waveguide section and the second waveguide section. Each modulated refractive index structure has at least one pair of regions of mutually different refractive index, adjacent to each other along the respective waveguide section. The regions of mutually different refractive index have a portion of the respective waveguide section and a gap formed in the waveguide section. The refractive indexes of the regions differ from each other by least approximately <NUM>%. The device can be used for optical multiplexers/demultiplexers, particularly for wavelength division multiplexing optical communications.

<CIT> describes, according to its abstract, optical systems including an optical switching device generally configured to control signal characteristic profiles over the pluralities of signal channels, or wavelengths, to provide desired signal characteristic profiles at the output ports of the device. Various signal characteristics that can be controlled include power level, cross-talk, optical signal to noise ratio, etc. The optical switching devices can include balanced demultiplexer/multiplexer combinations and switches that provide for uniform optical loss through the devices. In addition, low extinction ratio switches can be configured to provide higher extinction ratios.

The invention to which the present European patent relates is defined in the appended claims.

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, a multiplexer and/or demultiplexer comprises a plurality of Bragg gratings arranged in a plurality of stages. Using a WDM demultiplexer as an example, a first Bragg grating of a first stage is used to transmit a first two wavelengths and to reflect a second two wavelengths of a multiplexed optical signal. A second Bragg grating of a second stage transmits one of the first two wavelengths to a first receiver, and reflects the other of the first two wavelengths to a second receiver. A third Bragg grating of the second stage transmits one of the second two wavelengths to a third receiver, and reflects the other of the second two wavelengths to a fourth receiver. In some embodiments, the plurality of Bragg gratings is formed in optical waveguides of a silicon photonic chip. In some embodiments, the first Bragg grating comprises different sidewall corrugation periods, where each sidewall grating reflects a respective wavelength.

Beneficially, using the multiple stages of Bragg gratings provides the multiplexer and/or the demultiplexer with a relatively flat-top passband, and may be used to eliminate the temperature control on the laser source and/or to reduce the power consumption of the optical apparatus. Further, the 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> decibel (dB). Further, using the multiple stages of Bragg gratings allows the Bragg gratings of the second stage to have much wider passbands and greater fabrication tolerance.

<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 are 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 respective Bragg gratings that are arranged in multiple stages. Beneficially, using the multiple stages of 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, 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> dB.

<FIG> is a diagram <NUM> of an exemplary demultiplexer <NUM> with multiple stage Bragg gratings, according to one or more embodiments. In the diagram <NUM>, the demultiplexer <NUM> is communicatively coupled with four (<NUM>) receivers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> via respective optical links. Each of the receivers <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> may comprise an optical demodulator and/or circuitry for further processing of the received optical signal having a respective wavelength. As shown, the receiver <NUM>-<NUM> receives a first optical signal having a wavelength λ<NUM>, the receiver <NUM>-<NUM> receives a second optical signal having a wavelength λ<NUM>, the receiver <NUM>-<NUM> receives a third optical signal having a wavelength λ<NUM>, and the receiver <NUM>-<NUM> receives a fourth optical signal having a wavelength λ<NUM>.

The demultiplexer <NUM> comprises a first Bragg grating <NUM>-<NUM>, which comprises an input port IN, a drop port DROP, and an output port OUT. The input port IN is coupled with the optical link <NUM>. The drop port DROP is coupled with a second Bragg grating <NUM>-<NUM>, and the output port OUT is coupled with a third Bragg grating <NUM>-<NUM>.

The demultiplexer <NUM> further comprises a second Bragg grating <NUM>-<NUM>, which comprises an input port IN, an add port ADD, and an output port OUT. The input port IN is directly coupled with the drop port DROP of the first Bragg grating <NUM>-<NUM>. The drop port DROP is coupled with the receiver <NUM>-<NUM>, and the output port OUT is coupled with the receiver <NUM>-<NUM>.

The demultiplexer <NUM> further comprises a third Bragg grating <NUM>-<NUM>, which comprises an input port IN, an add port ADD, and an output port OUT. The input port IN is directly coupled with the output port OUT of the first Bragg grating <NUM>-<NUM>. The drop port DROP is coupled with the receiver <NUM>-<NUM>, and the output port OUT is coupled with the receiver <NUM>-<NUM>. In some embodiments, each of the first Bragg grating <NUM>-<NUM>, the second Bragg grating <NUM>-<NUM>, and the third Bragg grating <NUM>-<NUM> comprises a two-mode Bragg grating.

The first Bragg grating <NUM>-<NUM> is included in a first stage <NUM>-<NUM>, in which the four wavelengths λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM> carried on a single optical signal over the optical link <NUM> are demultiplexed onto two optical signals: a first optical signal comprising a first two wavelengths λ<NUM>, λ<NUM>, and a second optical signal comprising a second two wavelengths λ<NUM>, λ<NUM>. The first optical signal is output from the drop port DROP of the first Bragg grating <NUM>-<NUM>, and the second optical signal is output from the output port OUT of the first Bragg grating <NUM>-<NUM>. The second Bragg grating <NUM>-<NUM> and the third Bragg grating <NUM>-<NUM> are included in a second stage <NUM>-<NUM>, in which the first two wavelengths λ<NUM>, λ<NUM> are demultiplexed into individual wavelengths, and the second two wavelengths λ<NUM>, λ<NUM> are demultiplexed into individual wavelengths.

In some embodiments, the first two wavelengths λ<NUM>, λ<NUM> correspond to a first two passbands that are non-overlapping with each other, and the second two wavelengths λ<NUM>, λ<NUM> correspond to a second two passbands that are non-overlapping with each other. Beneficially, using wavelengths that have non-overlapping passbands for the first stage Bragg gratings (here, the first Bragg grating <NUM>-<NUM>) improves the SNR at each wavelength that is output from the first stage Bragg grating.

In some embodiments, one of the second two passbands occurs between the first two passbands. For example, according to a CWDM scheme, the first two passbands may be respectively centered at <NUM> and <NUM>, and the second two passbands may be respectively centered at <NUM> (occurring between the first two passbands) and <NUM>.

The input port IN of the second Bragg grating <NUM>-<NUM> receives the wavelengths λ<NUM>, λ<NUM>. As shown, the input port IN is directly coupled with the drop port DROP of the first Bragg grating <NUM>-<NUM>. The second Bragg grating <NUM>-<NUM> demultiplexes the wavelengths λ<NUM>, λ<NUM> into a first optical signal having the wavelength λ<NUM>, and a second optical signal having the wavelength λ<NUM>.

The drop port DROP of the second Bragg grating <NUM>-<NUM> is coupled with the receiver <NUM>-<NUM>, and the first optical signal having the wavelength λ<NUM> is provided from the drop port DROP to the receiver <NUM>-<NUM>. The output port OUT of the second Bragg grating <NUM>-<NUM> is coupled with the receiver <NUM>-<NUM>, and the second optical signal having the wavelength λ<NUM> is provided from the output port OUT to the receiver <NUM>-<NUM>.

The input port IN of the third Bragg grating <NUM>-<NUM> receives the wavelengths λ<NUM>, λ<NUM>. As shown, the input port IN is directly coupled with the output port OUT of the first Bragg grating <NUM>-<NUM>. The third Bragg grating <NUM>-<NUM> demultiplexes the wavelengths λ<NUM>, λ<NUM> into a third optical signal having the wavelength λ<NUM>, and a fourth optical signal having the wavelength λ<NUM>.

The drop port DROP of the third Bragg grating <NUM>-<NUM> is coupled with the receiver <NUM>-<NUM>, and the third optical signal having the wavelength λ<NUM> is provided from the drop port DROP to the receiver <NUM>-<NUM>. The output port OUT of the third Bragg grating <NUM>-<NUM> is coupled with the receiver <NUM>-<NUM>, and the fourth optical signal having the wavelength λ<NUM> is provided from the output port OUT to the receiver <NUM>-<NUM>.

The first Bragg grating <NUM>-<NUM>, the second Bragg grating <NUM>-<NUM>, and the third Bragg grating <NUM>-<NUM> may have any suitable implementation. In some embodiments, the first Bragg grating <NUM>-<NUM>, the second Bragg grating <NUM>-<NUM>, and the third Bragg grating <NUM>-<NUM> are formed in a waveguide layer comprising a semiconductor material. In some embodiments, the first Bragg grating <NUM>-<NUM>, the second Bragg grating <NUM>-<NUM>, and the third Bragg grating <NUM>-<NUM> comprises asymmetric sidewall gratings.

While the demultiplexer <NUM> has been described as a <NUM>-to-<NUM> (<NUM>:<NUM>) demultiplexer having two stages <NUM>-<NUM>, <NUM>-<NUM> of Bragg gratings, other configurations of the demultiplexer <NUM> are also contemplated. For example, the demultiplexer <NUM> may include a larger or smaller number of Bragg gratings, a larger or smaller number of stages, and so forth.

<FIG> is a graph <NUM> illustrating operation of exemplary Bragg gratings, according to one or more embodiments. The features of the graph <NUM> may be used in conjunction with other embodiments, for example, representing the output spectrum of the first Bragg grating <NUM>-<NUM> of <FIG>.

In the graph <NUM>, a plot <NUM> represents the first optical signal output from the drop port DROP of the first Bragg grating <NUM>-<NUM>, and the plot <NUM> represents the second optical signal output from the output port OUT of the first Bragg grating <NUM>-<NUM>. The first optical signal comprises a first passband <NUM> centered at a first wavelength λ<NUM> (here, <NUM>), and a second passband <NUM> centered at a second wavelength λ<NUM> (here, <NUM>). The second optical signal comprises a third passband <NUM> centered at a third wavelength λ<NUM> (here, <NUM>), and a fourth passband <NUM> centered at a fourth wavelength λ<NUM> (here, <NUM>).

As shown, the first two wavelengths λ<NUM>, λ<NUM> correspond to a first two passbands <NUM>, <NUM> that are non-overlapping with each other, and the second two wavelengths λ<NUM>, λ<NUM> correspond to a second two passbands <NUM>, <NUM> that are non-overlapping with each other. Beneficially, using wavelengths that have non-overlapping passbands for the first stage Bragg gratings (here, the first Bragg grating <NUM>-<NUM>) improves the SNR at each wavelength that is output from each first stage Bragg grating. For example, use of the non-overlapping passbands supports a wider passband for the Bragg gratings of the second stage <NUM>-<NUM>, which makes the different optical signals at the different wavelengths easier to distinguish, and is more tolerant of fabrication processes.

Further, as shown, one of the second two passbands (here, the passband <NUM>) occurs between the first two passbands <NUM>, <NUM> (e.g., an alternating arrangement of the passbands <NUM>, <NUM>, <NUM>, <NUM>). However, other implementations of the first Bragg grating <NUM>-<NUM> may have a different arrangement of the passbands <NUM>, <NUM>, <NUM>, <NUM>. For example, the first optical signal and/or the second optical signal may have two overlapping passbands, the passbands <NUM>, <NUM>, <NUM>, <NUM> may be non-alternating, and so forth.

<FIG> is a diagram <NUM> illustrating Bragg gratings with different sidewall corrugation periods, 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 first Bragg grating <NUM>-<NUM> of <FIG> to pass two wavelengths and reflect two wavelengths of light <NUM> propagating through an optical waveguide.

In the diagram <NUM>, a first sidewall <NUM>-<NUM> has a first grating pattern with a first corrugation period Λ<NUM> and a first depth d<NUM>, and a second sidewall <NUM>-<NUM> has a second grating pattern with a second corrugation period Λ<NUM> and a second depth d<NUM>. The first grating pattern and the second grating pattern may be formed, e.g., by deep etching into an edge of an optical waveguide to create the periodic grating patterns along the length of the optical waveguide. In this way, the first sidewall <NUM>-<NUM> is dimensioned to reflect a particular wavelength, and the second sidewall <NUM>-<NUM> is dimensioned to reflect another wavelength. Using <FIG> as an example, the first sidewall <NUM>-<NUM> may reflect the first wavelength λ<NUM> and the second sidewall <NUM>-<NUM> may reflect the second wavelength λ<NUM>.

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 the first Bragg grating <NUM>-<NUM> 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). In some embodiments, the first Bragg grating <NUM>-<NUM> comprises more than two sidewall gratings to reflect more than two wavelengths.

<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. The thickness of the waveguide layer may range from less than <NUM> to greater than a micron. More specifically, the waveguide layer 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 discussed above, grating patterns may be etched along the sidewalls of the optical waveguide <NUM> to form the Bragg gratings of the multiplexer <NUM> and/or demultiplexer <NUM>.

<FIG> are graphs <NUM>, <NUM> illustrating operation of exemplary Bragg gratings, according to one or more embodiments. The features of the graphs <NUM>, <NUM> may be used in conjunction with other embodiments. For example, the graph <NUM> represents the output spectrum of the second Bragg grating <NUM>-<NUM> of <FIG>, and the graph <NUM> represents the output spectrum of the third Bragg grating <NUM>-<NUM>.

In the graph <NUM>, a plot <NUM> represents the first optical signal output from the drop port DROP of the second Bragg grating <NUM>-<NUM>, and the plot <NUM> represents the second optical signal output from the output port OUT of the second Bragg grating <NUM>-<NUM>. The first optical signal comprises a first passband <NUM> centered at a first wavelength λ<NUM> (here, <NUM>), and a second passband <NUM> that includes a second wavelength λ<NUM> (here, <NUM>).

In the graph <NUM>, a plot <NUM> represents the first optical signal output from the drop port DROP of the third Bragg grating <NUM>-<NUM>, and the plot <NUM> represents the second optical signal output from the output port OUT of the third Bragg grating <NUM>-<NUM>. The first optical signal comprises a first passband <NUM> centered at a third wavelength λ<NUM> (here, <NUM>), and a fourth passband <NUM> that includes a fourth wavelength λ<NUM> (here, <NUM>).

<FIG> is a diagram <NUM> of an exemplary demultiplexer <NUM> with multiple stage Bragg gratings, according to one or more embodiments. More specifically, the demultiplexer <NUM> comprises a fourth Bragg grating <NUM>-<NUM> in the first stage <NUM>-<NUM>. In some embodiments, the fourth Bragg grating <NUM>-<NUM> comprises a two-mode Bragg grating. The fourth Bragg grating <NUM>-<NUM> comprises an input port IN coupled with the drop port DROP of the first Bragg grating <NUM>-<NUM>. The fourth Bragg grating <NUM>-<NUM> further comprises a drop port DROP coupled with the input port IN of the second Bragg grating <NUM>-<NUM>.

The input port IN of the fourth Bragg grating <NUM>-<NUM> receives a first optical signal comprising the wavelengths λ<NUM>, λ<NUM>. A second optical signal is output from the drop port DROP of the fourth Bragg grating <NUM>-<NUM>, the second optical signal comprising the wavelengths λ<NUM>, λ<NUM>. Beneficially, including the fourth Bragg grating <NUM>-<NUM> helps to mitigate a crosstalk susceptibility of the demultiplexer <NUM>. The fourth Bragg grating <NUM>-<NUM> further comprises an output port OUT, which according to the claimed invention is coupled with an optical absorber <NUM>. Beneficially, the optical absorber <NUM> mitigates reflections of optical signals, which can further improve the SNR of the second optical signal output from the drop port DROP.

<FIG> is a graph <NUM> illustrating operation of exemplary Bragg gratings, according to one or more embodiments. The features of the graph <NUM> may be used in conjunction with other embodiments, for example, representing the output spectrum of the Bragg gratings of the first stage <NUM>-<NUM> (i.e., the first Bragg grating <NUM>-<NUM> and the fourth Bragg grating <NUM>-<NUM>) of <FIG>.

In the graph <NUM>, a plot <NUM> represents a first optical signal output from the drop port DROP of the fourth Bragg grating <NUM>-<NUM>, and the plot <NUM> represents the second optical signal output from the output port OUT of the first Bragg grating <NUM>-<NUM>. The first optical signal comprises a first passband <NUM> centered at the first wavelength λ<NUM> (here, <NUM>), and a second passband <NUM> centered at the second wavelength λ<NUM> (here, <NUM>). The second optical signal comprises a third passband <NUM> centered at the third wavelength λ<NUM> (here, <NUM>), and a fourth passband <NUM> centered at the fourth wavelength λ<NUM> (here, <NUM>).

As shown, the output spectrum illustrated by the graph <NUM> indicates a mitigated crosstalk susceptibility of the demultiplexer <NUM>. For example, the graph <NUM> indicates a susceptibility beyond about -<NUM> dB to -<NUM> dB, compared with a susceptibility beyond about -<NUM> dB to -<NUM> dB as in the graph <NUM> of <FIG>.

<FIG> illustrates a method <NUM> of demultiplexing using a plurality of Bragg gratings, according to one or more embodiments. The method <NUM> may be performed in conjunction with other embodiments, e.g., using the demultiplexer <NUM> of <FIG>, and <FIG>.

The method <NUM> begins at block <NUM>, where an optical signal is received comprising a plurality of wavelengths. At block <NUM>, a first Bragg grating (e.g., the first Bragg grating <NUM>-<NUM> of <FIG>, <FIG>) is used to output (i) a first optical signal comprising a first wavelength and a second wavelength of the plurality of wavelengths, and (ii) a second optical signal comprising a third wavelength and a fourth wavelength of the plurality of wavelengths.

At block <NUM>, a second Bragg grating (e.g., the third Bragg grating <NUM>-<NUM> of <FIG>, <FIG>) is used to output (i) a third optical signal comprising the first wavelength to a first receiver, and (ii) a fourth optical signal comprising the second wavelength to a second receiver.

At block <NUM>, a fourth Bragg grating (e.g., the fourth Bragg grating <NUM>-<NUM> of <FIG>, <FIG>) is used to output (i) a sixth optical signal comprising the third wavelength, and (ii) a seventh optical signal comprising the fourth wavelength. At block <NUM>, a third Bragg grating (e.g., the second Bragg grating <NUM>-<NUM> of <FIG>, <FIG>) is used to output (i) a fifth optical signal comprising the third wavelength to a third receiver, and (ii) a sixth optical signal comprising the fourth wavelength to a fourth receiver. The method <NUM> ends following completion of the block <NUM>.

<FIG> is a diagram <NUM> of an exemplary multiplexer <NUM> with multiple stage Bragg gratings, according to one or more embodiments useful to understand the claimed invention. The features of the diagram <NUM> may be used in conjunction with other embodiments.

In the diagram <NUM>, the multiplexer <NUM> is communicatively coupled with four (<NUM>) transmitters <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> via respective optical links. Each of the transmitters <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> may comprise a respective laser source generating a respective optical signal having a respective wavelength, as well as a modulator. As shown, the transmitter <NUM>-<NUM> outputs a first optical signal having a wavelength λ<NUM>, the transmitter <NUM>-<NUM> outputs a second optical signal having a wavelength λ<NUM>, the transmitter <NUM>-<NUM> outputs a third optical signal having a wavelength λ<NUM>, and the transmitter <NUM>-<NUM> outputs a fourth optical signal having a wavelength λ<NUM>.

The multiplexer <NUM> comprises a first Bragg grating <NUM>-<NUM>, which comprises an input port IN, an add port ADD, and an output port OUT. The input port IN is coupled with the transmitter <NUM>-<NUM>, and the add port ADD is coupled with the transmitter <NUM>-<NUM>. The output port OUT outputs a first optical signal comprising the wavelengths λ<NUM>, λ<NUM>.

The multiplexer <NUM> further comprises a second Bragg grating <NUM>-<NUM>, which comprises an input port IN, an add port ADD, and an output port OUT. The input port IN is coupled with the transmitter <NUM>-<NUM>, and the add port ADD is coupled with the transmitter <NUM>-<NUM>. The output port OUT outputs a second optical signal comprising wavelengths λ<NUM>, λ<NUM>.

The first Bragg grating <NUM>-<NUM> and the second Bragg grating <NUM>-<NUM> are included in a first stage <NUM>-<NUM>, in which the four wavelengths λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM> carried on four separate optical signals are multiplexed onto two optical signals: a first optical signal comprising a first two wavelengths λ<NUM>, λ<NUM>, and a second optical signal comprising a second two wavelengths λ<NUM>, λ<NUM>. In some embodiments, the first two wavelengths λ<NUM>, λ<NUM> correspond to a first two passbands that are non-overlapping with each other, and the second two wavelengths λ<NUM>, λ<NUM> correspond to a second two passbands that are non-overlapping with each other. Beneficially, using wavelengths that have non-overlapping passbands for the first stage Bragg gratings (i.e., the first Bragg grating <NUM>-<NUM> and the second Bragg grating <NUM>-<NUM>) improves the SNR at each wavelength that is output from each first stage Bragg grating.

The multiplexer <NUM> further comprises a third Bragg grating <NUM>-<NUM>, which comprises an input port IN, an add port ADD, and an output port OUT. The input port IN receives the wavelengths λ<NUM>, λ<NUM>. As shown, the input port IN is directly coupled with the output port OUT of the second Bragg grating <NUM>-<NUM>. The add port ADD receives the wavelengths λ<NUM>, λ<NUM>. As shown, the add port ADD is directly coupled with the output port OUT of the first Bragg grating <NUM>-<NUM>.

The output port OUT of the third Bragg grating <NUM>-<NUM> outputs a third optical signal comprising the wavelengths λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM> onto the optical link <NUM>. Thus, the third Bragg grating <NUM>-<NUM> is included in a second stage <NUM>-<NUM>, in which the four wavelengths λ<NUM>, λ<NUM>, λ<NUM>, λ<NUM> carried on two separate optical signals are multiplexed onto a single optical signal that is output onto the optical link <NUM>.

While the multiplexer <NUM> has been described as a <NUM>-to-<NUM> (<NUM>:<NUM>) multiplexer having two stages <NUM>-<NUM>, <NUM>-<NUM> of Bragg gratings, other configurations of the multiplexer <NUM> are also contemplated. For example, the multiplexer <NUM> may include a larger or smaller number of Bragg gratings, a larger or smaller number of stages, and so forth.

<FIG> is a diagram <NUM> of an exemplary multiplexer <NUM> with multiple stage Bragg gratings, according to one or more embodiments. The features of the diagram <NUM> may be used in conjunction with other embodiments.

In the diagram <NUM>, the multiplexer <NUM> further comprises a fourth Bragg grating <NUM>-<NUM> in the second stage <NUM>-<NUM>, which comprises an input port IN, an add port ADD, and an output port OUT. As shown, the input port IN of the fourth Bragg grating <NUM>-<NUM> is directly coupled with the output port OUT of the second Bragg grating <NUM>-<NUM>. The output port OUT of the fourth Bragg grating <NUM>-<NUM> is directly coupled with the add port ADD of the third Bragg grating <NUM>-<NUM>.

Beneficially, including the fourth Bragg grating <NUM>-<NUM> helps to mitigate a crosstalk susceptibility of the multiplexer <NUM>. According to the claimed invention, an optical absorber <NUM> is coupled with the add port ADD of the fourth Bragg grating <NUM>-<NUM>. Beneficially, the optical absorber <NUM> mitigates reflections of optical signals, which can further improve the SNR of the second optical signal output from the output port OUT of the fourth Bragg grating <NUM>-<NUM>.

<FIG> illustrates a method <NUM> of multiplexing using a plurality of Bragg gratings, according to one or more embodiments. The method <NUM> may be performed in conjunction with other embodiments, e.g., using the multiplexer <NUM> of <FIG>, <FIG>, and <FIG>.

The method <NUM> begins at block <NUM>, where a plurality of optical signals are received from a plurality of transmitters. At block <NUM>, a first Bragg grating (e.g., the first Bragg grating <NUM>-<NUM> of <FIG>, <FIG>) is used to output a first optical signal comprising a first two wavelengths from a first two optical signals of the plurality of optical signals. At block <NUM>, a second Bragg grating (e.g., the second Bragg grating <NUM>-<NUM> of <FIG>, <FIG>) is used to output a second optical signal comprising a second two wavelengths from a second two optical signals of the plurality of optical signals.

At block <NUM>, a fourth Bragg grating (e.g., the fourth Bragg grating <NUM>-<NUM> of <FIG>) is used to output a fourth optical signal comprising the first two wavelengths. In some embodiments, the output port of the first Bragg grating is directly coupled to the fourth Bragg grating. At block <NUM>, a third Bragg grating (e.g., the third Bragg grating <NUM>-<NUM> of <FIG>, <FIG>) is used to output a third optical signal comprising the first two wavelengths and the second two wavelengths. The method <NUM> ends following completion of block <NUM>.

Aspects of the present disclosure are described with reference to flowchart illustrations and/or block diagrams of methods.

Claim 1:
An optical apparatus (<NUM>) comprising:
a plurality of transmitters (<NUM>);
a multiplexer (<NUM>) comprising:
a first Bragg grating (<NUM>-<NUM>) having a first input port coupled with a first transmitter (<NUM>-<NUM>) of the plurality of transmitters, a first add port coupled with a second transmitter (<NUM>-<NUM>) of the plurality of transmitters, and a first output port configured to output a first optical signal comprising a first two wavelengths;
a second Bragg grating (<NUM>-<NUM>) having a second input port coupled with a third transmitter (<NUM>-<NUM>) of the plurality of transmitters, a second add port coupled with a fourth transmitter (<NUM>-<NUM>) of the plurality of transmitters, and a second output port configured to output a second optical signal comprising a second two wavelengths; and
a third Bragg grating (<NUM>-<NUM>) having a third input port configured to receive the first two wavelengths, a third add port configured to receive the second two wavelengths, and a third output port configured to output a third optical signal comprising the first two wavelengths and the second two wavelengths;
characterized in that the multiplexer further comprises
a fourth Bragg grating (<NUM>-<NUM>) directly coupled with the first output port, wherein a fourth output port of the fourth Bragg grating is directly coupled with one of the third input port and the third add port; and
an optical absorber (<NUM>) coupled with a fourth add port of the fourth Bragg grating.