Patent Description:
Broad-band optical communications typically require high-speed electro-optical modulators (EOM) to modulate light at a desired data rate. One common type of a broad-band EOM is a Mach-Zehnder modulator (MZM), which uses an optical waveguide structure forming an Mach-Zehnder interferometer (MZI), with RF-driven optical phase modulators in each waveguide arm. The waveguide arms of the MZM are typically formed in a suitable semiconductor or other electro-optic material such as LiNbOs, so that light propagation in the waveguide arms may be controlled by applying a voltage. Such a waveguide modulator may be implemented in an opto-electronic chip as a photonic integrated circuit (PIC).

A silicon photonics (SiP) platform, e.g., based on a Silicon-on-Insulator (SOI) technology, may be particularly attractive for implementing broad-band EOMs, which can, e.g., enable a natural integration with CMOS-based high-speed electronic drivers. In the SiP platform, voltage-controlled optical phase modulators may be formed by incorporating pin junctions into optical waveguides, such as a waveguide arm of an MZM. An electrode extending along a length of the optical waveguide in an electrical contact with the pin junction may be used to either inject electrical charge carriers (forward bias) or deplete carriers (reverse bias) in the optical waveguide core, thereby modulating the effective refractive index of the waveguide. At modulation rates in the range of tens of gigabits per second (Gb/s), electrical transmission lines formed with properly terminated travelling-wave (TW) electrodes are typically used.

The greater is the length of the electrode that interacts with light in the optical waveguide, the lower is the drive voltage needed for a same optical modulation depth. Thus in TW EOMs there is a tradeoff between required drive voltage and bandwidth, where making the length where a modulating radio frequency (RF) signal interacts with an optical signal longer may generally rise the modulator power efficiency, but at the cost of a decrease in the transmitter bandwidth and an increase in optical loss.

Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for providing high-bandwidth optical waveguide modulators.

<CIT> discloses a multi-section optical modulator and related method wherein two waveguide arms traverse a plurality of successive modulating sections. Specifically, a differential drive signal is applied separately to each waveguide arm of each modulating sections in synchronism with the transmission of light along the waveguide arms, effecting a dual differential driving of each section. By suitably selecting the number of modulating sections and the section length, a high modulation bandwidth and a high modulation efficiency may be achieved simultaneously for a given peak-to-peak voltage swing of the drive signal.

<CIT> discloses an optical modulator formed to include an adjustable drive arrangement for dynamically adjusting the effective length of the optical signals path(s) within the modulator. In particular, each modulator arm is partitioned into a plurality of segments, with each segment coupled to a separate electrical signal driver. Therefore, the effective length of each modulator arm will be a function of the number of drivers that are activated for each arm at any given point in time. A feedback arrangement may be used with the plurality of drivers to dynamically adjust the operation of the modulator by measuring the extinction ratio as a function of optical power, turning "on" of "off" individual drivers accordingly.

<CIT> discloses an optical modulator comprises K pieces of drive circuits S1 and S2 driving an optical modulator by one or more modulation electric signals, and a pair of optical waveguide arms comprises N pieces of areas equipped with a joint configured to include an area of a first doping polarity and an area of a second doping polarity opposite the first doping polarity. K pieces of the drive circuits are electrically connected to N pieces of the areas. In M pieces of areas out of N pieces of the areas, the optical waveguide of one arm is configured such that a side opposite other arm is the area of the first doping polarity and other arm side is the area of the second doping polarity, and the optical waveguide of the other arm is configured such that a side opposite the one arm is the area of the first doping polarity, and the one arm side is a first area serving as the area of the second doping polarity.

Some embodiments may implement a bandwidth/drive-voltage tradeoff in an TW EOM by incorporating multiple TW modulation segments along an individual optical waveguide of a EOM device. By combining the modulation response of multiple TW modulation segments in a cumulative way, e.g., a relatively low drive voltage can be used. While the modulation bandwidth of such devices may be increased by using a sequence of TW modulation segments along an individual optical waveguide, performance shortcomings may be associated with this approach. For example, it may be difficult to distribute well-timed RF drive signals to a sequence of spatially distinct regions of the optical waveguide. Furthermore, the quality of the RF drive signal delivered to later segments may be degraded, due to e.g. a frequency dependent loss resulting from the longer travel distance. Some of the embodiments herein may address such performance issues for optical modulators with segmented TW drive electrodes.

Accordingly, an aspect of the present disclosure relates to an apparatus comprising a segmented optical modulator, the segmented optical modulator comprising an optical waveguide along a planar surface of a substrate, a plurality of electrodes disposed along the planar surface of the substrate, and an electrical drive circuit; the plurality of electrodes comprises one or more first electrodes being located along a first segment of the optical waveguide to form a first modulator segment therewith, and one or more second electrodes being located along a separate second segment of the optical waveguide to form a second modulator segment therewith; the electrical drive circuit is configured to transmit RF drive signals to both the first and second electrodes to modulate an optical carrier propagating along the optical waveguide; wherein the segmented optical modulator is configured so that the first and second modulator segments modulate the optical carrier in distinct frequency regions. The electrical drive circuit may be configured to provide the RF drive signals to both the first and second electrodes to modulate a same data value onto the optical carrier in a same symbol period.

In some implementations the one or more first electrodes may have a different length than the one or more second electrodes.

In some implementations the plurality of electrodes may comprises a passband RF filter located in a signal path from the electrical drive circuit to the electrodes of the first or second modulator segment. In some implementations the passband RF filter may comprise at least one directional RF coupler. In some implementations the at least one directional RF coupler may comprise at least one dual directional RF coupler. In some implementations the at least one directional RF coupler includes an end segment of one of the first or second electrodes.

In any of the above implementations comprising a passband RF filter, the passband RF filter may be electrically connected between the first and second modulator segments. In some of such implementations, the first and second modulator segments may comprise, respectively, two first electrodes and two second electrodes, and the passband RF filter may comprise first and second dual directional RF couplers connecting output ends of the two first electrodes to input ends of the two second electrodes, wherein the first and second dual directional RF couplers cross between the first and second electrodes.

In any of the above implementations comprising a passband RF filter, the passband RF filter may have a passband extending beyond a <NUM> dB modulation bandwidth of the second modulator segment.

In some implementations, the one or more first electrodes may comprise one or more resonant stub electrodes having a resonance at an RF drive frequency of the electrical drive circuit. In some of such implementations, the first segment of the optical waveguide may comprise a modulating segment and a non-modulating segment, wherein the modulating segment has a length of at most one half of one of the resonant stub electrodes. In any of the above implementations wherein the one or more first electrodes comprise one or more resonant stub electrodes, the one or more second electrodes may be configured to operate as travelling wave electrodes for RF drive signals of the electrical drive circuit. In some implementations, the resonant-stub electrode may comprise a segment extending away from the optical waveguide. In any of the above implementations wherein the one or more first electrodes comprise one or more resonant stub electrodes, the plurality of electrodes may further comprise one or more third electrodes being located along a third segment of the optical waveguide to form a third modulator segment therewith, the segmented optical modulator further comprising a passband RF filter disposed in a signal path from the electrical drive circuit to the one or more third electrodes, wherein the one or more third electrodes are configured as travelling-wave electrodes for those of the RF drive signals provided thereto.

In some implementations, the apparatus may comprise a segmented Mach-Zehnder modulator (MZM), the segmented MZM comprising two instances of the segmented optical modulator in accordance with any of the above implementations thereof, with two instances of the optical waveguide along the planar surface of the substrate optically connected to form two arms of the MZM.

Embodiments disclosed herein will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, in which like elements are indicated with like reference numerals, and wherein:.

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and circuits are omitted so as not to obscure the description of the present invention.

Furthermore, the following abbreviations and acronyms may be used in the present document:.

Note that as used herein, the terms "first", "second" and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element from another, unless explicitly stated. Similarly, sequential ordering of method steps does not imply a requirement of sequential order of their execution, unless explicitly stated. The term "connected" may encompass direct connections or indirect connections through intermediate elements, unless explicitly stated otherwise. The term "electrically connected" and its derivatives encompasses both DC and AC connections, unless explicitly stated otherwise. Herein, the term pin junction encompasses both strict pin junctions having only contacting p-doped and n-doped regions and p/i/n junctions having a region of substantially intrinsic conductivity located between the p-doped and n-doped regions. The term "differential signal" refers to a signal that is transmitted using two single-ended signals having about complementary AC components. The term "inverted differential signal" refers to a differential signal having AC components of its constituent single-ended signals inverted relative to those of a reference differential signal, or refers to a differential signal having high-low and low-high transitions switched. The term "substrate" encompasses single and multi-layer structures.

A traveling wave (TW) electrode supports propagation of an RF operating signal there along. Typical TW electrodes usually are longer than about <NUM>/<NUM>th of a smallest operating wavelength for an RF signal propagating along the electrode. TW electrodes are often electrically terminated to, at least, partially suppress back reflections of the RF signal at their ends, or even to totally suppress such back reflections. For example, such an electrical termination may be a matched electrical connection to a chip ground or a DC source.

The term "EO interaction length" refers to a length of an optical waveguide along which an approximately co-propagating RF signal can vary the effective refractive index of the optical waveguide, e.g. by means of a Kerr effect, a Pockels effect, or injection and/or depletion of electric charge carriers.

The present disclosure describes examples of segmented electro-optic modulators (EOMs) and methods of operating such EOMs. In various examples, at least one of the segments of the EOM has a different RF spectral response to the driving signal than at least another one of the modulator segments. Different modulator segments of the EOM can be individually configured to modulate an optical carrier propagating therealong in distinct frequency regions, while modulating a same data value onto the optical carrier in a same symbol period. This may include the modulator segments providing contributions into the overall EO spectral response of the EOM that have recognizably different spectral profiles, which are different more than in overall amplitude, which may include substantially non-overlapping portions. Here, "substantially non-overlapping" may mean that there are sub-bands within a modulation bandwidth of the EOM, wherein the contributions of different modulator segments differ by at least <NUM> dB, or by at least <NUM> dB, or by <NUM> dB or greater in some embodiments. In various example described herein, one of the modulator segments may be configured to predominantly modulate the optical carrier at some lower frequencies, e.g. a lower-frequency sub-band of a target EOM bandwidth, while another modulator segment of the same EOM may be configured to predominantly modulate the optical carrier at some higher frequencies, or at a higher-frequency sub-band of the target EOM bandwidth. In some embodiments, different modulator segments of the EOM may be configured for a target performance metric in their respective modulation spectrum sub-bands. By way of example, an EOM may include one modulator segment configured to modulate input light within a <NUM> to <NUM> frequency band, and another modulator segment configured to modulate the light predominantly at frequencies of about <NUM> and smaller. This 'spectral stitching' of the RF spectral responses of different segments of the same EOM may be implemented such that their combined, or cumulative, effect results in a <NUM> RF modulation bandwidth, which exceeds the modulation bandwidth of each individual modulator segment. Optimizing the modulation efficiency of individual modulator segments within distinct, at least partially non-overlapping, RF sub-bands may enhance the bandwidth of the EOM, for example, at high frequencies. Embodiments of the multi-segment EOM may include one or more passband RF filters at driver input(s) of specific modulator segment(s) with low-frequency cut-off(s), typically in a GHz frequency range, and one or more other modulator segments of the EOM configured for modulating at the lower RF frequencies. Some embodiments may include modulator segments having one or more electrodes configured to operate as one of an RF bandpass filter or an RF resonator for RF drive signals of the EOM.

<FIG> schematically illustrates an example EOM <NUM> having different optical modulator segments (OMSs) configured to predominantly modulate an optical carrier, i.e. input light <NUM>, in different frequency regions, e.g., substantially distinct frequency sub-bands, as schematically indicated at <NUM> and <NUM> in <FIG>. These frequency regions may be at least partially complementary, so that the overall, or cumulative, modulation frequency band <NUM> of the EOM <NUM> may be greater than the modulation bandwidths of individual segments. In the illustrative example of <FIG>, the EOM <NUM> includes two OMSs, which are optically connected in series, a first optical modulator segment (OMS) <NUM> of length l<NUM> and a second OMS <NUM> of length l<NUM>. Each of the two OMS <NUM> and <NUM> includes at least one optical waveguide segment <NUM>, <NUM>, and one or more drive electrodes <NUM>, <NUM>. Each of the drive electrodes <NUM>, <NUM> has at least a segment of a corresponding length l<NUM> or /<NUM> extending adjacent to the at least one optical waveguide segment of the corresponding OMS to modulate light propagating therein. An electrical drive circuit <NUM>, <NUM> may be provided to deliver RF drive signals to the two drive electrodes <NUM>, <NUM> for modulating the input light <NUM> as it propagates through the respective OMSs. While <FIG> illustrates the drive electrodes <NUM>, <NUM>, as being paired, and the electrical drive circuits <NUM>, <NUM> as being connected to operate said paired electrodes <NUM>, <NUM> differentially, the electrical drive circuits <NUM>, <NUM> may be non-differential, and the drive electrodes <NUM>, <NUM> may be unpaired in other embodiments.

The optical waveguide segments <NUM> and <NUM> of the two OMSs are different segments of a same optical waveguide <NUM> disposed along a planar surface of a substrate <NUM> and allow the EO modulation of light propagating therein by RF signals applied to the corresponding adjacent electrodes <NUM> or <NUM>. The EO modulation may be effected, for example, by means of a Pockels effect, or Kerr effect. In some embodiments the EO modulation may be effected by injection or depletion of free charge carriers due to a so called plasma dispersion effect.

In some embodiments, the optical waveguide segments <NUM>, <NUM> may each be formed with an electro-optic material, such as but not exclusively LiNbO3. In some embodiments, parts, e.g., core regions, of the optical waveguide segments <NUM>, <NUM> may be formed with a semiconductor such as silicon (Si) or group III-V semiconductor. In such semiconductor embodiments, optical waveguide segments <NUM>, <NUM> have a pin junction extending there along, with pairs of electrodes <NUM> or <NUM> providing electrical contacts to the p-doped side (anode electrode) and n-doped side (cathode electrode) thereof along the electrode's lengths. Examples of pin junction configurations for such optical waveguide segments are illustrated in <FIG>. By varying a voltage across the pin junction, the refractive index in the waveguide may be modulated, so that an optical waveguide segment having a pin junction extending therealong may function as an optical phase modulator (OPM). In some embodiments the optical waveguide segments with a pin junction may be reversed-biased to operate as carrier-depletion OPMs, having a voltage-controlled carrier depletion region or regions in the optical waveguide. In the context of this specification, a segment of an optical waveguide which effective refractive index may be electro-optically controlled along the segment's length by electrical signals at adjacent electrodes may be referred to as an EO modulating segment. In the example embodiment illustrated in <FIG>, pin junction regions are indicated with shaded areas between the electrodes.

The electrodes <NUM>, <NUM> of the two OMS may be travelling-wave (TW) electrodes, or adjacent pairs thereof, for the RF drive signals provided thereto, and may have an input end to which a corresponding one of the RF drive signals is fed, and an output end connected to a suitable lumped electrical termination <NUM>. Terminations <NUM> may be configured to partially or substantially suppress back reflections of the RF signals. In the illustrated example the electrodes are terminated, e.g., by impedance matched electrical connections to ground or to one or more DC voltage sources to provide a DC bias.

The first OMS <NUM> may be configured for modulating input light <NUM> at higher frequencies than the second OMS <NUM>, e.g. in the upper modulation sub-band <NUM>, while the second OMS <NUM> may be configured for modulating the light predominantly in the lower-frequency sub-band <NUM>. In some embodiments the first OMS <NUM>, may be shorter than the second OMS <NUM>, i.e. l<NUM> < l<NUM>. Since higher RF frequencies experience comparatively greater attenuation when propagating along a TW electrode, a shorter OMS <NUM> may support a greater modulation bandwidth.

The electrical drive circuit of the EOM <NUM> may be configured to deliver RF drive signals to the two OMS <NUM>, <NUM> that differ in spectrum and/or bandwidth. For example, in embodiments with a target modulation bandwidth BWtotal of the EOM from fmin to fmax, the electrical drive signals delivered to the electrodes <NUM> of the first OMS <NUM> may contain predominantly high-frequency components in a frequency range from f<NUM> > fmin tofmax, with the lower-frequency components below f<NUM> suppressed or at least attenuated. The lower-frequency components fmin ≤ f ≤ f<NUM> of the target modulation band may be delivered to the electrodes <NUM> of the second OMS <NUM>.

In the illustrated embodiment, the electrical drive circuit of the EOM <NUM> includes first and second RF drivers <NUM> and <NUM> connected to separately drive the first and second OMS <NUM>, <NUM>, respectively. In some embodiments the RF drivers <NUM> and <NUM> may be differential RF drivers outputting differential RF signals, which single-ended components are fed to pairs of electrodes <NUM> or <NUM> for differentially driving the respective OMSs. In some embodiments the RF drivers <NUM> and <NUM> may be configured to provide RF drive signals that differ in spectrum. In some embodiments a passband RF filter <NUM> may be connected between the first RF driver <NUM> and the electrodes <NUM> of the first OMS <NUM>, while the second RF driver <NUM> may be connected to the electrodes <NUM> of the second OMS <NUM> directly, i.e. in the absence of a pass-band filter therebetween. In some embodiments a second, different passband RF filter (not shown) may be connected between the second RF driver <NUM> and the electrodes <NUM> of the second OMS <NUM>, so that the RF drive signals delivered to OMSs <NUM> and <NUM> differ in spectrum.

The passband filter <NUM> may have a passband at a high-frequency end of the signal bandwidth or a target modulation bandwidth of the EOM, e.g. it may correspond to a modulation sub-band <NUM> illustrated in <FIG>. In some embodiments the passband of filter <NUM>, defined e.g. at a -3dB level, may extend beyond a <NUM> dB modulation bandwidth of the second OMS <NUM>; i.e. it may include frequencies that the second OMS <NUM> substantially attenuates, e.g. by at least 3dB, or by at least <NUM> dB, or even <NUM> dB or greater in some embodiments, for example due the longer length of the electrodes of the second OMS <NUM>. Due to at least in part to this attenuation, at some frequencies within the passband of the passband RF filter <NUM>, the contribution of the first OMS <NUM> in the overall modulation response of the EOM <NUM> may be greater than that of the second OMS <NUM> by at least 3dB, or by at least <NUM> dB, or even by <NUM> dB or greater in some embodiments. At some other frequencies below the passband of the RF filter <NUM>, the contribution of the first OMS <NUM> in the overall modulation response of the EOM <NUM> may be smaller than that of the second OMS <NUM> by at least 3dB, or by at least <NUM> dB, or even by at least <NUM> dB in some embodiments. This is schematically illustrated in <FIG>, where the modulation transfer function <NUM> of the second OMS <NUM> only partially overlaps with a higher-frequency modulation transfer function <NUM> of the first OMS <NUM>, which may be defined at least in part by the passband RF filter <NUM>.

The electrical drive circuit of the EOM <NUM> may further include an electrical input port <NUM> for receiving an input data signal <NUM>, and an input RF signal circuit <NUM> that connects the electrical input port <NUM> to each of the RF drivers <NUM> and <NUM>. The RF drivers <NUM>, <NUM> may be configured to convert the input digital data signal <NUM> into the RF drive signals for driving the first and second OMS <NUM>, <NUM>. The RF drive signals generated by the first and second RF drivers <NUM>, <NUM> may thus be traceable to the same input digital data signal <NUM>, and in some embodiments may have a substantially same or similar bandwidth. In some embodiments, at least one of the RF drivers <NUM>, <NUM> may be linear, in the sense that output signals are linear functions of their input signals. In some embodiment at least one of the RF drivers <NUM>, <NUM> may be configured to modify the spectrum of the RF drive signal at its output, e.g. to emphasize a high-frequency portion of the signal's spectrum.

In the illustrated embodiment, the second OMS <NUM> is disposed optically downstream from the first OMS <NUM>, with the second RF driver <NUM> electrically downstream from the first RF driver <NUM>. The input RF signal circuit <NUM> may further be configured to provide the input data signal <NUM> to the second RF driver <NUM> with a time delay Δt, so as to synchronize the provisioning of the RF drive signals to the consecutive OMS <NUM>, <NUM> with the propagation of input light <NUM> along the waveguide <NUM>.

Since a longer RF signal path to the second OMS <NUM> may result in a relatively greater deterioration of the RF signal strength or quality at high frequencies, it may be advantageous to position the "high-frequency" OMS <NUM> to be upstream from the "lower-frequency" OMS <NUM>.

By way of example, an OMS may be fabricated in a SOI platform with a silicon waveguide using depletion-mode pin junctions for modulation, a 3dB modulation bandwidth of <NUM> may correspond to the electrode length of about <NUM> or smaller, while a 3dB modulation bandwidth of <NUM> may correspond to the electrode length of about <NUM> or smaller. Accordingly, an example EOM <NUM> fabricated using a SOI platform with a target modulation bandwidth of <NUM> may use a <NUM> to <NUM> passband filter for the first OMS <NUM>, with the lengths of the first and second OMS <NUM>, <NUM> of about <NUM> or less and about <NUM> or less, respectively.

Turning now to <FIG>, there is illustrated an example EOM <NUM> having first and second OMS <NUM>, <NUM> optically connected in series, as described above with reference to <FIG>. In <FIG> and <FIG>, same or similar elements are labeled with same reference numerals and may not be described here again. The EOM <NUM> may be viewed as a modification of the EOM <NUM>, in which a passband RF filter <NUM> is disposed electrically between the first OMS <NUM> and the second OMS <NUM>, electrically interconnecting their respective drive electrodes <NUM>, <NUM>, which may be properly terminated TW electrodes, i.e., to suppress RF signal reflections. The OMS <NUM> and <NUM> both receive the RF drive signals ultimately from a same RF driver <NUM>, which in turn may receive an input electrical digital data signal from an input electrical port of the EOM (not shown). The RF drive signals from the RF driver <NUM> are delivered to the first, input ends of the paired and substantially parallel electrodes <NUM> of the first OMS <NUM>, propagate along the paired electrodes <NUM>, and are delivered to the input ends of the paired and substantially parallel electrodes <NUM> of the second OMS <NUM> after being filtered by the passband RF filter <NUM>. The RF filter <NUM> may have a passband at the higher-frequency end of the target modulation frequency band of the EOM <NUM>, e.g. as described above with reference to the RF filter <NUM>. In this embodiment, the second OMS <NUM> may be shorter than the first OMS <NUM>, l<NUM> > l<NUM>. Although the high-frequency components of the RF drive signals may be attenuated by the propagation through the first OMS <NUM>, the second OMS <NUM> may be configured for efficient high-frequency modulation, for example by using suitably doped lateral p/n junctions as illustrated in <FIG>. Here, "lateral" refers to the direction, in-plane and relative to the waveguide's length, in which the doping changes from p-type to n-type. By removing or substantially attenuating lower-frequency components of the modulation signal in the RF filter <NUM>, the second OMS <NUM> may selectively enhance the combined modulation efficiency of the EOM <NUM> at high frequencies.

<FIG> and <FIG> illustrate example embodiments of the EOM <NUM> and EOM <NUM>, respectively, wherein the passband RF filters <NUM> and <NUM> are implemented using directional RF couplers, at <NUM> and <NUM> in <FIG> and <FIG>, respectively. Such RF filters may be disposed at least partially upon the same substrate as the OMS. Suitable directional RF filters are known in the art of microwave electronics, and may, e.g., be formed with two TW electrodes having adjacent segments with a small gap therebetween, so that an electromagnetic wave propagating in one of the electrodes can couple into the adjacently extending segment of the other. The spectral position, width and shape of the transmission passbands of such RF filters may be tuned by varying the length of the adjacent segments and the gap therebetween. Examples of a one-segment, two-segment, and three-segment directional RF couplers are illustrated in <FIG>, with corresponding transmission spectra schematically shown next to each of said RF couplers.

Turning first to <FIG>, an EOM <NUM> includes first and second OMS <NUM>, <NUM> that may be similar to the OMS <NUM> and <NUM> described above. In <FIG>, same or similar elements to those shown in <FIG> are labeled with same reference numerals. The passband RF filter <NUM> of the EOM <NUM> is implemented using two dual directional RF (DDRF) couplers <NUM>, each including an input electrode segment <NUM> and a separate coupling electrode segment <NUM>. The input and coupling segments <NUM>, <NUM> together form a first directional RF coupler <NUM>, and the coupling segments <NUM> and end segment of electrode <NUM> a, 113c together form a second, consecutive, directional RF coupler <NUM>. The input segment <NUM> may be aligned with the corresponding electrode <NUM>. The passband of the DDRF coupler <NUM> may be tuned e.g. by varying the lengths of the adjacent segments thereof and the gaps therebetween. In the illustrated embodiments the electrodes <NUM> of the OMS <NUM> are differentially driven, with two DDRF couplers <NUM> separately connecting the electrodes 113a, 113c to respective differential ports of the RF driver <NUM>. DDRF couplers <NUM> may be a part of the first OMS <NUM>, and in some embodiments, may be disposed on the same substrate as the optical waveguide.

An RF signal transmission through each of the directional RF couplers <NUM>, <NUM> may result in a <NUM>° phase shift thereby providing a <NUM>° phase shift by transmission over the DDRF coupler <NUM>. Accordingly, the first and second RF drivers <NUM>, <NUM> are typically of opposite polarity, so as to provide RF drive signals to input inputs of electrodes <NUM> and <NUM> with about the same phase. For example, in <FIG> the electrodes 113a, 123a shown above the optical waveguide <NUM> may be anode electrodes of the respective first and second OMS, with the electrodes 113c, 123c shown below the optical waveguide <NUM> may be cathode electrodes of the respective first and second OMS. Since only one of the OMS <NUM>, <NUM> is coupled to its respective RF driver through a <NUM>° phase-shifting coupler, one of the anode electrodes 113a, 123a may be connected to an inverting output port of the corresponding RF driver <NUM> or <NUM>, while the other of the anode electrodes 113a, 123a may be connected to the non-inverting output of the other of the two RF drivers. In <FIG>, the inverting and non-inverting outputs of the RF drivers <NUM>, <NUM> are labeled "<NUM>°" and "<NUM>°", respectively.

Turning now to <FIG>, an EOM <NUM> may be an embodiment of the EOM <NUM>, and includes first and second OMS <NUM>, <NUM> that may be similar to the OMS <NUM> and <NUM> described above. In <FIG>, same or similar elements to those shown in <FIG>, <FIG> are labeled with same reference numerals. The passband RF filter <NUM> of the EOM <NUM> is implemented using two coupling segments <NUM> and <NUM>, which form two DDRF couplers <NUM> that couple output ends of the electrodes <NUM> of the first OMS <NUM> to input ends of the electrodes <NUM> of the second OMS <NUM>. Since each of the DDRF coupler <NUM> inverts the phase of the RF signal transmitted therethrough by <NUM>° degrees, the coupling segments <NUM>, <NUM> may be disposed with a cross-over <NUM>. the DDRF couplers <NUM> may couple the anode and cathode electrodes 113a, 113c of the first OMS <NUM> to, respectively, the cathode and anode electrodes 123c, 123a of the second OMS <NUM>.

<FIG>, <FIG> show example EOMs <NUM>, <NUM>, <NUM> and <NUM> having a pair of electrodes in each OMS that are differentially driven, generally in counter-phase. In other embodiments one of the electrodes in each OMS may be connected to a ground, or a source of a DC voltage, with the RF driver providing an RF drive signal to the other electrode.

Furthermore, each of the EOMs <NUM>, <NUM>, <NUM> and <NUM> described above may include more than two OMSs, with at least one of the additional OMSs configured for modulating light in a different spectral region or sub-band, e.g. to further extend the cumulative modulation bandwidth of the EOM.

Furthermore <FIG>, <FIG> show example EOMs <NUM>, <NUM>, <NUM> and <NUM> with a single optical waveguide <NUM> extending therethrough. Two instances of such EOMs may be connected between an optical splitter and an optical combiner to form a segmented MZM, which may be differentially or dual-differentially driven to implement a push-pull modulation.

In some embodiment, the optical waveguide <NUM> of the EOMs <NUM>, <NUM>, <NUM> and <NUM> may represent segments of two waveguide arms of a Mach-Zehnder interferometer (MZI). Each of these waveguide arms may be electro-optically coupled to a different electrode <NUM> or <NUM> to form a segmented MZM, which may be push-pull modulated by one or more differential RF drivers. In such MZM, each of the first and second OMS corresponds to a different MZM segment.

<FIG> schematically illustrates an EOM <NUM> in the form of a segmented MZM having first and second OMS <NUM>, <NUM> optically connected in series and commonly driven by a differential RF driver <NUM>. The EOM <NUM> may be referred to herein as the MZM <NUM>. The optical waveguides <NUM>, <NUM> are connected between an input optical splitter <NUM> and an output optical combiner <NUM> to form two waveguide arms of the MZM <NUM>. The first OMS <NUM> includes electrodes 713a and 713b that extend adjacent to respective first segments <NUM>, <NUM> of the first and second waveguide arms <NUM> and <NUM>, respectively. The second OMS <NUM> includes electrodes 723a and 723b that extend adjacent to respective second segments <NUM>, <NUM> of the first and second waveguide arms <NUM> and <NUM>, respectively.

The electrodes <NUM>, <NUM> are operatively coupled to the corresponding adjacent segments of the optical waveguides <NUM>, <NUM> for modulating light propagating therein, with electrodes 713a and 723a coupled to the optical waveguide <NUM>, and electrodes 713b and 723b coupled to the optical waveguide <NUM>. In some embodiments, the waveguide segments <NUM>, <NUM>, <NUM>, <NUM> may include pin junctions extending or distributed along the segment's length, or at least a portion thereof. In some embodiments, each OMS <NUM>, <NUM> may include semiconductor material that is, for example, p-doped between the laterally adjacent segments of the waveguide arms <NUM> and <NUM>, and n-doped between the waveguide segments and the adjacent electrode <NUM> or <NUM> with pin junction(s) between the laterally adjacent p-type and n-type doped region. Alternately, n-doped region(s) may be between the laterally adjacent segments of the waveguide arms <NUM> and <NUM>, and p-doped region(s) may be between the waveguide segments and the adjacent electrodes. In some embodiments, each OMS may include a third electrode disposed between the optical waveguides, which may be connected to ground or a DC bias voltage source. It will be appreciated that other embodiments may have different electrode configurations in each OMS, for example with a signal electrode between the waveguides in one or both modulator segments.

Similarly to the EOMs <NUM> and <NUM> described above, the two OMS <NUM> and <NUM> are commonly driven by a differential RF driver <NUM>, which may be an embodiment of the differential RF driver <NUM>. The differential RF driver <NUM> may receive an input digital data signal from an input electrical port of the EOM (not shown). The differential RF drive signals from the RF driver <NUM> are delivered to the first, input ends of the electrodes <NUM> of the first OMS <NUM>, propagate along the electrodes <NUM>, and are then transmitted to the input ends of the electrodes <NUM> of the second OMS <NUM> through two DDRF couplers <NUM>, which may be formed using RF coupling waveguide segments <NUM> and <NUM>, generally as described above with reference to <FIG> and DDRF couplers <NUM>. Since each of the DDRF couplers <NUM> inverts the phase of the (single-ended) RF signal transmitted therethrough by about <NUM>° degrees, the coupling segments <NUM>, <NUM> may be disposed with a cross-over, to cross-couple output ends of the electrodes 713a, 713b of the first OMS <NUM> to input ends of the electrodes 723b, 723a, respectively, of the second OMS <NUM>. As a result, the push-pull modulation of light propagating through the OMS <NUM> and <NUM> may be phase-synchronized and generally in-phase relative to the propagation of light along the MZM arms. The lengths of the coupling segments <NUM>, <NUM> may be adjusted to synchronize the propagation of light through the MZM <NUM> with the propagation of the RF drive signal in the OMSs. Similarly to the EOM <NUM> and <NUM>, in some embodiments the electrodes <NUM> of the second OMS <NUM> may be shorter than the electrodes <NUM> of the second OMS <NUM>. In some embodiments, the pin junctions in the optical waveguide segments <NUM>, <NUM> of the second OMS <NUM> may differ from the pin junctions in the optical waveguide segments <NUM>, <NUM> of the first OMS <NUM>, for example to facilitate the modulation of light at higher frequencies. The differences may include differences in doping concentrations in the respective pin junctions and/or the geometry of the pin junctions. For example, in some embodiments the optical waveguide segments <NUM>, <NUM> of the second OMS <NUM> may have lateral pin junctions, as schematically illustrated in <FIG>, while the optical waveguide segments <NUM>, <NUM> of the first OMS <NUM> may have interdigitated pin junctions, as schematically illustrated in <FIG>.

<FIG> schematically illustrates an example EOM <NUM> embodied as a segmented MZM having three OMSs <NUM>, <NUM>, and <NUM> that are optically connected in series and individually driven by differential RF drivers <NUM>, <NUM>, and <NUM>, respectively. The OMSs <NUM> and <NUM> are connected to corresponding RF drivers <NUM>, and <NUM> through respective passband RF filters <NUM> and <NUM> formed with DDRF couplers. The passband RF filters <NUM> and <NUM> may be viewed as parts of the respective OMSs <NUM> and <NUM>, and may be configured for modulating predominately in two distinct modulation sub-bands <NUM> and <NUM> (<FIG>), corresponding to the passbands of the RF filter <NUM> and <NUM>, respectively.

The third OMS <NUM> may be connected to the corresponding RF driver <NUM> without directional RF couplers therebetween, and configured for modulation in the lower-frequency sub-band <NUM>. In some embodiments the RF driver <NUM> may be AC-coupled to the electrodes <NUM> of the OMS <NUM>, e.g. by means of a high-pass filter having a cut-off frequency fmin that defines a low-frequency edge of the modulator's bandwidth BWtotal. The frequency fmin is smaller than the lower-frequency cut-offs f<NUM> and f<NUM> of the passband filters <NUM> and <NUM>, e.g. at least <NUM> times, or <NUM> times smaller, or at least <NUM> times smaller. By way of example, fmin may be smaller than <NUM>, and f<NUM> may be greater than <NUM>. The OMSs <NUM>, <NUM>, and <NUM> and their respective RF drivers and passband filters may be configured to provide a cumulative spectral response <NUM> of the EOM, which may be composed of their respective modulation responses <NUM>, <NUM>, and <NUM>, and have a total bandwidth <NUM> that exceeds the modulation bandwidth of each individual OMS.

Example embodiments of the segmented EOMs described above may be composed of TW-OMSs that include TW electrodes in each modulator segment, wherein some or all of the TW electrodes may be suitably terminated to eliminate, or at least suppress, RF back reflections from the electrodes' ends. In some embodiments, one or more RF-resonant modulator segments, or OMSs, may be provided, which utilize RF resonator(s) to modulate light propagating in adjacent optical waveguide segments at frequencies near a selected RF resonance frequency fr. Such RF resonators may be for example in the form of a non-terminated electrode segment, termed resonant-stub electrode, that is configured to operate as a standing-wave RF resonator with the RF resonance frequency fr selected within the modulation frequency bandwidth of the EOM. An electric length of such a resonant-stub electrode may be about equal to λr/<NUM>, where λr is the wavelength of an RF signal of the resonance frequency fr propagating in the segment. Embodiments with resonant electrodes having the electric length equal to about an integer multiple of λr/<NUM> may also be envisioned.

When driven by a broad-band RF signal, a resonant-stub electrode may accumulate more energy, in a standing wave at RF frequencies near fr, than at significantly lower or higher frequencies, e. g f < fr-Δf and f > fr+Δf, and thus may operate effectively as a passband RF filter with a passband of width ~ 2Δƒ centered at the resonant frequency fr. Here, 2Δf may be the FWHM (full width at half maximum) width of the RF spectral response of the resonant electrode, which depends on the Q-factor thereof. An RF-resonant OMS that employs one or more resonant-stub electrodes may be configured to selectively modulate light in a specific sub-band of the modulation spectral band of the EOM, e.g. at the high-frequency end of the EOM bandwidth.

Since a resonant-stub electrode accumulates the RF energy in a standing wave, an RF-resonant OMS using such electrodes may be configured to have an electro-optic interaction length lEO smaller than the length of the resonant stub electrode(s), which may lessen undesirable effects of the velocity mismatch between the standing RF wave and the traveling optical signal upon the modulation efficiency of the RF-resonant OMS.

<FIG> illustrates an example multi-segment EOM <NUM> that includes a first OMS <NUM> and a second OMS <NUM> optically connected in series, with at least one optical waveguide <NUM> extending through the two OMS. The EOM <NUM> may be an embodiment of the EOM <NUM>, in which one of the OMSs employs resonant-stub electrodes instead of DDRF couplers to provide modulation frequency selectivity. In the illustrated embodiment, the first OMS <NUM> may be a TW-OMS that includes a pair of TW electrodes <NUM> extending adjacent to a first segment or segments of the at least one optical waveguide <NUM>. The second OMS <NUM> is an RF-resonant OMS that includes a pair of resonant-stub electrodes <NUM> extending adjacent to a second segment or segments of the at least one optical waveguide <NUM>. As illustrated, the OMS <NUM> is positioned optically downstream of the OMS <NUM>. In some embodiments, the positions of the OMS <NUM> and OMS <NUM> may be switched, so that the resonant OMS <NUM> is optically upstream of the TW OMS <NUM>. The TW electrodes <NUM> may be longer than the resonant-stub electrodes <NUM>. The TW electrodes <NUM> may have a 3dB transmission response roll-off at a first RF frequency f<NUM>, and the resonant-stub electrodes <NUM> may be configured to resonate at the resonant frequency fr ≥ f<NUM>, so as to extend the overall modulation band of the EOM <NUM> to frequencies beyond f<NUM>.

By way of example, f<NUM> may be in the <NUM> to <NUM> range, and fr may be in the <NUM> to <NUM> range, or f<NUM> may be in the <NUM> to <NUM> range, and fr may be in the <NUM> to <NUM> range. Further by way of example, for a SOI-based EOM employing silicon-channel optical waveguide(s) with pin junctions, the length l<NUM> of the resonant-stub electrode <NUM> may be in the range from about <NUM> microns to about <NUM> microns, corresponding to the fr in the range from <NUM> to <NUM>.

In embodiments with modulating pin junctions, the EO interaction length lEO is the length of an electrode-adjacent segment <NUM> of the optical waveguide <NUM> with the pin junction present, the corresponding segment termed the EO modulating segment. In <FIG>, the shaded areas between the electrodes schematically indicate the p-doped and n-doped regions for an example embodiment, similarly to <FIG>, <FIG>, <FIG>, <FIG>, and <FIG>. Segments <NUM><NUM> of the electrodes <NUM> extending along the EO modulating segment <NUM>, and being in electrical contact with the p-doped and n-doped regions along their length, may be referred to as active, or pin junction loaded, electrode segments. Segments <NUM><NUM> of the electrodes <NUM> in the un-doped regions may be referred to as passive, or unloaded electrode segments. The OMS <NUM> also includes a non-modulating segment of the optical waveguide <NUM> lacking the pin junctions.

<FIG> illustrates a multi-segment EOM <NUM> embodied as a segmented MZM having a TW-OMS <NUM> and two RF-resonant OMSs, OMS <NUM>, and OMS <NUM>. The EOM <NUM> may be an embodiment of the EOM <NUM> with an added second resonant OMS and the at least one optical waveguide <NUM> replaced with two optical waveguides <NUM>, <NUM> connected in parallel to form two waveguide arms of an MZI. The EOM <NUM> may also be an embodiment of the EOM <NUM> using resonant-stub electrodes instead of DDRF couplers in two OMSs to support modulation within distinct sub-bands. The OMS <NUM>, <NUM>, and <NUM> may be electrically connected to corresponding RF drivers <NUM>, <NUM>, and <NUM>, respectively, with or without directional RF couplers therebetween, and in operation may receive the RF drive signals having substantially same frequency content or somewhat different frequency content. The RF drivers <NUM>, <NUM>, and <NUM> may be configured to provide differential RF signals to the electrode pairs of the OMS <NUM>, <NUM>, and <NUM> to implement a push-pull modulation of input light <NUM>.

The TW-OMS <NUM> may include suitably terminated TW electrodes <NUM>, and may be configured to modulate input light <NUM> in a low-frequency sub-band or segment <NUM> of a target modulation band (fmin, fmax) <NUM> of the EOM (<FIG>), e.g. at RF frequencies about or smaller than a first frequency f<NUM>. The first frequency f<NUM>, which may be smaller than fmax, may correspond to a <NUM>-dB roll-off of the RF frequency response of the TW electrodes <NUM>, which is schematically illustrated in <FIG> at <NUM>.

The second OMS <NUM> may be an RF-resonant OMS, with the electrodes <NUM> configured to exhibit an RF resonance at an RF frequency fr<NUM> > f<NUM>, to modulate input light <NUM> in a mid-frequency sub-band <NUM> (<FIG>) of the target modulation band <NUM>; the RF frequency response associated with the resonant electrodes <NUM> of the second OMS <NUM> is schematically indicated at <NUM> in <FIG>. The OMS <NUM> may be configured to operate as a "passband element" for RF drive signals at the resonant frequency fr<NUM>. The EO interaction length l<NUM>EO in the OMS <NUM> may be smaller than the length l<NUM> of the electrodes <NUM>, i.e. to lessen the effect of the EO velocity mismatch as described above with reference to <FIG>. In some embodiments, the EO interaction length l2EO in the OMS <NUM> may be about half of the electrode length l<NUM> or smaller, l<NUM>EO ≤ <NUM>. The EO interaction length l<NUM>EO in the OMS <NUM> may thus be about <NUM>/<NUM>th of the wavelength of the resonant RF wave in the electrode, or less, and the OMS <NUM> may act substantially as a lumped EO element, although the electrodes <NUM> are not lumped. The electrodes <NUM> may each include a passive segment <NUM><NUM> of length (l<NUM> - l2EO) that is not coupled, or only weakly coupled, to any of the optical waveguides <NUM>, <NUM>. In some embodiments the passive segment <NUM><NUM> of each electrode <NUM> is at least half of the electrode in length.

The first OMS <NUM> may also be an RF-resonant OMS, with the electrodes <NUM> configured to exhibit an RF resonance at an RF frequency fr<NUM> > fr<NUM>, to modulate input light <NUM> in a high-frequency segment <NUM> of the target modulation band <NUM>; the RF frequency response associated with the resonant electrodes <NUM> of the first OMS <NUM> is schematically indicated at <NUM> in <FIG>. The OMS <NUM> may be configured to operate as a "passband element" for RF drive signals near the resonant frequency fr<NUM>, with an EO interaction length l<NUM>EO in the OMS smaller than the length l<NUM> of the electrodes <NUM>. In some embodiments, the EO interaction length l<NUM>EO in the OMS <NUM> may be about half of the electrode length l<NUM> or smaller, l<NUM>EO ≤ <NUM> /<NUM>. The electrodes <NUM> may each include a passive segment <NUM><NUM> of length (l<NUM> - l<NUM>EO) that is not coupled, or only weakly coupled, to any of the optical waveguides <NUM>, <NUM>. In some embodiments the passive segment <NUM><NUM> of each electrode <NUM> is at least half of the electrode in length.

The passive electrode segments <NUM><NUM> and <NUM><NUM> may be rendered "passive", i.e. electro-optically decoupled from the optical waveguides <NUM> and <NUM>, by being disposed substantially away from the corresponding optical waveguides, and/or in a region lacking modulating pin junctions. <FIG> illustrates an example embodiment wherein the active electrode segments are disposed in the p-doped or n-doped contact regions of the modulating pin junctions. The p-doped and n-doped contact regions of the modulating pin junctions are shown by the shaded areas between the electrodes, extending through the optical waveguides <NUM> and <NUM> to indicate the EO-modulating segments thereof.

In some embodiments of the example multi-segment EOMs described above, the RF drivers associated with different OMSs may be a part of an RF driving circuit that is configured to receive input digital data signal and distribute it to the RF drivers of the respective OMS, preferably in synchronization with the propagation of light therein, e.g. as illustrated in <FIG>. The input digital data signal may contain a range of RF frequencies distributed in a signal frequency band of a width that may depend on the symbol rate chosen for modulation. According to an aspect of the present disclosure, the RF drive signals provided to different OMSs modulate a same data value onto the optical carrier in a same symbol period, with different OMSs of a same multi-segment EOM configured to modulate the optical carrier in different frequency regions of the signal frequency band, which may be at least partially non-overlapping as described above. In some embodiments, an OMS configured to operate at relatively higher RF drive frequencies may be disposed optically upstream of another OMS configured to operate at relatively lower RF drive frequencies, e.g. as illustrated in <FIG>, <FIG>, <FIG>, and <FIG>; in other embodiments the order in which higher RF drive frequency and lower RF drive frequency OMSs are disposed along the optical path in the EOM may be different.

Embodiments in which one or more higher RF drive frequency OMSs is/are disposed optically upstream of one or more lower RF drive frequency OMSs may have an advantage of shorter transmission distances for higher RF drive frequencies, which may be more susceptible to transmission related deterioration. In some embodiments, the modulating segments of the optical waveguide(s) in different OMSs may be configured differently, depending on the target frequency sub-band of the particular OMS. For example, in some embodiments an OMS configured for a comparatively higher RF drive frequency sub-band may employ lateral pin junctions, while an OMS configured for a comparatively lower RF drive frequency sub-band may employ interdigitated pin junctions, or vice versa. In some embodiments, the pin junctions of a lower RF drive frequency OMS may have different doping levels than the pin junctions of a higher RF drive frequency OMS.

Each of the example EOMs <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> described above may be embodied using one or more chips. In some embodiments, the optical waveguides and the electrodes coupled thereto in various modulator segments may be located in a photonic chip along a planar surface of a substrate, such as substrate <NUM> illustrated in <FIG> and <FIG>. The photonic chip may be for example a silicon photonic (SiP) chip, for which various optical waveguides are silicon waveguides, e.g., ridge waveguides. In some embodiments, the RF driving circuit may be at least partially incorporated in a second chip, such as an RF integrated circuit (IC) chip, which may be flip-chip bonded to the photonic chip to provide electrical contacts to the RF drive electrodes and/or RF filter(s) in the OMSs of the EOM.

In some embodiments, the IC chip may include "passive" parts of the EOM's electrodes whose RF waves are not intended to directly contribute to the modulation of light. For example, in some embodiments with RF-resonant OMSs, active segments of the resonant electrodes may be disposed in a photonic chip, while the passive segments thereof may be at least partially disposed in the second, RF IC chip. This is schematically illustrated in <FIG>, showing an RF IC chip <NUM> that is flip-chip bonded to a phonic chip <NUM>. A resonant-stub electrode <NUM> of an OMS has an active segment that extends along a planar surface <NUM> of the photonic chip's substrate adjacent to an optical waveguide (not shown). A passive segment 1333p of the resonant-stub electrode <NUM> is disposed upon a surface of the IC chip <NUM> facing the photonic chip, and is electrically connected to the active segment 1333a, e.g. with a metallic solder "bump" bond <NUM>. The resonant electrode <NUM> may represent any of the electrodes <NUM>, <NUM>, <NUM> of the multi-segment EOMs <NUM> and <NUM> illustrated in <FIG> and <FIG>.

In some embodiments, the RF IC chip may also include the passband RF filters as described above, or some parts thereof. One possible configuration is illustrated in <FIG>, showing an RF IC chip <NUM> that is flip-chip bonded to a phonic chip <NUM>. A DDRF coupler <NUM> includes an input electrode segment <NUM>, which may be connected to an RF driver on the IC chip by e.g. a metallic solder "bump" bond. An end segment of the input electrode <NUM> is electro-magnetically coupled to a first end of a coupling electrode <NUM>, which is disposed upon a surface of the RF IC chip <NUM> facing the photonic chip <NUM>, forming a first directional RF coupler therewith. The second end of the coupling electrode <NUM> is electro-magnetically coupled to an input end of a modulating electrode <NUM>, which is disposed upon a surface of the photonic chip <NUM> and extends adjacent to an optical waveguide (not shown). The DDRF coupler <NUM> may represent any of the DDRF couplers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> of the multi-segment EOMs illustrated in <FIG>, <FIG>, <FIG>, and <FIG>.

The examples of multi-segment EOMs described above are not intended to be limiting, and many variations will become apparent to a skilled reader having the benefit of the present disclosure. For example, various features described above with reference to a specific embodiment or embodiments may be combined with other embodiments. For example, a same multi-segment EOM may combine one or more OMS incorporating passband RF filters with one or more other OMSs incorporating resonant electrodes. By way of example, the passband-driven OMS <NUM> of EOM <NUM> of <FIG> may be replaced with the RF-resonant OMS <NUM> of the EOM <NUM> of <FIG>. Furthermore, additional TW OMS, passband-driven OMS, or RF-resonant OMS may be added to any of the multi-segment EOMs described above. Furthermore, in any of the described examples and their variations, different OMSs may provide distinct contributions into the EOM modulation spectrum that are substantially non-overlapping in some frequency regions thereof, while modulating a same data value onto the optical carrier in a same symbol period. Furthermore, in any of the described examples and their variations, a higher-frequency OMS may be shorter, or have a smaller EO interaction length, than a lower-frequency OMS, or may be of a substantially the same length.

Furthermore, passband RF filters other than DDRF couplers may be used in various embodiments of the multi-segment EOM in according with the present disclosure such as for example a passband RF filter in the form of a rat-race coupler.

Furthermore, multi-segment EOMs such as those described above may be implemented in material systems other than those based on silicon, and may use electro-optical effects other than those based on free carrier injection or depletion. Non-limiting example of materials and material systems that may be used to implement one or more modulator segments of an EOM include photonic-crystals, integrated graphene layers, various compound semiconductor materials such as those of the III-V group, or thin-film LiNbO3 integrated on a suitable, e.g. silicon, substrate.

Furthermore in the description above, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. In some instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. Thus, for example, it will be appreciated by those skilled in the art that block diagrams herein can represent conceptual views of illustrative circuitry embodying the principles of the technology.

Claim 1:
An apparatus comprising:
a segmented optical modulator (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>), the segmented optical modulator comprising:
an optical waveguide (<NUM>) along a planar surface (<NUM>) of a substrate (<NUM>);
a plurality of electrodes (<NUM>, <NUM>) disposed along the planar surface of the substrate, the plurality of electrodes comprising:
one or more first electrodes (<NUM>) located along a first segment (<NUM>) of the optical waveguide to form a first modulator segment (<NUM>) therewith;
one or more second electrodes (<NUM>) located along a second segment (<NUM>) of the optical waveguide to form a second modulator segment (<NUM>) therewith; and,
an electrical drive circuit (<NUM>, <NUM>) configured to transmit RF drive signals to both the first and second electrodes (<NUM>, <NUM>) to modulate an optical carrier (<NUM>) propagating along the optical waveguide;
wherein the segmented optical modulator is configured so that the second modulator segment (<NUM>) is optically connected in series with the first modulator segment (<NUM>) and characterized in that the first and second modulator segments (<NUM>, <NUM>) modulate the optical carrier in distinct frequency regions (<NUM>, <NUM>), wherein a spectral response (<NUM>, <NUM>) of the first modulator segment and a spectral response (<NUM>, <NUM>) of the second modulator segment are at least partially non-overlapping in the distinct frequency regions.