WO2003001282A1

WO2003001282A1 - Compound optical modulator

Info

Publication number: WO2003001282A1
Application number: PCT/IL2001/000580
Authority: WO
Inventors: Amnon Manassen; Avner Sander
Original assignee: 3Dv Systems, Ltd.
Priority date: 2001-06-26
Filing date: 2001-06-26
Publication date: 2003-01-03
Also published as: WO2003001282A8

Abstract

There is therefore provided in accordance with an embodiment of the present invention a modulator for modulating light comprising: a quantum well structure (QWS), comprising narrow-bandgap semiconductor layers interleaved with wide-bandgap semiconductor layers which QWS has an operating band of wavelengths for which transmittance for light is controllable by an electric field generated in the QWS; a configuration of electrodes electrifiable to generate an electric field perpendicular to the semiconductor layers of the QWS in each of a plurality of localized non-overlapping voxels of the QWS independent of an electric field in any of the other voxels; a different optical filter coupled to each of at least two of the voxels, each different filter transmitting light in a different sub-band of the operating band of the QWS and being substantially opaque to light having a wavelength in the operating band that is outside of the sub-band; and a power supply controllable to apply voltage to the configuration of electrodes so as to generate the electric field.

Description

COMPOUND OPTICAL MODULATOR FIELD OF THE INVENTION

The invention relates to optical modulators and in particular to optical modulators used for generating optical signals that are multiplexed for transmission in communication networks.

BACKGROUND OF THE INVENTION High speed wavelength division multiplexing (WDM) communication networks provide a plurality of optical communication channels each of which uses a different narrow band of wavelengths in a limited bandwidth of wavelengths for signal transmission. Optical signals generated for different optical communication channels of a WDM network are generally multiplexed and transmitted simultaneously via a same optic fiber to their destinations.

To support these networks and assure their reliability there is a need for devices that can generate, amplify and route optical signals encoded in light at the different WDM carrier wavelengths of a WDM network. In particular, there is a need for modulators that can modulate light at the different WDM wavelengths to provide optical signals suitable for multiplexhig and transmission by WDM communication networks.

Typical high-speed optical modulators often comprise a multiple quantum well (MQW) structure or a superlattice (SL) structure through which light to be modulated passes. An MQW or SL structure, hereinafter referred to generically as quantum well (QW) structures, comprises a stack of thin layers (hereinafter, "narrow-bandgap layers") foπned from narrow-bandgap semiconductor material alternating with layers (hereinafter, "wide- bandgap layers") formed from wide-bandgap semiconductor material. The alternating structure generates a series of quantum wells located in the narrow-bandgap layers. The wide- bandgap layers on each side of a narrow-bandgap layer provide walls of the quantum wells and act as barriers that tend to confine conduction band electrons and valence band holes in the QW structure to the quantum wells. In MQW structures, confinement is relatively strong, whereas in SL structures confinement is relatively weak. The narrow-bandgap layers are hereinafter referred to as "quantum well layers" and the wide-bandgap layers are hereinafter referred to as "barrier layers".

Thickness of the layers and materials from which they are made determine a shape and an absorption edge for an absorption spectrum of the QWS. Depending upon the type of modulator and its mode of operation, the absorption edge is red shifted or blue shifted by controlling a voltage applied to the QWS that generates an electric field in the QWS.

Transmittance of the QWS for light having a wavelength in a relatively narrow band of wavelengths, hereinafter referred to as an "operating band", near the absorption edge changes when the absorption edge is red shifted or blue shifted. The width of the operating band is referred to as the bandwidth of the QWS and a wavelength in the operating band is referred to as an "operating wavelength" of the QWS. Light having a wavelength that is an operating wavelength of the QWS that passes through the modulator is modulated by changes in transmittance of the QW. Modulators that comprise a QWS are described in US Patents 4,525,687 and

5,194,983, and PCT Publication WO 99/40478, the disclosures of which are incorporated herein by reference. US Patent 5,412,499 describes a spatial light modulator (SLM) that includes an integral, slab-like, optical medium that comprises a MQW, which is subdivided into picture elements. Each picture element comprises electrodes that can be electrified independently of the other electrodes to change the transmittance of the picture element.

SUMMARY OF THE INVENTION An aspect of some embodiments of the present invention relates to providing an optical modulator that can simultaneously modulate light at different wavelengths with optionally different modulation patterns. The optical modulator can therefore simultaneously provide optical signals for an optical communication network at different wavelengths that are optionally modulated by different modulation patterns.

According to an aspect of some embodiments of the present invention, light at the different wavelengths that is modulated by the compound modulator is provided by a same single light source According to an aspect of some embodiments of the present invention the different wavelengths of light are different carrier wavelengths of a WDM network.

A compound modulator, in accordance with an embodiment of the present invention comprises a plurality of component modulators that share a common QWS. Each component modulator comprises a configuration of electrodes that can be electrified to generate an electric field substantially perpendicular to the layers of the QWS in a different localized volume region of the QWS. The electric field is substantially confined to the localized volume region, hereinafter referred to as a "modulation voxel" of the component modulator and changes in transmittance of the modulation voxel for light at an operating wavelength of the QWS are produced by changing the electric field. In accordance with an embodiment of the present invention, the electrodes of each component modulator are controlled independently of the electrodes of the other component modulators. Consequently, in accordance with an embodiment of the present invention, transmittance of the modulation voxel of each component modulator is controllable independently of the transmittance of the modulation voxels of the other component modulators.

Each component modulator also comprises an optical filter that transmits light for a different portion, hereinafter referred to as an "operating sub-band", of the operating band of wavelengths of the QWS. The component modulator's filter is optionally opaque to light at operating wavelengths of the QWS that lie outside the sub-band of the component modulator. The filter is positioned and has a size and shape so that it preferably filters substantially all and substantially only light that is transmitted through the modulation voxel of the component modulator.

As a result of its electrode configuration and optical filter, each component modulator of the compound modulator is controllable to modulate light in a different operating sub-band of the operating band of the QWS simultaneously with and independent of modulation of light by the other component modulators. Assume that a light source, in accordance with an embodiment of the present invention, illuminates the compound modulator in a direction perpendicular to the modulator's QWS layers with light at substantially all operating wavelengths of the compound modulator. The compound modulator can then simultaneously provide optical signals at wavelengths in different sub-bands of the compound modulator with different modulation patterns.

For some applications, the bandwidth of a QWS may not be large enough to be partitioned into a desired number of different modulation bands. For example the International Telecommunications Union (ITU) has established standardized "grids" of optical channels for a bandwidth between 1530 to 1570 nm for DWDM (Dense Wavelength Division Multiplexing) communication networks. The grids provide for between 50 and 100 optical communication channels at standard communication channel wavelengths, which are spaced at intervals of 100 GHz (0.8 nm) and 50 GHz (0.4 nm) respectively. A practical operating bandwidth for many QWSs is about 8 nm. Assume that each operating sub-band of a compound modulator, in accordance with an embodiment of the present invention, is to provide optical signals for a WDM communication channel. The available operating bandwidth of the compound modulator is sufficient to provide optical signals for a maximum of only about 10 DWDM communication channels at the 0.8 nm spacing and about 20 DWDM channels at the 0.4 nm spacing.

To provide a compound modulator that can provide more than a number of channels that are provided by a compound modulator comprising a single QWS, in some embodiments of the present invention a compound modulator comprises a plurality of QWSs stackecLone on top of the other. The QWSs in the stack have different optionally non-overlapping operating bands. Each QWS has a different area, where area of a QWS refers to an area of a cross section of the QWS parallel to its layers. Larger area QWSs in the stack are designed to have operating bands characterized by wavelengths that are shorter than wavelengths that characterize operating bands of smaller area QWSs in the stack. An operating band of the stack of QWSs, in accordance with an embodiment of the present invention, is substantially equal to a sum of the operating bands of its constituent QWSs and can be used to provide more channels than a compound modulator comprising a single QWS.

In some embodiments of the present invention the stack, hereinafter referred to as a "terraced QWS", of QWSs is terraced. In a terraced QWS, in accordance with an embodiment of the present invention, the QWSs are stacked so that from a bottom to a top of the stack the QWSs are successively smaller.

The stack of QWSs is partitioned, in accordance with an embodiment of the present invention to provide a plurality of component modulators. Assume that the QWS stack comprises "N" QWSs and that the QWSs are identified in accordance with decreasing size by numbers 1 through N {i.e. smaller QWSs are identified by larger numbers). Each component modulator comprises a different non-overlapping portion of the stack volume. The portion of the volume comprised in a component modulator optionally includes regions of "n" QWSs identified by numbers from 1 to n and does not include regions of any QWSs identified by any numbers from (n+1) to N. The number n depends upon the location of the component modulator with respect to the stack and optionally can assume any value that satisfies the relation l≤ n ≤ N.

A component modulator comprising regions from n QWSs will be substantially opaque to light having a wavelength in any of the operating bands of QWSs 1 to (n-1), independent of an electric field in the regions of the QWSs comprised in the component modulator. In addition, the component modulator will be substantially transparent to light having a wavelength in any of the operating bands of QWSs (n+1) to N. The component modulator therefore cannot be used to modulate light having wavelengths in any of the operating bands of QWSs 1 to (n-1) and (n+1) to N.

However, fight having a wavelength in the operating band of the n-th QWS will be affected by changes in transmittance of the region of the n-th QWS that is located in the component modulator. The component modulator can therefore be used to modulated fight in the operating band of wavelengths of the n-th QWS by generating appropriate electric fields in the region of the n-th QWS comprised in the component modulator. The region of the n-th QWS comprised in the component modulator is a modulation voxel of the component modulator. The compound modulator comprising the stack of QWSs comprises an electrode structure that can be electrified to generate a desired electric field perpendicular to the layers in the modulation voxel of each component modulator, independent of an electric field in the modulation voxel of any other of the component modulators. Optionally, the compound modulator comprises at least one component modulator for each value of n. It is therefore seen that the compound modulator comprising the stack of QWSs, in accordance with an embodiment of the present invention, can be used to modulate light in all of the wavelength bands of the QWSs in the stack. The compound modulator and the QWS stack that it comprises have an operating band that is substantially equal to the sum of the operating bands of the QWSs comprised in the stack. In some embodiments of the present invention the at least one component modulator for each value of n comprises a plurality of component modulators. If there are "k" component modulators for each value of n the compound modulator comprising the stack of N QWSs provides kN independent optic channels. In some embodiments of the present invention, each of the plurality of component modulators for a value of n is coupled to a filter that transmits light in a different sub-band of the operating band of the n-th QWS. In such a case, each of the kN independent channels provides modulated light at a different wavelength. .

Optionally, each of the voxels is coupled to a different optical filter each filter transmitting light in a different sub-band of the operating band of the QWS and each being substantially opaque to light having a wavelength in the operating band that is outside of the sub-band. Additionally or alternatively the sub-bands are non-overlapping.

There is further provided, in accordance with an embodiment of the present invention, a modulator for modulating light comprising: a semiconductor structure comprising a plurality of quantum well structures (QWSs), each of which comprises narrow-bandgap semiconductor layers interleaved with wide-bandgap semiconductor layers, and each of which has an operating band of wavelengths for which transmittance for light is controllable by an electric field generated in therein, wherein the QWSs are stacked in a direction perpendicular to the layers; a configuration of electrodes electrifiable to generate an electric field perpendicular to the planes of the semiconductor layers in each of a plurality of localized non-overlapping voxels of the semiconductor structure, wherein an electric field in a voxel is controllable independently of an electric field in any of the other voxels, and wherein each of the voxels comprises a region of the semiconductor layers from a single QWS of the plurality of QWSs; and a power supply controllable to apply voltage to the configuration of electrodes to generate said electric field in each of the voxels.

Optionally, the operating bands of the QWSs are non-overlapping. Optionally, substantially any line through a voxel perpendicular to the layers in the voxel does not intersect a QWS having an operating band at wavelengths longer than wavelengths of the operating band of the voxel.

Optionally, each of the plurality of QWSs has a different size cross sectional area in the plane parallel to its semiconductor layers. Optionally, the operating wavelength bands of QWSs of the plurality of QWSs having larger cross-sectional areas parallel to the semiconductor layers, are located in shorter wavelength regions of the spectrum than QWSs having smaller cross-sectional areas parallel to the semiconductor layers. From a bottom of the stack to a top of the stack, the QWSs optionally have successively smaller cross-sectional areas parallel to the semiconductor layers.

Optionally, substantially any line through a voxel perpendicular to the layers in the voxel intersects all QWSs of the plurality of QWSs having operating bands at wavelengths shorter than wavelengths of the operating band of the voxel.

Optionally, there is at least one voxel for each QWS.

Optionally, each of the at least one voxel of a particular QWS is coupled to an optical filter that transmits light in a sub-band of the operating band of wavelengths of the particular QWS and is substantially opaque to light haying a wavelength in the operating band of the particular QWS outside of the sub-band.

Optionally, the filter is substantially opaque to light having a wavelength in operating bands of QWSs of the plurality of QWSs characterized by wavelengths longer than wavelengths in the operating band of the particular QWS.

Additionally or alternatively, the sub-bands are optionally non-overlapping. In some embodiments of the present invention, the bandwidth of the sub-bands is less than or equal to about 0.8 nanometers.

In some embodiments of the present invention, the bandwidth of the sub-bands is less than or equal to about 0.4 nanometers.

In some embodiments of the present invention, the filter is an optic fiber filter. In some embodiments of the present invention, the filter is an integral part of the modulator formed on a layer of the modulator.

In some embodiments of the present invention, the filter is a dielectric filter.

In some embodiments of the present invention, the configuration of electrodes comprises a first and second electrode for each voxel that sandwiches substantially only the voxel between them.

Optionally, the first electrode of each voxel has a perimeter substantially the same as a perimeter of a cross sectional area of the voxel parallel to the semiconductor layers.

Optionally, the first electrode comprises a conducting frame surrounding an open area.

Optionally, the first electrode comprises a uniform conducting layer that is substantially transparent to the sub-band of wavelengths of the filter coupled to the voxel.

In some embodiments of the present invention, the second electrode of each voxel has a perimeter substantially the same as a perimeter of a cross sectional area of the voxel parallel to the semiconductor layers. A modulator according to claim 25 wherein the second electrode comprises a conducting frame surrounding an open area.

A modulator according to claims 25 wherein the second electrode comprises a uniform conducting layer that is substantially transparent to the sub-band of wavelengths of the filter coupled to the voxel.

In some embodiments of the present invention, the second electrode of each voxel is a different region of an electrode common to at least two of the voxels. Optionally, the common electrode comprises a uniform conducting layer that is substantially transparent to wavelengths in the operating wavelength bands of the at least two voxels. In some embodiments of the present invention, each QWS comprises a multiple quantum well structure. In some embodiments of the present invention, each QWS structure comprises a superlattice structure.

In some embodiments of the present invention, the operating band of each QWS is in a region of wavelengths from 1530 to 1570 nm. In some embodiments of the present invention, the operating band of each QWS is in a region of wavelengths from 830-875 nm.

In some embodiments of the present invention, the modulator comprises a single light source that illuminates all the voxels with light having a same spectrum in a direction substantially perpendicular to the semiconductor layers.

In some embodiments of the present invention, the modulator, comprises a different optic fiber for each voxel that receives light modulated by the voxel.

BRIEF DESCRIPTION OF FIGURES Non-limiting examples of embodiments of the present invention are described below with reference to figures attached hereto. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below.

Fig. 1A schematically shows a perspective view of a compound modulator, in accordance with an embodiment of the present invention; Fig. IB shows a schematic cross section view of the compound modulator shown in

Fig. 1A, in accordance with an embodiment of the present invention; Fig. 2 schematically shows a cross section view of the modulator shown in Figs. 1 A and IB simultaneously providing modulated optical signals at different light wavelengths, in accordance with an embodiment of the present invention;

Fig. 3A schematically shows a terraced compound modulator, in accordance with an embodiment of the present invention; and

Fig. 3B schematically shows a cross section view of the terraced modulator shown m Fig. 3 A, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Figs. 1A and IB respectively show a perspective view and a cross-section view of a compound modulator 20, in accordance with an embodiment of the present invention. The cross section view in Fig. IB is taken through a line A-A shown in Fig. 1 A.

Compound modulator 20 comprises, by way of example a QWS 22 comprising a plurality of thin, quantum well layers 24, which are shown shaded, formed from narrow- bandgap semiconductor material alternating with thin, barrier layers 26, formed from wide- bandgap semiconductor material. Quantum well layers 24 and barrier layers 26 are optionally rectangular. Material from which quantum well layers 24 and barrier layers 26 are formed and their thickness determine operating wavelengths and an operating band for QWS 22, as is known in the art.

QWS 22 is sandwiched between an optionally highly n-doped semiconductor layer 30 on one side thereof and an array 31 of optionally square, highly p-doped semiconductor plates

32, formed on the other side thereof. Each highly p-doped plate 32 is optionally capped with a highly p-doped cap layer 38 that protects layer 32 from oxidation and functions as a contact layer on which an electrode structure 40 is formed. By way of example, electrode structure 40 is a "window-frame" electrode surrounding an opening 44. Highly n-doped layer 30 is optionally covered by a highly n-doped layer 34, which protects layer 30, on which layer 34 an electrode structure 36 is formed. In some embodiments of the present invention, electrode structure 36 comprises a plurality of window-frame electrodes. Each window-frame electrode is directly opposite a different one of window-frame electrodes 40 and is a mirror image of the window-frame electrode 40. In some embodiments of the present invention, electrode structure 36 is single large electrode that covers substantially all of cap layer 34 and is transparent to light modulated by compound modulator 20. For convenience electrode structure 36 is shown as a single large electrode. Quantum well layers 24 and barrier layers

26 are preferably formed from intrinsic semiconductor layers and highly p-doped plates 32 and their respective cap layers 38 and electrodes 40 are optionally electrically isolated from each other by trenches 41.

Optionally, on electrode 36, opposite each highly p-doped plate 32 an optical filter 46 is formed. Each optical filter 46 transmits light having wavelengths in a sub-band of the operating band of QWS 22 and blocks light in the operating, band of the QWS that has a wavelength that is outside of the sub-band. Optionally, the sub-band of each filter 46 is different from the sub-bands of the other filters 46. Optionally the sub-bands are substantially non-overlapping. Each highly p-doped plate 32, its cap layer 38 and electrode 40, associated filter 46 and a localized volume region 50 of QWS 22, i.e. a modulation voxel 50, between the p-doped plate and electrode 36 define a component modulator 52 of compound modulator 20. Opening 44 in electrode 40 of a component modulator 52 defines an aperture for the component modulator through which light enters or leaves the component modulator. Modulation voxels 50 for some component modulators 52 are outlined by dashed lines 54 extending from the modulators' respective p-doped plates 32. By way of example, compound modulator 20 comprises nine component modulators.

It is noted that electrode 40 can have a shape different from that shown in Fig. 1 A and Fig. IB. For example, electrode 40 can be a transparent electrode that covers substantially all of cap layer 38 or have a shape similar to a shape of an electrode shown for optical modulators in PCT Publication WO 99/40478, referenced above. It is also noted that filters 46 can be formed in apertures 44 of window-frame electrodes 40 instead of on conducting layer 36. In some embodiments of the present invention, filters 46 are not integral parts of the structure of compound modulator 20. For example, a filter 46 of a component modulator 52 can be a fiber filter such as a Bragg filter formed in an optic fiber that receives light modulated by the component modulator. Other types of filters suitable for use with component modulators 52 and methods of coupling appropriate filters to the component modulators will occur to a person of the art.

In accordance with an embodiment of the present invention, electrode 36 functions as a common electrode for all component modulators 52. When a voltage difference is applied between an electrode 40 of a particular component modulator 52 and common electrode 36, an electric field is generated in the component modulator's modulation voxel 50. The electric field is substantially localized to modulation voxel 50 and controls transmittance of the modulation voxel for light having a wavelength that is an operating wavelength of QWS 22. As a result, light of any wavelength. in the operating band of wavelengths of QWS 22 that enters the particular component modulator will be modulated by changes in the transmittance generated by the electric field.

However, filter 46 of particular component modulator 52 transmits light that has a wavelength in a sub-band of the operating band and blocks light in the operating band that has a wavelength outside of the sub-band. Consequently, the only light modulated by modulation voxel 50 that exits the particular component modulator 52 is light in the operating band of QWS 22 that has a wavelength in the sub-band of filter 46 of the particular component modulator. Therefore by controlling voltage applied to its electrode 40, each component modulator 52 of compound modulator 20 is controllable to provide modulated light in a different sub-band of the operating band of QWS 22, which sub-band is determined by the component modulator's filter 46.

In accordance with an embodiment of the present invention, voltage of each electrode 40 with respect to common electrode 36 is controlled independently of voltage on the other electrodes 40. Therefore, if compound modulator 20 is illuminated with light characterized by a spectrum having wavelengths in all the sub-bands of component modulators 52, the compound modulator can simultaneously provide independently modulated optical signals at wavelengths in all the sub-bands.

By way of a example, a compound modulator, in accordance with an embodiment of the present invention, similar to compound modulator 20 shown in Figs. 1A and IB and described above may comprise a QWS 22, that is an MQW structure. The MQW structure optionally comprises 120 narrow-bandgap quantum well layers 24 formed from GaAs and wide-bandgap layers 26 formed from Al_xGaπ __x)As where, optionally, x = 0.36. Thickness of quantum well GaAS layers 24 is optionally about 8 nanometers and thickness of Al_xGaπ _ _x)As barrier layers 26 is optionally about 10 nanometers. Highly n-doped layer 30 is optionally a 200 nanometer thick highly n-doped layer of Al_xGaπ __x)As with x = 0.3 and cap layer 36 is optionally a 20 nanometer thick layer of highly n-doped GaAs. Each highly p- doped plate 32 is optionally a square plate about 10,000 nanometers on a side and 200 nanometers thick formed from highly p-doped Al_xGan __x)As with x - 0.3. Cap layer 38 is^' optionally a 20 nanometer thick layer of highly p-doped GaAs. For the above specifications, compound modulator 20 has an operating band having a bandwidth of about 8 nanometers centered at a wavelength of about 860 nanometers. Filters 46 are optionally narrow band optical filters such as narrow band Bragg filters, having a transmission band of about 0.8 nanometers, which would provide for between 8 and 10 sub-bands and a corresponding number of component modulators 52.

Fig. 2 schematically shows a cross section view of compound modulator 20 simultaneously providing optical signals at different wavelengths, wherein optical signals at different wavelengths are modulated by different modulation patterns, in accordance with an embodiment of the present invention, hi the cross section view only three of component modulators 52 comprised in compound modulator 20 are shown.

Light that is modulated by compound modulator 20 is provided^~by a light source 60 that optionally provides light, represented by waveforms 62, at all the operating wavelengths in the operating band of compound modulator 20. Different wavelengths of light provided by light source 60 are schematically represented by different "wavelengths" of waveforms 62.

Optical filter 46 of each component modulator 52 transmits a different sub-band of the operating band.

Compound modulator 20 is connected to a controller 61 that applies a potential difference between electrode 40 of each component modulator 52 relative to common electrode 36 to control transmittance of the component modulator. The potential difference has time dependence that is a function of a desired modulation pattern that is transcribed to light in a sub-band of wavelengths transmitted by the component modulator's filter 46.

Modulated light transmitted by each component modulator 52 is schematically represented by a modulated waveform 66 shown exiting the component modulator. Wavelength of a waveform 66 exiting a component modulator 52 represents light having a wavelength in the sub-band of the component modulator. Waveforms 66 exiting different modulators 52 are schematically shown modulated by different modulation patterns.

Fig. 3A and 3B schematically show a perspective view and a cross section view respectively of a terraced compound modulator 70, in accordance with an embodiment of the present invention. The cross-section view shown in Fig. 3B is taken through a line B-B shown in Fig. 3A. As noted above, a terraced compound modulator, which comprises a plurality of

QWSs having different operating bands, can generally provide more independent modulation channels than a compound modulator comprising a single QWS. Terraced modulator 70 is by way of example a two-terraced compound modulator that comprises a first, large area QWS 72 on which a second smaller area QWS 74 is formed. A surface region 76 of first QWS 72 that is not covered by second QWS 74 forms a first terrace of terraced compound modulator 70 and a surface region 78 of second modulator 74 forms a second terrace of the terraced modulator. By way of example,, surface regions 76 and 78 are square and have equal areas. Surface region 76 is referred to as first terrace 76 and surface region 78 is referred to as second terrace 78. Shapes of first and second terraces 76 and 78 other than those shown in Figs. 3 A and 3B can be used in the practice of the present invention and such other shapes can be advantageous.

First QWS 72 comprises quantum well layers 80 (shown shaded) interleaved with barrier layers 82 (shown non-shaded). Second QWS 74 is formed from (shaded) quantum well layers 84 interleaved with (non-shaded) barrier layers 86. First QWS 72 is characterized by a first operating band of operational wavelengths which is generally determined by thickness of quantum well layers 80 and barrier layers 82 and materials from which they are formed. Similarly, second QWS 74 is characterized by a second operating band of operational wavelengths which is generally determined by thickness of quantum well layers 84 and barrier layers 86 and by materials from which they are formed. First and second operating bands are preferably non-overlapping and the second operating band is located at a longer wavelength region of the spectrum than the first operating band.

Optionally, quantum well layers 80 and 84 are formed from a same semiconductor material and barrier layers 82 and 86 are formed from a same semiconductor material. A difference in spectral location of the first and second operating bands is determined by differences between thickness of quantum well layers 80 and quantum well layers 84. For example, in order for the second operating band to be located at a region of the spectrum having longer wavelengths than the wavelengths of the first operating band, quantum well layers 84 are formed thicker than quantum well layers 80.

First QWS 72 is formed on a highly doped semiconductor layer 90 which is protected by a highly doped cap layer 92 on which an electrode structure 94 is formed. Optionally, highly doped layer 90 and its cap layer 92 are highly n-doped. Optionally, electrode structure

94 comprises a single large electrode that is substantially transparent to light modulated by compound modulator 70 and substantially completely covers cap layer 92.

A first array 95 of thin, optionally square plates 96 of highly, optionally, p-doped semiconductor material is formed on first terrace 76. Each plate 96 is optionally covered with a cap layer 98 on which an electrode structure, optionally comprising a single window-frame electrode 100, is formed. Square plates 96 and their respective cap layers 98 are formed by suitably etching layers of appropriate highly p-doped semiconductor material that are grown on QWS 72. Portions of the layers that remain after etching form highly p-doped layers 97 that are located between QWS 72 and QWS 74. (In manufacturing terraced compound modulator 70 and similar compound modulators, in accordance with embodiments of the present invention, all layers required for producing the compound modulator are optionally formed having a same cross-sectional area parallel to the layers. The semiconductor layers form a relatively uniform "block" having a constant cross-sectional area parallel to the layers. Appropriate regions of the various layers are then etched away to provide the desired terraces, e.g. first and second terraces 76 and 78 in compound modulator 70.)

Similarly, a second array 101 of thin optionally square plates 102 of highly n-doped semiconductor material, each of which is covered with an n-doped cap layer 104 having thereon a window-frame electrode 106 is formed on second terrace 78. (Plates 102 and their cap layers 104 are n-doped since layers 97 are p-doped.)

Optionally, an optical filter 110 is formed on electrode 94 opposite each p-doped plate 96 on first terrace 76. Each optical filter 110 transmits light having a wavelength in an optionally different optionally substantially non-overlapping ^■ sub-band of operating wavelengths of the first operating band {i.e. the operating band of wavelengths of first QWS 72). Each highly p-doped plate 96, its cap layer 98 and electrode 100, associated filter 110 and a region 112 of QWS 72 located between the p-doped plate and electrode 94 define a component modulator 120 of terraced compound modulator 70. Region 112 of a component modulator 120 is the component modulator's modulation voxel. Some component modulators 120 are outlined by dashed lines 121 extending from the modulators' respective p-doped plates 96. By way of example, compound modulator 70 comprises nine component modulators 120.

By controlling voltage of electrode 100 of a component modulator 120 with respect to electrode 94 light transmitted by the component modulator's filter 110, {i.e. light having a wavelength in the component modulator's sub-band), can be modulated in accordance with a desired modulation pattern.

Similarly, an optical filter 128 is optionally formed on electrode 94 opposite each n- doped plate 102 on second terrace 78. Each optical filter 128 transmits light having wavelengths in an optionally different optionally substantially non-overlapping sub-band of operating wavelengths of the second operating band. Each highly n-doped plate 102, its cap layer 104 and electrode 106, associated filter 128 and portions 127, 129 and 131 of QWS 72 and 74 and layers 97 respectively located between the n-doped plate and electrode 94 define a component modulator 130 of terraced compound modulator 70. Region 129 of a component modulator 130 is the component modulator's modulation voxel. For some component modulators 130, dashed lines 133 extending from the modulators' respective n-doped plates 102 outline the shape and size of the component modulators. By way of example, compound modulator 70 comprises nine component modulators 130. In operation, layers 97 of compound modulator 70 are preferably electrically connected to electrode 94 so that there is substantially no voltage difference between layers 97 and electrode 94. Voltage between electrode 106 of a component modulator 130 and layers 97 is controlled to control transmittance of modulation voxel 129 of the component modulator and modulate thereby light having a wavelength in the operating band of second QWS 74 that is transmitted by the component modulator's filter 128.

It is noted that whereas each component modulator 130 comprises a region 127 of QWS 72, as well as a region 129 of QWS 74 that functions as a modulation voxel of the component modulator, the presence of region 127 does not affect functioning of the component modulator. Wavelengths in the operating band (the first operating band) of QWS 72 are shorter than wavelengths of the operating band (the second operating band) of QWS 74. Therefore, region 127 of component modulator 130 is substantially transparent to light in the second operating band and modulation voxel 129 of the component modulator is substantiality opaque to light in the first operating band. As a result, irrespective of an electric field in region 127, the region does not affect light modulated by component modulator 130 and irrespective of an electric field in modulation voxel 129 or region 127, the component modulator cannot be used to modulate light in the first operating band.

From the above discussion it is seen that terraced modulator 70 can be used to modulate light having a wavelength in either the first operating band or the second operatmg band and has an operating bandwidth that is the sum of the bandwidths of the first and second operating bands.

It is noted that whereas terraced modulator 70 comprises two QWSs, terraced modulators, in accordance with an embodiment of the present invention, comprising more than two QWSs can be formed and such terraced modulators will generally have larger operating bandwidths than two terraced modulators. In general, the bandwidth of a terraced modulator, in accordance with an embodiment of the present invention, is the sum of the bandwidths of the QWS terraces it comprises.

By way of example, assume that a terraced modulator, in accordance with an embodiment of the present invention, similar to compound modulator 70 comprises a second QWS 74 that has quantum well layers 84 and barrier layers 86 formed from materials and having thickness as specified above for compound modulator 20. Then the terraced compound modulator will be useable to modulate light in a band of wavelengths 8 nanometers wide and centered at a wavelength of about 860 nanometers. Assume further that quantum well layers 80 and barrier layers 82 in first QWS 72 are formed from the same materials respectively that are used to formed quantum well and barrier layers 84 and 86 of second QWS 74. However, assume that thickness of quantum well layers 80 is 7 nanometers instead of 8 nanometers. First QWS 72 will then have an operating band of about 8 nanometers centered at about 852 nanometers. Terraced compound modulator 70 will therefore also be useable to modulate light having a wavelength in the operating band of first QWS 72. As a result, the terraced optical modulator, in accordance with an embodiment of the present invention, will have an operating band of wavelengths about 16 nanometers wide centered at 856 nanometers. It is noted that this bandwidth is used in local WDM networks for which transmission distances are generally moderate and less than about 1000 m. For long distance WDM networks that typically have operating wavelengths in a range from 1530 nm to 1570 nm, compound modulators similar to compound modulators 20 and 70 can be provided using QWS structures having operating bands in appropriate regions of the spectrum.

It is noted that whereas examples of the compound modulators, in accordance with an embodiment of the present invention, shown in Figs. 1 A-3B have shapes that are square or rectangular and comprise component modulators having square apertures, other shapes for compound modulators and component modulators are possible. For example, a terraced compound modulator can comprise a plurality of circular QWS terraces. The circular QWS terraces are optionally stacked one on top of the other with from bottom to top of the stack, each succeeding terrace in the stack having a radius smaller than the radius of the QWS terrace on which it lies. As seen from the top the QWS terraces form a "bulls-eye" pattern of concentric annuli. Apertures of component modulators of the bulls-eye compound modulator are located on the annuli.

In the description and claims of the present application, each of the verbs, "comprise" "include" and "have", and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb. The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The . described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.

Claims

1. A modulator for, modulating light comprising: a quantum well structure (QWS), comprising narrow-bandgap semiconductor layers interleaved with wide-bandgap semiconductor layers which QWS has an operating band of wavelengths for which transmittance for light is controllable by an electric field generated in the QWS; a configuration of electrodes electrifiable to generate an electric field perpendicular to the semiconductor layers of the QWS in each of a plurality of localized non-overlapping voxels of the QWS independent of an electric field in any of the other voxels; a different optical filter coupled to each of at least two of the voxels, each different filter transmitting light in a different sub-band of the operating band of the QWS and being substantially opaque to light having a wavelength in the operating band that is outside of the sub-band; and a power supply controllable to apply voltage to the configuration of electrodes so as to generate the electric field.

2. A modulator according to claim 1 wherein each of the voxels is coupled to a different optical filter each filter transmitting light in a different sub-band of the operating band of the QWS and each being substantially opaque to light having a wavelength in the operating band that is outside of the sub-band.

3. A modulator according to claim 1 or claim 2 wherein the sub-bands are non- overlapping.

4. A modulator for modulating light comprising: a semiconductor structure comprising a plurality of quantum well structures (QWSs), each of which comprises narrow-bandgap semiconductor layers interleaved with wide- bandgap semiconductor layers, and each of wliich has an operating band of wavelengths for which transmittance for light is controllable by an electric field generated in therein, wherein the QWSs are stacked in a direction perpendicular to the layers; a configuration of electrodes electrifiable to generate an electric, field perpendicular to the planes of the semiconductor layers in each of a plurality of localized non-overlapping voxels of the semiconductor structure, wherein an electric field in a voxel is controllable independently of an electric field in any of the other voxels, and wherein each of the voxels comprises a region of the semiconductor layers from a single QWS of the plurality of QWSs; and a power supply controllable to apply voltage to the configuration of electrodes to generate said electric field in each of the voxels.

5. A modulator according to claim 4 wherein the operating bands of the QWSs are non- overlapping.

6. A modulator according to claim 5 wherein substantially any line through a voxel perpendicular to the layers in the voxel does not intersect a QWS having an operating band at wavelengths longer than wavelengths of the operating band of the voxel.

7. A modulator according to claim 6 wherein each of the plurality of QWSs has a different size cross sectional area in the plane parallel to its semiconductor layers.

8. A modulator according to claim 7 wherein the operating wavelength bands of QWSs of the plurality of QWSs having larger cross-sectional areas parallel to the semiconductor layers, are located in shorter wavelength regions of the spectrum than QWSs having smaller cross-sectional areas parallel to the semiconductor layers.

9. A modulator according to claim 8 wherein from a bottom of the stack to a top of the stack, the QWSs have successively smaller cross-sectional areas parallel to the semiconductor layers.

10. A modulator according to claim 9 wherein substantially any line through a voxel perpendicular to the layers in the voxel intersects all QWSs of the plurality of QWSs having operating bands at wavelengths shorter than wavelengths of the operating band of the voxel.

11. A modulator according to any of claims 4-10 wherein there is at least one voxel for each QWS.

12. A modulator according to claim 11 wherein each of the at least one voxel of a particular QWS is coupled to an optical filter that transmits light in a sub-band of the operatmg band of wavelengths of the particular QWS and is substantially opaque to light having a wavelength in the operating band of the particular QWS outside of the sub-band.

13. A modulator according to claim 12 wherein the filter is substantially opaque to light having a wavelength in operating bands of QWSs of the plurality of QWSs characterized by wavelengths longer than wavelengths in the operating band of the particular QWS.

14. A modulator according to claim 12 or claim 13 wherein the sub-bands are non- overlapping.

15. A modulator according to any of claims 1-3, or 12-14 wherein the bandwidth of the sub-bands is less than or equal to about 0.8 nanometers.

16. A modulator according to any of claims 1-3, or 12-14 wherein the bandwidth of the sub-bands is less than or equal to about 0.4 nanometers.

17. A modulator according to any of claims 1-3 or 12-14 wherein the filter is an optic fiber filter.

18. A modulator according to any of claims 1-3 or 12-14 wherein the filter is an integral part of the modulator formed on a layer of the modulator.

19. A modulator according to any of claims 1-3, 12-14 or 17 wherein the filter is a dielectric filter.

20. A modulator according to any of claims 1- 19 wherein the configuration of electrodes comprises a first and second electrode for each voxel that sandwiches substantially only the voxel between them.

21. A modulator according to claim 20 wherein the first electrode of each voxel has a perimeter substantially the same as a perimeter of a cross sectional area of the voxel parallel to the semiconductor layers.

22. A modulator according to claim 21 wherein the first electrode comprises a conducting frame surrounding an open area.

23. A modulator according to claim 21 wherein the first electrode comprises a uniform conducting layer that is substantially transparent to the sub-band of wavelengths of the filter coupled to the voxel.

24. A modulator according to any of claim 20-23 wherein the second electrode of each voxel has a perimeter substantially the same as a perimeter of a cross sectional area of the voxel parallel to the semiconductor layers.

25. A modulator according to claim 24 wherein the second electrode comprises a conducting frame surrounding an open area.

26. A modulator according to claims 24 wherein the second electrode comprises a uniform conducting layer that is substantially transparent to the sub-band of wavelengths of the filter coupled to the voxel.

27. A modulator according to any of claims 20-23 wherein the second electrode of each voxel is a different region of an electrode common to at least two of the voxels.

28. A modulator according to claim 27 wherein the common electrode comprises a uniform conducting layer that is substantially transparent to wavelengths in the operating wavelength bands of the at least two voxels.

29. A modulator according to any of claims 1-28 wherein each QWS comprises a multiple quantum well structure.

30. A modulator according to any of claims 1-28 wherein each QWS structure comprises a superlattice structure.

31. A modulator according to any of claims 1-30 wherein the operating band of each QWS is in a region of wavelengths from 1530 to 1570 nm.

32. A modulator according to any of claims 1-23 wherein the operating band of each QWS is in a region of wavelengths from 830-875 nm.

33. A modulator according to any of claims 1-32 and comprising a single light source that illuminates all the voxels with light having a same spectrum in a direction substantially perpendicular to the semiconductor layers.

34. A modulator according to any of claims 1-33 and comprising a different optic fiber for each voxel that receives light modulated by the voxel.