Wavelength Division Multiplexed Optical Modulator and Multiplexing Method Using Same

This invention relates generally to an integrated optic device, and more particularly to a method and apparatus for externally modulating a plurality of wavelengths generated by a single multi-longitudinal mode laser in order to wavelength division multiplex the signals over a transmission carrier (e.g., an optical fiber).

The impact of photonics technology in the area of communication systems has been dramatic. In view of the advances, new communication system architectures have been proposed based on the photonics technology. The communication architectures take advantage of the ability of optical fibers to carry very large amounts of information with very little marginal cost once the optical fiber is in place.

With these architectures, switching operations are located at the edges of the network, large amounts of information are distributed throughout the network, and the information is filtered at gateways at the edges of the network. Further, these approaches often build on optical wavelength division multiplexing or optical frequency division multiplexing (coherent techniques) to deliver these large bundles of information throughout the network. The information carrying potential of the optical fiber systems is further enhanced with the use of external modulators for higher speed modulation of the guided light beam.

In the past, external modulators have been used to modulate the entire spectrum which is generated by the laser. Therefore, all of the wavelengths emitted by the laser carried the same information through the optical fiber. Since the different wavelengths travel at different speeds through the fiber, at large distances, interference between the wavelengths carrying the same information occurred.

Also in the past, multi-mode fibers have been utilized to carry different wavelength signals. However, different lasers have been utilized to generate the signals. Due in part to the ability to control the output wavelength of the lasers, the wavelengths have been selected to utilize two or more of the fiber optic windows of absorption. However, since a single window is typically the optimum transmission range, these devices suffer a drawback of not transmitting all information at the optimum wavelength window of absorption for the optical fiber over which the information will be transmitted.

Therefore, there arises a need for a photonic method and apparatus which enables external modulation of two or more longitudinal modes of a multi-longitudinal mode semiconductor laser in which each of the longitudinal modes can be independently and simultaneously modulated. By providing this combination of wavelength division multiplexing and external modulation, increased optical fiber information transmission capacity results. The present invention directly addresses and overcomes the shortcomings of the prior art.

The present invention provides a simple, relatively inexpensive and yet reliable method and apparatus for increasing the amount of data which can be carried over an optical wave carrier. In the preferred embodiment, the invention accomplishes this result by physically separating previously non-utilized wavelengths from a single multi-longitudinal mode laser, externally modulating the physically separated wavelengths, and coupling the wavelengths into a single optical wave carrier (i.e., the wavelengths are "multiplexed" into a single optical fiber).

In a preferred embodiment constructed in accordance with the principles of the present invention, the wavelength division multiplexor ("WDM") is an integrated optic based device which provides for both multiplexing and independent external modulation of three wavelengths emitted by a multi-longitudinal mode laser. The wavelengths are physically separated from one another by a diffraction grating and the separated wavelength beams are individually focussed onto corresponding directional coupler modulators. A multi-mode optical fiber is butt coupled to the integrated optic device chip to transmit the information coded wavelengths to a receiver.

One feature of the present invention is the use of an integrated optics device chip to enable coupling of the dispersed laser wavelengths into separate electro-optic modulators which encode each of the carrier wavelengths with a different signal. Therefore, the device enables a larger data volume to be transmitted through an optical fiber network. This increase in the volume of data which can be transmitted is useful and often required for data systems that have an integrated form (e.g., where video, voice, and data are simultaneously distributed and processed). This improved system performance is in part achieved at relatively low cost because of the need for only one laser source.

Another feature of the present invention is that through utilizing the primary and secondary mode wavelengths emitted by a single laser, narrow line widths are utilized (which reduces dispersion) and the minimized fiber optic window of absorption can be utilized.

Therefore, according to one aspect of the invention, there is provided an apparatus for wavelength division multiplexing a plurality of signals over an optical fiber, comprising: a single laser for generating a plurality of optical signals of different wavelengths; external modulator means for modulating the optical signals in accordance with control signals, whereby the optical signals carry information; and wavelength separation means, for spatially separating the different wavelength signals and directing the signals from the laser to the integrated optic modulator.

According to another aspect of the invention, there is provided an apparatus for receiving and decoding a plurality of wavelength division multiplexed signals from a single optical fiber, comprising: wavelength separation means, for spatially separating the different wavelength signals from the single optical fiber; and a light detector array for detecting the modulated information in the different wavelength signals, wherein transmitted information may be processed, transmitted or stored.

According to still another aspect of the invention, there is provided a system for transmitting and receiving wavelength division multiplexed signals over a single optical fiber, comprising: transmitter means, including: i) a single laser for generating a plurality of optical signals of different wavelengths; external modulator means for modulating the optical signals in accordance with control signals, whereby the optical signals carry information; and wavelength separation means, for spatially separating the different wavelength signals and directing the signals from the laser to the integrated optic modulator; a multi-mode or single mode optical fiber; and receiver means, including: wavelength separation means, for spatially separating the different wavelength signals transmitted by said optical fiber; and a light detector array for detecting the modulated information in the different wavelength signals, wherein transmitted information may be processed, transmitted or stored.

According to another aspect of the invention, there is provided a method of wavelength division multiplexing a plurality of signals over an optical fiber, comprising the steps of: generating a plurality of optical signals of different wavelengths from a single laser diode; spatially separating the different wavelength signals and directing the signals from the laser to an integrated optic modulator; and modulating the optical signals with the integrated optic modulator in accordance with control signals, whereby the optical signals carry information.

While the invention will be described with respect to a preferred embodiment configuration and with respect to particular devices used therein, it will be understood that the invention is not to be construed as limited in any manner by either such configuration or components described herein. Also, while the particular types of laser, physical wavelength separating device and integrated optic modulator are described herein, it will be understood that such particular components are not to be construed in a limiting manner and the functionality of those devices should be appreciated. Further, while the preferred embodiment of the invention will be described in relation to transmit information over an optical fiber, it will be understood that the scope of the invention is not to be so limited. The principles of the invention apply to the physical separation of inherent wavelengths in a laser device and individual external modulation of the physically separated wavelengths in order to increase the amount of information which can be transmitted utilizing the wavelengths. These and other variations of the invention will become apparent to those skilled in the art upon a more detailed description of the invention.

These and various other advantages and features which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages and objectives obtained by its use, reference should be had to the drawing which forms a part hereof and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

Referring to the drawing, wherein like numerals represent like parts throughout the several views:

FIG. 1 is a functional block diagram of a wavelength division multiplexed optical modulator transmitter and receiver practicing the principles of the present invention;

FIG. 2a is a diagrammatic illustration of the longitudinal mode spectra of a semi-conductor laser with a Fabry-Perot structure;

FIG. 2b is a diagrammatic illustration of the absorption characteristics of a fiber optic.

FIG. 3 is a diagrammatic cross-section of a diffraction grating and illustrating the angles of incidence (I) and diffraction (D), and the grating groove spacing (a) are illustrated;

FIG. 4 is an illustration of the diffraction of monochromatic light into different orders m;

FIG. 5 is an illustration of the diffraction of polychromatic light and the dispersion of the different wavelength components over an angular region for each of the diffraction orders m>1;

FIG. 6 is a diagrammatic illustration of the ratio of the beam widths d/c equals the anamorphic magnification;

FIG. 7 is a diagrammatic illustration of the integrated optic device chip with three optical intensity modulators based on directional couplers;

FIG. 8 is a diagrammatic illustration of a first alternative embodiment of the modulator of FIG. 7, wherein single mode optical fibers are butt coupled; and

FIG. 9 is a diagrammatic illustration of a second alternative embodiment of the modulator of FIG. 7, wherein single mode optical fibers are butt coupled.

As noted above, the principles of the present invention apply to the external modulation of a plurality of wavelengths of light generated by a single laser source, wherein a larger amount of information may be transmitted over an optical wave carrier, such as one or more optical fibers. In the preferred embodiment, the invention accomplishes this result by physically separating previously non-utilized wavelengths from a single multi-longitudinal mode laser, externally modulating the physically separated wavelengths, and coupling the wavelengths into a single optical wave carrier (i.e., the wavelengths are "multiplexed" into a single optical fiber). However, it will be appreciated that the modulated wavelengths may be transmitted over one or more optical fibers.

A preferred embodiment of the device constructed according to the principles of the present invention includes the following components: a multi-longitudinal mode laser; a diffraction grating; collimating optics; focusing optics; and an integrated optic modulator. Referring first to FIG. 1, the WDM transmitter is illustrated generally at 20, the optical fiber is illustrated generally at 21, and the WDM receiver is illustrated generally at 22. Also illustrated in FIG. 1 are the laser power and temperature circuitry functional block 23, the directional coupler modulator controller functional block 24, and the detector array controller functional block 25.

In order to more clearly describe the present invention, the operation of the device will be deferred pending a discussion of the various elements which will be presented in the following paragraphs.

Still referring to FIG. 1, preferably, the WDM system 26 utilizes very closely spaced optical wavelengths. A series of closely spaced optical wavelengths centered about 1.3 microns wavelength can be produced with a conventional semiconductor diode laser 26. FIG. 2a shows a typical optical spectra and line widths of a laser diode with a Fabry-Perot structure. The wavelength separation between the longitudinal modes is .delta.w=0.0007 microns-7.ANG..

By using a single multi-longitudinal mode laser, the wavelength spectral separation of the three carrier beams remains fixed. However, since the entire longitudinal mode pattern can shift at a rate of 0.0005 microns=5.ANG. per degree Centigrade (C) temperature variation, stable and accurate temperature control of the laser diode 26 is required. Laser diode temperature controllers are commercially available which can be used to stabilize the laser output with respect to temperature induced spectral shifting. These temperature controllers offer 0.2 degrees C accuracy with 0.01.degree. C. stability (e.g., the model LDT-5412 temperature controller by ILX Lightwave of Bozeman, Montana). Thus, the semiconductor laser output could be stabilized to reduce spectral shifting to <<0.05 .ANG. wavelength. As noted above, the laser power and temperature circuitry functional block is illustrated at 23.

FIG. 2b illustrates the absorption characteristics of an optical fiber which illustrates the first window (designated at "A") , the second window (designated at "B") , and the third window (designated at "C").

A diffraction grating is used to spatially separate light of different wavelengths (i.e., to disperse light). Referring to FIG. 3, a typical diffraction grating consists of a substrate 40 with a large number of parallel grooves 41 ruled or replicated in its surface and overcoated with a reflecting material such as aluminum. Referring to the grating geometry shown in FIG. 3, the grating equation relating the angle of incidence I of the incoming light to the angle D at which the diffracted light exits is:

where

a=groove spacing,

w=wavelength of light, and

m=an integer.

When a parallel beam of monochromatic light is incident on a grating, the light is diffracted from the grating in directions corresponding to m=0, 1, 2, etc., as shown in FIG. 4. If polychromatic light is incident on the grating, then each wavelength is diffracted into a different angular direction to form a fan of light for each order m, as shown in FIG. 5.

The resolving power R of a grating is defined in terms of wavelengths w and w+.delta.w, where w+.delta.w is the closest wavelength to w which can be resolved. The resolving power is given by:

where N is the number of grating lines illuminated. To resolve a wavelength separation of .delta.w=0.0007 microns (i.e., the longitudinal mode separation achievable with a semiconductor laser), the number of gratings which must be illuminated for m=1 is:

Therefore, if the laser beam diameter is 0.3 cm, then the grating groove spacing must be:

This groove spacing corresponds to 6190 grooves per cm. Gratings with this number of grooves per cm are commercially available.

If monochromatic light strikes a grating, a fraction of it is diffracted into each order m. Gratings are not equally efficient at all wavelengths. The efficiency can be tuned by changing the groove facet angles, or shape and depth. The optimization of efficiency by appropriate groove shaping is known as blazing. In the present invention, the angles I and D, together with the grating blaze angle, are determined by the required spatial separation between the integrated optic modulator 31 input ports for the three different wavelengths.

Referring again to FIG. 1, components 28 and 29 must be specified to yield both the required linear dispersion of the laser longitudinal mode wavelengths at the waveguide input face, and also the required laser beam spot size at the waveguide input face. The linear dispersion achieved with the diffraction grating 27 must be great enough to enable spatial location of the different longitudinal modes on the input coupling face of three different optical modulators. Also, the collimated laser beam must be appropriately shaped to optimize the input coupling efficiency to the integrated optic waveguide modulator 31. The focused laser beam spot size must match the guided wave mode profile to achieve good input coupling efficiency.

Since the cross-section of the guided light beam in the integrated optic waveguide has more of an elliptical shape, the optical beam incident on the second optical element must also have an elliptical cross-section. However, the spatial direction of the major and minor axes are reversed for the collimated input beam and the guided beam.

To satisfy these requirements, optical elements 28 and 29 can be selected as follows:

1. Collimating element: focal length=8 mm, N.A.=0.5.

2. Focusing element: focal length=8 mm, N.A.=0.5.

The elliptical beam shape required for efficient input coupling to the waveguide is established upon diffraction of the laser beam by the holographic grating 27. As shown in FIG. 6, the laser beam cross section undergoes an anamorphic magnification upon diffraction. The laser beam diameter which lies in the plane defined by the incident beam and the diffraction grating normal, is changed according to:

For the specified holographic grating, the anamorphic magnification is:

Design calculations for the waveguide design for the directional coupler 31 indicates that the guided beam is elliptically shaped with a minor axis to major axis ratio of about 0.23.

Collimating optical element 29' in FIG. 1 must collimate the light exiting the multi-mode optical fiber 21 to permit separation of the modulated laser diode longitudinal modes by the holographic grating 27' at the receiver. The angular divergence of the collimated beam formed by element 29' must be less than the angular dispersion of the longitudinal mode wavelengths upon diffraction by the holographic grating 27'. For an extended source, such as the output of a 100 micron diameter optical fiber, the full divergence angle of the collimated beam formed by element 29' is:

where f.sub.OE3 is the focal length of element 29'. The angular dispersion between two adjacent longitudinal modes achieved with the grating specified for the WDM optical modulator is:

To obtain .delta.D>.PHI..sub.CB, the focal length of OE must be:

A lens with a focal length of 10 cm was used for focusing lens 28' in FIG. 1. This lens 28' focused the dispersed longitudinal mode wavelengths to spots easily resolved with a video camera during experimentation. In application, however, a detector array 32 will be fused to monitor the dispersed longitudinal modes, and focusing lens 28' is selected to provide the proper spot size and separation.

Referring to FIGS. 1 and 7, the integrated optic device 31 utilized in the preferred embodiment incorporates three separate optical intensity modulators 51, 52, 53. However, the number of optical intensity modulators incorporated on the integrated optic device chip could be increased to match the number of high intensity longitudinal modes in the laser diode 26 output beam. The lateral separation of the modulator structures on the integrated optic chip must:

1. match the spatial separation of the focused input beams, and

2. be sufficiently separated to avoid crosstalk among the modulators.

To meet these requirements, the preferred modulator 31 is based on a directional coupler integrated optic structure. FIG. 7 illustrates the modulator structure. Each modulator 51, 52, 53 includes two parallel channel waveguides 54 and 55. Since each modulator is constructed in a similar manner to one another, it will be appreciated that when referring to modulator 51, the structure of modulators 52 and 53 is equivalent and so they will not be described further herein.

The waveguide 54 and 55 is a multilayer structure of GaAs and AlGaAs epitaxial films. Ridges are etched into the cover clad layer of the multilayer waveguide to define the channel guides. A semiconductor pin heterojunction in the multilayer waveguide permits application of a signal electric field within the region where the guided light propagates. Through the electro-optic effect, this signal electric field modulates the waveguide propagation constant, enabling control of the coupling coefficient between the two channel guides 54 and 55 of the modulator 51.

Modulator chip 31 includes a substrate 59 and includes electrodes 57 which overlay the channel waveguides 54 and 55. Electrical contact pads 58 are provided for each of the electrodes 57. The external modulator controller block 24 is operatively connected to the electrical contact pads 58 in order to modulate the light so as to impose the desired information, data, or other signals into the light beam for subsequent transmission to receiver block 22.

In operation, light from the laser diode 26 is coupled into the waveguide 54 with a light absorbing region 56. The length of the ridge guides 54, 55 is such that the light couples from ridge guide 54 to ridge guide 57 in propagating the length of the guide. Therefore, the light exits guide 55 when no signal voltage is applied to the semiconductor pin heterojunction in the ridge guide. By applying a voltage to the heterojunction, the coupling between guides A and B is modified so that less light couples to guide B and exits the modulator chip. Guided light which is not coupled from guide A to B is absorbed by a metal film covering the light absorbing region 56 at end of ridge guide 54. Thus, the intensity of the light exiting ridge guide 54 is modulated by the signal voltage applied to the semiconductor pin heterojunction. The directional coupler modulators are sufficiently separated on the integrated optic device chip to permit independent modulation of the three wavelengths W.sub.1, W.sub.2, and W.sub.3 (see FIG. 7).

Design details such as:

1. doping levels in the semiconductor pin heterojunction,

2. electrode configuration,

3. device length, and

4. channel guide separation

can be specified for the preferred operation at a wavelength of 1.3 microns. However, it will be appreciated by those skilled in the art that the integrated optic device 31 could also be designed for use at other laser diode wavelengths. Also, electro-optic materials other than AlGaAs could also be used. For example, if LiNbO.sub.3 is used for the electro-optic waveguide material, then the electrode configurations are different from that used for semiconducting AlGaAs--so as to reflect the ferroelectric nature of LiNbO.sub.3.

The WDM optical modulator waveguide structure can be deposited on GaAs substrates using molecular beam epitaxy (MBE) or metal organic vapor phase epitaxy (MOVPE). Both p type and n type dopants can be added to the AlGaAs material during growth of the epitaxial layers.

The ridge waveguides and electrode are fabricated using standard photolithography techniques, thin film deposition techniques, and wet chemical and reactive ion etching techniques.

Device processing involves the following basic steps set forth in TABLE 1:

1. fabricating an electrical contact to the substrate clad layer;

2. defining the channel waveguides 54 and 55 by etching a ridge in the cover clad layer;

3. fabricating the optical attenuator region 56 at the end of the one guide 54 in each directional coupler 51, 52, 53;

4. forming a planarizing layer over the cover clad layer; and

5. fabricating the metal electrodes 57 on the channel waveguide, the metal contact pads 58, and the metal interconnects between the channel guide electrodes and the contact pads.

An electrical contact must be made to both the cover clad layers and the substrate clad layers in order to reverse bias the pn junction in the multilayer AlGaAs semiconductor waveguide structure. Fabricating an electrical contact to the substrate clad layer involves etching a via through both the cover clad layer and the guide layer. Once the substrate clad layer is exposed at the via cut, a metal contact pad is fabricated using photolithography lift off techniques and thin film deposition processes.

Channel waveguides 54, 55 are then defined in the multilayer, planar waveguide film structure using photolithography and reactive ion etching (RIE) procedures. An optical attenuator region 56 is formed at the end of one channel waveguide in each directional coupler modulator 51, 52, 53 by etching away the ridge and depositing a film of an absorbing material over the etched region.

The coverclad surface is then planarized by depositing a thick photoresist layer which is hard baked at about 170.degree. C. to form a hard, durable, polymer layer. The photoresist layer is patterned to provide openings to the coverclad layer at the channel guides prior to the high temperature hard bake. Thin metal film deposition techniques and photolithography techniques are then used to fabricate (1) electrodes 57 to the cover clad layer of the channel guides (2) electrical contact pads 58 on the photoresist planarizing layer, and (3) electrical interconnects between the electrodes 57 and the contact pads 58.

In use, the preferred wavelength division multiplexor and optical modulator is a building block on which new telecommunication system architectures can be developed. These new telecommunication system architectures will distribute large amounts of information throughout the network. Wavelength division multiplexing and high speed external modulation of the laser light provide for the generation of the large bundles of information.

The operation of the transmitter portion 20 will be next described. First, the laser diode 26 is maintained at a constant temperature to minimize wavelength variations by temperature and power control block 23. Second, the output beam from the laser diode 26 is directed to collimating lens 28. The output of lens 28 is reflected off of mirror 30 to diffraction grating 27. The diffraction grating 27 spatially orients the differing wavelengths of light apart from one another. As utilized herein, this may be referred to as "physically" separating the wavelengths of light. It will be appreciated by those skilled in the art that other devices which separate differing wavelengths from one another, such as prisms, might also be used. The physically separated wavelengths are in turn provided to collimating lens device 29 for injection into the external modulator 31.

Voltages are applied to pads 58 of the modulator 31 by controller block 24 in accordance with predetermined control signals to either attenuate or propagate the differing wavelengths of light through each of the modulators 51, 52, or 53. Accordingly, differing modulated signals are provided to a transmission guide 21 which is preferably a multi-mode optical fiber. Accordingly, large bundles of information may be provided over single optical fibers. A receiver 22 receives the differing wavelengths at collimating device 29' which is provided to diffraction grating 27' and collimating lens 28'. The information transmitted by the resulting physically separated modulated wavelengths are detected by detector array 32 and the detected information from the plurality of wavelengths is gathered by detector array control device 25 for further processing, transmission, and/or storage.

Controller block 24 can be either a general purpose computer (e.g., a 486 based personal computer) with appropriate interface boards or may be a special purpose communications device or computer. Similarly, detector array control device 25 can also be either a general purpose computer with appropriate interface boards or may be a special purpose communications device or computer.

Turning now to FIG. 8, a first alternative embodiment modulator 31' is illustrated wherein the exiting output beams are butt-coupled to single mode optical fibers 60. These single mode optical fibers 60 can subsequently be combined into a one single mode optical fiber or can carry the modulated information individually.

FIG. 9 illustrates a second alternative embodiment, wherein the channel waveguides 51 and 53 are not angled to provide more spacing between the exit area (as shown in FIG. 8), but instead the waveguides 51 and 53 are offset through use of reflective devices 61 to exit into butt-coupled single mode optical fibers 60.

It will be appreciated that the principles of this invention apply not only to the circuitry used to implement the invention, but also to the method in general of automatically utilizing the plurality of wavelengths to transmit information over a single fiber optic device.

While a particular embodiment of the invention has been described with respect to its application, it will be understood by those skilled in the art that the invention is not limited by such application or embodiment or the particular circuits disclosed and described herein. It will be appreciated by those skilled in the art that other circuit configurations that embody the principles of this invention and other applications therefor other than as described herein can be configured within the spirit and intent of this invention. The circuit configuration described herein is provided as only one example of an embodiment that incorporates and practices the principles of this invention. Other modifications and alterations are well within the knowledge of those skilled in the art and are to be included within the broad scope of the appended claims.