Abstract:
A method for fabricating a magneto-optic modulator, such as for use with a solid state ring resonator gyroscope. The method includes inducing a magnetic field at a substrate holder as a layer of magnetic material is being deposited on a substrate. The magnetic field helps to optimally orient the deposited magnetic material layer to improve the characteristics of the magneto-optic modulator. In addition to inducing a magnetic field, a low energy ion beam may be applied to optimize orientation. The method can be used to fabricate a magneto-optic modulator on a substrate containing a partially fabricated ring resonator without destroying previously fabricated components.

Description:
This is continuation of application Ser. No. 09/201,247 filed Nov. 30, 1998, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Optical waveguides are presently being used for a number of applications including communications, interconnects between optical circuits, and certain optical resonator applications. For varying reasons, there are needs to have phase modulators associated with these optical waveguides. The phase modulators create selected phase shifts for any number of reasons. One particular instance where a phase modulator is necessary is in conjunction with an optical resonator. Additionally, there are numerous instances where phase shifts may be required, for example, in optical communication networks, or in conjunction with optical circuits. 
     Optical modulators of several types are presently used. Examples of these optical modulators include acousto-optic modulators, as well as electro-optic modulators. Acousto-optic modulators are devices wherein an acoustic wave traveling in a bulk medium is used to modulate an optical signal which is traveling in an associated medium. The disadvantage of acousto-optic modulators is their use of acoustic waves, or sound waves, which are fairly slow and require a large bulk medium to support their transmission. 
     Another type of modulator is a magnetic modulator. These modulators require the creation of some type of magnetic field which interacts with the optical signal traveling through the modulator. The interaction with the magnetic field alters the amplitude and/or frequency of the optical signal. 
     A third type of modulator is an electro-optic modulator wherein the optical signals interact with an electric field. The field interaction alters the characteristics of the optical signal, thus varying the frequency amplitude and/or phase of the optical signal. 
     All of the above-mentioned modulators require precise alignment of the differing components. Specifically, the modulator itself must be aligned and positioned to receive an incoming optical signal and must be situated to appropriately transmit an output optical signal. This alignment can often become very tedious and exacting work which is both time consuming and costly. Furthermore, the modulators require the use of specific materials (i.e., electro-optic materials, magnetic materials, and acousto-optic materials) which display the appropriate characteristics. 
     Also, many times the application of material on the modulators is uncontrollable. The composition as well as the amount of crystallinity has not been able to be controlled in the past. Further, in the magnetic modulators, the orientation of the magnetic axis of the material is poor and many times, improperly aligned which results in loss. It would be desirable to optimize the operation of the phase modulators by controlling material deposition and optimizing the alignment so that loss could be reduced as well. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a magnetic modulator and system of fabrication, such that the modulator exhibits very low losses and very high efficiency. 
     A further object of the present invention is to provide a system of fabricating a magnetic modulator which can be used in conjunction with the fabrication of other components on a single substrate. Specifically, it is an object of the invention to fabricate a magnetic modulator upon a substrate without destroying components which already exist on the same substrate. 
     Another object of the invention is to provide a system of fabricating a magneto-optic modulator which is capable of depositing the necessary thin films of magnetic materials. 
     The present invention provides a system for the fabrication of a magneto-optic modulator. The method can be used to easily and economically fabricate a magneto-optic modulator which is easily integrated into other devices. Furthermore, the present invention can be used to fabricate a modulator which is situated on a single substrate along with accompanying waveguides. The modulators fabricated are very efficient and high speed modulators. Furthermore, due to the materials used and the purity/consistency of the materials, very low power is required to achieve the necessary modulation. 
     In accordance with the above-mentioned goals and objectives, the magnetic modulator of the present invention is fabricated using the processes of ion beam and magnetron deposition. It is now possible to use these processes to fabricate thin films of magnetic materials having the necessary magneto-optic characteristics to form a magneto-optic modulator. The deposition processes can be used at differing points in the fabrication of optical devices because the deposition processes are non-destructive methods of fabrication. More specifically, these methods of thin film deposition can be used to fabricate films on a single substrate without destroying previously fabricated structures that already exist upon that substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 displays one possible application for the magneto-optic modulator wherein the modulator is used in conjunction with a solid state ring resonator to sense inertial rotation; 
     FIGS. 2 a  and  2   b  are a perspective view and a side view, respectively, of one embodiment of a magneto-optic modulator fabricated using the process of the present is invention; 
     FIG. 3 is diagram illustrating the deposition system of the present invention; 
     FIG. 4 shows the magnetic aligning aspect of the deposition system of the present invention; and 
     FIGS. 5 a  and  5   b  show the alignment of the material deposited using previously known methods compared to the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One application of a modulator fabricated using the process of the present invention is in conjunction with a ring resonator. A more specific application of the ring resonator is its use to measure rotation (i.e., a gyroscope). 
     Referring to FIG. 1, there is shown an application of a modulator  10  fabricated using the principles of the present invention. The solid state gyroscope  20  shown in FIG. 1 has an optical ring resonator  22  which supports two counterpropagating optical signals. Optical signals are provided to ring resonator  20  by an optical signal source  24 . Optical signal source  24  may be a laser diode or other similar light producing device. 
     Optical signal source  24  provides two identical optical signals which are transmitted to a first modulator  10  and a second modulator  12  via a first waveguide  26  and a second waveguide  28 , respectively. First modulator  10  and second modulator  12  receive the transmitted optical signal from optical signal source  24  and provide a modulated optical signal to a first optical coupler  30  and a second optical coupler  32 . These modulated optical signals are then coupled into ring resonator  22  and thus counterpropagate therein. 
     Located adjacent to ring resonator  22  is a first detection coupler  40  and a second detection coupler  42 . The counterpropagating optical signals which are resonating within ring resonator  22  are coupled into first detection coupler  40  and second detection coupler  42 . The signal coupled into first detection coupler  40  is provided to a first detector  44  which is capable of sensing the received optical signal. Similarly, the optical signal coupled into second detection coupler  42  is communicated to second detector  46  which is also capable of detecting the characteristics of the received optical signal. In conjunction, first detector  44  and second detector  46  are then capable of sensing the characteristics of the two counterpropagating optical signals. More specifically, first detector  44  and second detector  46  are capable of detecting the frequency of the two optical signals counterpropagating within ring resonator  22 . Electrical signals are produced by first detector  44  and second detector  46  which are indicative of their received optical signals. The electrical signal produced by first detector  44  is connected to a control means  50  via connection  52 . Similarly, the electrical signal produced by second detector  46  is communicated to control means  50  via connection  54 . 
     In summary, solid state gyroscope  20  detects rotation by detecting a shift in the resonant frequency of the optical signals counterpropagating within ring resonator  20 . When ring resonator  22  sits stationery in inertial space, a set difference in frequency is produced between the optical signals so that the counterpropagating optical signals are less sensitive to each other&#39;s backscatter. Alternatively, when ring resonator  22  is rotated about its central axis, the resonant frequency of the optical signals resonating within ring resonator  22  is shifted. The resonant frequency in one signal will be increased while the resonant frequency in the other signal will be decreased. It is this shift in resonant frequencies which is indicative of rotation. First detector  44  and second detector  46  are used to sense this shift in resonant frequency. In response to the shift in resonant frequency, controller  50  provides appropriate signals to first modulator  10  and second modulator  12 . These modulators can then shift the frequency of the optical signals being provided to ring resonator  20  and thus maintain equivalent difference in resonant frequencies in each direction. These frequency shifts create a closed-loop system wherein the resonant frequency of the optical signals within ring resonator  22  are maintained at a constant differential frequency and the amount of frequency shift required to maintain this equilibrium is indicative of rotation. 
     Referring now to FIGS. 2 a  and  2   b,  there is shown a perspective view and a side view, respectively, of a modulator fabricated in accordance with the method of the present invention. Reference will be specifically made to first modulator  10 ; however, it is understood that all statements apply equally to second modulator  12  and all other components related thereto. 
     First modulator  10 , along with first waveguide  26  and first optical coupler  30 , are all situated upon a single substrate  60 . Substrate  60  could be fabricated from any number of materials including, but limited to, silica, silicon dioxide, or gallium arsenide. The desirable characteristics of substrate  60  are low coefficient of thermal expansion and good adhesion qualities, thus providing a good base upon which materials can be deposited. 
     Upon substrate  60  is an optical containment layer  62  which is fabricated of a material displaying the necessary characteristics to contain any optical signals within first waveguide  26 , first modulator  10  and first optical coupler  30 . 
     First waveguide  26  is shown as a single block of material. It will be understood by those skilled in the art that first waveguide  26  could take on many forms. The necessary requirements for first waveguide  26  are its ability to efficiently transmit optical signals. The optical signals carried by first waveguide  26  are then coupled into first modulator  10 . In order to efficiently couple these optical signals, it is required that modulator  10  and first waveguide  26  be precisely aligned. Similar alignment is required to transmit optical signals from modulator  10  to first optical coupler  30 . When dealing with optical components of the size contemplated by the present invention (i.e., a waveguide approximately 10 mils across and a modulator approximately 1-2 cm long), it is recognized that appropriate alignment can be a very tedious and difficult task. 
     Upon optical containment layer  62  is the structure making up first modulator  10 . Directly deposited upon optical containment layer  62  is a magnetic material  66 . Examples of appropriate magnetic materials are iron permalloys and garnets while it is understood that other materials exist. Magnetic material  66  must have the desired characteristic of changing its optical properties in the presence of a magnetic field. 
     Upon the upper surface of magnetic material  66  is deposited a metallic film  70 . Metallic film  70  provides the necessary means for carrying electrical current, thus creating a magnetic field. Electrical leads  70  and  74  are connected to metallic film  70  to provide the desired electrical current, thus creating the desired magnetic fields. 
     Although it is not shown in FIGS. 2 a  and  2   b,  it will be understood that an overcoating could be deposited upon all exposed surfaces of first waveguide  26 , modulator  10  and optical coupler  30  to provide further optical containment. To achieve this function it is necessary that the material used for the overcoating have the appropriate optical qualities, i.e., the appropriate index of refraction. Furthermore, such an overcoating will shield the waveguides and optical elements from external optical sources. 
     The modulator shown in FIGS. 2 a  and  2   b  is manufactured by a deposition system in a coating chamber as shown in FIG.  3 . The deposition technique uses a low energy ion beam means  80  along with a magnetron deposition means  82  to deposit the magnetic material on the substrates  84  held by a substrate holder  86  connected to a planet apparatus  87 . The planet apparatus  87  has multiple substrate holders  86  and rotates the substrate holders  86  above the deposition source  82  as material is deposited onto the substrate  84 . The planet apparatus is known in this area of technology and will not be discussed in any further detail. Further, multiple deposition sources can be used to deposit multiple layers of different materials on the substrates  84 , but the detailed description will describe the use of only one deposition source as an example of the preferred embodiment. 
     FIG. 4 a  shows the substrate holder  86  in an enlarged view. Magnets  88  connected with a pole piece  90  create a magnetic field to orient the materials deposited on the substrate  84 . As stated in the background of the invention, the deposited mirrors are not optimally deposited and result in disoriented clustered deposition as seen in FIG. 5 a  which causes losses and shifts in operation. The magnetic field created by the magnets  88  cause rotation of the deposited material components so that orientation of the deposited material is optimally oriented and deposited as seen in FIG. 5 b  thus changing the optical characteristics of that material. Due to the orientation of the material, changes in optimal operation without loss and shift results. 
     The use of low energy ion beam deposition used in the fabrication of the modulator structure has many advantages. Of these advantages, the most important is the ability to provide the material with proper energy and time to orient and then attach optimally. Deposited magnetic materials do not usually have enough energy to orient, but attach to the substrate  84  too quickly. The low energy ion beam means  80  provides more energy to the deposited material so that the material has more time to orient properly and optimal deposition is attained. A low energy ion beam means is used since it provides just enough energy to allow the optimal orientation to be achieved, but not too much energy so that destruction of the deposited material is prevented. An example of a low energy ion beam means would be a Hall effect ion source. However, the present invention is not limited to the use of this source, but any low energy deposition means could be used in the present invention. 
     Another advantage of ion beam deposition is to fabricate a magnetic modulator without destroying existing structures that may exist on the substrate  60 . In the present embodiment, first waveguide  26  and first optical coupler  30  already exist upon substrate  66  before magnetic modulator  10  is fabricated thereon. Using ion beam deposition allows magnetic modulator  10  to be deposited on substrate  60  without destroying the existing structures of first waveguide  26  and first optical coupler  30 . Furthermore, ion bean deposition is a very efficient method by which thin films of material are fabricated. The thin films fabricated using this method replicate the target material very well and are free of impurities which may be induced by the process of fabrication. Furthermore, ion beam deposition allows the effective fabrication of a thin film of magnetic material which is not possible using other methods of deposition. 
     Coating chambers are known in this area of technology for depositing material on substrates. The ion beam means and magnetron deposition means are well known in this area of technology and can be interchanged with other deposition means to fulfill the spirit of the present invention. Further, magnets and pole pieces are used and described as one embodiment of the present invention. However, it is understood that many other means can be used to orient the deposited material and the magnets and pole pieces are used purely as examples of the present invention. 
     It will be understood by those skilled in the art that different structures can be used to fabricate a modulator which will operate similarly to that disclosed in the preferred embodiment. For example, a similar magnetic material could be used in conjunction with a dielectric mirror to form a bounce-type magnetic modulator. While the structures may vary, the principles of operation and fabrication remain the same. 
     Having described the present invention in considerable detail, it will be understood that the method of the present invention can be altered without departing from the scope of the present invention. We claim all modifications and alterations coming within the scope and spirit of the following claims.