|Publication number||US5023944 A|
|Application number||US 07/402,508|
|Publication date||Jun 11, 1991|
|Filing date||Sep 5, 1989|
|Priority date||Sep 5, 1989|
|Publication number||07402508, 402508, US 5023944 A, US 5023944A, US-A-5023944, US5023944 A, US5023944A|
|Inventors||Eric M. Bradley|
|Original Assignee||General Dynamics Corp./Electronics Division|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (54), Non-Patent Citations (4), Referenced by (42), Classifications (14), Legal Events (10)|
|External Links: USPTO, USPTO Assignment, Espacenet|
I. Technical Field
The present invention relates to optical electronics. More specifically, the present invention relates to integrated optical electronics for use in optical communication and computing applications, wherein the integrated optical electronics are primarily fabricated in the form of monolithic semiconductor structures.
II. Background Art
Conventional integrated optics apparatus for communications or computation typically comprise active devices such as emitters, modulators, and detectors as well as passive devices such as resonators for filtering. These devices may range in size from relatively large glass reflectors and mirrors to microscopic semiconductor laser diodes. In any case, each device is generally a separate, discrete component. Therefore, assembling a complete optical communications or computation system comprising a combination of such devices requires physical integration at the component level of many different optical devices. Many disadvantages arise as a result of such component-level integration.
Most obvious is the difficulty encountered in assembling components to provide the extremely accurate alignments necessary for proper operation. The resulting misalignments greatly complicate coupling the plurality of devices to a single optical beam.
A further disadvantage of the prior art component-level architecture is its susceptibility to disalignment due to vibration and temperature fluctuation. Because the optical devices are merely interconnected and do not share a sufficiently substantial common structural support, they are likely to become disaligned or detuned from mechanical vibration or thermal expansion or contraction. Also, the materials of which resonant devices themselves are constituted expand and contract with temperature variations and also show variations in their indices of refraction. These properties cause detuning of resonators and result in wavelength variations in emitted and detected light.
To overcome the disadvantages of the prior art, it is therefore desirable to have a method and apparatus which provides for the integration of optical electronics in primarily monolithic dielectric and/or semiconductor architecture during the fabrication process.
It is further desirable to provide monolithic integrated optics whereby each optical device is very closely aligned to a common optical .axis so as to facilitate the coupling of all devices to a single optical beam or single mode optical fiber.
The novel method and apparatus of the present invention provides integrated optics which overcome the disadvantages of the prior art and further provide many significant new features and advantages.
The present invention provides a novel and improved method and apparatus of integrated optics which overcome the disadvantages of the prior art, and further provides significant new advantages.
In preferred embodiments, the present invention comprises a plurality of optical resonators which may be active resonators such as emitters or detectors and/or passive resonators such as resonators for filtering, for example. The resonators are successively formed primarily by epitaxial growth for semiconductors or sputtering for dielectrics, stacked immediately adjacent to one another, typically vertically on top of one another, all originating from a semiconductor substrate. Such a stack of resonators can be fabricated simply and precisely with methods already known in the art such as Vapor Phase Epitaxy (VPE) or Molecular Beam Epitaxy (MBE) combined with well known deposition, etching, and photolithographic techniques.
It is a notable novel feature of the present invention that all resonators are closely aligned to a common optical axis without implementing any alignment procedures.
Due to novel and improved structure, devices in accordance with the present invention provide many advantages. The stacked resonators can be fabricated consistently with conventional techniques such as VPE and MBE and Ion Beam Sputtering, and they require no assembly or alignment past the fabrication stage. Further, because of the stacked architecture, they are wafer space efficient and can be coupled to a single optical beam or single mode optical fiber by Holographic Optical Elements (HOE's) being developed elsewhere. Therefore, many resonator stacks may be easily integrated into a two dimensional array of stacks for use in appropriate systems. Additionally, because all of the resonators are monolithic, they will remain tuned under significant mechanical vibration and temperature variation.
Temperature stability is particularly enhanced when dielectric materials which have specially temperature stabilized refractive indices are employed. This feature can be accomplished by choosing the materials according to their coefficients of thermal expansion and the variation of refractive index with temperature. By proper selection and processing of materials, one can fabricate structures with alternating layers having thermal variations which cancel each other, or one can fabricate mixtures of such materials (composite layers) which have thermal variations which are greatly reduced. The active material can be a nonlinear glass which can achieve optic-optic modulation since this material might be easier than single crystal semiconductor materials to fabricate between two multilayer dielectric mirrors. Dielectric materials are suggested since this material type shows the greatest promise for temperature stabilized properties.
Various configurations of active and/or passive resonator stacks may be produced in accordance with the present invention merely by varying the composition and doping profiles while growing the devices, and/or further altering the semiconductor properties with techniques such as diffusion or ion implantation. In addition, sputtering and evaporation deposition techniques may be employed in situ, creating layers comprised of materials substantially different from the semiconductor material.
These and other features and advantages of the invention will become fully apparent from the detailed description set forth below when taken in conjunction with the drawings wherein:
FIG. 1 illustrates in the schematical form a cross section of an exemplary resonator structure formed upon a substrate;
FIG. 2 is a graph of cavity resonance with respect to wavelength relative to the exemplary structure as disclosed with reference to FIG. 1;
FIG. 3 illustrates a structure similar to FIG. 1 configured for wavelength multiplexing/demultiplexing and emission/detection;
FIG. 4 is a graph illustrating transmittance versus wavelength characteristics of the resonators of FIG. 3;
FIG. 5 illustrates a single resonator FM to AM converter structure;
FIG. 6 is a graph illustrating the characteristics of the FM to AM converter of FIG. 5;
FIG. 7 illustrates an alternate embodiment to an FM to AM converter;
FIG. 8 is a graph illustrating the response characteristics of the structure of FIG. 7;
FIG. 9 illustrates in schematical form a surface emitting laser formed upon a substrate and having a feedback grating formed upon the laser;
FIG. 10 is a graph illustrating the relative index of refraction versus height above the substrate for the laser of FIG. 9;
FIG. 11 is an alternate embodiment of the structure of the surface emitting laser of FIG. 9 incorporating a phase control resonator;
FIG. 12 illustrates in schematical form a surface-emitting laser similar to that of FIG. 9 having formed on top thereof an electronically adjustable cavity used for coupling light from the surface emitting laser;
FIG. 13 is a graph corresponding to FIG. 12 illustrating the index of refraction with respect to height above the substrate for the laser of FIG. 11;
FIG. 14 is a graph illustrating for a surface-emitting laser the index of refraction versus height above substrate;
FIG. 15 is a graph illustrating a variation in the index of refraction profile for the active region of FIG. 14;
FIG. 16 is a graph illustrating the index of refraction versus height above substrate for a variation of a surface emitting laser;
FIG. 17 is the emission in a structure constructed according to the index of refraction of FIG. 16;
FIGS. 18-19 are variations of the active region as described with reference to FIGS. 14-16; and
FIG. 20-21 are representative implementations of the integrated optic devices described with reference to FIGS. 1-19.
FIG. 1 illustrates a first preferred embodiment of the present invention in which at least one surface emitting (SE) laser is coupled to at least one cavity or resonator structure for optical devices.
This first embodiment comprises Fabry-Perot resonators 10, 12 and 14 which represent a plurality of resonators stacked immediately adjacent to one another. This embodiment further comprises substrate 16 which may be formed of a conductive or semi-insulating material upon which the stack of resonators is formed. The structure is typically fabricated by epitaxial growth of III-V semiconductor materials, such as the Alx G1-x As family for example, upon a substrate. The layers comprising the resonators are each subsequently formed by monolithic fabrication techniques known in the art such as epitaxial growth or sputtering combined with photolithographic, deposition and etching techniques.
Resonators 10, 12, and 14 include corresponding optical cavities 11, 13, and 15 respectively. The optical cavities may be passive, for filtering for example. In the alternative the optical cavities may be active, containing doping profiles providing saturable absorbers, electro-absorption media, optically bi-stable material, or lasing or light emitting junctions, for example.
Resonators 10, 12, and 14 further comprise multilayer dielectric mirror structures represented by dielectric mirrors 18, 20, 22 and 24. Each resonator has an associated pair of multilayer mirrors, i.e. mirrors 18 and 20 being associated with resonator 10, mirrors 20 and 22 with resonator 12, and mirrors 22 and 24 with resonator 14. The dielectric mirrors are fabricated of a plurality of alternating layers of high and low refractive index dielectric or semiconductor material. The semiconductor mirrors are lattice matched to the semiconductor material of the optical cavity between the mirrors so that high-quality crystal growth can be achieved throughout the structure. The number, dimensions and composition of the dielectric or semiconductor layers comprising the dielectric or semiconductor mirrors and optical cavities may be varied as prescribed by optical wave theory, well understood in the art, so as to provide desired wavelength selectivity and reflectance/transmittance variation. In the first preferred embodiment the resonators are typically tuned near the same wavelength to provide enhanced propagation and coupling at that wavelength.
In the first preferred embodiment, at least one of optical cavities 11, 13 or 15 comprises a semiconductor doping profile such that an active region is formed within the optical cavity. The active region, in cavity 15 for example, is capable of being pumped by electron/hole injection so that resonator 14 forms a surface-emitting laser or a light-emitting diode (LED). The direction of light emission, as indicated by arrow 17, is perpendicular to a plane common to mirrors 22, 24 and cavity 15. The remaining resonator/cavity layers of the device may be passive, acting as filters for example, or active, containing doping profiles providing saturable absorbers, electro-absorption media, or optically bi-stable materials, for example.
The first preferred embodiment of the present invention allows surface-emitting lasers, such as the SE laser of resonator 14, which comprise at least one of its resonators, to be coupled to cavities, such as cavities 11 and 13, containing other active or passive materials. Examples of highly-useful implementations of such cavity coupling are cavities having saturable absorbers for mode locking, electro-absorption media for modulation, or optically bi-stable materials for logic functions. It is understood that any number, combination, and ordering of such resonators can be constructed in accordance with the generic principles of the present invention.
Furthermore, according to the first preferred embodiment of the present invention, resonators 10, 12 and 14 have their resonant frequencies very near each other so that the light emitted by one resonator is efficiently coupled to other resonators. Efficient coupling is accomplished by fabricating each resonator's associated mirrors, as prescribed by thin film optical theory understood in the art, so as to provide each resonator with a bandpass filter characteristic.
An exemplary plot of cavity resonance is shown in FIG. 2. Curve 30 shows the cavity resonance characteristics of resonator 10, curve 32 shows the characteristics of resonator 12, and curve 34 shows the characteristics of resonator 14. Given such a resonator configuration, the present invention allows wavelengths in a highly reflected range, area 36, to be coupled and propagate throughout the device while other undesirable frequencies are rejected. This feature provides for exceptional longitudinal mode discrimination.
The first preferred embodiment further provides for exceptional mode discrimination because surface emitting lasers are employed rather than the more conventional edge emitter. Surface emitting lasers have a cavity length typically two orders of magnitudes shorter than conventional edge emitters. In the conventional edge emitter diode laser, the longitudinal modes are typically a few angstroms apart. However, because the surface emitting laser has a cavity length much shorter than the edge emitter, the longitudinal modes are separated by as much as several hundred angstroms. Under these circumstances, the passband of a series of mirrors similar to those with characteristics corresponding to FIG. 2 provides useful mode discrimination.
The present invention as implemented as disclosed with reference to FIG. 1 further provides for accurate control of coupling strength between resonating cavities over a wide range. This aspect of the invention is due to the fact that the dielectric mirror or semiconductor reflectivity can be accurately controlled by altering the number and thicknesses of various layers in the mirrors which separate the resonators.
It is further possible to avoid the need for coupled cavities in particular instances by growing, for example, a layer of saturable absorber material (for mode locking) in the same resonator as the active layer of the laser. This can be viewed as the limiting case of strongly coupled cavities, in which the reflectivity of the mirror separating the two cavities goes to zero.
A second preferred embodiment is shown in FIG. 3. The second embodiment is similar in structure to the first preferred embodiment, however, the second embodiment is designed and suited for wavelength multiplexing/demultiplexing and emission/detection. To provide these functions, each successive resonator in the second embodiment is tuned to a resonator at a longer wavelength which is still in the passband of the preceding resonator. This differs from the first embodiment in which certain resonators are tuned slightly off a common wavelength to provide coupling between signals emitted by the emitter or emittees of the other cavities.
As illustrated in FIG. 3, the second embodiment, like the first embodiment, comprises vertically stacked Fabry-Perot resonators 40, 42, and 44 which represent a plurality of resonators stacked immediately adjacent to one another, as well as conductive or semi-insulating substrate 46. This device is also fabricated primarily by epitaxial growth of III-V semiconductor materials, such as the Alx G1-x As family for example, in combination with deposition, etching, and photolithographic techniques.
Resonators 40, 42 and 44 include corresponding optical cavities 41, 43, and 45, respectively. The cavity layers may be passive for filtering, or active, containing doping profiles providing saturable absorbers, electro-absorption media, optically bi-stable material, or lasing or light-emitting junctions, for example.
Resonators 40, 42, and 44 further comprise multilayer dielectric mirror structures represented by multilayer dielectric mirrors 48, 50, 52 and 54. Each resonator has an associated pair of multilayer mirrors; mirrors 48 and 50 being associated with resonator 40, mirrors 50 and 52 with resonator 42, and mirrors 52 and 54 with resonator 44. As in the first embodiment, the semiconductor mirrors are fabricated of a plurality of alternating layers of high and low index semiconductor material. These layers are lattice matched to the semiconductor material in between the mirrors so that high-quality crystal growth can be achieved throughout the structure. The number, dimensions and composition of the semiconductor layers comprising the mirrors and cavities may vary as prescribed by optical wave theory, understood in the art, so as to provide wavelength selectivity and reflectance/transmittance variation.
The second preferred embodiment allows separate generation and detection of separate optical signals which have been impressed on optical beams of separate wavelength in the case where all those signals are traveling in the same beam, for example on the same optical fiber.
Resonators 40, 42, and 44 have their resonant frequencies separated from one another so that the light emitted or detected by one resonator is transparent to another resonator. This aspect is accomplished by fabricating each resonator's associated mirrors so as to provide each resonator with a low-pass filter characteristic. The low-pass filter characteristics results in high optical frequencies (shorter wavelengths) being resonated, and low optical frequencies (longer wavelengths) being transmitted. An example of such a resonator transmittance configuration is shown in FIG. 4.
Referring to FIGS. 3 and 4, mirrors 48 and 50 of resonator 40 have dimension and composition configuration such that their interaction provides resonator 40 with a cut off wavelength λ1, illustrated by curve 60. Furthermore, the active material of optical cavity 41 is formed of a high bandgap material such that it absorbs the shorter wavelength signals and is thus sensitive to a wavelength less than the cutoff wavelength λ1. Resonator 42 of FIG. 3 comprises mirrors 50 and 52 and optical cavity 43 with the active material in optical cavity 43 having a lower bandgap than optical cavity 41. Resonator 42 has a cut off wavelength λ2 illustrated by curve 62 and or is responsive to wavelengths between the cutoff wavelengths λ1, typically nearer, the cutoff wavelength λ2. Similarly, resonator 44 comprises mirrors 52 and 54 and optical cavity 45 with the active material in optical cavity 45 having a lower bandgap than optical cavity 43. Resonator 44 has a cut off wavelength λ3, illustrated by curve 64 and is responsive to wavelengths between cutoff wavelengths λ2 and λ3, typically nearer the cutoff wavelength λ3. Such a scheme may be continued down a stack having a greater number of resonators than illustrated in FIG. 3.
Any combination and number of resonators having wavelength selectivity as described may be constructed in accordance with the present invention. If the active layers of the resonators are all detectors, as in, for example, a structure used to demultiplex the output of an optical fiber carrying a wavelength multiplexed signal, then each of the optical cavities can be made of a ternary alloy such as Alx Ga1-x As where the composition is changed in each detector layer so that the higher band gap materials are on the top of the stack and tend to be transparent to the longer wavelengths in the multiplexed signal. The optical cavities can be made relatively thin because the resonator causes the beam having the desired wavelength to make multiple passes through the detector medium, thus increasing the interaction length. The thin layers also provide reduced residual absorption of the longer wavelengths in the higher band gap layers on top so as to reduce crosstalk in the detector/receiver.
Additionally, a stack of surface-emitting lasers may be fabricated in accordance with the second embodiment of the present invention. Such a configuration could be realized with the SE laser on top having the shortest wavelength of emission and the laser on the bottom having the longest wavelength of emission.
Also, combination modules having one or more resonators containing detector material and one or more resonators having a lasing medium may be fabricated. In such a configuration the detector layer or layers could operate as horizontal p-i-n structures with p and n regions only on the edges of the layer. This would require good intrinsic material for the growth layer. One might observe that such a structure is similar to a photoconductive detector. Alternatively, the detector layer or layers may be grown intrinsic having ohmic contacts provided at the layer's edges. Thus configured, the layer may operate as a photoconductive device instead of a p-i-n detector.
The doping for the SE lasers in a combination module typically requires a p-n junction, and high-band gap layers surrounding the active region for injected current confinement. Such a doping profile may be achieved by doping n type during growth and opening a small diffusion window, with photolithographic and etching techniques known in the art so that a p-type doping may be introduced.
A specific implementation of the second preferred embodiment is particularly suited to be a frequency modulation (FM) to amplitude modulation (AM) converting detector for coherent optical communications. In such an implementation, cavities 41, 43 and 45 of FIG. 3 each comprise active detector material, each having a different bandgap. Resonator 40 passes wavelengths greater than the cutoff wavelength λ1 x illustrated by curve 60, and has detector material having a bandgap responsive to wavelength less than the cutoff wavelength of curve 60. Resonator 42 passes wavelengths greater than the cutoff wavelength λ2 illustrated by curve 62 and has bandgap material responsive to wavelengths between the cutoff wavelengths λ1 and λ2, typically nearer cutoff wavelength λ2. Resonator 44 passes wavelengths greater than the cutoff wavelength λ2 and has a bandgap detector material responsive to wavelengths between the cutoff wavelengths λ2 at curve 62 and λ3 at curve, 64, typically nearer the cutoff wavelength λ3. Thus, each successive resonator is responsive to longer wavelengths than the previous resonator and shorter wavelengths than the following resonator. Therefore, each resonator is capable of converting a selected frequency band from the incoming optical beam to an electrical amplitude. Such a configuration simplifies the required structure and increases the performance in the detection of coherent FM optical signals for fiber optic data links or other optical data processing systems.
Slight variation of the second preferred embodiment further provides stacks of optical devices suitable for FM to AM converting. One such configuration is shown in FIG. 5. This configuration comprises a single Fabry-Perot resonator 70. Resonator 70 comprises optical cavity 71 and multilayer dielectric mirrors 74 and 76. Optical cavity 71 is formed primarily of semiconductor material having a doping profile providing photodetection properties. The material of cavity 71 is responsive optimally to wavelengths near the center of wavelength of curve 80 of FIG. 6. Resonator 70 may be formed upon a substrate 72 or a layer 73 between substrate 72 and resonator 70.
Cavity 71 together with mirrors 74 and 76 are configured to provide a resonator having wavelength response as illustrated in FIG. 6. So configured, out-of-band frequencies carried by a received optical beam are reflected by resonator 70. Frequencies in band, near the center wavelength λc of curve 80, enter the resonator and are absorbed by the detector medium of cavity 71. Thus, the configuration illustrated in FIG. 5 is capable of converting frequencies in the wavelength range near the center wavelength to an electrical amplitude, while rejecting all other frequencies. Such a "single resonator FM to AM converter" can be incorporated with other structure to provide a device in accordance with the present invention.
An additional variation of the second preferred embodiment suitable for FM to AM converting is shown in FIG. 7. This variation comprises resonator 90 formed upon resonator 92 which is formed upon substrate 94. Resonator 92 comprises optical cavity 93 and broadband multilayer semiconductor mirrors 96 and 98. Cavity 93 is formed primarily of a semiconductor material having a doping profile which provides photodetection properties. The material of cavity 93 is responsive optimally to wavelengths near wavelength λ2 FIG. 8. Cavity 93, together with mirrors 96 and 98, are configured to provide a Fabry-Perot resonator having wavelength response according to curve 102 as illustrated in FIG. 8.
Resonator 90 of the FM to AM converter of FIG. 7 further comprises multilayer dielectric mirrors 94 and 96, and optical cavity 91. Cavity 91, together with mirrors 94 and 96 are configured to provide resonator 90 with a mirror 94 having a wavelength response according to curve 100 as shown in FIG. 8. Mirror 94 may thus be considered as a longwave pass filter. Cavity 91 is formed primarily of semiconductor material having a doping profile which provides photodetection properties and is a higher bandgap material than cavity 93. The material of cavity 91 is responsive optimally to wavelengths lower than wavelength λ1, but near wavelength λ1.
An optical signal, input to the device of mirror 94, having a wavelength of λ1 sees high reflectivity at mirrors 94 and 96 so it appears as a resonator, but only to λ1 wavelength signals. The detector material of optical cavity 91 absorbs the energy at λ1 but not at λ2. Signals at λ2 wavelength pass through mirror 94 and cavity 91 where it sees mirrors 96 and 98 as forming a resonator.
This variation of the second preferred embodiment as illustrated in FIG. 7 converts frequencies having a wavelength less than wavelength λ1 but , near wavelength λ1, to a first amplitude corresponding electrical signal, while converting frequencies having a wavelength near λ2 to a second amplitude corresponding electrical signal. Thus, differential detection may be accomplished between two different frequency bands of a received optical signal which is shifted between the wavelength region less than wavelength λ1 and the wavelength region near wavelength λ2. It is notable that this system does not require a local oscillator. It is understood that the number of resonators and specific wavelength responses of the resonators may be varied according to the present invention while still providing FM to AM converting.
A third preferred embodiment of the present invention is shown in FIG. 9. FIG. 9 illustrates an external grating feedback surface-emitting laser in accordance with the present invention. This device comprises substrate 108, laser active region 110, multilayer dielectric mirrors 112 and 114, and feedback grating region 116. Mirror 112 is formed on substrate 108, with laser active region 110 disposed between mirrors 112 and 114, and feedback grating region 116 formed on mirror 114.
Substrate 108 is typically a III-V material which may be conductive or semi-insulating. Semiconductor mirrors 112 and 114 are fabricated of a plurality of alternating layers of high and low index semiconductive material. These layers are lattice matched to the semiconductor material in between the mirrors, so that high-quality crystal growth can be achieved throughout the structure. The number, dimensions, and composition of the semiconductor layers comprising the mirror and laser cavity may vary so as to provide frequency selectivity and reflectance/transmittance variation. Laser active region 110 further comprises a semiconductor doping profile such that an active region is formed in the optical cavity. Laser active region 110 is capable of being pumped by electron/hole injection so that the cavity forms a surface-emitting laser. Feedback grating region 116 may be formed by methods such as epitaxial crystal growth, sputtering, electron beam evaporation, or thermal evaporation techniques, for example. Feedback grating region 116 comprises a material which has a periodic variation in its refractive index.
The index of refraction profile as a function of height above the substrate of the device of FIG. 9 is shown in FIG. 10. Region 120 is an exemplary refractive index profile of multilayer dielectric or semiconductor mirror 112. Region 122 is an exemplary profile of the index of refraction of the laser active region 110. Region 124 is an exemplary profile of multilayer dielectric or semiconductor mirror 114. Region 126 is an exemplary profile of the refractive index of feedback grating region 116.
The device of the third preferred embodiment of the present invention uses optical resonators containing index variations, feedback grating region 116 of FIG. 9, to feedback selected spectral components into the laser resonator. The effect of this is to encourage lasing action at the wavelength which has been fed back. Such a configuration of the present invention serves to control the wavelength of emission of a surface emitting laser. The external grating, feedback grating region 116, can be passive thus stabilizing the wavelength, or it can be active to allow wavelength tunability.
The surface-emitting laser of the third embodiment is contained between the lower two multilayer dielectric or semiconductor mirrors 112 and 114. Above the laser is grown or deposited a material which has a periodic variation in its refractive index to form a grating. This grating reflects light at the Bragg wavelength, which is near the wavelength that is twice the peak-to-peak spacing of the periodic index variation. The reflected light is fed back into the laser resonator so as to lock the wavelength of emission to the Bragg wavelength.
If the grating is fabricated by a process such as MBE, it is very easy to shorten the period of the index variation to less than 1,000 angstroms, thus allowing feedback gratings for visible wavelength lasers to be easily fabricated.
If the grating is constructed of semiconductor materials (e.g., GaAs/AlGaAs) the index of refraction can be modified by injection of current into the grating material at electrodes formed upon the grating. Current injection will change the Bragg wavelength of the grating. Since the laser's wavelength is locked to the Bragg wavelength of the grating. The wavelength of the laser's emission will change accordingly. As a result a tunable wavelength laser is created. The very wide spacing and wavelength of the longitudinal modes of the surface-emitting laser will allow a wider range of continuous tunability than has been possible with conventional edge-emitting structures.
As in conventional active feedback diode laser structures, it may be necessary to insert a third resonator with electrically-controlled index to provide phase control. Such a variation of the third preferred embodiment of the present invention is illustrated in FIG. 11. FIG. 11 shows phase control resonator 130 disposed between feedback grating region 116' and the surface-emitting laser, the laser comprising laser-active region 110' and multilayer dielectric or semiconductor mirrors 112' and 114'. Phase control resonator 130 comprises a cavity of semiconductor material 132 and multilayer dielectric or semiconductor mirror 134. Phase control resonator 130 utilizes mirror 114 with cavity 132 disposed between mirrors 114 and 132.
A variation of the third preferred embodiment of the present invention provides electronically adjustable cavity coupling for surface-emitting lasers and is illustrated in FIG. 12. This feature is accomplished by coupling the external grating feedback surface-emitting laser to an adjacent cavity, comprising multilayer dielectric or semiconductor coupling mirror 136, cavity 138 and mirror 140.
The index of refraction profile of such a configuration is shown in FIG. 13. Region 142 is identical to the profile of the external grating feedback surface-emitting laser as shown in FIG. 10. Regions 144, 146, and 148 comprise the adjacent resonator to be coupled to the surface-emitting laser. Region 144 is a tunable coupling grating or mirror 136 having a periodic index of refraction that is identical to the periodic index variation of the grating 114" of the external grating feedback surface-emitting laser of FIG. 12. Region 146 is either active or passive semiconductor material corresponding to cavity 138. Region 148 is a multilayer dielectric or semiconductor mirror corresponding to mirror 140.
The coherent light emitted from the surface-emitting laser is coupled to the resonator comprising regions 144, 146, and 148 by tunable coupling grating 136. The coupling grating's reflectivity at the laser's emission wavelength can be controlled by injecting current into the coupling grating at electrodes coupled thereto. Thus, coupling grating 136 can be tuned or detuned so as to reflect or transmit the light generated by the surface-emitting laser. If coupling grating 136 strongly reflects the light at the laser's emission wavelength, the laser will be minimally coupled to the second cavity. If the tunable coupling grating 136 transmits light at the laser's emission wavelength, the laser will be strongly coupled to the second cavity.
Like other preferred embodiments of the present invention, the number, composition, order, and combination of resonators and related structures may vary in accordance with the third preferred embodiment.
Utilizing the basic teaching presented above many variations in formation of the cavities, mirrors and gratings may be generated. Each of these structures may be represented easily by a graph illustrating index of refraction versus height above the substrate. For purposes of convenience, the formation of these various layers is presented in the form of these charts.
Many embodiments of the present invention may incorporate surface-emitting lasers having their emission wavelength stabilized by variably spaced quantum wells. The index of refraction profile of one such wavelength stabilized surface-emitting laser is shown in FIG. 14. Regions 150 and 152 have periodically varying index such that they have optimum reflectance at a single frequency which has wavelength twice the distance of the periodic variation. Mirrors 150 and 152 serve as the reflecting mirrors for the resonating cavity 154 of active material which includes quantum well layer regions 156.
Quantum wells are formed in the active layer by "shuttering" or selectively introducing an aluminum component, along with the Ga and As components during the epitaxy technique formation of the active layer. The introduction of quantum wells into the optical cavity active layer 154 will lower the lasing threshold current due to the reduced density of states in the well. Quantum wells formed in the optical cavity active layer also increase the wavelength of emission due to the reduced bandgap in the active layer. Increased emission wavelength allows the lowest bandgap in the multilayer semiconductor mirrors to be lowered. A reduction in the lowest bandgap in the multilayer semiconductor mirrors will permit the index of refraction to increase and thus improve reflectivity of the mirrors.
FIG. 14 shows evenly "chirped" spacing of quantum wells. Alternatively, the active cavity region 154 including quantum well layer regions 156 may be replaced by quantum wells varying sinusoidally such as illustrated in FIG. 15. Furthermore, sinusoidally varying index of refraction mirrors 150 and 152 may be alternatively replaced with conventional multilayer mirrors having stepped index of refraction profiles such as multilayer dielectric or semiconductor mirrors 150' and 152' shown in FIG. 16.
As illustrated in FIG. 17, such index of refraction profiles stabilize the emission wavelength of a surface-emitting laser because the gain is significantly greater in the regions with a high density of quantum wells than in the regions without quantum wells. The wavelength experiencing the greatest gain will be equal to twice the spacing of the peak gain regions (i.e., twice the spacing of the groups of quantum wells). Other wavelengths are not as highly favored and therefore will tend not to oscillate.
Alternative index of refraction profiles for the active region comprising active layer region 154 of FIG. 14 are shown in FIGS. 18 and 19. FIG. 18 illustrates an active layer region 154" having regions 158 between the quantum wells regions 156 smoothly graded. FIG. 19 illustrates the active layer region 154'" comprising the quantum well regions 156 having regions 160 between the quantum wells stepped in index of refraction. It is understood that many combinations and variations of the index of refraction profiles illustrated will provide a wavelength stabilized surface-emitting laser which may be incorporated in embodiments in accordance with the present invention.
It is an important novel feature of the present invention that optical devices comprising resonators and related structures are stacked vertically on top of one another, all successively grown epitaxially and originating from a semiconductor substrate. Yet another important novel feature of the present invention is that the properties of the optical devices and related structures comprising embodiments in accordance with the present invention have operating characteristics which are established merely by varying doping profiles while growing the devices, and/or further altering semiconducting properties with techniques such as diffusion or ion implantation, for example. Additionally, it is another important novel feature of the present invention that the optical devices comprising embodiments of the present invention are integrated during fabrication as stacked combinations of structures which all are closely aligned to a common optical axis perpendicular to the epitaxial layers comprising the device.
It is understood that variations in number, order, compositions, and operating characteristics of the individual optical devices and related structures stacked to comprise the described preferred embodiments of the present invention may be made freely in accordance with the novel features of the present invention. It is further understood that individual optical devices and structures comprising the described preferred embodiments may be integrated with other optical devices and structures not described herein in accordance with the novel features of the present invention.
It should be noted that the drawings of the preferred embodiments enclosed herein illustrate devices having ideally flat sides. However, owing to conventional chemical etching processes which may be employed in producing devices in accordance with the present invention, the sides of the devices may typically taper from substrate toward the top of the device.
FIG. 20 illustrates an exemplary implementation of devices in accordance with the present invention. Integrated optical devices 162 and 164 are devices in accordance with the present invention such as those described herein as preferred embodiments. Lenses 166 and 168 are optical lenses capable of focusing light into a narrow optical beam. Optical fiber 170 is a waveguide capable of carrying signals of optical frequency. A system as shown in FIG. 19 could employ devices in accordance with the present invention capable of wavelength multiplexing/demultiplexing and emission/detection for communication of information at optical frequencies.
FIG. 21 illustrates devices in accordance with the present invention adapted for use in parallel array processing structures/architectures involving spatial light modulators. Grids 172 and 174 represent a two-dimensional array of integrated optical devices in accordance with the present invention. Integrated optical devices 172a and 174arepresent two individual devices, one from each of grids 170 and 172 respectively. Lens 176 represents a lens or other beam-handling equipment.
Integrated optical devices 172a and 174a and lens 178 comprise a single channel of the parallel array processing structure which requires the transmission and reception of multiple wavelengths along one optical beam. Therefore, many embodiments of integrated optical devices in accordance with the present invention could be employed to generate, receive, and process optical signals required for the parallel array processing architecture.
The previous descriptions of the preferred embodiments are provided to enable any person skilled in the art to use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive facility. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features enclosed herein.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US3471802 *||Feb 1, 1967||Oct 7, 1969||Texas Instruments Inc||Modulated laser using a solid fabry-perot etalon having a birefringent center core|
|US3549236 *||Sep 30, 1968||Dec 22, 1970||Us Army||Optical frequency discriminator with dual frequency resonator|
|US3622911 *||Dec 2, 1968||Nov 23, 1971||Bell Telephone Labor Inc||Laser oscillator with mode selector|
|US3656836 *||Jun 26, 1969||Apr 18, 1972||Thomson Csf||Light modulator|
|US3766393 *||Nov 22, 1971||Oct 16, 1973||Rca Corp||Optical data transmission system employing polarization-shift, multiple-cavity laser|
|US3813605 *||Nov 7, 1972||May 28, 1974||Massachusetts Inst Technology||Bistable optical device|
|US3930718 *||Apr 12, 1974||Jan 6, 1976||The United States Of America As Represented By The Secretary Of The Navy||Electro-optic modulator|
|US3982207 *||Mar 7, 1975||Sep 21, 1976||Bell Telephone Laboratories, Incorporated||Quantum effects in heterostructure lasers|
|US4009453 *||Oct 14, 1975||Feb 22, 1977||Siemens Aktiengesellschaft||Narrow-band interference filter and its application in a laser system|
|US4023993 *||Aug 28, 1975||May 17, 1977||Xerox Corporation||Method of making an electrically pumped solid-state distributed feedback laser|
|US4024466 *||Aug 18, 1975||May 17, 1977||Werkzeugmaschinenfabrik Oerlikon-Buhrle Ag||Laser containing a laser resonator|
|US4115747 *||Dec 27, 1976||Sep 19, 1978||Heihachi Sato||Optical modulator using a controllable diffraction grating|
|US4196396 *||May 3, 1978||Apr 1, 1980||Bell Telephone Laboratories, Incorporated||Interferometer apparatus using electro-optic material with feedback|
|US4233569 *||Mar 19, 1976||Nov 11, 1980||General Electric Company||High power laser with tuning and line narrowing capability|
|US4240696 *||Nov 13, 1979||Dec 23, 1980||Rockwell International Corporation||Multilayer electro-optically tunable filter|
|US4269481 *||Jul 6, 1979||May 26, 1981||Rockwell International Corporation||Multiple-cavity electro-optic tunable filter|
|US4274706 *||Aug 30, 1979||Jun 23, 1981||Hughes Aircraft Company||Wavelength multiplexer/demultiplexer for optical circuits|
|US4318059 *||Jun 13, 1980||Mar 2, 1982||Nippon Electric Co., Ltd.||Semiconductor laser device having plural active regions|
|US4328468 *||Jan 16, 1980||May 4, 1982||Sperry Corporation||Multipass laser optics for light deflector|
|US4395769 *||Mar 3, 1981||Jul 26, 1983||Bell Telephone Laboratories, Incorporated||Tunable semiconductor laser|
|US4438447 *||Jan 18, 1982||Mar 20, 1984||Bell Telephone Laboratories, Incorporated||Multilayered optical integrated circuit|
|US4498179 *||Jul 30, 1982||Feb 5, 1985||The Unites States Of America As Represented By The Secretary Of The Army||Modulated infrared laser with two coupled cavities|
|US4503447 *||Jul 16, 1982||Mar 5, 1985||The United States Of America As Represented By The Secretary Of The Army||Multi-dimensional quantum well device|
|US4504950 *||Mar 2, 1982||Mar 12, 1985||California Institute Of Technology||Tunable graded rod laser assembly|
|US4532632 *||Jul 30, 1982||Jul 30, 1985||Omron Tateisi Electronics Co.||Tunable semiconductor laser|
|US4546244 *||Mar 14, 1984||Oct 8, 1985||At&T Bell Laboratories||Nonlinear and bistable optical device|
|US4550410 *||Apr 16, 1982||Oct 29, 1985||United Technologies Corporation||Coupled-cavity laser|
|US4558449 *||Jul 8, 1983||Dec 10, 1985||At&T Bell Laboratories||Semiconductor laser with coupled loss modulator for optical telecommunications|
|US4573156 *||Sep 16, 1983||Feb 25, 1986||At&T Bell Laboratories||Single mode laser emission|
|US4575851 *||Dec 7, 1982||Mar 11, 1986||Nippon Electric Co., Ltd.||Double channel planar buried heterostructure laser with periodic structure formed in guide layer|
|US4583226 *||Jul 28, 1983||Apr 15, 1986||At&T Bell Laboratories||Coupled cavity injection laser|
|US4583820 *||Dec 23, 1982||Apr 22, 1986||Instruments S.A.||Wavelength multiplexer/demultiplexer using optical fibers|
|US4585491 *||Sep 2, 1983||Apr 29, 1986||Xerox Corporation||Wavelength tuning of quantum well lasers by thermal annealing|
|US4589115 *||Sep 9, 1983||May 13, 1986||Xerox Corporation||Wavelength tuning of quantum well heterostructure lasers using an external grating|
|US4599728 *||Jul 11, 1983||Jul 8, 1986||At&T Bell Laboratories||Multi-quantum well laser emitting at 1.5 μm|
|US4602370 *||May 12, 1983||Jul 22, 1986||At&T Bell Laboratories||Large optical cavity laser having a plurality of active layers|
|US4603420 *||Jul 1, 1983||Jul 29, 1986||Handotai Kenkyo Shinkokai||Optical integrated circuit|
|US4618959 *||Oct 12, 1983||Oct 21, 1986||Nec Corporation||Double heterostructure semiconductor laser with periodic structure formed in guide layer|
|US4622674 *||May 7, 1984||Nov 11, 1986||Nec||Single longitudinal mode semiconductor laser|
|US4658402 *||Oct 5, 1984||Apr 14, 1987||Nec Corporation||Optical bistable semiconductor laser producing lasing light in direction normal to semiconductor layers|
|US4660206 *||Jul 2, 1984||Apr 21, 1987||Hughes Aircraft Company||Chirp laser stabilization system|
|US4665527 *||Nov 6, 1984||May 12, 1987||Kokusai Denshin Denwa Kabushiki Kaisha||Distributed feedback semiconductor laser|
|US4671830 *||Jun 9, 1986||Jun 9, 1987||Xerox Corporation||Method of controlling the modeling of the well energy band profile by interdiffusion|
|US4674094 *||Feb 7, 1985||Jun 16, 1987||Mitsubishi Denki Kabushiki Kaisha||Semiconductor laser with an active layer having varying thicknesses|
|US4680768 *||Jul 15, 1985||Jul 14, 1987||Sanyo Electric Co., Ltd.||Semiconductor laser with a quantum well layer including a disordered region|
|US4705361 *||Nov 27, 1985||Nov 10, 1987||Texas Instruments Incorporated||Spatial light modulator|
|US4709371 *||Oct 18, 1985||Nov 24, 1987||West Fred D||Variable wavelength laser diode|
|US4718070 *||Jan 22, 1985||Jan 5, 1988||Massachusetts Institute Of Technology||Surface emitting diode laser|
|US4796269 *||May 22, 1987||Jan 3, 1989||Oregon Graduate Center||Two-dimensional phase-locked surface emitting semiconductor laser array|
|US4869780 *||Apr 7, 1988||Sep 26, 1989||Trw Inc.||Ion milling method|
|US4881236 *||Apr 22, 1988||Nov 14, 1989||University Of New Mexico||Wavelength-resonant surface-emitting semiconductor laser|
|US4881237 *||Aug 26, 1988||Nov 14, 1989||Massachusetts Institute Of Technology||Hybrid two-dimensional surface-emitting laser arrays|
|US4899361 *||Apr 13, 1988||Feb 6, 1990||Nec Corporation||Optical filter device|
|US4901327 *||Oct 24, 1988||Feb 13, 1990||General Dynamics Corporation, Electronics Division||Transverse injection surface emitting laser|
|1||Bachtold et al., "Integrated Bidirectional Fiber Optic Repeater", 4/81, IBM Tech. Dis. Bull., vol. 23, #11.|
|2||*||Bachtold et al., Integrated Bidirectional Fiber Optic Repeater , 4/81, IBM Tech. Dis. Bull., vol. 23, 11.|
|3||*||Lozes Duony et al., Recent Developments . . . Diode Lasers , J. Opt. (France), vol. 20, 2, pp. 65 70, 3 4/89.|
|4||Lozes-Duony et al., "Recent Developments . . . Diode Lasers", J. Opt. (France), vol. 20, #2, pp. 65-70, 3-4/89.|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US5262660 *||Aug 1, 1991||Nov 16, 1993||Trw Inc.||High power pseudomorphic gallium arsenide high electron mobility transistors|
|US5365540 *||Sep 22, 1993||Nov 15, 1994||Nec Corporation||Vertical-to-surface optical semiconductor device and apparatus for coupling optical signals|
|US5401983 *||Apr 7, 1993||Mar 28, 1995||Georgia Tech Research Corporation||Processes for lift-off of thin film materials or devices for fabricating three dimensional integrated circuits, optical detectors, and micromechanical devices|
|US5527732 *||Jul 14, 1994||Jun 18, 1996||Nec Corporation||Method for fabricating semiconductor laser and photo detecting arrays for wavelength division multiplexing optical interconnections|
|US5617250 *||Jun 28, 1993||Apr 1, 1997||Jenoptik Gmbh||Radiation projection arrangement with integrated radiation indicator|
|US5999322 *||Jun 28, 1995||Dec 7, 1999||Cushing; David Henry||Multilayer thin film bandpass filter|
|US6180990||Mar 26, 1999||Jan 30, 2001||Lockheed Martin Corporation||Hyperspectral radiation detector|
|US6215592||Mar 19, 1999||Apr 10, 2001||Ciena Corporation||Fabry-perot optical filter and method of making the same|
|US6355939||Nov 3, 1998||Mar 12, 2002||Lockheed Martin Corporation||Multi-band infrared photodetector|
|US6665459 *||May 17, 2002||Dec 16, 2003||Marconi Uk Intellectual Property Limited||Wavelength selective optical filter|
|US6897447||Dec 5, 2002||May 24, 2005||Lockheed Martin Corporation||Bias controlled multi-spectral infrared photodetector and imager|
|US7135698||Dec 5, 2002||Nov 14, 2006||Lockheed Martin Corporation||Multi-spectral infrared super-pixel photodetector and imager|
|US7310454 *||May 24, 2004||Dec 18, 2007||Massachusetts Institute Of Technology||Photonic bandgap modulator, amplifier, demux, and TDM devices|
|US7521658 *||Dec 1, 2005||Apr 21, 2009||Aptina Imaging Corporation||Pixel having photoconductive layers to absorb different ranges of wavelengths|
|US8567960 *||Sep 16, 2009||Oct 29, 2013||Ostendo Technologies, Inc.||Quantum photonic imagers and methods of fabrication thereof|
|US8644649||May 27, 2011||Feb 4, 2014||Micron Technology, Inc.||Optical waveguide with cascaded modulator circuits|
|US8705898||Jun 16, 2008||Apr 22, 2014||The Trustees Of Columbia University In The City Of New York||Systems, devices and methods for tuning a resonant wavelength of an optical resonator and dispersion properties of a photonic crystal waveguide|
|US8909000||Jan 13, 2014||Dec 9, 2014||Micron Technology, Inc.||Optical waveguide with cascaded modulator circuits|
|US8995489 *||Mar 8, 2013||Mar 31, 2015||Ricoh Company, Ltd.||Semiconductor stack and vertical cavity surface emitting laser|
|US9030278||Aug 2, 2012||May 12, 2015||Cts Corporation||Tuned dielectric waveguide filter and method of tuning the same|
|US9030279||Dec 3, 2011||May 12, 2015||Cts Corporation||Dielectric waveguide filter with direct coupling and alternative cross-coupling|
|US9051178 *||Sep 14, 2012||Jun 9, 2015||Gwangju Institute Of Science And Technology||Transmissive image modulator including stacked diode structure having multi absorption modes|
|US9130255||Nov 25, 2013||Sep 8, 2015||Cts Corporation||Dielectric waveguide filter with direct coupling and alternative cross-coupling|
|US9130256||May 29, 2014||Sep 8, 2015||Cts Corporation||Dielectric waveguide filter with direct coupling and alternative cross-coupling|
|US9130257||Aug 25, 2014||Sep 8, 2015||Cts Corporation||Dielectric waveguide filter with structure and method for adjusting bandwidth|
|US9130258||Sep 18, 2014||Sep 8, 2015||Cts Corporation||Dielectric waveguide filter with direct coupling and alternative cross-coupling|
|US9209904||Nov 6, 2014||Dec 8, 2015||Micron Technology, Inc.||Optical waveguide with cascaded modulator circuits|
|US20030174423 *||Mar 15, 2002||Sep 18, 2003||Hart Brian T.||Monolithic filter array|
|US20030174424 *||Jun 28, 2002||Sep 18, 2003||Hart Brian T.||Monolithic filter array|
|US20040108461 *||Dec 5, 2002||Jun 10, 2004||Lockheed Martin Corporation||Bias controlled multi-spectral infrared photodetector and imager|
|US20040108564 *||Dec 5, 2002||Jun 10, 2004||Lockheed Martin Corporation||Multi-spectral infrared super-pixel photodetector and imager|
|US20050259922 *||May 24, 2004||Nov 24, 2005||Shoji Akiyama||Photonic bandgap modulator, amplifier, DEMUX, and TDM devices|
|US20070125934 *||Dec 1, 2005||Jun 7, 2007||Matthews James A||Pixel having photoconductive layers|
|US20100066921 *||Mar 18, 2010||Ostendo Technologies, Inc.||Quantum Photonic Imagers and Methods of Fabrication Thereof|
|US20100270481 *||Jun 16, 2008||Oct 28, 2010||Columbia University||Systems, devices and methods for tuning a resonant wavelength of an optical resonator and dispersion properties of a photonic crystal waveguide|
|US20130175500 *||Sep 14, 2012||Jul 11, 2013||Gwangju Institute Of Science And Technology||Transmissive image modulator including stacked diode structure having multi absorption modes|
|US20130243024 *||Mar 8, 2013||Sep 19, 2013||Ricoh Company, Ltd.||Semiconductor stack and vertical cavity surface emitting laser|
|CN103311805A *||Mar 13, 2013||Sep 18, 2013||株式会社理光||Semiconductor stack and vertical cavity surface emitting laser|
|CN103311805B *||Mar 13, 2013||Sep 2, 2015||株式会社理光||半导体层叠板和垂直腔面发射激光器|
|EP0595469A1 *||Sep 27, 1993||May 4, 1994||Nec Corporation||Vertical-to-surface optical semiconductor device and apparatus for coupling optical signals|
|WO1999047956A1 *||Mar 19, 1999||Sep 23, 1999||Ciena Corporation||Fabry-perot optical filter and method of making the same|
|WO2008157484A1 *||Jun 16, 2008||Dec 24, 2008||The Trustees Of Columbia University In The City Of New York||Systems, devices, and methods for tuning a resonant wavelength of an optical resonator and dispersion properties of a photonic crystal waveguide|
|U.S. Classification||359/107, 348/343, 398/201, 398/164, 359/890|
|International Classification||G02F1/21, H01S5/183|
|Cooperative Classification||H01S5/4087, H01S5/18302, H01S5/426, H01S5/18361, G02F1/218|
|European Classification||H01S5/42B2, G02F1/21S|
|Sep 5, 1989||AS||Assignment|
Owner name: GENERAL DYNAMICS CORPORATION, A CORP. OF DE, CALIF
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:BRADLEY, ERIC M.;REEL/FRAME:005131/0383
Effective date: 19890830
|Dec 17, 1991||AS||Assignment|
Owner name: TACAN CORPORATION, CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:GENERAL DYNAMICS CORPORATION ELECTRONICS DIVISION;REEL/FRAME:005947/0158
Effective date: 19911203
|Jan 17, 1995||REMI||Maintenance fee reminder mailed|
|Feb 3, 1995||FPAY||Fee payment|
Year of fee payment: 4
|Feb 3, 1995||SULP||Surcharge for late payment|
|Jan 4, 1999||AS||Assignment|
Owner name: UNION BANK OF CALIFORNIA, N.A., CALIFORNIA
Free format text: SECURITY AGREEMENT;ASSIGNOR:TACAN CORPORATION;REEL/FRAME:009703/0068
Effective date: 19981230
|Jan 5, 1999||REMI||Maintenance fee reminder mailed|
|Jun 13, 1999||LAPS||Lapse for failure to pay maintenance fees|
|Aug 24, 1999||FP||Expired due to failure to pay maintenance fee|
Effective date: 19990611
|Jun 6, 2000||AS||Assignment|
Owner name: TACAN CORPORATION, CALIFORNIA
Free format text: SECURITY INTEREST;ASSIGNOR:UNION BANK OF CALIFORNIA N.A.;REEL/FRAME:010892/0422
Effective date: 19990708