Abstract:
A resonant coupler has a coupling region having first and second ends, a coupling length, and a tapered variable width, such that a phase matching condition for the waveguide is met within the coupling length. In an exemplary embodiment the first and second ends each have a corresponding variable width which varies at a rate greater than the variable width of the coupling length. In yet another embodiment the coupling region has an electrically sensitive phase matching characteristic and an electrode in operative relation with the coupling region for varying the phase matching characteristic.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/110,015 filed Nov. 25, 1998, and U.S. Provisional Application No. 60/116,076 filed Jan. 15, 1999, the disclosures of which are hereby incorporated by reference in their entirety. 
    
    
     This invention was made with Government support under Contract Number MDA 90495C2037 awarded by the National Security Agency. The Government has certain rights in the invention. 
     This invention was made with government support pursuant to contract No. MDA90495C2037 awarded by the National Security Agency. The National Security Agency has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention pertains to optical coupling, and in particular, to optical resonant coupling between two waveguides. 
     There is a need for monolithically integrating various active and passive optical devices to obtain highly functional optical modules. The platform technology to this monolithic integration should be as simple as possible to keep costs low. Currently, the technologies used for monolithic integration, like selective area growth, or regrowth, are not only very expensive but also do not allow for enough freedom in designing the various active and passive devices. 
     The modes of a laser and an optical fiber are poorly matched in size and shape, leading to poor coupling efficiency therebetween. By integrating a mode expander with a laser, it is possible to obtain efficient coupling to an optical fiber. The invention describes a general technique by which a mode can be coupled from a tightly confined waveguide to a loosely confined waveguide. In particular, the invention employs resonant coupling to achieve mode expansion over a relatively short distance to efficiently couple a tightly confined waveguide in an active region of a semiconductor laser/amplifier and a loosely confined mode in a passive waveguide such as an optical fiber. 
     Various mode expanders have been demonstrated based on a concept called adiabatic mode transformation. In this approach, there are two separate sections in the device: a section optimized for high gain; and a section optimized for maximum coupling to a fiber. The sections are linked through a mode expander region, which adiabatically transforms, i.e. expands, the mode from the first section to the second section. For minimum losses to occur, adiabatic mode transformation must take place gradually over a relatively long length, e.g. 500 microns. In an exemplary device, as shown in FIGS. 1A and 1B, the adiabatic mode expander is formed as an extension to the active device such as a laser. 
     Mode transformation can be achieved by means of a technique known as resonant coupling, sometimes hereinafter referred to as phase matching. When two waveguides are approximately matched in their refractive indices and dimensions, and are located in close proximity, there is a transfer of power between the two waveguides in an oscillatory fashion. In other words, the mode in one waveguide is coupled to the proximate waveguide. This phenomenon is illustrated in FIGS. 1C and 1D wherein two waveguides I &amp; II, each 1 μm wide and each with a refractive index of 3.21 are proximately located in side by side relationship. Waveguide I may be an active device having an electrical current input, and waveguide II may be a passive device. The power transfer between the two waveguides is theoretically 100%. If the refractive index of waveguide I is changed, for example, to 3.23 keeping the width unchanged, weak coupling results as the waveguides are no longer phase matched. However, if the width of the waveguide I is changed, for example, to 2 μm, strong coupling is once again observed. This implies that the phase matching condition is re-established. Thus, the phase matching condition depends both on the refractive index and the dimensions of the waveguide. The device, shown in FIGS. 1C and 1D, comprises two rectangular waveguides: active waveguide I and coupling waveguide II. Waveguide I has a higher refractive index than waveguide II, but has a smaller size in both the horizontal (lateral) and vertical (transverse) directions. On the other hand, the waveguide II has a lower refractive index than waveguide I, but has a larger size in both dimensions. The two waveguides can be designed so that their effective refractive indices are nearly equal. This close match of the effective refractive indices forces the power to couple back and forth between the two waveguides in an oscillatory fashion over a characteristic overlapping distance along the length of the waveguides, known as the coupling length L c . 
     In principle, by cleaving the device within the coupling length L c  at the exact point where the mode resides in the lower coupling waveguide II, i.e., where transfer takes place, a mode expander can be realized. However, the refractive indices of materials are not known with great precision. Further, the refractive indices also depend on the current injection level in the active device I. Given this uncertainty in the refractive indices, it is not possible to design the waveguides for optimum power transfer. In addition, the point where the mode in waveguide I couples to waveguide II is not known a priori. Therefore, the practical feasibility of the method has not been demonstrated so far. In addition, there is an oscillatory power transfer along the device between the two waveguides due to the phase-matched condition being met along the entire length of the device. Because, there is a power loss during each oscillatory cycle, the device length is usually limited to L c . This limits the length of the gain (active) region of waveguide I, which in turn, affects the performance of the device. 
     SUMMARY OF THE INVENTION 
     The invention pertains to an approach for resonant coupling from a waveguide to another waveguide positioned in the horizontal or vertical plane, using an horizontal or a vertical taper, or a combination of both. The optical coupling between the two waveguides occurs over a very short taper with low optical loss. The invention enables monolithic integration of various passive and active optical devices and allows for improved coupling efficiency and alignment tolerances between various waveguides such as a laser and an optical fiber. 
     Multiple waveguides can be placed in close proximity to each other. Each waveguide can be optimized, for a specific optical function (e.g. active waveguide optimized for gain, passive waveguide optimized for ease of coupling, passive waveguide optimized for splitting, directional coupling or other passive devices). Using a properly designed taper, the optical mode can be moved from one waveguide to another as many times as required, therefore, achieving monolithic integration of several optical devices performing different functions. 
     The above-mentioned limitations of existing approaches are overcome in the device according to the invention by using a mode transformer having a selected tapered geometry for the active waveguide I. The horizontal dimension of waveguide I is tapered from a initial large width (W i ) to a small final width (W f ). To have arbitrarily long gain sections integrated into the device, it is necessary to have a portion where the two waveguides are off resonance, ensuring that the mode is resident almost entirely in waveguide I. This condition is achieved by choosing a suitably large initial width W i . The tapering of waveguide I between two extreme widths ensures that the mode matching condition is met somewhere along the taper. When this condition is met, the dimensions of the waveguide I change slowly so that the optical power is transferred resonantly to waveguide II. It does not matter where exactly it occurs as long as it is in the range expected. In addition, the taper is such that waveguide I achieves a cutoff condition over a length smaller than the coupling length L c , thus forcing the mode to be resident in waveguide II without oscillation. This makes the point of cleaving non-critical. In addition, the characteristic lengths can be designed to be much smaller than the lengths needed for adiabatic mode transformers. These short characteristic lengths directly translate to shorter lengths for the overall mode expanders. 
     In an exemplary embodiment, the mode transformer for coupling modes between first and second waveguides includes a coupling region having first and second ends, a coupling length, and a variable tapered width, such that, a phase matching condition is met within the coupling length defined in said coupling region. A tapered inlet for each end of the coupling region transmits a mode between each waveguide and the corresponding end of the coupling region. The inlets each have a first variable width which varies at a rate greater than the variable width of the coupling region. 
     In another embodiment, the coupling region includes an electrode means for varying a cut off characteristic of the coupling region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A &amp; 1B are schematic illustrations of an adiabatic mode transformer; 
     FIGS. 1C &amp; 1D are generalized schematic illustrations of two waveguides exhibiting resonant coupling; 
     FIG. 2 is a sectional view of resonantly coupled waveguides according to the invention; 
     FIG. 3 is a plain view of a taper shape for the resonantly coupled waveguides shown in FIG. 2; 
     FIG. 4 is a graphical representation of light intensity as a function of distance along the waveguide in FIG. 2; 
     FIG. 5 is a plain view of an exemplary taper shape; 
     FIG. 6 is a graphical representation of the light intensity as a function of distance in a waveguide formed with the taper of FIG. 5; 
     FIG. 7A is a perspective illustration showing a device with active and passive monolithic integration; 
     FIG. 7B is a cross-section of FIG. 7A taken along line  7 B— 7 B thereof; 
     FIG. 8 is a plain view illustrating side-by-side coupling of phase matched waveguides; 
     FIG. 9 is a plain view of side-by-side coupling of off resonant waveguides where one waveguide is tapered; 
     FIG. 10A is a plain view of a waveguide employing a separate cutoff electrode; 
     FIG. 10B is a perspective illustration of the arrangement shown in FIG. 10A; 
     FIG. 10C is a sectional view of the waveguide shown in FIG. 10A along line  10 C— 10 C thereof; 
     FIG. 11 is a graphical representation of light v current with bias on a device having a second electrode; 
     FIG. 12 is a perspective illustration of a device employing active-passive monolithic integration; 
     FIG. 13 is an illustration of a multiple waveguide stack showing active and passive integration with confined and expanded mode sections; 
     FIGS. 14A and 14B are schematic plan and sectional views respectively of an active-passive monolithic optical piece with multiple channels; 
     FIGS. 15A-15E are schematic illustrations of various waveguides which may be employed in any desired combination in accordance with the invention; 
     FIG. 16 shows the absorption vs. wavelength for a polarization sensitive waveguide; and 
     FIG. 17 shows a vertical section of a coupling region of a resonant coupler in accordance with the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The following is a presentation of results from a set of computer simulations to demonstrate the use of resonant coupling between two waveguides for realizing a mode-expanded device according to the invention. 
     A schematic diagram of a basic device  20  used in the simulations is shown in FIG.  2 . Table I sets forth the layer structure of the device  20  including active device  22  having an upper ridge width W R    24  and an active region  26 , and a passive device  28  formed of a waveguide  30  on a substrate  32 . Electrodes  34  and  36  disposed on the top and bottom respectively, provide the current terminals. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Layer Name 
                 Thickness 
                 Refractive index 
               
               
                   
                   
               
             
             
               
                   
                 1. Inp Upper Cladding 
                  1.5 μm 
                 3.169 
               
               
                   
                 2. Active region 
                 0.11 μm 
                 3.460 
               
               
                   
                 3 Coupling Waveguide 
                  3.0 μm 
                 3.210 
               
               
                   
                 4. Inp Substrate 
                   
                 3.169 
               
               
                   
                   
               
             
          
         
       
     
     The active region  26  is a 5-Quantum well/4 barrier stack with 400 A° SCH regions (QW thickness=100 A°, Barrier thickness=100 A°), designed for optimum gain at 1.55 μm, the low-loss window of standard telecommunication optical fibers. The refractive index of the 5 QW/4 barrier active layer stack  26  has an approximated equivalent index, which takes a weighted square average of the refractive index. For the simulations described herein, an equivalent index of 3.460 (a weighted average of 3.54 for the QW and 3.38 for the barrier) and an equivalent thickness of 0.11 μm (which represents the total thickness of the stack  26 ) were used. The arrangement of the active region  26  is discussed hereinafter. 
     The simulations were carried out using a commercial 3-D Beam Propagation Method (3-D BPM)software from Optiwave Corporation. This software is capable of propagating a given input field along any arbitrarily defined waveguide. To obtain the initial mode supported at the start of the propagation, a 2-D eigen solver provided by the software package was used. The mode obtained from this solver was propagated across the waveguides using the 3-D beam propagation software. The following set of simulations demonstrates resonant coupling. 
     For the above specified layer structure and a device width (W D ), shown in the plan view of FIG. 3, we first determine the ridge width W R  at which the phase matching condition is met. At this point the mode couples from the upper active region  26  to the lower coupling waveguide  30  over a characteristic coupling length L c  along the device  20  as shown. To determine this point, an input mode was propagated along the device  20 . The exemplary ridge width W R  of the active device  22  is linearly tapered as shown from an initial width W i  of 2.0 μm to a final width W f  of 0.3 μm over a length of 500 μm. 
     The light intensity distribution in the transverse direction as a function of the propagation distance is shown in FIG.  4 . In the exemplary embodiment, the observed resonant coupling between the active region and the coupling waveguide takes place where W R =1.4 μm. The characteristic length L c  is about 50 μm. Two oscillations are observed in which the power is coupled from one waveguide to the other. This leads to undesirable power losses which are obviated according to an embodiment of the invention discussed below. 
     FIG. 5 illustrates such an embodiment of the invention, wherein a waveguide  40  having a variable taper shape prevents oscillations and confines the mode to the lower waveguide after resonant coupling takes place. The device  40  shown in FIG. 5 has a moderately tapered coupling region  42  formed with respective opposite ends  44  and  46  and tapered walls  48  that converge slowly over the coupling length  42  (L c ). The larger end  44  has a more severe taper  50  to couple the active region  52  to the coupling region  42 . The small end  46  is terminated by a cut-off region  54  as shown. In the arrangement shown, the walls  48  are tapered from about 1.4 μm at the large end  44  to a ridge width W c  where the active waveguide cannot support any mode (cut-of condition), over a length smaller than the characteristic length L c . This ensures that the mode does not couple back to the active waveguide. Electrode  56  shown in dotted line covers the active region  52  and the coupling region  42  and  54 , and severe taper region  58 . 
     For the device design shown, the cut-off condition for the active waveguide is achieved at a ridge width of W C =0.7 μm. Accordingly, the ridge width is tapered from about W R =1.4 μm to about 0.3 μm over a length of about 100 μm, as shown. The taper  50  varies from about 2 μm to about W R =1.4 μm over a length of about 100 μm. The overall length of about 200 μm is considerably less than the length of the adiabatic device shown in FIG.  3  and can be further reduced with minimal optical losses. 
     The simulation results are shown in FIG. 6 where it can be observed that the mode is resonantly coupled to the underlying waveguide at a taper width of 1.4 μm. However, as the active waveguide width is decreased, below the cut-off width before the mode gets resonantly coupled back to the active region, i.e. the mode resides only in the coupling waveguide. Thus, by using the technique according to the invention, efficient mode transformation over very short lengths can be achieved. 
     FIG. 7A is a perspective illustration of another exemplary embodiment of the invention featuring active-passive monolithic integration. In the arrangement, a resonant coupler  60  is shown coupling an active gain section  62 , a passive section  64  and an intermediate mode transformer section  66 . The coupler  60  resonantly couples modes bidirectionally between an active waveguide  68  and a passive waveguide  70 . The gain section  62  has a uniform ridge width of about 2.5 μm as shown. Likewise, the top waveguide in the passive section  64  has a ridge width of about 0.3 μm to ensure cutof. The intermediate mode transformer section  66  has an overall length of about 100 μm which is long enough to keep loss low but short enough to ensure coupling without oscillations. Taper lengths as short as 40 μm can be designed for a slightly higher optical loss. The dimensions and taper shape shown in FIG. 7A are exemplary and depend on the type of materials and the functions implemented. In the exemplary embodiment, the active section  62  employs an active solid state laser waveguide with a confined mode. The passive section  64  employs a passive waveguide  70  with a confined mode. It should be understood, however, that the passive waveguide  70  may be less confined. Also, if desired, the arrangement shown may include a pair of resonantly coupled, confined active waveguides. 
     The shape of the tapered ridge  72  in mode transformer section  66  may be tailored for various waveguides having different characteristics. In the arrangement shown, the region  72  employs an exponential configuration including a first exponentially tapered section  74  at an interface  74  coupled to the gain section  62  and having an initial width W i −G, and second exponentially tapered section  76  starting at interface  78  and ending at the passive section  64  and having a final width W f −P, as shown. In the region where the first exponential section  72 , and the second exponential section  76  meet, the ridge width tapers gradually in such a way that the optical mode gets resonantly transferred from one waveguide to the other over the length Lc which corresponds to the coupling length. The width of ridge  80  within the coupling length L c  corresponds to the resonant ridge width W R . All useful coupling occurs within the coupling length L c . It should be understood that the device  60  shown in FIG. 7 employs an electrode  82  on the top surface, and an electrode  84  on the lower surface, as shown. 
     The layer structure of the device is shown in FIG.  7 B. The active layer  61  is formed of undoped four 1% compressively strained 10 nm thick quantum wells  63  for emission at 1.55 μm, and three lattice matched 10 nm barriers (λ g =1.25 μm)  65 . The active layer is surrounded by a 1.5 μm InP upper cladding (5×10 17  cm −3  p-doped)  67  and a 0.8 μm Inp lower cladding  69  (5×10 17  cm −3  n-doped). A cap layer  71  is disposed over the upper cladding  67  to establish ohmic contact with electrode  82 . The lower waveguide comprises a  20  layer stack of alternating InP ( 62 A)  73  and lattice matched quaternary  75  ( 138 A), with an equivalent refractive index of 3.32 and a total thickness of 0.4 μm. 
     In accordance with the invention, if the indices of refraction of the active and passive waveguides  68  and  70  are not close, the ridge taper in the coupling region  80  should gradually change between the initial to final width W i −W f  such that the coupling width W c  is between W i −W f . Similarly, if the indices of refraction of the active and passive waveguides are similar, coupling width W c  should likewise be between W i −W f . However, the taper may be less gradual because the coupling location is more readily determined. 
     Various alternative forms of the invention may be implemented as follows: 
     1. Different taper shapes: 
     a) Two linear tapered regions, the first one being adiabatic to approach the coupling width and the second region being a rapid taper to prevent coupling back. 
     b) Exponential+Linear Taper. 
     c) Linear+Exponential Taper. 
     d) Linear+Polynomial Taper or combinations thereof. 
     e) Exponential−Exponential. 
     f). Other functional dependence. 
     2. Different mode sizes in the two waveguides: 
     a) For mode transformer applications couple from the tightly confined mode to the large mode. 
     b) For other applications, e.g. involving integration of active and passive waveguides, the two waveguides can be of similar or dissimilar designs. 
     3. Different types of waveguides: 
     a) One waveguide may be active (bulk or quantum wells), the other one may be passive. 
     b) Both waveguides may be active. 
     c) Both waveguides may be passive. 
     4. 3-D Integration: 
     a) The method and arrangement employing two waveguides may be extended to multiple waveguides in the transverse direction, making it suitable for 3-D integration as shown in FIG.  13 . 
     5. Waveguide Tapers: 
     The waveguide can be of various types including ridge, buried, loaded, voltage induced or combinations thereof. 
     6. Functionality Integration: 
     The resonant tapers can be used several times to couple light back and forth between different waveguides of different functionality. This leads to monolithic integration of several optical devices, e.g., loss-less splitters, Mach-Zehnder interferometers based on electro-absorption modulators or semiconductor optical amplifiers, cross-connect switches, distributed bragg grating lasers, modulators integrated with lasers, and the like. 
     7. Polarization: 
     The optical devices can be polarization sensitive or insensitive. 
     8. Taper Direction: 
     To achieve resonant coupling, the thickness of the waveguides, instead of its width, can be varied appropriately; or both the width and the thickness can be varied. 
     9. Multiple Tapers: 
     Both the waveguides can have the tapers in the same section to achieve resonant coupling. 
     10. Materials: 
     The optical waveguides resonantly coupled can be in either semiconductors, glass or polymers. 
     11. Waveguide Orientation: 
     Tapered geometry may be used for coupling between two or more similar or dissimilar waveguides disposed side by side in the lateral direction instead of the transverse direction arrangements described above. 
     The conventional approach for coupling between two waveguides illustrated in FIG. 8, involves bringing the two phase matched waveguides IA-IIA into close proximity, for the resonant coupling to take place. For off-resonant waveguides, cumbersome grating assisted coupling schemes are required. In the approach using tapered ridge geometry according to the invention, it is possible to effect resonant coupling between two off-resonant waveguides IB-IIB as illustrated in the FIG.  9 . The tapered geometry of waveguide IIB ensures that phase match occurs at some point along the tapered portion. 
     In the above arrangements, a mode-expanded laser uses resonant coupling between two waveguides. A tightly confined active waveguide is preferred for optimized gain performance of the device, and a loosely confined larger waveguide is preferred for efficient fiber coupling. The two waveguides of different dimensions are phase-matched by adjusting their refractive indices. If the two waveguides are in close proximity to effect evanescent field coupling, power from one waveguide can be resonantly coupled to the second waveguide as discussed. According to the invention, the active waveguide is tapered from a wide initial ridge width W i  to a narrow final ridge width W f . The coupling width W c  is between the two values. The initial width W i  is chosen such that the two waveguides are off resonance and the power resides mainly in the active waveguide. The final width W f  is such that less than 1% of the power resides in the active region and the bulk of the power is in the larger waveguide. The taper is designed, such that the taper width varies very slowly so that optical power can propagate through the characteristic length L c  required for resonant coupling at the coupling width W c  where phase matching occurs. The taper is then reduced sharply so that the power does not couple back. 
     An exemplary device according to another embodiment of the invention uses an 100 μm exponential taper  88  long followed by a 100 μm linear taper. The region of the taper over which the mode is guided into the underlying waveguide is very small. Separate electrodes are employed in the active region and in the mode transformation region. An applied bias on the second electrode region changes the refractive index of the quantum wells appreciably by a phenomenon called electrorefraction. Having two electrodes on the active region, one in the active region and one in the mode transformation region, causes the laser to act like a switch. The first electrode is on the linear section of the device and is forward biased. This electrode provides the carriers required for the optical gain necessary for lasing or amplification. The second electrode is positioned on the section of the taper where the mode is coupled from the active waveguide to the underlying passive waveguide. By applying an appropriate negative bias on the second electrode, the refractive index of the active region may be increased so that the waveguides go off resonance and the coupling is destroyed or inhibited. The optical power instead of coupling into the underlying waveguide is radiated and the lasing may likewise be suppressed. In this device, electroabsorbtion also plays a role. 
     The device may be used as an inexpensive, directly driven laser source or as an external amplifier switch. As the switching is done using negative bias in the absence of injected carriers, the chirp (i.e. rise time) of the device should be low, permitting high-speed devices. Also, the taper which is used for mode expansion is also used for switching. Thus, there is no increase in the cavity length. As the device inherently has expanded mode, fiber coupling is simplified. Such an arrangement thus enables passive packaging of devices in a batch processing environment. Also these devices may use conventional fabrication techniques without regrowth. Thus, the devices are ideal for inexpensive optoelectronic circuits useful in various applications. 
     FIGS. 10A,  10 B and  10 C are respective top plan, perspective and sectional views of such a switchable device  90  according to the invention employing separate electrodes  92  and  94  on the top surface in the active region  96  and the mode transformation region  98  respectively. The device  90  shown in cross-section in FIG. 10C comprises active layer  102  formed of undoped five 1% compressively strained 10 nm thick quantum wells  104  for emmission at 1.55 μm, and four lattice matched 10 nm barriers (λ g =1.25 μm)  96 . The active layer is surrounded by a 1.5 μm InP upper cladding (5×10 17  cm −3  p-doped)  108  and a 3.0 μm lower cladding  110  (5×10 17  cm −3  n-doped). A cap layer  111  is disposed over the upper cladding  108  to establish ohmic contact with electrodes  92  and  94 . The lower cladding/coupling waveguide comprises of a  20  layer stack of alternating InP ( 1200 A)  112  and lattice matched quaternary  113  ( 300 A), such that the equivalent refractive index was 3.21. 
     The waveguides are grown on InP substrate  115  with a buffer layer  117 . The coupling width W c  of the active region where phase matching occurs is 1.4 μm. For efficient coupling between the two waveguides. A tapered ridge  120  has an optimized shape in the form of a third order 100 μm long exponential taper  122  followed by another 100 μm linear taper  124  from initial width of W i −1 of 2.0 μm to a final width W F  of 1.4 μm and another linear taper from a width W i −2 1.4 μm to a final width W i −2 of 0.3 μm. The two electrodes  92  and  94  are defined on top of the ridges. The first electrode  92  is 630 μm long and is positioned over the straight 2.0 μm ridge. The second electrode  94  is 150 μm long and is positioned over the taper  120 . 
     The device  90  is mounted p-side up on copper heat sinks (not shown). The first electrode  92  maybe pumped in pulsed mode and different reverse biases may be applied on the second electrode  94 . The light output from the expanded facet in the passive region has characteristic LI curves for various reverse biases as shown in FIG.  11 . When no bias is applied on the second electrode  94 , the device  90  lases at 70 mA. The lasing threshold is increased by ˜20 mA and ˜30 mA for reverse biases of 0.2 V on the second electrode respectively. With bias of 1.5 V, the device does not lase. Also, good modulation depth, even with very low bias of 0.2 V around the threshold may be seen. This shows the good switching response of the device. In FIG. 11, it can be appreciated that switching voltages is reduced from about 1 V to about 0.1 V. 
     Farfield measurements of the device  90  using a rotating stage and a pin-hole detector were made. The transverse and lateral farfields resulted in full width half maximum (FWHM) angles 24° and 10° respectively for the transverse and lateral sides. The FWHM angles from the control facet are 39° and 34° respectively for the transverse and lateral directions. This illustrates the expansion of the mode, which would be helpful for efficient fiber coupling. A butt coupling efficiency of 3.7 dB is expected to a standard cleaved single mode fiber at 1.55 μm. 
     In the arrangement of FIGS. 10A-10C the device  90  uses a single region for both mode expansion and switching. The cavity length is thus not increased and hence, the device is good for high speed applications and for making hybrid fiber DBR lasers. Also, as the switching is done using reverse bias. Carriers are not involved, and hence, the associated problems of chirp in the laser are minimized. 
     The device may be antireflection coated on both end faces and used in a traveling wave amplifier configuration as a switch. The external modulators like electroabsorption modulators have the disadvantage of high losses and need to be integrated with amplifiers either monolithically or in a hybrid arrangement. Hybrid integration would require complex packaging technology. To have passive packaging of the devices, it may be necessary to add mode expanders on the amplifiers or modulators. Monolithic integration typically requires growth of two active regions with different bandgaps. The device has the integrated mode expander along with providing good switching contrast. It also has a gain section, which can be tailored. Thus, the device performs the function of the amplifier and the external modulator reducing the number of devices required. 
     FIG. 12 shows an embodiment of the invention featuring active passive monolithic integration. The device  130  includes input fiber  132  coupled to a ridge waveguide  134  according to the invention, and a splitter  136 . Monolitically integrated amplifiers  138  are integrally coupled to the splitters at one end, and are aligned with output fibers  140  as shown. 
     FIG. 13 shows a three stage device  140  according to the invention. In the arrangement shown, a confined active region  142  is coupled to a confined passive region  144  by a tapered ridge arrangement  146  described hereinabove. An expanded mode section  148  is coupled to the passive section  144  by tapered ridge  150 . 
     FIGS. 14A and 14B are respective schematic plan and sectional views of an active-passive monolithic optical device  160  wherein an optical input  162  is divided or split into N channels  164 - 1  . . .  164 -N on a passive waveguide  166 . In each of the channels (e.g., channel  164 -N, as shown) the optical power or mode  170  is coupled to an active region  171  of active waveguide  168  using resonant coupling to achieve an optical gain therein. In this way a 1×N lossless splitter may be implemented. The mode  170  is then coupled back to the passive waveguide  166  to achieve further optical functionality. In channel  164 -N, an integrated Mach Zender  172  is implemented to achieve phase modulation. In the Mach Zender  172 , the passive waveguide  166  is divided into channels  164 -N 1  and  164 -N 2  to thereby divide the optical mode  170 , which is then coupled to the active region  174  in the active waveguide  168 . Biasing electrodes  176  and  178  are provided on the respective arms or channels  164 -N 1  and  164 -N 2 . By biasing electrodes  176  and  178  differently, a phase difference may be introduced into each arm or channel  164 -N 1 ,  164 -N 2  or the Mach Zender. The phase shifted modes resulting therefrom, namely  170 -S 1  and  170 -S 2 , are then coupled back to the passive waveguide  166  and combined using mode combiner  180 . In this way, the optical modes in the channel  164 -N can be phase modulated. 
     FIGS. 15A-15E illustrate various types of waveguides which may be employed in various combinations in accordance with the present invention. For example, FIG. 15A shows a deep etch ridge waveguide  190 A including core  192 A and cladding  194  A. FIG. 15B shows a shallow edge waveguide  190 B comprising core  192 B, cladding  194 B and residual cladding  196 B. FIG. 15C shows a buried waveguide  190 C having core  192 C and cladding  194 C all surrounding the core. FIG. 15D shows a loaded waveguide  190 D having loading in the form of dielectric or metal depositions  180 D which result in an effective core  192 D and an effective cladding  194 D. FIG. 15E shows a voltage-induced waveguide  199 E with electrodes  200 E and  202 E which receive a bias voltage  204 E resulting in a voltage-induced core  192 E and a voltage-induced cladding  194 E. 
     FIG. 16E shows absorption vs. wavelength for TE and TM modes of a polarization sensitive waveguide. In a polarization insensitive waveguide the absorption vs. wavelength characteristics for the TE and TM modes are essentially the same. 
     FIG. 17 shows a vertical section of a coupling region  210  between an active waveguide  212  and a passive waveguide  214 , wherein the thickness T of the coupling region  110  of the active waveguide  212  is tapered in the vertical direction to achieve resonance of the optical mode  216 . It should be understood that the coupling region of either or both waveguides may be so tapered. 
     While there have been described what as present are considered to be exemplary embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and is it intended in the appended claims to cover such changes and modifications which fall within the spirit and score of the claims.