Abstract:
An electro-absorption modulator is rendered capable of withstanding increased optical input power by one of the following means: incorporating a heat sink into the electro-absorption modulator structure to conduct heat away from the optical waveguide; incorporating a resistive member into the upper electrode of the electro-absorption modulator, producing a voltage drop that reduces absorption near the optical input end of the waveguide; making the bandgap energy of the absorbing layer of the waveguide higher at the optical input end than at the optical output end; and placing the electrode pad of the upper electrode near the optical input end.

Description:
BACKGROUND OF THE INVENTION 
     This application is a counterpart of Japanese Patent Application Ser. No. 326685/1999, filed Nov. 17, 1999, the subject matter of which is incorporated herein by reference. 
     The present invention relates to an electro-absorption modulator, and to a method of manufacturing an optical device from semiconductor materials. 
     An electro-absorption modulator is an opto-electronic device that modulates light intensity by modulating an electric field controlling absorption of the light. Electro-absorption modulators are used for various types of optical signal processing. In particular, the output of a semiconductor laser diode can be modulated more rapidly by an electro-absorption modulator than by modulation of the driving power of the laser diode itself. Electro-absorption modulators can be fabricated from semiconductor materials, enabling a modulator and laser to be integrated into the same semiconductor chip. Integrated laser-modulator chips with a distributed-feedback (DFB) laser are useful as transmitters in high-bandwidth fiber-optic communication systems. 
     Since the absorption of light generates heat, electro-absorption modulators are vulnerable to thermal damage. The optical input power level at which thermal damage occurs is referred to as the damage level. Thermal damage is discussed in “Reliability Study of InGaAlAs/InAlAs MQW Electro-absorption Modulator,” a paper presented by H. Kamioka et al. at the Optoelectronics and Communications Conference (OECC) held at Makuhari Messe in Japan in July 1998, published in the OECC Technical Digest (pp. 452-453). This paper studies damage levels of different electro-absorption modulator structures and their reliability below the damage level, concluding that damage level and reliability are related to the ability of the structure to dissipate the heat generated by absorption. The high mesa structure, for example, is found to dissipate heat more effectively than the buried heterostructure and thus to have a higher damage level. 
     The buried heterostructure, using semi-insulating iron-doped indium phosphide (Fe-doped InP), has been employed in integrated laser-modulator chips designed for broadband communication applications, but in addition to its low damage level, this structure is comparatively difficult to fabricate, because of the difficulty of growing crystalline semi-insulating InP with sufficiently high electrical resistance. An alternative structure is the ridge waveguide structure described in “76-km transmission over standard dispersion fiber at 10 Gbit/s using a high-power integrated laser modulator and a PIN receiver without any optical amplifier” by D. Lesterlin et al. in a paper presented at the Wednesday afternoon poster session of the 1997 Optical Fiber Communication (OFC) conference, published in the OFC Technical Digest, pp. 199-200. 
     With the ridge waveguide structure, it is comparatively easy to achieve a broadband electro-absorption modulator, and the device can be fabricated with fewer crystal growth steps than are needed for a buried-heterostructure waveguide. The ridge structure is intermediate between the buried heterostructure and the high mesa structure, however, so its thermal damage level can be expected to be intermediate between the damage levels found in those two structures. 
     There is a general need to enable electro-absorption modulators to withstand higher levels of optical power, so that signals can be transmitted over greater distances in optical communication systems. In particular, electro-absorption modulators with a ridge waveguide structure need to have higher damage levels if the full benefits of the ridge structure are to be realized. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide electro-absorption modulators with higher damage levels. 
     The invented electro-absorption modulator has an optical waveguide with an optical input end and an optical output end. A first electrode is disposed above the optical waveguide. A second electrode is disposed below the optical waveguide. An electric field applied to the optical waveguide from the first and second electrodes modulates the absorption of light in the optical waveguide as the light travels from the input end to the output end. 
     According to one aspect of the invention, the electro-absorption modulator also has a heat sink running parallel to the optical waveguide on one or both sides. The heat sink cools the optical waveguide by conducting heat away, thereby preventing overheating of the optical waveguide as a whole. By conducting heat in the lengthwise direction of the optical waveguide, the heat sink also prevents localized hot spots from forming. These features improve the ability of the electro-absorption modulator to withstand high optical input levels. 
     The heat sink is preferably formed as a thin metal film, metals in general being good conductors of heat. The optical waveguide may have an inverted mesa structure, in which case the overhanging part of the optical waveguide can function as a spacer when the heat sink is formed. 
     According to another aspect of the invention, the electro-absorption modulator is structured so as to reduce optical absorption near the optical input end of the optical waveguide, where most of the optical absorption and heating occur in a conventional electro-absorption modulator. This reduction also improves the ability of the modulator to withstand high optical input levels. 
     One way to obtain the desired absorption reduction is to reduce the electric field applied to the optical input end of the waveguide. The first electrode typically includes a stripe running parallel to the optical waveguide, and a pad connected to an external power source. In this configuration, the electric field can be reduced by an electrical resistance that produces a voltage drop between the electrode pad and the end of the electrode stripe disposed above the optical input end of the waveguide. 
     For example, the stripe can be divided into two or more segments, which are coupled in series through interconnecting members offering a higher electrical resistance than the stripe itself. A voltage drop equal to the product of the higher resistance and the current flowing through the resistance is produced. The resistive interconnection is preferably offset to one side of the stripe, away from the optical waveguide, thereby protecting the optical waveguide from joule heating that occurs in the interconnection resistance. The offset distance can be selected to obtain the necessary degree of protection, provided the voltage drop remains within an acceptable limit. The interconnection may include a section having an electrical conductance that can be adjusted to obtain a desired voltage drop. In particular, the interconnection may include a thin-film resistor, which can be formed easily by standard semiconductor fabrication techniques such as vacuum evaporation, lift-off, and photolithography. The thin film may have a different composition from the stripe itself; the thin-film material or materials can be selected to obtain a desired electrical resistance. 
     Alternatively, light absorption can be reduced at the optical input end of the waveguide by providing the absorbing layer of the waveguide with a higher bandgap energy at the optical input end than at the optical output end. For example, the optical waveguide can be fabricated by selective crystal growth. 
     In another aspect of the invention, the pad of the first electrode is disposed near the optical input end of the optical waveguide, to reduce the amount of heat generated by current flow in the stripe of the first electrode, particularly near the optical input end of the waveguide. Thermal damage to the electro-absorption modulator is thereby prevented, in that the temperature of the optical input end of the optical waveguide is lowered. This configuration is conditional on the modulation frequency, in that the pad must be located so that the high-frequency component of the applied voltage propagates through the entire stripe, but this condition does not appear to present problems at the modulation frequencies used in communication systems at present. 
     In all of the above aspects of the invention, the optical waveguide is preferably a ridge waveguide. A ridge waveguide is easier to fabricate than a buried-heterostructure waveguide, and is better than a high-mesa waveguide at providing the conditions for single-mode wave propagation. An electro-absorption modulator with a ridge waveguide can also be adapted comparatively easily for broadband operation. 
     The invented electro-absorption modulator can be integrated with a semiconductor laser into a monolithic device. 
     The invention also provides a general method of fabricating a semiconductor optical device. The invented method includes the formation of an optical waveguide having an inverted-mesa channel, and the formation of a heat sink by use of the inverted-mesa channel as a spacer, so that the heat sink extends parallel to the optical waveguide. The heat sink is thereby formed without the need for a separate masking pattern, and is positioned to cool the optical waveguide by conducting heat away from the optical waveguide, without being so close to the waveguide as to interfere with wave propagation in the waveguide. The spacing between the optical waveguide and the heat sink can be adjusted by adjustment of the height of the channel. 
     The semiconductor optical device may have an upper cladding layer that is partly doped with an impurity element to form a contact layer. In this case, the heat sink and a thin impurity-source film can be formed in the same step, simplifying the fabrication process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
     FIG. 1 is a perspective view of a monolithic laser-modulator device including an electro-absorption modulator illustrating a first embodiment of the invention; 
     FIG. 2 is a sectional view through line  2 — 2  in FIG. 1; 
     FIG. 3 is a sectional view through line  3 — 3  in FIG. 1; 
     FIG. 4A is a flowchart of a manufacturing process for the device in FIG. 1; 
     FIG. 4B is a series of sectional views showing the device at corresponding stages of the manufacturing process in FIG. 4A; 
     FIGS. 5A,  5 B,  5 C, and  5 D are plan views illustrating various stages of the manufacturing process in FIG. 4A; 
     FIG. 6 is a sectional view illustrating a metalization step performed in the manufacturing process in FIG. 4A; 
     FIG. 7 is a perspective view of a monolithic laser-modulator device including an electro-absorption modulator illustrating a second embodiment of the invention; 
     FIG. 8 is a plan view of the device in FIG. 7; 
     FIG. 9 is an equivalent circuit diagram of the electro-absorption modulator in FIG. 7; 
     FIG. 10 is a perspective view of a monolithic laser-modulator device including an electro-absorption modulator illustrating a third embodiment of the invention; 
     FIG. 11 is a sectional view through line  11 — 11  in FIG.  10 . 
     FIG. 12 is a perspective view of a monolithic laser-modulator device including an electro-absorption modulator illustrating a fourth embodiment of the invention; and 
     FIG. 13 is a plan view of the device in FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the invention will now be described in detail with reference to the attached drawings, in which similar elements are indicated by three-digit reference numerals having the same last two digits, the first digit being used to distinguish occurrences of these elements in different embodiments. Redundant descriptions of identical elements will be omitted. 
     FIG. 1 shows a monolithic integrated laser-modulator device  100  including an electro-absorption modulator zone  102 , an isolation zone  104 , and a DFB laser zone  106 . The electro-absorption modulator zone  102  and the DFB laser zone  106  will also be referred to simply as the electro-absorption modulator and DFB laser, respectively. The electro-absorption modulator  102  embodies the present invention. 
     The device  100  has a substrate  108  that also serves as a lower cladding layer. A partly etched upper cladding layer  110  is disposed above the substrate  108  in all three zones  102 ,  104 ,  106 . A lower electrode  112  is disposed below the substrate  108 . The etched portions of the upper cladding layer  110  are filled in with filler layers  114 . A thin metal film  182  is disposed at the bottom of each filler layer  114 , extending at least for the entire length of the electro-absorption modulator zone  102 . The thin metal film  182  may also extend through the DFB laser zone  106 . 
     An optical waveguide of the ridge type, having an inverted mesa structure, extends lengthwise through the center of the device, parallel to the Z-axis indicated in the drawing. The central upper cladding layer  110  provides the channel of the waveguide. In the electro-absorption modulator zone  102 , the optical waveguide  120  is centered in an absorbing layer  122  disposed between the substrate  108  and upper cladding layer  110 . The absorbing layer  122  has a bulk crystalline structure, which is more easily fabricated than the multiple-quantum-well (MQW) structure used in many conventional electro-absorption modulators. 
     An upper electrode  124  is formed on the upper surface of the electro-absorption modulator zone  102 . The upper electrode  124  includes an electrode stripe  124   a  extending parallel to the optical waveguide  120 , and an electrode pad  124   b  connected to the electrode stripe  124   a  and to an external power source (not visible). The electrode pad  124   b  extends sideways away from the electrode stripe  124   a , so that a bonding wire (not visible) can be attached to the electrode pad  124   b  without stressing the optical waveguide  120 . A contact layer  126 , disposed below the upper electrode  124 , provides an ohmic contact between the upper electrode  124  and the upper surface of the upper cladding layer  110 . A separate upper electrode  164  is formed on the upper surface of the DFB laser zone  106 . 
     FIG. 2 shows a longitudinal or Z-axis cross section through the center of the device  100 . The DFB laser  106  also has a contact layer  166 , disposed below the upper electrode  164 , providing an ohmic contact between the upper electrode  164  and the upper surface of the upper cladding layer  110 . No contact layer is present in the isolation zone  104 , which provides electrical isolation between the upper electrode  124  of the electro-absorption modulator  102  and the upper electrode  164  of the DFB laser  106 . 
     In the isolation zone  104 , the optical waveguide  140  includes a core layer  142 . In the DFB laser zone  106 , the optical waveguide  160  includes an active layer  162 . The absorbing layer  122 , core layer  142 , and active layer  162  are mutually aligned. The active layer  162  is a semiconductor layer configured as a multiple quantum well. The multiple-quantum-well structure enables electrical and optical properties differing from those of the bulk semiconductor material to be created artificially, to enhance the lasing performance of the DFB laser  106 . The active layer  162  has a separate-confinement heterostructure (SCH), meaning that light is confined to waveguide layers disposed above and below a carrier confinement layer, by differing refractive indices of the layers. A grating  162   a  is formed in the lower surface of the active layer  162  to provide distributed feedback during lasing operation. 
     Light P generated in the DFB laser  106  leaves the optical output end  160   a  of the optical waveguide  160  in the DFB laser  106 , travels through the optical waveguide  140  in the isolation zone  104 , enters the optical waveguide  120  in the electro-absorption modulator  102  at its optical input end  120   a , and leaves the optical waveguide  120  at its optical output end  120   b . The front facet of the device has an anti-reflection (AR) coating to prevent reflection of light emerging from the optical output end  120   b.    
     FIG. 3 shows a lateral or X-axis cross section through the electro-absorption modulator  102 . The inner surfaces of the filler layers  114  are coated with a dielectric film  118 . The thin metal film  182  overlies this dielectric film  118 , separated from the base of the upper cladding layer  110  by a distance L corresponding to the overhang of the inverted mesa. The distance L should be sufficient to assure that the thin metal film  182  has substantially no effect on light propagation in the optical waveguide  120 . For single-mode devices, L should be adequate to position the thin metal film  182  where the single-mode propagation intensity is low enough to render the effect of the thin metal film  182  negligible. The thin metal film  182  functions as a heat sink for the optical waveguide  120 , cooling the optical waveguide  120  by conducting heat away from it. 
     Next, a process for manufacturing the above opto-electronic device  100  will be described. 
     Referring to FIG. 4A, the manufacturing process has six main steps: a grating formation step S 1 ; a first crystal growth step S 2 ; an island etching step S 3 ; a second crystal growth step S 4 , followed by a third crystal growth step S 4 ′; an upper electrode formation step S 5 ; and a lower electrode formation step S 6 . Several of these steps comprise two or more sub-steps, as will be described. These steps are carried out on a wafer in which many devices  100  are formed simultaneously. The manufacturing process also includes a few well-known steps, such as a wafer cleavage step, an anti-reflection coating step, and a carrier mounting step or wire bonding step, which will not be described. 
     The grating formation step S forms the grating  162   a  on the surface of the substrate  108  in the DFB laser zone  106 . 
     The first crystal growth step S 2  forms the active layer  162  and a growth-base film  110 ′, forming these layers on the entire surface of the substrate  108 , by metal-organic vapor-phase epitaxial growth (MOVPE), for example. The active layer  162  is formed as a multiple-quantum-well layer with the separate-confinement heterostructure noted above. The growth-base film  110 ′ comprises the same material as the upper cladding layer  110 , providing a base on which the upper cladding layer  110  can be grown easily in the second crystal growth step, described below. 
     The island etching step S 3  is a selective etching step that etches the electro-absorption modulator zone  102  and isolation zone  104  down to the substrate  108 , leaving the active layer  162  and growth-base film  110 ′ present only in the DFB laser zone  106 . A slight amount of the substrate  108  is removed in the electro-absorption modulator zone  102  and isolation zone  104 . The etching is preceded by the formation of a mask layer  116  of, for example, silicon dioxide (SiO 2 ), covering the DFB laser zone  106  and exposing the electro-absorption modulator zone  102  and isolation zone  104 . FIG. 5A illustrates the surface of the wafer after the island-etching step S 3 . 
     The second crystal growth step S 4  forms the absorbing layer  122 , the core layer  142 , and another growth-base film  110 ″. First, an adjustment film  108 ′, comprising the same material as the substrate  108 , is grown on the exposed surfaces of the electro-absorption modulator  102  and isolation zone  104 , by MOVPE, for example, to planarize the surface of the substrate  108 . The planarized surface of the substrate  108  is substantially level with the lower surface of the active layer  162 . Next, the absorbing layer  122  and core layer  142  are grown on the planarized surface of the substrate  108 , by MOVPE, for example. The second growth-base film  110 ″ is then grown on the surfaces of the absorbing layer  122  and core layer  142 . 
     The third crystal growth step S 4 ′ forms the upper cladding layer  110 , the contact layers  126 ,  166 , and various other features. After the mask layer  116  is removed, the upper cladding layer  110  is initially formed, by MOVPE, for example, as a layer covering the entire wafer surface. Part of this layer is then removed, by an anisotropic etching process, for example, to form the inverted mesa structure. As illustrated in FIG. 5B, cladding material is removed in parallel strips down to the absorbing layer  122  and the other layers level therewith. The remaining upper cladding layer  110  has the form of a series of ridges with inverted-mesa cross sections. Next, the dielectric film  118  is formed on the entire wafer surface, excepting the upper surface of the upper cladding layer  110 . The dielectric film  118  is formed as, for example, a silicon dioxide film. 
     Referring to FIG. 6, a thin metal film is now deposited on the wafer surface, by vacuum evaporation, for example. Where the upper cladding layer  110  has been removed, the metal is deposited on the dielectric film  118 , forming the metal heat-sink films  182  separated from the base of the upper cladding layer  110  by the distance L described earlier. The distance L can be adjusted by adjusting the height of the upper cladding layer  110 ; that is, by controlling the thickness of the upper cladding layer  110  in the third crystal growth step S 3 . Where the metal is deposited on the upper cladding layer  110 , a thin metal film  180  is formed. This metal film  180  includes substances that will be introduced into the upper cladding layer as impurity elements to create the contact layers  126 ,  166 . The metal films  180 ,  182  are, for example, gold-zinc (Au—Zn) thin films. 
     Metal film  180  is not formed on the isolation zone  104 . (Alternatively, the metal film  180  is formed on, then removed from the isolation zone  104 ). At the end of the metalization process, the wafer has the appearance shown in FIG.  5 C. The wafer is now heated, causing impurity atoms to diffuse from metal film  180  into the upper cladding layer  110 , forming the contact layers  126 ,  166 . Following this heat treatment, the filler layers  114  are formed, filling in the spaces between the inverted-mesa cladding ridges. The metal heat-sink films  182  are buried beneath the filler layers  114 . At the end of this step, the wafer has the appearance shown in FIG.  5 D. 
     In the upper electrode formation step S 5 , the upper electrodes  124 ,  164  are formed by, for example, the lift-off method. The upper electrode  124  in each electro-absorption modulator zone  102  has the configuration shown in FIG. 1, comprising an electrode stripe  124   a  and an electrode pad  124   b.    
     In the lower electrode formation step S 6 , the lower electrode  112  is formed on the entire underside of the wafer. 
     Next, the operation of the first embodiment will be described. 
     During operation, the lower electrode  112  is coupled to ground, and a continuous current is injected from upper electrode  164  into the optical waveguide  160  in the DFB laser  106 , while a modulated electric field is applied from upper electrode  124  to the optical waveguide  120  in the electro-absorption modulator  102 . In optical communication applications, the applied electric field may have a very high frequency, such as a frequency of several tens of gigahertz. The light P generated by laser action in the DFB laser  106  is absorbed in the electro-absorption modulator  102 , to a degree controlled by the instantaneous strength of the applied electric field. A modulated light beam is thus emitted from the optical output end  120   b  of the optical waveguide  120 . 
     Light being a form of energy, the absorption of light generates heat in the optical waveguide  120 , but the heat is conducted away from the optical waveguide  120  by the thin metal heat-sink film  182 , which has a much higher thermal conductivity than do the constituent materials of the substrate  108 , upper cladding layer  110 , and filler layers  114 . The metal heat-sink film  182  also distributes heat evenly along the length of the optical waveguide  120 , compensating for the fact that light absorption and heating occur predominantly near the optical input end  120   a  of the optical waveguide  120 . Thus the optical waveguide  120  is cooled both locally and globally by the thin metal film  182 . 
     This cooling effect enables the DFB laser  106  to operate at power levels that would damage the electro-absorption modulator  102  by overheating if the optical waveguide  120  were not cooled. The cooling effect also improves the reliability of the device  100  at all power levels. 
     Next, a second embodiment will be described, with reference to FIGS. 7 to  9 . 
     FIG. 7 shows an integrated laser-modulator device  200  that is divided into an electro-absorption modulator zone  202 , an isolation zone  204 , and a DFB laser zone  206 , with filler layers  214 , a lower electrode  212 , an optical waveguide  220 , and an absorbing layer  222  that is present only in the electro-absorption modulator zone  202 . Except for the upper electrode in the electro-absorption modulator zone  202 , these elements are similar to the corresponding elements in the first embodiment. The isolation zone  204  and DFB laser  206  have exactly the same internal structure and functions as in the first embodiment. 
     The upper electrode of the electro-absorption modulator  202  differs from the upper electrode in the first embodiment in that the electrode stripe paralleling the optical waveguide  220  is divided into two parts: a first part  224   a   1  disposed near the isolation zone  204 , and a second part  224   a   2  extending to the output end of the optical waveguide  220 . The electrode pad  224   b  is connected to the second part  224   a   2  of the stripe, and to an external power source (not visible). The two parts  224   a   1 ,  224   a   2  of the stripe are interconnected by a resistive member  224   c  disposed on the surface of one of the filler layers  214 , extending sideways from the stripe. The resistive member  224   c  includes a thinfilm resistor  224   c   1 , disposed at the end of the sideways extension. 
     The DFB laser  206  has an upper electrode  264  similar to the upper electrode in the first embodiment. 
     Referring to FIG. 8, the first part  224   a   1  of the electrode stripe in the electro-absorption modulator zone  202  extends to the optical input end  220   a  of the optical waveguide  220 . The resistive member  224   c  is arranged so that all current flowing between the two parts  224   a   1 ,  224   a   2  of the electrode strip passes through the thin-film resistor  224   c   1 . The thin-film resistor  224   c   1  has a rectangular shape with the long axis of the rectangle substantially parallel to the axis of the optical waveguide  220  and the electrode stripe. The thin-film resistor  224   c   1  comprises a metal material having a lower electrical conductivity than the other parts of the resistive member  224   c . These other parts of the resistive member  224   c  are formed from the same material as the electrode stripe, but are narrower in width. Consequently, the entire resistive member  224   c  offers greater electrical resistance per unit length than does the electrode stripe. The total electrical resistance of the resistive member  224   c  can easily be adjusted by adjusting its width, thickness, and total length, by adjusting the dimensions of the thin-film resistor  224   c   1  , and by selection of a material with suitable electrical conductivity for the thin-film resistor  224   c   1 . 
     The electrode stripe  224   a   1  ,  224   a   2  and pad  224   b    2  comprise, for example, a highly conductive metal such as gold (Au). The thin-film resistor  224   c   1  comprises, for example, a low-conductivity metal such as titanium (Ti) or tungsten (W). 
     The integrated laser-modulator  200  can be manufactured by a modification of the process illustrated in FIG.  4 A. The upper electrode formation step S 5  is now divided into two sub-steps, one sub-step forming the thin-film resistor  224   c   1 , the other sub-step forming the other parts of the upper electrodes. Both sub-steps can be performed by the lift-off method. 
     The operation of the second embodiment will be described with reference to FIG.  9 . 
     The electric field between the upper electrode  224  and lower electrode  212  is created by a variable voltage V applied to the electrode pad  224   b . The applied voltage V also reverse-biases a pn junction in the optical waveguide  220 , represented in the drawing as a pair of diodes, and determines the wavelength at which strong absorption begins in the absorbing layer  222 . When this wavelength, referred to as the absorption edge, is longer than the wavelength of the light P entering the optical input end  220   a , the light is greatly attenuated in the optical waveguide  220 . When the absorption-edge wavelength is shorter than the wavelength of the entering light P, the light is only slightly attenuated in the optical waveguide  220 . 
     When light is absorbed, photocurrent flows from the optical waveguide  220  to the upper electrode  224 . Current I flowing into the first part  224   a   1  of the electrode stripe must pass through the thin-film resistor  224   c   1  to reach the electrode pad  224   b . A voltage drop V 1  equal to IR occurs, where R is the resistance of the thin-film resistor  224   c   1 . The voltage applied to the first part  224   a   1  of the upper electrode stripe is accordingly V—IR, instead of the voltage V applied to the second part  224   a   2 . The electric field at the optical input end  220   a  of the optical waveguide  220  is therefore weaker than the electric field at the optical output end  220   b , so absorption at the optical input end  220   a  is reduced. 
     The reduced absorption generates less heat at the optical input end  220   a  of the optical waveguide  220 , enabling higher optical input power levels to be tolerated without damage. In the part of the optical waveguide  220  below the second part  224   a   2  of the electrode stripe, although the electric field is not reduced, some of the light P has already been absorbed near the optical input end  220   a , so the amount of light absorption and consequent heating that take place below the second part  224   a   2  of the electrode stripe also remains below the damage level. 
     Compared with an electro-absorption modulator lacking the resistive member  224   c , the electro-absorption modulator  202  can accordingly withstand higher optical power levels, enabling the laser-modulator  200  as a whole to generate more light for long-distance communication or other purposes. 
     Next, a third embodiment will be described, with reference to FIGS. 10 and 11. 
     These drawings show an integrated laser-modulator device  300  divided into an electro-absorption modulator zone  302 , an isolation zone  304 , and a DFB laser zone  306 , with a All lower electrode  312  below all three zones. The isolation zone  304  and DFB laser zone  306  have the same internal structure and functions as the corresponding zones in the preceding embodiments. The electro-absorption modulator zone  302  includes an upper electrode  324  as in the first embodiment, an optical waveguide  320 , and an absorbing layer  322 . The absorbing layer  322 , while generally similar to the absorbing layers in the preceding embodiments, differs as follows. 
     The difference is that the bandgap energy of the absorbing layer  322  is not constant, but increases from the optical output end  320   b  to the optical input end  320   a . The bandgap energy gradient is not visible in the drawings. An increased bandgap energy hinders the absorption of light. The increasing bandgap energy thus counteracts the tendency for more light to be absorbed near the optical input end  320   a  of the optical waveguide  320 . The bandgap energy profile of the absorbing layer  322  is preferably arranged so that light absorption is substantially uniform throughout the length of the optical waveguide  320 . The total range of variation of the bandgap energy of the absorbing layer  322  should be calculated according to the length of the optical waveguide  320 , the wavelength of the light produced in the DFB laser  306 , and the total extinction ratio that must be produced by absorption in the absorbing layer  322 . The bandgap energy may increase continuously, or in a series of steps. 
     The integrated laser-modulator  300  can be manufactured by the process shown in FIG. 4A, with a modification of the second crystal growth step S 4 . For example, the absorbing layer  322  can be grown by a selective growth method. 
     By reducing light absorption near the optical input end  320   a  of the optical waveguide  320 , the third embodiment produces substantially the same effect as the second embodiment, enabling the electro-absorption modulator  302  to withstand higher levels of input optical power. Equalization of light absorption throughout the length of the optical waveguide  320  can maximize the allowable input power level by eliminating local overheating. 
     Next, a fourth embodiment will be described, with reference to FIGS. 12 and 13. 
     FIG. 12 shows an integrated laser-modulator device  400  having a lower electrode  412 , an optical waveguide  420 , and an upper electrode  424 . The upper electrode  424  comprises an electrode stripe  424   a  and an electrode pad  424   b . The device  400  is designed for use in optical communications. As shown in FIG. 13, the device comprises an electro-absorption modulator zone  402 , an isolation zone  404 , and a DFB laser zone  406 . The isolation zone  404  and DFB laser zone  406  have the same internal structure and functions as in the preceding embodiments, but the electro-absorption modulator zone  402  differs from the preceding embodiments in the positioning of the electrode pad  424   b , which is now disposed near the end of the electrode stripe  424   a  above the optical input end  420   a  of the optical waveguide  420 . 
     In optical communications, a high-frequency voltage waveform applied to the electrode pad  424   b  must propagate throughout the electrode stripe  424   a . This requirement suggests that the electrode pad  424   b  should be disposed in the conventional position near the center of the electrode stripe  424   a , but in the inventor&#39;s view, the frequency levels used in optical communication systems at present are not so high as to prevent satisfactory voltage waveform propagation, even if the electrode pad  424   b  is disposed at one end of the electrode stripe  424   a.    
     During operation of the integrated laser-modulator  400 , the absorption of light in the optical waveguide  420  is accompanied by the above-described flow of current from the optical waveguide  420  to the electrode stripe  424   a , through the electrode stripe  424   a  to the electrode pad  424   b , and from the electrode pad  424   b  to the external power source. The current flow causes resistive heating in the electrode stripe  424   a , but because most of the light absorption takes place near the optical input end  420   a  of the waveguide  420 , and because the electrode pad  424   b  is disposed at this end of the upper electrode  424 , most of the current reaches the electrode pad  424   b  without having to traverse the electrode stripe  424   a  for any appreciable distance, so the electrode stripe  424   a  is not heated as much as it would be if the electrode pad  424   b  were disposed near the middle of the electrode stripe  424   a . This reduction of the heat generated in the electrode stripe  424   a  reduces the temperature of the optical waveguide  420 . 
     The fourth embodiment accordingly raises the power level at which the DFB laser  406  can operate without damage to the electro-absorption modulator  402  caused by overheating. 
     The invention is not limited to the embodiments as described above. The first embodiment, for example, is not limited to the use of a thin metal film as a heat sink. The heat sink may be a thick metal film or layer, a metal plate, or any other thermally conductive body, including certain known types of ceramics having high thermal conductivity. 
     The second embodiment can be modified by dividing the upper electrode stripe into more than two parts, which are coupled in series by a plurality of resistive interconnecting members. Alternatively, the two or more parts of the electrode stripe may be coupled in parallel to the electrode pad through interconnecting members having different electrical resistances. 
     When the upper electrode stripe is divided into two or more parts that are coupled in series, the resistive interconnecting members may be disposed in line with the separate parts of the stripe, above the optical waveguide, possibly in a higher layer than the electrode stripe layer, instead of extending to one side. 
     The resistive member does not need to incorporate a material having a lower electrical conductivity than that of the electrode stripe. The entire resistive member and the electrode stripe may be formed from the same material, in the same metalization step, the desired voltage drop being produced by appropriate design of the length, width, and thickness dimensions of the resistive member. The manufacturing cost of the device can thereby be reduced, by reducing the number of separate fabrication steps, and manufacturing yields can be improved. 
     Various other structures can be used to produce a voltage drop at the end of the electrode stripe near the optical input end of the waveguide. For example, the electrical resistance of the electrode stripe itself may be varied, by varying the width, thickness, or composition of the stripe; a comparatively low-resistance interconnecting stripe and a higher-resistance electrode stripe may be interconnected in parallel at a plurality of points, excluding points near the input end; or the electrode stripe may comprise a comparatively high-resistance layer extending the full length of the stripe, and a low-resistance layer extending from the optical output end partway toward the optical input end. 
     The invention is not limited to the use of a bulk-crystal absorbing layer. The absorbing layer may have a multiple-quantum-well structure, a single-quantum-well structure, a superlattice structure, a strained-quantum-well structure, or any other type of structure. 
     Similarly, when the electro-absorption modulator is integrated with a DFB laser, the active layer of the laser is not limited to the multiple-quantum-well structure mentioned in the embodiments. The active layer of the DFB laser may have a bulk structure, a single-quantum-well structure, a superlattice structure, a strained-quantum-well structure or various other structures. 
     The invention is not limited to the use of a ridge waveguide with an inverted mesa structure. The optical waveguide may have an ordinary (non-inverted) mesa structure, a high mesa structure, a buried heterostructure, or any other structure. The invention can be practiced in electro-absorption modulators having a planar waveguide, as well as in electro-absorption modulators having a stripe-geometry waveguide. In electro-absorption modulators having a buried-heterostructure waveguide or high-mesa waveguide, the waveguide channel may be formed in the upper cladding layer and absorbing layer, or in the upper cladding layer, the absorbing layer, and a lower cladding layer, as is well known. 
     The invented electro-absorption modulator may be integrated with various optical and opto-electronic components other than DFB lasers, including both active and passive components. Examples of active semiconductor components include other types of semiconductor lasers, such as distributed Bragg reflection (DBR) lasers; light-emitting diodes (LEDs) and other light-emitting elements; photodiodes and other light-receiving elements; optical amplifiers; and other electro-absorption modulators. Examples of passive optical components with which the invented electro-absorption modulator may be integrated include optical branching couplers, polarizers, mode splitters, wavelength splitters, wavelength combiners, lenses, prisms, directional couplers, and various other optical waveguide devices. 
     The invented electro-absorption modulator need not be part of a monolithic integrated device. The invented electro-absorption modulator can be employed as a discrete device, or as part of a hybrid planar lightwave circuit (PLC) module, or in optical equipment in which different optical components are interconnected by optical fibers. 
     The embodiments have been described separately, but various combinations of the embodiments are possible. 
     Those skilled in the art will recognize that further variations are possible within the scope claimed below.