Electroabsorption Modulator

Abstract

To provide an electronic field absorption modulator that is excellent in high-speed response property.SOLUTION: An electronic field absorption type modulator comprises: a substrate; a mesa structure that includes a first electric conduction type clad layer, multiple quantum well layer, and second electric conduction clad layer, which are provided in a first plane of the substrate; a first electric conduction type electrode that is electrically connected to the first electric conduction type clad layer; and a second electric conduction type electrode that is provided in a second plane of the substrate. The first electric conduction type electrode has: a mesa upper electrode that is arranged along a direction where the mesa structure extends; a pad electrode to which an electric signal is externally input; a drawing line electrode that connects between the mesa upper electrode and the pad electrode, in which the mesa structure includes: a light input terminal to which light is externally input; and a light output terminal that is on a side opposite the light input side. A connection position of a center position in a short-length direction of the drawing line electrode to the mesa upper electrode is close to a light output terminal side in a longitudinal direction of the mesa upper electrode, and the connection position is a position less than 50% from the light output terminal side with respect to a length in the longitudinal direction of the mesa upper electrode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to electroabsorption modulators, and in particular to electrode shapes. [Background technology]

[0002] Optical modulation technology is used in many applications, including optical fiber communications, free-space optical communications, and optical pulse-based information processing, creating a demand for compact optical modulators with high-speed response. One such optical modulator is the electroabsorption modulator (EA (Electro-Absorption) modulator), which utilizes the electroabsorption effect of semiconductors. EA modulators have a pin structure with a multi-quantum well layer as the i-layer, and light intensity modulation can be achieved by applying a voltage to the pin structure. EA modulators are known to have a structure based on an InP substrate with a multi-quantum well layer made of InGaAsP or other materials. Other known structures include a Si-based structure with an i-layer made of SiGe, for example. EA modulators are also often used as semiconductor optical devices integrated with semiconductor lasers.

[0003] Patent Document 1 discloses a structure in which the p-side electrode of an EA modulator is composed of a mesa-top electrode along the mesa, a pad electrode to which an externally connected wire is bonded, and a lead-out electrode that connects the pad electrode and the mesa-top electrode. Patent Document 2 discloses that carriers (photocurrents) generated when an EA modulator absorbs light have a distribution in the direction along the mesa of the EA modulator. Furthermore, a structure is shown in which the connection between the lead-out electrode and the mesa-top electrode is located on the light input end side to improve heat dissipation at that time. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-145973 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-222965 [Non-patent literature]

[0005] [Non-Patent Document 1] T. Ido, S. Tanaka, M. Suzuki, M. Koizumi, H. Sano, H. Inoue, "Ultra-high-speed multiple-quantum-well electro-absorption optical modulators with integrated waveguides," Journal of Lightwave Technology, vol. 14, no. 9, pp. 2026-2034, Sept. 1996. [Non-patent document 2] R. Sahara, K. Morito, K. Sato, Y. Kotaki, H. Soda, N. Okazaki, "Strongly improved frequency response at high-optical input powers from InGaAsP compensated strain MQW electroabsorption modulators," IEEE Photonics Technology Letters, vol. 7, no. 9, pp. 1004-1006, Sept. 1995. Summary of the Invention [Problem to be solved by the invention]

[0006] Non-Patent Document 1 reports that the main cause of a decrease in the response speed of an EA modulator is the electrostatic capacitance of the modulator. Furthermore, Non-Patent Document 1 reports that shortening the modulator length improves the response speed. On the other hand, shortening the modulator length reduces the total amount of optical absorption, lowering the extinction ratio, which can contribute to an increase in the communication error rate in optical transmission. Therefore, to achieve a high extinction ratio in an EA modulator with a short modulator length, an increase in the amount of optical absorption per unit length is necessary. However, an increase in the amount of optical absorption leads to an increase in the number of carriers (photoexcited carriers) generated in the EA modulator. While this would not be a problem if the generated photoexcited carriers were extracted smoothly from the EA modulator, there are cases in which they are not extracted sufficiently. In such cases, the carriers remain within the EA modulator, particularly in the multiple quantum well layer. As a result, carrier pileup occurs in the multiple quantum well layer. Carrier pileup leads to a decrease in the EA modulator's characteristics, such as its extinction characteristics and response speed. Non-Patent Document 2 shows that the extinction characteristics and response speed decrease depending on the input light intensity and the amount of optical absorption to the EA modulator.

[0007] As disclosed in Patent Document 2, the photocurrent is larger on the optical input side of an EA modulator than on the optical output side. Assuming that a substantially constant voltage is applied in the optical axis direction of the EA modulator, the photocurrent distribution can be said to reflect the distribution trend of photoexcited carriers. This indicates that there are more carriers to be extracted from the multiple quantum wells on the optical input side than on the optical output side. Even if carriers are extracted smoothly when a DC voltage or a low speed of less than kHz is applied to the EA modulator, carriers may not be extracted when an AC voltage of GHz or higher is applied. If carriers are not extracted quickly, the extinction characteristics deteriorate, and further, frequency characteristics, such as bandwidth, deteriorate. This effect is particularly pronounced when the amount of light absorption per unit length is large, such as in short modulators or at high temperatures where light absorption increases.

[0008] The present invention has been made in view of the above problems, and has as its object to provide an electroabsorption modulator that is excellent in high-speed response. [Means for solving the problem]

[0009] (1) An electro-absorption modulator according to the present disclosure comprises a substrate, a mesa structure provided on a first surface of the substrate, the mesa structure including a first-conductivity-type cladding layer, a multiple quantum well layer, and a second-conductivity-type cladding layer, a first-conductivity-type electrode electrically connected to the first-conductivity-type cladding layer, and a second-conductivity-type electrode provided on a second surface of the substrate, the first-conductivity-type electrode having a mesa-top electrode arranged along an extension direction of the mesa structure, a pad electrode to which an external electrical signal is input, and a lead-out electrode connecting the mesa-top electrode and the pad electrode, the mesa structure having an optical input end to which light is input from the outside and an optical output end opposite the optical input end, the connection position of the center position of the lead-out electrode in the lateral direction and the mesa-top electrode being close to the optical output end in the longitudinal direction of the mesa-top electrode, and the connection position being less than 50% of the longitudinal length of the mesa-top electrode from the optical output end side. [Effects of the Invention]

[0010] The present invention provides an electroabsorption modulator with excellent high-speed response. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a top view of an electro-absorption modulator integrated laser according to a first embodiment of the present invention. [Figure 2] 2 is a cross-sectional view taken along line AA of the electroabsorption modulator shown in FIG. [Figure 3] This shows the distribution of photocurrent in the optical axis direction of the electroabsorption modulator shown in FIG. [Figure 4] 2 shows the results of a simulation of the electric field strength in the multiple quantum well layer of the electroabsorption modulator shown in FIG. [Figure 5] 2 shows the results of a simulation of the electric field strength in the multiple quantum well layer of the electroabsorption modulator shown in FIG. [Figure 6A] The electric field intensity ratio and photocurrent distribution at 10 GHz in the optical axis direction. [Figure 6B]The electric field intensity ratio and photocurrent distribution at 20 GHz in the optical axis direction. [Figure 6C] The electric field intensity ratio and photocurrent distribution at 40 GHz in the optical axis direction. [Figure 6D] The electric field intensity ratio and photocurrent distribution at 60 GHz in the optical axis direction. [Figure 7] 10 is a simulation result showing the average electric field strength ratio for each length of the electroabsorption modulator. [Figure 8] FIG. 10 is a top view of an electro-absorption modulator integrated laser according to a second embodiment of the present invention. [Figure 9] FIG. 10 is a top view of an electro-absorption modulator integrated laser according to a third embodiment of the present invention. [Figure 10] 10 is a cross-sectional view of the electroabsorption modulator shown in FIG. 9 taken along line CC. [Figure 11] FIG. 10 is a top view of an electro-absorption modulator integrated laser according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all drawings for explaining the embodiments, components having the same functions are designated by the same reference numerals, and repeated explanations thereof will be omitted. The drawings shown below are merely for explaining examples of the embodiments, and the size of the drawings does not necessarily correspond to the scale described in the examples.

[0013] [First embodiment] FIG. 1 is a top view of an electroabsorption modulator-integrated laser 1 according to a first embodiment of the present invention. The electroabsorption modulator-integrated laser 1 is a semiconductor integrated device in which an EA modulator 2 (electroabsorption modulator), a waveguide connection section 3, and a semiconductor laser 4 are integrated on a common substrate. The electroabsorption modulator-integrated laser 1 is an integrated device in which the semiconductor laser 4, the waveguide connection section 3, and the EA modulator 2 are optically connected to each other in this order. The semiconductor laser 4 emits continuous light, and the waveguide connection section 3 transmits the light emitted from the semiconductor laser 4 to the EA modulator 2. The EA modulator 2 includes a multiple quantum well layer 10 that absorbs light corresponding to the oscillation wavelength of the semiconductor laser 4. The continuous light that passes through the waveguide connection section 3 and enters the EA modulator 2 is intensity-modulated by the EA modulator 2 and converted into a modulated optical signal such as a binary or quaternary signal. The modulated optical signal output from the EA modulator 2 is output from a front end face 101. A dielectric non-reflective coating (not shown) is formed on the front end facet 101. A dielectric highly reflective coating (not shown) is formed on the rear end facet 102, which is the end face on the opposite side of the semiconductor laser 4. Note that although an example of an integrated type is shown in this embodiment, the effects of the present invention can be obtained even when the present invention is applied to a standalone EA modulator.

[0014] The semiconductor laser 4 is, for example, a DFB (Distributed Feedback) laser that oscillates in the 1.3 μm band. The oscillating wavelength band may be the 1.55 μm band or another wavelength band. Furthermore, the semiconductor laser 4 is not limited to a DFB laser, and may be an FP (Fabry-Perot) laser, a DBR (Distributed Bragg Reflector) laser, or a DR (Distributed Reflector) laser. The semiconductor laser 4 is provided with a semiconductor laser p-side electrode 5 (first conductivity type electrode) for conducting current. The semiconductor laser 4 oscillates continuous light by injecting current between an n-side electrode 14 (second conductivity type electrode) (described later) and the semiconductor laser p-side electrode 5.

[0015] The EA modulator 2 includes a p-side electrode 105 for the EA modulator. FIG. 2 is a schematic diagram of the AA cross section of the EA modulator 2, perpendicular to the optical axis. A mesa structure is provided on an n-type n-InP substrate 9, and semi-insulating InP burying layers 12 are disposed on both sides of the mesa structure. The mesa structure includes a layer structure in which, from the bottom, a portion of the n-InP substrate 9, an i-type multiple quantum well layer 10, and a p-type InP cladding layer 11 are stacked in this order. These layers form a pin structure. Although not shown, an n-type optical confinement layer may be disposed between the multiple quantum well layer 10 and the n-InP substrate 9. Similarly, a p-type optical confinement layer may be disposed between the upper side of the multiple quantum well layer 10 and the p-type InP cladding layer 11. Although not shown, a p-type contact layer is disposed on the p-type InP cladding layer 11. A passivation film 13 is disposed on the top surface of the burying layer 12. The p-side electrode 105 for the EA modulator is in contact with a p-type contact layer (not shown), allowing current to flow through the pin structure. An n-side electrode 14 is disposed on the surface of the n-InP substrate 9 opposite the surface on which the mesa structure is formed. The n-side electrode 14 is connected to a GND potential. As shown in FIG. 1, the p-side electrode 105 for the EA modulator is composed of a mesa-top electrode 6 disposed along the mesa structure, a pad electrode 8 to which an external current-carrying means (e.g., a wire) is connected, and a lead-out electrode 7 connecting the mesa-top electrode 6 and the pad electrode 8. An external electrical signal is input to the pad electrode 8, and the electrical signal is input to the mesa-top electrode 6 via the lead-out electrode 7. The electrodes constituting the p-side electrode 105 for the EA modulator are integrally formed and all have the same configuration. The p-side electrode 105 for the EA modulator is composed of three layers: Ti / Pt / Au, from the p-type contact layer side. The lead-out electrode 7 is connected to the tip of the mesa-top electrode 6 on the light output end side.

[0016] The multiple quantum well layer 10 is provided in a region extending to the end of the mesa-top electrode 6 on the light output end side, and a waveguide having a mesa structure (not shown) is provided between the end and the front end face 101. Note that instead of a waveguide having a mesa structure, a window structure without a mesa structure may be provided.

[0017] Figure 3 shows the distribution of photocurrent along the optical axis when the EA modulator 2 absorbs light. The photocurrent is the current that flows when a reverse bias is applied to the EA modulator 2 while the output light from the semiconductor laser 4 is being transmitted to the EA modulator 2. The magnitude of the photocurrent is roughly proportional to the amount of light absorbed by the EA modulator 2. In other words, the left side of Figure 3 represents the semiconductor laser 4 side, and the right side represents the front facet 101 side. The photocurrent is larger at position B, i.e., the semiconductor laser 4 side. This indirectly indicates that a large number of photoexcited carriers are generated at the light input end. The photocurrent decreases from the light input end to the light output end. In other words, in the EA modulator 2, the photocurrent distribution is not constant along the optical axis. The photocurrent is proportional not only to the intensity of the input light but also to the magnitude of the voltage applied to the EA modulator 2. The higher the voltage applied to the EA modulator 2, the greater the photocurrent. The voltage applied to the EA modulator 2 is applied to the pad electrode 8 via an external current-carrying means. The voltage applied to the pad electrode 8 is transmitted to the mesa electrode 6 via the lead electrode 7. The voltage is then transmitted from the mesa electrode 6 to the p-type contact layer and the p-type InP cladding layer 11, and is applied to the multiple quantum well layer 10.

[0018] The electric field strength along the optical axis of the mesa structure when a voltage is applied is constant when the applied voltage is a DC voltage. However, we found that the electric field strength along the optical axis of the mesa structure is not constant when a high-speed AC signal is applied. Figure 4 shows the results of a simulation of the electric field strength distribution in the multiple quantum well layer 10 at the B-B' cross section of this structure obtained by electromagnetic field analysis. The simulation conditions were a length of 150 μm and a width (narrower width) of the lead electrode 7 along the optical axis of 10 μm. In this case, the center of the lead electrode 7 is located 3% of the length of the entire mesa electrode 6 from the optical output end (B' side). As a comparative example, we also performed a simulation of a structure in which the center of the lead electrode 7 coincides with the center of the mesa electrode 6. In this structure, the center of the lead electrode 7 is located 50% of the length of the entire mesa electrode 6 from the optical output end (B' side). Hereinafter, the connection position of the lead electrode 7 will be referred to as the 3% position, the 50% position, etc., based on the ratio of the distance from the optical output end to the center of the lead electrode 7. FIG. 4 shows the simulation results when a 40 GHz high-frequency electrical signal is applied to the pad electrode 8. Here, the length of the mesa electrode 6 and the length of the multiple quantum well layer 10 are assumed to be the same. The solid line represents the electric field intensity distribution of this embodiment (3% position), and the dotted line represents the electric field intensity distribution of the comparative example (50% position). As shown in FIG. 4, the electric field intensity of the comparative example is generally constant at any position, whereas the electric field intensity of the EA modulator 2 of this embodiment varies depending on the position. Specifically, the electric field intensity at the optical input end side (side B) is higher than that of the comparative example, and the electric field intensity at the optical output end side (side B') is lower than that of the comparative example. The distribution in electric field intensity is due to the fact that the connection position between the lead electrode 7 and the mesa electrode 6 is on the optical output end side (side B'). A high-frequency electrical signal applied to the pad electrode 8 is transmitted through the lead electrode 7 to the mesa electrode 6. An electrical signal applied to the mesa electrode 6 is transmitted to the mesa electrode 6 and also to the multiple quantum well layer 10 via the p-type contact layer.At this time, the mesa electrode 6 is considered to function electrically as an open stub when viewed from the contact point between the mesa electrode 6 and the lead electrode 7. As a result, it is estimated that a distribution of electric field strength occurs on the mesa electrode 6. At this time, a strong electric field can be applied to the optical input end, where the photocurrent (i.e., photoexcited carriers) is large. The electric field strength gradually decreases from the optical input end to the optical output end. However, the amount of photoexcited carriers (amount of photocurrent) also decreases. Because there are few carriers to be extracted, carriers can be sufficiently extracted even when the electric field strength decreases, and carrier pileup is unlikely to occur. As a result, carrier pileup can be suppressed throughout the EA modulator 2. The higher the applied voltage, the more photoexcited carriers can be extracted from the multiple quantum well layer 10. The EA modulator 2 according to this embodiment has a structure with excellent carrier extraction capabilities. Because of its excellent carrier extraction capabilities, it is possible to provide an EA modulator 2 that suppresses carrier pileup and has excellent extinction characteristics and high-frequency characteristics.

[0019] Figure 5 shows the simulation results of the electric field strength of the EA modulator 2 when high-frequency electrical signals ranging from 10 GHz to 60 GHz are applied to the pad electrode 8. Note that the absolute value of the electric field strength varies with frequency, so it is normalized based on the position B'. At 10 GHz, the electric field strength does not vary significantly between the optical input terminal B and the optical output terminal B'. However, as the frequency increases, the electric field strength at the optical input terminal B increases relative to the optical output terminal B'. In other words, the effect of applying a stronger electric field to the optical input terminal B, which has a larger photocurrent, increases with increasing frequency. Pileup tends to occur more easily at higher drive frequencies, and the present invention can suppress pileup, particularly at high drive frequencies, making it effective for EA modulators supporting 56 Gbps.

[0020] The electric field strength distribution due to differences in the connection position of the lead electrode 7 was examined. The connection positions for this embodiment were set to 3% (the forefront), 10%, 20%, and 33% (see the fourth embodiment and FIG. 11 ), and for the comparative example, the 50% position (center), and a similar simulation was performed. The other conditions were the same as above. Figure 6A shows the results for 10 GHz, Figure 6B shows the results for 20 GHz, Figure 6C shows the results for 40 GHz, and Figure 6D shows the results for 60 GHz. The horizontal axis in each figure indicates the position in the optical axis direction. The line plotted as Iph indicates the photocurrent when a DC voltage is applied, and corresponds to the right axis. The other plots correspond to the normalized electric field strength ratio for each of the above-mentioned connection positions, with the electric field strength when the connection position is at 50% as the reference, and correspond to the left axis.

[0021] For the 10 GHz electrical signal shown in Figure 6A, the electric field strength tends to be roughly equivalent regardless of the connection position, being roughly equivalent at any position along the optical axis compared to the 50% position. Furthermore, there is little difference in electric field strength due to the connection position. For the 20 GHz electrical signal shown in Figure 6B, the electric field strength ratio tends to be larger at the optical input end side B and smaller at the optical output end side B'. This tendency becomes more pronounced as the frequency increases, as is clear from Figure 6C for 40 GHz and Figure 6D for 60 GHz. Furthermore, as for differences depending on the connection position, the electric field strength ratios at the 3%, 10%, and 20% positions, which are closer to the optical output end side B', are roughly equivalent, but the rate of increase with frequency at the 33% position is smaller. These results indicate that the electric field strength ratio at the optical input end side B can be increased by positioning the connection position between the lead electrode 7 and the mesa electrode 6 closer to the optical output end side B' than to the center of the mesa electrode 6. In particular, the effect is remarkable when the ratio is set to 33% or less. In order to obtain a stable effect taking into account manufacturing variations, it is more preferable to set the ratio to 20% or less, at which the electric field strength ratio becomes roughly the same.

[0022] Regardless of the connection position, the electric field intensity ratio is less than 1 at a position slightly closer to the optical output terminal side B' than the center position in the optical axis direction. The region where the electric field intensity ratio is less than 1 indicates a decrease in the electric field intensity compared to the conventional 50% connection, resulting in a decrease in the extraction efficiency of photoexcited carriers. For example, as shown in Figure 6D, the electric field intensity ratio becomes less than 1 at a certain position. A field intensity ratio less than 1 indicates a decrease in the electric field intensity compared to the conventional structure with a 50% connection position. A low electric field intensity indicates a decrease in the extraction efficiency of photoexcited carriers. Therefore, it can be said that the high-frequency characteristics may be degraded compared to the conventional structure in the region where the electric field intensity ratio is less than 1. However, as shown in Figure 6D, the photocurrent amount is not constant from the optical input terminal side B to the optical output terminal side B' but decreases toward the optical output terminal side B'. The total photocurrent amount in the region where the electric field intensity ratio is greater than 1 accounts for 95% of the total photocurrent amount. Conversely, the total photocurrent amount in the region where the electric field intensity ratio is less than 1 accounts for only 5% of the total photocurrent amount. Even if the electric field strength in the region where 5% of the total photocurrent is distributed is smaller than in the conventional structure, the effect on the photocurrent discharge performance is small. In fact, increasing the electric field strength in the region where 95% of the total photocurrent is distributed significantly improves the photocurrent (proportional to photoexcited carriers) extraction performance, resulting in overall improvements in extinction characteristics and high-frequency characteristics. In other words, the feature of the present invention is that the connection position of the extraction electrode 7 is shifted from the center (50%) of the mesa electrode 6 to the optical output end side B', thereby changing the distribution of the electric field strength along the optical axis of the mesa structure. While the electric field strength along the optical axis of the mesa structure in the conventional structure is approximately constant, in the present invention, the electric field strength at the optical output end side B', where the photocurrent is small, is reduced, and instead the electric field strength at the optical input end side B, where the photocurrent is large, is increased. As a result, an EA modulator with excellent high-speed response can be provided.

[0023] Next, we investigated the relationship between the length of the multiple quantum well layer 10, i.e., the modulator length Lmod, and the effects of the present invention. To support high-speed driving of 25 Gbaud or more, it is effective to shorten the modulator length and reduce parasitic capacitance. Taking into consideration driving of 56 Gbaud or more, simulations were performed for modulator lengths of 100 μm and 125 μm in addition to the above-mentioned 150 μm. Furthermore, as in Figures 6A to 6D, simulations were performed for each connection position between the lead electrode 7 and the mesa electrode 6. The width of the lead electrode 7 was not changed.

[0024] Figure 7 shows the simulation results. The results are shown when a 60 GHz electrical signal is applied. The horizontal axis indicates the connection position of the lead electrode 7. The vertical axis, as in Figures 6A to 6D, indicates the electric field intensity ratio normalized with respect to the electric field intensity when the connection position is 50%. Furthermore, as is clear from Figures 6A to 6D, the electric field intensity ratio varies with the position in the optical axis direction. Therefore, in Figure 7, the vertical axis represents the average electric field intensity ratio in the region on the optical input end side B of the mesa electrode 6, where 95% of the total photocurrent is distributed. As shown in Figure 7, an improvement in electric field intensity can be achieved regardless of the modulator length. Furthermore, the improvement in the average electric field intensity ratio is generally proportional to the modulator length. Therefore, at least for modulator lengths in the range of 100 μm to 150 μm, the above effect can be achieved by moving the connection position of the lead electrode 7 closer to the optical output end side B'. However, when the connection position of the lead electrode 7 is 33% and the modulator length is 100 μm, the improvement effect of the average electric field intensity ratio is small. In order to obtain the improvement effect of the average electric field intensity ratio regardless of the modulator length, it is preferable that the connection position of the lead electrode 7 is 20% or less. Furthermore, when it is 10% or less, the improvement effect of the average electric field intensity ratio reaches its peak, so it is also preferable to connect it to this position.

[0025] In this embodiment, the calculations are performed assuming that the multiple quantum well layer 10 and the mesa electrode 6 have the same length, but in reality, the multiple quantum well layer 10 may be slightly longer in consideration of manufacturing variations. Even in this case, the effects of the present invention can be fully obtained.

[0026] [Second embodiment] FIG. 8 is a top view of an electroabsorption modulator-integrated laser 201 according to a second embodiment of the present invention. The only difference from the electroabsorption modulator-integrated laser 1 shown in the first embodiment is the structure of the EA modulator 202. In the optical axis direction, the center of the pad electrode 8 coincides with the center of the mesa electrode 6. The lead electrode 7 is tilted at 45 degrees with respect to the optical axis. The mesa electrode 6 is rectangular, and the mesa electrode 6 and the lead electrode 7 are connected at a position where the top right vertex of the mesa electrode 6 (the front end face 101 side and the pad electrode 8 side of the mesa electrode 6) coincides with the top vertex of the lead electrode 7. The length of the mesa electrode 6 is 150 μm, and the narrower width of the lead electrode 7 is 10 μm. Therefore, the center of the lead electrode 7 is connected to a position approximately 5% away from the optical output end of the mesa electrode 6. A window structure is formed between the mesa electrode 6 and the front end face 101. It goes without saying that this structure also provides the same effects as the first embodiment. Furthermore, compared to the electroabsorption modulator integrated laser 1 shown in the first embodiment, the pad electrode 8 can be positioned closer to the inside of the element (lower side in FIG. 8), thereby shortening the overall length of the electroabsorption modulator integrated laser. Note that although the lead-out electrode 7 is inclined at 45 degrees with respect to the optical axis here, this is not a limitation and the angle of inclination is arbitrary. Furthermore, the lead-out electrode 7 is not limited to a straight line and may be curved (see the fourth embodiment). However, because the lead-out electrode 7 is a factor in generating parasitic capacitance, it is preferable that the area be as small as possible.

[0027] [Third embodiment] FIG. 9 is a top view of an electroabsorption modulator-integrated laser 301 according to a third embodiment of the present invention. FIG. 10 is a schematic diagram of a CC cross section perpendicular to the optical axis of an EA modulator 302. The only difference from the electroabsorption modulator-integrated laser 1 shown in the first embodiment is the structure of the EA modulator 302; the semiconductor laser 4 has the same structure. Of the n-type electrode 305 for the EA modulator, which is an electrode on the front side of the EA modulator 302, the mesa-top electrode 6 is located on top of the mesa structure. The pad electrode 8 has a rectangular shape. The lead electrode 7 connects the mesa-top electrode 6 and the pad electrode 8 and is substantially L-shaped. As shown in FIG. 10, the lead electrode 7 is connected to the mesa-top electrode 6 along the side of the mesa structure. An insulating film (not shown) is located between the side of the mesa structure and the lead electrode 7.

[0028] As shown in FIG. 10 , the multilayer structure is composed of a p-InP substrate 109 (p-type substrate), a multiple quantum well layer 10, and an n-InP cladding layer 110 (n-type cladding layer). A p-side electrode 314 is formed on the back surface side where the mesa structure is not formed. Here, the p-InP substrate 109 also functions as a p-type cladding layer. The n-InP cladding layer 110 also functions as a contact layer. The mesa structure is composed of a portion of the p-InP substrate 109, the multiple quantum well layer 10, and the n-InP cladding layer 110. Unlike the EA modulator 2 shown in the first embodiment, no buried layer 12 is disposed on the sides of the mesa structure. While the minimum configuration is shown here, a contact layer may be disposed on the top layer of the mesa structure to reduce optical confinement or resistance, and a passivation film to protect the semiconductor layer may be disposed as needed.

[0029] In this embodiment, too, the lead electrode 7 is connected to the mesa-upper electrode 6 closer to the optical output end side B' than the center. Even with this configuration, the effect of increasing the electric field intensity at the optical input end side B shown in the first embodiment can be obtained. That is, in an EA modulator having a pin structure, the effect of the invention can be obtained if the EA modulator electrode connected to either the p-side or n-side semiconductor has the following configuration. That is, the EA modulator electrode includes a mesa-upper electrode and a lead electrode arranged on the mesa structure, and the lead electrode is connected to the optical output end side of the mesa-upper electrode. Furthermore, because the lead electrode 7 is substantially L-shaped, it can be easily manufactured by photolithography.

[0030] [Fourth embodiment] FIG. 11 is a top view of an electroabsorption modulator-integrated laser 401 according to a fourth embodiment of the present invention. The only difference from the electroabsorption modulator-integrated laser 1 shown in the first embodiment is the structure of the EA modulator 402. The lead electrode 7 of the p-side electrode 405 for the EA modulator is curved toward the optical input end. The connection position of the lead electrode 7 is 33% from the optical output end. The length of the mesa electrode 6 of the p-side electrode 405 for the EA modulator is 150 μm, and the narrower width of the lead electrode 7 is 10 μm. A window structure is formed between the mesa electrode 6 and the front facet 101. As shown in FIGS. 6C and 6D , this structure with a 33% connection position can obviously achieve the same effects as the first embodiment. Furthermore, compared to the electroabsorption modulator-integrated laser 1 shown in the first embodiment, the pad electrode 8 can be positioned closer to the inside of the element (the lower side in FIG. 11 ), thereby shortening the overall length of the electroabsorption modulator-integrated laser. However, since the lead electrode 7 is a factor that generates parasitic capacitance, it is preferable that the area thereof be as small as possible.

[0031] The present invention is not limited to the above-described embodiments and is applicable to EA modulators in general. If the lead electrode is connected to the mesa electrode not at the center but offset toward the optical output end, the electric field intensity distribution changes, resulting in bandwidth improvement. In other words, this invention improves characteristics by the connection position to the mesa electrode, regardless of the shape of the pad electrode or the lead electrode. The p-type and n-type may be reversed. Furthermore, the above description assumes that the connection position is at the end of the mesa electrode on the optical output end side when it is 3% from the optical output end side. However, this ratio is determined by the width of the lead electrode 7 and the length of the mesa electrode 6. Therefore, the ratio is not limited to 3% as long as the end of the lead electrode 7 and the end of the mesa electrode 6 on the optical output end side are aligned. [Explanation of symbols]

[0032] 1,201,301,401 Electroabsorption modulator integrated laser, 2,202,302,402 EA modulator, 3 Waveguide connection portion, 4 Semiconductor laser, 5 p-side electrode for semiconductor laser, 6 Mesa electrode, 7 Lead electrode, 8 Pad electrode, 9 n-InP substrate, 10 Multiple quantum well layer, 11 p-type InP clad layer, 12 Buried layer, 13 Passivation film, 14 n-side electrode, 101 Front end facet, 102 Rear end facet, 105,205,405 p-side electrode for EA modulator, 109 p-InP substrate, 110 n-InP clad layer, 305 n-side electrode for EA modulator, 314 p-side electrode.

Claims

1. A substrate; a mesa structure provided on a first surface of the substrate, the mesa structure including a first conductivity type cladding layer, a multiple quantum well layer, and a second conductivity type cladding layer; a first conductivity type electrode electrically connected to the first conductivity type clad layer; a second conductivity type electrode provided on a second surface of the substrate, the first conductivity type electrode includes a mesa electrode disposed along an extension direction of the mesa structure, a pad electrode to which an external electric signal is input, and a lead electrode connecting the mesa electrode and the pad electrode; the mesa structure includes an optical input end to which light is input from the outside, and an optical output end on the opposite side of the optical input end; the mesa-top electrode has a first end closer to the optical input end than to the optical output end, and a second end closer to the optical output end than to the optical input end; a connection position between the lead electrode and the mesa-up electrode is an end of the mesa-up electrode on the light output end side; the electric field strength at the first end of the mesa electrode is greater than the electric field strength at the second end; a portion of the pad electrode is disposed between the second end of the mesa electrode and the light output end; the lead-out electrode extends in a direction perpendicular to a longitudinal direction of the mesa electrode and is connected to the pad electrode; a width of the lead-out electrode in the longitudinal direction of the mesa electrode is narrower than a width of the pad electrode; Electroabsorption modulator.

2. 2. The electro-absorption modulator according to claim 1, the length of the multiple quantum well layer in the longitudinal direction is 100 μm or more and 150 μm or less; Electroabsorption modulator.

3. 3. The electro-absorption modulator according to claim 1, Further, buried layers are provided so as to sandwich the mesa structure. Electroabsorption modulator.

4. 2. The electro-absorption modulator according to claim 1, the mesa structure further comprises a contact layer on the first conductivity type cladding layer. Electroabsorption modulator.

5. 2. The electro-absorption modulator according to claim 1, The external electrical signal includes an electrical signal of 20 GHz or higher. Electroabsorption modulator.

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