JP4941175B2 - Optical waveform shaping element - Google Patents

Description

  The present invention relates to an optical waveform shaping element.

For the realization of a future large-capacity photonic network, as shown in FIG. 13, there is a demand for a technology for reproducing a waveform deteriorated due to dispersion of an optical fiber or noise caused by a relay amplifier as it is. Has been.
Such a technique is usually called 2R (Reamplification and Reshaping), or 3R (Reamplification, Reshaping, and Retiming) with retiming added thereto.

  Conventionally, as a technique for waveform shaping by an optical semiconductor element, for example, as shown in FIGS. 14A to 14C, a semiconductor optical amplifier (SOA) and an optical waveguide are monolithically integrated, and a Mach-Zehnder (MZ) : Mach-Zehnder interferometer [see FIG. 14A], Symmetric Mach-Zehnder interferometer [see FIG. 14B], delay interferometer [see FIG. 14C] It has been proposed to use what has been done. An example in which a signal of 40 to 80 Gb / s is reproduced using such an element has been reported (see Non-Patent Documents 1 and 2).

Further, as a device using a single element, for example, as shown in FIG. 14D, a gain saturation method that suppresses 1 (ON) level noise using the gain saturation of the SOA has been proposed.
Furthermore, since SOAs with bulk and quantum well active layers have a slow gain response speed and the bit rate is greatly limited, use quantum dots with a fast gain response speed in the active layer and integrate saturable absorbers. Thus, it is suggested that noise of 0 (OFF) and 1 (ON) levels of a 40 Gb / s signal can be compressed (see Non-Patent Document 3).

Furthermore, since sufficient nonlinearity cannot be obtained with a single-stage amplification unit and saturable absorption unit, an optical nonlinear amplification element in which the amplification unit and saturable absorption unit are arranged in multiple stages has been proposed (see, for example, Patent Document 1). .
JP-A-5-75212 D. Wolfson et al, "40-Gb / s All-Optical Wavelength Conversion, Regeneration, and Demultiplexing in an SOA-Based All-Active Mach-Zehnder Interferometer", IEEE Photon. Technol. Lett. 12, no. 3, pp .332-334 (2000) Y. Ueno et al, "Penalty-Free Error-Free All-Optical Data Pulse Regeneration at 84 Gb / s by Using a Symmetric-Mach-Zehnder-Type SemiconductorRegenerator", IEEE Photon. Technol. Lett. 13, no. 5, pp. 469-471 (2001) Tomoyuki Akiyama et al., “40Gb / s signal regeneration using quantum dot semiconductor optical amplifier”, Proceedings of the 2005 Spring Conference of Applied Physics

However, in the method using the interferometer as shown in FIGS. 14A to 14C, the wavelength of the input / output signal is different, so that wavelength management is required and the configuration is complicated. There is a problem that leads to an increase in cost.
Further, in the method using the gain saturation of the SOA as shown in FIG. 14D described above, there is a problem that 0 (OFF) level noise cannot be suppressed although the wavelengths of the input and output signals are the same.

  Further, in the element proposed in Patent Document 1, an amplification unit that generates a gain by flowing a current in the forward direction from a light incident side and a saturable absorption unit that absorbs light by applying a reverse bias are alternately arranged. The length of the optical amplification unit at the final stage is longer than the lengths of the other optical amplification units so that amplification saturation occurs (see FIG. 7). However, in such an element structure, the input power (operating input power) serving as a threshold value becomes very large, and thus 2R and 3R that perform amplification and waveform shaping on a signal whose light intensity is reduced. It is disadvantageous to operation.

  Further, for example, as shown in FIG. 15, it is conceivable to alternately arrange the gain regions 100 and the absorption regions 101 and perform 2R and 3R signal reproduction (2R and 3R operations) using gain saturation and absorption saturation. In FIG. 15, reference numeral 102 is an n-type substrate, reference numeral 103 is an n-type cladding layer, reference numeral 104 is an active layer, reference numeral 105 is a p-type cladding layer, reference numeral 106 is a p-type contact layer, reference numeral 107 is a p-side electrode, Reference numeral 108 denotes an n-side electrode.

  However, when 2R and 3R reproduction of a signal is performed using gain saturation and absorption saturation, the signal is used at a high power level at which gain saturation and absorption saturation occur. In this case, simply by providing the gain region and the absorption region of the same optical waveguide structure, amplification is performed using a region where the gain characteristic is deeply saturated in the gain region (see, for example, the solid line A in FIG. 2). In the absorption region, absorption is performed using a region where the absorption characteristic is deeply saturated (see, for example, the solid line A in FIG. 3), and thus a pattern effect occurs. As a result, the output waveform is distorted without being shaped, and it is difficult to realize 2R and 3R reproduction of the signal.

  The present invention was devised in view of such problems, and even when used at a high power level where gain saturation and absorption saturation occur, the pattern effect can be suppressed and 2R and 3R reproduction of signals can be realized. An object of the present invention is to provide an optical waveform shaping element.

Therefore, optical waveform shaping device of the present invention includes a semiconductor optical waveguide having an active layer, a light amplification region having a single electrode provided in the semiconductor optical waveguides, is connected to the output side of the optical amplification region, and a light absorption region having a single electrode provided in the semiconductor optical waveguides, semiconductor optical waveguides, optical confinement factor is reduced at the output side of light than the input side of the light in the optical amplification region, the light absorption The optical confinement factor is smaller on the light input side than on the light output side in the region, and the light confinement factor is the same at the output end of the light amplification region and the input end of the light absorption region. It is a feature.

  Therefore, according to the optical waveform shaping element of the present invention, the pattern effect can be suppressed and 2R and 3R reproduction of signals can be realized even when used at a high power level where gain saturation and absorption saturation occur. There is an advantage.

Hereinafter, an optical waveform shaping element according to an embodiment of the present invention will be described with reference to FIGS.
The optical waveform shaping element according to the present embodiment includes a semiconductor optical waveguide 10 having an active layer 3 as shown in FIG. 1B, for example, along the direction in which the semiconductor optical waveguide 10 extends (optical waveguide direction), A light amplification region 20A is provided on the incident end face side [left side in FIG. 1B] of the semiconductor optical waveguide 10, and a light absorption region 20B is provided on the emission end face side [right side in FIG. 1B]. ing. As will be described later, the present optical waveform shaping element includes a light guide layer (SCH layer) above and below the active layer 3, but the light guide layer is not shown in FIG.

  That is, as shown in FIG. 1B, the present optical waveform shaping element is arranged in order on the active layer 3 on the semiconductor optical waveguide 10 formed on the semiconductor substrate 1 of one conductivity type (here, n-type). Other conductive materials are formed so that a region (light amplification region; SOA region) 20A for injecting current with a bias and a region (light absorption region; saturable absorption region) 20B for applying a reverse bias voltage to the active layer 3 are formed. A type (here, p-type) contact layer 7 (7A, 7B) and electrode 8 (8A, 8B) are divided into waveguide type active optical semiconductor elements (semiconductor optical active elements).

Here, the optical amplification region 20A and the light absorption region 20B are configured by a semiconductor optical waveguide 10 having the same active layer 3, as shown in FIG. Further, as shown in FIG. 7, the active layer 3 is a quantum dot active layer (here, a multi-layered quantum dot layer). Thereby, signal reproduction at a high bit rate of 40 Gb / s or more can be realized.
In the present embodiment, as shown in FIG. 1B, the p-type contact layer 7 and the p-side electrode 8 formed on the surface of the semiconductor optical waveguide 10 (semiconductor laminated structure) having the same active layer 3 are provided. Separately, an optical amplification region 20A having the p-type contact layer 7A and the p-side electrode 8A and an optical absorption region 20B having the p-type contact layer 7B and the p-side electrode 8B are arranged in one optical waveguide direction on one element. It forms in series along.

Here, a part of the p-type contact layer 7 is removed by etching so that the separated p-type contact layers 7A and 7B are arranged in series along the optical waveguide direction, and these separated p-type contacts are formed. The p-side electrodes 8A and 8B are formed independently on the layers 7A and 7B, respectively.
Further, as shown in FIG. 1B, antireflection coating is applied to both end faces of the incident end face and the exit end face of the optical waveform shaping element (that is, the entrance end face and the exit end face of the semiconductor optical waveguide 10). An antireflection film (nonreflective structure) 11 is formed.

Further, as shown in FIG. 8, the semiconductor optical waveguide 10 is configured as an oblique waveguide that is inclined with respect to the emission end face in order to prevent reflection. That is, the active layer 3 constituting the semiconductor optical waveguide 10 is formed obliquely so as to be inclined with respect to the emission end face.
In particular, in the present embodiment, when the optical signal (transmission signal; input optical signal) incident on the semiconductor optical waveguide 10 is off level (0 level), the power level is low, and therefore the gain characteristic is linear in the optical amplification region 20A. Amplification is performed using the region (linear gain), and absorption is performed using the linear region (linear absorption) of the absorption characteristics in the light absorption region 20B, whereas in the case of the on level (1 level), the power level is Therefore, amplification is performed using the saturation region (gain saturation) of the gain characteristic in the optical amplification region 20A, and absorption is performed using the saturation region (absorption saturation) of the absorption characteristic in the light absorption region 20B. For this reason, the optical amplification region 20A is a saturable gain region with a gain characteristic having a saturation region, and the light absorption region 20B is a saturable absorption region having an absorption characteristic with a saturation region.

In the present embodiment, the length of the optical amplification region 20A is such that a desired amplification factor can be obtained when the power of the input optical signal is on level (1 level) (here, the input power is 0 dBm). Is set to
Here, the optical amplification region 20A can amplify the power level (here, 0 dBm) of the input signal light to a desired power level (here, 15 dBm) that is amplified using the saturation region of the gain characteristic when it is on. As such, its length is set.

  In this case, at the ON level, the power level of the optical signal output from the optical amplification region 20A on the incident end face side (the power level of the optical signal amplified using the saturation region of the gain characteristic; gain saturated light output) becomes high. As a result, the power level of the optical signal input to the light absorption region 20B on the emission end face side is increased, so that the light absorption region 20B is absorbed using the saturated region of the absorption characteristics. Here, the optical confinement factor, the length of the light absorption region 20B, the material / structure of the active layer 3, the optical waveguide so that the absorption saturation state is maintained in the process of guiding the optical signal through the light absorption region 20B. Structure etc. are set.

As described above, the gain saturation state is maintained in the light amplification region 20A, the absorption saturation state is maintained in the light absorption region 20B, and light is transmitted well so that one level noise (on level noise) is reduced. It is suppressed and S / N is improved.
Further, in the present embodiment, when the power of the input optical signal is off level (0 level) (here, the input power is −10 dBm), a linear region of the absorption characteristic is used in the light absorption region 20B on the emission end face side. Are absorbed (linear absorption). Here, the length of the light absorption region 20B on the emission end face side is set so that the power level of the output optical signal is equal to or lower than the power level of the input optical signal at the off level. Thereby, 0 level noise can be suppressed.

By the way, in this embodiment, as shown in FIGS. 1A and 1B, the semiconductor optical waveguide 10 has an optical confinement coefficient in the vicinity of the incident end face and the exit end face, and the optical confinement in the element central area in the optical waveguide direction. It is comprised so that it may become larger than a coefficient.
Specifically, as shown in FIGS. 1A and 1B, in the optical amplification region 20A, the semiconductor optical waveguide 10 is configured such that the optical confinement factor decreases stepwise along the optical waveguide direction. ing. On the other hand, in the light absorption region 20B, the semiconductor optical waveguide 10 is configured such that the optical confinement coefficient increases stepwise along the optical waveguide direction. In FIG. 1B, the state of the optical confinement coefficient of the semiconductor optical waveguide 10 is changed and the pattern of the active layer 3 is changed.

Here, as shown in FIG. 1A, the length of the optical amplification region 20A is L _SOA , the optical confinement factor of the input half region (length L _SOA / 2) is Γ, and the output The optical confinement factor of the half region (length L _SOA / 2) on the side is Γ / 2. On the other hand, the length of the light absorption region 20B is L _SA , the light confinement factor of the half region on the input side (length L _SOA / 2) is Γ / 2, and the half region on the output side (length L The optical confinement factor of _SOA / 2) is Γ.

Here, FIG. 2 shows the result of calculating the optical output P _OUT (power level of the optical signal output from the optical amplification region 20A) and the gain of the optical amplification region 20A.
In FIG. 2, the solid line A shows the calculation result of the same active layer structure and the same (uniform) light confinement factor over the entire length of the optical amplification region 20A (light confinement factor ratio 1), and the solid line B is the same. Calculation result of the active layer structure in which the optical confinement factor in the output half of the optical amplification region 20A is half the optical confinement factor in the input half region (optical confinement factor ratio 0.5) Is shown.

As shown by the solid line B in FIG. 2, the linear gain is reduced by changing the optical confinement factor, but the gain saturation power level [the saturation depth of the gain characteristic is equal to or greater than a predetermined value (here 2 dB)). It can be seen that the light output becomes larger.
For example, assuming that the optical output P _OUT at the on-level is 15 dBm, a gain characteristic with a same optical confinement coefficient (solid line A in FIG. 2) has a gain characteristic saturation depth of 2 dB or more and a deep gain characteristic saturation. In contrast to the amplification using the region, the saturation depth of the gain characteristic is about 1.5 dB in the case where the optical confinement coefficient is changed in the optical waveguide direction (solid line B in FIG. 2). Therefore, amplification is performed using a region where the gain characteristic is not saturated.

Here, FIG. 3 shows the result of calculating the optical input P _IN (the power level of the optical signal input to the light absorption region 20B) and the amount of absorption (negative gain) of the light absorption region.
In FIG. 3, the solid line A indicates the calculation result of the same active layer structure and the same (uniform) light confinement factor over the entire length of the light absorption region 20B (light confinement factor ratio 1), and the solid line B is the same. Calculation result of the active layer structure in which the light confinement coefficient of the half region on the input side of the light absorption region 20B is half the light confinement factor of the half region on the output side (light confinement factor ratio 0.5) Is shown.

As shown by a solid line B in FIG. 3, the amount of absorption is reduced by changing the optical confinement coefficient. As a result, the light output from the light absorption region 20B increases. In addition, as shown by a solid line B in FIG. 3, it can be seen that the absorption saturation power level [light input at which the saturation depth of the absorption characteristic is equal to or greater than a predetermined value (3 dB in this case)] is also increased.
For example, if the optical input _PIN at the on-level is 15 dBm, the one with the same optical confinement factor (solid line A in FIG. 3) has a saturation depth of the absorption characteristic of about 3 dB, and the absorption characteristic is deeply saturated. In contrast to absorption using a region, the saturation depth of the absorption characteristic is about 2 dB when the optical confinement coefficient is changed in the optical waveguide direction (solid line B in FIG. 3). Absorption is performed using a region with a shallow characteristic saturation.

  Thus, as shown in FIGS. 1A and 1B, in the optical amplification region 20A, by changing the optical confinement factor stepwise so that the optical confinement factor decreases along the optical waveguide direction, In FIG. 2, as indicated by a solid line B, the gain saturation power level is increased, and amplification is performed using a region where the saturation of gain characteristics is shallow. On the other hand, as shown in FIGS. 1A and 1B, in the light absorption region 20B, the optical confinement factor is changed stepwise so that the optical confinement factor increases along the optical waveguide direction. In the middle, as shown by a solid line B, the absorption saturation power level is increased, and absorption is performed using a region where the absorption characteristic is shallow.

That is, in the present embodiment, the optical confinement factor is set so that the semiconductor optical waveguide 10 is amplified using a region with shallow gain characteristics saturation in the optical amplification region 20A at the on-level. Further, the optical confinement coefficient is set so that the semiconductor optical waveguide 10 is absorbed by using a region having a shallow saturation of the absorption characteristic in the light absorption region 20B at the on-level.
As a result, as shown in FIGS. 2 and 3, the gain saturation power level and the absorption saturation power level are appropriately set at the on-level, and the amplification is performed using the region where the gain characteristic is shallow, and the absorption characteristic is obtained. Since the absorption is performed using the shallowly saturated region, waveform distortion (noise) due to the pattern effect can be suppressed, and as a result, 2R and 3R operations are performed even at high power levels where gain saturation and absorption saturation occur. Is possible. Since 2R operation can be realized with one active element in this way, the configuration is simplified and the cost is reduced as compared with the above-described method using an interferometer [see FIGS. 14A to 14C]. Is possible.

  In this embodiment, as shown in FIG. 4, the semiconductor optical waveguide 10 is configured to include light guide layers (SCH layers) 12 and 13 on the upper side and the lower side of the active layer 3, and the light of the semiconductor optical waveguide 10 is formed. The light guide layer (upper SCH layer) 12 on the upper side of the active layer 3 has an entrance end face and an exit end face so that the confinement factor is larger in the vicinity of the entrance end face and the exit end face than the element central area in the optical waveguide direction. The thickness of the element central region in the optical waveguide direction is made thicker than the thickness of the region in the vicinity of. Here, in the upper SCH layer 12, the thickness of the element central region in the optical waveguide direction is increased stepwise along the optical waveguide direction with respect to the thickness in the vicinity of the incident end surface and the output end surface.

In this case, when the upper SCH layer 12 is formed, for example, as shown in FIG. 5, the mask opening becomes wide on the active layer 3 in the vicinity of the incident end face and the outgoing end face, and in the element central area in the optical waveguide direction. A mask (for example, a SiO ₂ mask) 14 in which the mask opening changes stepwise so as to narrow the mask opening may be formed, and the upper SCH layer 12 may be formed using this mask 14. That is, when the upper SCH layer 12 is formed using such a mask 14, the film thickness of the upper SCH layer 12 is reduced at a wide portion of the mask opening and is increased at a narrow portion of the mask opening. As shown, the upper SCH layer 12 is formed such that the thickness of the central region of the element in the direction of the optical waveguide is stepwise thicker than the thickness of the region near the incident end surface and the outgoing end surface (for example, Japanese Patent Laid-Open 7-283490 publication etc.).

  Here, light guide layers (SCH layers) 12 and 13 are provided above and below the active layer 3 and the thickness of the light guide layer 12 above the active layer 3 is changed, but this is not limitative. It is not a thing. For example, the thickness of the light guide layer (lower SCH layer) 13 below the active layer 3 may be changed, or the thickness of both the upper and lower light guide layers 12 and 13 of the active layer 3 may be changed. May be. That is, an optical guide layer is provided on the upper and lower sides of the active layer, and at least one of the upper and lower optical guide layers is located in the element central region in the optical waveguide direction with respect to the thickness in the vicinity of the incident end surface and the exit end surface. What is necessary is just to make it thick. Further, for example, a light guide layer is provided on the upper side or the lower side of the active layer, and the thickness of the central region of the element in the optical waveguide direction is greater than the thickness of the light guide layer in the vicinity of the entrance end face and the exit end face. Anyway.

In particular, in the present embodiment, as shown in FIG. 1B, the p-type contact layer 7 and the p-side electrode 8 formed on the surface of the semiconductor optical waveguide 10 (semiconductor laminated structure) having the same active layer 3 are The semiconductor optical waveguide 10 is separated in the region where the optical confinement coefficient is small.
Hereinafter, the present optical waveform shaping element will be described more specifically.
In the present optical waveform shaping element, as shown in FIG. 6, a mesa structure including an active layer 3 is formed on an n-type InP substrate 1 with an n-type InP cladding layer 2 interposed therebetween. Therefore, this mesa structure is a structure in which a p-type InP layer 4 and an n-type InP layer 5 are buried by a buried layer formed by a pn junction. As described above, the optical waveform shaping element includes the light guide layer (SCH layer) above and below the active layer 3, but the light guide layer is not illustrated in FIG.

A p-type InP cladding layer 6 is formed on the n-type InP buried layer 5, and a p-type InGaAs contact layer 7 is laminated on the p-type InP cladding layer 6.
A p-side electrode (electrode metal) 8 is formed on the p-type InGaAs contact layer 7. On the other hand, the n-side electrode (electrode metal) 9 is formed on the back surface of the n-type InP substrate 1.
Here, the active layer portion is formed of InGaAsP-SCH layers 12 and 13 above and below the active layer (in this case, the multi-layered quantum dot active layer) 3 as shown in an enlarged schematic sectional view of the active layer portion in FIG. It has a sandwiched structure.

  Here, as shown in FIG. 7, the multi-layered quantum dot active layer 3 is composed of a quantum dot layer 3C in which InAs quantum dots 3A are embedded with an InGaAsP barrier layer 3B, and the quantum dots 3A of each quantum dot layer 3C are joined vertically. As shown, a plurality of layers (here, 22 layers) are stacked. That is, as shown in FIG. 7, the multi-stacked quantum dot active layer 3 has a structure in which a multi-stack quantum dot 3D formed by stacking a plurality of quantum dots 3A is embedded with an InGaAsP barrier layer 3B.

Therefore, according to the optical waveform shaping element according to the present embodiment, the pattern effect is suppressed and 2R and 3R reproduction of the signal can be realized even when used at a high power level where gain saturation and absorption saturation occur. There are advantages.
[Second Embodiment]
Next, an optical waveform shaping element according to a second embodiment of the present invention will be described with reference to FIGS.

  The optical waveform shaping element according to the present embodiment is different from that of the first embodiment described above, as shown in FIGS. 9A and 9B, in the optical amplification region 20A, the semiconductor optical waveguide 10 is optically confined. The coefficient is configured to continuously decrease along the optical waveguide direction. In the light absorption region 20B, the semiconductor optical waveguide 10 has an optical confinement factor that continuously increases along the optical waveguide direction. It is different in that it is configured to go. As will be described later, the present optical waveform shaping element includes a light guide layer (SCH layer) on the upper and lower sides of the active layer 3, but the light guide layer is not shown in FIG. 9B. In FIG. 9, the same components as those in the first embodiment (see FIG. 1) are denoted by the same reference numerals.

  Thus, as shown in FIGS. 9A and 9B, in the optical amplification region 20A, by continuously changing the optical confinement factor so that the optical confinement factor decreases along the optical waveguide direction, The gain saturation power level is increased, and amplification is performed using a shallow region of gain characteristics saturation. On the other hand, as shown in FIGS. 9A and 9B, in the light absorption region 20B, absorption saturation is achieved by continuously changing the light confinement coefficient so that the light confinement coefficient increases along the optical waveguide direction. The power level is increased so that absorption is performed using a region where the absorption characteristics are shallow.

  In this embodiment, as shown in FIG. 10, the semiconductor optical waveguide 10 is configured to include light guide layers (SCH layers) 12 </ b> X and 13 on the upper side and the lower side of the active layer 3. The light guide layer (upper SCH layer) 12X above the active layer 3 has an entrance end face and an exit end face so that the confinement factor is larger in the vicinity of the entrance end face and the exit end face than in the central area of the element in the optical waveguide direction. The thickness of the element central region in the optical waveguide direction is made thicker than the thickness of the region in the vicinity of. Here, the upper SCH layer 12X is made to continuously increase along the optical waveguide direction from the vicinity of the incident end face and the outgoing end face toward the element central region in the optical waveguide direction. In FIG. 10, the same components as those in the first embodiment (see FIG. 4) are denoted by the same reference numerals.

In this case, when the upper SCH layer 12X is formed, for example, as shown in FIG. 11A, the mask opening becomes wide on the active layer 3 in the vicinity of the entrance end face and the exit end face, and the optical waveguide direction element is formed. A mask (for example, SiO ₂ mask) 14X in which the mask opening continuously changes is formed so that the mask opening becomes narrow in the central region, and the upper SCH is formed using this mask 14X as shown in FIG. 11B, for example. The layer 12X may be formed. That is, when the upper SCH layer 12X is formed using such a mask 14X, the film thickness of the upper SCH layer 12X is reduced at a wide portion of the mask opening and is increased at a narrow portion of the mask opening. As shown in B), the upper SCH layer 12X is formed so as to be continuously thick along the optical waveguide direction from the vicinity of the incident end face and the exit end face toward the element central region in the optical waveguide direction. (See, for example, JP-A-7-283490). In FIG. 11B, a region used as the semiconductor optical waveguide 10 is surrounded by a dotted line X.

  Here, light guide layers (SCH layers) 12X and 13 are provided above and below the active layer 3, and the thickness of the light guide layer 12X above the active layer 3 is changed, but this is not limitative. It is not a thing. For example, the thickness of the lower light guide layer (lower SCH layer) of the active layer 3 may be changed, or the thickness of both the upper and lower light guide layers of the active layer 3 may be changed. That is, an optical guide layer is provided on the upper and lower sides of the active layer, and at least one of the upper and lower optical guide layers is located in the element central region in the optical waveguide direction with respect to the thickness in the vicinity of the incident end surface and the exit end surface. What is necessary is just to make it thick. Further, for example, a light guide layer is provided on the upper side or the lower side of the active layer, and the thickness of the central region of the element in the optical waveguide direction is greater than the thickness of the light guide layer in the vicinity of the entrance end face and the exit end face. Anyway.

In particular, in the present embodiment, as shown in FIG. 9B, the p-type contact layer 7 and the p-side electrode 8 formed on the surface of the semiconductor optical waveguide 10 (semiconductor laminated structure) having the same active layer 3 are The semiconductor optical waveguide 10 is separated at the portion where the optical confinement coefficient is the smallest (the portion where the optical confinement coefficient is minimized) and the vicinity thereof.
The details of the other configurations are the same as those of the first embodiment described above, and thus the description thereof is omitted here.

Therefore, according to the optical waveform shaping element according to the present embodiment, the pattern effect is suppressed even when used at a high power level where gain saturation and absorption saturation occur, as in the first embodiment described above. Thus, there is an advantage that 2R and 3R reproduction of the signal can be realized.
[Others]
In each of the above-described embodiments, the case where the present invention is applied to an optical waveform shaping element including one light amplification region and one light absorption region is described as an example. However, the present invention is not limited thereto. Instead, for example, the present invention can also be applied to an optical waveform shaping element that includes a plurality of light amplification regions and a plurality of light absorption regions, which are alternately provided along the optical waveguide direction.

In each of the above-described embodiments, the optical confinement coefficient is changed by changing the film thickness of the light guide layer (SCH layer) along the optical waveguide direction. However, the present invention is not limited to this.
For example, when the active layer is a quantum dot active layer, the light confinement factor may be changed by changing the dot density of the quantum dot active layer along the optical waveguide direction. That is, as shown in FIGS. 12A and 12B, the quantum dots are set so that the optical confinement coefficient of the semiconductor optical waveguide 10 is larger in the vicinity of the incident end face and the outgoing end face than in the element central area in the optical waveguide direction. The active layer 3X may be configured such that the density of the quantum dots 3A in the element central region in the optical waveguide direction is lower than the density of the quantum dots 3A in the region near the incident end surface and the output end surface. In FIGS. 12A and 12B, the same components as those in the first embodiment (see FIG. 4) are denoted by the same reference numerals. As described above, as a method for changing the quantum dot density, for example, there is a method disclosed in Japanese Patent Application Laid-Open No. 2003-309322.

Further, a method of changing the film thickness of the light guide layer and a method of changing the density of the quantum dots may be combined. That is, the optical confinement factor may be changed by changing the film thickness of the light guide layer along the optical waveguide direction and changing the dot density of the quantum dot active layer.
Furthermore, for example, when the semiconductor optical waveguide is provided with a cladding layer on the upper side and the lower side of the active layer, the composition of the cladding layer may be changed along the optical waveguide direction. In other words, the composition of at least one of the upper and lower clad layers is changed so that the optical confinement coefficient of the semiconductor optical waveguide is larger in the vicinity of the incident end face and the outgoing end face than in the element central area in the optical waveguide direction. May be. Further, the method of changing the composition of the cladding layer may be combined with the method of changing the film thickness of the light guide layer and the method of changing the density of the quantum dots.

In each of the above-described embodiments, quantum dots are used for the active layer. However, the present invention is not limited to this, and even if a bulk active layer or a quantum well active layer is used as the active layer, the same as in each of the above-described embodiments. The effect is obtained. For example, when the signal bit rate is low, a bulk active layer or a quantum well active layer can be used.
In each of the above-described embodiments, the waveguide embedded structure device using the InP-based material is described as an example. However, the present invention is not limited to this. For example, the present invention may be used even when a GaAs-based material is used. The optical waveform shaping element according to the above can be realized.

Further, in each of the above-described embodiments, the pn buried structure is used as the buried structure, but the present invention is not limited to this. For example, a semi-insulating buried structure may be used. In each of the above-described embodiments, the waveguide embedded structure is used. However, the present invention is not limited to this. For example, the waveguide structure may be a ridge structure.
In each of the above embodiments, the light amplification region and the light absorption region are separated by etching and separating a part of the contact layer and forming an electrode on the separated contact layer. The electrode separation method (electrode separation structure) is not limited to this. For example, electrode separation may be performed by increasing the resistance of a part of the contact layer by proton implantation (ion implantation).

The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.
Hereinafter, additional notes will be disclosed regarding each of the above-described embodiments.
(Appendix 1)
A semiconductor optical waveguide having an active layer;
An optical amplification region provided on the incident end face side of the semiconductor optical waveguide;
A light absorption region provided on the emission end face side of the semiconductor optical waveguide,
An optical waveform shaping element, wherein the semiconductor optical waveguide is configured such that a light confinement coefficient in a region in the vicinity of an incident end face and an output end face is larger than a light confinement coefficient in a central area of the element in an optical waveguide direction.

(Appendix 2)
In the optical amplification region, the semiconductor optical waveguide is configured such that the optical confinement factor is continuously reduced along the optical waveguide direction,
2. The optical waveform shaping element according to appendix 1, wherein in the light absorption region, the semiconductor optical waveguide is configured such that an optical confinement factor increases continuously along the optical waveguide direction.

(Appendix 3)
Each of the light amplification region and the light absorption region includes an electrode,
The electrode of the light amplification region and the electrode of the light absorption region are separated in a portion where the light confinement coefficient of the semiconductor optical waveguide is the smallest and a region in the vicinity thereof. Optical waveform shaping element.

(Appendix 4)
In the optical amplification region, the semiconductor optical waveguide is configured such that the optical confinement factor decreases stepwise along the optical waveguide direction,
2. The optical waveform shaping element according to appendix 1, wherein in the light absorption region, the semiconductor optical waveguide is configured such that an optical confinement factor increases stepwise along the optical waveguide direction.

(Appendix 5)
The semiconductor optical waveguide is characterized in that an optical confinement factor is set so that the optical amplification region is amplified using a shallow region of saturation of gain characteristics when the power of an input optical signal is on level. The optical waveform shaping element according to any one of appendices 1 to 4.
(Appendix 6)
The semiconductor optical waveguide is characterized in that an optical confinement factor is set so that the light absorption region is absorbed using a shallow region of absorption characteristics when the power of the input optical signal is on level. The optical waveform shaping element according to any one of appendices 1 to 5.

(Appendix 7)
The light amplification region and the light absorption region are constituted by a semiconductor optical waveguide having the same active layer,
The light amplification region is configured such that a forward bias current is injected into the active layer,
The optical waveform shaping element according to any one of appendices 1 to 6, wherein a reverse bias voltage is applied to the active layer in the light absorption region.

(Appendix 8)
The semiconductor optical waveguide includes a light guide layer,
The light guide layer includes a region near the incident end surface and the exit end surface so that a light confinement coefficient of the semiconductor optical waveguide is larger in a region near the entrance end surface and the exit end surface than an element central region in the optical waveguide direction. The optical waveform shaping element according to any one of appendices 1 to 7, wherein the thickness of the element central region in the optical waveguide direction is greater than the thickness of the optical waveguide.

(Appendix 9)
The active layer is a quantum dot active layer;
In the vicinity of the incident end face and the exit end face, the quantum dot active layer has an optical confinement factor of the semiconductor optical waveguide that is larger in the vicinity of the entrance end face and the exit end face than in the central region of the element in the optical waveguide direction. 9. The optical waveform shaping element according to any one of appendices 1 to 8, wherein the density of quantum dots in the central region of the element in the optical waveguide direction is lower than the density of quantum dots in the region.

(Appendix 10)
The semiconductor optical waveguide includes a cladding layer,
The composition of the cladding layer is changed so that the optical confinement coefficient of the semiconductor optical waveguide is larger in the vicinity of the incident end face and the outgoing end face than in the element central area in the optical waveguide direction. The optical waveform shaping element according to any one of appendices 1 to 9.

(Appendix 11)
11. The optical waveform shaping element according to any one of appendices 1 to 8, wherein the active layer is a quantum dot active layer.
(Appendix 12)
The optical waveform shaping element according to any one of appendices 1 to 11, wherein a non-reflective coating is applied to an incident end face and an exit end face of the semiconductor optical waveguide.

(Appendix 13)
The optical waveform shaping element according to any one of appendices 1 to 12, wherein the semiconductor optical waveguide is configured as an oblique waveguide inclined with respect to the emission end face.

(A), (B) is a figure which shows the structure of the optical waveform shaping element concerning 1st Embodiment of this invention, (A) is a figure which shows the change of the optical confinement coefficient along an optical waveguide direction. , (B) is a schematic cross-sectional view (cross-sectional view taken along the line AA ′ in FIG. 6) showing a laminated structure. In the optical amplification region constituting the optical waveform shaping element according to the first embodiment of the present invention, the relationship between the optical output and gain when the optical confinement factor is the same along the optical waveguide direction, and along the optical waveguide direction. It is a figure which shows the relationship between the optical output at the time of changing an optical confinement coefficient, and a gain. In the light absorption region constituting the optical waveform shaping element according to the first embodiment of the present invention, the relationship between the optical output and gain when the optical confinement factor is the same along the optical waveguide direction, and along the optical waveguide direction. It is a figure which shows the relationship between the optical input at the time of changing an optical confinement coefficient, and a gain. FIG. 3 is a schematic cross-sectional view showing a specific configuration example for changing the optical confinement coefficient in the optical waveform shaping element according to the first embodiment of the present invention. It is a typical perspective view for demonstrating the method for implement | achieving the specific structural example for changing an optical confinement coefficient in the optical waveform shaping element concerning 1st Embodiment of this invention. It is typical sectional drawing which shows the structure of the optical waveform shaping element concerning 1st Embodiment of this invention. It is typical sectional drawing which shows the active layer which comprises the optical waveform shaping element concerning 1st Embodiment of this invention. FIG. 7 is a schematic cross-sectional view (cross-sectional view taken along the line BB ′ in FIG. 6) showing the configuration of the optical waveform shaping element according to the first embodiment of the present invention. (A), (B) is a figure which shows the structure of the optical waveform shaping element concerning 2nd Embodiment of this invention, (A) is a figure which shows the change of the optical confinement coefficient along an optical waveguide direction. (B) is typical sectional drawing which shows a laminated structure. It is typical sectional drawing which shows the specific structural example for changing an optical confinement factor in the optical waveform shaping element concerning 2nd Embodiment of this invention. (A), (B) is a schematic perspective view for explaining a method for realizing a specific configuration example for changing the optical confinement coefficient in the optical waveform shaping element according to the first embodiment of the present invention. FIG. (A), (B) is a schematic diagram which shows the other specific structural example for changing an optical confinement coefficient in the optical waveform shaping element concerning each embodiment of this invention. It is a figure for demonstrating the waveform shaping technique using an optical waveform shaping element. (A)-(D) are the figures for demonstrating the conventional waveform shaping technique. It is typical sectional drawing for demonstrating the subject of the optical waveform shaping element concerning this invention.

Explanation of symbols

1 n-type InP substrate (semiconductor substrate)
2 n-type InP cladding layer 3, 3X active layer (quantum dot active layer)
3A InAs quantum dot 3B InGaAsP barrier layer 3C quantum dot layer 3D multi-layered quantum dot 4 p-type InP layer 5 n-type InP layer 6 p-type InP clad layer 7, 7A, 7B p-type InGaAs contact layer 8, 8A, 8B p side Electrode 9 N-side electrode 10 Semiconductor optical waveguide 11 Antireflection film 12, 12X, 13 InGaAsP-SCH layer 14, 14X Mask 20A Optical amplification region (saturable gain region)
20B light absorption region (saturable absorption region)

Claims (6)

A semiconductor optical waveguide having an active layer;
A light amplifying region having a single electrode provided in the semiconductor optical waveguides,
Connected to the output side of the optical amplification region, and a light absorption region having a single electrode provided in the semiconductor optical waveguides,
The semiconductor optical waveguide has a smaller light confinement factor on the light output side than the light input side in the light amplification region, and a smaller light confinement factor on the light input side than the light output side in the light absorption region. The optical waveform shaping element is configured to have the same optical confinement coefficient at the output end of the light amplification region and the input end of the light absorption region . In the optical amplification region, the semiconductor optical waveguide is configured such that the optical confinement factor is continuously reduced along the optical waveguide direction,
2. The optical waveform shaping element according to claim 1, wherein in the light absorption region, the semiconductor optical waveguide is configured such that an optical confinement factor continuously increases along an optical waveguide direction. . In the optical amplification region, the semiconductor optical waveguide is configured such that the optical confinement factor decreases stepwise along the optical waveguide direction,
2. The optical waveform shaping element according to claim 1, wherein in the light absorption region, the semiconductor optical waveguide is configured such that an optical confinement factor increases stepwise along the optical waveguide direction.   The semiconductor optical waveguide is characterized in that an optical confinement factor is set so that the optical amplification region is amplified using a shallow region of saturation of gain characteristics when the power of an input optical signal is on level. The optical waveform shaping element of any one of Claims 1-3.   The semiconductor optical waveguide is characterized in that an optical confinement factor is set so that the light absorption region is absorbed using a shallow region of absorption characteristics when the power of the input optical signal is on level. The optical waveform shaping element of any one of Claims 1-4. The optical waveform shaping element according to claim 1, wherein the optical amplification region and the light absorption region are configured by a semiconductor optical waveguide having the same active layer.

Images