JP2009158602A - Light waveform shaping device and light waveform shaping element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a good waveform for a fast light signal faster than response speed of a gain saturation region and an absorption saturation region to enable waveform shaping when shaping the waveform by means of a light waveform shaping element having the two regions connected in series. <P>SOLUTION: The light waveform shaping device includes the light waveform shaping element 1 having the gain saturation region 4 for causing gain saturation and the absorption saturation region 5 for causing absorption saturation connected in series, and a driving circuit 20 for driving the element 1. The device is constituted so that response speed of the gain saturation region 4 coincides with that of the absorption saturation region 5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical waveform shaping device and an optical waveform shaping element that are used in an optical communication system, for example, and shape a waveform of an optical signal deteriorated during transmission or the like.

In the current optical communication system, after converting an optical signal into an electrical signal and shaping the waveform of the electrical signal, a method of converting the electrical signal into an optical signal and shaping the waveform of the optical signal again is taken. .
On the other hand, in order to reduce the size and power consumption of the waveform shaping device by omitting the electric circuit, there is an optical waveform shaping method by all-optical signal processing that processes an optical signal as it is without converting the electric signal into an optical signal. It has been studied in many institutions.

As one method for realizing optical waveform shaping by all-optical signal processing, a method of suppressing mark level noise using gain saturation of a semiconductor optical amplifier (SOA) has been studied.
In this method, when high intensity light is input to the SOA, gain saturation occurs and the power level of the output light becomes constant, so that the mark level power is constant and the mark level (1 level) noise is obtained. Suppress.

This method has a feature that an apparatus can be realized with a simple structure as compared with an optical waveform shaping method using other all-optical signal processing using the nonlinear input / output characteristics of a Mach-Zehnder interferometer. It also has the feature that it can be reproduced without changing the wavelength of the input signal.
For example, an SOA having a bulk or quantum well active layer has a slow gain response speed and greatly limits the bit rate. Therefore, a quantum dot having a high gain response speed is used in the active layer and a saturable absorber is integrated. Thus, it is suggested that noise of 0 (OFF) and 1 (ON) levels of a 40 Gb / s signal can be compressed (see, for example, Non-Patent Document 1).

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 Tomoyuki Akiyama et al., “40Gb / s signal regeneration using quantum dot semiconductor optical amplifier”, Proceedings of the 2005 Spring Conference of Applied Physics

By the way, in the optical waveform shaping element, a gain saturation region that causes gain saturation and an absorption saturation region that causes absorption saturation are connected in series, thereby suppressing mark level (1 level) noise by gain saturation and absorption. Saturation can suppress space level (zero level) noise.
However, if the gain saturation region and the absorption saturation region are set to have the best characteristics, and the gain saturation region and the absorption saturation region thus set are simply connected in series, these two regions It was found that a good waveform could not be obtained with a high-speed optical signal faster than the above response speed.

  Therefore, when waveform shaping is performed using an optical waveform shaping element in which a gain saturation region and an absorption saturation region are connected in series, a good waveform is obtained even in a high-speed optical signal that is faster than the response speed of these two regions, for example. I want to be able to obtain and shape the waveform.

For this reason, the optical waveform shaping device includes a gain saturation region that causes gain saturation and an absorption saturation region that causes absorption saturation, and an optical waveform shaping element in which the gain saturation region and the absorption saturation region are connected in series, And a drive circuit that drives the optical waveform shaping element, and the response speed of the gain saturation region and the response speed of the absorption saturation region are configured to be the same.
This optical waveform shaping element includes a gain saturation region that causes gain saturation and an absorption saturation region that causes absorption saturation. The gain saturation region and the absorption saturation region are connected in series, and the saturation level of the gain saturation region is And the saturation depth of the absorption saturation region are required to be the same.

  Therefore, according to the present optical waveform shaping device and optical waveform shaping element, when waveform shaping is performed using an optical waveform shaping element in which a gain saturation region and an absorption saturation region are connected in series, for example, the response of these two regions There is an advantage that a good waveform can be obtained even in a high-speed optical signal faster than the speed, and waveform shaping becomes possible.

Hereinafter, an optical waveform shaping device and an optical waveform shaping element according to the present embodiment will be described with reference to FIGS.
As shown in FIG. 1, the optical waveform shaping device according to the present embodiment includes a gain saturation region 4 that causes gain saturation and an absorption saturation region 5 that causes absorption saturation. The gain saturation region 4 and the absorption saturation region 5 include An optical waveform shaping element 1 connected in series (cascade connection) and a drive circuit (electric circuit; control circuit) 20 for driving the optical waveform shaping element 1 are provided.

In this embodiment, as shown in FIG. 1, the optical waveform shaping element 1 includes a semiconductor optical waveguide 3 having an active layer 2, and includes a gain saturation region (light amplification region) 4 and an absorption saturation region (saturable absorption region; The light absorption regions 5 are alternately provided along the direction in which the semiconductor optical waveguide 3 extends (along the optical waveguide direction).
In other words, as shown in FIG. 1, the present optical waveform shaping element 1 is formed in the order of the active layer 2 on the semiconductor optical waveguide 3 formed on one conductive type semiconductor substrate (here, n-type InP substrate) 6. A contact layer of another conductivity type (here, a region in which current is injected by bias (gain saturation region) 4 and a region (absorption saturation region) 5 in which a reverse bias voltage is applied to the active layer 2 are alternately formed. In the figure, a p-type InGaAs contact layer 7 and an electrode (here, p-side electrode) 8 are divided into waveguide type optical semiconductor elements.

As described above, the gain saturation region 4 and the absorption saturation region 5 are connected in series (here, because the gain saturation region 4 and the absorption saturation region 5 are repeated in one element), Mark level (1 level) noise is suppressed by gain saturation, and space level (0 level) noise is suppressed by absorption saturation.
In this embodiment, the gain saturation region 4 and the absorption saturation region 5 are constituted by the semiconductor optical waveguide 3 having the same active layer 2.

  Here, the active layer 2 is a quantum dot active layer (here, a multi-stacked quantum dot active layer). For example, the multi-layered quantum dot active layer has a structure in which a plurality of quantum dot layers in which InAs quantum dots are embedded with an InGaAsP barrier layer are stacked so that the quantum dots of each quantum dot layer are joined vertically. Yes. That is, the multi-stacked quantum dot active layer has a structure in which a multi-stack quantum dot formed by stacking a plurality of quantum dots is embedded with an InGaAsP barrier 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. 1, an electrode (here, p-side electrode) 8 formed on the surface of a semiconductor optical waveguide (semiconductor laminated structure) 3 having the same active layer 2 is separated, On one element, the gain saturation region 4 and the absorption saturation region 5 are formed in series along the optical waveguide direction.
Hereinafter, the optical waveform shaping element 1 will be described more specifically.
As shown in FIG. 1, the optical waveform shaping element 1 has a mesa structure including an active layer 2 on an n-type InP substrate 6 via an n-type InP clad layer (lower clad layer; not shown). In order to confine current and confine light, this mesa structure is buried by a buried layer formed by pn junction of the p-type InP layer 9 and the n-type InP layer 10. In addition, you may comprise this optical waveform shaping element 1 as a thing provided with the light guide layer (for example, InGaAsP-SCH layer) on the upper side and the lower side of the active layer 2. FIG.

A p-type InP clad layer (upper clad layer) 11 is formed on the n-type InP buried layer 10, and a p-type InGaAs contact layer 7 is laminated on the p-type InP clad layer 11.
A p-side electrode 8 is formed on the p-type InGaAs contact layer 7. On the other hand, an n-side electrode (not shown) is formed on the back surface of the n-type InP substrate 6.

By the way, the gain saturation region and the saturable absorption region (absorption saturation region) are set to have the best characteristics, and the gain saturation region and the saturable absorption region thus set are simply connected in series. Thus, it has been found that a good waveform cannot be obtained with a high-speed optical signal that is faster than the response speed of these two regions.
Here, FIG. 3 shows the response characteristics of the saturable absorption region set so that the characteristics of the saturable absorption region are the best.

As shown in FIG. 3, it takes a predetermined time from when light is input to the saturable absorption region until absorption saturation occurs in the saturable absorption region and light can be completely transmitted. That is, the response time of the saturable absorption region [time until the transmittance of the saturable absorption region becomes 1 / e (e: the base of natural logarithm)] is t ₁ . For this reason, the optical output waveform becomes a waveform as shown in FIG.

On the other hand, FIG. 4 shows the response characteristics of the gain saturation region set so that the characteristics of the gain saturation region are the best.
As shown in FIG. 4, when light that causes gain saturation enters the gain saturation region, the optical gain in the gain saturation region decreases with time due to gain saturation, and the optical output also decreases with time. . In this case, the gain response time of the saturation region [amplification factor of the gain saturation region 1 / e (e: base of natural logarithm) on until the time] is t _2.

FIG. 5 shows the response characteristics of an element in which such a saturable absorption region and a gain saturation region are simply connected in series. For this reason, the optical output waveform becomes a waveform as shown in FIG.
Thus, the response time t ₁ in the saturable absorption region is longer than the response time t ₂ in the gain saturation region, and the response speed in the saturable absorption region is slower than the response speed in the gain saturation region. The response speed is limited by the response speed of the saturable absorption region, and a high-speed response cannot be realized.

  Therefore, in the present optical waveform shaping device, the current that flows through the gain saturation region is intentionally reduced and the response speed in the gain saturation region is slowed down, as shown in FIGS. 2 (A) and 2 (B). By configuring so that the response speed of the region matches the response speed of the saturable absorption region, the gain rising characteristic of the gain saturation region and the rising characteristic of the saturable absorption region are offset, and a high-speed signal is obtained. However, it is possible to realize good characteristics that can be operated.

That is, in the present embodiment, as shown in FIG. 2 (A), (B) , an optical waveform shaping device, the response time t ₂ of the gain saturation region 4 and the response time t ₁ of the absorption saturation region 5 matches And the depth of saturation of the gain saturation region 4 [the change width of the gain (amplification factor) until reaching the deep region of gain saturation (amplification factor saturation); in FIG. 2A, the amplification factor is 1/10 Change width until gain] and the saturation depth of the absorption saturation region 5 [change width of the absorption rate (transmittance) until reaching a deep region of absorption saturation (transmittance saturation); FIG. 2 (B) Then, the change width of the transmittance until the transmittance becomes 10 times] is matched.

Specifically, the optical waveform shaping element 1 includes the length of the gain saturation region 4 and the length of the absorption saturation region 5 so that the saturation depth of the gain saturation region 4 matches the saturation depth of the absorption saturation region 5. Is set (see, for example, FIG. 1).
As shown in FIG. 1, the drive circuit 20 is connected to the gain saturation region 4, connected to the current source 21 that supplies current to the active layer 2 in the gain saturation region 4, and to the active layer 2 in the absorption saturation region 5, And a voltage source 22 for applying a voltage to the absorption saturation region 5. 2A and 2B, the current value supplied from the current source 21 to the active layer 2 in the gain saturation region 4 and the voltage source 22 are applied to the active layer 2 in the absorption saturation region 5. that the voltage value, the response time t ₂ of the gain saturation region 4 and the response time t ₁ of the absorption saturation region 5 is set to match.

  In this way, by setting the response time and the depth of saturation of the gain saturation region 4 and the absorption saturation region 5, the response speed of the gain saturation region matches the response speed of the absorption saturation region 5, and the gain saturation region 5 The gain rise characteristic and the rise characteristic of the saturable absorption region are canceled out, and the response characteristic of the optical waveform shaping device is as shown in FIG. 2A, and it is possible to realize a good characteristic that can operate even with a high-speed signal. become.

Hereinafter, a more specific configuration example will be described with reference to FIG.
As shown in FIG. 6, on the n-InP (n = 1 × 10 ¹⁸ cm ⁻³ ) substrate 6, a layer containing InAs quantum dots (surface density 4 × 10 ¹⁰ cm ⁻² ) naturally formed by MOCVD, for example. 5 are grown to form a quantum dot active layer 2 (active layer width 2 μm), and both sides of the active layer 2 are buried with a p-type InP layer 9 and an n-type InP layer 10. In the optical waveform shaping element 1 having a structure in which the absorption saturation region 5 is repeated in multiple stages, the saturation depths of the gain saturation region 4 and the absorption saturation region 5 are both 3 dB, and the response time of the gain saturation region 4 and the absorption saturation region 5 is To 25 ps, the length of each gain saturation region 4 is set to 6 mm, the length of each absorption saturation region 5 is set to 500 μm, and a forward bias current of 2.5 A is supplied to each gain saturation region 4 to absorb each absorption. A reverse bias voltage of 0.7 V (saturation region 5) 0.7V) may be applied to.

  That is, if the length of the gain saturation region 4 is 6 mm, the saturation depth is 3 dB and the gain is 18 dB. When a forward bias current of 2.5 A is passed through the gain saturation region 4, the response time of the gain saturation region 4 is 25 ps. On the other hand, when the length of the absorption saturation region 5 is 500 μm, the saturation depth is 3 dB. When a reverse bias voltage (−0.7 V) of 0.7 V is applied to the absorption saturation region 5, the response time of the absorption saturation region 5 is 25 ps.

  A signal light (input signal light) having a peak power of 2 dBm (1.6 mW), which is a high-speed signal of 40 Gbps, is applied to the optical waveform shaping element 1 set in this manner from the outside through the optical fiber 30 and the condenser lens 31. , A gain saturation of 3 dB occurs in the gain saturation region 4 having a length of 6 mm, a gain of 15 dB is obtained, and the signal light has a peak power of 17 dB. When the signal light having a peak power of 17 dB is input to the absorption saturation region 5, the absorption saturation of 3 dB occurs in the absorption saturation region 5 having a length of 500 μm.

  Thus, the saturation depths of the gain saturation region 4 and the absorption saturation region 5 are the same, and the response time of the gain saturation region 4 and the response time of the absorption saturation region 5 are both 25 ps. Therefore, the gain rise characteristic of the gain saturation region 4 and the rise characteristic of the saturable absorption region 5 are canceled out, and even a high-speed signal of 40 Gbps exceeding the response time 25 ps of the gain saturation region 4 and the absorption saturation region 5 A waveform shaping effect can be obtained, and good characteristics capable of operating even with high-speed signals can be realized.

Then, the signal light (output signal light) from which the waveform shaping effect is obtained is output to the outside through the condenser lens 32 and the optical fiber 33.
Although the waveform shaping effect is increased by making the gain saturation region 4 and the absorption saturation region 5 repeated in multiple stages as described above, even one stage is effective.
Therefore, according to the optical waveform shaping device and the optical waveform shaping element according to the present embodiment, when performing waveform shaping using the optical waveform shaping element 1 in which the gain saturation region 4 and the absorption saturation region 5 are connected in series, for example, Even in a high-speed optical signal that is faster than the response speed of these two regions 4 and 5, there is an advantage that a good waveform can be obtained and waveform shaping can be performed.

Further, since the optical waveform shaping element 1 in which the gain saturation region 4 and the absorption saturation region 5 are connected in series is used, an optical waveform shaping device capable of operating at high speed can be realized with a simple configuration, and the optical communication device can be reduced in size and power consumption. Can be realized.
In the above-described embodiment, the optical waveform shaping element 1 including a plurality of gain saturation regions 4 and a plurality of absorption saturation regions 5 and alternately provided along the optical waveguide direction will be described as an example. However, the present invention is not limited to this. For example, the present invention can be applied to an optical waveform shaping element having one gain saturation region and one absorption saturation region.

In the above-described embodiment, the waveguide embedded structure device using the InP-based material has been described as an example. However, the present invention is not limited to this. For example, the present invention can be applied to a GaAs-based material. Such an optical waveform shaping element can be realized.
Furthermore, in the above-described embodiment, 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 the above-described embodiment, 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 the above-described embodiment, the p-side electrode 8 formed on the surface of the semiconductor optical waveguide (semiconductor laminated structure) 3 having the same active layer 2 is separated, and the gain saturation region 4 and The absorption saturation region 5 is formed in series along the optical waveguide direction, but is not limited to this. For example, the absorption saturation region 5 is formed on the surface of the semiconductor optical waveguide (semiconductor laminated structure) 3 having the same active layer 2. The contact saturation layer 4 (here, p-type contact layer) 7 and the electrode (here p-side electrode) 8 are separated, the gain saturation region 4 having the contact layer 7 and the electrode 8 on one element, and the contact layer The absorption saturation region 5 having the electrodes 7 and the electrodes 8 may be formed in series along the optical waveguide direction. In this case, a part of the contact layer 7 is removed by etching so that the separated contact layers 7 are arranged in series along the optical waveguide direction, and each of the separated contact layers 7 is independently formed. The electrode 8 may be formed. Further, the gain saturation region and the absorption saturation region are separated by etching and separating a part of the contact layer and forming an electrode on the separated contact layer. The 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 configuration described in the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

It is a schematic diagram which shows the structure of the optical waveform shaping apparatus concerning one Embodiment of this invention. (A) shows the response characteristics of the gain saturation region provided in the optical waveform shaping device according to one embodiment of the present invention, and (B) is applicable to the optical waveform shaping device according to one embodiment of the present invention. The response characteristics of the saturated absorption region are shown, and (C) shows the response characteristics of the optical waveform shaping device according to the embodiment of the present invention. It is a figure for demonstrating the subject of the optical waveform shaping device which is set so that each characteristic of a gain saturation area | region and a saturable absorption area | region may become the best, and these are simply connected in series, A saturable absorption area | region It is a figure which shows the response characteristic. It is a figure for explaining the problem of the optical waveform shaping device that is set so that the respective characteristics of the gain saturation region and the saturable absorption region become the best, and these are simply connected in series. It is a figure which shows a response characteristic. It is a figure for explaining the subject of the optical waveform shaping device which set up so that each characteristic of a gain saturation field and a saturable absorption field may become the best, and these were simply connected in series, and an optical waveform shaping element It is a figure which shows the response characteristic. It is a schematic diagram which shows the specific structural example of the optical waveform shaping apparatus concerning one Embodiment of this invention.

Explanation of symbols

1 Optical Waveform Shaping Element 2 Active Layer (Quantum Dot Active Layer)
3 Semiconductor optical waveguide 4 Gain saturation region 5 Absorption saturation region (saturable absorption region)
6 Semiconductor substrate (n-type InP substrate)
7 Contact layer (p-type InGaAs contact layer)
8 electrodes (p-side electrode)
9 p-type InP layer 10 n-type InP layer 11 p-type InP clad layer 20 drive circuit 21 current source 22 voltage source 30, 33 optical fiber 31, 32 condenser lens

Claims (6)

An optical waveform shaping element comprising a gain saturation region causing gain saturation and an absorption saturation region causing absorption saturation, wherein the gain saturation region and the absorption saturation region are connected in series;
A drive circuit for driving the optical waveform shaping element,
An optical waveform shaping device configured so that a response speed of the gain saturation region matches a response speed of the absorption saturation region.   The gain saturation region response time and the absorption saturation region response time match, and the gain saturation region saturation depth and the absorption saturation region saturation depth match. The optical waveform shaping device according to claim 1, wherein:   In the optical waveform shaping element, the length of the gain saturation region and the length of the absorption saturation region are set so that the saturation depth of the gain saturation region matches the saturation depth of the absorption saturation region. The optical waveform shaping device according to claim 1, wherein the optical waveform shaping device is provided. The drive circuit is
A current source connected to the gain saturation region and supplying a current to the gain saturation region;
A voltage source connected to the absorption saturation region and applying a voltage to the absorption saturation region;
The current value supplied from the current source to the gain saturation region and the voltage value applied to the absorption saturation region by the voltage source match the response time of the gain saturation region and the response time of the absorption saturation region. The optical waveform shaping device according to claim 1, wherein the optical waveform shaping device is set as follows.   5. The optical waveform shaping element according to claim 1, wherein the gain saturation region and the absorption saturation region are alternately provided along the semiconductor optical waveguide. 6. Optical waveform shaping device. A gain saturation region that causes gain saturation;
With an absorption saturation region that causes absorption saturation,
The gain saturation region and the absorption saturation region are connected in series, and the saturation depth of the gain saturation region is configured to match the saturation depth of the absorption saturation region. Optical waveform shaping element.

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