JP3979108B2 - Optical signal processing device - Google Patents

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to an optical signal processing device, and more specifically to an optical signal processing device that can be used as a wavelength conversion device, a waveform shaping device, an optical arithmetic device, an optical gate device, and the like.
[0002]
[Prior art]
With the spread of the Internet, the demand for traffic flowing through the network is expanding without knowing that it will stop. In such a large-capacity network, a communication method using an optical fiber is applied to a transmission line connecting nodes, and the transmission speed of the transmission line is about to exceed 10 Gbit / s.
[0003]
In a network configured with a transmission speed of 40 Gbit / s or higher, the wavelength of the optical signal is converted without converting the optical signal into an electrical signal from the viewpoint of cost reduction, equipment installation space saving, and low power consumption. The function to do is desired. This is because complicated conversion processing between the optical signal and the electric signal can be eliminated. In terms of operation, it is difficult to prepare an electric circuit that operates at 40 Gbit / s or more.
[0004]
Among various wavelength converters proposed, the wavelength converters described in Japanese Patent Laid-Open No. 10-78595 and the corresponding US Pat. No. 5,959,764 have a small polarization dependency and a simple configuration. It is considered promising because of its stable operation.
[0005]
Also, this type of wavelength conversion device can be used as a device for shaping a deteriorated waveform, as an optical gate device that gates signal light in a specific time slot, or as an optical arithmetic device that performs logical operation on a plurality of pulse signal lights. It is known that the signal can be transferred between optical carriers of the same wavelength as an extreme case.
[0006]
[Problems to be solved by the invention]
In the wavelength conversion device described in Japanese Patent Laid-Open No. 10-78595 and US Pat. No. 5,959,764, the discharge of carriers generated inside the absorption type optical modulator by the pump light during wavelength conversion is the static of the absorption type optical modulator. Limited by capacity. Therefore, the operating frequency of wavelength conversion is limited by the capacitance of the absorption optical modulator. Since a normal absorption type optical modulator has an element length of about 200 μm and a capacitance of about 0.4 pF, the wavelength conversion operation is limited to a maximum of about 20 Gbit / s. At this time, the on / off ratio of the output light (wavelength converted light) of the wavelength converter is about 11 dB.
[0007]
[Problems to be solved by the invention]
According to the technique disclosed in OFC2001 ME4, “60 Gbit / s WDM-OTDM Transplexing Using an Electric-Abstraction Modulator”, the element length of the absorption-type optical modulator portion is shortened and the capacitance is reduced. The frequency can be improved, and the wavelength conversion operation up to 60 GHz has been confirmed. However, it has been observed that the on / off ratio of wavelength-converted light is reduced to 7.5 dB due to the shortening of the element length. Since an on / off ratio of about 10 dB required for transmission cannot be obtained, a method of shortening the element length of the absorption optical modulator portion is not suitable for practical use.
[0008]
An object of the present invention is to provide an optical signal processing device capable of operating at a high speed exceeding 20 Gbit / s.
[0009]
[Means for Solving the Problems]
An optical signal processing apparatus according to the present invention is an optical modulation unit that receives signal light of a signal wavelength, and includes an optical modulation unit that includes a plurality of absorption optical modulators arranged in series in the propagation direction of the signal light. A reverse bias circuit for applying a reverse bias voltage to each of the plurality of absorption type optical modulators, wherein the absorption type optical modulator positioned on the input side of the signal light is positioned on the output side of the signal light A reverse bias circuit that applies a reverse bias voltage equal to or higher than the reverse bias voltage to the absorption optical modulator, and probe light having a probe wavelength is input to the optical modulation unit in the same direction or in the reverse direction to the signal light. A probe light input device and probe light extraction means for extracting the probe light transmitted through the light modulation unit are provided.
[0011]
The light modulation unit includes an integrated element in which the plurality of absorption light modulators are integrated. Thereby, a light modulation unit can be reduced in size. By providing the first semi-insulating semiconductor that embeds the light propagation layer on the input side of the signal light of the integrated element, the input optical power resistance is improved. Of the integrated device, the output side of the signal light, by providing the second semi-insulating semiconductor burying a light propagation layer, can be accurately controlled modulation region length of the optical modulator on the output side.
[0012]
【Example】
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0013]
(First embodiment)
FIG. 1 shows a schematic configuration diagram of a first embodiment of the present invention. Signal light having a wavelength of 1535 nm is input to the input terminal 10, and CW probe light having a wavelength of 1558 nm is input to the input terminal 12. The optical coupler 14 combines the signal light from the input terminal 10 and the probe light from the input terminal 12. In the modulator integrated element 16, two electroabsorption optical modulators 16A and 16B each having a modulation layer made of InGaAsP are arranged in series. That is, the electrode 18 under the modulator integrated element 16 is connected to ground. On the integrated element 16, there are two electrically separated electrodes 20A, 20B. The electrode 20A is located above the light modulator 16A, and the electrode 20B is located above the light modulator 16B. That is, in this embodiment, a bias voltage can be applied independently to the optical modulators 16A and 16B.
[0014]
Such an integrated element 16 is, for example, H.264. Tanaka et al., “OPTICAL SHORT PULSE GENERATION AND DATA MODULATION BY A SINGLE-CHIP InGaAsP TANDEM-INTEGRATED ELECTROBSORPTION MODUL icter 198” 29, no. 11, pp. 1002-1003.
[0015]
The output light of the optical coupler 14 is input to the electroabsorption optical modulator 16A of the modulator integrated element 16. The output light of the electroabsorption optical modulator 16A is input to the electroabsorption optical modulator 16B. The output light of the electroabsorption optical modulator 16B is input to the optical bandpass filter 22 having the wavelength λp (1558 nm) as the central transmission wavelength. The optical bandpass filter 22 extracts only the component of the wavelength λp, that is, the probe light component from the output light of the electroabsorption optical modulator 16B, and supplies it to the output terminal 24.
[0016]
The electroabsorption optical modulators 16A and 16B are reverse-biased independently of each other. That is, the DC voltage source 26A has a positive electrode connected to the ground, and a negative electrode connected to the ground via the inductance 28A, the capacitor 30A, and a termination resistor 32A for terminating the high frequency signal. The voltage at the connection point between the inductance 28A and the capacitor 30A is applied as a reverse bias voltage to the electroabsorption optical modulator 16A via the electrode 20A. A portion including the inductance 28A and the capacitor 30A serves as a so-called bias tee. Similarly, the DC voltage source 26B has a positive electrode connected to the ground, and a negative electrode connected to the ground via an inductance 28B, a capacitor 30B, and a termination resistor 32B for terminating the high frequency signal. The voltage at the connection point between the inductance 28B and the capacitor 30B is applied as a reverse bias voltage to the electroabsorption optical modulator 16B via the electrode 20B. A portion including the inductance 28B and the capacitor 30B serves as a so-called bias tee.
[0017]
In the electroabsorption optical modulators 16A and 16B, the absorption loss of the probe wavelength λp changes according to the intensity change of the signal light, and as a result, the amplitude of the probe light changes according to the intensity change of the signal light. . As a result, the waveform of the signal carried by the signal light having the wavelength λs is transferred to the probe light. In other words, the wavelength of the optical carrier that carries the signal changes from λs to λp. Details of this operation are described in Japanese Patent Application Laid-Open No. 10-78595 and US Pat. No. 5,959,764, and further description thereof is omitted.
[0018]
In this embodiment, the reverse bias voltage of the electroabsorption optical modulator 16A on the signal light input side among the electroabsorption optical modulators 16A and 16B connected in series is used as the electroabsorption optical modulator on the signal light output side. The reverse bias voltage is higher than 16B. The reason will be described below.
[0019]
In the case of using a single electroabsorption optical modulator, the relationship between the optical power of the signal light, the reverse bias voltage, the element length, and the on / off ratio of the converted light was investigated. Specifically, when the optical power of the signal light input to the absorption optical modulator is +15 dBm and +10 dBm, the probe light component included in the output light of the absorption optical modulator with respect to the reverse bias voltage and the element length The ratio (on / off ratio) of the maximum power and the minimum power (wavelength converted light) was measured. FIG. 2 shows the relationship between the reverse bias voltage, the element length, and the on / off ratio of the wavelength converted light when the optical power of the signal light input to the absorption optical modulator is +15 dBm, and FIG. The relationship between reverse bias voltage, element length, and on / off ratio of wavelength converted light when the optical power of signal light input to the modulator is +10 dBm is shown. 2 and 3, the horizontal axis represents the reverse bias voltage Vb (V), and the vertical axis represents the on / off ratio (dB). The element length is 25 μm, 50 μm, 75 μm and 200 μm.
[0020]
The on / off ratio of the wavelength-converted light increases as the reverse bias voltage increases, but tends to decrease as the reverse bias voltage increases. In addition, the longer the element length, the larger the maximum value of the on / off ratio of the wavelength converted light, and the lower the reverse bias voltage at which the on / off ratio of the wavelength converted light is maximized. Furthermore, the higher the input power of the signal light, the larger the maximum value of the on / off ratio of the wavelength converted light, and the higher the reverse bias voltage at which the on / off ratio of the wavelength converted light becomes maximum.
[0021]
The higher the reverse bias voltage, the larger the on / off ratio of the wavelength converted light. However, wavelength conversion does not occur when a certain reverse bias voltage is exceeded, and wavelength conversion occurs in a region where the signal light power is sufficiently high. However, it shows that wavelength conversion does not occur when the signal light power decreases. As a result, in order to increase the on / off ratio of the wavelength-converted light, a reverse bias voltage is set in the vicinity of the signal light input end face where the power of the signal light is sufficiently high along the light propagation direction inside the absorption modulator. This indicates that the bias voltage should be gradually lowered in the vicinity of the signal light output end face where the wavelength conversion efficiency is increased and the signal light power is lowered.
[0022]
In accordance with this view, in this embodiment, the two electroabsorption optical modulators 16A and 16B are arranged in the propagation direction of the signal light, and the reverse bias voltage of the rear electroabsorption optical modulator 16B is used as the front electroabsorption. The reverse bias voltage of the type optical modulator 16A was set to be equal to or lower. In other words, the electroabsorption optical modulator is separated into the first half and the second half, and the reverse bias voltage in the second half is made equal to or lower than the reverse bias voltage in the first half.
[0023]
Even if the reverse bias voltage is the same, the individual electroabsorption optical modulators 16A and 16B are separated by separating the conversion region length necessary for the target wavelength conversion operation into the two electroabsorption optical modulators 16A and 16B. The element length becomes shorter. Thereby, the frequency band is widened, and the signal quality of the wavelength-converted light is improved. Of course, by making a difference in the reverse bias voltage, the frequency band is further expanded and the signal quality of the wavelength-converted light is improved.
[0024]
A prototype example will be described. The element length of each electroabsorption type optical modulator 16A, 16B was set to 75 μm. According to the measurement by the inventors, the capacitance is 0.16 pF at an element length of 75 μm. In this case, if the electroabsorption optical modulators 16A and 16B are used in a 50Ω high-frequency line, the 3 dB bandwidth is 40 GHz, and a wavelength conversion operation of 40 Git / s is possible. The input power of signal light in the modulator integrated element 16 was set to +15 dBm, and the reverse bias voltages of the electroabsorption optical modulators 16A and 16B were set to -3.5V and -2.5V, respectively. At this time, the intensity of the probe light is modulated by the electroabsorption optical modulator 16A with an on / off ratio of 7.7 dB, and further, the intensity of the probe light is modulated by an electroabsorption optical modulator 16B with an on / off ratio of 2.8 dB. Is done. Therefore, the intensity of the probe light output from the modulator integrated element 16 is modulated at 10.5 dB. Therefore, the wavelength converter shown in FIG. 1 can operate at 40 Gbit / s, and can achieve an on / off ratio of 10 dB or more with respect to the wavelength-converted light.
[0025]
(Second embodiment)
FIG. 4 shows a schematic configuration diagram of the second embodiment of the present invention. The same components as those in the embodiment shown in FIG.
[0026]
In the first embodiment shown in FIG. 1, two electroabsorption optical modulators 16A and 16B are connected in series, but in the embodiment shown in FIG. 4, three electroabsorption optical modulators are connected in series to the optical modulator integrated element 116. 116A, 116B, and 116C are formed, and the reverse bias voltage values are applied in the order of the electroabsorption optical modulators 116A, 116B, and 116C.
[0027]
That is, on the integrated element 116, there are three electrodes 120A, 120B, and 120C that are electrically separated, the electrode 120A is located above the light modulator 116A, and the electrode 120B is located above the light modulator 116B. And the electrode 120C is positioned on top of the light modulator 116C.
[0028]
The DC voltage source 126A has a positive electrode connected to the ground, and a negative electrode connected to the ground through an inductance 128A, a capacitor 130A, and a termination resistor 132A for terminating a high-frequency signal. The voltage at the connection point between the inductance 128A and the capacitor 130A is applied to the electroabsorption optical modulator 116A through the electrode 120A as a reverse bias voltage. A portion including the inductance 128A and the capacitor 130A serves as a so-called bias tee.
[0029]
Similarly, the DC voltage source 126B has a positive electrode connected to the ground, and a negative electrode connected to the ground via an inductance 128B, a capacitor 130B, and a termination resistor 132B for terminating a high-frequency signal. The voltage at the connection point between the inductance 128B and the capacitor 130B is applied as a reverse bias voltage to the electroabsorption optical modulator 116B via the electrode 120B. A portion composed of the inductance 128B and the capacitor 130B serves as a so-called bias tee.
[0030]
The DC voltage source 126C has a positive electrode connected to the ground, and a negative electrode connected to the ground via an inductance 128C, a capacitor 130C, and a termination resistor 132C for terminating a high-frequency signal. The voltage at the connection point between the inductance 128C and the capacitor 130C is applied as a reverse bias voltage to the electroabsorption optical modulator 116C via the electrode 120C. A portion composed of the inductance 128C and the capacitor 130C serves as a so-called bias tee.
[0031]
In the embodiment shown in FIG. 4, the element length of each electroabsorption optical modulator 116A, 116B, 116C can be shortened to about 50 μm. When the element length of each of the modulators 116A, 116B, and 116C is 50 μm, the capacitance is about 0.12 pF, which is smaller than that in the first embodiment, the 3 dB bandwidth is 53 GHz, and the bit rate is higher. Wavelength conversion operation becomes possible. Note that the reverse bias voltages to the electroabsorption optical modulators 116A, 116B, and 116C are, for example, −4.5V, −3.5V, and −1.5V, respectively. Since the reverse bias voltage to the electroabsorption optical modulators 116A, 116B, and 116C can be set to a value that can obtain an optimum wavelength conversion operation according to each element length and signal light power, it is higher than that of the first embodiment. An on / off ratio can be achieved.
[0032]
(Other)
FIG. 5 shows an external perspective view of the modulator integrated element 16, and FIG. 6 shows a central sectional view thereof. From the light input side, it consists of a waveguide region 210, a modulation region 212, an insulating region 214, a modulation region 216, and a waveguide region 218. The modulation region 212 corresponds to the electroabsorption optical modulator 16A, and the modulation region 216 corresponds to the electroabsorption optical modulator 16B.
[0033]
An InGaAsP modulation layer 222 and an InGaAsP buffer layer 224 are stacked on the n-InP substrate 220. On the buffer layer 224, n-InP layers 226A and 226B and p-InGaAsP layers 228A and 228B are stacked on the portions corresponding to the modulation regions 212 and 214, and correspond to the waveguide regions 210 and 218 and the insulating region 214. In this portion, semi-insulating InP layers 230, 232 and 234 doped with iron are laminated.
[0034]
Electrodes 236A and 236B are disposed on the p-InGaAsP layers 228A and 228B, respectively, and glass (SiO ₂ ) 240 is disposed on the InP layers 230, 232 and 234. Further, an electrode 242 is disposed on the entire surface under the substrate 220. The electrode 242 corresponds to the electrode 18.
[0035]
In the integrated configuration shown in FIGS. 5 and 6, since the semi-insulating InP layer 230 doped with iron is embedded in the waveguide region 210 on the input side, the input power resistance of signal light can be improved. In the actual measurement example, the improvement was 2 dB. As a result, the input optical power to the modulator integrated element 16 can be further increased, and as a result, the on / off ratio of the wavelength-converted light can be increased. Further, since the semi-insulating InP layer 232 doped with iron is embedded in the waveguide region 218 on the output side, the length of the modulation region 216 can be accurately controlled.
[0036]
In the above embodiment, the reverse bias voltage applied to the electroabsorption optical modulator on the signal light input side is set higher than the reverse bias voltage applied to the electroabsorption optical modulator on the signal light output side. However, even if the reverse bias voltage is the same, a higher operating frequency than the conventional example is possible. For example, when the reverse bias voltages of the two absorption optical modulators 16A and 16B in the first embodiment are set to -3.5 V, the on / off ratio is reduced by 0.01 dB, but the operating frequency is the conventional example. Can be higher.
[0037]
Although an embodiment of a modulator integrated element in which two or three absorption optical modulators are integrated on the same substrate has been described, a signal is obtained using a modulator integrated element in which four or more electroabsorption optical modulators are integrated. Even with a configuration in which the reverse bias voltage is the same or low from the light input side, it is possible to improve the on / off ratio of the wavelength-converted light and / or to obtain a high operating frequency.
[0038]
Although the embodiment in which the signal light and the probe light propagate in the same direction has been described, the same effect can be obtained even in a configuration in which the signal light and the probe light propagate in the opposite directions. In this case, the optical bandpass filter 22 can be omitted. An optical circulator that extracts the probe light output from the element 16 may be disposed between the input terminal 10 and the element 16.
[0039]
As the electroabsorption optical modulators 16A, 16B, 116A, 116B, and 116C, InGaAsP electroabsorption optical modulators are typical, but any absorption optical modulator that generates absorption by applying a reverse bias voltage can be used. Can be used in the present invention. For example, an optical modulator using the quantum Stark effect of a semiconductor quantum well (QW) may be used.
[0040]
Although an embodiment has been described in which a plurality of electroabsorption optical modulators are integrated, it is not essential for the present invention to integrate them as one element. An absorption type optical modulator composed of individual elements may be optically coupled. In that case, it is preferable to arrange an optical amplifier between the absorption type optical modulators, if necessary.
[0041]
Although an example of application to a wavelength converter has been described, it can also be used as a waveform shaping device or a part thereof. In addition, it is obvious that the present embodiment can also operate as an optical gate device or an optical arithmetic unit by using the probe light as clock pulse light or pulse light carrying another signal.
[0042]
【The invention's effect】
As can be easily understood from the above description, according to the present invention, the on / off ratio of output light can be increased and / or the operating frequency can be increased.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a first embodiment of the present invention.
FIG. 2 is a diagram showing the wavelength conversion efficiency when the signal light input power is +15 dBm.
FIG. 3 is a graph showing wavelength conversion efficiency when the signal light input power is +10 dBm.
FIG. 4 is a schematic block diagram of a second embodiment of the present invention.
5 is a perspective view of an optical modulator integrated device 16. FIG.
6 is a central sectional view of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10, 12: Input terminal 14: Optical coupler 16: Modulator integrated element 16A, 16B: Electroabsorption type optical modulator 18,20A, 20B: Electrode 22: Optical band pass filter 24: Output terminal 26A, 26B: DC voltage source 28A, 28B: Inductance 30A, 30B: Capacitors 32A, 32B: Termination resistor 116: Optical modulator integrated elements 116A, 116B, 116C: Electroabsorption optical modulators 120A, 120B, 120C: Electrodes 126A, 126B, 126C: DC voltage source 128A, 128B, 128C: Inductance 130A, 130B, 130C: Capacitors 132A, 132B, 132C: Terminating resistor 210: Waveguide region 212: Modulation region 214: Insulation region 216: Modulation region 218: Waveguide region 220: n-InP substrate 222: InGaAsP modulation layer 224: nGaAsP buffer layer 226A, 226B: n-InP layer 228A, 228B: p-InGaAsP layer 230, 232, 234: iron-doped InP layer 236A, 236B: electrode 240: Glass _(SiO 2)
242: Electrode

Claims (5)

An optical modulation unit to which signal light of a signal wavelength is input, and is composed of a plurality of absorption optical modulators (16A, 16B; 116A, 116B, 116C) arranged in series in the propagation direction of the signal light (16, 116),
A reverse bias circuit for applying a reverse bias voltage to each of the plurality of absorption type optical modulators, wherein the absorption type optical modulator positioned on the input side of the signal light has an absorption position positioned on the output side of the signal light; Reverse bias circuits (26A to 32A, 26B to 32B; 126A to 132A, 126B to 132B, 126C to 132C) for applying a reverse bias voltage equal to or higher than the reverse bias voltage to the optical modulator,
A probe light input device (12, 14) for inputting probe light having a probe wavelength in either the same direction or the opposite direction to the signal light to the light modulation unit;
Probe light extraction means (22) for extracting the probe light transmitted through the light modulation unit
An optical signal processing device comprising: The optical signal processing device according to claim 1, wherein the optical modulation unit (16, 116) includes an integrated element in which the plurality of absorption optical modulators are integrated. The optical signal processing apparatus according to claim 2, wherein the integrated element has a first semi-insulating semiconductor (230) that embeds a light propagation layer on the input side of the signal light. The optical signal processing apparatus according to claim 2, wherein the integrated element includes a second semi-insulating semiconductor (234) that embeds a light propagation layer on the output side of the signal light. The optical signal processing device according to claim 1, wherein the probe light input device is an optical coupler (14) that combines the signal light and the probe light and applies them to the optical modulation unit.

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