JP2018092100A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmitter with low power consumption by allowing a general differentially driven semiconductor MZ modulator to be driven with an open collector type driver IC.SOLUTION: A semiconductor MZ modulator 200 including a GSGSG (GND-SIGNAL-GND-SIGNAL-GND) line, an open collector type (open drain type) driver IC 250 having input/output PADs 251, 252 of the GSGSG, and a digital/analogue converter (DAC) 260 having an output PAD of the GSGSG are connected to each other. Instead of an earth electrode 142 originally having 0 V, Vbias (for example, in the order of +5 V to +20 V) is applied to form a reference electrode 242. The driver IC 250 and the semiconductor MZ modulator 200 are connected only via a signal electrode 241. An application voltage for the driver IC 250 is applied to the signal electrode 241 from a power source Vcc via a termination resistance 270 adjusted to coincide with impedance of the semiconductor MZ modulator 200.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an optical transmitter using a semiconductor Mach-Zehnder modulator that converts an electrical signal into an optical signal.

  With the recent explosive increase in data communication volume, an increase in capacity of an optical communication system has been demanded, and the optical components used have been integrated, complicated, and signal speeded up. Examples of such optical components include an optical modulator. Recently, in order to increase transmission capacity, two optical I / Q modulators (for example, Patent Document 1) based on Mach-Zehnder (MZ) modulators that support multilevel modulation such as QPSK and 16QAM are used. Two polarization multiplexed optical I / Q modulators (a configuration in which four Mach-Zehnder modulators are integrated for convenience) that are integrated for two optical polarizations have come to be used.

As a typical MZ modulator, an LN modulator using LiNbO ₃ (LN) is widely used. This operates using an electro-optic effect in which the refractive index of the medium changes according to the electric field applied to the LN. However, the LN modulator has a relatively long element length due to the physical constant of the material. In recent years, miniaturization and low drive voltage of optical transmitter modules have become issues, and miniaturization and low drive voltage of optical modulators are indispensable issues.

  In order to meet these demands, research on a semiconductor MZ modulator that is small and can be driven at a low driving voltage has been actively conducted. 1A shows the appearance of a semiconductor MZ modulator 100 having a conventional structure, and FIG. 1B shows a cross section around one arm waveguide (for example, Non-Patent Document 1 or Non-Patent Document 1). Reference 2).

  The configuration (differential drive type) and operation principle of the general semiconductor MZ modulator 100 shown in FIGS. 1A and 1B will be described. The signal light incident on the semiconductor MZ modulator 100 propagates through the optical waveguide 110, is branched into two by the optical demultiplexer 120, and is guided to the two arm waveguides. A traveling wave electrode 140 is formed around the arm waveguide, and an electric field is induced in the arm waveguide by applying a potential difference between the signal electrode 141 and the ground electrode 142.

  As shown in FIG. 1B, the arm waveguide is formed of a mesa stripe in which an n-type InP layer 102, a semiconductor core layer 103, and a p-type InP layer 104 are stacked on a semi-insulating (SI) InP substrate 101. A dielectric buried layer 105 is formed on both sides thereof. A signal electrode 141 is formed on the p-type InP layer 104 and the dielectric buried layer 105, and a ground electrode 142 is formed outside the dielectric buried layer 105 on the n-type InP layer 102.

  When an electric field is applied to the semiconductor core layer 103 by applying a voltage between the signal electrode 141 and the ground electrode 142, the refractive index of the semiconductor core layer 103 changes due to the electro-optic effect. As a result, the semiconductor core layer 103 The phase of the light propagating through the light changes. Thus, the output light intensity combined and outputted by the optical multiplexer 130 is changed by the phase difference of the light propagating through the two arm waveguides whose refractive indexes are controlled. This is the principle of operation of the semiconductor MZ modulator.

  Since the traveling-wave electrode 140 is applied with a reverse bias voltage, the ground electrode 142 is 0 V. Therefore, in order to apply an electric field to the semiconductor core layer 103, it is necessary to apply a negative potential to the signal electrode 141. .

  When considering a combination of a driver IC and a semiconductor MZ modulator, the driver IC may be differentially driven. In general, the differential drive type is more advantageous in terms of power consumption than the single-end drive type. It is. In addition, the open collector type can reduce power consumption because the back termination is not required and the amount of current can be halved compared to the conventional back termination type. Therefore, recently, studies have been actively conducted to reduce power consumption by combining an open collector (drain) type driver IC and a semiconductor MZ modulator (see, for example, Non-Patent Document 3).

  As in Non-Patent Document 3, the capacitively loaded electrode structure can be easily connected to an open collector driver IC.

JP-T-2004-516743

C. Rolland et al, “10 Gbit / s, 1.56 μm multiquantum well InP / InGaAsP Mach-Zehnder optical modulator,” Electron. Lett., Vol. 29, no. 5, pp. 471-472, 1993 K. Tsuzuki “Low Driving Voltage 40 Gbit / s n-i-n Mach-Zehnder Modulator Fabricated on InP Substrate” IEICE TRANS. ELECTRON., Vol. E88-C, no. 5, 2005 N. Wolf et al, “Electro-Optical Co-Design to Minimize Power Consumption of a 32 GBd Optical IQ-Transmitter Using InP MZ-Modulator,” CSICS 2015

  However, the general differential driving type semiconductor MZ modulator as described above and the open collector type driver IC cannot be connected by a normal driving method. In the driving method of the differential drive type semiconductor MZ modulator, it is necessary to apply a reverse bias voltage for driving the semiconductor MZ modulator to the signal electrode, while the open collector (drain) type is used. This is because when connecting to the driver, it is necessary to apply a driving voltage to the driver through the signal electrode section on the semiconductor MZ modulator side. That is, it is necessary to apply two kinds of voltages to the signal electrode at the same time.

  The present invention has been made in view of such a problem, and an object of the present invention is to enable driving of a general differential drive type semiconductor MZ modulator by an open collector type driver IC. It is to provide an optical transmitter with power consumption.

  In order to solve the above problems, the present invention is an optical transmitter having a differential drive type semiconductor Mach-Zehnder modulator having a signal electrode and a reference electrode, and an input / output pad comprising a signal pad and a ground pad An open collector type driver IC for driving the semiconductor Mach-Zehnder modulator, wherein the driver IC has a ground pad connected to the ground, the signal pad connected to the signal electrode, and applied to the signal electrode. The semiconductor Mach-Zehnder modulator is driven by one power source, and is connected to a second power source so that the reference electrode has a potential different from that of the ground pad.

  The invention according to claim 2 is the optical transmitter according to claim 1, wherein the signal pad is connected to one end of the signal electrode, and the first power source is connected to the other end of the signal electrode. It is characterized by being.

  According to a third aspect of the present invention, in the optical transmitter according to the first or second aspect, the first and second power supplies apply a reverse bias voltage to the semiconductor Mach-Zehnder modulator.

  According to a fourth aspect of the present invention, in the optical transmitter according to any one of the first to third aspects, the signal electrode and the first power source have a termination resistor that matches the impedance of the semiconductor Mach-Zehnder modulator. It is characterized by being connected via.

  According to a fifth aspect of the present invention, in the optical transmitter according to any one of the first to fourth aspects, the semiconductor Mach-Zehnder modulator includes a first conductive semiconductor cladding layer and a non-doped semiconductor core layer on a semiconductor substrate. And a second conductive semiconductor clad layer are sequentially laminated to form a Mach-Zehnder interferometer with an optical waveguide having two arm waveguides made of an optical waveguide, and the reference electrode is formed of the first conductive layer. The signal electrode is formed on the second conductive semiconductor cladding layer. The signal electrode is formed on the conductive semiconductor cladding layer.

  According to a sixth aspect of the present invention, in the optical transmitter according to the fifth aspect, in the semiconductor Mach-Zehnder modulator, which of the first conductive semiconductor clad layer and the second conductive semiconductor clad layer is selected. One of them is an n-type semiconductor, and the other is a p-type semiconductor.

  According to a seventh aspect of the present invention, in the optical transmitter according to the fifth or sixth aspect, the semiconductor Mach-Zehnder modulator includes both the first conductive semiconductor clad layer and the second conductive semiconductor clad layer. Is an n-type semiconductor, and a p-type third conductivity is provided between the non-doped semiconductor core layer and the first conductive semiconductor clad layer or at least one of the second conductive semiconductor clad layer. A semiconductor clad layer is inserted.

  The invention according to claim 8 is the optical transmitter according to any one of claims 5 to 7, wherein the semiconductor Mach-Zehnder modulator has at least a part of the non-doped semiconductor core layer having a multiple quantum well structure. Features.

  The present invention realizes low power consumption of an optical transmitter by enabling a general differential drive type semiconductor MZ modulator to be driven by an open collector type driver IC.

It is a figure explaining the conventional semiconductor Mach-Zehnder optical modulator. It is a figure which shows the structure of the optical transmitter which concerns on one Embodiment of this invention. It is a figure which shows the cross section of one arm waveguide periphery of the semiconductor MZ modulator of the optical transmitter which concerns on one Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail.

  FIG. 2 shows the configuration and connection method of an optical transmitter according to an embodiment of the present invention. The optical transmitter according to the present invention includes a semiconductor MZ modulator 200 formed of a GSSGSG (GND-SIGNAL-GND-SIGNAL-GND) line, an open collector type (open drain type) driver IC 250 having GSSG I / O PADs 251 and 252, and GSSGSG. Are connected to a digital-to-analog converter (DAC) 260 having an output PAD. Note that the DAC 260 that performs digital-analog conversion of the modulation signal is not an essential configuration in the present invention, and may be omitted.

  FIG. 3 shows a cross section around one arm waveguide of a semiconductor MZ modulator 200 according to an embodiment of the present invention. The semiconductor MZ modulator 200 is a mesa stripe in which a conductive lower cladding layer 202 made of InP, a non-doped semiconductor core layer 203, and a conductive upper cladding layer 204 made of InP are sequentially stacked on an SI-InP substrate 201. The waveguide is provided. The semiconductor core layer 203 functions as an optical waveguide layer. For example, a material system such as InGaAsP or InGaAlAs is used, and the semiconductor core layer 203 is composed of a single-component quaternary mixed crystal bulk layer or multiple quantum well layer, or a multiple quantum well layer. In addition, a structure having an optical confinement layer having a band gap larger than that of the multiple quantum well layer and lower than that of the upper clad layer 204 and the lower clad layer 202 can be used.

  The band gap wavelengths of the quaternary mixed crystal bulk layer and the multiple quantum well layer are set so that the electro-optic effect acts effectively and the light absorption does not become a problem at the light wavelength used.

  Further, the semiconductor MZ modulator 200 of the present invention is not limited to an InP material, and for example, a material system that matches a GaAs substrate may be used.

  One of the upper cladding layer 204 and the lower cladding layer 202 in FIG. 2 may be an n-type semiconductor and the other may be a p-type semiconductor.

  On the other hand, both the upper cladding layer 204 and the lower cladding layer 202 are n-type semiconductors, and a third p-type is interposed between the upper cladding layer 204 and the semiconductor core layer 203 or between the lower cladding layer 202 and the semiconductor core layer 203. A structure in which a cladding layer is inserted can also be adopted.

  The semiconductor MZ modulator 200 includes a traveling wave type electrode 240, a signal electrode 241 is formed on the upper cladding layer 204, and a reference electrode 242 is formed on the lower cladding layer 202. In addition, a dielectric buried layer is formed on both sides of the mesa stripe of each arm waveguide in the region where the signal electrode 241 and the reference electrode 242 are formed, similarly to the semiconductor MZ modulator 100 shown in FIG. You can also

  As shown in FIG. 1, the general driving method of the conventional differential drive type semiconductor MZ modulator is to drop the ground electrode 142 to 0V and apply a negative voltage to the signal electrode 141 (for example, about −5V to −15V). It was driven by applying. However, since the open collector type driver IC needs to apply the voltage VCC (VDD) via the signal electrode 141 of the semiconductor MZ modulator 100, the conventional driving method of the semiconductor MZ modulator 100 uses the open collector type driver IC. It is impossible to use.

  Therefore, in the connection method of the present invention, the semiconductor MZ modulator 200 has a structure in which the upper cladding layer 204 is a p-type semiconductor, or a third p-type cladding layer is inserted between the upper cladding layer 204 and the semiconductor core layer 203. When the structure is adopted, a voltage (for example, about +5 V to about +20 V) is applied from the power source Vbias instead of the ground electrode 142 which has been conventionally set to 0 V, and the reference electrode 242 is obtained. Then, the driver IC 250 and the semiconductor MZ modulator 200 are connected only by the signal electrode 241, and the voltage for the driver IC 250 is supplied to the signal electrode 241 through the termination resistor 270 that matches the impedance of the semiconductor MZ modulator 200. Apply from.

  In this case, by applying the reverse bias voltage to the semiconductor MZ modulator 200 by making the voltage of the power supply Vbias higher than the voltage of the power supply Vcc so that the potential of the reference electrode 242 becomes higher than the potential of the signal electrode 241, A drive voltage can be applied from the signal electrode 241 to the open collector type driver IC 250.

  In the present invention, the ground electrode of the driver IC 250 and the reference electrode 242 of the semiconductor MZ modulator 200 are not connected to make a common potential, but the reference electrode 242 of the semiconductor MZ modulator 200 and the ground pad of the driver IC 250 are thus connected. Even if not, there is no problem in characteristics.

  When an operation that can be adjusted between the arms is desired, the Vbias potential needs to be adjusted between the arms. In that case, another potential can be applied by separating the lower cladding layer 202 of the semiconductor MZ modulator 200 between the arms.

  In addition, unlike a conventional back termination type driver IC, in the case of an open collector type driver IC, the termination resistor 270 of the semiconductor MZ modulator 200 at the subsequent stage may have an arbitrary impedance (for example, differential 40Ω to 100Ω). Considering the electrical characteristics of the semiconductor MZ modulator 200 itself, it is desirable that the differential impedance is low, but from the viewpoint of power consumption, the differential impedance is preferably high.

  For simplification in FIG. 2, only a single semiconductor MZ modulator 200 and a one-channel driver IC 250 are provided. However, in reality, an IQ modulator including two single semiconductor MZ modulators 200 and a two-channel driver IC 250 are arranged. The use in the driver IC, and similarly the use in the polarization multiplexed IQ modulator and the 4-channel driver IC arranged in four, can be implemented by the same connection method only by increasing the number of connections. is there.

  For simplification, the line on the input side of the semiconductor MZ modulator 200 is not made equal in length, but because it is a differential line, there is actually a concern about characteristic deterioration due to common mode, skew, etc. It is necessary to equalize the length of the track. From the viewpoint of characteristic deterioration, it is preferable that the wire length of the connection portion between the driver IC and the semiconductor MZ modulator 200 is as short as possible. In view of characteristics, it is desirable to use a wedge wire or a ribbon wire.

  Further, in order to realize a wider-band modulator by combining the driver IC 250 and the semiconductor MZ modulator 200, the driver IC 250 has a specific frequency peaking (for example, 15 to 25 GHz for 32 GBd and 64 GBd for 64 GBd). 30-40 GHz is desirable). This is because, when connected to the driver IC 250 without peaking, the loss in the driver IC 250 and the wire simply contributes to the deterioration of the modulation band, resulting in a lower band. In this way, by combining the driver IC 250 and the semiconductor MZ modulator 200, a wider band can be achieved.

DESCRIPTION OF SYMBOLS 100 Semiconductor MZ modulator 101 SI-InP substrate 102 n-type InP layer 103 Semiconductor core layer 104 p-type InP layer 105 Dielectric buried layer 110 Optical waveguide 120 Optical demultiplexer 130 Optical multiplexer 140 Traveling wave type electrode 141 Signal electrode 142 Ground electrode 200 Semiconductor MZ modulator 201 SI-InP substrate 202 n-InP layer 203 Lower cladding layer 204
205 Upper cladding layer 206
210 Optical waveguide 220 Optical demultiplexer 230 Optical multiplexer 240 Traveling wave type electrode 241 Signal electrode 242 Reference electrode 250 Open collector type driver IC
260 DAC
270 Terminating resistor

Claims (8)

A differentially driven semiconductor Mach-Zehnder modulator having a signal electrode and a reference electrode;
An open collector type driver IC for driving the semiconductor Mach-Zehnder modulator having an input / output pad comprising a signal pad and a ground pad;
With
The driver IC is driven by a first power source applied to the signal electrode with the ground pad grounded, the signal pad connected to the signal electrode,
The optical transmitter, wherein the semiconductor Mach-Zehnder modulator is connected to a second power source so that the reference electrode has a potential different from that of the ground pad.   The optical transmitter according to claim 1, wherein the signal pad is connected to one end of the signal electrode, and the first power source is connected to the other end of the signal electrode.   The optical transmitter according to claim 1, wherein the first and second power supplies apply a reverse bias voltage to the semiconductor Mach-Zehnder modulator.   4. The optical transmitter according to claim 1, wherein the signal electrode and the first power source are connected to each other through a termination resistor that matches an impedance of the semiconductor Mach-Zehnder modulator. 5. . The semiconductor Mach-Zehnder modulator has two arms composed of an optical waveguide formed by sequentially laminating a first conductive semiconductor clad layer, a non-doped semiconductor core layer, and a second conductive semiconductor clad layer on a semiconductor substrate. A Mach-Zehnder interferometer is formed with an optical waveguide having a waveguide,
The reference electrode is formed on the first conductive semiconductor cladding layer,
The optical transmitter according to claim 1, wherein the signal electrode is formed on the second conductive semiconductor clad layer.   In the semiconductor Mach-Zehnder modulator, one of the first conductive semiconductor clad layer and the second conductive semiconductor clad layer is an n-type semiconductor and the other is a p-type semiconductor. The optical transmitter according to claim 5.   In the semiconductor Mach-Zehnder modulator, both the first conductive semiconductor clad layer and the second conductive semiconductor clad layer are n-type semiconductors, the non-doped semiconductor core layer and the first conductive semiconductor clad layer, 7. The light according to claim 5, wherein a p-type third conductive semiconductor cladding layer is inserted between at least one of the second conductive semiconductor cladding layers. Transmitter.   8. The optical transmitter according to claim 5, wherein in the semiconductor Mach-Zehnder modulator, at least a part of the non-doped semiconductor core layer has a multiple quantum well structure.

Images