JP2018136396A - Light modulator - Google Patents

Abstract

An optical modulator capable of adjusting an operating point of an optical modulator and improving modulation efficiency using only a DC voltage applied together with a modulation high-frequency signal without using a heater. An optical modulator of the present invention includes a phase modulation section provided in at least one of two optical waveguides extending in parallel between an optical splitter and an optical coupler. The modulation unit includes traveling wave electrodes 7 and 8 provided along the optical waveguide, and one end of the traveling wave electrode is connected to the high-frequency signal source RF via the first bias tee circuit 101, and other traveling wave electrodes are provided. The end is connected to the high-frequency termination resistor R through the second bias tee circuit 102. [Selection] Figure 3

Description

  The present invention relates to an optical modulator, and more specifically to a Mach-Zehnder (MZ) type optical modulator.

  A Mach-Zehnder interferometer (MZI) having a wide band and high extinction characteristics is widely used as a configuration of a phase modulation unit of an optical modulator. Patent Document 1 discloses an MZ type optical modulator using a conventional lithium niobate optical substrate. In FIG. 1 of Patent Document 1, a high-frequency signal source for modulation and a bias circuit for supplying a DC voltage for setting an operating point of the modulator are connected to one end of electrodes on two arm waveguides. And the structure which connects termination resistance to the other end is indicated. Further, FIG. 3 of Patent Document 1 also discloses a configuration in which the setting of the operating point of the modulator is performed using a thermo-optic effect by heating a heater provided on the arm waveguide.

  Non-Patent Document 1 discloses an MZ type optical modulator using a silicon optical waveguide. In the MZ type optical modulator of Non-Patent Document 1, a lateral pn junction is formed in the rib-shaped optical waveguides of both arms, and a DC reverse bias is input to the coplanar traveling wave electrode connected to the pn junction together with a modulated electric signal. Then, high-speed optical modulation is performed by modulating the phase of the guided light according to the change in carrier density.

  In the setting of the operating point of an optical modulator using a heater as exemplified in FIG. 3 of Patent Document 1, it is necessary to additionally provide a heater and its heating circuit, and a space for that is required, so that light is required. This hinders downsizing of the modulator. In particular, when the present invention is applied to a minute MZ type optical modulator using a silicon optical waveguide, the problem becomes remarkable. In addition, the manufacturing cost increases accordingly.

  In setting the operating point of an optical modulator using a DC voltage (reverse bias) applied together with a modulated electric signal (high frequency signal) as illustrated in FIG. 1 of Patent Document 1 or Non-Patent Document 1, Since only a uniform DC bias voltage is applied in the light propagation direction, the modulation efficiency in the traveling direction of the modulator cannot be adjusted. In other words, the modulated electrical signal (high-frequency signal) attenuates as it propagates, so conventional optical modulators cannot provide uniform modulation over the entire phase modulation region, resulting in degradation of modulation efficiency and high-frequency characteristics. Resulting in.

International Publication WO2010 / 064417

Kensuke Ogawa, IEICE Journal, Vol.96, No.3, pp.195-199, March 2013

  An object of the present invention is to provide an optical modulator capable of adjusting the operating point of the optical modulator and improving the modulation efficiency using only a DC voltage applied together with the modulation high-frequency signal without using a heater. It is to be.

  The present invention provides an optical modulator. The optical modulator includes a phase modulation unit provided in at least one of two optical waveguides extending in parallel between the optical branching unit and the optical coupler, and the phase modulation unit is provided along the optical waveguide. A traveling wave electrode is included, and one end of the traveling wave electrode is connected to a high frequency signal source via a first bias tee circuit, and the other end of the traveling wave electrode is connected to a high frequency termination resistor via a second bias tee circuit. .

  The present invention provides a method for controlling an optical modulator. In the control method, the optical modulator includes a phase modulation unit provided in at least one of two optical waveguides extending in parallel between the optical branching unit and the optical coupler, and the phase modulation unit extends along the optical waveguide. (A) applying a first DC bias voltage to one end of the traveling wave electrode; (b) applying a high frequency signal for modulation to one end of the traveling wave electrode; (c) applying a second DC bias voltage larger than the first DC bias voltage to the other end of the traveling wave electrode.

  According to the present invention, the operating point of the optical modulator can be adjusted in the same manner as when the heater is used with only two DC bias voltages applied to both ends of the traveling wave electrode. As a result, it is possible to reduce the size and manufacturing cost of the optical modulator. At the same time, by controlling the DC bias voltage, it is possible to prevent the modulation efficiency (modulated light output) from decreasing in the light propagation direction of the optical modulator and make it uniform.

It is a top view which shows the structure of the optical modulator of one Embodiment of this invention. It is sectional drawing which shows the structure of the optical waveguide of one Embodiment of this invention. It is a circuit diagram which shows the structure of the phase modulation part of one Embodiment of this invention. It is a figure which shows the example of the operating point control of the optical modulator of one Embodiment of this invention. It is a figure for demonstrating the modulation efficiency of the optical modulator of one Embodiment of this invention. It is a figure for demonstrating the modulation efficiency of the conventional optical modulator. It is a figure which shows the control flow of the optical modulator of one Embodiment of this invention.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a plan view showing a configuration of an optical modulator according to an embodiment of the present invention. The optical modulator 100 of FIG. 1 is an example of an optical modulator using a Mach-Zehnder interferometer (MZI). The optical modulator 100 includes two optical waveguides 5 and 6 extending in parallel between an optical splitter 2 connected to the input port 1 and an optical coupler 3 connected to the output port 4. The optical waveguides 5 and 6 are also called arms. The lengths of the two arms 5 and 6 may be either equal length or unequal length. CW (Continuous Wave) light is input to the input port 1, and modulated light after modulation is output from the output port 4. CW light means light having no time dependency of amplitude, and is supplied by, for example, a continuous wave laser light.

  In the phase modulation section, one of the optical waveguides 5 is sandwiched between a pair of electrodes constituting a traveling wave electrode, that is, an electrode S (7) to which a signal is applied and a ground G (8). Similarly, the optical waveguide 6 is configured to be sandwiched between a pair of electrodes S (9) and a ground G (8) that constitute a traveling wave electrode. FIG. 1 shows a configuration (S / G / S from the top) in which the ground G (8) is disposed between the two optical waveguides 5 and 6 as one common ground. It is also possible to adopt a configuration (S / G / G / S from the top) arranged as the ground, or a configuration (G / S / S / G from the top) replaced with the electrodes S (7, 9). The electrode arrangement can be arbitrarily set according to the specifications (space, wiring, etc.) of the optical modulator.

  A circuit 10 for applying a high frequency (RF) input and a direct current (DC) bias 1 is connected to at least one of the traveling wave electrodes, specifically, for example, one end of the electrode S (7) in FIG. Connected to is a circuit 11 for applying and terminating a direct current (DC) bias 2. Although the wiring is not shown, the circuits 10 and 11 are also electrically connected to the ground electrode G (8) due to the nature of the circuit. A circuit similar to the circuits 10 and 11 can be provided on the electrode S (9) forming the other traveling wave electrode. The contents of the circuits 10 and 11 will be further described later.

  FIG. 2 is a cross-sectional view showing the configuration of the optical waveguide 6 according to the embodiment of the present invention (cross section aa ′ in FIG. 1). The configuration of the optical waveguide 5 is the same. FIG. 2 shows an example in which a silicon rib type optical waveguide including a PN junction is used. In FIG. 2, a p-type (p +) region and an n-type (n +) are formed so as to form a lateral pn junction in the silicon layer (SOI layer) 12 on the buried oxide film (BOX layer) in a predetermined region of the SOI substrate. A region is formed. The BOX layer functions as a first cladding (lower cladding) of the phase modulation unit, and the SOI layer 12 functions as a core (part thereof). A p ++ region having a high dopant concentration is provided at the end of the p + region, and is in resistive junction with the electrode G (8) thereon. Similarly, an n ++ region having a high dopant concentration is provided at the end of the n + region, and is resistively joined to the electrode S (9) thereon. The circuits 10 and 11 of FIG. 1 are connected to the electrode S (9).

A rib 14 including a PN junction is provided at the center of the SOI layer having a flat surface. The rib 14 functions as a core (a part thereof). The ribs 14 and the SOI layer 12 are covered with a second clad (upper clad) 13 made of a material different from silicon. The second cladding 13 may be made of a material such as silicon oxide (SiO ₂ ), SiOCH, or SiON.

The relationship between the refractive index n1 of the SOI layer 12 and the rib 14 forming the core and the refractive index n2 of the first and second claddings including the core is basically required for the core and the cladding of the conventional optical waveguide. It is the same as the relationship of the refractive index, and at least n1> n2 needs to be satisfied. In the example of FIG. 2, the refractive index of Si of the SOI layer 12 is about 3.5, and the refractive index of silicon oxide (SiO ₂ ), which is an example of the first and second claddings, is about 1.4. These satisfy the above-described refractive index relationship n1> n2.

  The basic modulation operation of one embodiment of the present invention illustrated in FIGS. 1 and 2 is as follows as in the conventional case. In an MZI type optical modulator using a silicon optical waveguide, a pn junction is formed in the silicon optical waveguides 12 and 14 constituting the arms 5 and 6 constituting the MZ interferometer. Coplanar traveling wave electrodes S and G are arranged so as to be connected to the pn junction. By applying a reverse bias voltage to the pn junction, the width of the depletion layer changes. As a result, the carrier density in the optical waveguide changes, and the phase of light propagating due to the electro-optic effect (refractive index change) can be changed.

  Such a portion that gives a phase shift of light is also called a phase shifter. When an MZ interferometer is constructed using this principle and a high frequency electric signal is input to the traveling wave electrode of one arm or both arms, the phase of the CW input light from the input port 1 branched by the optical splitter 2 is As a result, the interference condition of light from both arms in the optical coupler 3 on the exit side of the MZ interferometer changes, and amplitude modulation and phase modulation can be applied to the output light from the output port 4.

FIG. 3 is a circuit diagram showing a configuration of the phase modulation unit of the optical modulator according to the embodiment of the present invention. Circuits 10 and 11 are connected to a set of electrodes S (7) and a ground G (8) constituting a traveling wave electrode. A block 103 surrounded by a broken line is an equivalent circuit when the optical waveguide is formed of a silicon optical waveguide as illustrated in FIG. The diode D of the equivalent circuit 103 shows rectification at the PN junction, and the resistance r shows the resistance of the electrode S (7). The circuit 10 includes a first bias tee circuit 101 connected to a high-frequency signal source RF and a first DC bias voltage source V _bias1 . The circuit 11 includes a second bias tee circuit 102 connected to the high-frequency termination resistor R and the second DC bias voltage source V _bias2 . Each of the two bias tee circuits 101 and 102 has a configuration including, for example, conventional capacitors C1 and C2 and inductors L1 and L2.

The capacitor C1 of the first bias tee circuit 101 cuts the direct current and applies only the high frequency signal from the high frequency signal source RF to the electrode S (7). The inductor L1 cuts the high-frequency signal and applies only the DC voltage from the first DC bias voltage source V _bias1 to the electrode S (7). The capacitor C2 of the second bias tee circuit 102 cuts the direct current and guides only the high-frequency signal from the electrode S (7) to the terminating resistor R. The inductor L2 cuts the high-frequency signal and applies only the DC voltage from the second DC bias voltage source V _bias2 to the electrode S (7). In the configuration of FIG. 3, it is one feature of the present invention that a bias tee circuit is provided not only at one end (input side) but also at the other end (output side) of the electrode S (7).

  Next, control of the operating point of the optical modulator according to one embodiment of the present invention, more specifically control of the initial operating point of the MZ interferometer, will be described with reference to FIG. Although the basic modulation operation of the optical modulator is as described above, it is necessary to adjust the modulation operation point (crossing point of MZ interference) in advance for the modulation. The reason for this is that, due to manufacturing, there are subtle variations in the dimensions and composition of both arms (silicon optical waveguides), so it is necessary to optimize the phase modulation output (efficiency) by absorbing the effects of these variations. .

FIG. 4 is a diagram illustrating an example of operating point control of the optical modulator according to the embodiment of the present invention. FIG. 4A shows an example of the present invention. For reference, FIG. 4B shows an example using a conventional heater. In either case, a silicon optical modulator is used, and the length of the phase modulator is about 2 mm. Bias difference of the horizontal axis of the graph _{(a) (V bias2 -V bias1} ) includes a second DC bias voltage source V _bias2 DC bias voltage V _bias2 supplied by the circuit 11 of FIG. 3, the circuit 10 It means the difference (ΔV) from the DC bias voltage V _bias1 supplied by the first DC bias voltage source V _bias1 . The horizontal axis of the graph (b) means the voltage (V) supplied to the heater (resistance) for heating the arm. The vertical axis of both graphs means the modulated light output (dBm) output from the optical coupler (output port) of the optical modulator.

  In the case of the present invention shown in FIG. 4 (a), the intersection A between the optical output as the cross port CP1 obtained at a bias difference of about 0.6 V and the optical output as the bar port BP1 is defined as the operating point of the optical modulator. Can be selected. This operating point A is compared with the case where the intersection A ′ of the optical output of the cross port CP2 and the optical output of the bar port BP2 when the conventional heater of (b) is used is selected as the operating point of the optical modulator. It is possible to obtain an operating point with a light output of almost the same size. By using the present invention, it is possible to adjust the operating point of the optical modulator (the initial operating point of the MZ interferometer) as in the case of using a heater without an individual heater as in the prior art. Further, the potential difference that gives the phase difference π is 1 V or less, which is lower than that of the prior art, and the configuration of the control circuit is simplified.

If there is a bias difference on the output side and the input side electrode S of the traveling wave electrode, i.e., given a different DC bias voltage (V _bias1 <V _bias2), can be a DC current to the electrodes S. In that case, since the electrode S has a finite electric resistance determined by a large (L / S) ratio, it can function as a kind of heater, and thereby the operating point of the optical modulator can be adjusted. Thus, if the present invention is used, an individual heater for adjusting the operating point of the optical modulator becomes unnecessary, and the manufacturing cost, the manufacturing period, and the development period can be reduced. Further, since a heater is not required, the optical modulator can be miniaturized.

Next, adjustment (improvement) of modulation efficiency in the light propagation direction of the optical modulator according to the embodiment of the present invention will be described with reference to FIGS. FIG. 5 is a diagram for explaining the modulation efficiency in the length direction (light propagation direction) of the phase modulation unit of the optical modulator according to the embodiment of the present invention. FIG. 6 is a diagram for explaining the modulation efficiency in the length direction (light propagation direction) of a conventional optical modulator, shown for comparison. FIG. 5 (a) shows that the magnitude (RF voltage) of the high frequency signal applied to the electrode S of the traveling wave electrode is along the length L (μm) of the optical waveguide of the phase modulation section unless any countermeasure is taken. It shows how it decays. The same applies to FIG. FIG. 5 (b), by providing the input and output sides in different DC bias voltages of electrodes S of the traveling-wave electrodes (V _bias1 <V _bias2), along the length of the optical modulator (the light propagation direction) The DC bias voltage (V) is gradually increased.

  In this case, although not shown, the phase modulation efficiency also increases as the bias voltage increases. As a result, by giving an appropriate DC bias voltage difference between the input side and the output side of the electrode S, as shown in FIG. 5C, the phase modulation efficiency (radian / V · μm) is reduced to the length L of the optical waveguide. (μm) can be increased. As shown in FIG. 5D, the total optical modulation amount (optical phase shift amount) is the magnitude (RF voltage (V)) and phase of the high-frequency signal at the length L (μm) of the optical waveguide of the phase modulation unit. It can be obtained as the integral value (area 1) of the product of the modulation efficiency (radian / V · μm). It is possible to improve the total light modulation amount at the length L (μm) of the optical waveguide of the phase modulation unit. That is, the modulation efficiency (modulated light output amplitude) can be improved.

  On the other hand, in the conventional optical modulator of FIG. 6, a constant (same) DC bias is applied to the input side and output side of the traveling wave electrode S as shown in FIG. As a result, as shown in FIG. 6C, the phase modulation efficiency (radian / V · μm) remains constant along the length L (μm) of the optical waveguide. In this case, the total light modulation amount (optical phase shift amount) decreases along the length L (μm) of the optical waveguide, as indicated by area 2 in FIG. That is, the conventional optical modulator cannot prevent a decrease in modulation efficiency (modulated light output amplitude) in the length direction (light propagation direction) of the phase modulator.

  FIG. 7 is a diagram showing a control flow of the optical modulator according to the embodiment of the present invention. The control flow in FIG. 7 can be executed, for example, in an MZI type optical modulator having the configuration in FIGS. In step S1, a DC bias voltage is set. Specifically, in order to control the operating point of the above-described optical modulator and improve the modulation efficiency in the light propagation direction, two DC bias voltages (first voltages applied to the input side and the output side of the electrode S of the traveling wave electrode) Set 1 and 2).

  More specifically, for example, the relationship between the DC bias voltage difference and the optical output in FIG. 4A is obtained in advance as data corresponding to the conditions of the phase modulation unit (material, length, doping concentration of the PN junction, etc.). Keep it in the computer's memory. Using a computer, the DC bias voltage difference at the operating point A of the optical modulator and the corresponding two DC bias voltages (first and second) are automatically selected from the graph of FIG. At this time, the magnitude of the DC bias voltage (second) on the output side of the electrode S can be determined using (referring to) the phase modulation efficiency data of FIG. That is, the second DC bias voltage is set so that both optimization of the operating point A of the optical modulator and improvement of the total optical modulation amount can be achieved simultaneously.

In step S2, the first DC bias voltage set in step S1 is applied to the input side of the traveling wave electrode S. As described above, the first DC bias voltage can be supplied by, for example, the first DC bias voltage source V _bias1 shown in FIG. In step S3, a high frequency signal is applied to the electrode S of the traveling wave electrode. As described above, this high-frequency signal can be supplied by, for example, the high-frequency signal source RF shown in FIG. In step S4, the second DC bias voltage set in step S1 is applied to the output side of the traveling wave electrode S.

As described above, the second DC bias voltage can be supplied by, for example, the second DC bias voltage source V _bias2 of FIG. In FIG. 7, step 3 and step 4 may be interchanged, and the application of the second DC bias voltage in step S4 may be performed before the application of the high-frequency signal in step S3. Alternatively, in step S2, a high-frequency signal may be applied first, and then the first DC bias voltage and the second DC bias voltage may be applied (continuously in time series).

  Embodiments of the present invention have been described with reference to the drawings. However, the present invention is not limited to these embodiments. Furthermore, the present invention can be implemented in variously modified, modified, and modified forms based on the knowledge of those skilled in the art without departing from the spirit of the present invention.

  The optical modulator of the present invention can be used as a part of an optical circuit, an optical integrated circuit, or the like as an optical modulator used in an optical communication device such as an optical transceiver device.

1: Input port 2: Optical splitter 3: Optical coupler 4: Output port 5, 6: Optical waveguide (arm)
7, 8, 9: Traveling wave electrode 10: Circuit for applying radio frequency (RF) input and direct current (DC) bias 1 11: Circuit for applying and terminating direct current (DC) bias 2 12: Silicon layer (SOI layer)
13: Second cladding (upper cladding)
14: rib 100: optical modulator 101: first bias tee circuit 102: second bias tee circuit 103: equivalent circuit of an optical waveguide

Claims (9)

A phase modulation section provided in at least one of two optical waveguides extending in parallel between the optical splitter and the optical coupler;
The phase modulation unit includes a traveling wave electrode provided along the optical waveguide, and one end of the traveling wave electrode is connected to a high-frequency signal source via a first bias tee circuit. The end is an optical modulator that is connected to a high-frequency termination resistor via a second bias tee circuit.   2. The optical modulator according to claim 1, wherein the first bias tee circuit is connected to a first DC voltage source, and the second bias tee circuit is connected to a second DC voltage source.   The first bias tee circuit applies a first bias voltage to one end of the traveling wave electrode, and the second bias tee circuit applies a second bias voltage larger than the first bias voltage to the other end of the traveling wave electrode. The optical modulator according to claim 1, wherein the optical modulator is applied.   The optical modulator according to claim 1, wherein the traveling wave electrode includes a pair of signal electrodes and a ground electrode located on both sides of the optical waveguide.   The optical modulator according to claim 1, wherein the optical modulator constitutes a Mach-Zehnder (MZ) type optical modulator.   The optical modulator according to claim 5, wherein the optical waveguide includes a silicon rib optical waveguide including a PN junction. An optical modulator control method comprising:
The optical modulator includes a phase modulation unit provided in at least one of two optical waveguides extending in parallel between the optical splitter and the optical coupler, and the phase modulation unit is provided along the optical waveguide Traveling wave electrodes,
Applying a first DC bias voltage to one end of the traveling wave electrode;
Applying a high frequency signal for modulation to one end of the traveling wave electrode;
Applying a second DC bias voltage larger than the first bias voltage to the other end of the traveling wave electrode.   The control method according to claim 7, wherein an operating point of the optical modulator is set by a difference between the first DC bias voltage and the second DC bias voltage.   9. The control method according to claim 7, wherein the second DC bias voltage is set so that modulation efficiency in the light propagation direction of the optical modulator is improved.

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