JP5432043B2 - Optical transmitter - Google Patents

Description

  The present invention relates to an optical transmitter.

  With the spread of optical communication networks in recent years, adaptation to the backbone optical communication system having a transmission capacity exceeding 40 Gbit / sec (gigabit per second) has been actively promoted. These ultra-high-speed communication systems have higher frequency utilization efficiency and optical signal-to-noise ratio (optical signal-to-noise ratio) than the NRZ (Non Return to Zero) modulation method applied in the conventional optical communication system. OSNR) various light modulation methods having excellent proof stress and non-linear proof strength have been adopted one after another. Against this background, the multi-bit information transmission method in one symbol time is considered as the most effective means.

For example, in the case of differential quadrature phase shift keying (DQPSK), which is a scheme capable of assigning 2 bits of data to each of the four modulated phases, the transmitter has at least four high-speed transmissions. A device configuration in which a phase modulator is integrated is required. Currently, as these high-speed phase modulators, modulators composed of lithium niobate (LiNbO ₃ ; LN) are widely used (see Patent Document 1 below). By using the LN waveguide, there is an advantage that these modulators can be realized with low loss.

  However, when dealing with a more complicated information transmission format using the LN waveguide, there is a demerit that the device size is increased. For example, in the case of DP-QPSK (Dual Polarization Phase Shift Keying) that allocates 4-bit data including polarization multiplexing that realizes 100 Gbit / sec (see Non-Patent Document 1 below), the configuration of DQPSK When the LN is used, the number of couplers is increased, and the number of phase modulation units is doubled to at least eight, so that the element size is very large.

  In the future, the more complex the information transmission format is supported, there is a problem that the device size becomes larger in the modulator using the LN. In order to avoid this problem, the modulator using LN adopts a folding structure to reduce the size (see Non-Patent Document 2 below). However, it is hard to say that it is small enough.

  Along with modulators using these LNs, attention is being focused on as optical modulators made of semiconductors. By applying an electric field to a part of the semiconductor waveguide, the refractive index is changed, and the input electrical signal Is a semiconductor Mach-Zehnder type phase light modulator (semiconductor MZM phase light modulator).

  The greatest merit of this semiconductor MZM phase light modulator is that it is possible to realize a smaller modulator while exhibiting characteristics equivalent to those of the LN modulator. In the case of the LN modulator, the chip size reaches about 5 cm. However, in the case of the semiconductor MZM phase light modulator, the same function as that of the LN modulator can be expressed with a chip size of at least about 5 to 6 mm. In addition, since the laser serving as the light source is also formed from a semiconductor, it is possible to integrate and manufacture the laser on the same substrate, and thus it is possible to provide a smaller phase modulator.

JP 2009-204753 A JP 2008-107468 A JP 2009-060533 A JP 2008-107468 A

A. H. Gnauck, 9 others, “25.6-Tb / s WDM Transmission of Polarization-Multiplexed RZ-DQPSK Signals”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 1, January 2008, p. 79-84 Kenji Aoki, 6 others, "High-Speed X-Cut Thin-Sheet LiNbO3 Optical Modulator With Folded Structure", JOURNAL OF LIGHTWAVE TECHNOLOGY. 25, NO. 7, July 2008, p. 1805-1810

  As described above, the semiconductor MZM phase optical modulator made of a semiconductor can be greatly reduced in size, but is inferior to the LN modulator at the present time. That is the power consumption for temperature adjustment. The insertion loss is also inferior at present, but this can be solved by optimizing the element structure and the coupling method.

  However, the fact that the power consumption generated by the temperature adjustment mechanism is large is a drawback of the semiconductor MZM phase light modulator compared to the LN modulator. Currently, as a measure against global warming, there is a strong demand for lower energy consumption and communication capacity is increasing. Before all the WDM wavelength band is used up, communication capacity is reduced by power consumption. It is said that the temperature adjustment mechanism that consumes power is a serious problem.

  The LN modulator can basically realize high-speed modulation with a constant voltage without using a temperature control device such as a Peltier element. That is, even if the temperature changes, the modulation efficiency hardly changes, so that it can be used without temperature control.

  On the other hand, in the semiconductor MZM phase light modulator, since the band gap changes with temperature, the modulation efficiency changes in principle. Therefore, a temperature adjustment mechanism is provided and driving at a constant temperature is unavoidable. That is, when the temperature changes, there is a problem that the voltage for modulation driving must be changed.

  Normally, changing the modulation drive voltage with respect to temperature changes is not performed because a special drive driver is used. In general, based on the input from the temperature sensor provided around the element, the Peltier element is driven so that the element temperature is constant, so that the modulation efficiency is constant even if the outside air temperature fluctuates, Constant voltage drive is performed. The power consumption required to maintain a constant temperature depends on the size of the element and the like, but about 1 W is consumed in the worst case.

  On the other hand, in the case of the LN modulator, it is 0 W, and the difference is large. Since one optical modulator is used for each channel, power consumption of 10 W is generated for 10 users and 1 KW is generated for 1000 users, resulting in a large difference in power consumption compared to the case of using an LN modulator.

  From the above, according to the present invention, even in the semiconductor MZM phase optical modulator, it is possible to drive with the same modulation voltage with respect to a change in environmental temperature, omitting a temperature adjustment mechanism using a Peltier element or the like, An object of the present invention is to provide an optical transmitter capable of reducing power consumption.

An optical transmitter according to a first invention for solving the above-described problem is
a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator,
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependency of the temperature and the bias voltage is obtained, and the relationship between the temperature and the bias voltage at which the modulation amplitude voltage is constant is obtained from the dependency,
When the input from the temperature sensor changes , the bias circuit changes the bias voltage and drives the semiconductor MZ modulator with a constant modulation amplitude voltage based on the relationship .

An optical transmitter according to a second invention for solving the above-described problem is
a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator;
An electronic storage medium for storing an input from the temperature sensor and an array of the bias voltage;
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependence of the temperature and the bias voltage is obtained, and from the dependence, the relationship between the temperature and the bias voltage that makes the modulation amplitude voltage constant is obtained, and the array of the electronic storage medium is obtained. Remember,
The bias circuit drives the semiconductor MZ modulator with a constant modulation amplitude voltage by changing the bias voltage with reference to the arrangement stored in the electronic storage medium based on an input from the temperature sensor. It is characterized by.

An optical transmitter according to a third invention for solving the above-described problem is
a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator;
An arithmetic circuit that calculates the bias voltage by a polynomial function of an input from the temperature sensor,
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependency of the temperature and the bias voltage is obtained, and the polynomial of the temperature and the bias voltage that makes the modulation amplitude voltage constant is derived from the dependency,
The bias circuit drives the semiconductor MZ modulator with a constant modulation amplitude voltage by changing the bias voltage according to a calculation result in the calculation circuit.

An optical transmitter according to a fourth invention for solving the above-described problem is
A tunable laser;
a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator;
An electronic storage medium that stores an operating wavelength of the tunable laser, an input from the temperature sensor, and a three-dimensional array of the bias voltage;
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependency of the operating wavelength, the temperature, and the bias voltage is obtained, and the relationship between the operating wavelength, the temperature, and the bias voltage that makes the modulation amplitude voltage constant from the dependency, Stored in the three-dimensional array of the electronic storage medium;
The bias circuit drives the semiconductor MZ modulator with a constant modulation amplitude voltage by changing the bias voltage with reference to the three-dimensional array stored in the electronic storage medium according to a change in temperature or wavelength. And

An optical transmitter according to a fifth invention for solving the above-described problem is
A tunable laser;
a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator;
An arithmetic circuit that calculates the bias voltage by a polynomial functioning an operation wavelength of the wavelength tunable laser and an input from the temperature sensor;
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependency of the operating wavelength, the temperature, and the bias voltage is obtained, and the operating wavelength, the temperature, and the bias voltage that make the modulation amplitude voltage constant are derived from the dependency. Every
The bias circuit drives the semiconductor MZ modulator with a constant modulation amplitude voltage by changing the bias voltage according to a calculation result in the calculation circuit.

  The present invention also enables a semiconductor MZM phase light modulator to be driven with the same modulation voltage with respect to changes in environmental temperature, omits a temperature adjustment mechanism using a Peltier element, etc., and reduces power consumption. An optical transmitter capable of performing the above can be provided.

It is the schematic diagram which showed the structure of the semiconductor MZ modulator which concerns on a 1st Example. It is the schematic diagram which showed the manufacturing method of the semiconductor MZ modulator which concerns on a 1st Example. It is the schematic diagram which showed the arrangement position of the isolation | separation groove | channel in the semiconductor MZ modulator which concerns on a 1st Example. FIG. 3 is a diagram illustrating a basic operation of the semiconductor MZ modulator according to the first embodiment. It is the figure which showed the bias voltage dependence of the modulation electrode extinction characteristic in the semiconductor MZ modulator which concerns on a 1st Example. It is the figure which showed the temperature dependence of the modulation electrode quenching characteristic in the semiconductor MZ modulator which concerns on a 1st Example. It is the figure which showed the bias voltage dependence of the modulation amplitude voltage in the semiconductor MZ modulator which concerns on a 1st Example. It is the block diagram which showed the structure of the optical transmitter which concerns on a 1st Example. It is the figure which showed the modulation-electrode quenching characteristic of -5 degreeC and 75 degreeC environment in the optical transmitter which concerns on a 1st Example. It is the block diagram which showed the structure of the optical transmitter which concerns on a 2nd Example. It is the figure which showed the temperature dependence of the bias voltage Vbias in the optical transmitter which concerns on a 2nd Example. It is the block diagram which showed the structure of the optical transmitter which concerns on a 3rd Example. It is the figure which showed the wavelength dependence of the modulation amplitude voltage in the optical transmitter which concerns on a 3rd Example. It is the figure which showed the bias voltage dependence of the modulation | alteration amplitude voltage in the optical transmitter which concerns on a 3rd Example. It is the block diagram which showed the other structure of the optical transmitter which concerns on a 3rd Example.

  Hereinafter, an optical transmitter according to the present invention will be described with reference to the drawings.

The optical transmitter according to the first embodiment of the present invention will be described below.
The optical transmitter according to the present embodiment includes an npin semiconductor MZ modulator, a temperature sensor that monitors the temperature of the semiconductor MZ modulator, and a bias circuit that applies a bias voltage to the modulation electrode of the semiconductor MZ modulator. The transmitter is characterized in that when the input from the temperature sensor changes, the bias voltage is changed to drive the modulation amplitude voltage to be constant. In this example, an optical transmitter according to this example was manufactured and evaluated.

First, the configuration of the semiconductor MZ modulator according to the present embodiment will be described.
FIG. 1 is a schematic diagram illustrating a configuration of a semiconductor MZ modulator according to the present embodiment.
As shown in FIG. 1, the semiconductor MZ modulator 1 according to this embodiment includes a 3 dB coupler 10 a disposed on the input port side, a 3 dB coupler 10 b disposed on the output port side, and two 3 dB couplers 10 a and 10 b. Arm waveguides 11a and 11b for connecting the electrodes, modulation electrodes 12a and 12b for performing high-speed modulation installed on the arm waveguides 11a and 11b, and a phase at the time of manufacture installed on the arm waveguides 11a and 11b. It is comprised by balance adjustment electrode 13a, 13b for adjusting an error. The layer structure of the semiconductor MZ modulator 1 according to the present embodiment is the npin structure, which is the same structure as the InP structure semiconductor MZ modulator found in Patent Document 2 above.

  In the semiconductor MZ modulator 1 according to the present embodiment, the path passing through the lower arm waveguide 11b is longer by a half wavelength (wavelength / effective refractive index / 2 length of the arm waveguides 11a and 11b). The asymmetric MZ is designed. The semiconductor MZ modulator 1 according to the present embodiment is normally OFF when it is ideally manufactured, and input from the input port when no voltage is applied to the modulation electrodes 12a and 12b and the balance adjustment electrodes 13a and 13b. The light is not output from the output port.

  However, a device that is actually manufactured does not become normally OFF due to a phase error at the time of manufacture, and thus a mechanism for adjusting the phase error at the time of manufacture is required. For this reason, balance adjusting electrodes 13a and 13b are provided at the subsequent stage of the arm waveguides 11a and 11b. Of course, since the phase error at the time of manufacture can be adjusted, the design is originally made as a symmetric MZ instead of the asymmetric MZ, and the extinction point is adjusted using the balance adjusting electrodes 13a and 13b after the manufacture. Good. Also, modulation electrodes 12a and 12b for placing communication signals are provided in front of the arm waveguides 11a and 11b.

  Note that FIG. 1 shows a configuration in which push-pull driving is performed, and a modulation electrode voltage Vpai is applied to the arm waveguides 11a and 11b. Further, a bias voltage Vbias having a direct current (DC) component is applied to the modulation electrodes 12a and 12b. A termination resistor 14 and a capacitor 15 are inserted on the termination side of the modulation electrodes 12a and 12b so that a DC component does not pass.

Next, a manufacturing method of the semiconductor MZ modulator according to the present embodiment will be described.
FIG. 2 is a schematic view showing a method for manufacturing the semiconductor MZ modulator according to the present embodiment.
As shown in FIG. 2A, the semiconductor MZ modulator 1 according to this embodiment includes a first n-type electrode layer (n + −) on a semi-insulating (SI) -InP substrate 20. InP) 21 is grown. A first n-type cladding layer (n-InP) 22 is grown on the first n-type electrode layer 21. A first intermediate layer (i-InGaAsP) 23 is grown on the first n-type cladding layer 22. A core layer 24 is grown on the first intermediate layer 23. A second intermediate layer (i-InGaAsP) 25 is grown on the core layer 24 in this order.

  A first low-concentration cladding (i-InP) 26 is grown on the second intermediate layer 25. A p-type cladding layer (p-InP) 27 that functions as an electron barrier is grown on the first low-concentration cladding 26. A second n-type cladding layer (n-InP) 28 is grown on the p-type cladding layer 27. A second n-type electrode layer (n + -InP) 29 is grown on the second n-type cladding layer 28.

  Here, the core layer 25 is configured so that the electro-optic effect works effectively at the operating light wavelength. For example, in the case of a device in the 1500 nm band, the layers in which the Ga / Al composition of InGaAlAs is changed are respectively quantum well layers. And a multi-quantum well structure with a quantum barrier layer. The first intermediate layer 23 functions as a connection layer for preventing carriers generated by light absorption from being trapped at the heterointerface.

FIG. 3 is a schematic diagram illustrating the arrangement positions of the separation grooves in the semiconductor MZ modulator according to the present embodiment.
As shown in FIG. 3, in order to manufacture the semiconductor MZ modulator according to the present embodiment, the separation groove 16 is formed on the arm waveguides 11a and 11b in order to electrically separate the two arm waveguides 11a and 11b. Form.

  In the case of the MZ structure in which the modulation electrodes 12a and 12b and the balance adjustment electrodes 13a and 13b are separated, a separation groove 16 is also provided between them to perform electrical separation. This is because the voltage applied to one arm waveguide for modulation affects the modulation of the other arm waveguide if electrical isolation is not performed.

  The isolation groove 16 is formed by performing standard photolithography and patterning on a part from the second n-type electrode layer 29 to the p-type cladding layer 27 functioning as an electron barrier, and using a wet etching technique, the arm waveguide 11a. , 11b is removed as a groove having a width of several microns. In the present embodiment, electrical separation is performed by the separation groove 16, but electrons from the second n-type electrode layer 29 are formed using a quartz hard mask in areas other than the periphery of the modulation electrodes 12a and 12b that are in contact with the electrodes. After removing up to the p-type cladding layer 27 functioning as a barrier, it may be regrown and replaced by semi-insulating InP.

  Next, as shown in FIG. 2B, the layers from the second n-type electrode layer 29 to the first intermediate layer 23 are etched by using a dry etching technique, so that a high-mesa waveguide structure is obtained. Form. Subsequently, the first n-type cladding layer 22 is etched to expose the first n-type electrode layer 21.

  Then, as shown in FIG. 2 (c), the modulation electrodes 12a and 12b and the n-type electrode 30 to be the balance adjustment electrodes 13a and 13b are placed on the second n-type electrode layer 29 to become the second n which becomes GND. A mold electrode 31 is formed on each first n-type electrode layer 21. In this embodiment, the n-type electrode 30 and the second n-type electrode 31 are made of gold. If necessary, a passivation film may be deposited to protect the mesa surface, or the high mesa structure may be protected using a polymer or the like.

Next, a method for mounting the semiconductor MZ modulator according to this embodiment on a module will be described.
After forming the n-type electrode 30 and the second n-type electrode 31 as described above, the back surface of the SI-InP substrate 20 was polished to such an extent that the cleavage performed later can be performed. A metal film was deposited on the back surface of the SI-InP substrate 20 so that the fixed solder was adhered, and each element was cleaved. Then, an end surface with an input / output waveguide was coated with an antireflection coating.

  Then, the semiconductor MZ modulator 1 according to this example is mounted on a submount made of aluminum nitride with a standard chip bonder and then fixed by heating. Subsequently, a terminal resistor, a capacitor, and a change in resistance are changed. A thermistor and a wiring board for transmitting high-speed signals as temperature sensors that detect changes in temperature were also mounted using a chip bonder and fixed by the reflow method.

  After the wiring and elements on the fixed submount and the modulation electrodes 12a and 12b and the balance adjustment electrodes 13a and 13b of the semiconductor MZ modulator 1 are connected by wire bonding, the submount is reflowed again to the mount made of CuW. The lens which collimates input / output light was fixed to the tip of the input / output waveguide using a YAG laser.

  As described above, what is mounted on the mount is mounted in an airtight package that can be fixed to both ends, and an I / O fiber with a lens is fixed with a YAG laser. A GPPO connector as an RF input connector was attached to the dynamic signal input terminal to form a module.

  In this embodiment, the lens is fixed using a YAG laser, but may be performed by other methods. Further, although the GPPO connector is used as the RF input connector, any other RF input terminal that can input a signal of a desired frequency may be used.

Next, based on the performance measurement result of the manufactured module, the non-temperature-controlled driving method of the semiconductor MZ modulator according to the present embodiment will be described together with the characteristics of the semiconductor MZ modulator according to the present embodiment.
FIG. 4 is a diagram illustrating the basic operation of the semiconductor MZ modulator according to the present embodiment.
FIG. 4 schematically shows the intensity of light output from the output port when a voltage is applied to the modulation electrodes 12a and 12b.

  Here, in the graph shown in FIG. 4, the negative side indicates a state in which the bias voltage Vbias (<0 V) is applied to the arm waveguide 11a and the modulation electrode voltage Vpai is applied as a negative voltage. At this time, for push-pull drive, the same bias voltage Vbias as that of the arm waveguide 11a is applied to the arm waveguide 11b, and the modulation electrode voltage Vpai having a sign opposite to that of the arm waveguide 11a is applied. . At this time, there is a relationship of | Vpai | <| Vbias |.

  Similarly, the positive side of FIG. 4 shows the difference between the arm waveguides to which the voltage is applied, and is plotted positive in the graph for convenience. However, the signs of the applied bias voltage Vbias and the modulation electrode voltage Vpai are the arm guides. It is the same code | symbol as the waveguide 11a. Subsequent measured graphs are plotted in the same way.

  As shown in FIG. 4A, in the characteristics of the module after manufacture, the extinction point does not appear at a zero voltage due to a phase error generated during manufacture. Therefore, a voltage is applied to the balance adjusting electrodes 13a and 13b (this voltage is set to Vnull) so that the extinction point is positioned at the zero voltage. Normally, a voltage is applied to the balance adjustment electrodes 13a and 13b whose Vnull is lower.

  In this embodiment, Vnull is automatically biased so that the extinction point of the modulation electrodes 12a and 12b is always zero when no voltage is applied, even if long-term fluctuation occurs on the time axis. Adjustment is performed using a control circuit (refer to Patent Document 3). Of course, when there is no long-term fluctuation, the automatic bias control circuit is not necessary.

  Thereafter, when a voltage is swept across the modulation electrodes 12a and 12b, symmetrical characteristics are obtained as shown in FIG. 4B. Here, the voltage up to the point at which the first positive and negative maximum transmission intensity is obtained is the modulation amplitude voltage Vpai when the semiconductor MZ modulator 1 is used as DPSK. The feature of the semiconductor MZ modulator 1 according to the present embodiment is that the modulation amplitude voltage Vpai can be arbitrarily selected to some extent.

FIG. 5 is a diagram showing the bias voltage dependence of the modulation electrode extinction characteristic in the semiconductor MZ modulator according to the present embodiment.
FIG. 5 shows, as an example, the Vbias dependence of the extinction characteristics of the modulation electrodes 12a and 12b when the bias voltage of the manufactured semiconductor MZ modulator 1 is 8V to 16V. FIG. 5 shows the result of measurement at a wavelength of 1560 nm while keeping the temperature constant at −5 ° C. Further, in FIG. 5, the standardized transmission intensity is shifted by 10 dB for each bias voltage Vbias for easy viewing.

  As shown in FIG. 5, when the bias voltage Vbias (that is, the DC component applied to the modulation electrodes 12a and 12b) is increased in a state where the Vnull is applied and the extinction point is adjusted, the zero voltage is moved toward the center of the symmetry axis. The modulation amplitude voltage Vpai is reduced. In the example shown in FIG. 5, when the bias voltage Vbias is changed by 1V, a change in the modulation amplitude voltage Vpai of 0.2V is obtained, and the change is approximately proportional to the bias voltage Vbias.

  By the way, the semiconductor MZ modulator 1 can make a drive voltage low, so that the length of a phase modulator part is long. This is because the bandgap always has temperature dependence, so if it is a semiconductor, there is always wavelength dependence of the driving voltage, and in order to make this constant driving voltage, bias voltage Vbias is applied and compensated. is there.

  A conventional optical modulator mainly has a pin structure. However, in the configuration of a general InP structure semiconductor MZ modulator (pin structure), the p layer has a high loss, and the loss increases when the length of the modulation region is increased. Therefore, the operating wavelength and the band gap wavelength are set close to each other so that modulation can be performed in a short modulation region. That is, the detuning amount (difference between the operating wavelength and the band gap wavelength) is reduced.

  Here, the band gap wavelength and length are set so that Vπ (modulation amplitude) is reduced to some extent and the loss in the entire C band (1530 nm to 1560 nm) is within an allowable range (for example, about 0.5 dB). Designing. This design is designed on the assumption that a Peltier element is used so that the temperature is constant.

  In the semiconductor MZ modulator having the InP structure having such an element structure, if the wavelength change is compensated by the bias voltage Vbias and further the temperature compensation is performed by applying the bias voltage Vbias, the increase in loss cannot be realized. .

  On the other hand, the structure of the semiconductor MZ modulator 1 according to the present embodiment is an npin structure (see Patent Document 2). This structure is the same in that the wavelength dependence must be compensated by the bias voltage Vbias, but the loss is smaller than that of the conventional structure. This is because the p layer is extremely thin and the loss is small as compared with the conventional structure, so that the length of the modulation region can be made sufficiently long without increasing the loss.

  Therefore, the semiconductor MZ modulator 1 according to the present embodiment has a feature that Vπ (modulation amplitude) is small without applying the bias voltage Vbias. This is because the adjustment range for the length of the modulation region is wide. For this reason, the range of the voltage which compensates wavelength dependence is also narrow. Further, there is a feature that temperature compensation can be sufficiently performed within the range of the bias voltage Vbias in which an increase in loss can be allowed.

  These features can be realized only by the semiconductor MZ modulator 1 having the npin structure according to this embodiment. Usually, in the case of a conventional LN modulator or the like, the modulation amplitude voltage Vpai derived from the physical property value and length of the LN modulator is determined. However, the semiconductor MZ modulator 1 according to the present embodiment is advantageous in that the modulation amplitude voltage Vpai can be arbitrarily selected to some extent.

FIG. 6 is a graph showing the temperature dependence of the modulation electrode extinction characteristic in the semiconductor MZ modulator according to this example.
FIG. 6 shows how the modulation amplitude voltage Vpai fluctuates when the temperature is changed. FIG. 6 shows extinction characteristics obtained by sweeping the modulation electrodes 12a and 12b when the temperature is changed from 5 ° C. to 75 ° C. In FIG. 6, the standardized transmission intensity is displayed with a shift of 10 dB at each temperature for easy viewing of the drawing. Here, the measurement was performed with the bias voltage Vbias = 5 V and the wavelength of 1560 nm constant.

  As shown in FIG. 6, the modulation amplitude voltage Vpai increases as the temperature decreases from the high temperature part. This is because the detuning amount increases as the temperature decreases, and the modulation efficiency decreases. Thus, in the semiconductor MZ modulator 1, the modulation amplitude voltage Vpai fluctuates due to a change in temperature. It is sufficient that the driver that drives the semiconductor MZ modulator 1 can change the amplitude voltage in accordance with the amplitude variation of the semiconductor MZ modulator 1 along with the temperature change, but usually such a driver is not available. .

FIG. 7 is a diagram showing the bias voltage dependence of the modulation amplitude voltage in the semiconductor MZ modulator according to the present embodiment.
In FIG. 7, the characteristics obtained by changing the bias voltage Vbias at the operating wavelength of 1560 nm shown in FIG. 5 are obtained in the temperature range of −5 ° C. to 75 ° C., and the relationship between the bias voltage Vbias and the modulation amplitude voltage Vpai is obtained. The temperature is plotted as a function.

  As shown in FIG. 7, at −5 ° C., the bias voltage Vbias = 16V, and the modulation amplitude voltage Vpai is 1.3V (indicated by a broken line in FIG. 7). It can be seen that as the temperature rises with the bias voltage Vbias kept constant, the modulation amplitude voltage Vpai decreases and decreases to 0.53 V at 45 ° C. It should be noted that at 55 ° C. or higher, the band edge is too close to the operating wavelength and an increase in loss is observed, so it is excluded from the plot.

  On the other hand, when the maximum ambient temperature is 75 ° C., the bias voltage Vbais is lowered, and when it is lowered to 4V, the modulation amplitude voltage Vpai can be raised to 2.8V. That is, it can be seen that the modulation amplitude voltage Vpai = 1.3 V achieved at Vbias = 16 V at −5 ° C. can be achieved even at 75 ° C. by setting the bias voltage Vbias = 7.1 V.

  At this time, the bias voltage Vbias = 7.1 V was obtained by linear interpolation of the previous and subsequent measurement points. As can be seen from FIG. 7, it is sufficient that the plot is almost linear. Of course, an appropriate bias voltage Vbias may be obtained by increasing the measurement accuracy and further narrowing the measurement interval of the bias voltage Vbias.

FIG. 8 is a block diagram illustrating the configuration of the optical transmitter according to the present embodiment.
As shown in FIG. 8, a bias voltage Vbias can be applied to the semiconductor MZ modulator 1 via bias tees 40a and 40b at terminals RF1 and RF2 to which an operation signal is input. The bias circuit 41 has a memory table 42 which is an array stored in an electronic storage medium, receives a detection temperature from a temperature sensor 43 installed in the vicinity of the semiconductor MZ modulator 1, and receives a bias voltage Vbias. Can be input in response to changes in temperature. In this embodiment, a standard thermistor is used as the temperature sensor 43.

  In the memory table 42, a bias voltage Vbias output in response to an input of the detected temperature from the temperature sensor 43 is stored, and a value measured in advance is stored as an array of the temperature and the bias voltage Vbias. A specific circuit has a circuit configuration in which the thermistor current is a constant current. After AD conversion of the output voltage, the bias voltage Vbias in the memory table 42 is determined and output by digital processing. Although not shown, an ABC circuit is connected to the balance adjustment electrodes 13a and 13b so that the extinction point can be adjusted.

  An optical transmitter was formed as described above, and an experiment was actually performed under no temperature control. The ambient temperature was simulated by mounting the semiconductor MZ modulator 1 according to this embodiment on the Peltier element and changing the module temperature. Similar results should be obtained even if the temperature is changed in an environmental furnace.

FIG. 9 is a diagram illustrating the modulation electrode extinction characteristics in the environments of −5 ° C. and 75 ° C. in the optical transmitter according to the present embodiment.
As shown in FIG. 9, both showed the same characteristics between -1.3V and 1.3V, and it was confirmed that non-temperature-controlled driving with a constant modulation amplitude voltage Vpai can be realized. When the bias voltage Vbias at this time was monitored, it was 16 V under the -5 ° C environment and 7.1 V under the 75 ° C environment.

  As described above, the optical transmitter according to the present embodiment can be driven while the modulation amplitude voltage Vpai is kept constant even when the temperature changes. In the optical transmitter according to the present embodiment, a memory table 42 is required for the bias circuit 41. However, temperature control by a Peltier element in the optical transmitter using the semiconductor MZ modulator 1 can be eliminated, An effect that power consumption can be reduced can be obtained.

An optical transmitter according to the second embodiment of the present invention will be described below.
In this embodiment, the optical transmitter drives a semiconductor MZ modulator that changes the bias voltage Vbias with respect to the input of the detected temperature from the temperature sensor 43, and the bias voltage Vbias is detected as the detected temperature from the temperature sensor 43. An optical transmitter having a semiconductor MZ modulator, which is determined by an arithmetic process for an input and is driven by a non-temperature-regulating element will be described.

  The semiconductor MZ modulator, its manufacturing method, and module assembling method used in this example are the same as those in the first example. Further, the semiconductor MZ element used in this example is an element having the same homogeneity as that of the first example, and the characteristics are the same as those of the element of the first example.

  In the first embodiment, the bias circuit 41 refers to the input of the detected temperature from the temperature sensor 43, determines the bias voltage Vbias by referring to itself or another memory table 42, and drives it. Temperature control was possible. However, when the memory table 42 is provided, data must be written to the memory table 42 and data written to the memory table 42 must be acquired.

FIG. 10 is a block diagram illustrating the configuration of the optical transmitter according to the present embodiment.
As shown in FIG. 10, in this embodiment, an arithmetic circuit 44 is prepared outside or inside the bias circuit 41 so that the modulation amplitude voltage Vpai can be driven constant even when the temperature changes. .

FIG. 11 is a diagram illustrating the temperature dependence of the bias voltage Vbias in the optical transmitter according to the present embodiment.
FIG. 11 is a plot of bias voltage Vbias and temperature that can achieve the modulation amplitude voltage Vpai = 1.3 V in FIG. 5 as a function.
FIG. 11 shows that this temperature range shows a very linear variation. As a result of fitting, it was found that this element has a relationship represented by the following formula (1).
Here, T is the environmental temperature.

The input of the detected temperature from the temperature sensor 43 may be given as T. However, since it is not usually given, T is expressed as a function of some input such as voltage or current. When a thermistor is used for the temperature sensor 43, the theoretical characteristics of the thermistor are expressed by the following equation (2).
Here, R is a resistance value, B is a thermistor constant, T is an actual temperature (K), Ta is a reference temperature, and Ra is a resistance value at the reference temperature. Actually, the temperature is measured by an equation to which the correction term of the above equation is added.

  The resistance value is a circuit configuration in which the thermistor current is a constant current, the output voltage is AD converted, digital processing is performed, and the bias voltage Vbias can be determined by applying T to the above equation (1). . In the element used in this example, the coefficient was 0.112 and the intercept was −15.50, but of course, the values differ depending on the element used.

In addition, although the element used in this example shows linear bias voltage Vbias dependence on temperature, even if it is non-linear, the modulation amplitude voltage is expressed using the polynomial shown in the following equation (3). Vpai can be driven constant.

  In this way, as in this embodiment, the coefficient indicated by the polynomial shown in the above equation (3) is stored in the electronic storage medium, and the detected temperature input from the temperature sensor 43 is used as an argument outside or inside the bias circuit 41. By calculating with the provided calculation circuit 44, the modulation amplitude voltage Vpai is determined, and the semiconductor MZ modulator 1 can be driven without the temperature adjustment mechanism via the bias circuit 41, so that power consumption is reduced. be able to.

Hereinafter, an optical transmitter according to a third embodiment of the present invention will be described.
FIG. 12 is a block diagram illustrating the configuration of the optical transmitter according to the present embodiment.
As shown in FIG. 12, in this embodiment, an optical transmitter using a semiconductor MZ modulator 1 is used. As a light source, a wavelength tunable laser (semiconductor tunable laser) 50, a laser drive circuit 51, and a semiconductor MZ modulator 1 are used. , A temperature sensor 43, a bias circuit 41 to which a detected temperature T from the temperature sensor 43 and an oscillation set wavelength λ from the laser drive circuit 51 are input, and the bias circuit 41 stores the bias voltage Vbias in an electronic storage medium referred to In the optical transmitter having the memory table 42 which is the arranged arrangement, the bias voltage Vbias is varied in the entire C band (1530 nm to 1560 nm), in the temperature range of −5 ° C. to 75 ° C., without a temperature adjusting element such as a Peltier element, An optical transmitter that drives the modulation amplitude voltage Vpai constant was manufactured.

  In the first and second embodiments, the modulation amplitude voltage Vpai of the semiconductor MZ modulator 1 has an array stored in the memory table 42 or an arithmetic circuit in order to drive a temperature-independent characteristic indicating temperature dependence. An optical transmitter that can be driven without depending on the temperature by varying Vbias 44 by using the detected temperature T from the temperature sensor 43 as an input function is shown. These are described for the case where a light source having a fixed wavelength is used.

  However, in general, the semiconductor tunable laser 50 is often used as the light source of the optical transmitter. The purpose of this is to reduce the stock of spare lasers with the semiconductor tunable laser 50 because of the problem of having stocks of many spare lasers for each wavelength when performing wavelength division multiplexing transmission (WDM transmission). Because.

  Although the semiconductor tunable laser 50 has a different method, the wavelength is controlled by current and temperature in order to change the wavelength. Therefore, in order to realize the operating wavelength specified by the user, the current and the wavelength input by the user are Some mechanism to set the temperature is installed.

  In the case of the semiconductor tunable laser 50, a desired characteristic is output by referring to the wavelength input by the user and adjusting the driving conditions to the preset values of the current and temperature stored in the memory table 42 in advance. It is what. That is, the semiconductor tunable laser 50 itself has a driving condition in the memory with respect to the oscillation wavelength.

  In this embodiment, the optical semiconductor MZ modulator 1 and the module are manufactured by the method shown in the first embodiment, and those having the same element characteristics are used. Further, the output from the semiconductor tunable laser 50 controlled by a board made of an electronic circuit was connected to the module by an optical fiber.

  Further, the wavelength λ is output as a digital signal from the board output driving the semiconductor tunable laser 50 to the bias circuit 41 of the semiconductor MZ modulator 1. The bias circuit 41 of the semiconductor MZ modulator 1 determines the bias voltage Vbias by referring to the input temperature T and wavelength λ from the arrangement stored in the memory table 42 in advance, and biases the semiconductor MZ modulator 1. The voltage Vbias is applied.

  Here, the semiconductor tunable laser 50 and the semiconductor MZ modulator 1 are formed by connecting modules that are separately packaged with fibers, but of course, they are housed in one package or formed on the same substrate. It is good. Further, although the semiconductor tunable laser 50 is used as a light source, a laser having another wavelength variable mechanism may be used.

FIG. 13 is a diagram illustrating the wavelength dependence of the modulation amplitude voltage in the optical transmitter according to the present embodiment.
In FIG. 13, the modulation amplitude voltage Vpai is plotted when the bias voltage Vbias = 12, the temperature T = 15 ° C. and the wavelength of the semiconductor tunable laser 50 is changed by an external input from 1530 to 1580 nm. is there.

  In the semiconductor MZ modulator 1 according to the present embodiment, the modulation amplitude voltage Vpai also depends on the wavelength, and the modulation amplitude voltage Vpai increases by 0.14 V each time the operating wavelength moves to the 10 nm long wave side. Yes. Thus, in the semiconductor MZ modulator 1, the modulation amplitude voltage Vpai fluctuates not only with respect to temperature but also with respect to the operating wavelength.

FIG. 14 is a diagram illustrating the bias voltage dependence of the modulation amplitude voltage in the optical transmitter according to the present embodiment.
In FIG. 14, the bias voltage Vbias dependency of the modulation amplitude voltage Vpai measured at an ambient temperature of −5 ° C. and a wavelength of 1560 nm, and the bias voltage Vbias dependency of the modulation amplitude voltage Vpai measured at an ambient temperature of 75 ° C. and a wavelength of 1530 nm. Is shown. Further, in FIG. 14, in order to avoid complication of the drawing, only low temperature and long wavelength, and high temperature and short wavelength, which are the most severe operating conditions, are shown.

  The modulation amplitude voltage Vpai is 1.3 V (indicated by a broken line in FIG. 14) when the environmental temperature is −5 degrees, the wavelength is 1560 nm, and the bias voltage Vbias is 16 V. This state corresponds to a state where the band gap is farthest from the operating wavelength when the bias voltage Vbias is not applied.

  On the other hand, when the ambient temperature is 75 ° C. and the wavelength is 1530 nm, the band gap is closest to the operating wavelength when the bias voltage Vbias is not applied. At this time, as shown in FIG. 14, when the bias voltage Vbias is 2V, the modulation amplitude voltage Vpai = 1.45V, and the modulation amplitude voltage when the bias voltage Vbias = 16V at the ambient temperature of −5 ° C. and the wavelength of 1560 nm. It exceeds Vpai = 1.3V.

  Therefore, if the bias voltage Vbias = 3.15V is set, the modulation amplitude voltage Vpai = 1.3V can be achieved when the ambient temperature is 75 ° C. and the wavelength is 1530 nm. Of course, if the temperature T and the wavelength λ are between them, the modulation amplitude voltage Vpai = 1.3V can be achieved by setting the bias voltage Vbias to a voltage between 3.15V and 16V.

  In other words, this result indicates that the temperature control element such as a Peltier element can be obtained by controlling the bias voltage Vbias in the wavelength range of the C band (1530 to 1560 nm) and between the environmental temperature of −5 ° C. and 75 ° C. This means that it is possible to drive with a constant modulation amplitude voltage Vpai without depending on temperature.

  In this way, by outputting the bias voltage Vbias stored in the memory table 42 based on the information on the temperature T and the wavelength λ, the Peltier driving at a constant modulation amplitude voltage Vpai can be performed regardless of the temperature T and the wavelength λ. Since it can be realized without a temperature adjusting element such as an element, power consumption can be reduced.

  In the above example, a three-dimensional array of temperature T, wavelength λ, and bias voltage Vbias is stored in the memory table 42 of the bias circuit 42 of the semiconductor MZ modulator 1, and semiconductor MZ modulation is performed by the input temperature T and wavelength λ. The bias voltage Vbias of the device 1 is determined. However, the semiconductor tunable laser 50 has an electronic storage medium (memory) that stores driving conditions of a target wavelength.

FIG. 15 is a block diagram illustrating another configuration of the optical transmitter according to the present embodiment.
As shown in FIG. 15, a three-dimensional array of the temperature T, wavelength λ, and bias voltage Vbias of the bias circuit 41 of the semiconductor MZ modulator 1 is stored in the memory table 52 on the semiconductor tunable laser 50 side, or the memory By sharing the table 52 with the bias circuit 42 side of the semiconductor MZ modulator 1, the electronic circuit can be simplified.

  As described above, the optical transmitter using the semiconductor MZ modulator 1 as in this embodiment, the semiconductor tunable laser 50, the laser driving circuit 51, the semiconductor MZ modulator 1, and the temperature sensor 43 as light sources. An optical transmitter having a bias circuit 41 to which the detected temperature T from the temperature sensor 43 and the oscillation setting wavelength λ from the laser driving circuit 51 are input, and memory tables 42 and 52 to which the bias circuit 41 refers to determine the temperature. In addition, driving with a constant amplitude voltage Vpai can be realized without a temperature adjusting element such as a Peltier element regardless of the temperature T and the wavelength λ, so that power consumption can be reduced.

  The present invention can be used for, for example, an optical transmitter including a semiconductor MZ modulator having a modulation region having an npin structure.

DESCRIPTION OF SYMBOLS 1 Semiconductor MZ modulator 10a, 10b 3 dB coupler 11a, 11b Arm waveguide 12a, 12b Modulation electrode 13a, 13b Balance adjustment electrode 14 Termination resistor 15 Capacitor 20 SI-InP substrate 21 1st n-type electrode layer 22 1st n Type cladding layer 23 first intermediate layer 24 core layer 25 second intermediate layer 26 first low-concentration cladding 27 p-type cladding layer 28 second n-type cladding layer 29 second n-type electrode layer 30 n-type electrode 31 Second n-type electrodes 40a, 40b Bias tee 41 Bias circuit 42 Memory table 43 Temperature sensor 44 Arithmetic circuit 50 Semiconductor tunable laser 51 Laser drive circuit 52 Memory table

Claims (5)

a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator,
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependency of the temperature and the bias voltage is obtained, and the relationship between the temperature and the bias voltage at which the modulation amplitude voltage is constant is obtained from the dependency,
When the input from the temperature sensor changes , the bias circuit changes the bias voltage to drive the semiconductor MZ modulator with a constant modulation amplitude voltage based on the relationship. . a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator;
An electronic storage medium for storing an input from the temperature sensor and an array of the bias voltage;
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependence of the temperature and the bias voltage is obtained, and from the dependence, the relationship between the temperature and the bias voltage that makes the modulation amplitude voltage constant is obtained, and the array of the electronic storage medium is obtained. Remember,
The bias circuit drives the semiconductor MZ modulator with a constant modulation amplitude voltage by changing the bias voltage with reference to the arrangement stored in the electronic storage medium based on an input from the temperature sensor. An optical transmitter characterized by. a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator;
An arithmetic circuit that calculates the bias voltage by a polynomial function of an input from the temperature sensor,
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependency of the temperature and the bias voltage is obtained, and the polynomial of the temperature and the bias voltage that makes the modulation amplitude voltage constant is derived from the dependency,
The optical transmitter, wherein the bias circuit drives the semiconductor MZ modulator by changing the bias voltage according to a calculation result in the calculation circuit and keeping a modulation amplitude voltage constant. A tunable laser;
a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator;
An electronic storage medium that stores an operating wavelength of the tunable laser, an input from the temperature sensor, and a three-dimensional array of the bias voltage;
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependency of the operating wavelength, the temperature, and the bias voltage is obtained, and the relationship between the operating wavelength, the temperature, and the bias voltage that makes the modulation amplitude voltage constant from the dependency, Stored in the three-dimensional array of the electronic storage medium;
The bias circuit drives the semiconductor MZ modulator with a constant modulation amplitude voltage by changing the bias voltage with reference to the three-dimensional array stored in the electronic storage medium according to a change in temperature or wavelength. And an optical transmitter. A tunable laser;
a semiconductor MZ modulator having a modulation region having an npin structure;
A temperature sensor for monitoring the temperature of the semiconductor MZ modulator;
A bias circuit for applying a bias voltage to the modulation electrode of the semiconductor MZ modulator;
An arithmetic circuit that calculates the bias voltage by a polynomial functioning an operation wavelength of the wavelength tunable laser and an input from the temperature sensor;
When the voltage applied to the modulation electrode is swept starting from the extinction point when the voltage applied to the modulation electrode of the semiconductor MZ modulator is zero, it is a positive / negative voltage that provides the first maximum transmission intensity. For a certain modulation amplitude voltage, the dependency of the operating wavelength, the temperature, and the bias voltage is obtained, and the operating wavelength, the temperature, and the bias voltage that make the modulation amplitude voltage constant are derived from the dependency. Every
The optical transmitter, wherein the bias circuit drives the semiconductor MZ modulator by changing the bias voltage according to a calculation result in the calculation circuit and keeping a modulation amplitude voltage constant.