JP5443314B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulator, and more particularly to an optical modulator including an LN modulator.

  An LN modulator, in which an optical waveguide is formed on a lithium niobate (LN) substrate using titanium (Ti) diffusion, is capable of high-speed modulation and is an important device in an optical communication system. For example, DQPSK for 40 Gbit / s Development of a modulator, a polarization multiplexing QPSK modulator for 100 Gbit / s, and the like is underway.

  On the other hand, the LN modulator has a problem of “DC drift”. A typical operating principle during intensity modulation of the LN modulator and a phenomenon called DC drift will be described below.

  FIG. 1 is an example of a typical LN modulator. Two optical waveguides 102 and 103 constituting a Mach-Zehnder interferometer (MZI) are formed on the LN substrate 101, and a high frequency (RF) electrode 104 and a DC bias adjusting electrode 105 are formed between them. Has been placed.

In FIG. 2 (see Non-Patent Document 1), the vertical axis of the two-dimensional coordinate axis represents the light output, and the horizontal axis represents the bias voltage. The modulation curve (transfer curve) of the LN intensity modulator is a function curve of cos square. Here, when V ₀ is added as a bias voltage, the light output becomes 0, and when V ₁ is added, the light output becomes maximum. However, these V ₀ and V ₁ are values when applied to the RF electrode 104 in FIG. 1, and when applied to the DC bias adjusting electrode 105, the phase change differs depending on the electrode length, so that different voltages are applied. Value. At the time of intensity modulation, the DC bias voltage V _B is set so that the operating point corresponding to the center of the maximum value and the minimum value of the electric digital signal coincides with the middle point of the cos square function as shown in FIG. . As a result, the electric digital signal is converted into an optical digital signal through the electro-optic effect, with the operating point in the figure as the center. The phenomenon called DC drift peculiar to the LN modulator is a phenomenon in which this modulation curve moves in the horizontal axis direction with time. As a result, the DC bias voltage also moves in the horizontal axis direction from the initial DC bias voltage V _B. The voltage Vπ in the figure corresponds to an applied voltage that changes the phase of the signal light propagating through the LN waveguide by a half wavelength.

  This DC drift is an inherent problem due to LN crystal defects and the like, and is known to be difficult to solve essentially. Therefore, at present, the long-term DC drift that increases as the operation time increases is designed to be a certain voltage or less, for example, ± 30 V or less, and the DC bias voltage applied to the DC bias adjustment electrode 105 is output. The light is monitored or used while being adjusted so as to be at the center of the modulation curve as shown in FIG.

  Specifically, a bias control circuit as shown in FIG. 3 (see Non-Patent Document 2) is provided, and the electronic feedback function is designed so that the operating point coincides with the midpoint of the function curve of cos square as described above. To do.

  In FIG. 3, since the RF electrode of the LN modulator is also used as the DC bias adjustment electrode, a bias T circuit is used. However, as shown in FIG. 1, the RF electrode 104 and the DC bias adjustment electrode are used. The electrode 105 may be divided. In that case, an electric signal is input independently from the driver and the DC bias application circuit without using the bias T circuit.

  Here, the characteristics of the DC bias voltage and the DC drift of the LN modulator will be described.

(1) The magnitude of the initial DC bias voltage V _B at the start of operation is within the voltage Vπ for changing the phase difference by π, and the value is caused by the phase error of both arms of the MZI at the time of manufacture. It is difficult to control this voltage V _B by making it to 0 V at the time of fabrication, and in reality, it becomes a random value between −Vπ and + Vπ.

(2) The DC bias voltage that fluctuates due to subsequent DC drift over time is divided into three stages as shown in FIG. 4 (see Non-Patent Document 4). The results shown in FIG. 4 were measured by the operating point locking method with the initial DC bias voltage V _B being 4.5 V and the subsequent DC bias voltage being continuously adjusted in an optimum state. The temperature is 130 ° C. The first stage ends with a large fluctuation in a relatively short time (see Non-Patent Documents 3 and 4). For example, in the example of FIG. 4, the initial bias voltage V _B is 4.5V, and when the first stage DC drift fluctuation amount V _B1 is added, a fluctuation of about 7V occurs in about 10 hours. This variation time varies considerably depending on the temperature and sample, and may end in a much shorter time. Here, it is known that the slope of the curve in FIG. 4, that is, the DC drift rate, is divided into three regions having different drift rates. The mechanism and the like have been discussed at academic societies and the like (see Non-Patent Document 3). The three regions having different DC drift rates are referred to as a first stage, a second stage, and a third stage of DC drift in this order.

(3) The DC bias voltage increases as the applied voltage increases. Therefore, as the initial bias voltage V _B and the first stage DC drift fluctuation amount V _B1 are larger, the subsequent DC drift becomes larger as shown in FIG. 5 (see Non-Patent Document 4). The results in FIG. 5 are obtained by measuring the DC drift when the temperature is 70 ° C. and the initial DC bias voltage is changed as a parameter. Note that FIG. 5 shows DC drift at 70 ° C., which is much lower than 130 ° C. in FIG. This variation corresponds to the first stage.

Sumitomo Osaka Cement Co., Ltd., “LN modulator”, page 2, [online], Internet <URL: http://www.socnb.com/product/hproduct/ln_appnote02.html> Sumitomo Osaka Cement Co., Ltd., “About LN modulator”, page 3, [online], Internet <URL: http: //www.socnb.com/product/hproduct/ln_appnote03.html> Miyazawa et al., “DC drift of Ti: LiNbO3 optical modulator”, IEICE, SA-9-3, 1994 H. Nagata, et. Al., ‘‘ Reliability of LiNbO3 based integrated optical waveguide devices for fiber communication systems, ’’ Mat. Res. Soc. Symp. Proc. 531, p.365. B. Malo, et. Al., ‘’ Enhanced photosensitivity in lightly doped standard telecommunication fiber exposed to high fluence ArF excimer laser light, ’Electron. Lett., Vol. 31, p. 879, 1995. J. Canning et al., Compensation of birefringence within integrated optical components using a CO2 laserElectron. Lettl vol.35 no.10 (1999) pp.812-814. T. Mizuno, et al., ‘’ Birefringence and path length adjustment of silica-based waveguide using permanent heater, ’Electron. Lettl vol.40 no.6 (2004) pp.371-372. Takuya Tanaka, “Study on external cavity laser with hybrid integration of UV-induced grating and LD”, University of Tokyo, Doctoral thesis, 2006 M. Abe, et al., ‘‘ Mach-Zehnder interferometer and arrayed-waveguide-grating integrated multi / demultiplexer with refractive wavelength tuning, ’’ Electronic Letters, Vol. 37, No. 6, pp. 376-377, 2001.

  The present invention has been made in view of such problems, and an object thereof is to reduce DC drift in an optical modulator including an LN modulator.

In order to achieve such an object, a first aspect of the present invention is an optical modulator including an LN waveguide and first and second PLCs connected to both ends of the LN waveguide. The LN waveguide and the first and second PLCs constitute an MZI having an upper arm and a lower arm, and a DC bias adjusting electrode is disposed in the LN waveguide, and the first or second PLC A phase trimming unit that trims the phase of the upper arm or the lower arm irreversibly so as to reduce the DC bias voltage is arranged in the PLC.

  In addition, according to a second aspect of the present invention, in the first aspect, the voltage applied to the LN waveguide by the phase trimming unit to change the phase difference between the upper arm and the lower arm by π. Is a refractive index adjusted to satisfy −Vπ / 10 <Vc <Vπ / 10, where Vπ is a DC bias voltage after DC drift in the first stage of the LN waveguide and Vc is Vc.

In addition, according to a third aspect of the present invention, in the first aspect, the voltage applied to the LN waveguide by the phase trimming unit to change the phase difference between the upper arm and the lower arm by π. Is a refractive index adjusted to satisfy −Vπ / 10 <V _B <Vπ / 10, where Vπ is an initial DC bias voltage when the MZI is operated as an intensity modulator and V _B. .

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the MZI is a nested MZI in which the upper arm and the lower arm each constitute an MZI.

  According to the present invention, in the PLC-LN modulator constituting the MZI, the DC drift is reduced by arranging the phase trimming unit that irreversibly trims the phase of the upper or lower arm of the MZI in the PLC portion. can do.

It is a figure which shows the example of a typical LN modulator. It is a figure for demonstrating operation | movement of the conventional LN modulator. It is a figure which shows the bias control circuit of the conventional LN modulator. It is a figure which shows the relationship between the DC bias voltage of the conventional LN modulator, and operation time. It is a figure which shows the relationship between the DC drift of the conventional LN modulator, and an initial stage DC bias voltage. 1 is a plan view showing an optical modulator according to a first embodiment of the present invention. It is a perspective view for demonstrating the connection relation of the optical modulator which concerns on the 1st Embodiment of this invention. It is a figure which shows the example of UV light irradiation time dependence of refractive index change (DELTA) n in quartz type PLC. It is a figure which shows the example which provided the phase trimming part with the QPSK modulator using the PLC-LN integration technique. It is a figure which shows the example of the PLC-LN modulator which took in a part of Y branch. It is a figure which shows the example of the DP-QPSK modulator using a PLC-LN integration technique.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 6 is a plan view showing the optical modulator according to the first embodiment of the present invention, and FIG. 7 is a perspective view for explaining the connection relationship of the optical modulator according to the present embodiment. The optical modulator 600 is a PLC-LN modulator in which first and second PLCs 602 and 603 are butt-connected to both ends of the LN modulator 601 (butt joint). An optical signal input side optical fiber 605 is connected to the input port of the first PLC 602 via the first fiber block 604, and the output port of the second PLC 603 is connected to the input port of the second PLC 603 via the second fiber block 606. The optical signal output side optical fiber 607 is connected. In FIG. 7, it is Yatoi that is arranged on the upper surfaces of the LN modulator 601, the first and second PLCs 602, 603 in the vicinity of the end faces. Here, “PLC” refers to a quartz lightwave circuit in which an optical waveguide mainly composed of SiO ₂ glass is formed on a Si substrate. In general, an LN modulator has a larger propagation loss and allowable bending radius than a PLC, and is not suitable for a complicated optical circuit configuration such as a variable coupler, loopback, and polarization synthesis circuit. However, the LN modulator and the PLC are combined. The PLC-LN modulator can utilize the characteristics of the excellent passive circuit of the PLC while maintaining the excellent characteristics of the LN modulator. For example, the entire circuit can be reduced in size or the entire loss can be reduced. Note that is possible.

  The optical modulator 600 actively aligns the optical waveguide of the LN modulator 601 and the optical waveguides of the first and second PLCs 602 and 603 at the end faces and connects them with UV adhesives, and then connects the first and It can be manufactured by connecting the second fiber blocks 604 and 606. If necessary, the produced PLC-LN-PLC chip is bonded and fixed to the module package.

The optical modulator 600 according to this embodiment includes a phase trimming unit 610 that irreversibly trims the phase of the upper or lower arm of the MZI in the second PLC 603 in order to reduce the DC drift of the LN modulator 601. Have Inventors of the present invention, not the single LN modulator but the PLC-LN modulator, use the phase adjustment function of the PLC portion to perform phase trimming, thereby obtaining the value of the initial DC bias voltage V _B or the initial DC bias. It has been found that the total value of the voltage V _B and the first stage DC drift variation V _B1 can be reduced. As a result, long-term drifts such as the second stage and the third stage described with reference to FIG. 4 can be reduced as compared with the conventional LN modulator. Hereinafter, the trimming unit 610 will be described in detail.

It is difficult to precisely match the phase between the upper and lower arms of the MZI included in the PLC-LN modulator. As described above, the initial DC bias voltage V _B is a value of −Vπ <V _B <+ Vπ from FIG. It becomes. For example, when Vπ = 4.0V, −4.0V <V _B <4.0V. In the above description of the DC drift, the LN modulator has been described, but the problem is similar in the PLC-LN modulator. This is because a DC drift due to the LN substrate portion exists in the same manner, but no new DC drift component is generated in the other portions, the PLC, or the connection portion between the PLC and the LN waveguide.

  In manufacturing the trimming unit 610, first, the first stage DC drift described with reference to FIG. 4 is caused. For example, in a state where the temperature is heated to 90 ° C., it is driven by an electric digital signal as described with reference to FIG. 2, and a bias control circuit is set so that the DC bias voltage at the center becomes the operating point of FIG. While continuing the adjustment, it is driven for a certain time until the first stage is completed. Note that the end of the first stage is determined by a change in the drift rate, which is the slope of the DC drift.

Next, the second PLC 603 is provided with a phase trimming unit 610 for trimming the DC drift generated by the driving for the predetermined time. The methods include (1) a method using UV light irradiation (see Non-Patent Document 5), (2) a method of irradiating a CO ₂ laser (see Non-Patent Document 6), and (3) a heater loaded on an optical waveguide. The heater trimming method (refer to Non-Patent Document 7) in which a current is passed through can be used.

  In the methods (1) and (2), a method has already been established in which the phase trimming portion 610 is formed by irradiating a part of the optical waveguide of the second PLC 603 with a laser. This is possible with a normal optical waveguide, and no special structure is required. In the method (3), the refractive index of the optical waveguide can be changed irreversibly by providing a heater for heating the optical waveguide in the vicinity of the optical waveguide, thereby changing the phase. Both are already well-proven technologies. The optical modulator according to the present invention constitutes a PLC-LN modulator instead of a single LN modulator and performs phase trimming in the PLC portion to reduce DC drift generated in the single LN modulator. Such a known technique may be used as a method for manufacturing the phase trimming unit 610.

Taking the method of (1) using UV light irradiation as an example, a method for manufacturing the phase trimming unit 610 will be described in detail. First, when an ArF excimer laser (wavelength: 193 nm) of 100 mJ / pulse is condensed to 1.1 J / cm ² by a cylindrical lens and irradiated to the optical waveguide at a pulse light repetition rate of 20 pulses / sec, the effective refractive index of the PLC is increased. , About 10 ⁻³ variation is known (Non-Patent Documents 5 and 8). Therefore, for example, by irradiating this UV light onto an optical waveguide having a length of 5 mm, a change of 4π or more in phase at a wavelength of 1.55 μm can be given. As a result, the DC bias voltage can be reduced. In order to reverse the direction of change of the DC bias voltage, that is, to reverse the direction of adjustment of the phase between the arms, the arm provided with the phase trimming unit 610 may be the other.

  As described above, it is possible to give a phase change of ± 2π. As a result, the refractive index of the phase trimming unit is adjusted by monitoring the output light or a part of the output light normally performed in the LN modulator, and the DC bias voltage Vc after the first stage is set to −Vπ / 10 <. It is possible to satisfy Vc <Vπ / 10. For example, for a PLC-LN modulator with Vπ = 4.0V, the DC bias voltage Vc can be 0.4V or less. In principle, it is desirable to make Vc close to 0, and from the viewpoint of mounting, it is desirable to make Vπ / 5 or less, more preferably Vπ / 10 or less.

Alternatively, Non-Patent Document 9 shows experimental values of refractive index changes in each of the TE mode and the TM mode. In the quartz-based PLC, the refractive index Δn of each of the TE mode and the TM mode changes as shown in FIG. 8 by irradiating UV light having a wavelength of 193 nm with an irradiation power of 125 mJ / cm ² / pulse and a repetition rate of 20 pps. . Here, considering the TM mode used in the LN phase modulator, for example, when UV light is irradiated for 200 minutes on a quartz PLC of 10 mm under the above-described conditions, Δn = 2.5 × 10 ⁻⁴ , so that the wavelength 1 The amount of phase change received by an optical signal of .5 μm is about 10. When UV light is irradiated to the phase shifter on the opposite side of the MZI, the phase can be changed by about 10 in the opposite direction. This means that a phase change of 2π or more can be given both in the positive direction and the negative direction as the amount of phase change caused by UV light irradiation. On the other hand, as is clear from FIG. 8, the phase continuously changes due to the UV light irradiation. From the above, it is theoretically possible to adjust the DC bias voltage, which is the operating point in FIG. 2, to 0 V by irradiating UV light while monitoring the light output light intensity of MZI.

In the above, the example in which the DC bias voltage Vc after the first stage is reduced by phase trimming is shown. However, even if only the initial DC bias voltage V _B at the start of operation is reduced by phase trimming, There is an effect of reducing long-term DC drift thereafter.

As described above, the UV trimming of (1) is taken as an example, and the reduction of the DC bias voltage by the phase trimming has been described in detail. (2) Phase trimming by irradiating the CO ₂ laser, (3) Loading to the PLC The same effect can be obtained by a heater trimming method in which a current is supplied to the heater.

  In the present embodiment, an example of a simple MZI intensity modulator has been described. This is a basic concept and is not restricted by the voltage of a fine operating point or the configuration of the modulator. For example, the voltage at the operating point of the bias control may correspond to the peak or bottom of the modulation curve in FIG.

(Second Embodiment)
The phase trimming unit of the first embodiment can also be applied to various multilevel modulators. This is because these multilevel modulators are basically constituted by a combination of interferometers such as MZI.

  In FIG. 9, DC drift due to LN exists in each of the I modulator, Q modulator, and nested MZI section. Therefore, the same DC bias voltage may be adjusted in the three MZIs.

  Here, the phase adjustment of the nested MZI section is not the adjustment of the modulation DC drift voltage, but the purpose of shifting the phase of the I modulator and the Q modulator by π / 2, but as a result, the phase adjustment is necessary. Are the same because they are the same.

  Since the phase of the arm of the nested MZI section can be adjusted also in the I modulator and Q modulator sections, the phase trimming of the nested MZI section can be performed by each phase trimming section of the I modulator and Q modulator. It is. At that time, the phase trimming portion of the nested MZI portion is not necessary.

  Further, this structure can be extended to a PLC-LN modulator that can obtain the same effect. For example, a PLC-LN modulator in which a part of the Y branch as shown in FIG. 10 is incorporated in the LN waveguide is also possible, but at that time, it can be applied if a phase trimming unit is provided on the PLC.

  Further, the present invention can be similarly applied to a 100G DP-QPSK modulator as shown in FIG. 11, which is promising as a modulator for a digital coherent communication system.

600 optical modulator 601 LN waveguide 602 first PLC
603 2nd PLC
604 First fiber block 605 Optical signal input side optical fiber 606 Second fiber block 607 Optical signal output side optical fiber 608 High frequency (RF) electrode 609 DC bias adjustment electrode 610 Phase trimming unit

Claims (4)

An LN waveguide;
An optical modulator comprising: first and second PLCs butt-connected to both ends of the LN waveguide,
The LN waveguide and the first and second PLCs constitute an MZI having an upper arm and a lower arm,
A DC bias adjusting electrode is disposed in the LN waveguide,
An optical modulator comprising: a phase trimming unit configured to irreversibly trim the phase of the upper arm or the lower arm so that a DC bias voltage is reduced in the first or second PLC . The phase trimming unit sets a voltage applied to the LN waveguide to change a phase difference between the upper arm and the lower arm by Vπ, and a DC after DC drift in the first stage of the LN waveguide. Assuming that the bias voltage is Vc,
−Vπ / 10 <Vc <Vπ / 10
The optical modulator according to claim 1, wherein the refractive index is adjusted so as to satisfy The phase trimming unit has an initial DC bias when operating the voltage applied to the LN waveguide as Vπ and the MZI as an intensity modulator in order to change the phase difference between the upper arm and the lower arm by π. Let V _{B be the} voltage
−Vπ / 10 <V _B <Vπ / 10
The optical modulator according to claim 1, wherein the refractive index is adjusted so as to satisfy   The optical modulator according to any one of claims 1 to 3, wherein the MZI is a nested MZI in which the upper arm and the lower arm each constitute an MZI.

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