JP2013050677A - Measuring method of optical interference element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measuring method of an optical interference element capable of obtaining optical characteristics of an arm.SOLUTION: An optical interference element comprises: an input coupler; first and second semiconductor arms connected to the input coupler; and an output coupler for interfering with output of the first and second semiconductor arms. A measuring method of the optical interference element includes the steps of: sweeping a bias voltage of the second semiconductor arm and obtaining an output state of the output coupler after causing a state of the first semiconductor arm to be switched to a light transmission state; and sweeping the bias voltage of the second semiconductor arm in a state that a bias voltage causing the first semiconductor arm to generate optical absorption characteristics greater in comparison with those in the light transmission state is applied thereto and obtaining the output state of the output coupler.

Description

  The present invention relates to a method for measuring an optical interference element.

  There is known an optical interference element including an input coupler, an output coupler, and a plurality of arms interposed between the input coupler and the output coupler (see, for example, Patent Document 1).

JP 2011-069911 A

  Ideally, the arms of the optical interference element have exactly the same optical length, but actually there are individual differences due to manufacturing variations. Obtaining the optical characteristics of the arm by grasping such variation contributes to simplification of man-hours for setting the driving condition of the optical interference element.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for measuring an optical interference element that can obtain the optical characteristics of an arm.

  An optical interference element measuring method according to the present invention includes an input coupler, first and second semiconductor arms connected to the input coupler, and an output coupler that interferes with outputs of the first and second semiconductor arms. A method of measuring an optical interference element comprising: a step of sweeping a bias of the second semiconductor arm after the first semiconductor arm is in a light transmitting state, and obtaining an output state of the output coupler; Sweeping the bias of the second semiconductor arm while applying a bias that produces a light absorption characteristic larger than that of the light transmitting state to the first semiconductor arm to obtain an output state of the output coupler; It is characterized by including. According to the method for measuring an optical interference element according to the present invention, the optical characteristics of the arm can be obtained.

  The measurement method further includes a step of sweeping a bias of the first semiconductor arm after the second semiconductor arm is set in a light transmission state to obtain an output state of the output coupler, and the second semiconductor arm. Performing a step of sweeping a bias of the first semiconductor arm in a state in which a bias generating a light absorption characteristic larger than that in the light transmission state is applied, and obtaining an output state of the output coupler, Obtaining a difference in optical characteristics of the first and second semiconductor arms using the output state of the output coupler obtained in the step. The method may further include a step of obtaining an initial bias value for reducing a difference in optical characteristics between the first and second semiconductor arms.

  In the measurement method, based on a difference in optical characteristics between the first and second semiconductor arms, a DATA signal is input to the first semiconductor arm and a differential signal between the DATA signal and the second semiconductor arm is obtained. When a BAR signal having a relationship is input, the phase difference between the light that has passed through the first semiconductor arm and the light that has passed through the second semiconductor arm when the DATA signal is ON is 2nπ, and the DATA signal The center bias of the DATA signal and the BAR signal is acquired so that the phase difference between the light that has passed through the first semiconductor arm and the light that has passed through the second semiconductor arm becomes (2n + 1) π at OFF input. The method may further include a step of:

  The optical interference element may be a Mach-Zehnder modulator. The light absorption property may be a light absorption rate of 20 dB or more compared to the light transmission state. The light transmission state may be realized by setting the arm to a non-biased state.

  Another method of measuring an optical interference element according to the present invention is to interfere with an input coupler, a plurality of semiconductor arms including first and second semiconductor arms connected to the input coupler, and outputs of the plurality of semiconductor arms. An output coupler comprising: an output coupler for measuring an optical interference element, wherein the first semiconductor arm is set in a light transmitting state and arms other than the first and second semiconductor arms are set in a light absorbing state. After sweeping the bias of the second semiconductor arm to obtain the output state of the output coupler, and after allowing the arm other than the first semiconductor arm and the first and second arms to be in a light absorbing state. Sweeping the bias of the second semiconductor arm to obtain the output state of the output coupler. According to another measuring method of the optical interference element according to the present invention, the optical characteristics of the arm can be obtained.

  Another method for measuring an optical interference element according to the present invention is an optical interference element comprising: an input coupler; a plurality of semiconductor arms connected to the input coupler; and an output coupler that interferes with outputs of the plurality of semiconductor arms. The bias of the first semiconductor arm is applied in a state where a bias that generates light absorption characteristics is applied to all of the semiconductor arms other than the first semiconductor arm among the plurality of semiconductor arms. Sweeping to obtain an output state of the output coupler; and a second semiconductor arm of the plurality of semiconductor arms in an unbiased state, and a first semiconductor arm and a second of the plurality of semiconductor arms. Sweeping the bias of the first semiconductor arm in a state in which a bias generating a light absorption characteristic is applied to all other semiconductor arms except the semiconductor arm of And obtaining the output state of the serial output coupler, characterized in that it comprises a. According to another measuring method of the optical interference element according to the present invention, the optical characteristics of the arm can be obtained.

  Another measuring method of the optical interference element according to the present invention includes an input coupler, first and second semiconductor arms connected to the input coupler, and an output for causing interference between outputs of the first and second semiconductor arms. A method of measuring an optical interference element comprising: a coupler; wherein the bias of the second semiconductor arm is swept in a state where a bias that generates a light absorption characteristic is applied to the first semiconductor arm, and the output of the output coupler A state of obtaining a state, and applying a bias to the first semiconductor arm that realizes a state with a loss of 20 dB or more less than when a bias that generates the light absorption characteristic is applied. Sweeping a bias to obtain an output state of the output coupler. According to another measuring method of the optical interference element according to the present invention, the optical characteristics of the arm can be obtained.

  According to the method for measuring an optical interference element according to the present invention, the optical characteristics of the arm can be obtained.

(A) is an example of the upper surface schematic diagram of an optical interference element, (b) is an example of the cross-sectional schematic diagram between AA of (a), (c) is between BB of (a). It is an example of a cross-sectional schematic diagram. It is a figure which shows the relationship between the reverse bias applied to a semiconductor arm, and the optical intensity of the light which passed the semiconductor arm. (A)-(d) is an example which shows the relationship between output light intensity and the swept reverse bias. It is a figure which shows the example of calculation of the relationship between the voltage applied to each semiconductor arm, and a phase. It is a figure which shows an example of the flow at the time of obtaining the difference of the optical characteristic of both semiconductor arms. (A)-(c) is a figure for demonstrating the logic of the output light by the phase difference of DATA side light and BAR side light.

  Hereinafter, modes for carrying out the present invention will be described.

  FIG. 1A is an example of a schematic top view of the optical interference element 100 that is a target of the measurement method according to the first embodiment. In this embodiment, the optical interference element 100 is a Mach-Zehnder modulator as an example. As shown in FIG. 1A, the optical interference element 100 is configured by combining paths of mesa-shaped optical waveguides on a semiconductor substrate. In FIG. 1A, each optical waveguide is seen through. 1B is an example of a schematic cross-sectional view between AA in FIG. 1A, and FIG. 1C is an example of a schematic cross-sectional view between BB in FIG. 1A. is there.

  With reference to FIG. 1B, the optical waveguide is formed on a semiconductor substrate 41. The optical waveguide has a structure in which a lower cladding layer 42a, a core 43, and an upper cladding layer 42b are stacked in this order on a semiconductor substrate 41. A passivation film 44 and an insulating film 45 are sequentially stacked on the upper surface of the semiconductor substrate 41 and the upper surface and side surfaces of the optical waveguide.

  The semiconductor substrate 41 is made of a semiconductor such as InP. The lower cladding layer 42a and the upper cladding layer 42b are made of a semiconductor such as InP. The core 43 is made of a semiconductor having a band gap energy smaller than that of the lower cladding layer 42a and the upper cladding layer 42b, and is an InGaAsP bulk layer, an AlGaInAsP quantum well structure layer, or the like. The light passing through the core 43 is confined by the lower cladding layer 42a and the upper cladding layer 42b. The passivation film 44 is made of a semiconductor such as InP. The insulating film 45 is made of an insulator such as SiN.

  Referring to FIG. 1A, the optical interference element 100 is provided with a first input optical waveguide 32a connected to the first input end 31a, and a second input optical waveguide connected to the second input end 31b. 32b is provided. The first input optical waveguide 32a and the second input optical waveguide 32b join at the input coupler 33, and branch to the first semiconductor arm 34a and the second semiconductor arm 34b. When the longitudinal direction of the optical interference element 100 is the axis of symmetry, the first semiconductor arm 34a is disposed on the same side as the first input end 31a, and the second semiconductor arm 34b is disposed on the same side as the second input end 31b. ing. In the present embodiment, the input coupler 33 is a 2 × 2 MMI (Multi Mode Interference).

  The first semiconductor arm 34a and the second semiconductor arm 34b join at the output coupler 35, and the first output optical waveguide 36a connected to the first output end 37a and the second output optical waveguide connected to the second output end 37b. Branch to 36b. When the longitudinal direction of the optical interference element 100 is the axis of symmetry, the first output end 37a is disposed on the same side as the second semiconductor arm 34b, and the second output end 37b is disposed on the same side as the first semiconductor arm 34a. ing. In this embodiment, the output coupler 35 is a 2 × 2 MMI.

  Each of the first semiconductor arm 34a and the second semiconductor arm 34b is provided with a phase adjustment electrode 46 and a modulation electrode 47. The phase adjustment electrode 46 and the modulation electrode 47 are separated from each other. The positional relationship between the phase adjustment electrode 46 and the modulation electrode 47 is not particularly limited, but in the present embodiment, the phase adjustment electrode 46 is disposed closer to the light input end than the modulation electrode 47. .

  Referring to FIG. 1C, the modulation electrode 47 is disposed on the upper cladding layer 42b via the contact layer 49. The contact layer 49 is made of a semiconductor such as InGaAs. Note that the passivation film 44 and the insulating film 45 are not provided between the upper cladding layer 42 b and the contact layer 49. The phase adjustment electrode 46 and the modulation electrode 47 are made of a metal such as Au. Similarly to the modulation electrode 47, the phase adjustment electrode 46 is also disposed on the upper cladding layer 42 b via the contact layer 49.

  When a voltage is applied to each phase adjustment electrode 46 and each modulation electrode 47, the refractive index of the core 43 changes in the first semiconductor arm 34a and the second semiconductor arm 34b, and the first semiconductor arm 34a and the second semiconductor The phase of the light passing through the arm 34b changes. When the optical interference element 100 is used as a modulator, differential signals (DATA signal and BAR signal) are input to each modulation electrode 47, and light that has passed through the first semiconductor arm 34a is input to each phase adjustment electrode 46. And a DC voltage for adjusting the phase difference between the light passing through the second semiconductor arm 34b.

  In using the optical interference element 100, it is necessary to perform cross point adjustment. In normal cross point adjustment, when 2 × 2 MMI is used for the output coupler, the bias voltage to each phase adjustment electrode 46 is adjusted so that the two output light intensities of the output coupler are equal. Specifically, the bias voltage for each phase adjustment electrode 46 is changed while detecting the two output light intensities of the output coupler. First, the phase adjustment electrodes of both arms are set to zero bias, and the arm to be adjusted is determined by obtaining the magnitude relationship between the two output light intensities obtained at that time. Adjust the value. In other words, since normal cross point adjustment is tuning while actually measuring, it takes time to increase accuracy. In this embodiment, a measurement method that facilitates initial state tuning such as cross-point adjustment by efficiently obtaining the optical characteristics of the arm will be described.

  By setting the voltage applied to the first semiconductor arm 34a to 0 V (no bias) and sweeping the voltage applied to the second semiconductor arm 34b, the light passing through the first semiconductor arm 34a and the second semiconductor arm 34b are Either the first output terminal 37a or the second output terminal 37b is obtained by mutually adding the interference effect due to the phase difference of the light that has passed and the effect of the change in electric field amplitude due to the loss caused by the voltage application. The light output from one side changes.

  On the other hand, by applying a sufficiently large reverse bias to the first semiconductor arm 34a so that the first semiconductor arm 34a absorbs light and sweeping the voltage applied to the second semiconductor arm 34b, the interference effect of the optical interference element 100 is obtained. Is eliminated. Thereby, the relationship between the applied voltage and the loss applied to the second semiconductor arm 34b is obtained in the light output from either the first output end 37a or the second output end 37b of the optical interference element 100.

  By comparing the relationship obtained in these two steps, the relationship between the applied voltage and the loss in the first semiconductor arm 34a, the relationship between the applied voltage and the phase, and the like can be obtained without assuming the fitting function. By performing the same two steps for the second semiconductor arm 34b, the relationship between the applied voltage and the loss in the second semiconductor arm 34b, the relationship between the applied voltage and the phase, etc. can be obtained without assuming a fitting function. can get. Further, by comparing the relationships obtained in these four steps, the contribution ratio from each arm can be calculated. Hereinafter, description will be made using specific mathematical expressions.

First, among the optical outputs from one of the first output end 37a and the second output end 37b of the optical interference element 100, the electric field E ₁ (V ₁ ) output through the first semiconductor arm 34a is as follows. It is expressed as equation (1). In the formula (1), V ₁ represents a voltage applied to the first semiconductor arm 34a. E ₀₁ represents the electric field amplitude when V ₁ = 0V. α ₁ (V ₁ ) represents a loss when the voltage V ₁ is applied to the first semiconductor arm 34a. Therefore, α ₁ (0V) = 0. θ ₁ (V ₁ ) represents the amount of phase change when the voltage V ₁ is applied to the first semiconductor arm 34a. Therefore, θ ₁ (0V) = 0.

Similarly, the electric field E ₂ (V ₂ ) output through the second semiconductor arm 34b is expressed as the following formula (2). In the formula (2), _{V 2} represents a voltage applied to the second semiconductor arm 34b. E ₀₂ represents the electric field amplitude when V ₂ = 0V. α ₂ (V ₂ ) represents a loss when the voltage V ₂ is applied to the second semiconductor arm 34b. Therefore, α ₂ (0V) = 0. θ ₂ (V ₂ ) represents the amount of phase change when the voltage V ₂ is applied to the second semiconductor arm 34b. Therefore, θ ₂ (0V) = 0.

Any of the above, the first semiconductor arm 34a of the voltages _{V 1} is applied to and the first output terminal 37a or the second output terminal 37b of the optical interference element 100 when the voltage _{V 2} is applied to the second semiconductor arm 34b The electric field E _out (V ₁ , V ₂ ) output from one of the _two is represented by the following formula (3). Therefore, the output light intensity P _out (V ₁ , V ₂ ) of the optical interference element 100 when the voltage V ₁ is applied to the first semiconductor arm 34 a and the voltage V ₂ is applied to the second semiconductor arm 34 b is expressed by the following equation: It is expressed as (4).

From the above, in order to inspect the characteristics of the optical interference element 100, E ₀₁ , α ₁ (V ₁ ), E ₀₂ , α ₂ (V ₂ ), and θ ₁ (V ₁ ) −θ ₂ (V _It is only necessary to grasp the five parameters of ₂ ). By applying a reverse bias to the semiconductor arm, the semiconductor arm can absorb light. FIG. 2 shows the relationship between the reverse bias applied to the semiconductor arm and the light intensity of light passing through the semiconductor arm. As shown in FIG. 2, by applying a sufficiently large reverse bias (for example, −10 V) to the semiconductor arm, light absorption in the semiconductor arm can be reduced to about −30 dB. Therefore, when a sufficiently large reverse bias (V ₁ = −10 V) is applied to the first semiconductor arm 34a, it can be approximated as exp (−α ₁ (V ₁ )) ≈0. Therefore, P _out (−10V, V ₂ ) can be expressed as the following formula (5).

From the above, regarding E ₀₂ and α ₂ (V ₂ ) among the above five parameters, the first output terminal when V ₁ is fixed to −10V and V ₂ is swept from 0V to −10V. Using the measured value P _out (−10 V, V ₂ ) of the light output from either one of 37a or the second output end 37b, it can be expressed as the following formula (6) and the following formula (7).

Similarly, for E ₀₁ , α ₁ (V ₁ ), the measured value P _out (V ₁ , -10V) of the optical output when V ₂ is fixed to −10V and V ₁ is swept from 0V to −10V. Can be represented as the following formula (8) and the following formula (9).

If the above formulas (6) to (9) are used, the remaining θ ₁ (V ₁ ) −θ ₂ (V ₂ ) among the five parameters uses the measured value P _out (V ₁ , V ₂ ). The following equation (10) can be expressed.

θ ₁ (V ₁ ) can be derived from the measured value by setting V ₂ to 0 V (no bias) and sweeping V ₁ (for example, 0 V to −10 V). θ ₂ (V ₂ ) can be derived from the measured value by setting V ₁ to 0 V (no bias) and sweeping V ₂ (for example, 0 V to −10 V). Therefore, the six parameters (E ₀₁ , α ₁ (V ₁ ), E ₀₂ , α ₂ (V ₂ ), θ ₁ (V ₁ ) that determine the characteristics of the optical interference element 100 are assumed without assuming a fitting function. , Θ ₂ (V ₂ )) can be obtained from the measured value of the output light intensity from one of the first output end 37a and the second output end 37b of the optical interference element 100. Therefore, the characteristics of the optical interference element 100 can be measured using actual measurement values.

From the above, the measurement values to be acquired in the present embodiment are the following four types.
(1) The optical interference element 100 when a sufficiently large reverse bias (for example, about −10 V) is applied to the first semiconductor arm 34 a and the voltage applied to the second semiconductor arm 34 b is swept (for example, 0 V to −10 V). Output light intensity from one of the first output end 37a and the second output end 37b.
(2) The first output terminal of the optical interference element 100 when a voltage of 0 V is applied to the first semiconductor arm 34a (no bias) and the voltage applied to the second semiconductor arm 34b is swept (for example, 0 V to −10 V). The output light intensity from either one of 37a or the second output end 37b.
(3) The optical interference element 100 when a sufficiently large reverse bias (for example, about −10 V) is applied to the second semiconductor arm 34 b and the voltage applied to the first semiconductor arm 34 a is swept (for example, 0 V to −10 V). Output light intensity from one of the first output end 37a and the second output end 37b.
(4) The first output terminal of the optical interference element 100 when a voltage of 0 V is applied to the second semiconductor arm 34b (no bias) and the voltage applied to the first semiconductor arm 34a is swept (for example, 0 V to −10 V). The output light intensity from either one of 37a or the second output end 37b.

  FIG. 3A to FIG. 3D are examples showing the relationship between the output light intensity of the above (1) to (4) and the swept reverse bias. 3A corresponds to the above (1), FIG. 3B corresponds to the above (2), FIG. 3C corresponds to the above (3), and FIG. Corresponds to 4). From the above four measured values, the relationship between the voltage applied to each semiconductor arm and the phase is calculated. FIG. 4 is an actual calculation example. Thus, when the relationship between the voltage applied to each semiconductor arm and the phase is obtained, a difference in optical characteristics between the two semiconductor arms can be obtained. By obtaining this difference in optical characteristics, it becomes easy to set a bias when the optical interference element 100 is differentially driven. For example, by acquiring an initial bias value for reducing the difference in the optical characteristics, cross-point adjustment is facilitated.

  FIG. 5 is a diagram illustrating an example of a flow for obtaining a difference in optical characteristics between the two semiconductor arms. As shown in FIG. 5, first, light is input to the first input end 31a of the optical interference element 100 (step S1). Next, a sufficiently large reverse bias (eg, about −10V) is applied to the phase adjustment electrode 46 of the first semiconductor arm 34a, and the voltage applied to the phase adjustment electrode 46 of the second semiconductor arm 34b is swept (eg, 0V). ˜−10 V), the output light intensity from one of the first output end 37a and the second output end 37b of the optical interference element 100 is measured (step S2). Next, the first output end 37a of the optical interference element 100 when the phase adjustment electrode 46 of the first semiconductor arm 34a is set to no bias and the voltage applied to the second semiconductor arm 34b is swept (for example, 0 V to −10 V). Alternatively, the output light intensity from one of the second output ends 37b is measured (step S3).

  Next, a sufficiently large reverse bias (eg, about −10V) is applied to the phase adjustment electrode 46 of the second semiconductor arm 34b, and the voltage applied to the phase adjustment electrode 46 of the first semiconductor arm 34a is swept (eg, 0V). ˜−10 V), the output light intensity from one of the first output end 37a and the second output end 37b of the optical interference element 100 is measured (step S4). Next, the optical interference element 100 when the phase adjustment electrode 46 of the second semiconductor arm 34b is set to no bias and the voltage applied to the phase adjustment electrode 46 of the first semiconductor arm 34a is swept (for example, 0 V to −10 V). The output light intensity from one of the first output end 37a and the second output end 37b is measured (step S5). Next, the optical characteristics of both semiconductor arms are obtained (step S6).

  In the above, the light transmission state is realized by making the arm non-biased. However, the light transmission state can also be realized by applying a certain amount of bias. For example, as shown in FIGS. 3A and 3B, the loss of the arm is almost the same between the case where -5 V is applied and the case where no bias is applied (0 V), and the side where the bias is applied. The difference in loss with the arm (−10V) is sufficiently large. Therefore, the desired effect of the present invention can be obtained even if a certain amount of reverse bias (for example, about −5 V) is applied instead of no bias.

  By the above procedure, the optical characteristics of the first semiconductor arm 34a and the second semiconductor arm 34b can be obtained. Further, by using the difference between the optical characteristics of the first semiconductor arm 34a and the optical characteristics of the second semiconductor arm 34b, it becomes easy to set a bias when the optical interference element 100 is differentially driven.

  Here, the logic of the output light based on the phase difference between the DATA side light and the BAR side light will be described. FIG. 6A shows the DATA signal. The DATA signal is a signal that repeats DATA ON (High) and DATA OFF (Low). The BAR signal has a differential relationship with the DATA signal. Therefore, when High is input to one semiconductor arm, Low is input to the other semiconductor arm, and when Low is input to one semiconductor arm, High is input to the other semiconductor arm. In this embodiment, the DATA signal is input to the modulation electrode 47 of the first semiconductor arm 34a, and the BAR signal is input to the modulation electrode 47 of the second semiconductor arm 34b. The light output from the first output end 37a is used as output light.

  FIG. 6B shows the phase change of the semiconductor arm to which the DATA signal is input and the phase change of the semiconductor arm to which the BAR signal is input. In FIG. 6B, the solid line represents the phase change corresponding to the DATA signal, and the broken line represents the phase change corresponding to the BAR signal. FIG. 6C shows the output light intensity output from the first output end 37 a of the optical interference element 100.

  As shown in FIGS. 6B and 6C, in order to make the logic of the input signal and the optical output coincide with each other, the light of the DATA signal input side arm and the light of the BAR signal input side arm when DATA ON is input. The phase difference between the light of the DATA signal input arm and the light of the BAR signal input arm must be (2n + 1) π (n is an integer) when DATA OFF is input. . Thus, in order to obtain a logical match, it is necessary to appropriately set the center bias of both arms. In the example of FIG. 6B, the phase difference between the light passing through the first semiconductor arm 34a and the light passing through the second semiconductor arm 34b at the time of DATA ON input is 0, and the phase difference at the time of DATA OFF input. This shows a case where the logic of the input signal and the optical output match when π becomes π.

Therefore, the drive amplitude voltage V _pp of the semiconductor arm constant, an example of obtaining a good eye pattern by setting each of the bias of the arms. The DATA signal ON level voltage V _{DATA_ON} and the BAR signal ON level voltage V _{BAR_ON} in the case of differential driving are the amplitude voltage V _pp , the DATA signal center bias voltage V _DATA, and the BAR signal center bias voltage V _BAR . Can be expressed as the following formula (11) and the following formula (12).

Similarly, the voltage _{V BAR_OFF} the OFF level OFF level voltage _{V DATA_OFF} and BAR signal DATA signal can be expressed by the following equation (13) and the following equation (14).

If the phase of light after passing through the semiconductor arm to which the DATA signal is applied is θ _DATA (V) and the phase of light after passing through the semiconductor arm to which the BAR signal is applied is θ _BAR (V), then ON The phase of the level can be expressed by the following formula (15) and the following formula (16), respectively.

Since the output light of the optical interference element 100 is strengthened at the ON level, θ _DATA (V _{DATA_ON} ) = θ _BAR (V _{BAR_ON} ). In this embodiment, the ON level phase θ _ON is defined as in the following equation (17). On the other hand, since the output light of the optical interference element 100 is weakened at the OFF level, the following formula (18) is derived.

From the above, when the relationship between the applied voltage and the phase of each of the semiconductor arm is obtained in advance, it is possible to determine the center bias voltage V _BAR center bias voltage V _DATA and BAR signal DATA signal in the procedure described below it can. Thereby, the logic of the input signal and the optical output can be matched.

First, it is assumed that a certain phase is an ON level phase θ _ON . Next, V _{DATA_ON} and V _{BAR_ON} are obtained from the relationship between the voltage applied to each semiconductor arm and the phase to obtain the phase θ _ON . Next, from V _{DATA_ON} and V _{BAR_ON} , V _{DATA_OFF} = V _{DATA_ON} -V _pp and V _{BAR_OFF} = V _{BAR_ON} + V _pp are obtained. Thereby, the phase θ _DATA (V _{DATA_OFF} ) and θ _BAR (V _{BAR_OFF} ) of the OFF level of each semiconductor arm are obtained.

Then, the θ _{DATA (V DATA_OFF)} and _{θ BAR (V BAR_OFF), θ} DATA (V DATA_OFF) -θ BAR (V BAR_OFF) = π looks for θON as established. Next, from V _{DATA_OFF} and V _{BAR_OFF} where θ _DATA (V _{DATA_OFF} ) −θ _BAR (V _{BAR_OFF} ) = π is established, the center bias voltage V _DATA = V _{DATA_OFF} + V _pp / 2 and V _BAR = V _BAR = V _{BAR — OFF} −V _pp / 2 is obtained.

(Other examples)
In the above embodiment, the cross-point adjustment is described in the initial state tuning. However, the initial state tuning includes other tunings. For example, when 2 × 2 MMI is used for the output coupler, tuning is also included in which one output is maximum and the other output is minimum (zero) in the initial state. In addition, when 2 × 1 MMI is used for the output coupler, when the modulation electrode is differentially driven, tuning is also included so that the output (one) of the MMI becomes a maximum and minimum intermediate value. Of course, in 2 × 1 MMI, a state where the output is controlled to be the maximum value or the minimum value may be set as the initial state. Since the present invention can accurately obtain the difference in the optical characteristics of both arms, the present invention can be applied to these other tunings.

  Note that the bias value obtained by applying the present invention can be optimized while confirming an actual cross point. In this case, since the adjustment is performed from a bias close to the cross point, the adjustment becomes easy.

  In the above embodiment, the Mach-Zehnder modulator is used as the optical interference element. However, the present invention can be applied to any optical interference element that includes an input coupler, a plurality of semiconductor arms, and an output coupler. For example, the present invention can be applied to an optical frequency doubler. Furthermore, the present invention can be applied to an optical interference element having more than two arms. In that case, according to the present invention, all the arms other than the arm that controls the light transmission state and the light absorption state may be controlled so as to be in the light absorption state. In the above embodiment, the MMI is used as the input coupler and the output coupler, but a directional coupler may be used.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

33 Input coupler 34a First semiconductor arm 34b Second semiconductor arm 35 Output coupler 42a Lower cladding layer 43 Core 42b Upper cladding layer 100 Optical interference element

Claims (12)

A method for measuring an optical interference element, comprising: an input coupler; first and second semiconductor arms connected to the input coupler; and an output coupler that interferes with outputs of the first and second semiconductor arms. ,
Sweeping the bias of the second semiconductor arm after the first semiconductor arm is in a light transmissive state to obtain the output state of the output coupler;
Sweeping the bias of the second semiconductor arm while applying a bias that produces a light absorption characteristic larger than that of the light transmitting state to the first semiconductor arm to obtain an output state of the output coupler; A method for measuring an optical interference element, comprising: And sweeping the bias of the first semiconductor arm after the second semiconductor arm is in a light transmitting state to obtain the output state of the output coupler;
Sweeping the bias of the first semiconductor arm in a state where a bias generating a large light absorption characteristic is applied to the second semiconductor arm compared to the light transmission state, and obtaining an output state of the output coupler; Carried out,
The optical interference according to claim 1, further comprising: obtaining a difference in optical characteristics between the first and second semiconductor arms using the output state of the output coupler obtained in the four steps. Element measurement method.   3. The method of measuring an optical interference element according to claim 2, further comprising a step of obtaining an initial bias value for reducing a difference between optical characteristics of the first and second semiconductor arms.   Based on a difference in optical characteristics between the first and second semiconductor arms, a BAR signal having a DATA signal input to the first semiconductor arm and having a differential relationship with the DATA signal is input to the second semiconductor arm. When the DATA signal is ON, the phase difference between the light that has passed through the first semiconductor arm and the light that has passed through the second semiconductor arm is 2nπ, and the DATA signal is OFF when the DATA signal is OFF. A step of obtaining a center bias of the DATA signal and the BAR signal so that a phase difference between the light passing through one semiconductor arm and the light passing through the second semiconductor arm becomes (2n + 1) π. The method of measuring an optical interference element according to claim 2.   The method of measuring an optical interference element according to claim 1, wherein the optical interference element is a Mach-Zehnder modulator.   The method for measuring an optical interference element according to claim 1, wherein the light absorption characteristic is a light absorption rate of 20 dB or more compared to the light transmission state.   The method of measuring an optical interference element according to claim 1, wherein the light transmission state is realized by setting the arm to an unbiased state. A method for measuring an optical interference element, comprising: an input coupler; a plurality of semiconductor arms including first and second semiconductor arms connected to the input coupler; and an output coupler that interferes with outputs of the plurality of semiconductor arms. There,
After the first semiconductor arm is in a light transmission state and the arms other than the first and second semiconductor arms are in a light absorption state, the bias of the second semiconductor arm is swept, and the output coupler Obtaining an output state;
Sweeping the bias of the second semiconductor arm after allowing the first semiconductor arm and the arm other than the first and second arms to be in a light absorbing state, and obtaining an output state of the output coupler; A method for measuring an optical interference element, comprising:   9. The method of measuring an optical interference element according to claim 8, wherein the light absorption state has a light absorption rate of 20 dB or more as compared with the light transmission state.   10. The method of measuring an optical interference element according to claim 8, wherein the light transmission state is realized by setting the arm to an unbiased state. An optical interference element measuring method comprising: an input coupler; a plurality of semiconductor arms connected to the input coupler; and an output coupler that interferes with outputs of the plurality of semiconductor arms,
The bias of the first semiconductor arm is swept in a state in which a bias that generates a light absorption characteristic is applied to all of the semiconductor arms other than the first semiconductor arm among the plurality of semiconductor arms. Obtaining an output state;
Of the plurality of semiconductor arms, the second semiconductor arm is in an unbiased state, and light is absorbed by all other semiconductor arms except the first semiconductor arm and the second semiconductor arm among the plurality of semiconductor arms. And a step of sweeping a bias of the first semiconductor arm to obtain an output state of the output coupler in a state where a bias generating a characteristic is applied. A method for measuring an optical interference element, comprising: an input coupler; first and second semiconductor arms connected to the input coupler; and an output coupler that interferes with outputs of the first and second semiconductor arms. ,
Sweeping the bias of the second semiconductor arm in a state in which a bias generating a light absorption characteristic is applied to the first semiconductor arm to obtain an output state of the output coupler;
The bias of the second semiconductor arm is swept in a state where a bias that realizes a state of less loss of 20 dB or more than when a bias that generates the light absorption characteristic is applied to the first semiconductor arm, Obtaining an output state of the output coupler, and a method of measuring an optical interference element.

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