JP2012252260A - Optical modulation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulation circuit that achieves suppression of nonlinearity of an output optical signal.SOLUTION: An optical modulation circuit relating to one embodiment in the invention comprises: an optical branching circuit 401 for branching an input optical signal; a first optical modulation part 410 and a second optical modulation part 420 that are connected in parallel to the optical branching circuit; and an optical coupling circuit 402 for coupling optical signals that are modulated by each of the optical modulation parts. Each response period ratio of the first optical modulation part 410 and the second optical modulation part 420 is set as 1:1/3, and an electric field intensity ratio of branching to the first optical modulation part 410 and the second optical modulation part 420 by the optical branching circuit 401 is set as 1:1/3=9:1. And, the electric field intensity ratio of coupling by the optical coupling circuit 402 is set as 1:1/3=9:1. The response period ratio and the electric field intensity ratio are determined on the basis of Fourier expansion of a triangular wave. An addition of a secondary component and a high-order Fourier component to a primary component realizes suppression of nonlinearity of a response curve to a drive voltage.

Description

  The present invention relates to an optical modulation circuit.

  In order to improve the utilization efficiency of the optical spectrum, multi-level modulation such as QAM (Quadrature Amplitude Modulation) and OFDM (Orthogonal Frequency Division Multiplexing) has been actively studied.

One method for obtaining a multilevel optical signal is to drive a push-pull drive type Mach-Zehnder modulator (MZM) that applies a drive voltage in reverse phase with a multilevel electrical signal. FIG. 1 shows a conventional MZM. An input optical signal is branched into a first optical waveguide 102 and a second optical waveguide 103 by an optical branch circuit 101 such as a directional coupler, an MMI, or a Y-shaped 1 × 2 coupler. As φ = (π / V _π ) · V, the first electrode 104 and the second electrode 105 give + φ / 2 and −φ / 2 phase changes to the respective optical waveguides. Here, V is a driving voltage, and _Vπ is an applied voltage for changing the phase of an optical signal propagating through the optical waveguide by a half wavelength. The optical signals modulated by the respective optical waveguides are combined by the optical coupling circuit 106 and output as an output optical signal. At this time, the electric field E of the output optical signal is represented by cos (φ / 2).

  FIG. 2 shows a response curve of the output optical signal with respect to the drive voltage. Since the response curve with respect to the drive voltage is non-linear, a deviation from the ideal equally spaced output optical signal obtained when the response curve is a linear triangular wave occurs when driving with a multi-valued electrical signal.

On the other hand, squeeze the amplitude of the drive voltage in order to suppress the signal distortion from 2V _[pi optical loss is minimized, (see Non-Patent Document 1) that occur loss of light as shown in FIG.

Shogo Yamanaka, Takayuki Kobayashi, Akihide Sano, Hiroji Masuda, Eiji Yoshida, Yutaka Miyamoto, Tadao Nakagawa, Munehiko Nagatani, and Hideyuki Nosaka, “11 x 171 Gb / s PDM 16-QAM Transmission over 1440 km with a Spectral Efficiency of 6.4 b / s / Hz using High-Speed DAC, ”Proc. ECOC 2010, paper We. 8. C. 1 (2010).

  The present invention has been made in view of such problems, and an object thereof is to provide an optical modulation circuit in which nonlinearity of an output optical signal is suppressed.

In order to achieve such an object, according to a first aspect of the present invention, there is provided an optical branch circuit for branching an input optical signal and N pieces (N is an integer of 2 or more) connected in parallel to the optical branch circuit. And an optical coupling circuit that combines the optical signals modulated by the respective optical modulation units, the first optical modulation unit and the nth (n is an arbitrary integer between 2 and N) The response period ratio of the optical modulation unit is 1: 1 / (2n−1), and the optical branching circuit supplies the input optical signal to the first optical modulation unit and the nth optical modulation unit. The optical coupling circuit branches at an electric field intensity ratio of 1: 1 / (2n−1) ² , and the optical coupling circuit converts the optical signals from the first and nth optical modulators into an electric field intensity ratio of 1: 1 /. (2n-1) ² is an optical modulation circuit characterized by coupling.

  According to a second aspect of the present invention, in the first aspect, each optical modulator is a push-pull drive type Mach-Zehnder modulator.

  According to a third aspect of the present invention, in the second aspect, each Mach-Zehnder modulator has a modulation electrode, and each modulation electrode is connected in cascade.

  In the fourth aspect of the present invention, two optical modulation circuits according to any of the first to third aspects are connected in parallel, and an optical phase difference π / 2 is given to the optical signal from each optical modulation circuit. The vector modulator includes an optical phase difference providing unit.

  According to the present invention, it is possible to provide an optical modulation circuit in which nonlinearity of an output optical signal is suppressed by generating a high-order Fourier component and adding it to the primary Fourier component.

It is a figure which shows the conventional MZM. It is a figure for demonstrating the signal distortion which arises in the conventional MZM. It is a figure for demonstrating the optical loss which arises in the conventional MZM. It is a figure which shows the optical modulation circuit by this invention. It is a figure which shows the response curve obtained by the light modulation circuit of FIG. It is a figure for demonstrating the deviation evaluation value D from the ideal point of a signal level. It is a figure which shows the relationship between the parallel number N and the evaluation value D of a light modulation part. 1 is a diagram illustrating an optical modulation circuit according to a first embodiment. It is a figure which shows the response curve obtained by the light modulation circuit of FIG. 6 is a diagram illustrating an optical modulation circuit according to a second embodiment. FIG. 6 is a diagram illustrating an optical modulation circuit according to a third embodiment. FIG. It is a figure which shows the response curve obtained by the light modulation circuit of FIG. It is a figure which shows the vector modulator which made the optical modulation circuit of Example 2 2 parallel. It is a figure which shows the vector modulator using the conventional 2 parallel MZM. It is an output signal diagram at the time of 4-value drive of a vector modulator.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, the basic concept of the present invention will be described. Thereafter, specific examples will be described.

(Basic concept of the present invention)
The basic concept of the present invention will be described with reference to FIG. An optical modulation circuit according to an embodiment of the present invention includes an optical branch circuit 401 that branches an input optical signal, and a first optical modulation unit 410 and a second optical modulation unit 420 that are connected in parallel to the optical branch circuit. And an optical coupling circuit 402 for coupling optical signals modulated by the respective optical modulation units. The response cycle ratio of the first light modulation unit 410 and the second light modulation unit 420 is 1: 1/3, and the first light modulation unit 410 and the second light modulation unit 420 are separated by the optical branch circuit 401. The branching electric field strength ratio is 1: 1/3 ² = 9: 1. The electric field strength ratio of coupling by the optical coupling circuit 402 is 1: 1/3 ² = 9: 1. The response period ratio and the electric field strength ratio are based on the following expression which is a Fourier expansion of a triangular wave.

  In this way, by adding a second-order Fourier component, nonlinearity of the response curve with respect to the drive voltage can be suppressed. As shown in FIG. 5, the linearity is improved.

In the above description, only the second-order component has been examined and described, but similarly, by adding a higher-order Fourier component to the first-order component, nonlinearity of the response curve with respect to the drive voltage can be suppressed. When generalized, the optical modulation circuit according to the present invention has N (N is an integer of 2 or more) optical modulation units connected in parallel. The nth optical modulation unit changes the electric field E ₀ of the optical signal branched to the optical modulation unit by the optical branch circuit with respect to the driving voltage V as follows.

Then, the optical signals modulated by the respective optical modulation units are coupled by the optical coupling circuit. Here, an optical branch circuit is designed in the first and n-th optical modulators so that the optical signal is branched at an electric field strength ratio of 1: 1 / (2n-1) ² . The optical coupling circuit is designed so that the optical signals from the nth optical modulation unit are coupled at an electric field intensity ratio of 1: 1 / (2n-1) ² . As can be seen from Equation (2), the response period ratio of the first and nth light modulation units is 1: 1 / (2n−1).

  Note that the present invention does not depend on the selection of the substrate material (X-cut LN or Z-cut LN, or an oxide crystal having an EO effect, a semiconductor, a polymer, or the like), or the selection of single drive or dual drive. Note that it is effective.

  Here, the evaluation value of the deviation from the ideal point of the signal level will be described. The evaluation value of deviation is defined as follows.

Here, M modulation level, E _k the actual output optical signal _level, E k, _ideal is the ideal level, E _MAX is the maximum signal level. E _MAX is normalized to 1.00 in this specification (see FIG. 6).

  FIG. 7 is a plot of changes in the evaluation value D against the parallel number N of the light modulation units for the modulation multi-level number M = 4, 8, and 16. Basically, the evaluation value D decreases as the parallel number N increases. That is, increasing the parallel number N improves the linearity and reduces the level shift.

  However, when the modulation multi-level number is small, it does not necessarily decrease monotonously. For example, when M = 4, D (N = 2) = 0.8%, whereas D (N = 3) = 1.1%. That is, N = 2 is optimal for quaternary modulation. For 8-level modulation and 16-level modulation, it is necessary to select a configuration in consideration of a trade-off between a level shift decrease due to an increase in the parallel number N and a configuration complexity.

  Examples will be described below.

Example 1
The light modulation circuit shown in FIG. 8 was formed on an X-cut LN substrate. The first light modulation unit and the second light modulation unit are each a single electrode type MZM. The second modulation electrode was designed to be about three times as long as the first modulation electrode, so that the response E ₁ of the first light modulation unit and the response E _{2 of} the second light modulation unit satisfy the following.

The first optical modulator and the second optical modulator were connected using an optical coupler with an intensity branching ratio of 9: 1 as an optical branch circuit and an optical coupler with an intensity coupling ratio of 9: 1 as an optical coupling circuit. The response of the entire optical modulation circuit according to the first embodiment is E = 0.9E ₁ + 0.1E ₂ . The phase adjustment unit was provided in order to make the phase difference between the two light modulation units zero.

Equally spaced drive voltage, when driven in the four-level amplitude 2V _[pi, ideal output optical signal level -1.00, -0.33, + 0.33, to + 1.00, in the conventional single MZM, -1.00, -0.50, +0.50, +1.00, while in this example, -1.00, -0.35, +0.35, +1.00, and the response linearity is The signal level was improved and a signal level closer to the ideal was obtained (see FIG. 9).

Example 2
The difference from the first embodiment is that the modulation electrodes are connected in cascade to form one (see FIG. 10). As a result, the drive system can be integrated. Of course, a configuration may be adopted in which a multi-valued electrical signal is first input to the second modulation electrode. Further, if the length of the second modulation electrode is about three times that of the first modulation electrode, they may be arranged separately.

Example 3
As an example of N = 3, the light modulation circuit shown in FIG. 11 was manufactured. The ratio of the modulation electrode lengths of the first, second, and third light modulation units was set to about 1: 3: 5, and the response was designed to satisfy the following.

An optical coupler having an intensity branching ratio of 225: 25: 9 (= 1: 1/9: 1/25) was connected as an optical branching circuit, and an optical coupler having an intensity coupling ratio of 225: 25: 9 was used as an optical coupling circuit. The response of the entire optical modulation circuit according to Example 3 was E = (225E ₁ + 25E ₂ + 9E ₃ ) / 259, and a response closer to linear than those of Examples 1 and 2 was obtained (see FIG. 12).

Example 4
A vector modulator (IQ modulator) shown in FIG. 13 was configured by arranging two optical modulation circuits of Example 2 in parallel. For comparison, FIG. 14 shows a vector modulator using a conventional two-parallel MZM.

  Compared with the conventional vector modulator, as shown in FIG. 15, there is little deviation from the ideal point of the signal point at the time of multilevel driving.

For example, when driven by four-valued I and Q channel signals (amplitude 2V _π ), a 16QAM signal diagram indicated by ◯ in FIG. 15 is obtained. Due to the improved linearity, it is closer to the ideal point (♦) than the result (Δ) obtained with the conventional vector modulator.

401 optical branching circuit 402 optical coupling circuit 410 first optical modulation unit 420 second optical modulation unit

Claims (4)

An optical branch circuit for branching an input optical signal;
N optical modulation units (N is an integer of 2 or more) connected in parallel to the optical branch circuit;
An optical coupling circuit for coupling optical signals modulated by each optical modulation unit,
The response period ratio of the first light modulation unit and the nth light modulation unit (n is an arbitrary integer of 2 or more and N or less) is 1: 1 / (2n−1),
The optical branching circuit branches the input optical signal into a first optical modulation unit and an nth optical modulation unit with an electric field intensity ratio of 1: 1 / (2n-1) ² ,
The optical coupling circuit couples optical signals from the first and n-th optical modulation sections with an electric field intensity ratio of 1: 1 / (2n-1) ^2. .   2. The optical modulation circuit according to claim 1, wherein each optical modulation unit is a push-pull drive type Mach-Zehnder modulator.   3. The optical modulation circuit according to claim 2, wherein each Mach-Zehnder modulator has a modulation electrode, and each modulation electrode is connected in cascade. Two optical modulation circuits according to any one of claims 1 to 3 are connected in parallel,
A vector modulator comprising: an optical phase difference providing unit that applies an optical phase difference π / 2 to an optical signal from each optical modulation circuit.

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