JP3461877B2 - Light modulation circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator circuit used in an optical transmitter or the like for optical fiber communication.

In a high-speed optical communication system, it is necessary to suppress the waveform deterioration of an optical pulse caused by the spectrum spread of an optical signal and the chromatic dispersion of an optical fiber and improve the transmission distance.

[0003]

2. Description of the Related Art As one of the optical modulation circuits for the above-mentioned purpose, there is an interference type optical modulation circuit such as a Mach-Zehnder interferometer type optical modulation circuit. In a Mach-Zehnder interferometer type optical modulation circuit, incident light is split and passed through two optical waveguides, and electric fields of opposite phases are applied to these two optical waveguides to phase-modulate the light propagating in the opposite directions. Then, the combined interference light is subjected to optical modulation by changing the light intensity by giving a phase difference and then synthesizing again, whereby optical modulation without wavelength chirping is possible.

[0004]

However, when the transmission speed is further increased to several Gbit / sec or more, even if the wavelength chirping is set to zero, the spectrum spread due to the modulation sideband, which is the Fourier component of the modulation waveform, occurs. There is a problem that the waveform deterioration of the optical pulse due to the chromatic dispersion of the optical fiber serving as the transmission path cannot be ignored, and the optical fiber having a large chromatic dispersion limits the transmission distance and cannot perform long-distance transmission.

That is, FIG. 9 shows a configuration example of a conventional Mach-Zehnder interferometer type optical modulation circuit. In the figure, 1 is a light source of a DFB (distributed feedback) structure that generates incident light, 2 is a light branching portion that splits incident light into two, and 3 and 4 are to change the refractive index of a substance having an electro-optical effect by an electric field. The optical waveguide 5 for changing the phase of the propagating light is an optical coupling portion for combining the light emitted from the optical waveguides 3 and 4 to cause interference.
6 is a bifurcating unit that bifurcates the input electric signal; 7 is an inverting unit that inverts the polarity of one of the bifurcated electric signals;
Reference numeral 9 is an electrode for applying an electric field to the optical waveguides 3 and 4.

FIG. 10 is an operation explanatory diagram of this conventional Mach-Zehnder interferometer type optical modulation circuit. The inputted electric pulse signal (A in FIG. 10) is branched into two, one of which is inverted in polarity by the inversion unit 7 and applied to the electrode 8 (B in FIG. 10),
The other is directly applied to the electrode 9 (C in FIG. 10). As a result, during the period of the electric pulse signal, the light wave phase φ1 of the light emitted from the optical waveguide 3 is shifted to the delay side of the phase φ0 of the incident light (D in FIG. 10), and the light wave phase φ2 of the light emitted from the optical waveguide 4 is changed. Shifts to the leading side of the phase φ0 of the incident light (F in FIG. 10).

In the portion where the phase of the emitted light of the optical waveguides 3 and 4 changes (the rising / falling portion of the phase change), the wavelength λ of the emitted light eventually changes, and the light wave phase of the emitted light changes. The wavelength becomes long at the part changing to the delay side (falling part in the figure), and becomes short at the part changing to the advance side (rising part in the figure) (E in FIG. 10).
And G).

Light emitted from these two optical waveguides 3 and 4 is coupled by the optical coupling portion 5 and the light wave phase difference becomes φ1-φ2 (H in FIG. 10), and this light wave phase difference is set to 0 or π by modulation. By doing so, the incident light incident on the optical branching portion 2 strengthens the electric pulse signal period due to interference, and the other periods cancel each other out, so that the interference light is output from the optical coupling portion 5 as a pulsed light intensity waveform. (I in FIG. 10).

When the output light of the optical modulator propagates through the optical fiber transmission line, it is affected by the chromatic dispersion of the optical fiber. For example, the long wavelength component of the propagated light has a delayed arrival time and a short wavelength. As for the component, the arrival time advances or lags with respect to the length of the wavelength depending on the characteristics of the optical fiber such as the arrival time advances. As a result, the output light component of the optical waveguide 3 is
After the fiber transmission, as shown in (J) and (K) of FIG. 10, the long wavelength component is delayed and the short wavelength component is advanced so that the pulse width of the phase modulation pulse is compressed. The pulse width of the emitted light component is extended as shown in (L) and (M) of FIG. The waveform obtained by synthesizing the pulse components by compression and expansion, that is, the optical intensity waveform of the optical signal after fiber transmission, causes waveform distortion as shown in (O) of FIG.

As a method for solving such a problem, the present applicant has previously filed Japanese Patent Application No. 1-61534 (Japanese Patent Application Laid-Open No. 2-269).
309) proposed an optical modulation method. As shown in FIG. 11, this optical modulation method outputs the pulse amplitude of the electric signal applied to the electrodes 8 and 9 by enlarging one of the pulse amplitudes by the pulse amplitude width expanding unit 14 and the pulse amplitude reducing unit 15 and reducing the other. The light is pulse-compressed to compensate for waveform deterioration caused by the modulation sideband and chromatic dispersion of the fiber and improve the transmission distance limit.

An object of the present invention is to improve the transmission distance limit by suppressing the generation of the waveform distortion of the optical signal caused by the wavelength dispersion of the optical fiber as in the case of the optical modulation method.

[0012]

FIG. 1 is a diagram illustrating the principle of the present invention. In order to solve the above-mentioned problems, the optical modulation circuit according to the present invention, as one form, splits incident light into two optical waveguides 50 and 51 to which electric fields of opposite phases are applied, and then combines them again. In the optical modulation circuit having the configuration, one of the electric pulse signals applied to the electrodes 52 and 53 of the two optical waveguides 50 and 51 is enlarged in pulse width by one of the magnitudes to compensate for chromatic dispersion of the optical fiber 60 serving as a transmission line. , A pulse width control circuit 5 for reducing the pulse width of the other
4, 55 are provided.

As another form of the optical modulation circuit according to the present invention, in the above-described optical modulation circuit, a rise / fall control circuit for controlling the rise time and the fall time of the electric pulse signal to be constant is provided. It is further characterized by being provided.

[0014]

In order to remove the waveform distortion of the optical pulse after being transmitted through the optical fiber 60, the phase modulation of the light emitted from the optical waveguides 50 and 51 is previously performed on the transmitting side by the amount of the chromatic dispersion received by the optical fiber 60. The waveform is pulse compressed / expanded. Therefore, after fiber transmission, the pulse compression / expansion of the phase modulation waveform caused by the fiber chromatic dispersion is canceled, so that the waveform distortion can be removed.

At this time, in order to make the influence of chromatic dispersion on the optical fiber transmission line constant, it is desirable to keep the rise / fall time of the electric pulse signal constant on the receiving side.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows an optical modulation circuit as an embodiment of the present invention. The optical modulation circuit of this embodiment is a so-called Mach-Zehnder interferometer type interferometric optical modulation circuit described in the above-mentioned prior art, and includes a light source 1, an optical branching section 2, optical waveguides 3, 4, an optical coupling section 5, and a bifurcating section. The part 6, the inversion part 7, the electrodes 8 and 9 have the same configuration. As this Mach-Zehnder interferometer type optical modulation circuit, for example, a Li Nb O ₃ Mach-Zehnder interferometer type or a semiconductor Mach-Zehnder interferometer type can be used.

The difference is that one electric pulse signal output from the inversion unit 7 is applied to the electrode 8 via the pulse width expansion unit 10 and the rising / falling control unit 12, while being output from the bifurcating unit 6. The other electric pulse signal thus generated is transmitted through the pulse width reduction unit 11 and the rising / falling control unit 13 to the electrode 9
Is configured to be applied to.

Here, the pulse width expansion unit 10 and the pulse width reduction unit 11 respectively expand and reduce the pulse width of the input electric pulse signal by a size that compensates for the influence of the chromatic dispersion of the optical fiber serving as the transmission path. . Further, the rising / falling control units 12 and 13 determine the rising time t of the input electric pulse signal.
Control is performed so that _r and the fall time t _f have a constant magnitude.

The operation of the embodiment apparatus will be described below with reference to FIG. The basic operation of this optical modulation circuit is the same as that of the above-described conventional example, except that the pulse width of the electric pulse signal applied to the electrode 8 is expanded by the pulse width expansion unit 10 (see FIG. 3). B) On the other hand, the pulse width of the electric pulse signal applied to the electrode 9 is reduced by the pulse width reduction unit 11 (C in FIG. 3). The magnitude of expansion and contraction of the pulse width depends on the phase-modulated light (output light component from the optical waveguide 3 and output light component from the optical waveguide 4) of the phase-modulated light that has undergone wavelength dispersion of the optical fiber and has been transmitted through the optical fiber. The pulse width is made to match the pulse width of the electric pulse signal before the pulse width expansion / reduction on the transmission side (J, K, L, M in FIG. 3).

In this way, the light emitted from the optical waveguide 3 undergoes pulse expansion, and the light emitted from the optical waveguide 4 undergoes pulse compression, so that the output pulsed light from the optical modulator circuit on the transmission side that interferes with them synthetically interferes with the light. Although the intensity waveform is distorted (I in FIG. 3), after being transmitted through the optical fiber transmission line, it is subjected to fiber chromatic dispersion, so that the respective output light components of the optical waveguides 3 and 4 are respectively input electrical pulses on the transmission side. Since the signal is pulse-compressed and pulse-expanded so as to have the pulse width of the signal, the optical intensity waveform of the pulsed light after transmission is eventually shaped by removing the distortion by compensating the influence of fiber chromatic dispersion. (O in FIG. 3).

[0021] Note that the transmission was a constant rise / fall control section 12, 13 an electrical pulse signal rise / fall time to be applied to the electrodes 8 and 9 by the side, this rise / fall time t _r, When t _f becomes a disjoint value, this eventually results in a change amount Δ of the light wave wavelength of the light emitted from the optical waveguides 3 and 4.
λ is a discrete value (i.e., because the rise / fall time t _r, the phase change amount of the time derivative in the t _f becomes wavelength variation [Delta] [lambda]), as a result, the size of the wavelength dispersion which receives the optical fiber This is because it is not constant and the compensation becomes troublesome.

The relationship between the fiber dispersion amount and the pulse width and various application limits in the case where the optical modulation circuit is constructed as described above are as follows.

(A) Relationship between fiber dispersion amount and pulse width The pulse compression / expansion amount τ (see FIG. 8) is determined as follows. τ = m · L Δλ (1) where Δλ = −dφ / dt · λ ² / (2πC) ≈−π / 2 / t _r · λ ² / (2πC) = −λ ² / (4C・ T _r ) ・ ・ ・ (2) where m is the fiber dispersion coefficient (ps / nm / km), L is the length of the optical fiber (km), Δλ is the wavelength change amount, φ is the phase, C is the speed of light, _tr is a rise time (fall time). Therefore, | τ | = m · L · λ ² / (4C · _tr ) (3)

(B) Applicable limit As the applicable limit, as can be seen from FIG. 8, t _r ≦ T (4) | τ | + t _r / 2 ≦ T / 2 (5) Here, T is the repetition time of the input electric pulse signal, and assuming that the bit rate is B, there is a relation of T = 1 / B. (1) to (5) ^{below, m · L · λ 2 /} 4C = | τ | · t r ≦ t r · (T-t r) / 2 ≦ T 2/4 ≦ 1 / (4B 2) (When the equal sign is _tr = T / 2) ... (6) Therefore, the application limit for the bit rate B and the dispersion is m · L ≦ 4C / (λ ² · 4B ² ) (7), and various parameters may be selected so as to satisfy this condition.

Even when the interferometric optical modulation circuit of the present invention is used under the condition that these application limits are not satisfied, the present invention is used as a means for mitigating waveform deterioration of an optical pulse after transmission due to fiber wavelength dispersion. Needless to say, is effective.

In the above-described embodiment, a drive circuit (two-branching unit 6, inverting unit 7, pulse width expanding unit 10, pulse width reducing unit 1) for generating electric pulse signals applied to the electrodes 8 and 9.
The circuit corresponding to 1) can be configured as shown in FIGS. 4 to 7, for example.

That is, as shown in FIG.
The AND circuit 22 divides the input electric pulse signal by the circuit 21 and delays the non-inverted output Q of the non-inverted output Q of the other one as it is
To the AND circuit 23 by delaying one of the inverted output * Q and the other, and
The polarity of the ND output is controlled by the polarity control circuits 24 and 25 to obtain the drive voltages V1 and V2. As a result, as shown in FIG. 4B, the pulse width to be enlarged / reduced can be adjusted by the amount of delay given above, and the polarity control circuits 24 and 25 can set the polarity of the mode A and the mode B. Thus, the positive and negative of the drive voltages V1 and V2 can be inverted.

Further, as shown in FIG. 5, the input electric pulse signal is branched by the circuit 26 and its non-inverted output Q is input to the AND circuit 27 with one being delayed as it is and the other being delayed.
Further, the non-inverted output Q is input to the OR circuit 28 while delaying the other one as it is, and the polarities of the respective outputs are controlled by the polarity control circuits 29 and 30, so that the drive voltages V1 and V2 are obtained. Good.

Further, as shown in FIG. 6, the input electric pulse signal is branched into the non-inverted output Q and the inverted output * Q in the circuit 31, and each is filtered by the low-pass filters 32 and 33 to round the waveform, By slicing the waveform with the threshold value Vht1 by the slicing circuits 34 and 35, an enlarged / reduced pulse width is created, and the polarity of the pulse output is controlled with the polarity control circuits 36 and 37 to obtain the drive voltages V1 and V2. You may do it.

Further, as shown in FIG. 7, the input electric pulse signal is branched to the non-inverted output Q by the circuit 38, each is filtered by the low-pass filters 39 and 40 to round the waveform, and the waveform is sliced. 41, 42 threshold Vht1, Vht
The pulse width may be enlarged / reduced by slicing at 2, and the pulse output may be polarity-controlled by the polarity control circuits 43 and 44 to be the drive voltages V1 and V2.

If a circuit for stabilizing the operating point of the modulator (for example, the circuit described in Japanese Patent Application No. 2-50189 (Japanese Patent Application Laid-Open No. 3-251815)) is incorporated in the above embodiment, stable operation can be obtained. To be Further, the rising / falling control units 12 and 13 are not limited to the case of being arranged in the latter stage of the pulse width expanding unit 10 and the pulse width contracting unit 11 as in the embodiment, but may be arranged in the former stage thereof, or may be divided into two. The rising / falling time of the electric pulse signal input to the unit 6 may be controlled to be constant.

[0032]

As described above, according to the present invention, it is possible to suppress the occurrence of waveform distortion of an optical signal caused by chromatic dispersion of an optical fiber, improve the transmission distance limit, and increase the transmission distance. Can be realized.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a diagram showing an optical modulation circuit as an embodiment of the present invention.

FIG. 3 is a diagram for explaining the operation of the apparatus according to the embodiment.

FIG. 4 is a diagram showing an example of a drive circuit in an embodiment.

FIG. 5 is a diagram showing another example of the drive circuit in the embodiment.

FIG. 6 is a diagram showing still another example of the drive circuit according to the embodiment.

FIG. 7 is a diagram showing still another example of the drive circuit according to the embodiment.

FIG. 8 is a diagram for explaining the relationship between T, _tr and τ in the example.

FIG. 9 is a diagram showing a conventional example of an optical modulation circuit.

FIG. 10 is a diagram for explaining the operation of the conventional device.

FIG. 11 is a diagram showing another conventional example of the light modulation circuit.

[Explanation of symbols]

1 light source 2 Optical branch 3, 4 optical waveguide, 5 Optical coupling section 6 bifurcation 7 Inversion section 8, 9 electrodes 10 Pulse width expansion section 11 Pulse width reduction section 12, 13 Rise / fall control unit 14 Pulse amplitude expansion section 15 Pulse amplitude reduction unit

Claims (2)

(57) [Claims] 1. An optical modulation circuit configured to split incident light into two optical waveguides (50, 51) to which electric fields having opposite phases are applied and then combine the incident light again. , 52) respectively, the pulse width control circuit (54, 55) for enlarging the pulse width of one of the electric pulse signals and the other for reducing the pulse width by a size that compensates the chromatic dispersion of the optical fiber serving as the transmission line.
An optical modulation circuit comprising: 2. The optical modulation circuit according to claim 1, further comprising a rising / falling control circuit for controlling the rising time and the falling time of the electric pulse signal to be constant.