JPH02269309A - Optical modulating system - Google Patents

Abstract

PURPOSE:To execute a long distance transmission by a fiber whose fiber wavelength dispersion is large even at a transmission rate of >= several giga-bit/sec by modulating asymmetrically a phase of light and moving the center wavelength of a rise part of an output light of a modulator to a long wavelength side, and moving the center wavelength of a fall part to a short wavelength side. CONSTITUTION:By modulating asymmetrically phases of each waveguide 1, 2 of a modulator, the center wavelength of an output light is moved to a long wavelength side and a short wavelength side in a rise part and a fall part, respectively. Accordingly, by wavelength chirping generated by this method, the rise part of an optical pulse is delayed and the fall part advances by a fiber dispersion, and a pulse compression is generated. It works in the direction for compensating the waveform divergence by a modulation side band and the wavelength dispersion of a fiber, and executes an action for improving the transmittable fiber length. In such a way, a long distance transmission an be executed by the fiber whose fiber wavelength dispersion is large even at a transmission rate of >= several giga-bit/sec.

Description

[Detailed Description of the Invention] [Summary] This invention relates to an optical modulation method used in a transmission device that transmits high-speed digital signals over long distances in an optical communication system using an optical fiber as a transmission path, even at transmission speeds of several gigabits/second or more. The purpose of this study is to provide an optical modulation method that can transmit long distances through fibers with large fiber wavelength dispersion, and by asymmetrically modulating the phase of light propagating through two optical waveguides in the Matsuhatsu Enda interferometer type optical modulator, The center wavelength of the rising portion of the modulator output light is moved to the longer wavelength side, and the center wavelength of the falling portion of the modulator output light is moved to the shorter wavelength side.

[Industrial application field]

The present invention relates to an optical modulation method used in a transmission device that transmits high-speed digital signals over long distances in an optical communication system using an optical fiber as a transmission path.

In high-speed optical communication systems, it is required to minimize the spectral spread of a light source to avoid waveform deterioration of optical pulses caused by spectral spread and wavelength dispersion of fibers.

For this reason, external modulation methods that can reduce the spectral spread are attracting attention, but even when using a two-wire method, at transmission speeds of several gigabit/s or more, transmission is caused by the spectral spread caused by the modulation sidebands. Since the distance is limited, it is necessary to devise ways to further extend the transmission characteristics.

[Conventional technology]

One of the optical modulation methods that has the smallest spectral spread and is therefore less susceptible to fiber wavelength dispersion is a modulation method that uses a Matsuhatsu Enda interferometer type optical modulator. The Matsuhatsu Enda interferometer type optical modulator modulates the phases of light propagating through two optical waveguides to the same magnitude and in opposite directions, allowing modulation without wavelength chirping. That is, the spectral spread can be reduced to the extent due to modulation sidebands, which are Fourier components of the modulation waveform.

Therefore, in the optical modulation method using the conventional Matsuhatsu Enda interferometer type optical modulator, the phases of the light propagating through the two optical waveguides of the modulator are modulated with the same magnitude and in opposite directions, resulting in modulation without wavelength chirping. I did it again (F, ni OY
AM^et, al,, JOLIRNAL 0FLI
GHTWAVT! TECIINOLOGY, VOL
, 6. NO, 1, 1988゜PP, 87-93)
.

However, at transmission speeds of several gigabits/second or higher, even if wavelength chirping is reduced to zero, spectral broadening due to modulation sidebands and deformation of optical pulses due to fiber wavelength dispersion cannot be ignored.

[Problem to be solved by the invention]

Therefore, even if the spectral spread due to modulation is reduced to the spread due to only the modulation sideband using an external modulation method, long-distance transmission is not possible in a fiber with large fiber chromatic dispersion at transmission speeds of several gigabits/second or more. This caused a problem.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical modulation method that allows long-distance transmission through a fiber with large fiber chromatic dispersion even at transmission speeds of several gigabits/second or more.

[Means to solve the problem]

FIG. 1 and FIG. 2 are diagrams explaining the principle of the present invention. 1st
The figure shows the electric field of light at each part of the Matsuhatsu Enda interferometer type optical modulator. In the figure, E0 is the amplitude of the electric field of input light, ω.

is the angular frequency of the electric field, t is the time, φ, , φ represent the modulated phase in the optical waveguides A and B, respectively, 1
Eout(t) is the electric field of output light, details of which are shown in equation (1).

EOut(t)=EO/2 (COS(ωot +φA
)+C05(ω.t÷φ1)) =Eo/2(X”+Y”) ””C05(ωot-ta
n-' (Y/X)) However, X=CQS (φa)
+ C03(φ,)Y=SIN(φA) + 5I
N(φm) (1) Eou t (t)
is subjected to phase modulation of jan-' (Y/X). This results in wavelength chirping as shown below.

ω. If t-tan-' (Y/X) is Φ, the angular frequency is ω(t)=dΦ/dt ” ωe-d(tan
-'(Y/X))/dt, and the wavelength is λ・2πC/
From ω(t) (c is the speed of light).

Δω=−d(tan −′(Y/X))/dt causes wavelength chirping.

Here, phase modulation is performed as follows.

φ, 〉0. φ, < O, ABS (φA) >
ABS (φ,) ABS (φ, -φA) NO (light output old g
h).

ABS(φ, -φm) #Z (Low optical output) However, ABS(φ) represents the absolute value of φ.

FIG. 2 shows the operating waveforms of each part at this time.

The phase of the output light is delayed in the portion where the output light intensity rises as shown in f, and the phase advances in the portion where it falls.

Correspondingly, the center wavelength moves to the long wavelength side in the rising portion and to the short wavelength side in the falling portion, as shown in g.

Conventionally, modulation was performed under the conditions of φ, .-φ. In this case, Eout(t) is expressed as equation (2).

Bout(t)=EoCO5(φ) C05(ωat)
---(2) However, φ=φ^·−φ layer In this case, modulation of φ only modulates the amplitude of the optical electric field, and no wavelength fluctuation occurs due to the modulation.

[Effect]

In the present invention, by asymmetrically modulating the phase of each waveguide of the modulator, the center wavelength of the output light is shifted to the longer wavelength side at the rising edge, as shown in g in FIG.

Also, it was made to move toward the shorter wavelength side at the falling edge.

On the other hand, the chromatic dispersion of the fiber is large when a 1.3μ-band zero-dispersion single mode fiber is used in the 1.55μ-band, which has the lowest loss.
0 ps, and the longer the wavelength, the slower the speed of propagation through the fiber.

Therefore, due to the wavelength chipping produced by the present invention, the rising portion of the optical pulse is delayed due to fiber dispersion, and the falling portion is advanced, resulting in pulse compression. This works to compensate for waveform distortion caused by modulation sidebands and fiber wavelength dispersion, and works to improve the fiber length that can be transmitted.

〔Example〕

Matsuhatsu Enda interferometer type optical modulator uses electro-optic effect for optical phase modulation. In other words, by changing the refractive index of a substance that has an electro-optical effect using an electric field,
Change the phase of light.

Therefore, in the Matsuhatsu Enda interferometer type optical modulator, several methods can be considered for asymmetrically modulating the phase of the light propagating through the two optical waveguides. One method is to modulate each waveguide with a different drive voltage. The second method uses the same driving voltage but makes the cross-sectional structure of the electrode asymmetrical, thereby making the modulation electric field applied to the optical waveguide asymmetrical. The third method is to change the length of the electrodes in each waveguide to change the waveguide length in which the light confuses the change in refractive index.

FIG. 3 shows a first embodiment. This is an example of applying the drive voltage amplitude asymmetrically, and assumes a Z-plate electro-optic crystal. The same method can be applied to X-plate and Y-plate electro-optic crystals. In the figure, 1 is an optical waveguide to which a large phase modulation is applied, and 2 is an optical waveguide to which a small phase modulation is applied.

3 is a modulation electrode, and 4 is a ground electrode. 3 and 4
This constitutes a traveling wave electrode. 5 is a terminating resistor that matches the characteristic impedance of the traveling wave electrode. 6 is a drive circuit for performing phase modulation of the optical waveguide 1, and 7 is a drive circuit for performing phase modulation of the optical waveguide 2.

FIG. 4 is a time chart showing the operation of the first embodiment. vl is a drive waveform that performs phase modulation of the optical waveguide 1, and V2 is a drive waveform that performs phase modulation of the optical waveguide 2. By reversing the polarities in (1) and (2) and making the driving voltage amplitude larger in (1), phase modulation is applied asymmetrically.

FIG. 5 shows a second embodiment. This is to apply phase modulation asymmetrically by making the electrode length asymmetric, and assumes a Z-plate electro-optic crystal. The meanings of symbols 1 to 4 in FIGS. 5 to 11 are the same as those in FIG. 3.

A third embodiment is shown in FIG. 6, in which the lengths of the two electrodes are made asymmetric to apply phase modulation asymmetrically, and an X-plate or Y-plate electro-optic crystal is assumed.

FIG. 7 shows a fourth embodiment. FIG. 7 shows a cross-sectional structure of a modulator. In this example, phase modulation is applied asymmetrically by making the cross-sectional structure of the electrode asymmetric. In this example, a two-plate electro-optic crystal is assumed, the electrode is slightly shifted from the optical waveguide 2. are placed.

FIG. 8 shows a fifth embodiment. In this example, phase modulation is applied asymmetrically by making the cross-sectional structure of the electrode asymmetric. thick (has).

FIG. 9 shows a sixth embodiment. This embodiment applies asymmetric phase modulation by making the cross-sectional structure of the electrode asymmetric, and assumes an X plate or a Y pseudoelectro-optic crystal. The optical waveguide 1.2 is modulated by one modulation electrode, and the distance between the optical waveguide 2 and the electrode is increased.

A seventh embodiment is shown in FIG. 10, in which only the optical waveguide 1 is modulated, and a Z-plate electro-optic crystal is assumed.

FIG. 11 shows an eighth embodiment. This modulates only the optical waveguide 1, and assumes an X-plate or Y-plate electro-optic crystal.

〔Effect of the invention〕

FIG. 12 shows the calculation result of the deterioration of the minimum received light power caused by chromatic dispersion, that is, the power penalty. The allowable value of the power penalty caused by fiber transmission is set to 0.
When setting the wavelength dispersion to 5 dB, the allowable wavelength dispersion value in conventional modulation methods is 500 to 700 ps/nm, whereas when the two-phase modulation ratio is set to 5:1, it is 1500 ps/nm.
It is improved to more than ps/nm. FIG. 13 shows similar calculations performed using different phase modulation ratios, and it can be seen that it is sufficient if the modulation ratio is 2:1 or more.

As can be seen from the above calculation results, the present invention improves fiber transmission characteristics as compared to conventional modulation systems, and greatly contributes to improving the performance of high-speed optical communication devices.

[Brief explanation of drawings]

FIG. 1 and FIG. 2 are diagrams showing the principle of the present invention. FIG. 3 is a configuration diagram of the first embodiment of the present invention. FIG. 4 is a time chart showing the operation of the first embodiment. FIG. 5 is a diagram showing a second embodiment of the present invention. FIG. 6 is a diagram showing a third embodiment of the present invention. FIG. 7 is a diagram showing a fourth embodiment of the present invention. FIG. 8 is a diagram showing a fifth embodiment of the present invention. FIG. 9 is a diagram showing a sixth embodiment of the present invention. FIG. 1O is a diagram showing a seventh embodiment of the present invention. FIG. 11 is a diagram showing an eighth embodiment of the present invention. FIGS. 12 and 13 are diagrams showing calculation results for improving fiber transmission characteristics according to the present invention. In the figure. l: Optical waveguide that applies large phase modulation. 2. Optical waveguide that reduces the phase modulation. 3. Electrode for modulation. 4. Earth electrode. 5. Termination resistor. 6. Drive circuit for performing phase modulation of optical waveguide 1. 7. Phase modulation of optical waveguide 2. This is a drive circuit for performing the following. Engraving of the drawing (no change in content) O 77CoSCw, t+φA) φA>o・φδkuQノ /φAI> /φEl・/me−φAl MisakiQ (high)lφδ - φ4/Negative (尤10w じ本4e-輯f) Hara 1ri 1 diagram 0) 1st f! ! Book4egR'),! , Diagram (2) showing the principle 1

Claims (1)

[Claims] In a Mach-Zehnder interferometer type optical modulator, by asymmetrically modulating the phase of light propagating through two optical waveguides, the center wavelength of the rising part of the modulator output light is moved to the longer wavelength side, and the center wavelength of the falling part is shifted to the longer wavelength side. An optical modulation method characterized by moving the wavelength toward shorter wavelengths.