JPH0886991A - Optical transmission method, optical transmission device, and optical transmission system - Google Patents

Abstract

(57) [Abstract] [Problem] Transmission speed of several gigabits per second or more,
Also, to provide an optical transmission method capable of long-distance transmission even in an optical fiber transmission line having large wavelength dispersion. In an optical transmission method for transmitting an optical pulse generated by an optical modulator, the optical pulse is generated, and the central wavelength of the rising portion of the optical pulse is moved to the long wavelength side, and the central wavelength of the falling portion is moved. To move to the short wavelength side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulation system used in a transmission device for transmitting a high speed digital signal over a long distance in an optical communication system using an optical fiber as a transmission line. FIG. 14 is a diagram showing a configuration of an optical communication system, which shows an optical communication system well known in the related art. In the figure, 10 and 20 are optical transmission devices on the transmitting side and the receiving side, respectively. Reference numeral 30 denotes an optical fiber transmission line.

The optical transmission device 10 on the transmission side generates an optical signal intensity-modulated by an electric signal carrying a high-speed digital signal, and the generated optical signal is introduced into an optical fiber transmission line 30. The optical signal propagates through the optical fiber transmission line 30, is transmitted to the optical transmission device 20 on the receiving side, is converted into an electric signal by a photoelectric conversion element (not shown) in the optical transmission device 20, and the original digital signal is obtained. It is what is received.

FIG. 15 is a diagram for explaining the chromatic dispersion characteristic of an optical fiber transmission line. In FIG. 15, the horizontal axis represents wavelength, the vertical axis represents chromatic dispersion, and λ ₀ represents zero-dispersion wavelength. The optical fiber transmission line has wavelength dispersion characteristics as shown in FIG. That is, even in the same optical fiber transmission line, the propagation speed of light varies depending on the wavelength. That is, the wavelength dispersion coefficient (ps / nm / km) takes a positive value in the wavelength region longer than the zero dispersion wavelength λ ₀ , and takes a negative value in the short region.

On the other hand, in a well-known direct modulation system in which a light source such as a semiconductor laser device is directly driven by a digital signal, it is known that the center wavelength varies within the pulse waveform due to the physical characteristics of the light source. That is, in the rising portion and the falling portion of the light pulse in which the light source is driven around the threshold voltage, the emission wavelength becomes unstable near the predetermined center wavelength. Such a phenomenon is called wavelength chirping or simply chirping, and causes a pulse waveform deterioration due to a synergistic effect with the wavelength dispersion characteristic of the transmission line.

FIG. 16 is a diagram for explaining the deterioration of the optical pulse waveform. Here, considering the case where the chirped optical pulse is transmitted over a considerable distance in the optical fiber transmission line, there are different wavelength components in the optical pulse. Transmission delay difference occurs. Then, the initial optical pulse waveform as shown in FIG. 16 (a) collapses in the transmission line because the components having different propagation delays are combined, and finally the waveform of FIG. This causes waveform deterioration as shown in (b).

On the other hand, the chirping at this time can be regarded as a spectrum spread within the optical pulse waveform, and it is considered that the waveform deterioration can be suppressed to some extent by reducing such a spectrum spread. . That is, if a modulation method that can eliminate the wavelength chirping due to the modulation is adopted, the spectrum spread is alleviated, and the waveform deterioration can be expected to be suppressed.

For this reason, it is possible to perform modulation with a small spectrum spread by generating an optical pulse by an interferometric optical modulator provided outside without directly driving the light source itself with a signal. Attention has been paid to an optical transmission method based on the method.

[0008]

2. Description of the Related Art An optical modulator such as a Mach-Zehnder interferometer type optical modulator to which optical interference is applied is one of the optical modulation methods that have the smallest spectrum spread and are therefore less susceptible to the chromatic dispersion of an optical fiber. , Interferometric optical modulator). In a well-known general interferometric modulator, light introduced from a light source is branched and propagated in two optical waveguides provided with control electrodes to increase or decrease the phase of each light by the same amount in opposite directions. As a result, a phase difference of 0 or π is given, and by making them interfere with each other, an intensity-modulated optical pulse with little wavelength chirping is generated.

That is, the interferometric modulator can reduce the wavelength chirping to the extent due to the modulation sideband that is the Fourier component of the modulation waveform (F.KOYAMA et al.,
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.6, NO.1, 198
8, PP.87-93). However, at a transmission speed of several gigabits / second or more, even if the wavelength chirping is set to zero, the spectrum spread due to the modulation sideband is essentially present, so the optical pulse waveform due to the chromatic dispersion characteristic of the optical fiber transmission line. Can not be completely ignored.

[0010]

Therefore, even if the spectral spread of the optical pulse is reduced to that due to the modulation sideband by adopting the external modulation method as described above, it is possible to achieve a high speed of several gigabits / second or more. In the area of transmission speed, for example, there is a problem that long-distance transmission cannot be performed in an optical transmission system using an optical fiber transmission line that does not exhibit zero dispersion with respect to the center wavelength of the light source used.

It is an object of the present invention to provide an optical transmission method capable of long-distance transmission even at a transmission speed of several gigabits / second or more and even on an optical fiber transmission line having a large chromatic dispersion.

[0012]

1 and 2 are explanatory views of the principle of the present invention. FIG. 1 shows the electric field of light at each part of the interferometric optical modulator. In the figure, E ₀ is the amplitude of the electric field of the input light,
ω ₀ represents the angular frequency of the electric field, t represents time, and φ _A and φ _B represent the phases modulated in the optical waveguides A and B, respectively.
E _out (t) is the electric field of the output light, and details are shown in the following Expression 1.

[0013]

[Equation 1]

As can be seen from equation 1, E _out (t) has ta
n ^-1 (Y / X) phase modulation is applied. This results in wavelength chirping as shown below. ω ₀ t- tan ^-1 (Y / X)
Where Φ is the angular frequency,

[0015]

[Equation 2]

Since the wavelength λ = 2πc / ω (t) (c is high speed), Δω = d (tan ⁻¹ (Y / X)) / dt causes wavelength chirping. Here, the phase modulation is performed, for example, as in Expression 3.

[0017]

(Equation 3)

The operation waveforms of the respective parts at this time are shown in FIG. As shown in (f) of the figure, the phase of the output light is delayed at the portion where the output light intensity rises, and advanced at the portion where the output light intensity falls. Correspondingly, the center wavelength moves to the long wavelength side at the rising portion and to the short wavelength side at the falling portion as shown in (g). On the other hand, in the above-described conventional method, the modulation is performed under the condition of φ _A = −φ _B. In this case, E _out (t) is as shown in Expression 4.

[0019]

[Equation 4]

It should be noted that the amplitude of the optical electric field is only modulated by the modulation of φ, and the wavelength fluctuation due to the modulation does not occur. In the present invention, by modulating the phase of each waveguide of the modulator asymmetrically, the center wavelength of the output light is increased to the long wavelength side at the rising portion and to the falling portion as shown in (g) of FIG. So that it moves to the short wavelength side.

On the other hand, the chromatic dispersion coefficient of the optical fiber transmission line is, as shown in FIG. 15, a single mode fiber showing a zero dispersion wavelength in the 1.3 μm band is used for the optical transmission line, and the chromatic dispersion coefficient is used as the signal light wavelength. It becomes a large value when it is used in the positive region of, for example, the 1.55 μm band with less loss. The wavelength dispersion coefficient at this time is, for example, 20 ps / nm / km at maximum, and the longer the wavelength, the slower the speed of propagation in the optical fiber.

Therefore, since the wavelength chirping generated by asymmetrically modulating the phase as described above and the effect of the positive chromatic dispersion of the optical fiber transmission line, the wavelength becomes long at the rising portion of the optical pulse. Since the propagation speed is relatively delayed and the wavelength is shortened at the falling portion, the propagation speed is relatively increased on the contrary. According to the above operation, the optical pulse generated by the present invention causes pulse compression while propagating through the optical fiber transmission line.

This pulse compression works in the direction of compensating for the waveform broadening caused by the chromatic dispersion of the modulation sideband and the optical fiber transmission line, and therefore has the effect of improving the transmittable optical fiber length.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION An interferometric optical modulator is one to which a so-called electro-optical effect is applied. That is, the refractive index of a substance having an electro-optical effect is changed by applying an electric field to control the phase of light propagating in the substance. Therefore, in the interferometric optical modulator, some forms can be considered as specific means for asymmetrically modulating the phase of light propagating through the two optical waveguides.

The first method is to modulate each optical waveguide with a different drive voltage. The second is a method in which the driving voltage is also made asymmetric in the cross-sectional structure of the electrodes so that the modulation electric field is applied to the optical waveguide asymmetrically. The third is a method in which the electrode length is made different for each optical waveguide and the waveguide length at which light feels a change in the refractive index is changed.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 3 is a diagram showing a main part of the first embodiment.
In the figure, reference numeral 1 is an optical waveguide for which phase modulation is largely applied, and 2 is an optical waveguide for which phase modulation is small.
Reference numeral 3 is a modulation electrode, and 4 is a ground electrode. 3 and 4
And constitute a traveling wave type electrode. Reference numeral 5 denotes a terminating resistor, which is matched with the characteristic impedance of the traveling wave electrode. Reference numeral 6 is a drive circuit for performing the phase modulation of the optical waveguide 1, and 7 is a drive circuit for performing the phase modulation of the optical waveguide 2.

This embodiment is an example in which the drive voltage amplitude is applied asymmetrically, and a Z-plate electro-optic crystal is assumed. Needless to say, the same can be applied to the X-plate and Y-plate electro-optic crystals. FIG. 4 is a time chart diagram showing the operation of the first embodiment. V1 is a drive waveform for performing phase modulation of the optical waveguide 1, and V2 is a drive waveform for performing phase modulation of the optical waveguide 2. Phases are asymmetrically applied by reversing the polarities of V1 and V2 and increasing the drive voltage amplitude of V1.

FIG. 5 shows a second embodiment. In this embodiment, the electrode length is made asymmetric and the phase modulation is made asymmetric, and a Z-plate electro-optic crystal is assumed. It is assumed that the symbols 1 to 4 in FIGS. 5 to 11 are the same as those in FIG. FIG. 6 shows a third embodiment.
In this embodiment, the electrode length is made asymmetric and the phase modulation is made asymmetric, and an X-plate or Y-plate electro-optic crystal is assumed. The positional relationship of the electrodes with respect to the optical waveguide is different from that in FIG.

FIG. 7 shows a fourth embodiment. FIG. 7 shows a cross-sectional structure of the interferometric optical modulator. In the present embodiment, the phase modulation is asymmetrically applied by making the sectional structure of the electrode asymmetrical, and a Z-plate electro-optic crystal is assumed. In this embodiment, the positions of the electrodes corresponding to the optical waveguide 2 are arranged with a slight shift. FIG. 8 shows a fifth embodiment. In this embodiment, the phase modulation is applied asymmetrically by making the sectional structure of the electrode asymmetrical,
An X-plate or Y-plate electro-optic crystal is assumed. In this embodiment, the distance between the optical waveguide and the electrode is increased.

FIG. 9 shows a sixth embodiment. In the present embodiment, the phase modulation is applied asymmetrically by making the sectional structure of the electrode asymmetrical, and an X-plate or Y-plate electro-optic crystal is assumed. In this embodiment, the optical waveguides 1 and 2 are modulated by one modulation-requiring electrode, and the distance between the optical waveguide 2 and the electrodes is increased. FIG. 10 shows a seventh embodiment. In this embodiment, only the optical waveguide 1 is modulated, and a Z-plate electro-optic crystal is assumed.

FIG. 11 shows an eighth embodiment. In this embodiment, only the optical waveguide 1 is modulated, and an X plate or Y plate electro-optic crystal is assumed.

[0032]

FIG. 12 is a diagram showing the deterioration of the minimum received power caused by chromatic dispersion, that is, the power penalty calculation value. When the allowable value of the power penalty caused by optical fiber transmission is 0.5 dB, the chromatic dispersion amount that can be tolerated by the conventional modulation method is 500 to 700 ps / nm, whereas the phase modulation ratio of the present invention is 5: 700 ps / nm. When set to 1, it is improved to 1500 ps / nm or more.

FIG. 13 shows the same calculation with different phase modulation ratios, and a sufficient effect is recognized when the modulation ratio is 2: 1 or more. As can be seen from the above, according to the present invention,
Compared with the conventional optical transmission method using the optical modulation method, the transmission characteristic is improved, and it largely contributes to the performance improvement of the high-speed optical communication device.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of the present invention (1)

FIG. 2 is a diagram for explaining the principle of the present invention (2)

FIG. 3 is a configuration diagram of a first embodiment of the present invention.

FIG. 4 is a time chart diagram explaining the operation of the first embodiment of the present invention.

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. 10 is a diagram showing a seventh embodiment of the present invention.

FIG. 11 is a diagram showing an eighth embodiment of the present invention.

FIG. 12 is a diagram showing a calculation result of transmission characteristic improvement according to the present invention (1).

FIG. 13 is a diagram showing a calculation result of transmission characteristic improvement according to the present invention (2).

FIG. 14 is a block diagram of an optical communication system.

FIG. 15 is a diagram illustrating chromatic dispersion characteristics of a transmission line.

FIG. 16 is a diagram illustrating deterioration of a pulse waveform.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... 1st optical waveguide 2 ... 2nd optical waveguide 3 ... Modulation electrode 4 ... Earth electrode 5 ... Termination resistance 6 ... 1st signal drive circuit 7 ... 2nd signal drive circuit

Continuation of front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location H04B 10/142 10/04 10/06 (72) Inventor Tadashi Okiyama 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Fujitsu Incorporated (72) Inventor Minoru Seino 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Fujitsu Limited

Claims (4)

[Claims] 1. An optical transmission method for transmitting an optical pulse generated by an optical modulator, wherein the optical pulse is generated, and the center wavelength of the rising portion of the optical pulse is moved to a long wavelength side, and the center of the falling portion is moved. An optical transmission method characterized by moving a wavelength to a short wavelength side. 2. An optical transmission method for producing an optical pulse modulated by an interferometric optical modulator, introducing the produced optical pulse into an optical fiber transmission line, and transmitting the optical pulse, wherein the interferometric optical modulator is used for the optical pulse. Is generated and the central wavelengths of the rising and falling portions of the optical pulse are moved in a direction in which pulse compression occurs due to chromatic dispersion of the optical fiber transmission line. 3. An optical transmission device comprising a light source for supplying light having a predetermined center wavelength and an interferometric modulator for modulating the light of the light source to generate an optical pulse, wherein the optical pulse is generated. At the same time, the optical transmission device is provided with the interferometric optical modulator that moves the center wavelength of the rising portion of the optical pulse to the longer wavelength side than the predetermined center wavelength and moves the falling portion to the shorter wavelength side. 4. A first optical transmission device that emits an optical pulse modulated by an interferometric optical modulator, an optical fiber transmission line that transmits the optical pulse from the first optical transmission device, and the optical device. In an optical communication system having a second optical transmission device that introduces the optical pulse via a fiber transmission path, the first optical transmission device is configured such that the center wavelengths of rising and falling portions of the optical pulse are the optical wavelengths. An optical communication system comprising the interferometric optical modulator, which is moved in a direction in which pulse compression is generated due to wavelength dispersion of a fiber transmission line.