JPH10303826A - Transmission characteristic evaluation method, optical modulator, optical transmitter and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an α-parameter and the fiber transmission characteristic from the quenching characteristic of a field absorption type (EA) modulator with no execution of a transmission experiment, deciding the relation between the quenching characteristic and the α-parameter, i.e., the fiber transmission characteristic, and performing the unitary management of both characteristics. SOLUTION: A transmitter 1 transmits the data input 11 to an EA modulator 17 via a flip-flop 13 and an EA modulation driving circuit 14 and in the timing of a clock 12. The modulator 17 modulates the output of a laser diode 16 and converts it into an optical signal 18. The modulator 17 also controls the optical output and phase based on the applied voltage. A specific relation is secured among a quenching ratio, an α-parameter, an optical output and an optical phase respectively. Thereby, the waveform of the optical signal that is transmitted via a transmission line having the group velocity dispersion can be calculated from the optical output 18 modulated by the modulator 17. Thus, it is possible to evaluate the transmission characteristic of a modulated signal by calculating the α-parameter from the quenching characteristic of the modulator 17.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electro-absorption (hereinafter abbreviated as EA) modulator, an optical transmitter using an EA modulator in combination with a laser diode (hereinafter abbreviated as LD), and an optical transmission system. .

[0002]

2. Description of the Related Art Conventionally, a single mode fiber used as a transmission line in an optical transmission system is a normal dispersion fiber in which the group velocity dispersion of light becomes zero at a wavelength of 1.3 μm.
In recent years, in order to extend the relay distance, it has become common to perform optical transmission with light in the 1.5 μm band that minimizes fiber loss. However, the group velocity dispersion of the dispersion fiber is usually large in the 1.5 μm band, and in high-speed optical transmission, the large group velocity dispersion is combined with the chirp of the optical pulse, so that the transmission degradation of the optical pulse occurs. Chirp is phase modulation that occurs with intensity modulation of light, so that the frequency of light fluctuates, and the wavelength fluctuates accordingly.

For this reason, a dispersion-shifted fiber in which the group velocity dispersion becomes zero in the 1.5 μm band has been put to practical use. However, in the existing fiber, the dispersion fiber usually accounts for an overwhelming proportion at present, and the transmission system can be speeded up by replacing the optical transceiver with a high-speed one using the existing normal dispersion fiber transmission line. Highly required. In this case, in order to realize high-speed optical transmission, it is important to reduce chirp in the optical transmitter.

As the transmission speed required for an optical transmission system increases, an LD (laser diode) is required for an optical transmitter.
It is indispensable to adopt an external modulation method which has a lower chirp property than the direct modulation method. The external modulation system uses an optical modulator as an electric / optical conversion device separately from a light source. In this external modulation method, an EA modulator is one of the electric / optical conversion devices used as the optical modulator. Further, an EA / DFB integrated light source in which this is integrated with a light source has been put to practical use.

Conventionally, an EA modulator or an EA / DFB
The size of the chirp of an integrated light source is usually controlled by a parameter called α-parameter. For example,
FirstOptoelectronics and Communications Conference
(OECC '96) Technical Digest (June 1996), 18B3-3
(Pp. 332-333) describes a method for determining transmission characteristics from α-parameters in an EA / DFB integrated light source.

On the other hand, the extinction characteristic, which is the relationship between the light output intensity and the voltage applied to the modulator, has been managed separately from the chirp characteristic.

In designing an optical transmission system,
The relationship between the optical output waveform of the modulator element and the transmission characteristics is not clear. In practice, an optical transmission system has been constructed by selecting, by experiments, elements that satisfy both simultaneously.
Specifically, as shown in Fig. 1, a transmission line for evaluation that simulates an actual transmission line is constructed, and transmission characteristics are grasped by conducting a transmission experiment using an EA modulator to select modulator elements. Was. In this method, the α-parameter was merely an auxiliary evaluation method.

[0008]

In the conventional EA modulator element characteristic management method and transmission design method in which the extinction characteristic and the α-parameter, that is, the fiber transmission characteristic are managed as separate characteristics, the two characteristics are independent. Was treated as something. As a result, many characteristic management items were required.
In addition, transmission design cannot be performed in consideration of essential performance limitations based on the operation principle of the EA modulator, and an evaluation transmission line that simulates an actual transmission path is constructed, and a transmission experiment is performed using the EA modulator. As a result, the transmission characteristics have to be grasped and modulator elements must be selected.

It is an object of the present invention to remedy the disadvantages of the prior art described above. Specifically, the transmission characteristics of the modulated signal after passing through the transmission path are estimated from the extinction characteristics of the EA modulator, and the extinction characteristics and the transmission characteristics are unified, and applied to element design and element characteristic management. It is in. It is another object of the present invention to obtain a transmission characteristic evaluation method that does not require modulator element selection, an optical modulator suitable for a transmission path, an optical transmitter, and an optical transmission system having excellent transmission characteristics.

[0010]

According to the inventors' consideration,
In an EA modulator that operates based on light absorption in a medium,
The extinction characteristic and the α-parameter should each be considered as two closely related phenomena. That is, the extinction characteristic and the α-parameter are based on the change in the light absorption coefficient and the change in the refractive index of the medium when an electric field is applied. According to this consideration, it is suggested that the extinction characteristic and the α-parameter, that is, the fiber transmission characteristic of the EA modulator are not independent, and that there is a close relationship between the two.

In order to solve the above-mentioned problem, in the present invention, the relationship between the extinction characteristic and the α-parameter is determined, and both characteristics are managed in an integrated manner. With this management method, the α-parameter and the fiber transmission characteristic can be obtained from the extinction characteristic without performing a transmission experiment by the relationship between the extinction characteristic and the α-parameter. Also, the modulator element specifications required for transmission on a given line can be determined. Specifically, the first-order Sta at the absorption edge is used as the extinction mechanism of the EA modulator.
Assuming the rk shift, the change in the refractive index is obtained from the extinction characteristic, that is, the change in the absorption coefficient, based on the Kramers-Kronig relationship. The ratio between the change in the refractive index and the change in the absorption coefficient is the α-parameter, from which the transmission characteristics can be estimated.

[0012]

FIG. 2 is a block diagram showing an embodiment of the present invention.
This will be described with reference to FIG.

First Embodiment First, a transmission characteristic evaluation method according to a first embodiment of the present invention will be described with reference to FIGS. Here, FIG. 2 is a block diagram showing the configuration of the optical transmission system, and FIG. 3 is a block diagram showing the configuration of the optical transmitter 1.

FIG. 2 shows the configuration of the simplest optical transmission system. The optical transmission system comprises an optical transmitter 1 and a fiber for transmitting an optical signal.
2, composed of an optical receiver 3. In the transmitter 1, a data input 11 is input to an EA modulator 17 which is an electric / optical conversion device via a flip-flop 13 and an EA modulator driving circuit 14 at the timing of a clock input 12, and a driving current source
The output of a laser diode (hereinafter abbreviated as LD) 16, which is a light source having 15, is modulated and converted into an optical signal 18.

In the EA modulator 17, the optical output I (V) and the optical phase φ (V) are controlled by the applied voltage V. Here, dB display of light output

[0016]

(Equation 1)

Is called an extinction ratio. Normally, the extinction ratio is expressed by using the optical output I (0) at V = 0 as a reference (0 dB) as in Expression (1). Also, the ratio α of the intensity modulation amount dI / I and the phase modulation amount dφ by the applied voltage V

[0018]

(Equation 2)

Is called the α-parameter. Equation (2)
Is written in integral form,

[0020]

(Equation 3)

It can also be expressed as

Equations (1) and (3) show the relationship between the extinction ratio, α-parameter, optical output I (V), and optical phase φ (V).

The optical carrier from the LD has an optical frequency ω0
And using the proportionality constant k

[0024]

(Equation 4)

When modulated by an EA modulator applied with a voltage V (t) and given by

[0026]

(Equation 5)

## EQU1 ## here,

[0028]

(Equation 6)

Are the amplitude and phase of the photoelectric field modulated by the EA modulator, respectively.

The optical output E modulated by the EA modulator
From (t), it is possible to calculate the waveform E ′ (t) of the optical pulse transmitted through the transmission line having the group velocity dispersion. Here, for simplicity, using the equivalent low-pass representation of E (t),

[0031]

(Equation 7)

This is numerically Fourier-transformed into an optical spectrum E (f), and the transfer function of the fiber having group velocity dispersion in E (f) is represented by an equivalent low-pass expression.

[0033]

(Equation 8)

Is multiplied by Where L0 is the total loss [dB], D
Is the total group velocity dispersion [s / m = 103 ps / nm], c is the light speed, and λ is the light wavelength.

E ′ (f) = H (f) E (f) By performing an inverse Fourier transform on the E ′ (f), an equivalent low-frequency representation E ′ (t) of the optical waveform after fiber transmission is obtained. Can be. Receiving sensitivity can be calculated from a waveform that has been received by a photodiode, which is a converter that converts a light waveform after transmission through an optical fiber into a photocurrent proportional to the square of an electric field, and that has passed through a Bessel filter. The determination as to whether transmission is possible for a given optical fiber transmission line is made based on a transmission penalty, which is a ratio between the reception sensitivity during no transmission and the reception sensitivity during fiber transmission. According to the present embodiment, it is possible to evaluate the transmission characteristics of the modulated signal by calculating the α-parameter from the extinction characteristic, which is the DC characteristic of the EA modulator.

In the above discussion, the response of the fiber is assumed to be linear. However, in practice, when a high-level light is input to an optical fiber using an optical transmission amplifier, etc., the self-phase modulation based on the Kerr effect, which is a nonlinear optical effect that changes the refractive index of the optical fiber due to the input light. (Self-Phase Modulation) is known to occur. In this case, as a method of analyzing the self-phase modulation effect, for example, Nonlinear Fiber Optics, (Academic Pre
ss, 1989), the optical waveform after optical fiber transmission can be calculated.

Second Embodiment An optical modulator and an optical transmitter according to a second embodiment of the present invention will be described with reference to FIGS. Here, FIG. 4 is an explanatory diagram showing a frequency shift of the absorption spectrum a (ω) in proportion to the applied voltage V. FIG. 5 is an explanatory diagram showing a frequency shift of the refractive index spectrum η (ω) in proportion to the applied voltage V.

First, the relationship between the extinction characteristic and the α-parameter will be determined. The operation principle of the EA modulator is based on the change in the absorption coefficient a of the medium when an external electric field is applied. Assuming that the effective complex refractive index of the medium is n = η-i κ, the absorption coefficient a corresponds to its imaginary part κ,

[0039]

(Equation 9)

There is the following relationship. Here, ω is the light frequency, and c is the light speed. Therefore, the optical output I (V) is

[0041]

(Equation 10)

## EQU3 ## Here, I0 is the light input intensity to the modulator, and L is the modulator length.

On the other hand, the real part η of n is the refractive index, the optical phase φ,

[0044]

[Equation 11]

There is the following relationship. Kramers-Kronig relation between real part η and imaginary part κ of n

[0046]

(Equation 12)

Therefore, the Kramers-Kronig relationship also exists between the absorption coefficient spectrum and the refractive index spectrum of the electroabsorption medium for an arbitrary applied voltage V.

[0048]

(Equation 13)

Holds. Equation for change in applied voltage V
Since (13) must always hold, differentiate by V

[0050]

[Equation 14]

The following holds.

Here, if the range in which the absorption spectrum changes according to the voltage applied to the EA modulator is limited to ω1 <ω <ω2, the integration range can be limited to this section. further,
The change in the absorption coefficient of the EA modulator is based on the Strk shift of the frequency proportional to the electric field of the absorption spectrum, that is, a first-order frequency shift with respect to the electric field is assumed as the change in the absorption coefficient. Under these conditions, the absorption coefficient a of the EA modulator and the spectra a (ω) and η (ω) of the refractive index η show a frequency shift proportional to V with respect to the applied voltage V as shown in FIGS. If -kV occurs, then a (ω0, V) = a (ω0 + kV, 0) η (ω0, V) = η (ω0 + kV, 0). Here, k is a proportional constant representing the magnitude of the frequency shift with respect to the applied voltage. ω = ω0 + kV

[0053]

(Equation 15)

By executing the integration, da / dV
Dη / dV can be obtained from

That is, the product of the absorption coefficient and the modulator length a (V) ·
Since L is obtained from the extinction characteristic, the product η (V) · L of the refractive index and the modulator length can be obtained by multiplying the left side of Expression (6) by the modulator length L. Further, from equation (11), the optical phase φ (V),
The α-parameter α (V) can be calculated from Expression (10) and Expression (2).

According to the second embodiment, the α-parameter could be calculated from the extinction characteristic. As described in the first embodiment, if the α-parameter is obtained, it is possible to calculate the optical waveform and the reception sensitivity after passing through a given optical fiber transmission line. Then, it can be determined whether the EA modulator having the extinction characteristic can be applied to a given optical fiber transmission line. That is, according to the present embodiment, the α-
Since the parameters can be obtained, it is possible to determine the modulator element specifications required for a given transmission path. As a result, if the extinction characteristics of the optical modulator or the optical transmitter are measured, an optical modulator and an optical transmitter suitable for the transmission path can be obtained.

In the above discussion, the modulator length L is used in the calculation, but the calculation results φ (V) and α (V) do not depend on L. Because when the extinction characteristic is decided, a is L-1
Has the L-dependence, and η calculated from this a is also L-
This is because L has an L-dependency of 1 and L cancels out in the calculation of φ and α. Therefore, the value of L can be set arbitrarily in the calculation.

[Embodiment 3] A transmission system according to a third embodiment of the present invention will be described with reference to FIGS. 2, 6 to 10. Here, FIG. 6 is a diagram illustrating a relationship between the voltage V and the extinction characteristic R (V). FIG. 7 is a diagram showing a result of calculating α-parameters for the extinction characteristics A and B. FIG.
FIG. 4 is a diagram comparing extinction characteristics with optical waveforms obtained before and after fiber transmission obtained by calculation. FIG. 9 is a flowchart showing the procedure of the calculation. FIG. 10 is a diagram illustrating a range where the transmission penalty is 2 dB or less.

Here, a design method in the case where a normal dispersion fiber having a transmission distance of 40 km, 1.3 μm, and no group velocity dispersion of 0 is used in the 1.5 μm band will be described. Generally, optical fiber loss is
Since there is 0.2 dB to 0.3 dB per km, the total fiber loss for 40 km is 8 dB to 12 dB. This transmission distance is typically limited to the optical output level of a 10 Gb / s optical transmitter
Considering that the minimum receiving sensitivity of the optical receiver can be -14 dBm or less, the transmission system can be configured without using an optical amplifier. From the typical value of group velocity dispersion in the 1.5 μm band of a dispersion optical fiber of 15 ps / nm to 18 ps / nm per km, the total group velocity dispersion is 600 ps / nm to 720 ps / nm. . This value is E
The transmission design is implemented with the goal of achieving transmission without a dispersion compensator by optimizing the EA modulator by approaching the theoretical limit for the α-parameter value α ≒ 0 that can be realized by the A modulator. . The parameters that can be adjusted by the designer of the transmission system including the optical transmitter 1, the optical fiber 2, and the optical receiver 3 shown in FIG. 2 are the extinction characteristic and the fiber input light level. Normal optical output of optical transmitter 0 dBm
To some extent, it is known that the nonlinear optical effect of the fiber is not significant. Therefore, it is not necessary to consider the fiber input light level, and the parameter to be adjusted is the extinction characteristic. Let us determine the extinction characteristic specifications of the EA modulator that enables transmission for a 40 km non-repeater optical transmission system with a normal dispersion fiber at a transmission speed of 10 Gb / s. Incidentally, the total group velocity dispersion amount of the normal dispersion fiber is from 600 ps / nm to 720 ps / nm as described above.
And

The extinction characteristic of the EA modulator is calculated as follows: R (V) = 10 log1
0 (I (V) / I (0))

[0061]

(Equation 16)

Here, Expression (16) is an example, and is not limited to this type of function. Any function that can be differentiated by V may be used. Here, R1, V1, and n are parameters that describe extinction characteristics, and specific extinction characteristics are determined by giving R1, V1, and n.

FIG. 6 is a graph showing the relationship between the voltage V and the extinction characteristic R (V), which specifically shows the equation (16).
A is R for R1 = -5 dB, V1 = 1 V, and n = 1 in equation (16).
In the example of (V), the extinction characteristic B is an example of R (V) for R1, = -5 dB, V1 = 2 V, and n = 2. From this extinction characteristic, the α-parameter is calculated by the above method. FIG. 7 shows the results of calculating α-parameters for the extinction characteristics A and B shown in Expression (16) and FIG.

Next, an equivalent low-frequency expression E (t) of the light output is obtained. Here, 0.5V when the optical signal is ON, and 0.5V when the optical signal is OFF
Suppose a voltage of 2.5V is input to the EA modulator. Rise time and fall time are each 45 ps (10% -90%)
A trapezoidal wave such that the input voltage waveform to the EA modulator is V
(t). Light output I is R, R is a function of input voltage V, and V is a function of time t. Since each function form is as shown above, the light output I can be calculated as a function I (t) of t. Also, the optical phase
φ is a function of V, and V is a function of t. φ (V)
Is as described in the second embodiment. V (t)
Since the example of the function form is shown above, the optical phase φ can be calculated as a function φ (t) of t from these. From I (t) and φ (t) obtained in this way, the equivalent low-pass expression E (t) of the light output is calculated by equation (7). FIG.
FIG. 4 is a diagram comparing extinction characteristics with optical waveforms shown by eye patterns before and after fiber transmission. Extinction characteristics A, B
For each, EA which is the optical waveform before fiber transmission
The calculation result of the optical output | E (t) | 2 is shown in the left half of FIG.

Next, a change in the optical waveform when E (t) is transmitted through an optical fiber which is a transmission line having group velocity dispersion is calculated. That is, E (t) is numerically Fourier-transformed into an optical spectrum E (f), and E (f) is multiplied by equation (8) which is a transfer function of a fiber having group velocity dispersion. The product E '(f) of E (f) and H (f) was subjected to inverse Fourier transform to calculate the optical waveform E' (t) after transmission. The calculation results of the optical waveform | E '(t) | 2 after fiber transmission for each of the extinction characteristics A and B are shown in the right half of FIG.

Finally, a reception equalization waveform using a Bessel filter was calculated from the optical waveforms before and after the fiber transmission, and a transmission penalty was calculated as a ratio of the minimum reception sensitivity. For extinction characteristic A, minimum reception sensitivity before transmission -14.
6 dBm, the minimum receiving sensitivity after transmission was -13.8 dBm. Therefore, the transmission penalty was 0.8 dB. For the extinction characteristic B, the minimum reception sensitivity before transmission was -13.9 dBm, and the minimum reception sensitivity after transmission was -11.7 dBm. Therefore, the transmission penalty was 2.2 dB. Generally, a transmission penalty is used as a criterion for determining the possibility of transmission.If this criterion is set to 2 dB or less, the extinction characteristic A can be transmitted for 40 km, and the extinction characteristic
Regarding B, it can be determined that transmission is impossible for 40 km.

From the above calculation, R1 = −5 dB, −10 dB, −15
The transmission penalty is calculated for a total of 60 combinations of four combinations of three types of dB, V1 = 0.5 V, 1 V, 1.5 V, 2 V, and 2.5 V, and n = 1, 1.5, 2, and 2.5. FIG. 9 is a flowchart showing a calculation flow. For the combination of R1, V1, and n (step 9-1), calculate the extinction characteristic R (V) (step
9-2), α (V) and φ (V) are calculated from the Kramers-Kronig relational expression (step 9-3). The optical output waveform E (t) of the optical transmitter is calculated from R (V) and φ (V) (step 9-4). E (t)
Is subjected to Fourier transform (step 9-5), and is multiplied by the transfer function of the fiber having group velocity dispersion (step 9-6) to perform inverse Fourier transform to obtain an optical waveform E ′ (t) after fiber transmission (step 9-5). 9-7). The minimum reception sensitivity is calculated for each of the optical waveform E (t) before fiber transmission and the optical waveform E '(t) for fiber transmission (step 9-8). Step 9-1 to step 9-9
Are repeated by the number of combinations of the extinction characteristics.

The ratio of the minimum reception sensitivity before transmission and after transmission is the transmission penalty. In this embodiment, the transmission penalty is 2 dB
It was determined that transmission was possible when: As a result, the combinations with a transmission penalty of 2 dB or less are: R1 = 5 dB, V1 = 1 V, n = 1 R1 = 5 dB, V1 = 1.5 V, n = 1 R1 = 5 dB, V1 = 2 V , N = 1. By interpolating n and R1, the transmission penalty is
When calculating the range of 2 dB or less, when R1 = 5 dB, V1 = 1 V, n ≤ 1.11 R1 = 5 dB, when V1 = 1.5 V, n ≤ 1.38 R1 = 5 dB, V1 = 2 V When n ≤ 1.29 n = 1, V1 = 1.5 V, R1 ≤ 9.13 dB When n = 1 and V1 = 2 V,-and R1 ≤ 8.72 dB are obtained, and the extinction characteristic parameters R1, V1, and n are It was found that being within the range was a necessary condition for the transmission penalty to be 2 dB or less.

At this time, the extinction characteristic of the EA modulator is I (0.
5) = 1 (0 dB), -3.1 dB <I (1 V) <-1 dB and -6.1 dB <I (1.5 V) <-2 dB and -8.5 dB <I (2 V) <−3 dB and −10 dB <I (2.5 V) <− 4 dB. If this is normalized with the voltage VON at the time of light emission, the voltage VOFF at the time of light extinction, and I (VON) = 0 dB, -3.1 dB <I ((3VON + VOFF) / 4) <-1 dB and -6.1 dB <I ((VON + VOFF) / 2) <-2 dB and -8.5 dB <I ((VON + 3VOFF) / 4) <-3 dB and -10 dB <I (VOFF) <-4 dB is there.

FIG. 10 shows a range in which the transmission penalty is 2 dB or less. Note that, in FIG. 10, the extinction characteristics A and B are set so that I (0) = 0 dB, and the voltages are further normalized and displayed. From FIG. 10, it can be seen that the extinction characteristic A, for which the transmission penalty calculation value is 0.8 dB and transmission is determined to be possible, satisfies the above condition regarding the extinction characteristic. Also, the extinction characteristic B, where the transmission penalty calculation value is 2.2 dB and transmission is determined to be impossible, is I (0.25) = -0.
Since 94 dB> -1 dB, it can be reconfirmed that the above condition regarding the extinction characteristic is violated.

As described above, the range of the extinction characteristic of the EA modulator for enabling the transmission at the transmission speed of 10 Gb / s has been determined for the ordinary dispersion fiber 40 km repeaterless optical transmission system. In summary, for each value of the extinction characteristic parameter, the extinction characteristic determined by the value was obtained. The α-parameter was determined from the determined extinction characteristics, and the optical penalty was evaluated by calculating the optical waveform after fiber transmission by optical transmission simulation. Transmission penalty
Based on 2 dB or less, it was determined whether transmission was possible. With this method, the range of the extinction characteristic parameter that enables transmission can be determined, and the range of the extinction characteristic can be determined.

According to the third embodiment, it is possible to obtain a transmission system excellent in transmission characteristics without the necessity of selecting an optical modulation element.

In the third embodiment, the extinction characteristic range is determined for the 40 km transmission system. However, the EA required for configuring the transmission system for different transmission distances by the same calculation method.
It is possible to determine the extinction characteristics of the modulator. Also,
Here, the case where the EA modulator is used as an electric / optical conversion device has been described.
An EA modulator integrated optical device, such as an EA / DFB integrated light source integrated with D or the like, can be designed in exactly the same manner when used as an electric / optical conversion device.

[Embodiment 4] Another design method of the transmission system according to the third embodiment of the present invention will be described with reference to FIGS.
Let's explain using 2. FIG. 11 is a diagram illustrating a long-distance transmission system using an optical amplifier. FIG.
5 is a flowchart showing a method for designing a long-distance transmission system using an optical amplifier. In the third embodiment described above, the transmission distance is
Although the case of 40 km has been described, if the transmission distance is increased, the total loss in the optical fiber also increases, so that it is necessary to use an optical amplifier. In the transmission system shown in FIG. 11, the transmission end immediately after the transmitter 1, the transmission end just before the receiver 3, the transmission fiber 2
By installing the optical amplifier 4 in the middle or the like, the loss of the fiber is compensated and the transmission distance is extended. Further, in a normal dispersion fiber, the waveform of an optical pulse is greatly deteriorated due to the group velocity dispersion of the fiber. Therefore, a dispersion compensator 5 for generating a dispersion opposite to that of the transmission fiber 2 is provided as necessary.

In such an optical transmission system as well, when an EA modulator is used as the electrical / optical conversion device in the optical transmitter 1, the Kramers-Kronig relational expression can be calculated from the extinction characteristic as in the above-described embodiment. It is possible to perform transmission design by utilizing this. When an optical amplifier is used, self-phase modulation based on the Kerr effect, in which the refractive index changes due to nonlinear optical effects, by introducing high-power light into the fiber
(SPM) is a problem, but as mentioned earlier, Split-Step
By dividing the transmission analysis by the Fourier method into a number of sections, it is possible to perform transmission analysis in consideration of the self-phase modulation effect.

FIG. 12 is a flowchart showing a flow of a transmission analysis calculation in consideration of the self-phase modulation effect. R1, V1, n
(Step 12-1), the extinction characteristic R (V) is calculated (step 12-2), and α (V) and φ (V) are calculated from the Kramers-Kronig relational expression (step 12-). 3). R
An optical output waveform E (t) of the optical transmitter is calculated from (V) and φ (V) (step 12-4). Optical fiber transmission line is divided into many sections ΔZ
(Step 12-5), and for each of the sections ΔZ, E (t) is
The optical waveform E '(t) after transmission in the section is obtained by performing ourier transformation (step 12-6) and multiplying by the transfer function of the fiber having group velocity dispersion (step 12-7) and performing inverse Fourier transformation (step 12-6). 12-8). Due to the effect of the SPM, phase modulation eiΔφ is applied to E ′ (t), so that E ″ (t) = E ′ (t) eiΔφ. Here, Δφ is a phase change in the section,
Nonlinear refractive index n2, optical frequency ω0, optical velocity c, effective area Aeff, position of distance z from fiber entrance, light intensity at time t as I (z, t)

[0077]

[Equation 17]

Is as follows.

Steps 12-5 to 12-10 are performed
Repeat for the number of sections. As a result, an optical waveform E '''(t) of the fiber output is obtained (step 12-11). The minimum receiving sensitivity is calculated for each of the optical waveform E (t) before fiber transmission and the optical waveform E ″ ′ (t) for fiber transmission (step 12-12).
Steps 12-1 to 12-13 are repeated by the number of extinction characteristic combinations.

As in the third embodiment, the transmission penalty is 2 dB.
It is sufficient to determine that transmission is possible based on the following. According to the fourth embodiment, it is possible to obtain a transmission system having excellent transmission characteristics without the need to select an optical modulation element.

When a dispersion compensator is used, for example, the dispersion compensator is treated as a device having a group velocity dispersion of the opposite sign to that of a normal dispersion fiber in the same manner as a fiber, so that the same design method as described above is used. Is applicable. More precisely, the above Nonlinear Fiber Optic
s, (Academic Press, 1989)
Analysis accuracy can be improved by considering the effects of higher-order dispersion on the dispersion compensator and the ordinary dispersion fiber.

Further, in the above examples, the case where the ordinary dispersion fiber is used as the transmission line is shown. However, the transmission performance grasping method using the relationship between the extinction characteristic of the EA modulator and the α-parameter in the present invention is as follows. The present invention is not limited to the ordinary dispersion fiber transmission system, but can be applied to the line design of the dispersion shift fiber transmission system as it is.

[Summary] The above embodiment will be summarized with reference to FIG. FIG. 13 is a diagram for explaining a method of ascertaining the transmission characteristics of an EA modulator and an optical transmitter using the same. As shown in the first to fourth embodiments, according to the present invention, as shown in FIG. 13, the α-parameter is calculated from the extinction characteristic which is the DC characteristic of the EA modulator, and the transmission simulation is performed. Thus, the transmission characteristics can be grasped.

Further, by this, the extinction characteristic and the transmission characteristic can be grasped in a unified manner and applied to element design and element characteristic management. This makes it possible to obtain a transmission characteristic evaluation method that does not require modulator element selection, an optical modulator suitable for a transmission path, an optical transmitter, and an optical transmission system having excellent transmission characteristics.

[0085]

As described above, according to the present invention,
Since the α-parameter can be estimated from the extinction characteristic, the extinction characteristic and the fiber transmission characteristic of the EA modulator element and the EA / DFB integrated light source element can be centrally managed.

It is no longer necessary to construct a transmission line for evaluation that simulates an actual transmission line and conduct a transmission experiment using an EA modulator or a transmitter. By employing the present invention, not only the transmission experiment requiring a great amount of time can be omitted, but also the optical modulator can be designed on the contrary from the required transmission characteristics.

Further, a method of evaluating transmission characteristics which does not require modulator element selection, an optical modulator suitable for a transmission line, an optical transmitter, and an optical transmission system having excellent transmission characteristics were obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a method for ascertaining the transmission characteristics of an EA modulator and an optical transmitter using the same in a conventional technique.

FIG. 2 is a configuration diagram of a repeaterless optical transmission system explaining a first embodiment and a third embodiment of the present invention.

FIG. 3 is a configuration diagram of an optical transmitter using an EA modulator to explain the first embodiment of the present invention.

FIG. 4 is a diagram schematically illustrating a frequency shift of an absorption spectrum a (μ) proportional to an applied voltage V according to a second embodiment of the present invention.

FIG. 5 is a diagram schematically illustrating a frequency shift of a refractive index spectrum η (ω) in proportion to an applied voltage V according to a second embodiment of the present invention.

FIG. 6 is a diagram illustrating an example of extinction characteristics of an EA modulator according to a third embodiment of the present invention.

FIG. 7: EA calculated from the extinction characteristic of Example 3 of the present invention
FIG. 3 is a diagram illustrating an example of an α-parameter of a modulator.

FIG. 8 is an eye pattern diagram of an optical waveform of an optical transmitter before and after fiber transmission calculated from extinction characteristics and α-parameters according to Embodiment 3 of the present invention.

FIG. 9 is a flowchart illustrating a method of designing an optical transmission line using an optical transmitter equipped with an EA modulator according to a third embodiment of the present invention.

FIG. 10 is an explanatory diagram illustrating extinction characteristics of an EA modulator for a 40 km non-repeated transmission system according to a third embodiment of the present invention.

FIG. 11 is a configuration diagram illustrating an optical transmission system according to a fourth embodiment of the present invention.

FIG. 12 is a flowchart illustrating a method of designing an optical transmission line using an optical transmitter equipped with an EA modulator according to a fourth embodiment of the present invention.

FIG. 13 is a diagram illustrating a method for ascertaining the transmission characteristics of an EA modulator and an optical transmitter using the same according to an embodiment of the present invention.

[Explanation of symbols]

1 ... optical transmitter using EA modulator, 2 ... optical fiber, 3
... optical receiver, 4 ... optical amplifier, 5 ... dispersion compensator, 11 ... data input, 12 ... clock input, 13 ... flip-flop, 14 ...
EA modulator drive circuit, 15 ... LD current source, 16 ... LD, 17 ...
EA modulator, 18 ... light output.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI H04B 10/152 10/142 17/00 (72) Inventor Hideyuki Serizawa 216 Totsukacho, Totsuka-ku, Yokohama-shi, Kanagawa-ken Hitachi, Ltd. Information Telecommunications Division

Claims (11)

[Claims] 1. A method for evaluating transmission characteristics of an optical modulator, an optical fiber transmitting a signal modulated by the optical modulator, and a combination, wherein the extinction characteristic of the optical modulator is converted into an α-parameter. A transmission characteristic evaluation method for evaluating the fiber transmission characteristic of the signal. 2. The transmission characteristic evaluation method according to claim 1, wherein the conversion of the extinction characteristic to the α-parameter is performed by Kramers-K.
A transmission characteristic evaluation method characterized by using a ronig relational expression. 3. A method of simulating the transmission characteristics of a fiber transmission line by converting the extinction characteristics into α-parameters, and setting the extinction characteristics so that the transmission characteristics fall within a predetermined range. An optical modulator characterized by: 4. The optical modulator according to claim 3, wherein the conversion of the extinction characteristic to the α-parameter is performed using a Kramers-Kronig relational expression. 5. An optical transmitter comprising: a laser diode; and an optical modulator for modulating light from the laser diode, wherein the extinction characteristic of the optical modulator is converted into an α-parameter to obtain a fiber. An optical transmitter which simulates transmission characteristics of a transmission path and sets the extinction characteristics so that the transmission characteristics fall within a predetermined range. 6. The optical transmitter according to claim 5, wherein a Kramers-Kronig relational expression is used to convert the extinction characteristic into the α-parameter. 7. An optical transmission system comprising a laser diode, an optical modulator for modulating light from the laser diode, an optical fiber transmission line, and an optical receiver, wherein: The extinction characteristic of the device is converted into an α-parameter, thereby simulating the transmission characteristic of the fiber transmission line, and setting the extinction characteristic so that the transmission characteristic falls within a predetermined range. Optical transmission system. 8. The optical transmission system according to claim 7, wherein the conversion of the extinction characteristic into the α-parameter is performed by Kramers-Kro.
An optical transmission system characterized by using a nig relational expression. 9. An optical transmission system comprising a laser diode, an optical modulator for modulating light from the laser diode, an optical fiber transmission line, and an optical receiver, wherein: The extinction characteristic of the optical transmitter is converted into an α-parameter to simulate the transmission characteristic of the fiber transmission line, and a first optical signal at the optical transmitter exit and a second optical signal at the optical receiver entrance are simulated. Wherein the extinction characteristic is set such that the ratio of the receiving sensitivity of the first optical signal to the receiving sensitivity of the second optical signal falls within a certain range. 10. The optical transmission system according to claim 9, wherein the conversion of the extinction characteristic to the α-parameter is performed by Kramers-K.
An optical transmission system characterized by using ronig's relational expression. 11. An optical transmitter comprising a laser diode and an optical modulator for modulating light from the laser diode at a rate of 9953.28 Mb / s, and the total group velocity dispersion D is 600 ps / nm <D.
An optical transmission system comprising an optical fiber transmission line in the range of <720 ps / nm and an optical receiver, wherein the voltage VON at the time of light emission of the optical modulator, the voltage VOFF at the time of light extinction, I
When (VON) = 0 dB, the extinction characteristic of the optical modulator is -3.1 dB <I ((3VON + VOFF) / 4) <-1 dB, -6.1 dB <I ((VON + VOFF) / 2) <-2 dB, -8.5 dB <I ((VON + 3VOFF) / 4) <-3 dB, -10 dB <I (VOFF) <-4 dB .