JP2000028934A - Variable dispersion optical equalizer - Google Patents

Abstract

(57) [Problem] To realize a dispersion-variable optical equalizer that can vary the amount of dispersion in a wide wavelength range of 1 nm or more and has a relatively small insertion loss. SOLUTION: A three-port optical circulator in which a first chirped fiber grating that receives light from a long wavelength side of a reflected wavelength and a second chirped fiber grating that receives light from a short wavelength side is cascaded. A transmission filter is constructed by connecting to each second port in any order, and a tuning means is provided for moving the reflection wavelength range of each chirped fiber grating equally in the same direction and changing the wavelength dispersion of light transmitted through the transmission filter. . The distribution λ (z) of the reflection wavelength in the longitudinal direction of the first chirped fiber grating is λ (z) = λ _S + Δλ (z /
L) ^1/2 and the distribution λ (z) of the reflected wavelength in the longitudinal direction of the second chirped fiber grating is λ (z) = λ _S
+ Δλ {1- (1-z / L) ^1/2 }.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical equalizer for correcting distortion of an optical signal transmitted through an optical fiber. In particular, the present invention relates to a variable dispersion optical equalizer that can vary the amount of dispersion.

[0002]

2. Description of the Related Art In an ordinary optical fiber transmission system, light having a wavelength near a zero dispersion wavelength is used in order to avoid the influence of chromatic dispersion. Generally, the deviation of the signal light wavelength from the zero-dispersion wavelength is accompanied by an increase in the total dispersion amount. However, in the case of long-distance transmission or high-speed transmission, an increase in the total amount of dispersion causes deterioration of transmission characteristics. Therefore, it is necessary to reduce the total amount of dispersion by bringing the signal light wavelength as close to the zero dispersion wavelength as possible, but on the other hand, there is also the effect of the nonlinear optical effect (for example, four-wave mixing). It doesn't mean that it should be done. That is, there is an optimum value for the total dispersion amount, and when starting up the system, it is necessary to set the signal light wavelength to a value at which the total dispersion amount is optimal.

When a new optical cable is added to an already installed optical cable, it may be necessary to readjust the signal light wavelength because the total dispersion changes.
In addition, when the optical power in the optical fiber is changed, the optimum total dispersion also changes. Therefore, it is necessary to adjust the signal light wavelength similarly.

However, in the optical fiber transmission system which has already been set up, when the signal light wavelength is changed, the transmission wavelength of an optical filter inserted into a large number of optical transmission lines must be changed, which involves a very complicated operation. Will be. Therefore, a method has been considered in which a dispersion compensating fiber is inserted into an optical transmission line to obtain an optimum total dispersion amount for a signal light wavelength. However, even in that case, it is necessary to precisely measure the chromatic dispersion of the optical transmission line.

When the temperature of the optical transmission line fluctuates periodically, the total dispersion also fluctuates periodically. In such a case, it is difficult to cope with the dispersion compensating fiber. In such a case, a method of optimizing the total dispersion amount of the fluctuating optical transmission line by using a variable dispersion optical equalizer capable of varying the dispersion amount has been considered. As this variable dispersion optical equalizer,
A dispersion equalizer (group delay equalizer) using a planar waveguide (PLC) has been proposed (reference: K. Takiguchi et.
al., "Variable group-delay dispersion equalizer usi
ng lattice-form programmable optical filter on pla
nar lightwavecircuit ", IEEE Journal of Selected To
pics in Quantum Electronics, vol.2, no.2, pp.270-27
6, 1996).

[0006]

By the way, the variable dispersion optical equalizer using the planar waveguide has a problem in that the operating band is narrow, at about 10 GHz, and the insertion loss is as large as 10 dB. .

An object of the present invention is to provide a variable dispersion optical equalizer which can vary the amount of dispersion in a wide wavelength range of 1 nm or more and has a relatively small insertion loss.

[0008]

A dispersion-variable optical equalizer according to the present invention comprises a first chirped fiber grating for entering light from a long wavelength side of a reflection wavelength and a second chirped fiber grating for entering light from a short wavelength side. A chirped fiber grating is connected to a second cascaded three-port optical circulator
Connected to the ports in random order or connected to the second and third ports of the four-port optical circulator in random order to form a transmission filter, and move the reflection wavelength range of each chirped fiber grating equally in the same direction; This is a configuration including tuning means for changing the wavelength dispersion of light transmitted through the transmission filter.

The distribution λ (z) of the reflection wavelength in the longitudinal direction of the first chirped fiber grating has a length L, a shortest reflection wavelength λ _S , a longest wavelength λ _L , and Δλ = λ _L −λ.
_S , when the coordinate of the coordinate axis taken in the longitudinal direction with the shortest wavelength part as the origin is z,

[0010]

(Equation 1)

## EQU1 ## The distribution λ (z) of the reflected wavelength in the longitudinal direction of the second chirped fiber grating is similarly expressed as

[0012]

(Equation 2)

## EQU1 ## The tuning means fixes the two chirped fiber gratings to the piezo element and controls the voltage applied to the piezo element, or fixes the two chirped fiber gratings to the heater, and supplies a current to the heater to reduce the amount of heat generated. A control configuration or the like can be used.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A fiber grating irradiates the core of an optical fiber with ultraviolet light, induces a periodic change in refractive index, forms a Bragg diffraction grating, and emits light having a wavelength corresponding to the period (pitch). This is a reflection type filter that reflects light. This pitch is constant in a normal fiber grating, but the chirped fiber grating used in the variable dispersion optical equalizer of the present invention has a pitch changed in the length direction of the optical fiber. This chirped fiber grating can provide a different delay time for each wavelength because the area of reflection changes depending on the wavelength, and functions as a dispersion compensation medium.

FIG. 1 shows a model of a chirped fiber grating represented by equation (1). The distribution λ (z) of the reflection wavelength when the coordinate of the coordinate axis taken in the longitudinal direction with the length L, the shortest wavelength λ _S of the reflection wavelength, the longest wavelength λ _L , and the shortest wavelength portion as the origin is represented by the following equation (1). It is represented by Although the pitch of the grating changes in a stepwise manner, here, a state where the short wavelength side is fine and the long wavelength side is rough is schematically shown.

When light having a wavelength λ is incident from the long wavelength side of such a chirped fiber grating, a group delay characteristic as shown in FIG. 2 is obtained. The group delay in this case is

[0017]

(Equation 3)

## EQU1 ## Here, Δλ = λ _L −λ _S (4) τ ₀ = 2 nL / c (5), where n is the effective refractive index of the core and c is the speed of light. FIG.
Shows the wavelength dispersion characteristic obtained by differentiating the equation (3) with respect to the wavelength.

FIG. 4 shows a model of a chirped fiber grating represented by the equation (2). However, the notation is the same as that of the first chirped fiber grating shown in FIG.

When light having a wavelength λ is incident from the short wavelength side of such a chirped fiber grating, a group delay characteristic as shown in FIG. 5 is obtained. The group delay in this case is

[0021]

(Equation 4)

## EQU2 ## FIG. 6 shows a chromatic dispersion characteristic obtained by differentiating equation (6) with respect to wavelength. Next, this 2
When the group delay characteristics of the two chirped fiber gratings are added, an upwardly convex quadratic curve as shown in FIG. 7 is obtained. The group delay in this case is

[0023]

(Equation 5)

## EQU1 ## The chromatic dispersion function obtained by differentiating equation (7) with respect to wavelength is

[0025]

(Equation 6)

The wavelength dispersion characteristic is as shown in FIG. In the chromatic dispersion characteristics shown here, the chromatic dispersion linearly changes from positive to negative as the wavelength becomes longer, which is opposite to the chromatic dispersion characteristics of a normal optical fiber as shown in FIG. In FIG. 9, λ ₀ is a zero-dispersion wavelength.

The maximum value of the chromatic dispersion shown in FIG. 8 is 2τ ₀ / Δ
λ, and the minimum value is −2τ ₀ / Δλ. In order to actually obtain such characteristics, the chirped fiber gratings represented by the equations (1) and (2) may be coupled via an optical circulator as shown in FIG. 10 or FIG.

The configuration shown in FIG. 10 will be described. First
To the second port of the three-port optical circulator 11
The long wavelength side of the first chirped fiber grating 12 represented by (1) is connected. Further, the short wavelength side of the second chirped fiber grating 14 represented by the equation (2) is connected to the second port of the second three-port optical circulator 13. The third port of the first three-port optical circulator 11 is connected to the first port of the second three-port optical circulator 13, and the light transmitted through the optical fiber is transmitted to the first three-port optical circulator. When the light enters the first port 11, it is possible to take out the dispersion-compensated light from the third port of the second three-port optical circulator 13.

A first three-port optical circulator 11 to which the first chirped fiber grating 12 is connected, and a second chirped fiber grating 14
The order of connection of the second three-port optical circulators 13 to which are connected may be changed. Alternatively, the short wavelength side of the second chirped fiber grating 14 is connected to the second port of the first three-port optical circulator 11, and the first chirped fiber is connected to the second port of the second three-port optical circulator 13. The same is true even if the longer wavelength side of the grating 12 is connected.

The configuration shown in FIG. 11 will be described. The second port of the four-port optical circulator 15 is connected to the long wavelength side of the first chirped fiber grating 12 represented by the equation (1), and the third port is connected to the second port represented by the equation (2). The short wavelength side of the chirped fiber grating 14 is connected. Then, by inputting the light propagating through the optical fiber to the first port of the four-port optical circulator 15, it is possible to take out the dispersion-compensated light from the fourth port.

The long-wavelength side of the first chirped fiber grating 11 is connected to a four-port optical circulator 15.
And the short wavelength side of the second chirped fiber grating 12 may be connected to the second port of the four-port optical circulator 15.

The configuration shown in FIGS. 10 and 11 operates as a transmission filter, and its wavelength dispersion characteristics are as shown in FIG. The transmission band of the transmission filter can be linearly tuned by moving the reflection wavelength ranges of the two chirped fiber gratings equally in the same direction and using tuning means for changing the chromatic dispersion of transmitted light. At this time, the wavelength dispersion characteristic moves in parallel as shown in FIG. 12 or FIG.

In FIG. 12, the broken line is the initial characteristic,
The solid line shows the characteristics when the initial characteristics are tuned to the short wavelength side by Δλ / 2. In FIG. 13, the broken line indicates the initial characteristic, and the solid line indicates the characteristic when the initial characteristic is tuned to the longer wavelength side by Δλ / 2. Focusing on a certain wavelength λsig, the chromatic dispersion of λsig is 0 in the initial state, but the chromatic dispersion is −2τ ₀ by the tuning of FIG.
/ Δλ. Further, the chromatic dispersion changes by 2τ ₀ / Δλ by the tuning of FIG.

[0034] Thus, by tuning the transmission band of the transmission filter, it is possible to change the wavelength dispersion at a certain wavelength λsig from -2τ ₀ / Δλ to 2τ ₀ / _Δλ. Therefore, the dispersion compensation amount for the wavelength λsig required for a certain optical transmission line is −2τ ₀ / Δ
If it is between λ and 2τ ₀ / Δλ, the compensation amount can be realized by tuning the transmission band of the chirped fiber grating of the present variable dispersion optical equalizer.

For example, if the reflection wavelength band Δλ of the chirped fiber grating is 10 nm, the length L is 100 mm, and the effective core refractive index n is 1.46, −2τ ₀ / Δλ = −1
95 ps / nm, 2τ ₀ / Δλ = 195 ps / nm,
The chromatic dispersion can be changed from about -200 ps / nm to about 200 ps / nm. Since the dispersion fluctuation of a normally installed dispersion-shifted fiber is within ± 2 ps / nm / km at a wavelength of 1550 nm, the dispersion-variable optical equalizer of the present invention can cope with a dispersion-shifted fiber having a length of about 100 km. it can.

[0036]

(First Embodiment) FIG. 14 shows a first embodiment of the tuning means. The first chirped fiber grating 12 and the second chirped fiber grating 14 shown in FIGS.
Is fixed by using an epoxy adhesive or the like. By applying a voltage to the piezo element 16, the reflection wavelength range of each chirped fiber grating can be moved equally in the same direction, and the chromatic dispersion of transmitted light can be changed as shown in FIG. 12 or FIG.

FIG. 15 shows an example of a configuration in which a tuning means (piezo element 16) is combined with a transmission filter using a three-port optical circulator shown in FIG. Further, the four-port optical circulator 1 shown in FIG.
5 can be similarly applied.

(Second Embodiment) FIG. 16 shows a second embodiment of the tuning means. The first chirped fiber grating 12 and the second chirped fiber grating 12 shown in FIGS.
Is fixed on a heater 18 placed on a pedestal 17. By controlling the amount of heat generated by adjusting the current flowing through the heater 18, the reflection wavelength range of each chirped fiber grating is moved equally in the same direction, and the wavelength dispersion of the transmitted light is reduced as shown in FIG. 12 or FIG. Can be changed.

(Third Embodiment) One chirped fiber grating represented by the formula (1) or (2) is connected to the second port of a three-port optical circulator to form a transmission filter. The chirped fiber grating may be fixed to the piezo element 16 or the heater 18.

However, when the chirped fiber grating 12 represented by the equation (1) is used, the chromatic dispersion can be changed from 0 to -2τ ₀ / Δλ as shown in FIG. When the chirped fiber grating 14 represented by the equation (2) is used, the chromatic dispersion can be changed from 2τ ₀ / Δλ to 0 as shown in FIG.

[0041]

As described above, the variable dispersion optical equalizer of the present invention can electrically control the wavelength dispersion of transmitted light within a certain range from negative to positive. This makes it possible to compensate in a relatively short time for excessive chromatic dispersion of the optical transmission line, which becomes a problem during high-speed transmission or long-distance transmission.

[Brief description of the drawings]

FIG. 1 is a diagram showing a model of a chirped fiber grating represented by equation (1).

FIG. 2 is a diagram showing a group delay characteristic when light is incident on a chirped fiber grating represented by Expression (1) from a long wavelength side.

FIG. 3 is a diagram showing chromatic dispersion characteristics when light is incident on a chirped fiber grating represented by the formula (1) from the long wavelength side.

FIG. 4 is a diagram showing a model of a chirped fiber grating represented by Expression (2).

FIG. 5 is a diagram showing a group delay characteristic when light is incident on a chirped fiber grating represented by Expression (2) from the short wavelength side.

FIG. 6 is a diagram showing chromatic dispersion characteristics when light is incident on the chirped fiber grating represented by the equation (2) from the short wavelength side.

FIG. 7 is a diagram showing the sum of the group delay characteristics of FIGS. 2 and 5;

FIG. 8 is a diagram showing the sum of the wavelength dispersion characteristics of FIGS. 3 and 6;

FIG. 9 is a diagram showing chromatic dispersion characteristics of a normal optical fiber.

FIG. 10 is a diagram illustrating a configuration example of a transmission filter when a three-port optical circulator is used.

FIG. 11 is a diagram illustrating a configuration example of a transmission filter when a four-port optical circulator is used.

FIG. 12 shows that the transmission band of the transmission filter is Δλ on the short wavelength side.
FIG. 9 is a diagram illustrating a change in chromatic dispersion characteristics when tuning is performed by / 2.

FIG. 13 shows that the transmission band of the transmission filter is shifted to the longer wavelength side by Δλ.
FIG. 9 is a diagram illustrating a change in chromatic dispersion characteristics when tuning is performed by / 2.

FIG. 14 is a diagram showing a first embodiment of the tuning means.

FIG. 15 is a diagram showing a configuration example in which tuning means is combined with a transmission filter using a three-port optical circulator.

FIG. 16 is a diagram showing a second embodiment of the tuning means.

[Explanation of symbols]

 11, 13 3-port optical circulator 12, 14 chirped fiber grating 15 4-port optical circulator 16 piezo element 17 pedestal 18 heater

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Masataka Nakazawa 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Co., Ltd. F-term (reference) 2H041 AA00 AB12 AB18 AC07 AC08 5K002 BA02 BA21 CA01 FA01 5K046 AA08 CC01 CC02 CC03 CC06 CC21 EE05

Claims (5)

[Claims] 1. The distribution λ (z) of the reflected wavelength in the longitudinal direction of a chirped fiber grating having a length L is such that the shortest wavelength of the reflected wavelength is λ _S , the longest wavelength is λ _L , Δλ = λ _L −λ _S , When the coordinate of a coordinate axis taken in the longitudinal direction with the wavelength portion as the origin is z, the long wavelength side of the first chirped fiber grating represented by λ (z) = λ _S + Δλ (z / L) ^1/2 Is connected to a second port of the first three-port optical circulator, and a first transmission filter having the first port and the third port as input / output ports, and a longitudinal filter of a length L chirped fiber grating. The distribution of the reflection wavelength λ (z) is expressed as follows: λ (z) = λ _S + ΔλΔ1- (1-z / L) ^1/2をConnect to the second port of the three-port optical circulator, and connect the first port and A second transmission filter having three ports as input / output ports is connected in cascade in any order to form one transmission filter, and the reflection wavelength ranges of the chirped fiber gratings are moved equally in the same direction. A variable dispersion optical equalizer comprising tuning means for changing wavelength dispersion of light transmitted through one transmission filter. 2. The distribution λ (z) of the reflected wavelength in the longitudinal direction of a chirped fiber grating having a length L is such that the shortest reflected wavelength is λ _S , the longest wavelength is λ _L , Δλ = λ _L −λ _S , When the coordinate of a coordinate axis taken in the longitudinal direction with the wavelength portion as the origin is z, the long wavelength side of the first chirped fiber grating represented by λ (z) = λ _S + Δλ (z / L) ^1/2 Is connected to the second port of the four-port optical circulator, and the distribution λ (z) of the reflection wavelength in the longitudinal direction of the chirped fiber grating having the length L is expressed as follows: λ (z) = λ _S + Δλ ｛1- (1-z / L) ^1/2し The short wavelength side of the second chirped fiber grating is connected to the third port of the four-port optical circulator, and the first and fourth ports are used as input / output ports. Constituting a transmission filter, each of said chirped fiber grays A variable wavelength optical equalizer comprising tuning means for equally moving a reflection wavelength range of the light in the same direction in the same direction and changing wavelength dispersion of light transmitted through the transmission type filter. 3. The distribution λ (z) of the reflected wavelength in the longitudinal direction of a chirped fiber grating having a length L is such that the shortest reflected wavelength is λ _S , the longest wavelength is λ _L , Δλ = λ _L −λ _S , The second chirp represented by λ (z) = λ _S + Δλ {1- (1-z / L) ^1/2 } where z is the coordinate of a coordinate axis taken in the longitudinal direction with the wavelength portion as the origin. The short wavelength side of the fiber grating is connected to the second port of the 4-port optical circulator, and the distribution λ (z) of the reflected wavelength in the longitudinal direction of the chirped fiber grating having the length L is λ (z) = λ _S + Δλ ( The long wavelength side of the first chirped fiber grating represented by (z / L) ^1/2 is connected to the third port of the four-port optical circulator, and the first and fourth ports are used as input / output ports. Constituting a transmission filter, each of said chirped fiber grays A variable wavelength optical equalizer comprising tuning means for equally moving a reflection wavelength range of the light in the same direction in the same direction and changing wavelength dispersion of light transmitted through the transmission type filter. 4. The tuning device according to claim 1, wherein the tuning means has a configuration in which two chirped fiber gratings are fixed to a piezo element, and a voltage applied to the piezo element is controlled. Variable dispersion optical equalizer. 5. The tuning means according to claim 1, wherein the two chirped fiber gratings are fixed to a heater, and a current is supplied to the heater to control the amount of heat generated. Variable optical equalizer.