JPH11183748A - High-order dispersion compensator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-order dispersion compensator capable of compensating the wavelength dispersion of an optical fiber up to a high-order term of dispersion in a wide wavelength region. SOLUTION: The compensator determining the wavelength dispersion function through differentiation with respect to the wavelength of a series partial sum obtained by Taylor expansion, up to an arbitrary degree, of the group delay amount of the optical fiber in the wavelength with zero dispersion wavelength as the center inverting the sign of the wavelength dispersion function to form a sign inverted function and compensate the wavelength dispersion of the group delay amount of the opical fiber using a chirp fiber grating regards the functional relation expressed by the sign inverted function as wavelength dispersion characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-order dispersion compensator for an optical fiber.

[0002]

2. Description of the Related Art In ordinary optical fiber transmission, the influence of higher-order chromatic dispersion is small because the band is narrow, and it is sufficient to consider compensation for second-order dispersion. However, as the transmission speed increases, the bandwidth is widened and the band is greatly affected by higher-order dispersion. Also, in wavelength division multiplexing transmission, the channels used extend over a wide wavelength range, so that they are also greatly affected by higher-order dispersion. However, it is difficult to compensate for higher-order dispersion with a normal dispersion compensating fiber. Therefore, recently, a third-order dispersion compensator (dispersion slope compensator) using a planar waveguide (Planer Lightwave Circuit) has been proposed (K. Takiguchi, S. Kawanishi, H. Takara,
K. Okamoto, K. Jinguji and Y. Ohmori, "Higher orde
r dispersion equalizer of dispersion shifted fiber
using a lattice-form programmable optical fiber ",
Electron. Lett., Vol. 32, no. 8, pp. 755-757, 199
6.)

[0003]

However, with the increase in transmission capacity and transmission distance, it is expected that a higher-order dispersion compensator will be required. The third-order dispersion compensator using the above planar waveguide has a band of 200
The problem is that it is as narrow as less than GHz. Further, dispersion compensation of an optical fiber having a special dispersion characteristic such as a dispersion flat fiber requires higher-order dispersion compensation.

It is an object of the present invention to provide a high-order dispersion compensator that operates in a wide wavelength range.

[0005]

SUMMARY OF THE INVENTION In the present invention, the above object is achieved by performing a Taylor expansion of the group delay of an optical fiber with respect to a wavelength around a zero dispersion wavelength to an arbitrary order to obtain a series partial sum. A chirped fiber grating is obtained by differentiating the sum with respect to wavelength to obtain a chromatic dispersion function, inverting the sign of the chromatic dispersion function to obtain a sign inversion function, and setting the functional relationship represented by the sign inversion function to chromatic dispersion characteristics. The problem is solved by configuring a higher-order dispersion compensator provided as a means for compensating the chromatic dispersion of the group delay amount of the fiber.

In the present invention, the sign inversion function is
A chirped fiber grating that decomposes into a sum of constant sign functions that do not change sign in the wavelength region to be dispersion-compensated and has the functional relationship represented by each constant sign function as chromatic dispersion characteristics is connected in cascade via an optical circulator, etc. Thus, a higher-order dispersion compensator is formed.

A high-order dispersion compensator according to the present invention is different from a conventional planar waveguide compensator in that a chirped fiber grating is provided as a dispersion compensating means.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below.

The amount of group delay per unit length of the optical fiber at the wavelength λ, that is, the group delay time is represented by τ (λ), and when this is Taylor-expanded around the zero dispersion wavelength λ ₀ , τ (λ) = τ ( λ ₀ ) + τ ⁽¹⁾ (λ ₀ ) (λ−λ ₀ ) + (1/2!) τ ⁽²⁾ (λ ₀ ) (λ−λ ₀ ) ² + (1/3!) τ ⁽³⁾ (λ ₀ ) (λ−λ ₀ ) ³ + (1) where τ ^(k) (λ ₀ ) = d ^k τ (λ) / dλ ^k (when λ = λ ₀ ) (2) Become. The chromatic dispersion D (λ) is obtained by differentiating τ (λ) with respect to the wavelength λ. That is, D (λ) = dτ (λ) / dλ = τ ⁽¹⁾ (λ ₀ ) + τ ⁽²⁾ (λ ₀ ) (λ−λ ₀ ) + (1/2!) Τ ⁽³⁾ (λ ₀ ) (λ−λ ₀ ) ² +... (3) Since λ ₀ is a zero-dispersion wavelength, τ ⁽¹⁾ (λ ₀ ) = 0, D (λ ₀ )
= 0. Also, τ (λ ₀ ) = 0 may be set. Eventually, (1) and (2) are as follows.

Τ (λ) = (1/2!) Τ ⁽²⁾ (λ ₀ ) (λ−λ ₀ ) ² + (1/3!) Τ ⁽³⁾ (λ ₀ ) (λ−λ ₀ ) ³ + ... (4) D (λ) = τ ⁽²⁾ (λ ₀ ) (λ−λ ₀ ) + (1/2!) Τ ⁽³⁾ (λ ₀ ) (λ−λ ₀ ) ² + ... ( 5) The chirped fiber grating functions as a dispersive medium. Therefore, it is considered that the group delay characteristics and the dispersion characteristics represented by (4) and (5) are compensated by the chirped fiber grating.

As shown in FIG. 1, the compensation band in which chromatic dispersion is to be compensated is defined as λ _S ≦ λ ≦ λ _L (6). Also, as shown in FIG. 2, a coordinate axis is set on the chirped fiber grating, with the direction from the short wavelength side to the long wavelength side being the positive direction and the grating end on the short wavelength side being the origin. Chirped fiber grating When L _F the length of the optical fiber must have a group delay characteristic and dispersion characteristics as follows.

[0012] _{τ FG (λ) = -τ (} λ) L F + C (7) D FG (λ) = -D (λ) L F (8) , where chirped fiber grating, as shown in FIG. 3 Since only a delay function that monotonically increases (FIG. 3A) or decreases (FIG. 3B) can be realized, the sign of the dispersion must always be constant. However, since the group delay characteristic of an optical fiber does not always increase or decrease monotonically, the sign of the dispersion of the optical fiber can change. In order to compensate for such dispersion, D _FG (λ) whose sign changes within the compensation band is required. In such a case, the dispersion function D _FG (λ) is changed to a constant sign function D within the compensation band.
If the optical signal is decomposed into _FGj (λ) and a chirped fiber grating having dispersion characteristics represented by the respective functions is connected via an optical circulator, the incident optical signal propagates through the dispersion medium represented by (7) or (8). The effect is equivalent to That is, _assuming that D _FG (λ) = Σ _j D _FGj (λ) (9) (where Σ _j indicates the summation regarding _j ), D _FGj
If the sign of (λ) is positive, the short wavelength side of the chirped fiber grating corresponding to the D _FGj (λ) is connected to the port of the optical circulator, and if the sign of D _FGj (λ) is negative, the D _FGj (λ Connect the long wavelength side of the chirped fiber grating corresponding to ()) to the port of the optical circulator. That is, (9) is not the characteristic of a single chirped fiber grating, but the characteristic of a dispersion compensator obtained by combining the characteristics of several fiber gratings FGj.

The length and pitch of each chirped fiber grating at this time are determined as follows. First, each dispersion function D _FGj (λ) is integrated with respect to the wavelength λ to obtain τ _FGj (λ) = _{∫D FGj} (λ) dλ (10).

Here, if D _FGj (λ) ≦ 0, coupling is on the long wavelength side, and the group delay amount is 0 at the longest wavelength λ _L.
That is, the constant C is obtained from τ _FGj (λ _L ) = 0 (11). That is, the group delay function is determined. Also, the chirped fiber grating length L
For _FGj, a n _c is an effective refractive index of the core, when the c the velocity of light in vacuum τ _FGj (λ _S) = from a _{(2n c / c) L FGj} (12), as follows L _FGj Is required.

L _FGj = (c / 2n _c ) τ _FGj (λ _S ) (13) In order to obtain the pitch Λ _FGj , first, an inverse function of the group delay function is obtained. Since the long wavelength side coupling _{τ FGj (λ) = (2n} c / c) (L FGj -z) (14) is because, _FGj λ a τ _FGj ^-1 as an inverse function of _{τ FGj (z) = τ FGj} - ¹ When _{{(2n c / c) (} L FGj -z)} (15), λ FGj (z) denotes the wavelength distribution in the length direction of the fiber grating. And this wavelength distribution λ
_FGj (z) has the following relationship with pitch Λ _FGj (z).

Λ _FGj (z) = 2n _c Λ _FGj (z) (16) Therefore, the pitch Λ _FGj (z) becomes Λ _FGj (z) = (１n _c ) τ _FGj ⁻¹ {(2 n _c / c ) (L _FGj −z)} (17)

On the other hand, if D _FGj (λ) ≧ 0, the coupling is on the short wavelength side, and the constant C is obtained from τ _FGj (λ _S ) = 0 (18). That is, the group delay function is determined. Further, from _{τ FGj (λ L) = (} 2n c / c) L FGj (19), grating length L _FGj is determined as follows.

L _FGj = (c / 2n _c ) τ _FGj (λ _L ) (20) In order to obtain the pitch Λ _FGj , first, as described above, the inverse function of the group delay function is obtained. In this case, τ _FGj (λ) = a (2n _c / _c) z because it is _{(21) λ FGj (z)} = τ FGj -1 {(2n c / c) z} (22) (22) ( 16) because Λ _FGj (z) = become _{(1 / 2n c) τ FGj} -1 {(2n c / c) z} (23).

In principle, a dispersion compensator that can handle a group delay function approximated to an arbitrary order n can be obtained. However, as the order increases, it becomes difficult to separate the dispersion function. Further, the inverse function of the group delay function cannot be easily obtained. Therefore, the dispersion function of the n-th order group delay function is separated for each order i with respect to λ-λ ₀ , and further separated so that the code is constant within the compensation band, and the corresponding chirped fiber grating is formed. It is more practical to obtain these and combine them to form a dispersion compensator. That is, in this case,
(8) is separated for each order i as in the following equation.

D _FG (λ) = Σ _i D _FGi (λ) (24) (where Σ _i is a sum relating to i. The same applies to the following.) Further, with respect to each order term, the term When the sign changes, the term is decomposed into a sum of constant sign functions. That is, D _FGi (λ) = Σ _j D _FGij (λ) (25) (where Σ _j is the summation regarding _j . The same applies hereinafter), and the function D _FGij (λ) is λ _S <λ <λ _L Always have the same sign. For each D _FGij (λ), the chirped fiber grating length L _FGij and pitch Λ _FGij are _obtained as shown in (11) to (23). These L
The short wavelength side or long wavelength side of the chirped fiber grating FGij determined by _FGij and Λ _FGij is represented by D
_The connection is made via an optical circulator according to the positive or negative sign of _FGij (λ). Thereby, a dispersion compensator having the group delay amount τ _FG (λ) and the chromatic dispersion D _FG (λ) represented by the following equation can be configured.

Τ _FG (λ) = Σ _i Σ _j τ _FGij (λ) (26) D _FG (λ) = Σ _i Σ _j D _FGij (λ) (27) where τ _FGij (λ) is a chirped fiber This is a group delay amount of the grating FGij.

FIG. 4 is a flowchart showing the above process. In FIG. 4, D _FG (λ) is on the code inversion function according to claim 1, D _FGi (λ) is applicable to each term of the polynomial according to claim 3, D _FGij (λ) is claimed This corresponds to the constant sign function described in item 4.

FIG. 5 is a block diagram of a dispersion compensator constructed through the above-described process. In FIG. 5, incident light 1 passes through a series of three-port optical circulators 2 to become outgoing light 6. Each port of the optical circulator 2 has a chirped fiber grating (3, 4, 5 in the figure)
Is connected. The light is guided by the optical circulator 2 to a chirped fiber grating (exemplified by 3, 4, and 5 in the figure), reflected while generating a delay (dispersion characteristic) shown in FIG. Return.

[0024]

(Embodiment 1) In the case of a standard optical fiber, it is sufficient to compensate by approximating the group delay characteristic to the second order term. At this time, (4) and (5) are approximated as follows.

Τ (λ) = (1/2) τ ⁽²⁾ (λ ₀ ) (λ−λ ₀ ) ² (28) D (λ) = τ ⁽²⁾ (λ ₀ ) (λ−λ ₀ ) (29) Therefore, the dispersion characteristic and the group delay characteristic to be satisfied by the dispersion compensator are D _FG (λ) = − D (λ) L _F = −τ ⁽²⁾ (λ ₀ ) (λ−λ ₀ ) L _F (30 _{) τ FG (λ) = -} (1/2) τ (2) (λ 0) (λ-λ 0) 2 L F + C 1 (31) (a) first, as shown in FIG. 6, lambda ₀ ≦ Consider the case of λ _S. Since D _FG (λ) ≦ 0 and coupling on the long wavelength side, τ _FG (λ _L ) = 0
Since it is ^{_{C 1 = (1/2) τ (}} 2) (λ 0) (λ L -λ 0) way _{^{2 L F (32), τ}} FG (λ S) = (2n c / c) L FG ( because it is 33), grating length L _FG is _{L FG = (c / 4n c} ) τ (2) (λ 0) L F {(λ L -λ 0) 2 - (λ S -λ 0) 2} ( 34) given by

Τ _FG (λ) = (2n _c / c) (L _FG −z) (35), where λ _FG (z) and Λ _FG are the wavelength λ and the corresponding pitch as a function of z, respectively. (z) when the expressed by _{λ FG (z) = λ 0} + {(λ L -λ 0) 2 - (4n c / cτ (2) (λ 0) L F) (L FG -z)} 1 / _{^{2 (36) Λ FG (z}} ) = (1 / 2n c) [λ 0 + {(λ L -λ 0) 2 - (4n c / cτ (2) (λ 0) L F) (L FG -z )} ^1/2 ] (37).

(B) Next, as shown in FIG. 7, λ ₀ ≧
Consider the case of λ _L. Since D _FG (λ) ≧ 0 and coupling on the short wavelength side, τ _FG (λ _S ) = 0, so C ₁ = (1/2) τ ⁽²⁾ (λ ₀ ) (λ _S −λ ₀ ) ² L _F (38) Since τ _FG (λ _L ) = (2 _nc / c) L _FG (39), the grating length L _FG is L _FG = (c / 4 _nc ) τ ⁽²⁾ (λ ₀ ) L _F {(λ _S −λ ₀ ) ² − (λ _L −λ ₀ ) ² } (40)

Τ _FG (λ) = (2n _c / c) z (41), and the wavelength λ and its corresponding pitch are defined as λ _FG (z) and Λ _FG (z), respectively, as a function of z. represents the _{λ FG (z) = λ 0} + {(λ S -λ 0) 2 - (4n c / cτ (2) (λ 0) L F) z)} 1/2 (42) Λ FG (z) = (1 / 2n _c) - to become _{[λ 0 + {(λ S} -λ 0) 2 (4n c / cτ (2) (λ 0) L F) z)} 1/2] (43).

[0029] Then (C), as shown in FIG. 8, lambda _S <
<For lambda _L, a D _FG (λ)> 0 when _{λ <λ 0, λ> λ} 0 because it is D _FG (λ) <0 when λ _0, D _FG (λ)
Must be separated as follows:

[0030] ^{_{D FG (λ) = -τ (}} 2) (λ 0) (λ-λ 0) L F = D FG1 (λ) + D FG2 (λ) + D FG3 (λ) (44) Here, D _FG1 (λ) = - (1/2) τ (2) (λ 0) (λ-λ S) L F (45) D FG2 (λ) = - (1/2) τ (2) (λ 0) ( _{λ-λ L) L F (} 46) D FG3 (λ) = -τ (2) (λ 0) {λ 0 - (λ S + λ L) / 2} L F (47) first, D _FG1 (λ) The chirped fiber grating FG1 corresponding to (1) is long-wavelength-side coupling because D _FG1 (λ) ≦ 0. By integrating (45), τ _FG1 (λ) = − (１) τ ⁽²⁾ (λ ₀ ) (λ−λ _S ) ² L _F + C ₁ (48) is obtained. Here, since τ _FG1 (λ _L ) = 0, the constant C ₁ is C ₁ = (１) τ ⁽²⁾ (λ ₀ ) Δλ ² L _F (49) Here, Δλ = λ _L −λ _S. Since a long wavelength side bond, _{τ FG1 (λ S) = (} 2n c / c) L FG1 (50). Thus, grating length L _FG1 is determined to _{L FG1 = (c / 8n c} ) τ (2) (λ 0) Δλ 2 L F (51). Since a long wavelength side _{binding, τ FG1 (λ) = (} 2n c / c) (L FG1 -z) putting and _{(52), λ FG1 (z} ) = λ S + Δλ (z / L FG1) 1 / ² (53) Λ _FG1 (z) = (1 / 2n _c ) {λ _S + Δλ (z / L _FG1 ) ^1/2 } (54)

[0031] Next, the chirped fiber grating FG2 corresponding to D _FG2 (lambda) is, D _FG2 (lambda) ≧
Since it is 0, it is a short wavelength side coupling. By integrating (46), τ _FG2 (λ) = − (１) τ ⁽²⁾ (λ ₀ ) (λ−λ _L ) ² L _F + C ₂ (55) is obtained. Here, since τ _FG2 (λ _S ) = 0, the constant C ₂ is C ₂ = (１) τ ⁽²⁾ (λ ₀ ) Δλ ² L _F (56). Τ _FG2 (λ _L ) = (2n _c / c) L _FG2 (57) Thus, grating length L _FG2 is determined to _{L FG2 = (c / 8n c} ) τ (2) (λ 0) Δλ 2 L F (58). Since τ _FG2 (λ) = (2n _c / c) z (59) because it is the short-wavelength side coupling, λ _FG2 (z) = λ _S + Δλ {1- (1-z / L _FG2 ) ^{1 / _{2} (60) Λ FG2 (}} z) = (1 / 2n c) [λ S + Δλ {1- (1-z / L FG2) 1/2}] (61) is obtained.

(C-1) Finally, consider a chirped fiber grating FG3 corresponding to D _FG3 (λ).
First, when λ ₀ > (λ _S + λ _L ) / 2, since D _FG3 (λ)> 0, coupling is on the short wavelength side. (47) by integrating the ^{_{τ FG3 (λ) = τ (}} 2) (λ 0) {λ 0 - (λ S + λ L) / 2} L F λ + C 3 (62) τ because it is the shorter wavelength side coupling _FG3 (λ _S ) = 0, and C ₃ = −τ ⁽²⁾ (λ ₀ ) {λ ₀ − (λ _S + λ _L ) / 2｝ L _F λ _S (63) is obtained. Since a shorter wavelength side coupling _{τ FG3 (λ L) = (} 2n c / c) L FG3 (64) Therefore _{L FG3 = (c / 2n c} ) τ (2) (λ 0) {λ 0 - (λ S + Λ _L ) / 2｝ Δλ _{L F} (65) τ _FG3 (λ) = (2 n _c / c) z (66) because of the short-wavelength side coupling. Therefore, λ _FG3 (z) = λ _S + (Δλ / L _FG3 ) z (67) Λ _FG3 (z) = (１n _c ) {λ _S + (Δλ / L _FG3 ) z} (68)

(C-2) Next, when λ ₀ = (λ _S + λ _L ) / 2, since D _FG3 (λ) = 0, FG3 corresponding to D _FG3 (λ) is not required. FIG. 9 shows a configuration diagram of the dispersion compensator in this case. In FIG. 9, incident light 1 passes through two 3-port optical circulators 2 and becomes outgoing light 6. Chirped fiber gratings 7 and 8 are connected to respective ports of the optical circulator 2. Reference numeral 7 denotes a long-wavelength-side coupled chirped fiber grating shown by (51) and (54), and 8 denotes (58) and (6).
This is a short-wavelength-side coupled chirped fiber grating shown in 1). The light is guided to the chirped fiber gratings 7 and 8 by the optical circulator 2 and is reflected while generating a delay (dispersion characteristic).
Returns to the optical transmission path.

(C-3) Next, when λ ₀ <(λ _S + λ _L ) / 2, since D _FG3 (λ) <0, coupling is on the long wavelength side. (4
7) is integrated to obtain (62) in the same manner as described above. In this case, τ _FG3 (λ _L ) = 0 because of long-wavelength side coupling, and C ₃ = −τ ⁽²⁾ (λ ₀ ) {λ ₀ − (λ _S + λ _L ) / 2｝ L _F λ _L (69) Since a long wavelength side coupling _{τ FG3 (λ S) = (} 2n c / c) L FG3 (70) Therefore _{L FG3 = (c / 2n c} ) τ (2) (λ 0) {(λ S + λ L) _{/ 2-λ 0} ΔλL F} (71) from a long wavelength side _{binding, τ FG3 (λ) = (} 2n c / c) (L FG3 -z) can be placed (72), and C-1 The same result, namely (6
7) and (68) are obtained.

(Embodiment 2) Consider a case where the group delay characteristic of a standard optical fiber is compensated by approximating it to the third order term. At this time, (4) and (5) are approximated as follows.

Τ (λ) = (1/2!) Τ ⁽²⁾ (λ ₀ ) (λ−λ ₀ ) ² + (1/3!) Τ ⁽³⁾ (λ ₀ ) (λ−λ ₀ ) ³ (73) D (λ) = τ ⁽²⁾ (λ ₀ ) (λ−λ ₀ ) + (1/2) τ ⁽³⁾ (λ ₀ ) (λ−λ ₀ ) ² (74) characteristics compensator ^{_{D FG (λ) = -τ (}} 2) (λ 0) (λ-λ 0) L F - (1/2) τ (3) (λ 0) (λ-λ 0) 2 L _F (75) where (75) is the right side of (44),
_FG4 (λ) is added.

D _FG4 (λ) = − (１) τ ⁽³⁾ (λ ₀ ) (λ−λ ₀ ) ² L _F (76) A dispersion compensator having a function represented by (44) as a dispersion characteristic. Has been described in the first embodiment. Therefore, by connecting the dispersion compensator having the dispersion characteristic of D _FG4 (λ) shown in (76) to the dispersion compensator constructed in the first embodiment in a cascade manner, the function shown in (75) can be dispersed. A dispersion compensator having characteristics can be configured.

It is known from the chromatic dispersion measurement of an actual optical fiber that τ ⁽³⁾ (λ ₀ ) <0, so that D _FG4 (λ) ≧ 0 for an arbitrary λ. It is not necessary to divide the function, and the chirped fiber grating FG4 corresponding to D _FG4 (λ) is coupled to the incident light on the short wavelength side. (76) by integrating the tau _FG4 (lambda) = - than ^{(1/6) τ (3) (} λ 0) (λ-λ 0) 3 L F + C 4 (77) τ FG4 (λ S) = 0 The integration constant C ₄ is obtained as C ₄ = (1/6) τ ⁽³⁾ (λ ₀ ) (λ _S −λ ₀ ) ³ L _F (78) On the other hand, given that the shorter wavelength side coupling τ _FG4 (λ _L) = from a _{(2n c / c) L FG4} (79), the length L _FG4 of ^{_{FG4 L FG4 = (cτ (3}} ) ( _{λ 0) L F / 12n c} ) {(λ S -λ 0) 3 - (λ L -λ 0) 3} given by (80). Further, τ _FG4 (λ) = at the _{(2n c / c) z (} 81), λ FG4 (z) = λ 0 + {(λ S -λ 0) 3 - (12n c / cτ (3) (λ ₀ ) L _F ) z} ^1/3 (82) Λ _FG4 (z) = (１n _c ) [λ ₀ + {(λ _S −λ ₀ ) ³ − (12 n _c / cτ ⁽³⁾ (λ ₀ ) L _F ) z} ^1/3 ] (83).

FIG. 10 shows a configuration diagram of the dispersion compensator in this case. In FIG. 10, incident light 1 passes through four 3-port optical circulators 2 and becomes outgoing light 6. Chirped fiber gratings 7, 8, 9, and 10 are connected to respective ports of the optical circulator 2. 7 is a long-wavelength-side coupled chirped fiber grating shown by (51) and (54), and 8 is (5)
8) and (61) are chirped fiber gratings of the short wavelength side coupling, 9 is a chirped fiber grating shown in (65) and (68) (the coupling direction can be changed), and 10 is (80) ) And (83) are short-wavelength-side coupled chirped fiber gratings. The light is guided to the chirped fiber gratings 7 and 8 by the optical circulator 2 and is delayed (dispersion characteristic).
, And returns to the optical transmission path by the optical circulator 2 again.

(Embodiment 3) In Embodiment 1, λ ₀ ≦ λ _S
In this case, D _FG (λ ₀ ) ≦ 0 and the coupling is on the long wavelength side. In this case, the requirement for the chirped fiber grating is (30) or (31).
Let λ ₀ << λ _S and Δλ = λ _L −λ _S << λ _L , λ _S. For example, λ ₀ = 1300 nm, λ _S = 1550 nm, λ _L = 1551 nm, Δλ
= 1 nm. In such a case, it may be approximated by a linear chirp grating as shown below. First, (36) is _{λ FG (z) = λ 0} + {(λ L -λ 0) 2 - (4n c / cτ (2) (λ 0) L F) (L FG -z)} 1/2 _{≒ λ 0 + (λ L -λ} 0) [1 - {2n c / cτ (2) (λ 0) L F (λ L -λ 0) 2} (L FG -z)] = λ L - {2n ^{_{c / cτ (2) (λ}} 0) L F (λ L -λ 0)} is approximated as _{(L FG -z) (84)} . Here, since it is _{^{τ (2) (λ 0)}} L F (λ L -λ 0) = D (λ L) L F (85), λ FG (z) ≒ λ L - (2n c / cD ( _{λ L) L F) (L} FG -z) = λ S + Δλ- (2n c L FG / cD (λ L) L F) (1-z / L FG) (86) where, ΔλD (λ _L) Since L _F ≒ 2n _c L _FG / c (87), λ _FG (z) ≒ λ _S + (Δλ / L _FG ) z (88) Λ _FG (z) ≒ (1 / 2n _c ) {λ _S + (Δλ / L _FG ) z} (89).

(Embodiment 4) In the present invention, when a fiber grating FGij is actually manufactured, it is considered effective to use a step chirp phase mask (R.
Kashyap, A. Swanton and DJ Armes, "Simple techn
ique for apodizing chirped and unchirped fiber Bra
gg gratings ", Electron. Lett., vol. 32, no. 13, p
p. 1226-1228, 1996). The step chirp phase mask is _composed of N sections of equal width, and the k-th pitch is _defined as Λ _PMij
(k). At this time, Λ _PMij (k) is expressed as follows. That is, in (17) and (23), j is ij
＝ _FGij (k) obtained by substituting z = (k / N) L _FGij (90) into (17) and (23) may be doubled. That is, _{PMij (k) = 2 _{} FGij} (k) (91)

[0042]

As described above, according to the present invention,
A broadband high-order dispersion compensator is obtained. In other words, it is possible to compensate for arbitrary chromatic dispersion characteristics of various optical fibers. Such a broadband high-order dispersion compensator will be very useful in future terabit transmission systems expected to use optical pulses of 1 ps or less. In addition, according to the present invention, dispersion slope compensation becomes possible in a wavelength division multiplexing transmission system, and the present invention is very useful.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a group delay characteristic of an optical fiber.

FIG. 2 is a diagram showing a chirped fiber grating and coordinates set thereon.

3A is a diagram illustrating a group delay characteristic of the chirped fiber grating when coupled to the short wavelength side, and FIG. 3B is a diagram illustrating a group delay characteristic when the chirped fiber grating is coupled to the long wavelength side.

FIG. 4 is a flowchart of a method of configuring a higher-order dispersion compensator.

FIG. 5 is a configuration diagram of a general high-order dispersion compensator according to the present invention.

FIG. 6 is a diagram illustrating a case where an operation region of a higher-order dispersion compensator is on a longer wavelength side than a zero dispersion wavelength.

FIG. 7 is a diagram illustrating a case where an operation region of a higher-order dispersion compensator is on a shorter wavelength side than a zero dispersion wavelength.

FIG. 8 is a diagram illustrating a case where the operation region of the higher-order dispersion compensator includes a zero-dispersion wavelength.

FIG. 9 is a diagram illustrating a configuration example of a dispersion compensator in which up to the second order term among the group delay characteristics of an optical fiber is considered, and illustrates a case where a zero dispersion wavelength is at the center of the band of the compensator.

FIG. 10 is a diagram illustrating a configuration example of a dispersion compensator in which a third-order term of the group delay characteristic of the optical fiber is considered, in which the zero dispersion wavelength is within the band of the compensator.

[Explanation of symbols]

1 ... incident light, 2 ... 3-port optical circulator, 3 ... chirped fiber grating FG11, 4 ... chirped fiber grating FGij, 5 ... chirped fiber grating FGnm, 6 ... outgoing light, 7 ... (51), (54) Chirped fiber grating (long-wavelength side coupling) 8... (58), chirped fiber grating (short-wavelength side coupling) represented by (61) 9... (65), chirped fiber grating represented by (68) (The direction of the coupling can be changed.) 10... Chirped fiber gratings (coupling on the short wavelength side) represented by (80) and (83).

Claims (5)

[Claims] 1. A group partial sum is obtained by subjecting the group delay amount of an optical fiber to a finite order with respect to a wavelength centered on a zero dispersion wavelength to obtain a series partial sum, and the series partial sum is differentiated with respect to a wavelength to obtain a chromatic dispersion function. Means for inverting the sign of the chromatic dispersion function to obtain a sign inversion function, and for compensating for the chromatic dispersion of the group delay amount of the optical fiber by using a chirped fiber grating having a function relationship represented by the sign inversion function as chromatic dispersion characteristics. A high-order dispersion compensator, characterized in that: 2. The method according to claim 1, wherein when the sign of the sign inversion function changes in a wavelength region to be dispersion-compensated, the sign inversion function is decomposed into a sum of a plurality of constant sign functions whose signs do not change in the wavelength region. A high-order dispersion compensator characterized in that a chirped fiber grating having a function relationship represented by each of the constant sign functions and a chromatic dispersion characteristic is connected via an optical circulator. 3. The group delay amount of the optical fiber is Taylor-expanded to a finite order with respect to the wavelength centered on the zero dispersion wavelength to obtain a series partial sum, and the series partial sum is differentiated with respect to the wavelength to obtain a chromatic dispersion function. A sign inversion function is obtained by inverting the sign of the chromatic dispersion function, and a chirped fiber grating having a function relationship represented by each term of a polynomial representing the sign inversion function as a chromatic dispersion characteristic is connected via an optical circulator. A high-order dispersion compensator characterized by the following. 4. A method according to claim 3, wherein when some of the signs of the above term change in a wavelength range to be dispersion-compensated, the term is decomposed into a sum of a plurality of constant sign functions whose signs do not change in the wavelength range. A high-order dispersion compensator characterized in that chirped fiber gratings each having a function relationship represented by each of the constant sign functions and having a chromatic dispersion characteristic are connected via an optical circulator. 5. The chirp according to claim 1, wherein when the sign of the sign inversion function is positive in the wavelength region to be dispersion-compensated, the functional relationship represented by the sign inversion function is regarded as a chromatic dispersion characteristic. The short wavelength side of the fiber grating, the sign of the sign inversion function, the long wavelength side of the chirped fiber grating with the chromatic dispersion characteristic of the functional relationship represented by the sign inversion function when the sign is negative in the wavelength range to be dispersion compensated, Higher-order dispersion compensators, each connected to a port of an optical circulator.