JP3452235B2 - Chromatic dispersion compensation circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a dispersion compensation circuit for correcting a waveform distortion of propagating light due to chromatic dispersion of an optical fiber in a communication system using an optical fiber. . 2. Description of the Related Art Generally, a normal 1.3 μm band zero dispersion optical fiber has a minimum refractive index around a wavelength of 1.3 μm.
For light with a longer wavelength, the longer the wavelength, the higher the refractive index. Therefore, the 1.5 μm wavelength optical signal propagated through the ordinary dispersion optical fiber is subject to a large group delay as the wavelength component becomes longer due to the influence of the chromatic dispersion of the optical fiber, and a larger waveform as the signal with a higher bit rate is transmitted. Subject to distortion. In order to correct such wavelength distortion, a chromatic dispersion compensating circuit that gives a smaller group delay to long-wavelength light, which is the opposite of the above-described chromatic dispersion, is inserted to reduce the chromatic dispersion of the entire transmission system. A method for reducing this is conceivable. It has been known that an element for compensating waveform distortion of a transmission optical signal due to chromatic dispersion can be realized by changing the period of change in the refractive index of a fiber grating in the longitudinal direction. FIG. 10 is a schematic diagram showing a schematic configuration of a conventional example of a chromatic dispersion compensating circuit constituted by using the above-described chirped fiber grating, wherein 1 is a chirped fiber grating, 2 is an optical circulator, and 3 is a signal. An input optical fiber 4 for inputting light is an output optical fiber for outputting signal light. As shown in FIG. 11, the distribution of the grating pitch in the chirped fiber grating 1 in the longitudinal direction is λ ₁ / x at both ends (x = 0, x = L).
The pitch becomes 2n, λ ₂ / 2n, and changes continuously and linearly with x in the middle. Here, n is the average refractive index of the grating forming portion of the optical fiber. Generally, the grating pitch is λ / 2n
Is a light wavelength filter that reflects only light in a very narrow wavelength range of the center wavelength λ. Accordingly, a chirped fiber grating as shown in FIG. 11 is equivalently configured as shown in FIG. 12 in which a fiber grating having a constant pitch whose reflection wavelength is gradually shifted to the short wavelength side is connected in series in the longitudinal direction. You can see. For this reason, in FIG. 10, an optical signal incident on the chirped fiber grating 1 from the input optical fiber 3 via the optical circulator 2 is reflected at a position closer to the incident end of the grating as the wavelength component becomes longer. Output. Accordingly, the dispersion compensating circuit shown in FIG. 10 has a wavelength dispersion characteristic opposite to that of the ordinary dispersion optical fiber, and has an effect of reducing the influence of the wavelength dispersion on the optical signal propagated through the ordinary dispersion optical fiber. Understand. Further, in the dispersion compensating circuit of this configuration, by adjusting the amount of change in the grating pitch per unit length of the chirped fiber grating 1, the amount of dispersion of the compensating circuit itself can be adjusted. For this reason,
It is possible to easily realize a compensation circuit corresponding to the amount of chromatic dispersion to be corrected. In fiber gratings,
When viewed in the longitudinal direction, the refractive index changes in a cycle of the wavelength order, and the width of the change is called a relative refractive index difference. As described above, it is possible to realize a dispersion compensating circuit by using the chirped fiber grating 1. At this time, the high refractive index portion and the low refractive index portion of the grating inside the optical fiber are used. If the longitudinal distribution of the relative refractive index difference is used without optimization, the obtained dispersion compensation characteristics may be significantly deteriorated. FIG. 13 shows the result of precisely calculating the reflectance, the group delay time of reflected light, and the amount of chromatic dispersion (value) of a chirped fiber grating in which the relative refractive index difference takes a constant value over the entire grating. In FIG. 13, A is a reflectance curve, B is a group delay time curve of reflected light, and C is a chromatic dispersion amount (value) curve. FIG. 14 shows the relative refractive index difference of this grating, and FIG. 15 shows the longitudinal distribution of the grating pitch. At this time, the dispersion compensation amount of the chirped fiber grating calculated from the change rate of the grating pitch per unit length is 172 ps / nm.
However, as apparent from FIG. 13, according to the calculation result, the amount of group delay time decreases while vibrating finely in the reflection wavelength region. Along with this, the amount of dispersion vibrates sharply with an amplitude of ± 10000 ps / nm or more with respect to the wavelength. Thus, the waveform of the transmitted optical signal is greatly distorted by the oscillation of the dispersion value far exceeding the amount of dispersion to be compensated. FIG. 16 shows an original waveform of an optical signal modulated by a random pattern of 10 Gb / s, and FIG. 17 shows an eye pattern after passing through a dispersion compensation circuit using the chirped fiber grating four times. It is assumed that the wavelength of the signal light is 1550 nm and no chirp due to the modulation has been received. The chromatic dispersion amount of the dispersion compensation circuit itself is corrected. As apparent from FIGS. 16 and 17, the optical signal that has passed through the dispersion compensating circuit is greatly distorted, and the eye opening is greatly degraded. Understand. As described above, when a chirped fiber grating in which the relative refractive index difference of the grating is uniform in the longitudinal direction is used for the dispersion compensating circuit, there is a problem in that the waveform of the passing optical signal is greatly deteriorated. An object of the present invention is to provide a dispersion compensating circuit for correcting a waveform distortion of propagating light due to chromatic dispersion of an optical fiber in a communication system using an optical fiber and not giving it to other additional distortion propagating light. Is to do. The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings. The following is a brief description of an outline of typical inventions disclosed in the present application. (1) As a medium for giving wavelength dispersion to an optical signal, a waveguide grating formed by giving a minute and periodic change in refractive index in an optical fiber or an optical waveguide is used, and the waveguide grating is used. In the chromatic dispersion compensating circuit, which is a chirped fiber grating in which the period of change of the refractive index changes at a constant rate with respect to the longitudinal direction of the grating, the chirped fiber grating is formed at its end and inside the grating. The width of the obtained refractive index change has a region that gradually changes in the longitudinal direction from 0 to the value of the refractive index change in the central portion of the grating in the longitudinal direction. (2) In the chromatic dispersion compensating circuit according to (1), the change in the refractive index in a region where the width of the change in the refractive index gradually changes in the longitudinal direction is a function of the position in the longitudinal direction of the fiber grating. And the function
Is represented by the following equation (Equation 1) . (Equation 1) In the equation of [Equation 1], x is the distance from the end of the grating.
Position and Λ are constant values of relative refractive index difference from grating edge
And d ₀ is the distance to the position where
Relative refractive index difference. According to the above-mentioned means, a distribution is provided to the relative refractive index difference of the grating at the light incident end of the chirped fiber grating, and the difference is continuously changed from both ends of the grating toward the center. It is greatly improved. That is, the present invention corrects the waveform distortion of the propagating light due to the chromatic dispersion of the optical fiber and also applies the relative refraction of the chirped fiber grating to be applied to the dispersion compensating circuit which does not give the additional distortion propagating light. This is clearly different from the prior art in that the longitudinal distribution of the rate difference is specifically defined. Embodiments of the present invention will be described below in detail with reference to the drawings. In all the drawings for describing the embodiments, parts having identical functions are given same symbols and their repeated explanation is omitted. (Embodiment 1) As shown in FIG. 10, a chromatic dispersion compensating circuit according to Embodiment 1 of the present invention is used as a medium for giving chromatic dispersion to an optical signal, in an optical fiber or an optical waveguide. The chirped fiber grating 1 uses a waveguide grating formed by giving a refractive index change, and the period of the refractive index change of the waveguide grating changes at a constant rate in the longitudinal direction of the grating. In a chromatic dispersion compensating circuit, the width of the refractive index change formed inside the chirped fiber grating 1 at the end thereof and the inside of the grating ranges from 0 to the value of the refractive index change at the center in the longitudinal direction of the grating. , Which have regions that gradually change in the longitudinal direction. FIG. 1 shows the reflection of a chirped fiber grating having a region where the relative refractive index difference linearly increases toward the center of the grating at both ends of the grating in the chromatic dispersion compensation circuit of the first embodiment. rate,
It is a figure which shows the result of having calculated the group delay time of reflected light, and the amount of chromatic dispersion precisely, A is a reflectance curve, B is a group delay time curve of reflected light, and C is a chromatic dispersion amount (value) curve. . FIG. 2 shows the relative refractive index difference of the grating in the chromatic dispersion compensating circuit of the first embodiment, and FIG. 3 shows the distribution of the grating pitch in the longitudinal direction. Here, when FIG. 1 of the first embodiment is compared with FIG. 13 of the conventional example, no oscillation of the graph of the group delay time of the reflected light is observed within the reflection wavelength region. Also,
Accordingly, the amplitude of the oscillation of the chromatic dispersion amount is suppressed to 1/100 or less. By suppressing the chromatic dispersion vibration, waveform distortion given to a passing optical signal can be largely suppressed. FIG. 4 shows an eye pattern of an optical signal of 10 Gb / s after passing through the dispersion compensating circuit using the grating four times. It is assumed that the wavelength of the signal light is 1550 nm and no chirp due to the modulation has been received. The chromatic dispersion amount of the dispersion compensation circuit itself is corrected. As is clear from the comparison between FIG. 4 and FIG. 16 of the conventional example,
The waveform deterioration of the signal light due to the insertion of the dispersion compensation circuit is suppressed to such an extent that it is hardly observed. On the other hand, under more severe conditions, a slight waveform deterioration may be observed in the dispersion compensation circuit of this embodiment. FIG. 5 shows an eye pattern of an optical signal that has passed through the dispersion compensation circuit 100 times. The wavelength of the signal light is 1550
It is assumed that a chirp with a chirp coefficient of 1 has been received with modulation in nm. As is clear from FIG. 5, under such severe conditions, a certain degree of waveform distortion and accompanying deterioration of the eye are observed. The waveform distortion generated under such severe conditions can be further reduced by changing the longitudinal distribution of the grating relative refractive index difference at both ends of the chirped fiber grating more smoothly. Specifically, a function indicating the distribution in the longitudinal direction is set so that higher-order derivatives are continuous in the longitudinal direction. In general, in the region where the relative refractive index difference continuously changes between the end portion of the grating (the relative refractive index difference is 0) and the center portion (the relative refractive index difference is constant) and at both ends thereof, the relative refractive index difference is obtained. The derivative of the nth order or less becomes continuous, and a function composed of only terms of lower order than x ^{2n + 1} is expressed by the following equation (1). ## EQU1 ## Where x is the position from the end of the grating, Λ is the distance from the end of the grating to a position where the relative refractive index difference has a constant value, and d ₀ is the relative refractive index difference at the position where the relative value has a constant value. . By changing the relative refractive index difference in the region according to this function, a dispersion compensation circuit without waveform distortion can be realized. Since a known technique is used for the method of manufacturing the dispersion compensating circuit, the description thereof is omitted here. (Embodiment 2) FIG. 6 shows a dispersion compensating circuit according to Embodiment 2 (where the relative refractive index difference is n =
2 is a diagram showing the reflectance, the group delay time of reflected light, and the amount of chromatic dispersion of a dispersion compensating circuit using a fiber grating according to the case of 2, where A is a reflectance curve, B is a group delay time curve of reflected light, C is a wavelength dispersion amount (value) curve. In this case, the above function is represented by the following equation (2). [Mathematical formula-see original document] This function is composed only of terms of order less than or equal to x ^5, and the second order or lower derivative of the relative refractive index difference at the positions of x = 0 and x = Λ is set to 0, and at that position the second order or less is obtained. Let the derivative of be continuous. FIG. 7 shows the relative refractive index difference of the grating, and FIG. 8 shows the distribution of the grating pitch in the longitudinal direction. As shown in FIGS. 7 and 8, d ₀ is 0.
Λ and Λ are lengths corresponding to 40000 grating layers. In the dispersion value graph of FIG. 6, the oscillation with respect to the wavelength is further suppressed as compared with FIG. 1, and it is considered that the waveform deterioration under the severe conditions as described above can be further suppressed. FIG. 9 shows an eye pattern of an optical signal having a wavelength of 1550 nm and a chirp coefficient of 1 due to modulation that has passed through the dispersion compensation circuit 100 times. Almost no deterioration of the eye pattern due to the passage of the circuit is observed, and it can be seen that the dispersion compensating circuit of the second embodiment does not cause waveform deterioration even under such severe conditions. In the description of the first and second embodiments, the examination results specified for the fiber grating designed with the parameters shown in the figure are described. However, the present invention conforms to the above-mentioned equations (1) and (2). , And is not limited to a particular value of n, d ₀ or Λ. Further, the dispersion compensation circuits of the first and second embodiments can be realized by using a known manufacturing technique. As described above, the invention made by the present inventor is:
Although specifically described based on the embodiment, the present invention
It is needless to say that the present invention is not limited to the above-described embodiment, but can be variously modified without departing from the scope of the invention. The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows. According to the present invention, it is possible to obtain a dispersion compensating circuit that corrects waveform distortion of propagating light due to chromatic dispersion of an optical fiber and does not apply the additional distortion propagating light.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the reflectance and group delay time of reflected light of a chirped fiber grating according to Embodiment 1 of the present invention;
FIG. 9 is a diagram illustrating a calculation result of a chromatic dispersion amount. FIG. 2 is a diagram illustrating a longitudinal distribution of a relative refractive index difference of the grating according to the first embodiment. FIG. 3 is a diagram showing a longitudinal distribution of a grating pitch in the first embodiment. FIG. 4 is a diagram showing an eye pattern observed after a 10 Gb / s optical signal has passed through a dispersion compensation circuit four times according to the first embodiment. FIG. 5: 10 Gb / chirped with a chirp of 1
The optical signal of s is set to 10 by the dispersion compensation circuit according to the first embodiment.
It is a figure showing an eye pattern observed after passing 0 times. FIG. 6 is a diagram illustrating calculation results of the reflectance, the group delay time of reflected light, and the amount of chromatic dispersion of the dispersion compensation circuit according to the second embodiment of the present invention. FIG. 7 is a diagram illustrating a longitudinal distribution of a relative refractive index difference of the grating according to the first embodiment. FIG. 8 is a longitudinal distribution of a grating pitch in the second embodiment. FIG. 9: 10 Gb / Chirp with a chirp of 1
s is the optical signal of the dispersion compensating circuit according to the second embodiment.
It is an eye pattern observed after passing through 00 times. FIG. 10 is a schematic diagram showing a schematic configuration of a chromatic dispersion compensating circuit configured using a conventional chirped fiber grating. 11 is a diagram schematically showing the structure of a chirped fiber grating shown in FIG. 12 is a diagram illustrating the principle of the dispersion compensation effect by the chirped fiber grating shown in FIG. 13 is a calculation result of the reflectance, the group delay time of reflected light, and the amount of chromatic dispersion of a chirped fiber grating in which the relative refractive index difference of the chirped fiber grating of FIG. 11 takes a constant value over the entire grating. FIG. 14 is a diagram illustrating a longitudinal distribution of a relative refractive index difference of the grating of the chirped fiber grating of FIG. 11; FIG. 15 is a diagram showing a longitudinal distribution of a grating pitch of the chirped fiber grating of FIG. 11; FIG. 16 is an eye pattern of an original waveform of an optical signal modulated by a random pattern of 10 Gb / s. FIG. 17 is a diagram showing an eye pattern observed when an optical signal modulated with a random pattern of 10 Gb / s passes through a conventional dispersion compensation circuit four times. [Description of Signs] 1 ... Chirped fiber grating, 2 ... Optical circulator, 3 ... Input optical fiber, 4 ... Output optical fiber.

Continuation of the front page (56) References JP-A-8-338919 (JP, A) JP-B-5-7683 (JP, B2) WO 96/036895 (WO, A1) KASHYAP, R et al, Simple technique For apodizing chirp d and unchirped fibre Bragg gratings, ELECTRONICS LETTERS, UK, IEEE, June 20, 1996, Vol. 32 No. 13, pp. 1226-1228 Tetsuro Komukai and 3 others, Fiber grating applied to spectral filtering, IEICE Transactions on Communications CI, Japan, IEICE, January 25, 1997, Vol. J80-CINO. 1, pp. 32-40 (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 6/00-6/02 G02B 6/10 G02B 6/16-6/22 G02B 6/44

Claims (1)

(57) [Claim 1] A waveguide grating formed by giving a minute and periodic refractive index change in an optical fiber or an optical waveguide as a medium for giving wavelength dispersion to an optical signal. In a chromatic dispersion compensating circuit that is a chirped fiber grating that is used, and in which the refractive index change period of the waveguide grating changes at a constant rate in the longitudinal direction of the grating, the chirped fiber grating has an end. Portion, the width of the change in the refractive index formed inside the grating, from 0 to the value of the change in the refractive index of the central portion in the longitudinal direction of the grating,
Having a region that changes gradually in the longitudinal direction, the refractive index change
Index change in the region where the width of
In the longitudinal direction of the fiber grating.
It is a function of position, and the function is expressed by the following [Equation 1].
Chromatic dispersion compensation circuit which is characterized in that. (Equation 1) In the equation of [Equation 1], x is the distance from the end of the grating.
Position and Λ are constant values of relative refractive index difference from grating edge
And d ₀ is the distance to the position where
Relative refractive index difference.