JPH06276154A - Optical gain equalizing circuit - Google Patents

Abstract

PURPOSE:To obtain an optical gain equalizing circuit whereby a sufficient transmission characteristic is secured even when the number of stage in an optical amplifier is large and even when the umbalance of the gain is large. CONSTITUTION:First and second Much-Zehnder-shape filters 20 and 30 with mutually different change cycles on the wave-length (or optical frequency) axis of transmissivity are continuously connected. Thus, the nonuniformity of a gain wave-length (or optical frequency) characteristic which cannot be flattened by the first Much-Zehnder-shape optical filter 20 can be flattened by the second Much-Zehnder-shape optical filter 30 with the change cycle on the wave-length (or optical frequency) axis of transmissivity, being different from that of the above first Much-Zehnder-shape optical filter 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical gain equalization circuit used in an optical wavelength (or optical frequency) multiplex transmission system.

[0002]

2. Description of the Related Art An optical wavelength (or optical frequency) multiplex transmission system is a system in which a plurality of signal lights having different wavelengths (or optical frequencies) carrying information are multiplexed in one optical fiber and transmitted. is there. When an optical amplifier that collectively amplifies the multiplexed signal light is applied to the above method, the difference between the transmission and reception levels of the transmission system is improved, the transmission distance is expanded in the long distance transmission system, and the number of distributions in the information distribution transmission system is increased. It is possible to increase.

By the way, in general, the gain of an optical amplifier is not constant depending on the wavelength. Therefore, in the case of collectively amplifying wavelength-division-multiplexed signal light, the optical level after amplification differs depending on the wavelength, and particularly, it passes through a plurality of optical amplifiers. In the signal light, the difference in light level is accumulated. As a result, when demultiplexing and receiving the multiplexed signal light at the receiving end, the optical level differs depending on the wavelength, and especially in a channel with a low optical level, interchannel crosstalk increases and SN ratio deterioration occurs. However, there is a problem that the reception sensitivity is deteriorated.

As a method of compensating the wavelength dependence of the gain of the optical amplifier described above, conventionally, a tunable Mach-Zehnder type optical filter having a variable transmittance characteristic is used as an optical gain equalizing circuit to flatten the output of the optical wavelength multiplexed signal. There is an example (Reference 1: K. Inoue, T. Kominato, and H. Toba, “Tunable gai
n equalization using a Mach-Zehnder optical filter
in multi-stage fiber amplifiers "IEEE Photon.Tec
hnol.Lett., vol.3, No.8, pp.718-720, 1991, or literature 2: H.Toba, K.Nakanishi, K.Oda, K.Inoue, and T.Komi.
nato, “A 100-channel optical FDM in-line amplifier
system employing tunable gain equalizers ”ECOC'92
See Proceedings pp.113-116).

FIG. 2 shows the optical gain equalization circuit described in the above-mentioned document. FIG. 2 (a) shows a connection mode between the optical amplifier and the optical gain equalization circuit.
The optical gain equalization circuit 3 flattens the optical level imbalance of the optical wavelength division multiplexed signal input to the optical amplifier 2 and amplified and output.

FIG. 2 (b) shows a detailed structure of the optical gain equalization circuit. Directional couplers 6 and 7 are provided on the input side and the output side of two waveguides 4 and 5 having different lengths, respectively. It has a configuration of a Mach-Zehnder type optical filter connected by. The directional couplers 6 and 7 on the input side and the output side have a structure of a symmetric Mach-Zehnder interferometer, and their coupling efficiency is adjusted by adjusting the bias currents to the respective phase adjusting electrodes 6a and 7a. Can be changed. As the electrodes 6a and 7a, for example, a Cr heater can be used, and the phase can be adjusted by the thermo-optical effect generated by passing a current through the electrodes 6a and 7a.

In FIG. 2B, the optical fiber 8 on the input side
From the output side to the optical fiber 9 on the output side is T≈1-A · cos ² {(λ−λo) π / Δλ when the excess loss such as the waveguide loss and the connection loss between the fiber waveguides is excluded. } …… (1) is represented. Here, λ is the operating wavelength, λo is the central wavelength at which the transmittance of the present optical gain equalization circuit is minimum, and A is 0.
A real number that satisfies <A ≦ 1. This transmittance characteristic has a period Δ
It has a periodicity of λ.

FIG. 3 shows the transmittance characteristics of the optical gain equalization circuit. Here, the phase adjustment electrodes 6a and 7a
By adjusting the bias current of
Can be changed, and the gain compensation amount of the original transmittance characteristic shown by the solid line 11 can be adjusted as shown by the broken line 12. Further, by adjusting the bias current of the electrode 4a, it is possible to change λo in the equation (1), and it is possible to adjust the center wavelength of the transmittance characteristic as shown by the alternate long and short dash line 13.

FIG. 4 shows the state of gain equalization described in the above-mentioned document 2. Here, from the light wavelength of 1548 nm
Wavelength interval occupying 1556 nm 0.08 nm (optical frequency interval 1
Signal light of 100 channels multiplexed with 0 GHz)
The spectrum when amplified by the aluminum co-doped erbium addition optical amplifier connected in multiple stages is shown.

The left side of FIG. 4 shows the input spectrum of the optical amplifier and the output spectrum of the third and sixth optical amplifiers when the optical gain equalizing circuit is not used. It can be seen that, when the optical gain equalization circuit is not used, the imbalance of the optical level of the optical wavelength division multiplexed signal increases with the number of stages of the optical amplifier. The right side of FIG. 4 shows the output spectrum of the optical gain equalization circuit shown in FIG. 2 when the optical gain equalization circuit is inserted in each of the three stages of optical amplifiers. It can be seen that when the optical gain equalization circuit is used, the imbalance of the optical level of the optical wavelength division multiplexed signal can be compensated to some extent.

[0011]

However, when the spectrum with the optical gain equalizing circuit shown on the right side of FIG. 4 is examined in detail, the spectrum corresponding to the output of the third-stage optical amplifier shows a substantially flat characteristic. However, it can be seen that the flatness of the spectrum corresponding to the output of the sixth-stage optical amplifier is deteriorated and the vicinity of the center of the spectrum is bulged and has an upwardly convex characteristic. This tendency of deterioration of the flatness further increases with an increase in the number of stages of the optical amplifier, and there is a problem that the transmission characteristics are deteriorated even when the above-mentioned optical gain equalizing circuit is used.

In view of the above-mentioned conventional problems, it is an object of the present invention to provide an optical gain equalization circuit capable of ensuring good transmission characteristics even when the number of stages of the optical amplifier is large or the gain imbalance is large. To do.

[0013]

According to the present invention, in order to achieve the above object, a gain wavelength (or optical wavelength) of an optical amplifier that collectively amplifies a plurality of multiplexed signal lights having different wavelengths (or optical frequencies) from each other. In an optical gain equalization circuit for flattening frequency characteristics, at least two Mach-Zehnder optical filters having different change periods of transmittance on the wavelength (or optical frequency) axis are cascade-connected. To propose.

[0014]

The Mach-Zehnder type optical filter has a transmittance characteristic that changes sinusoidally on the wavelength (or optical frequency) axis. By the way, it is clear from the Fourier series expansion method that an arbitrary waveform can be formed by superimposing a plurality of sine waves having mutually different periods. Therefore,
In principle, it is possible to realize arbitrary transmittance characteristics by cascading a plurality of Mach-Zehnder optical filters having different change periods on the wavelength (or optical frequency) axis of the transmittance. , The number of cascaded Mach-Zehnder type optical filters will be determined according to the required flatness of the optical amplifier output.

According to the optical gain equalization circuit of the present invention, the non-uniformity of the gain wavelength (or optical frequency) characteristic which cannot be flattened by the conventional optical gain equalization circuit using only one stage of the Mach-Zehnder type optical filter. The first stage is flattened by the Mach-Zehnder type optical filters of the second and subsequent stages, which are different in the change cycle of the transmittance on the wavelength (or optical frequency) axis.

[0016]

EXAMPLES The present invention will be described in detail below with reference to examples. FIG. 1 shows an embodiment of an optical gain equalization circuit of the present invention, in which 20 is a first Mach-Zehnder type optical filter, 30 is a second Mach-Zehnder type optical filter, 41 is an input side optical fiber, 42 is an output side optical fiber.

Mach-Zehnder type optical filters 20 and 30
Are different in the change cycle of the transmittance on the wavelength (or optical frequency) axis from each other, and are connected in two stages. The configuration of each Mach-Zehnder optical filter 20, 30 is basically the same as that shown in FIG. 2, and the electrodes 21 and 22 in the Mach-Zehnder optical filter 20 and the electrodes 31 and 3 in the Mach-Zehnder optical filter 30 are shown.
2 is for adjusting the gain compensation amount of the transmittance characteristic, and the electrode 23 in the Mach-Zehnder optical filter 20 and the electrode 33 in the Mach-Zehnder optical filter 30 are for adjusting the central wavelength of each transmittance characteristic. belongs to.

The specific operation of this embodiment will be described using measured data. Fig. 5 shows an example of the measured values of the input / output spectrum of the optical amplifier. Here, from the wavelength of 1549.5 nm,
The spectrum when 128-channel signal light multiplexed with an optical frequency interval of 10 GHz having a band of 1559.8 nm is amplified by an aluminum co-doped erbium-doped optical amplifier is shown. According to the measurement result, the gain of the optical amplifier is larger in the longer wavelength channel in this used wavelength band,
Moreover, it can be seen that the rate of increase in gain tends to saturate as the wavelength becomes longer.

FIG. 6 shows a state of gain equalization by this circuit, and the gain spectrum of the optical amplifier read from FIG. 5 is shown by a curve G1. On the other hand, when the first Mach-Zehnder interferometer type optical filter 20 having a period Δλ = 25 nm is selected and this is adjusted to have a central wavelength λ o = 1546.570 nm and A = 1, its transmittance characteristic is shown by F1 in FIG. It becomes a curved line. As a result, the gain spectrum of the optical amplifier compensated by the first Mach-Zehnder interferometer type optical filter 20 becomes a curve indicated by G2 in FIG. 6, and the wavelength dependence of the gain on the upper right is improved, but it is convex upward. It has wavelength characteristics and a residual gain deviation of 2.7 dB occurs.

Here, a second Mach-Zehnder type optical filter 30 having a period Δλ = 7.5 nm is further selected,
When this is adjusted to have a central wavelength λ o = 1555 nm and A = 0.46, the transmittance characteristic becomes a curve indicated by F2 in FIG. As a result, the first Mach-Zehnder optical filter 20
The gain spectrum of the optical amplifier compensated by the second Mach-Zehnder optical filter 30 becomes a curve indicated by G3 in FIG. 6, and the residual gain deviation is suppressed to 0.7 dB.

Thus, the transmittance wavelength (or optical frequency)
By using two types of Mach-Zehnder optical filters having different on-axis change periods, the wavelength dependence of gain can be made extremely flat.

In the present embodiment, another Mach-Zehnder type optical filter is connected in series immediately after the Mach-Zehnder type optical filter, but there is no problem if they are arranged separately. Further, although the Mach-Zehnder type optical filter is arranged at the output stage of the optical amplifier in this embodiment, it may be arranged at the input stage.

Further, in the present embodiment, Mach-Zehnder type optical filters having different change periods are used in a two-stage cascade connection.
Depending on the wavelength dependence of the gain of the optical amplifier, it is also possible to cascade a plurality of Mach-Zehnder optical filters having different change periods to flatten the gain.

[0024]

As described above, according to the present invention, the wavelength dependence of the gain of the optical amplifier can be appropriately compensated regardless of the number of stages of the optical amplifier and the magnitude of its imbalance. The optical level of the wavelength division multiplexed signal can be made extremely flat, so that the number of stages of the optical amplifier in the optical wavelength (or optical frequency) multiplex transmission system can be increased as compared with the conventional one, and the long distance transmission system can be used. It is possible to increase the transmission distance and increase the number of distributions in the information distribution and transmission system. Further, not only the optical amplifier but also the wavelength dependence of the transmittance characteristics of other optical circuits in the transmission line existing between the transmitting and receiving circuits can be compensated by this optical gain equalization circuit, and the wavelength characteristics can be flattened. It goes without saying that it is possible.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical gain equalization circuit of the present invention.

FIG. 2 is a diagram showing an example of a conventional optical gain equalization circuit.

FIG. 3 is a diagram showing a transmittance characteristic of a conventional optical gain equalization circuit.

FIG. 4 is a diagram showing an amplification characteristic of an optical wavelength division multiplexed signal when a conventional optical gain equalization circuit is used.

FIG. 5 is a diagram showing an example of actually measured values of an input / output spectrum of an optical amplifier.

FIG. 6 is a diagram showing a state of gain equalization by this circuit.

[Explanation of symbols]

20 ... 1st Mach-Zehnder type optical filter, 30 ... 2nd
Mach-Zehnder type optical filter, 41 ... Input side optical fiber, 42 ... Output side optical fiber.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location H04B 3/06 A 7741-5K H04J 14/02

Claims (1)

[Claims] 1. An optical gain equalization circuit for flattening gain wavelength characteristics of an optical amplifier for collectively amplifying a plurality of multiplexed signal lights having different wavelengths, wherein a change cycle of transmittance on a wavelength axis is An optical gain equalization circuit comprising at least two Mach-Zehnder type optical filters different from each other connected in series.