JPH10215015A - Optical-fiber amplifier part, optical-fiber amplifier and gain equalization of optical-fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to equalize the gain readily for the signal light, which is optically amplified at an optical-fiber amplifier part, at the gain equalizing part having the specified transmission loss spectrum by specifying the deviation of the gains in respective minute wavelengths with a logarithm indication for the specified average gain value. SOLUTION: An optical-fiber amplifier part has 2 or more minimum gain values within the signal optical wavelength region at 1535nm or more and 1560nm or less and is adjusted so that the dispersion of the minimum gain values becomes 4% or less with the numerical indication for the specified average gain value. The optical amplifier part of the optical-fiber amplifier is constituted so as to include EDFs 40, 42 and 43, which are substantially connected in the longitudinal direction, and exciting laser beam sources 20-22, which supply the excited light to the EDF. When the inverted distribution is formed in the respective EDFs 40, 42 and 43, the respective EDFs 40, 42 and 43 amplify and output the signal light when the signal light in the wavelength band of 1.55μm is inputted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier for collectively amplifying multi-wavelength signal light in a WDM optical transmission system and an optical amplification / gain equalization method.

[0002]

2. Description of the Related Art Research and development on a wavelength division multiplexing (WDM) transmission system have been carried out with an increase in the capacity and speed of optical communication. In the WDM transmission system, one of the most important optical elements is an optical amplifier that collectively amplifies multi-wavelength signal light. This optical amplifier is required to have high gain and low noise and to have a flat gain spectrum in a signal light wavelength band. Conventionally, an amplifying optical fiber (EDF: Er-) doped with an Er (erbium) element has been conventionally used. Optical fiber amplifier (EDFA: Er-Doped Fiber Amplifi) using Doped Fiber
er) is used.

Generally, a regenerative repeater has a maximum relay distance of 10
It is provided every about 00 km, and the total amount of passing loss between repeaters is about 250 dB. On the other hand, the level difference of signal light input that can be tolerated on the receiver side is about 10 dB. Therefore, it is desired that the flatness of the gain spectrum required for the optical amplifier, that is, the deviation of the gain from the average gain value is about 4% (= 10/250) or less in the signal light wavelength region.

[0004] In response to the demand for higher capacity and higher speed,
Studies have been made to flatten the gain spectrum of an EDFA over a wider signal light wavelength range. For example, AMVe
ngsarkar, et al. (IOOC'95, PD-1) constructed a gain equalizer having a transmission loss spectrum having a characteristic opposite to that of an EDF gain spectrum by cascading two types of fiber gratings. It has been proposed to flatten the entire gain spectrum over a wide band by cascading the equalizer and the EDF (hereinafter referred to as "prior art 1"). Okuno et al. (IEICE Technical Report, OPE96-64, 1996) constructed a filter having a transmission loss spectrum having a characteristic opposite to that of an EDF gain spectrum by cascading two types of Fabry-Perot etalons. To flatten the entire gain spectrum over a wide band by cascade connection with the same (hereinafter, referred to as “prior art 2”).

Further, B. Clesca, et al. (Optical fiber
technology, vol.1, pp.135-157, 1995) is 25 nm
In order to realize the above signal light wavelength range, an EDFA using a fluoride EDF has been proposed (hereinafter, referred to as prior art 3). M. Fukuoka, et al. (OAA'96, FA4, 1996) also
In order to realize a signal light wavelength range of 5 nm or more, fluoride ED
In addition to F, it has been proposed to use a gain equalization unit composed of a cascade-connected six-stage lattice-type optical circuit (hereinafter referred to as prior art 4).

Further, M. Kakui, et al. (OAA'96, SaA3,
1996) is an E element to which Al (aluminum) element is added.
DF with DF, P (phosphorus) element and Al element added
F with a cascade connection to form a hybrid EDFA configuration. The gain spectrum of Al-doped EDF rising to the right at saturation (higher gain at longer wavelengths) is compared with the gain spectrum of P / Al co-doped EDF at lower right (lower at longer wavelengths). By compensating for the gain, the overall gain spectrum is flattened over a wide band, and the pumping efficiency is improved (hereinafter referred to as Conventional Technique 5). FIG. 9 is a graph showing the signal light output intensity of the optical fiber amplifier according to the related art 5. As shown in this figure, the signal light output intensity of the optical fiber amplifier according to the prior art 5 has a substantially flat shape in the wavelength range of 1543 nm to 1559 nm.

[0007]

However, each of the above prior arts has the following problems. That is, each of the prior arts 1 and 2 requires two types of fiber gratings or two types of Fabry-Perot etalons. This is because the transmission loss spectrum of each fiber grating or Fabry-Perot etalon has a smaller transmission loss as the wavelength is further away from a predetermined wavelength at which the transmission loss is maximum, and is a symmetrical shape around the predetermined wavelength. In contrast, the gain spectrum of an EDF is usually due to its rather complex shape rather than symmetry. That is, this ED
In order to equalize the complex gain spectrum of F, two kinds of fiber gratings or two kinds of Fabry-Perot etalons are combined to realize an asymmetric transmission loss spectrum as a whole. In particular, in the case of the prior art 1, 2
Two exposures are required to produce different types of fiber gratings. Further, in the prior art 2, the gain spectrum becomes flatter, but the signal light wavelength band is not expanded.

Further, in both the prior art 3 and the prior art 4, the mechanical strength of the fluoride EDF is weak and the fusion cannot be performed. There are also problems such as high cost. In particular, in the prior art 4, the configuration is complicated because the gain equalizing unit including the six-stage grating type optical circuit is used.

Further, the hybrid E according to prior art 5
The gain spectrum of DFA is 1543 nm from 1543 nm.
Although it has a substantially flat shape in the range up to 59 nm, it has a maximum at a wavelength of 1535 nm and a wavelength of 1540 nm.
And has a minimum. To further equalize such a gain spectrum to a signal light wavelength band width of 25 nm or more,
In accordance with the minimum gain near the wavelength of 1540 nm, the wavelength 1
In addition to flattening the gain around 535, the wavelength 154
A substantially flat gain in the range from 3 nm to 1559 nm also needs to be reduced substantially uniformly. Among them, the wavelength 154
It is possible to flatten the gain near the wavelength 1535 in accordance with the minimum gain value near 0 nm by using a fiber grating. However, the wavelength from 1543 nm to 155
The design and manufacture of the gain equalizer that reduces the substantially flat gain in the range up to 9 nm almost uniformly includes fiber gratings,
Even if a Fabry-Perot etalon, a grating type optical circuit, or other optical elements is used, it is almost impossible or extremely complicated.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is easy to design, manufacture and handle, and has an optical amplifier having a flat gain spectrum in a wide signal light wavelength region, an optical amplifier and a gain. The purpose of the present invention is to provide a conversion method.

[0011]

An optical fiber amplifier according to the present invention has a minimum gain value of 2 or more within a signal light wavelength range of 1535 nm to 1560 nm, and a variation in the minimum gain value is a predetermined gain average value. Is adjusted to be 4% or less in logarithmic representation. According to this optical fiber amplifier, the gain spectrum is 1535.
Within the signal light wavelength range of 1 nm to 1560 nm,
Since there are two or more minimum wavelengths at which the gain is minimum, and the deviation of the gain at each minimum gain wavelength is 4% or less in logarithmic representation with respect to a predetermined gain average value, the light is amplified by the optical fiber amplifier. The signal light is easily gain-equalized by a gain equalizer having a predetermined transmission loss spectrum.

Further, the optical fiber amplifier according to the present invention comprises:
It is characterized by including a quartz optical fiber to which Er element and Al element of 3 wt% or more are added. Further, the optical fiber amplifier according to the present invention includes a quartz optical fiber to which an Er element, an Al element, and a P element are added, and a composition ratio of the P element and the Al element is 6 or more. I do. In any case, the silica-based optical fiber has high reliability, is easy to handle in fusion or the like, is inexpensive, and an optical fiber amplifier including such a silica-based optical fiber has a desired structure. It is suitable for realizing a gain spectrum.

The optical fiber amplifier according to the present invention has the following advantages.
Within the signal light wavelength range of 535 nm to 1560 nm,
An optical fiber amplifier having two or more minimum gain values, wherein the variation of the minimum gain value is adjusted to be 4% or less in logarithmic representation with respect to a predetermined average gain value;
(2) It is substantially cascaded with the optical fiber amplifier, and within the signal light wavelength range, the gain variation becomes 4% or more logarithmically with respect to a predetermined gain average value, and the gain maximum wavelength is set. And a gain equalizer for reducing the variation of the gain of the signal light amplified by the optical fiber amplifier to 4% or less in logarithm with respect to the average gain value. .

According to this optical fiber amplifier, the transmission loss spectrum in the gain equalizing section is within the signal light wavelength range and the gain deviation in the optical fiber amplifying section is 4% or more in logarithm with respect to the average gain value. In each of the certain wavelength ranges, the transmission loss increases as the wavelength becomes closer to the wavelength at which the gain in the gain spectrum is maximized, and the signal light optically amplified by the optical fiber amplifier having the above-described gain spectrum has such a transmission light. The gain is equalized by the gain equalizer having the loss spectrum, and the gain deviation of the entire optical fiber amplifier becomes 4% or less in logarithmic representation.

A gain equalizing method for an optical fiber amplifier according to the present invention is characterized in that a signal has a minimum gain of 2 or more within a signal light wavelength range of 1535 nm or more and 1560 nm or less, and the variation of the minimum gain is smaller than a predetermined gain average value. The optical fiber amplifier is adjusted so as to be 4% or less in a logarithmic representation, and the gain is adjusted in a wavelength range where the variation in gain is 4% or more in a logarithmic representation with respect to a predetermined gain average value within a signal light wavelength range. A gain equalizer having a larger transmission loss at a wavelength closer to the maximum wavelength equalizes the gain of the signal light such that the variation of the gain of the signal light becomes 4% or less in logarithm with respect to the average gain value.

According to the gain equalization method of the optical fiber amplifier, the gain spectrum when optically amplifying the signal light has two or more minimum gain wavelengths at which the gain is minimized in the signal light wavelength range of 1535 nm to 1560 nm. The gain deviation at each of the minimum gain wavelengths is 4% or less in logarithmic representation with respect to a predetermined average gain value. on the other hand,
The transmission loss spectrum at the time of gain equalization shows the gain in the gain spectrum in each wavelength range within the signal light wavelength range where the gain deviation in the gain spectrum is 4% or more in logarithm with respect to a predetermined gain average value. The transmission loss increases as the wavelength becomes closer to the wavelength at which the maximum is reached. Then, the signal light is optically amplified by such a gain spectrum, and gain-equalized by such a transmission loss spectrum.
% Or less.

[0017]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description. Further, when describing the shape of the gain spectrum, the general shape is referred to, and fine gain changes are ignored. For example, if the gain deviation is about 1 dB or less in a certain wavelength range, the gain spectrum is said to be "flat" in that wavelength range.
The same applies when describing the wavelength dependence of the signal light output intensity from the optical fiber amplifier.

(First Embodiment) First, a first embodiment will be described. FIG. 1 is a configuration diagram of the optical fiber amplifier according to the first embodiment. This optical fiber amplifier is for inputting multi-wavelength signal light transmitted through the optical fiber transmission line 60, amplifying the signal light collectively, and outputting the amplified signal light to the optical fiber transmission line 61. The isolator 10, the EDF 40, the WD
M coupler 30, isolator 11, gain equalizer 51, W
DM coupler 31, EDFs 42 and 43, WDM coupler 32 and isolator 12 are cascaded,
Further, excitation laser light sources 20 to 22 for supplying excitation light to the EDFs 40, 42 and 43 are provided.

The optical amplifying section of the optical fiber amplifier comprises EDFs 40, 42 and 4 which are substantially cascaded.
3 and excitation laser light sources 20 to 22 for supplying excitation light thereto. These EDF4
Each of 0, 42, and 43 is a quartz-based optical fiber (EDF) to which Er element of about several hundred wt.ppm is added. In particular, the EDF 42 is an EDF to which a P element and an Al element are co-added, and the EDF 43 is an EDF to which an Al element is added. Among them, the first stage EDF 40 is for reducing the noise figure, and the population inversion is formed by the excitation light having the wavelength of 0.98 μm. The EDFs 42 and 43 connected in cascade in the second stage are for obtaining a large signal light output.
A population inversion is formed by the excitation light having a wavelength of 1.48 μm. When the population inversion is formed in each of the EDFs 40, 42 and 43, the wavelength 1.55 μm
When m-band signal light is input, EDFs 40, 42 and 4
3 amplifies and outputs the signal light. The overall gain spectrum of the optical amplifier is a sum of the gain spectra of the EDFs 40, 42, and 43, respectively.

The excitation laser light source 20 outputs excitation light (wavelength 0.98 μm) for exciting the Er element in the EDF 40 to form a population inversion. The excitation light output from the excitation laser light source 20 enters the WDM coupler 30 via the optical fiber 70. The WDM coupler 30 outputs the input pump light to the EDF 40 side, and inputs the signal light amplified by the EDF 40 and passes the signal light to the isolator 11 side.

The excitation laser light sources 21 and 22 respectively generate excitation light (wavelength 1.48 μm) for exciting the Er element in the EDFs 42 and 43 to form a population inversion.
m) is output. The excitation light output from the excitation laser light source 21 is input to the WDM coupler 31 via the optical fiber 71. The WDM coupler 31 outputs the input pump light to the EDF 42 side, and
1 is input and output to the EDF 42 side. On the other hand, the excitation light output from the excitation laser light source 22 enters the WDM coupler 32 via the optical fiber 72. The WDM coupler 32 converts the input pump light into EDF4
3 and the signal light sequentially amplified by the EDFs 42 and 43 is input and output to the isolator 12 side.

The isolator 10 provided on the input side of the optical fiber amplifier is connected to the EDF
Light is allowed to pass in the direction toward the side 40 but not in the opposite direction. Also, EDF40 and ED
F42 is provided between the WDM and the FDM.
Light is allowed to pass in the direction from the coupler 30 to the gain equalizer 51, but is not allowed to pass in the opposite direction. The isolator 12 provided on the output side of the optical fiber amplifier transmits light in the direction from the WDM coupler 32 to the optical fiber transmission line 61, but does not transmit light in the opposite direction. .

Accordingly, the pumping light (0.98 μm) output from the pumping laser light source 20 is supplied to the EDF 40, passes through the isolator 10, and passes through the optical fiber transmission line 6
It is not output to 0. In addition, the excitation laser light source 2
Excitation light (1.48 μm) output from the EDF 4
2 and 43 are sequentially supplied to the optical fiber 72 side by the WDM coupler 32, and are not output to the optical fiber transmission line 61 side. The excitation laser light source 22
Pump light (1.48 μm) output from the EDF 43
And 42 are sequentially supplied to the optical fiber 71 side by the WDM coupler 31 and are not output to the gain equalizer 51 side.

The gain equalizing section 51 provided between the EDF 40 and the EDF 42 has a predetermined transmission loss spectrum and equalizes the gain spectrum of the optical amplifying section including the EDFs 40, 42 and 43. To do
A combination of fiber gratings in which gratings at regular intervals are formed in the optical axis direction of the fiber is preferably used. Note that the gain spectrum of the optical amplification unit including the EDFs 40, 42 and 43 and the gain equalization unit 5
The details of the transmission loss spectrum 1 will be described later.

This optical fiber amplifier operates as follows. That is, the pumping light (wavelength 0.98 μm) output from the pumping laser light source 20 is supplied to the EDF 40 via the WDM coupler 30, and the pumping laser light source 21
Pump light (wavelength 1.4)
8 μm) is supplied to the EDFs 42 and 43 via the WDM couplers 31 and 32, respectively, and the multi-wavelength signal light (wavelength 1.55 μm band) transmitted through the optical fiber transmission line 60 is supplied to the optical fiber amplifier. When input, the multi-wavelength signal light passes through the isolator 10,
The data is input to the EDF 40 and is collectively amplified. Then, this E
The multi-wavelength signal light output from the DF 40 sequentially passes through the WDM coupler 30 and the isolator 11, and is subjected to wavelength-dependent attenuation by a gain equalizer 51 having a predetermined transmission loss spectrum. Further, the multi-wavelength signal light output from the gain equalizing section 51 is
, And are sequentially input to the EDFs 42 and 43 to be collectively amplified again. The multi-wavelength signal light output from the EDF 43 passes through the WDM coupler 32 and the isolator 12 sequentially, and is output to the optical fiber transmission line 61.

As described above, when the multi-wavelength signal light passes through the optical fiber amplifier, it is subjected to wavelength-dependent amplification according to the gain spectrum of the optical amplifier including the EDFs 40, 42 and 43, and The gain equalizer 51 receives attenuation depending on the wavelength in accordance with the transmission loss spectrum, so that the gain spectrum (dB) of the entire optical fiber amplifier becomes equal to that of the optical amplifier including the EDFs 40, 42 and 43. And the transmission loss spectrum (dB) of the gain equalizer 51.

Here, the EDFs 40, 42, and 43 each are preferably used are quartz-based EDFs which can be expected to have the widest signal light wavelength region, and Al-doped EDFs to which an Al element is co-doped. The Al-doped EDF has a different gain spectrum depending on the Al concentration and the state of the population inversion. FIG. 2 is a graph showing a gain spectrum of the Al-doped EDF in each population inversion. FIG. 2 (a) shows that the A concentration is 1.0 wt%.
FIG. 2B shows a gain spectrum of the l-doped EDF, and FIG.
4 shows a gain spectrum of an Al-doped EDF having an Al concentration of 5.9 wt%.

As can be seen from this figure, when the strong excitation light is supplied and the population inversion becomes about 80%, the Al-doped EDF
Has a gain spectrum of 1520 nm to 1540 n.
In the range up to m, the gain becomes maximum around the wavelength of 1530 nm, and the gain becomes larger as the wavelength is closer to 1530 nm, but is substantially flat in the range from 1540 nm to 1560 nm. When the population inversion is large and the Al concentration is high, the wavelength is 1540 nm to 1560 nm.
Although it is almost flat in the range up to, the gain maximum slightly occurs at each of the two wavelengths in this wavelength range.

FIG. 3 shows that in the Al-doped EDF,
11 is a graph showing the Al concentration dependency of the signal light wavelength band width at which the gain deviation becomes 0.5 dB or less with respect to the gain of 30 dB.
However, in any case of the Al concentration, the signal light wavelength band width when the population inversion is optimized is obtained. As can be seen from this figure, the higher the Al concentration, the wider the signal light wavelength band width, which is advantageous for flattening the gain spectrum. Particularly, when the Al concentration is 2 to 3 wt%
The signal light wavelength band width rapidly expands to about
When the concentration becomes 3 wt% or more, the wavelength width of the signal light becomes sufficiently wide.

FIG. 4 is a graph showing a gain spectrum when the population inversion of the Al-doped EDF is optimized. FIG. 4A shows a gain spectrum of an Al-doped EDF having an Al concentration of 2.0 wt%, and FIG. 4B shows a gain spectrum of an Al-doped EDF having an Al concentration of 3.1 wt%. As can be seen from this figure, in each case, the gain spectrum is substantially flat in the wavelength range from 1540 nm to 1560 nm, but when the Al concentration is high (FIG. 4).
In (b)), the maximum of the gain is slightly recognized at each of the two wavelengths in this wavelength range.

Therefore, the Al-doped EDF having such a gain spectrum is used for each of the EDFs 40, 42 and 43 in the optical fiber amplifier shown in FIG. 1, and the transmission for equalizing the gain near the wavelength of 1530 nm in the Al-doped EDF. Gain equalizer 5 having loss spectrum
Use 1. By doing so, the signal light wavelength range can be flattened over a wide range. Such a quartz-based EDF has high reliability, is easy to handle in fusion or the like, is inexpensive, and is suitable for realizing a desired gain spectrum.

By the way, in the prior art 5 (excluding the gain equalizing section 51 from the configuration shown in FIG. 1), the Al-added EDF is used.
And the P / Al co-doped EDF are cascaded to form an EDFA, so that the signal light wavelength range is increased from 1543 nm to 1
It was set in the range up to 559 nm, and the purpose was to flatten the gain spectrum within this range (FIG. 9).
Therefore, the Al concentration of the Al-added EDF is about 1 wt%, and the P / Al composition ratio of the P / Al co-added EDF is 6%.
It was about. The conditions of the optical fiber amplifier according to the related art 5 are as follows. EDF40 and 43
Each is an Al-doped EDF, the absorption loss at a wavelength of 1.53 μm band by Er ions is 5.5 dB / m,
The EDF 42 is a P / Al co-doped EDF and has a 1.53 μm wavelength absorption loss of 4.5 dB / m due to Er ions.
It is. The length of the EDF 40 is 9.0 m and the EDF 4
2 is 8.0 m, and the length of the EDF 43 is 12.
0 m. EDF output from the excitation laser light source 20
The intensity of the excitation light (wavelength 0.98 μm) supplied to 40 is 70 mW, and the intensity of the excitation light (wavelength 1.48 μm) output from excitation laser light sources 21 and 22 and supplied to EDFs 42 and 43, respectively. 70 mW.

However, the gain spectrum when the population inversion of the P / Al co-doped EDF is 80% differs depending on the composition ratio of P / Al. FIG. 5 is a graph showing the gain spectrum of the P / Al co-doped EDF at each value of the P / Al composition ratio. 5A shows a case where the P / Al composition ratio is 2.5, and FIG. 5B shows a case where the P / Al composition ratio is 6 in FIG.
(C) shows the gain spectrum of the P / Al co-doped EDF in each case where the P / Al composition ratio is 8. As can be seen from this figure, when the P / Al composition ratio is small (FIG. 5A), the wavelength 1 at which the gain characteristic is linear is obtained.
In the range from 543 nm to 1559 nm, P /
When the Al composition ratio becomes 6, gain maxima appear at wavelengths around 1543 nm and 1555 nm, respectively (FIG. 5).
(B)) When the P / Al composition ratio becomes 8, the gain maximum becomes clear (FIG. 5 (c)).

Therefore, as EDFs 40 and 43, ADF
Using an EDF with an Al concentration of 5.9 wt%,
42, a P / Al co-doped EDF having a P / Al composition ratio of about 8 was used.
A cascade of two types of fiber gratings having transmission loss spectra for equalizing the gain around 535 nm and around 1544 nm is used. By doing so, the signal light wavelength range can be flattened over a wide range.

Next, an example of optical amplification and gain equalization of the optical fiber amplifier according to this embodiment will be described. FIG.
(A) is a graph showing the signal light output intensity obtained by removing the gain equalizing unit 51 from the optical fiber amplifier according to the first embodiment (that is, the optical amplifying unit). FIG.
(B) is a graph showing the signal light output intensity by the optical fiber amplifier according to the first embodiment.

Here, each condition is as follows. E
DF40 and 43 both have an Al concentration of 5.9 wt%.
Al-added EDF having a wavelength of 1.
The absorption loss in the 53 μm band is 5.5 dB / m, and the EDF42
Is a P / Al co-doped EDF having a P / Al composition ratio of about 8, and has a 1.53 μm wavelength absorption loss by Er ions of 4.5 dB / m. The length of the EDF 40 is 8.5 m, and the length of the EDF 42 is 10.0 m.
The length of 3 is 11.0 m. Excitation light (wavelength 0.9) output from the excitation laser light source 20 and supplied to the EDF 40
8 μm) is 70 mW, and the excitation laser light source 2
EDFs 42 and 4 output from 1 and 22 respectively
The intensity of the excitation light (wavelength 1.48 μm) supplied to 3 is 75 mW.

The signal light is 16 waves, and each wavelength is a wavelength from 1532 nm to 1562 nm in steps of 2 nm. The intensity of each of the 16 input signal lights is-
17.0 dBm, and the overall intensity is -5.0 dB
Bm. The average intensity per wavelength of the output 16 signal light is +8.0 dBm, and the total intensity is +20.0 dBm. The intensity of the excitation light was determined so that signal light of such intensity was output.

The signal light output intensity of the optical fiber amplifier (not including the gain equalizing unit) having the above-described components (FIG. 6)
(A)) has a maximum near 1535 nm in the wavelength range from 1532 nm to 1540 nm, and has a larger gain as the wavelength is closer to 1535 nm, and has a maximum near 1544 nm in the wavelength range from 1540 nm to 1550 nm. , Its wavelength 1544nm
The gain is larger for wavelengths closer to
In the range from 1562 nm to 1562 nm, the maximum is obtained near the wavelength of 1557 nm, and the gain becomes larger as the wavelength is closer to 1557 nm. The maximum gain value is larger in the order of 1535 nm, 1544 nm, and 1557 nm. On the other hand, the minimum gain wavelength at which the gain is minimum is 154
Two wavelengths around 0 nm and around 1552 nm,
The respective signal light output intensities are about 7.8 dBm and about 7.7 dBm, and their average value is 7.8 dB.
m. Therefore, the deviation of each of the two minimum gain values from the average gain value is sufficiently smaller than 4%.

Therefore, in the present embodiment, the gain equalizer 51 is configured by cascading two types of fiber gratings having different grating periods, and the transmission loss spectrum T1 ( λ) is T1 (λ) = 3.3−3.3 · (λ−1535.5) ² /6.25 (1a) when the wavelength λ (nm) is in the range from 1533 nm to 1538 nm, and T1 ( λ) = 0 (1b), and the first fiber grating having such a transmission loss spectrum T1 (λ) gain-equalizes the wavelength range from 1533 nm to 1538 nm of the gain spectrum of the optical amplifier. .

Further, the transmission loss spectrum T 2 (λ) of the second fiber grating is set to 154 (wavelength λ (nm)).
In the range of from 0nm to 1548nm, and T2 (λ) = 1.5-1.5 · ( λ-1544) 2/16 ... (2a), which in the wavelength range other than, and T2 (λ) = 0 ... ( 2b), The wavelength range from 1540 nm to 1548 nm in the gain spectrum of the optical amplifier is equalized by the second fiber grating having such a transmission loss spectrum T2 (λ).

The transmission loss spectrum T (λ) of the entire gain equalizing section 51 is as follows: T (λ) = T1 (λ) + T2 (λ) (3) That is, the entire transmission loss spectrum T
(λ) indicates that in each wavelength region where the gain deviation in the optical amplifying unit is 4% or more with respect to the average gain value, the transmission loss increases as the wavelength is closer to the wavelength at which the gain in the gain spectrum is maximized. .

Therefore, the gain spectrum of the entire optical fiber amplifier (FIG. 6B) is the same as the gain spectrum (FIG. 6A) of the optical amplifier including the EDFs 40, 42 and 43. The transmission loss spectrum T (λ) of the entire conversion unit 51 is integrated. As can be seen from FIG. 6B, the minimum value of the signal light output intensity is within the signal light wavelength range of 1535 nm to 1560 nm.
It is about 8 dBm, the maximum value is about 8.5 dBm, the average value is about 8.2 dBm, and the input power is-
Since the gain is 17 dBm and the gain is 25 dB, the deviation of the gain of the entire optical fiber amplifier is 4% or less.

In the case where the signal light output intensity near the wavelength of 1540 nm is not provided with the gain equalizer 51 (FIG. 6A)
Compared to the case where the gain equalizer 51 is provided (FIG. 6B)
Is larger. This is EDF40 and EDF
42, the signal light is amplified by the EDF 40 until it reaches the saturation region,
In addition, the total signal light output intensity of the entire optical fiber amplifier is 20
This is the result of the signal light output intensity at the maximum gain wavelength being distributed to the signal light output intensity at the minimum gain wavelength.

In the optical fiber amplifier according to the present embodiment,
By employing the gain equalizer 51 including two types of fiber gratings having such transmission loss spectra T1 (λ) and T2 (λ), the signal light output intensity (FIG. 6 (b)) Compared to the case of the optical fiber amplifier without the equalizer (FIG. 6A), the signal light wavelength range (15
The flatness of the gain spectrum at 32 nm to 1562 nm is improved.

(Second Embodiment) Next, a second embodiment will be described. The configuration of the optical fiber amplifier according to this embodiment is substantially the same as that of the first embodiment (FIG. 1). However, in the present embodiment, as compared with the first embodiment, the point that the gain spectrum of the optical amplifier including the EDFs 40, 42, and 43 has three gain maxima more clearly, and The difference is that the gain equalizer 51 is composed of three types of fiber gratings corresponding to the three gain maxima.

In the present embodiment, in order for the gain spectrum of the optical amplification section including the EDFs 40, 42 and 43 to have three distinct gain maxima, the length of each of these EDFs 40, 42 and 43 is required. It can be realized by setting the appropriateness.

An example of optical amplification and gain equalization of the optical fiber amplifier according to this embodiment will be described below. FIG. 7A shows the optical fiber amplifier according to the second embodiment without the gain equalizer 51 (that is, the optical amplifier).
6 is a graph showing the signal light output intensity according to FIG. FIG.
(B) is a graph showing the signal light output intensity by the optical fiber amplifier according to the second embodiment.

Here, each condition is as follows. E
The composition of each of the DFs 40, 42 and 43 is the same as in the first embodiment, but the length of the EDF 40 is 9.
0 m, and the length of the EDF 42 is 8.0 m.
The length of F43 is 12.0 m. Excitation laser light source 2
The intensity of the excitation light (wavelength 0.98 μm) output from 0 and supplied to the EDF 40 is 70 mW, and the excitation light (wavelength 1.48 μm) output from the excitation laser light sources 21 and 22 and supplied to the EDFs 42 and 43, respectively. Each intensity is 70 mW. Further, the conditions regarding the input signal light and the output signal light are the same as in the case of the first embodiment.

The signal light output intensity (see FIG. 7) of the optical fiber amplifier (not including the gain
In (a)), in the range of wavelengths from 1532 nm to 1538 nm, the gain becomes maximum near the wavelength of 1535 nm, and the gain increases as the wavelength approaches 1535 nm. , Its wavelength 1544nm
The gain is larger for wavelengths closer to
In the range from 1562 nm to 1562 nm, the maximum is obtained near the wavelength of 1557 nm, and the gain becomes larger as the wavelength is closer to 1557 nm. The gain maximum values are approximately the same at around 1535 nm, around 1544 nm, and around 1557 nm, respectively, and appear more clearly than in the case of FIG. On the other hand, the minimum gain wavelength at which the gain is minimum is two wavelengths near 1540 nm and 1552 nm, and the signal light output intensity of each is 7.4 dB.
m and about 8.0 dBm, and their average value is about 7.7 dBm. Therefore, the deviation of each of the two minimum gain values from the average gain value is 4% or less.

Therefore, in the present embodiment, the gain equalizer 51 is configured by cascading three types of fiber gratings having different grating periods, and the transmission loss spectrum T1 ( λ) is T1 (λ) = 1.4−1.4 · (λ−1535.4) ² /5.76 (4a) when the wavelength λ (nm) is in the range from 1533 nm to 1537.8 nm, and in the other wavelength range, T1 (λ) = 0 (4b), and the first fiber grating having such a transmission loss spectrum T1 (λ) makes the gain spectrum of the optical amplifying section from 1533 nm to 1537.8n.
Gain equalization is performed for the wavelength range up to m.

Further, the transmission loss spectrum T 2 (λ) of the second fiber grating is set to 154 (wavelength λ (nm)).
In the range from 0.5nm to 1548.5Nm, and T2 (λ) = 1.2-1.2 · ( λ-1544.5) 2/16 ... (5a), in the wavelength range other than this, T2 (λ) = 0 ... ( 5b), and the second fiber grating having such a transmission loss spectrum T2 (λ) is used to convert 1540.5 nm to 1548.
Gain equalization in the wavelength range up to 5 nm.

Further, the transmission loss spectrum T 3 (λ) of the third fiber grating is set to 155 (wavelength λ (nm)).
In the range from 2.5 nm to 1561.5 nm, T3 (λ) = 1.1−1.1 · (λ-1557) ² /20.25 (6a), and in other wavelength ranges, T3 (λ) = 0 ( 6b), and the third fiber grating having such a transmission loss spectrum T3 (λ) converts the gain spectrum of the optical amplifier from 1552.5 nm to 1561.
Gain equalization in the wavelength range up to 5 nm.

The transmission loss spectrum T (λ) of the entire gain equalizing section 51 is as follows: T (λ) = T1 (λ) + T2 (λ) + T3 (λ) (7) That is, the entire transmission loss spectrum T
(λ) indicates that in each wavelength region where the gain deviation in the optical amplifying unit is 4% or more with respect to the average gain value, the transmission loss increases as the wavelength is closer to the wavelength at which the gain in the gain spectrum is maximized. .

Accordingly, the gain spectrum of the entire optical fiber amplifier (FIG. 7B) is the same as the gain spectrum of the optical amplification section including the EDFs 40, 42 and 43 (FIG. 7A). The transmission loss spectrum T (λ) of the entire conversion unit 51 is integrated. As can be seen from FIG. 7B, the minimum value of the signal light output intensity is 7.8 in the signal light wavelength range of 1535 nm or more and 1560 nm or less.
Since it is about 2 dBm, the maximum value is about 8.6 dBm, and the average value is about 8.4 dBm, the deviation of the gain of the entire optical fiber amplifier is about 2.4%.

In the optical fiber amplifier according to this embodiment,
By employing the gain equalizer 51 including three types of fiber gratings each having such a transmission loss spectrum T1 (λ), T2 (λ) and T3 (λ), the signal light output intensity (FIG. 7 (b) 7)), the flatness of the gain spectrum in the signal light wavelength region (1532 nm to 1562 nm) is improved as compared with the case of the optical fiber amplifier without the gain equalizing section (FIG. 7A).

Next, the performances of the optical fiber amplifiers according to the first and second embodiments described above will be compared. FIG. 8 is a chart showing a performance comparison of the optical fiber amplifiers according to the first and second embodiments. In addition,
This chart also shows the performance of the optical fiber amplifier according to the prior art 5 for comparison.

Second-stage EDF (EDFs 42 and 43)
The intensity of the pumping light (wavelength 1.48 μm) to be supplied to the first embodiment is 140 mW in the related art 5, but in each of the first and second embodiments, the pumping intensity is almost the same as that of the related art 5. Light intensity is sufficient.

The signal light wavelength range from the maximum gain to the gain 0.5 dB down and the signal light wavelength range from the gain down 1.0 dB to the gain are the optical fiber amplifiers according to the first and second embodiments. Is wider than that of the conventional technology 5. In particular, in the case of the optical fiber amplification according to the second embodiment, since the gain equalization unit 51 configured by cascade-connecting three types of fiber gratings each having a small transmission loss maximum value is used, Equalization can be delicately performed, and therefore, the signal light wavelength range 2
The gain deviation is always suppressed to 0.5 dB or less in the range of 8 nm, and good performance is obtained as compared with the optical fiber amplifier according to the first embodiment.

The present invention is not limited to the above embodiment, and various modifications are possible. For example, in any of the embodiments described above, the gain equalizer is provided inside the optical fiber amplifier, but a gain equalizer is provided separately without providing the gain equalizer inside the optical fiber amplifier. May be connected in cascade.

In any of the above embodiments,
Although the gain equalizer is provided between the first-stage EDF 40 and the second-stage EDF (EDFs 42 and 43), the position of the gain equalizer is not limited to this. May be before the EDF of the second stage or after the second stage.

[0061]

As described above in detail, according to the present invention, the gain spectrum in the optical amplifier is 1535n.
Within the signal light wavelength range of m or more and 1560 nm or less, there are two or more minimum gain wavelengths at which the gain becomes a minimum, and the deviation of the gain at each of the minimum gain wavelengths is 4% or less in logarithm with respect to the average gain value. . Therefore, the signal light optically amplified by the optical amplifying unit is easily gain-equalized by separately providing a gain equalizing unit having a predetermined transmission loss spectrum and cascading the same.

Further, the optical amplifying section is composed of an Er element and 3 wt.
It is preferable to provide a quartz-based optical fiber doped with Al element of at least%. In addition, the quartz optical fiber,
Further, it is preferable that the P element is co-added and the composition ratio of the P element and the Al element is 6 or more. Silica-based optical fibers are highly reliable and easy to handle in fusion, etc.
The optical amplifier, which is inexpensive and includes such a silica-based optical fiber, can realize a desired gain spectrum.

Further, in this optical amplifier, an optical amplifying unit and a gain equalizing unit are substantially connected in cascade, and the transmission loss spectrum in the gain equalizing unit is within the signal light wavelength range. In each of the wavelength ranges where the deviation of the gain in the optical amplifying unit is 4% or more in logarithmic form with respect to the average gain value, the transmission loss increases as the wavelength is closer to the wavelength where the gain in the gain spectrum is maximum. The signal light optically amplified by the optical amplifier having the above-described gain spectrum is gain-equalized by the gain equalizer having such a transmission loss spectrum, and the deviation of the gain of the entire optical amplifier is expressed in logarithmic representation. 4% or less.

Further, according to the optical amplification / gain equalization method of the present invention, the gain spectrum when optically amplifying the signal light has a minimum gain at which the gain is minimized within a signal light wavelength range of 1535 nm to 1560 nm. It has two or more wavelengths, and the gain deviation at each of the minimum gain wavelengths is 4% or less in logarithmic representation with respect to the average gain value and the average gain value. On the other hand, the transmission loss spectrum at the time of gain equalization shows the gain in the gain spectrum in each wavelength range within the signal light wavelength range, where the gain deviation in the gain spectrum is 4% or more logarithmically with respect to the average gain value. The transmission loss increases as the wavelength becomes closer to the wavelength at which the maximum is reached. Then, the signal light is optically amplified by such a gain spectrum and gain-equalized by such a transmission loss spectrum, so that the deviation of the entire gain is 4% or less in logarithmic representation.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical fiber amplifier according to a first embodiment.

FIG. 2 shows Al-added ED at each value of population inversion.
9 is a graph showing a gain spectrum of F.

FIG. 3 shows a signal light wavelength band width A at which a gain deviation is 0.5 dB or less for a gain of 30 dB in an Al-doped EDF.
It is a graph which shows 1 concentration dependence.

FIG. 4 is a graph showing a gain spectrum when the population inversion of Al-doped EDF is optimized.

FIG. 5: P / Al at each value of P / Al composition ratio
4 is a graph showing a gain spectrum of a co-doped EDF.

FIG. 6A is a graph showing signal light output intensity obtained by removing the gain equalizer 51 from the optical fiber amplifier according to the first embodiment, and FIG. 6B is a graph showing the signal light output intensity according to the first embodiment; 5 is a graph showing the signal light output intensity of the optical fiber amplifier according to the first embodiment.

FIG. 7A is a graph showing the signal light output intensity obtained by removing the gain equalizer 51 from the optical fiber amplifier according to the second embodiment, and FIG. 7B is a graph showing the second embodiment. 5 is a graph showing the signal light output intensity of the optical fiber amplifier according to the first embodiment.

FIG. 8 is a table showing a performance comparison of the optical fiber amplifiers according to the first and second embodiments.

FIG. 9 is a graph showing the signal light output intensity of the optical fiber amplifier according to the related art 5.

[Explanation of symbols]

10, 11, 12 ... isolator, 20, 21, 22, ...
Excitation laser light source, 30, 31, 32 ... WDM coupler,
40, 42, 43 ... EDF, 51 ... gain equalizer, 60,
61: optical fiber transmission line, 70, 71, 72: optical fiber.

Claims (5)

[Claims] In a signal light wavelength range of 1535 nm or more and 1560 nm or less, a minimum gain value is 2 or more, and a variation of the minimum gain value is adjusted so as to be 4% or less in logarithm with respect to a predetermined average gain value. An optical fiber amplifier. 2. The optical fiber amplifier according to claim 1, further comprising a silica-based optical fiber to which Er element and Al element of 3 wt% or more are added. 3. The light according to claim 1, further comprising a quartz optical fiber to which Er element, Al element and P element are added, and wherein the composition ratio of P element and Al element is 6 or more. Fiber amplifier. 4. Within a signal light wavelength range of 1535 nm or more and 1560 nm or less, the minimum gain value is 2 or more, and the variation of the minimum gain value is adjusted so as to be 4% or less in logarithm of a predetermined gain average value. An optical fiber amplifier, characterized in that the optical fiber amplifier is substantially cascaded with the optical fiber amplifier, and within the signal light wavelength range, the variation in gain is expressed in a logarithmic manner with respect to the predetermined gain average value. With respect to the wavelength range of 4% or more, the transmission loss increases as the wavelength is closer to the maximum gain wavelength, and the variation of the gain of the signal light amplified by the optical fiber amplifier is 4% or less in logarithm with respect to the average gain value. An optical fiber amplifier comprising: a gain equalizing unit; 5. In a signal light wavelength range of 1535 nm or more and 1560 nm or less, a minimum gain value is 2 or more, and a variation of the minimum gain value is 4% in a logarithmic representation with respect to a predetermined gain average value.
The optical fiber amplifier is adjusted so as to be less than or equal to the maximum gain wavelength for the wavelength range where the variation of the gain is 4% or more logarithmically with respect to the predetermined gain average value in the signal light wavelength range. An optical fiber amplifier, wherein a gain equalizer having a larger transmission loss as the wavelength becomes shorter equalizes the variation in the gain of the signal light so as to be 4% or less in logarithm with respect to the average gain value. Gain equalization method.