JP2507590B2 - Distributed constant type optical fiber amplifier - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a distributed constant type optical fiber amplifier which compensates a decrease in the intensity of signal light by an optical fiber doped with erbium at a low concentration.

(Prior Art) Conventionally, an Er-doped optical fiber in which erbium (hereinafter referred to as Er), which is a rare earth element, is added to an optical fiber at a concentration of, for example, about 1000 ppm, and the optical fiber itself has an amplifying function is used. Are known.

FIG. 2 is a graph showing the fiber length dependence of the gain as an optical amplifier in an Er-doped optical fiber doped with Er at the above concentration. In FIG. 2, the horizontal axis represents the fiber length and the vertical axis represents the gain. The curves shown in the figure show the characteristics of the optical fiber with an Er-doped concentration of 1300 ppm.
The curves shown by indicate the characteristics of the optical fiber with Er concentration of 1000 ppm, respectively. The excitation light intensity for exciting erbium is 32 mW at the optical fiber entrance end (see Japanese Patent Application No. 63-311910).

As can be seen from FIG. 2, the conventional Er-doped optical fiber has a gain of about 12 dB at a fiber length of several m, and a unit length gain of 5 dB / m in the region where the gain is not saturated. There is. However, when the fiber length becomes long, reabsorption from the ground state becomes large due to absorption attenuation of the pumping light, and the gain as an amplifier gradually starts to decrease.

Therefore, when inserting a conventional Er-doped optical fiber into, for example, an optical fiber transmission line, several meters of Er-doped optical fiber are used as a so-called lumped constant, as shown in FIG.

In FIG. 3, 1-1, 1-2, 1-3 are optical fiber transmission lines through which the signal light S is propagated, and 2-1 and 2-2 are optical fiber amplifiers, which are pumping light sources for generating pumping light P. 2a and transmission line 1-
The optical fiber coupler 2b for multiplexing the signal light S propagating 1, 1-2 and the pumping light P and the combined light are incident, the signal light S is amplified with a predetermined gain as described above, and the optical fiber transmission line Er-doped optical fiber 2 for emission to 1-2 and 1-3, respectively
It consists of c and.

In such a configuration, for example, the signal light S incident on one end of the transmission line 1-1, while propagating through the transmission line 1-1, due to the loss characteristics of the transmission line 1-1, in FIG. As indicated by S1, the light is emitted from the other end of the transmission line 1-1 in a state where the strength is reduced. Signal S1 emitted from this transmission line 1-1
Is combined with the excitation light P by the optical fiber coupler 2b and then is incident on the Er-doped optical fiber 2c. This combined light is
Although it propagates through the Er-doped optical fiber 2c, at this time, the erbium is excited by the pumping light P and the signal light S is subjected to an amplifying action. As a result, the intensity of the signal light S is S-in FIG.
As indicated by 1, the intensity is restored to the intensity at the time of incidence on the transmission line 1-1, and the light is emitted to the next transmission line 1-2.

Hereinafter, the signal light S-1, which has propagated through the transmission line 1-2 and has a reduced intensity as indicated by S2, is subjected to the same action as described above in the optical fiber amplifier 2-2, and in FIG. As indicated by S-2, the intensity at the time of incidence on the transmission line 1-2 is recovered and propagated to the next transmission line 1-3. In this way, the long distance transmission of the signal light is performed.

On the other hand, as described above, the rare earth element Er
It is known that signal light is amplified by utilizing stimulated Raman scattering in an optical fiber without adding. Since this has a gain over a fiber length of several km, it is configured as a distributed constant type optical fiber amplifier in which the gain is distributed over the entire transmission line.

With this distributed constant type, the optical loss of the optical fiber is 0.2 dB / km.
Since it is small, a gain of 0.2 dB per km has the advantage of realizing a transmission line without optical loss.

In the experiment using a 1.48 μm semiconductor laser, 60
It was reported that a gain of 5 dB was obtained using an optical fiber of km (Reference 1; N. Edagawa, K. Motiduki, Y. Iwamot).
o, “Amplification of wavelength division multiplex
ed signals by a high efficient fiber Raman amplifi
er pumped by high power semiconductor laser ", ELECT
RONICS LETTERS 23,196 (1987). reference).

(Problems to be Solved by the Invention) However, in the lumped-constant type configuration as shown in FIG. 3, when an optical soliton of a nonlinear wave is to be propagated, the optical soliton is lengthened by the optical loss in the optical fiber. Cannot be maintained over distance. For example, the dynamic range of N = 1 optical soliton is about 9.5 dB, but if there is more optical loss than this, optical soliton communication cannot be performed.

Further, in the amplification using stimulated emission, as shown in Fig. 4, the input light intensity of 0.8 W is required with the 1.34 µm YAG laser as the excitation light source, as shown in Fig. 4 (Reference 2; M. Nakazawa, “Highly efficient Raman amplif
ication in a polarization−preserving optical fiber
r ", APPLIED PHYSICS LETTERS 46 (7), 628 (1985).
reference).

Furthermore, as described above, in an experiment using a 1.48 μm semiconductor laser, although a gain of 5 dB can be obtained using an optical fiber of 60 km, the optical loss of the actual optical fiber is 12 dB.
Considering that, the optical loss of 7 dB is subtracted.

The present invention has been made in view of such circumstances,
It is an object of the present invention to provide a distributed constant type optical fiber amplifier capable of compensating for a decrease in intensity accompanying propagation of these optical fibers over a long distance from linear pulses to optical solitons.

(Means for Solving the Problem) In order to achieve the above-mentioned object, in claim (1), an excitation light source that generates excitation light in a wavelength band that matches an absorption wavelength region peculiar to erbium (Er) and 0.01% erbium. ppm to 10p
An erbium-doped optical fiber doped at a concentration of pm and an optical multiplexing means for multiplexing the excitation light and the 1.5 μm band signal light and making the multiplexed light enter one end of the erbium-doped optical fiber.

Further, in claim (2), the erbium-doped optical fiber is configured by adding erbium to an optical fiber having a zero dispersion wavelength on the shorter wavelength side than the wavelength of the signal light.

Further, in claim (3), there is provided means for making pumping light in a wavelength band that coincides with the absorption wavelength region peculiar to erbium enter the other end of the erbium-doped optical fiber.

(Operation) According to claim (1), the signal light in the wavelength band of 1.5 μm and the excitation light in the wavelength band that coincides with the Er-specific absorption wavelength region emitted from the excitation light source are combined by the optical combining means, This combined light is incident on one end of the Er-doped optical fiber.

Of the combined light that is incident on the Er-doped optical fiber and propagates through the optical fiber, the excitation light excites Er, and the signal light is thereby propagated while canceling the optical loss received by the optical fiber, and the intensity at the time of incidence The light is emitted from the other end of the Er-doped optical fiber while maintaining almost the same strength.

According to claim (2), an optical soliton that is a nonlinear wave is also obtained by using an Er-doped optical fiber that has a zero-dispersion wavelength on the shorter wavelength side than the signal light (1.5 μm band). After being combined with the pumping light as in the above-mentioned claim (1), the optical loss is canceled while propagating in the Er-doped optical fiber, and the light is propagated over a long distance while maintaining a predetermined intensity without causing collapse. It

Further, according to claim (3), the excitation light in the wavelength band that matches the absorption wavelength region peculiar to Er is incident from the other end side of the Er-doped optical fiber. As a result, the length of the Er-doped optical fiber can be set to be twice as long as that in the case of claim (1) or claim (2). Even in this case, the linear pulse or the optical soliton is propagated while canceling the influence of the optical loss of the optical fiber and maintaining a predetermined intensity.

(Embodiment) FIG. 1 is a configuration diagram showing a first embodiment of a distributed constant type optical fiber amplifier according to the present invention. In FIG.
Reference numeral 11 denotes a pumping light source, which is composed of, for example, a semiconductor laser having an oscillation wavelength of 1.48 μm, and which is capable of amplifying the signal light S by pumping erbium over substantially the entire length direction of the Er-doped optical fiber 13 described later. The excitation light P of sufficient intensity (about 100 mW) is emitted.

12 is an optical fiber coupler (optical multiplexing means), which is an incident end 12
The signal light S having a wavelength of 1.535 μm, which is incident from a, and the excitation light P from the excitation light source 11, which is incident on the incident end 12b.
And are combined and emitted from the emission end 12c. The excitation light source 1
1 pumping light P output part and optical fiber coupler 12 input end 1
It is connected to 2b by an optical fiber cord.

13 is an Er-doped optical fiber whose length is set to about 20 km based on the principle described later and is a rare earth element throughout
Er is added at a low concentration of, for example, 0.076 ppm per unit length, and one end is connected to the emitting end 12c of the optical fiber coupler 12 with good matching.

14 is a wavelength selection element, which propagates through the Er-doped optical fiber 13,
Of the light emitted from the other end, the signal light S is transmitted,
The excitation light P is blocked.

 Next, the operation of the above configuration will be described.

For example, the signal light S emitted from a signal light source (not shown)
Is from the incident end 12a, while the excitation light P emitted from the excitation light source 11 is from the incident end 12b.
After being combined therewith, and after being multiplexed there, the light is incident on one end side of the Er-doped optical fiber 13 from the emission end 12c.

Of the combined light incident on the Er-doped optical fiber 13, the excitation light P is a wavelength region (1.48 μm) that matches the absorption band of Er.
m band) is used. Therefore, the pumping light P excites Er, whereby the signal light S is transmitted while canceling the optical loss due to the optical fiber and maintaining a predetermined intensity.
This is because the Er-doped concentration in the Er-doped optical fiber 13 is low and the optical fiber 13 has a predetermined gain over the entire length direction.

In this way, the signal light S propagating through the Er-doped optical fiber 13 while canceling the optical loss and emitted from the other end of the Er-doped optical fiber 13 is transmitted through the wavelength selection element 14 and the distributed constant type optical fiber amplifier. Is output from.

As described above, the amplification Er-doped optical fiber 13 according to the first embodiment has a low Er-doped concentration per unit length, that is, a gain per unit length is reduced to obtain an optical fiber of By making the optical loss per unit length to the same level, the optical loss of the optical fiber itself is canceled over the entire fiber length of several kilometers to several tens of kilometers. The Er-doped optical fiber 13 itself can be used as a transmission line, and a so-called equivalent lossless optical fiber transmission line can be realized.

The length of the Er-doped optical fiber 13 can be set by adjusting the intensity of the excitation light P and the Er-doped concentration in the optical fiber to desired values. Next, this principle will be described.

In order to realize the distributed constant optical fiber amplifier, the concentration of Er added to the optical fiber is an important point. According to the above-mentioned reference 2, the concentration of 1370 ppm (7.0 × 10 ¹⁸ ions / c
(corresponding to m ³ ), a gain of 12.5 dB was obtained with a pump light input of 32 mW and a fiber length of 3 m using a 1.48 μm semiconductor laser as a pump light source. This gain is included in the total volume of the core part
Since it is proportional to the Er ion, if the core diameter of the optical fiber is the same and the product of the fiber length and the concentration is constant, a gain of 12.5 dB can be obtained with the same pump light intensity. That is, the lower the concentration, the longer the fiber inversely.

Since the theoretical formula of the gain length direction has already been derived in the following reference 3, the gain of the Er-doped optical fiber can be obtained by numerical calculation [reference 3; E. Desurvire, JR Simpso.
n, and PCBecker, “High−gain erbium doped travelin
g-wave fiber amplifier ", OPTICS LETTERS, Vol.12, No.
11,888 (1987). reference].

However, in this theoretical formula, since the fiber length is short, the optical loss of the pumping light P and the signal light S due to the optical fiber can be ignored, but in reality, the optical loss of the optical fiber unrelated to Er is 0.3 in the 1.5 μm band. dB / km, 0.5 dB / km in the 1.48 μm band. Therefore, when the fiber length becomes several kilometers to several tens of kilometers, it becomes necessary to consider the optical loss of the signal light S and the pumping light P.

The formula in which the optical loss of the optical fiber is taken into consideration in the theoretical formula of Document 3 is as follows.

Here, I _p , _s indicates the intensity of the pumping light P and the signal light S, and the relationship with the power is I _p , _s = P _p , _s / Aeff (2) ρ is the Er addition concentration, σ _p , _s is the absorption cross section and the stimulated emission cross section, which are 6.5 × 10 ^-21 cm ² and 1.0 × 10 ^-21 cm, respectively.
A value of ² was used. α _p and _s represent the optical loss when Er is not added.

As for the gain, the intensity of the incident signal light S is P _s (0), the fiber length is 1
The intensity of the signal light S at is represented by P _s (1) and is defined by the following equation.

G = 10log {P _s (1) / P _s (0)} (3) In Fig. 5, Er was added at concentrations of 0.076 ppm and 0.069 ppm.
The calculation result of the fiber length dependence of the gain of the Er-doped optical fiber 13 obtained from the coupling equation of the above equation (1) is shown. In FIG. 5, the horizontal axis represents the fiber length and the vertical axis represents the gain. In the figure, when the curve shown by the solid line is the Er-doped concentration of 0.076 ppm, the curve shown by the broken line shows the Er-doped concentration of 0.069 ppm. The figure shows the case of ppm. The excitation light P
Wavelength (λ _P ) is 1.48 μm, and the wavelength of signal light S (λ _S )
Is 1.535 μm. The intensity of the incident signal light S is 1.6 mW.
(+2 dBm), and continuous wave light was assumed.

As can be seen from FIG. 5, when the Er-doped concentration is 0.069 ppm and the intensity of the incident excitation light P is 80 mW, the fiber length is 18.5 km.
The gain is 0 dB, that is, the emitted light intensity and the incident light intensity become equal. At this time, a distributed constant type optical fiber amplifier having a maximum gain of 1.4 dB near a fiber length of 9 km and a gain of 18.5 km has been realized.

Here, if the intensity of the excitation light P is raised to 100 mW, it will be 21.8 km.
It is possible to extend the transmission distance of the signal light S. The maximum amplification degree at this time is 1.9 dB.

On the other hand, if the intensity of the pumping light P is raised to 100 mW, the transmission distance of the signal light S can be extended to 21.8 km. At this time,
The maximum gain is 1.9dB. When the Er-doped concentration is 0.076 ppm (solid line), the intensity of the pumping light P of 80 mW has a gain up to 20 km, and at 100 mW, the transmission distance can be extended to 23.2 km.

Thus, the length of the optical fiber can be set by adjusting the intensity of the pumping light P. From the above calculation results, it can be confirmed that the predetermined gain can be distributed over the length of 20 km by lowering the Er addition concentration.

In the first embodiment, if the optical fiber to which Er is added has a zero-dispersion wavelength on the wavelength side of the signal light S, that is, on the shorter wavelength side than the 1.5 μm band, only a linear pulse is obtained. First, optical solitons, which are nonlinear waves, can be transmitted over long distances without causing their collapse.

The concentration of Er added is preferably 0.01 ppm to 10 ppm, and if the concentration is lower than 0.01 ppm, the optical fiber cannot have the desired amplification function.
If the added concentration exceeds m, the fiber length cannot be lengthened (it will be shortened by about two digits).

Furthermore, in the first embodiment, the semiconductor laser that emits the pumping light P having a wavelength of 1.48 μm is used as the pumping light source 11, but the present invention is not limited to this, and the wavelength band matching the absorption band of Er is used. , 0.5μm band, 0.6μm band, 0.8μm
It goes without saying that a laser device that emits excitation light in the band of 0.98 μm can be applied to the present invention.

FIG. 6 is a block diagram showing a second embodiment of the distributed constant type optical fiber amplifier according to the present invention. The difference between the second embodiment and the first embodiment is that the signal light S is transmitted and the excitation light P is reflected instead of the optical combining means instead of the optical fiber coupler to combine the two lights. The dielectric multilayer filter 15 is provided, and the SELFOC lens 16 is provided on the incident side of the dielectric multilayer filter 15 for the signal light S and the excitation light P.
This is because a and 16b are respectively arranged, and a SELFOC lens 16c is arranged between the exit side of the combined light and one end of the Er-doped optical fiber 13. The other structure is the same as that of the first embodiment, and the same effect can be obtained.

FIG. 7 is a block diagram showing a third embodiment of the distributed constant type optical fiber amplifier according to the present invention. The difference between the third embodiment and the first embodiment is that an optical fiber coupler 17 is inserted between the other end of the Er-doped optical fiber 13 and the wavelength selection element 14, and its input / output end faces 17a and Er are inserted. The other end of the doped optical fiber 13 and the emission end face 17b are connected to the wavelength selection element 14, and a pumping light source 18 having the same structure as the pumping light source 11 is provided, and the pumping light P from the pumping light source 18 is incident on the optical fiber coupler 17. Edge 17
It is configured so that Er is excited from both ends of the Er-doped optical fiber 13 by being incident from c and incident on the other end of the Er-doped optical fiber 13.

With such a structure, in addition to the effect of the first embodiment, the fiber length can be doubled. As described above, in the calculation result of the first embodiment, since the gain is obtained over the fiber length of 23.3 km, the gain is distributed over 46.4 km in the third embodiment. It is possible to realize a distributed constant type optical fiber amplifier.

(Effect of the invention) As described above, according to claim (1), an excitation light source that generates excitation light in a wavelength band that matches the absorption wavelength region specific to erbium (Er), and erbium from 0.01 ppm to 10 p
Since the erbium-doped optical fiber doped at the concentration of pm and the optical multiplexing means for multiplexing the excitation light and the 1.5 μm band signal light and making it enter one end of the erbium-doped optical fiber are included While canceling the light loss,
The signal light can be propagated, and the length of the optical fiber can be set to several kilometers to several tens of kilometers by adjusting the excitation light intensity and the Er-doped concentration to desired values within the above range. Therefore, a so-called equivalent lossless optical fiber transmission line can be realized. Therefore, there is an advantage that the limitation of the transmission distance can be relaxed in the optical communication system.

Further, according to claim (2), in addition to the effect of claim (1), it is possible to propagate an optical soliton, which is a nonlinear wave, while keeping the amplitude of the optical soliton substantially constant, thereby increasing the distance of the optical soliton transmission. Can be achieved.

According to claim (3), in addition to the effect of claim (1) or (2), the transmission distance can be further doubled. For example, linear pulses or optical solitons can be propagated over 46 km or more with almost no reduction in intensity.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of a distributed constant type optical fiber amplifier according to the present invention, FIG. 2 is a graph showing the fiber length dependence of the gain of a conventional Er-doped optical fiber, and FIG. FIG. 4 is a diagram showing an example of using a conventional optical fiber amplifier as a lumped constant type, FIG. 4 is a graph showing amplification experiment results by stimulated Raman scattering, and FIG. 5 is a fiber length dependence of the gain of an Er-doped optical fiber according to the present invention. FIG. 6 is a configuration diagram showing a second embodiment of the distributed constant type optical fiber amplifier according to the present invention, and FIG. 7 is a third embodiment of the distributed constant type optical fiber amplifier according to the present invention. It is a block diagram which shows. In the figure, 11, 18 ... Excitation light source, 12 ... Optical fiber coupler (optical multiplexing means), 13 ... Er (erbium) -doped optical fiber, 14 ... Wavelength selection element, 15 ... Dielectric multilayer film filter ( Optical multiplexing means), 17 ... Optical fiber coupler.

Claims (3)

(57) [Claims] 1. An excitation light source that generates excitation light in a wavelength band that matches an absorption wavelength region specific to erbium (Er), an erbium-doped optical fiber doped with erbium at a concentration of 0.01 ppm to 10 ppm, and the excitation light. A distributed constant type optical fiber amplifier, comprising: an optical multiplexing unit that multiplexes 1.5 μm band signal light and makes it enter one end of the erbium-doped optical fiber. 2. The distributed constant type optical fiber amplifier according to claim 1, wherein the erbium-doped optical fiber is formed by adding erbium to an optical fiber having a zero dispersion wavelength on the shorter wavelength side than the wavelength of the signal light. . 3. A distribution constant according to claim 1, further comprising means for causing pumping light having a wavelength band matching with an absorption wavelength region peculiar to erbium to be incident on the other end of the erbium-doped optical fiber. Optical fiber amplifier.