JPH09185090A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve an amplification characteristic by effectively utilizing excitation light from an optical amplifier. SOLUTION: Excitation light λp projected from an excitation light source 13 is made incident upon an erbium dope optical fiber(EDF) 14 through an wavelength multiplexer (WDM) coupler 12 and absorbed to generate sufficient inversion distribution. When signal light λs is made incident upon the EDF 14 in the incident state of the excitation light λp, The light λs is gradually amplified by induction emission action and the amplified signal light is discharged as an output optical signal λS. When the EDF 14 is sufficiently short against the intensity of the excitation light λp, the excitation light λp may be discharged to the output end of the EDF 14 as remaining excitation light (r). The light (r) is made incident upon a total reflection film 19 successively through a WDM coupler 15 and a fiber 18 and reflected by the film 19 and the reflected light is returned into the EDF 14 successively through the fiber 18 and the coupler 15, so that the amplification characteristic of the EDF 14 is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier used for optical communication or the like.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing an example of a conventional optical amplifier. This optical amplifier inputs the signal light λs from a transmitter (not shown) from the port 3a via the isolator 1, and the pumping light λp generated by the pumping light source 2 from the port 3b to obtain the signal light λs. A wavelength multiplexer (Wavelength D) that multiplexes with the pumping light λp and outputs it from the port 3c.
ivision Multiplexer (hereinafter referred to as WDM coupler) 3
have. The excitation light source 2 is composed of, for example, a laser diode or the like. Port 3c of WDM coupler 3
Is connected to the input side of the isolator 5 via the optical amplification fiber 4. The optical amplification fiber 4 is, for example, an Erbium Doped Fiber (hereinafter, referred to as “Erbium Doped Fiber”).
DF) and other optical amplifier fibers. In this EDF 4, both the signal light λs and the pumping light λp propagate through the EDF 4 so that the pumping light λp
Is transferred to the signal light λs, the signal light λs is amplified, and the output optical signal λS is output.
The output side of the isolator 5 is connected to, for example, an optical transmission line (not shown).

FIG. 3 is a diagram showing the energy levels of the optical amplifying fiber. The operation of FIG. 2 will be described with reference to FIG. In this optical amplifier, the pumping light λ in the EDF 4
When p is propagated, the erbium ion absorbs the excitation light λp and is excited from the ground level E1 to the excitation level E3.
Next, the electrons move from the excitation level E3 to the upper level E2.
Then, the number of ions N2 of the upper level E2 exceeds the number of ions N1 of the ground level E1, and the population inversion occurs. The signal light λs is amplified and becomes the output light signal λS by the stimulated emission action caused by the incidence of the signal light λs in this state. Here, the gain coefficient g of the output optical signal λS
(Λ) is expressed by the following equation (1). g (λ) = σe (λ) · N2-σa (λ) · N1 (1) where λ: wavelength of output optical signal λS σe: stimulated emission cross section σa; induced absorption cross section In the above configuration, in addition to the forward pumping in which the traveling direction of the signal light and the traveling direction of the pumping light coincide with each other as shown in FIG. 2, the backward pumping in the opposite case, and the bidirectional pumping combining them. Etc.

[0004]

However, the conventional optical amplifier has the following problems. FIG.
The output power is plotted on the vertical axis and the EDF length is plotted on the horizontal axis. FIG. 4 shows the output optical signal power P [λS] that increases with respect to the length direction of the EDF 4 in the optical amplifier by the forward pumping shown in FIG. Here, the pumping light power P [λp] is ED
When it is sufficiently strong with respect to the length of F4, the pumping light power P [λp] that decreases in a distribution constant in the length direction of the EDF4 is not absorbed in the EDF4, and its energy is output to the output end of the EDF4. It may remain as the residual excitation light r of P [r]. Therefore, the energy of the excitation light λp may be wasted. Further, when this optical amplifier is used for wavelength division multiplexing transmission, there is a problem that the gain deviation between wavelengths becomes large when the energy of the pumping light λp is insufficient. This problem will be described with reference to FIGS. 5 and 6A and 6B. FIG. 5 is a diagram showing wavelength characteristics of the stimulated emission cross section σe and the stimulated absorption cross section σa of the optical amplification fiber. In this figure, in the part where the wavelength λ of the output optical signal λS is 1540 nm to 1560 nm, the wavelength dependence of the stimulated emission cross section σe is small,
It is shown that the wavelength dependence of the induced absorption cross section σa is large.

FIGS. 6 (a) and 6 (b) are diagrams showing the states of electrons in the respective excited states of the optical amplifying fiber, of which FIG. 6 (a) shows the states of electrons in the completely excited state of the optical amplifying fiber. It is a figure which shows a state. In this figure, it is shown that most electrons are present in the excitation level E3 in the completely excited state of the optical amplification fiber. Therefore, the effect of the stimulated emission cross-section σe on the gain of the signal light becomes significant, and the gain coefficient g (λ) shown in the equation (1) has little wavelength dependence. FIG. 6B is a diagram showing the state of electrons in the incompletely excited state of the optical amplification fiber. In this figure, in the incompletely excited state of the optical amplifying fiber, the ground level E
It is shown that electrons are present in both 1 and the excitation level E3. Therefore, the gain of the signal light is affected not only by the stimulated emission cross section σe but also by the stimulated absorption cross section σa. Referring to FIG. 5, it can be seen that the stimulated absorption cross section σa increases on the short wavelength side. This induced absorption cross section σa
Is large, the gain decreases, so the gain on the short wavelength side becomes smaller than the gain on the long wavelength side. Therefore, the gain coefficient g (λ) shown in the equation (1) has a large wavelength dependency. Therefore, when the optical amplifier is used for wavelength division multiplexing transmission, the gain dependence of the gain coefficient g (λ) becomes large when the energy of the pumping light λp is insufficient, so that the inter-wavelength gain deviation becomes large and And flat amplification characteristics cannot be obtained.

[0006]

In order to solve the above-mentioned problems, a first aspect of the present invention is to input a signal light composed of light of a predetermined wavelength in an optical amplifier and amplify the signal light by pumping light. An optical amplifying fiber, a pumping light source that enters the pumping light in the same direction as the signal light with respect to the optical amplifying fiber, and a component of the signal light in the output light of the optical amplifying fiber, and And a transmitting / reflecting means for reflecting the component of the pumping light in the output light in a direction opposite to that of the signal light and returning to the optical amplifying fiber. According to the first invention,
Since the optical amplifier is configured as described above, the pumping light emitted from the pumping light source enters the optical amplifying fiber and is absorbed to cause population inversion. When the signal light is incident on the optical amplification fiber in this state, the signal light is gradually amplified by the stimulated emission action, and the amplified signal light is emitted as an output optical signal. Here, when the optical amplification fiber is sufficiently short with respect to the intensity of the excitation light, the excitation light is emitted to the output end of the optical amplification fiber as residual excitation light. This residual excitation light is reflected by the incident light to the transmitting / reflecting means and then returns to the inside of the optical amplifying fiber. Therefore, the optical amplification fiber is sufficiently pumped by the residual pumping light reflected and returned by the transmissive reflection means, and the amplification characteristic is improved.

According to a second aspect of the present invention, in an optical amplifier, an optical amplification fiber for inputting signal light composed of light having a predetermined wavelength and amplifying the signal light by pumping light, and the optical amplification fiber are provided for the optical amplification fiber. A pumping light source that enters the pumping light in the opposite direction to the signal light, and a component of the pumping light that passes through the signal light and is input to the optical amplification fiber, and that is output from the optical amplification fiber is the signal light. And a transmitting / reflecting means for reflecting the light in the same direction and returning it to the optical amplification fiber. According to the second aspect of the present invention, the pumping light emitted from the pumping light source is incident on the optical amplification fiber and is absorbed to cause population inversion. When the signal light enters the optical amplifying fiber in this state, the signal light is gradually amplified by the stimulated emission effect,
The amplified signal light is emitted as an output optical signal.
Here, when the optical amplification fiber is sufficiently short with respect to the intensity of the excitation light, the excitation light is emitted to the input end of the optical amplification fiber as residual excitation light. The residual pumping light enters the transmissive / reflecting means and is then reflected back into the optical amplification fiber. Therefore, the optical amplification fiber is sufficiently pumped by the residual pumping light reflected and returned by the transmissive reflection means, and the amplification characteristic is improved. Therefore, the above problem can be solved.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIG. 1 is a configuration diagram of an optical amplifier according to a first embodiment of the present invention. This optical amplifier has a WDM coupler 12 that inputs a signal light λs from a transmitter (not shown) from a port 12 a via an isolator 11. The WDM coupler 12 has wavelength selectivity, the pumping light λp generated by the pumping light source 13 is input from the port 12b, the signal light λs and the pumping light λp are multiplexed, and output from the port 12c. It has a function. The excitation light source 13 is composed of, for example, a laser diode or the like. The port 12c of the WDM coupler 12 is connected to the WDM coupler 15 via the EDF 14.
Connected to port 15a. The EDF 14 has a function of amplifying the signal light λs by the pump light λp.
The WDM coupler 15 has substantially the same wavelength selectivity as the WDM coupler 12, and transmits the output optical signal λS in the output light S14 of the EDF 14 and outputs it from the port 15b.
It has a function of reflecting the component of the excitation light λp in 14 (that is, the residual excitation light r) and outputting it from the port 15c. W
The port 15b of the DM coupler 15 is connected to the input side of the isolator 17 via the fiber 16, and the output side of the isolator 17 is connected to, for example, an optical transmission line (not shown). The port 15c of the WDM coupler 15 is connected to the total reflection film 19 via the fiber 18. The total reflection film 19 has a function of reflecting the residual excitation light r and returning it to the EDF 14 via the WDM coupler 15. Then, the WDM coupler 15, the fiber 18 and the total reflection film 19 are provided.
The transmissive / reflecting means is constituted by. FIG. 7 is a block diagram of the transmissive / reflecting means in FIG.

In this transmission / reflection means, for example, gold (Au) is formed on the end faces of the core 18a and the clad 18b of the fiber 18.
The total reflection film 19 is formed by vapor deposition. However, in this transmission / reflection means, the residual excitation light r is approximately 10
Although it reflects 0%, the residual excitation light r
If the reflection strength is strong, the total reflection film 19 may melt. Further, the transmitting / reflecting means in FIG. 1 may be configured as shown in FIGS. 8 to 12 below. FIG. 8 shows another transmissive reflection means in FIG.
It is a block diagram of (i), and the common code | symbol is attached | subjected to the element common to the element in FIG. In this transmission / reflection means (i),
The fiber 18 is connected to the port 15c of the WDM coupler 15, and the collimator lens 21 is arranged between the end face of the fiber 18 and the total reflection film 20. The collimator lens 21 collimates the residual excitation light r emitted from the fiber 18 and irradiates the total reflection film 20 with the parallel excitation beam r, and the residual excitation light r reflected by the total reflection film 20 is reflected by the fibers 18 and W.
It has a function of returning to the EDF 14 via the DM coupler 15. Other configurations are the same as those in FIG. FIG. 9 is a configuration diagram of another transmissive / reflecting means (ii) in FIG. 1, and elements common to those in FIG. 7 are designated by common reference numerals. In this transmission / reflection means (ii), an interference film filter 22 is inserted between the EDF 14 and the fiber 16. The interference film filter 22 has a function of reflecting the residual excitation light r from the EDF 14 and returning it to the EDF 14. However, at present, the interference film filter 22 reflects the output optical signal S14 by about 0.1% in the direction opposite to the traveling direction, so the return loss (transmitted light amount / reflected light amount) is about 30 dB, and the NF (noise) of the optical amplifier is reduced. The exponential characteristic may be adversely affected (that is, noise light may be amplified). Therefore, in the future, the interference film filter 22
If it is improved and the reflection attenuation amount is secured to about 50 to 60 dB, the interference film filter can be used as the transmissive reflection means.

FIG. 10 shows another transmitting / reflecting means (ii) in FIG.
It is a block diagram of i), and the common code | symbol is attached | subjected to the element common to the element in FIG. This transmitting / reflecting means (iii) is
The bulk type WDM coupler 15A is provided. The bulk type WDM coupler 15A includes a collimator lens 15Aa,
It has a dielectric multilayer film 15Ad composed of 15Ab, 15Ac and TiO ₂ , SiO _2, or the like. The collimator lens 15Aa is connected to the EDF 14. The fiber 16 is connected to the collimator lens 15Ab. Further, the collimator lens 15Ac is connected to the total reflection film 19 via the fiber 18. The collimator lens 15Aa, the dielectric multilayer film 15Ad, and the collimator lens 15Ab are arranged in a straight line.
Ad reflects the residual excitation light r in the output light S14 emitted from the collimator lens 15Aa, enters the collimator lens 15Ac, and outputs the output optical signal λ in the output light S14.
It is arranged at a position where it passes through S and enters the collimator lens 15Ab. FIG. 11 is a configuration diagram of another transmissive / reflecting means (iv) in FIG. 1, and elements common to those in FIGS. 8 and 10 are designated by common reference numerals. In this transmission / reflection means (iv), the collimator lens 15Ac is connected to the fiber 18
And a collimator lens 21 is arranged between the end face of the fiber 18 and the total reflection film 20. Otherwise, the configuration is the same as in FIG.

FIG. 12 shows another transmitting / reflecting means (v) in FIG.
FIG. 8 is a configuration diagram of the above, and elements common to those in FIG. 7 are denoted by common reference numerals. The transmitting / reflecting means (v) includes a bulk type WDM coupler 15B. This bulk type WD
The M coupler 15B includes collimator lenses 15Ba and 15B.
b, a total reflection film 15B composed of TiO ₂ , SiO _2, etc.
c and the dielectric multilayer film 15Bd. The collimator lens 15Ba is connected to the EDF 14. The fiber 16 is connected to the collimator lens 15Bb. Collimator lens 15Ba, dielectric multilayer film 15B
d and the collimator lens 15Bb are arranged in a straight line, and the dielectric multilayer film 15Bd is
The residual excitation light r in the output light S14 output from Ba is reflected to enter the total reflection film 15Bc, and the output optical signal λS in the output light S14 is transmitted to collimator lens 15Bb.
It is located at the position where it is incident on. Next, the operation of FIG. 1 will be described. Excitation light λp emitted from the excitation light source 13
Enters the EDF 14 via the WDM coupler 12 and is absorbed by the EDF 14 to cause a sufficient population inversion. Excitation light λp is EDF
When the signal light λs is incident on the EDF 14 while being incident on 14, the signal light λs is gradually amplified by the stimulated emission effect, and the amplified signal light is emitted as the output optical signal λS.

Here, when the EDF 14 is sufficiently short with respect to the intensity of the pumping light λp, the pumping light λp may be emitted as the residual pumping light r to the output end of the EDF 14, as shown in FIG. 4 of the related art. . The residual pump light r is supplied to the WDM coupler 15
After passing through the fiber 18 and the fiber 18, the light is incident on the total reflection film 19, is reflected, and then returns through the fiber 18 and the WDM coupler 15 to the inside of the EDF 14. Therefore, in the optical amplifier of the present embodiment, the EDF 14 is sufficiently pumped by the residual pumping light r reflected and returned by the total reflection film 19, and the amplification characteristic is improved. Further, in the conventional optical amplifier, even when the pumping becomes insufficient near the output end of the optical amplifying fiber,
By changing to the optical amplifier of the present embodiment, the pumping near the output end of the optical amplification fiber is sufficiently performed and the amplification characteristic is improved. Further, when the optical amplifier of the present embodiment is used for wavelength division multiplexing transmission, the EDF 14 is strongly excited, so that it is possible to realize amplification characteristics with less gain deviation between wavelengths. As described above, in the first embodiment, the EDF1
4 is the residual excitation light r reflected by the total reflection film 19 and returned.
Since it is sufficiently excited by, the amplification characteristic is improved. Further, when the optical amplifier of this embodiment is used for wavelength division multiplexing transmission, the EDF 14 is strongly excited, so that an amplification characteristic with less gain deviation between wavelengths can be realized.

Second Embodiment FIG. 13 is a block diagram of an optical amplifier showing a second embodiment of the present invention. Elements common to those in FIG. 1 are designated by common reference numerals. In this optical amplifier, the WDM coupler 12
12b is connected to the total reflection film 19 via the fiber 18. Also, the port 15c of the WDM coupler 15
Is connected to the excitation light source 13. Then, these WDM coupler 12, fiber 18 and total reflection mirror 1
The transmitting / reflecting means is constituted by 9. Other configurations are the same as those in FIG. As the transmissive reflection means, the transmissive reflection means having the configuration shown in FIGS. 7 to 12 may be used as in the first embodiment. Next, the operation of FIG. 13 will be described. The pumping light λp emitted from the pumping light source 13 is incident on the EDF 14 via the WDM coupler 15 and is absorbed to cause a sufficient population inversion. When the signal light λs enters the EDF 14 while the pump light λp enters the EDF 14, the signal light λ
s is gradually amplified by the stimulated emission action, and the amplified signal light is emitted as the output optical signal λS.

Here, when the EDF 14 is sufficiently shorter than the intensity of the pumping light λp, the pumping light λp may be emitted to the input end of the EDF 14 as the residual pumping light r. The residual pumping light r is incident on the total reflection film 19 through the WDM coupler 12 and the fiber 18 in order, and then is reflected to the fiber 18.
And the WDM coupler 12, and then returns to the EDF 14.
Therefore, in the optical amplifier of this embodiment, the EDF 14 is
The residual pumping light r reflected and returned by the total reflection film 19 is sufficiently excited to improve the amplification characteristic. Further, when the optical amplifier according to the present embodiment is used for wavelength division multiplexing transmission, as in the first embodiment, the EDF 14 is strongly excited, so that it is possible to realize amplification characteristics with a small gain deviation between wavelengths. As described above, in the second embodiment, as in the first embodiment, the EDF 14 is reflected by the total reflection film 19.
The residual pumping light r returned to the pump is sufficiently pumped to improve the amplification characteristic. Further, when the optical amplifier of the present embodiment is used for wavelength division multiplexing transmission, the EDF 14 is strongly excited,
It is possible to realize an amplification characteristic with less gain deviation between wavelengths. The present invention is not limited to the above embodiment, and various modifications can be made. For example, there are the following modifications.

(A) In the embodiment, the EDF is used as the optical fiber amplifier, but for example, an optical fiber amplifier doped with praseodymium (Pr) or neodymium (N).
The present invention is also applied to optical fiber amplifiers doped with other rare earth elements such as optical fiber amplifiers doped with d). The present invention can also be applied to an optical amplifier having a pumping mechanism or pumping characteristics equivalent to these. (B) In the transmissive reflection means shown in FIGS. 7 to 12, a gold vapor-deposited total reflection film having no wavelength dependence is used. However, a wavelength-dependent reflection film (that is, a certain wavelength is reflected here) is used. Reflective film) may be used.

[0016]

As described in detail above, according to the first aspect of the invention, the optical amplification fiber is sufficiently excited by the component of the excitation light in the output light returned by the transmissive reflection means, so that the amplification characteristic is obtained. Is improved. Furthermore, when the optical amplifier of the present invention is used for wavelength division multiplexing transmission, the optical amplification fiber is strongly excited, so that amplification characteristics with less gain deviation between wavelengths can be realized. According to the second invention, similarly to the first invention, the optical amplification fiber is sufficiently pumped by the pumping light returned by the transmitting / reflecting means, so that the amplification characteristic is improved. Furthermore, when the optical amplifier of the present invention is used for wavelength division multiplexing transmission, the optical amplification fiber is strongly excited, so that amplification characteristics with less gain deviation between wavelengths can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical amplifier showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional optical amplifier.

FIG. 3 is a diagram showing energy levels of an optical amplification fiber.

FIG. 4 is a level diagram of FIG.

FIG. 5 is a diagram showing wavelength characteristics of σe and σa.

FIG. 6 is a diagram showing states of electrons in each excited state.

FIG. 7 is a configuration diagram of a transmissive reflection unit in FIG.

FIG. 8 is a configuration diagram of another transmissive / reflecting means (i) in FIG.

9 is a configuration diagram of another transmissive reflection means (ii) in FIG. 1. FIG.

10 is a configuration diagram of another transmissive / reflecting means (iii) in FIG. 1. FIG.

11 is a configuration diagram of another transmissive reflecting means (iv) in FIG. 1. FIG.

FIG. 12 is a configuration diagram of another transmissive-reflecting means (v) in FIG.

FIG. 13 is a configuration diagram of an optical amplifier showing a second embodiment of the present invention.

[Explanation of symbols]

12, 15 WDM coupler 13 Pumping light source 14 Optical amplification fiber 15A, 15B Bulk type WDM coupler (transmissive / reflecting means) 18 Fiber (transmissive / reflecting means) 19, 20 Total reflection film (transmissive / reflecting means) 21 Collimator lens (transmissive / reflecting means) ) 22 interference film filter (transmissive / reflecting means) λs, λS signal light λp excitation light

Claims (2)

[Claims] 1. An optical amplification fiber for inputting signal light composed of light of a predetermined wavelength and amplifying the signal light with pumping light; and the pumping in the same direction as the signal light with respect to the optical amplification fiber. A pumping light source for entering light, a component of the signal light in the output light of the optical amplifying fiber is transmitted, and a component of the pumping light in the output light is reflected in a direction opposite to the signal light, and An optical amplifier, comprising: a transmitting / reflecting means for returning to the optical amplifying fiber. 2. An optical amplifying fiber for inputting signal light composed of light of a predetermined wavelength and amplifying the signal light with pumping light; and the pumping in a direction opposite to the signal light with respect to the optical amplifying fiber. A pumping light source for entering light, and a component of the pumping light that is transmitted through the signal light and is input to the optical amplification fiber, and that is output from the optical amplification fiber is reflected in the same direction as the signal light. An optical amplifier, comprising: a transmitting / reflecting means for returning to the optical amplifying fiber.