JP3296195B2 - Laser, optical amplifier, and optical amplification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-noise, high-gain optical amplifier, an optical amplification method, and a laser.

[0002]

2. Description of the Related Art In recent years, for the purpose of application to the field of optical communication, an optical amplifier using an optical fiber having a core doped with a rare earth element as an optical amplification medium, particularly an Er (erbium) doped optical amplifier (EDFA) has been developed. Applications to optical communication systems are being actively pursued.

In recent years, in order to cope with the diversification of communication services expected in the future, research on optical communication systems using wavelength multiplexing for effectively using transmission media and expanding transmission capacity has been actively conducted. I have. One of the characteristics required for the EDFA used in the wavelength division multiplexing transmission system is that the fluctuation of the amplification gain due to the signal wavelength is small. The reason is that the optical signal that is relay-amplified by arranging the EDFAs in multiple stages causes a difference in intensity level between the optical signals, so that it is difficult to perform transmission with uniform characteristics over all the wavelengths used. is there. Therefore, research on EDFAs in which the gain is flat with respect to the wavelength is currently being conducted.

[0004] As one of the leading candidates of such EDFA,
An Er-doped fluoride fiber amplifier (EDFFA) using a fluoride fiber as the host of Er has attracted attention.
The features of this amplifier are: 1. Er in fluoride glass.
The gain spectrum obtained because the emission spectrum due to the ⁴ I _13/2 → ⁴ I _15/2 transition in the 55 μm band is wider than that in quartz glass and the wavelength dependence such as _dip is smooth without a sharp change. Is flatter than an EDFA using a quartz fiber. Recently, a wavelength division multiplexing transmission experiment using EDFFA in multiple stages has been performed.

[0005]

However, EDFFA has one disadvantage. That is, the noise figure (NF) cannot be reduced as much as the EDFA using the quartz fiber. Hereinafter, a description will be given with reference to FIG. 10 showing energy levels of erbium (Er). The reason why NF value cannot be reduced as much as EDFA using quartz fiber is that E
In the case of _{^{DFFA, 4 I 15/2 → 4 I}} 11/2 transition by ⁴ I
_{The point} is that excitation at the _13/2 level (FIG. 10A) cannot be performed. For a quartz fiber, the phonon energy is 110
Since it has a large value of about 0 cm ^−1, even if it is excited to the ⁴ I _11/2 level with light of 0.98 _μm, it is reduced by phonon emission relaxation.
⁴ I energy is relaxed to _13/2 level, ⁴ I _13/2 level is efficiently excited, form a good inversion between ⁴ I _13/2 level position and ⁴ I _15/2 level (FIG. 10A). Therefore, the NF can be lowered and the quantum limit (3 dB)
And a value of about 4 dB close to is obtained.

However, in the case of the fluoride fiber, the phonon energy is about 500 cm ^−1, which is less than half of that of the quartz fiber, so that the phonon emission relaxation from the ⁴ I _{11/2 level} to the ⁴ I _13/2 level is _suppressed. Is difficult to occur, and it is difficult to obtain a gain with 0.98 μm excitation. Therefore, the excitation wavelength is 1.4.
Direct excitation of the ⁴ I _13/2 level with light near 8 μm to 1.55
A gain in the μm band is obtained (FIG. 10B). However, since this excitation method is excitation within the amplification starting level,
A good population inversion state cannot be obtained, and the NF has a high value of 6 to 7 dB, and the EDFFA does not realize an amplifier having excellent noise characteristics as compared with an EDFA using a quartz fiber.

An object of the present invention is to solve the drawback of the conventional EDFFA having a high noise figure, and to provide an optical amplifier with low noise, high gain and flat gain.

[0008]

In order to solve the above problems SUMMARY OF THE INVENTION The laser according to the present invention, an optical amplifying medium which erbium is added, comprising at least an excitation light source of the optical amplification material and erbium ⁴ I _13/2 level to ⁴ I
_In a laser using stimulated emission to a _15/2 level,
The excitation light source comprises at least a first light source and a second light source having wavelengths different from each other, and the first light source is a ⁴ I _13/2 level of erbium and a level having higher energy than the level. wherein the equivalent to the energy difference is a light source of wavelength, and high the energy levels of the ⁴ I
Stimulated emission to _13/2 level causes ⁴ I _13/2 level
Characterized in that it is a light source that increases the excited state density .

Preferably, the first light source is an erbium ⁴ I _13/2 level, ⁴ I _11/2 level, ⁴ I _9/2 level, ⁴ F
A light source having a wavelength corresponding to the energy difference between the _9/2 level and the ⁴ S _3/2 level, and preferably the second light source is a ⁴ I _15/2 level of erbium and a ⁴ I _{11/2 level.} level or ⁴ F
_This is a light source corresponding to the energy difference from the _9/2 level.

Preferably, the first light source is a light source which emits light having a wavelength corresponding to an energy difference between the ⁴ I _13/2 level and the ⁴ S _3/2 level of erbium. The second light source is a light source that emits light having a wavelength corresponding to the energy difference between the erbium ⁴ I _15/2 level and the ⁴ I _11/2 level, and the third light source is erbium ⁴ I _15/2. It has a light source that emits light having a wavelength corresponding to the energy difference between the level and the ⁴ I _13/2 level.

Preferably, the wavelength of the first light source is 0.1.
82 μm to 0.88 μm, and the wavelength of the second light source is 0.96 μm to 0.98 μm.

Preferably, the erbium-doped optical amplification medium is selected from the group consisting of an erbium-doped fluoride optical fiber, an erbium-doped chalcogenide optical fiber, an erbium-doped telluride oxide optical fiber, and an erbium-doped halide crystal.

Next, an optical amplifier according to the present invention comprises: an optical amplifying medium to which erbium is added; means for introducing and extracting signal light having a wavelength of about 1.5 μm into the optical amplifying medium;
In an optical amplifier including at least a pumping light source of the optical amplification medium, the pumping light source includes at least a first light source and a second light source having different wavelengths, and the first light source is erbium ⁴ I. _13/2 is a light source of a wavelength corresponding to the energy difference between the high level of energy than level and the quasi-position, and from high the energy level
The stimulated emission to the ⁴ I _13/2 level causes ⁴ I
It is a light source that increases the excited state density of the _13/2 level .

Preferably, the first light source is an erbium ⁴ I _13/2 level, ⁴ I _11/2 level, ⁴ I _9/2 level, ⁴ F
A light source having a wavelength corresponding to the energy difference between the _9/2 level or the ⁴ S _3/2 level, and preferably the second light source is a erbium ⁴ I _15/2 level and a ⁴ I _{11 / 2} levels or ⁴ F
_This is a light source corresponding to the energy difference from the _9/2 level.

Preferably, the first light source emits light having a wavelength corresponding to the energy difference between the ⁴ I _13/2 level and the ⁴ S _3/2 level of erbium, and the second light source light source is a light source that emits light of a wavelength corresponding to an energy difference between ⁴ I _15/2 level and ⁴ I _11/2 level of Er, further ⁴ I _15/2 level of Er as the third light source And ⁴ I _13/2
A light source that emits light having a wavelength corresponding to the energy difference from the energy level;

Preferably, the wavelength of the first light source is 0.1.
82 μm to 0.88 μm, and the wavelength of the second light source is 0.96 μm to 0.98 μm.

Preferably, the erbium-doped optical amplification medium is selected from the group consisting of erbium-doped fluoride optical fibers, erbium-doped chalcogenide optical fibers, erbium-doped telluride oxide optical fibers, and erbium-doped halide crystals.

The optical amplifier of the present invention, in the optical amplifier for the erbium amplification active elements, ⁴ I of said erbium
_15/2 and at least one light corresponding to an energy difference between level and ⁴ I _11/2 level, the energy difference between the energy level and ⁴ I _15/2 located higher than ⁴ I _15/2 level entering the at least one light and, ⁴ I _13/2 and the signal light amplified by stimulated emission transition from level to ⁴ I _15/2 level, amplifying medium in which the erbium is added in the same direction that corresponds to the It is characterized by doing.

According to the optical amplification method of the present invention, there is provided an optical amplifier using erbium as an amplification active element, wherein at least one of the erbium and the _ionic energy corresponding to the energy difference between the ⁴ I _15/2 level and the ⁴ I _11/2 level. Light and ⁴ I
_Energy levels higher than the _13/2 level ⁴
And at least one light corresponding to an energy difference between I _13/2 ^level, and a signal light is amplified by stimulated emission transition from 4 _{I 13/2} level to ⁴ I _15/2 level, from the same direction It enters the amplification medium in which the erbium is added, before
Serial ⁴ I _13/2 energy level which is located higher than the level
Position and energy of the ⁴ I _13/2 energy level
By at least one light corresponding to over difference, the ⁴ I
Before the energy level located higher than the _13/2 level
( ⁴⁾ _Inducing stimulated emission to the I _13/2 level .

Preferably, the ⁴ I _15/2 level of erbium and
⁴ I _13/2 light with a wavelength corresponding to the energy difference from the level
Light having a different wavelength from the signal light is incident on the amplification medium from a direction different from the same direction.

The optical amplification method according to the present invention, in the optical amplification method using erbium as an amplification active element, corresponds to an energy difference between the ⁴ I _15/2 level and the ⁴ I _11/2 level of erbium. Light with a wavelength of ⁴ S _3/2
And light having a wavelength corresponding to the energy difference between the level and the ⁴ I _13/2 ^level, 4 _{I 13/2} signal light and amplified by stimulated emission transition from level to ⁴ I _15/2 level Incident on the erbium-doped amplification medium from the same direction,
Between the ⁴ S _3/2 level and the ⁴ I _13/2 level
By the light having the wavelength corresponding to the energy difference, the ⁴ S
Stimulated emission from the _3/2 level to the ⁴ I _13/2 level
And characterized in that.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, at least one third light Er corresponding to the energy difference between the ⁴ I _13/2 level and the level located higher than the same level, together with the pump light and the signal light, is provided. The most significant feature is that the light enters the doped fiber.

FIG. 1 is a diagram for explaining the energy level of Er. In the figure, (a) to (d) are excited at mutually different levels. As shown in this figure, 0.98 μm
Pump excited state absorption by the ⁴ I _11/2 → ⁴ F _7/2 transition
SA) excites the ⁴ F _7/2 level, and relaxation occurs from this level to the ⁴ S _3/2 level due to phonon emission relaxation, and part of the Er ions is excited in this state. Therefore, light (0.85 μm) having an energy corresponding to the energy difference between the ⁴ S _3/2 level and the ⁴ I _13/2 level is incident,
⁴ S _3/2 → ⁴ I _If the stimulated release of _{13/2 occurs} , ⁴ S
The population density of the _3/2 level can be reduced, and the density of the excited state of the ⁴ I _13/2 level can be increased. as a result,
Than when excited by 0.98 μm light alone
The population inversion density between the ⁴ I _13/2 and ⁴ I _15/2 levels increases, and the gain efficiency improves ((a) in FIG. 1).

Further, levels excited by the pump ESA not only ⁴ S _3/2 level, ⁴ I _9/2, ⁴ F _9/2 also excited (in FIG. 1 (b), (c) ). 0.98 μm
The ⁴ I _11/2 level, which is directly excited by, also has a large excited state density (FIG. 1 (d)). Therefore _{^{4 I 9/2, 4 F 9/2,}} 4
Light having an energy that opposes the energy difference between the I _11/2 level and the ⁴ I _13/2 level (1.65 μm, 1.16, respectively)
μm, 2.7 μm), the excited state density of the ⁴ I _13/2 level can be increased in the same manner as when 0.85 μm light is incident.

Therefore, even in the EDFFA, a gain can be obtained even with the use of the 0.98 μm pump, which is easy to reduce the noise, and the low noise and the high gain of the EDFFA can be achieved.

Hereinafter, an optical amplifier and a laser according to the present invention will be specifically described based on embodiments. But,
The present invention is not limited to the following examples.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0028]

【Example】

Embodiment 1 FIG. 2 is a schematic diagram for explaining an embodiment of an optical amplifier according to the present invention. In the figure, reference numerals 1 and 2 are excitation light sources, 3 and 4 are optical couplers, and 5 is Er.
Is an optical fiber for amplification, and 6 is an optical isolator, and arrow A indicates an output (laser oscillation) direction.

In this embodiment, the light source 1 is 0.98 μm.
An m oscillation semiconductor laser and a 0.85 μm oscillation semiconductor laser were used as the light source 2. Light source 1 by optical coupler 3
After the excitation light from the light source 2 is coupled, the optical coupler 4 couples the excitation light output from the optical coupler 3 with the signal light input from the direction of arrow A in the figure. Then, the output light of the optical coupler 4 was incident on the Er-doped amplification optical fiber 5.

The Er-doped amplification optical fiber 5 has a glass component of ZrF ₄ -BaF ₂ -LaF ₃ -YF ₃ -ALF ₃
-PbF ₂ -LiF-HfF ₄ , with an Er concentration of 1,000 ppm, a length of 10 m, and a core diameter of 2.5
μm, and the cutoff wavelength is 1 μm. When only 0.98 μm of excitation light is incident at 200 mW, 1.55 μm
Was 5 dB. When 0.85 μm pumping light was incident at 50 mW, the gain increased to 30 dB.
In this case, when the noise figure (NF) of this amplifier was measured, a value of 4 dB was obtained.

Using the Er-doped fluoride optical fiber used in this example, the NF value was determined by pumping at 1.48 μm, and a value of 6 dB was obtained at 1.55 μm with a gain of 30 dB. . Therefore, according to the present embodiment, 30 dB which could not be obtained by the conventional 0.98 μm excitation.
NFFA can be reduced by about 2 dB as compared with 1.48 μm pumping, and the value is about the same as that obtained with 0.98 μm pumping EDFA using quartz fiber. .

[Embodiment 2] In the embodiment 1, ⁴ S _3/2 →
Light of 0.85 μm was incident as light corresponding to the ⁴ I _13/2 transition. Instead, light corresponding to the ⁴ I _11/2 → ⁴ I _13/2 transition was incident from the Er: YAG laser 2. . The wavelength was 2.7 μm. This translates into a population of ⁴ I _11/2 levels excited at 0.98 μm from ⁴ I _{11/2 to} ⁴ I
_{This is for} directly lowering by stimulated emission by the _13/2 transition and raising the population of the ⁴ I _13/2 level. Again,
It was confirmed that the gain was increased by 1.55 μm and the NF was decreased, as compared with the case of the pumping of 0.98 μm alone. Further, even when light of 1.16 μm was incident using the semiconductor laser as the light source 2, improvement in amplification characteristics was confirmed.

[Embodiment 3] In this embodiment, the light source 2 (semiconductor laser) corresponds to the transition from ⁴ I _{9/2 to} ⁴ I _13/2 .
Light of 65 μm was incident. Also in this case, the gain is increased by 1.55 μm and the NF
Was confirmed to decrease.

In the above embodiment, the incident light from the light source 2 is 0.85 μm, 2.7 μm, 1.16 μm,
1.65 μm light was used, but ⁴ S _3/2 , ⁴ I _9/2 , ⁴
The transition energy from the F _9/2 , ⁴ I _11/2 level to the ⁴ I _13/2 level has a width, and it is effective if the light incident from the light source 2 is within the energy width. Needless to say.

The light source 2 actually used is not only a solid-state laser such as a semiconductor laser or an Er: YAG laser, but also a light source that emits light of, for example, 2.7 μm.
A fiber laser such as an additive fluoride fiber laser may be used.

Note that there are other energy levels above the ⁴ I _13/2 level in addition to the above three energy levels, which correspond to the energy difference between the level and the ⁴ I _13/2 level. The amplification characteristics can be improved even if light having the energy of the incident light is incident.

Further, the light that causes the transition from the upper level to the ⁴ I _13/2 level is not limited to one type, and light having a plurality of wavelengths may be incident simultaneously with the excitation light, or may be used as the excitation light. Is ⁴ I
Level located in an upper level of the _9/2 level position, for example, ⁴ F _9/2 level position may be one to excite directly the ⁴ S _3/2 level.

Example 4 In Examples 1 to 3 above,
Although an Er-doped ZrF ₄ -based fluoride fiber was used as an amplification medium, it is difficult to obtain a gain of 1.55 _{μm by} 0.98 μm pumping ( ⁴ I _11/2 level pumping) only in the above-mentioned fiber. not the same as Er-doped ZrF ₄ -alF ₂ based fluoride fiber, an Er-doped InF ₃ based fibers, Er-doped chalcogenide glass fiber, in the Er-doped oxide telluride glass fibers. Therefore, it was confirmed that the present invention is also effective when an amplification medium using such a substance having a low phonon energy as a host is used.

Further, the level whose energy is higher than that of the ⁴ I _11/2 level is excited not only by 0.98 μm excitation, but also by:
The same applies to the case 0.8μm excitation ⁽⁴ F _9/2 level excitation), also in this case, the transition from the upper energy level of ⁴ I _13/2 level position to ⁴ I _13/2 level 1.55 even if the light of the corresponding energy is incident simultaneously with the excitation light (0.8 μm light).
An increase in gain of μm and a decrease in NF were confirmed.

[Embodiment 5] FIG. 3 is a schematic diagram for explaining an embodiment of a laser according to the present invention. In the figure,
Reference numerals 1 and 2 are light sources, 3 is an optical coupler, 7 is a resonator mirror, 8 is a crystal as an amplification medium, and arrow C indicates an output direction. Crystals that can be used as an amplification medium include Er-doped LaF ₃ , BaF ₂ , LaCl ₃ , YF ₃
And the like. In this example, the characteristics of amplification and laser oscillation of 1.5 μm were evaluated using such a halide crystal.

Also in these cases, 0.8 μm, 0.98 μm
When the light which causes stimulated emission from the upper level to the ⁴ I _13/2 level is simultaneously incident in addition to the pump light of m, the gain increases,
An increase in laser oscillation efficiency was confirmed.

Embodiment 6 FIG. 4 is a schematic diagram for explaining another embodiment of a laser according to the present invention. In the figure, reference numerals 1 and 2 denote light sources, 3 denotes an optical coupler, 5 denotes an optical fiber for Er-doped amplification, and 7 denotes a resonance mirror.
Arrow D indicates the output (laser oscillation) direction. An Er-doped optical fiber is used as an amplification medium and ZrF ₄ is used as a glass component.
_{-BaF 2 -LaF 3 -YF 3 -AlF 3} -PbF 2 -
Prepared as LiF-HfF _4, confirmed the laser oscillation of 1.5μm to produce a laser of FIG. Excitation light source 2, 3
A light source having a wavelength of 0.98 μm and a wavelength of 0.85 μm was used.

When the light from the light source having a wavelength of 0.85 μm was shielded, the intensity of laser oscillation was significantly reduced.

Embodiment 7 FIG. 5 is a schematic diagram for explaining another embodiment of the laser according to the present invention. In the figure, reference numeral 1 denotes a light source composed of a semiconductor laser oscillating at 0.98 μm, 2 denotes a semiconductor laser oscillated at 0.85 μm, 9 denotes a light source composed of a semiconductor laser oscillating at 1.47 μm,
Reference numerals 4 and 4 'denote optical couplers, and reference numeral 5 denotes an Er-doped amplification optical fiber.

After the excitation light from the light sources 1 and 2 is coupled by the optical coupler 3, the excitation light output from the optical coupler 3 and the signal light input from the direction of arrow E in the figure are coupled by the optical coupler 4. . The output light from the optical coupler 4 enters the Er-doped amplification optical fiber 5. Excitation light from the light source 9 is transmitted through the optical coupler 4 'to Er.
The light enters the additive amplification optical fiber 5.

The Er-doped amplifying optical fiber 5 as an amplifying medium used in this embodiment is made of ZrF ₄ -BaF ₂ -LaF ₃ -YF _3- as a glass component similarly to the sixth embodiment.
AlF ₃ -PbF ₂ -LiF-HfF ₄ is included. Also,
The Er concentration is 1,000 ppm, the length is 10 m, the relative refractive index difference between the core and the clad is 2.5%, and the cutoff wavelength is 1 μm. 200 mW of 0.98 μm excitation light only
When incident, the gain at 1.55 μm was 5 dB.
When 30 mW of 0.85 μm excitation light is incident,
The gain was 15 dB, and when 100 mW of 1.48 μm pumping light was incident on the gain, the gain was 40 dB. At this time, the noise figure of this amplifier was measured.
A value of 8 dB was obtained.

The noise figure was determined by using the Er-doped fluoride optical fiber used in this example and pumping at 1.48 μm. As a result, a value of 6 dB was obtained at a gain of 40 dB at 1.55 μm. Therefore, according to the present embodiment, a high gain of 40 dB, which cannot be obtained by the conventional pumping of 0.98 μm, can be obtained. At the same time, the noise figure is 2d compared to 1.48 μm excitation.
0.98μm using quartz fiber
It was confirmed that the value was almost the same as that obtained by the excitation EDFA.

Embodiment 8 In Embodiment 7, ⁴ S _3/2 → ⁴ I
_Although light of 0.85 μm was incident as light corresponding to the _13/2 transition, light corresponding to the ⁴ I _11/2 → ⁴ I _13/2 transition was incident from the light source 2 in this embodiment. That is, a 2.7 μm oscillation Er: YAG laser was used as a light source. In this case as well, compared to the case of 0.98 μm excitation alone, 1.5 times
An increase in gain of 5 μm and a decrease in noise figure could be confirmed. Further, it was confirmed that the amplification characteristics were improved even when the semiconductor laser was used as the light source 2 and light having a wavelength of 1.16 μm was incident.

Embodiment 9 In Embodiment 7, ⁴ S _3/2 → ⁴ I
Light of 0.85 μm was incident as light corresponding to the _13/2 transition, and light corresponding to the ⁴ I _11/2 → ⁴ I _13/2 transition was incident from the light source 2 in Example 8. However, in this embodiment, 1.65 μm light corresponding to the ⁴ I _9/2 → ⁴ I _13/2 transition was incident from the light source 2 (semiconductor laser). Again, 0.9
It was confirmed that the gain was increased by 1.55 μm and the NF was reduced, as compared with the case of the excitation of 8 μm alone.

In the above embodiments 7 to 9, the light incident from the light source 2 is 0.85 μm, 2.7 μm, 1.1 μm,
6 μm, 1.65 μm light was used, but ⁴ S _3/2 , ⁴ I
_{^{9/2, 4 F 9/2, 4 I}} 11/2 has a width in the transition energy from level to ⁴ I _13/2 level, as the light incident from the light source 2 not within the energy width Needless to say, it is effective.

The light source 2 actually used is not only a solid-state laser such as a semiconductor laser or an Er: YAG laser, but also a light source that emits light of, for example, 2.7 μm.
A fiber laser such as an additive fluoride fiber laser may be used.

Note that there are other energy levels above the ⁴ I _13/2 level in addition to the above three energy levels, which correspond to the energy difference between the level and the ⁴ I _13/2 level. The amplification characteristics can be improved even if light having the energy of the incident light is incident.

Further, the light that causes the transition from the upper level to the ⁴ I _13/2 level is not limited to one type, and light of a plurality of wavelengths may be incident simultaneously with the excitation light, or may be used as the excitation light. Is ⁴ I
Level located in an upper level of the _9/2 level position, for example, ⁴ F _9/2 level position may be one to excite directly the ⁴ S _3/2 level.

Embodiment 10 In this embodiment, a semiconductor laser having an oscillation wavelength of 0.97 μm was used as the light source 1 and a semiconductor laser having an emission wavelength of 0.855 μm was used as the light source 2 (FIG. 2). After the excitation light from the light source 1 and the light source 2 are coupled by the optical coupler 3, the excitation light output from the optical coupler 3 is coupled by the optical coupler 4 to the signal light input from the direction of arrow A in the figure. The output light of the optical coupler 4 was incident on the Er-doped amplification optical fiber 5. Although not shown in FIG. 2, the signal light was input via an optical isolator.

The Er-doped amplification optical fiber 5 has a glass component of ZrF ₄ —BaF ₂ —LaF ₃ —YF ₃ —AlF _3.
-PbF ₂ -LiF-NaF-HfF ₄ , the Er concentration in the core was 100 ppm, the length was 10 m, the relative refractive index difference between the core and the clad was 2.5%, and the cutoff wavelength was 1 μm. Was. Only 200 m of 0.97 μm excitation light
When W is incident, the gain at the signal wavelength of 1.55 μm is 1
dB, but the light of 0.855 μm was irradiated with 10 mW.
Upon incidence, the signal gain was 40 dB. At this time, when the noise figure was measured, it was 3.8 dB.

In this embodiment, as shown in FIG.^Four
I_15/2→^FourI_11/2,^FourI_11/2→^FourF _7/2Two steps by transition
By excitation of the floor^FourS_3/2After exciting the level,^FourS_3/2→
 ^FourI_13/2Using stimulated emission transition^FourI_13/2Excite a level
Is taking the process. Therefore,^FourI_13/2Excite levels
In order to do this, first make two-step excitation efficient^Four
S_3/2It is necessary to excite a level. For that, the excitation wave
It is necessary to choose a length. Figure 6 changes the excitation wavelength
When^FourS_3/2Changes in the excitation density of the energy level
It is. This depends on the excitation wavelength^FourS_3/2→^FourI_13/2Departure
It is obtained from the light intensity change. You can see from Figure 6
Excitation at wavelengths from 960 nm to 980 nm
By^FourS_3/2The levels are well excited. In particular, 969
At a wavelength near nm, the excitation efficiency is good. Therefore, the excitation wavelength
Use a wavelength between 960 nm and 980 nm.
Are particularly efficient, and particularly those having a wavelength of around 969 nm
It is effective as an efficient excitation wavelength.

The wavelength of light that induces stimulated emission from the ⁴ S _3/2 level to the ⁴ I _13/2 level ranges from 0.82 μm to 0.1 μm.
A wavelength of 88 μm can be used. This is due to the presence of the emission cross section of the ⁴ S _3/2 → ⁴ I _13/2 transition of the finite at that wavelength band as shown in FIG. In particular, since the emission cross section exceeds the absorption cross section from 0.839 μm to 0.88 μm, the efficiency of the ⁴ S _3/2 level to the ⁴ I _13/2 level can be _reduced by using light having a wavelength between them. Good stimulated emission can be caused.

[Embodiment 11] In this embodiment, in addition to the light source 1 and the light source 2 used in the first embodiment, a third light source 9 is connected to the Er-doped amplification optical fiber 5 from the rear via an optical coupler 4 '. Incident (FIG. 5). The excitation wavelength of the light source 9 is 1.48μ.
m. The configuration of the present embodiment _aims to directly excite the ⁴ I _13/2 level by applying 1.48 μm excitation and to obtain a low noise figure and high output even when a large signal is incident. I have.

In the Er-doped quartz optical fiber, 0.98 μm excitation light is incident from the front and 1.48 from the rear.
An optical amplifier having a high output and a low noise figure was able to be constructed by injecting pump light of μm. However, when an Er-doped fluoride optical fiber is used,
If only 0.98 μm excitation light is incident, the excitation efficiency of the ⁴ I _13/2 level is significantly deteriorated. Therefore, 0.97 μm and 0.855 μm excitation light were simultaneously incident as in this embodiment.

An excitation light of 0.97 μm is applied at 100 mW and at a power of 0.1 mW.
20 mW of 855 μm pump light and 200 mW of 1.48 μm pump light are applied, and 0 dB signal light is applied to 1 mW.
When the light was incident and amplified over a wavelength range of 525 μm to 1.570 μm, an amplification gain of 15 dB or more and a noise figure of 5 dB or less could be confirmed in the above wavelength range. Although not shown in FIG. 5, signal light input was performed via an optical isolator. Since only a gain of 1 dB or less could be confirmed when only the pumping light of 0.97 μm was made incident, it can be seen that the amplification characteristics were significantly improved by the present pumping method.

Therefore, it can be said that the present excitation method is effective for forming a high-output, low-noise amplifier.

[Embodiment 12] FIG. 8 is a schematic perspective view for explaining main components of a waveguide type optical amplifier according to the present invention. In the figure, reference numeral 10 denotes a core portion, 11 denotes a clad portion, and 13 denotes a substrate portion. In this embodiment, the core 10 and the clad 11 are made of fluoride glass, and the core 10 is further doped with 10% by weight of Er. The light of 1.48 μm and the light of 0.86 μm were combined as pumping light into the optical amplifier having such a configuration, and the combined light was input from the core unit 10.

As the concentration of Er added to the core portion 10 increases, the distance between Er in the fluoride glass decreases in proportion thereto, and energy transfer between Er by electric dipole interaction occurs.

FIG. 9 is an energy level diagram for explaining an excited state when the interaction between Er is considered. When the ⁴ I _13/2 level is excited by the excitation light having a wavelength of 1.48 μm, when the Er concentration is high, ( ⁴ I _13/2 → ⁴ I _15/2 ) →
_{^{(4 I 13/2 → 4 I 9/2}} ) frozen Pele Legislative upconversion occurs due to transition, ⁴ after I _9/2 level is excited, ⁴ from ⁴ I _9/2 level I _11/2 The level is relaxed by polyphonon emission, and the ⁴ I _11/2 level is excited. Next,
When the excited state density of the ⁴ I _11/2 level increases, ( ⁴ I
_{^{11/2 → 4 I 15/2) → (}} 4 I 11/2 → 4 I 7/2) occur freeze Pele Legislative up-conversion by the transition, ⁴ F
_7/2 level is excited, and finally ⁴ S _3/2, directly will be energy levels that are not excited is excited by the excitation light of 1.48μm, such as ⁴ F _9/2. Therefore, ⁴ I
_Since the pumping efficiency to the _13/2 level is reduced, 1.55
Light amplification in the μm band does not occur. Therefore, in this embodiment,
Stimulated emission from the ⁴ S _3/2 level to the ⁴ I _11/2 level was caused to increase the excitation density of the ⁴ I _13/2 level.

As a result, the excitation light intensity of 1.48 μm was 1
When the pump light intensity at 50 mW and 0.86 μm was 20 mW, an amplification gain of 30 dB was obtained at 1.55 μm. Since amplification gain could not be obtained only by pumping at 1.48 μm, it was found that the incidence of 0.86 μm light had a remarkable effect on improvement of amplification efficiency.

Here, light for causing stimulated emission of ⁴ S _3/2 → ⁴ I _13/2 was incident. However, in the process described above, ⁴
Since the F _9/2 , ⁴ I _9/2 , and ⁴ I _11/2 levels are also excited, the light that induces stimulated emission from those levels to the ⁴ I _13/2 level is excited by 1.48 μm excitation light. Thus, the effect of improving the amplification efficiency was confirmed even when the light was incident.

[Embodiment 13] The waveguide type optical amplifier of the present embodiment has the structure shown in FIG. 8 similarly to the twelfth embodiment. The core and the cladding are made of telluride oxide glass. 20% by weight of Er was added to the core. When the operation of the optical amplifier having such a configuration was examined, the following results were obtained. That is,
When telluride oxide glass is used for the waveguide material, ⁴
When the I _13/2 level is photoexcited, ⁴ S
_{^{3/2, 4 F 9/2, 4 I}} 9/2, 4 I 11/2 level and the like is excited through interaction between Er. Therefore, when light is incident from the outside and stimulated emission is caused from these levels to the 4 _13/2 level, the ⁴ I _13/2 level can be excited efficiently. Here, 0.8 together with 1.48 μm excitation light.
Light of 75 μm was simultaneously incident, and light amplification of 1.55 μm was performed. As a result, the excitation light intensity of 1.48 μm was 15
When the excitation light intensity at 0 mW and 0.875 μm was 20 mW, an amplification gain of 30 dB was obtained at 1.55 μm. Since the amplification gain was not obtained only by pumping at 1.48 μm, it was found that the incidence of 0.875 μm light had a remarkable effect on the improvement of the amplification efficiency.

In this embodiment, light of 1.48 μm and 0.8
Although 75 μm light is incident on the waveguide from the same direction, it goes without saying that it may be incident on the waveguide from an opposite direction.

[Embodiment 14] A waveguide type optical amplifier according to this embodiment has the structure shown in FIG. 8 similarly to Embodiments 12 and 13, and the core and cladding are made of quartz glass. 1% by weight of Er was added to the core. When the operation of the optical amplifier having such a configuration was examined, the following results were obtained. That is, even when silica glass is used as the waveguide material, when the ⁴ I _13/2 level is optically excited and the Er concentration is high, ⁴ S _3/2 ,
^{The 4} F _9/2 , ⁴ I _9/2 , ⁴ I _11/2 levels and the like are excited through the interaction between Er. Therefore, when light is incident from the outside and stimulated emission is caused from these levels to the ⁴ I _13/2 level, the ⁴ I _13/2 level can be efficiently excited. Here, 0.87 together with 1.48 μm excitation light
Light of μm was simultaneously incident, and light amplification of 1.55 μm was performed. As a result, the excitation light intensity of 1.48 μm was 150 m.
W, when the excitation light intensity of 0.875 μm is 20 mW,
An amplification gain of 30 dB was obtained at 1.55 μm. Since the amplification gain was not obtained only by pumping at 1.48 μm, it was found that the incidence of 0.87 μm light had a remarkable effect on the improvement of the amplification efficiency.

[0070]

As described above, an optical amplifier and a laser according to the present invention use an excitation light source comprising a first light source and a second light source having different wavelengths, and the first light source is erbium. ⁴ I _13/2 is a light source of a wavelength corresponding to the energy difference between the level and the quasi-position higher level of energy than and before the high the energy levels of the
Serial ⁴ I _13/2 ⁴ and to cause a stimulated emission to the level I
It is a light source that increases the excited state density of the _13/2 level . Therefore, even when an infrared transmitting fiber such as a fluoride fiber, which has been difficult in the past, is used as the host of Er, amplification in the 1.55 μm band by so-called 0.98 μm excitation, which can reduce noise, becomes possible. As a result, an optical amplifier having high-bandwidth, flat gain, and low-noise characteristics can be obtained.If the optical amplifier is applied to a communication system, transmission capacity can be increased and the system configuration can be diversified. It can contribute to the spread of optical communication and cost reduction.

[Brief description of the drawings]

FIG. 1 is an energy level diagram of erbium (Er) applied to an optical amplifier and a laser according to the present invention.

FIG. 2 is a block diagram for explaining an embodiment of an optical amplifier according to the present invention, and an optical isolator in the same direction as that shown in the drawing may be interposed in a signal light input unit.

FIG. 3 is a block diagram for explaining an embodiment of a laser according to the present invention.

FIG. 4 is a block diagram for explaining another embodiment of the laser according to the present invention.

FIG. 5 is a block diagram for explaining another embodiment of the optical amplifier according to the present invention, wherein an optical isolator in the same direction as that shown in FIG. 2 may be interposed in the signal light input unit.

FIG. 6 is an energy level diagram of erbium (Er) applied to a conventional optical amplifier and laser.

FIG. 7 is a diagram showing the emission and absorption cross sections between the ⁴ S _3/2 level and the ⁴ I _13/2 level of erbium.

FIG. 8 is a schematic diagram for explaining main components of a waveguide type optical amplifier according to the present invention.

FIG. 9 shows a waveguide type optical amplifier according to the present invention.
FIG. 3 is an energy level diagram for explaining an excited state when an interaction between Er is considered.

FIG. 10 is a diagram showing the excitation density of erbium at the ⁴ S _3/2 level.

[Explanation of symbols]

 1, 2 Excitation light source 3, 4 Optical coupler 5 Er-doped amplification optical fiber 6 Optical isolator 7 Resonant mirror 8 Crystal

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Shoichi Sudo 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-8-116117 (JP, A) JP-A-5-129683 (JP, A) JP-A-6-318754 (JP, A) JP-A-5-181176 (JP, A) Table 6-6-508961 (JP, A) IEICE Spring Conference Lecture Transactions Pt. 4 (1993) p. 4-73 "B-935 Er-doped light (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 3/00-3/30 H04B 10/00 JICST file (JOIS)

Claims (17)

(57) [Claims] 1. An optical amplification medium to which erbium is added ,
A laser including at least an excitation light source for the optical amplifier and utilizing stimulated emission of erbium from the ⁴ I _13/2 level to the ⁴ I _15/2 level, wherein the excitation light sources have at least different wavelengths from each other. A first light source and a second light source , wherein the first light source is ⁴ I _{13/2 of} erbium _;
A light source having a wavelength corresponding to an energy difference between a level and a level having higher energy than the level , and
Stimulated emission from a high level to said ⁴ I _13/2 level
To increase the excited state density of the ⁴ I _13/2 level
Laser, characterized in that that. 2. The method according to claim 1, wherein said first light source is ⁴ I of erbium.
_13/2 level and ⁴ I _11/2 level ^position, 4 _{I 9/2} level ^position, it is a light source of a wavelength corresponding to an energy difference between 4 _{F 9/2} level position or ⁴ S _3/2 position, The laser according to claim 1, wherein: 3. The method according to claim 1, wherein the second light source is ⁴ I of erbium.
_15/2 level and ⁴ I _11/2 level or ⁴ F _9/2
3. The laser according to claim 1, wherein the laser is a light source corresponding to an energy difference from a level. 4. The method according to claim 1, wherein the first light source is ⁴ I of erbium.
A light source that emits light having a wavelength corresponding to the energy difference between the _13/2 level and the ⁴ S _3/2 level, wherein the second light source is the ⁴ I _15/2 level and the ⁴
A light source that emits light having a wavelength corresponding to the energy difference from the I _11/2 level; and a third light source that emits light with a difference in energy between the ⁴ I _15/2 level and the ⁴ I _13/2 level of erbium. 4. A light source for emitting light of a corresponding wavelength.
A laser according to claim 1. 5. The wavelength of the first light source is 0.82 μm to 0.88 μm, and the wavelength of the second light source is 0.96 μm to 0.98 μm. 5. The laser according to claim 4. 6. The erbium-doped optical amplification medium is selected from the group consisting of an erbium-doped fluoride optical fiber, an erbium-doped chalcogenide optical fiber, an erbium-doped telluride oxide optical fiber, and an erbium-doped halide crystal. The laser according to any one of claims 1 to 5, characterized in that: 7. An optical amplification medium to which erbium is added,
An optical amplifier including at least means for introducing and extracting signal light having a wavelength of about 1.5 μm into the optical amplification medium, and an excitation light source for the optical amplification medium, wherein the excitation light sources are at least first light sources having different wavelengths from each other. If consists of a second light source, further, the first light source ⁴ is erbium I _13/2
A light source having a wavelength corresponding to an energy difference between a level and a level having higher energy than the level , and
Stimulated emission from a high level to said ⁴ I _13/2 level
To increase the excited state density of the ⁴ I _13/2 level
An optical amplifier, characterized in that that. 8. The method according to claim 1, wherein the first light source is erbium ⁴ I.
_13/2 level and ⁴ I _11/2 level ^position, 4 _{I 9/2} level position ^is the wavelength of the light source corresponding to an energy difference between 4 _{F 9/2} level position or ⁴ S _3/2 level, The optical amplifier according to claim 7, wherein: 9. The method according to claim 6, wherein the second light source is ⁴ I of erbium.
_15/2 level and ⁴ I _11/2 level or ⁴ F _9/2
The optical amplifier according to claim 7, wherein the optical amplifier is a light source corresponding to an energy difference from a level. 10. The method according to claim 1, wherein the first light source is ⁴ I of erbium.
A light source that emits light having a wavelength corresponding to the energy difference between the _13/2 level and the ⁴ S _3/2 level, wherein the second light source is the ⁴ I _15/2 level and the ⁴
A light source that emits light having a wavelength corresponding to the energy difference from the I _11/2 level; and a third light source that emits light having a wavelength corresponding to the energy difference between the ⁴ I _15/2 level and the ⁴ I _13/2 level of erbium. 8. A light source for emitting light of a corresponding wavelength.
10. The optical amplifier according to any one of claims 9 to 9. 11. The wavelength of the first light source is 0.82 μm.
11. The optical amplifier according to claim 7, wherein a wavelength of the second light source is 0.96 μm to 0.98 μm. 12. 12. The method according to claim 1, wherein the second light source is ⁴ I of erbium.
The optical amplifier according to claim 7, wherein the light source has a wavelength corresponding to an energy difference between _15/2 and ⁴ I _13/2 . 13. The erbium-doped optical amplification medium is selected from the group consisting of erbium-doped fluoride optical fiber, erbium-doped chalcogenide optical fiber, erbium-doped telluride oxide optical fiber, and erbium-doped halide crystal. Claim 7
13. The optical amplifier according to any one of claims 12 to 12. 14. A photomultiplier comprising erbium as an active element for amplification.
In the breadth container, the erbium⁴I_15/2Levels and⁴I_11/2Associate
At least one light corresponding to the energy difference between the ⁴ I _13/2  Energy levels higher than
⁴ I _13/2 With levelLess than the energy difference
And one light,⁴ I_13/2From level⁴I_15/2Stimulated emission transition to level
The signal light amplified by the transfer from the same direction.
Injected into the amplifying medium added with BiI,Said ⁴ I _13/2 Energy higher than the level
Level and the said ⁴ I _13/2 Energy with energy level
At least one light corresponding to the energy difference ⁴
I _13/2 From energy levels higher than the energy level
Said ⁴ I _13/2 Causes stimulated emission to levels An optical amplification method, comprising: 15. An optical amplification method for amplification active element erbium, and light having a wavelength corresponding to an energy difference between ⁴ I _15/2 level and ⁴ I _11/2 level of said ^erbium, 4 S
_3/2 the level and ⁴ I _13/2 of a wavelength corresponding to the energy difference between the level of ^{light, 4} 4 _{I 13/2} level I
_15/2 a signal light and amplified by stimulated emission transition to the state, entering from the same direction to the amplifying medium in which the erbium is added, and the ⁴ S _3/2 level, the ⁴ I _13/2 With level
By light having a wavelength of you corresponds to the energy difference between the ⁴ S
Stimulated emission from the _3/2 level to the ⁴ I _13/2 level
A method for amplifying light, comprising: 16. The ⁴ I _15/2 level of erbium and ⁴
A light having a wavelength corresponding to an energy difference from the I _13/2 level and having a wavelength different from the signal light is incident on the amplification medium from a direction different from the same direction. Item 16. The optical amplification method according to Item 14 or 15. 17. An optical amplification method for amplification active element erbium, and light to a wavelength no 0.82 .mu.m 0.88 .mu.m, and the light is 0.98μm to wavelength is not 0.96 .mu.m, erbium ⁴ I _{13 / 17.} A signal light amplified by a stimulated emission transition from a _two level to a ⁴ I _15/2 level and incident on the erbium-doped amplification medium from the same direction. The optical amplification method according to claim 1.