JPH06232490A - 1.3mum band amplifying optical fiber and optical fiber amplifier - Google Patents

Abstract

PURPOSE:To provide an optical fiber for optical direct amplification in a specific mum band and an optical fiber amplifier using it. CONSTITUTION:Amplifying optical fiber 7 composed of a core formed of optical glass that contains neodymium (Nd<3+>) and thulium (Tm<3+>) and a clad which surrounds the core and has lower diffraction factor than the core is used. When thulium is added, since all 1.05mu beams are absorbed by thulium ion, induced emission is not allowed and a 1.3mum band transition probability is kept in the initial condition. Thus, saturating phenomenon is prevented and amplification gain is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 1.3 .mu.m band amplifying optical fiber and an optical fiber amplifier using the same.

[0002]

2. Description of the Related Art For application to the optical communication field of 1.3 μm band,
Research has been conducted on functional fibers such as fiber amplifiers and fiber lasers using optical fibers doped with neodymium, which is a rare earth element. For example, in the literature (Y. Miyajima,
T.Komukai, and T.Sugawa, "1.31-1.36μm optical a
mplification in Nd ³⁺ -doped fluorozirconate fibr
e, ", Electronics Letters, 1990, vol.26, pp.194-195)
Is a fluoride optical fiber doped with neodymium.
It is reported that the amplification gain of was obtained. In this report, the amplification gain continues to increase in proportion to the output of the pump laser, but when the pump power exceeds 100 mW, the amplification gain saturates, and even if the pump power is further increased, the gain increases. Not reported.

After that, the literature (Y. Miyajima, T. Suga
wa, and T. Komukai, "Efficient 1.3 μm band amplif
ication in Nd ³⁺ -doped single-mode fluoride fibr
e, "Electronics Letters, 1990, vol.26, pp.1397-139
In (8), it was reported that a gain of 5.5 dB could be obtained not only for a small signal but also for a relatively strong signal of about 1 mW.
It has been pointed out that the gain tends to saturate above 50 mW. For this reason, in this document, the fluorescence with a wavelength of 1.05 μm generated at the same time as the fluorescence with a wavelength of 1.3 μm gradually increases up to an excitation power of about 40 mW, but the emission mechanism changes from the spontaneous emission process to the stimulated emission process near the excitation power of 50 mW. , The emission intensity of 1.05 μm increases rapidly,
With this, it is explained that the fluorescence intensity of 1.3 μm does not increase any more.

[0004]

The above problems will be described in more detail.

FIG. 2 is an energy level diagram showing the excited state of neodymium. When used as an amplifier in the 1.3 μm band, as shown in FIG. 2, the energy state is changed from the ground level ⁴ I _9/2 to ⁴ by an excitation laser in the 0.8 μm band.
Excited to F _5/2 . The energy state of ⁴ F _5/2 is
It transits to ⁴ F _3/2 state through non-radiative transition process. ⁴ F _3/2
The transition from 1 to ⁴ I _13/2 gives fluorescence in the 1.32 μm band. At this time, from ⁴ F _3/2 , in addition to the above, ⁴ I _15/2 , ⁴ I
_11/2, and transition to ⁴ I _9/2 occurs simultaneously. They correspond to light emission in the 1.85 μm band, 1.05 μm band, and 0.85 μm band, respectively. The ratio of each transition is 0.85 μm: 1.05 μm: 1.32
It is said that μm: 1.85 μm = 10: 55: 10: 25 (%). That is, it can be seen that the other fluorescence intensities are larger than the desired fluorescence in the 1.32 μm band, and particularly, the fluorescence in the 1.05 μm band is sufficiently larger than that in the 1.32 μm band. When the intensity of the pumping laser is increased in the neodymium-doped fiber having such characteristics, the stimulated emission phenomenon first occurs in the 1.05 μm band because the amplification coefficient in the 1.05 μm band is the largest. Since the transition probability is further increased in the stimulated emission process, the above transition ratio is even higher at 1.05 μm. Therefore, 1.32
In order to increase the fluorescence intensity at μm, even if the intensity of the excitation light is increased, the 1.05 μm transition is intensified, and the transition probability in the 1.32 μm band is reduced unless the device for weakening the 1.05 μm fluorescence is taken.

An object of the present invention is to provide an optical fiber for performing direct optical amplification in the 1.3 μm band and an optical fiber amplifier using the same.

[0007]

Means for Solving the Problems In order to achieve the above object, in claim 1, neodymium (Nd
³⁺ ) and thulium (Tm ³⁺ ) ions are included in the core, and the cladding surrounds the core and has a lower refractive index than the core. Further, in claim 2, the amplification optical fiber according to claim 1, which propagates the signal light in the wavelength band of 1.3 μm, the pumping light source in the wavelength band of 0.8 μm, and the pumping light from the pumping light source in the optical fiber. And optical means for making the light incident on. Further, in claim 3, the wavelength is 1.3 μm.
The amplification optical fiber according to claim 1, which propagates a band signal light, two pumping light sources of wavelength 0.8 μm band and wavelength 1.58 μm band, and pumping light from the pumping light source is made incident on the optical fiber. And optical means.

[0008]

SUMMARY OF] The present inventors, as a result of intensive studies for solving the above problems, codoped thulium Some 1.3 [mu] m band amplifying neodymium doped fiber, ³ of thulium ions H
_It was found that 1.05 μm light generated in neodymium can be absorbed by absorption accompanying the transition of ₄ → ³ F ₂ , and this is extremely effective for improving the amplification gain of a 1.3 μm amplifier.

That is, in the case where thulium is not co-doped, an incidentally generated 1.05 μm light propagates in the fiber on the side where the excitation light is incident, and acts on the next neodymium ion to produce an ion in the excited state. Emits 1.05 μm of light. Such phenomena occur one after another, and 1.05 μm light continues to be amplified in the fiber. Finally, laser oscillation is reached at 1.05 μm. On the other hand, when thulium is added together, 1.
Since all of the 05 μm light is absorbed by thulium ions, it does not lead to stimulated emission, and the 1.3 μm band transition probability keeps the initial state, which can prevent the saturation phenomenon and improve the amplification gain.

This will be described in detail with reference to the energy level diagram. FIG. 3 shows an energy level diagram of neodymium and thulium ions. Regarding neodymium, as described in the explanation of Fig. 2, by excitation in the 0.8 µm band.
Emissions of 1.32 μm and 1.05 μm occur (in the present description, the 0.85 μm band and the emission of 1.85 μm are ignored). On the other hand, with respect to thulium, the ³ F ₄ level is excited by the excitation in the 0.8 μm band. In this state, the light emission of 2.3 μm or 1.47 μm is caused to transit to the ³ H ₄ level. This ³ H ₄ level thulium ion is
It absorbs light in the 1.05 μm band and transitions to the ³ F ₂ or ³ F ₃ level. This absorption transition is very strong and naturally absorbs 1.05 μm light generated by neodymium ions. By adding neodymium and thulium ions to the fiber at a certain predetermined concentration, 1.05 μm light can be absorbed at the same time as it is generated, and the stimulated emission effect of 1.05 μm light disappears, so the amplification gain in the 1.3 μm band decreases. Can be prevented.

Further, the thulium ion which has absorbed 1.05 μm light and transited to the ³ F ₂ or ³ F ₃ level transits to the ³ F ₄ level by phonon relaxation, and then again emits 2.3 μm or 1.47 μm to emit ³ μm. Transition to H ₄ level.
At this time, part of it is 0.82μ from ³ F ₄ level directly to ³ H ₆ .
The 0.82 μm light contributes to the absorption of the neodymium ion from the ground level to the ⁴ F _5/2 level, though the transition occurs with the emission of the m band.

[0012]

1 shows a first embodiment of the present invention. In the figure, 1 is a signal input connector, 2 is a pumping semiconductor laser module, 3 is a 0.82 μm band semiconductor laser, 4 is a polarization beam splitter, 5 is a lens, 6 and 8 are dichroic mirrors, 7 is an amplification fiber, 9 is an end of the obliquely polished fiber, 10 is a narrow band filter, and 11 is a signal output connector terminal.

Signal light is made incident from the signal input connector terminal 1, and the light and the light from the pumping semiconductor laser module 2 are combined by the dichroic mirror 6 via the lens 5 and neodymium / thulium co-doped from the fiber end 9. In addition to being incident on the fiber 7, the light from the pumping semiconductor laser module 2 was also incident from the rear end 9 of the fiber 7 via the lens 5 and the mirror 8. The inside of the pumping semiconductor laser module 2 is two 0.82 μm band semiconductor lasers 3.
The light from the above is multiplexed by the polarization beam splitter 4, and an output of maximum 100 mW can be obtained after the multiplexing. The neodymium / thulium co-doped fiber 7 as an amplification medium is a fluoride glass whose glass material is ZrF4-BaF2-LaF3-AlF3-NaF, and has a neodymium addition amount of 500 ppm and thulium addition amount of 2000 ppm, a core diameter of 6.5 μm, and a cladding diameter. 125
μm, relative refractive index difference is 0.5%, and length is 4 m. Fiber 7
Both ends 9 are obliquely polished to prevent reflection. The signal light after amplification is 1.3 μm narrow band filter 10
The μm band spontaneous emission light is cut and the amplified signal light is output from the output connector terminal 11.

FIG. 4 shows the obtained amplification characteristic. Three types of neodymium-doped fibers were used as amplification fibers. That is, (A) fiber doped with only 500ppm neodymium, (B) 2000ppm neodymium and thulium.
Fiber doped with 2000ppm, and (C) neodymium
3 for fiber doped with 500ppm and 2000ppm thulium
It is a kind. As the test conditions, 50 mW of pumping light was guided into the fiber from the front and the back, respectively. Signal wavelength is 1.33μm, signal input intensity in fiber is -20d
Bm. As can be seen from the figure, amplification characteristics are dramatically improved by co-adding thulium. The reason why the amplification gain of fiber (C) is higher than that of fiber (B) is that the absorption coefficient of 0.8 μm light is larger for neodymium than thulium, and when 0.8 μm excitation light is used for most of the 0.8 μm excitation light. Is considered to be absorbed only by neodymium and the effect of co-adding thulium disappears.

FIG. 5 shows a second embodiment of the present invention.
The basic configuration is the same as in FIG. 1, but an excitation light source with a wavelength of 1.58 μm is used as the excitation light from the rear. That is,
The pumping semiconductor laser module 2'on the rear side has two units.
1.58 μm band semiconductor laser 3'and these semiconductor lasers 3 '
And a polarization beam splitter 4'which multiplexes the light from the. Although a high-power semiconductor laser was used as the pumping light source here, an Er-doped fiber laser using a 1.48 μm band high-power semiconductor laser as the pumping light source, or 1.58 μm
Light obtained by amplifying m light with a 1.55 μm band optical amplifier may be used.

As a light source in the 1.58 μm band, light in the range from 1.55 μm band to 1.63 μm band is effective, but 1.58 μm
m is the most efficient.

As a preliminary examination before confirming amplification, fluorescence was measured by exciting with 0.82 μm from the front and 1.6 μm from the rear. The results are shown in FIG.

FIG. 6 (a) shows the case where only 50 mW 0.82 μm pumping light was used for the neodymium / thulium co-doped fiber, and at this time, the output level of 1.05 μm was -29.18 dBm. Next, from the rear at the same time as 50mW 0.82μm excitation light
Fluorescence when 5 mW of 1.6 μm light is incident is shown in (b). At this time, the output level of 1.05 μm light is -45.67 dBm
Is. By injecting 1.6 μm light, the output level of 1.05 μm light is reduced by about 16.5 dB.

FIG. 7 is a diagram for explaining the mechanism. Light of 1.58 μm (here, 1.6 μm) causes Tm ³⁺
Excite the ions to the ³ H ₄ level. ³ H ₄ level Tm ³⁺
Ions absorb 1.05μm light generated from Nd ^{3+, 3}
Further excitation to the F ₂ level. In this process, Nd
^Since Tm ³⁺ ions absorb 1.05 μm light generated by ³⁺ ,
1.3 μm amplification will be unaffected. Furthermore, comparing with Fig. 6 (a) and (b), the output level of 1.32μm light is
It is higher in (b), which confirms that 1.3 μm light becomes stronger by attenuating 1.05 μm light in Tm ³⁺ ions.

With this experimental configuration, an amplification experiment using 1.33 μm light as a signal revealed that a gain of about 20 dB or more could be obtained.

From the above results, it was found that a maximum of 18 dB can be obtained with an excitation input of about 100 mW by co-adding neodymium and thulium at an appropriate concentration.

[0022]

As described above, according to the present invention,
With a 0.8 μm band laser diode with an output of about 100 mW,
It was revealed that a maximum gain of about 18 dB can be obtained. In the examples reported in conventional papers and the like, the maximum gain is about 10 dB, and by using the present invention, an amplification gain higher than the conventional one can be achieved.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the energy level of a neodymium-doped fiber.

FIG. 3 is a diagram for explaining how 1.05 μm light generated by neodymium is absorbed by thulium.

FIG. 4 is a diagram showing gain with respect to pump power.

FIG. 5 is a diagram showing a configuration of an optical fiber amplifier according to a second embodiment of the present invention.

FIG. 6 is a diagram showing a result of measuring fluorescence.

FIG. 7 is a diagram explaining how 1.05 μm light generated by neodymium is absorbed by thulium.

[Explanation of symbols]

1 ... Signal input connector, 2 ... Excitation semiconductor laser module, 3 ... 0.82 μm band semiconductor laser, 4 ... Polarization beam splitter, 5 ... Lens, 6 ... Dichroic mirror,
7 ... Amplifying fiber, 9 ... Obliquely polished fiber end, 10 ...
Narrow band filter, 11 ... Connector terminal for signal output.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical indication location H01S 3/07 8934-4M 3/094 3/10 Z 8934-4M (72) Inventor Fukasaku Yoshito 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. An active substance, neodymium (Nd ³⁺ ).
An optical fiber for 1.3 μm band amplification, comprising: a core made of optical functional glass containing thulium (Tm ³⁺ ) ions; and a clad surrounding the core and having a refractive index lower than that of the core. 2. The amplification optical fiber according to claim 1, which propagates a signal light in the wavelength band of 1.3 μm, a pumping light source in the wavelength band of 0.8 μm, and pumping light from the pumping light source in the optical fiber. An optical fiber amplifier, comprising: an optical means for making the light incident. 3. The amplification optical fiber according to claim 1, which propagates a signal light having a wavelength of 1.3 μm band, two pumping light sources of wavelength 0.8 μm band and wavelength 1.58 μm band, and pumping light from the pumping light source. And an optical unit for making the light incident on the optical fiber.