JPH0758377B2 - Optical fiber type optical amplifier - Google Patents

Description

The present invention relates to an optical fiber type optical amplifier which is small in size, has a high gain, and directly amplifies a 1.5 μm band signal light.

[Prior Art] Conventionally, as an excitation light source for an Er-doped optical fiber type optical amplifier, one that matches the absorption wavelength region generated by the energy level peculiar to Er (erbium) as shown in FIG. 7 has been used. , The optical excitation of the excitation light source _causes ⁴ I _13/2 in this figure.
Amplification is caused by forming an inversion distribution between the energy levels of − ⁴ I _15/2 . Here, as the wavelength that can be excited, the Ar ion laser (514.5 nm) indicated by the arrow in FIG. 7 (see the following Document 1), the color difference laser (650
nm) (see Reference 2 below), color center laser (1.49 μm ~
1.50 μm) (see Reference 3 below) and 0.8 μm band semiconductor lasers (see References 4 and 5 below) have been known.

Reference 1: E. Desurvire et al., “High-gain erbium-dope
d traveling−wave fiber amprifier ", Optics Letters
vol.12, p.888 (1987).

Reference 2: RJMears et al., “Low−noise erbium−doped
fiber amplifier operating at 1.54μm ", Electronics
Letters vol.23, p.1026 (1987).

Reference 3: E. Snitzer et al., “Erbium fiber laser amplif
ier at 1.55μm with pump at 1.49μm and Yb sensiti
zed Er oscillation ", OFC'88 (New Orleans), Postdead
line Paper PD2 (1988).

Reference 4: TJ Whitley et al. “1.54 μm Er ³⁺ −doped fiber
r amplifier potically pumped at 807nm ", European Co
nference on Optical Communication 88, p.58 (1988
Year).

Reference 5: DC Hanna et al., “Efficient operation of an
Yb−sensitized Er fiber laser pumped in 0.8 μm re
gion ", Electronics Letters vol.24, p.1068 (1988
Year).

For example, when pumping with Ar ion laser,
It _{makes a} transition from ⁴ I _{15/2 in the} ground state to ² H _11/2 , then emits phonons and _{makes a} transition to ⁴ I _13/2 . ² H _11/2 and ⁴ I in this process
The energy difference of the _13/2 level is lost as heat and passes through each level, which causes a time delay in the formation of the population inversion. That is, the closer to the signal light wavelength, the more efficiently there is no time delay and the population inversion can be excited. Therefore, excitation at 1.49 μm to 1.50 μm is suitable.
Up to now, there have been only reports on the color center laser of the above-mentioned reference 3. From the point of downsizing, except for the semiconductor laser in the 0.8 μm band, all of them are not practical because the pump light source becomes large. Therefore, in order to realize a compact and efficient optical amplifier, the 0.8 μm band is required. There was no other way than using the semiconductor laser of.

[Problems to be Solved by the Invention] However, in the above-mentioned conventional example, from the viewpoint of the propagation mode, since the excitation efficiency is proportional to the energy density of the excitation light, it is desirable to have a single mode at the excitation wavelength. The Er-doped optical fiber has a cutoff wavelength of 1.1 μm to 1.4 μm so that a single mode is produced at 1.5 μm of the signal light.
Had been set between. In this case, since the semiconductor laser light in the 0.8 μm band described above is excited to a higher mode, the propagating light beam spreads and the energy density of the excitation light becomes low. On the other hand, in an optical fiber that becomes a single mode at an excitation wavelength of 0.8 μm, on the contrary, the bending loss in the 1.5 μm band cannot be ignored, so that the gain with respect to the bending of the fiber becomes unstable. Therefore, the excitation wavelength is
Since an optimum fiber structure cannot be obtained with a 0.8 μm band semiconductor laser, a compact and efficient optical amplifier could not be actually realized.

Furthermore, due to the energy level peculiar to Er, when excited at 0.8 μm, a part of the excited electrons of ⁴ I _13/2 are excited to a _higher level, and excitation absorption occurs in which the population inversion is reduced. Was not optimal, and therefore it was necessary to take a method such as co-adding Yb (ytterbium) (see the above-mentioned document 5 and the following document 6). Even in this case, since the energy transfer between Yb and Er ions is used, the loss always occurs.

Reference 6: ME Fermann et al., “Efficient operation of
an Yb−sensitized Er fiber laser at 1.56μm ", Elect
ronics Letters vol.24, p.1135 (1988).

Further, according to the experimental result by E. Snitzer et al. Of the above-mentioned reference 3 using the color center laser as the excitation light source of the excitation wavelength of 1.49 μm to 1.50 μm, as shown in FIG. No gain was obtained. For this reason, in the Er-doped optical fiber amplifier, pumping by the semiconductor laser in the wavelength region shorter than 1.49 μm has not been attempted until now, and pumping between two levels is physically unsuccessful. Was considered possible.

In view of the above-mentioned problems, an object of the present invention is to provide an optical fiber type optical amplifier which is compact and highly efficient and has an amplification degree of 10 dB or more.

[Means for Solving the Problems] In order to achieve such an object, the present invention provides an excitation light source having an oscillation wavelength region of 1.4 μm within a range of 1.40 μm to 1.49 μm.
A semiconductor laser module (1) which outputs m-band semiconductor laser light and is modularized so that the output can be taken out by a fiber, and an optical fiber coupler for combining the semiconductor laser light and the 1.5 μm band signal light, or It consists of a waveguide type coupler (2), a core part with a relative refractive index difference that inputs the output of this coupler (2) and Er (erbium) of rare earth element added, and a clad part that does not contain Er.
An optical fiber coupler or a waveguide coupler (Er-doped optical fiber (3)) for demultiplexing the pumping light and the signal light from or at the end of the Er-doped optical fiber (3). 2A) is obtained by using the above Er-doped optical fiber (3) as an amplification medium to obtain amplified signal light.

[Operation] In the present invention, the oscillation wavelength region output from the semiconductor laser module is 1.4 μm within the range of 1.40 μm to 1.49 μm.
The band semiconductor laser light is used as the excitation light source, and the semiconductor laser light and the 1.5 μm band signal light are combined by the optical fiber coupler or the waveguide type coupler, and the combined light of the excitation light and the signal light is subjected to relative refraction. Since it is incident on an Er-doped optical fiber consisting of a core portion doped with Er of rare earth element and a cladding portion not containing Er, the Er-doped optical fiber is used as an amplifying medium and the end of the Er-doped optical fiber is terminated. Or the amplified signal light is obtained from the optical fiber coupler or the waveguide type coupler that is connected to this end and splits the pumping light and the signal light.

As described above, in the present invention, since the high-power semiconductor laser of 1.4 μm band is used as the pumping light source, it is possible to solve the problem of miniaturization of the pumping light source, which has been a problem in the related art, and to prevent the decrease of the amplification degree due to the pumping absorption. A possible Er-doped optical fiber type optical amplifier can be easily constructed.

In addition, because the semiconductor laser module modularized so that the output light can be taken out by the fiber and the optical fiber coupler are combined, each fiber can be fused or simply connected by a connector. Can be configured easily. Furthermore, 1.4μ
The excitation by m band is possible, ⁴ I _13/2 - is caused by more complex than the distribution of energy levels previously thought between ⁴ I _15/2, Part ⁴ I ₁₃ It is significantly different from the prior art in that it uses the splitting of the level of _{/ 2} .

Embodiments Embodiments of the present invention will be described in detail below with reference to the drawings.

A. First Embodiment FIG. 1 shows the configuration of the first embodiment of the present invention.

In Fig. 1, 1 is modularized so that the output can be taken out by a fiber, and the oscillation wavelength region for excitation is 1.40 μm.
From 1.49μm to 1.4μm semiconductor laser module, 2 is the output pump light of this semiconductor module 1 and 1.5μm
An optical fiber coupler or a waveguide type coupler for multiplexing the m-band signal light, and 3 are Er-doped optical fibers for amplification in which Er is added to the core portion for inputting the output light of the coupler 2. The cladding of the optical fiber 3 does not contain Er.

Output fiber 4 of 1.4 μm semiconductor laser module 1
Is connected to the input terminal of the optical fiber coupler or the waveguide coupler 2, and the excitation light is made incident on the Er-doped optical fiber 3. On the other hand, 1.5 introduced from the input fiber 5
The μm band signal light is fed from the other input terminal of the optical fiber coupler or waveguide type coupler 2 to the Er-doped optical fiber 3
Incident on. Thereby, the amplified signal light is obtained from the terminal end of the Er-doped optical fiber 3.

FIG. 2 shows the oscillation spectrum of the 1.4 μm band semiconductor laser module 1 and the absorption spectrum of the Er-doped optical fiber 3 used in this example. It can be seen that when a semiconductor laser beam having the center of the excitation wavelength at the wavelength of 1.48 μm shown by the solid line is used, it overlaps with the intrinsic absorption of Er shown by the broken line. Wavelength 1.48 μm,
From the absorption peaks at 1.53 μm and 1.55 μm, there are 3 transitions at room temperature, 1.48 μm from the ground state.
It is considered that three levels are formed between the levels corresponding to 1.53 μm or 1.55 μm. Therefore, 1.4 μm
The population inversion is formed in the Er-doped optical fiber 3 by the excitation light of the band, the stimulated emission by the signal light occurs, and the amplified signal light is obtained from the terminal end of the optical fiber 3.

FIG. 3 shows the coupling characteristics of the optical fiber coupler 2 of this embodiment used for multiplexing the pumping light and the signal light of the 1.4 μm band semiconductor laser module 1. As shown in Fig. 3, 1.48μ
80% in m band, 20-30% between 1.53μm and 1.55μm
Has a coupling efficiency of. Ideally, the excitation wavelength is 100
%, A coupling efficiency of 0% at the signal light wavelength is desirable, but it is difficult in practice.

FIG. 4 shows the experimental results regarding the fiber length dependence of the amplification degree obtained in this example. In this experiment, fibers with different Er concentration were prepared. As an example, fiber No. 1 was 1300 pp.
m, Fiber No. 2 has 1000 ppm Er added to its core. Excitation light intensity is No.1 and No.2 Er-doped optical fiber 3
Was 32 mW at the incident end of. The amplification degree (dB) is the ratio of the signal light intensity at the entrance end and the exit end of the Er-doped optical fiber.

As is clear from Fig. 4, fiber No. 1 and fiber No. 2
A gain of about 12 dB is obtained. Further, the fiber length dependency of the amplification degree depends on the Er concentration, and the saturation of the amplification degree found in the fiber No. 1 occurs because the excitation light is absorbed by Er and attenuates on the emission side. From the above experimental results, it was confirmed that a compact and easy-to-handle optical fiber type optical amplifier having an amplification degree of 10 dB or more can be easily realized in the present embodiment.

B. Second Embodiment FIG. 5 shows the configuration of the second embodiment of the present invention. In this embodiment, an optical fiber coupler or a waveguide coupler 2A is further connected to the terminal end of the Er-doped optical fiber 3 in the configuration of the above-mentioned first embodiment so that signal light and pumping light can be separated. belongs to.

The amplification operation of this embodiment is the same as that of the first embodiment, and the optical fiber coupler or the waveguide type coupler 2A having the same characteristics as the coupler 2 of the preceding stage is used. This coupler
Signal light that is amplified and separated from the pumping light is obtained from the 2 A output fiber 6.

C. Third Embodiment FIG. 6 shows the configuration of the third embodiment of the present invention. In this embodiment, the pumping light of the semiconductor laser module 1A similar to that in 1 is made incident from the signal light emitting side of the coupler 2A in the configuration of the above-mentioned second embodiment through the fiber 7, and the This is an example of the configuration shown in FIG. 4 in which saturation of the amplification degree due to excitation on only one side does not occur.

[Effects of the Invention] As described above, according to the present invention, since the 1.4 μm band semiconductor laser module, the optical fiber coupler and the Er-doped optical fiber are used, the amplification factor is 10 dB or more without insertion loss. Further, it is possible to provide an optical fiber type optical amplifier which is compact, easy to handle, and highly efficient.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the configuration of the first embodiment of the present invention, and FIG. 2 shows the oscillation spectrum of the pumping 1.4 μm band semiconductor laser and the absorption characteristics of the Er-doped optical fiber in the first embodiment of the present invention. FIG. 3 is a graph showing the coupling characteristics of the optical fiber coupler for demultiplexing and multiplexing in the first embodiment of the present invention, and FIG. 4 is the fiber length dependence of the amplification degree in the first embodiment of the present invention. FIG. 5 is a schematic diagram showing the configuration of the second embodiment of the present invention, FIG. 6 is a schematic diagram showing the configuration of the third embodiment of the present invention, and FIG. 7 is the excitation energy level of Er ions. And FIG. 8 is a graph showing the experimental results of the conventional Snitzer et al. 1,1A: 1.4 μm band semiconductor laser module, 2,2A: Optical fiber coupler or waveguide coupler, 3 ... Er-doped optical fiber, 4-7 ... Fiber.

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Claims (1)

[Claims] 1. A semiconductor laser module as a pumping light source, which is modularized so as to output a 1.4 μm band semiconductor laser light having an oscillation wavelength range of 1.40 μm to 1.49 μm, and the output can be taken out by a fiber, An optical fiber coupler or a waveguide type coupler for combining semiconductor laser light and signal light of 1.5 μm band, and a core portion to which Er (erbium) of a rare earth element having a relative refractive index difference for inputting the output of the coupler is added. And an Er-doped optical fiber consisting of a cladding portion not containing Er, and an optical fiber coupler or waveguide for demultiplexing the pumping light and the signal light from the end of the Er-doped optical fiber or connected to the end. An optical fiber type optical amplifier, wherein signal light amplified by using the Er-doped optical fiber as an amplification medium is obtained from a mold coupler.