JPH04235534A - Fiber type 1.3mum band light amplifier - Google Patents

Abstract

PURPOSE:To provide a fiber type light amplifier which directly amplifies light within 1.3mum band. CONSTITUTION:A fiber type 1.3mum band light amplifier uses single mode optical fiber 7 as an amplification medium in the core of which neodymium being a rare earth element having fluorescent characteristics within 1.3mum band is added, and exciting light is irradiated on the fiber 7 together with signal light of wavelength near 1.3mum and light is directly amplified by utilizing an induced emission effect due to the signal light of the excited neodymium and an optical fiber 8 allowing only light of 1.3mum band and the wavelength of the excited light to pass therethrough is installed inside the optical fiber 7 serving as an amplification medium.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to optical fiber type 1.3
This relates to a μm band optical direct amplifier.

0002

BACKGROUND OF THE INVENTION For application to the field of 1.3 μm band optical communications, research is being carried out on functional fibers such as fiber amplifiers and fiber lasers using optical fibers doped with neodymium, which is a rare earth element. For example, literature (E
electronics Letters. 1990,
Vol. 26, pp. 194-195),
Using fluoride optical fiber doped with neodymium,
10d for a small signal at a wavelength of 1.33μm
It has been reported that an amplification gain of B was obtained. In this report, the amplification gain initially continues to increase in proportion to the pump laser output, but when the pump power exceeds 100 mW, the amplification gain saturates, and no increase in gain is observed even if the pump power is increased further. It has been reported that.

[0003] Further, later, in the literature (Electron
ics Letters. 1990, Vol. 26
, pp. 1397-1398), it has been reported that a gain of 5.5 dB can be obtained not only for a weak signal but also for a relatively strong signal of about 1 mW, but in this case as well, the gain is small when the excitation power is 50 mW or more. It has been pointed out that there is a tendency for saturation. Regarding this reason,
In this document, the fluorescence with a wavelength of 1.05 μm that is generated simultaneously with the fluorescence with a wavelength of 1.3 μm gradually increases until the excitation power reaches about 40 mW, but at around 50 mW of excitation power, the emission mechanism changes from a spontaneous emission process to a stimulated emission process. It is explained that due to the transition, the emission intensity at 1.05 μm increases rapidly, and that the fluorescence intensity at 1.3 μm does not increase any further.

The above problems will be explained in more detail below. FIG. 4 is an energy level diagram showing the excited state of neodymium. When used as a 1.3 μm band amplifier, as shown in Figure 4, the 0.82 μm
The energy state is set to the ground level 4 by the band excitation laser.
It is excited from I9/2 to 4F5/2. 4F5/
The energy state of 2 becomes 4F3/ through a non-radiative transition process.
2. Move to state. The transition from 4F3/2 to 4I13/2 provides fluorescence in the 1.32 μm band. At this time, from 4F3/2, in addition to the above, 4I15/2, 4
The transitions to I11/2 and 4I9/2 occur simultaneously. They correspond to emission in the 1.85 μm band, 1.05 μm band, and 0.85 μm band, respectively. The proportion of each transition is
0.85μm: 1.05μm: 1.32μm:
1.85 μm = 10: 55: 10: 25
It is said that That is, other fluorescence intensities are higher than the desired fluorescence in the 1.32 μm band, especially in the 1.05 μm band.
It can be seen that the fluorescence in the μm band is sufficiently large compared to the 1.32 μm band. When the intensity of the excitation laser is increased in a neodymium-doped fiber with such characteristics, the stimulated emission phenomenon first occurs in the 1.05 μm band because the amplification coefficient is the largest in the 1.05 μm band. In the stimulated emission process, the transition probability increases further, so
The above transition ratio becomes even higher at 1.05 μm. Therefore, in order to increase the fluorescence intensity at 1.32 μm,
Even if the intensity of the excitation light is increased, the transition at 1.05 μm will only become stronger, and unless measures are taken to weaken the fluorescence at 1.05 μm, the transition probability in the 1.32 μm band will decrease.

[0005]

[Problems to be Solved by the Invention] The present invention provides a
An object of the present invention is to provide a fiber-type optical amplifier that directly amplifies light in the m band.

[0006]

[Means for Solving the Problems] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that
1.3 μm in the middle of neodymium fiber for band amplification
By inserting a filter that has an optical characteristic of transmitting a band of light but not transmitting other light, 1.
Suppressing light emission based on stimulated emission by preventing light other than 3 μm from propagating in the length direction of the fiber is effective in preventing the saturation phenomenon of fluorescence intensity in the 1.32 μm band. I found out. In other words, if such a filter is not inserted, when the incidentally generated 1.05 μm light propagates through the fiber on the side where the excitation light is incident, and acts on the next neodymium ion, the excited state ion Light of 1.05 μm is emitted from the Such phenomena occur one after another, and the 1.05 μm light continues to be amplified within the fiber. In the end, 1.05
Laser oscillation occurs in μm. When a filter is inserted, all light other than 1.3 μm is absorbed at the insertion point, so the amplification effect is cut off and stimulated emission does not occur.
The 1.3 μm band transition probability is maintained at its initial state, saturation phenomenon can be prevented, and amplification gain can be improved.

[0007]

Embodiments Hereinafter, embodiments of the present invention will be explained in detail with reference to the drawings. FIG. 1 shows the configuration of an embodiment of the present invention. Signal light is input from the terminal 6 of the signal input connector 1, and the light and the light from the excitation semiconductor laser module 2 are multiplexed by the dichroic mirror 6, and input to the neodymium-doped fluoride fiber 7, as well as to the rear of the fiber. Excitation laser light was also input from the terminal side. The inside of the excitation laser module has a configuration in which two 0.82 μm band semiconductor lasers 3 are combined by a polarization beam splitter 4, and after combining, a maximum output of 100 mW can be obtained. Neodymium-doped fluoride fiber as amplification medium 7
The glass material is ZrF4-BaF2-LaF3-Al
F3-NaF, neodymium addition amount 2000p
pm, core diameter 6.5 μm, cladding diameter 125 μm
, the relative refractive index difference is 0.5%, and the length is 4 m. Both ends of the fiber are obliquely polished fiber ends 9 to prevent reflection. Filters 8 are inserted into this fiber at three locations every 1 m. The signal light after amplification is 1
．． 3 μm band narrow band filter 10, 1.3 μm
The m-band spontaneous emission light is cut and output as amplified signal light from the signal output connector terminal 11.

FIG. 2(a) shows the configuration of the filter used here, and FIG. 2(b) shows the transmission characteristics of the filter. This filter 8 has a structure in which a dielectric multilayer filter with a thickness of about 10 μm is inserted between fibers that are obliquely polished to prevent reflection. The insertion loss is 0.8 because the filter film thickness is thinner and the fiber spacing is closer.
It is about dB. The transmission wavelength characteristic of the filter is that it has good transparency at the excitation wavelength and signal wavelength, and has a transmission wavelength of 1.05 μm.
It is characterized by poor transmittance.

FIG. 3 shows the amplification characteristics obtained. In evaluating this amplification characteristic, three types of neodymium-doped fluoride fibers were used. That is, there are three types: one with three filters inserted, one with one filter inserted, and one with no filter inserted. The test conditions were: forward,
Pumping light with an output of 50 mW was guided into the fiber from the rear. As can be seen from FIG. 3, it can be seen that the amplification characteristics are dramatically improved by inserting the filter. It was also found that even if the number of filters is increased further, the gain does not increase any further. This means that if the amplification factor at the fiber length corresponding to the spacing between filters is smaller than the loss value of the filter, even if more filters are inserted, it will be useful to improve the amplification factor in the 1.3 μm band. It means that it will not stand. In fact, even if the number is increased, the insertion loss increases, which has the opposite effect. From the above results, by appropriately inserting filters, 10
It was found that a maximum of 18 dB can be obtained with an excitation input of about 0 mW.

0010

As explained above, the fiber-type 1.3 μm band amplifier of the present invention uses a 0.8 μm band laser diode of about 100 mW to generate a
It became clear that a gain of 8 dB could be obtained.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

FIG. 2(a) is a configuration diagram of a filter used in an embodiment of the present invention. (b) is a characteristic diagram of the filter shown in (a).

FIG. 3 is a diagram showing gain versus pump power.

FIG. 4 shows energy levels of neodymium-doped fluoride fibers.

[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 Neodymium-doped fluoride fiber 8 Filter 9 Obliquely polished fiber end 10 Narrowband filter 11 Signal output connector Terminal 12 Narrow band filter plate 13 Fixing member

Claims (1)

[Claims] [Claim 1] A single-mode optical fiber whose core is doped with neodymium, a rare earth element that has fluorescence characteristics in the 1.3 μm band, is used as an amplification medium, and the optical fiber is pumped with signal light having a wavelength of around 1.3 μm. In an optical amplifier that directly amplifies the light by inputting the optical beam and utilizing the stimulated emission effect of the signal light of neodymium in the excited state, only the light in the 1.3 μm band and the wavelength of the pumping light are transmitted to the optical fiber, which is the amplification medium. A fiber-type 1.3 μm band optical amplifier characterized by having a built-in optical filter that can pass through.