JP3053255B2 - Optical fiber amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of mixing signal light and pump light with an optical coupler (optical multiplexer) and then mixing the mixed light with Er (erbium).
The present invention relates to an optical fiber amplifier which guides a rare earth-doped fiber doped with a rare earth element such as the above to amplify signal light by a stimulated emission effect.

[0002]

2. Description of the Related Art FIG. 4 is a schematic diagram showing a conventional optical fiber amplifier of this type.

In the figure, reference numeral 2 denotes a signal light incident connector,
Is an optical fiber for signal light incidence, and 6 is a semiconductor laser (LD)
Pump light source 8; pump light incidence optical fiber 10; optical coupler 10 for mixing signal light propagating through signal light incident optical fiber 4 with pump light propagating through pump light incident optical fiber 8; An optical isolator that almost completely blocks reflected light and allows light to pass only in one direction; 14 is a rare-earth doped fiber having a function of amplifying signal light by stimulated emission effect; 16 is also an optical isolator; and 18 is the wavelength of excitation light A band-pass filter that cuts off the component and the fluorescent component and allows only the wavelength component of the signal light to pass therethrough. Reference numeral 20 denotes a signal light emitting connector.

In this example, it is assumed that the rare earth element doped into the rare earth doped fiber 14 is Er (erbium). In the case of Er, the signal light having a wavelength of 1.55 μm is considered to have the highest amplification factor, and the pump light in that case is in the wavelength of 1.48 μm. The optical fiber comprises a core and a cladding on the outer periphery thereof, and Er is doped on the inner and outer peripheral surfaces of the core. Also, 1.55 μm
The wavelength range of the pump light is 1.48 μm for the signal light in the wavelength range.
And arrange both wavelength ranges as close as possible.
I have. This is due to the following reasons. That is,
In rare-earth doped fibers, the
And the wavelength range of the excitation light
However, it has the characteristic that its amplification efficiency is improved.
It is.

Next, the operation will be described.

[0006] Excitation light in the 1.48 μm wavelength band emitted from the excitation light source 6 is incident on the rare earth doped fiber 14 via the excitation light incidence optical fiber 8, the optical coupler 10 and the optical isolator 12. Er doped in the rare earth doped fiber 14 is pumped (excited) by the excitation light.

In such an excited state of Er, the modulated signal light in the 1.55 μm wavelength band is incident on the signal light incident optical fiber 4 from the transmission optical fiber (not shown) via the signal light incident connector 2, Further, the light is mixed with the pump light by the optical coupler 10 and is incident on the rare-earth doped fiber 14 via the optical isolator 12. The signal light is directly emitted by the stimulated emission effect in the process of traveling in the rare-earth doped fiber 14 (without being converted into an electric signal, and remains in a light state).
Amplified. The amplified signal light and a part of the pump light are passed through the optical isolator 16 to the bandpass filter 1.
Enter 8. The band-pass filter 18 cuts off the excitation light and the fluorescence component, passes only the signal light, and guides the signal light to a transmission optical fiber (not shown) via the signal light output connector 20.

The optical isolators 12 and 16 prevent laser oscillation due to reciprocal reflection of light at the junction between the rare-earth doped fiber 14 and a normal optical fiber.

[0009]

The problem to be solved by the present invention is that the rare-earth doped fiber 14 amplifies not only the signal light but also a part of the pump light as a result of the noise of the signal light. The point is that the characteristics are deteriorated. Hereinafter, this will be described in detail.

As shown in FIG. 5A, the spectrum of the signal light is sharp within a very narrow wavelength band even in the 1.55 μm wavelength band. On the other hand, when the pumping light source 6 is a semiconductor laser and viewed in the order of 1/100 μm, as shown in FIG. 5B, the spectrum of the pumping light in the 1.48 μm wavelength band is considerably wide. It spreads in the range, and its tail reaches 1.55 μm, which is the central wavelength of the signal light.

When the signal light and the pump light having such spectra are mixed in the optical coupler 10, the spectrum of the mixed light becomes as shown in FIG. FIG. 5D is an enlarged view of the spectrum around 1.55 μm. Most of the 1.55 μm component of the mixed light is signal light, but slightly contains pump light.

When the light component near 1.55 μm is amplified by the stimulated emission effect in the rare earth doped fiber 14, the result is as shown in FIG. At just 1.55 μm, S ₁ is the amplification of the signal light, S ₂ is the amplification of the tail of the excitation light, and S ₃ is the amplification of the fluorescent component (ASE). The sum (S ₁ + S ₂ + S ₃ ) of these is regarded as the amplified signal light. This is not separated by the band-pass filter 18 and is output from the signal light output connector 20 to the transmission optical fiber as it is. This is because the band-pass filter 18 passes the 1.55 μm component.

As a result, (S ₂ + S ₃ ) is mixed as an unnecessary noise component. That is, the noise characteristics (S / N ratio and noise figure NF) of the signal light are degraded. In order to solve such problems, the wavelength of the excitation light
The region should be located as far as possible from the wavelength range of the signal light.
However, for rare-earth doped fibers,
As described above, the wavelength range of the pump light and the signal light
The higher the amplification efficiency, the higher the amplification efficiency.
In rare earth doped fiber having such features as:
Even if the noise characteristics of signal light are improved,
Place the light wavelength range as far as possible from the signal light wavelength range
This can be done to reduce the amplification efficiency.
There was no.

The present invention has been made in view of such circumstances, and eliminates the influence of the trailing component of the pump light on the central wavelength of the signal light without deteriorating the amplification efficiency. It is an object of the present invention to improve the noise characteristics in the signal light of the above.

[0015]

According to the present invention, a signal light incident optical fiber and an excitation light incident optical fiber derived from an excitation light source are coupled via a melt-stretched optical coupler, and the optical coupler is poor. Ri Na and rare-earth doped fiber for amplifying the signal light to the side is connected to the wavelength band of the signal light excited
In an optical fiber amplifier arranged as close as possible to a wavelength region of light emission, a wavelength selection filter that cuts a component near a central wavelength of signal light and passes a wavelength component of excitation light in the optical fiber for excitation light incidence. Is interposed.

[0016]

The pumping light emitted from the pumping light source to the pumping light-injecting optical fiber in the state of a spectrum having a tail component to the signal light center wavelength is selected by the wavelength selection interposed in the pumping light-injecting optical fiber. When passing through the filter, a component near the central wavelength of the signal light is cut, and the light travels from the optical coupler to the rare-earth-doped fiber. It is the signal light center wavelength component that is amplified by the stimulated emission effect in the rare-earth-doped fiber, but this does not include the trailing component of the pump light and is substantially only the signal light component.

Since the wavelength selection filter for cutting off the component near the central wavelength of the signal light is interposed in the optical fiber for incidence of the pumping light, the trailing component of the pumping light to the central wavelength of the signal light is not contained in the rare-earth doped fiber. As a result, only the signal light component is amplified in the rare-earth-doped fiber, so that the noise characteristics (S / N
Ratio and noise figure NF) can be improved.

[0018]

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

1.55 from a transmission optical fiber (not shown)
A signal light incident optical fiber 4 is led out from a signal light incident connector 2 for introducing a signal light in the μm wavelength band, and an optical isolator 12 is provided near an end of the signal light incident optical fiber 4. 1.4 composed of semiconductor laser (LD)
Excitation light incident optical fiber 8 is derived from the excitation light source 6 for emitting excitation light of 8μm wavelength band, so-called melt-drawing type is coupled to the middle end thereof of the signal light incident optical fiber 4
Of the optical coupler 10 of FIG.

One end of a rare earth-doped fiber 14 doped with Er (erbium) as a rare earth element is joined to the end face of the optical fiber 4 for signal light incidence on the optical isolator 12 side. An optical isolator 16 is provided near the end of the rare-earth doped fiber 14, and 1.55 μm near the central wavelength of the signal light is provided in a subsequent ordinary optical fiber.
A band pass filter 18 that passes only the m wavelength component is interposed, and a signal light emitting connector 20 is provided at the last end.

The above configuration is the same as that of the conventional example. However, in the present embodiment, in the middle of the pumping light incidence optical fiber 8, that is, at any position from the pumping light source 6 to the optical coupler 10, A wavelength selection filter 22 having a filter characteristic as shown in FIG. 2 is interposed. The wavelength selection filter 22 has a wavelength between 1.55 μm, which is the central wavelength of the signal light, and 1.58 μm, which is between the central wavelength of the pumping light of 1.48 μm.
The cut-off wavelength f _C is set to 1.52 μm, and wavelength components longer than that are almost completely cut, and wavelength components smaller than that are passed. 1.5 of the wavelength selection filter 22
The transmittance at 5 μm is only about 0.1%.

As shown in FIG. 1 by a broken line,
The excitation light source 6 and the wavelength selection filter 22 may be integrally assembled to form a low noise LD module 24, or the excitation light source 6 and the wavelength selection filter 22 may be independent from each other. It is also conceivable to integrate the wavelength selection filter 22 with the optical coupler 10.

Next, the operation will be described.

FIG. 3A shows the spectrum of the pumping light in the 1.48 μm wavelength band emitted from the pumping light source 6, which is spread over a considerably wide wavelength band, and is 1.55 μm, which is the central wavelength of the signal light. Has a tailing component that reaches up to. However, when passing through the wavelength selection filter 22, trailing component, the cut-off wavelength f _C (= 1.51~1.52
μm), most of the wavelength components longer than that of FIG.
(B).

When the pump light whose tail component has been cut in this way and the extremely sharp signal light of the 1.55 μm wavelength band are mixed by the optical coupler 10, the spectrum of the mixed light is shown in FIG. Thus, the wavelength band of the signal light and the wavelength band of the pump light are clearly distinguished. When this mixed light is amplified in the rare-earth doped fiber 14, it becomes as shown in FIG. This includes the amplification component S ₃ amplification component S ₁ and the fluorescent component of the signal light, the amplifying component S ₂ of trailing component of the excitation light was seen in FIG. 5 of the prior art (e) contains Not.

The light emitted from the optical isolator 16 contains the amplified signal light and the pump light which is not amplified.
When the light passes through the band-pass filter 18, the pump light is cut off, and only the amplified signal light is emitted from the signal light output connector 2.
The light exits from 0 to the transmission optical fiber. The emitted light contains a fluorescent component but excludes a tailing component of the excitation light, so that noise characteristics (S / N ratio and noise figure NF) are improved as compared with the conventional example.

In the above embodiment, a rare earth-doped fiber doped with Er (erbium) is exemplified. However, the present invention is not limited to this, and the doped rare earth element is Nd (neodymium). You may. In this case, since the wavelength bands of the signal light and the pump light are also different from Er (erbium), the cutoff wavelength of the wavelength selection filter is adjusted accordingly.

In the above embodiment, the band pass filter (18) for passing only the signal light component is connected to the lower side of the rare earth doped fiber, but this band pass filter (18) is not always necessary.

[0029]

According to the present invention, since the wavelength selection filter for cutting off the components near the central wavelength of the signal light is interposed in the optical fiber for inputting the pump light, the tail of the pump light to the central wavelength of the signal light is provided. The component does not enter the rare-earth-doped fiber, and only the signal light component is amplified in the rare-earth-doped fiber. Therefore, the noise characteristics (S / N ratio and noise figure NF) of the amplified signal light are reduced. Can be improved. Moreover, the signal light wavelength range and the pump light wavelength range
Noise characteristics without increasing the wavelength spacing
The noise characteristics are improved, but the amplification efficiency is improved.
There is no inconvenience such as deterioration.

[Brief description of the drawings]

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

FIG. 2 is a characteristic diagram of a wavelength selection filter in the embodiment.

FIG. 3 is an explanatory diagram of an operation of the optical fiber amplifier according to the embodiment.

FIG. 4 is a schematic configuration diagram showing a conventional optical fiber amplifier.

FIG. 5 is a diagram illustrating the operation of a conventional optical fiber amplifier.

[Explanation of symbols]

 Reference Signs List 4 optical fiber for signal light incidence 6 pumping light source 8 optical fiber for pumping light incidence 10 optical coupler 14 rare earth doped fiber 18 bandpass filter 22 wavelength selection filter

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masataka Nakazawa 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Yasuo Kimura 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Japan (56) References JP-A-2-23322 (JP, A) JP-A-3-1355081 (JP, A) JP-A-3-11321 (JP, A) JP-A-3-25985 ( JP, A) JP-A-2-153327 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 3/06 H01S 3/094 H01S 3/10 G02F 1/35 G02B 6 / 00

Claims (1)

(57) [Claims] An optical fiber (8) for injecting excitation light derived from an optical fiber (4) for injecting signal light and an excitation light source (6).
Are bonded via a melt-stretched optical coupler (10),
Ri Na rare-earth doped fiber for amplifying the signal light on the downstream side of the optical coupler (10) (14) is connected, the signal
As close as possible to the wavelength range of light and the wavelength range of the excitation light
In the optical fiber amplifier described above, a wavelength selection filter (22) that cuts a component near the central wavelength of the signal light and passes the wavelength component of the excitation light is interposed in the optical fiber (8) for incidence of the excitation light. Optical fiber amplifier.