JPH057036A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE:To eliminate the influence of trailing components of pumping light upon signal light center wavelength, and improve noise characteristics in the signal light after amplification. CONSTITUTION:An optical fiber 4 for signal light incidence and an optical fiber 8 for pumping light incidence from a pumping light source 6 are coupled by an optical coupler 10. A rare earth doped fiber 4 is connected with the downstream side of the optical coupler 10. A bandpass filter 18 for passing only the signal light wavelength components is connected with the downstream side of the fiber 14. A wavelength selection filter 22 is interposed between the pumping light source 6 and the optical coupler 10, i.e., in the middle part of the optical fiber 8 for pumping light incidence. The wavelength selection filter 22 has filter characteristics for cutting off the components in the vicinity of signal light center wavelength and passing the pumping light wavelength components.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to Er (erbium) after mixing signal light and pumping light by an optical coupler (optical multiplexer).
The present invention relates to an optical fiber amplifier which is guided to a rare earth-doped fiber doped with a rare earth element, and which amplifies signal light by the 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, 2 is a signal light incident connector and 4 is a connector.
Is an optical fiber for signal light incidence, 6 is a semiconductor laser (LD)
A pumping light source such as 8; an optical fiber 8 for pumping light incidence; an optical coupler 10 for mixing signal light propagating in the optical fiber 4 for signal light incidence and pumping light propagating in the optical fiber 8 for pumping light incidence; An optical isolator that almost completely blocks reflected light and allows light to pass in only one direction, 14 is a rare-earth-doped fiber having a function of amplifying signal light by the stimulated emission effect, 16 is an optical isolator, and 18 is a wavelength of pumping light Reference numeral 20 denotes a bandpass filter that cuts the component and the fluorescent component and passes only the wavelength component of the signal light, and 20 denotes a signal light emitting connector.

In this example, the rare earth element doped in the rare earth-doped fiber 14 is Er (erbium). In the case of Er, the signal light having the wavelength band of 1.55 μm is said to have the highest amplification factor, and the excitation light in that case has the wavelength band of 1.48 μm. Although the optical fiber is composed of a core and a clad on the outer periphery thereof, Er is doped inside or on the outer peripheral surface of the core.

Next, the operation will be described.

The pumping light in the 1.48 μm wavelength band emitted from the pumping light source 6 enters the rare earth-doped fiber 14 via the pumping 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 Er excited state, the modulated signal light in the wavelength band of 1.55 μm 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, it is incident on the rare earth-doped fiber 14 via the optical isolator 12 in a state of being mixed with the excitation light by the optical coupler 10. The signal light is directly caused by the stimulated emission effect in the process of traveling through the rare earth-doped fiber 14 (in the state of light without being converted into an electric signal).
Is amplified. The amplified signal light and a part of the pumping light are transmitted through the optical isolator 16 to the bandpass filter 1
Enter 8. The bandpass filter 18 cuts off the excitation light and the fluorescent component, passes only the signal light, and the signal light is guided to a transmission optical fiber (not shown) via the signal light emitting connector 20.

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

[0009]

The problem to be solved by the present invention is that not only the signal light is amplified in the rare earth-doped fiber 14 but also a part of the pumping light is amplified, resulting in noise of the signal light. The point is that the characteristics deteriorate. This will be specifically described below.

As shown in FIG. 5 (a), the spectrum of the signal light is sharp within the extremely narrow wavelength band even though it is the 1.55 μm wavelength band. On the other hand, even if the pumping light source 6 is a semiconductor laser, when viewed on the order of 1/100 μm, the spectrum of the pumping light in the 1.48 μm wavelength band is considerably wide as shown in FIG. 5B. It spreads in the range, and the bottom reaches 1.55 μm, which is the center wavelength of the signal light.

When the signal light and the pumping light having such a spectrum are mixed in the optical coupler 10, the spectrum of the mixed light is as shown in FIG. 5 (c). FIG. 5D is an enlarged view of the vicinity of 1.55 μm in this spectrum. Most of the 1.55 μm component of the mixed light is signal light, but a small amount of pumping light is also included.

When the light component near 1.55 μm is amplified by the stimulated emission effect in the rare earth-doped fiber 14, it becomes as shown in FIG. 5 (e). At exactly 1.55 μm, S ₁ is an amplified component of the signal light, S ₂ is an amplified component of the tail portion of the excitation light, and S ₃ is an amplified component of the fluorescent component (ASE). The sum of these (S ₁ + S ₂ + S ₃ ) is regarded as the signal light after amplification. This is not separated even by the bandpass filter 18, and is directly emitted from the signal light emitting connector 20 to the transmission optical fiber. This is because the bandpass filter 18 passes the 1.55 μm component.

As a result of the above, (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 deteriorated.

The present invention was devised in view of such circumstances, and eliminates the influence of the trailing component of the pumping light on the central wavelength of the signal light, and improves the noise characteristics of the signal light after amplification. The purpose is to

[0015]

According to the present invention, an optical fiber for inputting a signal light and an optical fiber for inputting a pumping light derived from a pumping light source are coupled via an optical coupler, and the lower side of the optical coupler is doped with a rare earth element. In an optical fiber amplifier in which fibers are connected, in the middle of the pumping light incident optical fiber, a wavelength selection filter that cuts components near the signal light center wavelength and passes pumping light wavelength components is interposed. To do.

[0016]

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

Since the wavelength selection filter for cutting the component near the center wavelength of the signal light is interposed in the middle of the optical fiber for pumping light incidence, the tailing component of the pumping light to the center wavelength of the signal light is not contained in the rare earth-doped fiber. Since it does not enter and only the signal light component is amplified in the rare earth-doped fiber, the noise characteristic (S / N) of the signal light after amplification is increased.
The ratio and noise figure NF) can be improved.

[0018]

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

From an optical fiber for transmission (not shown), 1.55
An optical fiber 4 for signal light incidence is led out from a signal light incidence connector 2 for introducing signal light in the μm wavelength band, and an optical isolator 12 is provided near the end of the optical fiber 4 for signal light incidence. It consists of a semiconductor laser (LD) 1.4
An optical fiber 8 for pumping light is led out from a pumping light source 6 that emits pumping light in the wavelength band of 8 μm, and an end portion of the optical fiber 8 is coupled in the middle of the optical fiber 4 for signal light incidence to form an optical coupler 10.

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 signal light incident optical fiber 4 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 .mu. Near the center wavelength of the signal light is provided in the subsequent ordinary optical fiber.
A bandpass filter 18 that passes only the m-wavelength component is interposed, and a signal light emitting connector 20 is provided at the final end.

The above construction is similar to that of the conventional example, but in the present embodiment, in the middle of the optical fiber 8 for pumping light incidence, 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. This wavelength selection filter 22 has a center wavelength of 1.55 μm and a pumping light center wavelength of 1.48 μm.
The cutoff wavelength f _C is 1.52 μm, and wavelength components longer than that are almost completely cut, and wavelength components shorter than that are passed. 1.5 of the wavelength selection filter 22
The transmittance at 5 μm is only about 0.1%.

As shown by the broken line in FIG.
The pumping light source 6 and the wavelength selection filter 22 may be integrally assembled to form the low noise LD module 24, or the pumping 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.

The spectrum of the pumping light in the 1.48 μm wavelength band emitted from the pumping light source 6 is as shown in FIG. 3 (a), and the spectrum of the pumping light is 1.55 μm, which is the central wavelength of the signal light and spreads in a fairly wide wavelength band range. It has a hemming component that reaches up to. However, when passing through the wavelength selection filter 22, the trailing component has a cut-off wavelength f _C (= 1.51 to 1.52).
Most of the wavelength component longer than
The spectrum is as shown in FIG.

When the pumping light with the trailing component thus cut and the signal light in the extremely sharp 1.55 μm wavelength band are mixed by the optical coupler 10, the spectrum of the mixed light is shown in FIG. 3 (c). As a result, the wavelength band of the signal light and the wavelength band of the pumping 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 amplified component S ₁ of the signal light and the amplified component S _{3 of the} fluorescent component, but includes the amplified component S ₂ of the trailing component of the excitation light shown in FIG. It is not.

The light emitted from the optical isolator 16 includes the signal light after amplification and the excitation light which is not amplified.
When it passes through the bandpass filter 18, the excitation light is cut and only the amplified signal light is output as the signal light emitting connector 2.
The light is emitted from 0 to the transmission optical fiber. Since the emitted light contains the fluorescence component but excludes the tailing component of the excitation light, the noise characteristics (S / N ratio and noise figure NF) are improved as compared with the conventional example.

Although the rare earth-doped fiber doped with Er (erbium) is shown in the above embodiment, the present invention is not limited to this, and Nd (neodymium) is used as the rare earth element. May be. In that case, since the wavelength bands of the signal light and the pumping light are also different from Er (erbium), the cutoff wavelength of the wavelength selection filter is adjusted accordingly.

Further, 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 the band pass filter (18) is not always necessary.

[0029]

According to the present invention, since the wavelength selection filter for cutting the component near the center wavelength of the signal light is provided in the middle of the optical fiber for pumping light incidence, the tailing of the pumping light to the center wavelength of the signal light is performed. Since the component does not enter the rare earth-doped fiber and only the signal light component is amplified in the rare earth-doped fiber, the noise characteristics (S / N ratio and noise figure NF) of the amplified signal light are Can be improved.

[Brief description of drawings]

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

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

FIG. 3 is an operation explanatory diagram 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 an operation explanatory diagram of a conventional optical fiber amplifier.

[Explanation of symbols]

 4 Optical Fiber for Signal Light Injection 6 Pumping Light Source 8 Optical Fiber for Pumping Light Injection 10 Optical Coupler 14 Rare Earth Doped Fiber 18 Bandpass Filter 22 Wavelength Selection Filter

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical indication location H01S 3/07 7630-4M 3/106 7630-4M (72) Inventor Masataka Nakazawa Uchisaiwaicho, Chiyoda-ku, Tokyo 1-chome 1-6 Nihon Telegraph and Telephone Corp. (72) Inventor Yasuo Kimura 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corp.

Claims (1)

Claim: What is claimed is: 1. An optical fiber for signal light incidence (4) and an optical fiber for pumping light injection (8) derived from a pumping light source (6).
And are coupled via an optical coupler (10),
0) In an optical fiber amplifier in which a rare earth-doped fiber (14) is connected to the lower side, a component near the central wavelength of the signal light is cut in the middle of the optical fiber for pumping light injection (8) and a pumping light wavelength component An optical fiber amplifier characterized by interposing a wavelength selection filter (22) for passing light.