JPH07142796A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE:To reduce ASE noise without needing to use an optical filter for a noise reduction. CONSTITUTION:An optical fiber amplifier 30 is provided with an erbium doped fiber 15 for inputting signal beams of wavelength 1.55mum band, an exciting light source 11 of 1.48mum band for amplifying the signal beams, and a distribution coupling type wavelength split multiple coupler 31 for composing and separating beams and signal beams outputted from this exciting light source 11, and in which loss characteristics of a signal path are the maximum near 1.48mum and 1.536mum and the minimum near 1.55mum, so that ASE noise can be reduced without using an optical filter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier for amplifying light in the 1.55 .mu.m band by using an erbium-doped fiber, and more specifically, a light for amplifying signal light by using a pumping light source of 1.48 .mu.m. Regarding fiber amplifier.

[0002]

2. Description of the Related Art Optical fiber amplifiers are classified into three types according to the relationship between the signal light and the traveling direction of pumping light.
These are forward pumping, backward pumping, and bidirectional pumping (Ishio et al., "Optical amplifiers and their applications", p110, Ohmsha, 1992.), or "Optics," published by the Japan Society of Applied Physics, May 1990. Vol. 19, No. 5, p276-p282). this house,
The backward pumping optical fiber amplifier is an advantageous structure for obtaining a high output.

FIG. 3 shows an example of a conventional optical fiber amplifier having a backward pumping configuration. Optical fiber amplifier 1
0 includes a 1.48 μm band pumping light source 11, and the pumping light is input to a wavelength division multiplexing coupler (WDM) 12. The wavelength division multiplexing coupler 12 is the optical isolator 1.
This is for wavelength-synthesizing the signal light in the 1.55 μm band and the pumping light in the 1.48 μm band transmitted in the erbium (Er) ion-doped optical fiber 15 whose both ends are connected to 3 and 14. Further, the two optical isolators 13 and 14 provided on the input side and the output side of the signal light are for preventing oscillation due to reflected return light.

FIG. 4 shows an optical spectrum of spontaneous emission light (ASE) of an erbium ion-doped optical fiber. This is shown in Ishio et al., "Optical Amplifier and Its Application", p115, mentioned above. The spontaneous emission light of the erbium ion-doped optical fiber is 1.55 μm in addition to 1.55 μm band.
It also has a peak near 536 μm. Therefore, when the signal light wavelength is selected to be around 1.555 μm, the optical spectrum characteristic of the optical output of the optical fiber amplifier 10 has a peak of ASE noise near 1.536 μm as shown in FIG.

ASE noise increases the noise of optical fiber amplifiers. Therefore, in an optical transmission system using an optical fiber amplifier, a low noise optical fiber amplifier is necessary to maintain a high S / N (signal / noise) ratio. In order to meet this requirement, the optical fiber amplifier has AS
It is common practice to connect an optical filter to remove E noise.

FIG. 6 shows the structure of an optical fiber amplifier in which an optical filter for removing ASE noise is connected to the optical output section in this way. In this figure, the same parts as those in FIG. 3 are designated by the same reference numerals, and the description thereof will be appropriately omitted.
In this optical fiber amplifier 20, the wavelength division multiplexing coupler 1
The optical filter 21 is disposed further rearward (on the output side) than the optical isolator 14 disposed rearward of the optical filter 21.

FIG. 7 shows the loss wavelength characteristic of the optical filter of the optical fiber amplifier shown in FIG. 6, and FIG. 8 shows the optical spectrum characteristic of the optical output of the optical fiber amplifier shown in FIG. . Comparing FIG. 8 with FIG.
It can be seen that the peaks of ASE noise existing in the vicinity of μm are removed. As the optical filter 21, in addition to the band cut filter, a plurality of optical fiber couplers formed by fusion splicing two optical fibers within a predetermined length range as proposed in Japanese Patent Laid-Open No. 3-110506. Those connected in series have also been proposed.

[0008]

As described above,
The conventional optical fiber amplifier 20 shown in FIG. 6 requires an optical filter at its output in order to reduce noise. However, if an optical filter is placed at the output,
There is a problem that this causes an increase in loss and reduces the gain of the optical fiber amplifier. Further, in JP-A-2-23322, it is proposed that a means for removing light near the wavelength of the signal light from the spontaneous emission light contained in the pumping light is provided between the pumping light source and the wavelength multiplex combining means. Has been done. However, this proposal has a problem that such a new means is required and the configuration of the optical fiber amplifier becomes complicated.

Therefore, it is an object of the present invention to eliminate the need for using special components such as optical filters for noise reduction in AS.
An object is to provide an optical fiber amplifier capable of reducing E noise.

[0010]

According to a first aspect of the present invention, (a) an erbium-doped fiber for inputting a signal light having a wavelength of 1.55 μm band, and (b) 1.48 μm for amplifying the signal light. The excitation light source in the band, (c) the light output from the excitation light source and the signal light are combined and separated, and the loss characteristic of the signal path is 1.48 μm and 1.
An optical fiber amplifier is provided with a distributed coupling type wavelength division multiplexing coupler having a backward pumping configuration, which has a maximum around 536 μm and a minimum around 1.55 μm.

That is, according to the first aspect of the invention, the loss wavelength characteristic of the distributed coupling type wavelength division multiplexing coupler is sine (si).
n) Pay attention to the point that the curve is drawn. When the period is set to 50 nm, 1485 nm and 153
The loss of the signal light path becomes maximum at 5 nm (the pumping light path loss becomes minimum), and the loss of the signal light path becomes minimum at 1560 nm. Therefore, when such a distributed coupling type wavelength division multiplexing coupler is used for the wavelength division multiplexing coupler of the optical fiber amplifier of the backward pumping configuration, 1535 nm of the optical output light is output.
The light in the vicinity is blocked, and the ASE noise can be reduced.

[0012]

EXAMPLES The present invention will be described in detail below with reference to examples.

FIG. 1 shows the configuration of an optical fiber amplifier according to an embodiment of the present invention. In the optical fiber amplifier 30, the same parts as those in FIG. 3 are designated by the same reference numerals, and the description thereof will be appropriately omitted. As shown in FIG. 3, the optical fiber amplifier 30 includes a wavelength division multiplexing coupler 3 so that the 1.48 μm band pumping light source 11 has a backward pumping configuration.
Connected to 1. The wavelength division multiplexing coupler 31 uses a fiber fusion type WDM.

FIG. 2 shows the signal light path loss wavelength characteristics of the wavelength division multiplexing coupler used in this embodiment. The fusion-bonding type wavelength division multiplexing coupler 31 used in the optical fiber amplifier 30 of the present embodiment has a minimum loss at wavelengths of 1485 nm and 1535 nm, respectively.
The one having the maximum at 560 nm is used. The erbium ion-doped optical fiber 15 has an erbium (Er) ion and aluminum (Al) ion doping amount of 3000 and 5000 ppm, respectively. The 1.48 μm band pumping light source 11 has an oscillation center wavelength of 1485 nm. The two optical isolators 13 and 14 are polarization independent type and have a loss of 1d.
B, the isolation of 40 dB was used.

The optical fiber amplifier 30 having such a configuration
The wavelength of the input signal light is 1560 nm and the output is -5.
It is assumed that the optical output power of the pump light source 11 in dBm and the 1.48 μm band is 40 mW. In this case, +5 dBm was obtained as the optical output of the optical fiber amplifier 30. The optical output spectrum at this time is similar to that of the amplifier 6 shown in FIG. That is, AS around 1535 nm
E noise peaks are removed.

In the embodiment described above, the fiber fusion type is used as the wavelength division multiplexing coupler 31, but the same effect can be obtained by using the waveguide type wavelength division multiplexing coupler. is there.

[0017]

As described above, according to the invention of claim 1, the loss wavelength characteristics of the signal path are 1485 and 1.
Since a distributed-coupling type wavelength division multiplexing coupler that has a maximum around 535 nm and a minimum around 1560 nm is used,
A low-noise optical fiber amplifier with less loss can be configured without using an optical filter.

[Brief description of drawings]

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

FIG. 2 is a characteristic diagram showing signal light path loss wavelength characteristics of the wavelength division multiplexing coupler used in this embodiment.

FIG. 3 is a schematic configuration diagram showing an example of a conventional optical fiber amplifier having a backward pumping configuration.

FIG. 4 is an optical spectrum characteristic diagram of spontaneous emission light of an erbium ion-doped optical fiber.

FIG. 5 is an optical spectrum characteristic diagram of optical output of a conventional optical fiber amplifier that does not use an optical filter.

FIG. 6 is a schematic configuration diagram showing a configuration of a conventional optical fiber amplifier to which an optical filter for removing ASE noise is connected.

7 is a characteristic diagram showing a loss wavelength characteristic of the optical filter of the optical fiber amplifier shown in FIG.

8 is a characteristic diagram showing an optical spectrum characteristic of an optical output of the optical fiber amplifier shown in FIG.

[Explanation of symbols]

 11 1.48 μm band pumping light source 13, 14 Optical isolator 15 Erbium ion-doped optical fiber 30 Optical fiber amplifier 31 Wavelength division multiplexing coupler

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H01S 3/07 3/094

Claims (1)

[Claims] 1. An erbium-doped fiber for inputting signal light of wavelength 1.55 μm band, a pumping light source of 1.48 μm band for amplifying the signal light, light output from this pumping light source and the signal light. And are combined and separated, and the loss characteristic of the signal path is 1.48 μm.
And the maximum around 1.536 μm, 1.55 μm
An optical fiber amplifier, comprising: a distributed-coupling type wavelength division multiplexing coupler having a backward pumping configuration that has a local minimum.