JP2006294755A - Optical fiber amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber amplifier in which a manufacturing yield is good, in which low-pricing can be attained, and which has practical stability. <P>SOLUTION: The optical fiber amplifier includes an Er addition optical fiber 1 which is an amplification medium; a pump source 2 which generates an excitation light for exciting the amplification medium; and an optical combining/divider 3 which combines a signal light and an excitation light from the pump source 2, and is input into the Er addition optical fiber 1, and which amplifiers the signal light. In the optical fiber amplifier which amplifies the signal light, the pump source 2 outputs the excitation light of 0.66 μm band. The optical combining/divider 3 is a fiber type light coupler which makes it approach with a signal mode fiber with a cutoff wavelength shorter than the wavelength of the excitation light, and a fiber form light coupler which makes it approach with the single mode fiber longer than the wavelength of the excitation light and shorter than the wavelength of the signal light, and combines the light. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical fiber amplifier, and more particularly to an optical fiber amplifier that is used for optical communication, optical measurement, and the like, and uses an optical fiber doped with Er as an amplification medium.

  An Er-doped optical fiber amplifier (hereinafter referred to as EDFA) using an optical fiber doped with Er having a laser transition position in the core or cladding as an amplification medium is known. As an important component of an optical communication system that uses a communication wavelength band of 1.55 μm band or 1.58 μm band, EDFA has been vigorously researched and developed. Nowadays, it is widely applied not only to a trunk transmission system that transmits a digital signal but also to an optical CATV that transmits an analog signal.

  FIG. 1 shows a basic configuration of a conventional EDFA. In the EDFA, an optical isolator 8-1 for suppressing oscillation of an optical amplifier, an optical multiplexer / demultiplexer 7 for multiplexing signal light and pumping light, an Er-doped optical fiber 5 as an amplification medium, and an optical isolator 8-2 are sequentially arranged. It is connected. A pumping light source module 6 that supplies pumping light to the Er-doped optical fiber is connected to the other input of the optical multiplexer / demultiplexer 7. These optical components include an input / output optical fiber called a fiber pigtail for connection with other optical components. The black circles in FIG. 1 indicate the connection points of the fiber pigtails. The fiber pigtail is connected by fusion splicing or via an optical fiber connector.

  The pumping light source module 6 uses a semiconductor laser that outputs pumping light having a wavelength of 1.48 μm band or 0.98 μm band. However, it can be said that the semiconductor lasers in these wavelength bands are exclusively for EDFA, and are very expensive because there is little demand for other light sources.

  In recent years, in the 0.66 μm band which is the wavelength band of the excitation light of the Er-doped optical fiber, the output of the semiconductor laser has been increased, and the possibility of being practically used as an excitation light source has come out. Semiconductor lasers in this wavelength band are used as light sources for barcode readers, light sources for writing DVDs, and the like. The production amount of the 0.66 μm band semiconductor laser is much larger than that of the 1.48 μm band or 0.98 μm band semiconductor laser, the price is about 1/10, and further price reduction can be expected in the future. Therefore, by using a 0.66 μm band semiconductor laser as an excitation light source, it is possible to expect a dramatic price reduction of the EDFA.

JP-A-6-318750

  However, when a 0.66 μm band semiconductor laser is applied as an excitation light source for an EDFA, unlike the conventional EDFA, the wavelength band of the excitation light and the wavelength band of the signal light are largely separated from each other. there were.

  The cut-off wavelength of a fiber pigtail included in an optical component such as an Er-doped optical fiber, an optical multiplexer / demultiplexer, an excitation light source, or an optical isolator constituting the EDFA needs to be 660 nm or less. In order to manufacture an EDFA having a practical housing size, when the fiber pigtail is accommodated in the housing, the refractive index difference Δn between the core and the cladding must be increased in order to suppress an increase in bending loss.

  FIG. 2 shows the relationship between the winding diameter and loss of an optical fiber having a cutoff wavelength of 610 nm. It can be seen that the loss of the relative refractive index difference Δn2% and the winding diameter of 100 mm is as high as 5100 dB / km. Therefore, in order to make a practically sized EDFA, an Er-doped optical fiber having a relative refractive index difference Δn exceeding 2% and a fiber pigtail must be used. However, an Er-doped optical fiber having a relative refractive index difference Δn exceeding 2% is mainly used as a dispersion compensating fiber, and is more expensive than a normal optical fiber.

  The mode field diameter of an optical fiber having a cutoff wavelength of 610 nm and a relative refractive index difference Δn2% is 1.9 μm at a wavelength of 650 nm and 8 μm at a wavelength of 1550 nm. For this reason, there existed the subject shown below regarding manufacture of an optical multiplexer / demultiplexer, and manufacture of an excitation light source module.

  In general, as an optical multiplexer / demultiplexer, a fiber-type optical coupler that couples light by bringing two cores of the same type of optical fibers close to each other is widely used because it is small and easy to manufacture. Theoretically, fiber type optical couplers are known to have an unstable coupling ratio and increase loss unless the propagating / coupling light is single mode. For example, if a fiber-type optical coupler that multiplexes / demultiplexes 0.66 μm band pumping light and 1.55 μm band signal light using an optical fiber having a large relative refractive index difference Δn, the mode field diameter is greatly different. Therefore, the loss is large. As a specific example, when a fiber-type optical coupler is manufactured using an optical fiber having a cutoff wavelength of 630 nm and a relative refractive index difference Δn2%, the loss of excitation light in the 0.66 μm band is 1.3 dB and the signal light in the 1.55 μm band. Loss is about 1.8 dB, and a high-performance EDFA cannot be realized.

  Therefore, it is conceivable to apply a dielectric filter that does not require a single mode condition when coupling light. However, an optical multiplexer / demultiplexer using a dielectric filter increases the number of parts and requires man-hours such as a cutting / polishing process and an alignment process at the time of manufacture. Cannot be expected.

  In the excitation light source module, since the mode field diameter is very small, the fiber pigtail added to the excitation light source module must be mounted with high accuracy, and the yield cannot be improved.

  For example, Patent Document 1 discloses that a single-mode optical fiber whose cutoff wavelength is longer than the wavelength of pumping light and shorter than the wavelength of signal light is used as a method for solving the problems of the fiber-type optical coupler described above. Has been. However, in the fiber type optical coupler of Patent Document 1, the 0.66 μm band excitation light becomes multimode with respect to the optical fiber forming the optical coupler, and has short-term stability but long-term stability. Lack. For example, when the ambient temperature of the EDFA is changed from 20 degrees to 60 degrees and operated for 350 hours, the fluctuation of the signal gain is 2.6 dB.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical fiber amplifier that has a good manufacturing yield, can be reduced in price, and has practical stability. There is to do.

  In order to achieve the above object, according to the present invention, an invention according to claim 1 includes an Er-doped optical fiber that is an amplification medium, an excitation light source that generates excitation light for exciting the amplification medium, and signal light. And an optical multiplexer / demultiplexer that couples the pumping light from the pumping light source and inputs it to the Er-doped optical fiber. The optical fiber amplifier that amplifies the signal light includes a pumping light source of 0.66 μm band. The optical multiplexer / demultiplexer includes a single mode fiber having a cutoff wavelength shorter than the wavelength of the pumping light, and a single mode fiber having a cutoff wavelength longer than the wavelength of the pumping light and shorter than the wavelength of the signal light. It is a fiber type optical coupler that couples light by bringing them close to each other.

  The invention according to claim 2 is characterized in that the oscillation wavelength of the excitation light source according to claim 1 is in the range of 650 nm to 680 nm.

  The invention according to claim 3 is characterized in that the fiber pigtail of the pumping light source according to claim 1 or 2 is a single mode fiber having a cutoff wavelength shorter than the wavelength of the pumping light.

  According to a fourth aspect of the present invention, the fiber pigtail of the excitation light source according to the first or second aspect is a single mode fiber having a cutoff wavelength shorter than the wavelength of the excitation light and a relative refractive index difference Δn of 1% or less. It is characterized by being.

  According to a fifth aspect of the present invention, in the optical fiber amplifier according to any one of the first to fourth aspects, the signal light input side of the optical multiplexer / demultiplexer and the amplified signal of the Er-doped optical fiber An optical isolator is connected to the light output side, and the fiber pigtail of the optical isolator is a single mode fiber whose cutoff wavelength is longer than the wavelength of the pumping light and shorter than the wavelength of the signal light. .

  As described above, according to the present invention, an optical multiplexer / demultiplexer includes a single-mode fiber whose cutoff wavelength is shorter than the wavelength of pumping light, and whose cutoff wavelength is longer than the wavelength of pumping light and shorter than the wavelength of signal light. By using a fiber-type optical coupler that couples light close to a single mode fiber, it is possible to significantly reduce the combining loss, and it is not necessary to increase the refractive index difference Δn in order to suppress an increase in bending loss. Therefore, an inexpensive single mode fiber can be used.

  In addition, according to the present invention, the fiber pigtail of the excitation light source is a single mode fiber whose cut-off wavelength is shorter than the wavelength of the excitation light, so that long-term stability is achieved, and the coupling efficiency due to temperature or vibration is reduced. Change can also be improved. Furthermore, by using a single mode fiber having a relative refractive index difference Δn of 1% or less, the manufacturing yield is good and the price can be reduced.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present embodiment has the following characteristic points in configuring an Er-doped optical fiber amplifier using 0.66 μm band pumping light.
(1) The two optical fibers constituting the fiber-type optical coupler are single-mode fibers in which one cutoff wavelength is shorter than the wavelength of the pumping light, and the other cutoff wavelength is longer than the wavelength of the pumping light. It is a single mode fiber shorter than the wavelength of light.
(2) The cutoff wavelength of the fiber pigtail of the excitation light source module is shorter than the wavelength of the excitation light, and the relative refractive index difference Δn is 1% or less.

  Normally, a fiber-type optical coupler used in an EDFA couples light by bringing two similar optical fiber cores close to each other. On the other hand, a fiber-type optical coupler that realizes a tap function using different types of optical fibers has been known. However, as in the present embodiment, there is no known fiber-type optical coupler that multiplexes light in separate wavelength bands of 0.66 μm band and 1.55 μm band. By making the two optical fibers of the fiber-type optical coupler into the requirements described in (1) above, the loss of excitation light in the 0.66 μm band is 0.7 dB and the loss of signal light in the 1.55 μm band is 0.7 dB, and the multiplexing loss can be greatly reduced.

  In addition, the fiber pigtail of the pumping light source module is a single mode fiber shorter than the wavelength of the pumping light, and is connected to a single mode fiber shorter than the wavelength of the pumping light of the fiber-type optical coupler. Can be connected with low loss.

  Furthermore, a single mode fiber whose cutoff wavelength is longer than the wavelength of the excitation light (0.66 μm band) and shorter than the wavelength of the signal light, the signal light incident end of the 1.55 μm band and the signal light and the excitation light are combined. It becomes the combined end of waves. Therefore, when housed in an EDFA housing, it is not necessary to increase the refractive index difference Δn in order to suppress an increase in bending loss, so that an inexpensive single mode fiber can be used.

  In this embodiment, the 0.66 μm band excitation light is incident on a single mode fiber shorter than the wavelength of the excitation light, so that it has long-term stability and improves the change in coupling efficiency due to temperature or vibration. Can do.

  As described above, conventionally, the fiber pigtail of the excitation light source module has to have a cut-off wavelength of 660 nm or less, and in order to obtain an acceptable bending loss when manufacturing a practical EDFA, An optical fiber having a relative refractive index difference Δn exceeding 2% was used. At this time, the mode field diameter was 1.9 μm at a wavelength of 650 nm, and there was a problem that the coupling efficiency between the semiconductor laser of the excitation light source module and the fiber pigtail was low. In addition, the manufacturing tolerance of the excitation light source module is reduced, resulting in a poor manufacturing yield. For example, when a semiconductor laser having a wavelength of 650 nm and an optical fiber having a cutoff wavelength of 610 nm and Δn 2% are coupled using an aspheric lens, the coupling efficiency is 10%, which is not a practical excitation light source.

  FIG. 3 shows the relationship between the relative refractive index difference Δn and the coupling efficiency. The case of an optical fiber having a cutoff wavelength of 610 nm is shown, and it can be seen that by making Δn 1% or less, the coupling efficiency can be improved to 25% or more. For example, if a fiber pigtail of Δn 1% or more is used for a semiconductor laser chip having a wavelength of 660 nm and a power of 150 mW, which is currently being reduced in price, excitation light of 37 mW or more can be output. The ratio between the light amount of the excitation light having a wavelength of 660 nm and the light amount of the amplified output light is about 30%, and a practical EDFA with an output of 10 dBm or more can be realized.

  Further, since mounting with accuracy proportional to the mode field diameter is required, yield is improved by reducing Δn. For example, when an optical fiber having a cutoff wavelength of 610 nm and Δn 2% is used, the manufacturing yield of a pumping light source module having a coupling efficiency of 10% or more is about 20%. When an optical fiber having a cutoff wavelength of 610 nm and Δn 1% is used, the manufacturing yield of an excitation light source module having a coupling efficiency of 25% or more is about 50%.

  The Er-doped optical fiber used in the EDFA and the fiber pigtail of the optical isolator are also easily connected to optical components connected thereto by using a single mode fiber having a cutoff wavelength longer than the pumping light wavelength. .

  Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings. However, the embodiments disclosed below are merely examples of the present invention and do not limit the scope of the present invention.

  FIG. 4 shows a basic configuration of an EDFA according to an embodiment of the present invention. In the EDFA, an optical isolator 4-1 for suppressing oscillation of an optical amplifier, an optical multiplexer / demultiplexer 3 that combines signal light and pump light, an Er-doped optical fiber 1 that is an amplification medium, and an optical isolator 4-2 are sequentially arranged. It is connected. The other input of the optical multiplexer / demultiplexer 3 is connected to a pumping light source module 2 that supplies pumping light in the 0.66 μm band to the Er-doped optical fiber.

  FIG. 5 shows the structure of the optical multiplexer / demultiplexer. The optical multiplexer / demultiplexer 3 couples light by bringing the single mode fiber 11 whose cutoff wavelength is shorter than the wavelength of the pumping light and the single mode fiber 12 whose cutoff wavelength is longer than the pumping light wavelength and shorter than the signal light wavelength close to each other. This is a fiber type optical coupler.

  The Er-doped optical fiber 1 that is an amplification medium has a cutoff wavelength that is longer than the excitation light wavelength and shorter than the signal light wavelength. The excitation light source module 2 has a fiber pigtail with a cutoff wavelength shorter than the excitation light wavelength and Δn of 1% or less. The fiber pigtail of the optical multiplexer / demultiplexer 3 is a single mode fiber 11 on which the pump light is input, that is, the side connected to the pump light source module 2 has a cutoff wavelength shorter than the pump light wavelength. On the other hand, a single mode in which signal light is input, that is, a side connected to the optical isolator 4-1 and a side connected to the Er-doped optical fiber 1 have a cutoff wavelength longer than the excitation light wavelength and shorter than the signal light wavelength. Fiber 12. The fiber pigtails of the optical isolators 4-1 and 4-2 have a cutoff wavelength that is longer than the pumping light wavelength and shorter than the signal light wavelength.

The specifications of the optical fiber actually used are shown below.
[Er-doped optical fiber 1]
Δn 1.6%, cutoff wavelength 950 nm, Er addition concentration 2000 ppm, fiber length 5 m
[Fiber Pigtail of Excitation Light Source Module 2]
Δn 0.7%, cutoff wavelength 610nm
[Fiber-type coupler of optical multiplexer / demultiplexer 3]
Δn 1.6%, cutoff wavelength 950 nm and Δn 0.7%, cutoff wavelength 610 nm optical fiber 0.66 μm band pumping light loss 0.66 dB, 1.55 μm band signal light loss 0.57 dB
[Fiber pigtail of optical isolator 4]
Δn 0.7%, cutoff wavelength 1400 nm, fiber length 5 m.

  FIG. 6 shows the structure of an excitation light source module that outputs excitation light having a wavelength of 660 nm. The excitation light source module 2 includes an LD chip 21 that outputs light of 150 mW and 0.66 μm band, and the LD chip 21 and the fiber pigtail 23 are optically coupled via an aspheric lens 22. The output of the excitation light from the fiber pigtail 23 is 53 mW.

  With such a configuration, an EDFA having a signal gain of 20 dB, a noise of 6.5 dB, and an output of 11 dBm can be realized. FIG. 7 shows the relationship between the wavelength of the excitation light of the excitation light source module and the signal gain of the EDFA. The signal gain of the EDFA when the oscillation wavelength of the LD chip 21 of the excitation light source module 2 is changed is shown. It can be seen that a signal gain of 15 dB or more can be secured in the pump light wavelength range of 650 to 680 nm.

  FIG. 8 shows the stability of the signal gain of the EDFA according to the embodiment of the present invention. The time stability and temperature stability when the ambient temperature of the EDFA is changed from 20 degrees to 60 degrees and operated for 350 hours are shown. The dotted line represents the change in ambient temperature on the right vertical axis, and the solid line represents the signal gain of the EDFA on the left vertical axis. Under such conditions, the gain fluctuation is as small as 0.064 dB and is stable. As described above, the fluctuation of the signal gain of the EDFA disclosed in Patent Document 1 is 2.6 dB, whereas the fluctuation width is very small.

It is a figure which shows the basic composition of the conventional EDFA. It is a figure which shows the relationship between the winding diameter and loss of an optical fiber with a cut-off wavelength of 610 nm. It is a figure which shows the relationship between relative refractive index difference (DELTA) n and coupling efficiency. It is a figure which shows the basic composition of EDFA concerning the Example of this invention. It is a top view which shows the structure of an optical multiplexer / demultiplexer. It is sectional drawing which shows the structure of the excitation light source module which outputs the excitation light of wavelength 660nm. It is a figure which shows the relationship between the wavelength of the excitation light of an excitation light source module, and the signal gain of EDFA. It is a figure which shows stability of the signal gain of EDFA concerning the Example of this invention.

Explanation of symbols

1,5 Er-doped optical fiber 2,6 Excitation light source module 3,7 Optical multiplexer / demultiplexer 4,8 Optical isolator 11,12 Single mode fiber 21 LD chip 22 Aspheric lens 23 Fiber pigtail

Claims (5)

An optical doped / demultiplexed optical fiber that combines an Er-doped optical fiber that is an amplification medium, an excitation light source that generates excitation light for exciting the amplification medium, and a signal light and the excitation light from the excitation light source that are input to the Er-doped optical fiber. An optical fiber amplifier for amplifying the signal light,
The excitation light source outputs 0.66 μm band excitation light,
The optical multiplexer / demultiplexer includes a single mode fiber having a cutoff wavelength shorter than the wavelength of the pumping light and a single mode fiber having a cutoff wavelength longer than the wavelength of the pumping light and shorter than the wavelength of the signal light. An optical fiber amplifier, which is a fiber-type optical coupler for coupling light.   2. The optical fiber amplifier according to claim 1, wherein an oscillation wavelength of the excitation light source is in a range of 650 nm to 680 nm.   3. The optical fiber amplifier according to claim 1, wherein the fiber pigtail of the pumping light source is a single mode fiber having a cutoff wavelength shorter than the wavelength of the pumping light.   3. The optical fiber according to claim 1, wherein the fiber pigtail of the excitation light source is a single mode fiber having a cutoff wavelength shorter than the wavelength of the excitation light and a relative refractive index difference Δn of 1% or less. amplifier. An optical isolator is connected to the signal light input side of the optical multiplexer / demultiplexer and the amplified signal light output side of the Er-doped optical fiber, and the fiber pigtail of the optical isolator has a cutoff wavelength. 5. The optical fiber amplifier according to claim 1, wherein the optical fiber amplifier is a single mode fiber that is longer than the wavelength of the pumping light and shorter than the wavelength of the signal light.

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