JP2790520B2 - Optical fiber amplifier - Google Patents

Description

[Detailed description of the invention] Table of contents Overview Industrial application field Conventional technology Problems to be solved by the invention Means for solving the problem Action embodiment Effect of the invention Overview Signal light to a doped optical fiber doped with a rare earth element Optical fiber amplifier that directly amplifies signal light by injecting pumping light (pumping light) with the pumping light. The signal light and pumping light having a small wavelength difference can be made incident at right angles or parallel to each other and included in the pumping light. The objective of the present invention is to provide an optical fiber amplifier having an optical coupler having a wavelength separation degree capable of sufficiently blocking a signal light wavelength component to be mixed, and the signal light and the pumping light are multiplexed by a multiplexer to be doped with Er. In an optical fiber amplifier for directly amplifying signal light by making it incident on an Er dove optical fiber,
The first prism having a total reflection film formed on one surface defining the included angle of the first prism having an included angle of ゜ and a combined film formed on the other surface, and forming a combined film. A polarization coupling unit comprising a multiplexing prism unit having a triangular prism adhered to the other surface, and coupling the first pumping light and the second pumping light whose polarization directions are orthogonal to each other to be incident on the multiplexing prism unit. A device is provided, on at least one surface of the boundary surface with the air of the optical component disposed on the pumping light side with respect to the multiplexing film for multiplexing the signal light and the pumping light,
A filter film that reflects the signal light wavelength component and transmits the pumping light is disposed, and the signal light is totally reflected by the total reflection film and then enters the multiplexing film. The multiplexed and multiplexed signal light and pump light are made to enter the Er-doped optical fiber.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier for directly amplifying signal light by injecting signal light and pumping light (pumping light) into a doped fiber doped with a rare earth element according to the present invention.

In an optical fiber communication system currently in practical use, a repeater is inserted at every fixed distance in order to compensate for attenuation of an optical signal due to loss of an optical fiber. The repeater is configured to convert an optical signal into an electric signal using a photodiode, amplify the signal using an electronic amplifier, convert the signal into an optical signal using a semiconductor laser or the like, and send the signal again to an optical fiber transmission line. If this optical signal can be directly amplified with low noise as it is, the optical repeater can be reduced in size and economical.

Therefore, research on optical amplifiers that can directly amplify optical signals is being actively pursued, and the optical amplifiers targeted for training can be roughly classified into optical fibers doped with rare earth elements (Er, Nd, Yb, etc.). There are three types: a combination of pumping light, a rare-earth element-doped semiconductor laser, an induced Raman amplifier utilizing nonlinear effects in an optical fiber, and an induced Brillouin amplifier.

Among these, the optical amplifier combining the rare-earth-doped optical fiber and the pumping light has excellent characteristics such as no polarization dependence, low noise, and small coupling loss with the transmission line. It is expected that the transmission distance in a fiber transmission system will be dramatically increased and that an optical signal can be distributed to a large number.

For amplification in the 1.53 μm band used in optical fiber communication, an Er-doped optical fiber doped with Er as a rare earth element is generally used.

2. Description of the Related Art FIG. 6 shows the principle of optical amplification using a rare earth element-doped optical fiber. Reference numeral 2 denotes an optical fiber composed of a core 4 and a clad 6, and the core 4 is doped with Er (erbium). When pumping light (excitation light) having a wavelength of, for example, 1.48 μm is incident on such Er-doped optical fiber 2, Er atoms are excited to a high energy level. When signal light having a wavelength of, for example, 1.53 μm enters the Er atom in the optical fiber 2 excited to the high energy level, the Er atom transitions to a low energy level. The emission occurs, the power of the signal light gradually increases along the optical fiber, and the signal light is amplified.

FIG. 7 shows a schematic configuration of a conventional Er-doped optical fiber amplifier based on the amplification principle described above. Reference numeral 10 denotes a polarization coupler. The first pumping light having a horizontal polarization plane and the second pumping light having a vertical polarization plane are incident on the polarization coupler 10 via optical fibers 11 and 12, respectively. Are coupled by the polarization coupler 10 and emitted. The wavelength of both the first pumping light and the second pumping light is, for example, 1.48 μm. The polarization coupler 10 has a polarization coupling prism unit 16 having a configuration in which a polarization coupling film 15 is sandwiched between two triangular prisms 13 and 14.

First having a horizontal plane of polarization incident from optical fiber 11
The pumping light is collimated by the collimating lens 17 and then enters the polarization coupling prism unit 16 and passes through the polarization coupling film 15 as it is. On the other hand, the second pumping light having a vertical polarization plane which is incident from the optical fiber 12 is collimated by the collimating lens 18 and then is incident on the polarization coupling prism unit 16, and transmitted through the polarization coupling film 15 through the transmission optical path of the first pumping light. Are reflected on the same optical path. Therefore, the light emitted from the polarization coupling prism unit 16 is a combination of the first pumping light and the second pumping light, and the combined pumping light is condensed by the lens 19 and enters the optical fiber 20. . Optical fiber 21 is connection adapter 22
Are connected to the optical fiber 20, and the first and second pumping lights transmitted through the optical fibers 20 and 21 are incident on the multiplexer 23 and transmitted through the optical fiber 28, for example, 1.53.
It is multiplexed with the signal light in the μm band.

The multiplexer 23 includes a multiplexing prism 26 in which a multiplexing film 25 formed of a dielectric multilayer film or the like is formed on the inclined surface of the triangular prism 24, and the first and second pumping lights from the optical fiber 21 are provided. Is collimated by the collimating lens 27, then enters the multiplexing prism 26, passes through the multiplexing film 25, and exits. On the other hand, for example, the wavelength transmitted through the optical fiber 28
The signal light in the 1.53 μm band is collimated by the collimating lens 29, then enters the multiplexing prism 26, is reflected by the multiplexing film 25, and is emitted on the same optical path as the transmission optical path of the pumping light. The multiplexed signal light and pumping light are condensed by the lens 30 and made incident on the Er-doped optical fiber 31, and the signal light is amplified based on the above-described principle.

In the configuration of this conventional example, the first pumping light and the second pumping light
The configuration in which the pumping light is coupled by the polarization coupler 10 is employed when the output of the semiconductor laser (LD) that emits the pumping light is not sufficient and the two main conductor lasers are used to increase the pumping light. In order to output it,
In the case where the output of the semiconductor laser is sufficient, a redundant configuration using only one pumping light can be adopted.

Problems to be Solved by the Invention In the Er-doped optical fiber amplifier, as described above, laser light having a wavelength of 1.53 μm is used as signal light, and laser light having a wavelength of 1.48 μm is used as pumping light. If the wavelength difference between the signal light and the pumping light is small, the signal light and the pumping light cannot be sufficiently multiplexed unless the angle of incidence of the signal light on the multiplexing film is smaller than 45 °. In order to make the signal light incident on the multiplexing prism, it is impossible to make the signal light incident perpendicularly or parallel to the optical path of the pumping light, and there is a problem that the arrangement of optical components such as the multiplexing prism becomes complicated.

Therefore, the present applicant has previously proposed an optical fiber amplifier which has solved this problem by using a multiplexer having a special configuration (Japanese Patent Application No. 2-49254). The optical fiber amplifier described in the prior application can make the signal light and the pumping light having a small wavelength difference incident at right angles or parallel to each other, but can sufficiently block the signal light wavelength component included in the pumping light. It was later found that the signal light transmitted through the transmission line and having a reduced level is a cause of deteriorating the SN because the signal light has a lower level of wavelength separation.

This will be described with reference to FIG. In the figure, A is pumping light having a center wavelength of 1.48 μm, and its maximum light output is about 50 mW. B is a signal light having a center wavelength of 1.536 μm before being transmitted, and its maximum light output is about
1 mW. This signal B is attenuated during the long-distance transmission in the optical fiber, and its maximum light output is attenuated by about 0.01 W at the transmission end as shown by C.

If the wavelengths of the signal light and the pumping light are close to each other, the output of the pumping light is large, so that the EL (electroluminescence) component forms a wide base, and a part of the pumping light becomes part of the signal light. Will overlap with the wavelength. Since the signal light C after transmission has a very small optical output as described above, it is strongly affected by the EL component of the pumping light,
The S / N ratio of the signal light will be degraded.

The present invention has been made in view of such a point,
The purpose is to allow the signal light and the pumping light having a small wavelength difference to be incident at right angles or parallel to each other, and to have an optical coupling having a wavelength separation degree capable of sufficiently blocking the signal light wavelength component contained in the pumping light. The invention is to provide an optical fiber amplifier comprising a device.

Means for solving the problem In the optical fiber amplifier which multiplexes the signal light and the pumping light by the multiplexer and makes it incident on the Er-doped optical fiber doped with Er, and directly amplifies the signal light, 45
The first prism having a total reflection film formed on one surface defining the included angle of the first prism having an included angle of ゜ and a combined film formed on the other surface, and forming a combined film. And a multiplexing prism unit in which a triangular prism is attached to the other surface. Further, there is provided a polarization coupler for coupling the first pumping light and the second pumping light whose polarization directions are orthogonal to each other to be incident on the multiplexing prism unit, and a multiplexing film for multiplexing the signal light and the pumping light. Also, a filter film that reflects the signal light wavelength component and transmits the pumping light is disposed on at least one surface of the optical component disposed on the pumping light side and the boundary surface with the air. Then, the signal light is totally reflected by the total reflection film and then enters the multiplexing film,
The light is multiplexed with the pumping light by the multiplexing film, and the multiplexed signal light and the pumping light are made to enter the Er-doped optical fiber.

Operation By using the multiplexing prism unit having the above-described configuration, the optical path of the pumping light and the optical path of the signal light incident on the multiplexing prism unit can be perpendicular or parallel to each other, thereby simplifying the arrangement of the optical components. Can be

Furthermore, the signal light wavelength component is reflected on at least one surface of the interface between the optical component and the air, which is disposed closer to the pumping light than the multiplexing film that multiplexes the signal light and the pumping light, to transmit the pumping light. By providing the filter film, the signal light wavelength component of the pump light can be blocked with a high attenuation,
The SN ratio of signal light can be improved.

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention. 32 is a polarization combining prism unit, and two triangular prisms 33, 3
4, a polarization combining film 35 formed of a dielectric multilayer film or the like is formed by vapor deposition or the like. The transmission characteristics of the polarization combining film 35 are as shown in FIG.
The first pumping light having a horizontal polarization plane of m-polarized light is transmitted, and the second pumping light having a vertical polarization plane of 1.48 μm and being s-polarized light is reflected.

The first pumping light (excitation light) having a horizontal polarization plane and having a wavelength of 1.48 μm is emitted from an optical fiber 36 inserted into a ferrule 37, collimated by a rod lens 38, and incident on a polarization combining prism unit 32. The light passes through the synthetic film 35 and is emitted from the polarization combining prism unit 32 as it is. On the other hand, the second pumping light having a vertical polarization plane and having a wavelength of 1.48 μm is emitted from the optical fiber 39 inserted into the ferrule 40, collimated by the rod lens 41, and then incident on the polarization combining prism unit 32, and film
The first pumping light and the second pumping light are reflected by 35 and emitted on the same optical path as the transmission optical path of the first pumping light, and are combined.

Forming a filter film 56 having a property of reflecting the signal light wavelength component of the pump light and transmitting the pump light on the surface of the polarization combining prism unit 32 from which the combined first and second pump lights are emitted. Glass substrate
55 is bonded by an optical adhesive. Further, anti-reflection films 61 and 62 are formed on end faces of the polarization combining prism unit 32 on which the first pumping light and the second pumping light are incident.

The reason why the first pumping light and the second pumping light are combined by the polarization combining prism unit 32 is that when the output of the semiconductor laser (LD) that emits the pumping light is not sufficient, the two pumping lights are combined. This is to combine them to increase the output. When the output of the pumping light source is sufficient, only one of the pumping light sources can be driven to make the pumping light redundant.

Reference numeral 42 denotes a multiplexing prism unit that multiplexes the pumping light and the signal light. A prism 43 having a 45 ° included angle is formed by a dielectric multilayer film or the like on one surface that defines the included angle. In addition to forming a reflective film 44, the other surface of the prism 43 includes a prism 43 formed with a multiplexing film 45 also formed of a dielectric multilayer film or the like.
A 22.5 mm triangular prism 46 is attached. The transmission characteristics of the multiplexing film 45 are as shown in FIG. 3, and the first and second pumping lights having a wavelength of 1.48 μm are transmitted.
The signal light in the 1.53 μm band has a characteristic of being reflected.

Polarization combining prism unit of combining prism unit 42
A glass substrate 57 on which a filter film 58 having a property of reflecting the signal light wavelength component of the pumping light and transmitting the pumping light is adhered on the surface facing the surface 32 with an optical adhesive. On the other surface, antireflection films 63 and 64 are formed. The transmission characteristics of the filter films 56 and 58 are the same, and substantially match the transmission characteristics of the multiplexing film shown in FIG.

The first and / or second pumping light emitted from the polarization combining prism unit 32 enters the multiplexing prism unit 42, passes through the multiplexing film 45, and is emitted from the multiplexing prism unit 42 as it is. On the other hand, signal light having a wavelength of 1.53 μm is emitted from an optical fiber 47 inserted into a ferrule 48,
After being collimated by the rod lens 49, the light enters the multiplexing prism unit 42. This signal light is totally reflected by the total reflection film 44, enters the multiplexing film 45 at an incident angle of less than 45 °, for example, an incident angle of about 20 °, is reflected by the multiplexing film 45, and passes through the transmission optical path of the pumping light. The signal light and the first and / or second pumping light are emitted on the same optical path and are combined. In this embodiment,
Polarization combining prism unit 32 and combining prism unit 42
Thus, an optical coupler 60 for coupling the pump light and the signal light is configured.

The signal light and the pumping light multiplexed by the multiplexing prism unit 42 in this manner are condensed by the rod lens 50 and incident on the Er-doped optical fiber 52 inserted in the ferrule 51, and the signal light is formed based on the principle described above. Is amplified and sent out to the transmission path.

In this embodiment, the prism 43 having the included angle of 45 ° and the apex angle
Since the multiplexing prism unit 42 is formed by bonding the triangular prisms 46 each having 22.5 mm, when multiplexing the signal light and the pumping light having a small wavelength difference, the signal light is totally reflected by the total reflection film 48. Since the light can be incident on the multiplexing film 45, the incident optical path of the pumping light and the incident optical path of the signal light with respect to the multiplexing prism unit 42 can be set to be perpendicular to each other.

Further, a filter film 5 having the above-described characteristics is provided on the pumping light side of the multiplexing film 45 of the multiplexing prism unit 42.
Since 6, 58 are provided, the signal light wavelength component of the pump light can be blocked with a high attenuation. Filter membrane
In the configuration of only the multiplexing film 45 having no 56, 58, the wavelength separation degree is about 15 dB, but in the present embodiment, the filter films 56, 58
Is provided, the degree of wavelength separation can be improved to about 45 dB.

In the embodiment described above, the filter films 56 and 58 are
This is provided on the downstream side of 5, so that one filter film can filter both the first pumping light and the second pumping light. However, the arrangement position of the filter film is not limited to the illustrated embodiment. For example, the filter film may be provided on the surface of the polarization combining prism unit 32 on which the antireflection films 61 and 62 are formed. Alternatively, a filter film may be provided on the end surfaces of the rod lenses 38, 41.

FIG. 4 is a schematic view of another embodiment of the multiplexing prism unit. The multiplexing prism unit 42 'of this embodiment has a prism 43' having a sandwiching angle of 45 ° similar to that of the above-described embodiment.
Further, a quadrangular prism 54 whose apex angles α and β are 22.5 ° and 45 °, respectively, is adhered to the prism. On the surface of the multiplexing prism unit 42 'on the upstream side of the multiplexing film 45 with respect to the pumping light, a filter film 66 having a characteristic of reflecting the signal light wavelength component of the pumping light and transmitting the pumping light is formed. A glass substrate 65 is adhered by an optical adhesive. 67 and 68 are antireflection films. By employing the multiplexing prism unit 42 'having such a configuration,
The pumping light and the signal light can be made to enter the multiplexing prism unit 42 'in a parallel relationship with each other.

Similarly, by replacing the triangular prism 34 of the polarization combining prism unit 32 with a parallelogram prism, the first pumping light and the second pumping light can be made incident in parallel.

FIG. 5 is a diagram for explaining that the wavelength characteristic of the filter film is shifted by inclining the filter film with respect to the optical path of the pumping light, and the filter film is arranged perpendicular to the optical path of the pumping light. This is shown by the solid line, and when the filter film is inclined from this vertical relationship, the wavelength characteristic of the filter film shifts as shown by the broken line. By utilizing this principle, even if the characteristics of the formed filter film do not completely match the required characteristics, the polarization combining prism unit having the formed filter film
After adjusting the angle with respect to the optical axis of the multiplexing prism unit 32 or the multiplexing prism unit 42 and shifting the wavelength characteristics of the filter films 56 and 58 to optimize the wavelength separation degree, the polarization coupling prism unit 32
Alternatively, by fixing the multiplexing prism unit 42 and assembling the optical coupler 60, it is possible to achieve an optimum degree of wavelength separation even with a filter film having a characteristic shifted to some extent.

Effect of the Invention Since the optical fiber amplifier of the present invention is configured as described above in detail, it is possible to block the signal light wavelength component contained in the pump light with a high attenuation according to the number of insertion stages of the filter film. There is an effect that the SN ratio can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention, FIG. 2 is a transmission characteristic diagram of a polarization combining film, FIG. 3 is a transmission characteristic diagram of a multiplexing film, and FIG. FIG. 5 is an explanatory diagram for explaining a shift of the wavelength characteristic of the filter film, FIG. 6 is a schematic diagram showing a principle of optical amplification by a doped optical fiber doped with a rare earth element, and FIG. 7 is a conventional diagram. FIG. 8 is a schematic configuration diagram of an example, and FIG. 8 is an explanatory diagram showing a relationship between optical powers of signal light and pumping light. 32 ... Polarization synthesis prism unit, 35 ... Polarization synthesis film, 42,42 '... Combining prism unit, 43,43' ... Prism with 45 ° sandwiching angle, 46 ... Triangular prism, 52 ... Er-doped optical fiber, 54 ... quadrilateral prism, 56,58,66 ... filter film, 60 ... optical coupler.

Claims (5)

(57) [Claims] An optical fiber amplifier for multiplexing a signal light and a pumping light by a multiplexer to make it incident on an Er-doped optical fiber doped with Er and directly amplifying the signal light. Prism (4
3) forming the total reflection film (44) on one surface defining the included angle and forming a multiplexing film (45) on the other surface, and forming the multiplexing film (45) on the first surface. A multiplexing prism unit (42) having a triangular prism (46) attached to the other surface of the prism (43), and combining the first pumping light and the second pumping light whose polarization directions are orthogonal to each other. A polarization coupler (32) is provided to be incident on the multiplexing prism unit (42), and the air of the optical components disposed closer to the pumping light side than the multiplexing film (45) that multiplexes the signal light and the pumping light. A filter film (56, 58) that reflects the signal light wavelength component and transmits the pump light is disposed on at least one surface of the boundary surface of the filter, and the signal light is totally reflected by the total reflection film (44), and then the signal light is reflected. Incident on the wave film (45), is multiplexed with the pumping light on the wave film (45), and is multiplexed. Said signal light and pumping light
An optical fiber amplifier characterized by being incident on an Er-doped optical fiber (52). 2. The first prism in place of the triangular prism (46).
The optical fiber amplifier according to claim 1, wherein a quadrilateral prism (54) is attached to the other surface of the prism (43). 3. The filter film (58) on an end face of the multiplexing prism unit (42) facing the polarization coupler (32).
The optical fiber amplifier according to claim 1 or 2, further comprising: 4. A filter film (56) on an end face of the polarization coupler (32) facing the multiplexing prism unit (42).
The optical fiber amplifier according to claim 1 or 2, further comprising: 5. The filter film (56, 58) provided with the filter film (56, 58) by adjusting the angle with respect to the optical axis of the multiplexing prism unit (42) or the polarization coupler (32).
After optimizing the wavelength separation by shifting the wavelength characteristics of
Combining prism unit (42) or polarization coupler (32)
5. The method of assembling an optical fiber amplifier according to claim 3, wherein the optical fiber amplifier is fixed.