JP3282246B2 - Optical module for optical amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical module for supplying pumping light to an optical amplifier using a rare earth doped optical fiber. In recent years, practical use of optical fiber amplifiers has been promoted. In particular, in the case of an optical fiber amplifier intended for high output, a high output backward pumping method is effective. For this reason, a high-output and highly-reliable pumping optical module is required.

[0002]

2. Description of the Related Art FIG. 11 shows a configuration example of a conventional optical module.
FIG. 12 shows a configuration example of a conventional multiplexing prism. In the figure, 10 is an erbium-doped fiber, which has an amplifying function, 61 and 71 are pump laser diodes, which pump an erbium-doped fiber, 50 is a signal light sent by a pre-stage optical circuit, and 90 is an optical module In the configuration, the pump laser beam is combined and injected into the erbium-doped fiber, and the amplified signal light is separated.91 is a housing that fixes the coupling lens and the demultiplexing / multiplexing prism. A prism formed by bonding a plurality of glass prisms 41, 43 is a multiplexing / demultiplexing film, 44 is a polarization separating film,
It is responsible for multiplexing and demultiplexing.

[0003] Thus, conventionally, an optical module for optical amplifier using an optical fiber doped with a rare earth element such as Erubi c beam, in order to simplify the optical path configuration, optical bonding sandwiched between the glass prisms 41 The optical device was configured as an individual device using the optical film 42 (referred to as a short film).
However, since the output of the excitation light and the output of the signal light are high, the optical adhesive 45, which is an organic substance, may be deteriorated and damaged by light energy.

[0004]

For this reason, there has been a problem that the reliability of the device may be impaired, and the reliability of the entire optical repeater may be impaired. SUMMARY An advantage of some aspects of the invention is to employ a configuration in which an optical adhesive is not used in an optical module for optical amplification, and to avoid device deterioration due to damage of the optical adhesive by laser light.

[0005]

FIG. 1 is a diagram illustrating the principle of the present invention. In the drawing, reference numeral 21 denotes a birefringent crystal prism that refracts light, and 22 denotes an open film, that is, an optical film deposited on the surface of the birefringent crystal prism, which selectively reflects or transmits light. . The present invention makes use of the fact that the refractive indices for ordinary light and extraordinary light entering or exiting the birefringent crystal are different, that is, two excitation rays are applied to the prism (21) of the birefringent crystal, respectively. As an open film (22)
When the light is emitted from the prism after being reflected by the light source, the two excitation lights are combined so as to have the same emission position and angle.

[0006]

The optical module for an optical amplifier has the following optical functions.
A wavelength multiplexing / demultiplexing film (abbreviated as WDM film) or a polarization multiplexing / demultiplexing film (P
Abbreviated as BS film. ) Is often used, but if these open films are deposited on independent glass substrates and fixed independently to the main substrate, it is difficult to precisely control the position and angle between the glass substrates, and each glass substrate Angle deviation (due to temperature and aging) is added for the number of reflections,
The problem that the stability of the optical axis is low occurs.

When a light beam is incident on a birefringent crystal, the incident light beam is divided into a component perpendicular to the crystal axis (ordinary light) and a component parallel to the plane including the crystal axis (extraordinary light). Bend. Therefore, when the light exits from the birefringent crystal, the emission point differs between ordinary light and extraordinary light. Conversely, when ordinary light and extraordinary light enter from this emission point, it is expected that the combined light of the two will be emitted in one direction from one point along the reverse path.

The present invention provides a method of using the above principle to combine excitation light from two different points. That is, a WDM film is deposited on one surface of a birefringent crystal,
Due to the optical path configuration for reflection and refraction within the birefringent crystal, 2
One pump light is duplicated, and the function of combining and demultiplexing the signal light and the pump light is integrated into one crystal. This realizes a configuration having high reliability using the open film and having high stability of the optical axis due to the integrated structure.

In the present invention, as shown in FIG.
The WDM film is configured on one surface of the two, from the surface opposite to the surface that configures the WDM film, two excitation lights whose polarization planes are orthogonal to each other as ordinary light and extraordinary light for the crystal,
When the light is incident at an appropriate angle, the light is reflected by the WDM film and injected into the erbium-doped fiber as doubled excitation light from the facing surface. The signal light amplified by the erbium-doped fiber is incident on the WDM film, but the WDM film shows transmission characteristics with respect to the signal light, and is emitted out of the crystal at that point.

[0010]

FIG. 2 shows an entire configuration of an optical amplifier common to the following embodiments. The present invention relates to a configuration of a portion indicated by a broken line in the drawing. FIG. 3 shows the module structure of the first embodiment, and FIG. 4 shows the structure of the birefringent crystal plate. The signal light amplified by the erbium-doped fiber 10 is converted into a collimated beam by the lens As11 fixed to the housing 91 from the polarization plane preserving fiber 12, and is a non-emission film (abbreviated as an AR film) on the first surface of the birefringent crystal plate. .) (There is a case where there is no AR film, but it is assumed in the following description). Here, the vibration direction of the electric flux density vector D of the signal light is perpendicular (or parallel) to the main section (a plane including the light ray and the optical axis). Therefore, the signal light is emitted from the second surface in a single optical signal state in a certain direction outside the crystal, and is input to the next stage (such as an isolator). Further, even if the signal light has an arbitrary polarization state (that is, both ordinary light and extraordinary light), if the degree of separation between ordinary light and extraordinary light is low, there is a problem with the next stage (such as a lens assembly or an isolator). Can be combined without.

One of the two pumping lights enters the birefringent crystal 21 from the pumping laser 61 through the polarization preserving fiber 62 and the lens assembly (abbreviated as lens AS) 63 from the first surface. Here, the vibration direction of the electric flux density vector D is perpendicular (or parallel) to the main section, and the AR film 23 on the first surface is set.
Incident on. The excitation light incident on the birefringent crystal 21 is reflected by the multiplexing / demultiplexing film 22 on the second surface. Electric flux density vector D of signal light
The position and angle of the incident light are set so that the optical axis of the reflected light coincides with the optical axis that can be taken when the vibration direction is perpendicular (or parallel) to the main section.

Another pumping light passes from the pumping laser 71 to the AR film 23 on the first surface through the polarization preserving fiber 72 and the lens AS73. , And is reflected by the demultiplexing film 22 on the second surface similarly to the previous excitation light. The incident position and angle of this optical axis are set so that the signal light coincides with the optical axis that can be taken when the vibration direction of the electric flux density vector D is parallel (or perpendicular) to the main section.

In this way, the two excitation lights reflected by the multiplexing / demultiplexing film 22 on the second surface are refracted on the first surface and both are excited by the lens A.
The light enters the erbium-doped fiber 10 through s11 and excites erbium. FIG. 5 shows the configuration of the birefringent crystal plate of the second embodiment. A WDM film having signal light reflection / excitation light transmission characteristics is provided on a portion of the first surface where the signal light is incident, and a total reflection film (abbreviated as HR film) is provided on the second surface, and the HR film is reflected. An AR film is provided on a portion where the leakage signal light enters the first surface. Excitation light that has entered the birefringent crystal plate is totally reflected by the HR film on the second surface, passes through the WDM film on the first surface, and enters the erbium-doped fiber in the same manner as in the first embodiment. The signal light amplified by the erbium-doped fiber is reflected by the WDM film on the first surface, exits in a certain direction outside the crystal, and is input to the next stage (such as an isolator). The leaked signal light transmitted through the WDM film on the first surface is totally reflected by the HR film on the second surface and emitted out of the crystal from the AR film on the first surface.

A similar function can be realized by replacing the AR film on the second surface with a WDM film for transmitting signal light and reflecting excitation light with the same structure. In this case, the WD of the first surface
The leak signal light that has passed through the M film is transmitted through the WDM film on the second surface and is emitted outside the crystal, thereby preventing the leak signal light from entering the pump LD. Other operations are the same as in the case of using the HR film.

FIG. 6 shows the structure of a birefringent crystal plate according to a third embodiment. An AR film is provided on a portion of the first surface where the signal light is incident, and a WDM film for transmitting the signal light and reflecting the excitation light is provided on a portion where the signal light transmitted through the AR film is incident on the second surface.
An AR film is applied to a portion of the surface where the excitation light is incident, and the excitation light is reflected a plurality of times by the first surface and the second surface and is reflected by the WDM film of the second surface before reaching the signal light incidence portion of the first surface. An HR film is provided on the reflection portions of the first surface and the second surface of the optical path. Excitation light incident on the birefringent crystal plate is repeatedly total-reflected by the HR film on the second surface and the first surface in the same manner as in the first embodiment, and is reflected by the WDM film on the second surface. The light passes through the AR film and enters the erbium-doped fiber. The signal light amplified by the erbium-doped fiber passes through the AR film on the first surface and then passes through the WDM film on the second surface and exits in a certain direction outside the crystal.
It is input to the next stage (such as an isolator). In the case of this configuration, since the excitation light is repeatedly reflected a plurality of times, the interval between the beams at the incident points of the two excitation lights can be increased.

The same function can be realized by replacing the HR film for reflecting the excitation light on the first surface and the second surface with the WDM film for transmitting the signal light and reflecting the excitation light with the same structure.
In this case, since the second surface can use the same WDM film on the front surface, the film structure is simplified, and the leaked signal light is emitted out of the crystal at each reflection point, so that the leak of the signal light can be reduced. FIG. 7 shows the configuration of the birefringent crystal plate of the fourth embodiment. This configuration is different from the configuration of the birefringent crystal plate of the third embodiment in that
Instead of the AR film at the portion where the signal light is incident on the surface, a WDM film for signal light reflection / excitation light transmission is provided, and an HR film is provided on the second surface. The signal light amplified by the erbium-doped fiber is reflected by the WDM film on the first surface, exits in a certain direction outside the crystal, and is input to the next stage (such as an isolator). The propagation of the pump light is the same as in the case of the third embodiment.

The same function can be realized by replacing the HR film for reflecting the excitation light on the first and second surfaces with the WDM film for transmitting and reflecting the signal light with the same structure.
In this case, since the second surface can use the same WDM film on the front surface, the film structure is simplified, and the leaked signal light is emitted out of the crystal at each reflection point, so that the leak of the signal light can be reduced. FIG. 8 shows the configuration of the birefringent crystal plate of the fifth embodiment. This embodiment uses a birefringent crystal plate in which the first surface and the second surface are not parallel to each other.
The surface is provided with a WDM film that transmits signal light and reflects excitation light.

FIG. 9 shows the structure of a birefringent crystal plate according to a sixth embodiment. The present embodiment uses a birefringent crystal plate in which the first surface and the second surface are not parallel, and attaches a WDM film that reflects signal light and transmits excitation light to a portion of the first surface where light rays are incident. The HR film is provided on the surface. The same function can be realized by replacing the HR film on the second surface with a WDM film for transmitting signal light and reflecting excitation light with the same structure.

FIG. 10 shows a seventh embodiment. This configuration example uses the configuration of the birefringent crystal plate of FIG. 8 as the first birefringent crystal prism, and arranges a magneto-optical crystal on the signal output side to rotate the polarization plane of the optical signal by 45 °. This is a configuration in which a second birefringent crystal prism is arranged. The crystal axis of the second birefringent crystal prism is rotated by 45 ° with respect to the first. This results in a configuration with a built-in isolator.

Note that the first birefringent crystal prism and the second
In the configuration of the birefringent crystal prism described above, the same effect can be obtained by the configuration of the birefringent crystal plate shown in FIGS .
Also, in any of the above embodiments, a configuration in which an optical circuit using an optical film such as a narrow-band filter or a splitter is inserted on the optical signal output side can be adopted. The above explanation
As apparent from FIG.
Than,

SUMMARY OF THE INVENTION
Adopts a configuration that does not use an adhesive, and the optical adhesive
This avoids device degradation due to damage
”Is possible.

[0021]

According to the first to eighth aspects of the present invention,
With this, the polarization separation film was sandwiched between the glass prisms.
Optical adhesive, which was necessary for
Debonding due to damage to the adhesive by laser light
It is possible to avoid deterioration of the quality. Therefore,
According to the present embodiment, an optical module for backward pumping of a pumping light having a high output by polarization synthesis into an erbium-doped fiber
The optical module can be reduced in size and size without impairing the reliability of
The optical amplifier can be configured with low loss, and downsizing and high performance of the optical amplifier can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an example of the overall configuration of an optical amplifier.

FIG. 3 is a module structure of the first embodiment.

FIG. 4 shows a configuration of a birefringent crystal plate according to the first embodiment.

FIG. 5 shows a configuration of a birefringent crystal plate according to a second embodiment.

FIG. 6 shows a configuration of a birefringent crystal plate according to a third embodiment.

FIG. 7 shows a configuration of a birefringent crystal plate according to a fourth embodiment.

FIG. 8 shows a configuration of a birefringent crystal plate according to a fifth embodiment.

FIG. 9 shows a configuration of a birefringent crystal plate according to a sixth embodiment.

FIG. 10 shows a configuration of a birefringent crystal plate according to a seventh embodiment.

FIG. 11 is a configuration example of a conventional optical module.

FIG. 12 is a configuration example of a conventional multiplexing / demultiplexing prism. 10 Erbium-doped fiber 11, 63, 73 Lens AS 12, 62, 72 Polarization preserving fiber 21 Birefringent crystal 22 Optical film (open film) 23 AR film 40 Combining prism 41 Glass prism 42 Optical film (short film) 43 Demultiplexing film 44 Polarization separation film 45 Optical adhesive 50 Pre-stage circuit 61, 71 Excitation laser 90 Optical module configuration 91 Housing

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H04B 10/17 (72) Inventor Nobuhiro Fukushima 1015 Ueodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fujitsu Limited (56) References JP JP-A-3-252629 (JP, A) JP-A-4-127130 (JP, A) JP-A-5-341232 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 3 / 00-3/30 G02F 1/35 501 H04B 10/16 H04B 10/17 JICST file (JOIS)

Claims (8)

(57) [Claims] 1. An optical amplification module in which an optical functional component is inserted into a collimator beam to couple and branch a backward pumping light and a signal light of a rare-earth doped optical fiber amplifier. The birefringent crystal plate, which is disposed only at an angle, has an optical path formed by attaching an optical film that transmits signal light and reflects excitation light to a second surface facing the first surface as viewed from the signal light. Optical module configuration. 2. An optical film according to claim 1, wherein a wavelength demultiplexing film for reflecting the signal light and transmitting the excitation light is provided on a portion of the first surface where the light beam enters, and an optical film for reflecting at least the excitation light is provided on the second surface. Optical module configuration characterized by being attached. 3. The apparatus according to claim 1, wherein a wavelength demultiplexing film is provided at a portion where the light beam transmitted through the first surface is incident on the second surface, and the point of incidence of the excitation light on the first surface and the first surface of the signal light are provided. An optical module configuration, wherein an optical film that reflects at least the excitation light is provided on the first surface between the points of incidence on the first surface and on the second surface on which the excitation light reflected by the optical film is incident. 4. An optical module according to claim 3, wherein an optical film for reflecting signal light and transmitting excitation light is provided on a portion of the first surface where the signal light is incident. 5. An optical amplification module used for coupling and branching backward pumping light and signal light of a rare earth-doped optical fiber amplifier, wherein an optical functional component is inserted into a collimator beam. On the other hand, the optical path is formed by attaching an optical film that transmits the signal light and reflects the excitation light to the second surface facing the first surface as viewed from the signal light of the tapered birefringent crystal plate that is arranged to be inclined at a certain angle. Optical module configuration characterized by the following. 6. An optical film according to claim 5, wherein a wavelength demultiplexing film for reflecting the signal light and transmitting the excitation light is provided on a portion of the first surface where the light beam enters, and an optical film for reflecting at least the excitation light is provided on the second surface. Optical module configuration characterized by being attached. 7. The method according to claim 1, 3 or 5,
4. A magneto-optical crystal for rotating a polarization axis of a light beam by 45 degrees on an emission optical axis of the signal light, further comprising:
6. An optical module configuration, wherein a birefringent crystal whose crystal axis is rotated by 45 ° with respect to the birefringent crystal according to claim 5 is provided to have an isolation function for signal light. 8. In each of the first to seventh aspects, a non-reflection film is applied to a portion where neither the wavelength demultiplexing film nor the total reflection film is provided in the signal light incidence portion or the excitation light incidence portion. An optical module configuration characterized by adding.