JPH0246432A - Optical amplifier - Google Patents

Abstract

PURPOSE:To amplify only the incident light from one direction and output the same with the characteristic having no dependency on planes of polarization by dividing the incident light to both longitudinally and transversely polarized waves by a polarizing beam splitter, converting these waves to specific polarization states in respectively separate optical paths and amplifying the same. CONSTITUTION:The polarizing beam splitter 1 allows the transmission of the transversely polarized wave and reflects the longitudinally polarized wave perpendicularly. First and second Faraday rotating elements 2 and 3 rotate the plane of polarization of the respective incident polarized waves in positive and negative directions. First and second polarizers 6 and 7 respectively allow the transmission of only these polarized waves and are, therefore, positioned at both ends of a progressive wave amplifier 5 to form the optical paths together with total reflecting mirrors 4, 8, 9. The excellent optical amplifier with which the longitudinally polarized wave of the light separated by the splitter 1 after the incidence from the port X is transmitted and amplified in the arrow A direction and the longitudinally polarized wave in the direction B and are outputted from the port Y without having the dependency on the plane of polarization but the incident light from the port Y is blocked by the polarizers 6 and 7 is, therefore, constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical amplification device used for coherent optical transmission and the like, which amplifies an optical signal as it is.

(Prior art) Optical amplifiers can be used as optical device amplifiers to improve the level of received light, and can also be used as repeaters in coherent optical transmission to improve the performance of optical fiber transmission systems. be able to.

Conventionally, such optical amplification devices are capable of oscillating even at high injection currents by adding anti-reflection films to both end faces of a Fabry-Perot semiconductor laser element to significantly suppress the reflectance of both end faces. A so-called traveling wave optical amplifier is known which has a high gain without any interference (see Saito, Mukabe, and Noguchi, "1.5 μm Band Ga1nAsP Traveling Wave Optical Amplifier," Institute of Electronics and Communication Engineers Technical Research Report 0QE86-114).

(Problem to be Solved by the Invention) However, in the semiconductor laser amplifier constituting the traveling wave optical amplifier, the confinement coefficient is different for horizontally polarized waves and longitudinally polarized waves. However, the problem is that the signal gain is higher in the latter case, resulting in a gain difference between the two waves.

Furthermore, since the traveling wave optical amplifier described above has bidirectional amplification characteristics, when such traveling wave optical amplifiers are connected in multiple stages, a resonator is formed due to the influence of return light due to the residual reflectance of the end face. As a result, there is a problem in that the optical amplifier oscillates.

In view of the above-mentioned problems, an object of the present invention is to obtain a predetermined gain without depending on the polarization plane even if a traveling wave optical amplifier having gain dependence on the polarization plane and a bidirectional amplification function is employed. It is an object of the present invention to provide an optical amplification device that can perform amplification of incident light in only one direction.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a traveling wave optical amplifier in which an antireflection film is added to both end faces of a semiconductor laser element;
First and second polarizers are disposed on both end faces of this optical amplifier, and transmit only polarized waves whose planes of polarization have been rotated by a predetermined angle (7). a polarizing beam splitter that transmits either the horizontally polarized wave or the vertically polarized wave and reflects the other in a predetermined direction; and between the incident light transmitting side of this polarizing beam splitter and the polarizer of a first Faraday rotation element arranged between the incident light reflecting side of the polarizing beam splitter and the second polarizer, which rotates the plane of polarization by a predetermined angle; The second Faraday rotation element rotates by a predetermined angle in the opposite direction.

(Function) According to the present invention, when light with an arbitrary polarization state enters the polarization beam splitter through one input/output port, one of the horizontal polarization and the vertical polarization of the incident light is generated. Polarized waves, for example horizontally polarized waves, are transmitted through the polarizing beam splitter, and longitudinally polarized waves are reflected in a predetermined direction different from the incident direction. The horizontally polarized wave that has passed through the polarizing beam splitter enters the first Faraday rotator, whose plane of polarization is rotated by a predetermined angle, and after further passing through the first polarizer, it is sent to a traveling wave optical amplifier at one end thereof. incident from The polarized wave incident on the optical amplifier is amplified, exits from the other end of the optical amplifier, passes through the second polarizer, and then enters the second Faraday rotation element. The amplified polarized wave is transmitted by this second Faraday rotation element,
In the opposite direction to the rotation direction by the first Faraday rotation element,
The light is rotated by a predetermined angle, that is, it is returned to horizontally polarized waves and returned to the polarizing beam splitter, and further transmitted through the polarizing beam splitter and outputted from the optical amplification device.

On the other hand, the longitudinally polarized wave reflected by the polarizing beam splitter enters the second Faraday rotation element, and its plane of polarization is rotated by a predetermined angle in the direction opposite to the direction of rotation by the first Faraday rotation element. After passing through the polarizer, the light enters the optical amplifier from the other end face. The polarized wave incident on the optical amplifier is amplified, exits from one end face of the optical amplifier, passes through the first polarizer, and then enters the first Faraday rotation element.

The amplified polarized wave is rotated by a predetermined angle by this first Faraday rotation element, that is, returned to a longitudinally polarized wave, returns to the polarizing beam splitter, is further reflected by the polarizing beam splitter, and is transmitted from the optical amplification device. Output.

In addition, in this state, when light having an arbitrary plane of polarization enters the polarization beam splitter from the output direction of the horizontally polarized wave and the vertically polarized wave described above, the horizontally polarized wave passes through the polarized beam splitter.
After entering the second Faraday rotation element and having its plane of polarization rotated by a predetermined angle in the direction opposite to the direction of rotation by the first Faraday rotation element, the wave enters the second polarizer or cannot be passed through. Similarly, the longitudinal polarization is
After being reflected by the polarizing beam splitter and having its plane of polarization rotated by a predetermined angle by the first Faraday rotator, it enters the first polarizer, but cannot pass through the first polarizer. Therefore, neither the horizontally polarized wave nor the vertically polarized wave of the light incident from the output side of the optical amplifier is amplified by the optical amplifier and is not output from the optical amplifier.

(Embodiment) FIG. 1 is a simplified configuration diagram showing an embodiment of an optical amplification device according to the present invention. In FIG. 1, X and Y are input/output ports orthogonal to each other, and 1 is a polarization beam splitter, which transmits horizontally polarized waves of incident light and reflects vertically polarized waves in a direction perpendicular to the direction of incidence. Reference numeral 2 denotes a first Faraday rotation element, which rotates the plane of polarization of incident polarized light by (+) 45 degrees, and is disposed on the optical path to the polarizing beam splitter 1 facing port X. Reference numeral 3 denotes a second Faraday rotation element, which rotates the plane of polarization of the incident polarized light by (-) 45 degrees, and creates a reflected optical path by a total reflection mirror 4 provided in the optical path to the polarizing beam splitter 1 facing port Y. It is placed on the way. Reference numeral 5 denotes a traveling wave optical amplifier for amplifying incident light, which is constructed by adding anti-reflection films to both end faces of a semiconductor laser element, and allows input and output from both end faces. 6 is a polarizer disposed to face one end surface of the optical amplifier 5; 7 is a polarizer disposed to face the other end surface of the optical amplifier 5; Only the plane of polarization rotated by (+)45° ((−)45° from the longitudinal polarization) is transmitted. Also,
The optical amplifier 5 and the polarizers 6 and 7 are arranged in the middle of the optical path reflected by the total reflection mirrors 8 and 9 (both coincident).

Next, the operation of the above configuration will be explained. First, light with an arbitrary polarization state passes through port X to the polarizing beam splitter 1.
The horizontally polarized wave of the incident light is transmitted through the polarizing beam splitter 1, and the vertically polarized wave is reflected.

The horizontally polarized wave transmitted through the polarizing beam splitter 1 is transferred to the first Faraday rotation element 2, as shown by the solid arrow A in FIG.
The light beam enters the optical amplifier 5, its plane of polarization is rotated by (+) 45 degrees, is totally reflected by the total reflection mirror 8, passes through the first polarizer 6, and then enters the optical amplifier 5 from one end face thereof. Optical amplifier 5
The incident polarized wave is amplified, exits from the other end face of the optical amplifier 5, passes through the second polarizer 7, and then enters the second Faraday rotation element 3. The amplified polarized wave changes its plane of polarization to (
-) rotated by 45° to return to transverse polarization. This amplified horizontally polarized wave is totally reflected by the total reflection mirror 4, returns to the polarizing beam splitter 1, and is further transmitted through the polarizing beam splitter 1 and output to port Y.

On the other hand, the longitudinally polarized wave reflected by the polarizing beam splitter 1 is
As shown by the solid line arrow B in FIG. It is reflected and enters the second polarizer 7. The polarized wave incident on the second polarizer 7 rotates the plane of polarization of the vertically polarized wave by (-)45 degrees, but this polarization direction rotates the plane of polarization of the horizontally polarized wave by (+)45 degrees. It is the same as the rotated polarization direction. Therefore, it can pass through the second polarizer 7 and enter the optical amplifier 5 from the other end face. The polarized wave incident on the optical amplifier 5 is amplified and output from one end face of the optical amplifier 5, passes through the first polarizer 6, is totally reflected by the total reflection mirror 8, and undergoes first Faraday rotation. incident on element 2. The plane of polarization of the amplified polarized wave is rotated by (+) 45° by the first Faraday rotation element 2 and returns to a longitudinally polarized wave. This amplified longitudinally polarized wave returns to the polarizing beam splitter 1, is further reflected by the polarizing beam splitter 1, and is output to (B)Y.

Furthermore, when light with an arbitrary polarization plane enters from port Y in the above state, the horizontally polarized wave of this incident light passes through the polarizing beam splitter 1, is totally reflected by the total reflection mirror 4, and is reflected at the second is incident on the Faraday rotation element 3, and its polarization plane is (
-) The light is rotated by 45 degrees, is totally reflected by the total reflection mirror 9, and enters the second polarizer 7, but cannot be transmitted.

Similarly, the vertically polarized wave of the incident light from port Y is reflected by the polarizing beam splitter 1, enters the first Faraday rotation element 2, and after the plane of polarization is rotated by (+) 45 degrees,
Although it is totally reflected by the total reflection mirror 8 and enters the first polarizer 6, it cannot be transmitted. Therefore, the light incident from port Y is not amplified by the optical amplifier 5 and is not output from the optical amplifier.

As described above, according to this embodiment, the polarizing beam splitter 1 separates the horizontally polarized wave and the vertically polarized wave of the incident light, and the horizontally polarized wave and the vertically polarized wave after the separation are transmitted to the first and second Faraday polarized waves. First and second polarizers that are converted into a fixed polarization state by the rotating elements 2 and 3, respectively, and that can transmit only the polarized light in this state.6.
Since the amplification is performed by the optical amplifier 5 via the optical amplifier 7, it is possible to realize an excellent optical amplification device that has optical amplification characteristics without polarization dependence and can amplify and output only incident light from one direction.

In addition, in this example, total reflection mirrors 4, 8.9 are used,
Although the direction of the optical path is changed in this embodiment, the invention is not limited to this, and it goes without saying that, for example, a polarization-maintaining optical fiber may be used as a means for propagating polarized waves between each element.

(Effects of the Invention) As explained above, according to the present invention, there is provided a traveling wave optical amplifier formed by adding an antireflection film to both end faces of a semiconductor laser element, and a traveling wave optical amplifier formed by adding an antireflection film to both end faces of a semiconductor laser element, and a traveling wave optical amplifier provided on both end faces of the optical amplifier. first and second polarizers that transmit only polarized waves whose polarization planes have been rotated by a predetermined angle, and transmit either horizontally polarized waves or vertically polarized waves of incident light from one input/output port. , a polarizing beam splitter that reflects the other in a predetermined direction, and a first Faraday rotation that is disposed between the incident light transmitting side of the polarizing beam splitter and the first polarizer, and that rotates the plane of polarization by a predetermined angle. a second Faraday element, which is disposed between the incident light reflecting side of the polarizing beam splitter and the second polarizer, and rotates the plane of polarization by a predetermined angle in a direction opposite to that of the first Faraday rotation element. Since it is equipped with a rotating element, it is possible to separate horizontally polarized waves and vertically polarized waves into separate optical paths and then convert them to a constant polarized state for amplification, and it has optical amplification characteristics that are independent of polarization plane. In addition, there is an advantage that an excellent optical amplification device that can amplify and output only incident light from one direction can be provided.

[Brief explanation of the drawing]

FIG. 1 is a simplified configuration diagram showing an embodiment of an optical amplifying device according to the present invention. In the figure, 1... Polarizing beam splitter, 2... First Faraday rotation element, 3... Second Faraday rotation element, 4, 8.9... Total reflection mirror 5... Traveling wave type optical amplifier, 6... first polarizer, 7... second polarizer. Patent applicant Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] A traveling wave optical amplifier formed by adding an antireflection film to both end faces of a semiconductor laser element, and a polarized wave whose plane of polarization is rotated by a predetermined angle, which is disposed on each end face of the optical amplifier. a polarizing beam splitter that transmits only one of the horizontally polarized wave and the vertically polarized wave of the incident light from one input/output port and reflects the other in a predetermined direction. and a first Faraday rotation element that is disposed between the incident light transmitting side of the polarizing beam splitter and the first polarizer and rotates the plane of polarization by a predetermined angle, and the incident light of the polarizing beam splitter. A second Faraday rotation element is provided between the reflective side and the second polarizer and rotates the plane of polarization by a predetermined angle in a direction opposite to that of the first Faraday rotation element. Optical amplifier.