AN OPTICAL LAUNCHING DEVICE FOR OPTICAL DIRECT AMPLIFIER

Abstract: A high power pump light source for optical direct amplifier using the stimulated Raman effect is obtained by using a plurality of semiconductor lasers (11, 12) each of which provides low output power. More than two beams of the same wavelength generated by semiconductor lasers are combined (polarization multiplex) by using a polarization prism (15) block (15), which provides sufficient high power pump light for the stimulated Raman effect amplifier. The output of said polarization prism block is wavelength division multiplexed with signal light to be amplified by using an interference filter block (16), output of which is applied to a transmission optical fiber (13d) in which amplification of said signal light is effected.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical launching device, which is used as an optical source of an optical direct amplifier using the stimulated Raman scatter effect in an optical fiber. The Raman effect is the phenomenon that when light of the angle frequency (ω) illuminates a material, another light of the angle frequency (ω+Δω) is generated depending upon the transition of the vibration state and/or the spin state of the material. The light of (ω-Δω) is called Stokes light, and the light of (ω+Δω) is called anti-Stokes light. When a strong laser beam of the angle frequency (ω) is input to a material, the Stokes light of (ω-Δω) is generated through the Raman effect, and the vibration of the frequency (Δω) is generated by the beat between the input beam and the Stokes beam. The generated beam triggers another Stokes beam, and the beat. The stimulated Raman scatter is generated by above process. An optical direct amplification is carried out by using the stimulated Raman scatter phenomenon by non-linear effect of an optical fiber. For instance, such an amplifier is reported in n o 11115, 4-239 and n o 1113, 4-237 in the annual meeting in 1985 of the Institute of Electronics and Communication in Japan. Fig. 7 shows a prior optical direct amplifier using stimulated Raman scatter effect. In the figure, the numeral 71 is a pump light source, 72 is a light combiner (multiplexer), 73 is a wavelength separator, 74 is an optical fiber for pump light, 75 is an optical fiber for signal, and 76 is an optical fiber for transmission (Raman effect). In the figure, the pump light in the optical fiber 74 is combined with the signal light in the optical fiber 75. Then, the combined light is transmitted in the optical fiber 76, where the signal light is amplified by the stimulated Raman effect. The wavelength separator 73 separates only the desired wave length as an amplified output signal. In the optical direct amplifier using the stimulated Raman effect as shown in Fig. 7, the pump wavelength is selected so that the Stokes wavelength generated by the pump wavelength is almost the same as the wavelength of the signal to be amplified. Therefore, a high power pump light source 71 and an optical coupler 72 for coupling both pump light and signal light to a common optical fiber are essential. Conventionally, a pump light source 71 is a solid laser like a YAG laser. And an optical combiner 72 for combining a pump light with a signal light is implemented by a half mirror. By the way, it has been known that silica based optical fiber has a small attenuation in the wavelength 1.3 µm and 1.5 µm. In order to amplify those wavelengths, the preferable wave lengths of the pump light source are 1.2 µm and 1.4 µm, respectively. However, a conventional solid laser, like YAG laser and/or E r laser can generate only 1.06 µm or 1.55 µm, which is unfortunately different from said preferable wavelength. Further, those solid lasers (YAG laser and E r laser) have the disadvantage that the size is large and the operation is not easy. A semiconductor laser which generates said preferable wavelength is now available in the commercial market. A semiconductor laser is small in size as compared with a solid laser, and it is easy to operate and the wavelength is adjustable. However, a semiconductor laser has the disadvantages that the output power is insufficient for a pump light source 71. The apparatus of Fig. 7 has another disadvantage that the combiner 72 which is implemented by a half mirror has relatively large optical attenuation, since a half mirror reflects a part of pump light and signal light. Documents EP-A-177 800, EP-A-153 722 and JP-A-56 137303 disclose optical wavelength multiplexer/demultiplexer without reference to an optical device for providing high power light for an optical direct amplifier using stimulated Raman effect. SUMMARY OF THE INVENTION It is an object, therefore, of the present invention to overcome the disadvantage and limitation of a prior optical source for an optical direct amplifier by providing a new and improved optical device. It is also an object of the present invention to provide an optical device which provides a high power optical beam for optical direct amplifier with the desired wavelength. It is also an object of the present invention to provide an optical device for an optical direct amplifier in which both pump light and signal light are combined into a common optical fiber with small attenuation. The above and other objects are attained by an optical launching device to be used with an amplifier using the stimulated Raman effect, the optical device comprises : two laser diodes both emitting light at the same wavelength corresponding to a pump wavelength of the Raman amplifier and each providing a linearly polarized output beam, the respective states of polarization being orthogonal to each other ; a first polarization prism arranged to combine the output beams of the two laser diodes coherently and provide a combined pump beam ; a first wavelength multiplexer arranged to receive said combined pump beam and superpose a signal light beam having a different wavelength from said combined pump beam, thereby to create a combined pump-signal beam ; and means to launch said pump-signal beam into a transmission fibre. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features, and attendant advantages of the present invention will be understood by means of the following description and accompanying drawings wherein : Fig. 1 shows a structure of an optical coupler according to the present invention, Fig. 2 shows a structure of another embodiment of an optical coupler according to the present invention, Fig. 3 shows a structure of still another embodiment of the present optical coupler, Fig. 4 shows a structure of still another embodiment of the present optical coupler, Fig. 5 shows a structure of still another embodiment of the present optical coupler, Fig. 6 shows a structure of still another embodiment of the present optical coupler, and Fig. 7 shows a prior optical direct amplification system using the stimulated Raman scatter effect. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows the first embodiment of the present invention, which is used in an optical direct amplifier using stimulated Raman effect. In the figure, the numerals 11 and 12 are respectively the first semiconductor laser diode module and the second semiconductor laser diode module for optical source. Those two modules provide the same wavelength as each other. The numerals 13a and 13b are polarization maintaining optical fibers located at the outputs of said laser modules 11 and 12, respectively, for accepting light from the laser modules 11 and 12. A pair of collimating lenses 14a and 14b are located at the outputs of said optical fibers 13a and 13b, respectively, for collimating the optical beams. The polarization prism block 15 combines (polarization combination) the beams from the collimating lenses 14a and 14b in coherenet condition, and provides a pump output light. The numeral 13c is an optical fiber for signal light to be amplified. The collimating lens 14c is located at the output of the optical fiber 13c to collimate the output beam of the optical fiber 13c. The interference filter block 16 effects the wavelength multiplex combination of the pump beam from the polarization prism block 15 and the signal light from the collimating lens 14c. The collimating lens 14d and the transmission optical fiber 13d are provided at the output of the interference filter block 16. The numeral 17 is the boundary plane of the polarization prism block 15, and the numeral 18 is the interference filter plane of the interference filter block 16. The output beam of the first semiconductor laser diode module 11 applied to the collimating lens 14a through the optical fiber 13a is converted to a parallel beam by the collimating lens 14a, and is applied to the transmission optical fiber 13d through the polarization prism block 15, the interference filter block 16 and the collimating lens 14d. It is supposed that the polarization plane of the above beam is P state wave, in which the polarization plane is parallel to the plane 17 of the polarization prism block 15. On the other hand, the output beam of the second semiconductor laser diode module 12 through the optical fiber 13b and the collimating lens 14b is reflected by the boundary plane 17 of the polarization prism block 15, and then, that beam is applied to the transmission optical fiber 13d through the block 16 and the collimating lens 14d. The beam in this case is supposed to be S state wave which is perpendicular to the plane 17. The signal light is interposed with the pump beam in the interference filter block 16 simply using the fact that the wavelength band of the signal light differs from that of the pump light (for instance, the wavelength shift of a silica based optical fiber is about 425 cm⁻¹ in wave numbers. In the first embodiment of Fig.1, the high power pump beam is obtained since two semiconductor lasers are used, and the same wavelength beams of those lasers are combined by a polarization prism block. Further, the high power pump beam is directly multiplexed with signal light which is to be amplified by using an interference filter (wavelength multiplex combiner) without using an intermediate lens and an intermediate optical fiber. Furthermore, it should be noted that the interference filter block is optically coupled directly with the polarization prism block, without using an optical fiber and/or a collimating lens. So, the optical attenuation between those two blocks does not occur. Therefore, the coupling loss is very small. In one embodiment, two semiconductor lasers with output power of 100 mW each are used. The optical power of each laser when the light is introduced into an optical fiber is about 50 mW, and two lights of 50 mW are combined to provide 100 mW, which is enough for stimulated Raman effect. Fig.2 shows the second embodiment of the present invention. The feature of Fig.2 is that the polarization prism block 15 is attached directly to the interference filter block 16 by using transparent optical contact adhesive. The embodiment of Fig.2 has the advantage that the optical loss is smaller than that of the embodiment of Fig.1, which has spacing between two blocks. Fig.3 shows the third embodiment of the present optical coupler. The feature of that embodiment is that the semiconductor lasers 31 and 32 are directly coupled with the collimating lenses 14a and 14b, respectively, without using the optical fibers 13a and 13b of Fig.1. The polarization prism block 15 is directly coupled with the interference filter block 16 as is the case of Fig.2. The embodiment of Fig.3 has the advantage that the optical loss is smaller than that of the previous embodiments. Fig.4 shows the fourth embodiment of the present optical coupler. In the figure, two polarization prism blocks 43 and 44 are coupled with two interference filter blocks 45 and 46, as shown in the figure. The two linear polarization beams in the optical fibers 41a and 41b having the same wavelength (λ₁) as each other are applied to the polarization prism block 43 with the polarizations perpendicular, and are combined. The combined beam is reflected by the interference filter block 45 which transmits only the wavelength (λ₂) and reflects other wavelengths, and is further reflected by the other interference filter block 46 which transmits only the wavelength (λ s ) and reflects other wavelengths. The output of the interference filter block 46 is applied to the transmission optical fiber 41f through the collimating lens 42f. Similarly, the beams generated by other semiconductor lasers (not shown) coupled with the optical fibers 41c and 41d having the wavelength (λ₂) and the linear polarization are applied to the polarization prism block 44 through the collimating lenses 42c and 42d, and are combined. Then, the combined beam transmits the interference filter block 45, and the output of the block 45 is reflected by the interference filter block 46 the output of which is coupled with the transmission optical fiber 41f through the collimating lens 42f. Furthermore, the signal light having the wavelength (λ s ) in the optical fiber 41e transmits the interference filter block 46 and is coupled with the transmission optical fiber 41f through the collimating lens 42f. Therefore, all the linear polarization beams through the optical fibers 41a, 41b, 41c, 41d and 41e are coupled with the transmission optical fiber 41f with no attenuation theoretically. It should be noted that the wavelength of the pump light with respect to the wavelength of signal light to be amplified is not severe in an amplifier using stimulated Raman effect, but some error of wavelength of pump light is allowable. Therefore, a pump wavelength does not need to be a single wavelength, but a plurality of close wavelengths are possible. Fig.4 uses the above fact. The wavelength (λ₁) from the optical fibers 41a and 41b are combined (polarization multiplex) in the polarization prism blcok 43, and the lights of the wavelength (λ₂) in the optical fibers 41c and 41d are combined in the polarization prism block 44. The outputs of two polarization prism blocks 43 and 44 are wavelengths multiplexed in the interference filter block 45, which provides the final strong pump light. The above members (43, 44, 45) comprise a pump light source. The signal light in the optical fiber 41e with the wavelength (λ s ) is wavelength multiplexed with the wavelengths (λ₁ and λ ₂) in the interference filter block 46, output of which is coupled with the transmission optical fiber 41f. The embodiment of Fig.4 has the advantage that the higher pump light with small loss is obtained. In one embodiment of Fig.4, the first wavelength (λ₁) is 1440 nm, the second wavelength (λ₂) is 1460 nm, and the wavelength of the signal light is 1550 nm. Fig.5 shows the modification of the embodiment of Fig.4. The feature of Fig.5 is that three close wavelengthes (λ₁, λ₂ and λ₃) are combined (polarization multiplex) to provide a pump light source. Therefore, three polarization prism blocks 53a, 53b, and 53c, three interference filter blocks 54a, 54b and 54c, and a prism 55 are provided. Those members are attached directly with one another by using transparent optical contact adhesive. The numerals 51a through 51h are optical fibers, 52a through 52h are collimating lenses, 53a through 53c are polarization prism blocks, 54a through 54c are interference filter blocks. Fig.6 shows still another embodiment of the present invention, which is the modification of the embodiment of Fig.4. The feature of Fig.6 is that the polarization prism blocks are separated from the interference filter blocks to allow the freedom of the design of the apparatus. Therefore, the polarization prisms 63 and 64 are separated from the interference filter block 65 as shown in the figure. The interference filter block 65 is provided with the interference filters 66, 67, and 68. In the figure, the numerals 61a through 61d are optical fibers, 61e is an optical fiber for signal light, 61f is a transmission optical fiber, 62a through 62f are collimating lenses. As mentioned above, according to the present invention, two linear polarization beams with the same wavelength as each other are combined (polarization multiplex) by using a polariza tion prism block. Therefore, high power pump light source is obtained even when low power semiconductor lasers are used. Furthermore, the pump light beam is directly combined with signal light to be amplified without using a lens and/or an optical fiber. Therefore, the coupling loss is small. Additionally, as all the members including a polarization prism block and an interference filter block are mounted in a single housing, the size of the apparatus may be small, and the operation of the apparatus is simple. Also, when more than two close wavelengthes are combined, the higher pump light source is obtained as well. From the foregoing it will now be apparent that a new and improved optical coupler has been found. It should be understood of course that the embodiments disclosed are merely illustrative and are not intended to limit the scope of the invention. Reference should be made to the appended claims, therefore, rather than the specification as indicating the scope of the invention.

Claims: An optical launching device to be used with an amplifier using the stimulated Raman effect, the optical device comprises : two laser diodes (11, 12), both emitting light at the same wavelength corresponding to a pump wavelength of the Raman amplifier and each providing a linearly polarized output beam, the respective states of polarization being orthogonal to each other ; a first polarization prism (15, 43) arranged to combine the output beams of the two laser diodes coherently and provide a combined pump beam ; a first wavelength multiplexer (16, 46) arranged to receive said combined pump beam and superpose a signal light beam having a different wavelength from said combined pump beam, thereby to create a combined pump-signal beam ; and means (14d) to launch said pump-signal beam into a transmission fibre (13d). An optical device according to claim 1, wherein the wavelength of said pair of semiconductor lasers is 1.2 µm, and the wavelength of said signal light is 1.3 µm. An optical device according to claim 1, wherein the wavelength of said pair of semiconductor lasers is 1.4 µm and the wavelength of said signal light is 1.5 µm. An optical device according to claim 1, wherein said polarization prism is directly coupled with said wavelength multiplexer. An optical device according to claim 1, wherein said wavelength multiplexer is an interference filter. An optical device according to claim 1, further comprising a second polarization prism (44) multiplexing two linearily polarized beams of a second wavelength (λ₂) which is close to said pump wawelength, generated by two semiconductor lasers, a second wavelength multiplexer (45) combining the outputs of said first polarization prism (43) and said second polarization prism and providing said pump-beam, said first polarization prism (43), and said second polarization prism (44) being directly coupled to said second wavelength multiplexer, which is directly coupled optically with said first wavelength multiplexer (46).

Inventors: MOCHIZUKI, KIYOFUMI, TAMURA, YASUAKI
Applicants: KOKUSAI DENSHIN DENWA CO., LTD, OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1987
Main classification: G02B6
IPC codes: G02B27, G02B6, G02F1, H01S3, H01S5