Optical fiber amplifier

Abstract: Optical fiber amplifier in which a signal light source is integral with an exciting light source. The optical fiber amplifier comprises Fabry-Proit semiconductor laser element 1 which is a light emitting element having a wide spectrum, fiber grating 2 having diffraction grating which reflects only the light having a wave length in the signal light which can be amplified by the optical fiber, on the longer wave length side of the Fabry-Proit type semiconductor laser element 1, an inline isolator 3 having a high isolation in the same wave length range as that of the signal light and an optical fiber 4 to stimulatedly amplifying the signal light. Signal light source 8 transmitting the signal light and an exciting light source 9 exciting the signal light are formed in the same Fabry-Perot semiconductor laser element 1 . Accordingly, since the signal light from the same emitting element can be stimulatedly amplified, reduction in size and cost of the optical fiber and enhancement in noise factor characteristics can be achieved.

Description: TECHNICAL FIELD The present invention relates to an optical fiber amplifier having a signal light source and exciting light source which are integral with each other. BACKGROUND OF THE INVENTION A pri or art opti cal fiber amplifier has a si gnal light source for generating a signal light and an exciting light source for generating an exciting light which are separately provided. Fig. 3 is a block diagram showing an exemplary configuration of a prior art optical fiber amplifier 1. The pri or art opti cal fiber amplifier 1 comprises an inline isolator 3, a rare earth-doped opti cal fiber 4, an external modulator 5, a wave length combining coupler (multiplexer) 7, a signal light source 8 and an exciting light source 9. In Fig. 3, the signal light source for generating the signal light is separated from the exciting light source for generating the exciting light and they are disposed in different positions. The signal light which is generated from the signal light source 8 is combined with the exciting light generated from the exciting light source 9 by the wave length combining coupler 7 and the combined signal light is passed through the inline isolator 3 and the rare earth-doped optical fiber 4 and is incident upon the external modulator 5. The prior art optical fiber amplifying device disclosed in Japanese Patent Kokai Publication JP-A-9-160085 which is shown as an exemplary prior art 2 comprises two discrete optical fiber amplifiers so that an optical fiber amplifying device of a two stage type or an independent bidirectional transmitting type is formed. In this prior art optical fiber amplifier 2, reduction in current flowing through the exciting light source, enhancement in gain and increase in output are achieved by increasing the exciting efficiency. A pri or art opti cal fiber amplifying device disclosed in Japanese Patent Kokai Publication JP-A-9-222621 which is shown as an exemplary pri or art 3 comprises a composite optical component having a capability of receiving the power of input signal light which is provi ded on the input side of the optical fiber amplifier. This optical fiber amplifying device is configured so that it has an exciting mechanism for transmitting the exciting light in a direction opposite to that of the signal light. However, the following problems have been encountered in the course of investigations toward the present invention. Namely, since the signal light source is separated from the exciting light source in both the above-mentioned prior arts, a high cost is a problem, and the size of the entire of the optical fiber amplifier becomes larger since accommodating spacing in which the light sources are separately disposed is required. It is therefore an object of the present invention to provide a novel optical fiber amplifier free of the aforementioned drawbacks. Particularly, it is a further object of the present invention to provide an optical fiber amplifier comprising signal light source and exciting light source which are integral with each other. Other objects of the present invention will become apparent in the entire disclosure. The present invention as defined in claim 1 provides an optical fiber amplifier having a signal light source emitting a signal light and an exciting light source emitting an excitation light exciting the optical fiber which amplifies said signal light, characterized in that said optical fiber amplifier comprises: a light emitting element comprising said exciting light source; a grating (2) for establishing an external resonator with said light emitting element, adapted to reflect light having a wavelength which is so much longer than a center wavelength of said light emitting element, said wavelength being in a range that can be amplified by said optical fiber amplifier so, that laser oscillation occurs both at said wavelength and another shorter wavelength of the light emitted by said light emitting element, said wavelength and said shorter wavelength being the wavelengths of said signal excitation lights correspondingly an inline isolator for substantially isolating light in a wavelength range including the wavelengths of said signal and excitation lights, and an optical fiber adapted to stimulatedly amplify said signal light. According to a preferred embodiment of the present invention, there is provided an optical fiber amplifier having a signal light source transmitting a signal light and an exciting light source exciting the signal light, characterized in that the optical fiber amplifier comprises: a light emitting element having a wide spectrum range, a fiber grating having a diffraction grating adapted to reflect only signal light having a wave length which is on the longer wave length side than that (i.e. a center wave length) of the light emitting element, and the former wave length is in a range that can be amplified by the optical fiber amplifier, an inline isolator having a high isolation ability in the same wave length range as that of said signal light, and an optical fiber adapted to stimulatedly amplify the signal light. The signal light source and the exciting light source are formed of the same light emitting element. Preferably, the light emitting element is a Fabry-Perot semiconductor laser element. The Fabry-Perot semiconductor laser element may have an oscillating wave length characteristic in the 1.48 µm band and the oscillated light may have a wide spectrum of the 1.48 µm band. The optical fiber may be a rare earth doped optical fiber. The rare earth doped optical fiber may be an optical fiber which is formed by adding any of the rare earth elements into a host glass of quartz glass or fluoride glass. The optical fiber amplifier may comprise a 1.55 µm band signal light source module transmitting the signal light, and a 1.48 µm band exciting light source exciting the rare earth doped optical fiber. The signal light source module and the exciting light source may be formed as an integral module. The optical fiber amplifier may further comprise an external modulator for modulating the signal light. The external modulator may be disposed at a rear stage of the optical fiber, the external modulator modulating the signal light which has been amplified by the optical fiber. The external modulator may be disposed between the fiber grating and the inline isolator. The optical fiber amplifier may further include a wave length-division demultiplexer and a wave length-division multiplexer which are disposed at upstream and downstream of said external modulator, respectively. The demultiplexer may be optically connected with the multiplexer via a bypass circuit so that the separated lights which have been separated by the demultiplexer are combined with one another in the multiplexer via the bypass circuit. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a circuit block diagram showing the configuration of the first combined embodiment of the optical fiber amplifier of a present invention. Fig.2 is a circuit block diagram showing the configuration of a second embodiment of the optical fiber amplifier of the present invention. Fig. 3 is a block diagram showing an exemplary prior art circuit configuration. PREFERRED EMBODIMENTS Now, modes of embodying an optical fiber amplifier of the present invention will be described in detail with reference to annexed drawings. Referring now to Figs. 1 and 2, an embodiment of the opti cal fiber amplifier of the present invention is shown. Figs. 1 and 2 show the optical fiber amplifiers of first and second embodiments of the present invention, respectively. In Fig. 1, the optical fiber amplifier which is the first embodiment of the present invention comprises a Fabry-Perot type semiconductor laser element 1, fiber grating 2, inline isolator 3, rare earth doped optical fiber 4 and an external modulator 5 in this order. The optical fiber amplifier of the present embodiment comprising the above-mentioned components is formed as a module in which a 1.55 µm band signal light source which transmits a si gnal light is integral with a 1.48 µm band exciting light source which excites the rare earth doped optical fiber. The Fabry-Perot type semiconductor laser element 1 is capable of generating light having an oscillation characteristic of 1.48 µm band wave length using a crystal of indium In, garium Ga, arsenic As, phosphorus P and the like. Accordingly, the light excited by this Fabry-Perot type semiconductor laser element 1 has a wide spectrum range of 1.48 µm band(i.e., having a broad spectrum band with a center (peak) wave length of 1.48µm). The fiber grating 2 comprises a diffraction grating for the signal light having a wave length of the 1.55 µm band in a range on the longer wave length side than that (i.e., the center wave length) of the Fabry-Perot type semiconductor laser element 1, provided that the former range can be amplified by the opti cal fiber amplifier. The fiber grating 2 is a narrow band type fiber grating in which a Bragg diffraction grating is formed in a fiber core by utilizing the change in ultra-violet light induction diffraction index. The diffraction grating has a wave length band of the signal light range, on the side of longer wave length than that (i.e., center peak spectrum) of the Fabry-Perot type semiconductor 1, provided that the former wave length band can be amplified by the optical fiber amplifier. The diffraction granting has a reflectivity of 0.1 to 50 %, particularly 2 to 10-%. The half value width of the fiber grating 2 is preset to a desired value in such a manner that the Fabry-Perot type semiconductor laser element 1 is desired to be oscillated in a narrow band. The inline isolator 3 is an inline isolator using Faraday rotator and the like and has a high isolation ability in the same wave length range as that of the signal light. The rare earth doped opti cal fiber 4 is formed by addition of quartz glass or fluoride glass with rare earth elements such as erbium Er and/or neodium Nd so that the ions of the rare earth elements added to the glasses are activated by the exciting light in the 1.48 µm band to provide an inverted population. This causes the signal light in the 1.55 µm band to be subjected to stimulated amplification. The external modulator 5 may include EA (electro-absorption) modulator, LN modulator etc. using effects such as electro-optical effect or A0 (acousto-optic) modulator using effects such as acoustic optical effect. In the optical fiber amplifier comprising the above-mentioned components in such a manner, an "external" resonator is established by and between the Fabry-Perot semiconductor laser element 1 and the fiber grating 2 having a wave length different (apart) from that of the former laser element 1. The resonation of this resonator enables to oscillate the light in a narrow band, that is 1.55 µm band of the signal light wave length range, which band corresponds to the range of the diffraction grating of the fiber grating 2, that is the wave length range extending on the longer wave length side from that of the Fabry-Perot type semiconductor laser element 1, and which band can be amplified by the opti cal fiber amplifier. As mentioned above, the wave length of the Fabry-Perot type semiconductor laser element 1 is separated from that of the diffraction grating of the fiber rating 2 by about 20 or 30 nm or more. Accordingly, not the entire of the wave length range of the single Fabry-Perot type semiconductor laser element 1 can be drawn (taken over) into a wave length range of the fiber grating 2. Thus, in this system oscillation may also occur in the wave length range of the single Fabry-Perot type semiconductor laser element 1. Therefore, two wave lengths such as a wave length which is determined by the fiber grating 2 and another wave length of the Fabry-Perot type semiconductor laserelement 1 are taken out. In the present embodiment, the lights having two wave lengths are used as si gnal light and exciting light for the optical fiber amplifier. Referring now to Fig. 1 again, the optical fiber amplifier of the present embodiment takes the signal light and the exciting light out of the same Fabry-Perot type semiconductor laser element 1. After the amplifier amplifies the taken out signal light in the rare earth doped optical fiber 4, the signal light is modulated by the external modulator 5. Now, operati on in the first embodiment of the present invention shown in Fig. 1 will be described with reference to the annexed drawings. The light which is emitted from the Fabry-Perot type semiconductor laser element 1 is collected to an optical fiber having the fiber grating 2 in which the Bragg diffraction grating is formed. Part of the light is reflected on the fiber grating 2 so that an external resonator is formed between the fiber grating 2 and the rear side of the Fabry-Perot type semiconductor laser el ement 1. The external resonator is capable of oscillating the light having only a desired wave length which is determined by the diffraction grating of the fiber grating 2 in the wave length range of the Fabry-Perot type semiconductor laser element 1 which has a wide oscillation wave length width of 50 nm (40 dB down width) or more. That is, the external resonator is capable of oscillating light in a narrow band having a half value width of 1 nm or less within the wide wave length range of the Fabry-Perot type semiconductor laser element. As mentioned above, the wave length of the Fabry-Perot type semiconductor laser element 1 is separated from that of the diffraction grating of the fiber grating 2 by several tens nm. Accordingly, not all the wave lengths of the single Fabry-Perot type semiconductor laser element 1 can be taken over into the wave length range determined by the fiber grating 2. As a result, two wave lengths, i.e. a wave length which is determined by the fiber grating 2 and another wave length of the Fabry-Perot type semiconductor laser element 1 can be taken out. The light beams having two wave lengths pass through the inline isolator 3 and is incident upon the rare earth doped optical fiber 4. The signal light in 1.55 µm band is amplified by the stimulated emission of 1.48 µm exciting light in the rare earth doped optical fiber 4. The amplified signal light is modulated to a high frequency by the external modulator 5 and is taken out of the optical fiber end (not shown). ASE (amplified spontaneous emission) which is generated in the rare earth doped optical fiber 4 is prevented from entering the Fabry-Perot type semiconductor laser el ement 1 by the i nl i ne isolator 3. A first effect of the first embodiment of the present invention resides in that the optical fiber amplifier can be made compact. The reason is that the signal light source is integral (unison) with the exciting light source in contrast to the prior art in which they had to be separate. A second effect of the first embodiment of the present invention resides in that an economical optical fiber amplifier can be provided. The reason is that the signal light source is integral with the exciting light source and necessity of the wave length combining coupler (multiplexer) for combining the signal light with the exciting light can be omitted, in contrast to the prior art in which the multiplexer is essential. A third effect of the first embodiment of the present invention resides in that the NF (noise factor) of the optical fiber amplifier can be lowered. The reason is that since no wave length combining coupler (wave length-division multiplexer) is required, the loss at the wave length combining coupler is eliminated so that the loss at an input stage upstream of the rare earth doped optical fiber can be reduced. Now, a second embodiment of the present invention will be described with reference to the annexed drawings. In Fig. 2, the optical fiber amplifier of the second embodiment of the present invention comprises a Fabry-Perot type semiconductor laser element 1, fiber grating 2, wave length-division coupler (demultiplexer) 6, external modulator 5, wave length combi ni ng coupler (wave length-division multiplexer) 7, inline isolator 3 and rare earth doped opti cal fiber 4, which are in series connected in a serial manner. The wave length-division demultiplexer 6 is coupled to the wave length combining coupler 7 so that the lights, the wave lengths of which are separated by the wave length-division demultiplexer 6 are combined with each other in the wave length combining coupler 7. In the second embodiment of the present invention, the light which exits from the fiber grating 2 is separated into the signal light and the exciting light by the wave length-division demultiplexer 6 and only the signal light is modulated by the external modulator 5. After the modulation, the signal light is combined (multiplexed) with the exciting light by the wave length combining coupler 7 and the signal light is amplified by the rare earth doped optical fiber 4. Although preferred embodiments of the present invention have been described, the invention is not limited thereto. A number of various modifications can be made without departing from the gist of the invention. The meritorious effect of the present invention are summarized as follows. As is apparent from the foregoing description, in the optical fiber amplifier of the present invention, light in the wave length band of the si gnal light range, on the longer wave length side than that (center spectrum) of the light emitting element having a wide spectrum range, which can be amplified by the optical fiber amplifier is oscillated by the fiber grating. Since the wave length of the light emitting element is separate from that of the fiber grating by several tens nm, not all the wave lengths of the light from the light emitting element can be taken over into the diffraction wave length range of the fiber grating so that oscillation may al so occur in the wave length range of the light emitting element. Therefore, the signal light from the light emitting element having the same signal light and exciting light source can be stimulatedly amplified. It should be noted that other objects of the present invention will become apparent in the entire disclosure and that modifications may be done without departing the scope of the present invention as disclosed herein and appended herewith. Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features. Statements in this specification of the "objects of the invention" relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims. The description of the invention with reference to the drawings is by way of example only. The text of the abstract filed herewith is repeated here as part of the specification. Optical fiber amplifier in which a signal light source is integral with an exciting light source. The optical fiber amplifier comprises Fabry-Proit semiconductor laser el ement 1 which is a light emitting el ement having a wide spectrum, fiber grating 2 having diffraction grating which reflects only the light having a wave length in the signal light which can be amplified by the optical fiber, on the longer wave length side of the Fabry-Proit type semiconductor laser element 1, an inline isolator 3 having a high isolation in the same wave length range as that of the signal light and an optical fiber 4 to stimulatedly amplifying the signal light. Signal light source 8 transmitting the signal light and an exciting light source 9 exciting the signal light are formed in the same Fabry-Perot semiconductor laser element 1. Accordingly, since the signal light from the same emitting element can be stimulatedly amplified, reduction in size and cost of the optical fiber and enhancement in noise factor characteristics can be achieved.

Claims: An optical fiber amplifier having a signal light source emitting a signal light and an exciting light source emitting an excitation light exciting the optical fiber which amplifies said signal light, characterized in that said optical fiber amplifier comprises: a light emitting element (1) comprising said exciting light source; a grating (2) for establishing an external resonator with said light emitting element, adapted to reflect light having a wavelength which is so much longer than the center wavelength of said light emitting element, said wavelength being in a range that can be amplified by said optical fiber amplifier, that laser oscillation occurs both at said wavelength and another shorter wavelength of the light emitted by said light emitting element, said wavelength and said shorter wavelength being the wavelengths of said signal and excitation lights correspondingly, an inline isolator (3) for substantially isolating light in a wavelength range including the wave lengths of said signal and excitation lights, and an optical fiber (4) adapted to stimulatedly amplify said signal light. An optical fiber amplifier according to Claim 1, wherein said grating (2) comprises a diffraction grating of a fiber grating. An optical fiber amplifier according to Claim 1 or 2, characterized in that said light emitting element (1) is a Fabry-Perot semiconductor laser element. An optical fiber amplifier according to Claim 3, characterized in that said Fabry-Perot semiconductor laser element (1) has a spectrum band with a centre wave length of 1.48 µm. An optical fiber amplifier according to any preceding claim, characterized in that said optical fiber (4) is a rare earth doped optical fiber. An optical fiber amplifier according to Claim 5, characterized in that said rare earth doped optical fiber (4) is an optical fiber which is formed by adding any of the rare earth elements into a host glass of quartz glass or fluoride glass. An optical fiber amplifier according to any preceding claim, characterized in that said optical fiber amplifier comprises a 1.55 µm band signal light source module transmitting said signal light, and a 1.48 µm band exciting light source exciting said rare earth doped optical fiber, said signal light source module and said exciting light source being formed as an integral module. An optical fiber amplifier according to any preceding claim, characterized in that said optical fiber amplifier further comprises an external modulator (5) modulating said signal light. An optical fiber amplifier according to Claim 8, characterized in that said external modulator (5) is disposed at a rear stage of said optical fiber (4), said external modulator (5) modulating said signal light which has been amplified by said optical fiber (4). An optical fiber amplifier according to any preceding claim, characterized in that said light emitting element (1), said grating (2), said inline isolator (3), and said optical fiber (4) are optically coupled in that order. An optical fiber amplifier according to Claim 8, characterized in that said external modulator (5) is disposed between said grating (2) and said inline isolator (3), that said optical fiber amplifier further includes a wave length demultiplexer (6) and a wavelength multiplexer (7) which are disposed upstream and downstream of said external modulator (5), respectively, and that said demultiplexer (6) is connected with said multiplexer (7) through a bypass circuit so that the separated lights which have been separated by said demultiplexer (6) are combined with one another in said multiplexer (7) via said bypass circuit. An optical fiber amplifier according to Claim 11, characterized in that said light emitting element (1), said grating (2) and said demultiplexer (6) are optically coupled in that order, and wherein said multiplexer (7), said inline isolator (3) and said optical fiber (4) are optically coupled in that order. An optical fiber amplifier according to any preceding claim, characterized in that said grating (2) is of a substantially narrow wave length as compared to the spectrum range of the light emitting element (1). An optical fiber amplifier according to any preceding claim, characterized in that said grating (2) is capable of oscillating a band of 1 nm or less in terms of half width value.

Inventors: KANDA MASAHIRO, NAMIWAKA MASAHIKO, KANDA, MASAHIRO, NAMIWAKA, MASAHIKO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1999
Main classification: H01S3
IPC codes: H01S3, H01S5