1. Field of the Invention
This invention relates to an optical fiber amplifier and a dispersion compensating fiber module for use with an optical fiber amplifier.
2. Description of the Related Art
In recent years, research and development of an optical communication system has been and is being performed energetically, and the importance of booster amplifiers, repeaters or preamplifiers which make use of the technique of optical amplification in which an erbium (Er) doped fiber (an erbium-doped-fiber may be hereinafter referred to as xe2x80x9cEDFxe2x80x9d) is employed has become apparent.
Further, due to the appearance of optical amplifiers, attention is drawn to an optical-amplifier-repeated transmission system since the transmission system plays a very important role in achievement of economization of a communication system in the multimedia society.
By the way, in an ordinary rare earth doped fiber optical amplifier which particularly amplifies a wavelength of a signal, the length of the doped fiber is set to a value at which a maximum gain is obtained in order to assure a high conversion efficiency from pump power to signal power.
Meanwhile, in a wavelength division multiplexing (WDM) optical amplifier which amplifies many channels at the same time, it is important to keep the wavelength dependency of the gain as flat as possible. As a result, the rare earth doped fiber (which will be hereinafter discussed in connection with a representative EDF) must operate in a condition wherein the degree of the saturation of the gain is low. To this end, where the concentration of high level ions is represented by N2 while the concentration of all ions is represented by N1 and N2/N1 is defined as pump ratio, in order to raise the average pump ratio N2/N1 of the doped fiber over the entire length, the length of the doped fiber must be set short.
However, if the doped fiber is formed short in this manner, then much residual pump power will leak out from the other end of the doped fiber, resulting in degradation of the conversion efficiency. Nevertheless, since the required pump power increases as the number of signal wavelengths increases, the output power of a semiconductor pump laser must be raised.
In particular, although it is apparent from the conservative law of energy that the pump power increases as the number of wavelengths increases, a wavelength multiplexing optical amplifier cannot be used in a condition in which it exhibits a high efficiency of conversion from pump power to signal power. This is because, since the rare earth doped fiber is intentionally formed short so as to prevent saturation in order to obtain a gain over a wide bandwidth or to make the gain flat, pump power which has not been converted into a signal will leak out from the other end of the doped fiber.
Accordingly, while high pump power is required originally when comparing with ordinary amplification of only one signal channel, the rare earth doped fiber must be used in a condition wherein the pump power leaks out therefrom.
Thus, in order to effectively make use of thus leaking out residual pump light, a technique has been proposed wherein a reflecting mirror is provided at the other end of a doped fiber so that residual pump light is reflected by the reflecting mirror so as to be introduced back Into the doped fiber so that it may be used for optical amplification again. The technique is disclosed In Japanese Patent Laid-Open Application No. Heisei 3-25985 or Japanese Patent Laid-Open Application No. 3-166782.
However, where residual pump light is reflected by the reflecting mirror in this manner, the pump light is returned not only to the doped fiber but also to the pump source. This pump light may possibly give rise to unstable operation of the pump source such as interference.
By the way, while, due to the appearance of optical amplifiers, attention is drawn to an optical-amplifier-repeated transmission system which includes a plurality of repeating and amplifying optical amplifiers since it plays a very important role in achievement of economization of a communication system in the multimedia society as described above, the transmission system has subjects to be solved in terms of the dispersion compensation, reduction in nonlinear effects (effects having a bad influence on the transmission quality) in an optical fiber serving as a transmission line and economic wide bandwidth wavelength multiplexing transmission.
Generally, an optical fiber serving as a transmission line has a dispersion characteristic and accumulates a dispersion amount in proportion to the length thereof. Usually, however, in an optical fiber transmission system which employs regenerative repeaters, the dispersion amount is reset at the regenerative repeaters. Consequently, the accumulation of the dispersion amount does not make a problem.
However, in an optical-amplifier-repeated transmission system, since a transmitted optical signal is repeated by a kind of analog amplification, the dispersion amount is accumulated. Accordingly, in order to eliminate the accumulation, the signal wavelength used for transmission should be set to a zero dispersion wavelength. This, however, provides the following subjects to be solved:
1-1) Optical fibers have already been laid by a large amount, and unfortunately, those optical fibers have a zero dispersion wavelength at 1.3 xcexcm while an optical amplifier which is expected to be put into practical use soon can amplify only a signal of the 1.55 xcexcm band;
1-2) It has been reported recently that, even if optical fibers whose zero dispersion wavelength is 1.55 xcexcm are laid newly to transmit a signal of 1.55 xcexcm, nonlinear effects occur actively in the optical fibers. This signifies that, if a signal wavelength equal to a zero dispersion wavelength is used for transmission, then undesirable nonlinear effects occur; and
1-3) Particularly in wavelength multiplexing transmission, since a plurality of different signal wavelengths are involved, the concept that the signal wavelengths are set equal to a zero dispersion wavelength cannot be applied.
Accordingly, it has been proposed recently to intentionally displace the signal wavelength from the zero dispersion wavelength suitably and compensate for the dispersion, for example, at the repeater.
While research of dispersion compensators has been and is being performed actively in recent years in this manner, one of dispersion compensators which is expected to be most likely put into practical use is a dispersion compensating fiber (which may be referred to as xe2x80x9cDCFxe2x80x9d; here the term DCF is the abbreviation of Dispersion Compensating Fiber). The DCF, however, has the following subjects to be solved:
2-1) Where fibers (transmission lines) laid already are utilized, a dispersion compensating fiber must be interposed as a device at each repeating point in order to perform dispersion compensation collectively at such each repeating point. Therefore, research and development is being directed to reduction in length of dispersion compensating fibers.
2-2) When fibers are to be laid newly, it is a possible idea not to interpose a dispersion compensating fiber as a device but to lay a dispersion compensating fiber as part of a transmission line. For example, a transmission line of 40 km may be formed from a fiber of 20 km and a dispersion compensating fiber of 20 km. However, research and development of such a novel dispersion compensating fiber as Just mentioned makes overlapping development with research and development of a dispersion compensating fiber for the application described in paragraph 2-1) above.
In summary, in wavelength multiplexing transmission, a wavelength dispersion must be compensated for, and since the compensation for a wavelength dispersion is expected to be most likely put into practical use where a dispersion compensating fiber is employed, it is prospective to use a dispersion compensating fiber. Further, it is investigated to incorporate a dispersion compensating fiber as a part into an optical amplifier repeater. Generally, however, the mode field diameter of a dispersion compensating fiber (DCF) is set small In order to compensate for a dispersion, and consequently, nonlinear effects are liable to occur and, as the dispersion amount to be compensated for increases, also the loss increases.
Thus, it is a possible method to compensate also for the loss of a dispersion compensating fiber using an optical amplifier. In this instance, the loss must be compensated for so that a transmission optical signal may not be influenced by nonlinear effects which degrade the quality of a signal such as self-phase modulation (SPM) and cross-phase modulation (XPM) occurring in the dispersion compensating fiber. Accordingly, the possible method has a problem in that designing of a level diagram is difficult. Further, while a flat and wide optical amplification bandwidth is required for an optical amplifier for WDM, also a rare earth doped fiber optical amplifier has a wavelength dependency of the gain. Accordingly, there is a subject to be solved in that it is difficult to realize a flat and wide amplification bandwidth.
Meanwhile, a rare earth doped fiber optical amplifier having a high gain sometimes suffers from unnecessary oscillations which are produced when it performs optical amplification. If such unnecessary oscillations are produced, the rare earth doped fiber optical amplifier operates but unstably.
For example, in an erbium-doped-fiber optical amplifier, spontaneous emission light (ASE) of 1.53 to 1.57 xcexcm in wavelength is generated when optical amplification is performed, and since the ASE is repetitively reflected from reflection points in the erbium-doped-fiber optical amplifier, unnecessary oscillations are liable to be produced. Particularly with an erbium-doped-fiber optical amplifier adjusted for multiple wavelength collection amplification (that is, an erbium-doped-fiber. optical amplifier having a high pump rate), since it has a high gain In the proximity of 1.53 xcexcm, unnecessary oscillations are liable to be produced at this wavelength. When such unnecessary oscillations are produced, the erbium-doped-fiber optical amplifier operates but unstably.
It is an object of the present invention to provide an optical fiber amplifier wherein stable operation of a pump source (pump light source) is assured and residual pump power which is produced when the average pump ratio is raised is utilized efficiently to Improve the conversion efficiency.
It is another object of the present invention to provide an optical fiber amplifier and a dispersion compensating fiber module for an optical fiber amplifier employing a dispersion compensating fiber wherein the loss of the dispersion compensating fiber by Raman amplification can be compensated for making use of the fact that the threshold value of the Raman amplification is low because the mode field diameter of the dispersion compensating fiber is small.
It is a further object of the present invention is to provide an optical fiber amplifier wherein, where a silica-type-optical-fiber having a Raman amplification function similarly to a dispersion compensating fiber Is employed, the loss of the silica-type-optical-fiber by Raman amplification can be compensated for similarly to the case where a dispersion compensating fiber is used.
It is a still further object of the present invention to provide an optical fiber amplifier which minimizes unstable operation of a rare earth doped fiber optical amplifier having a high gain or a rare earth doped fiber optical amplifier adjusted for multiple wavelength collective amplification.
In order to attain the objects of the present invention described above, according to an aspect of the present invention, there is provided an optical fiber amplifier including a rare earth doped fiber, which comprises first means for introducing pump light into one end of the rare earth doped fiber by way of a first optical coupler, second means for demultiplexing residual pump light originating from the pump light introduced into the one end of the rare earth doped fiber by the first means and arriving at the other end of the rare earth doped fiber by a second optical coupler and reflecting the demultiplexed residual pump light by reflection means so as to be introduced back into the rare earth doped fiber, and third means for preventing the residual pump light introduced back into the rare earth doped fiber by the second means from being introduced into a pump source, from which the pump light to be introduced into the rare earth doped fiber by the first means is produced, by optical isolation means so as to prevent unstable operation of the pump source.
In the optical fiber amplifier, when pump light is introduced into the one end of the rare earth doped fiber by way of the first optical coupler, residual pump light arrives at the other end of the rare earth doped fiber and is then demultiplexed by the second optical coupler, whereafter it is reflected by the reflection means so that it is introduced back into the rare earth doped fiber. In order to prevent unstable operation of the pump source caused by interference of the residual pump light introduced back into the rare earth doped fiber, the optical isolation means is interposed between the pump source and the first optical coupler. Consequently, the optical fiber amplifier is advantageous in that it makes use of the pump power with a high efficiency while assuring stabilized operation of the pump source.
According to another aspect of the present invention, there is provide an optical fiber amplifier including a rare earth doped fiber, which comprises a pump source, a first optical coupler for introducing pump light from the pump source into one end of the rare earth doped fiber, a second optical coupler for demultiplexing residual pump light originating from the pump light introduced into the one end of the rare earth doped fiber by way of the first optical coupler and arriving at the other end of the rare earth doped fiber, a reflecting mirror for reflecting the residual pump light demultiplexed by the second optical coupler so as to be introduced back into the rare earth doped fiber by way of the second optical coupler, and an optical isolator interposed between the pump source and the first optical coupler for preventing unstable operation of the pump source arising from interference of the residual pump light introduced back into the rare earth doped fiber.
In the optical fiber amplifier, when pump light is introduced into the one end of the rare earth doped fiber by way of the first optical coupler, residual pump light arrives at the other end of the rare earth doped fiber and is demultiplexed by the second optical coupler, whereafter it is reflected by the reflecting mirror so that it is introduced back into the rare earth doped fiber. In order to prevent unstable operation of the pump source caused by interference of the residual pump light Introduced back into the rare earth doped fiber, the optical isolator is interposed between the pump source and the first optical coupler. Consequently, the optical fiber amplifier is advantageous in that it makes use of the pump power with a high efficiency while assuring stabilized operation of the pump source.
According to a further aspect of the present invention, there is provided an optical fiber amplifier including a first rare earth doped fiber and a second rare earth doped fiber disposed at front and rear stages, which comprises first means for introducing pump light into one end of one of the first rare earth doped fiber and the second rare earth doped fiber by way of an optical circulator having three or more ports and a first optical coupler, second means for demultiplexing residual pump light originating from the pump light introduced into the one end of the one rare earth doped fiber by the first means and arriving at the other end of the one rare earth doped fiber by a second optical coupler and reflecting the demultiplexed residual pump light by reflection means so as to be introduced back into the one rare earth doped fiber, and third means for causing the residual pump light reflected from the reflection means and introduced back into the one rare earth doped fiber by the second means to follow, after passing the one rare earth doped fiber, a different optical path by the optical circulator and multiplexing the residual pump light in the different optical path with an output of the other one of the first rare earth doped fiber and the second rare earth doped fiber by a third optical coupler.
In the optical fiber amplifier, pump light is first passed through the optical circulator having three or more ports and then introduced into the one end of the rare earth doped fiber at the front stage or the rear stage by the first optical coupler. Then, residual pump light originating from the pump light and arriving at the other end of the rare earth doped fiber is demultiplexed by the second optical coupler and then reflected by the reflection means so that it is introduced back into the rare earth doped fiber. The residual pump light is then introduced, after passing the rare earth doped fiber, into the different optical path by the optical circulator and is multiplexed with an output of the other rare earth doped fiber by the third optical coupler. Consequently, the optical fiber amplifier of the two stage construction just described is advantageous in that it makes use of the pump power with a high efficiency.
According to a still further aspect of the present invention, there is provided an optical fiber amplifier including a first rare earth doped fiber and a second rare earth doped fiber disposed at front and rear stages, which comprises a pump source, a first optical coupler provided at one end of one of the first rare earth doped fiber and the second rare earth doped fiber, a second optical coupler provided at the other end of the one rare earth doped fiber, a third optical coupler provided at one end of the other one of the first rare earth doped fiber and the second rare earth doped fiber, a reflecting mirror for reflecting residual pump light demultiplexed by the second optical coupler so as to be introduced back into the one rare earth doped fiber by way of the second optical coupler, and an optical circulator having three or more ports connected to the pump source, the first optical coupler and the third optical coupler, and wherein pump light from the pump source is introduced into one end of the one rare earth doped fiber by way of the optical circulator and the first optical coupler, and residual pump light originating from the pump light introduced into the one end of the one rare earth doped fiber and arriving at the other end of the one rare earth doped fiber is demultiplexed by the second optical coupler and reflected by the reflecting mirror so as to be introduced back into the one rare earth doped fiber, whereafter the residual pump light is introduced, after passing the one rare earth doped fiber, into a different optical path by the optical circulator so that the residual pump light is thereafter multiplexed with an output of the other rare earth doped fiber by the third optical coupler.
The optical fiber amplifier of the two stage construction just described is advantageous in that it makes use of the pump power with a high efficiency.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier including a first rare earth doped fiber and a second rare earth doped fiber disposed at front and rear stages, which comprises first means for branching pump power at a ratio of n:1, n being a real number equal to or greater than 1, by an optical branching element, multiplexing the pump light from a port of the optical branching element by a first optical coupler and introducing the multiplexed light into one end of one of the first rare earth doped fiber and the second rare earth doped fiber, second means for extracting residual pump power originating from the pump light introduced into the one end of the one rare earth doped fiber by the first means and arriving at the other end of the one rare earth doped fiber by a second optical coupler connected to the other end of the one rare earth doped fiber, multiplexing the extracted residual pump power by a third optical coupler and introducing the multiplexed power into one end of the other one of the first rare earth doped fiber and the second rare earth doped fiber, and third means for multiplexing the pump power from another port of the optical branching element branched by the optical branching element and Introducing the multiplexed power into the other end of the other rare earth doped fiber by a fourth optical coupler.
In the optical fiber amplifier, the pump power is branched at the ratio of n:1, and the pump light from a port of the optical branching element is multiplexed by the first optical coupler and then introduced into the rare earth doped fiber at the front stage or the rear stage. Then, residual pump power is extracted by the second optical coupler connected to the other end of the rare earth doped fiber and is then multiplexed by the third optical coupler. Then, the output light of the third optical coupler is introduced into the one end of the other rare earth doped fiber. Meanwhile, the branched pump power from another port of the optical branching element is introduced into the other end of and multiplexed in the other rare earth doped fiber by the fourth optical coupler. Consequently, the optical fiber amplifier of the two stage construction just described is advantageous in that it makes use of the pump power with a high efficiency.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier including a first rare earth doped fiber and a second rare earth doped fiber disposed at front and rear stages, which comprises a pump source, an optical branching element for branching pump power from the pump source at a ratio of n:1, n being a real number equal to or greater than 1, a first optical coupler for multiplexing the pump light from a port of the optical branching element and introducing the multiplexed light into one of the first rare earth doped fiber and second rare earth doped fiber, a second optical coupler for extracting residual pump power outputted from the one rare earth doped fiber, a third optical coupler for multiplexing the residual pump power extracted by the second optical coupler and introducing the multiplexed power into the other one of the first rare earth doped fiber and the second rare earth doped fiber, and a fourth optical coupler for multiplexing the pump power from another port of the optical branching element branched by the optical branching element and introducing the multiplexed power into the other rare earth doped fiber.
In the optical fiber amplifier, the pump power is branched at the ratio of n:1, and the pump light from a port of the optical branching element is multiplexed by the first optical coupler and then introduced into the rare earth doped fiber at the front stage or the rear stage. Then, residual pump power is extracted by the second optical coupler connected to the other end of the rare earth doped fiber and is then multiplexed by the third optical coupler. Then, the output light of the third optical coupler is introduced into the one end of the other rare earth doped fiber. Meanwhile, the branched pump power from another port of the optical branching element is introduced into the other end of and multiplexed in the other rare earth doped fiber by the fourth optical coupler. Consequently, the optical fiber amplifier of the two stage construction just described is advantageous in that it makes use of the pump power with a high efficiency.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier including a rare earth doped fiber, which comprises a pump source, an optical circulator having three or more ports one of which is connected to the pump source, a first optical coupler for multiplexing pump light introduced thereto from the pump source by way of the optical circulator and introducing the multiplexed light into one end of the rare earth doped fiber, a second optical coupler for demultiplexing residual pump light originating from the pump light introduced into the one end of the rare earth doped fiber by the first optical coupler and arriving at the other end of the rare earth doped fiber, a reflecting mirror for reflecting the residual pump light demultiplexed by the second optical coupler so as to be introduced back into the rare earth doped fiber by way of the second optical coupler, a residual pump light detector for detecting the residual pump light introduced back into the rare earth doped fiber by the reflecting mirror and inputted from the one end of the rare earth doped fiber to the optical circulator by way of the first optical coupler, and a controller for controlling the pump source so that the residual pump light detected by the residual pump light detector may be constant.
In the optical fiber amplifier, pump light is first passed through the optical circulator having three or more ports and is then introduced into the one end of the rare earth doped fiber by the first optical coupler. Then, residual pump light originating from the pump light and arriving at the other end of the rare earth doped fiber is demultiplexed by the second optical coupler and then reflected by the reflecting mirror so that it is introduced back into the rare earth doped fiber. The residual pump power comes out from the one end of the rare earth doped fiber and is then inputted by way of the first optical coupler to the optical circulator, by which it is introduced into the different optical path so that it is monitored by the residual pump light detector. Then, the residual pump power is kept constant under the control of the controller. Consequently, the wavelength characteristic of the gain of the optical fiber amplifier can be controlled so that it may not be varied irrespective of any variation of the input level. Consequently, the optical fiber amplifier is advantageous in that it can be realized readily as a multiple wavelength collective amplifier.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a rare earth doped fiber optical amplification element formed from a rare earth doped fiber, and a Raman optical amplification element which is pumped with pump light to cause Raman amplification to occur, the rare earth doped fiber optical amplification element and the Raman optical amplification element being connected in cascade connection.
The optical fiber amplifier is advantageous in that it makes use of the pump power with a high efficiency while it has a two stage construction.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a rare earth doped fiber optical amplification element formed from a rare earth doped fiber, a Raman optical amplification element which is pumped with pump light, which is capable of pumping the rare earth doped fiber optical amplification element, to cause Raman amplification to occur, the rare earth doped fiber optical amplification element and the Raman optical amplification element being connected in cascade connection, and a pump source for supplying pump light for pumping the rare earth doped fiber optical amplification element and the Raman optical amplification element.
In the optical fiber amplifier, since it includes the pump source for supplying pump light for pumping the rare earth doped fiber optical amplification element and the Raman optical amplification element, the pump power can be utilized with a high efficiency and the number of pump sources to be used can be reduced, which contributes to simplification in construction and reduction in cost.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a rare earth doped fiber optical amplification element formed from a rare earth doped fiber, and a Raman optical amplification element formed from a dispersion compensating fiber which is pumped with pump light to cause Raman amplification to occur, the rare earth doped fiber optical amplification element and the Raman optical amplification element being connected in cascade connection at two front and rear stages.
The optical fiber amplifier just described is advantageous in that it makes use of the pump power with a high efficiency while it has a two stage construction.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a rare earth doped fiber and a dispersion compensating fiber disposed at two front and rear stages, a first pump source for producing pump light of a first wavelength band for the rare earth doped fiber, a first optical coupler for introducing the pump light from the first pump source into the rare earth doped fiber, a second pump source for producing pump light of a second wavelength band for the dispersion compensating fiber, and a second optical coupler for introducing the pump light from the second pump source into the dispersion compensating fiber, the dispersion compensating fiber being pumped with the pump light of the second wavelength band from the second pump source to cause Raman amplification to occur.
With the optical fiber amplifier, compensation for the loss of the dispersion compensating fiber by Raman amplification can be achieved while optical amplification is performed by the rare earth doped fiber.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises an erbium-doped-fiber and a dispersion compensating fiber disposed at two front and rear stages, a pump source for producing pump light, and an optical coupler for introducing the pump light from the pump source into the erbium-doped-fiber, the dispersion compensating fiber being pumped with residual pump light from the erbium-doped-fiber to cause Raman amplification to occur.
With the optical fiber amplifier, compensation for the loss of the dispersion compensating fiber by Raman amplification can be achieved while optical amplification is performed by the erbium-doped-fiber.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises an erbium-doped-fiber and a dispersion compensating fiber disposed at two front and rear stages, a pump source for producing pump light, and an optical coupler for introducing the pump light from the pump source Into the dispersion compensating fiber, the erbium-doped-fiber being pumped with residual pump light from the dispersion compensating fiber.
With the optical fiber amplifier, compensation for the loss of the dispersion compensating fiber by Raman amplification can be achieved while optical amplification is performed by the erbium-doped-fiber.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a dispersion compensating fiber doped with a rare earth element, a pump source for producing pump light for the dispersion compensating fiber, and an optical coupler for introducing the pump light from the pump source into the dispersion compensating fiber.
With the optical fiber amplifier, since the dispersion compensating fiber used is doped with a rare earth element, dispersion compensation can be performed by the dispersion compensating fiber, and the loss of the dispersion compensating fiber can be reduced simultaneously. The optical fiber amplifier with the dispersion compensation function is advantageous also in that it can optically amplify signal light sufficiently.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises an erbium-doped-fiber and a dispersion compensating fiber disposed at two front and rear stages, a pump source for producing pump light for the erbium-doped-fiber, an optical coupler for introducing the pump light from the pump source into the erbium-doped-fiber, and an optical filter interposed between the erbium-doped-fiber and the dispersion compensating fiber for intercepting residual pump light coming out from the erbium-doped-fiber.
With the optical fiber amplifier, leakage pump power Raman amplifies the dispersion compensating fiber. Consequently, the optical fiber amplifier is prevented from unstable operation or from variation of the wavelength dependency of the amplification band thereof.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a rare earth doped fiber optical amplification element formed from a rare earth doped fiber, and a Raman optical amplification element formed from a silica-type-optical-fiber which causes, when pumped with pump light, Raman amplification to occur, the rare earth doped fiber optical amplification element and the Raman optical amplification element being connected in cascade connection at two front and rear stages.
The optical fiber amplifier just described is advantageous in that it makes use of the pump power with a high efficiency while it has a two stage construction.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a silica-type-optical-fiber and an erbium-doped-fiber provided at a front stage and a rear stage, respectively, a silica-type-optical-fiber pump source for producing pump light of a wavelength band for the silica-type-optical-fiber, an optical coupler for introducing the pump light from the silica-type-optical-fiber pump source into the silica-type-optical-fiber, an erbium-doped-fiber pump source for producing pump light of a wavelength band for the erbium-doped-fiber, and another optical coupler for introducing the pump light from the erbium-doped-fiber pump source into the erbium-doped-fiber, the silica-type-optical-fiber being pumped with the pump light from the silica-type-optical-fiber pump source to cause Raman amplification to occur.
With the optical fiber amplifier, compensation for the loss of the silica-type-optical-fiber by Raman amplification can be performed while optical amplification by the erbium-doped-fiber is performed.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises an erbium-doped-fiber having a low noise figure and a silica-type-optical-fiber provided at a front stage and a rear stage, respectively, a silica-type-optical-fiber pump source for producing pump light of a wavelength band for the silica-type-optical-fiber, an optical coupler for introducing the pump light from the silica-type-optical-fiber pump source into the silica-type-optical-fiber, an erbium-doped-fiber pump source for producing pump light of a wavelength band for the erbium-doped-fiber, and another optical coupler for introducing the pump light from the erbium-doped-fiber pump source into the erbium-doped-fiber, the silica-type-optical-fiber being pumped with the pump light from the silica-type-optical-fiber pump source to cause Raman amplification To occur.
With the optical fiber amplifier, compensation for the loss of the silica-type-optical-fiber by Raman amplification can be performed while optical amplification by the erbium-doped-fiber is performed.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a rare earth doped fiber optical amplification element formed from a rare earth doped fiber and having a low noise figure, the rare earth doped fiber optical amplification element being disposed as a front stage amplification element, a Raman optical amplification element for causing Raman amplification to is occur when pumped with pump light the Raman optical amplification section being disposed as a middle stage amplification element, and another rare earth doped fiber optical application element formed from a rare earth doped fiber and disposed as a rear stage amplification element.
With the optical fiber amplifier, the compensation effect of the Raman optical amplification element can be increased. Consequently, a wide bandwidth optical amplifier can be realized while achieving simplification in structure and reduction in cost.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a first erbium-doped-fiber having a low noise figure, a dispersion compensating fiber and a second erbium-doped-fiber provided at a front stage, a middle stage and a rear stage, respectively, a first erbium-doped-fiber pump source for producing pump light of a wavelength band for the first erbium-doped-fiber, an optical coupler for introducing the pump light from the first erbium-doped-fiber pump source into the first erbium-doped-fiber, a dispersion compensating fiber pump source for producing pump light of a wavelength band for the dispersion compensating fiber, another optical coupler for introducing the pump light from the dispersion compensating fiber pump source into the dispersion compensating fiber, a second erbium-doped-fiber pump source for producing pump light of a wavelength band for the second erbium-doped-fiber, and a further optical coupler for introducing the pump light from the second erbium-doped-fiber pump source into the second erbium-doped-fiber, the dispersion compensating fiber being pumped with the pump light from the dispersion compensating fiber pump source to cause Raman amplification to occur.
With the optical fiber amplifier, the compensation effect of the Raman optical amplification element can be increased. Consequently, a wide bandwidth optical amplifier can be realized while achieving simplification in structure and reduction in cost.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a first erbium-doped-fiber having a low noise figure, a silica-type-optical-fiber and a second erbium-doped-fiber provided at a front stage, a middle stage and a rear stage, respectively, a first erbium-doped-fiber pump source for producing pump light of a wavelength band for the first erbium-doped-fiber, an optical coupler for introducing the pump light from the first erbium-doped-fiber pump source into the first erbium-doped-fiber, a silica-type-optical-fiber pump source for producing pump light of a wavelength band for the silica-type-optical-fiber, another optical coupler for introducing the pump light from the silica-type-optical-fiber pump source into the silica-type-optical-fiber, a second erbium-doped-fiber pump source for producing pump light of a wavelength band for the second erbium-doped-fiber, and a further optical coupler for introducing the pump light from the second erbium-doped-fiber pump source into the second erbium-doped-fiber, the silica-type-optical-fiber being pumped with the pump light from the silica-type-optical-fiber pump source to cause Raman amplification to occur.
With the optical fiber amplifier, the compensation effect of the silica-type-optical-fiber can be increased. Consequently, a wide bandwidth optical amplifier can be realized while achieving simplification in structure and reduction in cost.
According to a yet further aspect of the present invention, there is provided a dispersion compensating fiber module for an optical fiber amplifier, which comprises a dispersion compensating fiber, and a pump source for pumping the dispersion compensating fiber to cause Raman amplification to occur.
Where an optical fiber amplifier is constructed using the module wherein the dispersion compensating fiber is pumped to cause Raman amplification to occur, it exhibits a reduced loss due to reduction of the loss by the dispersion compensating fiber.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier including a dispersion compensating fiber, which comprises a pump source, and an optical coupler for introducing pump light from the pump source into the dispersion compensating fiber, the dispersion compensating fiber being pumped with pump light from the pump source to cause Raman amplification to occur.
Also the optical fiber amplifier is advantageous in that the loss of the dispersion compensating fiber can be reduced.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a pump source, and an optical coupler for introducing pump light from the pump source into the silica-type-optical-fiber, the silica-type-optical-fiber being pumped with the pump light from the pump source to cause Raman amplification to occur.
The optical fiber amplifier is advantageous in that the loss of the silica-type-optical-fiber can be reduced.
According to a yet further aspect of the present invention, there is provided an optical fiber amplifier, which comprises a rare earth doped fiber optical amplification element formed from a rare earth doped fiber, and an optical fiber attenuation element formed from an optical fiber or an optical fiber with an optical isolator for suppressing unstable operation of the rare earth doped fiber optical amplification element.
The optical fiber amplifier is advantageous in that stabilized optical amplification can be achieved with unstable operation of the rare earth doped fiber optical amplification element suppressed.
According to a yet further aspect of the present, invention, there is provided an optical fiber amplifier, which comprises an optical amplification unit including a front stage optical amplification element and a rear stage optical amplification element each formed as a rare earth doped fiber optical amplification element formed from a rare earth doped fiber, and an optical fiber attenuation element formed from an optical fiber or an optical fiber with an optical isolator interposed between the front stage optical amplification element and the rear stage optical amplification element of the optical amplification unit for suppressing unstable operation of the optical amplification unit.
The optical fiber amplifier is advantageous in that stabilized optical amplification can be achieved with unstable operation of the optical amplification unit suppressed.
Further objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference characters.