WO2002079851A1

WO2002079851A1 - An gain-clamped erbium-doped fiber amplifier for long wavelength band

Info

Publication number: WO2002079851A1
Application number: PCT/KR2002/000550
Authority: WO
Inventors: Donghan Lee
Original assignee: Neotek Research Co Ltd
Current assignee: Neotek Research Co Ltd
Priority date: 2001-03-28
Filing date: 2002-03-28
Publication date: 2002-10-10
Anticipated expiration: 2003-09-28
Also published as: KR20020077061A

Abstract

The present invention relates to an erbium doped optical fiber amplifier for a long wavelength band which can improve an amplification gain of a long wavelength band optical signal by reflecting a portion of backward amplified spontaneous emission generated from the pumped erbium doped fiber back to the pomped erbium doped fiber; wherein a pump light is divided in two directions by a coupler, and the erbium doped fiber is pumped in two directions, and a reflecting means is arranged between an input terminal and the erbium doped optical amplification optical fiber to apply the backward amplified spontaneous emission of a conventional band to the erbium doped optical amplification optical fiber together with the long wavelength band signal light.

Description

AN GAIN-CLAMPED ERBIUM-DOPED FIBER AMPLIFIER FOR LONG WAVELENGTH BAND

Background of the invention Field of the invention

The present invention relates to an optical fiber amplifier for a long wavelength band, and more particularly, to an optical fiber amplifier for a long wavelength band which divides a pump light into two directions to pump an erbium-doped fiber amplifier from both directions, and arranges a reflecting means between an input terminal and a pumped erbium-doped fiber amplifier to apply an backward amplified spontaneous emission of a conventional band to the erbium- doped fiber amplifiers together with a signal light of a long wavelength band, thereby greatly improving an amplification gain of a light signal of a long wavelength band. Also, the present invention relates to an optical fiber amplifier for a long wavelength band which automatically fixes a gain by maintaining the population inversion of the amplifier constant even though the number of input channel is varied and obtains a stable gain clamping and a high amplification efficiency by arranging a reflecting means on a front end of a gain clamping amplifying portion.

Description of Related Art

In order to cope with a current rapid increase of a communication capacity, there are many researches for a wavelength division multiplexing (hereinafter, WDM) system which increases the transmission capacity by transmitting several channels having different wavelengths through one piece of optical fiber. Up to now, an optical communication system has used a band of 1525 nm to 1565nm as a signal wavelength band, and currently a variety of trials have been made to use a long wavelength band of 1565nm 1615nm so as to cope with an increase of transmission capacity. The long wavelength band is a portion that a gain coefficient of an erbium-doped optical fiber amplifier is small, and a lengthy erbium-doped optical fiber and a relatively large pumping light are required to use the long wavelength band. However, this has a problem in that an efficiency is low and a noise figure is increased, and thus an active research to improve this has been tried.

A band of 1550 nm is usually used in current optical communication systems. The 1550nm band is very efficient and thus can use a relatively small pump light. However, since there is a large gain difference on wavelength, a gain flattening filter is required. As a result, it is very difficult to lower the cost of the amplifier.

There is an expectation that demands for a long-distance transmission system and a metro WDM/DWDM system are rapidly increased. In the metro system which is relatively short in distance, a loss resulting from an insertion of elements has to be compensated. In addition, there is a need for a low price optical amplifier which does not require a large gain and output but has a basic characteristic.

FIG. 1 is a view illustrating a first example of a conventional erbium-doped optical fiber amplifier. In the conventional erbium-doped optical fiber amplifier, a signal light input from a first isolator 101 and a pumping light input from a laser diode 102 are coupled in a wavelength division multiplexer 103 and then are input to an erbium-doped fiber (104; hereinafter, EDF). The pumping light is added to the EDF 104 to excite an erbium which is a rare earth ion. As a result, the input signal light is amplified by a stimulated emission and is output through a second isolator 105. The conventional erbium-doped optical fiber amplifier uses a short wavelength band without causing a big problem. However, when it uses a long wavelength band, since a length of the EDF 104 becomes lengthy and an absorption of the pumping light is increased, an amplification is possible in a front portion of the EDF 104 while an absorption occur in a rear portion thereof. That is, there are many problems in that gain is typically low and an amplification efficiency is bad in a long wavelength band.

FIG. 2 is a view illustrating a second example of the conventional erbium- doped optical fiber amplifier implemented by Massicott et al to solve the problems of the first example [J.F. Massicott, R. Wyatt, and B. J. Ainslie, "Low noise operation of Er doped silica fibre amplifier around 1.6μm", Electronic Letters, 28, pp1924-25 (1992)].

The second example of the conventional erbium-doped optical fiber amplifier, as shown, has a wavelength division multiplexer 107 and a1550 nm band laser diode 106 in addition to the configuration of the first example of FIG. 1. Therefore, a light signal of the laser diode 106 output from the wavelength division multiplexer 107 is added to a signal light input through the first isolator 101 , and then amplified and output. In the case of the second example, since a 1550 nm band light is included, a 1550 nm band light having a high gain is efficiently amplified in even a short length, and an absorption occurs while the light proceeds along a long EDF in a forward direction to continuously supply a gain to a long wavelength band, whereupon a gain and an efficiency are increased in comparison to the first example.

However, the second example has a problem in that it not only requires the wavelength division multiplexer 107 and the signal laser diode 106 which are high in price but also increase a noise factor of the long wavelength band signal light due to an insertion loss.

A third example of FIG. 3 shows an optical amplifier using a 980 nm laser diode 108 as a pumping light which is low in price and is easily obtained. An absorption length of the 980nm laser diode 108 is shorter than that of the 1480 nm laser diode, and thus there is a problem in that an efficiency becomes lower in a long wavelength band.

A fourth example of FIG. 4 shows a structure of an optical amplifier that a signal light is not only amplified in a very short length but also can be amplified continuously along the long EDF.

That is, in the fourth example, a short-length EDF 104 is placed at the first front end and is pumped by a 980nm laser diode 108. Thereafter, an amplification gain of a long-wavelength band light signal is improved by applying a long- wavelength band signal light which is amplified by the EDF 104 and at the same time a backward amplified spontaneous emission of a 1550 nm band generated from the EDF 104 excited by the laser diode 108 to the long length of EDF 112[H.S. chung, M. S. Lee, N. Park, and D. J. DiGiovanni, Electronics Letters, 35, pp1099- 1100(1999)]. Although the fourth example can improve a noise factor and an output characteristic, but it requires a 980nm band laser diode 108, the wavelength division multiplexer 109 and the isolator 105 which are high in cost. Therefore, the fourth example is not suitable for a low cost long wavelength band optical amplifier.

FIG. 5 shows a fifth example having an economical structure that an amplified spontaneous emission of a 1550 nm band and a signal light are together applied to the EDF 104.

Here, a part of 1550 nm band among the backward amplified spontaneous emission coming out of the EDF 104 is reflected back by an optical fiber Bragg grating that reflects only a selected wavelength band and does not causes a loss in other wavelength bands. An amplification gain of the long wavelength band light signal is improved by applying a part of the reflected 1550 nm band together with the input long wavelength band signal light to the pumped EDF 104.

The fifth example is a structure including merely only a fiber Bragg grating in addition to the structures of the examples described above and thus can be manufactured at a low cost. However, the fifth example can improve a performance but cannot provide a sufficient efficiency required in the metro system. Therefore, there is a problem in that a high power laser diode which is high in price is required in order to generate a desired output power.

In the WDM network, the number of channel can be varied due to changes of components, failures of components thereof and frequent add/drops of channel. A variation of a total input signal intensity coming from this channel number variation generates a transient phenomena for the output of the rest channels and so increases transmission errors momentarily due to gain variations. Therefore, in the WDM network, a gain variation due to the power variation of an input signal has to be minimized.

A method of automatically controlling a gain of such an optical amplifier includes a method by controlling a pump light and a method by applying a compensating signal. FIG. 6 is a block diagram illustrating a gain fixing optical amplifier according to a conventional art, and particularly shows a structure that applies a compensating signal. As shown, the gain fixing optical amplifier includes a first isolator 21 passing an input signal through one side thereof, a first optical coupling portion 11 coupling outputs of the first isolator 21 and a filter 31 , a laser diode 71 generating a population inversion in an EDF 41 , a first wavelength division multiplexer 12 coupling outputs of the first optical coupling portion 11 and the laser diode 71 , the EDF 41 amplifying a signal received from the first wavelength division multiplexer 41 , a second isolator 22 blocking a light reflected to an output of the EDF 41 and transmitting light transmitted to the output of the EDF 41 , a second optical coupling portion 15 dividing an output of the second isolator 22, a third isolator 23 isolating the signal divided by the second optical coupling portion 15, a filter 31 passing a certain wavelength from an output of the third isolator 23 to the first optical coupling portion 11 , and a fourth isolator 24 receiving and isolating the signal divided by the second optical coupling portion 15 to output a light signal to which a compensating signal is applied.

A gain of the conventional gain fixing optical amplifier is determined by a loss in two optical coupling portions 11 and 15 that forms a ring shaped structure. Also, such a conventional technique underwent many researches in a C-band amplifier, but a little research documents exist in a L-band amplifier. When the gain fixing optical amplifier is configured in a long wavelength band and only the conventional method is used, there is a problem in that performance thereof becomes greatly lowered in comparison to the existing C- band. In this case, when the pumping is performed using a 980 nm LD, the 980 nm light source is absorbed in a very short portion of the front portion, and there is no pumping light source in the rest portions so that absorption occurs, whereupon amplification efficiency is bad. Thus, a compensating signal does not increase continuously and it cannot perform a desired gain fixing function.

In addition, for the sake of the metro system communication, gain and efficiency must be high in reality, and the manufacturing cost must be low. Therefore, there is a need for an optical amplifier to meet the demands.

SUMMARY OF THE INVENTION

To overcome the problems described above, an object of the present invention is to provide an optical fiber amplifier for a long wavelength band which divides a pump light into two directions to pump an erbium-doped fiber amplifier from both directions, and arranges a reflecting means between an input terminal and a pumped erbium-doped fiber to apply an backward amplified spontaneous emission of a conventional band to the erbium-doped fiber amplifier together with a signal light of a long wavelength band, thereby improving greatly an amplification gain of a light signal of a long wavelength band.

Another object of the present invention is to provide an erbium-doped fiber amplifier for a long wavelength band which automatically fixes a gain by maintaining a population inversion of the amplifier constant even though the number of input channel is varied and obtains a stable gain fixation and a high amplification efficiency by arranging a reflecting means in a front end of a gain fixation amplifying portion.

Another object of the present invention is to provide an erbium-doped fiber amplifier for a long wavelength band which arranges a reflecting means between an input terminal and a pumped erbium-doped fiber amplifier to apply an backward amplified spontaneous emission of a conventional band to the erbium-doped fiber amplifier together with a signal light of a long wavelength band, so that without adding a high-cost pumping laser diode and an isolator, an efficiency of the optical fiber amplifier can become high, a stable gain fixing characteristic can be obtained, and an amplification gain can improved although it is economically configured.

In order to achieve the above object, the preferred embodiments of the present invention provide an erbium-doped optical fiber amplifier for a long wavelength band for amplifying a signal light, comprising: a laser diode for generating a pumping laser light amplifying the signal light; a coupler for receiving the pumping laser light from the laser diode and dividing the pumping laser light at a certain ratio to be output; first and second wavelength division multiplexers for receiving the divided pumping light from the coupler to apply the pumping light in directions opposite to each other, the first wavelength division multiplexer located in an incident light direction, the second wavelength division multiplexer located in an output light direction; an erbium-doped fiber amplifier located between the first and second wavelength division multiplexers, receiving the signal light from an incident side and the pumping lights from the first and second wavelength division multiplexers, and transmitting the signal light through a light pumping; and a reflecting means located in front of the first wavelength division multiplexer to output the signal light and re-transmitting a backward amplified spontaneous emission of a conventional region from the first wavelength division multiplexer to the erbium-doped optical amplificaiton optical fiber.

The present invention further provides an erbium-doped fiber amplifier for a long wavelength band for amplifying an incident signal light, comprising: a laser diode module including at least one laser diode for generating a pumping laser light amplifying the signal light; a coupler receiving the pumping laser light from the laser diode module and dividing the pumping laser light at a certain ratio to be output; first and second wavelength division multiplexers for receiving the divided pumping light from the coupler to apply the pumping light in directions opposite to each other, the first wavelength division multiplexer located in an incident light direction, the second wavelength division multiplexer located in an output light direction; an erbium-doped fiber amplifier located between the first and second wavelength division multiplexers, receiving the incident signal light and the pumping lights from the first and second wavelength division multiplexers, and transmitting the signal light through a light pumping; and a reflecting means located in front of the first wavelength division multiplexer to output the signal light and re-transmitting a backward amplified spontaneous emission from the first wavelength division multiplexer to the erbium-doped optical amplificaiton optical fiber.

The present invention further provides a gain fixing optical fiber amplifier for a long wavelength band of an optical communication system for amplifying an incident signal light through at least one pumping laser diode for amplifying the incident signal light, first and second wavelength division multiplexers, and an erbium-doped fiber amplifier located between the first and second wavelength division multiplexers, the amplifier comprising: a first optical fiber Bragg grating for outputting the incident signal light and re-transmitting a backward amplified spontaneous emission from the first wavelength division multiplexer to the erbium-doped fiber amplifier; and second and third optical fiber Bragg gratins including a resonator to cause a laser oscillation using a part of an amplification emission generated from the erbium-doped fiber amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which like reference numerals denote like parts, and in which:

FIG. 1 is a view illustrating a first example of a conventional erbium-doped optical fiber amplifier for a long wavelength band;

FIG. 2 is a view illustrating a second example of a conventional erbium- doped optical fiber amplifier for a long wavelength band;

FIG. 3 is a view illustrating a third example of a conventional erbium-doped optical fiber amplifier for a long wavelength band;

FIG. 4 is a view illustrating a fourth example of a conventional erbium- doped optical fiber amplifier for a long wavelength band;

FIG. 5 is a view illustrating a fifth example of a conventional erbium-doped optical fiber amplifier for a long wavelength band; FIG. 6 is a block diagram illustrating a gain fixing optical amplifier according to a conventional art;

FIG. 7 is a view illustrating an erbium-doped optical fiber amplifier for a long wavelength band according to one embodiment of the present invention;

FIG. 8 is a view illustrating a characteristic of the erbium-doped optical fiber amplifier according to one embodiment of the present invention;

FIG. 9 is a characteristic view of a gain fixing erbium-doped optical fiber amplifier for a long wavelength band according to another embodiment of the present invention;

FIG. 10 is a configuration view illustrating an erbium-doped optical fiber amplifier for the long wavelength band according to another embodiment of the present invention;

FIGs. 11 A to 11 D are views to describe examples that the reflecting means of the FIG. 7 is varied;

FIG. 12 is a view illustrating an erbium-doped optical fiber amplifier for a long wavelength band according to still another embodiment of the present invention

FIG. 13 is a characteristic view of the erbium-doped optical fiber amplifier for a long wavelength band according to still another embodiment of FIG. 12;

FIG. 14 is a configuration view of the gain fixing erbium-doped optical fiber amplifier for a long wavelength band according to still another embodiment of the present invention;

FIG. 15 is a block diagram of a gain fixing erbium-doped optical fiber amplifier for a long wavelength band according to an embodiment of the present invention in an L-band;

FIG. 16 is a configuration view of a gain fixing erbium-doped optical fiber amplifier for a long wavelength band according to still another embodiment of the present invention in an L-band; and

FIG. 17 is a view illustrating a result data obtained from measuring a characteristic of a gain fixing erbium-doped optical fiber amplifier for a long wavelength band according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFFERED EMBODIMENTS

Reference will now be made in detail to preferred embodiments of the present invention, example of which is illustrated in the accompanying drawings.

FIG. 7 is a view illustrating an erbium-doped optical fiber amplifier for a long wavelength band according to one embodiment of the present invention.

As shown, the erbium-doped optical fiber amplifier for the long wavelength band according to one embodiment of the present invention includes first and second isolators 301 and 308, a laser diode 302, a coupler 303, an optical fiber

Bragg grating 304, two wavelength division multiplexers 305 and 307, and an EDF

306.

The erbium-doped optical fiber amplifier of the present invention has a structure which places the EDF 306 at a central portion and performs a pumping from two sides through the two wavelength division multiplexers 305 and 307. A signal light to be amplified is input from the first isolator 301. A pumping light generated from the laser diode 302 is divided into two directions at a certain ratio through the coupler 303 and then are input to the two wavelength division multiplexers 305 and 307. Therefore, a pumping is carried out from two sides with the EDF 306 centered.

The optical fiber Bragg grating 304 functions as a reflecting means. The optical fiber Bragg grating 304 applies a C-band backward amplified spontaneous emission together with the input signal light of the long wavelength band to the EDF 306 again.

FIG. 10 is a configuration view illustrating an erbium-doped optical fiber amplifier for the long wavelength band according to another embodiment of the present invention. The optical fiber Bragg grating 304 is located between the first wavelength division multiplexer 305 and the EDF 306 to function as a reflecting means. A gain fixing characteristic is obtained by selecting a reflecting wavelength of the optical fiber Bragg grating 304 long close to a 1560 nm.

FIG. 8 is a view illustrating a characteristic of the erbium-doped optical fiber amplifier according to one embodiment of the present invention. Here, measured characteristics of one embodiment of the present invention and the third and fifth examples of the conventional art are shown.

In the drawing, a characteristic curve A denotes a characteristic of one embodiment of the present invention, a characteristic curve B denotes a characteristic of the fifth example of the conventional art, and a characteristic curve C denotes a characteristic of the third example of the conventional art. An intensity (Pin) of the input light is -4dB, an x axis denotes a wavelength [nm], and a y axis denotes an amplification gain [dB].

As shown in the characteristic curve C, the third example of the conventional art cannot achieve a flat gain condition even though it uses a 170mW. In the characteristic curve B, in order to obtain a gain almost similar to one embodiment of the present invention using the fifth example of the conventional art, a pumping of 170mW is required. On the other hands, it is understood that as shown in the characteristic curve A the optical amplifier according to the first example of the present invention shows a good gain characteristic even at a small pumping light intensity of 103 mW. Also, a noise figure is less than 6 dB over the entire section. That is, it is understood that even at a small pump light intensity, a good noise characteristic is shown.

In the erbium-doped optical fiber amplifier shown in FIG. 7, a performance of the amplifier according to a division ratio of the coupler 303 is searched. As a result, as the ratio of a pump light intensity divided into a backward direction becomes large, the amplifier has showed the almost same characteristics at lower pump light intensity. However, noise figure increased significantly when the ratio is higher than 60%. When an efficiency and a noise figure are considered together, a division ratio of the coupler 303 is preferably between 60:40 and 50:50. But, the range of the division ratio that satisfies system specifications is wide.

Like a characteristic curve A, a performance according to a wavelength and band of the optical fiber Bragg grating 304 is compared, and it is understood that an efficiency becomes higher as a reflecting wavelength is close to 1555nm. There may occur an application problem in the case that a light intensity in a reflected wavelength in one embodiment of the present invention is significantly large not to be ignored in comparison to an intensity of an amplified signal. However, in this case, when the optical fiber Bragg grating close to 1545 nm is used, a light intensity in a reflected wavelength is lower than a amplified spontaneous noise level, and thus the problem is solved. In consideration of this point, an acceptable characteristic is shown between 1520nm and 1568nm, and in the WDM system which selects respective wavelength elements after amplification to measure, even though an optical fiber Bragg grating of a longer wavelength is used, little problem occurs. In the case of a wide band, a better characteristic is shown, but there is only a little difference, and thus it can be used in various situations.

One embodiment of the present invention has a coupler 303 and one more wavelength division multiplexer than the fifth example of the FIG. 5. However, the two components can use a simple optical fiber fusion type and are low in cost. On the other hands, the laser diode 302 is very economical since a low power type can be used compared to the conventional art.

The optical fiber Bragg grating functioning as a reflecting means applies the backward amplified spontaneous emission to the EDF 306 again together with the a signal light of the long wavelength band. At this time, when the reflecting wavelength of the optical fiber Bragg grating 304 is set to be close to 1560 nm, it can function to greatly improve an amplification gain of the long wavelength band and maintaining a gain constantly in regardless of a size of the input signal.

FIG. 9 is a characteristic view of the a gain fixing erbium-doped optical fiber amplifier for a long wavelength band, and shows a measurement result of an amplification gain and a noise factor according to the input signal intensity when the optical fiber Bragg grating 304 reflecting a relatively long wavelength and the optical fiber Bragg grating 304 reflecting a relatively short wavelength are used.

Here, a characteristic A shows a characteristic of a long wavelength (1560nm) reflection, and a characteristic B shows a characteristic of a short wavelength (1545nm) reflection. Also, a pump light has an intensity of 98mW/25mW and is a two-way pumping, and a pump wavelength is 980nm. An x- axis denotes an input signal intensity [dBm], and a y-axis denotes an amplification gain and a noise factor gain/NF [dB], respectively. When the optical fiber Bragg grating of a short wavelength (1545nm) reflection like the characteristic curve B is used, it shows a tendency that in a small input signal a gain is reduced gradually as an amplification gain is large and a signal is large.

On the other hands, the optical amplifier according to one embodiment of the present invention like the characteristic curve A shows that a gain of 22dB is hardly varied in the input signal of from -30dBm to -12dBm, and thus it can be used as a gain fixing optical amplifier in the case that the input signal is less than 12dBm. Since a gain is fixed to 22dB even in the higher input signal when the pump light intensity is increased, it can be used in various system conditions. Also, it is understood that since a gain is fixed even in a higher input signal by increasing a reflecting ratio at the reflecting wavelength, various conditions can be applied.

In the present invention, an excellent characteristic is shown in that by using the 980nm pump laser diode and sending the existing band amplification emission of the optical fiber Bragg grating to the reflecting amplifying terminal, an amplification efficiency is higher while performing a gain fixation. In addition, since the laser resonator is not used for a gain fixation, a relaxation oscillation phenomenon does not occur, and thus the transmission quality is not deteriorated. FIG. 10 is a configuration view that the optical fiber Bragg grating, the reflecting means of the erbium-doped optical amplifier for the long wavelength band according to the present invention, is placed between the first wavelength division multiplexer 305 and the EDF 306. It operates in the same way as the optical fiber Bragg grating is placed between the first isolator 301 and the first wavelength division multiplexer 305 as shown in FIG. 7.

FIGs. 11 A to 11 D are views to describe examples that the reflecting means of the FIG. 7 is varied.

As shown in FIG. 11A, as the reflecting means, an additional coupler 311 and a reflecting mirror 312 are used. In this case, it is preferred that a division ratio of the reflecting mirror 312 coupled to the coupler is less than 10% to reduce a loss of the input long wavelength band signal and a reflection of the backward amplified spontaneous emission is adjusted appropriately.

As shown in FIG. 11 B, as the reflecting means, a coupler 321 and a reflecting optical fiber Bragg grating 322 reflecting a 1550nm band are used. As shown in FIG. 11 C, an air gap is used as the reflecting means. In this case, it is preferred that the air gaps 331 of a capacitor type opposite to each other are arranged to reflect the backward amplified spontaneous emission by about 4% and re-input it in a direction of a pumped EDF 306. Hence, a loss of the input long wavelength band signal light is sufficiently small and thus hardly affects a noise figure.

As shown in FIG. 11 D, a facing reservoir 341 by a liquid having a refractive index is used as the reflecting means. In this case, a reflectance is lower than 4%, so that the backward amplified spontaneous emission re-input into the EDF is decreased, whereupon a loss of the input long wavelength band signal light is sufficiently small not to affect the noise figure.

FIG. 12 is a view illustrating an erbium-doped optical fiber amplifier for a long wavelength band according to another embodiment of the present invention. As shown, the erbium-doped optical fiber amplifier for a long wavelength band has a laser diode module 309 including two pumping laser diodes 309a and 309b in addition to a configuration of that of FIG. 7. This is to obtain an amplification gain higher than that of FIG. 7, and the laser diode module 309 can be configured by coupling a plurality of laser diodes if required.

In the another embodiment of the prevent invention of FIG. 12, outputs from the two laser diodes 309a and 309b of the laser diode module 309 are divided into two directions according to a ratio determined in the coupler 303 and then pump the EDF 306 in two directions.

Even in this case, as the reflecting means the optical fiber Bragg grating 304 is placed between the input terminal and the forward pumped EDF 306. Therefore, the backward amplified spontaneous emission is applied to the EDF 306 together with the signal light. As a result, an amplification gain of the long wavelength band is significantly improved, and a flat gain can be obtained even at the large input.

The optical fiber Bragg grating 304 functions as the reflecting means. The optical fiber Bragg grating 304 applies the backward amplified spontaneous emission and the signal light of the input long wavelength band to the EDF 306 again. At this time, when the reflecting wavelength of the optical fiber Bragg grating 304 is set to be close to 1560nm, it functions to improve an amplification gain of a long wavelength band while maintaining a gain constant regardless of the intensity of the input signal.

FIG. 13 is a characteristic view of the gain fixing erbium-doped optical fiber amplifier for a long wavelength band according to another embodiment of FIG. 11.

As shown, 980nm laser diodes 309a and 309b of 170/110mW are used respectively, and it is understood that a high output is obtained over a long wavelength band section.

FIG. 14 is a configuration view that in the gain fixing erbium-doped optical fiber amplifier for a long wavelength band according to another embodiment of FIG. 12, the optical fiber Bragg grating 304 functioning as the reflecting means is placed between the first wavelength division multiplexer 305 and the EDF 306. It operates in the same way as the optical fiber Bragg grating 304 is placed between the first isolator 301 and the first wavelength division multiplexer 305 as shown in FIG. 11.

In addition, in another embodiment of the present invention of FIG. 12, an amplification characteristic can be improved by verifying the reflecting means of FIGs. 11 A to 11 D in various methods, and it is possible to couple various reflecting means which is not shown in the drawings.

Also, it is possible to couple the laser diodes which constitute the laser diode module 309 to improve a gain by adding necessary diodes. FIG. 15 is a block diagram of the erbium-doped optical fiber amplifier for a long wavelength band according to an embodiment of the present invention. The erbium-doped optical fiber amplifier includes a first isolator 301 receiving a signal light to be amplified, a first optical fiber Bragg grating 411 functioning as a reflecting means which is located in front of a first wavelength division multiplexer 305 to reflects an incident light from the first isolator 301 and re-transmits a backward amplified spontaneous emission from the first wavelength division multiplexer 305 to an erbium-doped fiber amplifier 306, second and third optical fiber Bragg gratings 412 and 412 constituting a resonator to cause a laser oscillation using a part of the amplification emission generated in the EDF 306, the first and second laser diodes 302 and 312 generating a pumping laser light to amplify the signal light, the first and second wavelength division multiplexers 305 and 307 receiving the pumping light and the signal light from the first and second laser diodes 302 and 312 wherein the first wavelength division multiplexer 305 is located in an incident light direction and the second wavelength division multiplexer 307 is located in an emitting light direction, the erbium-doped fiber amplifier 306 located between the first and second wavelength division multiplexers 305 and 307 to receive the incident light from an incident side through the first isolator 301 and the pumping light from the first and second wavelength division multiplexers 305 and 307 so as to amplify and transmit the signal light through a pumping, and a second isolator 308 outputting the signal light transmitted from the third optical fiber Bragg grating 413.

Here, the second optical fiber Bragg grating 412 is located between the first optical fiber Bragg grating 411 and the first wavelength division multiplexer 305, and the third optical fiber Bragg grating 413 is located between the second wavelength division multiplexer 307 and the second isolator 308. Operation of the present invention having an above-described configuration will be explained in detail with reference to the attached FIG. 16.

First, the present invention adds a reflecting means between an input terminal and the erbium-doped fiber amplifier to apply a backward amplified spontaneous emission together with a long wavelength band signal light to the erbium-doped fiber amplifier.

FIG. 15 is a configuration view of an erbium-doped optical fiber amplifier for a long wavelength band according to an embodiment of the present invention. A pair of second and third optical fiber Bragg gratings 412 and 413 are configured to cause a reflection at the same wavelength. The optical amplifier having the above described configuration causes an laser oscillation and generates a compensating signal when an intensity of the input signal varies. In this case, a population inversion of an amplifying terminal is maintained constant due to a compensating signal, and thus a gain according to a channel is also fixed, and a total output including a compensating signal is also fixed.

For the first optical fiber Bragg grating 411 , in order to reflect a C-band amplified spontaneous emission, a wavelength between 1520 nm and 1565nm which is in C-band band is selected. There may be a some difference in amplification efficiency depending on wavelength. The C-band amplified spontaneous emission serves as an intermediate pump which is a medium which makes a pumping light be used in a signal of a long wavelength band having a small gain coefficient value, thereby increasing an amplification efficiency. Hence, a compensating signal fixing a gain of the amplifier performs a stable laser oscillation. Here, for the 980nm pumping, the amplifier cannot have a fix gain when there is no first optical fiber Bragg grating 411.

As an another embodiment, the second optical fiber Bragg grating 412 is located in the rear end of the first isolator 301 , and the first optical fiber Bragg grating 411 is located between the second optical fiber Bragg grating 412 and the first wavelength division multiplexer 305, and the third fiber Bragg grating 413 is located between the second wavelength division multiplexer 307 and the second isolator 308.

As still another embodiment, the first optical fiber Bragg grating 411 is located between the first wavelength division multiplexer 305 and the erbium- doped fiber amplifier 306.

As still yet another embodiment, the second optical fiber Bragg grating 412 is located between the first wavelength division multiplexer 305 and the erbium- doped fiber amplifier 306. As another embodiment, the third optical fiber Bragg grating 413 can be located between the erbium-doped fiber amplifier 306 and the second wavelength division multiplexer 307.

That is, as described above, the first optical fiber Bragg grating 411 can be located selectively between the first isolator 301 and the second optical fiber Bragg grating 412 or between the second optical fiber Bragg grating 412 and the first wavelength division multiplexer 305 or between the first wavelength division multiplexer 305 and the erbium-doped fiber amplifier 306.

Also, the second optical fiber Bragg grating 412 can be located selectively in front of the first optical fiber Bragg grating 411 or between the first optical fiber Bragg grating 411 and the first wavelength division multiplexer 305 or between the first wavelength division multiplexer 305 and the erbium-doped fiber amplifier 306.

Also, the third optical fiber Bragg grating 413 can be located selectively between the second wavelength division multiplexer 307 and the second isolator 308 or between the erbium-doped fiber amplifier 306 and the second wavelength division multiplexer 307.

FIG. 16 is a configuration view of an erbium-doped optical fiber amplifier for a long wavelength band according to still another embodiment of the present invention. The first optical fiber Bragg grating 411 is located between the first isolator 21 and the first optical coupling portion 11.

At this time, the gain fixing optical amplifier of FIG. 6 functions to fix a gain in an existing short wavelength band of from 1525nm to 1565nm, but in a long wavelength band all the a pump light source is absorbed in a very short front section. Therefore, an oscillation of a compensating signal is difficult to perform a gain fixation. On the other hands, when the first optical fiber Bragg grating 411 according to an embodiment of the present invention is inserted, a c-band amplification emission is reflected to be efficiently used for a signal of a long wavelength band having a small gain coefficient value, whereupon it functions as an intermediate pump which is a medium to increase an amplification efficiency. As a result, a compensating signal fixing a gain of the amplifier serves to help stabilizing an oscillation.

As another embodiment, the first optical fiber Bragg grating 411 can be located between the first light coupling portion 11 and the first wavelength division multiplexer 12. As still another embodiment, the first optical fiber Bragg grating 411 can be located between the first wavelength division multiplexer 12 and the erbium-doped fiber amplifier 41.

As still yet another embodiment, the third optical fiber Bragg grating 413 can be located between the erbium-doped fiber amplifier 41 and the second wavelength division multiplexer 13.

FIG. 17 is a view illustrating a result data obtained from measuring a characteristic of the erbium-doped optical fiber amplifier for a long wavelength band according to an embodiment of the present invention. An x-axis denotes a wavelength [nm], and a y-axis denotes an amplification gain and a noise factor [dB]. The first optical fiber Bragg grating 411 used has a wavelength of 1545nm and a reflectance of 90%, and the second and third optical fiber Bragg gratings 412 and 413 have a wavelength of 1600nm of 30% and 10%.

The input signal is varied from -6dBm to -20dBm, and when a pump power of "98 + 25mW" is applied, a gain is 17dB and a noise figure is less than 5dB. It is understood that at the input signal variation of 14dB the gain is fixed to 17dB, and a gain variation is within 0.5dB.

As described above, embodiments of the present invention are explained, but modifications are possible for example by changing a location of each reflecting means. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

As described hereinbefore, the erbium-doped optical fiber amplifier for a long wavelength band according to the present invention has great advantages in that an efficiency is improved significantly and a gain fixation is achieved by adding a low-cost coupler, wavelength division multiplexer and optical fiber Bragg grating, thereby improving a performance and raising a price competitiveness.

In addition, by dividing a pump light into two directions through a coupler to pump the erbium-doped fiber amplifier in two directions and placing a reflecting means between an input terminal and the erbium-doped fiber amplifier to apply a backward amplified spontaneous emission of a conventional band together with a long wavelength band signal light to the erbium-doped fiber amplifier, an efficiency of the optical fiber amplifier is improved, and also a gain fixation function is performed without causing a gain transient phenomenon.

Claims

What is claimed is:

1. An erbium-doped optical fiber amplifier for a long wavelength band for amplifying a signal light, comprising: a laser diode for generating a pumping laser light amplifying the signal light; a coupler for receiving the pumping laser light from the laser diode and dividing the pumping laser light at a certain ratio to be output; first and second wavelength division multiplexers for receiving the divided pumping light from the coupler to apply the pumping light in directions opposite to each other, the first wavelength division multiplexer located in an incident light direction, the second wavelength division multiplexer located in an output light direction; an erbium-doped fiber located between the first and second wavelength division multiplexers, receiving the signal light from an incident side and the pumping lights from the first and second wavelength division multiplexers, and transmitting the signal light through a light pumping; and a reflecting means located in front of the first wavelength division multiplexer to output the signal light and re-transmitting a backward amplified spontaneous emission of a conventional band from the first wavelength division multiplexer to the erbium-doped optical amplificaiton optical fiber.

2. The amplifier of claim 1 , wherein the reflecting means is an optical fiber Bragg grating which reflects a conventional wavelength band among a wavelength between 1520nm and 1568 and transmitting the rest wavelength band.

3. The amplifier of cliam 1 , wherein the reflecting means is a mirror which is coupled to a reflecting coupler and reflects a backward amplified spontaneous emission in the other direction of the reflecting coupler.

4. The amplifier of claim 1 , wherein the reflecting means is a reflecting optical fiber Bragg grating which is coupled to a reflecting coupler and reflects a backward amplified spontaneous emission in the other direction of the reflecting coupler.

5. The amplifier of claim 1 , wherein the reflecting means is air gaps of a capacitor type which are opposite to each other.

6. The amplifier of claim 1 , wherein the reflecting means is a facing reservoir by a liquid having a refractive index.

7. The amplifier of claims 1 to 6, wherein the reflecting means is located between the first wavelength division multiplexer and the erbium-doped optical amplifier.

8. The amplifier of claim 1 , wherein the coupler have various division rates to the first and second wavelength division multiplexers.

9. The amplifier of claim 1 , further comprising, a first isolator added in front of the reflecting means to receive the signal light transmitted from an external portion; or a second isolator for outputting the signal light transmitted from the second wavelength division multiplexer.

10. An erbium-doped optical fiber amplifier for a long wavelength band for amplifying an incident signal light, comprising: a laser diode module including at least one laser diode for generating a pumping laser light amplifying the signal light; a coupler receiving the pumping laser light from the laser diode module and dividing the pumping laser light at a certain ratio to be output; first and second wavelength division multiplexers for receiving the divided pumping light from the coupler to apply the pumping light in directions opposite to each other, the first wavelength division multiplexer located in an incident light direction, the second wavelength division multiplexer located in an output light direction; an erbium-doped fiber located between the first and second wavelength division multiplexers, receiving the incident signal light and the pumping lights from the first and second wavelength division multiplexers, and transmitting the signal light through a light pumping; and a reflecting means located in front of the first wavelength division multiplexer to output the signal light and re-transmitting a backward amplified spontaneous emission from the first wavelength division multiplexer to the erbium- doped optical amplificaiton optical fiber.

11. The amplifier of claim 10, wherein the reflecting means is an optical fiber Bragg grating which reflects a conventional wavelength band among a wavelength between 1520nm and 1568 and transmitting the rest wavelength band.

12. The amplifier of cliam 10, wherein the reflecting means is a mirror which is coupled to a reflecting coupler and reflects a backward amplified spontaneous emission in the other direction of the reflecting coupler.

13. The amplifier of claim 10, wherein the reflecting means is a reflecting optical fiber Bragg grating which is coupled to a reflecting coupler and reflects a backward amplified spontaneous emission in the other direction of the reflecting coupler.

14. The amplifier of claim 10, wherein the reflecting means is air gaps of a capacitor type which are opposite to each other.

15. The amplifier of claim 10, wherein the reflecting means is a facing reservoir by a liquid having a refractive index.

16. The amplifier of claims 10 to 15, wherein the reflecting means is located between the first wavelength division multiplexer and the erbium-doped optical amplifier.

17. The amplifier of claim 10, wherein the coupler varies a division rate by the first and second wavelength division multiplexers.

18. The amplifier of claim 10, further comprising, a first isolator added in front of the reflecting means to receive the signal light transmitted from an external portion; or a second isolator for outputting the signal light transmitted from the second wavelength division multiplexer.

19. The amplifier of claim 1 or 10, wherein a reflecting wavelength of the reflecting means is in a range between 1558nm and 1568nm.

20. A gain fixing optical fiber amplifier for a long wavelength band of an optical communication system for amplifying an incident signal light through at least one light pumping laser diode for amplifying the incident signal light, first and second wavelength division multiplexers, and an erbium-doped fiber located between the first and second wavelength division multiplexers, the amplifier comprising: a first optical fiber Bragg grating for outputting the incident signal light and re-transmitting a backward amplified spontaneous emission from the first wavelength division multiplexer to the erbium-doped fiber amplifier; and second and third optical fiber Bragg gratings to form a resonator for a laser oscillation by using a part of an amplification emission generated from the pumped erbium-doped fiber.

21. The amplifier of claim 20, wherein the first optical fiber Bragg grating is a mirror which is coupled to a reflecting coupler and reflects a backward amplified spontaneous emission in the other direction of the reflecting coupler.

22. The amplifier of claim 20, wherein the first optical fiber Bragg grating is located selectively between a signal light incident terminal and the second optical fiber Bragg grating or between the second optical fiber Bragg grating and the first wavelength division multiplexer or between the first wavelength division multiplexer and the erbium-doped fiber amplifier.

23. The amplifier of claim 20 or 22, wherein the second optical fiber Bragg grating is located selectively in front of the first optical fiber Bragg grating or between the first optical fiber Bragg grating and the first wavelength division multiplexer or between the first wavelength division multiplexer and the erbium- doped optical amplificaiton optical fiber.

24. The amplifier of claim 20 or 22, wherein the third optical fiber Bragg grating is located selectively between the second wavelength division multiplexer and an output terminal or between the erbium-doped fiber amplifier and the second wavelength division multiplexer.

25. A gain fixing optical fiber amplifier for a long wavelength band of an optical communication system for amplifying an incident signal light through a light pumping laser diode for amplifying the incident signal light, first and second wavelength division multiplexers, a filter transmitting a certain wavelength among signals output from the second wavelength division multiplexer, a first optical coupling portion coupling a signal output from the filter to the incident signal light, and an erbium-doped fiber amplifier located between the first and second wavelength division multiplexers, the amplifier comprising: an optical fiber Bragg grating for outputting the incident signal light and retransmitting a backward amplified spontaneous emission from the first wavelength division multiplexer to the erbium-doped fiber amplifier.

26. The amplifier of claim 25, wherein the optical fiber Bragg grating is located between the first optical coupling portion and the first wavelength division multiplexer.

27. The amplifier of claim 25, wherein the optical fiber Bragg grating is located between the first wavelength division multiplexer and the erbium-doped fiber amplifier.

28. The amplifier of claim 20, 22, and 25 to 27, further comprising, an isolator in an input portion or an output portion of the gain fixing erbium-doped optical fiber amplifier for a long wavelength band.