DE19926811A1 - Light source for optical information transmission has selection device, in doped optical fiber, for partial selection of light in wavelength range with relatively lower emission spectrum - Google Patents

Abstract

A coupler (14) connects a pumping light into a doped optical fiber (16) through which a laser medium is excited. A selection device in the doped optical fiber (16) is used for the partial selection of light in the wavelength range with relatively lower emission spectrum by which this part of the spectrum is amplified. An Independent claim is included for: (a) a doped fibre inserts for amplifying an optical input signal

Description

The invention relates to a light source for optical information transmission and a fiber amplifier for amplifying an optical input signal.

Optical fibers offer enormous capacity for optical information transmission in binary form. Most such systems in operation use lasers, which emit light with a wavelength of approximately 1300 nm. The laser light is used with the transmitting information is modulated and coupled into monomode fibers, which the Infor Transport the mation over the desired distance. Known techniques for light sources, With this technology, modulators, detectors, amplifiers etc. allow a transmission rate of 10 GHz.

In order to further increase the transmission rates or capacities, a single one Optical fiber is used for the simultaneous transmission of several information channels. Here becomes light with a different wavelength for each individual information channel used. This technique is called Wavelength Domain Multiplexing (WDM). Here each individual information channel retains a transmission capacity of 10 GHz. The The total transmission capacity of the system is then a multiple of 10 GHz.

The greatest possible variety of wavelengths are used to implement WDM needed at the same time. A corresponding number of i. d. R. mono chromatic lasers, e.g. B. semiconductor lasers, operated simultaneously side by side. Here each laser needs its own control and its own power supply. Overall, one increases Large number of individual lasers, the overall system's susceptibility to failure and the costs.

The object of the invention is a single, simple and inexpensive light source or to create such a fiber amplifier for optical communication using WDM fen.

This object is achieved by a light source with the features of Claim 1 and a fiber amplifier with the features of claim 10 solved.

The light source according to the invention has a pump light source for generating Pump light at a first wavelength. This pump light source can be a suitable one  Lamp. I. d. However, the pump light source will usually be a laser, preferably a half conductor laser.

The central element of the light source is an optical fiber using an active laser is medium doped. Rare earth ions such as come in particular as the active laser medium Erbium or neodymium. The pump light source must be selected such that the Laser medium can be excited at the wavelength of the pump light source.

Most laser media, especially the rare earth ions, produce emission light with a wavelength spectrum with at least one wavelength range of high lei stung, typically two maxima, and at least one wavelength range with relative lower power, typically a minimum between the maxima.

The light source has a device so that the laser medium can be excited to couple the pump light into the doped optical fiber. These can be concave mirrors which direct the light from the side onto the doped optical fiber, or it can be an optical one Arrangement for the final pumping of the doped optical fiber.

The active laser medium absorbs part of the pump light and goes in an electronically excited state. The active laser medium then emits spontaneous broadband radiation, sometimes at such flat angles that the Radiation is partially captured in the doped optical fiber. The captured spon tane radiation is amplified in the doped optical fiber by stimulated emission (amplified spontaneous emission ASE).

Most laser media have an emission spectrum that of maximum power and performance minima is marked, far from being even. According to the invention, a selection device is used in or on the doped light Fiber optic light in the wavelength range with relatively lower power of the emission spec partially selected. As a result, this part of the spectrum becomes the further through preferably reinforced by the doped optical fiber.

In an overall system for optical information transmission is then the broad spectrum is broken down into individual wavelengths. This can be known to various people Happen so, for example with downstream narrow-band interference filters or with the help of grids, e.g. B. in a so-called demultiplexer. The individual wavelengths are each modulated by itself to transmit data, thus forming a transfer supply channel. Then the different wavelengths are brought together again, for example in a so-called multiplexer, and coupled together in an optical fiber and transmitted through this.  

The broadband light source according to the invention is extremely simple construction. It is therefore less prone to failure, robust and can be produced at low cost.

At least one Fabry-Perot filter or an interference can be used as the selection device filters are inserted into a section of the doped optical fiber. A perio The modulation of the refractive index can be provided in the core of the doped optical fiber.

In an advantageous development of the invention, the doped optical fiber is a Single mode fiber. As a result, the broadband light is generated with a low divergence. It can also be easily coupled into other fibers.

A particularly handy structure of the broadband light source according to the invention results when the pump light source is a semiconductor laser, the radiation of which is in a second Single mode fiber is injected. An optical coupler can also be used which has three inputs, the first input coupled to the second single-mode fiber , the second input is coupled to one end of the doped optical fiber, and the third input is coupled to a third single-mode fiber. The coupler is like this is formed such that the pump light from the second monomode fiber into the doped light guide fiber is coupled. The doped optical fiber is thus finally pumped. Furthermore, the Coupler designed such that the emission light from the doped optical fiber in the third monomode fiber is coupled. The emission light is therefore without power losses to the output of the broadband light source. This structure is also particularly good suitable for series production and assembly.

In order to obtain an even wider spectrum, the doped optical fiber can be used with a Mixture of different rare earth ions can be doped.

In an advantageous development, the light source according to the invention additionally has Lich a device for selecting a plurality of individual wavelength ranges from the emission spectrum. This device can be a Fabry-Perot filter or a Be a demultiplexer. Or it can by periodic modulation of the refractive index in the Core of the doped optical fiber can be formed (Bragg grating), the periodic modula tion is designed such that it has a plurality of individual wavelength ranges reflected.

Through such an additional device, the total radiation energy is effective lines or spectral ranges used. The radiation energy is thus better exploited.

By using a Fabry-Perot filter as the additional device mentioned, the Line width as well as the distance or the number of lines can be freely selected and changed  the. A system which can be reconfigured in a simple manner is thus obtained. The Wel lenlength of the lines are regulated, for. B. by comparison with a reference wavelength.

Overall, you get a simple light source that differentiates individual lines Lichen wavelengths generated with the same intensity.

The inventive ideas for the light source according to the invention can also be advantageous can be used for a fiber amplifier for amplifying an optical input signal. Such a fiber amplifier has a pump light source for generating pump light a first wavelength. Furthermore, an optical fiber with an active laser medium is doped, which can be excited at the first wavelength, making it emission light with a wavelength spectrum with at least one high power wavelength range and can emit at least one wavelength range with relatively lower power. The pump light source and the optical input signal are converted into a first via a coupler Coupled end of the doped optical fiber.

A selection device is usually located in the doped optical fiber immediately after the coupler. It selects light in the wavelength range with relative lower power of the emission spectrum, which makes this part of the spectrum in the further passage through the doped optical fiber is preferably amplified.

An optical isolator is located at the second end of the doped optical fiber Allows light to pass that enters the optical fiber from the first end.

The fiber amplifier according to the invention offers a smooth gain profile for the Most of the wavelengths that can be amplified with the doped fiber.

A particularly simple construction of the amplifier is obtained when as a selection device at least one suitably set Fabry-Perot filter in a section of the doped Optical fiber is inserted.

Similar effects can be achieved with an interference filter that goes beyond that has the advantage of leading to a monolithically compact structure.

A periodic modulation of the refractive index can also be used as a selection device be provided in the core of the doped optical fiber.

Further advantageous developments of the inventions are in the subclaims featured.

The invention is explained in more detail below with the aid of exemplary embodiments which: are shown schematically in the figures. The same reference numbers in the individual figures denote the same elements. In detail shows:

Figure 1 is a schematic representation of the structure of the broadband light source.

FIG. 2 shows the spontaneous emission spectrum of erbium;

Fig. 3 which achieved with the broadband light source uniform spectrum;

Fig. 4 uses a schematic representation of the structure of a selection device, the polarization effects; and

Fig. 5 is a schematic representation of the structure of a fiber amplifier.

In the structure of the broadband light source shown schematically in FIG. 1, the pump light source 10 is a semiconductor laser at a wavelength of 980 nm with a power of approximately 90 mW. The laser radiation from the pump laser is coupled into a single-mode fiber 12 .

The monomode fiber 12 is coupled via an optical coupler 14 to a second monomode fiber 16 in such a way that the radiation from the first ionomer fiber 12 passes essentially losslessly into the second. As a coupler 14 z. B. Components are used as they are used in WDM to merge two different wel lenlängen in a fiber (multiplexer).

The coupler 14 is set in the exemplary embodiment according to FIG. 1 in such a way that it couples the radiation of the pump laser 10 at 980 nm as completely as possible into the second monomode fiber 16 . By contrast, radiation at approximately 1550 nm (see below) entering the coupler 14 from the second monomode fiber 16 is not coupled back into the first monomode fiber 12 . Rather, the coupler 14 is set such that this radiation is coupled into a third monomode fiber 18 , the end of which is designated A.

The second monomode fiber 16 is an optical fiber amplifier which is doped with approximately 300 ppm erbium ions, a so-called EDFA (erbium doped fiber amplifier). The latter is between 7 and 20 m long. The end C of the EDFA 16 facing the coupler 14 is made of a material whose refractive index corresponds to that of ordinary glass fibers. This reduces reflections at the coupling point.

The light from the pump laser 10 is then partially absorbed by the erbium ions in the EDFA 16 . The latter spontaneously emit light at a wavelength between approximately 1530 and 1570 nm, sometimes at angles that are captured in the single-mode fiber. The captured radiation is amplified in the EDFA 16 by stimulated emission. This leads to increased spontaneous emission (ASE amplified spontaneous emission).

At each end C, B of EDFA 16 , weak broadband radiation first occurs, the spectrum of which is characterized by strong maxima and minima. The observed spontaneous spectrum of an erbium-doped fiber laser is shown in FIG. 2. Fig. 2 shows the power (power) per 0.5 nm wavelength in decibels as a function of the wavelength (wave length) in nanometers. The spectrum has a first maximum at 1532 nm and a second maximum at 1556 nm, which is about 2 to 3 dB weaker. In between there is a minimum at about 1538 nm, which is about 8 dB weaker than the maximum at 1532 nm. The amplification of the spontaneous emission leads to a concentration of the radiation around that at the maxima. The entire emission spectrum has a bandwidth of around 40 nm.

Erbium ions belong to the class of the so-called three-level systems. Part of the along The radiation traveling through the optical fiber is reabsorbed by the erbium itself. This applies more strongly ker for the short-wave portion of the radiation. Longer fibers therefore have a spectrum that has an increased proportion of long-wave radiation compared to shorter fibers.

In a first embodiment, the emission spectrum is flattened Light with a wavelength of 1538 nm is selectively transmitted through a filter device.

A Fabry-Perot filter 20 is used for this purpose . A Fabry-Perot filter is an optical arrangement of two transparent, plane-parallel, partially reflecting plates, which are arranged at a distance d from each other, which corresponds to a multiple m of half the wavelength λ / 2 of the desired light passing through. So it applies

m is called the order of the Fabry-Perot filter.

In the present case, the Fabry-Perot filter 20 is formed by the space or the surfaces of two head-to-head, smoothly cut fibers that are inserted into ceramic connector sleeves (ferrules).

A maximum transmission around the minimum of the spectrum at 1538 nm is desired, corresponding to a frequency ν of 195 THz. The half width FWHM of the transmission line should be approx. 5 nm or 0.63 THz. It results from

where FSR (ν) is the so-called free spectral range, ie the distance between adjacent lines that are transmitted by the Fabry-Perot filter. It results from

where c is the speed of light between the reflecting plates. The free one Spectral range FSR (ν) should be chosen so that the neighboring transmitted Lines are outside the gain spectrum of the EDFA, i. H. at about 50 nm or  6.3 THz. The distance d then results from formula (3) to be approximately d ≈ 23 μm. By a exact setting of the distance d according to formula (1) ensures that the maximum the transmission line lies exactly on the spectral minimum of 1538 nm.

F is the finesse of the Fabry-Perot filter and is given by

where R is the reflectivity of the mirror surfaces. In the present example, the free spectral range 50 nm and the half width of the transmission line 5 nm. Aus Formula (2) thus results in F = 10. Formula (4) gives R = 73%. The surfaces of the In the uncoated state, fibers have a reflectivity R of approximately 4% for the radiation in the EDFA. However, the reflectivity R can be suitably chosen by coating. The coating should reflect the area around 1538 nm as homogeneously as possible, while the pump wavelength of 980 nm from the coating should be transmitted.

This selectively transmitted spectral range around 1538 nm is additionally amplified during the further passage through the EDFA 16 . If, at the end B of the EDFA 16, the light is reflected back into the fiber 16 by a reflector 32 , the selectively transmitted spectral range is amplified over a longer section of the EDFA 16 as light that is not transmitted by the Fabry-Perot filter 20 but was reflected.

Another possibility is to use the Fabry-Perot filter not for selective transmission, but for selective reflection. For this purpose, the distance between the surfaces of the Fabry-Perot filter is set such that there is a maximum reflection or minimum transmission for light with a wavelength of 1538 nm. Transmission maxima can be integer multiples of 6 nm away: one at 1538 nm-6 nm = 1532 nm and one at 1538 nm + 3.6 nm = 1556 nm, i.e. exactly at the power maxima to be weakened. The distance between the transmission maxima or the FSR would then be 12 nm or 1.5 THz, and the distance d between the two surfaces of the Fabry-Perot filter would be approximately 98 μm from formula (3). In such a case, a reflector could be omitted at end B of EDFA 16 .

Both options can be used in combination. In principle, with a The majority of Fabry-Perot filters optimize the spectrum of the light source.

The Fabry-Perot filter 20 can be placed anywhere in the EDFA 16 . In the presently preferred embodiment, the Fabry-Perot filter 20 was installed in the EDFA 16 one meter from the end B.

The flattening of the spectrum can be optimized by varying the position of the Fabry-Perot filter 20 , the distance d and the reflectivity R of the mirror surfaces.

Further, may optionally additionally, a reflector 32 is inserted at the end B of the optical fiber 16, the exiting light into the optical fiber 16 is reflected back. The reflector 32 can be designed as a selective reflector which reflects light of the wavelength of 1538 nm back into the optical fiber 16 more strongly than light of other wavelengths, e.g. B. in the form of a dielectric coated mirror or a suitable Bragg grating in the fiber 16th A plurality of Bragg gratings arranged one behind the other is also possible, each reflecting individual wavelength ranges with a predetermined reflection coefficient in order to generate a flattened spectrum. In the latter cases, the reflector 32 can also be used alone, without the Fabry-Perot filter 20 .

The end of the EDFA 16 labeled C is located in the coupler 14 mentioned above. In the coupler 14 there is also the third monomode fiber 18 provided for the optical transmission of data, at the end labeled A the radiation emerges. In order to avoid disturbing reflections at the end A of the fiber 18 , which could change the shape of the spectrum, a conventional optical isolator 22 is built into the fiber 18 before the end A.

With the arrangement described, a power of the broadband laser of achieved about 10 mW.

Fig. 3 shows the power in decibels as a function of wavelength for a flattened spectrum. In Fig. 3 one when a selectively transmissive Fabry-Perot filter 20 is placed one meter from the end B of the EDFA 16 detects a significant flattening of the minimum. The distance between the two plates of the Fabry-Perot filter 20 was set such that there is an optimal transmission of the radiation in the range of the minimum, ie radiation with a wavelength of approximately 1538 nm. In the present exemplary embodiment, a depth of the minimum of only 2.5 dB between 1535 and 1555 nm could be achieved. By further optimizing the position and finesse F of the Fabry-Perot filter 20 , a depth of up to 0.5 dB can be achieved.

2nd embodiment

A flattening of the spectrum of the light source can also be achieved using polarization effects. To this end, beyond the end B of the optical fiber 16 , a linearly polarizing fiber section 24 follows which linearly polarizes the ASE. The polarized light is coupled into one of the two intrinsic directions of a polarization-maintaining fiber 26 . Then part of the light spectrum, e.g. B. the part rotated by 1538 nm, selectively in the polarization direction by 90 °. This can be achieved by a suitable periodic disturbance of the position of the optical axis 28 of the optical fiber (micro bends). For this purpose, the optical fiber is pressed in a known manner against a sequence of rollers 28 which are aligned in parallel and which lie against each other and have a diameter which corresponds to the beat length of the polarization-maintaining fiber (about 1-3 mm). The direction of the pressing force is 45 ° inclined with respect to the natural axis of the polarization-maintaining fiber 26 .

Another linearly polarizing fiber section 30 follows, which serves as an analyzer and only allows the part of the light spectrum to pass, which in this example is close to 1538 nm. At the very end of the fiber there is a reflector 32 which reflects the selected light of the desired wavelength back into the fiber. It passes the listed components in the opposite direction and thus reaches end B of optical fiber 16 again . From B it is passed through the reinforcing fiber section EDFA before it reaches the end C and after passing the coupler 14 it leaves the fiber at A.

3rd embodiment

A flattening of the spectrum of the light source can also be caused by a periodic Modulation of the refractive index in the core of the fiber in the form of a long-wave grating can be achieved. Such a grating couples light from the mode guided in the fiber radiated modes from, d. H. selective loss of light occurs. With a suitable choice of The coupling bandwidth is a few nanometers, so that the radiation measure ximum can be coupled out at 1532 nm. The strength of the decoupling of the be selected in such a way that the maximum power at 1532 nm is weakened in such a way that there is a uniform spectrum.

Furthermore, a second grating can be integrated in the fiber core in order to maximize the Attenuate 1556 nm.

By a suitable choice of these long-wave gratings, the spectrum can be completely reduced become flat.

4th embodiment

Instead of the Fabry-Perot filter according to the first embodiment, minde at least one interference filter is used, the appropriate transmission or reflection  has properties. The resulting structure can be made stable and compact. With such a structure, low manufacturing tolerances can easily be achieved.

5th embodiment

In the presently preferred exemplary embodiment, a second Fabry-Perot filter 42 is additionally used, which selects a plurality of individual wavelength ranges from the emission spectrum. In order to select the plurality of individual wavelengths at the beginning of the amplification process in the EDFA 16 , the Fabry-Perot filter 42 is arranged approximately 0.5 to 2 m from the end C of the EDFA 16 .

When choosing the suitable parameters for the Fabry-Perot filter 42 , it should be noted that the transmission rate of an optical data line is determined by the minimum pulse width of the optical pulses. This essentially results from the relation

Transfer rate = 1 / pulse width (5)

however, a factor of 2 may have to be taken into account, depending on the type of modulation selected for the transmission. However, the pulse width is connected to the spectral half-width FWHM according to

Pulse width.FWHM (ν) ≧ 1 (6)

which gives the following limitation on the transmission rate

Transmission rate = 1 / pulse width ≦ FWHM (ν) (7)

With a target transmission rate of currently 10 GHz, the FWHM (v) is also 10 GHz. If a distance of the individual lines of FSR = 50 GHz is additionally sought in accordance with the ITU standard in order to be able to transmit as many lines as possible simultaneously, formula (3) results in a distance of d = 3 mm. In addition, according For mel (2) has a finesse of F = 5 and from the formula (4) has a reflectivity of R = 54%.

Within the approximately 40 nm or 5 THz usable bandwidth of the EDFA amplifier 16 , 100 lines can thus be separated from one another and 100 transmission channels, each with a 10 GHz transmission rate, can thus be created. Overall, according to the preferred embodiment there is a transmission rate of 1 THz.

This is a prerequisite for the application of the given formulas (1) to (4) of parallel light on the reflective surfaces of the Fabry-Perot filter. When out however, the light diverges from a fiber. It may therefore be advantageous to use lenses, e.g. B. self-focusing lenses (selfoc lenses) or fiber lenses, to parallelize the light before the Fabry-Perot filter. Diverging light enters through a Fabry-Perot Filter, there is a slightly increased spectral bandwidth FWHM. This can be done through a  Increasing the reflectivity R can be compensated again. In addition, the EDFA works in such a way that it slightly reduces the FWHH of the lines because it amplifies higher signal components better.

Numerous modifications and developments of the described embodiments can be realized.

The selection mechanisms described can also be used to flatten the spectrum of fiber amplifiers (EDFA). Without such a selection mechanism, EDFA show an inhomogeneous amplification, whereby light with wavelengths around 1532 and 1556 nm is better amplified. Fig. 5 shows an EDFA with a pump laser 34, an optical input signal 36, a coupler 14, an amplifying fiber section 38 and an optical insulator 22. In addition, the EDFA has one of the selection devices 40 described.

Instead of selectively transmitting or reflecting means, one can generally also refer to selectively absorbing agents can be used.

Furthermore, the techniques described cannot only be used for those described in the embodiments play wavelengths are used, but for any wavelength. The same applies to the rare earth ions mentioned.

The so-called L-band can also be used in addition. H. Wavelengths in the range from 1565 to approx. 1610 nm. B. by filters or couplers, optically from the so-called C- Band separated between approx. 1530 to 1565 nm. Signals in the L-band can then be separated separately Reinforcement in z. B. an EDFA.

The Fabry-Perot filter can also be implemented using Bragg gratings in the light guide that serve as reflectors. A change in the properties of this Fabry-Perot Filters can be achieved by stretching and compressing the fibers.

The Bragg grids do not have to be formed in the EDFA 16 itself. They can also be formed in one or a plurality of light guides which are coupled to the EDFA 16 via an optical coupler. Such a structure could be modular.

The components mentioned can also be used in ring lasers that consist of Light guides are constructed.

In addition to the monomode fibers mentioned, double-core fibers can also be used. They have an outer core which is designed as a multi-mode light guide and an inner core which is designed as a single-mode light guide. The outer core carries the pump light, which typically comes from a powerful semiconductor laser. The inner core carries the generated fluorescent light and the ASE of the rare earth ions. The coupler 14 must be chosen appropriately for such a double core fiber.

The reflectivity of the two reflecting surfaces of the Fabry-Perot filter can also be different. A surface can also be almost completely reflective.

So-called confocal Fabry-Perot filters can also be used as Fabry-Perot filters, ie those with two focusing mirrors whose radii of curvature correspond to their mutual distance d. The distance d is then a multiple m of a quarter of the wave length λ of the light passing through. So it applies

The free spectral range FSR (ν) then results from

where c is the speed of light between the reflecting plates. The finesse F of a confocal Fabry-Perot filter is given by

where R is the reflectivity of the mirror surfaces.

All of the components mentioned can also be used in several amplifiers in succession be set.

The light source according to the invention is likely to have its preferred application in the optical transmission of information. However, the light source can also be beneficial for Sen sensors are used, the smallest movements of buildings with the help of light guides such as detecting bridges, tunnels and high-rise buildings as well as the formation of small cracks monitor in these structures.

Reference list

10th

Pump light source

12th

first single mode fiber

14

Coupler

16

second single mode fiber

18th

third single mode fiber

20th

Fabry-Perot filter

22

optical isolator

24th

linearly polarizing fiber section

26

polarization maintaining fiber

28

roll

30th

linearly polarizing fiber section

32

reflector

34

Pump laser

36

optical input signal

38

reinforcing fiber section

40

Selection device

42

second Fabry-Perot filter

Claims (15)

1. Light source for optical information transmission with

• a) a pump light source ( 10 ) for generating pump light at a first wavelength;
• b) an optical fiber ( 16 ) doped with an active laser medium that can be excited at the first wavelength, thereby emitting emission light with a wavelength spectrum with at least one high power wavelength range and at least one relatively low power wavelength range;
• c) a device ( 14 ) for coupling the pump light into the doped optical fiber ( 16 ), whereby the laser medium is excited, the emission light of the laser medium partially propagating in the doped optical fiber; and with
• d) a selection device in or on the doped optical fiber ( 16 ) for partially selecting light in the wavelength range with a relatively lower power of the emission spectrum, as a result of which this part of the spectrum is preferably amplified in the further passage through the doped optical fiber.

2. Light source according to claim 1, characterized in that at least one Fabry-Perot filter ( 20 ) is inserted as a selection device in a section of the doped optical fiber ( 16 ). 3. Light source according to claim 1, characterized in that a periodic modulation of the refractive index in the core of the doped optical fiber ( 16 ) is provided as a selection device. 4. Light source according to claim 1, characterized in that an interference filter is inserted as a selection device in a section of the doped optical fiber ( 16 ). 5. Light source according to claim 1, characterized in that the selection device has the following:
a linearly polarizing fiber section ( 24 ) followed by
a polarization-maintaining fiber ( 26 ) which is coupled to the linearly polarizing fiber section ( 24 ) in such a way that it can absorb and transmit the linearly polarized light as completely as possible, followed by
means ( 28 ) for selectively rotating the direction of polarization of the at least one wavelength region with relatively lower power by 90 °, followed by
a second linearly polarizing fiber section ( 30 ), which serves as an analyzer and only allows the part of the light spectrum rotated through 90 ° in the direction of polarization to be passed, followed by
a reflector ( 32 ) which reflects the selected light of the desired wavelength back into the fiber. 6. Light source according to one of claims 1 to 5, characterized in that the doped optical fiber ( 1 b) is a single-mode fiber;
that the pump light source ( 10 ) is a semiconductor laser, the radiation of which is coupled into a second monomode fiber ( 12 );
that an optical coupler ( 14 ) is provided with at least a first input, a second input and a third input,
the first input being coupled to the second single-mode fiber ( 12 ),
the second input being coupled to a second end (C) of the doped optical fiber ( 16 ),
the third input being coupled to a third single-mode fiber ( 18 ),
wherein the coupler ( 14 ) is designed such that the pump light from the second monomode fiber ( 12 ) is coupled into the doped optical fiber ( 16 ), whereby the doped optical fiber is finally pumped, and
wherein the coupler ( 14 ) is also designed such that the emission light from the doped optical fiber ( 16 ) is coupled into the third single-mode fiber ( 18 ). 7. Light source according to one of claims 1 to 6, characterized in that the doped optical fiber ( 16 ) is doped with a mixture of different rare earth ions. 8. Light source according to one of claims 1 to 7, characterized by a device device for selecting a plurality of individual wavelength ranges from the emis sion spectrum.   9. Light source according to claim 8, characterized in that the device for Selecting a plurality of individual wavelength ranges from the emission spec is a Fabry-Perot filter or a demultiplexer or by a periodic module tion of the refractive index is formed in the core of the doped optical fiber, the periodi cal modulation is designed such that it has a plurality of individual wavelengths areas reflected. 10. Fiber amplifier for amplifying an optical input signal ( 36 ) with

• a) a pump light source ( 34 ) for generating pump light at a first wavelength;
• b) an optical fiber ( 38 ) doped with an active laser medium that can be excited at the first wavelength, whereby it can emit emission light with a wavelength spectrum with at least one high power wavelength range and at least one relatively low power wavelength range;
• c) means ( 14 ) for coupling the pump light and the optical input signal into a first end of the doped optical fiber ( 38 );
• d) an optical isolator ( 22 ) at the end facing away from the first end of the doped optical fiber ( 38 ) which allows light to pass through which enters the optical fiber from the first end; and with
• e) a selection device ( 40 ) in or on the doped optical fiber ( 16 ) partially selecting light in the wavelength range with a relatively lower power of the emission spectrum, whereby this part of the spectrum is preferably amplified during the further passage through the doped optical fiber.

11. Fiber amplifier according to claim 10, characterized in that at least one Fabry-Perot filter is inserted as a selection device ( 40 ) in a section of the doped optical fiber ( 38 ). 12. A fiber amplifier according to claim 10, characterized in that a periodic modulation of the refractive index in the core of the doped optical fiber ( 38 ) is provided as a selection device ( 40 ). 13. Fiber amplifier according to claim 10, characterized in that an interference filter is inserted as a selection device ( 40 ) in a section of the doped optical fiber ( 38 ). 14. Fiber amplifier according to claim 10, characterized in that the selection device ( 40 ) has the following:
a linearly polarizing fiber section ( 24 ) followed by
a polarization-maintaining fiber ( 26 ) which is coupled to the linearly polarizing fiber section ( 24 ) in such a way that it can absorb and transmit the linearly polarized light as completely as possible, followed by
means ( 28 ) for selectively rotating the direction of polarization of the at least one wavelength region with relatively lower power by 90 °, followed by
a second linearly polarizing fiber section ( 30 ), which serves as an analyzer and only allows the part of the light spectrum rotated through 90 ° in the direction of polarization to pass. 15. Fiber amplifier according to one of claims 10 to 14, characterized in that the doped optical fiber ( 16 ) is doped with a mixture of different rare earth ions.