Optical fiber


G02B6/16 -

 

(57) Abstract:

The invention relates to optical fibers, alloyed resonant substances for modifying the characteristics passing through it ultrashort light pulses. The inventive optical fiber supplements resonant substance is distributed with the formation of the transverse profile of the concentration of resonant additives, the type of which is determined by the formula:

< / BR>
where n0is the refractive index on the axis of the waveguide; n is the relative difference between the refractive indices in the core of the waveguide; - detuning of the carrier frequency of the pulse from the center of the absorption line; the width of the inhomogeneously broadened spectral line absorption; C is the speed in vacuum;0- the minimum value of pulse width; I = I() - transverse intensity profile at the peak; I0- the maximum value of the intensity of the pulse; =r/r0- transverse coordinate r, the regulation on the core radius ro.

The invention relates to optical devices, in particular to the optical fibres, containing alloying resonant substances for modifying the characteristics of the pipeline radiation in the form of ultrashort pulses area wavelengths 1520-1570 nm, doped with ions of trivalent erbium, aluminum, and samarium (RF patent N 2015125, C 03 C 13/04 was investigated, publ. 29.10.90).

The disadvantage of this optical fiber is that it is not appropriate to form the desired transverse intensity profile pipeline radiation. In the absence of an external pumping optical fiber trivalent ions play the role of resonance absorbing substance, and the fiber loses its ability to stabilize an ultrashort light pulse, provided that the radius of the beam and the concentration of resonant ions satisfy the relationship of LabLdwhere Labthe length of the absorption and Ldthe diffraction length.

Closest to the proposed optical fiber is an optical fiber doped with ions of erbium, which formed the transverse profile of the refractive index (Nakazawa M. Kimura Y. Kurokawa K, Suzuki K. Phys. Rev. A, 1992, 45, R23). In this optical fiber is made to stabilize the structure of the beam is an ultrashort light pulse. When this stabilization is obtained through the use of waveguide properties of the transverse profile of the refractive index, since the absorption length Laba lot more diffraction length Ld: Lab>Ld. This ncture beam, that is, the distortion of the field, made resonant environment, effectively suppressed due to the rigid shape of the waveguide fashion. The shape of the waveguide fashion is determined by the geometrical parameters of the waveguide without alloying additives. The disadvantage of this optical fiber is impossible to obtain the stable propagation of ultrashort pulse lengths of the resonant absorption of a substance, relatives and/or smaller than the diffraction length LabLdthat is, in particular, for large concentrations resonant substances and/or multimode optical waveguides, and it cannot be formed in a given transverse profile of the field.

The basis of the invention is the development of doped additives resonant substances optical fiber, in which the use of non-linear characteristics of the resonant substances, provided LabLdinstead used in a conventional optical fiber waveguide properties of the profile of the refractive index will allow to stabilize the propagation of an ultrashort light pulse with the desired transverse intensity profile at distances greatly exceeding the limit of L>15-20Lab(L is the length of the doped >/P>Achieving the above technical result is ensured by the fact that in the optical fiber supplements resonant substance is distributed with the formation of the transverse profile of the concentration of resonant additives defined by the formula:

< / BR>
where n0the refractive index on the axis of the waveguide;

n the relative difference of the refractive indices in the core of the waveguide;

the detuning of the carrier frequency of the pulse from the center of the absorption line;

DW the width of the inhomogeneously broadened spectral line absorption;

with the speed of light in vacuum;

t0the minimum value of the pulse duration (pulse having a bell-shaped form in cross-section,0must be chosen equal to the time duration value on the axis of the fiber);

I I (a) transverse intensity profile at the peak, i.e. in the time maximum pulse;

I0the maximum value of the intensity of the pulse (for pulse having a bell-shaped form in cross-section, I0must be chosen equal to the value of the intensity on the axis of the fiber: I0I( 0);

r r/r0the transverse coordinate r, normalized by the radius of the core, ro.

The distribution of additives resonant substances with a transverse intensity profile in the case when there is the following relation between the diffraction length LDand the absorption length Lab: LabLd. For this you have to set a desired shape intensity I(rin the peak of the pulse and, substituting into the formula to obtain the distribution of the doped resonant additives F (r).

Given the shape of the transverse profile of the concentration of resonant additives defined by the above formula allows you to control the transverse profile of the beam by changing the profile of the refractive index nonresonant atoms simultaneously with the change of the concentration profile of the resonant atoms. The feature of the proposed optical fiber is that it may not have the profile of the refractive index, while the concentration profile of the resonant atoms to provide you. This proposed optical fiber is fundamentally different from the commonly used optical fibers, a necessary attribute of which is the profile of the refractive index. It is this fundamental difference allows to obtain an optical fiber with new properties that allow formation of the desired transverse profile of the intensity of the infrared frequency switch is supplied with optical fibers can be explained as follows. First, refer to the description of the General principles of interaction of optical radiation with resonant absorbing substance. To the input optical fiber is radiation pulse, the carrier frequency which is in close proximity to the selected optical transition, so that the conditions of resonance interaction between the pulse and the alloying absorbing substance (e.g., trivalent ions of the rare earths). When spread on such optical fiber long low-intensity pulses of their energy decreases exponentially and, having length L 2-3 Labthe pulse is almost completely absorbed.

Qualitatively different behavior is detected, if the input optical fiber is low pulse duration (duration must be much less than the relaxation times of both resonant levels and the relaxation time of the polarization of the pulse is an ultrashort pulse) and a large enough intensity so that the area under the envelope of the input formula

< / BR>
satisfy the relation: > . Then after a short transient (distance L 2-3 Lab) momentum, losing a small part of the energy ( 10%), transform the Eton self-induced transparency will be distributed over optical fiber without changing the shape and without energy loss at arbitrarily large distances.

The physical mechanism responsible for the formation of self-induced transparency solitons, pronounced nonlinear threshold character entering the absorbing medium, its leading edge excites the atoms, translating it from the lower energy level to the upper, rear edge of the pulse triggers stimulated emission from the upper energy level and, thus, returns energy back to the field pulse. The main property of self-induced transparency soliton that these two processes are completely balance each other, and after the passage of a soliton self-induced transparency absorbing environment remains largely energy, and therefore energy loss field is not happening. Such a pattern of distribution of soliton assumes that its duration is much shorter than all of the relaxation times, so that the residence time of atoms in the upper excited state dissipative processes do not have time to work.

A natural limitation on the length of the propagation in real optical fibers associated with the inevitable loss of energy due to the mechanisms of different nature: waveguide losses, incoherent losses due to relaxation is hernych (that is unlimited in the transverse direction) solitons, the shape of which depends on the longitudinal coordinate z and time t. It is also known that in the practical use of coherent resonant properties of the substance is required to take into account the views of the transverse structure of the field, as any real momentum is limited in the transverse direction. It turns out that when considering the limited beam in the transverse direction, the propagation of the soliton self-induced transparency in a homogeneous (i.e. not having volnovedushchie properties) resonant environments is accompanied by uncontrolled change in the transverse and temporal structure of the field due to the action of the diffraction mechanism of instability. Moreover, passing in the distance L, 15-20 Labspatial and temporal pulse shape unpredictable change, its energy is absorbed, and ultimately pulse is completely falling apart. The physical mechanism of collapse is associated with the difference of the velocities of propagation of the different parts of the beam so, if momentum is in the transverse direction of the bell-shaped form with a maximum on the axis, then piaceva part of the beam will move with greater speed than the peripheral, which in the end will lead to a "blurring" of the pulse. In practice, this simple picture u is hraneniya and to make a significant contribution to the distortion of the pulse shape and, therefore, to accelerate its decay.

The proposed optical fiber is intended to stabilize the soliton self-induced transparency, which will reach length distribution is not less 100-200 L Lab, which is now limited to only a weak dissipative processes in optical fibers. Physically, the mechanism of suppression of diffraction volatility is based on the alignment of the velocity distribution of all parts of the beam. For example, consider the soliton self-induced transparency with the transverse intensity distribution in the form of a Gaussian function. The nonlinear nature of the interaction of the pulse with the environment determines the propagation velocity of the pulse, which decreases with increasing amplitude of the field and with increasing concentration of absorbing atoms (ions). Till now used the medium was prepared with a uniform distribution of atoms (ions) in cross-section so that the periphery of the pulse had a smaller amplitude and, as a consequence of the spread at a slower rate. Serves to smoothly decrease the density of absorbing atoms (ions) in an optical fiber as the distance from the axis, so as to increase the speed of the peripheral part. Thus, viravnivaet which the concentration profile of the resonant additives used as volnovatsa environment for self-induced transparency soliton. Described physical picture based on the dominant role of nonlinear processes in the formation of the transverse profile of the beam. This property differs described optical fiber from existing conventional optical fibers, based on the use of waveguide properties of the profile nonresonant refractive index.

It follows from the above restriction on the limits of applicability of the presented method: LabLdthat is , when the nonlinear resonant interaction may take quite strongly. Also from the principle of the proposed optical fiber directly follows that by varying the transverse profile of the alloying additives, we dictated that only the intensity profile, for which the propagation velocity of all parts of the beam across the same. Thus, the optical fiber acts as a device that specifies the transverse profile of the beam, which can be modified as desired by selection of the desired profile of alloying elements.

The proposed optical fiber, which helps build the self-induced transparency solitons, can find application in fiber-optic communication lines. JJA, which is formed soliton self-induced transparency, serves as a natural filter for noise radiation, and thereby prevent undesired operation of the equipment. Thus, the proposed optical fiber can increase the efficiency of data transmission in fiber-optic communication lines. The application of the described optical fiber based on multimode fibers with core diameter 200-100 μm makes it easy to provide the desired concentration profile of the resonant substances, which can be entered into the fiber, for example, by the method of "doping solution", is well known in the art, or other well known methods in accordance with specific needs. Such a multimode optical fiber can be effectively used for the stabilization and formation of high-energy pulses.

As an example, consider a multimode fiber with a diameter of 300 μm. Erbium contained in the fiber in the form of oxide (Er2O3), is 9 parts per thousand (by weight); and the absorption length is Lab3.610-2m at the frequency corresponding to the resonant transition between levels4I15/2and01.7. The relative difference between the refractive indices in the core of the waveguide Dn 0.05. The resonance line is inhomogeneously broadened, and the half-width of the spectral contour is an amount equal to 1.41012s-1. To obtain the transverse intensity distribution described by the Gauss law, I=I0exp(-2), you need to create the transverse profile of the concentration described by the law:

F()=[1-1,610-22]exp(-2/2)

Optical fiber containing a core with the addition of resonant material, characterized in that the additive resonant substance is distributed with the formation of the transverse profile of the concentration of resonant additives defined by the formula

< / BR>
where naboutthe refractive index on the axis of the current frequency of the transmitted fiber pulse from the center of the absorption line;

the width of the inhomogeneously broadened spectral line absorption;

- the duration of the transmitted fiber impulse;

c speed of light in vacuum;

0- the minimum duration value transmitted by the fiber impulse;

I = I () is the transverse intensity profile of the transmitted fiber pulse peak;

I0the maximum value of the intensity transmitted through the fiber impulse;

= r/r0- transverse coordinate r, normalized by the radius of the core, r0.

 

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