DE2418534C3 - Mode mixer for mode mixing in a dielectric optical waveguide - Google Patents

Description

uud is less than

With

tric optical waveguide is called dielectric multimode optical waveguide.

Inside a multimode dielectric fiber optic cable originally in a fashion sprawling

In this mode, light propagates until there is a disturbance in the geometry or structure of the dielectric optical waveguide occurs. Such a disturbance can affect the electromagnetic Change fields of light and cause all light to move into one or more of its mode of propagation several new fashions changes. This change in the mode of propagation is called mode mixing, and one The arrangement which effects this mode mixing is called a mode mixer. A fashion mixer is not at one limited discrete point in a dielectric optical waveguide, rather it can be a length section of the dielectric optical waveguide, in which a disturbance of the structure either periodically or gradually continuously over a

a = radius of the core,

M ₁ = refractive index of the core,

n ₂ = refractive index of the cladding,

Z ₀ = free space wavelength of the light.

2. Mode mixer according to claim 1, characterized in that 30 considerable length occurs characterized in that the curved portion (2) One of the main problems when using a dieinen to a figure eight curved section of electrical multimode fiber optic door the of the dielectric optical waveguide (4,5) contains message transmission is the limitation of the band (Fig. 4c). width caused by the group delay dispersion,

3. mode mixer according to claim 1, characterized 35 gently by a change in the group vibrations, that the radius of curvature (2) speed for different modes of propagation. As a result, a meandering section of the can be a logical spreading in a fashion dielectric light wave conductor (4, 5) contains "one" with a preceding one, which is in another (Fig. 4a). logical »zero« spreading through the fashion

4. mode mixer according to claim 1, characterized in 40 cut. This effect can be characterized by specific modes that the curved section (2) mixing can be mastered. With continuous

Mode mixing or with mode mixing in equal

a section of the dielectric optical waveguide (4,5) bent as a helix with a variable pitch contains (Fig. 4b).

The invention relates to a mode mixer for propagation mode mixing of itself in a dielectric Multimode fiber optic cable from core and cladding propagating light by means of a guide through a curved portion of the dielectric optical waveguide.

The terms "light" and "optical" used here refer to those areas of the electromagnetic Spectrum, commonly referred to as the infrared, visible, and ultraviolet regions.

When light travels along a dielectric light A given energy packet remains the same length in every mod ^ when it is spreading This ensures that each energy packet when passing through the entire dielectric optical waveguide takes the same time, thereby minimizing the influence of group delay dispersion will.

The propagation of light in curved dielectric waveguides with rectangular cross-sections and in particular the radiation losses that occur have already been investigated (cf. Bell System Techn. J., Vol. 48 [Sept. 1969], p. 2103 2132). It was only incidentally found that curved dielectric Optical waveguides with a large cross-sectional area that can transmit many modes, more like a mode wall are subject to radiation losses, d. that is, the curvature of a multimode light conductor to

Waveguide spreads, it can lead in the dielectric 60 of a mode mixing. Like one

Optical waveguides in general, several different mode mixing could be done in a targeted manner, will not

have electromagnetic field distributions. That phy- stated.

Physical behavior of light in the dielectric It is also known that mode mixing is caused by Optical fiber depends on the special electro-periodic change in the axial direction of the light wave magnetic Field distribution, i.e. can be brought about by the propagation conductor (see US Pat. No. 3,687,514 fashion, off. So e.g. B. the group speed and US-PS 36 66 348). Essentially, a of the scattering along a dielectric optical waveguide in the multimode optical waveguide propagating light depends on the propagation mode. reduces mode conversion in the waveguide

j is increased. For this purpose, especially the radius or diameter of the waveguide is changed by changing the Pulling speed modulated during its production. In addition, there is also a shift the axial direction of the optical waveguide possible, creating a helix of constant pitch and constant radius of curvature of the waveguide arises. The period of curvature of the waveguide corresponds to the difference in the propagation constants of the two modes to be coupled (cf. Equation (2) of U.S. Patent 3,687,514). When the amplitude of the displacement of the optical fiber axis should be sufficient to cause mode mixing, but creates such a pitch of the helix a curvature of the optical waveguide, the radius of curvature of which is disadvantageous to a considerable Attenuation of the light passing through the optical waveguide leads. One carried out in this way: Mixture of modes thus causes energy losses through radiation from the waveguide.

It is the object of the invention to provide a practically applicable mode mixer in which one Circular cross-section - light waveguide passing light can be transmitted practically without attenuation.

The object is achieved according to the invention in a mode mixer of the type mentioned at the beginning solved that at least a part of the curved section has a continuously changing radius of curvature that is greater than anywhere

R) -

4a ² «,

l / T "

and is less than

6.4α ³ π?

K ₁ = J

a - radius of the core,
H ₁ = refractive index of the core,
n ₂ = refractive index of the cladding,
Z ₀ = free space wavelength of the light.

The mode mixing is carried out according to the invention with the aid of a curved section of the dielectric Optical waveguide carried out, the at least partially continuously changing radius of curvature having. The curved portion of the dielectric optical waveguide has a path difference between that traveled by the wavefront on the inside and outside of the curvature Distance on. This path difference corresponds to a difference in the phase velocity, which causes a transition of light between modes.

If the radius of curvature is larger than the value R ₁ , no mode mixing takes place. since the difference in phase velocity created by the bending is insufficient to compensate for the phase velocity difference between modes. On the other hand, if the radius of curvature is smaller than the value R ₂ , propagation modes, ie modes in which the light is bound to the dielectric light waveguide, are coupled into radiation modes, ie in modes in which the light is not bound to the dielectric light waveguide This results in a strong attenuation of the light passing through the dielectric optical waveguide, which is not permissible for a communication link.

If, according to the invention, the radius of curvature is greater than R ₂ , a certain damping must be expected, but this is negligibly small.

The mode mixer according to the invention thus contains

ίο a curved section of a dielectric optical waveguide, namely for continuous mode mixing with a continuously changing one Radius of curvature. This can be done in several advantageous ways, e.g. B. by a bend with slowly increasing curvature, followed by an area of constant curvature and an area with a slowly decreasing curvature, like a figure-of-eight loop, or through a meandering one Section of the dielectric light wave pile or by one as a helix with a variable Slope bent section of the dielectric optical waveguide.

The invention is explained in more detail with reference to the exemplary embodiments shown in the drawing.

It shows

F i g. 1 schematically shows an optical or light communications link with a mode mixer,

Fig. 2 shows a curved portion of a dielectric Optical fiber,

F i g. 3 shows a light beam path of light propagating through the core of a curved portion of a dielectric optical waveguide;
F i g. 4a to 4c different types of curvature of a dielectric optical waveguide for mode mixing and

F i g. 5 and 6 signal responses from an optical communications link with a Fashion mixer.

Referring to Figure 1, light from a source 1 is passed into a section of a multimode dielectric optical fiber 4 fed in. The light then passes through a mode mixer 2 and another dielectric Multimode optical waveguide 5 to a detector 3. The mode mixer 2 contains a curved section of the dielectric optical waveguide. In practice, the entire connection between the light source 1 and the detector 3 essentially consist of curved portions of the dielectric Optical waveguide exist that the mode mixer 2 represents the entire connection. How the Mode mixer results from the following theory. A wave in a straight dielectric fiber optic has a single phase velocity

With:

= 2.t / = circular frequencies /.

2.

= Wave expansion constant.

Ladder wavelength.

When the fiber optic cable moves to r-1 «. 1 ^ u a Radius is bent, there is a difference in

the paths of the phase front on the outer and inner radius of curvature of the waveguide. The length on the outer path of the wave front is (r + α) α and on the inner path (/ - - a) n. The path difference is .Ir- «= 2aa, with a mean path r η and a normalized path unler

differed - = -. This standardized path difference

1 V is a standardized difference - ^ ¹ - the phase-

speed equivalent.

With a fixed frequency <· < , each phase velocity corresponds to a value of the propagation constant / (. Because of:

d V _p _ <n_ _ ^ _1

ate " ~ ψ ~ * ~ v" 7th

is. with ί = speed of light in free space or vacuum.

For continuous mode mixing, the light wave letter must alternately be bent and not bent be to result in a meandering path / u. The radius of curvature of the bends must be small be enough to mix all the fashions / u, but not so small that the fashions of the highest order are in that Radiation field is coupled. The critical radius of curvature corresponds to a critical l / i.

fi can be written as

ν = - β

1 / i ¹ ^ = - -, where

so that at the end of the optical fiber with a given radius of curvature there are two values for β for each mode:

\ ß _ 2a

The secondary modes generated by this splitting of / i are closely coupled and degenerate. This is equivalent to calculating the curved optical waveguide in terms or expressions of the straight optical waveguide, provided that the radius of curvature of the bend is always substantially greater than the radius of the fiber; this condition is determined exclusively by material quantities. The energy swings continuously from one side mode to the other as the wave advances around the bend. By gradually inserting a bend in the fiber, all modes that are less than t / i apart are c-coupled, ie. By running into a bend or by running out of it, a mode mixing is effected up to the size of the limit radius of curvature of the bend. However, if the light wave conductor is bent with a constant radius of curvature. no further mode mixing occurs. In this sense, bending an optical waveguide to a radius has the same effect as introducing a spatial frequency into the fiber dimensions, whereby all modes are synchronized whose frequencies differ by the spatial frequency. If the length of the space or the slope of the change causing coupling /. _r is. is applicable:

1/'

t>

2a

= ν

Ύα lac

j

A =

Λ _ " ² V

/ tu \ ² . - —J is.

With:

u = argument of ./ “(u). the Bessel function representing the fields within the core.

For the modes of the highest order u * V and Tür the difference between successive zeros of J “(u) we have Iu ^.-7 It follows that

AL

you

V Ύαΰ) Ϋ ^π

2ηΛ

M Λ

35 .-τ Λ

for small Λ,

with R ₂ = critical radius of curvature. If r <R, then a coupling into the Strahlungsfclc with r> R ₁ takes incomplete Modenmisehcr Substituting V results

whereby a spatial frequency /, = - " ^{C is} represented 55 For the modes of the lowest order it follows:

.03 Π

^! -M

where U ₁ and W _{2 are} the values of u for the lowest and far lowest order modes. At high V-values, M ₁ and H ₂ can be represented by:

M ₁ -

With

- M

- M

M ₀ , = M _n2 =

₀₂

, M ₂ - Mj

j ₂

first zero of J _n M = 2.4048.
second zero of J ₀ (μ) = 5.5201.

It follows

5,783

For large V we have

(30.47 - 5,

= 12.34 c " ² " · - ¹ ^.
and since

.783) 1

30.47

With OB = r + α and OA = r - u it follows:

/ ■ + α r - a

ίο cos H cos (H + ii)

cos

cos <-)

cos H ■ cos f <- sin Π ■ sin <i
cos <->

lh = ■

2.89 -e-- ■ '' 'λ

α (Λ)

'' ■ = 12.34 -e- ^2/1 · ',

For the usual capital I and for multimode fibers small Λ results

By equating sin «= <i for small angles follows:

= 1 - (cos .i - α ■ lan H)

² + ti tan Π.

ti ² + «■ tan <-) - ----- = 0.

= - tan H -ii tan- <■) -f

- ^ = 12.34 ■ c

Wecen

Aa λ ^{1 2}

r J

- tan w

Ϊ, 45 "■ == - ·" ^IM "Γ« ■ '· If «= 0 is concerned) (-> - 1

where I (-.

40

or, by substituting 1 and Λ

mil: R ₁ - minimum radius of curvature before a 4s mismatching of the lowest order modes begins.

Let us now observe the geometric path of a light beam in the core of a dielectric light wave guide (see FIG. 3>. A beam in a light fiber 50 cos .. is reflected at an angle <-) from the inner surface of the core. When the fiber is bent. the angle of reflection is changed. Once the beam travels in a waveguide that is curved with a constant radius of curvature, there is no further overall change of (-) (no further mode change). The angles of reflection on the inner and outer surfaces 5> nd different for each yoke, and a ray is alternately reflected from each surface at different angles. This corresponds to the degenerate modes with the two values for ß.

In the triangle OAB (see Fig. 3) , 1 - \ <-> is the angle by which W is changed, and the following applies:

the angle is. with which an axial beam passes through the Bend is rotated. The angle of an axial ray can only be increased (since it is always from dci Outside of the bend), d. h.dcr fashion lowest order can only be in fashions higher! Order to be converted.

In order for a ray to be reflected, the fasci be bent around a minimal arc. For an axial beam (see Fig. 3) the following applies:

r + α

[la ir + = a r 2 (i . sin r + + a

sin .1

sin (90 + H)

OA
sin 5 SUBSCRIPTION

65

SUBSCRIPTION

With continuous mode mixing, the fiber must therefore be curved through a series of arcs, each of which has at least one angle u . The best results are obtained for a radius of curvature that is exactly equal to R ₂ . Because of this meandering, the fiber is lengthened somewhat. The elongation is calculated from the length of the arc. Length of the fiber = </ sin «.

BH of a bend is the intermodc coupling distance, i.e. the distance over which the energy exudes one of the two modes merges into the other equal to the distance between two successive reflections from the inside to the outside dci Curvature (or vice versa), since the angle (or mode) starts from a value / um for each reflection whose is changed. This distance /. “Is equal to>

in Fig. 3. Neglecting the influences on L _{1n of} the radius of curvature of the bend, the following applies:

- ² _f - = lan <-) * sin W * · - (<>) " ² .

Mn

example

For a fiber with the following characteristics:
Core diameter = 50 μΐη = la: n ₂ = 1.5100:

/ I ₁ = 1.5563, free space wavelength = 0.85 μΐη = / _Λ)

_{Λ =} i-fJ ^ .V = 0.58;

V = - ~; ^T - ^£ - / ι, (Λ) ¹ ' ² = 69.26,

R ₂ = - ⁴ ^ i ¹ Z ₂ = 1.90 cm: L _n , = 0.207 mm
(for modes of the highest order, u = s = V),
R, = 0.162 · --'- · e ^{2 / r} = 34.47:

L _n , = 5.98 mm

(for modes of the lowest order, u = 2.405),

For the increase in length for a fiber that meanders with a radius R ₂ over a series of arcs with the angle α . is applicable:

- 1 = 4.38 x 10 " ⁴ = 0.0438%.

sin a

A dielectric optical waveguide can be brought into a suitable meandering curve according to FIG. 4a, by embedding it in a plastic substrate. This creates continuous mode mixing ensured along the entire length of the dielectric optical waveguide. Otherwise if a cable formed from a bundle of dielectric optical waveguides can be a continuous Change in the radius of curvature of each dielectric optical waveguide by changing the twist of the dielectric optical waveguide in the cable, so that each dielectric optical waveguide

ίο the cable has the shape of a helix with changeable Has slope according to Fig. 4b. If at a discreet If mode mixing is to take place in the cable, this can be achieved by adding the dielectric Optical waveguide is brought into a suitable geometric arrangement. e.g. by forming a Figure eight on a suitable support according to FIG. 4c.

When mode shuffling occurs across all non-radiant modes, one in one end has one dielectric lightwave conductor fed narrow impulse which in F i g. 5 pulse shape shown. When mode mixing takes place in a mode group of fewer modes, the number of which is smaller than the number of the non-radiative modes, such as e.g. B. in a continuous meander fiber with a maximum Meander radius of curvature, which is much larger. than is required for complete mode mixing is. the output pulse has a double hump, as in FIG. 6 is shown. This double stool is caused by a mode mixing pulse 6 which is shorter than the mean arrival time is centered, followed by the later arriving block of unmixed modes 7. As it is in practice is difficult. mix all modes and get a maximum bandwidth without loss of energy from the dielectric To obtain light wave lciter. i..uss damil It can be calculated that the output pulse has a double hump characteristic. because the radii of curvature are significantly larger than the critical radius, but especially with digital signals such as ζ. Β with pulse code modulation (PCM), the second stool has only a minor influence. So it is possible the actual bandwidth of a multimode fiber without undue loss or restriction of the effective fiber aperture.

For this purpose 4 sheets of drawings

Claims (1)

Patent claims: 1. Mode mixer for propagation mode mixing of itself in a dielectric multimode optical fiber Light propagating from core and cladding by means of guidance through a curved section of the dielectric optical waveguide, characterized in that when the dimensions of the dielectric optical waveguide are sufficiently small, the light is on when passing through the dielectric optical waveguide limited to only one single propagation mode; Such a dielectric optical waveguide is called dielectric Single mode fiber optic cable. In the case of larger dielectric optical waveguides, however, propagate the light in several modes, to which the light when passing through the dielectric light- is limited in the least part of the curvature section of the T ^ ίο waveguide: such a dielectric continuously changing radius of curvature that is greater than anywhere