RU2031363C1 - Method of measuring optical lengths and delays of fiber light guides and other fiber-optic elements - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: linear dependence of phase shift of modulated optical signal in light guide on modulation frequency f is used for realization of the method. As phase difference meters are able to measure phase shift only in 2π boundaries, the electric signal frequency should be changed according to the method which modulates intensity of optical radiation passing through the light guide. Two values f₁ and f₂ of frequencies are fixed which correspond to two sequent "zero" countings of phase differences of signals in channels of two-channel measuring device; in this case channels of light guide measured or other passive fiber-optic element are disconnected. After that two sequent countings f₃ and f₄ are fixed which correspond to two sequent countings of phase differences of signals in channels but taking into account that one channel of light guide measured is connected to the circuit. Optical length of the light guide to be found and measured as well as signal delay in passive fiber-optic element are calculated from the relation given in the description of the invention. EFFECT: simplified process of measurement: improved precision. 3 dwg

Description

 The invention relates to measuring equipment, namely, measuring the characteristics of optical fibers, and can be used in the manufacture of optical fibers, in fiber optic technology, in measuring technology for creating and calibrating delays based on fiber optical fibers and passive fiber optic elements (optical couplers, fiber optical switches, etc.), when creating, researching and calibrating fiber-optic sensors of different physical quantities, it can also be used in optical Measurements for measuring the refractive index of various transparent media (liquid, gaseous, etc.).

 Closest to the invention may be a method according to which the intensity-modulated harmonic electrical signal of the optical signal is divided into two optical signals. Each of these two signals is introduced into a separate optical channel, and the output of each of these two optical channels is the input of a null-organ (comparator), fixing the phase equality of the envelopes of the modulated signals at the outputs of these optical channels.

 The measured optical fiber is included in the first of the optical channels, and it is possible, by introducing a special reflector, to install this reflector at the first measurement stage at the input end of the measured aircraft, and at the second measurement stage, at the output end of the measured aircraft.

 The second channel is an adjustable variable optical delay - a combination of movable and fixed reflective prisms, providing a change in the optical length of the second channel. At the same time, it is possible to accurately measure changes in the length of the second channel, for which a Michelson interferometer (with a counter of interference fringes) is used, the reflective mirror of one of the arms of which is rigidly connected with the movable prism of the second channel.

 The measurement of the optical length of the aircraft by this device is as follows. First, the reflector is installed at the input end of the measured aircraft, by moving the movable elements of both channels, a zero reading is achieved on the zero-organ installed at the channel outputs (the optical lengths of both channels are aligned). In this case, the readings of the Michelson interferometer counter associated with the second channel are set to zero.

 Then, the reflector is installed at the output end of the measured fiber, as a result of which the readings of the null organ in the general case will differ from zero due to a change in the optical length of the first channel by the double optical length of the measured aircraft. Then, by moving the movable prism of the second channel, a zero reading on the null organ is again achieved (i.e., the optical lengths of the first and second channels are aligned again), and the Michelson counter determines the change in the optical length of the second channel by changing the interference pattern, i.e. measured fiber.

 The accuracy of this method is actually limited only by the sensitivity of the null organ, fixing the coincidence of the phases of the envelopes of the modulated signals in the channels.

 The disadvantage is the use of an adjustable optical delay in the second channel.

 The method of measuring the optical lengths (delays) of optical fibers and other passive fiber-optic components according to the invention is based on the use of optical radiation modulated in intensity by a harmonic electric signal, which is divided into two light fluxes, each of which is introduced into its optical channel. In one of these channels during the measurement process include the measured fiber; the output radiation of each channel is introduced into a phase comparator, fixing zero values of the phase differences of the envelopes of the modulated signals in the channels.

 The technical result from the use of the invention is to simplify the process of measuring and maintaining high accuracy in a wide range of optical lengths of optical fibers.

-

где С - скорость света.This is achieved by the fact that for measuring the optical lengths of the optical fibers, the equivalent optical difference of the channel lengths is first pre-set without connecting the measured element, for which the modulating signal frequency is changed and two values of the modulating signal frequency f ₁ and f _{2 are} stored, at which the phase comparator fixes two adjacent consecutive zero values of phase differences. Then, the measured passive fiber-optic element is included in one of the channels, while increasing the optical length of this channel by the length of the measured element, again, the frequency of the modulating signal is changed and two new values of the frequency f ₃ and f _{4 of the} modulating signal are stored, at which the phase comparator fixes already two new adjacent consecutive zero counts of phase differences. The value of the desired optical length of the passive fiber optic element is calculated by the expression:
L = C

-

where C is the speed of light.

-

Способ заключается в следующем. Сущность способа измерения оптических длин и задержек пассивных волоконно-оптических элементов можно пояснить следующим образом.The signal delay value in the measured passive fiber optic element is calculated by the expression:
τ =

-

The method is as follows. The essence of the method of measuring the optical lengths and delays of passive fiber optic elements can be explained as follows.

The dependence of the intensity of the modulated optical radiation on time t can be represented by the expression:
I = I ₌ + I _o sin (ωt + φ _o ) = I ₌ + I _o sin (2 πf + φ), (1) where I ₌ is the constant component of optical radiation;
I _o - the amplitude of the modulated radiation;
φ _o - the initial phase of the modulated signal;
f is the frequency of the modulated signal.

Moreover, the ratio is true:
λf = C, (2) where λ is the wavelength of the modulating signal (electrical signal);
C is the propagation velocity of electromagnetic waves.

 In addition, the optical length L of an optical element is the product of the geometric length of the light path in the element and the refractive index of the medium n of this element.

 L = nl, (3) where l is the geometric length of the light path in the element.

 The equivalent optical length of an optical channel is determined by the sum of the optical lengths of all the elements making up this channel.

 The invention is illustrated in figures 1-3.

 According to the diagram shown in Fig. 1, the optical radiation modulated in intensity by a harmonic signal, the intensity of which is determined by expression (1), is sent to a beam splitter D (for example, a translucent mirror, a prism cube or a 1x2 light guide coupler, etc.), i.e. divide the input radiation into two light flux modulated by the same expression (1), but with lower intensities and amplitudes compared to the input radiation.

Each of these two intensity-modulated light fluxes is introduced into a separate optical channel (two channels in total). Channels can be created on the basis of passive elements, both purely optical and on the basis of fiber-optic elements, and can have a difference of equivalent optical lengths in a very wide range (for example, from 0 to 10 ³ -10 ⁴ m).

 The output radiation of each of the channels is introduced into the phase comparator K, which ensures fixing with high accuracy the coincidence of the phases of the envelopes (equal to zero phase differences) of the modulated signals arriving at it. In the channel (for example 1) include the measured element through the light contacts a and b (light connectors), as shown in figure 2, or connect these contacts shortly with each other, as shown in figure 1.

 Consider first the case shown in FIG. 1.

 If the optical channels I and II have equal optical lengths, then the intensity-modulated signals in optical form arrive at the comparator K in the same phases, and the comparator reads zero. If the optical channels I and II have unequal lengths, then the output signals will go to the comparator K with a phase shift Δφ, the value of which can be associated with the difference in the channel lengths and the frequency of the modulating signals from the following proportion.

(где f - частота модулирующего излучения) соответствует разность фаз сигналов, поступающих на компаратор, равная 2 π= 360^о.The difference of the optical channel lengths L ₁ , equal in magnitude to the wavelength of the modulating radiation λ =

(where f is the frequency of the modulating radiation) corresponds to the phase difference of the signals supplied to the comparator, equal to 2 π = 360 ^about .

__→ 2π= 360^o.The arbitrary difference of the optical lengths of the channels L corresponds to the phase difference of the signals supplied to the comparator, equal to Δφ, i.e. L ₁ = λ =

__ → 2π = 360 ^o .

=

или
Δφ =

2π =

2π =

2π т.е. Δφ =

2π (4)
Согласно выражению (3) L=ln, то
Δφ =

2π
(5)
Выражения (4) и (5) связывают значения разностей фаз сигналов на выходах каналов с эквивалентной оптической разностью длин или просто геометрических длин каналов и частотой модулирующего сигнала. На практике по-видимому удобнее пользоваться выражением (4), тем более в случаях, когда требуется определять задержку в каналах.From here you can make the proportion:
_

=

or
Δφ =

2π =

2π =

2π i.e. Δφ =

2π (4)
According to the expression (3) L = ln, then
Δφ =

2π
(5)
Expressions (4) and (5) connect the values of the phase differences of the signals at the outputs of the channels with the equivalent optical difference in the lengths or simply the geometric lengths of the channels and the frequency of the modulating signal. In practice, it is apparently more convenient to use expression (4), especially in cases where it is necessary to determine the delay in the channels.

 Expression (4) shows the linear dependence of Δφ on f, if L - is a constant. As follows from expression (4), the numerical value of L determines the angle of inclination of the dependence of the phase difference Δφ on frequency f. The difference in the lengths of the channels can be formed, for example, by a long fiber included in one of the channels.

Let the channel length difference be formed by a quartz fiber waveguide with a geometric length l of the order of 1.05 km. If we take into account that for quartz-quartz fiber optical fibers, the refractive index of the fiber core is n ≈1.5, then this means that the optical length of this segment of the fiber is L ≈1575 m. Let the modulation frequency of optical radiation be f = 1 MHz = 10 ⁶ Hz.

=

= 300 м Т.е. длина световода в несколько раз больше длины волны модулирующего сигнала. Согласно выражению (4) разность фаз Δφ, вносимая рассматриваемым световодом, составляет:
Δφ =

2π =

2π=5,25·2π=10,5π=1890^°
Но все существующие измерители разностей фаз измеряют фазовые сдвиги лишь в пределах 0-360^о (или -180^о-+180^о). В результате фазометр компаратора, не улавливая значения 10π=1800^о, будет индицировать значение разностей фаз каналов, равное 90^о, что может в равной степени соответствовать сдвигам фаз, равным и 90, и 450, и 810 и 1170^о и т.д., т.е. возникает неоднозначность отсчета фазового сдвига.This corresponds to the wavelength of the modulating signal.
λ =

=

= 300 m i.e. the fiber length is several times the wavelength of the modulating signal. According to expression (4), the phase difference Δφ introduced by the fiber under consideration is:
Δφ =

2π =

2π = 5.25 · 2π = 10.5π = 1890 ^°
But all existing phase difference meters measure phase shifts only within the range of 0-360 ^о (or -180 ^о - + 180 ^о ). As a result, the comparator phasometer, without detecting 10π = 1800 ^о , will ^{indicate the} channel phase difference value equal to 90 ^о , which can equally correspond to phase shifts equal to 90, 450, 810 and 1170 ^о , etc. , i.e. the ambiguity of the phase shift count occurs.

=

≈ 250 м
Т.е. длина волны уменьшилась, и этих длин волн на рассматриваемом отрезке световода отложится еще большее число по сравнению с предыдущим случаем. Согласно выражению (4) разность фаз Δφ, вносимая рассматриваемым световодом (L ≈1575 м) в этом случае составляет
Δφ =

2π =

2π=6,3·2π=22,6π=2268^°
В этом случае фазометр компаратора будет регистрировать значения разности фаз, равное 108^о, что может означать одновременно и 108^о, и 468^о и 828^о и т.д., т.е. опять неопределенность и неоднозначность отсчета.If we change the frequency f of the modulating signal and make it, let us say, f = 1.2 MHz = 1.2 × 10 ⁶ Hz. This corresponds to the wavelength of the modulating radiation:
λ =

=

≈ 250 m
Those. the wavelength has decreased, and these wavelengths in the considered segment of the fiber will be postponed even more than in the previous case. According to expression (4), the phase difference Δφ introduced by the fiber under consideration (L ≈1575 m) in this case is
Δφ =

2π =

2π = 6.3.2π = 22.6π = 2268 ^°
In this case, the comparator phase meter will record the values of the phase difference equal to 108 ^о , which can mean both 108 ^о , and 468 ^о and 828 ^о , etc., i.e. again the uncertainty and ambiguity of the reference.

 The authors measured the dependences of Δφ on f for a multimode optical fiber of the quartz-quartz type 50x125 μm with a gradient profile of the refractive index for different geometric lengths l of the optical fiber. These dependences, shown in figure 3, fully confirmed the validity of the linear dependence on for different values of l (which is the same as L) for any passive optical elements.

 This circumstance, as well as the possibility, according to expression (4), of changing the Δφ value with a given value of the change in f and achieving at the same time Δφ values that are multiples of 2 π, which the comparator phase meter registers with a high degree of accuracy as "zero" values, eliminates all measurement ambiguities phase shifts, and thus measuring the optical length of a fiber waveguide or other passive fiber optic element as follows.

2π
(6) где K - неопределенная величина; L - оптическая разность длин каналов.We turn again to the conditional circuit shown in figure 1. We first pre-set the difference in the lengths of the optical channels into which the measured element is not yet included. To do this, we introduce optical radiation modulated according to expression (1) to the beam splitter D with the optical connectors a and b short-circuited. Suppose that the phase meter of the comparator K shows some (different from zero) value of Δφ. Changing the value of the frequency f of the modulating signal (for example, increasing it), we can find the first value of the frequency f ₁ at which the phase comparator K fixes the first “zero” value, which can correspond to some value of the phase difference Δφ ₁ = 2 πK, where K = 0,1,2,3 .... (K are integers). According to expression (4), the equality
Δφ ₁ = 2πK =

2π
(6) where K is an indefinite quantity; L is the optical difference between the channel lengths.

2π
(7)
Взяв разность выражений (7) и (8) получим:
Δφ₂ - Δφ₁=2 π(K+1)-2 πK=2π . Но в то же время:
Δφ₂-Δφ₁ =

(f₂-f₁)2π т.е. 2π =

(f₂-f₁)2π (8) или же L =

Выражение (8) дает значение эквивалентной оптической разности длин I и II каналов, что необходимо при предварительной установке.Continuing a smooth change in the frequency in the same direction, you can find the following (adjacent) sequential value of the frequency f ₂ corresponding to the following sequential "zero" reading of the comparator:
Δφ ₂ = 2π (K + 1) =

2π
(7)
Taking the difference of expressions (7) and (8) we get:
Δφ ₂ - Δφ ₁ = 2 π (K + 1) -2 πK = 2π. But at the same time:
Δφ ₂ -Δφ ₁ =

(f ₂ -f ₁ ) 2π i.e. 2π =

(f ₂ -f ₁ ) 2π (8) or L =

Expression (8) gives the value of the equivalent optical difference in the lengths of the I and II channels, which is necessary during pre-installation.

Then, in one of the channels, for example in channel I, a measured passive fiber-optic element with the desired optical length L _meas . To this end, this element is included between the optical connectors a and b (figure 2), i.e. the optical difference of the channel lengths L increased by the value of L _ISM , i.e. became L + L _rev . Let the phase meter of comparator K again show some new (different from zero) value Δφ.

By changing the frequency f of the modulating signal (for example, increasing it), we can again find the frequency f ₃ at which the phase comparator K fixes a new “zero” value corresponding to some value of the phase difference Δφ ₃ = 2π m, where m = 0 , 1,2,3 ....

2π
(9) где m - неопределенная величина.According to the expression (4), the equality holds:
Δφ ₃ = 2πm

2π
(9) where m is an indefinite quantity.

2π
(10)
Взяв разность выражений (10) и (9), получим
Δφ₄ =2π (m+1)-2 πm=2 π.Continuing a smooth change in the frequency f in the same direction, we can find the following (adjacent) sequential value of the frequency f ₄ corresponding to the following sequential "zero" reading of the comparator:
Δφ ₄ = 2π (m + 1) =

2π
(10)
Taking the difference of expressions (10) and (9), we obtain
Δφ ₄ = 2π (m + 1) -2 πm = 2 π.

(f₄-f₃)·2π
т.е. 2π =

(f₄-f₃)·2π или же L+L_изм =

(11)
Взяв разность выражений (11) и (8) получим для искомой оптической длины L_изм измеряемого элемента:
L_изм=C

-

.And at the same time
Δφ ₄ -Δφ ₃ =

(f ₄ -f ₃ ) 2π
those. 2π =

(f ₄ -f ₃ ) · 2π or L + L _meas =

(eleven)
Taking the difference of expressions (11) and (8) we obtain for the desired optical length L _{meas from the} measured element:
L _ISM = C

-

.

(12)
As can be seen from expression (12), L _ISM is determined only by the values of the measured frequencies f ₁ , f ₂ , f ₃ and f ₄ , the value of the fundamental constant C and the error in fixing the “zeros” by the phase comparator.

Real measurements were made of the optical length of the optical fibers using a phase comparator, fixing the values of the phase differences at the resonant frequency of 1 MHz (after frequency conversion) with a phase error of less than 0.001 ^about when choosing frequencies f ₁ -f ₄ in the range of 1-10 MHz. It was found that the change in phase difference ^of 0.001, even at large lengths of optical fibers corresponds to the frequency change ≈3,5 Hz.

Such a change in frequency in this frequency range is easily fixed by modern electronically counted frequency meters. The value of the speed of light C is known with an error of the order of 3 10 ^-0 , which does not limit the accuracy of measurements when calculated by expression (12).

 According to the assessment, the error in determining the optical lengths according to expression (12) does not exceed 0.005 m. However, for the implementation of the proposed method there is no need for a precision optical delay that is adjustable in length.

The method allows to measure the optical lengths of the measured passive fiber optic elements in a wide range from 0 to 10 ⁴ m

The method can be used in the research and development of fiber-optic sensors that change their optical length under external influences. The method can also be used to estimate the refractive index of transparent liquids, if instead of the measured fiber-optic element between the optical contacts a and b in figure 2, turn on a cuvette with a previously known geometric length l, measure the frequencies f ₁ and f ₂ with an empty cuvette, and the frequencies f ₃ and f ₄ - when the cell is filled with liquid. Then, according to expression (12), the value of L _meas can be calculated, and the refractive index n in this case can be calculated from expression (3) with the known value of l.

-

Это выражение является расчетным для вычисления времени задержки τ в измеряемом пассивном волоконно-оптическом элементе.Since it is possible to write a well-known relation when light passes through the optical length of a fiber-optic element C = L _ISM / τ, with this in mind it can be obtained from expression (12):
τ =

-

This expression is calculated to calculate the delay time τ in the measured passive fiber-optic element.

Claims (1)

METHOD FOR MEASURING OPTICAL LENGTHS AND DELAYS OF FIBER FIBERS AND OTHER PASSIVE FIBER-OPTICAL ELEMENTS, namely that the optical radiation is modulated in intensity by a harmonic electric signal, the resulting radiation is divided into two light fluxes, each of which is introduced into two optical fluxes, each of which is introduced into the optical channel channels during the measurement process include the measured element, the output radiation of each channel is introduced into the phase comparator, zero values of the phase differences of the envelopes of the modules are fixed nnyh signals in the channels, characterized in that, to facilitate the measurement process, after changing the frequency of the modulating signal, are fixed two adjacent consecutive zero phase difference values and the corresponding values of the frequencies f ₁ and f ₂ of the modulating signal set to a value equivalent to the optical path difference of the lengths of optical channels without connecting the measured element, after being included in one of the optical channels of the measured fiber-optic element, the length of the modulating signal is changed, two and new adjacent successive zero values of the phase difference and remember the two values of the frequencies f ₃ and f _{4 of the} modulating signal, and the value of the desired optical length L of the fiber optic element is calculated by the expression

where C is the speed of light,
and the value of the signal delay τ in the measured passive fiber optic element is calculated by the expression

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