UDC 535.3
Nikolay Gorlov
Siberian state University of telecommunications and Informatics,
Novosibirsk, Russia
[email protected]
Ali Mehtiyev
Karaganda state technical University, Karaganda, Kazakhstan
[email protected]
Elena Neshina
Karaganda state technical University, Karaganda, Kazakhstan
[email protected]
Arkadiy Bilichenko
Karaganda state technical University, Karaganda, Kazakhstan
[email protected]
THE RESULTS OF INVESTIGATIONS OF LIGHT WAVES
SCATTERING AND REFLECTION IN OPTICAL FIBER
The article presents the research results of optical radiation parameters. It was calculated the
maximum power level of the reflected light and inverse scattering light. It was made an estimate of
the receiver sensitivity by averaging the signals. It was carried out the diagram of the levels of light
wave power reflection in the fiber. The testing is performed using a reflectometer Optical Time
Domain Reflectometer. It was estimated the influence of the index of reflection of backscattered
and reflected signals.
Keywords: optical fiber, optical radiation, reflection, signal, impulse, power, noise level,
scattering, range, reflectometer.
Now the optical fiber is considered the most perfect physical medium for
information transmission, as well as the most promising medium for transmission of
large information flow over significant distances. The reasons to consider optical
fiber as the most promising medium for transmission of large information flows are
derived from the number of features typical for optical waveguides. The optical fiber
is a waveguide typically of circular cross-section. The fiber is made of certain
dielectric materials such as polymer or silica glass. The optical fiber works by means
of light signals transmitting instead of electric ones, as those that is transmitted over
conventional copper wires. The fiber optic cables do this by acting as the wave
conductors for the light waves of a certain frequency. It is possible thanks to the
physical phenomenon such as refraction. The refraction is the change of wave
direction (in this case a light wave) due to the changes of speed. [1]
It is commonly known that the main parameters of the considered devices are:
the maximum range of discontinuity detection, minimum resolution allowing
identifying two neighboring discontinuities of the fiber, and also the accuracy of
discontinuities identification. To evaluate these parameters it is first necessary to
determine the minimum level of the detected reflected signal and to establish the
factors affecting the decrease of its value.
On exposure, for example, of the pulse source of optical radiation with power Р0
and duration of At = 100ns to the fiber with the following typical parameters
S = 3.8-10-3, Vг –c/n = 2.85-108 m/s , а = 2.5 dB/km = 6 • 10-4 Np/m as a result of
these values substituting to the last formula, we’ll get
or what is the same,
where the value of x is given in km and а —in dB/km.
At the point with an abrupt change of refractive index, Frenel reflection takes
place which is determined by the reflection coefficient
(1)
and on the frontier line, for example, glass (n1= 1.5) — air (n2= 1)? R2= 0.04
corresponds to 4% of the light reflection from this boundary.
Typically, the first peak of the sharp change of refraction index occurs in the
inlet section of the optical fiber in the time of inputting the radiation to it that on the
signal level it is always higher than that required to measure by the reflectometer.
Therefore, such a signal generally overloads the photoreceiver and its restoration
takes some time. This time interval at which the reflectometer is insensitive to other
signals of reflection during the transition to a distance is called the "dead zone" of the
reflectometer. Currently many techniques are used to reduce the influence of this
effect; however, it fails to eliminate it completely.
To assess the impact of this index it is required to determine the difference
between back-scattered and reflected signals. Let Рс — the power reflected from the
end surface at the beginning of the fiber, and Рг — the power scattered in the reverse
direction,
(2)
Then, with values taken in the previous calculation, we’ll get
Or
101og(Pc/Pr) = 32d£.
Therefore, the difference in the level of capacities of the Рс, and Рг in the present
case is 32 dB and the radiation power reflected from the discontinuity of the fiber is
approximately 1/100 000 of the light wave power propagating in the forward
direction at the point of reflection, which requires extremely sensitive methods for its
detection. In Fig. 1 there is a diagram that illustrates the range of power levels with
which OTDR works. From this diagram it can be defined both the highest and the
lowest signal levels of the reflection and inverse scattering, and also the maximum
equivalent noise level (NEP) at the receiver input, which provides the desired
dynamic range of OTDR.
To calculate the maximum power level of the reflected light and inverse
scattering light, let’s take a laser source of radiation with a maximum pulse power of
+13 dBm. Then, taking the losses of the connector on the OTDR connector equal
3 dB, the initial power at its output (the input of fiber) will be +10 dBm. In the case
of using on the front panel the non-contact connector, on the nearest end of the fiber
4% reflection appears caused by glass-air transition. It corresponds to a reflected light
pulse which is approximately 14 dB below than the information pulse and it is equal
to -4 dBm. For single-mode fiber when λ = 1310 nm the power level of inverse
scattering will be for 49 dB/µs below than the maximum power level that at the pulse
duration of 10 μs it will correspond to backscattered signal approximately equal to 30 dBm and for 100 μs is about -50 dBm. Taking into account the attenuation of
optical fiber, the backscattered signal decreases with increasing of distance and
eventually becomes weaker than the noise level of the receiver. Obviously, to test at
large distances it is necessary to use more powerful source of radiation. On the other
hand, as will be shown below, to ensure optimum resolution capability and dynamic
range of OTDR the working frequency range of the receiver must be adapted to the
selected measurement range, enabling the use of higher level of NEP for short
distances.
Figure 1 – Level diagram of OTDR
In addition, the receiver sensitivity can be improved by averaging the signals. To
assess this possibility, let’s consider an optical fiber of length 20 km which is tested
using OTDR with the conversion efficiency of the pulse width/distance, equal to 10
μs/km, and determine the degree of noise reduction by averaging the signals in a
single phase of measurement equal to 1 s and 3 min, respectively. In view of the fact
that 10% of the time will be lost, and the time required for signal propagation in the
forward and backward directions, is defined as
the corresponding degree of noise reduction and Nls and N3min will be equal to
As the level of noise reduction is proportional to the square root of N, bilateral
SNR improvement will be
It is obvious that the noise reduction is mainly achieved during the first second
of the measurement, and after 3 minutes the noise level on screen is reduced to
(29.5 - 18.3)/2 dB as compared to the first display updating.
List of references:
1. Оптические кабели / И. И. Гроднев, Ю. Т. Ларин, И. И. Теумен. - М.: Энергоиздат,
1991.
УДК 535.3
Горлов Николай Ильич
Телекоммуникациялар және информатика Сібір мемлекеттік университеті,
Россия
[email protected]
Мехтиев Али Джаванширович
Қарағанды мемлекеттік техникалық университеті, Қарағанды, Қазақстан
Республикасы
[email protected]
Нешина Елена Геннадьевна
Қарағанды мемлекеттік техникалық университеті, Қарағанды, Қазақстан
Республикасы
[email protected]
Биличенко Аркадий Петрович
Қарағанды мемлекеттік техникалық университеті, Қарағанды, Қазақстан
Республикасы
[email protected]
ОПТИКАЛЫҚ ТАЛШЫҚТАР ЖЕҢІЛ ТОЛҚЫНДАР НӘТИЖЕЛЕР
ШАШЫРАУ ЖӘНЕ КӨРІНІС
Мақала оптикалық сәулелену параметрлерін зерттеу нәтижелерін ұсынылады. Артқа
шарықтың шашырау мен шағылысқан жарықтын максималды қуаты деңгейін есептеу.
Сигналдарды орташаланған, ресивердің сезімталдығын бағалау. Талшықты жарық
толқынының көрініс қуатын диаграмма деңгейлері болып табылады. Тестілеу OTDR
Оптикалық Time Domain көрсетуін пайдаланып жүзеге асырылады. Қайта ұмытшақ пен
сигналдар көрінісінің әсерін бағалау.
Түйінді сөздер: талшықты-оптикалық, оптикалық сәулелену, шағылу, сигнал, серпін,
қуат, шу деңгейі, шашаырау, диапазон, рефлекторметр.
УДК 535.3
Горлов Николай Ильич
Сибирский государственный университет телекоммуникаций и информатики,
Новосибирск, Россия
[email protected]
Мехтиев Али Джаванширович
Карагандинский государственный технический университет, Караганда,
Республика Казахстан
[email protected]
Нешина Елена Геннадьевна
Карагандинский государственный технический университет, Караганда,
Республика Казахстан
[email protected]
Биличенко Аркадий Петрович
Карагандинский государственный технический университет, Караганда,
Республика Казахстан
[email protected]
РЕЗУЛЬТАТЫ ИССЛЕДОВАНИЙ РАССЕЯНИЯ И ОТРАЖЕНИЕ
СВЕТОВОЙ ВОЛНЫ В ОПТИЧЕСКОМ ВОЛОКНЕ
В статье приводятся результаты исследований параметров оптического излучения.
Приведен расчёт максимального уровня мощности отраженного света и света обратного
рассеяния. Произведена оценка чувствительности приемника путем усреднения сигналов.
Приведена диаграмма уровней отражения мощности световой волны в волокне.
Тестирование выполняется с использованием рефлектометра Optical Time Domain
Reflectometer. Выполнена оценка влияния показателя отражения обратно-рассеянного и
отраженного сигналов.
Ключевые слова: оптическое волокно, оптическое излучение, отражение, сигнал,
импульс, мощность, уровень шума, рассеяние, диапазон, рефлектометр.