Measuring of optical output and attenuation

Measuring of optical output and attenuation THEORY

Measuring of optical output is the fundamental part of measuring in optoelectronics. The importance of an optical power meter can be compared to an ammeter in electronics.

Wavelengths of 850nm, 1300nm and 1550nm (transmission “windows”) are used for

telecommunication transmission. It is an invisible radiation so it is not light (360nm – 780nm) but an optical radiation (200nm – 1000nm). In the visible range of a spectrum we speak about photometric variables and photometric measuring, that take into account the spectral sensitivity of the human eye.

The optical radiation output P[W] represents the output transferred by radiation, i.e. it is

determined by the amount of energy passing through a tracked area in a unit of a time. In optoelectronics, more usually the optical output L is measured in absolute decibels [dBm] which conveys a level of optical output in relation to 1mW.

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By means of this variation L attenuation A in relative decibels [dB] can be easily determined by the following equation

1 2 L L

A= −

where L1 and L2 are output levels obtained for example by measuring at the beginning and end of an

optical line. Optical power meters have got a reference function that allows the storing of a reference value P1 and then can measure the relative level Lr of arbitrary optical output against this reference. The

attenuation A is then

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and can be read (except the sign) on the instrument display.

Photodiodes (Si, Ge, InGaAs) are used as detectors. They achieve high sensitivity and low dark current. Their sensitivity is spectrally dependent; we must then set a power meter to the wavelength of the light source used. A detector area must be large enough to detect the entire radiation leaving the fibre (conventionally 2 – 3mm). Adapters to various types of optical connectors are used for fixing the fibre end. The adapter must ensure the alignment of the fibre and the detector and it must not obstruct the optical beam. Any impurity and dust will distort the measuring (diffusion and absorption) → CLEAN! MEASURING

Measure the optical power transmitted by a cable. Use a multimode cable GI 50/125 (orange) for λ = 850nm a 1300nm, and a single mode cable 1E9/125 (yellow) for λ = 1310nm and 1550nm.

instruments: light source, power meter, coupling elements GI 50/125 (orange), 1E9/125 (yellow)

process:

1. join the source (connector MM) to the power meter using the coupling element GI 50/125 (the left hand connector at the source, first clean the connector faces of the element)

2. turn on the source and set the wavelength to 850nm (λ button) 3. set λ = 850nm at the power meter

4. measure optical output L1, P1

5. set λ = 1300nm at the source 6. measure optical output L2, P2

7. set λ = 1300nm at the power meter 8. measure optical output L3, P3

9. set λ = 1310nm at the source, use 1E9/125 coupling element (first clean the faces), join it to the SM connector

10. set λ = 1310nm at the power meter and measure optical output L4, P4

11. set λ = 1550nm at the power meter and at the source and measure optical output L5, P5

TABLE OF MEASURED VALUES

No of meas. 1 2 3 4 5

Li[dBm]

Pi [mW]

Measuring of attenuation by inserting loss method THEORY

Three standard methods are used for measuring attenuation of optical fibres. Two of them are transmission methods, one is a reflection method.

a) method of two lengths b) method of inserting loss

These two are transmission methods utilizing a regulated radiation supply and a power meter. The real attenuation of a signal passing through a fiber is measured (direct methods). When measuring by the method a, an optical output P2 is measured at the exit of a fibre of length L. Saving the coupling

conditions, the fibre is broken next at a distance of 2m from the beginning. After finishing the fibre end, an optic output P1 is measured. The attenuation in dB is then A = 10log(P1/P2). This method is the most

accurate, yet destructive. c) OTDR method

This is a reflective method based on measuring the back diffusion with an optical refractometer. The time dependence of a back-diffracted optical power of the signal is evaluated; a narrow impulse is used. This method gives information about attenuation distribution along the fiber, about joints positions and about contingent fiber distortions, so it is suitable for fibre and line checking. Reproducibility of this method is not better than 0,5dB.

MEASURING

Determine the attenuation of a 1km optical link by the inserting loss method. Use the cable GI 50/125 for measuring and the cable GI 62,5/125 for reference measuring.

instruments: light source, power meter, 3 pieces of GI 50/125 cable (orange), GI 62,5/125 cable (blue), optical link 1km

process:

1. attach a cable GI 50/125 to each end of optical link 2. connect a cable GI 50/125 to the source and meter 3. turn on the source and set λ = 850nm (use λ button) 4. set λ = 850nm at the meter

5. read reference power Lref1 on the meter

6. remove reference cable and attach the optical link to the source and meter 7. measure the power level L1 an calculate attenuation A1 = Lref1 – L1 [dB]

8. disconnect the optical link and repeat the measuring with reference cable GI 62,5/125, read the value Lref2 on the meter display

9. disconnect the reference cable GI 62,5/125 from the meter, attach the optical link to the source and meter using GI 62,5/125 cables

TABLE OF MEASURED VALUES

GI 50/125 GI 62,5/125

Lref1 [dBm] L1 [dBm] A1 [dB] Lref2 [dBm] L2 [dBm] A2 [dB]

Question 1: What value of attenuation is right and why? Question 2: Why is the value A2 higher than A1?

Influence of excitation to fiber attenuation THEORY

As the signal travels through the fibre optic cable, an energy loss occurs. The measure of total loss for a given wavelength is attenuation

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where P1 and P2 is optical power measured at the beginning and the end of a cable, respectively. In

practice, a variable specific attenuation α(λ) is used, which is attenuation related to cable length unit

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where L is cable length (in km in this case). Attenuation and specific attenuation are fundamental and they are most important transmission parameters of fibre cable or link. They are a scale of cable quality and they allow us to estimate the maximum reach of the cable.

There are two types of energy loss mechanisms

a) material loss – radiation absorption and diffusion on impurities; spectrally dependent → telecommunication “windows” (minimums of attenuation)

b) radiation loss – dependent on conditions of excitement. Optical signals travel through fibre in a finite number of modes. The higher modes travel at higher angles, go along longer optical paths and are more attenuated, easily escape a fibre for example on bends. If more higher modes are excited, the cable seems more lossy.

MEASURING

Compare attenuation of optical link without and with mode filter. Use GI 50/125 cable.

instruments: light source, power meter, launch and receive reference cables GI 50/125 (orange), optical link 1km, mating adapter, mode filter

process:

1. Attach launch reference cable GI 50/125 to the source and meter 2. set λ = 850nm on the source (λ button)

3. set λ = 850nm on the meter

4. read power L1 [dBm] on the meter

5. disconnect launch cable from the meter and attach it to the link tested 6. connect receive cable GI 50/125 to the link and meter

7. read power L2 on the meter and calculate loss of the link A = L1 – L2 [dB]

9. wrap launch cable around mode filter 5 times, attach launch cable to the meter and read L3

10. disconnect launch cable from the meter and attach it to the link, attach receive cable to the meter 11. read power L4, calculate attenuation Af = L3 – L4

During the measuring, don’t disconnect the launch cable from the source. The LED in the source may couple a slightly different amount of power into the launch cable when reconnected, ruining your reference set.

TABLE OF MEASURED VALUES

without filter with filter

L1 = L2 = L3 = L4 =

A = Af =

Question: Compare values A and Af. Which of them is higher

and why? source