JDS Uniphase RX3 Series, Руководство пользователя

Страница: 15 / 55

2
Backreflection Measurements
Reflections in optical systems can come from a number of sources. Primary sources include the
fiber (Rayleigh backscatter) and Fresnel reflections that occur at the planar junction of two
materials having different refractive indices, for example, connector and fiber endfaces, splices,
bulk optic interfaces, and detector surfaces.
Typically, only Fresnel reflections are significant because transmitters are relatively insensitive
to distributed reflections such as Rayleigh backscatter. Backreflection caused by Rayleigh
backscatter varies with the length and type of fiber, and is only significant when measuring
components with backreflections below -40 dB or with very long pigtails (multimode), or -80 dB
(single-mode). However, Rayleigh backscatter can be a large contributor of backreflection in
installed systems; therefore, proper fiber termination techniques must be applied to separate the
backreflection of the test component from the Rayleigh backscatter of the fiber.
An internal switch and coupler enable the RX3 meter to measure the internal light source signal
(P
in
), the signal offset with no light (P
dark
), and the total signal level from internal and external
backreflections (P
br
).
The meter first calculates the total backreflection from internal and external sources (BR
tot
)
using the following equation, where CAL and USERCAL are the factory- and user-set calibration
factors, respectively:
BR
tot
= 10 log (P
br
- P
dark
)/(P
in
- P
dark
) + CAL + USERCAL [dB]
When the RX3 meter is calibrated at the factory, USERCAL is set to 0 dB.
The backreflection from external sources (BR) is then calculated using the following equation,
where BR
0
is the stored value of the total backreflection up to the device under test (DUT), and
OFFS is the offset used to compensate for losses between the meter and the DUT:
BR = 10 log (10
BR
tot
/10 - 10
BR
0
/10) + OFFS
[dB]
The measurement cycle takes approximately one second to complete. The meter then displays
the value of BR. The preset value of OFFS is set at twice the average insertion loss of the
connection between the RX3 meter and a measurement jumper.
The exact values of BR
0
and OFFS can be measured after a measurement jumper is connected
to the meter.
Loss and Power Measurements
The RX3 meter is equipped with a front-panel InGaAs or Ge detector for loss measurements
and both relative and absolute power measurements. Because the meter is capable of storing
the dark signal from the detector, high-accuracy power measurements as low as -80 dBm
(single-mode)/-60 dBm (multimode) can be obtained.
Output Port
The output ports of the RX3 meter are equipped with ultra-low backreflection FC/APC
connectors.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com