光学反射仪的课件

10.3.5 Coherent vs. Direct Detection
direct detection Optical reflectometry techniques coherent detection
10.3.5 Coherent vs. Direct Detection
Direct detection: I d Ps : responsitivity of the detector Ps : reflected optical power Coherent detection: Id [ Ps PLO 2 Ps PLO cos ] : phase difference between Ps and the local osicllator PLO
concept：
Total return loss=total reflectivity power / input total power
Total return-loss technique: measure the total fractional power that is reflected from a test device
3dB Test arm Optical terminator Power meter Optical terminator
cw Source
power reading 3
Measurement Example
1 power reading 1 power reading 2 power reading 3 power reading 2 - power reading 3 -12.41dBm -17.2dBm(19.1uW) -37dBm(199nW) 2 -12.6Fra Baidu bibliotekdBm -18dBm(15.8uW) -36.7dBm (213.8nW) 3 -12.72dBm -18.10dBm(15.5uW) -36.7dBm(213.8nW)
cw Source
3dB
Test arm
Test device E.g.,pigtailed photodiode Connector or Fusion splice
Power meter
Terminated end
power reading 2
Measurement Example
3.Subtracting background reflections The fiber between the connector and PD is terminated or wrapped many times around a small-diameter mandrel to attenuate all reflections from PD.
cw Source
3dB
Test arm FC/PC connector
Power meter
Optical terminator
power reading 1
Measurement Example
2.Measuring total reflectivity PD is connected to the test arm of 3 db coupler.
-17.24dBm(18.9uW) -18.07dBm(15.59uW) -18.16dBm(15.29uW)
reflectivity
19.53dB
20.08dB
20.14dB
-14.7db-(-17.24dbm-(-12.41dbm))=-19.53db 参考：陈妍-20090720-器件的回波损耗（背向反射）测试方法.doc
3. Measurement of reflectivity performed on long fiber span (spaced >1 m)
10.1 Introduction
The goal of spatially resolved optical reflectometry：
measure optical reflectivity as a function of distance
10.3.1 Spatial Resolution
10.3.1 Spatial Resolution
The relationship between spatial resolution and pulse width:
1 zr vg ts 2 vg :the group velocity(c/n g，n g is the group index) ts :system response time
10.3.1 Spatial Resolution
This section will describe the relationship between resolution and pulse width (or frequency span) of the probe signal.
Spatial resolution (“two-point” spatial resolution): The minimum distance between two reflectors that can still be resolved by the measure system.
Coherent detection The dynamic range Minimum signal sensitivity Polarization sensitivity Optical source Direct detection Good(square root of Bad the reflected power) Good(PLO amplifies signal ) Sensitive(overcomed by ploarizationdiversity receiver) Special characteristics Bad
Ch10
Optical Reflectometry for Component Characterization
Outline
10.1 Introduction 10.2 Total Return-Loss Technique 10.3 Basic Concepts for Spatially Resolved Reflectometry 10.4 Optical Low Coherence Reflectometry 10.5 Survey of Different Techniques 10.6 Comparasion of Techniques
10.2 Conclusion
A few general considerations on total return-loss measurements: 1.The measurement is simple and inexpensive 2.Measurable reflectivities need to be larger than about -70db This limit is set by the level of background reflections caused by Rayleigh backscatter . 3.inconsistent measurements can occur when the test device contains several reflecting surfaces.
70db/meter
PC connecter： 40db APC connecter ： 60db
possible backscatter
10.2.2 Multiple reflections
Multiple reflections can add interferometrically resulting in the total return loss being wavelength dependent.
10.1 Introduction
Motivation for high resolution measurements:
1.Trouble-shooting 2.Characterization of future optical components and subsystems
10.2 Total return-loss technique
Spatially resolved reflectometry can overcome above problems!
10.3 Basic concepts for spatially resolved reflectometry
Spatial Resolution
Dispersion Limit Rayleigh Backscatter and Spatial Resolution Rayleigh Backscatter and Coherent Speckle Coherent vs. Direct Detection
OTDR:
FMCW：
optical time-domain reflectometry
frequency-modulated continuous wave
10.1 Introduction-Theme
1. Measurement of Total Return Loss 2. Measurement of spatially resolving reflections with high resolution (spaced <1 cm)
Abbreviation
DUT：
CW：
OCWR: LO: OLCR: PBS:
device under test
continuous wave
optical continuous wave reflectometry local oscillator optical low coherence reflectometry polarizing beam splitter
10.2.1 Reflection Sensitivity
The unwanted reflections limit the reflection sensitivity of the total return loss measurement.
connection to the test device controlled terminated end unwanted reflections Rayleigh backscatter uncontrolled
10.3.5 Coherent vs. Direct Detection
Coherent vs. Direct Detection:
Direct detection: Id Ps Coherent detection: Id [ Ps PLO 2 Ps PLO cos ]
Assuming the receiver time: r and the pulse time :τ p ts τ p r
2 2
τp r
if τ p r if τ r p
The response time of the detector usually limits the minimum spatial resolution
Optical continuous wave reflectometry(OCWR)： send a cw optical signal to the test device and measuring the total reflected cw power
10.2 Total return-loss technique
Measurement Example
1.Calibrating the power meter Calibrate the power meter using a known reflectivity so that measured power can be related to optical reflectivity. Fresnel reflection from the fiber-to-air interface: -14.7db