Return Loss and Insertion Loss Calibration and Measurement of Optical and RF Power.

Optical MeasurementWhat is return loss?When light passes through an optical component most of it travels in the intended

direction, but some light is reflected or scattered. In many applications these reflections are unwanted, because they can affect the emission characteristics of any laser in the system. In such systems it is important to measure the reflections for the components of the system.

The reflection factor for the component is the measure of how much light the component reflects. It the ratio of the power reflected by the device to the power incident on the device. More normally we talk about the return loss of a component. The return loss has units of dB. Return loss is given by:

Return Loss (dB) = -10 log (Reflection Factor) (dB) Return Loss (dB) = -10 log (Reflected Power/Incident Power) (dB)

What is insertion loss?Light that is absorbed, scattered, or reflected by a component also affects how much

light a component transmits. The transmission factor of a component is a measure of how much light the component transmits. It is a ratio of the power transmitted by the device to the power incident on the device. More normally we talk about the Insertion Loss of a component. The insertion loss has units of dB. Insertion loss is given by:

Insertion Loss (dB) = -10 log (Transmission Factor) dBInsertion Loss (dB) = -10 log (Transmitted Power/Incident Power) (dB)

Measurement ModuleThe 8161XA series Return Loss modules are compact modules for use with the Agilent

8164A Lightwave Measurement System. They are used for making return loss measurements and, in conjunction with a power sensor module can be used to measure insertion loss.

A return loss module measures the light reflected and scattered as light passes through an optical component. Agilent 81612A return loss module was used in this project. It includes a power sensor, monitor diode, two couplers and an internal laser source in one module as shown in Figure 1.

Output 9/125 mInternal laser

source

External laser source

contact MDUT

PDUT

The diodes shown compensate for power variation in the light source.

Calibration Measurements

Before measuring the insertion loss or return loss of a device under test, calibration is important as it eliminates wavelength dependencies, coupler directivity, insertion losses, backscattering and other non-ideal characteristics of the system.

Calibrating the Return Loss Module

The mainframe always displays a value for the return loss if it has a return loss module. It is therefore important to calibrate the return loss module against a component of known reflectance. I used Agilent 81610CC reference cable for calibration. The procedure is described in the user’s guide. Attach the reference cable to the output of the 81612 A. This return loss module has an internal laser. Attach the high return loss connector of the reference cable to the output. For best results and higher repeatability fix the cable. Now on the module make sure that the value of [RLref] is correct (14.6dB). If not then edit the value. Then press menu and select reflectance calibration. The instrument now measures the power reflected by the reference cable. The [RL] value changes to the same value as entered for [RLref]. This completes the calibration of the module.

Calculating the Return Loss and Insertion Loss of the DUT

Figure 1. The contents of Agilent 81612A Return Loss module.

Mref

Pref

Pin

Pr

t1

t2

Psrc

K1

K2

The reflected power measured by the instrument from the component with the known reflection factor, is given by the sum of:

The part of the power reflected by the component, which is transmitted through the coupler, and

The reflections due to the measurement system.

That is:

P=t 2 K1 K2 Psrc R+ t2 sM where:

M=t1 Psrc

c1=t 2

t 1k1 k 2

c2=t2

t1s

⇒P=c1 MR+c2 M

The constants t1, t2, k1, k2, are multipliers giving the proportion of power transmitted through the coupler form the input port to the output port and from the output port to the sensor port respectively. What this means is that when optical power is input at the output port, K2 times that power is output at the sensor port. Calibration eliminates these constants.

The constant s is a multiplier giving the scattering factor. This factor accounts for the directivity of the second coupler, backscatter in the fiber, and the reflections of the connectors. The calibration procedure helps eliminate the effects of these on return loss measurements.

We calibrate the return loss with a component with a known return loss, Rref.

Pref =c1 M ref R ref +c2 M ref

Measurement patchcord Emeas

Power meter

Mref

Pref

t1

t2

Psrc

K1

K2

When there are no reflections all the power measured by the sensor is due to the non-ideal nature of the measurement system. This is the termination parameter.

Ppara=c2 M para

for a DUT,

PDUT=c1M DUT RDUT+c2 M DUT

Substituting equations into equation,

Pref =c1 M ref Rref +M ref

M paraPpara

PDUT=c1 M DUT R DUT +M DUT

M paraPpara

RLDUT=−10 logM ref

M DUT

PDUT−M DUT

M paraP para

Pref −M ref

M paraPpara

−10 log Rref

DUT

Measurement patch cord EIL

Power meter

MILPsrc

The insertion loss is given by the following equation,

ILDUT=−10 logE IL

EMeas

Results

To measure the return loss, connect the device under test directly to the return loss module. The value displayed in the RL field in dB is the return loss of the DUT. The return loss measurement for different cables is

Type Return loss(RL)FC-FC cable 8.1 dB

FC-SC cable 8 dB

FC-SC with FC-FC 7.3 dB

FC-FC with FC-SC 7.7 dB

012345678910

1500 1520 1540 1560 1580 1600 1620

wavelength (nm)

RL (d

B)

7.35

7.4

7.45

7.5

7.55

7.6

7.65

7.7

7.75

-6 -4 -2 0

Retu

rn L

oss

(dB)

The return loss varies with wavelength as well as with laser power. With wavelength the difference in the value of return loss is about 4 dB and with laser power the variation in its value is about 0.3dB. Since return loss is a measure of the amount of power reflected back at a discontinuity hence it is also dependent on the refractive index of the fiber. As the wavelength changes the refractive index of the fiber also changes causing the return loss value to be different at different wavelengths. Ideally the return loss should be the same for any value of laser power but this is not the case as can be seen from the measurements. All these effects should be kept in mind for accurate measurements.

The insertion loss of the FC-SC cable was measured. First the 81640A tunable laser source was connected directly to the 81635A power sensor using SC-SC cable in order to measure the power transmitted directly. Then the FC-SC cable was connected to the SC-SC cable and then connected to the power sensor. The difference in the power measured is the insertion loss of the FC-SC cable. The measurements for different wavelengths are given in the following table

Wavelength Power (SC-SC) dBm Power (FC-SC) dBm Insertion Loss (dB)1510 -12.1 -12.22 0.121520 -8.1 -8.23 0.131530 -6.21 -6.25 0.041540 -5.5 -5.51 0.011550 -5 -5.1 0.11560 -4.6 -4.7 0.11570 -4.3 -4.37 0.071580 -4.1 -4.2 0.11590 -4 -4.1 0.11600 -4.2 -4.37 0.171610 -6.2 -6.22 0.02

The insertion loss of the FC-SC cable is about .1dB and is dominantly due to the connector loss. This value of loss is reasonable for the connector.

RF Measurements

HP 85106C Millimeter-wave Network Analyzer System

7.35

7.4

7.45

7.5

7.55

7.6

7.65

7.7

7.75

-6 -4 -2 0

Retu

rn L

oss

(dB)

System Description:

The 85106C mm-wave system consists of an HP 8510C, two sources, a band independent HP 85105 mm-Wave Test Set Controller, and a pair of band dependent HP 85104 series mm-Wave test set modules covering the 75 to 110 GHz waveguide band i.e. W band. One source provides the RF (stimulus) signal and the second source provides the Local Oscillator (LO) signal.The HP 8510 uses its multiple source control feature over the system bus to tune both the RF and LO source over the entire frequency sweep. Calibration:The calibration procedure for two port devices consists of measuring a zero-length thru connection, short circuits at each port, terminations at each port for isolation, and the waveguide shim as the Line standard. This calibration produces 12-term error correction and provides best accuracy for 2-port devices. Refer to System Manual page 2-12 for calibration procedure.

Measurement of Insertion loss and Reflection loss:

The system is ready for use in measurement after the calibration is completed successfully. Measurement of s parameters for a device is used for calculating the above losses. To calculate the insertion loss of a waveguide connect port 1 and port 2 directly and measure S21 i.e. the forward transmission coefficient. Now connect the waveguide between port 1 and port 2 and again measure S21. The difference in the measurements is the insertion loss of the waveguide. S11 and S22 are the input and output reflection losses respectively. The 20 dB and 50 dB attenuators were used and verified during the measurement.