E&CE 477 - Lab #1 - ATTENUATION MEASUREMENT

Introduction

Designing an optical fiber communication system usually consist of a power budget analysis which measures the overall system loss (attenuation). A power budget analysis will determine whether or not a system is loss-limited, that is, if the received power is sufficient to meet the bit error rate specification. Although system performance can also be delay-limited, the performances of most short haul and LAN systems are limited by the amount of losses in the systems.
The aim of this experiment is to measure the losses in several types of optical fibers and connectors. The two basic types of fiber are singlemode (SM) and multimode (MM).

Sources of Attenuation

  1. Source-fiber coupling loss
    The number of modes propagating in the fiber is significantly smaller than the number of modes generated by the source at the launching end of the fiber, therefore, only a portion of light can be coupled into the fiber. The amount of loss depends mainly on the launching condition, source type and the numerical aperture of the fiber.
  2. Connector and Splice losses
    Both the connector and splicer are used to connect fibers together. While the splicer gives a permanent joint, the connector is a demountable device used to conveniently disconnect and join fibers. The losses are due to misalignment of the fiber cores.
  3. Fiber loss
    Loss in fibers is due primarily to Rayleigh scattering, absorption and bends. Microscopic nonuniformities and impurities in the glass scatter and absorb light energy. Bends in the fiber cause light to escape from the core. Unlike the previous losses, fiber loss is frequency dependent. It is important to have equilibrium mode distribution, EMD, in the fiber before measuring the fiber loss. Because cladding and leaky modes will also be excited during launching, the measured loss without EMD can be above or below the steady-state fiber loss.
  4. Fiber-detector coupling loss
    Loss during fiber detector coupling are usually assumed to be zero due to the large area of the detector.

Fiber Loss Measurement Method

The most common techniques for measuring fiber loss are the cutback method, the reference fiber and the optical time domain reflectometry (OTDR) method. The cutback method is a destructive method and requires the removal of a length of fiber from the test fiber. The reference fiber method and OTDR are two non-destructive loss measurement methods. In this experiment, the reference fiber method will be used.

In the reference fiber method, one first determines the launching condition and the input power level by inserting a short length of reference fiber between the source pigtail and the detector (figure 1). Next, the output power level is measured by replacing the reference fiber with the text fiber (figure 2). The fiber loss is then:

L fiber = 10 log 10 ( P in / P out ) ( 1 / fiber length )    typically in dB per km


Figure 1: Reference Measurement             Figure 2: Loss Measurement


Legend

Equipment

  1. Optical Signal Generator: The 7700 XR and 7750 XR optical signal generators use LED sources to provide optical power output signals at 820 nm (35 nm BW) and 1300 nm (40 nm BW), respectively. The output power should be at maximum. The output is measured in dBm units where:
    dBm = 10 log 10 ( power / 1mW )
  2. Optical Power Meter: The 22 XLC measures the optical radiations at 820 nm and 1300 nm with the 150 and 550 sensor heads, respectively. The bandwidth of these devices is quite wide and they are only calibrated at the center frequency. Direct measurement of optical power in dBm will be made with the 22 XLC. To measure the optical power, simply couple the source into the `A' optical port and depress [dBm RCVRA ONLY].
    Model 150: sensitive from 400 - 1150 nm, ñ6% from 400 - 900 nm, calibrated at 850 nm.
    Model 550: sensitive from 800 - 1800 nm, ñ12% over the full range, calibrated at 1300nm
  3. Elastomeric Splicer: This splicer is a plastic sleeve, with a drop of index matching fluid inside, that allows bare fibers to be inserted from both ends. By rotation and proper insertion, of the fibers, this can give a very low loss connection.
  4. SMA Connector: These connectors allow a connection of bare fibers to each other by inserting the fiber into a metal connector and then aligning connectors with a plastic sleeve. They were the standard in the 1980's, are quite robust, but do not align the fibers very accurately. This means that, although low loss connections are possible, consistency is hard to achieve. These connectors can not be used reliably for SM fibers.
  5. FC/PC Connector: This type of connector does not have the rotation problems of SMA connectors, is more accurately built and the fiber is normally epoxied into place, cleaved and then polished. The multimode type typically has a metal tip while the singlemode has a white ceramic tip. On some of these connectors it is hard to push the fiber through. If you push too hard, the fiber will break and the lab staff can try to repair it. Keep in mind, that these connectors cost $50 each. If you are having problems with a connector, point it out to the lab staff, do not try to fix it yourself.
  6. Pigtails: There are at most two pigtails used in this experiment. One is for coupling light from the source to the fiber and the other, from the fiber to the sensor head.
    The 1230-050B Amphenol SMA cable is a jacketed pigtail using graded index fiber. It has a SMA type connecting terminal.
  7. Reference fiber and Test fiber: lengths of bare fiber.

PROCEDURE

You are to investigate the coupling losses in two types of fiber to fiber connections, which are (1) SMA, (2) FC. In addition, you will measure the transmission loss (attenuation) for SM and MM fiber at three commonly used communications wavelengths, 850nm, 1300nm and 1550nm.
These simple experiments are intended to give you practice in basic fiber preparation and measurement. In addition, you will appreciate the (non-negligible) magnitudes of the losses involved in fiber connections, and how they can affect overall system performance, and experimental measurements.

Repeat the following procedures for:

  1. SMA MM cables at 850 or 1300 nm for the one splice to see how the loss changes with rotation. The first fiber must be a bare fiber with a FC connector on one end and an SMA on the other. This experiment will be already set up for you. Simply rotate the connector to see how the loss varies with rotation. Note the maximum power measurement on the power meter [typically around -13 dB for MM fiber] as a reference for how much power was injected into the fiber.
  2. FC SM and MM cables at one frequency (850, 1300 or 1550 nm) for the reference, 1 and 2 splices. Measure the spools at as many frequencies as possible. Use MM spool #1 only as MM spool #2 is damamged (consider the risetime data (1 GHz detector) for spool #2 at the end of lab 2).

    You must use cable fiber types which match the spool ie use MM cables for a MM spool and SM patch cables for a SM spool.

Hints

To achieve optimum throughput at a splice, keep the following items in mind.
  1. Check the cleaved ends of each bare fiber under the microscope before connection, to ensure that the cleave is free from aberrations.
  2. As the last step before a connection is made, clean the fiber end with acetone and tape.
  3. Optimize the output, where possible, by rotating the two coupled fibers with respect to each other.
    Optical connections will only work if the fibers are clean. Before you try to complete a connection clean it with tape and acetone. Dirt on a connector will easily shatter both fibers which you are trying to connect.

Figure 3: One Series of Measurements at a Single Wavelength

A) Coupling and splice losses measurements

  1. Ensure that the optical output is set to D.C. at maximum level.
  2. Connect a fiber between the generator and the sensor head. The connection to the sensor head must be made with the proper (SMA, FC) adaptor. Do not upset the connection to the source. The connection to the sensor can be made with a SMA or FC connector as well as a bare fiber. Measure the power.
  3. Insert another fiber between the detector and the first fiber. Adjust the fiber connections at the splice (if possible) until a maximum reading is shown. Record that value.
  4. Connect any additional fibers between the last fiber and the source (Figure 3). Again record the maximum value. From these three readings calculate the loss per splice in dB. This loss is partially due to coupling (ie transmitting the energy from one fiber to the other) and also to engery lost at the glass-air interface. Calculate the loss an air air-glass boundary.
    NOTE: These three measurement will permit you to calculate the loss due to a joint (SMA, FC connector, etc.) or a length of fiber. Extract as much information as you can to demonstrate the accuracy of your results by calculating the average loss as well as the std. deviation.

B) Transmission loss Connect a spool of fiber between the source and detector. Using the measured coupling and splice losses (in part A) calculate the fiber loss in dB/km. Why would you want to minimize the number of splices and the number of connections which are changed in such a measurement?

Report

Questions

  1. The loss of a fiber is measured with two test sets at 1300nm. One uses a laser source and one uses an LED source. Would you expect the measured loss to be the same or different with both test sets? Why and why not? Note: LED BW (bandwidth) = 50 nm, laser BW < 1 nm.
  2. Given the information in question #1, what is the BW of the modulation of the optical source and how does this compare to the frequency of the source? For simplicity assume AM modulation and that both the signal and modulator are perfect sinewaves.
  3. A monochromatic source illuminates the end face of a polished fiber as shown in the following figure:

The source radiates in a uniform cone of half angle (theta), such that the base of the cone at the fiber face is uniformly illuminated. The source is always positioned some distance from the fiber end such that for any specific (theta), the edges of the light cone coincide with the fiber edges as shown in the figure. Using ray optics, (i.e. assume infinite number of modes in this multimoded fiber) calculate the power guided in the core as a fraction of the power guided in the whole fiber (core plus cladding), as (theta) is varied from 0 to pi/2.

Hint: Assume that to be guided, a ray path in the core or cladding need only be smaller than the critical angle in the core or cladding (Theta co or Theta cl ) respectively. Plot or sketch Pcore / Pfiber vs theta.