E&CE 477 - Lab #3 - Spectral Attenuation Measurements

Introduction

The optimum wavelength of an optical fiber communication system requires determination of the spectral loss of the fiber. The minimum loss of the fiber should occur at the operating wavelength to maximize the repeater spacing. A spectral loss measurement which measures the loss of the fiber over a range of wavelengths will locate this optimum operating wavelength.

In addition to minimum loss, the singlemode system should operate at a wavelength near the fiber's next higher mode effective cutoff to enhance fundamental mode confinement. The spectral bending loss of the singlemode fiber will indicate this cutoff.

In this experiment, we will determine the intrinsic loss and scattering parameters of the fibers. We will also determine the effective cutoff of a singlemode fiber.

Spectral Loss

The loss of an optical fiber (in dB/km) can be modeled as:

where A is the Rayleigh scattering coefficient, B is the wavelength independent loss due to waveguide imperfections, microbends and bends. C(l) is due to both the impurities and intrinsic absorptions. While the intrinsic absorption, such as the UV and IR absorption tails are monotonic functions, the impurity absorptions are narrow band functions that occur at the resonance frequencies of the impurities. The reduction of impurities present in the fiber material and an improvement in the fabricating technique can reduce the total loss of the fiber. The amount that can be reduce is limited by the Rayleigh scattering coefficient and the absorption tails.

As lapproaches infinity, the Rayleigh scattering loss approaches to zero. If C(l) is due mainly to impurities, the various parameters in equation 1 can be determined from a plot of l vs l -4.

Experimentally, one measures the loss of the fiber over a range of wavelengths. Using the reference fiber method, the spectral loss is determined from,


Spectral Bending Loss

When a mode propagates near its cutoff, it becomes weakly guided and a large portion of its power propagates outside the core. As the frequency increases, the radial decay factor g increases and the power is more confined within the core.

A weakly guided mode will suffer more loss due to bends and waveguide imperfections than a stronger guided mode. The spectral bending loss of a fiber B(l), defined as


measures the loss due to the additional curvature in a fiber can be used to determine the higher order mode cutoff. Near cutoff, the bent fiber will no longer support the weakly guided higher order mode and B(l) increases shapely. For an ideal singlemode fiber without any waveguide imperfections, the LP11 cutoff should occur at l c with V = 2.405. However, in practice, waveguide imperfections are always present. The measured cutoff wavelength l e will always be smaller than l c.

The effective cutoff l e can than be defined as the wavelength above which the second order mode is below a given level compared to the fundamental mode. Presently, the industry standard (EIA) for cutoff measurement of a singlemode fiber is to determine l e of a 2m fiber with a 28cm diameter loop.

Equipment Setup

A halogen, white light, source is filtered by a monochromator (bandpass filter) allowing the user to select a very narrow bandwidth, typically 3.2 nm/mm, centered at a wavelength between 400nm and 1700 nm. However, the actual dispersion of wavelength in nm/mm varies with wavelength and the size of the slit openings. Losses in the mirrors, gratings and fiber limit usefull measurements to the 800nm to 1700 nm range for our equipment.

The narrow spectrum is then passed thru the fiber under test and measured by a wideband detector. The HP 815x optical power meters are calibrated from 850 nm to 1700 nm and can detect light to -70 dBm (HP 8152) or -110 dBm (HP 8153). Typically the noise floor is about -100 dBm due to stray light. Also the light meters have a limited resolution and accuracy. For the 8152 a typical reading is -65.5 dBm. That will be ±5% ±0.15 dB + 100 pW. It is obivous that the number of significant digits decreases as you approach -70 dBm (100pW).

By using a computer to control the sweeping of the bandpass filter (monochromator) and measure the resulting power it is possible to easily get plots of light power versus wavelength. By taking a reference measurement at the same time (ie modern microwave test equipment does this) or very soon before an actual measurement it is possible to get absolute measurements of loss versus wavelength within the repeatability of the connections.

Procedures

A) Spectral Loss Measurement

    The Lab. Staff will do the following:

  1. Make sure that the power meter, halogen source and other equipment is turned on.
  2. Use the fiber holder assembly to launch the optical output from the monochromator into a short (1m) multimode fiber. This fiber will serve as the source pigtail of the system.
  3. Set the wavelength setting near the center of the measuring range.
  4. Adjust the fiber holder assembly to obtain a maximum power output from the pigtail.

    Your responsibility is to:

  5. Ensure that the equipment is working (ie enough light power is getting thru to get good measurements). Secure the launching setup and do not disturb the setup until the final measurement has been taken.
  6. Connect a reference fiber (2m) between the pigtail and the detector.
  7. Do a frequency sweep to acquire the reference power PREF (l).
  8. Without upsetting the launching condition, replace the reference fiber with a fiber spool.
  9. Now do another frequency sweep to obtain the spectral loss of the fiber.
  10. Store the measured data into a file. Notice that the stored data is the spectral loss in dB. The value must be divided by the spool length to obtain the loss per unit length.

B) Spectral Bending Loss Measurement

    Assuming that the monochromator setup is operating properly:
  1. Launch the monochromator output directly into the test fiber (2m).
  2. Make sure that the smallest loop in the fiber has a diameter >= 15cm.
  3. Acquire the power output as the reference.
  4. Using the bending jig, get frequency sweeps with loops of 1cm to 6cm.
  5. Store the bending loss data.

Measurements

  1. Measure the multimode and singlemode spectral losses using the range 850 to 1550nm in steps of 10nm with the MM fiber (old monochromator) and 4.0nm with the SM fiber (Digikrom monochromator).
  2. Using the same range, measure the spectral bending losses of a singlemode fiber. When obtaining the data, use diameters of >= 15 cm (ie straight), 6cm, 4cm, 2cm and 1cm.

Report

  1. Locate the OH absorption peaks.
  2. Comment on the common fiber optics system's operating wavelengths of 850nm, 1300nm and 1550nm (e.g. relative repeater spacing etc. given a 0 dBm source with a signal strength of >= -30 dBm required).
  3. Compare the multimode and singlemode spectral losses. Explain the differences.
  4. Find the effective cutoff of the singlemode fiber from the spectral bending loss curves.
  5. Which spectral bending loss curve should provide the best result on the effective cutoff?
  6. Suggest a method to estimate the actual cutoff of the singlemode fiber.