Dispersion Measurement

Dispersion MeasurementUnderstanding, processing, and recommending1ObjectivesAfter completing this program you will be able to:Understand origin of the dispersion.Explain Importance of dispersion measurement.Characterize some dispersion measurement techniques include:Hilbert transform method,Interferometric methods, andConventional phase-shift technique.Estimate expectation of dispersion measurements.

2DispersionThere are different opinions about who first discovered dispersion.The dispersion question is originally from the rainbow.The most popular experiment of dispersion discovery was done by Newton at 1666.Dispersion is one of the properties of the light.Thediscoveryofdispersionphenomenon wasof greatsignificancetothedevelopmentof the current photographicoptics.3Newtons experimentWhile studying the images of heavenly bodies formed by a lens, Newton found that the images were colored at the edges.To investigate this, he performed an experiment using a prism.Newton darkened his room and allowed a beam of sunlight to pass through a small circular hole.He then placed a triangular prism in the path of the beam of light, see next slide.4Newtons experimentFig. 1. Prism spreading white light into spectrum of light

Then, the white light was split into seven colors which resembled the colors of a rainbow.This process of splitting of white light into its constituent colors when it is passed through a transparent medium is known as Dispersion.With such an astonishing experiment done by newton, yet he failed to recognize the material dependence of the dispersion.5Dispersion after NewtonLater on people realized that dispersion occurs whenever there is a process that changes the direction of light in a manner that depends on wavelength.For a given medium, n increases as wavelength decreases and is greatest for violet light. People are studying this phenomenon with respect to the material, for example Dispersion of waves on water was studied byPierre-Simon Laplacein 1776.In order to evaluate the amount of dispersion we are facing, given a material, we need a measurement.6Importance of dispersion measurementThe growing number of femtosecond lasers in industry, medicine, and communications.Femtosecond lasers are particularly sensitive to dispersion, even beyond first order.Pulse duration is a very important parameter in femtosecond laser applications, for example, laser eye surgery.All the laser surgeries can be greatly improved using the shortest femtosecond laser pulses.Accuracy has the highest rank in the surgery.7Dispersion measurementsDispersion measurements give an indication of the distortion to optical signals as they propagate down optical fibers.It limits the information-carrying capacity of the fiber.It is the most important transmission characteristic of an optical fiber.There are many techniques to measure the dispersion of an optical fiber. In general, these techniques can be classified into three categories:The time-of-flight (TOF) method,The modulation phase shift (MPS) method, andThe optical interferometry-based methods.8Dispersion measurementsThe TOF method is the most direct way to measure dispersion.The pulse source is filtered by a tunable optical filter or a monochromator. Electrical pulse with different time delays are measured. Which are used to calculate the dispersion of the fiber under test.The MPS method is a well-established technique and has become an industrial standard for dispersion measurement. It requires expensive vector network analyzer to measure the electrical phase response.9Dispersion measurementsThe optical-interferometry-based method has the highest measurement accuracy.Here, the length of the fiber under test is typically limited to a few meters.Abedin et al. proposed a method to measure dispersion of a long fiber.Now, we are going to focus on three basic techniques for dispersion measurement. These are:Hilbert transform method,Interferometric methods, andConventional phase-shift technique

10Hilbert transform methodThe amplitude and the phase response of an optical filter are related via the Hilbert transform.To assure this relation is valid, the filter must fulfill a so-called minimum phase condition.Phase response and then the dispersion can be calculated directly from the measured amplitude response.However, the minimum phase condition does not hold for component including various types of apodized or chirped fiber.Reconstruction of the phase information from the measured amplitude response is not possible.11Interferometric methodsHere measuring the transmission time through the components are applied.The measurement setups are typically based on Michelson or Mach-Zehnder interferometers.The principle of Mach-Zehnder interferometer (MZI) based dispersion measurement is shown in Fig. 2.Input light is divided into two beams by an input beam splitter(BS). One beam is launched into a reference fiber, whose group delay spectrum is carefully calibrated. The second beam is launched into the test fiber of approximately same length as the reference fiber.

12Theory of MZIAt the output, the two beams are combined into a single beam, which is fed through a detector. The detector cross-correlates the test beam relative to the reference beam. The cross-correlation produces an interference pattern between the test and reference beams, which is measured to determine the group delay spectrum of the test fiber relative to reference fiber.

1313Realizing MZI

Fig. 3. Realizing Mach-Zehnder interferometer usingoptical intensity modulators.14Realizing MZI1515Realizing MZI1616Experimental set up

Fig. 4. Experimental setup to measure dispersion using intensity modulators. The second modulator .beats. the signal from the first modulator creating an interference pattern. By measuring the power with and without modulation, the group delay spectrum is estimated.17Conventional phase-shift techniqueA basic measurement setup for the phase-shift technique is outlined in Fig. 5.

Fig. 5. Experimental setup to measure dispersion using the phase-shift technique18Conventional phase-shift techniqueThe light from a tunable laser is intensity modulated with a sinusoidal signal. The modulation generates sidebands on both sides of the optical carrier. The sidebands will experience a phase shift when the modulated light passes through the device under test. The phase shift of the detected signal allows the group delay of the component to be determined. The basic setup and variations of it using different light sources have been utilized for measurements of the dispersion of an optical fiber for years.19ConclusionDispersion is major problem in optical communications.It is highly dependent on the material.People have been studying this phenomena for long time with respect to the materials.To solve this problem we need to understand it first through measuring.There are many different techniques for dispersion measurements.All techniques have in common calculating the group delay velocity.Try to think it by new way?20ReferencesL. G. Cohen, Comparison of single mode fiber dispersion measurement techniques, J. Lightwave Technol., LT-3.B. J. Ainslie and C. R. Day, A review of single-mode fibers with modified dispersion characteristics, J. Lightwave Technol.JOHN M. SENIOR, third edition, Optical Fibre Communications Principles and Practice21End22