Construction of a Mach-Zehnder Interferometer and Applications in Fiber Sensors

A Senior Design Proposal By: Ryan Sherry '97

For: Dr. Susan M. Lord

Summary:

Interferometers are very versatile optical instruments that can be adapted numerous ways to perform a myriad of measurements pertaining to light. The advent of glass fiber has revolutionized the sensor industry by allowing fiberoptic interferometers to be constructed. The principle behind the fiber interferometer is based upon interference patterns that occur when two beams of light are coupled together into two fibers and then rejoined and focused on the same point. Shifting interference rings of the combined light are indicators of changing physical parameters around the sensor. Equations can be derived to correlate the amount of physical change such as temperature or pressure to the amount of shifting in the interference pattern. The basic configuration for a fiber interferometer includes two fibers of equal length, into which light is coupled. One of the fibers is used as a reference and the other is exposed to the physical phenomena to be sensed. As the refractive index of the sensing fiber changes, the interference pattern after the light from each fiber is recombined is shifted. The proposed project will entail constructing such an interferometer to be used in sensing various physical phenomena including temperature, pressure, and ultimately acceleration. If an accelerometer can be achieved in one dimension a fiber gyroscope will be a final objective to achieve in the design of the various sensors. With the approval of Professor S.M. Lord, the interferometer will be constructed in her Optoelectronics Lab where we will be able to obtain the extremely high degree of vibrational isolation necessary for proper operation of the instrument.

Objectives:

The main objective will be to successfully construct a fiber interferometer where a sharp diffraction pattern is in fact observable. Once this has been accomplished, there will be many possibilities for adapting the device to make various measurements. This will require some practice in coupling a laser into single mode fiber, which is no trivial issue. Various precision optical mounts and translational stages that have extremely fine adjustment are used to couple light into the fiber. The vibrational isolation afforded by the optical table will also be absolutely essential for accomplishing this.

The first sensor will be used to sense temperature changes. In order to make actual measurements, the setup will need to be calibrated (probably with an ice cube since we can be sure it is 0 degrees C) and equations will need to be derived to correlate the amount of shifting in the interference pattern to the amount of temperature change. The second sensor will be implemented in a similar fashion to sense and measure pressure.

These temperature and pressure sensors are described in Experiment #10 of Newports Projects in Fiber Optics(1), and will hopefully go rather smoothly since some equations are already derived and system setups are described. Barring any unusually difficult problems, these two sensors will hopefully be completed somewhere about half way through the semester or a little later. Successful completion of the temperature and pressure sensors will lead to the ultimate goal of implementing three accelerometers in a fiber gyroscope design during the final five to seven weeks of the semester.

Succcessful implementation of any or all of these fiber sensors will lead to the development of a laboratory exercise for EE341. Development of the lab will have already begun during documentation throughout the project, but steps will be taken to create a lab handout similar to those for other labs in EE341. The lab handout will cover basic background of fiber sensors, detailed setup procedures, as well as lab questions to be addressed in the student writeup. When the lab exercise has been developed, it will be important to find some volunteer students to test the lab for errors or ambiguities in the setup or instructions. The lab will be adapted according to feedback from both students and Professor Lord.

Plan of Action:

The progressive stages for this project will tentatively be to:

1. Acquire reasonable skill in coupling laser light into single mode fiber.
2. Study the theory behind the physical design of the interferometer, and to understand how and why it works.
3. Get right into the actual construction and or redesign of the interferometer (Mach-Zehnder for temperature and pressure sensors, and Sagnac for the gyroscope).
4. Perform an initial test of the instrument upon completion to see if any diffraction pattern is noticeable at all.
5. Proceed to make necessary alterations to the setup in order to make the desired measurements.
6. Derive any equations necessary to correlate the amount of interference shifting to the amount of the change in the sensed physical phenomena.
7. Develop Lab exercise and have student volunteers test it.
8. Report findings in presentation.

Equipment:

An inventory of parts for completion of the various designs is included in the attached Appendix 1. It should be noted that many of the components are already available in the optoelectronics lab. It seems highly unlikely that the Newport Corporation has actually tested the setups they describe in the Projects in Fiber Optics, because the setup diagrams don't indicate any type of adjustment stages which would make it nearly impossible to align the optics and couple light into single mode fiber. So it is possible that a few items such as extra translation stages, for example, may be necessary. The lab is already equipped with some of these stages and mounts, but since Shala Powell will be performing similar experiments it will be necessary for us to implement at least two or more setups. I would suspect that expenditures may be between $0 and $2000. The estimate is quite high because translation stages with multiple axis adjustment can cost as much as $600. Also I want to account for parts that may be necessary to implement the Sagnac interferometer, as I am not yet sure of the exact setup for that type of interferometer. Professor Lord has indicated that she is willing to supply a reasonable amount of funding available to her through grants to provide equipment that is proven absolutely critical to success.

Closing:

The feasibility of this senior design is based primarily on a description of experiments with interferometric fiber sensors in a project manual entitled "PROJECTS IN FIBER OPTICS: Applications Handbook" which has been supplied by the Newport Corporation. Experience in the optoelectronics lab performing labs for EE341 (an optoelectronics elective) as well as some other experience over the summer in the lab will prove very useful in the success of this design.

Appendix 1: Parts List

Appendix 2: Time Schedule

1 Newport Corporation, PROJECT IN FIBER OPTICS: Applications
Handbook.  Fountain Valley, CA., p.82, 1986.  
 

Pictures of the lab