ENGINEERING TRIPOS PART IIA - 2012/2013

Project SB2 - Optical Filters

Leaders: Prof J.E. Carroll email: jec1000 and Dr T.D.Wilkinson email: tdw13

Project Type: Standard
Project Category: Design
Prerequisites: 3B5 & 3B6 essential and some experience of MATLAB programming.
Timing: Fridays 9-11am, plus afternoons and Tuesdays 11-1pm

SUMMARY

Modern long distance terrestrial communication systems are dominated by the techniques of sending modulated laser light along optical fibres that can be as thin as a human hair. In such systems, different sets of information are simultaneously transmitted at different optical wavelengths along a single optical fibre - a process that is called wavelength division multiplexing (WDM). To avoid possible confusion of data, it is essential to design optical filters that either select (bandpass) or block (bandstop) a particular wavelength (i.e. a particular set of information). Filters also are essential to control the light emitted by the optical sources (lasers) so that these lasers generate light only at the required specific wavelengths for any one set of information. This project then examines some key design features that are required in modern optical communication systems.

The project will design two types of filter using the principles of wave interference - explained during the project. The theory for the first type, known as multi-layer dielectric interference filters, is based on plane waves travelling through several layers of glasses with different refractive indices. These are passive optical filters because they add no power to the incoming light. Typical applications: (a) wavelength selective mirrors for lasers, (b) minimising unwanted reflections (eg blooming of lenses in optical systems - for example binoculars).

A second type of filter (an active filter) is based on similar theory but now the waves grow in amplitude as they travel through an optically active medium such as a laser. The student will be introduced to frequency selection in distributed feedback (DFB) lasers for modern telecommunications. No prior knowledge of laser physics is required.

Project students will write programs to run under Matlab to design a variety of bandpass and bandstop passive optical filters and one or two designs for a DFB.

This design exercise uses the basic Matlab and does not allow for the use of Simulink or the many Matlab packages.

AIMS

1. To understand electromagnetic wave propagation through different materials.
2. To design specific interference filters in both passive and active optical systems.
3. To give practice in using Matlab as a numerical tool for analysis and design.
4. To encourage clear presentations of results and graphical data.

OBJECTIVES

By the end of the project students should be able to:

1. Explain how plane waves propagate through different materials;
2. Calculate the optical transmittance/reflectance through multi-layers of materials;
3. Write appropriately annotated Matlab programs with inputs, outputs and parameter changes to (ii) above.  Strong emphasis is placed on individual work developing code and making appropriate checks for code and designs.
4. Present clear graphical data with appropriately concise descriptions of the work that has been carried out.

FORMAT

Students will work individually in the EIETL using departmental computer systems. They must write their programs in Matlab and must write up their reports using OpenOffice (Writer and Draw), Microsoft Office or other compatible software. All reports and Matlab programs must be stored in a storage space designated to each student where staff running the project will have access and be able to run these programs and annotate any reports or programs. The interim report will be submitted electronically in OpenOffice compatible format (not PDF) along with Matlab code that must be able to be run independently by the marker. The final report will be submitted in both paper and electronic format with code again ready to be run independently by any examiner using the computer system. Encouragement is given 'to write the report as you work' using the PC as an electronic notebook.

ACTIVITIES

Week 1 : Introduction to waves and interference. Software/hardware familiarisation.
Week 2 Write and test programs for multi-layer dielectric filters.
Week 3  Introduction to active filters. Design/testing of code for active filters.
Week 4  Analysis and design of simple DFB lasers.

ASSESSMENT

Format: Submission Date - Marks

Interim report: 4pm Tuesday 21 May 2013 - 20 marks

Final report: 4pm Friday 7 June 2013 - 60 marks

Last updated: November 2012