Joensuu Summer School on Optics: Optical Engineering: Understanding Optical System by Experiments

Campus Joensuu

Read more about the course also here

Dates: 14-25 August 2017
Credits: 5 ECTS
Teaching language: English
Teachers:  Dr. Toralf Scharf and Dr. Myun-Sik Kim, EPFL, Switzerland
Level: Master, Doctoral
Max. number of attendees: 20 students

Coordinator: Noora Heikkila, noora.heikkila(at)uef.fi
Responsible department: Department of Physics and Mathematics
Learning outcomes: The main objective is to familiarize the student with basic design and manufacturing problems in realizing miniaturized optical systems. A theoretical introduction is followed by experiments and analysis of the results in form of a report to be delivered by the students. A modular experimental system is used that contains basic optical components and serves as experimental kit. The design of the experiments allows to fit all practical tasks into a short time slot of only 3-5 h. As a result students gather practical experience on subjects in optical micro-engineering which would not be accessible elsewhere and difficult to teach without “hands on”. An important aspect is the link between measurements and its evaluation by suitable software. We use MATLAB as the standard software within the course and all results were analyzed with MATLAB scripts that were provided to the students. Beside the theoretical and practical part there is always an analysis part based on software use. Students will be familiarized with basic features of the software and whenever possible methods to improve measurement quality by software treatment (averaging for example) were introduced to show the link between these two parts of an experiment.

After the course students should be able to

  • Explain with its own words the contents of each experience
  • Define the most important factors and parameters of optical components and systems
  • Understand the performance limits of optical systems
  • Be able to evaluate results and measurements with Matlab, visualize and document results
  • Apply optical measurement principles to unknown tasks

Transversal skills

  • Assess one's own level of skill acquisition, and plan their on-going learning goals.
  • Use a work methodology appropriate to the task.
  • Access and evaluate appropriate sources of information.
  • Use both general and domain specific IT resources and tools

Contents: Read more about the course also here

Twelve different modules are available divided in two classes: basic knowledge and optical systems.

Each experiment can be characterizes by a set of keywords where the list is given below. The idea is that students after each experiment have a precise idea on what each keyword means and how it is linked to optical system design.

To provide an idea of the technical learning outcomes selected exam questions are given below to illustrate better what will be  teached:

1) Imaging

  • What is the meaning of the main parameters of the camera (gain, exposure) and what are the other parameters like brightness, contrast etc. good for?
  • When is it appropriate to use automatic exposure?
  • What is a collimated laser beam, how do you realize it?
  • How one can construct an image made by a thin lens?
  • Describe the main parameters of a camera lens and explain their meanings! (focal lengths, F# number, numerical aperture, magnification)
  • What is the relation between the F# number, the resolution and the optimal pixel size for an optical system?

Keywords: Imaging with a thin lens, magnification, F#, numerical aperture, beam collimation

2) Detector and noise

  • What different sources of noise exist for detection of electromagnetic radiation?
  • What is the software control under contrast and brightness doing in detail?
  • What is the difference between contrast enhancement in a single image and HDR technique?
  • When is the high dynamic range technique useful?

What is a limitation of HDR technique?

Keywords: Parameters: gain and exposure, dynamic range, detection chain and sources of noise, software based image improvements (brightness, contrast, HDR)

3) Sources

  • How one can adjust the intensity of polarized light with a polarizer and what is Malus’ law?
  • What is a solid angle?
  • What is the brightness?
  • What is the brightness theorem?
  • Which aspects have to be considered to define the properties of sources?
  • What defines the spectral width of the Halogen lamp, the LED and the laser?
  • Why has the webcam an infrared filter and what does it do?
  • What limits the focussing of a source?
  • Why is the spot size of the laser always very small independent of the imaging conditions (objective or lens cap)?

Keywords: Operation principles of Halogen-LED-Laser, parameters of sources: size-solid angle-spectrum, focussing of sources, brightness

4) Mulimode fibres

  • What different types of fibres exist?
  • What are typical parameters for multimode fibers?
  • What are step index and gradient index fibres?
  • What limits the injection efficiency of light into fibres?
  • For a given fibre and a coupling lens find which idealized source has the highest coupling efficiency?
  • What are applications for multimode fibres and why?

Keywords: Types of fibres: multimode and single-mode, operation principle and modes, parameters: numerical aperture - size, efficient coupling

5) Monomode fibres

  • What is the origin of the modes in fibres?
  • What are typical parameters for monomode fibers?
  • Which sources are used with monomode fibres that have high coupling efficiency?
  • What limits the injection of light into fibres?
  • What is the main application for monomode fibres and why?

Keywords: Types of fibres: multimode and single-mode, operation principle and modes, parameters: numerical aperture - size, efficient coupling

6) Pinhole camera

  • What is a pinhole camera and what are its characteristics?
  • What parameter is used in the modulation transfer function to determine the quality of imaging?
  • What is a spatial frequency?
  • What is the relation between pinhole diameter and the MTF curve?
  • How large is the focal length of a pinhole camera?
  • Explain the optimal conditions of operation of a pinhole camera!

Keywords: Geometrical image construction, geometric and diffraction limited resolution, spatial frequency, contrast and MTF, intensity distribution over the field

7) Spectrometer

  • What is the spectral overlap in a grating spectrometer?
  • What is the dependence of the grating shape (profile) and its efficiency?
  • What is the slit good for?
  • What is imaged onto the detector?
  • What limits the resolution in a spectrometer?
  • What types of calibrations are used in spectrometers?
  • What limits the performance (signal to noise ratio) of grating spectrometer?

Keywords: Spectrometer design and main components: grating - slit - optics, grating diffraction, spectral resolution, calibration procedures: spectral and amplitude

8) Interferometer

  • Under which conditions interference appears?
  • Where do the shapes of the fringes come from?
  • What is so special about the zero optical path difference?
  • In a single fringe situation and when the fringe is getting dark, explain where the energy is found!
  • Explain the interdependence between coherence length and fringe contrast qualitatively? (No calculations!)

Keywords: Origin of interference, Michelson interferometer, coherence length, two wavelengths interference and beat frequency, phase shifting technique

9) Speckle sensor

  • What is the origin of speckles?
  • In a concrete setup with a source size and distance of observation plane and source, what determines the size of speckles?
  • Under which condition appears incoherent superposition?
  • How is autocorrelation in Matlab calculated?
  • What limits the maximum measurable movement distance of a speckle displacement sensor?
  • What is speckle boiling?

Keywords: Origin of speckles, parameters determining the speckle size, correlation of images, spatial noise, limitations of motion detection with speckles

10) Abberation

  • What are the origins of geometrical aberrations?
  • How one can describe aberrations mathematically?
  • What are the origins of chromatic aberrations?
  • How longitudinal chromatic aberrations can be corrected and what is an achromat?
  • What is the ideal orientation for a plano convex lens to focus a collimated laser beam?
  • What configuration/orientation of a planconvex lens should be used in imaging?

Keywords: Imaging with a thin lens, magnification, F#, numerical aperture, aberrations, resolution limit, Airy disk

11) Diffraction

  • What determines the maximum diffraction angle?
  • How one can calculate the diffraction pattern for gratings (e.g. plane wave illumination)?
  • Under which conditions does the Fourier transform describes the propagation of light?
  • Explain the relationship between the size of the object and the angular spreading of light?

Keywords: Diffraction patterns after apertures (slit, circle, square, arbitrary), grating equation, parameters dependence of the diffraction pattern of a binary grating – period – slit width

12) Shack Hartman sensor

  • How lenses are tested by an interferometer?
  • How the lenses are tested in a Shack Hartman sensor?

Explain the relationship between the size of the microlens array, its focal lengths and the precision of the measurement.

Teachers:  Dr. Toralf Scharf and Dr. Myun-Sik Kim, EPFL, Switzerland