Optoelectronics 1: Devices for Optical Communications
Laser Beam Profiling Using a Scanning Knife Edge Technique
Objective:
To investigate spatial characteristics of a He-Ne Laser, using Scanning Knife Technique. To
measure the FWHM (full width half maximum) and 1/e2 diameters of the beam.
Equipment:
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He-Ne laser, emitting at wavelength of 632.8 nm;
A blade, mounted on a translational X-Y stage with a coordinate scale;
Cased PIN photodiode;
Oscilloscope.
Background:
See attached notes
Method:
Connect the PIN photodiode to the oscilloscope and set the Oscilloscope measurements to a MEAN
value of the photo-response.
Starting from the position, when the laser beam is fully unblocked, move the blade across the beam
and take readings for the MEAN value of the photo-detector response. As the blade moves across
the beam, light is blocked from reaching a photo-detector and the measured signal continuously
decreases to zero.
Position of the blade, mm
Photo-detector response, mV
x1
I1
x2
I2
.
.
.
.
xn-1
In-1
xn
In
Plot the data for the receiver response as a function of the blade position.
Differenciate the data. You may use any suitable graphical software or perform the differenciating
manually. To do this you need to fill in the following table, based on your measurements:
Position of the blade, mm
Derivative fo the Photo-response, a.u.
x2
(I2-I1)/(x2-x1)
x3
(I3-I2)/(x3-x2)
.
.
.
.
xn-1
(In-1-In-2)/(xn-1-xn-2)
xn
(In-In-1)/(xn-xn-1)
Plot the calculated data as a function of the blade position. Comment on the results in terms of TEM
modes profiled.
Fit the data by a Gaussian curve (if working with a standard graphical software).
From the last plot calculate the FWHM and 1/e2 diameters of the laser beam (see attached notes).
Results:
Record all the results listed in the paragraphs above. Comment on your results.
Background information
Introduction
In every laser application, whether in medical, industrial, laser printing, marking, welding and cutting, or fiber optics,
the beam profile provides valuable information for the most efficient use of the laser. Although profiling is often not part
of the Theoretical Optics curriculum taught in universities, beam profiles are very commonly the principal measurement
in practical applications found in industry. The beam profile tells all about the beam’s spatial characteristics, which in
turn describe the propagation, beam quality and utility of the beam. In addition, it can tell how effectively optics are
succeeding in modifying and shaping the laser’s output. Profiling is particularly helpful in building optical systems for
laser printers and fiber optic collimators. Until you know the beam profile, it is difficult or even impossible to put the
laser light to use.
What is Beam Profiling?
Spatial characteristics describe the distribution of radiant energy across the wave front of an optical beam. The radiation
can be shown as a plot of the relative intensity of points across a plane that intersects projected path of the beam. The
most basic measurement of the beam’s irradiance is a single number defining its width or diameter. Since optical beams
do not actually have sharp physical edges, the beam width is made between two points that contain a selected percentage
of the “useful” energy. When beams are Gaussian, or at least approximately Gaussian, the common value for this
measurement is at the 1/e2 diameter. This is the point at which the beam contains 4-sigma of the energy distribution and
occurs where the beam’s power is at 13.5% of the maximum height. Another common measurement is at the full-widthhalf maximum (FWHM) level, where the power drops to one half of the maximum.
Overview of Scanning Knife Edge Technique
Schematic of knife-edge beam profiler: As the blade moves across the beam, light is blocked from reaching a photodetector and the measured signal continuously decreases to zero. The light
that passes through the aperture is measured by a detector and correlated
with the position of the aperture. The signal is then differentiated to obtain
the beam profile.
•One of the better methods for profiling using mechanical parts
•Knife is scanned perpendicularly across the beam’s axis of propagation,
and a photodetector measures the intensity of the unmasked portion of the
beam
•Differentiates each of the scans and makes an overall profile
•Helps to determine spatial characteristics
Understanding the TEM mode
Transverse modes are stable modes of oscillation involving light waves propagating perpendicular to the oscillator axis
in a laser. In a HeNe system where you can adjust the position of one of the mirrors, the transverse modes are seen by
misaligning (making slightly unparallel) the two mirrors that define the laser cavity.
Transverse modes are designated by ordered integral subscripts appearing after the acronym TEM (Transverse
Electromagnetic Mode), specifying the number of nodes along the perpendicular axes. Some various transverse modes
follow.