Faraday Optical Isolator Quality
People have achieved ~40dB isolation under stable conditions with the
Leysop Faraday Optical Isolator. If set up optimally and held within a
narrow temperature range it should certainly exceed the nominal 30dB
specification. The device has been designed to produce an extremely
linear magnetic field in the TGG crystal. This is accomplished by using
a larger magnet size than other FOIs. The Leysop FOI is bigger because
of this. Leysop prepares the TGG rod for minimum residual surface strain
and mounts it to induce as little extra strain as possible. The
photoelastic constant of TGG is reasonably high so this special mounting
is important to minimize strain induced birefringence. All of these
extra features pay off. Power levels can be up to 10 – 15 W before there
is a loss of isolation.
Ultra-Wide
Tuning Range Faraday Isolator
In
response to customer demands for isolators with increasingly broad
tuning ranges, Leysop has introduced a new design of isolator based
on the FOI5/57 design but using a slightly
longer than normal TGG rod and greater strength magnet assembly to
increase the maximum wavelength of operation. More importantly, it
provides a new, more convenient wavelength tuning assembly.
The photograph at right shows the device at the two extremes of
the tuning range. It also shows clearly the helicoidal tuning
mechanism which produces pure translation of the TGG rod into the
magnet assembly, without rotation of either the rod or alteration of
the relative orientation of the polarizers. The tuning mechanism is
extremely smooth and allows very precise adjustment of the tuning to
maximize isolation over the extraordinarily wide wavelength range of
650-1,100 nm, all in one device.
Faraday optical tunable isolators / rotators
These are tunable Faraday rotators in which
the plane of polarization of a laser beam is rotated through 45° as it
passes through a high Verdet constant material, positioned in an axial
magnetic field produced by permanent magnets.
Some optical glasses (e.g.
FS57 and special grades of glass selected for their Verdet constant) may
be used, but Leysop only uses the highest quality single crystal TGG (terbium
gallium garnet). This offers the highest Verdet constant for the visible
to near infrared region and provides high power handling with low optical
distortion.
Rotation is additive for reflectors
passing back through the isolator giving 90° special difference between
the input and reflected polarization states at the input to the isolator.
An input polarizer will therefore reject back reflections. An output
polarizer is necessary to select the correct polarization state for
transmission back through the isolator (see diagram).
Product Specifications
Isolators are wavelength tunable by a
screw thread adjustment.
Input and output polarizers can be
supplied and are mounted onto the isolator forming an integral system.
Relative position of polarizers can be adjusted.
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Type
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FOI 5/57
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FOI 5/711
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Aperture (mm)
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5
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5
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Wavelength (mm)
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500 - 830
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870 - 1100
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Isolation (db)
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> 30
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> 30
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Insertion Loss (dB)
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< 0.5
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< 0.5
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Tunable
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Two Ranges
500 - 750 and 750 - 870
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±5° rotation about
fixed wavelength
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Dimensions (mm)
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Excluding Polarizers
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60 dia x 58 long
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75 dia x 75 long
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With Polarizers
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60 dia x 100 long
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75 dia x 120 long
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Application Notes
The
Faraday or Magneto-Optic Effect
Applications of Faraday Devices
Faraday Optical Isolator Quality
Broadband rotators/isolators for Ti:Sapphire now
available
Leysop tunable isolators as described above are ideal for fixed
wavelength operation or where only occasional adjustment is required
between experiments when the source wavelength has been changed.
These adjustments however are relatively time consuming to make and
are not convenient where a broadband or rapidly tunable source such
as a dye laser or one of the new classes of vibronic mode lasers
such as Ti:Sapphire are used. There is not much point in having a
laser source which can be tuned to a different wavelength within
seconds if the Faraday isolator in the system requires removing from
the bench/equipment and set up off line for optimum performance.
This is where our new broadband units come into their own.
By inserting a passive optical element (Quartz rotation plate)
into the device in addition to the TGG rod, the combination of the
two components is an isolator in which the the output polarization
is now at the more convenient angle of 90° with respect to the
input. More importantly, it can be shown that the isolation of the
unit is now maintained over a wide bandwidth instead of at just one
wavelength.
Leysop's first offering is designed for use over the range of
wavelengths covered typically by the very important Ti:Sapphire
laser source, that is 700nm to 900nm. Over this range, the isolation
should remain greater than 30dB without any retuning being required.
There is a small penalty to pay in that the transmission over the
band is not uniform due to the varying degree to which the combined
rotation of the Faraday rod and the Quartz rotator matches the
outgoing polarization axis to the preferred axis of the output
polarizer.
The price for this new broadband isolator (or rotator if you
prefer) is very similar to that of the wavelength tunable units. If
you wish to discuss how this unit could improve your optical system
or to look at alternative wavelength options, please contact us.
Quartz Optical Rotators
When plane polarized light propagates
along the optic axis of quartz, it is rotated clockwise or anticlockwise
depending on whether it is right or left hand quartz. The rotation is
dependent on the wavelength and the distance traveled, as shown below.
Specific Rotary Power
Quartz rotators can be supplied to give
90° rotation or any angle between 0-90°. They are ordered as standard
components with the same aperture and mount dimensions as those shown for
retardation plates but by using the suffix, QOR.
