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.
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.
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).
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.
|Type||FOI 5/57||FOI 5/711|
|Wavelength (mm)||500 - 830||870 - 1100|
|Isolation (db)||> 30||> 30|
|Insertion Loss (dB)||< 0.5||< 0.5|
500 - 750 and 750 - 870
+5° rotation about
60 dia x 58 long
75 dia x 75 long
60 dia x 100 long
75 dia x 120 long
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 Leysop's 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 FOI 5/57BB Broadband Faraday Optical Isolator (or Rotator) 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. No tuning is required!
The price for these new broadband isolators (or rotators 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.
|FOI 5/57BB||Broadband Faraday Optical Isolator / Rotator||700nm to 900nm||No tuning required|
|FOI 5/57UWB||Ultra Wide Band Faraday Optical Isolator / Rotator||650nm to 1100nm||Easily tuned|
In response to customer demands for isolators with increasingly broad tuning ranges, Leysop has introduced a new design of isolator (rotator), the FOI 5/57UWB. This is based on their FOI5/57 design but uses a slightly longer than normal TGG rod and greater strength magnet assembly to increase the maximum wavelength of operation and a new more convenient wavelength tuning assembly.
The pictures below show the two extremes of the tuning range. It also shows the helicoidal tuning mechanism which produces pure translation of the TGG rod into the magnetic assembly without rotation of the rod or alteration of the relative orientation of the polarizers. The tuning mechanism is extremely smooth and allows very precise adjustments of the tuning to maximum isolation over the extremely wide range of 650nm to 1100nm, all in one device.
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.