Newsletter #23.qxp - Newport Corporation

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Page 2 Fall 2006Application NoteNew Kit For Variable VAttenuation Of Laser PowerNewport’s Technology and Applications Center hasdeveloped a cost effective solution for the continuousattenuation of laser output power. The solution is nowavailable for sale as a kit and is described in detail in anapplication note available for download atwww.newport.com/AppsNote26The attenuator kit described is based on a half waveplate and polarizer. It is suitable for attenuation of alinearly polarized laser beam with a wavelength in the690–1040 nm range and average power up to 100 watts.The schematic of the device is shown in Fig.1.LaserPrinciple of Operation1/2 Wave Plate PolarizerBeamStopFigure 1: Schematic of variable attenuator for lasers.SPI = cos 2 (2φ) (2)I owhere ø is the azimuth of the 1/2 wave-plate. Full rangeof attenuations from minimum to maximum is achievedby 1/2 wave-plate rotation from 0° to 45°.Which Components do I Use?The choice of optical components is defined by thebandwidth and the power of the laser to be controlled.For a 1/2 wave-plate the Newport model 10RP52-2 isrecommended. This achromatic zero-order wave-plateconsists of two different birefringent crystals: crystallinequartz and magnesium fluoride (MgF 2 ), in an air-spaceddesign for high power. Compared to zero-order quartzwave-plates, achromatic quartz MgF 2 wave-plates offerexcellent retardation accuracy over broad wavelengthranges. Achromatic quartz MgF 2 wave-plates are lessexpensive than achromatic polymer film wave-plates andalso feature higher damage thresholds. Fig. 2 shows the10RP52-2 half wave-plate mounted in a 25.4 mm housingto allow easy mounting in any of our manual rotationstages, such as the RM-25 or the RSP-1T.The principle of operation is based on Malus’ Law. Theintensity I of the polarized light that passes through thepolarizer is given by where I 0 is the input intensity, and θis the angle between the beam’s initial direction ofpolarization and the axis of the polarizer. Rotation of a1/2 wave-plate mounted to a rotational stage allowscontinuous rotation of the beam’s polarization direction.When combined with a polarizer, it provides variableattenuation according to (1).I = cos 2 ( θ ) (1)I owhere I 0 is the input intensity, and θ is the anglebetween the beam’s initial direction of polarization andthe axis of the polarizer. Rotation of a 1/2 wave-platemounted to a rotational stage allows continuousrotation of the beam’s polarization direction. Whencombined with a polarizer, it provides variableattenuation according to (1).Rotation of the 1/2 wave plate by ø degrees leads to 2ødegrees of polarization rotation. Intensity at the outputof the attenuator is then given by (2)Figure 2: Newport 1/2 Wave-plate, model 10RP52-2. The line on the mount defines theazimuth of the wave-plate. If aligned at 45° relative to the input polarization direction,the output beam polarization is rotated by 90°.For a polarizer, the Newport model 10GL08AR.16 Glan-Laser Calcite Polarizer (Fig. 3b) is recommended. Itconsists of two calcite cubes with the optical axisoriented parallel to each other (Fig. 3a) and is designedfor demanding applications requiring extremepolarization purity and high resistance to laser damage.www.newport.com
Fall 2006 Page 3PSetup and AlignmentPSS(3a)(3b)Figure 4 shows an experimental setup for output powercontrol of a Spectra-Physics Mai Tai ® Ultrafast Oscillator.The two steering mirrors at the output of the laser arerecommended in order to make the alignment as easy aspossible and get the laser output to desired beam height.If more than a 10-20 mm of beam height adjustment isneeded, the Newport BSD-2A beam steering kit is easilyutilized in addition to these two turning mirrors.Reflectance (%)2.01.51.00.5AR.16Broadband NIRAR Coating0°Typical0600 700 800 900 1000 1100Wavelength (nm)Figure 3: Newport 1/2 Wave-plate, model 10RP52-2. The line on the mount defines theazimuth of the wave-plate. If aligned at 45° relative to the input polarization direction,the output beam polarization is rotated by 90°.For more information on the attenuator set up and laserpower considerations, please refer to the ApplicationNote downloadable at: www.newport.com/AppsNote26As a single-source provider of solutions to make, manageand measure light, Newport can also supply powermeters and detectors optimized for your applications.For ultrafast oscillators our 841-PE power meter and 818P-020-12 detector are recommended. If the outputpower of the laser is known, as is the case with theMai Tai Laser, then the attenuated power is equal to thedifference between the laser power and the measuredpower. Total insertion loss of the attenuator, on average,is about 7%.Figure 4: Setup for controlling the output ofthe Spectra-Physics Mai Tai ultrafast oscillator.Beam DumpPL15RSP-1TRM251/2 Wave-Plate10RP52-2Glan-LaserPolarizer10GL08AR.16VPH-2-PSP-2PS-Fwww.newport.com
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Newsletter #23.qxp - Newport Corporation

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