Laser Power Control Units
Laser sources with polarized output provide subtile attenuation option – output power control through manipulation of the polarization (Fig.1). Such method incorporates polarization rotator and polarizer. Polarization rotator is usually a waveplate or electrooptic (EO) modulatorthat rotates input polarization. Then the beam passes onto the polarizer, where orthogonal polarizations S and P are separated into two separate S and P polarized beams. Intensity ratio between the two beams is controlled by polarization rotator which can be either a waveplate or an EO modulator. The EO modulatorcan turn input beam polarization by 90° within tenths of picosecond , which makes it applicable in pulse picking/cavity dumping applications, but this comes at cost of complicated controller which must cope with voltage of several kV, thus it is used only for applications were fast open-shut rates are mandatory. If the waveplate is used as polarization rotator (Fig.2), it must be physically turned which slows down the polarization rotation but such scheme can be realized in both – manual and motorized control and the latter could provide MIN-MAX attenuation within few hundredths of a millisecond. Polarizing optics come in a huge variety readily available on the market, which allows to assemble attenuators with distinct features serving particular applications.
For instance: separation of 90° angle between S and P polarizations can be realized by using polarizing beamsplitter cube, if parallel output beams are needed, two parallel Brewster polarizers could be used, for femtosecond lasers – broadband optics like achromatic waveplates and "ultrafast" 72° angle of incidence polarizers would help to preserve pulse duration, mid – IR region can be accessed by employing appropriate materials and optics production technologies, e. g. mid-IR waveplates and Yttrium orthovanadate (YVO4) or Tellurite (TeO2) Gian polarizers. Attenuators, operating under polarization control principle provide numerous advantages over other methods, firstly – the dynamic attenuation range 0.04–99% can be achieved, and no less important is laser damage threshold of the optics which can be up to 20 J/cm 2 . However, only polarized sources can be managed in this principle, which means, that unpolarised lasers need to befitted with another power control solution.
Unpolarised laser sources also have several options for attenuation. Attenuator principle relying on Fresnel reflection will be reviewed. Knowing the Power – Amplitude relation P = (A)2 reflected power at the interface between two transparent homogeneous media is defined by equations :
were Rs and Rp is reflected power of S and P polarizations, n, and n2 are refractive indices of intersecting media, θ1, is angle of incidence, θ2 is angle of refraction.
Such attenuator comprises two uncoated glass plates which are positioned in a chevron configuration (Fig.3).
As Fresnel reflections are dependent on angle of incidence, changingthe angle between the plates also changes the power of transmitted radiation.
Glass plates being uncoated determine that laser induced damage threshold of the attenuator is equal to damage threshold of the glass material which makes it, in principle, the most damage resistant external attenuator. Another advantageous feature of this product is operation in broadband spectral range, which means that even the power of supercontinuum sources can be effectively controlled by the solution. However, the main disadvantage of the product is that it cannot be used with ultrafast laser sources due to relatively thick dispersive media which leads to pulse temporal elongation. Another drawback is that the minimum losses of the setup are rather high. As Fresnel reflections cannot be avoided (for instance UVFS reflects >3% at 1 064 nm at normal incidence (Fig.4)  thus the maximum transmission of such system is usually no more than 90%. Also it is worth mentioning, that the beam after this attenuator becomes polarized and the higher the attenuation, the more dominant is the P polarization at the output.
In summary, external laser output control units are:
• Incident polarization insensitive:
• Variable neutral density filters
• Fresnel reflections based attenuator
• Incident polarization sensitive:
• Electro optical modulator+ polarizer
• Waveplate + polarizer
Altechna offers the following continuously variable attenuation solutions:
• Circular Variable Neutral Density Filters
• Manually variable waveplate + polarizer attenuators
• Motorized wavepalte + polarizer attenuators