Laser & Variable Attenuators Application Note

Laser Optics Laser Attenuators — Polarization Insensitive Laser Attenuator

Part 1: Controlling the Output Power using Laser Attenuators

A laser attenuator is an optical device capable of reducing the optical power or intensity of the incoming laser beam. This is especially useful for laser sources with fixed output powers. There are both fixed and variable laser attenuators depending on the intended attenuation required. Variable attenuators are readily available with either manual adjustment or motorized adjustment. The laser attenuator in no way affects the direction of the beam propagation after attenuation.

Operation Principle

The laser attenuator optics works on the principle of reflection where a percentage of light is reflected off the surface of the optics, which is dielectric coated and the remainder is transmitted through. The angular rotation of the optics changes the angle of the incident light on the optics which in turn causes a change in the transmission ratio which ultimately changes the attenuation level. Due to the angular rotation of the optics, the laser beam passing through the optics undergoes refraction which changes the path of the outgoing laser beam. In order to correct this issue, a second optic is used in sequence and the two optics are moved in tandem to not alter the direction of the beam propagation as seen in Figure 1.

Item No.	ATTN-10600-WC-V1
Wavelength	10.6μm
Clear Aperture	Up to 19mm (Customizable)
Transmission Range	10 – 90%
Damage Threshold	1MW/cm^2
Resolution	5%

Table 1. Key specifications of ATTN-10600-WC-V1

*The attenuator is available for operation at 9.4μm.

The architecture of the laser attenuator allows for larger beam input without the worry of beam clipping during transmission and the water cooling channels allow for constant cooling throughout the operation to prevent any damage or beam manipulation at any point in time. The design of the laser attenuator can be seen in Figure 2.

Laser Attenuator 3 — Figure 2. Design features of ATTN-10600-WC-V1

Applications

The internal design of the laser attenuator allows for efficient absorption of reflected light. It is designed for attenuation of collimated beam outputs and not focusing beams. The laser attenuator can be implemented into any laser system with the purpose of precise control or reduction of the output beam power.

Laser Attenuator 5 — Figure 3. ATTN-10600-WC-V1 outline

The compact design, as seen in Figure 3, allows for ease of integration into laser systems and the base plate allows for easy manufacturing of holders or adapters for mounting the laser attenuator. A motorized version is also available upon request.

Part 2: Precision Control by the Principle of Polarization using Variable Attenuators

Precision laser applications require fine power control. A variable attenuator with a large dynamic range and precision control is designed to fulfill this purpose. It is suitable for intensity attenuation over a wavelength range from UV to IR.

Operation Principle

This variable attenuator consists of a specially designed optomechanical adapter and a precision optomechanical holder. The key optics involved are a half-wave plate and a thin film polarizer. The half-wave plate is usually made of birefringent crystal cut parallel to the optical axis. It is used to change the polarization direction of the incident beam.

Laser Attenuator 7 — Figure 1. Principle of polarization attenuator

A thin film Brewster-type polarizer placed after the half-wave plate reflects s-polarized light while transmitting p-polarized light. The intensity ratio of s-polarized to p-polarized beams may be continuously varied by rotating the wave plate. The intensity of either the exit beam or their intensity ratio can be controlled over a wide dynamic range. P-polarization can be selected for maximum transmission. A full range of attenuation from maximum to minimum can be achieved by rotating the half-wave plate from 0 to 45 degrees. For constant monitoring of power, the beam dump can be replaced with a power meter.

Laser Attenuator 9 — Figure 2. The layout of the polarization attenuator

The key specifications of the polarization attenuator at different operating wavelengths are listed below. Compared to similar products in the market, we offer a large dynamic range of attenuation, which is useful for precision laser process control.

Wavelength	355/532/1064nm
Type	Transmission Mode
Clear Aperture	14mm
Beam Shift	0.5mm
Extinction Ratio	>200:1
Attenuation Range	0.5%-95%
Damage Threshold	>5J/cm2@1064nm, 20ns, 20Hz
Weight	<300g

Table 1. Key specifications of polarization attenuator

Applications

The following listed characteristics enable the polarization attenuator to precisely control the laser intensity with fine adjustment steps. Using a suitable type of polarizer, this principle can be realized at very high-power levels.

Divides laser beam into two parallel beams of the manually adjustable intensity ratio
Large dynamic range
Negligible transmitted beam deviation
High Optical damage threshold
Transmission attenuation range 0.5% – 95.0%

The application of polarization optics design is not limited to attenuation of laser intensity; other applications include phase control (phase retarding), interferometer, etc.