TUTORIAL 1: How to choose a galvo scanner?

800px-Plexi_Galvohalter

There are several methods for the laser beam steering. The most common method would be the nanopositioner/ piezo-translation stage. It can move with high precision but low speed due to the large inertial. The counterpart galvo- scanner ( or simply galvo) offers high speed scanning and less expensive solution with the tradeoff of precision. In this tutorial, I will summarize the features that one should notice for choosing the galvo scanner ( 1D or 2D).

What the hell is galvo?

In a Nutshell, galvo scanner is high speed moving mirrors. A galvo system mainly consist of 3 components: motor, mirror and driver board. If the user would like to control the system with digital signals, then they will include a D/A converter to do the job. Otherwise, they can control the movement using analog control (voltage).

scanner CTIsource: Cambridge Technology

Motor/ galvanometer selection

Three important components in a galvanometer: rotor, stator and position detector (PD). The performance of the rotor + stator dictate the torque efficiency whereas the position detector dictate the performance of the system. Here we will not cover the topics of rotor/stator performance and user should refer the “Handbook of optical and laser scanning” by Gerald F. Marshall for more information.

The PD plays an important role in the performance of the galvanometer. The latest design of the PD involves the illumination diode(s),mask and photodector(s). There are several specifications of PD that one should aware of:

i) Linearity: how the PD behaves as an ideal linear transfer function. Typical values: 99.5%- 99.9%

ii) Repeatability:  describe the consistency of PD when return to a specific position, typical values: 1µrad – 8µrad.

iii) long term stability (drift): describe how the system behaves over time and temperature. Usually described by Offset drift (deviate from the 0,0 position) and Gain drift (deviation between set and real position).

Mirrors selection

The basic function of the mirrors is to accommodate the input laser beam and cover the entire beam over the scan angles.  The mirror thickness, size, shape, material and the moment of inertial of the mirror play an important on the performance of system. Especially when the system is subjected to the high acceleration during operation, the mirror must remain rigid or the speed or the accuracy of the system would be compromised. So what should you look at when searching for the galvo mirrors?

i) shape and size:  the shape and size of the mirror are normally defined / fixed by the manufacturers, since the tunings are based  on the geometry of the mirror .

ii) substrate materials: common used materials are silicon, fused silica, beryllium or silicon carbide. Each material has their own pro and cons. For more information, we would recommend the read the following article.:

http://www.laserfocusworld.com/articles/print/volume-45/issue-3/features/optics-for-scanning-rapid-scanning-applications-drive-mirror-design.html

iii) Optical coating: each mirror must designed and customized according to the laser wavelength and laser power. Beside the broadband metallic coating, the commonly used Laser mirrors consist of multilayers dielectric layers and could provide >99% of reflectivity. Other than the reflectivity, there are several parameters that one should look at when shopping for the appropriate laser mirrors. The most important properties are surface quality, surface flatness and the laser damage threshold:

Optical Surfaces 

Surface Quality

The surface quality of an optic is described by its surface figure and irregularity. Surface figure is defined as peak-to-valley deviation from flatness, including any curvature (also known as power) present. Surface irregularity is represented by the peak-to-valley deviations when power is subtracted. The front-surface figure is typically guaranteed flat to less than λ/10 at 633 nm over the clear aperture. Our 2″ mirrors have a typical figure of λ/4 over the clear aperture. When preservation of wavefront is critical, choose a flatness of λ/10 or better.
As for surface quality, the smaller the scratch-dig specification, the lower the scatter. Our metal mirrors offer a scratch-dig of 25-10; our dielectric mirrors, 15-5; and our UV mirrors, 10-5, which is ideal
for the most demanding laser systems where low scatter is critical.
dig: a defect on the surface of an optic as defined in average diameter in 1/100 of a millimeter.
scratch: a defect on an optic that is many times longer than it is wide.

The mirror application drives the requirements for surface flatness and surface quality. When preservation of wavefront is critical, a λ/10 to λ/20 mirror should be selected; when wavefront is not as important as cost, a λ/2 to λ/5 mirror can be used. For surface quality, the tighter the scratch-dig specification, the lower the scatter. For demanding laser systems, 20-10 to 10-5 scratch-dig is best. For applications where low scatter is not as critical as cost, 40-20 to 60-40 scratch-dig can be used.

Surface Flatness

Figure Cost Applications
λ/2 Low Used where wavefront distortion is not as important as cost
λ/5 Moderate Excellent for most general laser and imaging applications where low wavefront performance must be balanced with cost
λ/10 Moderate For laser and imaging applications where low wavefront distortion, especially in systems with multiple elements
λ/20 High For the most demanding laser systems where maintaining accurate wavefront is critical to performance

Surface Quality

Scratch-Dig Cost Applications
60-40 Low Used for low-power laser and imaging applications with unfocused beams where scatter is not critical
40-20 Moderate Ideal for laser and imaging applications with collimated beams where scatter begins to affect system performance
20-10 High Excellent for laser systems with focused beams that can tolerate little scattered light
10-5 High For the most demanding laser systems where low scatter is critical to performance

Source: Newport

LASER DAMAGE THRESHOLD  (LDT)

Coating selection (1)

Normally the specification of laser damage threshold of a mirror is specified by the manufacturers and should not be exceeded, otherwise delamination or burn out could occur (Figure below)

OE_52_8_086103_f005source: SPIE Library

Depending on the laser operation mode, the laser induced damage mechanism are different. In the case of continuous wave (CW) or Quasi- CW operation, the damage is usually caused by the thermal effect due to the absorption. As for the case of pulsed operation, especially pulsed duration in the pico- or femto seconds range, the damage is caused by the dielectric breakdown. So when assessing the LDT of the mirrors, it is essential to ask the following questions:

  1. CW or pulsed operation?
  2. What is the power /energy density of your beam (total power/energy divided by 1/e2area)
  3. Pulse length of your lase
  4. Pulse repetition frequency (prf) of your laser
  5. Beam diameter of your laser (1/e2)

For more information please refer to the Thorlabs LDT Tutorial:

https://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=7040

Driver boards selection

There are not much of choices when come the driver boards/servos. It can either be digital or analog control. Analog servos do the job with lower price, however, the tunings have to be performed manuals and is normally coupled to the particular galvos. Digital servos are more expensive but offer better performance and extra features.

 

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