High Power Laser Apertures

The recent diversification in the laser usage industry can only be described as astronomical. Just about anyone can purchase a laser and learn to use it for OEM fabrication or value-added products. There are a lot of small companies in the field that are successfully using lasers. They do not have scientists or experienced research technicians on their staff. In contrast, the use of a laser by a large corporation with nearly unlimited financial and technical resources is analogous to General Motors using an automobile. 

High energy and high power apertures are intended to be used in the optical transfer assembly of a system using a large laser as a source. The laser power and energy and the time duration of the beam pulse are also important.

For applications where the beam pulse is continuously or intermittently generated in groups, the repetition rate and time between pulses is important.  A repetition rate should be selected that provides a packet separation time that is long enough to allow sufficient heat to be conducted away and thus avoid damage to the aperture disc. The aperture substrate material should be one that it is not deformed or partially vaporized by the laser beam. The material of a high energy or high power aperture disc should both conduct away sufficient heat and have a highly reflective surface facing the laser. In concurrence with this, it is advantageous to mount the aperture disc in a heat conducting holder. Table 1 outlines the melting temperature for a few of the more common pinhole aperture substrate materials.

Units of Measure

A joule is the unit of radiant energy and is defined as 1 watt-second (1 joule = 1 watt-second). In radiometric terms, radiant power is the rate at which energy is produced or the time rate flow of radiant energy and is expressed in watts. It is important to understand the relationship between power, energy and pulse width. Table 2 outlines the power-energy-pulse duration function. With reference to Table 2, if the energy (joules) remains constant, the power (watts) increases as the pulse width (time) decreases. Accordingly, if the power (watts) remains constant, the energy (joules) increases as the pulse width (time) increases. The total energy per pulse is the integration of power over time and is inversely proportional to the length of the pulse. The average peak power per pulse decreases linearly as the length of the pulse is increased. Laser beam power is often expressed in terms of watts per unit area or watts/cm2 within the laser beam. Laser beam average power density is also expressed in terms of power/area.

Table 2

Energy:
1 joule =10 watts x 0.1 second
1 joule = 100 watts x 0.01 second

Power:
10 watts= 1 joule:0.01 second

10 watts= 10 joules: 1.0 second

 If the rated laser power is 10 watts and the output beam size is 1 cm2 (equivalent to a 1.13 cm diameter beam), the beam may be defined as having a power of 10 watts/cm2. To substantially increase the we use a series of lenses to focus the beam to a much smaller diameter. Assume that our 10 watt laser is focused to a 0.01 cm diameter spot size. This translates into a 7.9 x 10~ cm2 area. There are several methods of calculating the approximate watts/cm2 of this spot. One is to use the incoming (laser output) beam area to focused spot area ratio. For example:.

1 cm2 ÷ 7.9 x 10~ cm2 = 12,658.2 (ratio)

Then:
Laser Output Power watts/cm2 x ratio = Focused Spot Power watts/cm2

Or:
10 watts/cm2 x 12,658.2 = 126,582.2 watts/cm2

Another method to achieve the same result is to calculate the laser beam average power density (power/unit area) by dividing the laser output beam watts by the focused spot area. For example:

10 watts ÷ 7.9 x 105 cm2 = 126,582.2 watts/cm2

Power density defines how much power can be pumped through a
given aperture or spot size.

For illustrative purposes, Figure 1 outlines laser power levels for 1 joule of energy for pulse widths of 1 and 1000 ms. With reference to Figure 1, we see that a millisecond pulse of one joule renders 1000 watts. The same one joule pulse integrated over one second will render only one watt.

Aperture Material Characteristics

For high power/high energy apertures, the ideal substrate would be the one with the highest melting point, highest thermal conductivity for heat sinking and the highest reflectivity at the laser wavelength for the polished surface. Most aperture applications disallow this ideal condition and compromise is in order. For reference, Table 3 outlines the thermal conductivity of a few aperture substrate or aperture coating materials. Table 4 outlines the reflectivity for select materials in the Nd: YAG wavelength range of 1.064 micron for the standard (#10) or polished finish of the material. It should be noted that silver oxidizes rapidly when exposed to air and reacts to airborne sulfides in a similar manner. The surface is then no longer silver, it is silver oxide or silver sulfide and the reflectivity is degraded considerably. Before exposure to air, silver must be coated with a substance that is transparent in the laser wavelength. In this manner, the high reflectivity can be maintained. A similar degradation occurs when copper is exposed to air however, the reaction time is considerably longer than the near instantaneous reaction of silver. A degradation in system performance provides an indicator for the condition of the copper surface. If maximum surface reflectivity is a requirement for your system, you should either coat the surface or re-polish the surface on a regular basis. To maintain high surface reflectivity, spare copper units should be stored in an inert atmosphere.

 Aperture substrate discs with a highly reflective surface facing the laser provide the advantage of reflecting away most of the energy so that the disc is not damaged by heat. The disadvantage of the reflective surface is the possible introduction of scattered light in the optical axis of the system. The use of optical baffles throughout the system will eliminate or significantly reduce scattered light.

 *Cal/clx.Jsec/ C

Above values at room temp.

In the laser field, experience and heritage are often the guidelines for selection of pinhole aperture substrate materials. There is no direct means by which one may calculate the expected temperature of a given aperture disc when irradiated with a laser beam of known power or energy. Modeling based on the first principles of Physics is not a reliable method to use for this endeavor. High resolution, detailed matrix modeling could produce useful thermal predictions. However, this is a labor, computer time and software intensive effort. As an alternative, there is empirical measurement and the trial and error method. For empirical temperature measurement, there are some moderately accurate high temperature optical pyrometers on the market today. Pyrometer response time is generally significantly slower than the laser pulse duration and this may disallow a viable measurement. Given the relatively low cost of precision apertures, the trial and error method is practical and less labor-intensive. Select several aperture materials with melting points above that of the estimated temperature of the laser beam environment. One face of the aperture disc should be polished so as to reflect away a large percentage of the laser power. The aperture disc holder should be fabricated of a high heat conducting material. Empirical testing of the apertures in the laser environment will generate an aperture material performance data base that is unique to your system.

Small Diameter Apertures

For aperture diameters of 25 micron in diameter or smaller, the "tunnel" effect is the largest problem. It is helpful to describe this effect by referencing dimensions at the macroscopic level. With reference to Figure 2, imagine a 2” diameter hole (d) through an 12” thick wall (t) as compared to a 2” diameter diameter hole through a wall only 1" thick. If you peer through the 2” diameter hole in the thick wall, it will appear as a 12” long pipe or tunnel. At the microscopic level, when the pinhole aperture diameter approaches or becomes less than the dimensional thickness of the substrate disc, this same “tunnel” effect takes place. Essentially, the aperture becomes a narrow pipe through the disc with an entrance aperture finitely separated 

from its exit aperture. At this point, the aperture is no longer an aperture. It becomes an optical element with an entrance and exit aperture with tunnel walls in between that scatter the energy and introduce significant power loss. A round hole may be defined as a cylinder that extends between the front surface and back surface of the substrate sheet. The aperture disc thickness to hole diameter ratio is an important parameter. A substrate disc thickness to hole diameter ratio of 2:1 or 3:1 is acceptable. However, empirical measurement shows that a 30% power loss takes place between the ratios of 3:1 and 5:1. It is best to keep this ratio less than 3:1. With reference to Figure 2, the view on the left approximates a 5:1 ratio and the view on the right indicates a more desirable ratio of 0.5:1.

For hole diameters less than 4 microns, the <12 micron disc substrate thickness becomes a fabrication feasibility problem. The thin aperture disc is impractical to fabricate, handle or mount. A practical solution is the tapered thickness disc with the center thinner than the perimeter. For high power requirements, the tapered thickness aperture disc offers the added advantage of more efficient heat sinking.

Acknowledgement

 The author wishes to gratefully acknowledge the assistance of Dr. David Palmer, USRA, Code 661, NASA Goddard Space Flight Center, Greenbelt, Md. who provided much in the form of review and discussion of the technical and scientific aspects of the

Figure 2