Diffractive Optics and Other Products

Diffractive Optics

Overview

What if you need to measure the performance of the beam in real-time while your application is online? You cannot insert or remove optical components or interrupt the beam either. How do you do that? One way is to divert a small fraction of the beam that is an exact replica of the main beam. Sampling is also a solution if the beam is too "hot" for your diagnostic instruments to handle. In either case, almost all of the main beam is available to do its job.

Sampling

One approach is to use something like a moveable mirror to sample in time. The beam is interrupted and diverted for small slices of time. A big disadvantage though is that the time sampler provides average values so it is not well suited for real-time operation, especially with pulsed lasers. The other way is to continuously divert a tiny fraction of the beam to another angle while the main beam passes through, along its original line. However it is done, the sample needs to be a low power replica of the main beam.

Real-Time Beam Sampling

The components or methods suitable for real-time beam sampling are summarized in Table 1. The particular requirements of an application will narrow down the list of acceptable methods. All of these methods provide a decent replica of the main beam in the sampled beam and all can withstand reasonably high power. If it is important that the polarization of the sample be the same as the beam, then the Mirror and the Frustrated Total Internal Reflection are not good choices due to mixing of the horizontal and vertical polarizations. Low sampling fraction is very good. This means that most of the beam passes on to the application with a tiny percentage being diverted. The Mirror and Hole Matrix remove much more energy from the beam than the other components which is a problem for many applications.

Issues in beam sampling

Environmental Issues

If the relative position of the beam on the sampler is critical, vibration will have a disruptive effect. The methods employing refraction to some degree (eg. prisms and wedges) are highly sensitive to vibration. This includes vibration in the beam as well as the sampler. You will prefer other choices in rough environments or for laser beams that do not have great pointing stability. Changing temperature and humidity can be a problem for all but the Holographic sampler and Cascaded Wedges. This is an important consideration for industrial environments.

Application Issues

Cartesian main beam: if it is important that the main beam continue either parallel or perpendicular to the incident beam, you would have to rule out the Cascaded Wedges or the Wedge. The need for that depends on the design of the optical train. The other problem for the Cascaded Wedges is its unusable losses. Compared to the other methods, a significant fraction of the energy that is diverted from the main beam is not useful for sampling and must be dissipated. That can be substantial in high power applications. From the table you can see that the holographic sampler is suited to the widest range of applications.

Comparison of Real-Time Sampling Methods

	Spatial Profile Available	Polarization Insensitive	Low Sampling Fraction	Sustain High Power	Vibration Insensitive	Environment Insensitive	Cartesian Main Beam	Small Unusable Losses	Commercial Product Available
Reflective Samplers Wedge: Low-Reflective dielectric coatings on both faces (Rmin)
Mirror: Leakage from high reflectivity mirror (Rmax)
Refractive Samplers Cascaded Wedges
Frustrated Total Internal Reflection (FTIR): Adjustable gap between two prisms
Transmission Samplers Hole matrix: Periodic array of holes machined in highly reflective mirror or transparent substrate
Holographic: Relief hologram etched on transparent substrate