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1. What type of diffraction gratings (a.k.a. gratings) does Wasatch Photonics manufacture?
2. How does your Volume Phase Holographic Grating (VPHG) technology differ from surface relief grating technology?
3. What are some of the other properties of VPHGs?
4. How do you clean VPHGs?
5. What information is available on your gelatin recording medium and is it stable over time?
6. What is a Dickson Grating® and why is it different from other gratings?
7. What kind of performance characteristics can be expected from a Dickson Grating®?
8. Can gratings with different dispersion angles be made?
9. Over what wavelengths can your gratings operate?
10. What physical sizes can be made?
11. What wavefront uniformity/distortion can be expected?
12. What environmental tests have been performed on your gratings?
13. What are the thermal and aiming characteristics?
14. How are DCG gratings affected by continuous wave (CW) and pulsed laser energy density conditions?  How about DWDM and input signal conditions?
15. Is grating performance affected at temperature extremes or as a result of temperature cycling?
16. Are your gratings affected by radiation?
17. Holographically recorded gratings normally have a spatial efficiency dependence across the grating caused by a laser's Gaussian beam profile.  Do your holographic gratings have this spatial dependence?

Q.  What type of diffraction gratings (a.k.a. gratings) does Wasatch Photonics manufacture?
A.  We make volume phase holographic gratings (VPHGs) recorded interferometrically in DiChromated Gelatin (DCG) sealed in glass.   There are many other optical recording media.  DCG possesses the required combination of characteristics to produce very low polarization dependent loss (PDL), very high efficiencies, and wide bandwidths.   These translate into benefits that enable telecom and other companies to leverage and capitalize on their own technologies.

Q. How does your Volume Phase Holographic Grating (VPHG) technology differ from surface relief grating technology?

A. VPHGs have a constant thickness (T) with a periodic, typically sinusoidal, change in refractive index (n). Surface relief gratings have a periodic change in thickness with a constant refractive index. Most VPHGs are made in transmission geometry while most surface relief gratings are made in reflection geometry. Master VPHGs can usually be made for spatial frequencies >900 lpmm.   These master VPHGs may be copied optically without either the master or copy VPHG degrading over time. Surface relief masters are copied mechanically and both the surface relief master and copy will degrade over time as the number of copies increases.

Q. What are some of the other properties of VPHGs?

A. VPHGs possess low stray light and low absorption in the visible and NIR. They are sealed in glass making them long-lived, easy to handle, scratch resistant, and easy to clean. VPHGs usually have anti-reflection (AR) coatings applied to one or both surfaces to minimize Fresnel reflections and increase overall output.

Q. How do you clean VPHGs?

A. VPHGs, whether they are AR coated or not, can be easily be cleaned just as you would any other glass optic to remove fingerprints, dust, and contaminants.  Use the standard optics cleaning method of "drop and drag".  Gently wipe the grating with a clean, lint-free cloth/wipe using acetone or alcohol without much pressure; reagent-grade methanol or Windex may also be used. Do not use MEK, polishing compounds, etc.  Hard rubbing with such chemical or polishing compounds may damage the AR coating. AR coated surfaces will pass the "Scotch Tape" test.

Q. What information is available on your gelatin recording medium and is it stable over time?
A. Our gelatin recording medium is very similar to the gelatin that has been used in the photography industry for more than 100 years. As you may know, properly processed photographs are very stable and will retain their properties for decades. One of the major reasons for this excellent stability of photographs is the stability of the gelatin base. The discoloration of very old black and white photographs is a result of insufficient removal of fixer from the paper base, which results in a yellowing of the paper. It has nothing to do with the gelatin.

Wasatch Photonics uses a dichromated gelatin (DCG) mixture as its recording medium of choice. Once this mixture is processed and the chromium removed, it is essentially pure gelatin and is very stable.  During the past thirty years, there has been no measurable degradation in gelatin gratings that are properly sealed. The DCG holographic scanning disks used in some of the early optical bar code scanners still function exactly as they did over 20 years ago. 

Q.  What is a Dickson Grating® and why is it different from other gratings?
A.  A Dickson Grating® is a highly specialized volume phase transmission grating.   It can diffract both orthogonal linear polarizations with equal and high efficiency at included angles greater than 90 degrees over 40 nm bandwidths.  Other gratings designed to be used at these high diffraction angles will have much greater PDL, lower diffraction efficiency, and a greater variation in diffraction efficiency across a 40 nm bandwidth. The 40 nm bandwidth and favorable high dispersion angles combined with efficiencies near 95% make the Dickson Grating® highly desirable for Telecom applications such as Mux/Demux/Remux, OSA, monitors, filters and Raman Spectroscopy. The grating is named after the grating designer, and Wasatch Photonics Co-Founder, Lee Dickson. 

Q.  What kind of performance characteristics can be expected from a Dickson Grating®?
A.  Dickson Gratings® can easily be designed for almost any spectral range.  For instance, a 940 lpmm Dickson Grating® designed for 1525-1565nm will have these high performance characteristics:

	High Dispersion		Extremely High Efficiency (>90%)
	Extremely Low PDL (<0.25dB)		Flat Response Across C, S, or L Bands

Q.  Can gratings with different dispersion angles be made?
A.  VPHGs can be made at almost any angle.  Dickson Gratings® perform in narrow angular ranges, it is this limitation that allows them to work as well as they do. There are however two classes of realizable Dickson Gratings® with the first working near a half angle of 46.5 degrees in air and the other working at internal angles too high to escape from a flat substrate which must therefore be used with prisms. This latter design is termed the Dickson Grism® and has nearly TWICE the dispersion per pass of a Dickson Grating®.  In January of 2005, we delivered samples of our latest high dispersion product, the Dickson 1350® for 1525-1565nm which produces >2 times the dispersion of a 940 lpmm Dickson Grating®. 

Q.  Over what wavelengths can your gratings operate?
A.  We cover 350 -2400 nm (the entire visible range and NIR) with a single recording material on your choice of glass substrates and prisms. 

Q.  What physical sizes can be made?
A.  Typical ruggedized sizes with perfect hermetic seals begin at approximately 20 mm x 30 mm.  Smaller sizes have to be externally sealed. Gratings as large as 400 mm x 400 mm have been made by the Wasatch Photonics team. 

Q.  What wavefront uniformity/distortion can be expected?
A.   1/4 wave is typical.  Wavefront uniformity <lambda/40 rms at 633 nm can be achieved for additional costs using superior substrates, polishing techniques, etc. See this link as an example of a Zygo interferometer measurement for a very low wavefront error Dickson Grating®.

Q.  What environmental tests have been performed on your gratings?
A.  Our telecom-grade, epoxy sealed gratings have survived boiling water for several days with no degradation and 95^o C at saturated humidities for more than 2 weeks. A collapse of the grating structure has been observed after several days at 120^o C.  Some of our similar, but non-Dickson design, gratings are currently in commercial multiplexers and are surviving real world tests as well as having passed standard compliance testing.  The  DCG is hermetically sealed between various substrates such as BK-7, Borofloat, low iron sheet glass and fused silica, making it impervious to everything but extreme heat.