At any given writing velocity, there exists an erasure-threshold-intensity IETh below which there is no change in optical density of LDW-glass plates even with multiple retraces. Using a write-beam intensity above the erasure-threshold-intensity IETh, the optical density of LDW-glass plates reduces with each additional retrace and the LDW-glass plates can be erased to a transparent state with multiple retraces. As the write-beam intensity increases further above the erasure-threshold-intensity IETh, retraces needed to bring about the transparent state decrease in number. LDW-glass plates are made transparent in one laser sweep i.e., no retraces at a full-erasure-intensity IFE.
At any given writing velocity, there also exists an abrasion-threshold-intensity IATh at and above which the LDW-glass plates are abraded or damaged on the glass surface due to excessive temperature (>800°C) at the laser focused spot. However, the abrasion is not a pure thermal effect, since the abrasion-threshold-intensity IATh is lower using a write beam of a shorter wavelength.
At a given writing velocity, the write-latitude is defined as the difference IATh - IFE between the abrasion-threshold-intensity and the full-erasure-intensity. The write-latitude increases with decreasing writing velocity and also increases with a write-beam of a longer wavelength.
At a writing velocity of 1 to 4 meter/sec the required writing energy density for full erasure is 2 to 4 joule/cm2 using a write-beam whose wavelength in the spectral range of 488 nm to 1060 nm, provided the optical density of the LDW-glass plate is in excess of 0.5 at the wavelength of the write-beam.
The values of the writing energy density cited are based on experimental data using write-beams having a Gaussian intensity profile at the focused laser spot. One can expect the required writing energy density to reduce by a factor of more than 2 and the write-latitude increases, when a flat top intensity profile is utilized.
Multigray levels were written in LDW-glass plates using the writing velocity (e.g., the clock rate) or laser beam intensity or multiple retraces or a combination thereof as variable parameters.
A LDW-glass photomask with multi-gray levels is ideally suited for fabrication of diffractive optical elements
(DOE), Micro-electro-mechanical (MEM) devices, Micro-Opto-electro-mechanical (MOEM) devices. A mask for 32 phase levels of DOE is made by exposing in a laser beam writer with predetermined energy density levels according to a depth versus optical density calibration curve such as that shown in Fig. 1 and Fig. 2. Fig. 1 shows the remaining thickness after development of Shipley S1650 photoresist versus optical density at 436 nm wavelength of LDW-HR plate. Fig. 2 shows the remaining thickness after development of OCG OeBR-514 photoresist versus optical density of LDW-HR plate at 436 nm wavelength. Both calibration curves were done with a vacuum contact aligner.