StockerYale's Phase Mask Product Line

Introduction to Phase Masks

Phase masks are surface relief gratings etched in fused silica (Figure 1). In most applications, a phase mask essentially serves as a precision diffraction grating (Figure 2) that divides an incident monochromatic beam into two outgoing beams. The incident radiation is usually in the UV range. By generating two outgoing beams, a phase mask creates an interference pattern (Figure 3). This pattern is located in the space where the outgoing beams overlap. Phase masks have a wide variety of applications, but most frequently StockerYale phase masks (which are themselves gratings) are used to record other gratings, such as planar waveguide gratings in integrated optics devices, and fiber Bragg gratings (FBGs). An FBG is a light frequency filter located in the core of an optical fiber.

StockerYale is a world leader in phase mask fabrication. The phase mask manufacturing operation, located at StockerYale Canada Inc. in Montreal, involves sophisticated and demanding techniques in microfabrication and holography.

Most phase masks are fabricated in UV-transparent fused silica of high purity, but other materials are available. The data sheets accessible on this webpage provide an overview of the ranges of the various phase mask parameters for our standard products. For inquiries concerning custom phase masks, please contact us at StockerYale Canada.

The "period" (or "pitch") of phase mask gratings range from a few hundred nanometers to almost 2000 nanometers (2 microns). The grating areas come in a wide variety of dimensions, ranging from a few millimeters square to 10 mm by 150

Figure 1. Phase masks are surface relief gratings etched in fused silica.

Figure 2. A phase mask essentially serves as a precision diffraction grating.

Figure 3. A phase mask creates an interface pattern.

mm. The silica substrates on which the phase mask gratings are etched are typically 1/8" thick. The grating profile is essentially binary (a rectangular wave) for the longer periods, and tends to be somewhat quasi-sinusoidal for the shorter periods.

Phase masks are typically employed in one of two configurations: +1/-1 or 0/-1. In the +1/-1 configuration, the UV radiation is directed with normal incidence at the phase mask, and the period of the fringe pattern generated by the interference of the outgoing beams is exactly one half of the period of the phase mask grating. In the 0/-1 configuration, the UV radiation is directed at the phase mask with a specially chosen angle of incidence, and the period of the fringe pattern is exactly equal to the period of the phase mask grating.

Figure 4. Phase masks, which are themselves gratings, are used to record other gratings, such as fiber Bragg gratings (FBGs).

The upper part of Figure 4 is a schematic depiction of the recording of an FBG in the +1/-1 configuration. UV radiation is normally incident on the phase mask. A pattern of fringes is generated by the interference of the two outgoing beams (not shown). The fringes (which are stationary alternating zones of high and low intensity) are represented in the picture by vertical lines. A piece of optical fiber (usually made of silica) is placed in the interference pattern. The fiber's core (which is typically doped with oxides of germanium, tin, boron, phosphorus and other elements) is photosensitive, in the sense that it's index of refraction is altered through exposure to UV radiation. Thus, exposure to the interference pattern causes a periodic modulation of the index of refraction in the core material. The result is a fiber Bragg grating (FBG), shown in the bottom part of the figure.

Figure 5. A Fiber Bragg Grating is a light-frequency filter located in the core of an optical fiber. Only the light of resonant frequency is reflected back. The other frequencies pass through. FBGs are located at the heart of many fiber sensor devices and telecom components. They are fundamental building blocks in many fiber systems.

When a phase mask is operated in the +1/-1 configuration, the UV light is normally incident on the grating. The angles of diffraction Θ₀ , Θ_-1 , Θ₊₁ , Θ_-2 , Θ₊₂ etc. are given in terms of the UV wavelength λ_UV and the phase mask period Λ_PM, by the formula

sinΘ_m= m λ_UV / Λ_PM

The period of the fringe pattern created by the interference of the +1 and -1 beams is exactly one half of the period of the phase mask, regardless of the wavelength of the incident radiation.

Λ_fringe = (1/2) Λ_PM

Figure 6. +1/-1 Phase Mask Configuration

StockerYale's phase masks are optimized so that the intensity of the +1 and -1 orders is maximized, while the intensity carried in the zeroth order is minimized. Also, the intensity in any higher orders (m =±2, ±3, etc.), if such orders are present, is minimized.

A phase mask can also be operated effectively in the 0/-1 configuration depicted in Figure 7. This configuration is defined by the condition |Θ₀|=|Θ_-1|, which ensures that the fringes are perpendicular to the phase mask surface. In order to satisfy this condition, the required angle of incidence is

|sin Θ_in| = |sin Θ₀|
= λ_UV /(2 Λ_PM )

Moreover, if the condition

(2/3) Λ_PM < λ_UV < 2 Λ_PM

Figure 7. +0/-1 Phase Mask Configuration

is satisfied, then there will be one and only one diffracted order (the -1 order) and no other orders (such as +1, ±2, ±3, etc.). In other words, there are two and only two outgoing beams: the 0 order and the -1 order. This guarantees a clean fringe pattern.

The period of the fringe pattern created by the interference of the 0 order and -1 order beams is exactly equal to the period of the phase mask period.

Λ_fringe = Λ_PM

This remains true regardless of the wavelength of the incident radiation, and regardless of whether the condition
|sin Θ_in| = λ_UV /(2 Λ_PM ) is perfectly achieved.