About Diffraction Gratings
WHAT IS A DIFFRACTION GRATING?
A diffraction grating is an optical element that diffracts energy into its constituent wavelengths.
The groove density, depth and profile of a diffraction grating dictate the spectral range, efficiency, resolution and performance of the diffraction grating.
There are typically two different types of diffraction grating – the ruled grating and the holographic grating.
A ruled diffraction grating is produced by a ruling engine that cuts grooves into the coating on the grating substrate (typically glass coated with a thin reflective layer) using a diamond tipped tool. A holographic diffraction grating is produced using a photolithographic technique.
A diffraction grating can be a reflection grating or a transmission grating. The most common type of diffraction grating are plane gratings and concave gratings although they can also be other profiles such as convex or toroidal depending on the application
Reflection gratings are normally coated with a reflective coating, usually aluminum with a protective overcoat for UV-VIS-NIR use or gold for IR use. Transmission gratings are usually supplied with an antireflection coating.
A diffraction grating can have a sinusoidal or blazed profile. A sinusoidal grating generally offers lower efficiency than a blazed grating, but often gives a broader spectral coverage. A blazed grating has a ‘saw tooth’ profile and normally offers higher efficiency.
A commercial diffraction grating is generally a replica grating produced from a sub-master, which may be a number of generations down from the master diffraction grating.
Typically, the cost of producing a master diffraction grating is expensive and by supplying replica gratings (which offer almost indistinguishable performance), one master may produce thousands of replicas, lowering the unit cost of the diffraction grating.
TYPES OF HOLOGRAPHIC GRATINGS
A holographic grating is a diffraction grating manufactured using a photolithographic technique that utilizes a holographic interference pattern. Two intersecting laser beams produce equally spaced interference fringes that are projected onto a photoresist material on the grating substrate. The photoresist dissolves in proportion to the intensity of the fringes resulting in a holographic grating that has a sinusoidal profile. The holographic grating is then coated with a reflective coating.
Because a holographic diffraction grating has no periodic errors or imperfections, it exhibits significantly lower stray light compared to a ruled grating and no ghosting effects. See Ruled Grating vs Holographic Grating for further information.
A holographic grating can be blazed to produce a blazed grating that has increased efficiency over a defined spectral region.
Blazed Holographic Grating
A blazed holographic grating is a diffraction grating where the sinusoidal profile has been transformed into a ‘saw tooth’ profile. This saw tooth profile effectively increases the efficiency of the blazed grating over the required wavelength region.
The blaze wavelength is the wavelength where the grating gives maximum efficiency. A blazed holographic diffraction grating can be manufactured using a number of techniques. The proprietary technique used by Spectrum Scientific creates a blazed grating which offers significantly lower stray light when compared to an ion-etched blazed grating.
A blazed holographic grating offers the same high efficiency as a blazed ruled grating, but significantly lower stray light and no ghosting.
The main advantage of a concave grating is that it can be used as the primary dispersive and focusing element in an instrument. The concave grating reduces the number of optical elements required, increasing throughput and instrument efficiency. There are typically three types of concave grating.
Aberration Corrected (Flat Field Imaging) Concave Grating
The first type of concave grating is an aberration corrected concave grating (or flat field imaging grating). On an aberration corrected concave grating, the grooves are neither parallel nor equidistant.
An aberration corrected concave grating is designed to eliminate astigmatism and allow the complete spectral range to be imaged on a plane.
This makes an aberration corrected concave grating ideal for use with planar array detectors such as photo diode arrays (PDA) or charge coupled device (CCD) detectors.
Constant Deviation Monochromator Concave Grating
The second type of concave grating is the constant deviation monochromator grating. This type of concave grating is used in a scanning monochromator where the grating is rotated and scans the signal from the entrance slit across the exit slit. The deviation angle between the incident signal and diffracted signal remains constant.
The main advantage of a constant deviation concave grating over a plane grating is that it removes the need for collimating and focusing optics, reducing the number of optical elements and increasing throughput. It also allows more compact instrument designs.
Rowland Type Concave Grating
The third type of concave grating is known as a Rowland type concave grating where the grooves are straight and equidistant.
This type of concave grating diffracts the spectrum onto a Rowland circle which is defined as a circle where the diameter of the circle is equal to the radius of curvature of the concave grating.
Rowland type concave gratings suffer from astigmatism but other types of aberration are small.
Blazed Holographic Concave Grating
A blazed concave grating is similar to a standard blazed grating where the groove profile has been modified to increase efficiency in a defined spectral region.
Unlike some other types of blazed holographic concave gratings, the blazed concave gratings produced by Spectrum Scientific are produced using a process that yields a blaze profile that varies across the grating surface, increasing efficiency across the full image plane. Typical efficiencies of >80% are achievable.
Sinusoidal Holographic Grating
A sinusoidal grating is a diffraction grating that has a sinusoidal groove profile. Typically, a sinusoidal grating offers a wider spectral coverage compared to a blazed grating but has lower efficiency. However, under certain conditions where the groove spacing and wavelength ratio are close to unity, a sinusoidal grating can exhibit the same efficiency as a blazed grating. An example of this is an 1800g/mm sinusoidal grating, which has very high efficiency in the visible region.
A transmission grating is produced in the same way as a reflection grating, but the grooves are designed to diffract transmitted light. Transmission gratings offer high efficiency and are generally easier to align than reflection gratings. To produce high efficiency, a transmission grating usually requires a deep groove profile.
Pulse Compression Grating
A pulse compression grating is a special type of sinusoidal grating that is used for Laser Chirped Pulse Compression and typically optimized for use at 1053nm. A pulse compression grating needs to have a very high damage threshold and high efficiency (>90% for Littrow incidence, S-polarization).
Modern telecommunications allow for vast amounts of information to be moved through optical fibers. Gratings allow for the management of the signals in the fibers by separating the individual wavelengths, allowing access to the information.
Telecom applications require gratings with very low polarization dependent loss (PDL) coupled with high diffraction efficiency. They also need to be highly environmentally stable with very good grating to grating repeatability.
RULED GRATING vs HOLOGRAPHIC GRATING
Ruled diffraction gratings by the nature of the manufacturing process cannot be produced without defects, which may include periodic errors, spacing errors and surface irregularities. All of these contribute to increased stray light and ghosting (false spectral lines caused by periodic errors).
The optical technique used to manufacture holographic diffraction gratings does not produce periodic errors, spacing errors or surface irregularities. This means that holographic gratings have significantly reduced stray light (typically 10x lower stray light compared to ruled gratings) and no ghosts.
Historically, ruled diffraction gratings offered higher efficiency than holographic diffraction gratings, but with the introduction of blazed holographic diffraction gratings this is no longer the case.
In addition, concave gratings present specific problems for ruled gratings compared to holographic gratings. Ruled concave gratings cannot be utilized for flat field imaging applications as the projected groove pattern of the grating always results in straight, equidistant lines and therefore additional optics are required to correct for aberration. Holographic concave gratings, however, can be designed and produced with curved grooves that produce aberration corrected images. Holographic concave gratings can also be produced with lower f-numbers than ruled concave gratings.
For nearly all applications a blazed holographic diffraction grating will offer significantly better overall performance when compared to a ruled diffraction grating. A ruled diffraction grating should only be used where groove density or spectral range requirements preclude the use of a blazed holographic diffraction grating.
MASTER DIFFRACTION GRATING vs REPLICA DIFFRACTION GRATING
In nearly all cases the optical performance of a replica diffraction grating is virtually indistinguishable from the performance of the master from which it has been produced. There are very few applications that benefit from using a master grating instead of a replica grating. Replica gratings are also generally lower in cost than master gratings (especially for volume production) and will exhibit better consistency from grating to grating.
SSI’s proprietary blazing technique also means that our replica blazed gratings exhibit significantly lower stray light compared to gratings produced using other types of blazing methods.