Diffraction gratings are a dispersive optical component. Similar to prisms, they split polychromatic light into its constituent wavelengths. Unlike prisms though, diffraction gratings offer dispersion according that follows a sinusoidal relationship according to the grating equation. The ability to precisely disperse light makes them a critical component in a variety of applications. Most notably, diffraction gratings are widely used in monochromators and spectrometers.
While the functional principles are similar, there are two types of diffraction gratings – reflection and transmission. A reflective diffraction grating has a highly reflective surface with a series of equally spaced parallel grooves. When the incoming light reflects off the grooves, the resulting wavefront division creates a dispersion pattern that is characteristic of the groove spacing and incident radiation. An in-depth discussion of performance characteristics can be found in our diffraction gratings knowledge base.
Diffraction gratings often have hundreds or thousands of grooves per millimeter. By comparison, the width of a spider’s silk is less than 4μm which is the size of each groove on a 250 grooves/mm grating. In contrast, the average width of a human hair is 70μm. The human eye cannot discern the grooves, so inspection and metrology of diffraction gratings require technologies like Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM).
Due to groove spacing and accuracy requirements, fabricating diffraction gratings is an arduous task requiring precision machines. Early diffraction gratings were made using complex, mechanical ruling engines run by expert operators in highly controlled environments. Cost, long manufacturing times, and part-to-part variation limited their application.
Ruling engines are still widely used today alongside systems that use monochromatic and coherent lasers to produce holographic diffraction gratings. Modern ruling engines use interferometry with closed-loop control technology and environmentally controlled chambers, which add a greater level of precision and control to the master grating.. Replication, which can produce identical diffraction gratings from a master grating, has improved repeatability, cost, and manufacturing time, making diffraction gratings more readily available for various applications.
What Are Master Gratings?
Master gratings are first-run diffraction gratings where a substrate, likely glass or copper, is polished to a finish better than one-tenth of a wavelength (λ/10) with a high degree of flatness. For ruled diffraction gratings, the surface is then coated with aluminum using vacuum deposition. Photosensitive (photoresist) coating is used for holographic diffraction gratings. The master grating is completed by grooving the surface with either a ruling engine or holographic system.
Before reliable replication methods were developed, all diffraction gratings were essentially master gratings. Making a master grating is a precise and laborious process that can take several weeks. Slight variations in groove shape, spacing, parallelism or processing can result in an out-of-spec grating. It is also an iterative process—fabricate, inspect, adjust—repeated until the target specification is met. Precision optical and metrological inspection is completed between each run.
The good news is, once a master grating meets specification, it can be used to make hundreds of thousands of replicated gratings—in reality though, a master grating is typically only replicated a few dozen times. Replicated gratings have identical optical properties to the master grating. Repeatability from one replica to the next is far superior to that from master to another master.
What Are Ruled Gratings?
Custom ruled gratings are manufactured using a precisely shaped diamond tool and a ruling engine. Although cut grooves can be used for very course rulings, typically, a diamond tool burnishes, rather than cuts, the grooves into the reflective coating. The ruling engine must be able to retrace the exact path of the diamond forming tool on each stroke and to index (advance) the substrate a predetermined amount after each cut. The stage of the ruling engine is mechanically isolated from external vibration, and the temperature is controlled within tenths of a degree to maintain thermal stability.
The groove profiles are sawtooth-shaped. The performance of the diffraction grating is primarily dictated by the angle of the incident light, the groove spacing, and the blaze angle, defined by the angle between the groove face and the surface (Figure 1). The surface and ruling quality determine the performance of the grating.
It is feasible for ruling engines to rule master gratings over 3,000 grooves/mm. However, it is faster and easier to produce gratings with groove spacing finer than 2,000 grooves/mm using using holographic methods.
What Are Holographic Gratings?
Even the best mechanical ruling engines can introduce mechanical errors in spacing and groove shape, especially for exceptionally fine grooves. Holographic gratings, on the other hand, have consistent groove shape and spacing because they are made optically rather than mechanically.
Polished substrates are first coated with a photoresist layer that interacts with light. The coated blank is placed between the intersecting beams of monochromatic and coherent light produced by a laser (such as a 488nm Argon laser). The photoresist is differentially exposed in parallel, equally spaced interference fringes with sinusoidally varying intensity. Following this exposure, the blank is processed to reveal the grooves and a final reflective coating is added (Figure 2).
Due to the sinusoidal grooved pattern, a holographic grating performs slightly differently than a ruled grating. While the groove spacing accuracy of holographic gratings eliminates ghosting and stray light caused by mechanical errors, there is usually some penalty in efficiency. Ruled gratings tend to have higher absolute efficiency at the peak wavelength within the specified range due to the precision of the blaze angle.
The Gratings Replication Process
Once a master grating is produced, numerous replicas can be made from them. The replication process is straightforward. Generally speaking, the basic procedure, shown in Figure 3, is as follows:
- Fabricate and verify a master grating.
- Apply a release agent to the surface of the master grating.
- Apply epoxy.
- Add a replica substrate.
- Cure the epoxy and release the replica.
- Apply a reflective coating to the surface (this step is not necessary in transmission grating)
If it is a concave grating, an additional replication procedure is needed to create the correct curvature.
The replication process can create high-quality replica gratings that do not stray from the master grating’s groove profile. As a result, their performance is identical to that of the master grating. Of course, the quality of the replica is dependent on the quality of the substrate, epoxy, coating, and overall process.
Metrology of Diffraction Gratings
Since the grooves on diffraction gratings are extremely fine and not visible to the naked eye, inspection and metrology relies upon the use of sophisticated techniques that include microscopic and optical performance verification. Geometry and diffraction efficiency are two of the most essential specifications of diffraction gratings.
Geometry: The groove spacing and groove profile of diffraction gratings is typically measured using a scanning electron microscope (SEM) or an Atomic Force Microscope (AFM). These scanning microscopes create topographic maps of the grating surface with nanometer-level resolution. The SEM or AFM can measure the grooves of a master grating then compare them to replicas to verify the replication process.
Diffraction Efficiency: This is a function of groove shape, angle of incidence, and the reflectance of the coating. Absolute efficiency is the percentage of incident radiation diffracted to the desired order. Relative efficiency is the ratio of the energy of the desired order to the energy of a first surface mirror. This mirror typically has a known good coating of the same reflective material used on the grating to be measured. Typically aluminum or gold..
The diffraction efficiency is determined by measuring the intensity of light at different wavelengths using a spectrophotometer. Some custom diffraction gratings require custom-built equipment to measure efficiency.
Other performance criteria such as stray light, resolution, wavefront, and imaging properties are typically verified in a test setup that mimics the final application.
Diffraction gratings are an important dispersive optical element. Early diffraction gratings were primarily used in specialized applications due to the difficulty of creating the precision grooves. Improvements in grating master fabrication technology, along with high quality optical replication has enabled replica gratings to be made quickly and cost effectively from master gratings. This has made possible optical instruments like monochromators and spectrometers that are widely used in industrial applications, life sciences, research, space exploration, and instrumentation.