The most commonly used gratings are ruled or etched using either a process involving the mechanical scratching of the surface of a substrate or exposure of a photoresist layer deposited on a substrate where the interference pattern produced by two coherent light beams is used to expose the photoresist. Both these processes create a fine, periodic relief pattern on the surface of a blank substrate; diffraction gratings produced by one of these processes are often called holographic. The relief structure can predictably modify the amplitude and/or phase of the incident light. The superposition on the effects of many such grooves allows for the angular separation of the spectral components of the incident light. Depending on whether the light is reflected from the grooved surface, or transmitted through it, the grating is called either a reflection or transmission diffraction grating.
PPO Gel Gratings
With PPO Gel Gratings, a photosensitive gel or polymer is deposited on a substrate. An interference pattern is then shone onto the gel which changes its index of refraction accordingly. A chemical process is then used to enhance the refractive index modulation and to “save” the pattern on the grating. As light passes through the different areas of the gel, it bends according to its wavelength, resulting in an angular separation of spectral components of the incident light. The diffraction efficiency of gel gratings depends on a greater number of factors than that of relief gratings. The gel thickness, its refractive index, the modulation depth of the refractive index and the distribution of the refraction index across the gel layer all contribute to the diffraction efficiency. This gives more degrees of freedom to produce a grating with much higher efficiency and lower polarization dependency. Unlike relief gratings, gel diffraction gratings can reach close to 100% efficiency, even for high spatial frequencies if a sufficiently thick layer of gel is used. Furthermore, PPO Gel Gratings produce less scattered light due to the absence of groove edges of standard gratings. Overall efficiency over a given spectral range can be greatly increased. Another advantage of PPO Gel Gratings is that more than one grating can be registered in the same layer of gel.
P&P Optica manufactures PPO Gel Gratings by exposing a photosensitive gel layer with a periodic interference field. After processing, this causes redistribution of material on a molecular level, producing smooth and periodic modulation of the index of refraction within the gel. Such a smooth, periodic structure scatters significantly less light than gratings produced as a result of a diamond ruling or chemical etching of the photo-resist. The unique, high-quality PPO Gel Gratings have significantly better optical qualities than traditional gratings, which allow for the construction of a single stage spectrometer, outperforming double or triple stage instruments with traditional diffraction gratings.
PPO Gel Gratings produce no ghost effects, which means that all photons of a particular wavelength are diffracted to only a single area of the detector. Ghost effects of other types of gratings often create confusion in spectral information, as a portion of photons of a particular wavelength is often diffracted to a detector area corresponding to a different wavelength. Other scatter effects often seen in scanning or reflective systems are eliminated by the P&P Optica design as well. This prevents non-diffracted photons from reaching the detector to produce noise. Finally, our transmission optics are always designed to produce as few aberrations as possible. This means that multiple wavelength photons are diffracted in the same way whether at the centre or edge of the detector. This eliminates washing-out of sharp peaks and peak overlap on the entire detector.
To capture and successfully diffract as many photons as possible, we use our PPO Gel Gratings which are made to very stringent specifications. PPO Gel Gratings offer the average absolute efficiency of over 75% with peak absolute efficiency reaching well above 95%. This is significantly higher than relief and even blazed gratings which can offer high efficiency in only a narrower band and typically report efficiencies for the impossible to reach Littrow configuration. Furthermore, the use of transmission design allows P&P Optica to build spectrometers which introduce very little scatter and loss of photons. All components are selected to reduce light loss and are coated by high-transmission, broadband anti-reflection (AR) coatings.
Comparison of PPO Gel Grating and Standard Relief Gratings (HRR)