Notch Filter, called notch filter in Chinese, is also called a band-stop filter or a negative filter. Its optical characteristics are as follows: within the application band, the filter has good transmittance for most wavelengths of light. , and can effectively cut off light within a specific wavelength range (stop band). It can remove certain wavelength bands from a spectrum, and its function is exactly the opposite of a bandpass filter.
Notch filters are particularly useful in applications that require filtering out the light emitted by a laser. For example, in order to obtain a good signal-to-noise ratio in Raman spectroscopy experiments, it is crucial to suppress the pump laser. This can be achieved by placing a notch filter in the detection channel. In addition to Raman spectroscopy test systems, notch filters are frequently used in laser-based fluorescence instruments and biomedical laser systems.
However, the main disadvantage of the standard Rugate-Notch thin film notch filter is the limited passband range due to the fundamental frequency and high harmonic spectral stopbands (see the blue curve in Figure 1 below).
Figure 1 Spectral comparison between Rugate design and Our design
To obtain a wider passband than standard thin film notch filters can provide, optical engineers have had to turn to “holographic” or “Rugate” notch filters. Unfortunately, holographic filters have low reliability, passband transmittance, high cost, poor system noise performance or high system complexity, and cannot be truly industrialized. The traditional Rugate notch filter, based on sinusoidal changes in refractive index or film thickness, is usually limited by low transmittance, resulting in a limited passband range at the fundamental frequency and high harmonic spectrum stopband, especially Applications at longer wavelengths.
Omeda Optoelectronics’ notch filters provide breakthroughs in Rugate optical notch filter technology, combining all the advantages of hard dielectric standard thin film notch filters with ultra-wide passband and high transmittance (see Figure 1 above blue curve).
The comparison of the design structures of Rugate-Notch and Omeda Notch is shown in Figure 2 below. In Figure 2, Rugate-Notch is based on sinusoidally varying film thickness. The greater the thickness difference, the narrower the FWHM of Rugate-Notch will be. In Figure 2, Omeda-Notch is based on a variety of sinusoidally varying film thicknesses. The thickness difference is very small, and a very narrow FWHM can be obtained. In particular, a higher transmittance and a wider transmission bandwidth can be obtained.
Figure 2 Comparison of Rugate-Notch and Our Notch design structures
Typical applications of notch filters in biomedicine, Raman analysis, etc. are shown in Figures 3 and 4.
