Advanced narrow band filters in PTR glass for Raman spectroscopy

Abstract

Volume Bragg gratings in Photo-ThermoRefractive (PTR) glass enable unmatched filter performance for RAMAN spectroscopy. High diffraction efficiency combined with ultra-narrow bandwidth provide simultaneous access to Stokes and anti-Stokes Raman modes with frequencies as low as 5 cm. Recent advances in the technology of the diffractive elements in PTR glass enabled fabrication of Raman filters with optical density OD>5. New holographic elements embedded in a photo-thermorefractive (PTR) glass were recently deployed in a widevariety of applications. This material allows recording of phase Bragg gratings with high efficiency for visible and near IR spectral regions. The refractive index modulation in PTR glass is achieved by controlled precipitation of a nano-crystalline phase in the whole volume of PTR glass exposed to UV radiation. Once recorded and developed, volume Bragg gratings (VBGs) cannot be erased by illuminating to optical and ionizing radiation or by heating unless the temperature exceeds 400C. Unlimited VBG life time without degradation of its parameters makes beneficial its usage for lasers and optical systems. High-efficiency VBGs provide unmatched optical filtering capabilities with diffraction efficiencies as high as 99.99% and spectral bandwidth below 100 pm. Such gratings can be used for manufacturing of laser cavity mirrors, deflectors and notch filters for RAMAN spectroscopy. VBG based notch filters and single stage spectrometers show unmatched capability providing simultaneous access to Stokes and anti-Stokes Raman modes with frequencies as low as 5 cm. Reflecting type gratings significantly reduce complexity of ultra-low frequency Raman measurements that widens its applications. In this work we present recent advances in technologies of high-efficiency gratings that led to development of unique optical filters with enhanced optical density. Usage of high quality PTR glass combined with the state of the art recording techniques enabled manufacturing of Bragg gratings with diffraction efficiency above 99.999% in visible and near IR spectral regions. Those gratings are based on reflecting type VBGs with periods of several hundred of nanometers and couple millimeter thickness. Such gratings provide unmatched spectral and angular selectivity. Angular dependences of optical density of VBG notch filters with various optical densities are shown in Figure 1. All filters have thickness of about 3 mm. It confirms that enhanced optical density is achieved due to improved quality of recording. Coupled wave theory has been used to model the diffraction efficiency of the filters. It was found that the refractive index modulation has been increased to 400 ppm. The spectral bandwidth of those filters is less than 7 cm (FWHM) that is essential for ultra-low frequency RAMAN spectroscopy. In this work we discuss properties of the novel filters that provide enhanced optical density OD>5 at different spectral regions. The achieved diffraction efficiency combined with low losses implies that such filters can be successfully used in various optical systems including RAMAN spectroscopy, astronomy instruments hyperspectral imaging, etc. Further developments of the technology will be discussed. 0.00001 0.0001 0.001 0.01 0.1 1 5 5.2 5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 7 Tr an sm itt an ce