A compact, low cost, seven channel polychromator for Thomson scattering measurements

A seven channel polychromator, utilizing high performance interference filters (transmission ≳80%, rejection at laser wavelength >10 ⁵), has been tested for use in the multi-Nd:YAG laser Thomson scattering system for the DIII-D tokamak. Unique features of this polychromator are the combination of high throughput, easy alignment, flexibility, compact size, and low cost when compared with other alternatives. Light is introduced to the polychromator (f/1.75) via a fiber optic bundle which permits the use of small (3.0 cm diam) optics and leads to a compact (44×24×8 cm) design, an important design consideration for multiple polychromator systems. The light is cascaded through a series of different bandpass interference filters and relay lenses which are mounted on two precision parallel rails in such a way that alignment is trivial. The relay lenses are positioned directly in front of the filters so that light reflected from the filter passes through the lens twice. This leads to an efficient, compact design and reduces the angle of incidence (4°) and the cone angle of light (4.5°) seen by the filter, an important factor for narrowband (3.0 nm) filters. The transmission (average 70%) was optimized for 700-1100 nm by using broadband coatings throughout. The output images of each channel (2.3 mm. diam) can be directly coupled to large format (3 mm diam) RCA silicon avalanche photodiode detectors, avoiding the losses caused by fiber optic coupling.