UV Written Blazed Chirped Bragg Grating Spectrometers

Abstract

Monolithic integrated spectrometers are ideal for use in portable testing equipment, however current research tends to focus on high-resolution platforms at near-infrared wavelengths. This thesis investigates a dispersive spectrometer platform based on blazed chirped Bragg gratings which has high resolution, large bandwidth, and low cost. As such it is suitable for portable Raman spectroscopy and optical coherence tomography.
Blazed chirped Bragg gratings diffract and focus light, with the angle of diffraction dependent on the wavelength of the input light. As such, the detected intensity distribution along a detector array can be used to measure the light spectrum. This thesis shows flexible fabrication and characterisation of these devices using small-spot direct UV writing, as well as methods of efficiently modelling their output intensity distribution.
Focussing aberrations of blazed chirped Bragg gratings are investigated and an ideal chirp function is derived to eliminate such aberrations. This is experimentally verified, enabling a device which operates over the 1440 nm to 1640 nm wavelength range. The device exhibited a peak resolution of 1.8 nm at 1560 nm and a typical resolution of 2.6 nm across a 100 nm range.
Scalar diffraction modelling was used to investigate the bandwidth of devices, showing 3 dB bandwidths of greater than 210 nm for devices operating at 1550 nm using a novel 45° detector mounting scheme. This mounting scheme also flattens the resolution response with wavelength, resulting in greater spectral resolution further from the design wavelength.
Finally a blazed chirped Bragg grating with variable blaze angle was demonstrated to offset fabrication effects and further increase device resolution and sensitivity. To our knowledge this is the first example of Bragg gratings with varying blaze angle along their length. Devices operating at wavelengths close to 780 nm achieved measured resolutions of 0.4 nm to 0.5 nm, though it is suspected that this is limited by aberrations inside the characterisation system. Modelling shows that fully optimised devices operating at 780 nm should achieve resolutions of 0.3 nm, as well as bandwidths exceeding 100 nm.

More information

Identifiers

ORCID for James Field: orcid.org/0000-0002-0985-6062

ORCID for Peter Smith: orcid.org/0000-0003-0319-718X

Catalogue record

Export record