Miniaturised, planar, integrated Bragg grating spectrometer

We present a new, direct laser-written integrated Bragg grating spectrometer, with large bandwidth due to the large blaze angle of the grating. Integrated cost-effective solutions to spectral monitoring are highly desirable for both quick and portable measurements in the lab and miniaturised rugged interrogation systems. Blazed, chirped, Bragg gratings in a planar silica-on-silicon platform provide a route to this. Blazed gratings direct light out of a waveguide mode into radiation modes, the direction of which is dependent on the wavelength of the light. Chirping the blazed grating correctly creates a focus with wavelength-dependent position. Spectra can thus be resolved by interrogating the focal plane using a CCD (Fig. 1). Planar waveguides formed by FHD glass layers ensure light is guided in the device plane, eliminating the requirement to refocus the other dimension. Small blaze-angle devices (<15º) have been previously shown [1,2]. In contrast this work uses large-angle devices (45º) which offer increased bandwidth and are polarisation sensitive. The direct UV-writing method [3] allows for a high grating period detuning range, as well as a high detuning rate, both of which are required to make high resolution devices with short focal length.This work aims to improve upon previous work [4] by increasing the resolution of devices and providing a more complete theoretical framework. Proposed devices have been investigated both theoretically and experimentally and are in good agreement. A simple ray-tracing model, as well as a full angular spectrum model of the response of devices have been used to optimise the focus created by the device.Bragg gratings (with a length of 1 mm) were fabricated using the direct UV writing technique [3] in FHD (flame hydrolysis deposited) silica on a silicon substrate. The grating was blazed at 45º and a period was chosen such that light from the TM mode was ejected perpendicular to the waveguide. The light ejected from the chip was collected using a 55x imaging system and observed on a linear infrared camera (Raptor Photonics OW1.7-VS-CL-640). A narrow linewidth tuneable laser (Agilent 8164B) was used to characterise the system and demonstrate the spectral response of the chip for its suitability as a spectrometer.Fig. 1 Left: Experimental data for the width and normalised intensity distribution of the focal spot for different wavelengths. Middle: Photograph of fabricated device and imaging system. Right: Schematic of the blazed grating spectrometer with an integrated detection system.The UV written Bragg grating produced a Gaussian focal spot with a width of 11.8 μm that moved by 5.2 μm per nm of input wavelength tuning (Fig. 1). The resolution of the device is 2.3 nm, though positions of single, well-resolved peaks can be measured to a precision of 10 pm. Theoretical models show that optical resolution is directly proportional to grating length, this can easily be increased in future devices. Device operation was observed over a 100 nm range, which was limited by the bandwidth of the tuneable laser used.The results of our latest work will be presented, including optimising the resolution and integrating the chip with the detection system (Fig. 1).References[1] C.K.Madsen, J. Wagener, T.A. Strasser, D. Muehlner, M.A. Milbrodt, E.J. Laskowski and J. DeMarco, “Planar Waveguide Optical Spectrum Analyzer Using a UV-Induced Grating,” IEEE J. Quantum Electron. 4(6) 925-929 (1998)[2] C. Koeppen, J.L. Wagener, T.A. Strasser and J. J. DeMarco , “High Resolution Fiber Grating Optical Network Monitor,” NFOEC Proceedings, Session 17 p13-19 (1998)[3] C. Sima, J.C. Gates, H.L. Rogers, P.L. Mennea, C. Holmes, M.N. Zervas and P.G.R. Smith, “Ultra-wide detuning planar Bragg grating fabrication technique based on direct UV grating writing with electro-optic phase modulation,” Opt. Express 21 (13) 15747-15754 (2013)[4] J.W. Field, M.T. Posner, S.A. Berry, R.H.S. Bannerman, J.C. Gates and P.G.R. Smith, “Fabricating a Prototype Spectrometer Using a Large-Angle Direct UV-Written Chirped Tilted Grating”, Advanced Photonics 2018, paper BW2A.