A wavenumber linearised spectrometer, the spectrometer comprising: a single-mode optical fiber as an electromagnetic radiation entrance element configured to receive electromagnetic radiation from an external source; collimating optics configured to collimate the received electromagnetic radiation; a dispersive assembly comprising: first and second diffractive gratings, wherein the first diffractive grating, having a spatial frequency of 1417 lines per mm, is configured in a first dispersive stage to receive and disperse the collimated electromagnetic radiation and wherein the second diffractive grating is configured in a second dispersive stage to receive and disperse the electromagnetic radiation dispersed by the first diffractive grating; wherein a first order diffracted beam of the center wavenumber of the bandwidth of the electromagnetic radiation, originating in the single-mode optical fiber, and the bandwidth ranging from 830 nm to 930 nm, makes an output angle at the first grating relative to an input beam at the first diffractive grating and is directed to the second diffractive grating such that an output angle at the second diffractive grating relative to the input beam at the first diffractive grating is reduced relative to the output angle at the first grating; a linear detection array; and an imaging lens assembly configured to image the electromagnetic radiation dispersed by the first and second diffractive gratings onto the linear detection array such that the variation in frequency spacing along the linear detection array is no greater than 10%. a single-mode optical fiber as an electromagnetic radiation entrance element configured to receive electromagnetic radiation from an external source; collimating optics configured to collimate the received electromagnetic radiation; a dispersive assembly comprising: first and second diffractive gratings, wherein the first diffractive grating, having a spatial frequency of 1417 lines per mm, is configured in a first dispersive stage to receive and disperse the collimated electromagnetic radiation and wherein the second diffractive grating is configured in a second dispersive stage to receive and disperse the electromagnetic radiation dispersed by the first diffractive grating; wherein a first order diffracted beam of the center wavenumber of the bandwidth of the electromagnetic radiation, originating in the single-mode optical fiber, and the bandwidth ranging from 830 nm to 930 nm, makes an output angle at the first grating relative to an input beam at the first diffractive grating and is directed to the second diffractive grating such that an output angle at the second diffractive grating relative to the input beam at the first diffractive grating is reduced relative to the output angle at the first grating; a linear detection array; and an imaging lens assembly configured to image the electromagnetic radiation dispersed by the first and second diffractive gratings onto the linear detection array such that the variation in frequency spacing along the linear detection array is no greater than 10%. first and second diffractive gratings, wherein the first diffractive grating, having a spatial frequency of 1417 lines per mm, is configured in a first dispersive stage to receive and disperse the collimated electromagnetic radiation and wherein the second diffractive grating is configured in a second dispersive stage to receive and disperse the electromagnetic radiation dispersed by the first diffractive grating; wherein a first order diffracted beam of the center wavenumber of the bandwidth of the electromagnetic radiation, originating in the single-mode optical fiber, and the bandwidth ranging from 830 nm to 930 nm, makes an output angle at the first grating relative to an input beam at the first diffractive grating and is directed to the second diffractive grating such that an output angle at the second diffractive grating relative to the input beam at the first diffractive grating is reduced relative to the output angle at the first grating; a linear detection array; and an imaging lens assembly configured to image the electromagnetic radiation dispersed by the first and second diffractive gratings onto the linear detection array such that the variation in frequency spacing along the linear detection array is no greater than 10%.