Schematic of the measurement setup. (a) White light transmitted through a source fiber (1500 μm core diameter) is filtered by two independently tunable optical filters. The detection unit consists of a detection fiber (1500 μm core diameter), which is coupled into a CCD spectrometer. (b) A fiber bundle with 7 linearly arranged fibers (200 μm core diameter) served as source fiber, and the image of the fiber bundle on a diffusing screen is recorded by a camera. The spectrum of each fiber is acquired by the spectrometer.
The transmission spectrum, relative to the transmission without any filter, is shown for the setup in Fig. 1(a) with one tunable thin-film optical band-pass filter inserted at the filter rotation angles θ = [0°, 20°, 30°, 40°, 50°] shown below each transmission spectrum, with λC = [563.2 nm, 553.2 nm, 541.6 nm, 527.2 nm, 512.0 nm] and λSB = [18.1 nm, 17.8 nm, 17.6 nm, 17.4 nm, 17.5 nm].
The central wavelength λC (left y-axis) and bandwidth λSB (right y-axis) of the transmission spectrum using one tunable thin-film optical band-pass filter at different angles are shown, as well as the theoretically predicted central wavelength. The bandwidth varies only between 17.3 and 18.1 nm, while the central wavelength could be tuned between 510 nm and 563 nm.
The transmission spectrum with the two tunable thin-film filters is shown relative to the transmission spectrum without filters (solid black line). The two filters were rotated independently to transmit light only at the desired central wavelengths (λ C = 520 nm and λ C = 540 nm) with the desired bandwidths (3 nm ≤ λ SB ≤ 15 nm, 4 nm increments).
The bandwidth of the transmitted light is plotted as a function of the transmitted central wavelength for five bandwidth settings, 3–15 nm (3 nm increments), demonstrating the freely adjustable central wavelength at defined bandwidth settings. The tuning range is limited by the bandwidth setting (maximum bandwidth setting range: 3 nm ≤ λ SB ≤ 16 nm). The transmitted intensity in arbitrary units is denoted for each bandwidth setting at the highest and lowest achievable central wavelength λ C.
The transmitted intensity, i.e., the sum over all pixel values of the transmission spectrum, is plotted as a function of the spectral bandwidth at four central wavelengths, in the range 520–550 nm. It is obvious that for a bandwidth λ SB > 6 nm, the intensity increases linearly with the bandwidth, and for λ SB < 6 nm with a larger slope.
The images of light transmitted through a linearly arranged fiber bundle focused onto a diffusing screen according to the setup in Fig. 1(b) is shown, for (a) counter-directional and (b) equidirectional rotating filters. The position-dependent central wavelength λ C and bandwidth λ SB of the filtered spectrum is shown for both cases. In (a), the central wavelength λ C remained virtually constant (range: 539.8–540.1 nm), while the bandwidth λ SB varied spatially (range: 2.3–12.2 nm). For the outermost left fiber, the transmitted intensity was too low for evaluation, so the bandwidth and central wavelength is indicated only by extrapolating the neighboring values. In (b), the central wavelength λ C varied spatially (range: 542.6–536.1 nm), while the bandwidth λ SB varied only by a smaller amount (range: 6.0–4.7 nm).
Minimum and maximum transmitted central wavelength λ C and bandwidth λ SB for each of the 7 fibers of the fiber bundle at different central wavelengths.
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