Fig. 3: Experimental layout and measured and calculated properties of the HCF and the homemade pump source.

a Experimental layout for 4 μm laser generation. b SEM images of the cross section of the HCF. c Comparison between simulated and measured loss of the HCF. d Measured spectrum of the pump system seeded by chosen absorption line wavelengths. e Output wavelength of the pump system changing with the Vtec of the R(5) diode laser. f Measured absorption line shapes of HBr under pressures of 2.2, 4.8 and 6.5 mbar seeded by the R(3) absorption line. g Variation in the measured residual pump power with time when tuned to the centre of the R(3) absorption line