Fig. 1. Schematic diagram of a microfiber and surface morphologies of the GaSe-transferred microfiber. (a) Schematic of the microfiber for exciting SH, with the crystal structure and phase matched modes shown in the inset. (b) Optical microscope image of the GaSe-transferred microfiber under bright field, with the scattering light from the GaSe coating shown in (c). (d, e) In-situ AFM images of the GaSe layer on the microfiber, with two different thicknesses (72 and 50 nm) found. (f) Raman spectrum of the used GaSe layer recorded with 532 nm light, with$A_{\text{1g}}^{\text{1}}$,$E_{\text{2g}}^{\text{1}}$ and$A_{\text{1g}}^{\text{2}}$ vibrational modes identified. (g) Measured polarization-dependent SH signal, fitted by the theoretically calculated SH under the 56% equivalent coverage ratio. SH, second-harmonic; AFM, atomic force microscope.

Fig. 2. Experimental setup for exciting and measuring harmonics. 532 nm SH emitted from the microfiber surface and the end face of the fiber are illustrated in the top and bottom insets, respectively. SH, second-harmonic; SMF, single-mode fiber; MMF, multi-mode fiber; DM, dichroic mirror; CWDM, coarse wavelength division multiplexer.

Fig. 3. (a) Spectra of SH at 775 nm and TH at 516.3 nm, pumped by a 0.868 mW picosecond laser. (b) Collected local SH intensity along the axial position of microfiber surface. (c) Power-dependence of SH pumped by the 1550 nm picosecond laser. (d) Power-dependence of SH when pump average power of the CW laser is tuned from 0.7 to 14.5 mW. SH, second-harmonic; TH, third harmonic; CW, continuous-wave.

Fig. 4. (a) Typical spectrum of SH and SF at six frequency conversion wavelengths (SH: 635/775/795 nm, SF: 698.05/706.05/784.87 nm), pumped by three CW lasers at 1270/1550/1590 nm simultaneously. (b) Intensity evolution when pump power of TL₁ is varied from 0 to 10 mW. (c) Log-log plotted intensities of three frequency conversion signals (SH: 635 nm, SF: 698.05/706.05 nm) that increased with the pump power of 1270 nm laser. Similar power dependences were investigated by changing pump power of TL₂ and TL₃ respectively, with their evolutions and log-log plotted slopes illustrated in Fig. 4(d–g). SH, second-harmonic; SF, sum-frequency; CW, continuous wave; TL, tunable laser.

Fig. 5. (a) Spectral evolution of broadband SH centering around 773 nm when the pump power of SLED source is varied from 1.8 to 13.9 mW, and corresponding power dependence is log-log plotted in (b). (c) The comparison of spectral widths between SH continuum and corresponding pump source of 7.3 mW. SH, second-harmonic; SLED, Superluminescent light-emitting diode.

Table 1. Comparison of SH efficiency or enhancement in different fibers.