Low-loss optical silica micro/nanofibers (MNFs), also called sub-wavelength-diameter fibers (SWDFs), have raised much attention in the areas of nonlinear optics [
1], optical-fiber communication [
2], microwave photonics [
3,
4], sensing [
5], physics [
6], and biology [
7] in the past few years. Due to the large difference of refractive index between air cladding and silica core, MNFs can confine the guided light on a sub-wavelength scale and show high nonlinear coefficients which are usually hundreds of times larger than those of conventional optical fibers. The high nonlinear coefficients will be helpful in various photonic applications [
1,
8-
11]. So far, several methods, which mainly rely on flame-blushing techniques [
12-
15], have been developed to fabricate low-loss silica MNFs. However, for the flame-blushing techniques, expensive and sophisticated mechanical system will be utilized in order to control the nanofiber diameter with high accuracy. As we know, MNF diameter is a determining factor which influences the effective refractive index of optical guided modes in these waveguides. On one hand, when propagating along air-cladding silica MNFs, optical signals experience large dispersion which is sensitive to MNF diameter [
16] and is harmful to those optical signal processing functions based on four-wave mixing (FWM) and cross-phase modulation (XPM). For instance, the dispersion reaches as high as 1622 and 604 ps/(nm
-1·km
-1) for 0.8 and 1-µm-diameter silica MNFs at 1.55 µm wavelength, respectively. Furthermore, the fabrication tolerance of several nanometers will shift zero-dispersion wavelength a few nanometers away, which will yield strong “walk-off” effects and greatly decrease the conversion efficiency of FWM and XPM. On the other hand, micro/nanofiber gratings have aroused wide interests recently [
17-
19]. For silica MNF Bragg gratings, central reflective wavelength (CRW) will be apparently changed with MNF diameter. Theoretically, the CRW will be blue-shifted as many as tens of nanometers if the diameter decreases 10 nm. Therefore, controlling the diameter accurately is of great importance. The etching technique is an effective method to fabricate micro/nano devices. However, it will take several hours to etch a 125-µm-diameter optical fiber into a nanofiber [
20,
21]. On the other hand, high-concentration hydrogen fluoride acid used to shrink optical fiber is volatile [
20]. The evaporation makes the etching process unstable over long etching period and thus it is not convenient to
in situ monitor MNF diameter.