Fig. 1. Schematic of the FFPC for lasing. (a) The FFPC is formed by a concave fiber mirror and a flat mirror. An Er3+/Yb3+ co-doped silica film (thickness of 35.1 μm) is set inside the cavity and bonded onto the flat mirror. (b) An interferometric image of the concave surface of the fiber mirror (with the scale bar of 30 μm), and the radius of curvature of the concave mirror is calculated as 100 μm from the concentric fringes.

Fig. 2. Measurement of cavity longitudinal modes. (a) The scheme to measure the transmission and reflection of the FFPC. A tunable laser is coupled into the fiber cavity, and the transmitted (reflected) light is detected by PD1 (PD2). A transmission (reflection) spectrum is obtained by scanning the laser wavelength from 1460 to 1570 nm. (b) A two-dimensional transmission spectrum by gathering 400 sets of transmission spectrum data as a function of the cavity length L. (c), (d) Measured transmission and reflection spectra of the bare cavity (film-in cavity) and corresponding FSR in length to 22.07 nm (17.6 nm), respectively. (e), (f) Close-up view of the peaks marked in (c) and (d), respectively. Lorentzian line shape is used to fit the data, and the corresponding fitted linewidth δν is 10 pm (17 pm).

Fig. 3. (a) Image of the assembled microlaser device. A thick stainless steel bracket is used to support the device, the fiber mirror is glued on a shear PZT, and the flat mirror is mounted on the right part of the bracket. (b) Schematic of the fiber cavity laser. A 980 nm laser is used as the pumping source, and a fiber-based WDM (980/1550 nm) is used to separate the 1550 nm laser output from the 980 nm pumping input. (c) Laser threshold measurement of the fiber cavity laser. The threshold is 210 μW with pump wavelength at 980 nm and lasing wavelength at 1538 nm. (d) The lasing spectrum beyond the threshold, which indicates a single longitudinal mode. (e) Tunable range measured by a wavelength meter; (f) is the zoomed chart of the shade range in (e), which shows that the laser tunable range without mode hopping can reach 10 nm.

Fig. 4. Waterfall diagram of emission spectrum of the cavity laser. It shows the intensity variation versus the tunable wavelength.

Fig. 5. Mechanical bandwidth of the laser device by measuring the driving response. (a) The envelope of the transmission spectrum in green and the spectrum response driven by the small sinusoidal signal. (b) Frequency response diagram of the assembled laser device. From both magnitude and phase variations, the first direct resonance is indicated at 60 kHz.

Fig. 6. Measurement of the laser linewidth. (a) Schematic of laser linewidth measurement, showing the Michelson interferometer with two arms: a short arm with a fiber phase shifter and a long arm producing optical path difference by different-length delay. Each arm is retro-reflected by the Faraday reflector and a fiber attenuator to guarantee the same intensity of the two arms. The interference fringes of the two arms are shown on an oscilloscope. (b) The interference fringes at the delay length of 20 m (fit overlaid in green). (c) The measured results (blue) are fitted by the theoretical curve (orange) corresponding to the laser linewidth of 3.1 MHz.

Fig. 7. Stability measurement of the laser intensity, corresponding to the stability of ±0.85% for 4 h.