A mode-locked all-fiber laser based on nonlinear multimode interference effect of the graded index fiber (GIMF) is proposed and demonstrated. The saturable absorber (SA) used in the laser consists of a step index multimode fiber connected by a graded index multimode fiber. Compared with the theoretically proposed SMF-GIMF-SMF structure, the introduction of the step index multimode fiber (SIMF) eliminates the restriction on the length of GIMF and provides a more flexible method of making a SA based on the NL-MMI. Through bending the device to a certain state, the modulation depth of the SA was measured to be 3.16%. The mode-locked laser output pulses have the pulse width of ~446 fs, bandwidth of 4.48 nm at the fundamental repetition rate of 11.73 MHz. This versatility and simplicity of the SA device combined with the possibility of scaling the pulse energy in large mode area double clad fiber laser make it highly attractive in ultrafast photonics.

A novel mode-locking method based on the nonlinear multimode interference in the stretched graded-index multimode optical fiber (GIMF) is proposed in this Letter. The simple device geometry, where the light is coupled in and out of the stretched GIMF via single-mode fibers, is demonstrated to exhibit the temporal intensity discrimination required for mode locking. The nonlinear saturable absorber (SA) characteristics of the device are controllable by simply adjusting the strength of the stretching applied. The modulation depth of the device, which consists of $\sim 23.5\mathrm{cm}$ GIMF, is tuned from 10.37% to 22.27%. Such a simple SA enables the wavelength-switchable mode-locking operation in a ring Er-doped fiber laser, and ultrafast pulses with a pulse width of 506 fs at 1572.5 nm and 416 fs at 1591.4 nm were generated. The versatility and simplicity of the SA device, together with the possibility of scaling the pulse energy, make it highly attractive in ultrafast photonics.

One of the efficient techniques to enhance the sensitivity of optical fiber sensors is by use of vernier effect. However, complex system structure, precisely controlled device fabrication or expensive materials required creates the difficulty in the application of the technique. Here we proposed a highly sensitive optical fiber strain sensor base on two cascaded Fabry-Perot interferometers and vernier effect. The two interferometers, one is for sensing and the other is for reference, are formed by two pairs of in-fiber reflection mirrors fabricated by inducing refractive-index-modified area in the fiber core by femtosecond laser pulse illumination. A relatively large distance between the two Fabry-Perot interferometers needs to be used to ensure the independent operation of the interferometers. The fabrication of the device is simple and the cavity length can be precisely controlled by the computer controlled three-dimensional micromachining platform. Moreover, as the device is based on the inner structure inside the optical fiber, good robustness of the device can be guaranteed. The experimental results obtained show that the strain sensitivity of the device is ~28.11 pm/°C while the temperature sensitivity achieved is ~278.48 pm/°C.

A micro-cavity fiber Fabry-Perot interferometer based on dual capillaries is proposed and demonstrated for gas pressure measurement. Such a device is fabricated by fusion splicing of a tiny segment of a main-capillary with a feeding-capillary on one end, and a single mode fiber on the other, to allow gas enters the main-capillary via the feeding-capillary. The reflection spectrum of the interferometer device shifts with the variation of gas pressure due to the dependence of gas refractive index on the pressure applied. During the device fabrication process, a core-offset fusion splicing method is adopted, which turns out to be highly effective for reducing the detection limit of the sensor. The experimental results obtained show that the proposed device exhibits a high gas pressure sensitivity of 4147 pm/MPa, a low temperature cross-sensitivity of less than 0.3 KPa/°C at atmospheric pressure, and an excellently low detection limit down to ~4.81 KPa. The robust tip structure, ultra-compact device size and ease of fabrication make the device an attractive candidate for reliable and highly sensitive gas pressure measurement in a precise location.

A hybrid no-core fiber (NCF)–graded index multimode fiber (GIMF) structure is used as a saturable absorber (SA) for mode-locked laser operation. Such an SA supports various types of soliton outputs. By changing the cavity parameters, not only the spatiotemporal mode-locking states with a stable single pulse but also tightly and loosely bound solitons are generated. Single 35.5 pJ solitons centered at 1568.5 nm have a 4 nm spectral full-width at half-maximum and an 818 fs temporal duration. Tightly bound soliton pairs with continuously tunable wavelength from 1567.48 nm to 1576.20 nm, featured with an $\sim 700\mathrm{fs}$ pulse train with a separation of 2.07 ps, have been observed by stretching the NCF-GIMF structured device. Meanwhile, several different pulse separations from 37.57 ps to 56.46 ps of loosely bound solitons have also been realized. The results provide help in understanding the nonlinear dynamics in fiber lasers.

An Er-doped mode-locked fiber laser with a saturable absorber based on single mode - graded index multimode - single mode fiber (SMF-GIMF-SMF) with inner micro-cavity is demonstrated. The modulation depth of the saturable absorber was measured to be 1.9% when the SMF-GIMF-SMF structure is bent to a certain state. Such a simple saturable absorber enables the mode-locking operation in a ring Er-doped fiber laser and ultrafast pulses with pulse energy of 0.026 nJ and pulse width of 528 fs at the fundamental repetition rate of 14.34 MHz can be generated. In addition, the harmonic mode-locking operation can also be achieved.