Chun Gu

Low-threshold single-wavelength all-fiber laser generating cylindrical vector beams using a few-mode fiber Bragg grating

We propose and demonstrate a low-threshold single-wavelength all-fiber laser generating cylindrical vector beams using a few-mode fiber Bragg grating. Both radially and azimuthally polarized beams have been generated with very good modal symmetry and polarization purity higher than 94%. The radially and azimuthally polarized modes can be switched by simply adjusting the polarization controllers built in the fiber laser cavity. This fiber laser operates at a single wavelength of 1053 nm with a 3 dB linewidth of less than 0.02 nm, signal-to-background ratio of more than 55 dB, and a threshold as low as 16 mW. A new method for the polarization purity measurement is also proposed.

Square-wave pulse with ultra-wide tuning range in a passively mode-locked fiber laser

We report the generation of ultrawide tunable square-wave pulse in an erbium-doped mode-locked fiber laser. The pulse width can be tuned in an ultrawide range of more than 1700 ns by simply increasing the pump power. The pulse-width tuning is 5.1 ns/mW. To the best of our knowledge, this is the widest pulse-width tuning range of any square-wave pulse in an all-fiber passively mode-locked fiber laser. Experimental results show that the fiber nonlinearity plays an important role in the tuning range of the output pulse width. The high nonlinearity helps to increase the tuning range of the pulse width.

An all-fiber laser generating cylindrical vector beam

We proposed and demonstrated an all-fiber laser to generate cylindrical vector beam. A pair of fiber-based collimators was used to select the cylindrical vector beam operating. The radially and azimuthally polarized modes can be switchable just by applying pressure to a section of fiber in our fiber laser system. A 70 cm long Yb-doped fiber was used as gain medium and the lasing wavelength was around 1030 nm.

Width and amplitude tunable square-wave pulse in dual-pump passively mode-locked fiber laser

We have proposed and demonstrated a figure-8 dual-pump passively mode-locked fiber laser to generate square-wave pulse tunable by both width and amplitude. Just by simply adjusting the power of the pumps, both the amplitude and width of the output square-wave pulse can be tuned independently and continuously. One pump is used to tune the output pulsewidth while the other is used to tune amplitude.

Wavelength-tunable Yb-doped passively Q-switching fiber laser based on WS 2 saturable absorber

We have fabricated an Yb-doped passively Q-switching fiber laser based on WS(2) saturable absorber. Both the operating wavelength and the repetition rate can be tuned in a wide range. The operating wavelength can be continuously tuned from 1027 nm to 1065 nm under the Q-switching state at a fixed pump power, while the repetition rate increases from 60.2 kHz to 97.0 kHz by varying pump power at a fixed wavelength of 1048.1 nm. The shortest pulse duration of 1.58 µs was observed. To the best of our knowledge, its the first demonstration of WS(2)-based passively Q-switching fiber laser with a wide tunable range at 1.0 μm band.

Mode-locked all-fiber laser producing radially polarized rectangular pulses

We propose and demonstrate a radially polarized mode-locked fiber laser through the use of a figure-8 cavity in combination with cascade fiber Bragg gratings (FBGs). The mode-locked laser emits rectangular pulses with width tunable from 2.8 to 23 ns under an increasing pump power at 1056.3 nm with 0.2-nm 30-dB linewidth. A polarization purity as high as 96% for the output transverse mode has been achieved simultaneously.

Actively mode-locked all fiber laser with cylindrical vector beam output.

We demonstrated an all fiber actively mode-locked laser that emits a cylindrical vector beam. An intra-cavity few-mode fiber Bragg grating inscribed in a short section of four-mode fiber is employed to provide mode selection and spectrum filtering functions. Mode coupling is achieved by offset splicing between the single-mode fiber and the four-mode fiber in the laser cavity. A LiNbO3 Mach-Zehnder modulator is used to achieve active mode-locking in the laser. The laser operates at 1547 nm with 30 dB spectrum width of 0.2 nm. The mode-locked pulses have a duration of 2 ns and repetition of 12.06 MHz. Through adjusting the polarization state in the laser cavity, both radially and azimuthally polarized beams have been obtained with high mode purity.

Tungsten disulphide based all fiber Q-switching cylindrical-vector beam generation

We proposed and demonstrated an all fiber passively Q-switching laser to generate cylindrical-vector beam, a two dimensional material, tungsten disulphide (WS2), was adopted as a saturable absorber inside the laser cavity, while a few-mode fiber Bragg grating was used as a transverse mode-selective output coupler. The repetition rate of the Q-switching output pulses can be varied from 80 kHz to 120 kHz with a shortest duration of 958 ns. Attributed to the high damage threshold and polarization insensitivity of the WS2 based saturable absorber, the radially polarized beam and azimuthally polarized beam can be easily generated in the Q-switching fiber laser.

Generation of Optical Vortices Using a Helical Fiber Bragg Grating

We present an effective method to generate optical vortices (OVs) by using a helical fiber Bragg grating (H-FBG) written directly on the few-mode fiber. The helical index modulation is obtained by rotating fiber during single-side UV exposure using a phase mask. A set of coupled-mode equations are formulated to analysis reflection characteristics of the H-FBG. We propose that stable OVs can be achieved with a proper design M-fiber, and the order of the OVs can be adjusted by tuning the resonant wavelength and the handedness of the H-FBG.

Speckle Suppression by Controlling the Coherence in Laser Based Projection Systems

In this paper, the relationship between the coherence and the speckle contrast in a laser-based projection display system is studied with the consideration of human visual percept. By using a varifocal liquid-crystal lens and a monochromatic charge-coupled device (CCD), a system which is used for simulating a human eye is set up to record the speckle pattern. An efficient method for controlling the spatial coherence by using a dielectric elastomeric actuator (DEA) is demonstrated. We find that there is a limitation for eliminating the speckle contrast in the laser projection system when only the method of reducing the spatial coherence is used. When the distance between the observer and the screen is sufficiently large ( > 3 m), the speckle contrast can be eliminated well ( ) by reducing the spatial coherence and the observer do not observe the speckle phenomenon. When the viewing distance is smaller than 3 m, however, the observer will feel the speckle all of the time. To further reduce the speckle contract, a method of wavelength modulation is used to reduce the temporal coherence while the spatial coherence is well reduced. The speckle contrast is reduced to 4% at the viewing distance of 1 m by reducing both the temporal and spatial coherence of the laser.