Zhishen Zhang

Narrow linewidth single frequency microfiber laser

A compact 2 kHz linewidth single frequency microfiber ring laser is demonstrated. Microfiber, with a diameter of 1.88 μm, which is drawn from an Er(3+)/Yb(3+) co-doped phosphate glass fiber, serves as the gain medium. By using this microfiber, a double-knot resonator with a total length of 1.75 mm is constructed. Based on this resonator, a narrow linewidth single frequency laser with output power higher than 0.95 μW is obtained at the wavelength of 1536.1 nm. The linewidth of this microfiber laser is as narrow as 2 kHz, and the side-mode-suppression ratio is higher than 38 dB.

Optical fiber design with orbital angular momentum light purity higher than 99.9

The purity of the synthesized orbital-angular-momentum (OAM) light in the fiber is inversely proportional to channel crosstalk level in the OAM optical fiber communication system. Here the relationship between the fiber structure and the purity is firstly demonstrated in theory. The graded-index optical fiber is proposed and designed for the OAM light propagation with the purity higher than 99.9%. 16 fiber modes (10 OAM modes) have been supported by a specific designed graded-index optical fiber with dispersion less than 35 ps/(km∙nm). Such fiber design has suppressed the intrinsic crosstalk to be lower than -30 dB, and can be potentially used for the long distance OAM optical communication system.

High Spatial Resolution BOTDR Based on Differential Brillouin Spectrum Technique

A novel differential-technique Brillouin optical time domain reflectometry with sub-meter spatial resolution is proposed and demonstrated. By analyzing the time-space related property of the pulse excited backward spontaneous Brillouin scattering light, a weighting factor distribution of the Brillouin spectrum along the fiber is obtained. Based on this distribution, a two-step-subtraction technique is proposed. A pulse pair with slightly width difference is employed as the probe pulse. At each corresponding location, for each pulse of the pulse pair a Brillouin-spectrum pair is extracted with two different time sequence length. By performing a two-step subtraction on these two Brillouin-spectrum pairs, the differential Brillouin spectrum is theoretically and experimentally proved to be spatially related with the width difference of the pulse pair. A spatial resolution of 0.4 m utilizing 60/56 ns pulse pair is experimentally achieved over 7.8-km sensing length with 4.1-MHz Brillouin frequency accuracy.

High spatial resolution distributed fiber strain sensor based on phase-OFDR

A novel method to realize high spatial resolution distributed strain measurement is proposed based on phase demodulation scheme of optical frequency domain reflectometry (OFDR). Strain information can be demodulated directly by analyzing the phase change of Rayleigh backscattered light. Strain location can be obtained with high spatial resolution by cross-correlation method using a wide scanning range of tunable laser source. Based on the above scheme, breakpoint detection with 0.1 mm spatial resolution has been demonstrated, static and dynamic strain up to 100 Hz could be distributedly measured with 10 cm spatial resolution over 200 m sensing fiber, and the minimum measurable strain is about 1 με.

Single-Frequency Microfiber Single-Knot Laser

A compact single-frequency microfiber laser was demonstrated in a single-knot Er3+/Yb3+ co-doped phosphate glass fiber ring with a diameter of 397.5 µm. The lasing wavelength is 1534.25 nm and the side-mode suppression ratio (SMSR) is higher than 27 dB. The linewidth is measured to be about 5 kHz and the relative intensity noise (RIN) is around -110 dB/Hz at frequencies over 50 kHz.

All-fiber mode-locked laser based on microfiber polarizer.

A novel all-fiber mode-locked fiber laser based on microfiber polarizer is proposed and demonstrated. The microfiber polarizer is composed of two pieces of microfibers that are finely manipulated to be partly overlapped. Because of the asymmetric cross section, the microfiber polarizer shows a strong birefringence that ultimately induces a high polarization-selective feature. Compared with other polarizers, the microfiber polarizer owns the merits of simpler fabrication, lower cost, broader band, and more compact size. The polarization extinction ratio of the microfiber polarizer is 26 dB, and the stable pulse sequence with the duration of 2.9 ps is generated from this microfiber polarizer based all-fiber mode-locked laser.

Tunable Dual-Wavelength Narrow-Linewidth Microfiber Laser

A compact and tunable dual-wavelength narrow-linewidth microfiber laser is demonstrated with a novel and high-Q double-knot resonator. Each wavelength laser spectrum delivers a 2 kHz linewidth and an optical side-mode suppression ratio of over 31 dB. The laser wavelength and frequency separation between the two wavelength lasers can be tuned freely by changing the coupling length and position between the taper-drawn fiber and the double-knot resonator.

Low-crosstalk orbital angular momentum fiber coupler design

A fiber coupler for low-crosstalk orbital angular momentum mode beam splitter is proposed with the structure of two separate and parallel microfibers. By properly setting the center-to-center distance between microfibers, the crosstalk is less than -20 dB, which means that the purity of the needed OAM mode in output port is higher than 99%. For a fixed overlapping length, high coupling efficiency (>97%) is achieved in 1545-1560 nm. The operating wavelength is tuned to the whole C-band by using the thermosensitive liquid. So the designed coupler can achieve the tunable coupling ratio over the whole C-band, which is a prospective component for the further OAM fiber system.

Refractive index fiber sensor based on Brillouin fast light

A new type of refractive index fiber sensor was invented by combining the evanescent-field scattering sensing mechanism with the Brillouin fast light scheme. Superluminal light was realized using Brillouin lasing oscillation in a fiber ring cavity. The refractive index of the solution around the microfiber within the cavity is related to the group velocity of the fast light. This fast light refractive index sensor offers an alternative for high-accuracy sensing applications.