For developing a continuous-wave (CW) tunable Terahertz-wave (THz-wave) source using difference-frequency generation (DFG) in highly nonlinear optical crystals, we proposed and demonstrated a dual-wavelength fiber ring laser system operating around 1060 nm based on wideband chirped fiber Bragg gratings (CFBGs) and semiconductor optical amplifier (SOA). Thermo-induced phase shift along the CFBG produces a very sharp transmission spike therefore two lasing wavelengths with single longitudinal mode operation are oscillating simultaneously within the fiber ring cavity. Due to the inhomogeneous gain broadening property of SOA, the wavelength spacing of our dual-wavelength fiber laser can be continuously adjusted from 0.3 to 9.5 nm. By using this single emitter dual-wavelength fiber laser to pump an organic nonlinear DAST crystal, type-0 collinear phase matching of DFG process can be fulfilled and monochromatic THz wave ranging from 0.5 to 2 THz has been successfully generated.

Tunable Terahertz-wave generation from DAST crystal pumped by a monolithic dual-wavelength fiber laser.

A broadband tunable, single-longitudinal-mode (SLM) ytterbium fiber laser based on a passive multiple-ring cavity (MRC) technique is proposed and demonstrated experimentally for the first time to our knowledge. Two different short ring cavities are inserted into the main ring cavity and serve as wideband mode filters to ensure SLM oscillation. With 1-m ytterbium-doped fiber as the gain medium, the SLM operation is achieved with over 60-nm wavelength tuning range at 100-mW pump power. The laser is very stable with output power of 6 dBm and an optical signal-to-noise ratio of higher than 53 dB in all the 60-nm tuning range.

60-nm-Wide Tunable Single-Longitudinal-Mode Ytterbium Fiber Laser With Passive Multiple-Ring Cavity

A novel tunable fiber Fabry-Perot (FP) filter is proposed and demonstrated by using a hollow-core photonic bandgap fiber (HC-PBF) and a micro-fiber. The interference cavity is a hollow core of HC-PBF. One of the reflection mirrors is the splicing point between a section of HC-PBF and a single mode fiber. The other reflection mirror is a gold-coated end of micro-fiber that uses chemical etching process to obtain the similar diameter as the core of HC-PBF. Hence the movable mirror can be adjusted with long distance inside the hollow core of HC-PBF. Tunable FP filter is used as a mode selecting component in the reflection mode to implement stable single longitudinal mode (SLM) operation in a ring laser. With FP cavity length of 0.25 ± 0.14 mm, the wavelength of SLM laser can be tuned over 1554-1562 nm with a tuning step of 0.2-0.3 nm, a side-mode suppression ratio (SMSR) of 32-36 dB and a linewidth of 3.0-5.1 kHz. With FP cavity length of 2.37 ± 0.37 mm, the SLM laser can be tuned over 1557.3-1560.2 nm with a tuning step of 0.06-0.1 nm, a SMSR of 44-51 dB and a linewidth of 1.8-3.0 kHz.

Tunable Fabry-Perot filter using hollow-core photonic bandgap fiber and micro-fiber for a narrow-linewidth laser.

We demonstrate a multiwavelength single-longitudinal-mode (SLM) ytterbium-doped fiber laser. Two Sagnac loop mirrors are inserted into the cavity to ensure multiwavelength and SLM operation, respectively. Three wavelengths with a wavelength spacing of 1.8 nm are generated under stable operation, and the linewidth of each wavelength is measured to be narrower than 100 kHz. A side-mode suppression ratio of greater than 56 dB is achieved for each wavelength. The maximum power fluctuation was less than 0.8 dB for each wavelength. This scheme has the potential to be employed as a useful source in optical communications, fiber optical sensing, and high resolution spectroscopy in the 1-μm wavelength band.

Multiwavelength Single-Longitudinal-Mode Ytterbium-Doped Fiber Laser

A 1,064 nm band Ytterbium doped fiber laser (YDFL) is proposed and investigated. Using 70 cm gain fiber and recycling pumping scheme, a high slope efficiency of 50 % and low threshold power of 53 mW are obtained. The power variation is less than 0.1 dB in 13 nm tuning span. Using the Vernier effect, near single frequency lasing is realized for the whole tuning range in 2 dBm output power. Have advantages of simpler structure, larger pump slope efficiency and possible shorter cavity, the YDFL may find potential applications in various ways.

Single longitudinal mode Ytterbium doped fiber lasers with large proposed tuning range

A compact single-frequency 1060 nm fiber laser based on a high-power semiconductor optical amplifier (SOA) has been proposed and demonstrated with over 40 nm wavelength tuning range. The inhomogeneous gain-broadening feature of the SOA and the unpumped ytterbium-doped fiber-based saturable absorber contribute to the single longitudinal mode lasing of the fiber laser. The maximum 3 dBm output power can be obtained with the optical signal to noise ratio larger than 50 dB. Stable single-frequency operation can be maintained more than 1 h with negligible power fluctuations and low relative intensity noise.

Xiaona Lu

Papers

Single-frequency 1060 nm semiconductor-optical-amplifier-based fiber laser with 40 nm tuning range.