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Due to large coherence length, high detection sensitivity and feasible design, a narrow-linewidth single-frequency laser in the blue spectral range is essential for underwater communication and high-resolution atomic/molecular spectroscopy. By exploiting the efficient frequency doubling of a 978 nm Yb-doped phosphate fiber laser in a fiber-coupled 10 mm long periodically poled LiNbO3, an all-fiber single-frequency blue laser at 489 nm with an ultra-compact and practical design is demonstrated. Over 68 mW of stable narrow-linewidth blue radiation is generated with a conversion efficiency of 30.3%. The estimated laser linewidth is less than 15 kHz, while the measured relative intensity noise is less than –139 dB Hz−1 at frequencies above 20 MHz. The obtained blue laser output is naturally stable with a power stability of ±1.5% over a period of 120 min. [ABSTRACT FROM AUTHOR]

An iterative method based on coupled mode theory and the steepest descent principle in a microwave waveguide is proposed in the optical waveguide for the design of a LP0m mode converter. After iterative design, a LP02-LP01 mode converter and a LP03-LP02 mode converter can achieve 99% conversion efficiency in our model. The 1dB bandwidth of a LP02-LP01 mode converter is measured as more than 130 nm. These optimum waveguides are simulated by RSoft software and indicate high conversion efficiency as well. Furthermore, we investigate the compatibility of the iterative method for mode converter design with initial waveguides of multiple taper shapes. The proposed method is promising for applications in mode division multiplexing systems. [ABSTRACT FROM AUTHOR]

The high order vector mode (HOVM) coupling mechanism is investigated based on the mode matching method (MMM). In the case of strong HOVM coupling where the weakly guiding approximation fails, conventional coupling analysis methods become invalid due to the asynchronous coupling feature of the horizontal and vertical polarization components of HOVM. The MMM, which uses the interference of the local eigenmodes instead of the assumptive modes to simulate the light propagation, is adopted as a more efficient analysis method for investigating HOVM coupling processes, especially for strong coupling situations. The rules of the optimal coupling length, coupling efficiency, and mode purity in microfiber directional coupler are firstly quantitatively analyzed and summarized. Different from the specific input modes, some special new modes would be excited at the output through the strong HOVM coupling process. The analysis of HOVM coupling mechanism based on MMM could provide precise and accurate design guidance for HOVM directional coupler and mode converter, which are believed to be fundamental devices for multi-mode communication applications. [ABSTRACT FROM AUTHOR]

An efficient low-noise, single-frequency 1033 nm master oscillator power amplifier (MOPA) Yb3+-doped phosphate fiber (YPF) laser system is demonstrated. A maximal output power of 612 mW with a laser linewidth of <1.2 kHz is obtained from a 3.7 cm long YPF. By suppressing the intensity noise of the short-cavity phosphate fiber oscillator with a semiconductor optical amplifier, a measured signal-to-noise ratio from the MOPA system of >65 dB and relative intensity noise (RIN) of <−120 dB Hz−1 at the relaxation oscillation frequency are achieved. Moreover, an RIN of <−152 dB Hz−1 is achieved for frequencies above 15.0 MHz, which approaches the shot noise limit. This laser system has applications in the fields of optical frequency standards and beam combining. [ABSTRACT FROM AUTHOR]

An all-fiber Yb-doped kHz-linewidth low-noise linearly polarized single-frequency master-oscillator power-amplifier (MOPA) laser with a stable CW output power of >52 W is demonstrated. By suppressing the intensity noise of the DBR phosphate fiber oscillator, the linewidth of MOPA laser is not noticeably broadened, and an ultra-narrow linewidth of <3 kHz is obtained. Furthermore, the low-noise behavior of MOPA lasers is investigated. A measured relative intensity noise of < −130 dB Hz−1 at frequencies of over 2 MHz, a phase noise above 1 kHz of <5 μrad/Hz1/2, and a signal-to-noise ratio of >63 dB are achieved. [ABSTRACT FROM AUTHOR]

A compact frequency-modulation Q-switched single-frequency fiber laser is demonstrated at 1083 nm. The short linear resonant cavity consists of a 12 mm long homemade Yb3+-doped phosphate fiber and a pair of fiber Bragg gratings (FBGs) in which the Q-switching and the frequency excursion is achieved by a tensile-induced period modulation. Over 375 MHz frequency-tuning range is achieved with a modulation frequency varying from tens to hundreds of kilohertz. The highest peak power of the output pulse reaching 6.93 W at the repetition rate of 10 kHz is obtained. [ABSTRACT FROM AUTHOR]

We demonstrate an optoelectronic feedback method to simultaneously reduce the intensity and frequency noise of a single-frequency phosphate fiber laser for the first time. Based on the fact that the intensity and frequency noise in this laser mainly originate from the pump power fluctuations, we implement a simple and compact feedback loop and achieve a maximum noise reduction of over 20 dB for the intensity noise at frequencies of 0.3–3 kHz and 10 dB for the frequency noise at around 1 kHz. In addition, the laser instability is improved from ±1.7% to ±0.1%, and the laser line shape is observed to be compressed at the central part. [ABSTRACT FROM AUTHOR]