Experimental demonstration of single-mode fiber coupling using adaptive fiber coupler

Coupling plane wave into a single-mode fiber （SMF） with high and steady coupling efficiency is crucial for fiber- based free-space laser systems, where random angular jitters are the main influencing factors of fiber coupling. In this paper, we verified a new adaptive-optic device named adaptive fiber coupler （AFC） which could compensate angular jitters and improve the SMF coupling efficiency in some degree. Experiments of SMF coupling under the angular jitter situation using AFC have been achieved. Stochastic parallel gradient descent （SPGD） algorithm is employed as the control strategy, of which the iteration rate is 625 Hz. In closed loop, the coupling efficiency keeps above 65% when angular errors are below 80/3tad. The compensation bandwidth is 35 Hz at sine-jitter of 15 ~rad amplitude with average coupling efficiency of above 60%. Also, experiments with simulated turbulence have been studied. The average coupling efficiency increases from 31.97% in open loop to 61.33% in closed loop, and mean square error （MSE） of coupling efficiency drops from 7.43% to 1.75%.

Wavelength dependent loss of splice of single-mode fibers

After reviewing three different definitions of mode field diameter of single-mode fibers, coupled efficiency calculation methods associated with lateral offset, longitude separation and wavelength, the effects produced by them, and the influences of splicing defects were discussed in detail. The regularities of the effects were studied according to the first order derivation of couple efficiency formula, and a simplified formula for couple efficiency calculation was presented under the circumstance of slight misalignment, with respect to wavelength, 2, and in a good agreement with the theoretical model. The simplified formula provides a new but simple approach to evaluate wavelength dependent couple efficiency of single-mode fibers. Theoretical analyses and numerical calculations show that, when those defects exist, the wavelength produces additional effects on the couple loss that growth of wavelength causes an increase on the couple efficiency for the lateral offset or longitude separation whereas lessens the couple efficiency due to angular misalignment or mode fields mismatching, and that the wavelength degrades the couple efficiency distinctly when λ≥2.5 μm whereas it distorts the couple slightly in range of λ≤2λ≤2 μm.

Research on the influence of ocean temperature on XCTD profiler single-mode fiber transmission channel

XCTD, as one of the most important instruments for the deep sea exploration, is an important device for deep-sea hydrological data acquisition. But some difficult technical problems of traditional metal enameled wire channel have become the major bottleneck for XCTD development. Aiming at this problem, this paper puts forward with using single-mode fiber as the transmission channel of XCTD. Firstly, this paper makes a brief analysis on the problems of using enameled wire as transmission channel faces. Secondly, it analyzes the advantages of the single mode fiber technology. Finally, it makes theoretical research and experimental verification of the influence of seawater temperature change on the optical fiber transmission channel. The experimentat results show that the error rate at the transmission rate of 10 MB/S is 0, and the loss can be negligible when the single-mode fiber channel is used in the sea water and the seawater temperature changes from 0 to 20℃. This method will greatly increase the rate of signal transmission and the transmission stability. And this paper shows that using the single-mode fiber as the transmission channel of XCTD has certain feasibility.

Frequency-stabilized Yb：fiber comb with a tapered single-mode fiber

We demonstrate a stable Yb：fiber frequency comb with supercontinuum generation by using a specially designed tapered single-mode fiber, in which a spectrum spanning from 500 nm to 1500 nm is produced. The carrier-envelope offset signal of the Yb：fiber comb is measured with a signal-to-noise ratio of more than 40 dB and a linewidth narrower than 120 kHz. The repetition rate and carrier-envelope offset signals are simultaneously phase locked to a microwave reference frequency.

Design of a Tapered Lens for Semiconductor Laser to Single-Mode Fiber Coupling

The theoretical investigation of the coupling efficiency of a laser diode to a single mode fiber via a hemispherical lens on the tip of the tapered fiber in the presence of possible transverse offset and angular mismatch is reported.Without the misalignment,coupling efficiency increases with the decreasing of taper length.With the misalignment,this relation is that the coupling efficiency decreases with each kind of offset.

Optical Pulse Compression Based on High-doped Erbium Fiber Amplifier and Standard Single-mode Fiber

Proposed is a novel optical pulse compression technique based on high-doped erbium fiber amplifier and standard single-mode fiber(SMF). We used the amplifier with the erbium ion concentration of 6.3×10-3 to amplify a hyperbolic secant pulse from a regeneratively mode-locked fiber laser. The central wavelength, pulsewidth and peak power of the pulse are 1 550 nm, 12.5 ps and 3 mW, respectively. Then the amplified pulse with peak power level corresponding to a higher-order soliton is compressed when it propagates through a 3-km-long single-mode fiber. Studied are the compressed pulses under different pump powers and fiber lengths. The results show that it can get a narrower pulse, and solve the difficulty that pulses at low power can not be compressed directly in the fiber. And the construct is compact.

Effects of Input Spectra on the Threshold of Modulation Instability in a Single-Mode Fiber

We investigate modulation instability (MI) in the presence of a monochromatic spectrum and on the background of a broadband amplified spontaneous emission spectrum induced by an erbium-doped fiber amplifier,respectively.For the former case,the MI threshold is ~110mW,which agrees well with the theoretical value.However,for the latter case,the threshold is as high as ~170mW,which indicates that the MI threshold depends on the shape of the input spectrum and that the broadband spectrum has a higher threshold than the monochromatic one.

Multi-Beam Interference Transmission Spectrum Observed from an Eccentric Core Single-Mode Fiber

An eccentric core single-mode fiber,whose core is away from the axis of the fiber,is fabricated by using the fiber preform goniometric-groove method and the stack-and-draw method.An eccentric core single-mode fiber without coating is spliced between two single-mode fibers.Fundamental mode and cladding modes are excited at one splicing point between the eccentric core single-mode fiber and the single-mode fiber,and propagate in the eccentric core single-mode fiber,then interfere with each other at the other splicing point.Multi-beam interference transmission spectrum is observed.An in-line fiber interferometric strain sensor based on the eccentric core single-mode fiber is realized.

Gain Improvement of Fiber Parametric Amplifier via the Introduction of Standard Single-Mode Fiber for Phase Matching

We investigate and experimentally demonstrate the gain improvement of single-pump fiber parametric amplifiers by using a suitable length of standard single-mode fiber(SSMF)inserted between two high-nonlinear fibers(HNLFs).The SSMF with different dispersion properties from the HNLF is employed for phase matching of the parametric processes,and the signal gain is improved as compared with the absence of the SSMF with experiment results.Furthermore,the effect of the input pump power and the fiber length of the SSMF on the output signal gain are discussed with experimental investigation.

Derivation of General Formula for Coupling Loss of Single-mode Fiber Collimators with Gradient-index Rod Lenses

Gradient-index rod lens (GRIN-lens) whose pitch is ordinary value with bevel ferrule coupling system is analyzed, an equivalent method which can be used to analyze this system is put forward, and a general formula for determining the coupling loss with axes mismatch, radial mismatch, and angular mismatch is derived by use of the Gaussian field approximation and mode-field coupling theory. The experimental results are in good agreement with the theory prediction. It indicates that these formulas are suitable to analyze the gradient-index rod lens coupling system with pigtail fiber.

2 kW single-mode fiber laser employing bidirectional-pump scheme

A 2 kW single-mode fiber laser with two cascade home-made cladding light strippers(CLSs) has been demonstrated by employing bidirectional-pump scheme.2.009 k W signal power is obtained when pump power is 2.63 kW and the slope efficiency is 76.6%.Raman Stokes light is less than-47 d B at 2.009 kW even with a 10-m delivery fiber with core/inner cladding diameter of 20 mm/400 mm.The beam quality M^2≤1.2 and the spectral FWHM bandwidth is 4.34 nm.There is no transverse mode instability and the output power stability of ±0.14% is achieved by special thermal management for a more uniform temperature distribution on the Yb-doped gain fiber.

5 kW-level single-mode fiber amplifier based on low-numerical-aperture fiber

A low-numerical-aperture(NA)concept enables large-mode-area fiber with better single-mode operation ability,which is beneficial for transverse mode instability and nonlinear effects suppression.In this contribution,we reported a high-power fiber amplifier based on a piece of self-developed large-mode-area low-NA fiber with a core NA of 0.049 and a core/inner cladding diameter of 25/400μm.The influence of the pump wavelength and fiber length on the power scaling potential of the fiber amplifier is systematically investigated.As a result,an output of 4.80 kW and a beam quality factor of~1.33 were finally obtained,which is the highest output power ever reported in a fiber amplifier exploiting the low-NA fiber.The results reveal that low-NA fibers have superiority in power scaling and beam quality maintenance at high power levels.

High efficiency of Brillouin scattering behavior in single-mode Ge–As–Se–Te fibers at 2μm

The Brillouin characteristics of step-index Ge–As–Se–Te(GAST)fibers at 2μm are designed and simulated on the basis of optical and acoustic properties.The refractive indexes of Ge_(20)As_(20)Se_(45)Te_(15)glass and Ge_(20)As_(20)Se_(43)Te_(17)glass serving as fiber core and cladding are 3.20 and 3.18 at 2μm,and their acoustic velocities are 2200 m/s and 2300 m/s,respectively.Numerical results indicate that the stimulated Brillouin scattering(SBS)efficiency is 248 m^(-1)·W^(-1),and the Brillouin threshold power is 66 m W when the core diameter of the 2-m-long GAST fiber is 4μm at 2-μm wavelength.The optic–acoustic coupling factor,the Brillouin frequency shift,and the Brillouin gain coefficient are 0.98,7.02 GHz,and 3.81×10^(-9)m/W,respectively.The SBS effect of GAST fibers simulated for the first time provides a new promising approach to selecting gain medium based on 2-μm-wavelength fiber laser.

A high-efficiency grating coupler between single-mode fiber and silicon-on-insulator waveguide

We present the design of a diffractive grating structure and get the optimal parameters which can achieve more than 75%coupling efficiency（CE） between single-mode fiber and silicon-on-insulator（SOI） waveguide through 2D finite-different time-domain（FDTD） simulation.The proposed architecture has a uniform structure with no bottom reflection element or silicon overlay.The structure,including grating couplers,adiabatic tapers and interconnection waveguides can be fabricated on the SOI waveguide with only a single electron-beam lithography（ICP） step,which is CMOS-compatible.A relatively high coupling efficiency of 47.2%was obtained at a wavelength of 1562 nm.

Transmission of 2.5 Gb/s WDM Channels Spaced at 5 GHz over 480 km of Single-Mode Fiber

We demonstrate the transmission of 2.5-Gb/s WDM signals spaced at 5 GHz over 480 km of single-mode fiber. The results indicate that it should be possible to transmit more than one thousand 2.5-Gb/s channels spaced at 5 GHz over long distance.

A fiber micro-vibration sensor based on single-mode fiber ring laser

A new micro-vibration sensor based on single-mode fiber ring laser is put forward. The Mach-Zehnder interferometric （MZI） detection technique is presented for interrogating laser frequency shift due to the measurand （piezoelectric transducer （PZT） is used to simulate the micro-vibration） induced laser cavity strain from both single- and multi-mode lasers. In the experiment, compared with multi-mode laser sensors, the single-mode laser sensor is proved to be a sensor with high resolution. When the PZT is driven by the analog signal （0.03 rad near 2 kHz）, the signal-to-noise ratio （SNR） of output signal from the single-mode laser sensor is close to 55 dB and the sensitivity of the sensor is about 5 ×10^-5 rad/Hz1/2.

Single-mode antiresonant terahertz fiber based on mode coupling between core and cladding

Based on the index-induced mode coupling between the higher-order mode in core and the fundamental mode in cladding tubes,the single-mode operation can be realized in any antiresonant fibers(ARFs)when satisfying that the area ratio of cladding tube and core is about 0.46:1,and this area ratio also should be modified according to the shape and the number of cladding tubes.In the ARF with nodal core boundary,the mode in core also can couple with the mode in the wall of core boundary,which can further enhance the suppression of high-order mode.Accordingly,an ARF with conjoint semi-elliptical cladding tubes realizes a loss of higher-order mode larger than 30 dB/m;simultaneously,a loss of fundamental mode loss less than 0.4 dB/m.

Modal interferometer based on tapering single-mode-multimode-single-mode fiber structure by hydrogen flame

A modal interferometer is experimentally demonstrated based on tapering a single-mode-multimode-single- mode （SMS） fiber structure heated by hydrogen flame. The interference fringe begins to form when tapering length is 19.8 mm, and becomes regular and clear when the tapering length is longer and the tapered waist diameter is smaller. Annealing process is undertaken to achieve a high extension ratio of approximately 17 dB with free spectral range of 1.5 nm when the tapering length is 33 mm and the tapered waist diameter is approximately 5 μm. The temperature and axial strain dependences of the tapered SMS structure are characterized, and the measured temperature and strain coefficients are ＋7 pm/℃ and -9.536 pm/με, respectively.

LP_(01) to LP_(11) mode convertor based on side-polished small-core single-mode fiber

An all-fiber LP_(01)-LP_(11) mode convertor based on side-polished small-core single-mode fibers(SMFs) is numerically demonstrated. The linearly polarized incident beam in one arm experiences π shift through a fiber half waveplate, and the side-polished parts merge into an equivalent twin-core fiber(TCF) which spatially shapes the incident LP_(01) modes to the LP_(11) mode supported by the step-index few-mode fiber(FMF). Optimum conditions for the highest conversion efficiency are investigated using the beam propagation method(BPM) with an approximate efficiency as high as 96.7%. The proposed scheme can operate within a wide wavelength range from 1.3 μm to1.7 μm with overall conversion efficiency greater than 95%. The effective mode area and coupling loss are also characterized in detail by finite element method(FEM).