Coherent beam combining of two fiber amplifiers using stochastic parallel gradient descent algorithm

We demonstrate the coherent beam combining of fiber amplifiers using a stochastic parallel gradient descent (SPGD) algorithm, producing a total output power of 23 W. Active phase control on each amplifier is performed by running the SPGD algorithm on a DSP chip with updating rate of almost 100 kHz. Experimental results show that power encircled in the target pinhole in closed-loop mode is 1.85 times more than that in open-loop mode, which is 92% of the ideal case. The fringe contrast of the far-field fringe pattern is as high as 92% when the system is in closed-loop mode. The whole system in closed-loop mode performs well in a long-time observation.     

Resolution and sensitivity enhancements in strong grating based fiber Fabry–Perot interferometric sensor system utilizing multiple reflection beams

We investigate an asymmetric intensive fiber Bragg grating (FBG) defined Fabry–Perot (F–P) sensor system decoded by a multiple-path-matched Michelson interferometer. The interrogation of higher order reflection beams cannot only solve the problem of the degraded resolution induced by the spectral mismatch of the FBGs, but also amplify the effect of the fiber strain on the phase of the light. We demonstrate multiple reflection beams in the F–P cavity based on the concept of the FBG effective length for constructing respective interrogation interferometers, and present a cost function with optimized system parameters to improve noise properties. The performances of interrogating the second, third and fourth order reflection beams are compared in a strain sensing experiment arrangement. Under the condition of the same optical path length mismatch, the interrogation of the fourth order reflection beam can achieve 9.8 dB sensitivity enhancement and 3 dB resolution promotion compared with the result using the second order reflection beam.     

Phase stepping methods based on PTDC for Fiber-Optic Projected-Fringe Digital Interferometry

Active homodyne control can be used to stabilize; π/2-rad phase steps in a Fiber-Optic Projected-Fringe Digital Interferometry. Two beams emitted from a fiber-optic coupler are combined to form an interference fringe pattern on a diffusely reflecting object. Fresnel reflections from the distal fiber ends undergo a double pass in the fibers and interference at the fourth port of the coupler which formed a Michelson interferometer. We suggested a method of PTDC (DC phase tracking) to maintain the interference intensity at quadrature by feedback control. Stepping between quadrature positions force a π/2-rad phase step. A method based on the ratio of harmonic of the interference signal is proposed to estimate phase step accuracy .A root-mean-square phase stability of 2 mrad and phase step accuracy of 13.8 mrad were measured with PTDC for the Fiber-Optic Projected-Fringe Digital Interferometry. It worked well in 2 h without resetting the integrator.     

Design and development of an optical beam splitter assembly and high precision colinearity measurements of laser beams

Laser beams with extremely high colinearity are often required where precision position monitoring is important. In order to achieve the said objective, a special type of Laser Beam Splitter Assembly (BSA) has been designed and fabricated in a very small volume due to space constraints. The main features and details of such a system are described here. This type of beam splitter assembly coupled with a diode laser through fibers can be remotely used for alignment or position monitoring of different medium to large size structures with a reconstruction accuracy of 10 μm. In this way, BSA generates two counter propagating laser beams from a single diode laser coupled to an optical fiber. In the present work, the colinearity between two beams within 1 mrad with the variation of 50 μrad has been achieved. The laser's power in the two arms may be controlled precisely, which is an important feature of this BSA. The BSA has been tested to work over a temperature range between ?20 °C to +40 °C. It has also been exposed to 1.0 MeV neutrons at a flux of ～5.0×10¹⁰ n/cm²/s and found compatible.     

A novel triple pump 1050 nm, 1400 nm, 800 nm pumping scheme for thulium doped fiber amplifier

Thulium doped fiber amplifier is a good candidate for S, and S+ band. This paper demonstrated a three pump pumping scheme for thulium doped fiber amplifier with 1050 nm co propagating pump and 1400 nm and 800 nm counter propagating pumps with a total pumping power 600 mW. This configuration yields up to 33 dB gain in 20 nm region from 1460 nm to 1480 nm, with noise figure <4 dB. To the knowledge of authors it is the highest gain achieved by thulium-doped amplifier in a single pass configuration with good power conversion efficiency.     

Dynamic evolution of Riemann–Silberstein vortices for Gaussian vortex beams

The dynamic evolution of Riemann–Silberstein (RS) vortices for Gaussian vortex beams with topological charges m = ± 1 in free space is studied. It is shown that for Gaussian on-axis vortex beams there exist both RS vortex with m = + 2 and circular edge dislocation. For Gaussian off-axis vortex beams the circular edge dislocation splits into two RS vortices with opposite topological charges m = ± 1 and the RS vortex with m = + 2 decays into two vortices with same topological charges m = + 1. The motion of RS vortices takes place by varying the propagation distance, waist width, off-axis parameter, or topological charge. RS vortices for Gaussian vortex-free beams can be treated as a special case. The results are illustrated analytically and numerically.     

A novel polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber

A kind of polarization splitter in ZnTe tellurite glass three-core photonic crystal fiber has been proposed. The polarization splitter is based on the phenomenon of resonant tunneling. We use the finite element method and the full-vector beam propagating method to analyze the characteristics of three-core photonic crystal fiber. Compare with the silica glass three-core PCF, the ZnTe tellurite glass three-core PCF have higher extinction ratios and lower coupling loss, the extinction ratios ERA = ? 164.2681 dB and ERC = ? 37.1742 dB at the wavelength λ = 1.55 μm, and the coupling loss is lower than 0.02 dB. The 8.7983-mm-long splitter is proposed to achieve extinction ratio better than ? 20 dB and a bandwidthof 20 nm.     

Influence of the nanoparticle sizes on the photo-induced absorption of La–Ga–S–O–Dy glass nanocomposites

We have established that the illumination by two coherent beams originating from nanosecond Nd:YAG laser at wavelengths 1064 nm and 532 nm in the La–Ga–S–O–Gd:PVA La-Ga-S-O-Dy polymer glass nanocomposites leads to substantial changes in the absorption. The effect is completely reversible and disappears after interruption of the optical treatment. The illumination power density was varied up to 0.6 GW/cm². All the samples show destruction less than 0.2% changes after more than 300,000 laser pulses. The beams were incident with the angles varying between 45° and 50° with respect to the nanocomposite surface. Moreover, the additional analysis of TEM did not demonstrate any additional aggregations. The role of light polarizations, beam stability, and light scattering is discussed.     

Experimental study of SBS suppression and coherence property of 1064 nm high power multi-tone fiber amplifier

Seeding high power fiber amplifier employing multi-tone fiber laser is an effective approach to suppress stimulated Brillouin scattering (SBS). In this paper, a two-stage 1064 nm high power fiber amplification system was set up. Single-, two- and three-tone fiber lasers were employed. SBS threshold powers and maximum output powers of the multi-tone cases are enhanced compared with the single-tone case. The multi-tone amplifiers also show comparable optical-to-optical efficiency to the single-tone amplifier. To demonstrate and validate coherence property of the two multi-tone fiber amplifiers, the output laser beams of the amplifiers were self-interfered in our self-made coherent beam combining system. The laser beams of the multi-tone cases showed good coherence property comparable to the single-tone case, which implied that the high power output laser light of the multi-tone fiber amplifiers could be used for coherent beam combining.     

Influences of geometry parameter on mode suppression ratio of fiber grating external cavity semiconductor laser

According to equivalent external cavity approximation model, after taking into account the joint contribution of semiconductor laser, external cavity and fiber grating (FG) to the phase condition, the mode distribution of the fiber grating external cavity semiconductor laser (FGESL) can be determined. As a result, the effect of the FG external cavity length (L) on the side mode suppression ratio (SMSR) of the FGESL is investigated theoretically. The results show that with the injected current and the coupling efficiency increase of the SMSR has taken on rise at all. For strong feedback (R₂ = 10^?4), the SMSR become more flattened with more than 40 dB, but, for weak feedback condition, The SMSR have lesser than 35 dB by an oscillation during rising course. Under the condition of short external cavity, the SMSR is in deep relation to the external cavity length, but the SMSR of longer external cavity is smaller than the SMSR of shorter external cavity on the whole and for 8–11 mm of the external cavity length, the SMSR of the FGESL has better (SMSR > 40.8 dB), and the SMSR become more flattened.     

Generation of tunable and narrow linewidth continuous-wave yellow laser by sum–frequency mixing of diode-pumped solid-state Nd:YAG ring lasers

We report a tunable, narrow linewidth and high beam quality continuous-wave (CW) yellow laser system at 589 nm. The system is an all solid-state design employing single-pass sum–frequency generation in a KTP crystal by mixing the 1064 nm with 1319 nm lines of two side-pumped Nd:YAG enforcing unidirectional ring lasers. With this method, a CW yellow laser at 589.159 nm with an output power of 0.8 W, a linewidth less than 1.5 GHz and a beam quality M² = 1.29 is obtained. The wavelength of the laser also can be precisely tuned from 589.112 to 589.181 nm in step-length of about 0.22 pm.     

An optical continuous phase FSK modulation scheme with an arbitrary modulation index over long-haul transmission fiber link

This paper presents an optical continuous phase frequency shift keying (CPFSK) modulation scheme with an arbitrary modulation index. The detailed principle on the optical CPFSK generation is derived and analyzed, which includes the special case of the minimum-shift keying (MSK) with a modulation index h = 1/2. The differential detection and the coherent detection of CPFSK are also depicted. The performances of the four kinds of the optical CPFSK modulated system with a 40 Gb/s modulation rate whose modulation index are h = 1/2, h = 2/3, h = 3/4 and h = 1 are simulated via the spectral efficiency and the receiver sensitivity over fiber link respectively. In addition, comparison with the differential phase shift keying (DPSK) is taken. Through the calculation of the spectral efficiency of each modulation formats, CPFSK has higher spectral efficiency than DPSK with the same optical devices. The transmission performances of our CPFSK over the fiber link change better as the modulation index increases under the condition of the first order dispersion of the fiber link is completely compensated. Through simulations, a 1200 km transmission distance can be achieved with a modulation index h = 1.     

External-cavity beam combining of 4-channel quantum cascade lasers

We demonstrate an external-cavity (EC) beam combining of 4-channel quantum cascade lasers (QCLs) with an output coupler which makes different QCL beams propagating coaxially. A beam combining efficiency of 35% (up to 75% near threshold) is obtained with a beam quality M² of 5.5. A peak power of 0.64 W is achieved at a wavelength of 4.7 μm. The differences of spot characteristic between coupled and uncoupled are also showed in this letter. The QCLs in this EC system do not have heat crosstalk so that the system can be used for high power beam combining of QCLs.     

Improvement of all-optical switching performance based on azo dye-doped polymer film using two cross-linearly polarized pump beams

We propose a method for improving the characteristics of all-optical switching based on azo dye-doped polymers. Using alternately two cross-linearly polarized beams (532 nm, continuous light wave (CW)) to pump azo dye–ethyl red (ER) doped polymer methyl methacrylate (PMMA) film, the modulation depth of the all-optical switching reached 96% at the pump powers of 4.8 mW and 1.6 mW and the modulation frequency of 1000 Hz. For comparison, we used respectively the single linearly polarized beam (4.8 mW) and the alternately linear–circular polarized beams (4.8 mW and 1.6 mW) to pump the film at the modulation frequency of 1000 Hz, the obtained modulation depths of the all-optical switching were 36% and 45.8%, respectively. Furthermore, the experimental measurement and analysis showed that the turn off speed of the all-optical switching could be obviously increased by use of our pump method.     

Laser microplasma as a tool to fabricate phase grating applied for laser beam splitting

In this paper, we present the method of phase gratings (PGs) formation on the fused silica by laser-induced black body heating (LIBBH) technology with irradiation of ytterbium fiber laser (λ=1.064 µm, τ ~ 4–200 ns, ν ~ 10–100 kHz). Formed PGs have sinusoidal profile with possible depth modulation of 0.5–2 µm. The PGs formation time, depending on its size and the period, ranged between 1 and 5 min. The optical characteristics of the PGs are studied and gained results are compared with the diffraction theory. This result shows that it is possible to fabricate different PGs with necessary optical characteristics by LIBBH technology. The potential application of such optical elements is beam splitting. Thus, the experiment with interference of laser beams has also been carried out in this work. The result of metal film processing by interference pattern is presented in the article.     

Propagation characteristics of Gaussian beams through 2 × 2 square matrix circular apertures

Based on the expansion of the hard aperture function into a finite sum of complex Gaussian functions, using Collins formula, the approximate analytical expression of Gaussian beams through 2 × 2 square matrix circular apertures is derived. Finally, some numerical examples are given to illustrate the analytical results by Matlab. It is shown that the propagation characteristics is related with the propagation distance z, the radius of circular aperture a and the distance d. The results can be directly used in other beams and square matrix apertures, and be applied to control beams and optical system designing.     

Multi-wavelength bismuth-based erbium-doped fiber laser based on four-wave mixing effect in photonic crystal fiber

A stable multi-wavelength erbium-doped fiber laser based on four-wave mixing (FWM) in a photonic crystal fiber (PCF) is demonstrated in this paper. The phase matching condition for four-wave mixing in the photonic crystal fiber has been enhanced using a seed signal and a polarisation controller to control the states of polarisation in the ring laser cavity. At a maximum pump power of 1480 nm, 5 lines are observed with nearly 2.15 nm spacing between the lines, and with a signal to noise ratio of more than 20 dB. The number of channels and wavelength spacing can be controlled by varying the output coupler ratio.     

Low-threshold continuous-wave Tm3+:YAlO3 laser

We describe a continuous-wave, low-threshold Tm:YAlO₃ (Tm:YAP) laser operating at 1945 nm with incident threshold pump powers in the 10–20 mW range. The z-cavity containing a 2-mm-long Tm:YAP crystal with 4 at.% Tm³⁺ concentration was end pumped by a continuous-wave Ti:sapphire laser at 795 nm. Tight focusing of the pump and the laser beams enabled low-threshold operation. The power performance of two different cavity configurations with 5-cm radius (R = 5 cm cavity) and 10-cm radius (R = 10 cm cavity) curved mirrors was tested. The best performance was obtained with the R = 10 cm configuration, where, the incident threshold pump power could be lowered to 10 mW after optimizing the polarization direction of the pump beam and by employing double pumping. Theoretical estimation of the threshold power was in good agreement with the experimental observations. The laser could be further tuned from 1842 to 1994 nm.     

The disinfection effect of a novel continuous-flow water sterilizing system coupling dual-frequency ultrasound with sodium hypochlorite in pilot scale

In this paper, a self-designed novel continuous-flow water disinfection system coupling dual-frequency ultrasound (US) with chemical disinfectant sodium hypochlorite (NaClO) was tested in a pilot scale using a simulated effluent containing Bacillus subtilis (B. subtilis), one of the indicators of water treatment efficiency. A suspension having a B. subtilis concentration of approximately 10⁴ CFU/mL was introduced into the system to (1) investigate disinfection efficiency of US pretreatment with NaClO (US + NaClO) and simultaneous US and NaClO (US/NaClO) disinfection under different single frequencies; (2) further examine the disinfection efficiency of these two processes with dual-frequency US; and (3) identify dosage reduction of chlorine in this disinfection system. The results demonstrated that lower dual-frequency (17 kHz + 33 kHz) US pretreatment with NaClO disinfection and simultaneous higher dual-frequency (70 kHz + 100 kHz) US and NaClO were beneficial to bacterial inactivation in terms of sterilizing efficiency. It has also been observed that US pretreatment with lower combination of 17 + 33 kHz frequencies showed better enhancement in which log reduction reached to 3.82 after 10 min chlorine reaction (chlorine alone was 0.22 log reduction), nearly 1 log reduction higher than single frequencies at the same constant power. Consequently, at equivalent power dissipation levels, US of lower frequencies combination pretreatment with NaClO disinfection performed such a promising process that one-thirds (from 12 mg/L NaClO reduced to 8 mg/L NaClO) of the required NaClO dosage was reduced for the ideal disinfection efficiency of 4 log reduction, namely 100% disinfection. And the utilization efficiency of NaClO was increased from 37.67% to 85.25% in 30 min of treatment time using an optimized combination of pretreatment and chlorination.     

Phase errors in FSE signals due to low frequency electromagnetic interference

ObjectiveTo quantitatively evaluate induced phase errors in fast spin echo (FSE) signals due to low frequency electromagnetic inference (EMI).MethodsSpecific form of Bloch equation is numerically solved in time domain for two different FSE pulse sequences (ETL = 8) with two different bandwidths. A single spin is modeled at x = 10 cm, EMI frequencies are simulated from 1 to 1000 Hz and phase errors at different echo times are calculated.ResultsPhase errors in the received echo signals induced by EMI are significantly higher at low frequencies (< 200 Hz) than at high frequencies and the phase errors at low frequencies can be effectively reduced by using high receiving bandwidth.ConclusionPulse sequence bandwidth can be used to control the phase errors in the FSE signals due to low frequency EMI.