Coherent beam combination of 137 W fiber amplifier array using single frequency dithering technique

We demonstrate coherent beam combination of 137 W fiber amplifier array using single frequency dithering technique. Four polarization-maintained fiber amplifiers are tiled into 2×2 array and the output power of each amplifier is about 35 W. Single frequency dithering algorithm is run on a signal processor based on field programmable gate array (FPGA) for phase control on the fiber amplifiers. When the phase control system goes into closed-loop, the fringe contrast of far-field intensity pattern is improved by more than 87% from 0 in open-loop, and the residual phase error is less than λ/20.     

Erbium-doped photonic crystal fiber laser with 49 mW

We have demonstrated a continuous-wave (CW) all fiber laser operation at 1558.4 nm of a diode-pumped erbium-doped PCF laser based on 9.6 m erbium-doped PCF. The maximum output power and the threshold of the fiber laser are 49.4 mW and 6.67 mW, respectively. We show that it is possible to achieve a high stability and beam quality laser, which has a great application potential in optical communication field in future.     

Tunable 19 × 10 GHz L-band FP-SOA based multi-wavelength mode-locked fiber laser

We present a multi-wavelength mode-locked fiber ring laser incorporating a semiconductor optical amplifier (SOA) and a Fabry-Perot semiconductor optical amplifier (FP-SOA). Because the gain of the SOA is depleted by an external injection optical signal, the SOA acts as a loss modulator. The FP-SOA serves as a tunable comb filter. The presented laser source can generate 19 synchronized wavelength channels with the extinction ratio of about 21 dB, each mode-locked at 10 GHz, and mode-locked pulse width is about 40 ps. Oscillation wavelengths band can be tuned by adjusting the bias current of the SOA, and wavelength spacing also can be changed by using a tunable optical delay line (ODL) or a temperature controller. The polarization-insensitive devices ensure that the output power is rather stable. This fiber laser has potential applications in longer waveband (L-band) within the low-attenuation window.     

Tunable 12×10 GHz mode-locked semiconductor fiber laser incorporating a Mach-Zehnder interferometer filter

A stable multiwavelength mode-locked semiconductor fiber ring laser incorporating a fiber Mach-Zehnder interferometer (MZI) filter was proposed and experimentally demonstrated. A semiconductor optical amplifier (SOA) serves as an optically controlled mode-locking element due to gain exhaustion caused by external injected optical pulses. Another SOA severs as a constant-gain medium. A fiber MZI filter with a temperature control is incorporated into the fiber ring cavity to acquire a stable and tunable multiwavelength oscillation. Twelve wavelengths are synchronously mode-locked at 10 GHz, pulse width of mode-locked pulse are about 30 ps. Proposed multiwavelength mode-locked semiconductor fiber ring laser has some distinct advantages, such as simple and compact structure, easy integration, convenient tuning, good stability, potential high repetition rate operating, which has potential application in the future WDM communication system.     

High-beam-quality, 5.1 J, 108 Hz diode-pumped Nd:YAG rod oscillator-amplifier laser system

A high efficiency, high beam quality diode-pumped Nd:YAG master oscillator power-amplifier (MOPA) laser with six amplifier stages is demonstrated. The oscillator with two-rod birefringence compensation was designed as a thermally determined near hemispherical resonator, which presents a pulse energy of 223 mJ with a beam quality value of M² = 1.29 at a repetition rate of 108 Hz. The MOPA system delivers a pulse energy of 5.1 J with a pulse width of 230 μs, a M² factor of 3.6 and an optical-to-optical efficiency of 38.5%. To the best of our knowledge, this is the highest pulse energy for a diode-pumped Nd:YAG rod laser operation with a high beam quality and a pulse width of hundreds of microseconds at a repetition rate of over 100 Hz.     

Vibrationally-assisted dissociative adsorption of oxygen on Ru(0 0 0 1)-p(2 × 1)-O

Supersonic molecular beam technique combined with high resolution X-ray photoelectron spectroscopy using synchrotron radiation was applied to the study of the dynamics of dissociative adsorption of oxygen on Ru(0 0 0 1) surface in high coverage region. The Ru(0 0 0 1) surface pre-covered with oxygen atoms of 0.5 monolayer, which corresponds to the p(2 × 1)-O structure, was dosed to oxygen molecules with translational energy of 0.5 eV. Oxygen uptake was compared between the cases with and without the beam source heated in order to verify the effects of internal energy of oxygen. We found drastic enhancement in initial sticking probability of oxygen when the beam source was heated to 1400 K. We concluded that the enhancement of sticking probability is mainly caused by molecular vibrational excitation, indicating that dissociation barrier is located in the exit channel on potential energy surface.     

A structural parallel between Ge- and Si-induced 4 × 4 and 3 × 3 reconstructions on SiC(0 0 0 1) drawn from comparative RHEED oscillations

The initial Ge growth stages on a (√3 × √3)R30°-reconstructed SiC(0 0 0 1) surface (√3) have been studied using a complete set of surface techniques such as reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), atomic force microscopy (AFM) and photoemission and compared with similar Si surface enrichments in place of Ge. The investigations essentially focus on the wetting growth-regimes that are favoured by the use of the √3 surface as a starting substrate, this surface being the closest to a smooth and ideally truncated Si-terminated face of hexagonal SiC(0 0 0 1). Depending on temperature and Ge or Si coverages, varying surface organizations are obtained. They range from unorganized layer by layer growths to relaxed Ge(1 1 1) or Si(1 1 1) island growths, through intermediate attempts of coherent and strained Ge or Si surface layers, characterized by 4 × 4 and 3 × 3 surface reconstructions, respectively. RHEED intensity oscillation recordings, as a function of Ge or Si deposited amounts, have been particularly helpful to pinpoint the limited (by the high lattice mismatch) existence domains of these interesting coherent phases, either in terms of formation temperature or surface coverages. Prominently comparable data for these two Ge- and Si-related reconstructions allow us to propose an atomic model for the still unexplained Ge-4 × 4 one. It is based on a same local organization in trimer and ad-atom units for the Ge excess as admitted for the Si-excess of the 3 × 3 surface, the higher strain nevertheless favouring arrangements, for the Ge-units, in 4 × 4 arrays instead of 3 × 3 Si ones. Admitting such models, 1.25 and 1.44 monolayers of Ge and Si, should, respectively, be able to lie coherently on SiC, with respective lattice mismatches near 30% and 25%. The experimental RHEED-oscillations values are compatible with such theoretical ones. Moreover, these RHEED coverage determinations (for layer completion, for instance) inform us in turn about the initial Si richness of the starting √3 reconstruction and help us to discriminate between earlier contradictory atomic models proposed in the literature.     

Limitation in the intrinsic method of EDFA gain optimization for 32 × 40 Gbit/s WDM systems

The gain flattening of the erbium doped fiber amplifier (EDFA) is one of the most important aspects in the EDFA which the gain is wavelength dependent. For the first time the limitation of EDFA gain optimizing for a 32-channel wavelength division multiplexing (WDM) systems is investigated and reported in this paper. In a 32-channel WDM system the most favorable flatness gain achieved was 23.16 ± 1.51 dB with an average noise figure of 5.70 dB. This outcome proposes that the method does not achieve a uniform spectral gain in a 32-channel WDM system that incorporates a bandwidth of around 25 nm. Based on the simulation results the intrinsic optimization of EDFA causes the poor SNR and peak signal power with great variation over a transmission distance of 480 km single mode fiber.     

Study of all-fiber flat-top passband interleaver based on 2 × 2 and 3 × 3 fiber couplers

An all-fiber interleaver of cascaded Mach-Zehnder interferometer (CMZI) using a 2 × 2 and three 3 × 3 fiber couplers with almost rectangular wavelength response is developed by using a new configuration. Based on the digital approximation method of Fourier series, the relationships among the coupling ratios of the couplers, the length difference of interference arms and the intensity transmission spectrum are analyzed. The results indicate that a uniform flat-top spectral response can be obtained by the configuration when the length difference of interference arms and the coupling coefficient of the couplers, with some certain values, are given. The passband and stopband of optical interleaver are improved remarkably. Compared with conventional CMZI using 2 × 2 fiber couplers, the most advantage of the present method is that the coupling ratio of couplers can be controlled accurately because the interference effect of the monitor light can be sheered away in the course of fabricating couplers. A novel structure of filter is fabricated by using the fused biconical taper technics in experiment. The experiment results are in good agreement with the analytical ones.     

Initial stages of the autocatalytic oxidation of the InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen

The initial stages of oxidation of the In-rich InAs(0 0 1)-(4 × 2)/c(8 × 2) surface by molecular oxygen (O₂) were studied using scanning tunneling microscopy (STM) and density functional theory (DFT). It was shown that the O₂ dissociatively chemisorbs along the rows in the [1 1 0] direction on the InAs surface either by displacing the row-edge As atoms or by inserting between In atoms on the rows. The dissociative chemisorption is consistent with being autocatalytic: there is a high tendency to form oxygen chemisorption sites which grow in length along the rows in the [1 1 0] direction at preexisting oxygen chemisorption sites. The most common site size is about 21-24 Å in length at ∼25% ML coverage, representing 2-3 unit cell lengths in the [1 1 0] direction (the length of ∼5-6 In atoms on the row). The autocatalysis was confirmed by modeling the site distribution as non-Poisson. The autocatalysis and the low sticking probability (∼10⁻⁴) of O₂ on the InAs(0 0 1)-(4 × 2)/c(8 × 2) are consistent with activated dissociative chemisorption. The results show that is it critical to protect the InAs surface from oxygen during subsequent atomic layer deposition (ALD) or molecular beam epitaxy (MBE) oxide growth since oxygen will displace As atoms.     

Comparative study of H2 adsorption on W(1 0 0)-c(2 × 2)Cu and W(1 0 0): Surface alloying effects

The interactions of H and H₂ with W(1 0 0)-c(2 × 2)Cu and W(1 0 0) have been investigated through density functional theory (DFT) calculations to elucidate the effect of Cu atoms on the reactivity of the alloy. Cu atoms do not alter the attraction towards top-W sites felt by H₂ molecules approaching the W(1 0 0) surface but make dissociation more difficult due to the rise of late activation barriers. This is mainly due to the strong decrease in the stability of the atomic adsorbed state on bridge sites, the most favourable ones for H adsorption on W(1 0 0). Still, our results show unambiguously that H₂ dissociative adsorption on perfect terraces of the W(1 0 0)-c(2 × 2)Cu surface is a non-activated process which is consistent with the high sticking probability found in molecular beam experiments at low energies.     

Transmission performance of 20 × 10 Gb/s WDM signals using cascaded optimized SOAs with OOK and DPSK modulation formats

We investigated 20 channels at 10 Gb/s wavelength division multiplexing (WDM) transmission over 1190 km single mode fiber and dispersion compensating fiber using cascaded inline semiconductor optical amplifier at a span of 70 km for RZ-DPSK (return zero differential phase-shift keying) modulation format by using same channel spacing, i.e. 100 GHz. We show for RZ-OOK (return zero on-off keying) format a transmission distance of up to 1050 km with Q factor more than 15 dB, without any power drops. We developed the SOA model for inline amplifier having minimum cross-talks and ASE (amplified spontaneous emission) noise power with sufficient gain. At optimal bias current of 400 mA, a high constant gain of 36.5 dB is obtained up to a saturation power of 21.36 mW. So reduction of cross-talk and distortion is possible by decreasing the bias current at appropriate amplification factor.The DPSK modulation format has less cross-talk as compared to OOK format for nonlinearities and saturation case. The impact of optical power received and Q factor at different distance for both RZ-OOK and RZ-DPSK modulation format has been illustrated. We have shown the optical spectrum and clear Eye diagram at the transmission distance of 1190 km in RZ-DPSK system and 1050 km in RZ-OOK systems.The bit error rate (BER) for all channels observed is less than 10⁻¹⁰ up to gain saturation for both DPSK and OOK systems. Finally, we investigated that the transmission distance decreases with a decrease in channel spacing of up to 20 GHz.     

An 8 × 10 Gbps parallel single-chip light-to-logic converter

An 8 × 10 Gbps direct light-to-logic converter for hybrid mounted Ge photodiodes is presented. The receiver is realized in standard 0.35 μm SiGe BiCMOS technology and the Ge photodetector is directly mounted on the top of the CMOS wafer. Each of the 8 channels includes a transimpedance amplifier, limiting amplifier stages and a 50 Ω CML output driver. The overall transimpedance is 275 kΩ and at a data rate of 10 Gpbs a sensitivity of − 23.1 dBm (BER = 10⁻⁹) is reached.     

Robustness of NRZ, RZ, CSRZ and CRZ modulation on fiber nonlinearities and amplifier noise at 40 Gbps for (OC-786) long haul link

In this paper, the robustness of NRZ, RZ, carrier-suppressed RZ (CSRZ) and chirped RZ (CRZ) modulation formats at 40 Gbps for (OC-786) long haul link on the amplifier noise figure and fiber nonlinearities has been investigated. The investigations reveals that highest Q²(dB) of the order of [20, 25]; [19, 20]; [18, 19] and [16, 18] has been obtained in case of RZ, CSRZ, CRZ and NRZ modulation formats, respectively, in the presence of fiber nonlinearities and with and without amplifier noise figure of 6 dB. It has been observed that Q²(dB) in case of CRZ fluctuate between 18 and 14 at power variation of −10 to 4 dBm and NF of 6 dB, while it is between 19-9, 20-0 and 16-0 in case of CSRZ, RZ and NRZ. It has been identified that CRZ and CSRZ have shown the robustness on fiber nonlinearity and noise at 40 Gbps up to the transmission distance of 450 km. The wide eye opening in case of CRZ modulation also proved the robustness on fiber nonlinearity and noise.     

Etching reaction of methylchloride molecule on the GaAs (0 0 1)-2 × 4 surface

Adsorption process of methylchloride (CH₃Cl) on the GaAs (0 0 1)-2 × 4 surface was studied by a scanning tunnelling microscopy (STM) measurement. The arsenic rich 2 × 4 surface, which was prepared by molecular beam epitaxy (MBE), was exposed to a supersonic molecular beam of CH₃Cl with a kinetic energy of 0.06 eV. New bright spots appeared on the CH₃Cl exposed surface. They were largely observed at the “B-type” step edge and divided into two types according to their locations. It was suggested that new spots were due to weakly adsorbed CH₃Cl molecules without any dissociation. The adsorption mechanism of CH₃Cl molecule was also studied by an ab initio Hartree-Fock calculation, which explained the experimental results well.     

Broadband 2 × 2 free-space optical switch using electronically controlled liquid lenses

To the best of our knowledge, proposed is the first liquid lens technology-based 2 × 2 free-space optical switch using a pair of Electronically Controlled Variable Focus Lenses (ECVFLs). By independently controlling the focal lengths of two cascaded liquid ECVFLs, the two input optical beams are spatially adjusted to couple to their respective output beam ports. At 633-nm, the experimental switch demonstrates 26.3 dB crosstalk, 23.0 dB within-channel isolation, 1.1 dB optical loss, and 0.2 dB Polarization Dependent Loss (PDL). A 0.2 dB Wavelength Dependent Loss (WDL) is measured over 633-nm and 514-nm wavelengths. A 1 × 2 optical-fiber coupled switch version is also tested. The proposed switch can be useful in free-space laser systems as well as fiber-based modules.     

Surface energy of M2AC(0 0 0 1) determined by density functional theory (M = Ti, V, Cr; A = Al, Ga, Ge)

We have studied the correlation between the valence electron configuration and the electronic structure of M₂AC(0 0 0 1) surfaces (M = Ti, V, Cr; A = Al, Ga, Ge) by density functional theory. The A surface termination is the most stable configuration for all systems studied according to our surface energy data. As the M valence electron population is increased, the surface energy increases by 22% and 12% for A = Al and Ga, respectively, while it decreases by 29% for A = Ge. This can be understood by evaluating the valence electron concentration induced changes in the surface density of states. Antibonding surface Md-Ap states are present as Ti is substituted by Cr in M₂AC(0 0 0 1) for A = Al and Ga, while antibonding surface Md-Ap states are not present as Ti is substituted by Cr in M₂GeC(0 0 0 1).     

Coherent beam combination of an optical array using adaptive fiber optics collimators

A novel adaptive-fiber-optics-collimator (AFOC) compensating both piston-type and tip/tilt-type phase errors of output beam is introduced, and has been employed in experiments of coherent beam combination (CBC) of a delta distributed fiber array. Feedback control is realized using stochastic parallel gradient descent (SPGD) algorithm. Excellent CBC effect has been achieved when piston and tip/tilt errors among beamlets corrected. The necessity of wavefront tip/tilt control in CBC is verified. Experimental results exhibit great potential applications of this kind of AFOC in fiber amplifier arrays.     

Waveform shaping of 1550 nm transmission through hollow quartz fiber

In this paper, we connected two single-mode fiber ends with a micron hollow quartz fiber. The hollow fiber was used to modulate the waveform of a 1550 nm light signal, resulting in a lower noise and a narrower linewidth. Specifically, the noise range and the linewidth decreased from 14% to 3% and 0.28–0.23 nm, respectively. The light field distribution and far-field intensity distribution of the hollow fiber are studied using finite difference time domain simulation. Furthermore, we discussed the research suggestions of this configuration in detail. Collectively, these results suggest that it can be used in the fields of optical signal processing, fiber laser and optical sensor.