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Exploiting the optical/magnetic bifunctional upconversion nanoparticles (UCNPs) with small size has become a trend in the high-resolution biological imaging. Here, we report the simultaneous size manipulation and color tuning in Er3+-sensitized perovskite KZnF3 UCNPs via Mn2+ doping. It was found that the introduction of Mn2+ into the KZnF3 host matrix facilitates the reduction of the nanoparticle size (from 1 μm to 50 nm). Under the excitation of both 980 and 808 nm continuous wave (CW) laser, these KZnF3:Er3+/Mn2+ (1/x mol%) UCNPs can efficiently generate tunable upconversion luminescence (UCL) and achieve pure red UCL as the Mn2+ concentration reaches 30 mol%. It is suggested that the energy transfer (ET) processes between Er3+ and Mn2+ ions contribute to the single-band red UC emission, which was well supported by investigating the dependence of UCL intensity on the pump power and the decay lifetimes of 4S3/2 and 4F9/2 states of Er3+ ions. In addition, the KZnF3:Er3+/Mn2+ UCNPs have been used as the T1-weighted magnetic resonance imaging (MRI) agents, which exhibits a high optical contrast with the background. These features make the KZnF3:Er3+/Mn2+ UCNPs promising for excellent dual-modal agents in UCL imaging and MRI.

Yttrium aluminum oxide YAlO3 is an attractive laser host material for lanthanide ions like Nd3+. In this paper, a dual liquid/vapor-fed flame synthesis (DLVF) system is developed for investigating the role of SiO2 coating on flame-made YAlO3:Nd3+ nanophosphors. The hexamethyldisiloxane (HDMSO) vapor is flexibly fed into the YAlO3:Nd3+ flame spray flow at different positions and molar ratios. We find that the introduction of SiO2 coating dramatically improves the crystallinity of YAlO3:Nd3+cores because of the heat-insulation effect and nearly doubles the fluorescence intensity. Through a timescale analysis, we demonstrate that the best timing for coating injection is when the small particles have already been formed but not heavily aggregated. An early injection could result in segregated SiO2 particles and some impurities like Al2SiO5, while a delayed injection leads to the coating of SiO2 on the entire aggregates instead of on individual particles. Experiments of different coating ratios show that the coating ratio 1:1 of Si:(Y+Nd) produces the best crystallinity and strongest fluorescence intensity. Further increase of the coating ratio does not show any improvements of the crystallinity, and may even causes the decrease of the relative fraction of Nd3+ ions. Compared with YAG:Nd3+ bulk materials (with a typical optimum doping concentration 1.1mol%) and uncoated YAlO3:Nd3+ nanophosphors (with an optimum doping concentration 2 mol%), the SiO2 coating promotes the optimum Nd3+ doping concentration up to 3 mol%.

We report a high power, narrow linewidth fiber laser based on oscillator one-stage power amplification configuration. A fiber oscillator with a center wavelength of 1080 nm is used as the seed, which is based on a high reflection fiber Bragg grating (FBG) and an output coupling FBG of narrow reflection bandwidth. The amplifier stage adopted counter pumping. By optimizing the seed and amplifier properties, an output laser power of 2276 W was obtained with a slope efficiency of 80.3%, a 3 dB linewidth of 0.54 nm and a signal to Raman ratio of 32 dB, however, the transverse mode instability (TMI) began to occur. For further increasing the laser power, a high-power chirped and tilted FBG (CTFBG) was inserted between the backward combiner and the output passive fiber, experimental results showed that both the threshold of Stimulated Raman scattering (SRS) and TMI increased. The maximum laser power was improved to 2576 W with a signal to Raman ratio of 42 dB, a slope efficiency of 77.1%, and a 3 dB linewidth of 0.87 nm. No TMI was observed and the beam quality factor M2 maintained about 1.6. This work could provide a useful reference for obtaining narrow-linewidth high-power fiber lasers with high signal to Raman ratio.

Line-by-line direct writing by femtosecond laser has been proved to be a simple and effective method for the fabrication of low-loss fiber Bragg gratings (FBGs), and is more flexible compared with the traditional ultraviolet exposure method. In this paper, the line-position-dependent characteristics of cladding modes coupling in line-by-line FBGs have been studied, to the best of our knowledge, for the first time. Both theoretical and experimental results show that off-center inscribing could compress the bandwidth of the Bragg resonance and excite more abundant cladding mode coupling, in which the core-guided fundamental mode would couple to the cladding-guided LP0n and LP1n simultaneously. By aligning the line positions across the core region, the first apodized line-by-line FBG was achieved. This work enriches the theories of line-by-line FBGs and provides an inscription guidance to meet different application requirements.

Chirped and tilted fiber Bragg gratings (CTFBGs) have attracted a lot of attention in stimulated Raman scattering (SRS) suppression of high-power fiber lasers. However, the laser power handling capacity seriously limits their applications. In this paper, by optimizing the inscription parameters and post-processing strategy, we fabricate a large-mode-area double-cladding CTFBG with a thermal slope of ~0.015 °C/W due to the low insertion loss of about 0.15 dB, which make it possible for direct kilowatt-level application. A 2 kW-level fiber laser oscillator is employed to test the CTFBG, and a series of experiments have been carried out to compare the effect of SRS mitigation in high-power fiber laser long-distance delivery. In addition, the influence of CTFBGs on laser beam quality is studied for the first time. Experimental results indicated that the CTFBG could effectively mitigate SRS and has no obvious influence on laser beam quality. This work opens a new opportunity for further power scaling and the delivery of high-power fiber lasers over longer distances.

Stimulated Raman scattering (SRS) is a major limiting factor for better performance of high-power fiber lasers. Several spectral filtering methods have been used for Raman suppression, among which long period fiber grating (LPFG) is relatively simple and effective due to its all-fiber structure and broadband rejection properties. Here, a broadband, low insertion loss and low thermal effect LPFG is easily fabricated using CO2 laser by galvanometer scanning, then is used for SRS suppression within a high-power fiber oscillator for the first time. Results show that a maximum SRS suppression ratio of ~16 dB was achieved and the extractable Raman-free output power was increased by about 13% with the insertion of LPFG. This work has demonstrated a potentially applicable method for Raman suppression in high-power fiber oscillators.

Side-pumping combiner is used for pumping double-clad fiber in various fiber laser schemes. However, its coupling efficiency and temperature characteristics suffer when pumped via a large numerical aperture (NA) pump light. We investigated the method of optimizing the coupling efficiency of a (2 + 1) ×1 combiner under a large NA pump light injection. After optimization of taper ratio and length of the pump fiber and fusion area between pump and signal fiber, the coupling efficiency increased and the temperature characteristic improved, which could be useful for fabrication of a side-pumping combiner for high-power fiber laser applications.

Chirped and tilted fiber Bragg gratings (CTFBGs) have attracted a lot of attention for stimulated Raman scattering (SRS) suppression in high-power fiber laser systems in past years. Here, we fabricate CTFBGs in large-mode-area double cladding fibers and demonstrate its application in a 2-kW fiber laser for SRS filtering at the system’s output. A maximum Raman suppression of about 17 dB is obtained at the central wavelength, and the insertion loss is smaller than 0.2 dB for signal laser. In addition, no obvious degradation of laser beam quality is observed. Better suppression effect could be achieved by broadening the rejection bandwidth of the CTFBGs in the future.

The one-stage master oscillator power amplifier (MOPA) is an efficient way to acquire high-power narrow-linewidth fiber lasers (NLFLs), which have wide usage in beam combination and detection. However, stimulated Raman scattering (SRS) is still one of the main obstacles for further power scaling. In this paper, we set up a counter-pumping kW-level narrowlinewidth one-stage MOPA fiber laser system and tried to use a chirped and tilted fiber Bragg grating (CTFBG) as a broadband rejection filter to suppress the Raman stokes light in the output laser. We finally acquired a 2.5 kW output with 3 dB linewidth of about 0.87 nm, and the SRS isolation ratio is ~42 dB on the spectrum, which is 12.7 dB higher compared with the situation without CTFBG.

A novel hybrid silicone foam was successfully prepared by mixing shear stiffening gel (STG) with polymethylvinylsiloxane (MVQ) using expandable microsphere as a blowing agent. Storage modulus of STG/MVQ composite foam increased from 0.045 MPa to 0.38 MPa, when the shear frequency shifted from 0.1 Hz to 100 Hz, presenting a remarkable shear stiffening characteristic. The tensile result demonstrated that the elongation at break of the hybrid foam was as high as 814%. Besides, the hybrid foam gained excellent self-healing performance due to the cold-flow feature and viscous state of STG. More importantly, the drop hammer test indicated the safeguarding performance enhanced as the STG contents increased, with the residual peak force decreasing from 36.72 KN to 31.50 KN. In addition, a possible mechanism was proposed that it was the dynamic “B O bonds” that played a significant role in shear stiffening.

Harnessing the color tuning capability of upconversion nanoparticles (UCNPs) is of great significance in the field of advanced bioimaging and color display. Here, we report the tunable size and upconversion luminescence (UCL) multicolor in CaF2:Yb3+/Ho3+/Ce3+ UCNPs, which were synthesized by a facile hydrothermal method. It was found that the size of these UCNPs could be controlled (from 600 to 30 nm) by varying the concentration of Ce3+ ions. Under the excitation of a 980 nm continuous-wave (CW) laser, the UCL color of these UCNPs can be tuned from green to red as the doped Ce3+ ions gradually increase from 0 to 10 mol% and the red-to-green (R/G) ratio is enhanced remarkably. It is suggested that the cross-relaxation (CR) processes between Ho3+ and Ce3+ ions contribute to the tunable multicolor and enhancement of the R/G ratio. The mechanism of these processes is well supported by the time-resolved decay and near infrared (NIR) emission measurements.

We study here the spectral characteristics of square-wave-modulated type II long-period fiber gratings (LPFGs) inscribed by a femtosecond laser. Both theoretical and experimental results indicate that higher-order harmonics refractive index (RI) modulation commonly exists together with the fundamental harmonic RI modulation in such LPFGs, and the duty cycle of a square wave has a great influence on the number and amplitudes of higher-order harmonics. A linear increase in the duty cycle in a series of square wave pulses will induce another LPFG with a minor difference in periods, which is useful for expanding the bandwidth of LPFGs. We also propose a method to reduce insertion loss by fabricating type II LPFGs without higher-order harmonic resonances. This work intensifies our comprehension of type II fiber gratings with which novel optical fiber sensors can be fabricated.

Recently, chirped and tilted fiber Bragg gratings (CTFBGs) have received great attention because they can realize suppression of stimulated Raman scattering (SRS) in high-power fiber lasers. In this study, the possible coupling between the core modes and cladding modes in CTFBGs inscribed in large-mode-area double-cladding fibers is investigated for the first time. Theoretical results show that the coupling between the LP11 mode and cladding modes would destroy the transmission spectra envelope only considering the coupling of LP01 for single-mode CTFBGs, which will degenerate the SRS suppression performance. This was confirmed experimentally by measuring the spectral response under different mode excitations. A reliable method is demonstrated to ease the LP11-excitation-induced spectral deterioration by choosing an appropriate chirp rate for the inscription of CTFBGs, which is useful for improving the Raman suppression effect of large-mode-area double-cladding CTFBGs in high-power fiber lasers.

Stimulated Brillouin scattering (SBS) is one of the mainly factors those limit the output power in narrow-linewidth highpower fiber laser systems. Here, we propose and demonstrate a novel method for the suppression of SBS in optical fibers using a tilted fiber Bragg grating (TFBG). With a TFBG being inserted between a single-mode fiber (SMF) amplifier and a 150-meter-length single-mode energy-transmitting fiber, not only the backward Stokes wave is rejected, but also an obvious increasing of the SBS threshold is observed with a value of 1.2 times that without the TFBG, which increases the effective laser output power by about 18 %. This work provides a new idea for SBS suppression in fiber. It is very useful for the further power scaling of high-power narrow bandwidth all-fiber lasers.

Stimulated Brillouin scattering (SBS) has significant influence on optical fiber communication (OFC), optical fiber sensing (OFS), and narrow-linewidth fiber laser (NLFL) systems. How to effectively suppress it has always been a challenge. In this Letter, we propose and demonstrate a versatile solution, for the first time, to the best of our knowledge, by using tilted fiber Bragg gratings (TFBGs). A specially designed and fabricated TFBG can be used as an ultra-narrow spectral filter, precisely matching with the operation laser wavelength and the tiny frequency shift due to SBS. Experimental results show that the backward Stokes can be strongly rejected with a filtering ratio of > 10 d B ; meanwhile, an obvious increasing of SBS threshold is observed with a maximum value of 1.7 times that without the TFBG, which enhances the effective transmission power by 33%. The operation stability of this method also is validated. This work opens new opportunities for SBS suppression in OFC, OFS, and high-power NLFL systems.

Er3+-sensitized upconversion nanoparticles (UCNPs) have attracted great attention due to their tunable upconversion (UC) emissions, low cytotoxicity, high resistance to photobleaching and especially multiple effective excitation wavelengths. However, detailed energy conversion between Er3+ and Tm3+ ions in Y2O3 UCNPs is still a problem, especially under multi-wavelength and variable pulse width excitation. In this work, we successfully fabricated a series of Er3+-sensitized Y2O3 nanocrystals by a spray flame synthesis method with a production rate of 40.5 g h&minus, 1. The as-prepared UCNPs are a pure cubic phase with a mean size of 14 nm. Excited by both 980 and 808 nm lasers, the tunable upconversion luminescence (UCL) from Er3+ ions was achieved by increasing the Er3+ doping concentration, co-doping Tm3+ ions and extending excitation pulse-width. The investigations of the lifetimes and the laser power dependence of UC emissions further support the proposed mechanism, which provides guidance for achieving effective color control in anticounterfeiting and multiplexed labeling applications. In addition, the red UC emission at about 5 mm beneath the tissue surface was observed in an ex vivo imaging experiment under the excitation of 808 nm laser, indicating that the Y2O3:Er3+/Tm3+ UCNPs have great prospects in further biological applications.

We firstly report here a novel method for suppression of stimulated Brillouin scattering (SBS) in optical fibers by using specially designed and fabricated tilted fiber Bragg gratings. It opens new opportunity for SBS suppression in fiber communication, fiber sensing, and narrow-bandwidth high-power fiber laser systems.

Here, the influence of rejection bandwidth of chirped and tilted fiber Bragg gratings on Raman suppression is studied in a 5 kilowatt-level direct LD pumped MOPA fiber laser. A better suppression could be achieved with a broader-band CTFBG, which is quite useful for further power scaling. © 2020 The Author(s)

We report here the effective suppression of stimulated Raman scattering (SRS) in a LD directly pumped 5 kW 1080 nm fiber amplifier using chirped and tilted fiber Bragg gratings (CTFBGs). The CTFBGs are carefully designed and fabricated to match the Raman gain spectrum, and experimental results show that the SRS suppression ratio is about 27.5 dB at 5 kW.

Side pumping combiners are widely used in fiber laser schemes for their high coupling efficiency, low insertion loss, and multi-point pumping capability. However, side pumping combiners perform differently in coupling efficiency when pumping with a laser diode (LD) and a high-brightness 1018 nm Yb-doped fiber laser (YDFL). In this paper, for the first time, to the best of our knowledge, we investigated the different parameters to fabricate the (2+1)×1 combiner with high coupling efficiency when pumping with an LD and a YDFL, respectively. After optimization, the maximum coupled pump power from one single-pump port of the combiner was 1200 W and 2730 W when pumping with a LD and a YDFL, respectively.

Long period fiber grating (LPFG) can be used as a band rejection component, providing a potential effective way for Stimulated Raman scattering (SRS) suppression in high power fiber laser. Here, an improved CO2 laser scanning system with a galvanometer is built up for fabrication of LPFGs, which greatly improves the speed and consistency of grating inscribing. The characteristics of the manufactured LPFG is measured in detail, then a direct laser diode pumped 5 kW-level fiber amplifier system is established to test the validity of the LPFGs for Raman suppression for the first time. Experimental results show that the SRS suppression ratio is about 14 dB at 5.1 kW with the LPFG being inserted between the seed and the amplifier stage, meanwhile the effective output signal laser power is increased. In contrast to the ultraviolet laser exposed band rejection components, such as chirped and tilted fiber Bragg grating (CTFBGs), this type of LPFG does not need photosensitive fibers, annealing treatment and has a much lower thermal effect, which is very useful for high-power fiber lasers applications.

How to charge high-temperature superconducting coils is a crucial problem for superconducting magnets. In recent years, flux pump is recommended as a promising method for current compensation since it can charge coils without electrical contact to current source. In this work, two types of charging methods, rotating magnet based flux pump and magnetically controlled persistent current switch, are discussed experimentally and numerically. Their output characteristics were tested and demonstrated. Charging performance on different types of loads including insulated HTS coil, no-insulation HTS coil and DC current source, is analyzed and compared. Results of this paper will prove the validity of those two charging methods in future superconducting magnet applications.

To the best of our knowledge, this is the first study to propose and verify a novel and simple direct inscription method for long-period fibre gratings (LPFGs) with an arbitrary grating period using a sinusoidal intensity modulated femtosecond laser. This method induces a sine refractive index modulation, which makes the LPFGs have no higher-order harmonic resonance that exists in the normal square-wave modulation method. This is beneficial for reducing the insertion loss of the LPFGs and optimising the transmission spectrum. Using this method, we also conveniently realise the inscription of superimposed, apodised, and chirped LPFGs. This work presents a simple but effective inscription scheme for various types of LPFGs using a femtosecond laser.

In this paper, we studied the basic characteristics of tilted fiber Bragg gratings (TFBGs), inscribed line-by-line. Experimental results showed that if the TFBGs were located within different planes parallel to the fiber axis, the spectra performed differently. For 2°TFBG, if it was located near the central plane, the Bragg resonance was stronger than ghost mode resonance, and the order reversed if it was located near the boundary between core and cladding. As the tilted angle increased, the range of cladding mode resonance increased. When the tilted angle was larger than 12°, the birefringence effect was observed. Based on the birefringence phenomenon, torsion characteristics were experimentally studied; the sensitivity was about 0.025 dB/degree in the linear variation range. The harmonic order of TFBGs also affected the transmission spectrum. Leaky mode resonance was observed in the 8th order TFBG, and torsion (or polarization) influenced the spectrum of the 8th order TFBG. Our research represented the theory of line-by-line inscribed TFBGs and provided an inscription guidance for TFBGs.

We propose the design of a high-power fiber oscillator with chirped and tilted fiber Bragg gratings (CTFBGs) to suppress the stimulated Raman scattering within the fiber cavity and in the delivery fibers. A high-power single-mode fiber oscillator system was constructed for demonstration. With a CTFBG inserted into the fiber cavity, obvious Raman suppression was observed and the output signal laser power increased by ∼7%. With a CTFBG inserted between the oscillator and the delivery fibers, a maximum Raman suppression ratio of >20 dB was achieved, which implies a longer delivery distance for the same laser power level. By further improving the performance of CTFBGs, including lower inversion loss at signal wavelength, wider rejection bandwidth and deeper rejection depth, meanwhile optimizing the positions of CTFBGs, better Raman suppression could be obtained, which is significant for the further power scaling and delivery distance increasing of high-power fiber oscillators.

Developing a rapid and continuous synthesis method for scalable production of upconversion nanoparticles (UCNPs) plays an important role in the applications of advanced color display, high-resolution biomedical imaging and precise optical thermometry. Here, for the first time, we report the multicolor tunable upconversion luminescence (UCL) in Y2O3:Yb3+/Er3+ (x/1 mol%) UCNPs with the average size of ∼14 nm, which were successfully fabricated by liquid-fed flame aerosol synthesis with a high production rate of ∼40 g h−1. Under the excitation of 980 nm continuous-wave (CW) laser, the UCL color can be efficiently modulated from green to red and the red-to-green (R/G) UCL intensity ratio is enhanced dramatically as the doping concentration of Yb3+ ions increases from 1 to 15 mol%. The UCL color manipulation can be ascribed to the cross-relaxation (CR) and the energy back transfer (EBT) processes between Yb3+ and Er3+ ions, and it was proved by investigating the dependence of UCL on the pump power and the decay lifetimes of 4S3/2 and 4F9/2 states. Moreover, the coefficients of CR and EBT processes were also calculated by a rate equation model, and the results suggest that they are efficient in Y2O3 host. In addition, the optical thermal sensing performance was also evaluated by analyzing fluorescence intensity ratio (FIR) of the thermally coupled levels (2H11/2, 4S3/2) of Er3+ ions in Y2O3:Yb3+/Er3+ (1/1 mol%) UCNPs, achieving an excellent thermal sensitivity of 0.0196 K−1 at 543 K.

In this article, 40-Gb/s all-optical 3R regeneration with a monolithic amplified feedback laser (AFL) as the all-optical clock recovery and a semiconductor optical amplifier delayed interferometer (SOA-DI) as the optical decision gate has been experimentally demonstrated. A 40-GHz optical clock with low timing jitter and uniform amplitude was successfully recovered from the polarization mode dispersion (PMD) and noise degraded signal by the AFL. The PMD degraded signal was restored by the 3R regenerator with the Q-factor and the amplitude uniformity improved. Bit-error rate improvement of the noise degraded signal has also been achieved using the 3R regenerator. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:433–436, 2016

The key remaining technological challenge to the realization of further power scaling for high-power fiber laser systems is overcoming the stimulated Raman scattering (SRS) effect. In past years, chirped and tilted fiber Bragg gratings (CTFBGs) have been demonstrated to be a simple and effective way to suppress SRS in high-power fiber amplifiers. However, the weak reflection at the Bragg wavelength could be strongly amplified, which not only limits the power and efficiency but also degrades the beam quality. We report here, for the first time to the best of our knowledge, the influence of the residual Bragg reflection of CTFBGs on SRS suppression in high-power fiber laser systems. Two groups of CTFBGs with different Bragg reflection wavelengths are fabricated and used for the comparison experiments. Test results show that the CTFBGs of longer Bragg wavelengths have a better suppression effect, in particular, at a higher power level. By further moving the Bragg wavelength of a CTFBG out of the Raman gain spectral range, a better suppression effect and a promotion in laser efficiency could be achieved, which is very useful for further power scaling.

The building-up and suppression of cladding-mode coupling in UV exposure fiber Bragg gratings (FBGs) is investigated experimentally and theoretically. Experimental results reveal that fundamental mode can simultaneously couple to cladding modes from different groups in the un-tilted FBGs as the transmission spectrum takes on multilevel. Theoretical results indicate that due to the non-uniform refractive index profile on the transverse plane of the FBGs, core-guided fundamental mode not only couples to axis-symmetrical cladding modes LP0n, but also transfers to circular-symmetrical cladding modes like LP1n if they distribute in the core region. Based on the symmetry property of the coupled cladding modes, the "ring-by-ring" inscription is theoretically proposed to suppress the cladding mode coupling for the first time, which plays a significant role in high-power fiber lasers research and application.

In this paper, we first propose and demonstrate a novel tracing atom based absorption spectroscopy method for the real-time measurement of the temperature rise inside the pump region of a pulsed diode pumped alkali laser (DPAL). By artificially adding potassium atoms into the gain medium of an operational rubidium laser, the information of the temperature rise can be obtained from the variation of the potassium absorption signal. Some important influencing factors are studied. Typical results show that, as the pump power (2 ms duration) increases from 22 W to 92 W, the temperature rise increases from 103 K to 227 K. As the pulse duration increases from 1ms to 5 ms, the temperature rise increases from 128 K to 314 K, and the heat relaxation time increases from 3.8 ms to 8.1 ms. The method is favored for its ability for real-time detection and high sensitivity, which provides a useful way for DPAL diagnostics.

Nd3+-sensitized nanoparticles have attracted attention recently due to their large absorption cross section and lower water absorption coefficient at 808 nm, which can potentially solve the laser-induced heating problem and is essential for labelling and imaging applications in living organisms. Here, we report a single-step hydrothermal synthesis of CaF2:Yb3+/Er3+/Nd3+ upconversion nanoparticles (UCNPs). The size of the as-prepared UCNPs decreases from ∼138 to ∼30 nm as the Nd3+ doping concentration increases from 0 to 3 mol%. Under the excitation of a 808 nm continuous-wave (CW) laser, these UCNPs exhibit typical UC emissions at 539 nm (green) and 656 nm (red) for Er3+, and the luminescence is strongest when Nd3+ doping concentration is 0.75 mol%. Moreover, with the increase of doped Nd3+ ions, the suppressing of green and the enhancing of red UC emissions can be observed, which leads to the luminescence color tuning from green to red. Further investigations suggest that the resonant cross-relaxations (CRs) between Er3+ and Nd3+ ions contribute to the UC luminescence color changing and the remarkable red UC emission for the highly Nd3+-doped UCNPs. These advances make the UCNPs potentially to be applied in vivo labelling, bioimaging and phototherapy.

In this paper, several applications in all-optical signal processing based on a semiconductor optical amplifier (SOA) and variable delayed interferometers (DIs) have been experimentally demonstrated. Wavelength converter based on a nonlinear polarization switch (NPS) and a DI is proposed and presented for the wavelength conversion of nonreturn-to-zero (NRZ) signals. An all-optical nonreturn-to-zero to return-to-zero (NRZ-to-RZ) format converter with tunable duty cycles is achieved by the DI with variable delays. The 40 Gb/s reconfigurable optical OR/NOR gate in a single SOA, followed a tunable optical bandpass filter (OBF) and a DI, optical 2R regeneration using an SOA-DI are investigated. It is found that this combinative realization of filters has been endowed with great flexibility and quality for 40 Gb/s optical logic and 2R regeneration.

A 1.55 mu m InGaAsP-InP partly gain-coupled two-section DFB self-pulsation laser (SPL) with a varied ridge width has been fabricated. The laser produces self-pulsations with a frequency tuning range of more than 135 GHz. All-optical clock recovery from 40 Gb/s degraded data streams has been demonstrated. Successful lockings of the device at frequencies of 30 GHz, 40 GHz, 50 GHz, and 60 GHz to a 10 GHz sidemode injection are also conducted, which demonstrates the capability of the device for all-optical clock recovery at different frequencies. This flexibility of the device is highly desired for practical uses. Crown Copyright

We demonstrate a multiwavelength 10 GHz pulse source using a dispersion-tuned actively mode-locked fiber ring laser incorporated with a semiconductor optical amplifier and an erbium-doped fiber amplifier. Simultaneous seven-wavelength operation of the laser is obtained. The side-mode suppression of all wavelengths is above 30 dB. Smooth wavelength tuning is achieved over more than 12 nm by changing the modulation frequency or the length of the optical delay line. Pulse characteristics are almost constant over the entire tuning span. Wavelength spacing can also be varied from 0.9 to 10 nm by adjusting the dispersion of the cavity. These experimental observations agree well with theoretical analyses.

Although the diode pumped alkali laser (DPAL) works in a three-level scheme, higher energy-state excitation and ionization processes exist during operation, which may lead to deleterious effects on laser performance. In this paper, we report the ionization degree measurement in the gain medium of an operational hydrocarbon-free Rb DPAL by using the optogalvanic method. The results show that, at the pulsed mode with a duration of ~1 ms, a maximal ionization degree of ~0.06% is obtained at a pump power of 140 W. While in the CW mode, the plasma reaches an ionization degree as high as ~2% at a pump power of 110 W, which is mainly due to the enough time for sufficient plasma development. A comparison with our previous work [Opt. Lett.39, 6501 (2014)] as well as modeling results is made and discussed. The influences of different population transfer channels on laser performance are simulated and analyzed. The results show that, for a typical hydrocarbon-free Rb laser (pump intensity of 15 kW/cmsup2/sup, helium pressure of 10 atm and cell temperature of 438 K), all the high-energy excitation effects give an overall negative influence on laser efficiency of ~3.78%, while the top two influencing channels are the photoionization (~1.8%) and the energy pooling (~1.53%). The work in this paper experimentally reveals the influence of the macroscopic ionization evolution process on an operational DPAL for the first time, which would be helpful for a more comprehensive understanding of the physics in DPALs.

In this report, we experimentally demonstrate efficient optical 3R regeneration of 40-Gb/s degraded data signals by using two schemes. The first 3R regenerator incorporates a dual-ring injection mode-locked fiber ring laser as the clock recovery module and an electro-absorption modulator (EAM) as the decision gate and the other 3R regenerator consists of an amplified feedback laser (AFL) for all-clock recovery and a semiconductor optical amplifier delayed interferometer (SOA-DI)as the optical decision gate. Optical clock with low timing jitter and uniform amplitude are successfully extracted with the use of the clock recovery units and significant improvement of the signal quality are obtained from 40-Gb/s RZ data signals degraded by polarization mode dispersion(PMD), chromatic dispersion or noise of amplified spontaneous emission (ASE).

All-optical wavelength converter (WC) is proposed and demonstrated based on a nonlinear polarization switch (NPS) followed by a SOA. The quality of the wavelength-converted signal can be improved by the SOA.

Synchronization of an optoelectronic oscillator (OEO) was realized by using the modulation sideband of an injected subharmonic optical signal. A stable 10 GHz microwave output with side mode suppression ratio (SMSR) of 57dB was obtained from a 1/3 harmonic injection at 3.3 GHz.

In this paper, 40-Gb/s all-optical 3R regeneration has been experimentally demonstrated based on semiconductor devices. The 3R regenerator incorporates an amplified feedback laser (AFL) for all-clock recovery and a semiconductor optical amplifier delayed interferometer as the optical decision gate. 40-GHz optical clock with low timing jitter and uniform amplitude is successfully recovered from the degraded signal by the AFL. The degraded signal is restored by the 3R regenerator with the Q-factor improved.

All-optical clock recovery is a key technology in all-optical 3R signal regeneration (Re-amplification, Retiming, and Reshaping) process. In this paper, a monolithic integrated three-section amplified feedback semiconductor laser (AFL) is demonstrated as an all optical clock regenerator. We fabricated a three-section AFL using quantum well intermixing process without regrowth instead of butt-joint process. The tunable characteristics of three-section AFL were investigated, and all optical clock recovery for 40Gb/s return to zero (RZ) 231−1 pseudorandom binary sequence (PRBS) is demonstrated experimentally using AFL with time jitter about 689.2fs.

A partly gain-coupled ridge varied two-section DFB self-pulsation laser for optical microwave generating has been fabricated. It produces microwave with a wide tuning range of more than 135GHz. A successful locked to the microwave frequencies of 30GHz, 40GHz, 50GHz, 60GHz are demonstrated and a timing jitter below 300fs is detected.

We propose and demonstrate a novel single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber ring laser incorporating a semiconductor optical amplifier. The SOA biased in its low-gain regime greatly reduces the gain competition of the two wavelengths. The stable SLM operation is guaranteed by a passive triple-ring cavity and a fiber Fabry-Perot filter. The dual-wavelength output with a 40 GHz wavelength spacing is switchable in the range of 1533-1565.4 nm.

A novel all-optical wavelength converter for nonreturn-to-zero systems based on a nonlinear polarization switch (NPS) and a delayed interferometer (DI) is proposed and demonstrated experimentally. The performance of the proposed wavelength converter is compared with the performance of a conventional wavelength converter that only incorporates a NPS. Results show that the quality of the converted signal is significantly improved by inserting a DI after the NPS. The transmission performance of the wavelength-converted signal is also improved by the DI.

Optical regenerative nonreturn-to-zero (NRZ) to return-to-zero (RZ) format conversion using a lithium niobate phase modulator and a lithium niobate intensity modulator is proposed and demonstrated. The key advantage of the proposed format converter is that the converted RZ signal has a very small pulse width, which can be multiplexed to a higher bit rate using optical time division multiplexing technology. The operation can greatly reduce the timing jitter of the degraded NRZ signal due to the regenerative property of the proposed scheme. Besides, the format converter can also support multi-channel operation. An experiment is performed with the feasibility of the scheme demonstrated.

All-optical regenerative wavelength converter is proposed and demonstrated based on the delayed interference signal wavelength converter followed by a SOA. Signal quality can be improved by the regenerative amplification of the SOA.

We propose and demonstrate a novel single-longitudinal-mode dual-wavelength erbium-doped fiber ring laser incorporating a semiconductor optical amplifier. The dual-wavelength output with 40-GHz wavelength spacing is switchable in the range of 1533 nm-1565 nm.