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Traditional wave kinetics describes the slow evolution of systems with many degrees of freedom to equilibrium via numerous weak non-linear interactions and fails for very important class of dissipative (active) optical systems with cyclic gain and losses, such as lasers with non-linear intracavity dynamics. Here we introduce a conceptually new class of cyclic wave systems, characterized by non-uniform double-scale dynamics with strong periodic changes of the energy spectrum and slow evolution from cycle to cycle to a statistically steady state. Taking a practically important example—random fibre laser—we show that a model describing such a system is close to integrable non-linear Schrödinger equation and needs a new formalism of wave kinetics, developed here. We derive a non-linear kinetic theory of the laser spectrum, generalizing the seminal linear model of Schawlow and Townes. Experimental results agree with our theory. The work has implications for describing kinetics of cyclical systems beyond photonics.

Effects of intensity interactions (pump power depletion and sequential saturation of cascade generation) have been studied in a two-stage phosphosilicate Raman fiber laser in experiment and modeling. An analytical model of the laser which takes into account power variation along the fiber and spectral broadening of the first Stokes wave has been developed that describes experimental data both qualitatively and quantitatively. Good agreement between calculated and experimental values of the thresholds, differential efficiency for the first and the second Stokes waves and saturation levels for the residual pump and the first Stokes wave powers is demonstrated.

A novel class of low-cost and high resolution spectrometer-on-chips has been developed that are suitable for numerous laser-monitoring applications. The technology is based on the integration of digital planar holograms and a full optical circuitry on the same photonic chip. The prototypes are fabricated at a low-cost by nanoimprint lithography and are packed into the size of a USB key. Nano-spectrometers with a resolution down to 0.5 nm and a spectral range up to 229 nm were successfully demonstrated. The original concept of an on-chip spectrometer integrated into commercial image sensors was demonstrated and opens the route to numerous applications. Our miniaturized spectrometers are defining the state-of-the-art for on-chip spectroscopy, as well as in terms of spectral resolution and bandwidth than for their miniaturized size.

FBGs inscribed through polyimide coating by femtosecond laser point-by-point direct writing may serve as robust laser mirrors and sensors. Proposed slit beam shaping method for LPGs writing enables creating efficient polarization-maintaining fiber filters

A review of recent results on generation, optimization and coherent combining of high-energy Raman dissipative solitons of different Stokes orders, obtained by synchronous pumping in PM fiber cavity of normal-dispersion dissipative soliton laser, is given.

Investigation of a self-sweeping fiber laser with regular dynamics shows that the phases of modes generated with different pulses are not random. A superposition of 18 consequent laser pulses inside an external ring fiber cavity highlights their coherent combining into a train of nanosecond pulses.

Dynamic population inversion gratings induced in an active medium by counter-propagating optical fields may have a reverse effect on writing laser radiation via feedback they provide. In this Letter we report, to the best of our knowledge, on the first demonstration of an open-cavity fiber laser in which the distributed feedback is provided by a dynamic grating "written" in a Yb-doped active fiber, either by an external source or self-induced via a weak (∼0.1%) reflection from an angle-cleaved fiber end. It has been shown that meters-long dynamic grating is formed with a narrow bandwidth (50 MHz) and a relatively high-reflection coefficient (7%) securing single-frequency operation, but the subsequent hole-burning effects accompanied by new grating formation lead to the switching from one longitudinal mode to another. providing a regular pulse-mode dynamics. As a result, periodically generated pulse trains cover a spectrum range of several terahertz delivering millions of cavity modes in sequent pulses.

We report on the first demonstration of an all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode fiber-coupled laser diodes. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the graded-index fiber core, results in high-efficiency conversion of a multimode (M2~26) pump at 915 nm into a high-quality (M2~2.6) output beam at 954 nm. About 50 W output power has been obtained with slope efficiency of 67%. The proposed development and integration of key multimode fiber technologies opens the door to new type of LD-pumped high-power high-beam-quality fiber lasers that may operate at almost any wavelength defined by available LDs.

A random lasing based on Rayleigh scattering in a passive fiber directly pumped by a high-power laser diode (LD) has been demonstrated. Owing to the random distributed feedback (RDFB) the low-quality LD beam (938 nm) is converted into the high-quality laser output (980 nm). Because of the relatively low excess above the threshold with the available LD, the RDFB laser output is limited in power on 0.5 W level. In the used gradient-index (GRIN) fiber the output beam has 4.5 lower divergence as compared with the pump beam thus demonstrating a new approach for high-power fiber lasers with high-quality output.

As recently revealed, chirped dissipative solitons (DSs) generated in a long cavity fiber laser are subject to action of stimulated Raman scattering (SRS). Here we present theoretical and experimental study of the DS formation and evolution in the presence of strong SRS. The results demonstrate that the rising noisy Raman pulse (RP) acts not only as an additional channel of the energy dissipation destroying DS, but on the contrary can support it that results in formation of a complex of the bound DS and RP of comparable energy and duration. In the complex, the DS affords amplification of the RP, whereas the RP stabilizes the DS via temporal-spectral filtering. Stable 25 nJ SRS-driven chirped DS pulses are generated in all-fiber ring laser cavities with lengths of up to 120 m. The DS with duration up to 70 ps can be externally dechirped to

The semiconductor maskmaking industry imposes challenging requirements for the uniformity of critical dimensions (CD) and placement error. In electron beam lithography (EBL), electrons backscatter from the resist and substrate, reach the bottom of objective lens and come back to the resist, causing undesirable exposure and charging far away from the point of exposure. This fogging effects both CD variation and placement accuracy in EBL. The Monte Carlo software CHARIOT was upgraded to be capable of simulating this fogging effect. The results of simulations are presented. It was found that Gaussian approximation is sufficient; the approximation parameters were found for various working distances. The results were used for the correction of charging placement error. Fogging is one of the major contributing factors to the charging placement error; the DISPLACE software tool predicts the displacement map for any layout, which allows for the correction of placement error in maskmaking.

Studying transition to a highly disordered state of turbulence from a linearly stable coherent laminar state is conceptually and technically challenging and immensely important, e.g. all pipe and channel flows are of that type. In optics, understanding how systems lose coherence with increase of spatial size or excitation level is an open fundamental problem of practical importance. Here we identify, arguably, the simplest system where this classical problem can be studied: we learnt to operate a fibre laser in laminar and turbulent regimes. We show that laminar phase is an analogue of a one-dimensional coherent condensate and turbulence onset is through a spatial loss of coherence. We discover a new mechanism of laminar-turbulent transition in laser operation: condensate destruction by the clustering of dark and grey solitons. This is important both for the design of devices exploiting coherent dynamics and for conceptually new technologies based on systems operating far from thermodynamic equilibrium., Comment: 27 pages, 12 figures

Recently, temporal and statistical properties of quasi-CW fiber lasers have attracted a great attention. In particular, properties of Raman fiber laser (RFLs) have been studied both numerically and experimentally. Experimental investigation is more challengeable, as the full generation optical bandwidth (typically hundreds of GHz for RFLs) is much bigger than real-time bandwidth of oscilloscopes (up to 60GHz for the newest models). So experimentally measured time dynamics is highly bandwidth averaged and do not provide precise information about overall statistical properties. To overpass this, one can use the spectral filtering technique to study temporal and statistical properties within optical bandwidth comparable with measurement bandwidth or indirect measurements. Ytterbium-doped fiber lasers (YDFL) are more suitable for experimental investigation, as their generation spectrum usually 10 times narrower. Moreover, recently ultra-narrow-band generation has been demonstrated in YDFL which provides in principle possibility to measure time dynamics and statistics in real time using conventional oscilloscopes.

We review recent progress in optical wave turbulence with a specific focus on the fast growing field of fibre lasers. Weak irregular nonlinear interactions between a large number of resonator modes are responsible for practically important characteristics of fibre lasers such as spectral broadening of radiation. Wave turbulence is a fundamental nonlinear phenomenon which occurs in a variety of nonlinear wave-bearing physical systems. The experimental impediments and the computationally intensive nature of simulating of hydrodynamic or plasma wave turbulence often make it rather challenging to collect a significant number of statistical data The study of turbulent wave behaviour in optical devices offers quite a unique opportunity to collect an enormous amount of data on statistical properties of wave turbulence using high-speed, high precision optical measurements during a relatively short period of time. We present recent theoretical, numerical and experimental results on optical wave turbulence in fibre lasers ranging from weak to strong developed turbulence for different signs of fibre dispersion. Furthermore, we report on our studies of spectral wave condensate in fibre lasers that make interdisciplinary links with a number of other research fields.

Summary form only given. Formation of dissipative solitons (DS) in a normal-dispersion cavity of passively mode-locked fiber laser oscillators shows a great promise for generation of stable high-energy femtosecond pulse trains [1]. All-fiber design of the laser cavity greatly improves a stability of the laser, see e.g. [2], but increasing energy of the DS pulses at lengthening of a laser cavity is limited by a onset of stimulated Raman scattering (SRS) converting part of DS energy to the Stokes-shifted wavelengths [3]. A natural question arises about stability of the dissipative solitons in a fiber oscillator at presence of strong SRS effect.

In this paper we present CW all-fiber optical parametric oscillator based on 18-m long birefringent photonic crystal fiber LMA5-PM pumped by a CW Ytterbium-doped fiber laser (YDFL) near 1050 nm.

Summary form only given. Tunable Raman fiber lasers have attracted great interest owing to their high efficiency and reliability important for applications, such as optical fiber communications and sensing, spectroscopy, and instrument testing. Their tuning range is defined by the Raman gain bandwidth amounting to about 40 nm in telecom spectral range (~1550 nm) for conventional silica single mode fibers (SMF). To increase the range, highly nonlinear fibers which broaden pump spectrum may be incorporated in the cavity of Raman fiber lasers, see e.g. [1]. Another approach is to involve Rayleigh scattering forming random distributed feedback in a relatively long fiber resulting in prominent flattening of the tuning curve [2]. In this paper we report on combination of these two techniques in tunable Raman fiber lasers thus providing great improvement of their output characteristics. In the experiments we use highly-nonlinear True Wave fiber with zero-dispersion wavelength of 1457 nm. The pump wavelength of 1455 nm is close to that value, so the pump wave becomes spectrally broadened at propagation through the fiber (at high pump power even a supercontinuum can be generated in a long fiber [3]). In Fig.1a one of the studied experimental configurations is shown. Here a ring cavity with True Wave fiber (TWF) is pumped by 1455 nm laser via WDM coupler, a tunable filter is introduced in the cavity to select the wavelength. Output signal is measured through an output coupler. Linear cavities formed by loop mirror and normally or cleaved fiber ends were also explored. At large TWF length (>10 km) Rayleigh scattering (RS) becomes important [2] in both the ring and linear configurations of the cavity, at that pure RS-based random distributed feedback lasing is possible only for cleaved end linear configuration. The tuning curve for the ring cavity at 2W pumping is shown in the inset of Fig. 1a and corresponding output spectra are shown in Fig.1b: the laser is continuously tuned in the range 1520 - 1600 nm, i.e. 80 nm that is >2 times wider that that in [1,2]. At that, the power variations in this range are below 2 dB, while the maximum power is achieved around 1570 nm. Note that not only the generated power is changed at the tuning, but also the optical signal to noise ratio (OSNR) as well (Fig.1b). The maximum OSNR value exceeds 40 dB at 1551 nm and minimum OSNR is 25 dB at wavelength 1520 nm. The tuning range is limited by filter characteristics and may be further increased (especially in long-wavelength part). Maximum output power of >2 W has been achieved in linear configuration. Thus, a possibility to greatly increase a tuning range and output power of the Raman fiber laser by combining effects of highly-nonlinear fiber and Rayleigh-scattering based feedback in the cavity has been demonstrated. A detailed presentation of the experimental results in different configurations will be done.

Summary form only given. Lasers with random distributed feedback (DFB) owing to Rayleigh scattering in optical fibers [1] have attracted a great interest: a number of papers demonstrating new laser schemes and applications have been proposed [2-7] recently. Moreover, the generation output power and, more generally, generation power distribution could be described both analytically and numerically within simple balance models [8-9]. However, spectral properties of random DFB fiber lasers are not studied except some attempt made in [10]. Generation spectrum of random DFB fiber laser is quite broad (more than 1 nm), and physical mechanisms of its formation and broadening are still unclear. There is no any practical solution up to date to minimize the generation spectrum width. Here we experimentally show the way to minimize the generation spectral width.We use the one-arm laser scheme having a single pump from a one fiber facet. Such design does not include any elements except the fiber and the pump source. We use polarized ytterbium doped fiber laser generating up to 10 W at 1064 nm and telecommunication TrueWave fiber of different length varying from 2 km to 15 km. The laser generates Stokes wave at 1118 nm in the backward direction. An increase of the fiber length results in decreasing of the laser generation threshold pump power while preserving the slope efficiency almost constant, Fig 1a. At the same time, spectral width is also changed. Namely, the longer laser, the narrower the generation spectrum (at the fixed generation power), Fig. 1b. However, increasing the length over the 13 km does not affect the spectrum width. If one fixes the pump power, the spectral width remains the same for any cavity length revealing universal linear spectral broadening law, Fig. 1c. Such behaviour may indicate that the generation spectrum width is mostly determined by cross-phase modulation with the pump wave.

CW all-fiber optical parametric oscillator (FOPO) with tuning range from 950 to 1010 nm is demonstrated using birefringent photonic crystal fiber pumped by an Ytterbium-doped fiber laser (YDFL) near 1 μm. CW parametric generation with spectral linewidth of 3.7 nm at 972 nm has been obtained with slope efficiency as high as 9.4% and output power of up to 460 mW. It is also shown that the FOPO slope efficiency reaches 25% after narrowing of the pump spectrum down to 40 pm. At that the generated power exceeds 1 W, but in this case the generated radiation is modulated with 48 ns period and 50% duty factor due to pump laser power modulation which is probably caused by stimulated Brillouin back scattering.

We report on the experimental realization of a highly-chirped dissipative soliton (DS) oscillator with all- bercavity consisting of a short single-mode ber part (for mode locking via nonlinear polarization evolution) and along PM ber part (for generation of highly-chirped DS) that enabled to increase cavity length to L 90 m.Stable DS pulses dechirped to 200 fs are generated with maximum energy of 20 nJ. The energy limit is shownto be de ned by the onset of Raman conversion of the DS spectrum. The Stokes pulse reaching comparableenergy inside the cavity and does not break the soliton stability. Higher DS energy is possible by means of acore enlargement, corresponding experiments are also performed.Keywords: Mode-locked lasers, Fiber Lasers, Ultrafast lasers, Dissipative solitons, Scaling, Raman scattering 1. INTRODUCTION Femtosecond lasers with all- ber design is stable and compact source of ultrashort pulses. Energy of femtosecondpulses is the most important parameter for applications and generate of highly-chirped pulses is the main route toincrease it. Such pulses generated in a cavity with normal dispersion and o er much higher energy than transform-limited soliton pulses generated in cavities with anomalous or nearly zero net dispersion, that is explored eitherin ber

We demonstrate for the first time a CW all-polarization maintaining (PM) all-fiber optical parametric oscillator (FOPO) based on a birefringent photonic crystal fiber pumped by a tunable linearly polarized ytterbium-doped fiber laser. The all-PM FOPO features polarization-adjustment-free tunable operation in wavelength range from 920 to 1000 nm for both the slow and the fast fiber axes with output power reaching 1.3 W.

Single-frequency operation of a 1.03 μm fiber laser with random distributed feedback (RDFB) is demonstrated. The laser cavity is based on a 4 cm long fiber Bragg grating (FBG) consisting of 10 homogeneous subgratings with random phase and amplitude of refractive index modulation inscribed in a polarization maintaining (PM) Yb-doped fiber. Such RDFB laser generates single longitudinal mode with output power up to 25 mW, which is 3.5 times higher than that for a DFB laser based on regular π-shifted FBG of the same length in the same fiber. The single-frequency linewidth is measured to be

We present a comprehensive study of power output characteristics of random distributed feedback Raman fiber lasers. The calculated optimal slope efficiency of the backward wave generation in the one-arm configuration is shown to be as high as ~90% for 1 W threshold. Nevertheless, in real applications a presence of a small reflection at fiber ends can appreciably deteriorate the power performance. The developed numerical model well describes the experimental data.

The dissipative solitons (DS) generated in fiber oscillators with mode-locking mechanism based on nonlinear polarization evolution in a single-mode fiber exhibit stability and energy limits at the cavity lengthening. We demonstrate an alternative approach that enables us to increase the cavity length of the DS oscillator up to 30 m, namely, by the use of a long section of polarization-maintaining (PM) fiber in an all-fiber cavity configuration. We have also identified the next limit of energy scaling related to the onset of Raman conversion of the DS spectrum. The maximum energy of the stable highly chirped DS realized with a 5.5 μm core PM fiber, amounts to ∼20 nJ in ∼200 fs pulses after a grating compressor. As a next step, energy scaling by means of a fiber core enlargement is discussed.

The required measurement precision for multilayered EUV mask metrology is set below 0.4 nm three sigma. In addition to limited precision of CD-SEM, there are fundamental physical factors that deteriorate the accuracy of the measurements, the most important of which is charging. It is widely believed that EUV masks are conductive. However, experiments have revealed noticeable charging in CD-SEM measurements of EUV masks that cannot be ignored. In this work, the results of the experiments and simulations of the SEM signals are presented. It was shown that charging affects the metrology in a few ways. The SEM signal shifts at each frame, changes with beam current and also depends on the wall angle of the absorber. The results of the simulations are compared to experimental results.

Placement error due to charging in electron beam lithography has been identified as the most important factor limiting placement accuracy in EBL, which is especially important in the fabrication of masks for double patterning. Published results from a few major companies demonstrated that the placement errors due to charging are far larger than 10 nm. Here, we will describe the results of predicting the charging placement error based on a significantly improved physical model. Specially designed patterns were used to characterize the details of the charging placement error. Reference marks were exposed before the exposure of the test pattern, during the exposure, and after the exposure was completed. The experimental results were used to calibrate the parameters of the physical model. Furthermore, the DISPLACE software was used to predict the placement error maps for other experiments. The results of the measurements and simulations are presented in this paper. The results produced by the software were in good agreement with the experimental measurements. When the amplitude and the direction of the placement error due to charging is predicted, it can be easily corrected using readily available software for mask data preparation, or directly in EBL writers.

We have measured the longitudinal power distribution inside a random distributed feedback fiber laser. Both analytic solution and results of direct numerical modeling are in excellent agreement with experimental observations.

Phase matching curves for parametric generation in four wave mixing (FWM) processes of different types are studied experimentally and numerically for a polarization maintaining photonic crystal fiber (PCF) pumped by a tunable continuous wave (CW) ytterbium doped fiber laser near 1 μm. Parametric frequency shifts up to 100 THz for scalar and pump-divided vector FWM processes are observed providing generation of idler wave with wavelengths as short as 765 and 758 nm for the two processes respectively. Explicit analytical solutions for scalar and polarization phase matching in vicinity of the zero dispersion wavelength have been also deduced. They are based on phase mismatch Taylor series expansion taking into account the polarization contribution. A good quantitative agreement between experimental and calculated frequency shifts is demonstrated.

A generation spectrum of a fiber laser becomes broader with increasing generation power. The spectra are rather narrow at low power and become comparable with fiber Bragg gratings (FBG) width at high power. It has been shown that the spectral broadening of a fiber laser can be described analytically if the generation spectrum is much narrower than the FBG width. The developed theory has been compared with experiment. Double clad Yb-doped fiber laser of up to 10 W output power is used in the experiment. Scanning Fabry-Perot interferometer with resolution down to 1.2 pm is applied for accurate spectral measurements. At power level less then 1 W a self-sustained pulsation regime accompanied by a narrow-line self-sweeping is observed. At higher power a quasi-CW generation regime with multiple longitudinal modes is established. Investigation of the regime shows linear increase of the generation width with generation power growth. Slope of the dependence has excellent agreement with the theory, but an additive quantity should be added to describe an absolute value that makes significant contribution at low powers. It has been shown that at low powers a spatial hole burning has to be considered. Theoretical model describing the hole burning effect for multimode cw generation is also developed. After inclusion of the hole burning effect the model starts to agree quantitatively with the linewidth measurements both at low and high powers.

We have measured the longitudinal power distribution inside a random distributed feedback Raman fiber laser. The observed distribution has a sharp maximum whose position depends on pump power. The spatial distribution profiles are different for the first and the second Stokes waves. Both analytic solution and results of direct numerical modeling are in excellent agreement with experimental observations. © 2012 Optical Society of America., The authors acknowledge support from the Russian Ministry of Science and Education, Russian Foundation for Basic Research, the Leverhulme Trust, the European Research Council, the Marie Curie FP7 Program IRSES, the Dynasty Foundation, Russian science support foundation, the Spanish Ministry of Science and Innovation (TEC2011-27314) and Consejo Superior de Investigaciones Científicas (2010RU0083).

Critical dimension measurement is the most essential metrology needed in nanofabrication processes and the practice is most commonly executed using SEMs for its flexibility in sampling, imaging, and data processing. In bit patterned media process development, nanoimprint lithography (NIL) is used for template replication and media fabrication. SEM imaging on templates provide not only individual dot size, but also information for dot size distribution, the location of dots, pitch and array alignment quality, etc. It is very important to know the SEM measurement limit since the feature nominal size is less than 20 nm and the dot feature size and other metrics will relate to the final media performance. In our work an analytical SEM was used. We performed and compared two imaging analysis approaches for metrology information. The SEM beam was characterized using BEAMETR test sample and software for proper beam condition setup. A series of images obtained on a 27 nm nominal pitch dot array patterns were analyzed by conventional brightness intensity threshold method and physical model based analysis using myCD software. Through comparison we identified the issues with threshold method and the strength of using model based analysis for its improvement in feature size and pitch measurement uncertainty and accuracy. TEM cross sections were performed as accuracy reference for better understanding the source of measurement accuracy deviation.

The uncertainty associated with scanning electron microscopy (SEM) metrology is significant because SEM image brightness is complexly related to the size and shape of the feature, its material, the geometry of the pattern, as well as SEM setup. While regularly used methods of extracting critical dimensions (CD) rely on image brightness analysis, the myCD software uses a physical model of the SEM in order to improve the accuracy of measurements. Metrology below 10 nm was studied in this paper. Patterns were fabricated using electron beam lithography and nanoimprint; they were imaged by SEM and examined using myCD. Factors that are important for metrology at the sub-10 nm size range were studied using advanced Monte Carlo software; the beam size, voltage, detector and linewidth were varied. SEM images were processed using myCD, which utilizes an analytic model of the SEM and so does not require any libraries. The top and bottom sizes, as well as wall angles and line width roughness were analyzed. The CD and profile results from top down SEM images were compared to the vertical crossections. The challenges of sub-10 nm metrology are discussed, mainly regarding the quality of SEM images and the physics of image formation.

The effect of broad-range self-sweeping in Yb-doped fiber laser has been demonstrated experimentally for the first time. The self-sweeping effect is observed in an all-fiber laser configuration with a double-clad Yb-doped fiber and a cavity formed by a broad-band fiber loop mirror and Fresnel reflection from one cleaved end. The sweep range is limited by the width of the broad-band reflector and reaches up to 16nm. It is found that the self-sweeping effect is related to selfpulsations. So the sweep rate is increased with an increase in pump power and is decreased with increasing cavity length. RF and optical spectra (linewidth is measured to be not more than 100 MHz) show that during the evolution of a single pulse a small number of longitudinal modes take a part in lasing. Based on these results we propose a model describing dynamics of the laser frequency. The model is based on the spatial hole burning effect and the gain saturation in Yb laser transition, and takes into account self-pulsations of the laser. Theoretical estimation for pulse to pulse change of lasing frequency is in good agreement with experimental data.

Results of characterization of digital optical spectrometers-on-chip are reported. We present high resolution (up to 4×10−5) spectrometers fabricated on SiO 2 :Ge and Si 3 N 4 planar waveguides covering the optical range from 440nm to760nm.

We present the optimization of power and spectral performances of the random DFB fiber laser using the balance equation set. The numerical results are in good in agreement with experiments.

An optical fiber is treated as a natural one-dimensional random system where lasing is possible due to a combination of Rayleigh scattering by refractive index inhomogeneities and distributed amplification through the Raman effect. We present such a random fiber laser that is tunable over a broad wavelength range with uniquely flat output power and high efficiency, which outperforms traditional lasers of the same category. Outstanding characteristics defined by deep underlying physics and the simplicity of the scheme make the demonstrated laser a very attractive light source both for fundamental science and practical applications.

We present the results of comparative numerical study of energy deposition in single shot femtosecond laser inscription for fundamental and second harmonic of Yb-doped fiber laser. We have found that second harmonic is more efficient in absorbing energy which leads to lower inscription threshold. Hence this regime is more attractive for applications in femtosecond laser microfabrication.

We experimentally demonstrate a Raman fiber laser based on multiple point-action fiber Bragg grating reflectors and distributed feedback via Rayleigh scattering in an ∼22-km-long optical fiber. Twenty-two lasing lines with spacing of ∼100 GHz (close to International Telecommunication Union grid) in the C band are generated at the watt level. In contrast to the normal cavity with competition between laser lines, the random distributed feedback cavity exhibits highly stable multiwavelength generation with a power-equalized uniform distribution, which is almost independent on power. © 2011 Optical Society of America., The authors acknowledge financial support of the Engineering and Physical Sciences Research Council (EPSRC) EP/E015646/1 and the Russian Ministry of Education and Science grants and thank I. N. Nemov for fabrication of the FBG array.

The fabrication of digital planar holograms by Step and Repeat UV nanoimprint lithography is reported. It opens a route for commercial development of new nanophotonic devices based on digital planar holography. Two first applications to be demonstrated are the high resolution spectrometer chips and the enhancement of brightness and power of laser diodes.

We present experimental demonstration of a 200-km-long, dual-wavelength Raman laser utilizing two slightly different-wavelength fiber Bragg gratings, one on each side of the fiber span. The obtained results clearly prove the generation of two independent Raman lasers with a distributed >random> Rayleigh scattering mirror forming a cavity together with each of the individual fiber Bragg grating reflectors. © 2010 Optical Society of America.

Methods of extracting information regarding critical dimensions (CD) in scanning electron microscopes (SEM) are currently based on image brightness. This brings significant uncertainty of the measured results because image brightness has a complex relation to the size and shape of feature, its material, geometry of the pattern as well as SEM setup. A model based extraction of CDs out of SEM images has been developed. The analysis is based on an understanding of physical principles involved in the formation of the SEM signal. Some parameters, such as beam voltage and materials, should be known to the operator as the input data along with the SEM image. The output results of the myCD software are contours of lines, linewidth at the top, bottom, and in the middle of the line, line edge roughnessess at each edge, line width roughness, and wall angles at each edge. In addition, averaged values over all lines present in the image are also displayed. The model based analysis of SEM images may considerably improve accuracy of CD measurements in SEM.

SEM metrology involves uncertainty of the linewidth measurement because the SEM signal formation is an extremely complex process. In this work, we used an analytical SEM for CD metrology applications on quartz nanoimprint template. The SEM was tuned first to find the best reasonable condition for consistent operation. Beam characterization was done using BEAMETR beam measurement technique. SEM images of templates were taken at optimum conditions. The measurements were done using a) regular imaging processing software and b) using physical model based processing tool myCD. The quartz template was then measured using TEM crossections at selected sites to reveal profile information as metrology comparison reference. The metrology capability and fundamental limitation of analytical SEM operation with regular imaging processing was identified. Information about SEM setup and materials was used. The considerable improvement using the physical modeling imaging process was found.

BEAMETR (BEAm METRology) technique is demonstrated as an attractive solution for automatic measurement of electron beam sizes in two coordinates. The method associates one software and one specially designed pattern chip. The fabrication of new BEAMETR design is performed by electron beam lithography and metal lift-off. A specific bi-layer resist system and proximity correction is used for achieving the requirements for the "pound-key" shape of BEAMETR pattern. Beam sizes in two coordinates (x,y) of Scanning Electron Microscope are measured for various operating conditions. This method allows measuring electron beam sizes down to 2 nanometers.

Researchers conducted investigations to demonstrate the advantages of random distributed feedback fiber laser. Random lasers had advantages, such as simple technology that did not require a precise microcavity and low production cost. The properties of their output radiation were special in comparison to those of conventional lasers and they were characterized by complex features in the spatial, spectral, and time domains. The researchers demonstrated a new type of one-dimensional laser with random distributed feedback based on Rayleigh scattering (RS) that was presented in any transparent glass medium due to natural inhomogeneities of refractive index. The cylindrical fiber waveguide geometry provided transverse confinement, while the cavity was open in the longitudinal direction and did not include any regular point-action reflectors. © 2010 Optical Society of America

We demonstrate lasing based on a random distributed feedback due to the Raman amplified Rayleigh backscattering in different types of cavities with and without conventional point-action reflectors. Quasistationary generation of a narrowband spectrum is achieved despite the random nature of the feedback. The generated spectrum is localized at the reflection or gain spectral maxima in schemes with and without point reflectors, respectively. The length limit for a conventional cavity and the minimal pump power required for the lasing based purely on a random distributed feedback are determined. © 2010 The American Physical Society., The authors acknowledge financial support of EPSRC EP/E015646/1 and Russian Ministry of Education and Science grants.

Green and yellow diode-pumped solid-state (DPSS) lasers (532 and 561 nm) have become common fixtures on flow cytometers, due to their efficient excitation of phycoerythrin (PE) and its tandems, and their ability to excite an expanding array of expressible red fluorescent proteins. Nevertheless, they have some disadvantages. DPSS 532-nm lasers emit very close to the fluorescein bandwidth, necessitating optical modifications to permit detection of fluorescein and GFP. DPSS 561-nm lasers likewise emit very close to the PE detection bandwidth and also cause unwanted excitation of APC and its tandems, requiring high levels of crossbeam compensation to reduce spectral overlap into the PE tandems. In this article, we report the development of a new generation of green fiber lasers that can be engineered to emit in the range between 532 and 561 nm. A 550-nm green fiber laser was integrated into both a BD LSR II cuvette and FACSVantage DiVa jet-in-air cell sorter. This laser wavelength avoided both the fluorescein and PE bandwidths and provided better excitation of PE and the red fluorescent proteins DsRed and dTomato than a power-matched 532 nm source. Excitation at 550 nm also caused less incidental excitation of APC and its tandems, reducing the need for crossbeam compensation. Excitation in the 550 nm range, therefore, proved to be a good compromise between 532- and 561-nm sources. Fiber laser technology is, therefore, providing the flexibility necessary for precisely matching laser wavelengths to our flow cytometry applications.

We analyze the physical mechanisms limiting optical fiber resonator length and report on the longest ever laser cavity, reaching 270 km, which shows a clearly resolvable mode structure with a width of ∼120Hz and peak separation of ∼380Hz in the radio-frequency spectrum. © 2009 The American Physical Society., Grant No. TEC2008-05791 from the Spanish Ministry of Science and Innovation.