transform-limited femtosecond pulses

Top PDF transform-limited femtosecond pulses:

Broadly tunable femtosecond pulses around 2 06 µm from a diode pumped Tm3+ doped solid state laser source

Broadly tunable femtosecond pulses around 2 06 µm from a diode pumped Tm3+ doped solid state laser source

Increasing the output coupling to 2% resulted in higher average output powers but with a narrower tuning range and longer pulse durations. Operating in the 1st order, a tuning range of 2070–2102 nm was recorded, with output power and pulse durations varying from 78 fs to 119 mW and 435–670 fs, respectively [Fig. 5(d)]. It can be seen that the pulse durations and output power followed a similar profile to that seen with the 1% output coupler, with the maximum output power and minimum pulse duration found around 2090 nm. Figures 5(e) and 5(f) show the optical spectrum and intensity autocorrelation traces for a pulse recorded at 2090 nm, respectively. A pulse duration of 435 fs with an associated optical bandwidth of 11 nm were measured, giving a time-bandwidth product of 0.33. Moving to the 2nd order with the 2% output coupler gave a similar tuning range of 2072–2108 nm, with output powers varying between 74 mW and 103 mW and near-transform-limited pulses ranging from a maximum pulse duration of 811 fs to a minimum pulse duration of 563 fs. As with the 1% output coupler, clean RF spectra and widespan autocorrelation traces confirmed stable, single pulse mode-locked operation throughout the tuning range at a pulse repetition frequency of 114.3 MHz.
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Broadly tunable femtosecond mode locking in a Tm:KYW laser near 2 mu m

Broadly tunable femtosecond mode locking in a Tm:KYW laser near 2 mu m

operating around 2060 nm. It must be recognized though, that Tm-Ho codoped materials can suffer from the presence of increased up-conversion processes [15] compared to single Tm- doping and this in turn can lead to additional thermal loading inside the gain medium, resulting in reduced laser efficiency and limited output powers during continuous wave operation at room temperature. For this reason, direct generation of ultrashort pulses from Tm-doped materials looks more attractive for further development of laser diode pumped high-power mode-locked lasers that operate around 2 μm. Recently, we have demonstrated Tm-doped fluorogermanate glass laser producing near-transform-limited pulses of 410 fs duration centered at 1997 nm [16]. Average output power of 84 mW was, however, limited by poor thermo-mechanical properties of the glass.
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Broadly tunable femtosecond pulses around 2.06 µm from a diode-pumped Tm 3+ -doped solid-state laser source

Broadly tunable femtosecond pulses around 2.06 µm from a diode-pumped Tm 3+ -doped solid-state laser source

Increasing the output coupling to 2% resulted in higher average output powers but with a narrower tuning range and longer pulse durations. Operating in the 1st order, a tuning range of 2070–2102 nm was recorded, with output power and pulse durations varying from 78 fs to 119 mW and 435–670 fs, respectively [Fig. 5(d)]. It can be seen that the pulse durations and output power followed a similar profile to that seen with the 1% output coupler, with the maximum output power and minimum pulse duration found around 2090 nm. Figures 5(e) and 5(f) show the optical spectrum and intensity autocorrelation traces for a pulse recorded at 2090 nm, respectively. A pulse duration of 435 fs with an associated optical bandwidth of 11 nm were measured, giving a time-bandwidth product of 0.33. Moving to the 2nd order with the 2% output coupler gave a similar tuning range of 2072–2108 nm, with output powers varying between 74 mW and 103 mW and near-transform-limited pulses ranging from a maximum pulse duration of 811 fs to a minimum pulse duration of 563 fs. As with the 1% output coupler, clean RF spectra and widespan autocorrelation traces confirmed stable, single pulse mode-locked operation throughout the tuning range at a pulse repetition frequency of 114.3 MHz.
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Ueffing, Moritz
  

(2018):


	Direct amplification of femtosecond pulses.


Dissertation, LMU München: Fakultät für Physik

Ueffing, Moritz (2018): Direct amplification of femtosecond pulses. Dissertation, LMU München: Fakultät für Physik

Non-linear pulse compression constitutes the key technology in state of the art la- ser systems to produce optical pulses well below durations that are supported by the amplification gain medium [22, 164]. Different schemes based on bulk com- pression [165], cascaded second order non-linear effects [153, 166] and hollow core fibers have been presented. All of which have their limitations. Bulk compressors using a single pass through the non-linear medium usually suffer from bad beam quality requiring a spatial filter that leads to high losses. Cascaded second order non-linear effects provide better beam qualities but are not useful for pulse energies above 1 mJ because of the large crystal apertures required for such systems. Hol- low core fibers [22] still reach unprecedented pulse durations and high beam quality factors. For high average powers this scheme has recently been adopted to a fiber laser system resulting in 6 . 3 fs at 216 W and 170 µ J [30]. However, scaling this con- cept to higher pulse energies and average powers in the kW scale is complicated. Other pulse compression techniques for pulse energies of more than 1 mJ at average powers > 100 W providing reasonably good conversion efficiency maintaining high beam quality factors have been missing so far.
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Femtosecond Spin Current Pulses Generated by the Nonthermal Spin Dependent Seebeck Effect and Interacting with Ferromagnets in Spin Valves

Femtosecond Spin Current Pulses Generated by the Nonthermal Spin Dependent Seebeck Effect and Interacting with Ferromagnets in Spin Valves

Optimization and control of spin currents (SC) and their interaction with magnetic constituents in heterostructures on a femtosecond time scale is key for future terahertz spintronics applications. Although electronic transport through a ferromagnet (FM), as described by Mott’s two current model [1], generates a spin-polarized current, its density is intrinsically limited by Joule losses. The discovery of the spin-dependent Seebeck effect (SdSE), where thermal gradients over a bulk FM [2] or across an interface to a normal metal [3] generate SCs, opened a path towards overcoming such limitations. Indeed, short- lived thermal gradients can produce short ( ∼100 ps) SC pulses at densities exceeding the static Joule limit, as recently demonstrated upon laser excitation of spin-valve structures [4].
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Fernández González, Alma
  

(2007):


	Chirped Pulse Oscillators: Generating microjoule femtosecond pulses at megahertz repetition rate.


Dissertation, LMU München: Fakultät für Physik

Fernández González, Alma (2007): Chirped Pulse Oscillators: Generating microjoule femtosecond pulses at megahertz repetition rate. Dissertation, LMU München: Fakultät für Physik

Femtosecond (fs) light pulses can successfully modify transparent material on a µm and even nm scale [54, 56, 57]. Among different structures, waveguides are of great interest for real- izing all-optical chips, sensors, etc. They are a basic component of µm-scale devices written in bulk media. The waveguides can be written with light sources of 2 types: laser systems at a kHz repetition rate (oscillator + amplifier) and laser oscillators at a repetition rate of 1-100 MHz. The difference between these two approaches lies in the accumulated thermal effects. In the case of MHz repetition rates, the residual heat effects from a previous light pulse are present for the next pulses, modifying the pulse interaction with the medium [58– 61]. Usually, thermal effects lead to production of smoother and broader transversal structures than those obtained in the absence of these effects. Additionally, MHz repetition rates provide the further important advantage of high-speed material modification. This allows writing of high-quality structures due to the stable pulse energy and good beam pointing stability. The difference in the speed of waveguide fabrication between these two approaches can be as high as 3 orders of magnitude. Thermal effects also help to produce waveguides with larger diame- ter, thus decreasing their losses and increasing the coupling efficiency. Experiments show that the waveguide losses decrease as the repetition rate of the writing laser increases [61, 62]. The lowest losses demonstrated in fused silica with MHz repetition rate so far are ∼1 dB/cm in waveguides written at a speed of 15 mm/s [61, 62]. The pulse energies hitherto used in this regime range from 5 nJ to 5 µJ. In a recent publication [63] waveguides with losses ∼0.2 dB/cm were demonstrated. They were written at kHz repetition rate. All waveguides fabricated and investigated so far look like smooth structureless objects, except for the ones reported in the in [63], where complex waveguide structures were formed by single pulses (losses >1 dB/cm). This chapter reports the fabrication and characterization of a novel type of waveguide, written at a repetition rate of 10 MHz with high energy - up to 40 nJ, sub-30 fs pulses. These waveguides look like a chain of connected pearls and can be written at a high speed with an optimum at 1 mm/s. They have several interesting features that can be useful for photonics and for the production of micro-channels with modulated diameter. Traditional smooth waveguides were also written at the lower pulse energies - below 26 nJ.
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ISAR Imaging of Non-Uniform Rotation Targets with Limited Pulses via Compressed Sensing

ISAR Imaging of Non-Uniform Rotation Targets with Limited Pulses via Compressed Sensing

As a novel time-frequency analysis method, the completely time- domain analysis and linear transform property of FRFT make it having particular superiority in the processing of LFM signal [29]. FRFT can be viewed as a rotation of the time-frequency plane, the rotation angle α and the transform order p satisfy α = pπ/2. The FRFT of a LFM signal with finite length can achieve the best energy concentration in a proper fractional Fourier domain, this property has already been used in the separation of multi-component LFM signal [19]. As mentioned before, the range cell echo of uniformly accelerated rotation targets with multiple scattering centers is multi- component LFM signal, and both the initial frequency and chirp rate of each component are different, thereby appears as multiple noncrossing slant lines in the time-frequency plane, the echoed signal corresponding to each scattering center will concentrate to a peak of sinc function in the fractional frequency spectrum perpendicular to it. When one component achieves the best energy concentration in a certain fractional Fourier domain, the energy of other components will disperse, as shown in Figure 4. For clarity purpose, only the projections with the optimal energy concentration in the fractional frequency domain u 1 and u 2 are depicted.
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Zeidler, Dirk
  

(2002):


	Coherent Control of Molecular Dynamics with Shaped Femtosecond Pulses.


Dissertation, LMU München: Fakultät für Physik

Zeidler, Dirk (2002): Coherent Control of Molecular Dynamics with Shaped Femtosecond Pulses. Dissertation, LMU München: Fakultät für Physik

To overcome these difficulties, Judson and Rabitz proposed that the optimal electric field be determined recursively within a feedback-controlled experiment [10]. In their approach, an ultrashort laser pulse excites a sample, and a signal which is characteristic of the system (e.g. a target state) is monitored, as depicted in Fig. 2 [11, 12]. The amplitude of this probe signal serves as feedback for the optimization algorithm, which controls a pulse shaper and proposes a new pulse. The feedback signal of this modified pulse serves again as input for the algorithm, which proposes yet another pulse, and so forth until some convergence criterion is met. Since no a-priori knowledge of the physical system is necessary, this feedback scheme is applicable even if a theoretical investigation of the system is intractable on a quantum mechanical level. From the optimized laser field, insight into the physics of the system under investigation can be gained. The hardware for the implementation of such a feedback loop experiment comprises a source for ultrashort laser pulses, a computer-controlled pulse shaper and an optimization algorithm. The temporal shaping of ultrashort pulses is realized by complex filtering of their spectrum in the Fourier domain. Of critical importance in this concept is a reliable optimization algorithm which efficiently spots the global optimum in a multidimensional parameter space under the influence of experimental noise. Evolutionary algorithms have proven robust under such conditions. Pioneering examples of such optimizations were reported by the groups of Gerber [13] and Wilson [14], who reported control of the dissociation of a complex organometallic compound and of the fluorescence excitation of a dye molecule, respectively. Subsequent experiments on different types of systems followed:
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Synthesis of 3D nanostructured metal alloy of immiscible materials induced by megahertz repetition femtosecond laser pulses

Synthesis of 3D nanostructured metal alloy of immiscible materials induced by megahertz repetition femtosecond laser pulses

For the current scenario, as shown in Figure 9, the laser was focused so as to ablate the microparticle layer and the aluminum foil simultaneously. The particles from the foil and the microparticle layer were ejected into the plume and, upon subsiding of the laser pulses, formed into nanoparticle networks. The generated networks showed two types of generated nanostructured metal alloys as follows:

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New science exploration from XFEL: a new paradigm for structural visualisation of macromolecules

New science exploration from XFEL: a new paradigm for structural visualisation of macromolecules

One final challenge will be dealing with the unexpected ways that matter will interact with the XFELs. This is the first time in history that such single-shot experiments have been feasible with hard x-rays. Exposing biological molecules to such intense pulses, far beyond the tolerable radiation dose will have a wide variety of new consequences. These will need to be properly modelled and observed to help understand the data that is acquired, and how it should best be interpreted. Such unexpected effects, such as the diffuse intensity recorded around Bragg peaks in SFX, have already led to novel analysis schemes (Chen et al, 2014b). Better characterisation of the damage dynamics that occur following exposure will be important in guiding methodology development to diminish such effects. For example the observation that primary density loss occurred through a positively charged surface in simulated single-shot experiments is what led to the potential of a sacrificial tamper being recognised (Hau-Riege et al, 2010). These studies are also crucial for benchmarking the ideal photon energy and pulse length to be used in a given experiment to provide both the maximum resolution, with the minimum sample damage effects (Spence et al, 2012).
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Ultrafast double magnetization switching in GdFeCo with two picosecond delayed femtosecond pump pulses

Ultrafast double magnetization switching in GdFeCo with two picosecond delayed femtosecond pump pulses

dependence of the energy required to induce a single switch- ing event and an empirical model based on the rate of change of the equilibrium magnetization. Furthermore, our results open the possibility to induce further switching events. Although a number of parameters would have to be further optimized, such as fluence, Gd concentration, heat diffusion, electro-phonon coupling, and separation between pulses. Whilst quantitatively our results are restricted to GdFeCo, this effect should not be restricted to this material alone. Several studies have recently investigated the possibility of switching in other types of RE-TM alloys, 26,27 as well as syn- thetic ferrimagnetic structures. 28,29 We hope that our findings will invoke new experimental measurements on the possibil- ity of inducing multiple rapid switching events, which could potentially be used for magnetic writing schemes.
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Nanoimprinted distributed feedback lasers of solution processed hybrid perovskites

Nanoimprinted distributed feedback lasers of solution processed hybrid perovskites

To achieve high performance operation in perovskite lasers, it is important to create films with good optical performance and low scattering losses. As such, deposition methods which produce large crystallites such as thermal evaporation are less suitable for making DFB lasers [19]. Here we demonstrate DFB lasing by using the lead acetate deposition method to form a low loss optical waveguide on top of high fidelity polymer micro pillar arrays made by UV- NIL where sub-nanometre lasing spectra are observed at the band edge of the photonic dispersion. We compare laser thresholds under nano- and femto- second optical pumping and show that the perovskite lasers are very stable compared with organic semiconductor lasers, even at high repetition rates of 20 kHz, dropping to half their initial output after ~10 8 pulses.
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Holzwarth, Ronald
  

(2001):


	Measuring the Frequency of Light using Femtosecond Laser Pulses.


Dissertation, LMU München: Fakultät für Physik

Holzwarth, Ronald (2001): Measuring the Frequency of Light using Femtosecond Laser Pulses. Dissertation, LMU München: Fakultät für Physik

Although self phase modulation is likely the dominant mechanism of spectral broaden- ing there are other processes like stimulated Raman and Brillouin scattering or shock wave formation that might spoil the usefulness of these broadened frequency combs. Indeed in an experiment using 8 cm of PCF and 73 fs pulses at 75 MHz repetition rate from a Mira 900 system (Coherent Inc.) we have seen an exceptionally broad spectrum from 450 to 1400 nm as shown in Fig. 3.22 but with excessive broadband noise. We did not observe these problems with the 25 fs pulses at 625 and 750 MHz repetition rate utilize for all other spectra shown in this section. Our colleagues in Boulder used in their experiments a 12 fs Ti:sapphire laser (KMLabs) and a fiber from Bell Labs as mentioned above. They report the creation of an optical octave at 25 mW power through the fiber. Further increase of the power also generated broadband noise. The exact source of this noise is still not completely understood. Taking further the heavily structured spectrum into account that might have a deep hole where one wants to measure an optical frequency there is still a certain amount of “art” connected with the use of these fibers.
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Some Aspects of Analysis of Dolphins’ Acoustical Signals

Some Aspects of Analysis of Dolphins’ Acoustical Signals

differ from each other by the waveform in time domain and by the set of spectral components in frequency do- main (Figure 3). In this connection it is possible to as- sume, that each non-coherent pulse represents the pho- neme of the speech of a dolphin spoken language and every pack of the non-coherent pulses is a word, then sequence of the packs of the non-coherent pulses is the sentence. For the best understanding, we shall compare speech of a dolphin spoken language with speech of a human spoken language. In dolphins the phonemes spec- trum covers almost all frequencies band of a hearing, from 6 - 15 kHz up to 160 kHz. Frequencies approxi- mately below 6 - 15 kHz (Figure 2(b)), apparently, are excluded from a dolphin speech for increasing a speech noise-immunity, because for the frequencies below ap- proximately 10 kHz both a dolphin’s hearing thresholds and a level of environmental noise significantly increase. Phonemes spectrum of a human speech also covers al- most all frequencies band of a human hearing, but ap- proximately 0.3 − 4 kHz is necessary and sufficient for speech intelligibility; however this frequencies band is located primarily beyond coverage of a dolphin speech. The duration of dolphins phonemes are relatively very short, 80 - 600 µsec, that on 2 - 3 orders shorter than duration of a human phonemes. In addition, between dolphins phonemes there are relatively long-duration interpulse intervals (the characteristic value nearby of 150 msec). This interval can widely vary and, apparently, is necessary for phonemes immunity from reverberations. The phonemes of a human spoken language also consist of spectral components, but as opposed to a dolphin a human is producing phonemes of one word inseparably, as a solid word. In this connection duration of a dolphin word and a human word can be approximately identical at identical quantity of phonemes. Probably, in the free-ranging dolphins the interpulse interval will be con- siderably less in this case words duration will decrease. Certainly, it is hardly likely, but every the non-coher- ent pulse of a dolphin may be represents the word and a pack of pulses represents the sentence, but at present this is inessential as the answer to this question will be given by the further researches.
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Electrophysiology and Co ordinated Behavioural Responses in the Colonial Bryozoan Membranipora Membranacea (L )

Electrophysiology and Co ordinated Behavioural Responses in the Colonial Bryozoan Membranipora Membranacea (L )

It seems probable that in Membranipora the spread of response is limited in some way by the reduction in peak frequency and increase in total duration of a burst of T i pulses as it trav[r]

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Uniphasic insulin responses to secretin stimulation in man

Uniphasic insulin responses to secretin stimulation in man

Secretin-stimulated insulin release was studied in normal subjects. In response to rapid intravenous injections (pulses) of secretin, insulin levels reached a peak between 2 and 5 min and returned to basal levels with 15 min. In contrast to large glucose pulses, increasing secretin pulses did not elicit sustained or prolonged insulin responses. In addition, insulin responses to a pulse and infusion were essentially identical with that of a pulse alone. Increasing secretin pulses given in 1 day were associated with decreasing insulin

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On the relationship between elzaki transform and new integral transform "zz transform"

On the relationship between elzaki transform and new integral transform "zz transform"

In this paper we discusses some relationship between Elzaki transform and the newintegral transform called ZZ transform, we solve first and second order ordinarydifferential equations with constant and non-constant coefficients, using both transforms,and showing ZZ transform is closely connected with Elzaki transform.

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Characterisation and performance of a Terfenol-D coated femtosecond laser inscribed optical fibre Bragg sensor with a laser ablated microslot for the detection of static magnetic fields

Characterisation and performance of a Terfenol-D coated femtosecond laser inscribed optical fibre Bragg sensor with a laser ablated microslot for the detection of static magnetic fields

The sensor was made using a three step process. First a low insertion loss fibre Bragg grating, FBG, was inscribed point by point [15,16] in Corning single mode fibre-28 using a femtosecond laser (a High Q femtoREGEN laser operating at 1035nm, with pulse duration of 300fs, 1kHz pulse repetition rate with the pulse energy being controlled by a variable attenuator.). A Mitutoyo x50 NIR lens was used as it provides a long working distance with a moderate NA thus generating a small and highly accurate focal spot that the structures written required. The accurate nature of the spot through the sample due to the large NA and working distance are critical to the direct write nature of the work. The fibre was suspended between fibre mounts on a high precision Aerotech air bearing 2D translation stage, Fig. 1a. Once the focal spot of the laser beam had been aligned the stage were then translated at a velocity of 1.07mms − 1 creating a 2nd order grating with a pitch of 1.07µm over 30mm in the centre of the fibre core in order to be resonant at approximately 1550nm. The grating inscribed had an effective optical strength of 1dB as measured in transmission, typical of femtosecond inscribed low loss gratings written in this manner.
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Phonon spectroscopy with chirped shear and compressive acoustic pulses

Phonon spectroscopy with chirped shear and compressive acoustic pulses

Our results point the way to the development of techniques utilizing chirped acoustic pulses to measure the sub-THz and THz phonon spectra of single micro- and nano-objects, where, currently, the signal-to-noise ratio strongly limits the spectral resolution and the speed of obtaining the information. Using a chirped pulse, the measurement time window can be selected to exclude adjacent frequency components and, importantly, their phase and jitter noise contributions. The improvements, therefore, come from the reduction in integration or averaging time required at each sampling point. Sub- THz spectroscopy with TA phonons, whose spectrum is more sensitive to defects, could provide new information about biological objects like single cells [32], and become a new instrument for nanoscopy with spectral resolution. Heterodyne mixing of a single-frequency microwave probe with chirped acoustic pulses could be used for analyzing the spectra using GHz oscilloscopes and spectrum analyz- ers [33]. Using piezoelectric transformation of THz acous- tic waves to THz electromagnetic waves, e.g., Ref. [34], chirped acoustic pulses may be transformed into chirped microwave signals, becoming also an instrument for THz microwave spectroscopy.
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Support Limited Generalized Uncertainty Relations on Fractional Fourier Transform

Support Limited Generalized Uncertainty Relations on Fractional Fourier Transform

This paper investigates the generalized uncertainty principles of fractional Fourier transform (FRFT) for concentrated data in limited supports. The continuous and discrete generalized uncer- tainty relations, whose bounds are related to FRFT parameters and signal lengths, were derived in theory. These uncertainty principles disclose that the data in FRFT domains may have much higher concentration than that in traditional time-frequency domains, which will enrich the ensemble of generalized uncertainty principles.

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