Broad-area diode-pumped 1W femtosecond fiber system

Summary form only given. High-average-power 310-fs pulses were obtained with an all-fiber chirped pulse amplification (CPA) system. Both the oscillator and the power amplifier are based on Er/Yb-doped fiber for cladding-pumping with broad-area laser diodes. A single 10-cm-long fiber grating is employed in the CPA system as a compact femtosecond-pulse stretcher/compressor

Model Predictive Feeding Rate Control in Conventional and Single-use Lab-scale Bioreactors: A Study on Practical Application

A developed solution for fed-batch process modeling and model predictive control (MPC), facilitating good manufacturing practice (GMP) based on process elaboration, control, and validation, is presented in the paper. The step-by-step evolution of the so-called “golden batch” optimal biomass growth profile and its control during the process is demonstrated. The case study of an advanced fed-batch control was performed on the recombinant E. coli BL21 lab-scale (5.4 L) biomass production process using the conventional stirred tank glass reactor. Additionally, a test experiment for control reproducibility and applicability assessment of the proposed approach was carried out in a single-use stirred tank reactor (5.7 L). Four sequentially performed experiments are demonstrated as an example for desirable feeding profile evolution for E. coli BL21 biomass production in a glucose-limited fed-batch process. Under different initial biomass and glucose conditions, as well as for different reference feeding profiles selected in the explorative experiments, good tracking quality of preset reference trajectories by the MPC system has been demonstrated. Estimated and experimentally measured biomass mean deviations from the preset reference value at the end of the processes were 4.6 and 3.8 %, respectively. Biomass concentration of 93.6 g L–1 (at 24 h) was reached in the most productive run. Better process controllability and safer process run, in terms of avoiding culture overfeeding but still maintaining a sufficiently high growth rate, was suggested for the process with biomass yield of 79.8 g L–1 (at 24 h). Practical recommendations on the approach application and adaptation for fed-batch cultures of interest are provided

Cladding pumped erbium fibre amplifier generating femtosecond pulses with an average power of 0.26W

Femtosecond pulse amplification in a cladding pumped fiber amplifier is demonstrated for the first time. A cladding-pumped Er3+ doped fiber amplifier generates 380fsec near-bandwidth-limited pulses at repetition rates up to 50MHz with an average power up to 0.26.

Dispersive properties of quasi-phase-matched optical parametric amplifiers

The dispersive properties of non-degenerate optical parametric amplification in quasi-phase-matched (QPM) nonlinear quadratic crystals with an arbitrary grating profile are theoretically investigated in the no-pump-depletion limit. The spectral group delay curve of the amplifier is shown to be univocally determined by its spectral power gain curve through a Hilbert transform. Such a constraint has important implications on the propagation of spectrally-narrow optical pulses through the amplifier. In particular, it is shown that anomalous transit times, corresponding to superluminal or even negative group velocities, are possible near local minima of the spectral gain curve. A possible experimental observation of such effects using a QPM Lithium-Niobate crystal is suggested.Comment: submitted for publicatio

Diode-pumped high-average power femtosecond fiber laser systems

The recent progress in femtosecond laser technology is currently driven by the prospect of fully diode pumped systems, which promise the eventual replacement of the well-established Ti:sapphire laser in the field of ultrafast optics. Apart from more traditional diode-pumped solid-state lasers, fiber-based systems have received an increasing amount of attention due to the uniquely compact assemblies possible with fiber lasers. However, to date fiber lasers have replaced Ti:sapphire-based systems only in areas, were low power levels are required, such as the injection seeding of regenerative amplifiers. Here, we show that fiber lasers can also produce power levels and pulse widths that are sufficient for the pumping of optical parametric oscillators (OPOs) and amplifiers (OPAs)

Broadly tunable carrier envelope phase stable optical parametric amplifier pumped by a monolithic ytterbium fiber amplifier

In an effort to develop a robust and efficient front end for a chirped-pulse parametric amplification chain, we demonstrate a broadband difference-frequency converter driven by a monolithic femtosecond Yb-doped-fiber amplifier and emitting carrier-envelope-offset-free pulses with the energy of tens of nanojoules tunable in the wavelength range from 1200 nm to beyond 2 μm. Next to providing these seed pulses, the system enables direct optical synchronization of Nd- and Yb-doped pump lasers for subsequent parametric amplification. © 2009 Optical Society of America

Frequency-doubling of a cladding-pumped Er³⁺/Yb³⁺ femtosecond fiber laser system using a periodically-poled LiNbO₃

As real-world ultra-fast optical devices proliferate, there is a growing need for highly reliable and compact sources of femtosecond pulses [1]. Currently most of these applications require moderate power sources operating around 800 nm, which is ideally compatible with frequency-doubling of femtosecond Er3+-fiber lasers. Previously integrated high-power fiber laser systems were developed based on chirped-pulse amplification schemes relying on chirped fiber gratings for pulse stretching and compression to minimize the nonlinearities of femtosecond fiber amplifiers [2]. The component count of such systems can be considerably reduced and the optical efficiency increased by implementing aperiodically poled lithium niobate [3] (APPLN), as APPLN allows a unique integration of chirped pulse amplification with frequency-doubling. Here we demonstrate the first system application of a APPLN frequency-doubler in conjunction with a high-power cladding-pumped Er3+/Yb3+ fiber laser. The experimental set-up is shown in Fig. 1. The fiber seed system is based on an environmentally stable fiber soliton laser [1] and generates bandwidth-limited 250 fsec pulses with pulse energies of 300 pJ at a repetition rate of 40 MHz at a wavelength of 1.56µm. To operate the cladding pumped power amplifier in saturation the pre-amplifier is used, which boosts the average signal power to 35 mW. Prior to amplification in the cladding-pumped power amplifier the pulses are stretched to ~ 1.7 psec in a 2.9 m length of positive dispersion fiber. Using a coupled pump power of ~6 W at 980 nm into the power amplifier, we obtain a signal power of 600 mW. After frequency-doubling in a length of 2 cm of APPLN an average power of 180 mW is obtained at 780 nm. The frequency-doubled pulse energy is 4.5 nJ. Note that the crystal was not AR-coated and the internal SH power was ~210 mW. The internal SH conversion efficiency was 40 %. An autocorrelation trace and the corresponding pulse spectra at the frequency-doubled wavelength are shown in Fig, 2. The pulse width is 290 fsec and assuming a gaussian pulse shape the time bandwidth product is 0.51, i.e. the pulses were within 20% of the bandwidth limit. Since currently APPLN allows the recompression of pulses up to 15 psec in width [3], we can expect that this technology may be upscaled to producing femtosecond pulses at Watt-level powers at 780 nm. Fig. 1: High-power frequency-doubled Er/Yb fiber laser. Fig. 2: Autocorrelation and spectrum of the pulses generated at 780 nm. The pulse width is 290 fsec and the time-bandwidth product is 0.51 assuming a Gaussian shape

Cladding-pumped fibre laser/amplifier system generating 100µJ energy picosecond pulses

We report on a fibre amplifier system generating picosecond pulses with 100µJ energy. A mode-locked fibre oscillator is used to seed a multi-stage acousto-optically gated fibre amplifier. The final stage comprises a cladding pumped multimode Er3+/Yb3+ co-doped fibre pumped with 4 W of power at 982 nm and produces up to 250µJ pulse energy

Generation of dual-wavelength pulses by frequency doubling with quasi-phase-matching gratings

We demonstrate generation of two synchronized picosecond pulses at different wavelengths near 778 nm by frequency doubling of a femtosecond pulse. We use nonlinear frequency filtering with quasi-phase-matching gratings, which allow us to obtain second-harmonic spectral intensities that are higher than the spectral intensities of the pump. © 2001 Optical Society of America OCIS codes: 140.7090, 190.7110, 190.2620, 320.5540, 320.7110. Synchronized short pulses generated at two different wavelengths are required in such applications as pump -probe experiments, coherent control, and generation of short pulses in the mid infrared by difference-frequency generation. Over the past several years a number of dual-wavelength Ti:sapphire oscillators have been demonstrated that use relatively complex dual-cavity designs to eliminate timing jitter between the pulses. -4 Another approach to the generation of dual-wavelength pulses is to use linear frequency f iltering of a single pulse. 9,10 Here we use this QPM SHG pulse-shaping technique to demonstrate generation of synchronized dual-wavelength pulses. Using structures with a phase-reversal sequence superimposed upon a uniform grating, we produce two synchronized coherent picosecond pulses by QPM SHG spectral filtering of a single femtosecond FH pulse. The wavelengths of the secondharmonic (SH) pulses and their temporal lengths are determined by the grating design, subject to limitation by the bandwidth of the FH pulse. Because of the nonlinear nature of QPM SHG filtering, the energy efficiency is not limited by the passband of the filter and, in fact, the SH spectral intensity can exceed the FH spectral intensity over the same bandwidth. We note that QPM devices similar to those described here have already been used for a cw frequency conversion. 11 Under the assumptions of plane-wave interaction, an undepleted pump, slowly varying envelopes, and negligible group velocity and higher-order dispersion of the material, the QPM SHG process is described with a transfer-function relation 9,10 : where 2 ͑V͒ is the frequency-domain envelope of the output SH and d A 1 2 ͑V͒ is the self-convolution of the frequency-domain envelope of the input FH and hence is proportional to the spectrum of the nonlinear drive. In Eq. where g ϵ 2p͞l 1 n 2 , l 1 is the FH wavelength, and n 2 is the SH refractive index. In Eq. (2) Dk 0 2k 1 2 k 2 and dv 1͞u 1 2 1͞u 2 , where k i are the carrier k vectors and u i ͑dk͞dv͒ 21 j vvi are the group velocities for the FH ͑i 1͒ and the SH ͑i 2͒. We first consider a constant-duty-cycle uniform QPM grating of length L and period L 0 (and hence the grating k vector for first-order QPM K 0 2p͞L 0 ). Ultrashort-pulse SHG with such gratings was previously analyzed in the literature where jdj is related to the intrinsic nonlinear coefficient of the material, d eff , as jdj ͑2͞p͒d eff for firstorder QPM; we also assume that the QPM condition is satisfied: K 0 Dk 0 . Equation (3) gives a familiar sinc 2 tuning curve when jD 0 ͑V͒j 2 is evaluated. The tuning curve is centered at V 0 and has a FWHM of DV 0 5.57͑͞dvL͒, which, for a given dv (material parameter), is determined by the grating length. If the width ofD 0 ͑V͒ is much smaller than the bandwidth of the nonlinear drive d A 1 2 ͑V͒, DV, the spectrum of the generated SH essentially replicatesD 0 ͑V͒; hence in the time domain the SH is a long (compared with the FH) top-hat pulse of length dvL [se

High-power single-mode fiber amplifiers using multimode fibers

As the optical power levels extractable from current single-mode (SM) fiber amplifiers are reaching regions where optical nonlinearities become significant, even under cw operation, it is becoming increasingly important to develop methods for reducing these nonlinearities to further boost the amplifier performance. Here we demonstrate a simple new approach, namely the SM excitation of specially designed large-core multimode (MM) fibers. Cladding-pumped Er/Yb amplifier versions of such fibers allow the direct amplification of diffraction-limited optical soliton pulses with peak powers up to 12 kW, about one order of magnitude larger than possible with conventional fiber amplifiers. Under SM excitation of a MM mode fiber, the amount of power propagating in the fundamental mode as a function of fiber length decreases due to micro-bending-induced mode-coupling. The resulting M2-value as a function of fiber length, as calculated from mode-coupling experiments, in various 50µm-core diameter fibers is shown in Fig. 1. The excitation wave-length was 1.55µm. In this fiber #1 was made by the rod-in-tube technique, whereas fibers #2 and #3 where made using modified chemical-vapor deposition. The low-micro-bending fiber #3 indeed allows to obtain an M2-value <1.2 for fiber lengths up to 10m, allowing for essentially diffraction-limited amplification in cladding-pumped Er/Yb amplifiers, which typically vary in length between 1-10m. The fundamental mode is launched in these MM fibers with high accuracy by suppressing modal speckle by using broad-bandwidth excitation sources such as ultrashort pulses. As the temporal coherence length of broad-bandwidth sources is very short, temporal interference between the modes and thus the presence of speckle is prevented. The stability of the spatial beam profile was verified by coupling the output of a MM fiber into a SM dummy fiber and measuring the launched signal power as a function of time. The result for both single-frequency (SF) excitation and broad-band excitation of the MM fiber is shown in Fig. 2. The large random power fluctuations due to modal speckle for the case of SF excitation are clearly visible, whereas the power fluctuations for broad-band excitation are about a factor of 5 smaller. Figure 3 shows the autocorrelation and the spectrum of 340 fs pulses directly amplified to a peak power of 12 kW in a 1.5m length of 30-µm-core diameter cladding-pumped Er/Yb amplifier fiber. The pulses had an energy of 4 nJ and were near-transform-limited with a time-bandwidth-product of 0.29. The autocorrelation is displayed on a logarithmic scale to demonstrate the absence of a pedestal. Secondary autocorrelation peaks due to small residual excitation of higher-order modes are suppressed to better than 0.5%. Pulses amplified to similar peak powers in such Er/Yb fibers have recently also been frequency-doubled to generate 300 femtosecond pulses with peak powers of 25 kW and a record average power of 300 mW at 780 nm. In conclusion, we have demonstrated a new technique for greatly expanding the operation limits of fiber lasers. The very large-core, low micro-bending fiber amplifiers discussed here should allow the construction of a new generation of ultrahigh-power fiber laser systems