We report the amplification of 10pJ-100pJ, semiconductor diode pulses up to an energy of 158µJ and peak powers &gt;100kW in a multi-stage fibre amplifier chain based on a single-mode, large mode-area erbium doped amplifier design. These results represent the highest single-mode pulse energy ever extracted from any doped fibre system

Taverner, D.

Richardson, D.J.

Dong, L.

Caplen, J.E.

Williams, K.

Penty, R.V.

English

Southampton (e-Prints Soton)

1Southampton SO17 1BJ,  UKTelephone: +44 1703 593150e-mail:   light@orc.soton.ac.ukOptoelectronics Research CentreUniversity of Southampton158µJ Pulses From a Single Transverse Mode, Large Mode-Area EDFAD.Taverner, D.J.Richardson, L.Dong, J.E.Caplen. K.Williams* and R.V.Penty**Dept. Electrical Engineering, Bristol University, Bristol, DS8 1TR, UK.Abstract We report the amplification of 10pJ-100pJ, semiconductor diode pulses up to an energy of 158µJ and peakpowers >100kW in a multi-stage fibre amplifier chain based on a single-mode, large mode-area erbium dopedamplifier design. These results represent the highest single-mode pulse energy ever extracted from any dopedfibre system.Sources capable of delivering high energy pulses of nanosecond order duration at 'eye safe'wavelengths beyond 1500nm are required for a wide range of practical applications from LIDARthrough to laser marking. A semiconductor seed source followed by an erbium doped fibreamplifier (EDFA) cascade would seem an attractive means to such an end, providing for acompact, reliable system. Recently progress towards this solution to such applications has beenmade with >100µJ, 10ns pulses being reported from a multimode, diode-pumped Er/Yb fibreamplifier [1]. However, for many of these applications a single transverse mode-output beam isa desirable, if not fundamental requirement. To date conventional single-mode EDFA fibredesigns, optimised for small signal gain and high pump efficiency, have restricted the attainablepulse energies to less than 10µJ [2,3]. For applications requiring higher pulse energies sourcechoice has therefore been limited to more cumbersome and expensive bulk laser options.To obtain significantly increased output energy characteristics one needs to consider what can bedone in terms of fibre design to achieve improved energy storage within the EDFA.  Increasingthe mode-field and dopant areas of the EDFA should result in a greater number of ionsparticipating in the amplification process and a reduction of the small signal gain occurring withinthe amplifier, which can restrict the inverted population depletion through gain saturation byamplified spontaneous emission (ASE). The improvement such increases make has been shownthrough the numerical modelling of low repetition-rate high-energy EDFA pulse amplification inreference [4] and in the work of reference [5] wher  a multimode EDFA produced pulse energiesof 0.4mJ. With the restriction of maintaining a single transverse mode at the signal wavelength,greater single-mode areas can be achieved through a reduction in the fibre numerical aperturealong with an increased core size, though such changes are limited by the requirement ofmaintaining a practical level of bend-loss. We have recently reported the development anapplication of a large mode area EDFA in an experimental chirped-pulse amplification (CPA)system [6] where the increased mode area also reduces the nonlinear effects that limit pulserecompression in these systems. In this paper we report an examination of the limits to pulsedenergy extraction in these large mode-area fibres through the amplification of 10ns-100ns pulses,resulting in the highest single-mode pulse energies extracted from any doped fibre amplifiersystem, to our knowledge. igure 1 shows a schematic diagram of the experimental setup. A laser diode source, providing1mW cw at 1534nm, was directly modulated to produce 10ns-100ns square pulses with resultantpulse energies of 10pJ-100pJ. These were passed into a two stage, high gain EDF pre-amplifierand then finally aunched into the large mode-area EDFA stage. The three amplifier stages were2Figure 1: Experimental set-up. EDFA: Erbium-doped fibre amplifier, AOM:Acousto-optic modulator, ISO: Bulk isolator, BPF: Wavelength tunable band-passfilter, f ,f : Focussing optics matched to the NA of the large mode area EDFA.1 2Points 1, 2 and 3 correspond to the positions of temporal measurements presentedin figure 3.Figure 2: 100ns pulse output spectra from fibres A and B at a repetition rate of 1kHz,corresponding to pulse energies of 158µJ and 123µJ respectively.separated by two acousto-optic modulators (AOMs) with 100ns rise times, triggered to gatethrough the pulses whilst blocking ASE from passing between adjacent stages. A 1nm bandpassfilter was placed after the two pre-amp stages, prior to the large mode-area EDFA, to eliminatethe small amount of ASE in the time slot of the pulse. For a 100ns pulse at a repetition rate of1kHz this resulted in a pulse energy of 3µJ incident on the final stage. With the pulse widtreduced to 10ns pulse energies of 1.4µJ were still being extracted. The coupling optics betweenthe preamplifier and large mode area final stage were chosen to accommodate their large NAmismatch so that a coupling efficiency of 75% was achieved. The large mode area amplifier wasend pumped in a backward direction with up to 1.1W of 980nm radiation from an argon ionpumped Ti:Sapphire laser, and amplified output signal separated from the incident pump usinga dichroic beam splitter.3Figure 3: Temporal pulse forms obtained atpoints 1, 2 and 3 in the system (see figure 1) fora 10ns square input pulse.Figure 4: Output power (open data points) and pulseenergy (filled data points) versus input repetition rate for10ns (squares) and 100ns (circles) input pulses.Two fibres were tested as the final stageeach angle polished at both ends to reducereflections. The first fibre, A, was dopeto 400ppm erbium, had an NA of 0.066, acalculated field mode area of  .310µm2and a cutoff wavelength of 1450nm givinga single mode at the signal wavelength. A4.5m length of this fibre was used coilewith a 30cm diameter, observed to besufficient to alleviate the problems of bendloss. Figure 2A shows the outputspectrum obtained for a 100ns initial pulsewidth at a repetition rate of 1kHz (foundto be the optimum rate, see figur  4). Withan output power as high as 160mW theASE power generated during the gainrecovery period estimated to be less than2mW, corresponding to a pulse energy of158µJ. The spectrum from fibre A shows a significant proportion of the pulse energy has beenscattered into Raman generated spectral components. In order to reduce the interaction lengthsover which this nonlinear effect occurs a significantly shorter amplifier length was required, so asecond fibre was fabricated with higher erbium-ion dopant concentration. This fibre, B, had adopant concentration of 1500ppm, an NA of 0.07, 275µm field mode area, and a single mode2cutoff wavelength at 1400nm. With the amplifier length reduced to 1.45m of fibre B 123µJ outputpulses were obtained (130mW of output power with an estimated 7mW of ASE). Figure 2Bshows the resulting output spectrumwith no noticeable signs ofnonlinearly generated components.On reducing the input pulse widthto 10ns, we obtained estimatedpulse energies of 148µJ from fibreA and 118µJ from fibre B, valueswhich were only slightly reducefrom those obtained for 100ns inputpulses. The reasons for this becameclear when the pulse temporal formswere observed using a PIN diodeand oscilloscope after each amplifierin the system. These are shown infigure 3. The significant shapingeffects that result in the low level ofthe pulse trailing edge occur as thestored energy was depleted by thefront edge of the pulse passingthrough the amplifier, suggestingthat almost all of the extractable energy was being taken up by the pulse. The initially square 10nspulse was found to have narrowed to 6ns after the pre-amps and to almost 1ns at the final stage4Figure 5: Output pulse energy (black circles) and laststage gain (white squares) versus input pulse energy for10ns input pulses at 1kHz repetition rate.output. With an energy of 118µJ thiscorresponds to a peak power of over100kW.The variation of the output powerwith the pulse r petition rate in fibreA was recorded in figure 4. It can beseen that the greatest pulse energywas reached at a repetition rate of1kHz. At repetition rates higher thanthis the fibre is being pumped forshorter periods between pulses soless energy is stored. However,below 1kHz the ASE saturates thgain, preventing any furtherincreases in energy storage andhence energy extracted. Themaximum extracted averge powerfor each fibre, 310mW and 207mWrespectively, was achieved atrepetition rates above approximately20kHz although at this point thepulse energies had been reduced toless than 20µJ.By controlling the preamplifier it was possible to obtain a plot of incident pulse energy versuoutput energy and the corresponding amplifier gain, as shown in figure 5. The observed saturationof the output energy further confirms that most of the available stored energy was being extracted.It was noted that the final stage amplifier was producing a gain of over 19dB at the point ofmaximum energy extraction and that gains of 28dB were still being achieved for smaller signals.In conclusion, we have produced pulse energies of 158µJ at a repetition rate of 1kHz from anEDFA pumped with 1.1W at 980nm. 10ns pulses of 10pJ energy were amplified to energies of118µJ with a spectrally c ean output, demonstrating an input-to-output system gain greater than70dB. The resultant pulse narrowing to 1ns implies peak pulse powers over 100kW. Using thislarge mode-area EDFA we have achieved an increase of greater than 10dB over previouslyreported results from an erbium-doped amplifier [3], energies that to our knowlege represent thehighest extracted from any single mode doped fibre system. These high energies and peak powersfrom a system that is well suited to diode pumping schemes, suggest the potential of our designas a compact source for applications such as supercontinuum generation and LIDAR. This fibredesign also results in significantly reduced nonlinearities as compared to a standard EDF andhence would be particularly well suited to applicati ns su h as CPA, where self phase modulationcan be a limiting factor to the recompressed pulse energy.5References1. V. P. Gaspontsev, V. V. Fomin, I. E. Samartsev: Proc CLEO ‘96, 9, 209 (1996).2. D. Rafizadeh and R. M. Jopson: Electron. Lett., 30, 317 (1994).3. A. Galvanauskas: Generation, amplification and measurement of ultrashort laser pulsesII, SPIE Vol. 2377, 117 (1995).4. J. Nilsson and B. Jaskorzynska: Opt. Lett., 18, 2099 (1993).5. B. Desthieux, R. I. Laming, D. N. Payne: Appl. Phys. Lett., 63, 586 (1993).6. D. Taverner, A. Galvanauskas, D. Harter, D. J. Richardson, L. Dong: Proc. CLEO ‘96,9, 496 (1996).

Generation, amplification and measurement of ultrashort laser pulses II,

158µJ pulses from a single transverse mode, large mode-area EDFA

158µJ pulses from a single transverse mode, large mode-area EDFA

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