Objective　High-pulse　energy　femtosecond　lasers　with　high　repetition　rates　are　required　in　numerous　areas　such　as　high-field　physics,　high-energy　terahertz　pulse　generation,　high-fluence　attosecond　pulse　generation,　and　material　processing.　Femtosecond　pulses　with　energies　above　millijoules　are　generally　achieved　using　solid-state　lasers.　Heat-related　problems　limit　solid-state　laser　operation　at　high　average　powers.　By　contrast,　fiber　lasers　are　attractive　owing　to　their　compactness,　flexibility,　and　low　cost.　However,　the　pulse　energy　of　the　fiber　lasers　is　limited　by　their　nonlinearity　and　optical　damage.　Coherent　stacking　of　numerous　femtosecond　pulses　is　a　promising　technique　for　generating　high　pulse　energy　at　a　high　repetition　rate.　Temporal　stacking　can　be　achieved　using　cavity　stacking,　delay-line　stacking,　or　a　hybrid　of　the　two.　Delay　line　stacking　uses　pulses　with　a　repetition　rate　of　80‒100　MHz,　corresponding　to　a　pulse　separation　of　12.5‒10.0　ns,　or　several　delay　lines　with　the　lengths　of　multiple　times　of　3.75‒3.00　m　for　stacking,　which　makes　the　system　bulky　and　unstable.　Methods　We　propose　and　demonstrate　that　with　a　1　GHz　repetition　rate　pulse　train,　the　delay　lines　can　be　significantly　shortened　so　that　more　pulses　can　be　stacked　in　a　compact　structure.　The　system　includes　a　1　GHz　repetition　rate　femtosecond　fiber　laser,　a　chirped　fiber　Bragg　grating　(CFBG)　as　a　stretcher,　four　fiber　amplifiers　in　series,　eight　delay　line　stages,　and　a　grating　compressor.　The　pulses　experience　intensity　modulation　that　chops　the　pulse　train　into　bursts　of　128　pulses　using　an　electro-optical　modulator　(EOM).　Then,　the　128　pulses　are　stretched　using　a　CFBG　and　split　into　two　branches.　Two　EOMs　are　inserted　into　each　branch　to　modulate　the　phases　of　individual　pulses.　The　phase　modulations　in　0　or　π　are　added　to　individual　pulses　such　that　after　each　stacking,　the　pulse　train　maintains　two　polarizations　with　neighbor　pulses　that　are　crossly　polarized.　In　the　stacking　stage,　the　first　delay　line　is　used　to　compensate　for　the　path　difference　in　the　amplification　stages.　Then,　the　remaining　seven　stages　of　the　delay　lines　are　used　to　compensate　for　the　time　delay　between　pulses.　The　stacking　strategy　used　is　pulse-train　folding.　The　next　stacking　stage　folds　the　128　pulses　in　the　burst　into　64　pulses,　and　the　folding　process　continues　until　only　one　pulse　remains.　A　set　of　control　FPGA　software　and　hardware　is　employed　to　maintain　the　stability　of　the　delay　lines.　Results　and　Discussions　The　final　amplification　delivers　a　10　kHz　burst　train　with　an　average　power　of　2　W.　The　energy　of　each　pulse　during　a　burst　is　780　nJ.　After　stacking　and　compression,　the　pulse　energy　of　the　stacked　256　pulses　becomes　100.5　µJ,　with　a　pulse　width　of　437　fs.　The　average　power　of　the　combined　pulse　train　is　stabilized　within　a　fluctuation　of　0.45%.　The　stacking　efficiency　is　56%.　The　low　stacking　efficiency　is　attributed　to　the　contrast　and　transmission　of　the　polarization　beam　splitters.　Conclusions　A　coherent　stacking　of　256　femtosecond　pulses　is　shown　from　a　burst　of　a　1　GHz　pulse　train.　Although　the　pulse　energy　is　not　high,　the　stacking　technique　has　the　potential　to　stack　a　few　hundred　mJ　femtosecond　pulses　and　is　expected　to　find　broad　applications.　©　2024　Science　Press.　All　rights　reserved.