1._CRD Introduction_NN1 
Laser-absorption_NN1 spectroscopy_NN1 (_PUL LAS_NP0 )_PUR is_VBZ widely_AV0 used_VVN to_TO0 provide_VVI non-invasive_AJ0 measurements_NN2 of_PRF temperature_NN1 ,_PUN chemical_AJ0 species_NN0 ,_PUN pressure_NN1 ,_PUN and_CJC velocity_NN1 ._SENT The_AT0 demand_NN1 for_PRP LAS_NP0 measurements_NN2 in_PRP high_AJ0 pressure_NN1 and_CJC highly_AV0 transient_AJ0 combustion_NN1 and_CJC propulsion_NN1 environments_NN2 (_PUL e.g._AV0 ,_PUN detonation_NN1 engines_NN2 ,_PUN rocket_NN1 motors_NN2 )_PUR continues_VVZ to_TO0 propel_VVI innovations_NN2 in_PRP LAS_NP0 diagnostics_NN1 [_PUL 1_CRD –_PUN 5_CRD ]_PUR and_CJC data-processing_AJ0 techniques_NN2 [_PUL 6_CRD –_PUN 8_CRD ]_PUR ._SENT Several_DT0 of_PRF these_DT0 innovations_NN2 have_VHB focused_VVN on_PRP increasing_VVG the_AT0 usable_AJ0 spectral_AJ0 bandwidth_NN1 of_PRF LAS_NP0 diagnostics_NN1 while_CJS maintaining_VVG or_CJC improving_VVG temporal_AJ0 resolution_NN1 ._SENT 
LAS_NP0 diagnostics_NN1 utilizing_VVG large_AJ0 spectral_AJ0 bandwidth_NN1 benefit_NN1 from_PRP the_AT0 greater_AJC diversity_NN1 of_PRF spectral_AJ0 information_NN1 collected_VVN ._SENT This_DT0 helps_VVZ mitigate_VVI some_DT0 challenges_NN2 associated_VVN with_PRP determining_VVG the_AT0 non-absorbing_AJ0 baseline_NN1 and_CJC reduced_AJ0 levels_NN2 of_PRF differential_AJ0 absorption_NN1 at_PRP elevated_AJ0 pressures_NN2 ,_PUN as_CJS well_AV0 as_CJS improve_VVI the_AT0 dynamic-range_NN1 of_PRF thermometry_NN1 ._SENT Broadband_AJ0 LAS_NP0 diagnostics_NN1 must_VM0 ,_PUN however_AV0 ,_PUN achieve_VVB fast_AJ0 temporal_AJ0 resolution_NN1 in_PRP order_NN1 to_TO0 characterize_VVI highly_AV0 transient_AJ0 processes_NN2 such_DT0 as_PRP chemical_AJ0 or_CJC internal_AJ0 non-equilibrium_NN1 and_CJC to_TO0 avoid_VVI time-varying_AJ0 ,_PUN non-absorbing_AJ0 transmission_NN1 losses_NN2 such_DT0 as_PRP those_DT0 from_PRP beam_NN1 steering_NN1 or_CJC particle_NN1 scattering_NN1 ._SENT To_PRP date_NN1 ,_PUN broadband_AJ0 LAS_NP0 diagnostics_NN1 with_PRP fast_AJ0 time_NN1 resolution_NN1 can_VM0 be_VBI grouped_VVN into_PRP two_CRD classes_NN2 :_PUN (_PUL 1_CRD )_PUR those_DT0 employing_VVG broadly_AV0 tunable_AJ0 CW_NP0 light_NN1 and_CJC (_PUL 2_CRD )_PUR those_DT0 employing_AJ0 ultrashort_NN1 pulses_NN2 ._SENT Regarding_PRP the_AT0 former_DT0 ,_PUN vertical_AJ0 cavity_NN1 surface-emitting_NN1 lasers_NN2 (_PUL VCSELs_NP0 )_PUR and_CJC rapidly_AV0 tunable_AJ0 external-cavity_NN1 quantum-cascade_NN1 lasers_NN2 (_PUL EC-QCLs_NP0 )_PUR have_VHB been_VBN utilized_VVN in_PRP the_AT0 near-IR_AJ0 and_CJC mid-IR_AJ0 ,_PUN respectively_AV0 ,_PUN to_TO0 provide_VVI broadband_AJ0 (_PUL 10s_CRD to_PRP 100s_CRD of_PRF cm−1_CRD )_PUR measurements_NN2 of_PRF atomic_AJ0 and_CJC molecular_AJ0 absorbance_NN1 spectra_NN2 in_PRP combustion_NN1 environments_NN2 ;_PUN however_AV0 ,_PUN such_DT0 light_AJ0 sources_NN2 are_VBB currently_AV0 available_AJ0 at_PRP only_AV0 a_AT0 few_DT0 central_AJ0 wavelengths_NN2 ._SENT As_CJS a_AT0 result_NN1 ,_PUN here_AV0 we_PNP will_VM0 focus_VVI on_PRP diagnostics_NN1 employing_VVG broadband_AJ0 pulses_NN2 within_PRP various_AJ0 architectures_NN2 ,_PUN namely_AV0 ,_PUN supercontinuum_NN1 lasers_NN2 (_PUL SCLs_NN2 )_PUR ,_PUN dual-frequency-comb_AJ0 spectrometers_NN2 (_PUL DFCS_NP0 )_PUR ,_PUN optical-parametric_AJ0 oscillators_NN2 (_PUL OPOs_NP0 )_PUR ,_PUN and_CJC optical-parametric_AJ0 amplifiers_NN2 (_PUL OPAs_NP0 )_PUR ._SENT 
Regarding_PRP SCLs_NP0 ,_PUN Sanders_NP0 utilized_VVD a_AT0 femtosecond-laser-pumped_AJ0 SCL_NP0 producing_VVG 1000_CRD cm−1_CRD of_PRF bandwidth_NN1 near_PRP 1.56_CRD 𝜇m_NN2 to_TO0 acquire_VVI temperature_NN1 and_CJC species_NN0 measurements_NN2 at_PRP 50_CRD kHz_NN0 in_PRP gases_NN2 at_PRP 10_CRD bar_NN1 ._SENT Similarly_AV0 ,_PUN Werblinski_NP0 et_NP0 al._NP0 used_VVD an_AT0 SCL_NP0 with_PRP 500_CRD cm−1_CRD of_PRF bandwidth_NN1 near_PRP 1.4_CRD 𝜇m_NN2 to_TO0 acquire_VVI 10_CRD kHz_NN0 measurements_NN2 of_PRF temperature_NN1 ,_PUN pressure_NN1 and_CJC H2O_NN1 in_PRP a_AT0 rapid_AJ0 compression_NN1 machine_NN1 (_PUL RCM_NP0 )_PUR at_PRP pressures_NN2 up_AVP to_PRP 65_CRD bar_NN1 ._SENT While_CJS the_AT0 large_AJ0 bandwidth_NN1 and_CJC repetition_NN1 rate_NN1 of_PRF such_DT0 diagnostics_NN1 are_VBB extremely_AV0 attractive_AJ0 ,_PUN SCL-based_AJ0 diagnostics_NN1 are_VBB often_AV0 limited_VVN by_PRP pronounced_AJ0 shot-to-shot_NN1 intensity_NN1 variations_NN2 (_PUL requiring_VVG multi-shot_AJ0 averaging_NN1 )_PUR and_CJC complexities_NN2 associated_VVN with_PRP accurately_AV0 modeling_VVG various_AJ0 instrument-broadening_AJ0 sources_NN2 ,_PUN particularly_AV0 when_CJS dispersing_VVG fibers_NN2 are_VBB used_VVN ._SENT More_AV0 recently_AV0 ,_PUN DFCS_NP0 has_VHZ emerged_VVN as_PRP an_AT0 effective_AJ0 tool_NN1 for_PRP providing_VVG high-fidelity_AJ0 ,_PUN broadband_AJ0 measurements_NN2 of_PRF absorbance_NN1 spectra_NN2 with_PRP unprecedented_AJ0 spectral_AJ0 resolution_NN1 ._SENT For_PRP example_NN1 ,_PUN Draper_NP0 et_NP0 al._NP0 utilized_VVD a_AT0 DFCS_NN2 with_PRP 160_CRD cm−1_CRD of_PRF bandwidth_NN1 near_PRP 1660_CRD nm_NN2 to_TO0 acquire_VVI 1.4_CRD kHz_NN0 measurements_NN2 of_PRF temperature_NN1 and_CJC CH4_CRD at_PRP up_AVP to_PRP 30_CRD bar_NN1 inside_NN1 of_PRF an_AT0 RCM_NN1 ._SENT However_AV0 ,_PUN DFCS_NN2 also_AV0 typically_AV0 requires_VVZ time-averaging_AJ0 to_TO0 achieve_VVI high_AJ0 signal-to-noise_AJ0 ratio_NN1 (_PUL SNR_NP0 )_PUR and_CJC such_DT0 diagnostics_NN1 in_PRP the_AT0 mid-IR_NN1 are_VBB still_AV0 emerging_AJ0 ._SENT 
Last_ORD ,_PUN the_AT0 recent_AJ0 advent_NN1 of_PRF high-speed_AJ0 ,_PUN cryogenically_AV0 cooled_VVN ,_PUN mid-IR_NN1 cameras_NN2 has_VHZ ,_PUN in_PRP part_NN1 ,_PUN enabled_VVD the_AT0 first_ORD single-shot_AJ0 ,_PUN ultrafast_NN1 (_PUL i.e._AV0 ,_PUN sub-nanosecond_NN0 )_PUR absorption_NN1 measurements_NN2 in_PRP the_AT0 mid-IR_NN1 ,_PUN as_CJS well_AV0 as_CJS broadband_AJ0 absorption_NN1 measurements_NN2 with_PRP near-microsecond_NN1 time_NN1 resolution_NN1 at_PRP 10s_CRD of_PRF kHz_NN0 ._SENT Regarding_PRP the_AT0 latter_DT0 ,_PUN Loparo_NP0 et_NP0 al._NP0 employed_VVD a_AT0 mid-IR_NP0 OPO_NP0 which_DTQ produced_VVD ultrashort_AJ0 pulses_NN2 at_PRP 100_CRD MHz_NN0 to_TO0 provide_VVI 476_CRD cm−1_CRD of_PRF bandwidth_NN1 near_PRP 3.25_CRD 𝜇m_NN2 ._SENT This_DT0 enabled_VVD the_AT0 authors_NN2 to_TO0 acquire_VVI measurements_NN2 of_PRF CH4_CRD ,_PUN C2H4_CRD and_CJC C2H6_CRD at_PRP 21.5_CRD kHz_NN0 in_PRP a_AT0 shock_NN1 tube_NN1 at_PRP pressures_NN2 near_PRP 2.5_CRD bar_NN1 ._SENT To_PRP date_NN1 ,_PUN this_DT0 technique_NN1 has_VHZ demonstrated_VVN a_AT0 measurement_NN1 time_NN1 resolution_NN1 of_PRF 37_CRD 𝜇s_NN2 (_PUL set_VVN by_PRP the_AT0 integration_NN1 time_NN1 of_PRF the_AT0 camera_NN1 )_PUR ._SENT 
Here_AV0 we_PNP build_VVB on_PRP our_DPS recent_AJ0 development_NN1 of_PRF ultrafast_NN1 laser-absorption_NN1 spectroscopy_NN1 (_PUL ULAS_NP0 )_PUR to_TO0 provide_VVI the_AT0 first_ORD single-shot_AJ0 ,_PUN ultrafast_NN1 ,_PUN mid-IR_NN1 measurements_NN2 of_PRF temperature_NN1 and_CJC species_NN0 in_PRP high-pressure_AJ0 and_CJC multi-phase_AJ0 combustion_NN1 gases_NN2 ._SENT Further_AV0 ,_PUN we_PNP demonstrate_VVB for_PRP the_AT0 first_ORD time_NN1 that_CJT ULAS_NN2 can_VM0 provide_VVI 1D_NN0 ,_PUN single-shot_AJ0 ,_PUN ultrafast_NN1 measurements_NN2 of_PRF temperature_NN1 and_CJC species_NN0 in_PRP a_AT0 flame_NN1 which_DTQ represents_VVZ a_AT0 significant_AJ0 advancement_NN1 beyond_PRP previously_AV0 developed_VVN broadband_AJ0 LAS_NP0 diagnostics_NN1 ._SENT These_DT0 new_AJ0 capabilities_NN2 were_VBD made_VVN possible_AJ0 through_PRP advancements_NN2 in_PRP the_AT0 optical_AJ0 setups_NN2 employed_VVN and_CJC several_DT0 notable_AJ0 refinements_NN2 to_PRP the_AT0 spectral-modeling_AJ0 and_CJC -fitting_AJ0 procedures_NN2 ._SENT This_DT0 manuscript_NN1 describes_VVZ these_DT0 advancements_NN2 and_CJC the_AT0 key_AJ0 principles_NN2 of_PRF ULAS_NN2 in_PRP detail_NN1 ._SENT Additionally_AV0 ,_PUN a_AT0 supplemental_AJ0 document_NN1 discusses_VVZ important_AJ0 practical_AJ0 considerations_NN2 for_PRP spectral_AJ0 interpolation_NN1 and_CJC imaging_NN1 ultrashort_NN1 pulses_NN2 in_PRP the_AT0 mid-IR_NN1 ._SENT 
2._CRD Experimental_AJ0 setup_NN1 
2.1_CRD Optical_AJ0 setup_NN1 
Figure_NN1 1_CRD shows_VVZ a_AT0 schematic_AJ0 of_PRF the_AT0 optical_AJ0 setup_NN1 used_VVN for_PRP this_DT0 work_NN1 ._SENT Special_AJ0 attention_NN1 is_VBZ given_VVN to_PRP the_AT0 new_AJ0 additions_NN2 and_CJC modifications_NN2 to_PRP the_AT0 basic_AJ0 ULAS_NP0 optical_AJ0 setup_NN1 (_PUL described_VVN by_PRP Tancin_NP0 et_NP0 al._NP0 )_PUR which_DTQ enabled_VVD high-pressure_AJ0 and_CJC 1D_NN0 measurements_NN2 ._SENT The_AT0 ultrafast-laser_NN1 system_NN1 (_PUL described_VVN in_PRP previous_AJ0 work_NN1 )_PUR consists_VVZ of_PRF a_AT0 Ti_NP0 :_PUN sapphire_NN1 oscillator_NN1 which_DTQ produces_VVZ 55_CRD femtosecond_NN1 pulses_NN2 centered_VVN near_PRP 800_CRD nm_NN2 at_PRP 80_CRD MHz_NN0 ._SENT The_AT0 pulses_NN2 were_VBD selectively_AV0 amplified_VVN at_PRP 5_CRD kHz_NN0 to_PRP a_AT0 pulse_NN1 energy_NN1 of_PRF 2_CRD mJ_NP0 and_CJC subsequently_AV0 converted_VVN into_PRP the_AT0 mid-IR_NN1 using_VVG optical_AJ0 parametric_AJ0 amplification_NN1 (_PUL OPA_NP0 )_PUR and_CJC non-collinear_AJ0 difference-frequency_NN1 generation_NN1 (_PUL NDFG_NP0 )_PUR to_TO0 achieve_VVI a_AT0 center_NN1 wavelength_NN1 near_PRP 4.9_CRD 𝜇m_NN2 with_PRP a_AT0 pulse_NN1 energy_NN1 of_PRF ≈_NP0 5_CRD 𝜇J_NP0 ._SENT 
After_PRP wavelength_NN1 conversion_NN1 ,_PUN a_AT0 MgF2_AJ0 Rochon-prism_NN1 polarizer_NN1 was_VBD used_VVN to_TO0 attenuate_VVI the_AT0 pulse_NN1 energy_NN1 to_PRP ≈_NP0 2_CRD 𝜇J_NP0 ._SENT Next_ORD ,_PUN a_AT0 spatial_AJ0 filter_NN1 (_PUL optics_NN1 #1_CRD ,_PUN #2_CRD ,_PUN and_CJC #3_NN1 in_PRP Fig._NN1 1_CRD )_PUR was_VBD used_VVN to_TO0 improve_VVI beam_NN1 quality_NN1 and_CJC reduce_VVB the_AT0 1_CRD /_PUN 𝑒2_CRD beam_NN1 diameter_NN1 to_PRP ≈_NP0 2_CRD mm_ITJ ._SENT The_AT0 improved_AJ0 beam_NN1 quality_NN1 provided_VVN by_PRP the_AT0 spatial_AJ0 filter_NN1 improved_VVD the_AT0 spectral_AJ0 resolution_NN1 and_CJC SNR_NP0 of_PRF the_AT0 diagnostic_AJ0 ._SENT The_AT0 latter_DT0 of_PRF which_DTQ ultimately_AV0 enabled_VVD 1D_NN0 measurements_NN2 through_PRP eliminating_VVG the_AT0 need_NN1 for_PRP spatial_AJ0 (_PUL i.e._AV0 ,_PUN intrapulse_AJ0 )_PUR averaging_NN1 ._SENT The_AT0 laser_NN1 beam_NN1 was_VBD then_AV0 directed_VVN through_PRP the_AT0 test_NN1 gas_NN1 within_PRP either_AV0 a_AT0 heated-gas_NN2 cell_NN1 or_CJC propellant_NN1 flame_NN1 ._SENT The_AT0 transmitted_AJ0 pulses_NN2 were_VBD then_AV0 focused_VVN onto_PRP the_AT0 input_NN1 slit_NN1 of_PRF a_AT0 spectrograph_NN1 (_PUL Andor_NP0 Shamrock_NN1 500i_CRD )_PUR using_VVG a_AT0 CaF2_NN1 ,_PUN plano-convex_NN1 ,_PUN cylindrical_AJ0 lens_NN1 with_PRP a_AT0 focal_AJ0 length_NN1 of_PRF 75_CRD mm_NN0 ._SENT The_AT0 spectrograph_NN1 employed_VVD a_AT0 reflective_AJ0 diffraction_NN1 grating_AJ0 to_TO0 disperse_VVI the_AT0 light_NN1 onto_PRP the_AT0 InSb_AJ0 FPA_NN1 of_PRF a_AT0 high-speed_AJ0 ,_PUN infrared_AJ0 camera_NN1 (_PUL Telops_NP0 FAST-IR_NP0 2K_NP0 )_PUR ._SENT The_AT0 IR_NN1 camera_NN1 can_VM0 acquire_VVI at_PRP nearly_AV0 2_CRD kHz_NN0 at_PRP full-frame_AJ0 resolution_NN1 (_PUL 320_CRD x_ZZ0 256_CRD )_PUR and_CJC faster_AJC when_CJS the_AT0 resolution_NN1 is_VBZ reduced_VVN ._SENT A_AT0 frame_NN1 rate_NN1 of_PRF 10_CRD kHz_NN0 can_VM0 be_VBI reached_VVN at_PRP 256_CRD x_ZZ0 52_CRD resolution_NN1 (_PUL what_DTQ was_VBD used_VVN here_AV0 )_PUR and_CJC a_AT0 maximum_NN1 of_PRF 90_CRD kHz_NN0 can_VM0 be_VBI achieved_VVN at_PRP 64_CRD x_ZZ0 4_CRD resolution_NN1 ._SENT In_PRP high-pressure_AJ0 propellant_NN1 tests_NN2 ,_PUN images_NN2 of_PRF single-shot_AJ0 spectra_NN2 were_VBD averaged_VVN across_PRP the_AT0 spatial_AJ0 dimension_NN1 of_PRF the_AT0 image_NN1 to_TO0 improve_VVI the_AT0 measurement_NN1 SNR_NP0 ._SENT A_AT0 high-resolution_AJ0 (_PUL 300_CRD lines_NN2 /_PUN mm_NN0 )_PUR diffraction_NN1 grating_NN1 providing_VVG 10_CRD cm−1_CRD of_PRF bandwidth_NN1 (_PUL near_PRP 5_CRD 𝜇m_NN2 )_PUR and_CJC 0.3_CRD nm_AJ0 resolution_NN1 was_VBD used_VVN for_PRP propellant_NN1 tests_NN2 at_PRP atmospheric_AJ0 pressure_NN1 ._SENT A_AT0 low_AJ0 resolution_NN1 (_PUL 150_CRD lines_NN2 /_PUN mm_NN0 )_PUR grating_AJ0 providing_VVG 35_CRD cm−1_CRD of_PRF bandwidth_NN1 and_CJC 0.6_CRD nm_AJ0 resolution_NN1 was_VBD used_VVN for_PRP high-pressure_AJ0 propellant_NN1 tests_NN2 ._SENT Both_DT0 gratings_NN2 were_VBD used_VVN at_PRP all_DT0 pressures_NN2 for_PRP gas-cell_AJ0 tests_NN2 ._SENT 
Corrective_AJ0 optics_NN1 were_VBD required_VVN in_PRP order_NN1 to_TO0 reduce_VVI the_AT0 loss_NN1 of_PRF signal_NN1 from_PRP severe_AJ0 beam_NN1 steering_NN1 encountered_VVN in_PRP high-pressure_AJ0 propellant_NN1 flames_NN2 ._SENT Figure_NN1 2_CRD illustrates_VVZ that_CJT without_PRP corrective_AJ0 optics_NN1 ,_PUN complete_AJ0 loss_NN1 of_PRF signal_NN1 can_VM0 occur_VVI from_PRP beam_NN1 steering-induced_AJ0 angular_AJ0 deflections_NN2 ._SENT These_DT0 additional_AJ0 optics_NN1 consisted_VVD of_PRF a_AT0 lens_NN1 telescope_NN1 (_PUL optics_NN1 #4_CRD and_CJC #5_CRD in_PRP Fig._NN1 1_CRD )_PUR with_PRP focal_AJ0 lengths_NN2 of_PRF 𝑓_NN1 =_NN1 200_CRD mm_NN0 and_CJC 𝑓_NN1 =_NN1 25.4_CRD mm_NN0 and_CJC a_AT0 final_AJ0 lens_NN1 (_PUL optic_NN1 #6_CRD )_PUR with_PRP a_AT0 focal_AJ0 length_NN1 of_PRF 250_CRD mm_NN0 ._SENT The_AT0 focal_AJ0 point_NN1 of_PRF the_AT0 lens_NN1 telescope_NN1 was_VBD centered_VVN at_PRP the_AT0 propellant_NN1 flame_NN1 and_CJC acted_VVD to_TO0 partially_AV0 correct_VVI any_DT0 angular_AJ0 deflections_NN2 caused_VVN by_PRP beam_NN1 steering_NN1 ,_PUN minimizing_VVG the_AT0 loss_NN1 of_PRF signal_NN1 ._SENT The_AT0 final_AJ0 lens_NN1 aided_VVN in_PRP boosting_VVG signal_NN1 counts_NN2 by_PRP reducing_VVG the_AT0 beam_NN1 size_NN1 incident_NN1 on_PRP the_AT0 focal_AJ0 plane_NN1 array_NN1 (_PUL FPA_NP0 )_PUR of_PRF the_AT0 IR_NN1 camera_NN1 ._SENT The_AT0 improvements_NN2 provided_VVN by_PRP the_AT0 corrective_AJ0 optics_NN1 are_VBB illustrated_VVN by_PRP Fig._NN1 2_CRD ,_PUN which_DTQ shows_VVZ the_AT0 average_AJ0 signal_NN1 counts_VVZ with_PRP and_CJC without_PRP corrective_AJ0 optics_NN1 for_PRP a_AT0 variety_NN1 of_PRF pressures_NN2 ._SENT 
For_PRP 1D_NN0 ,_PUN intrapulse_AJ0 measurements_NN2 ,_PUN more_DT0 significant_AJ0 changes_NN2 were_VBD required_VVN in_PRP order_NN1 to_TO0 improve_VVI the_AT0 spatial_AJ0 resolution_NN1 and_CJC measurement_NN1 field-of-view_NN1 ._SENT First_ORD ,_PUN the_AT0 recollimation_NN1 lens_NN1 used_VVN for_PRP the_AT0 spatial_AJ0 filter_NN1 (_PUL optic_NN1 #3_CRD in_PRP Fig._NN1 1_CRD )_PUR was_VBD replaced_VVN with_PRP a_AT0 150_CRD mm_NN0 focal_AJ0 length_NN1 lens_NN1 such_DT0 that_CJT the_AT0 beam_NN1 was_VBD expanded_VVN to_PRP over_AV0 1_CRD cm_NN0 in_PRP diameter_NN1 ._SENT Next_ORD ,_PUN optic_NN1 #4_CRD was_VBD replaced_VVN with_PRP a_AT0 plano-convex_NN1 ,_PUN CaF2_NP0 cylindrical_AJ0 lens_NN1 with_PRP a_AT0 200_CRD mm_NN0 focal_AJ0 length_NN1 to_TO0 focus_VVI the_AT0 laser_NN1 beam_NN1 into_PRP a_AT0 vertically_AV0 oriented_AJ0 sheet_NN1 on_PRP the_AT0 centerline_AJ0 axis_NN1 of_PRF the_AT0 propellant_NN1 flame_NN1 ._SENT The_AT0 beam_NN1 was_VBD then_AV0 recollimated_AJ0 using_VVG a_AT0 similar_AJ0 cylindrical_AJ0 lens_NN1 with_PRP a_AT0 75_CRD mm_NN0 focal_AJ0 length_NN1 (_PUL optic_NN1 #5_CRD )_PUR ._SENT Finally_AV0 ,_PUN a_AT0 plano-convex_NN1 ,_PUN CaF2_NP0 cylindrical_AJ0 lens_NN1 (_PUL optic_NN1 #_NN1 6_CRD )_PUR with_PRP a_AT0 200_CRD mm_NN0 focal_AJ0 length_NN1 (_PUL oriented_VVN to_TO0 focus_VVI the_AT0 beam_NN1 in_PRP the_AT0 vertical_AJ0 direction_NN1 )_PUR was_VBD used_VVN to_TO0 control_VVI the_AT0 field_NN1 of_PRF view_NN1 of_PRF the_AT0 imaging_AJ0 system_NN1 ._SENT 
2.2_CRD Gas-cell_AJ0 experiments_NN2 
ULAS_NP0 measurements_NN2 were_VBD acquired_VVN in_PRP a_AT0 high-uniformity_NN1 ,_PUN heated_AJ0 ,_PUN static-gas_AJ0 cell_NN1 (_PUL described_VVN by_PRP Schwarm_NP0 et_NP0 al._NP0 )_PUR to_TO0 validate_VVI the_AT0 accuracy_NN1 of_PRF the_AT0 diagnostic_AJ0 at_PRP high_AJ0 temperature_NN1 and_CJC pressure_NN1 ,_PUN as_CJS well_AV0 as_CJS to_TO0 determine_VVI the_AT0 instrument_NN1 response_NN1 function_NN1 (_PUL IRF_NP0 )_PUR via_PRP a_AT0 multi-spectrum_AJ0 fitting_NN1 routine_NN1 (_PUL MSFR_NP0 )_PUR discussed_VVN in_PRP detail_NN1 in_PRP Section_NN1 4.2_CRD ._SENT Spectra_NN2 were_VBD measured_VVN in_PRP a_AT0 mixture_NN1 of_PRF 2_CRD %_NN0 CO_NN1 and_CJC 1.8_CRD %_NN0 CO2_NN1 by_PRP mole_NN1 with_PRP a_AT0 balance_NN1 of_PRF nitrogen_NN1 at_PRP 1000_CRD ±_NN0 20_CRD K_ZZ0 and_CJC pressures_NN2 of_PRF 0.3_CRD ,_PUN 1_CRD ,_PUN 3_CRD ,_PUN 10_CRD ,_PUN 20_CRD ,_PUN and_CJC 40_CRD bar_NN1 ._SENT The_AT0 gas_NN1 pressure_NN1 was_VBD measured_VVN using_VVG a_AT0 UNIK-5000_NN0 pressure_NN1 transducer_NN1 with_PRP a_AT0 full-scale_AJ0 range_NN1 of_PRF 0_CRD to_PRP 70_CRD bar_NN1 and_CJC an_AT0 accuracy_NN1 of_PRF ±_NN0 0.028_CRD bar_NN1 ._SENT Temperature_NN1 was_VBD measured_VVN using_VVG three_CRD type-K_NN1 thermocouples_NN2 with_PRP a_AT0 nominal_AJ0 accuracy_NN1 of_PRF 0.75_CRD %_NN0 (_PUL at_PRP ≥_NN1 600_CRD K_ZZ0 )_PUR fixed_VVN to_PRP the_AT0 outside_NN1 of_PRF the_AT0 gas_NN1 cell_NN1 as_CJS in_PRP ._SENT The_AT0 gas_NN1 cell_NN1 was_VBD equipped_VVN with_PRP 15_CRD cm_NN0 long_AJ0 ,_PUN wedged_AJ0 ,_PUN CaF2_AJ0 rods_NN2 which_DTQ provided_VVD an_AT0 absorbing_AJ0 path_NN1 length_NN1 of_PRF 9.4_CRD cm_NN0 at_PRP room_NN1 temperature_NN1 ._SENT 
2.3_CRD Laser-ignited_AJ0 propellant_NN1 flames_NN2 
Cylindrical_AJ0 propellant_NN1 strands_NN2 with_PRP a_AT0 height_NN1 of_PRF 4_CRD to_PRP 6_CRD mm_NN0 and_CJC a_AT0 diameter_NN1 of_PRF 7_CRD mm_NN0 were_VBD tested_VVN ._SENT The_AT0 particle_NN1 size_NN1 of_PRF ammonium_NN1 perchlorate_NN1 (_PUL AP_NP0 )_PUR was_VBD distributed_VVN bimodally_AV0 with_PRP mean_AJ0 diameters_NN2 of_PRF 200_CRD 𝜇m_NN2 and_CJC 20_CRD 𝜇m_NN2 in_PRP a_AT0 coarse-to-fine_AJ0 ratio_NN1 of_PRF 4_CRD :_PUN 1._CRD In_PRP the_AT0 aluminized_AJ0 propellants_NN2 ,_PUN Valimet_NP0 H-30_CRD aluminum_NN1 with_PRP a_AT0 mean_AJ0 particle_NN1 diameter_NN1 of_PRF 31_CRD 𝜇m_NN2 was_VBD used_VVN ._SENT The_AT0 AP-HTPB_NP0 propellant_NN1 consisted_VVD of_PRF 80_CRD %_NN0 AP_NN1 by_PRP mass_NN1 ,_PUN whereas_CJS the_AT0 AP-HTPB-Al_AJ0 propellant_NN1 was_VBD 68_CRD %_NN0 AP_NP0 ,_PUN 15_CRD %_NN0 aluminum_NN1 and_CJC 17_CRD %_NN0 binder_NN1 by_PRP mass._NP0 
The_AT0 propellant_NN1 strands_NN2 were_VBD ignited_VVN using_VVG a_AT0 CO2_AJ0 laser_NN1 providing_VVG an_AT0 optical_AJ0 intensity_NN1 of_PRF 38_CRD W_ZZ0 /_PUN cm2_CRD to_PRP the_AT0 surface_NN1 of_PRF the_AT0 strand_NN1 ._SENT The_AT0 data_NN0 acquisition_NN1 system_NN1 was_VBD triggered_VVN 100_CRD ms_NN1 prior_AJ0 to_PRP the_AT0 CO2_AJ0 laser_NN1 turning_VVG on_AVP to_TO0 allow_VVI for_PRP the_AT0 baseline_NN1 laser_NN1 intensity_NN1 (_PUL 𝐼0_CRD )_PUR to_TO0 be_VBI recorded_VVN ._SENT The_AT0 CO2_AJ0 laser_NN1 was_VBD turned_VVN on_AVP for_PRP 200_CRD ms_NN1 (_PUL 400_CRD ms_NN1 at_PRP 1_CRD bar_NN1 )_PUR to_TO0 ignite_VVI the_AT0 strand_NN1 ._SENT Video_NN1 of_PRF each_DT0 test_NN1 was_VBD recorded_VVN with_PRP a_AT0 Nikon_NP0 D3200_NN0 DSLR_NN1 camera_NN1 to_TO0 ensure_VVI that_DT0 ignition_NN1 and_CJC burning_NN1 of_PRF each_DT0 strand_NN1 was_VBD uniform_AJ0 ,_PUN and_CJC to_TO0 provide_VVI an_AT0 estimate_NN1 of_PRF the_AT0 absorbing_AJ0 path-length_NN1 ._SENT The_AT0 propellant_NN1 flames_NN2 reached_VVD a_AT0 quasi-steady_AJ0 state_NN1 approximately_AV0 100_CRD ms_NN1 after_PRP ignition_NN1 ._SENT Figure_NN1 3_CRD shows_VVZ images_NN2 of_PRF the_AT0 propellant_NN1 flames_NN2 acquired_VVD using_VVG the_AT0 DSLR_NN1 camera_NN1 ._SENT 
All_DT0 propellant_NN1 tests_NN2 were_VBD conducted_VVN in_PRP a_AT0 high-pressure_AJ0 combustion_NN1 chamber_NN1 (_PUL HPCC_NP0 )_PUR ,_PUN described_VVN in_PRP detail_NN1 by_PRP Tancin_NP0 et_NP0 al_NP0 ._SENT ._SENT The_AT0 ULAS_NP0 beam_NN1 was_VBD aligned_VVN through_PRP the_AT0 centerline_NN1 of_PRF the_AT0 propellant_NN1 for_PRP all_DT0 tests_NN2 ._SENT Measurements_NN2 were_VBD acquired_VVN at_PRP a_AT0 height_NN1 of_PRF 2_CRD mm_NN0 and_CJC 1_CRD cm_NN0 relative_AJ0 to_PRP the_AT0 initial_AJ0 surface_NN1 location_NN1 of_PRF the_AT0 propellant_NN1 strand_NN1 for_PRP non-aluminized_AJ0 and_CJC aluminized_AJ0 propellants_NN2 ,_PUN respectively_AV0 ._SENT Prior_AJ0 to_PRP each_DT0 test_NN1 ,_PUN the_AT0 chamber_NN1 was_VBD evacuated_VVN using_VVG a_AT0 vacuum_NN1 roughing_VVG pump_NN1 before_PRP being_VBG pressurized_VVN with_PRP an_AT0 argon_NN1 bath_NN1 gas_NN1 ._SENT A_AT0 time_NN1 history_NN1 of_PRF pressure_NN1 was_VBD recorded_VVN during_PRP each_DT0 test_NN1 enabling_VVG the_AT0 data-processing_AJ0 routine_NN1 to_TO0 use_VVI an_AT0 updated_AJ0 pressure_NN1 for_PRP each_DT0 subsequent_AJ0 laser_NN1 shot_NN1 ._SENT The_AT0 gas_NN1 pressure_NN1 increased_VVN by_PRP ≈_NP0 0.6_CRD bar_NN1 during_PRP each_DT0 test_NN1 ._SENT For_PRP tests_NN2 at_PRP atmospheric_AJ0 pressure_NN1 ,_PUN the_AT0 HPCC_NP0 vent_NN1 valve_NN1 was_VBD partially_AV0 opened_VVN immediately_AV0 prior_AJ0 to_PRP ignition_NN1 ,_PUN such_DT0 that_CJT no_AT0 pressure_NN1 rise_NN1 occurred_VVN during_PRP the_AT0 test_NN1 ._SENT 
Proof-of-concept_AJ0 ,_PUN 1D-resolved_AJ0 ,_PUN single-shot_AJ0 temperature_NN1 and_CJC CO_NN1 measurements_NN2 were_VBD acquired_VVN in_PRP an_AT0 HMX_NN1 (_PUL i.e._AV0 ,_PUN 1_CRD ,_PUN 3_CRD ,_PUN 5_CRD ,_PUN 7-tetranitro-1_CRD ,_PUN 3_CRD ,_PUN 5_CRD ,_PUN 7-tetrazoctane_CRD )_PUR flame_NN1 ._SENT The_AT0 HMX_NN1 was_VBD pressed_VVN into_PRP cylindrical_AJ0 pellets_NN2 which_DTQ were_VBD 6_CRD mm_NN0 in_PRP diameter_NN1 and_CJC 4_CRD to_PRP 5_CRD mm_NN0 tall_AJ0 ._SENT For_PRP this_DT0 test_NN1 ,_PUN the_AT0 CO2_AJ0 laser_NN1 was_VBD left_VVN on_AVP for_PRP the_AT0 full_AJ0 duration_NN1 of_PRF the_AT0 experiment_NN1 ._SENT The_AT0 HMX_NP0 experiment_NN1 was_VBD conducted_VVN in_PRP air_NN1 ._SENT 
3._CRD High-fidelity_AJ0 modeling_NN1 and_CJC spectral-fitting_NN1 of_PRF ULAS_NP0 spectra_NN2 
This_DT0 section_NN1 provides_VVZ a_AT0 detailed_AJ0 description_NN1 of_PRF the_AT0 data-processing_AJ0 and_CJC spectroscopic_AJ0 modeling_NN1 procedures_NN2 used_VVN to_TO0 infer_VVI gas_NN1 properties_NN2 from_PRP ULAS_AJ0 measurements_NN2 of_PRF transmitted_AJ0 intensity_NN1 spectra_NN2 ._SENT In_PRP essence_NN1 ,_PUN a_AT0 nonlinear_AJ0 least-squares_NN2 fitting_VVG routine_NN1 is_VBZ used_VVN to_TO0 fit_VVI simulated_AJ0 absorbance_NN1 spectra_NN2 to_PRP measured_AJ0 absorbance_NN1 spectra_NN2 ._SENT There_EX0 are_VBB two_CRD workflows_NN2 :_PUN (_PUL 1_CRD )_PUR processing_VVG measured_AJ0 data_NN0 and_CJC (_PUL 2_CRD )_PUR simulating_VVG absorbance_NN1 spectra_NN2 which_DTQ are_VBB described_VVN below_AV0 and_CJC illustrated_VVN in_PRP Fig._NN1 4._CRD 
The_AT0 procedure_NN1 for_PRP processing_VVG measured_AJ0 spectra_NN2 consists_VVZ of_PRF five_CRD key_AJ0 steps_NN2 ._SENT 
